Communication
Homoleptic Rhodium Pyridine Complexes for Catalytic Hydrogen
ACCESS
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ABSTRACT: The homoleptic rhodium pyridine complex [Rh(py) ] ([1] ) is prepared from simple precursors. Lacking good π-
4
+
2+
acceptor ligands but being sterically protected, [1] reversibly oxidizes to colorless [Rh(py) (thf) ] . This monomeric S = 1/2
Rh(II) complex activates H to give [HRh(py) L] , which can also be generated by protonation of [1] . The Rh(III)−H bond is
weak, being susceptible to H atom abstraction as well as deprotonation. These results underpin a novel catalytic system for the
4
2
2+
+
2
4
oxidation of H by ferrocenium.
2
n this paper, we describe homoleptic pyridine (py)
complexes of rhodium(I) and rhodium(II). Their chemistry
underpins well-defined catalysts for hydrogen oxidation and
of the 2,6-protons on pyridine precludes coplanarity of these
ligands. Also potentially relevant is the overlap of the π-systems
I
of the pyridine ligands with the filled dxz and d orbitals on
yz
reveal unexpected features of pyridine as a ligand. Pyridine
offers the following features of relevance to homogeneous
(py) array and Rh 4d orbitals (Figure S31). The D rotamer
4
4h
z
catalysis: (i) Complexes of the type [M(py) ] are labile for n
where the four pyridine rings are aligned with the C axis is
n
4
1
>
4, thus ensuring coordinative unsaturation. (ii) Complexes
of the type [M(py) ] are too encumbered to allow M−M
bonding, also relevant to unsaturation. (iii) In contrast to other
destabilized by 5.2 kcal/mol according to DFT calculations
(B3LYP, zora-def2-tzvp), consistent with poor π-accepting
ability of pyridine. For comparison, the pyridine orientation is
z
4
2
monodentate N-ligands, pyridines stabilize both low- and
similar to that for [RhCl (py) ]Cl (d
= 2.064(2) Å,
2
4
Rh−N(avg)
high-valent rhodium centers.
dihedral 40.76(9)°).
1
+
The electronic attribute that make pyridine attractive for
The H NMR spectrum of [1] is conventional. While
2
H O
+
low-valent complexes is its moderate basicity (pK
= 5.2)
a
separate signals are observed for [1] and excess pyridine,
3
+
coupled with its weak π-acceptor properties. The steric effects
prevent cis-coplanar geometry. This structural constraint
distinguishes pyridine from 2,2′-bipyridine, a noninnocent α-
5
F
(
4
4
1
3
+
toward oxidants: 2 equiv of trityl chloride converts [1] into
the well-known trans-[RhCl (py) ] .
The distinctive feature of [1] is its easy conversion into a
4
+
14
diimine.
2
4
The versatility of the new rhodium−pyridine system is
illustrated through reactions with a proton and a dihydrogen.
Exploiting dihydrogen as a green fuel requires innovations in
+
monomeric rhodium(II) derivative. Cyclic voltammetry (CV)
F
1
5
experiments using 0.1 M [NBu ]BAr electrolyte reveal a
4
4
catalysts that reveal new pathways for H oxidation. Nature
+/2+
+/0
2
reversible [1]
couple at E = −0.2 V vs Fc (Figure 2,
1
/2
5
,6
oxidizes H via hydrogenase enzymes, but there still remains
+
+
+
2
Fc = [Fe(C H ) ] ). Chemical oxidation of [1] by 1 equiv
5 5 2
intense interest in simple, manipulable systems for H redox.
FcBArF in THF followed by the addition of pentane
2
4
Most molecular catalysts for H oxidation require elaborate
F
2
precipitated the Rh(II) salt as a white solid. Excess FcBAr
7
−12
4
ligands.
does not oxidize the product further. Elemental analysis of the
+
The synthesis of [Rh(py) ] is straightforwardly achieved
F
4
oxidation product is consistent with a [Rh(py) ][BAr ] unit
4
4 2
from commercial reagents. Addition of >8 equiv of pyridine
with three thf molecules. The composition of the Rh(II)
(
py) to a solution of Rh Cl (C H ) in Et O led to
F
2
2
2
4
4
2
species is believed to be [Rh(py) (thf) ][BAr ] according to
4
2
4 2
precipitation of trans-RhCl(C H )(py) , which can be isolated
2
4
2
X-ray crystallographic analysis of the related 4-tert-butylpyr-
as a yellow solid. No further reaction with py is evident until
tBu
idine ( py) derivative (vide infra). The third thf does not
F
F
the addition of NaBAr (Ar = C H -3,5-(CF ) ), which
4
6
3
3 2
interact with the rhodium complex.
caused a rapid color change concomitant with precipitation of
F
F
4
solid NaCl. Orange crystals of [Rh(py) ]BAr ([1]BAr ) can
4
4
Received: May 16, 2021
Published: June 28, 2021
be isolated in 98% yield. Crystallographic analysis confirmed
+
+
that [Rh(py) ] ([1] ) ion is square-planar (Figure 1). The
4
pyridine ligands are arranged in a propeller-like fashion
resulting in D4 point group symmetry (dRh−N(avg)
.0387(13) Å). The average torsion between pyridine plane
and rhodium-centered square plane is 53.66°. The steric clash
=
2
©
2021 American Chemical Society
J. Am. Chem. Soc. 2021, 143, 10065−10069
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0065