μ-η6,η6-arene-bridged diuranium hexakisketimide complexes isolable in two states of charge

Article abstract of DOI:10.1021/ic202163m

Diuranium μ-η⁶,η⁶-arene complexes supported by ketimide ligands were synthesized and characterized. Disodium or dipotassium salts of the formula M₂(μ-η⁶, η⁶-arene)[U(NC^tBuMes)₃]₂ (M = Na or K, Mes = 2,4,6-C₆H₂Me₃) and monopotassium salts of the formula K(μ-η⁶,η⁶-arene)[U(NC ^tBuMes)₃]₂ (arene = naphthalene, biphenyl, trans-stilbene, or p-terphenyl) were both observed. Two different salts of the monoanionic, toluene-bridged complexes are also described. Density functional theory calculations have been employed to illuminate the electronic structure of the μ-η⁶,η⁶-arene diuranium complexes and to facilitate the comparison with related transition-metal systems, in particular (μ-η⁶,η⁶-C₆H₆)[VCp] ₂. It was found that the μ-η⁶,η⁶- arene diuranium complexes were isolobal with (μ-η⁶, η⁶-C₆H₆)[VCp]₂ and that the principal arene-binding interaction was a pair of δ bonds (total of 4e) involving both metals and the arene lowest unoccupied molecular orbital. Reactivity studies have been carried out with the mono- and dianionic μ-η⁶,η⁶-arene diuranium complexes, revealing contrasting modes of redox chemistry as a function of the system's state of charge.

Full text of DOI:10.1021/ic202163m

Article
pubs.acs.org/IC
μ-η6,η6-Arene-Bridged Diuranium Hexakisketimide Complexes
Isolable in Two States of Charge
Paula L. Diaconescu*,† and Christopher C. Cummins*,‡
†Department of Chemistry & Biochemistry, University of California, Los Angeles, California 90095, United States
‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: Diuranium μ-η6,η6-arene complexes supported by ketimide ligands were synthesized and characterized. Disodium
or dipotassium salts of the formula M2(μ-η6,η6-arene)[U(NCtBuMes)3]2 (M = Na or K, Mes = 2,4,6-C6H2Me3) and
monopotassium salts of the formula K(μ-η6,η6-arene)[U(NCtBuMes)3]2 (arene = naphthalene, biphenyl, trans-stilbene, or
p-terphenyl) were both observed. Two different salts of the monoanionic, toluene-bridged complexes are also described. Density
functional theory calculations have been employed to illuminate the electronic structure of the μ-η6,η6-arene diuranium
complexes and to facilitate the comparison with related transition-metal systems, in particular (μ-η6,η6-C6H6)[VCp]2. It was
found that the μ-η6,η6-arene diuranium complexes were isolobal with (μ-η6,η6-C6H6)[VCp]2 and that the principal arene-binding
interaction was a pair of δ bonds (total of 4e) involving both metals and the arene lowest unoccupied molecular orbital.
Reactivity studies have been carried out with the mono- and dianionic μ-η6,η6-arene diuranium complexes, revealing contrasting
modes of redox chemistry as a function of the system’s state of charge.
arene ligand; the δ back-bonds have been visualized with the aid
of density functional theory (DFT) calculations, which
reproduce rather accurately the key structural parameters.12
While the U−arene bonding renders the assignment of a formal
oxidation state to the U centers in such systems problematic,13
the complexes serve as synthons for divalent uranium.14,15
The inverted sandwich motif has been known both in
transition-metal chemistry,16−21 as exemplified by the triple-
decker (μ-η6,η6-C6H6)[CpV]2 with an average C−C distance of
1.443(5) Å,16 and in lanthanide chemistry, as exemplified by
INTRODUCTION
■
Metal complexes involving the π system of benzene and related
aromatic hydrocarbons have been of long-standing interest
because of the fundamental nature of such species.1 The
situation is particularly interesting when the metal, as is the case
for uranium, possesses both d and f valence orbitals capable of
covalent interactions.2 The first crystallographically character-
ized uranium benzene complex, U(η2-AlCl4)3(η6-C6H6), was
reported in 1971 by Cesari et al.,3 who observed an average
U−C distance of 2.91 0.01 Å and an average C−C distance
of 1.39 Å for the planar η6-bound benzene ring. The latter
distance compares well with the 1.397(1) Å distance for free
benzene.4,5 In 1987, Cotton and Schwotzer described the
analogous hexamethylbenzene complex, reporting an average
U−C distance of 2.93(2) Å;6 other arene complexes were also
reported.7,8 A related finding was a uranium(III) homoleptic
alkoxide complex obtained as a dimer by virtue of η6-arene
binding,9 with the observed U−C distances essentially identical
with those reported for Cesari’s benzene complex. Further-
more, actinide bis(arene) sandwich complexes including U(η6-
C6H6)2 have been studied by computational methods.10,11
In 2000, we reported a new structural motif for arene
coordination to uranium, namely, the inverted sandwich formed
between a bridging aromatic hydrocarbon and two uranium
centers via μ-η6,η6 interactions.12 The first such complexes were
neutral species of the formula (μ-η6,η6-arene)[U(N[R]Ar)2]2
Lappert’s anion [(Cptt La)2(μ-η6,η6-C6H6)]− (Cptt = η5-
2
1,3-tBu2C5H3).22,23 Lappert and co-workers formulated the
latter system as an ionic aggregate containing two LaII ions and
−
̇
the benzene radical anion C6H6 on the basis of the observed
benzene C−C distances that average 1.44(1) Å.22,23 In related
work involving anthracene as the bridging ligand, a μ-η3,η3-
bonding mode was observed for (Cp*2La)2(μ-C14H10) rather
than μ-η6,η6, similarly consistent with the ionic inclusion of a
reduced aromatic hydrocarbon amidst lanthanide cations.24
Before our work with uranium, covalent bonding was not
invoked to explain sandwiching of a benzene derivative by any
of the f elements.
The closest precedent for our arene-bridged diuranium
compounds is that of Ephritikhine and co-workers, who
reported a μ-η7,η7-cycloheptatrienyldiuranium sandwich com-
plex, the anionic component of [U(BH4)2(OC4H8)5][(μ-η7,η7-
C7H7)[U(BH4)3]2].25,26 Although a detailed description of the
electronic structure was not included, it was suggested that the
μ-η7,η7-C7H7 ring should be described as an aromatic trianion
and the metals as UIV ions. We believe that this suggestion is
consistent with the notion that the four electrons in the doubly
t
[R = Bu or 1-adamantyl (Ad); arene = benzene or toluene;
Ar = 3,5-C6H3Me2].12 The molecular structure of (μ-η6,η6-
toluene)[U(N[Ad]Ar)2]2 revealed both substantially shorter
U−C distances [average 2.594(9) Å] and longer C−C
distances [average 1.438(13) Å] than those previously observed
for any uranium complex of a benzene derivative. This large
structural perturbation has been explained in terms of covalent
4e, δ back-bonding from the two U centers to the bridging
Received: October 5, 2011
Published: February 16, 2012
© 2012 American Chemical Society
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3−
degenerate C7H7
highest occupied molecular orbital
Chart 1. Dianions Prepared as Disodium or Dipotassium
Salts by Reduction of 1-I(DME) in the Presence of
Stoichiometric Amounts of Arene
(HOMO; δ symmetry with respect to the U−U internuclear
axis) engage in covalent δ bonding with the two U centers; such
a description is equivalent to that advanced by us for the
isolobal (μ-η6,η6-arene)[U(N[R]Ar)2]2 complexes.12
a
Although bridging benzene or toluene uranium complexes
have been reported recently,27−32 analogous uranium com-
plexes of other aromatic hydrocarbons had not been forth-
coming. In order to extend the class of μ-η6,η6-arene diuranium
complexes, a new ancillary ligand, the ketimide NCtBuMes
(Mes = 2,4,6-C6H2Me3) was investigated and led to the report
of a reactive dianionic μ-η6,η6-naphthalene complex K2(μ-η6,η6-
C10H8)[U(NCtBuMes)3]2 (K2[μ-naph-12]).33 This system
uniquely incorporates a planar naphthalene ligand with both
U atoms interacting in a symmetrical η6 fashion to the same ring
of the naphthalene bridge. This structural motif is different
from that known for lanthanides, which coordinate to opposites
sides of the two aromatic rings.1,34−49 Herein we report that the
ketimide ligand allows the synthesis of other μ-η6,η6-arene
complexes as well; the new system contrasts with the amide
system’s neutral status by generating mono- and dianionic
complexes. A synthetic protocol was developed that does
not require the use of the arene as the solvent, allow-
ing expansion of this class to biphenyl, trans-stilbene, and
p-terphenyl derivatives. Finally, computational methods have
been used to probe the electronic structure of these
complexes; in addition, they allowed us to recognize the
isolobal relationship between diuranium inverted sandwiches
and prototypical triple-decker transition-metal sandwich
complexes.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Dianionic μ-η6,η6-
Arene Diuranium Complexes. As was reported previ-
ously, the iodide trisketimide uranium(IV) starting material
UI(NCtBuMes)3(DME) (1-I(DME), DME = 1,2-dimethoxy-
ethane)33 reacts smoothly with 4 equiv of KC8 and 0.5 equiv of
naphthalene in DME, leading to the naphthalene-bridged
complex K2[μ-naph-12] in 60% yield as a dark-brown powder.33
The μ-η6,η6 formulation for the C10H8 bridge was established
by X-ray crystallography.33 The dianion of K2[μ-naph-12] is
related to the neutral systems (μ-η6,η6-arene)[U(N[R]Ar)2]2
because it is comprised formally of two f4 [U(NCtBuMes)3]−
fragments conjoined by a neutral arene bridge. The structure of
K2[μ-naph-12] is unusual because known d-block dimetal
complexes with a bridging naphthalene ligand prefer coordina-
tion of the two metal centers to opposite ends of the aromatic
bridge. Systems of the latter type are referred to as “slipped
triple-decker” complexes.50,51
a
Isolated yields of crystalline substances: K2[μ-naph-12], 60%; K2[μ-
biph-12], 56%; Na2[μ-naph-12], 57%; K2[μ-stilb-12], 63%; K2[μ-terph-
12], 67%.
stabilize the dianionic inverted sandwiches. The disodium salts
have proven to be highly lipophilic, hampering their recry-
stallization from n-pentane and leading to relatively low isolated
yields.
