Organometallics
Article
6.9 Hz, 2H), 3.39 (s, 3H), 13C NMR (125 MHz, CD3CN, 294 K):
195.3, 187.4, 178.5, 150.1, 142.9, 140.1 138.8, 136.2, 135.3, 133.3,
130.1, 129.8, 129, 125.9, 124.9, 124.7, 118.9, 29.4. Anal. Calcd for
(C19H15I2N3O2Ru)(CH3CN)0.5: C, 34.61; H, 2.39; N, 7.07. Found:
C, 33.49; H, 2.28; N, 7.26. ESI-MS, m/z: 543.9171 [RuL1(CO)2I]+.
General Procedure for the Semihydrogenation of Internal
Alkynes. A cleaned and predried autoclave was charged with the
calculated amount of alkyne derivative (0.2 mmol), catalyst (0.006
mmol), and AgBArF (0.012 mmol) in 4 mL of deionized water. The
autoclave was then pressurized with H2 (5 bar), and the mixture was
stirred at 80 °C. After 2−9 h, the autoclave was cooled and the
hydrogen pressure was released carefully. The reaction mixture was
extracted with EtOAc (5 mL), and the extract was passed through a
short column of silica gel and subjected to GC-MS analysis. To
characterize the alkene via NMR spectroscopy, after completion, the
reaction mixture was extracted with EtOAc (3 × 10 mL). The
combined extracts were dried over Na2SO4 and concentrated under
vacuum, and the residue was purified by column chromatography
using silica gel (hexane/EtOAc 10/0 → 9/1) to afford the alkene.
General Procedure for the Semihydrogenation of Terminal
Alkynes. An identical procedure was followed for the semi-
hydrogenation of terminal alkynes, using the calculated amount of
alkyne derivative (0.2 mmol), catalyst (0.006 mmol), and AgBArF
after oxidative insertion of the hydride in an endergonic
reaction step. After the rotation of the C−C bond in H, I is
formed in a marginally endergonic step. A β-hydride
elimination followed by the expulsion of the H2 returns back
F for resumption of the catalytic cycle. The β-hydride
elimination proceeds with a low energy barrier with TS_βhe
lying only 1.2 kcal mol−1 above I. Overall, our computational
results are in line with the experimental observations.
CONCLUSIONS
■
An annelated chelate MIC and two cis carbonyls at the Ru sets
up a platform for alkyne semihydrogenation and subsequent Z
→ E isomerization. The catalytic system 1/BArF displays broad
substrate scope and excellent functional group tolerance to
afford (E)-alkenes from internal alkynes and molecular
hydrogen in water. The catalyst is further distinguished by
its ability to semihydrogenate terminal alkynes to the
corresponding alkenes in isopropyl alcohol. Reaction profile
and isomerization studies confirm the initial formation of (Z)-
alkenes and subsequent metal-catalyzed isomerization to the E
products. A two-cycle mechanism, a reduction cycle and an
isomerization cycle, is proposed on the basis of experimental
results and also supported by DFT calculations. This work
further underlines the general utility of the annelated π-
conjugated carbene ligand for hydrogenation chemistry.
i
(0.012 mmol) in 4 mL of PrOH. After 1−4 h, the reaction mixture
was diluted with EtOAc (5 mL), passed through a short column of
silica gel, and subjected to GC-MS analysis. To characterize the
alkene via NMR spectroscopy, after completion, the reaction mixture
was dried under vacuum and the residue was purified by column
chromatography using silica gel (hexane/EtOAc 10/0 → 9/1).
General Procedure to Obtain Reaction Profile. A cleaned and
predried autoclave was charged with the calculated amount of
diphenylacetylene (0.1 mmol), catalyst (0.003 mmol), AgBArF (0.006
mmol), and mesitylene (0.1 mmol) in 2 mL of deionized water. The
autoclave was then pressurized with H2 (5 bar) and the mixture was
stirred at 80 °C. After 5 min, the autoclave was cooled and the
hydrogen pressure was released carefully. The reaction mixture was
extracted with EtOAc (5 mL), passed through a short column of silica
gel, and subjected to GC-MS analysis. The same methodology was
repeated to find out the conversion at different time intervals.