Compounds [Na2(OEt2)][μ-biph-12] and K2[μ-stilb-12]
were structurally characterized by single-crystal X-ray diffrac-
tion (Figures 1 and 2). This study allows the comparison of
C−C distances in a sandwiched versus a pendant phenyl ring.
Table 1 shows distances of ca. 1.44 Å for the complexed ring12
and values close to that of unperturbed benzene [1.397(1) Å]
for the pendant ring.4,5 In the case of [Na2(OEt2)][μ-biph-12],
the tight and symmetrical μ-η6,η6-biphenyl complexation is
revealed clearly by the short interatomic distances [average
2.627(8) Å] between U1/U2 and C71−C76. Furthermore, the
C−C distances in the sandwiched ring are similar [average
1.442(10) Å] and longer than those in the pendant biphenyl
ring [average 1.387(16) Å]. Crystalline biphenyl is planar (D2h)
at 110 K, with aromatic C−C distances varying between
1.379(3) and 1.399(3) Å [average 1.390(3) Å].52−54 On the
A key result of the present work is the extension of the
synthetic strategy used to prepare K2[μ-naph-12] to other
aromatic hydrocarbons (Chart 1). Thus, the treatment of
1-I(DME) with 4 equiv of KC8 in the presence of stoichiometric
amounts of the desired arene is a general synthetic route for
obtaining dipotassium salts of the dianions [(μ-η6,η6-arene)[U-
(NCtBuMes)3]2]2− (arene = biphenyl, K2[μ-biph-12]; trans-
stilbene, K2[μ-stilb-12]; p-terphenyl, K2[μ-terph-12]). The
pentane solubility of these systems qualitatively increases in
the following order: naphthalene < biphenyl < trans-stilbene ≈
p-terphenyl (i.e., K2[μ-naph-12] is slightly soluble in n-pentane,
while K2[μ-terph-12] dissolves readily in it). The corresponding
disodium salts could also be obtained by using sodium as the
reductant, indicating that K ion incorporation is not required to
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Table 1. Selected Interatomic Distances and Angles for
Structures of New μ-η6,η6-Arene Diuranium Complexes
[Na2(OEt2)]
K2
[K2I]
[K(DME)]
[μ-biph-12] [μ-stilb-12] [μ-tol-12] [μ-tol-12]
C−C avg. for μ-η6,η6n 1.442(10)
1.439(14) 1.45(2)
1.365(22)
1.405(15)
arene
C−C avg. for pendant 1.387(14)
C6H5
U1−C avg.
U2−C avg.
U1−N avg., terminal
U1−N avg., μ-Na or K 2.331(6)
U2−N avg., terminal 2.233(6)
U2−N avg., μ-Na or K 2.319(6)
2.621(7)
2.633(7)
2.240(6)
2.670(9)
2.646(9)
2.208(7)
2.256(8)
2.222(8)
2.274(8)
98.5(3)
2.663(13) 2.594(10)
2.632(13) 2.641(10)
2.209(10) 2.239(7)
2.237(12) 2.245(8)
2.215(10) 2.209(8)
2.236(12)
NUN avg.
96.4(2)
99.1(4)
99.5(3)
UNC avg., terminal
162.1(5)
164.8(7)
169.0(8)
171.1(10) 174.3(8)
171.0(12) 164.1(8)
UNC avg., μ-Na or K 157.4(6)
Figure 1. Structural drawing of [Na2(OEt2)][μ-biph-12] with thermal
ellipsoids at the 50% probability level; all H atoms have been omitted
for clarity. See Table 1 for selected interatomic distances.
The same situation is found for N4 and N5, which, being
proximal to Na2, exhibit distances to U2 longer by ca. 0.1 Å
than the U2−N6 distance. Both Na1 and Na2 can be viewed as
approximately five-coordinate. In the case of Na1, in
addition to N2 and N3, there are close contacts to aryl
rings. In the case of Na2, the fifth interaction is to the diethyl
ether O1s. The interaction between Na1 and C81 (pendant
phenyl ipso-C) accounts for the gentle curvature of the
complexed biphenyl ligand. The electrostatic solvation of
alkali-metal cations by aromatic residues is a well-
documented phenomenon.56−59
The structural picture of K2[μ-stilb-12] (Figure 2 and
Table 1) is similar to that of [Na2(OEt2)][μ-biph-12],
presenting the following key features: (1) elongation of
C−C distances in the complexed ring relative to the pendant
phenyl; (2) inclusion of roughly five-coordinate K cations,
which each interact with two ketimide N atoms, two mesityl
rings, and the stilbene double bond; (3) U centers experiencing
a three-legged piano-stool coordination environment.
Synthesis and Characterization of Monoanionic μ-η6,η6-
Arene Diuranium Complexes. The optimum synthesis
of the dianions described above was found through trial and
error to consist of treatment of the soluble, molecular
uranium(IV) starting material 1-I(DME) with an excess of
potassium graphite (KC8, 4 equiv per U) along with a
stoichiometric amount of the arene to be complexed, in DME
solvent (Chart 1). Because of the production of the diuranium
inverted sandwiches from 1-I(DME) requires two electrons per
U center, 2 equiv of KC8 per U was used in an effort to
optimize the procedure. Those conditions resulted in the
formation of new, isolable, inverted sandwiches having only a
single negative charge, namely, the series of K(μ-η6,η6-
arene)[U(NCtBuMes)3]2 (K[μ-arene-12], Chart 2). The
formation of these complexes starting from 1-I(DME) is a
process requiring three electrons total; as such, K[μ-arene-12]
represent intriguing examples of mixed-valent diuranium
systems with short interuranium distances (see below).
Mixed-valent diuranium complexes are not common,60−63 and
the question of possible valence delocalization in such systems
represents a problem of considerable fundamental interest.
Burns and co-workers discussed this subject with reference to a
Figure 2. Structural drawing of complex K2[μ-stilb-12] with thermal
ellipsoids at the 50% probability level; H atoms have been omitted for
clarity. See Table 1 for selected interatomic distances.
other hand, biphenyl adopts a twisted structure (D2) in
solution, with a central torsion angle of 32 2°; the two phenyl
rings are only weakly conjugated favoring the planar structure,
while ortho H···H repulsions induce a twist.55 The biphenyl
ligand of [Na2(OEt2)][μ-biph-12] displays a minimal distortion
from planarity, as indicated by the values of the two torsion
angles C72−C71−C81−C82 and C76−C71−C81−C86 (−1.5
and 3.2°, respectively). The C71−C81 distance between the
two phenyl rings is 1.455(10) Å, as compared to 1.496(3) Å for
free biphenyl.54 The contraction of the inter-ring junction is
known for various biphenyl excited states as well as for the
biphenyl radical anion.55 In summary, the major structural
change experienced by biphenyl upon the formation of
[Na2(OEt2)][μ-biph-12] is expansion of the C−C distances
by 0.05 Å specifically in the ring undergoing η6,η6 complex-
ation.
Other [Na2(OEt2)][μ-biph-12] structural parameters of
interest include the U−N distances, of which two are long
and one is short for each U atom: the U1−N2 and U1−N3
distances are ca. 0.1 Å longer than U1−N1, likely because N2
and N3 are involved in a side-on interaction with the Na cation.
UV/VI system having a crystallographic center of symmetry
2
relating the two U nuclei and bridging imido ligands potentially
capable of mediating electronic delocalization.60
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Chart 2. Monoanions [μ-arene-12]− Prepared as Potassium
Salts by Reduction of 1-I(DME) with 2 equiv of KC8 in the
a
Presence of Arene
Figure 3. Structural drawing of complex [K(DME)][μ-tol-12] with
thermal ellipsoids at the 50% probability level. Only one of the two
crystallographically distinct but chemically equivalent molecules is
shown; H atoms have been omitted for clarity. See Table 1 for selected
interatomic distances.
a
Isolated yields of crystalline substances: K[μ-naph-12], 56%; K[μ-
biph-12], 42%; K[μ-stilb-12], 83%; K[μ-terph-12], 69%; [K(DME)][μ-
tol-12], 41%; [K2I][μ-tol-12], 44%; [K(DME)][μ-benz-12], 11%.
Interestingly, while unavailable in the series of K2[μ-arene-12],
inverted sandwiches with the bridging ring as benzene or
toluene could be obtained in the series of K[μ-arene-12]
(Chart 2). We speculate that highly conjugated arenes such
as trans-stilbene or naphthalene support the extra charge in
K2[μ-arene-12] by virtue of delocalization; this point merits
further study by computational means. In addition, toluene is
generally less oxidizing than the other arenes employed;64
stated alternatively, toluene is the least δ-acidic of the arenes
studied herein.
The toluene complexes were isolated with two different
formulations: [K(DME)][μ-tol-12] and [K2I][μ-tol-12], both
crystallographically characterized (Figures 3 and 4). In both
structures (Table 1), the toluene ligand bridges the two U
atoms in a μ-η6,η6 fashion, with all U−C distances close to 2.6
Å, as was the case for K2[μ-arene-12]. While the included K2I
cation provides a coordination environment identical for both
U atoms, the included K(DME) cation places the K ion close
to only one of the two U centers (U1 in Figure 3). Structural
parameters support this dichotomy: in [K(DME)][μ-tol-12],
the terminal U1−N (side that complexes K) distances are
longer by ca. 0.03 Å than the terminal U2−N distances. In
addition, the U1−C bonds are shorter than the U2−C bonds
Figure 4. Structural drawing of complex [K2I][μ-tol-12] with thermal
ellipsoids at the 50% probability level; H atoms have been omitted for
clarity. See Table 1 for selected interatomic distances.
by 0.05 Å. In the case of [K2I][μ-tol-12], C and N distances to
both U centers are essentially the same. Comparison of the two
structures leads to the notion that both the nature of the cation
and the mode of its inclusion may significantly influence the
degree of charge delocalization.
NMR Spectroscopic Characterization of Inverted
Sandwich Uranium Complexes. The chemical shifts for
the protons/deuterons of the bridging ring of μ-η6,η6-arene
diuranium complexes are diagnostic. For the parent system
(μ-η6,η6-C6H6)[U(N[R]Ar)2]2, the single signal for the bridg-
1
2
ing benzene ligand was found at ca. −80 ppm by H or H
NMR spectroscopy.12 For (μ-η6,η6-toluene)[U(N[R]Ar)2]2,
the bridging toluene methyl resonance was found near
+20 ppm, while the ring protons were characterized by three
resonances in the −60 to −80 ppm range.12 These data
generate an expectation that the μ-η6,η6 rings associate with
strong upfield chemical shifts for protons or deuterons directly
attached to the bridging ring.
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As was reported previously,33 disodium or dipotassium salts
of [μ-naph-12]2− evince four signals for the bridging
naphthalene ligand, showing that the U centers do not move
back and forth between the two fused six-membered rings on
the 1H and 2H NMR spectroscopy time scales. The same is also
true for the naphthalene-bridged monoanion [μ-naph-12]−. Of
further importance for [μ-naph-12]2− is the observation of a
single ketimide ligand environment, implying mobility of the asso-
ciated alkali-metal ions. The situation is less clear for [μ-naph-12]−,
ring junction. This means that the two U centers are statically
affixed to just one of the two phenyl rings of the bridging
biphenyl in solution at room temperature, consistent with the
solid-state structures discussed above. Only a single ancillary
ketimide ligand environment, identified by four peaks in the 1H
NMR spectrum of K2[μ-biph-12], is observed. Analogous to
1
K[μ-naph-12], the H NMR spectrum of K[μ-biph-12] is more
complicated than that observed for the corresponding dianion.
In the case of the bridging p-terphenyl systems K2[μ-terph-
12] and K[μ-terph-12], the central phenylene ring or the termi-
nal phenyl residues are, in principle, subject to diuranium
1
in that more H NMR resonances are found than the four
expected for a single ketimide ligand environment; unambig-
uous assignment of the ancillary ketimide ligand resonances for
monoanions [μ-arene-12]− is generally complicated by the
presence of broad and overlapping signals. Chemical shift
assignments for the bridging naphthalene in both [μ-naph-12]2−
and [μ-naph-12]− were facilitated by deuterium labeling and 2H
NMR studies (Chart 3).
2
complexation. Investigation of the H NMR spectrum (77
MHz, Et2O, 22 °C) of K2[μ-terph-12]-d14, prepared from
p-terphenyl-d14, revealed eight peaks at +22, +8, +6, +4, +3, −18,
−50, and −124 ppm. The observation of eight peaks is con-
sistent with terminal and static μ-η6,η6 complexation of a phenyl
residue, as pictured in Charts 1 and 2. If complexation were to
take place at the central phenylene portion of p-terphenyl, then
2
only four peaks would be observed. Similarly, the H NMR
Chart 3. Salts K2[μ-naph-12]-dn and K[μ-naph-12]-dn
spectrum (77 MHz, Et2O, 22 °C) of K[μ-terph-12]-d14
prepared from perdeuterio-p-terphenyl displayed exactly eight
peaks at +19, +12, +8, +7, −6, −88, −146, and −162 ppm.
These results are consistent with a structure in which the U
centers are bound to the terminal ring for both K2[μ-terph-12]
and K[μ-terph-12]. In the case of K2[μ-terph-12], a single
Prepared from the Corresponding Naphthalene Isotopomers
To Facilitate Chemical Shift Assignments
a
1
ketimide ligand environment (four signals) is observed by H
NMR spectroscopy, consistent with alkali-metal cation mobility
1
on the NMR time scale; the H NMR spectrum of K[μ-terph-
12] is not well resolved.
Interestingly, although the monoanionic [μ-arene-12]− salts
1
displayed complicated and not well resolved H NMR spectra,
the salts [K2I][μ-tol-12] and [K(DME)][μ-tol-12] display
simpler H NMR spectra, with the expected number of signals
a
2
1
Observed H NMR signals (ppm) are as indicated (20 °C). Signals
marked with an asterisk were relatively broad and were assigned to the
(four) for a single ketimide ligand environment in solution at
six-membered ring undergoing μ-η6,η6 complexation.
1
room temperature. To aid in assignment of the H NMR
spectrum of [K2I][μ-tol-12], the salt [K2I][μ-benz-12], with a μ-
η6,η6-benzene ligand, was synthesized and its 1H NMR
spectrum (500 MHz, C6D6, 22 °C) recorded. On the basis of
the chemical shift of −45 ppm for the bridging benzene protons
in [K2I][μ-benz-12], along with the relative intensities of peaks
(500 MHz, toluene-d8, 20 °C), the chemical shifts at +35, −38,
−43, and −45 ppm were assigned, respectively, to the protons
of the μ-η6,η6-toluene ligand in [K2I][μ-tol-12] as follows:
PhCH3, o- or m-PhCH3, p-PhCH3, and o- or m-PhCH3. The
1H NMR spectra of [K2I][μ-tol-12] and [K(DME)][μ-tol-12]
show largely different chemical shifts for the bridging toluene
ligand ([K(DME)][μ-tol-12]: 65, CH3-toluene; −109, o- or m-
toluene; −113, o- or m-toluene; −126, p-toluene). Although the
characterization of the corresponding benzene complexes is in
agreement with these chemical shifts, an explanation for this
difference is not apparent.
Isotopomers of the biphenyl-bridged anions K2[μ-biph-12]
and K[μ-biph-12] were also prepared for NMR spectroscopic
investigation, as indicated in Chart 4. The observation in both
Chart 4. K2[μ-biph-12]-dn and K[μ-biph-12]-dn Prepared
from the Corresponding Biphenyl Isotopomers To Facilitate
a
Chemical Shift Assignments
Redox Interconversion between Mono- and Dianionic
Inverted Sandwiches. As shown in Scheme 1, it was possible
to effect the chemical one-electron oxidation of dianions
[μ-arene-12]2− to the corresponding monoanions [μ-arene-12]−.
The oxidants [Cp2Fe][O3SCF3] and P4 were found competent
to carry out the indicated reactions on a preparative scale, with
isolated yields of K[μ-naph-12] being ca. 70%. The original
intent for the P4 reaction was to produce a uranium phosphide
complex; presumably, P4 was instead converted to a reduced
form such as (K2P4)n.65 The chemical reduction of [μ-naph-12]−
could be used preparatively to provide [μ-naph-12]2− in yields
averaging 80%; potassium anthracenide was employed as the
a
2
Observed H NMR signals (ppm) are as indicated (20 °C). Signals
marked with an asterisk were relatively broad and were assigned to the
phenyl ring undergoing μ-η6,η6 complexation.
2
cases of six H NMR signals associated with the bridging
biphenyl for the anions prepared from biphenyl-d10 reveals C2v
symmetry, with the C2 axis being coincident with the biphenyl
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Scheme 1. Chemical Interconversion between Mono- and
a
Dianionic Diuranium Inverted Sandwiches
Figure 5. Structural drawing of complex 2 with thermal ellipsoids at
the 50% probability level; H atoms were omitted for clarity. Selected
distances (Å): U−N (avg.), 2.171(13); U−C (avg.), 2.715(30); C−C
(avg.), 1.38(6).
a
In the example shown above, K[μ-stilb-12] was reduced to K2[μ-stilb-
12] with potassium anthracenide in 78% isolated yield. Alternatively,
K2[μ-stilb-12] was oxidized to K[μ-stilb-12] with either ferrocenium
triflate in 81% isolated yield or white phosphorus in 69% yield.
Interconversions involving the other K2[μ-arene-12]/K[μ-arene-12]
redox pairs proceeded similarly.
In addition to the reactions with 1,3,5,7-cyclooctatetraene,
it was found that Na2[μ-naph-12] reacted with diphenyl dis-
ulfide (2 equiv) to form a dark-yellow product, which was
isolated in 60% yield (Scheme 2). The product was deter-
mined to be a dinuclear trithiolate-bridged uranium(IV) deri-
vative, Na[(μ-SPh)3U2(NCtBuMes)6] (Na[3], 3 = [(μ-SPh)3U2-
(NCtBuMes)6]) by X-ray crystallography (Figure 6). Although
the redox reaction involved the transfer of four electrons, as
expected, it was found that the structure of the product is
organized such that one Na ion is retained.
reductant. Notably, in the reduction reactions, no anthracene
incorporation was observed. The reactions illustrated in
Scheme 1 show that the chemical interconversion between
the two states of the diuranium μ-η6,η6-arene inverted sandwich
complexes can be executed successfully.
Other Reactions of Arene-Bridged Diuranium Hex-
akisketimide Anions. As was reported previously, the
treatment of M2[μ-naph-12] (M = Na or K) with 2 equiv of
1,3,5,7-cyclooctatetraene afforded a mixture of two products:
M[(COT)U(NCtBuMes)3] (M[2], 2 = [(COT)U(NCtBuMes)3])
and (μ-η8,η8-COT)U2(NCtBuMes)6 (μ-COT-12).33 The latter
compound is remarkable in possessing a COT ligand
sandwiched between two trisketimidouranium fragments.66
The reactions of all dipotassium salts K2[μ-arene-12] with
1,3,5,7-cyclooctatetraene provided K[2]. As was mentioned
previously, Na2[μ-naph-12] forms Na[2] and μ-COT-12 in a
1:1 ratio.33 At the other extreme, Na2[μ-biph-12] forms only
μ-COT-12. It is important to note that while all of the COT
reactions discussed take place within 2 h, the latter example
requires 20 h for completion. Monitoring a diethyl ether
solution of pure K[2] at room temperature, no formation of
μ-COT-12 was observed even after 23 h. In the case of the odd-
electron species, a mixture of K[2] and μ-COT-12 was always
formed in different ratios. Therefore, K[2] can be synthesized
from any even-electron species, Na[2] from Na2[μ-naph-12],
and μ-COT-12 from the reaction of 1-I(DME) with M[2]
(M = Na or K).33
Rather surprising is the fact that the odd-electron, arene-
bridged species K2[μ-arene-12] form K[3] in similar reactions.