General Procedure for Alkene Isomerization Study. A
cleaned and predried autoclave was charged with the calculated
amount of cis or trans-stilbene (0.1 mmol), catalyst (0.003 mmol),
AgBArF (0.006 mmol), and mesitylene (0.1 mmol), in 2 mL deionized
water. The autoclave was then pressurized with H2 (5 bar), and the
mixture was stirred at 80 °C. After 7 h, the autoclave was cooled and
the hydrogen pressure was released carefully. The reaction mixture
was extracted with EtOAc (5 mL), and the extract was passed through
a short column of silica gel and subjected to GC-MS analysis.
Computational Details. All molecular geometries were fully
optimized in water (in accordance with the experiments) with the
M0634 functional using the 6-31+g(d,p) basis set for lighter atoms
and the LANL2DZ basis set along with the corresponding
pseudopotential for the Ru atom. Solvent effects (water) were
considered within the framework of the SMD model by Truhlar and
Cramer.37 All geometry optimizations were followed by a harmonic
vibrational analysis to characterize the nature of the stationary points:
transition states (one imaginary frequency) or minima (no imaginary
frequencies). Intrinsic reaction coordinate (IRC) calculations were
performed to verify the transition states. The computed enthalpies
and Gibbs free energy values include zero-point energy and thermal
corrections at 353.15 K and 1 atm pressure. The thermochemical
corrections were obtained within the quasi-harmonic approximation
by Cramer and Truhlar, in which all vibrational frequencies below 100
cm−1 were raised to 100 cm−1.38 All Gibbs free energy data reported
in the manuscript were computed at the SMD(water)/M06/{6-
31+G(d,p)+(LANL2DZ+ECP)} level of theory. All calculations were
performed using the Gaussian 16 suite of programs.39
EXPERIMENTAL SECTION
■
General Procedures. All reactions with metal complexes were
carried out under an atmosphere of purified nitrogen using standard
Schlenk-vessel and vacuum-line techniques. Glassware was flame-
1
dried under vacuum prior to use. H NMR spectra were obtained on
1
JEOL JNM-LA 400 and 500 MHz spectrometers. H NMR chemical
shifts were referenced to the residual hydrogen signal of the
deuterated solvents. The chemical shift is given as a dimensionless
1
δ value and is frequency-referenced relative to TMS for H and 13C
NMR spectroscopy. GC-MS experiments were performed on an
Agilent 7890A GC and 5975C MS system. Hydrogenation reactions
were performed at constant pressures using a stainless steel 25 mL
Parr hydrogenation reactor. ESI-MS measurements were recorded on
a Waters Micromass Quattro Micro triple-quadrupole mass
spectrometer. Infrared spectra were recorded on a Bruker Vertex 70
FTIR spectrophotometer in the range 400−4000 cm−1. Elemental
analyses were performed on a Thermoquest EA1110CHNS/O
analyzer. The crystallized compound was washed several times with
dry diethyl ether, powdered, and dried under vacuum for at least 48 h
prior to elemental analyses. TLC analyses were performed on
commercial TLC paper, and silica gel (100−200 mesh) was used for
column chromatography.
Materials. Solvents were dried by conventional methods, distilled
under nitrogen, and deoxygenated prior to use. RuCl3·xH2O was
purchased from Arora Matthey, India. All other chemicals were
purchased from Sigma−Aldrich. The ligand [L1 I]I,9
36
[Ru2(CH3CN)6(CO)4][BF4]2,35 and AgBArF were prepared
according to literature procedures.
Synthesis and Characterization of 1. The synthesis of complex
1 were reported previously by us.9 A dichloromethane solution (10
mL) of [L1I]I (56 mg, 0.11 mmol) was added dropwise to a
dichloromethane solution (15 mL) of [Ru2(CO)4(CH3CN)6][BF4]2
(40 mg, 0.054 mmol), and the solution was stirred for 12 h at room
temperature. A bright yellow solution appears during the reaction.
After completion of the reaction dichloromethane was evaporated and
the residue was washed with diethyl ether and dried under vacuum. X-
ray-quality crystals were grown again, and the cell parameters match
those of the earlier report. Yield: 52 mg (82%). 1H NMR (500 MHz,
CD3CN, 294 K): δ 9.01 (d, J = 4.6 Hz, 1H), 8.54 (d, J = 8 Hz, 1H),
8.11 (s, 1H), 7.93 (s, 1H), 7.87 (m, 1H), 7.59 (m, 3H), 7.49 (d, J =
G
Organometallics XXXX, XXX, XXX−XXX