In the case of K[μ-arene-12] reactions, 1.5 instead of 2 equiv of
PhSSPh is used and these reactions involve the transfer of three
electrons. It is probable that, in the reactions of M2[μ-arene-12]
(M = Na or K), the other alkali-metal cation is eliminated as
MSPh, although the identity of the byproducts was not
established.
The reaction with azobenzene proceeded cleanly to a single
major product in the case of the bridging toluene compounds
[K(DME)][μ-tol-12] and [K2I][μ-tol-12]. Even so, the reaction
is not straightforward because the isolated product is the
diuranium(V) complex (μ-NPh)2U2(NCtBuMes)6 (4; Figure 7),
and five electrons are transferred during this reaction.
K[PhNNPh], a known compound,69 is proposed to be the
byproduct, although its identity was not confirmed. This
proposal is in accordance with the fact that the reaction with 2
equiv of PhNNPh is cleaner than the reaction with only 1 equiv
of azobenzene.
Other K[μ-arene-12] compounds also give 4 as a major
product upon azobenzene treatment, but the reactions are
not as clean as those of the toluene-bridged complexes. The
reactions of K2[μ-arene-12] with PhNNPh have also been
surveyed, but they are not as straightforward as those for
[K(DME)][μ-tol-12] and [K2I][μ-tol-12], even when the
stoichiometry was adjusted to 3 equiv of PhNNPh.
Compounds M[2] can be oxidized to 2 (Figure 5) by
reaction with TiCl4(THF)2 (used initially to test the formation
of heterobimetallic COT complexes). As expected for a
formally uranium(V) compound, the average U−C distance is
shorter [2.715(30) Å] than that in μ-COT-12 [2.822(15) Å].
Similar UVCOT complexes have been reported; for example,
(COT)U(NR2)3 (R = Me or Et) was also obtained by
oxidation of the corresponding anion.67,68
General Bonding Considerations for μ-η6,η6-Arene-
Bridged Diuranium Complexes. The bonding model
proposed by us for the neutral amide model complexes
(μ-η6,η6-C6H6)[U(NH2)2]2 (Figure 8) considers them to be
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Scheme 2. Preparation of M[3] (M = Na or K) from Either
M2[μ-naph-12] or K[μ-naph-12], with Loss of Naphthalene
Figure 7. Structural drawing of complex 4 with thermal ellipsoids at
the 35% probability level; H atoms have been omitted for clarity.
respect to the axis of interaction with benzene). Similar
bonding considerations should apply to the dianionic ketimide
model systems {(μ-η6,η6-C6H6)[U(NCH2)3]2}2− (Figure 8).
On the basis of this bonding scheme, the correspond-
ing monoanionic ketimide complexes [μ-arene-12]− would be
3
S = /2 systems, with only three U-centered electrons. The
monoanions [μ-arene-12]− would also feature a pair of δ back-
bonds that characterize arene binding.
An intuitive way to assess the importance of δ back-bonding
in early actinide chemistry is by its analogy to π back-bonding
in transition-metal chemistry (Figure 9). The parallel between
π back-bonding in transition-metal complexes and δ back-
bonding in early actinide compounds is based on the fact that
each bond involves half of the total lobes in a specific orbital
(two out of four in the case of d orbitals and four out of eight in
the case of f orbitals) when interacting with LUMOs of the
ligand in question.
Bonding Considerations for Neutral μ-η6,η6-Arene-
Bridged Diuranium Complexes. The benzene-bridged
system (μ-η6,η6-C6H6)[U(NH2)2]2 has been adopted as a
computational model with reference to the crystallographically
determined structure of (μ-η6,η6-toluene)[U(N[Ad]Ar)2]2.
Amsterdam Density Functional (ADF)70−72 calculations were
carried out, specifying a spin polarization equal to four (four
unpaired electrons). Pictures of the key molecular orbitals
(MOs) for (μ-η6,η6-C6H6)[U(NH2)2]2 are shown in Figure 10.
A first noteworthy point is the excellent energy match between
the LUMOs of uncomplexed benzene and the manifold of
uranium-based orbitals (see Figure SX9 in the Supporting
Information for details). Together with good overlap, this
energy match leads to a strong stabilization of the four δ-
bonding electrons found in MOs formed by U f orbitals and the
LUMOs of benzene (Figure 10). The four unpaired electrons
are located at ca. −2.5 eV, immediately below a dense manifold
of virtual orbitals. There is a large energy difference of ca. 4 eV
between the benzene HOMO and the uranium valence
manifold such that electron donation from the benzene
HOMO to uraniuman interaction of π symmetry with
reference to the U−U axisaccounts for little of the bonding.
Weak interactions involving the benzene HOMO can be
discerned (PI and PI STAR in Figure SX9 in the Supporting
Information).
Figure 6. Structural drawing of complex Na[3] with thermal ellipsoids
at the 50% probability level; H atoms were omitted for clarity.
S = 2 systems, with the four unpaired electrons occupying
uranium-based orbitals. These orbitals are followed by one pair
of degenerate δ bonds formed by the overlap of benzene’s
lowest unoccupied molecular orbital (LUMO; π*, of δ
symmetry with respect to the axis of interaction with uranium)
and 5f orbitals of appropriate symmetry (of δ symmetry with
The model (μ-η6,η6-C6H6)[U(NH2)2]2 reproduces well the
observed structure of (μ-η6,η6-toluene)[U(N[Ad]Ar)2]2 (see
the Supporting Information for details). Because covalent
overlap of filled U orbitals with the benzene LUMOs
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Figure 8. Schematic representation of frontier MOs for (μ-η6,η6-C6H6)-bridged diuranium complexes emphasizing the similarity between the neutral
amide model systems (μ-η6,η6-C6H6)[U(NH2)2]2 (left) and the dianionic ketimide model systems {(μ-η6,η6-C6H6)[U(NCH2)3]2}2− (right); orbital
energies not drawn to scale.
Figure 9. Comparison between π back-bonding in transition-metal chemistry and δ back-bonding in early actinide chemistry.
constitutes the principal interaction, it is natural that the
C−C distances should be long relative to free benzene. The
magnitude of the calculated elongation is ca. 0.07 Å, while the
observed elongation is ca. 0.05 Å. Reproduced rather well are
the short U−C distances of ca. 2.6 Å. Another interesting result
from the DFT calculations is association of the spin density
with the atoms present: essentially, all of the spin density is
U-localized (Figure 8); very little spin density is associated with
the C atoms of the bridging benzene. Thus, (μ-η6,η6-
toluene)[U(N[Ad]Ar)2]2 cannot be construed as an ionically
sandwiched benzene radical anion; although DFT calculations
tend to overestimate the importance of covalency in organo-
metallic complexes,2,73,74 our results show that the mechanism
of charge transfer from U to C is one of covalent overlap, i.e., δ
back-bonding.
Bonding in Dianionic μ-η6,η6-Arene Diuranium Hex-
akisketimide Complexes. [Na(OH2)3]2(μ-η6,η6-C6H6)[U-
(NCH2)3]2 was chosen as a computational model for M2(μ-
η6,η6-arene)[U(NCtBuMes)3]2. This model reproduces the key
structural elements of the dianions [μ-arene-12]2−, including
the ion-pairing interactions characterized by contacts between
two ketimide N atoms per alkali-metal cation, and nominal five-
coordination at each alkali metal (see the Supporting
Information for details).
It should be mentioned that the ketimide ligand class,
represented generically by NC(R1)(R2), is expected to
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Figure 10. Selected orbitals for model system (μ-η6,η6-C6H6)[U(NH2)2]2. Top row from left to right: the F NONBONDING orbitals 32 B3 α, 34 B2
α, 31 B1 α, and 36 A α. Second row: the DELTA orbitals 33 B2 β, 35 A β, 35 A α, and 33 B2 α. Third row: the PI STAR orbitals 29 B1 β, 30 B3 β, 29
B1 α, and 30 B3 α. Bottom row: the PI orbitals 28 B1 β, 29 B3 β, 28 B1 α, and 29 B3 α. Note that each depicted orbital corresponds to a single
electron (spin-unrestricted formalism).
exhibit π-donor character in the trigonal plane defined by C and
its N, R1, and R2 substituents; in the plane perpendicular to this,
these ligands are expected to have π-acid character by virtue of
the NC π* orbital. Thus, this ligand type is well suited to the
stabilization of a negatively charged system. Accordingly, it was
interesting to find that the unpaired spin density associated
with [Na(OH2)3]2(μ-η6,η6-C6H6)[U(NCH2)3]2 (S = 2) was
not localized solely on the U centers but rather was delocalized
onto the ketimide C atoms by the mechanism of π back-
bonding (top row of MOs in Figure 11). These four MOs
correspond to the four unpaired electrons in the previous
systems, and because of the delocalization, their energies
(Figure SX13) are depressed such that they separate out clearly
below the dense manifold of low-lying virtual orbitals. The
multipole-derived (MD) spin density76 is essentially negligible
on all atoms except for the U centers (ca. −1.1 each) and
the ketimide ligand C atoms (ca. −0.5 each). For comparison,
the MD spin density76 per uranium in the neutral model system
(μ-η6,η6-C6H6)[U(NH2)2]2 was −1.8. Thus, the ketimide
ligands, while being strong π donors in one plane, also assist
in the stabilization of negatively charged systems by virtue of
their π-acid character.
these orbitals graphically, omitting those corresponding to
ketimide π donation.
The bonding model proposed by us for dianions [μ-arene-
12]2− considers them to be S = 2 systems, with the four
unpaired electrons resident in uranium-based orbitals that are
stabilized by π back-bonding to the peripheral ketimide ligands
(Figure 11). Accordingly, we expect the corresponding
3
monoanions [μ-arene-12]− to be S = /2 systems, with only
three such U-centered π back-bonding electrons. The
monoanions [μ-arene-12]− would retain a full complement of
four δ-bonding electrons for arene binding. Using the same
computational methods as those discussed above, unfortu-
nately, we have not been able to achieve convergence for a
model monoanion, e.g., salt [Na(OH2)3](μ-η6,η6-C6H6)[U-
(NCH2)3]2; therefore, detailed computational elucidation of
the electronic structure of monoanions [μ-arene-12]− will be
deferred to future work.
Isolobal Relationship with Vanadium Triple-Decker
Systems. Electronic structure calculations on (μ-η6,η6-C6H6)-
[VCp]2 were carried out by Chesky and Hall in 1984.75 They
pointed out that this system’s MOs are easily identified with
those of the cylindrical D∞h point group and may be labeled
accordingly as σ, π, or δ; furthermore, they remarked on the
strong stabilization of a doubly degenerate δ pair of CpV linear
combinations (containing four electrons) by the benzene
LUMOs, making δ back-bonding the primary interac-
tion between benzene and the two CpV fragments. As for
(μ-η6,η6-toluene)[U(N[Ad]Ar)2]2, the ground state for
(μ-η6,η6-C6H6)[VCp]2 was suggested to have S = 2.75
In the energy-level diagram for [Na(OH2)3]2(μ-η6,η6-C6H6)-
[U(NCH2)3]2 (Figure SX13 in the Supporting Information),
four manifolds of orbitals are separated clearly from each other:
metal-centered stabilized by π back-bonding to the ketimide
ligands (four high-lying electrons, denoted as F-NONBOND-
ING in Figure SX13 in the Supporting Information), covalent δ
overlap between benzene LUMO and the two U centers (next
highest-lying four electrons, denoted as DELTA), ketimide
π-donor functions (12 electrons denoted as KETIMIDE LP),
and the benzene HOMO that does not overlap covalently with
the U centers (four electrons denoted as PI). Figure 11 displays
In order to compare (μ-η6,η6-C6H6)[U(NH2)2]2 and
(μ-η6,η6-C6H6)[VCp]2, we performed new calculations on
(μ-η6,η6-C6H6)[VCp]2 using the same methods as those
employed for (μ-η6,η6-C6H6)[U(NH2)2]2. The calculated C−C
distances for the sandwiched benzene ring are ca. 1.45 Å, in
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Figure 11. Selected MOs for [Na(OH2)3]2(μ-η6,η6-C6H6)[U(NCH2)3]2 in order of decreasing energy from top left to bottom right. Top row: four
electrons of spin α that have no spin β counterpart; note that these are not completely metal-localized but engage in back-bonding to the ketimide
ligands. Middle row: four δ-bonding electrons in orbitals comprised of covalent uranium/benzene LUMO overlap. Bottom row: four electrons
derived from the benzene HOMOs in orbitals that do not show U−C covalent overlap.
Figure 12. Selected MOs as calculated for (μ-η6,η6-C6H6)[VCp]2 in C2h symmetry with the spin-unrestricted formalism and four spin-α electrons in
excess (each pictured orbital represents a single electron). Top row from left to right: 14 Bg α, 27 Ag α, 26 Bu α, and 26 Ag α. The top row
corresponds to the molecule’s four unpaired electrons. Middle row: 14 Au β, 25 Bu β, 14 Au α, and 25 Bu α. The middle row corresponds to the two δ
back-bonding orbitals constructed from the LUMOs of the bridging benzene and filled V d orbitals of appropriate symmetry. Bottom row: 24 Ag β,
12 Bg β, 24 Ag α, and 12 Bg α. The bottom row corresponds to the interaction of the benzene HOMOs with V-acceptor orbitals of π symmetry.
good agreement with those determined by X-ray crystallog-
raphy (see the Supporting Information for details).16 As found
for (μ-η6,η6-C6H6)[U(NH2)2]2, the frontier MOs contain the
four electrons involved in δ bonding and also the four electrons
of spin α that have no spin β counterpart (Figure 12). In
addition, the HOMO of free benzene, at lower than −6 eV, is a
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purchased from EM Science. Diethyl ether, toluene, benzene, n-pentane,
and n-hexane were dried and deoxygenated by the method of Grubbs
et al.77 THF and DME were distilled under nitrogen from purple
sodium benzophenone ketyl. Distilled solvents were transferred under
vacuum into glass vessels before being pumped into a Vacuum
Atmospheres drybox. C6D6 and toluene-d8 were purchased from
Cambridge Isotopes and were degassed and dried over 4 Å sieves.
Naphthalene-d8, biphenyl-d10, and p-terphenyl-d14 were purchased
from Cambridge Isotopes. Naphthalene, α-naphthalene-d1,78 naph-
thalene-d8, biphenyl, biphenyl-d10, o-biphenyl-d1,78 p-biphenyl-d1,78
p-terphenyl, and p-terphenyl-d14 were dissolved in THF, and their
solutions passed through alumina, concentrated, and placed in a −35 °C
refrigerator. The crystals obtained were dried extensively under
vacuum (at least 3 h) before use. 1,3,5,7-Cyclooctatetraene was passed
through alumina and stored in a refrigerator at −35 °C. Sieves (4 Å),
alumina, and Celite were dried in vacuo overnight at a temperature just
above 200 °C. Compounds KC8,79 1-I(DME),33 M2[μ-naph-12],33
M[(COT)U(NC[tBu]Mes)3] (M = Na or K),33 and (μ-η8,η8-
poor match energetically for the metal-based orbitals. Overall,
the four unpaired electrons occupy σg, σu, and δg orbitals, strong
covalent δu bonding (4e) is observed, and the four electrons
derived from the πg benzene HOMO enjoy little stabilization
by vanadium. In short, two d4 CpV fragments interact with a
μ-η6,η6-C6H6 ligand in a fashion isolobal with two f4 (H2N)2U
fragments. Both systems can be construed as enjoying a
maximum (4e) of δ back-bonding to the bridging ring and have
an S = 2 ground state, with little spin density on the bridging
ring.
CONCLUSIONS
■
The ketimide ligand NCtBuMes has facilitated the isolation of
several new arene-bridged diuranium complexes, all featuring
three ketimide ligands per U center and inclusion of alkali-
metal counterions. Reduction of the iodide trisketimide
complex 1-I(DME) with 4 equiv of KC8 in DME in the
presence of stoichiometric amounts of arene was found to be a
general route to a series of dipotassium salts, K2(μ-η6,η6-
arene)[U(NCtBuMes)3]2 (arene = naphthalene, biphenyl,
trans-stilbene, and p-terphenyl). Crystallographic character-
ization for arene = biphenyl and trans-stilbene revealed in both
cases a characteristic elongation of ca. 0.05 Å of arene C−C
distances in the bridging ring, in accordance with findings from
our DFT calculations. The magnitude of the C−C elongation
upon complexation is similar to that reported for the transition-
metal analogue (μ-η6,η6-benzene)[VCp]2, with which the
uranium complexes were found to be isolobal.
33
COT)U2(NC[tBu]Mes)6 were prepared according to literature
1
2
methods. Other chemicals were used as received. H and H NMR
spectra were recorded on Varian XL-300 or Varian INOVA-501
spectrometers at room temperature unless otherwise specified.
Chemical shifts are reported with respect to an internal or external
solvent, 7.16 ppm (C6D6). UV−vis spectra were recorded on a HP
spectrophotometer from 200 to 1100 nm using matched 1 cm quartz
cells; all spectra were obtained using a solvent reference blank.
Numerical modeling of all data was done using the program Origin 6.0.
CHN analyses were performed by H. Kolbe Mikroanalytisches
Laboratorium (Mulheim an der Ruhr, Germany).
̈
General Synthesis of Salts K2[μ-arene-12] with That of K2[μ-biph-
12] Given as a Representative Example. Compound 1-I(DME)
(0.532 g, 0.50 mmol, 2 equiv) and biphenyl (0.039 g, 0.25 mmol, 1
equiv) were dissolved in DME (15 mL), and the solution was frozen
by placing the vessel containing it into the glovebox cold well.
Separately, KC8 (0.271 g, 2.00 mmol, 8 equiv) was slurried in DME
(10 mL), and the slurry was also frozen. Both mixtures were removed
from the cold well and allowed to thaw. The thawing KC8 slurry was
added dropwise to the thawing biphenyl/1-I(DME) solution. After
complete addition, the reaction mixture was stirred at ca. 25 °C for
25 min and then filtered through Celite. After solvent removal from the
filtrate, the dark-brown solid was extracted with pentane (ca. 100 mL),
the extract was filtered through Celite, and the filtrate was taken to
dryness. The crude residue thus obtained was mixed with diethyl ether
(10 mL), whereupon the mixture was transferred to a vial and stored
in a −35 °C refrigerator for several days. Decantation of the mother
liquor and drying under reduced pressure gave a first crop of solid
K2[μ-biph-12] amounting to 0.223 g (0.23 mmol, 46% yield). A
second crop (ca. 0.050 g) was subsequently obtained, giving a total
yield of 56% (0.28 mmol). 1H NMR (500 MHz, toluene-d8, 20 °C): δ
34.04 (s, ν1/2 = 22 Hz, 1H, bound p-biphenyl), 8.30 (s, 12H, m-Ar).
3.74 (s, 18H, p-Me), 3.03 (s, 36H, o-Me), −3.09 (s, 54H, tBu), −5.78
(s, ν1/2 = 6 Hz, 1H, pendant p-biphenyl), −21.40 (s, ν1/2 = 17 Hz, 2H,
pendant o-biphenyl), −47.13 (s, ν1/2 = 48 Hz, 2H, pendant m-
biphenyl), −73.53 (s, very broad, 2H, bound m-biphenyl), −126.56 (s,
ν1/2 = 64 Hz, 2H, bound o-biphenyl). UV−vis (Et2O, 22 °C): λmax, nm
(ε × 10−2, M−1 cm−1) = 212 (2555.7 110.1), 244 (808.6 38.5),
This study also revealed that the μ-η6,η6-arene diuranium
hexakisketimide complexes are available in a higher oxidation
state. Specifically, salts of monoanions [(μ-η6,η6-arene)[U-
(NCtBuMes)3]2]− were synthesized for arenes such as
naphthalene, biphenyl, trans-stilbene, p-terphenyl, toluene,
and benzene. In cases where a 1:1 correspondence exists, i.e.,
with the exception of the benzene and toluene complexes, the
chemical interconversion of the mono- and dianionic forms was
successful.
Reactivity patterns established for the μ-η6,η6-arene diura-
nium hexakisketimide complexes show that they are susceptible
to oxidation by a variety of reagents, leading most frequently to
uranium(IV) and occasionally to uranium(V) products.
Reactions with diphenyldisulfide and azobenzene provided
new phenylthiolate- and phenylimido-bridged systems, respec-
tively, while reactions with cyclooctatetraene provided a new
entry into U(COT) half-sandwich chemistry. In all cases
studied thus far, redox reactions of μ-η6,η6-arene diuranium
hexakisketimide complexes give rise to arene expulsion,
consistent with the notion that the arene is held in place by
a pair of electron-rich U centers engaged in electron donation
to the arene LUMO. The observed reactivity suggests that the
dianionic μ-η6,η6-arene diuranium hexakisketimide complexes
can be construed as uranium(II) synthons.
282 (476.4
24.8), 395 (152.2
8.6). Anal. Calcd for
Overall, the μ-η6,η6-arene diuranium hexakisketimide motif
represents, by virtue of variability in the choice of both the
arene and state of charge, an example of a highly tunable
reactive organometallic entity.
C96H130N6K2U2: C, 59.98; H, 6.82; N, 4.37. Found: C, 60.06; H,
6.48; N, 4.11.
K2[μ-biph-12-d10]. This was synthesized as described for K2[μ-biph-
2
12], substituting biphenyl-d10 for biphenyl. H NMR (46 MHz, Et2O,
22 °C): δ 33.98 (s, 1D), −5.72 (s, 1D), −21.45 (s, 2D), −46.85 (s,
2D), −79.75 (s, 2D), −126.24 (s, 2D).
EXPERIMENTAL SECTION
K2[μ-biph-12-2-d1]. This was synthesized as described for K2[μ-
■
2
biph-12], substituting biphenyl-2-d1 for biphenyl. H NMR (46 MHz,
General Considerations. Unless stated otherwise, all operations
were performed in a Vacuum Atmospheres drybox under an
atmosphere of purified nitrogen or using Schlenk techniques under
an argon atmosphere. Anhydrous diethyl ether was purchased from
Mallinckrodt; n-pentane, n-hexane, and tetrahydrofuran (THF) were
Et2O, 22 °C): δ −21.05 (s, 1D), −125.43 (s, 1D).
K2[μ-biph-12-4-d1]. This was synthesized as described for K2[μ-
2
biph-12], substituting biphenyl-4-d1 for biphenyl. H NMR (46 MHz,
Et2O, 22 °C): δ 34.31 (s, 1D), −5.53 (s, 1D).
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Na2[μ-biph-12]. Sodium (0.030 g, 1.30 mmol, 3.5 equiv) was added
to a THF solution (6 mL) of biphenyl (0.172 g, 1.12 mmol, 3 equiv)
in a vial protected from light (with electrical tape), and the resulting
mixture was stirred at room temperature for 1 h, after which time the
vial was placed in the glovebox cold well. Separately, a solution of
1-I(DME) (0.400 g, 0.38 mmol) in THF (10 mL) was frozen. The
mixtures were then removed from the cold well and allowed to thaw.
The thawing sodium/biphenyl mixture was added dropwise to the
solution of 1-I(DME), including the piece of sodium. The reaction
mixture was allowed to reach room temperature and was stirred for
2 h, after which time the solution was decanted from the sodium piece
and filtered. After removal from the filtrate of all volatile materials
under reduced pressure, the crude solid residue thus obtained was
extracted with pentane. The extract was filtered through Celite, and
pentane was evaporated from the filtrate. Pentane extraction/filtration
was repeated, leading to a new filtrate (5 mL), which was placed in a
−35 °C refrigerator overnight. The mother liquor was then decanted
from the solid obtained (biphenyl with traces of Na2[μ-biph-12]) and
concentrated. The solid thus obtained was extracted with pentane
(3 mL) followed by diethyl ether, but the two filtrates were not combined
at this stage. Diethyl ether was removed from the filtered ether extract,
and the solid obtained was extracted with pentane. At this point, the
pentane extracts were combined, the solution was concentrated, and
the concentrate was placed in a −35 °C refrigerator. Salt Na2[μ-biph-
12] was obtained as a microcrystalline powder in 57% yield (0.204 g,
0.11 mmol) after 1 day. 1H NMR (300 MHz, C6D6, 22 °C): δ 9.92 (s,
1H, bound p-biphenyl), 8.05 (s, 12H, m-Ar), 3.48 (s, 18H, p-Me), 2.50
Both frozen mixtures were removed from the cold well and permitted
to thaw. The thawing KC8 slurry was added dropwise to the thawing
solution of 1-I(DME) and naphthalene, with vigorous magnetic
stirring. After complete addition, the reaction mixture was stirred for
20 min and then filtered through Celite. Solvent removal from the
filtrate provided a dark-brown solid, which was extracted with pentane
(ca. 100 mL). The pentane extract was filtered through Celite. Solvent
removal from the filtrate provided a solid that was next slurried in a 2:1
mixture of pentane and diethyl ether (total of 10 mL), transferred to a
vial, and stored in a −35 °C refrigerator for 1 day. The mother liquor
was decanted, and the microcrystalline solid thus obtained was dried
under reduced pressure, giving a first crop of K[μ-naph-12] (0.251 g,
1
0.13 mmol, 56% yield). H NMR (300 MHz, C6D6, 22 °C): δ 20.22
(s, 6H, m′-Ar), 9.34 (s, 6H, p′-Me), 8.93 (s, 8H, m-Ar), 4.49 (s, 12H,
t
o′- or p-Me), 3.65 (s, 12H, o′- or p-Me), 1.29 (s, 36H, Bu), −0.14
t
(s, 18H, Bu′), −1.19 (s, 2H, pendant α-naphthalene), −2.19 (s, 24H,
o-Me), −37.96 (s, 2H, pendant β-naphthalene), −121.17 (s, 2H,
bound α-naphthalene), −124.53 (s, 2H, bound β-naphthalene). UV−
vis (toluene, 22 °C): λmax, nm (ε × 10−2, M−1 cm−1) = 282 (376.7
28.4), 388 (134.9
12.1), 632 (47.5
7.4). Anal. Calcd
for C94H128N6KU2: C, 60.79; H, 6.95; N, 4.53. Found: C, 60.70; H,
6.86; N, 4.70.
K[μ-naph-12-d8]. The same preparation as that for K[μ-naph-12]
substituting naphthalene-d8 for naphthalene. 2H NMR (46 MHz,
Et2O, 22 °C): δ −0.46 (s, 1D), −38.97 (s, 1D), −117.73 (s, 1D),
−126.53 (s, 1D).
K[μ-naph-12-d1]. The same preparation as that for K[μ-naph-12]
2
t
substituting α-naphthalene-d1 for naphthalene. H NMR (46 MHz,
(s, 36H, o-Me), −3.31 (s, 54H, Bu), −1.23 (s, 1H, pendant p-
Et2O, 22 °C): δ −0.42 (s, 1D), −117.49 (s, 1D).
biphenyl), −14.97 (s, 2H, pendant o-biphenyl), −55.88 (s, 2H,
pendant m-biphenyl), −76.03 (s, 2H, bound m-biphenyl), −109.10 (s,
2H, bound o-biphenyl). Anal. Calcd for C96H130N6U2Na2: C, 61.00; H,
6.93; N, 4.45. Found: C, 60.90; H, 7.56; N, 4.34.
K[μ-biph-12]. The same preparation as that for K[μ-naph-12]
substituting biphenyl for naphthalene; yield 42% for the first crop
1
(collected after 3 days, from pentane/Et2O). H NMR (300 MHz,
C6D6, 22 °C): δ 16.86 (s, 6H, pendant m-biphenyl and m′-Ar), 8.96 (s,
K2[μ-stilb-12]. The same procedure as that described above for
K2[μ-biph-12], substituting trans-stilbene for biphenyl; the yield was
63% for the first crop of solid material, collected from n-pentane after
8H, m-Ar), 7.95 (s, 6H, p′-Me), 4.50 (s, 12H, o′- or p-Me), 3.91 (s,
t
t
12H, o′- or p-Me), 1.02 (s, 36H, Bu), 0.02 (s, 18H, Bu′), −2.17 (s,
24H, o-Me), −9.19 (s, 2H, pendant o-biphenyl), −14.18 (s, 1H,
pendant p-biphenyl), −83.57 (s, 1H, bound p-biphenyl), −140.72 (s,
2H, bound m-biphenyl), −163.87 (s, 2H, bound o-biphenyl). UV−vis
(toluene, 22 °C): λmax, nm (ε × 10−2, M−1 cm−1) = 285 (354.8
1
10 days. H NMR (500 MHz, C6D6, 22 °C): δ 71.48 (br s, trans-
stilbene), 8.76 (s, 12H, m-Ar), 8.58 (s, 18H, p-Me), 4.12 (s, 36H, o-
t
Me), −3.04 (s, 54H, Bu), −3.81 (br s, trans-stilbene), −24.25 (br s,
trans-stilbene), −33.30 (br s, trans-stilbene), −35.04 (br s, trans-
stilbene), −48.58 (br s, trans-stilbene), −144.18 (br s, trans-stilbene),
−153.84 (br s, trans-stilbene). UV−vis (Et2O, 22 °C): λmax, nm (ε ×
10−2, M−1 cm−1) = 210 (2554.0 305.6), 280 (648.4 99.1), 290
(350.7 14.8), 353 (153.0 8.5), 610 (93.0 10.0). Anal. Calcd for
C98H132N6K2U2: C, 60.41; H, 6.83; N, 4.31. Found: C, 60.21; H, 6.56;
N, 4.30.
23.3), 431 (188.4
17.9), 647 (45.2
4.1). Anal. Calcd for
C96H130N6KU2: C, 61.22; H, 6.96; N, 4.46. Found: C, 60.74; H, 6.48;
N, 3.94.
K[μ-biph-12-d10]. The same procedure as that for K[μ-naph-12]
2
substituting biphenyl-d10 for naphthalene. H NMR (46 MHz, Et2O,
22 °C): δ 17.02 (s, 2D), −9.14 (s, 2D), −13.87 (s, 1D), −83.62 (s,
1D), −141.20 (s, 2D), −164.92 (s, 2D).
K2[μ-terph-12]. The same procedure as that described above for
K2[μ-biph-12], substituting p-terphenyl for biphenyl; the yield was
67% for the first crop of solid material, collected from n-pentane after
7 days. 1H NMR (500 MHz, toluene-d8, 22 °C): δ 21.74 (s, ν1/2 = 93
MHz, 1H, p-bound p-terphenyl), 8.22 (s, 12H, m-Ar), 7.56 (m, 2H,
pendant p-terphenyl), 5.43 (m, 1H, p-pendant p-terphenyl), 3.76 (s,
18H, p-Me), 3.38 (m, 2H, unbound p-terphenyl), 2.88 (s, 36H, o-Me),
K[μ-biph-12-2-d1]. The same procedure as that for K[μ-naph-12]
2
substituting biphenyl-2-d1 for naphthalene. H NMR (46 MHz, Et2O,
22 °C): δ −9.20 (s, 1D), −164.46 (s, 1D).
K[μ-biph-12-4-d1]. The same procedure as that for K[μ-naph-12]
2
substituting biphenyl-4-d1 for naphthalene. H NMR (46 MHz, Et2O,
22 °C): δ −14.02 (s, 1D), −82.99 (s, 1D).
t
K[μ-stilb-12]. The same procedure as that for K[μ-naph-12]
substituting trans-stilbene for naphthalene; isolated yield 83% for the
2.23 (m, 2H, pendant p-terphenyl), −2.36 (s, 54H, Bu), −17.71 (s,
ν1/2 = 20 MHz, 2H, pendant p-terphenyl), −49.79 (s, ν1/2 = 61 MHz,
2H, bound p-terphenyl), −123.53 (s, ν1/2 = 76 MHz, 2H, bound
p-terphenyl). UV−vis (Et2O, 22 °C): λmax, nm (ε × 10−2, M−1 cm−1) =
214 (2646.8 136.0), 246 (1032.0 142.1), 273 (820.9 76.7), 494
(103.9 10.0). Anal. Calcd for C102H134N6K2U2: C, 61.30; H, 6.76; N,
4.21. Found: C, 61.51; H, 6.48; N, 4.12.
1
first crop of crystalline solid (1 day, pentane). H NMR (500 MHz,
C6D6, 22 °C): δ 16.64 (s, 4H, m′-Ar), 16.08 (br s, trans-stilbene), 9.47
(s, 6H, p′-Me), 8.01 (s, 8H, m-Ar), 4.81 (s, 12H, o′- or p-Me), 4.10 (s,
t
t
12H, o′- or p-Me), 0.63 (s, 36H, Bu), −0.44 (s, 18H, Bu′), −2.11 (s,
24H, o-Me), −46.02 (br s, trans-stilbene), −71.89 (br s, trans-stilbene).
UV−vis (Et2O, 22 °C): λmax, nm (ε × 10−2, M−1 cm−1) = 215 (2118.9
103.3), 282 (532.9 56.2), 382 (150.3 5.4), 568 (97.3 4.7).
Anal. Calcd for C98H132N6KU2: C, 61.65; H, 6.97; N, 4.40. Found: C,
61.68; H, 6.81; N, 4.40.
K2[μ-terph-12-d14]. The same procedure as that described above for
K2[μ-biph-12], substituting p-terphenyl-d14 for biphenyl. 2H NMR (76
MHz, Et2O, 22 °C): δ 22.28 (s, 1D), 8.16 (s, 2D), 6.00 (s, 1D), 3.84
(s, 2D), 2.89 (s, 2D), −17.84 (s, 2D), −49.76 (s, 2D), −123.71
(s, 2D).
General Synthesis of Salts K[μ-arene-12] with That of K[μ-naph-
12] Given as a Representative Example. Compound 1-I(DME)
(0.492 g, 0.46 mmol, 1 equiv) and naphthalene (0.030 g, 0.23 mmol,
0.5 equiv) were dissolved in DME (15 mL), and the solution was
frozen in the glovebox cold well. Separately, KC8 (0.125 g, 0.92 mmol,
2 equiv) was slurried in DME (12 mL), and the slurry was also frozen.
K[μ-terph-12]. The same procedure as that for K[μ-naph-12]
substituting p-terphenyl for naphthalene; isolated yield 69% for the
1
first crop of crystalline solid (1 day, pentane). H NMR (500 MHz,
toluene-d8, 22 °C): δ 18.11 (s, 2H, pendant p-terphenyl), 11.32 (m,
2H, pendant p-terphenyl), 8.88 (s, 8H, m-Ar), 7.97 (s, 6H, p′-Me),
7.51 (m, 1H, p-pendant p-terphenyl), 6.17 (m, 2H, pendant
p-terphenyl), 4.39 (s, 12H, o′- or p-Me), 3.95 (s, 12H, o′- or p-Me),
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0.89 (s, 36H, tBu), −0.12 (s, 18H, tBu′), −1.75 (s, 24H, o-Me), −7.35
(s, 2H, pendant p-terphenyl), −81.17 (s, ν1/2 = 106 Hz, 1H, p-bound
p-terphenyl), −142.44 (s, ν1/2 = 78 Hz, 2H, m- or o-bound p-terphenyl),
−163.31 (s, ν1/2 = 156 Hz, 2H, m- or -o-bound p-terphenyl). UV−vis
(toluene, 22 °C): λmax, nm (ε × 10−2, M−1 cm−1) = 285 (552.9
3.36 (s, 4H, CH2-DME), 3.15 (s, 6H, CH3-DME), 1.22 (s, 36H,
o-Me), −1.20 (s, 54H, Bu), −109.52 (s, 2H, o- or m-toluene), −113.84
(s, 2H, o- or m-toluene), −126.52 (s, 1H, p-toluene). UV−vis (Et2O,
22 °C): λmax, nm (ε × 10−2, M−1 cm−1) = 214 (2099.2 255.7), 267
(593.3 108.6), 432 (120.5 10.5), 541 (80.7 8.1). Anal. Calcd for
C95H138N6U2O2K: C, 59.70; H, 7.28; N, 4.40. Found: C, 60.10; H,
7.17; N, 4.59.
[K(DME)][μ-tol-12-d8]. The same procedure as that described for
[K(DME)][μ-tol-12], substituting toluene-d8 for toluene. 2H NMR
(76 MHz, pentane, 22 °C): δ 63.42 (s, 3D, CD3-toluene), −107.23 (s,
2D, o- or m-toluene), −114.43 (s, 2D, o- or m-toluene), −127.73 (s,
1D, p-toluene).
t
27.2), 447 (199.1
7.8), 517 (174.2
7.8). Anal. Calcd for
C102H134N6KU2: C, 62.52; H, 6.89; N, 4.29. Found: C, 62.57; H, 6.90;
N, 4.06.
K[μ-terph-12-d14]. The same procedure as that for K[μ-naph-12]
substituting p-terphenyl-d14 for naphthalene. 2H NMR (76 MHz,
toluene, 22 °C): δ 18.87 (s, 2D), 12.17 (s, 2D), 8.45 (s, 1D), 6.83
(s, 2D), −5.84 (s, 2D), −88.13 (s, 1D), −145.85 (s, 2D), −162.39
(s, 2D).
[K(DME)][μ-benz-12]. The same procedure as that described for
[K(DME)][μ-tol-12], substituting benzene for toluene. 1H NMR (300
MHz, C6D6, 22 °C): δ 8.43 (s, 12H, m-Ar), 3.98 (s, 18H, p-Me), 3.07
(s, 4H, CH2-DME), 2.85 (s, 6H, CH3-DME), 1.47 (s, 36H, o-Me),
Synthesis of [K2I][μ-tol-12]. A solution of 1-I(DME) (2.075 g, 1.95
mmol) in 60 mL of toluene and a slurry of KC8 (0.527 g, 3.90 mmol, 2
equiv) in 100 mL of toluene were frozen in the glovebox cold well,
then taken out, and allowed to thaw. To the thawing solution of
1-I(DME) was added dropwise the thawing KC8 slurry. The reaction
mixture was stirred for 2 h at ca. 25 °C and then filtered through
Celite. The solvent was removed from the filtrate, the resulting solid
was extracted with diethyl ether (100 mL), and the extract was filtered.
The filtrate was concentrated to 10 mL and then placed in a −35 °C
refrigerator for several days. Salt [K2I][μ-tol-12] was thereby obtained
as a dark-orange-brown microcrystalline solid (0.852 g, 0.43 mmol,
44% yield). 1H NMR (300 MHz, C6D6, 22 °C): δ 34.70 (s, 3H,
toluene-CH3), 6.98 (s, 12H, m-Ar), 2.53 (s, 18H, p-Me), 1.63 (s, 54H,
tBu), 1.50 (s, 36H, o-Me), −38.01 (s, 2H, o- or m-toluene), −43.05 (s,
1H, p-toluene), −44.63 (s, 2H, o- or m-toluene). UV−vis (Et2O, 22 °C):
λmax, nm (ε × 10−2, M−1 cm−1) = 214 (2099.2 255.7), 267 (593.3
t
−0.78 (s, 54H, Bu), −112.83 (s, 1H, C6H6).
Oxidation of K2[μ-arene-12] to K[μ-arene-12] Using Fc[OTf]. Note:
Only the oxidation of K2[μ-stilb-12] to K[μ-stilb-12] is described in
detail, with the reactions for the other three K2[μ-arene-12] being
similar. Solutions of K2[μ-stilb-12] (0.253 g, 0.13 mmol) and Fc[OTf]
(0.044 g, 0.13 mmol, 1 equiv) in diethyl ether were frozen. The
thawing K2[μ-stilb-12] solution was added dropwise to the thawing
Fc[OTf] solution, and the reaction mixture was allowed to reach room
temperature and stirred for 1 h. After that time, the mixture was
filtered through Celite, and the volatiles were removed. The solid
obtained was extracted with pentane, and the solution obtained was
filtered through Celite, concentrated, and placed in a −35 °C
refrigerator. After 8 days, K[μ-stilb-12] was obtained as a microcrystal-
line solid (0.200 g, 0.10 mmol) in 81% yield.
108.6), 432 (120.5
10.5), 541 (80.7
8.1). Anal. Calcd for
C84H120N6U2K2I: C, 55.03; H, 6.45; N, 4.23. Found: C, 54.45; H, 5.85;
N, 3.92.
[K2I][μ-tol-12-d8] was prepared as described for [K2I][μ-tol-12-d8],
substituting toluene-d8 for toluene. 2H NMR (76 MHz, pentane, 22 °C):
δ 34.42 (s, 3D, CD3-toluene), −37.76 (s, 2D, o- or m-toluene), −42.73
(s, 1D, p-toluene), −44.43 (s, 2D, o- or m-toluene).
Oxidation of K2[μ-arene-12] to K[μ-arene-12] Using P4. Note: Only
the oxidation of K2[μ-stilb-12] to K[μ-stilb-12] is described in detail,
with the reactions for the other three K2[μ-arene-12] being similar.
Solutions of K2[μ-stilb-12] (0.109 g, 0.06 mmol) and P4 (0.009 g, 0.07 mmol,
1.3 equiv) in diethyl ether were frozen. The thawing K2[μ-stilb-12]
solution was added dropwise to the thawing P4 slurry; the reaction
mixture was allowed to reach room temperature and stirred for 0.5 h.
After that time, the mixture was filtered through Celite and the
volatiles were removed. The solid obtained was extracted with
pentane, and the solution obtained was filtered through Celite,
concentrated, and placed in a −35 °C refrigerator. After 5 days, K[μ-
stilb-12] was obtained as a microcrystalline solid (0.074 g, 0.10 mmol)
in 69% yield.
Reduction of K[μ-arene-12]to K2[μ-arene-12] Using K/Anthra-
cene. Note: Only the reduction of K[μ-stilb-12] to K2[μ-stilb-12] is
described in detail, with the reactions for the other three K[μ-arene-
12] being similar. Potassium (0.100 g, 2.6 mmol, 5.2 equiv) was added
to a THF solution (3 mL) of anthracene (0.010 g, 0.05 mmol, 1 equiv)
in a vial protected from light (with electrical tape), and the resulting
mixture was stirred at room temperature for 1 h, after which time the
vial was placed in a cold well. Separately, a solution of K[μ-stilb-12]
(0.091 g, 0.05 mmol) in THF (2 mL) was frozen. The thawing K/
anthracene mixture was added dropwise to the K[μ-stilb-12] solution,
with the excess of potassium being left behind. The new mixture was
allowed to reach room temperature and stirred for 1 h, after which
time the solution was transferred to a filter flask. After removal of the
volatiles under reduced pressure, the solid obtained was extracted in
pentane, the extract filtered through Celite, and the pentane removed.
A new extraction in pentane and filtration through Celite led to a new
filtrate (5 mL), which was concentrated and placed in a refrigerator
overnight. Compound K2[μ-stilb-12] was obtained as a microcrystal-
line powder in 78% yield (0.073 g, 0.04 mmol) after several days.
Synthesis of (η8-C8H8)U(NC[tBu]Mes)3 (2). Solutions of K[2] (0.063 g,
0.06 mmol) and TiCl4(THF)2 (0.022 g, 0.06 mmol, 1 equiv) in
diethyl ether (3 mL each) were frozen. To the thawing TiCl4(THF)2
solution was added dropwise the K[2] solution. The reaction mixture
was stirred for 3 h at room temperature and then filtered through
Celite. The solvent from the filtrate was removed and the solid
extracted with pentane (5 mL) and then with diethyl ether. The
solution was concentrated and then placed in a refrigerator for several
Synthesis of [K2I][μ-benz-12]. A solution of 1-I(DME) (0.090 g,
0.08 mmol) in 8 mL of benzene and a slurry of KC8 (0.023 g, 0.17 mmol,
2 equiv) in 4 mL of benzene were frozen by placing the vessels
containing them into the glovebox cold well. Subsequently, said vessels
were taken out of the cold well and allowed to thaw. To the thawing
solution of 1-I(DME) was added dropwise the thawing KC8 slurry.
The reaction mixture was stirred for 20 min, after which it was filtered
through Celite and the volatiles were removed from the filtrate under
reduced pressure. After extraction of the residue with pentane, the
extract (20 mL) was concentrated to 3 mL and cooled to −35 °C. The
crystals of [K2I][μ-benz-12] (0.018 g, 0.009 mmol, 11% yield) so
obtained were used to prepare an NMR sample, leading to the
1
following data: H NMR (300 MHz, C6D6, 22 °C): δ 6.97 (s, 2H,
t
m-Ar), 2.57 (s, 3H, p-Me), 1.87 (s, 9H, Bu), 1.36 (s, 6H, o-Me),
−44.91 (s, 1H, C6H6).
Synthesis of [K(DME)][μ-tol-12]. Complex 1-I(DME) (0.163 g,
0.15 mmol) and toluene (3 drops, ca. 0.030 g, 0.33 mmol, 2.2 equiv) were
dissolved in DME (5 mL), and the solution was frozen by placing the
vessel containing it into the glovebox cold well. Separately, KC8 (0.062 g,
0.46 mmol, 3 equiv) was slurried in DME (3 mL) and the slurry was
also frozen. Both mixtures were then taken out of the cold well and
allowed to thaw. The thawing KC8 slurry was then added dropwise to
the thawing solution of complex 1-I(DME). After complete addition,
the reaction mixture was stirred for 20 min and then filtered through
Celite. After solvent removal from the filtrate, the resulting dark-brown
solid was extracted with pentane (ca. 15 mL), the pentane extract was
filtered through Celite, and the pentane was removed from the filtrate
by assisted evaporation. The resulting crude solid was extracted with
pentane (10 mL), and the extract was filtered through Celite,
concentrated to 3 mL, and stored in a −35 °C refrigerator for several
days. A first crop of [K(DME)][μ-tol-12] crystals (0.061 g, 0.03 mmol,
41% yield) was obtained by decanting the mother liquor and drying
1
under reduced pressure. H NMR (500 MHz, toluene-d8, 22 °C): δ
64.48 (s, 3H, CH3-toluene), 8.61 (s, 12H, m-Ar), 4.15 (s, 18H, p-Me),
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Article
days. 2 was obtained as an orange-brown microcrystalline solid
(0.035 g, 58% yield, 0.03 mmol). 1H NMR (500 MHz, C6D6, 22 °C):
δ 7.35 (s, 6H, m-Ar), 3.13 (s, 27H, tBu), 2.70 (s, 9H, p-Me), 1.53 (s, 18H,
o-Me), −14.84 (s, 8H, COT). Anal. Calcd for C50H68N3U: C, 63.28;
H, 7.22; N, 4.43. Found: C, 62.96; H, 7.22; N, 4.31.
National Science Board (Alan T. Waterman Award to C.C.C.,
1998). Permission to include material from P.L.D.’s Ph.D.
thesis, for which MIT has the copyright, has been received, and
it is given in the Supporting Information.
Reaction of Na2[μ-naph-12] with PhSSPh. Note: All of the
reactions with PhSSPh were performed in a similar manner, with the
results being different as discussed in the text. For K[μ-arene-12]
compounds, 1.5 equiv of PhSSPh was used instead of 2 equiv.
Solutions of Na2[μ-naph-12] (0.232 g, 0.12 mmol) and PhSSPh (0.054
g, 0.25 mmol, 2 equiv) in diethyl ether (6 mL each) were frozen. To
the thawing Na2[μ-naph-12] solution was added dropwise the PhSSPh
solution, and the reaction mixture was stirred for 1 h at room
temperature. After that time, the solvent was removed and the solid
extracted with pentane (10 mL). The solution was concentrated and
then placed in a refrigerator for several days. Na[3] was obtained as a
dark-brown microcrystalline solid (0.225 g, 0.10 mmol, 83% yield). 1H
NMR (500 MHz, C6D6, 60 °C): δ 8.80 (s, 2H, o- or m-PhS), 8.62 (s,
2H, o- or m-PhS), 7.80 (s, 4H, m-Ar), 6.65 (s, 1H, p-PhS), 3.97 (s, 6H,
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, details of X-ray
crystallography, and DFT calculations. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: pld@chem.ucla.edu (P.L.D.), ccummins@mit.edu
(C.C.C.).
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
For support of this work, the authors are grateful to the
National Science Foundation (Grant CHE-9988806) and the
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