Carbon Monoxide Cleavage by (silox)3Ta (silox = tBu3SiO(-)): Physical, Theoretical, and Mechanistic Investigations

Article abstract of DOI:10.1021/ja00207a011

Reduction of (silox)3TaCl2 (1, silox = ^tBu3SiO(-)) with Na/Hg in THF leads to a three-coordinate, Ta(III) siloxide, (silox)3Ta (2).Derivatization of 2 with excess O2 or H2 provides (silox)3Ta=O (3) and (silox)3TaH2 (4), respectively.At 25 deg C, carbonylation of 2 (0.1-1.0 atm) generates 1/2 3 and 1/4<(silox)3Ta>2(μ-C₂) (5), consistent with a CO uptake of 0.47 equiv.X-ray (P<*>, R = 9.6percent) structural, IR, and Raman studies of dicarbide 5 manifest a near-linear μ-C₂ bridge (<*>TaCC = 173 (3) deg), a C=C double bond (1.37 (4) Angstroem, ν(C=C) = 1617 cm^-1) and typical TaC double bonds (1.95 (2) Angstroem, ν(Ta=C) = 709 cm^-1), respectively.EHMO calculations of a linear μ-C₂-bridged D_3d 5 indicate that the e_g² HOMO (³A_2g, ¹E_g, ¹A_1g) is ca. 80percent Ta (d_xz, d_yz) and ca. 20percent C (p_x, p_y).Magnetic susceptibility measurements of 5 from 2 to 300 K reveal a large temperature-independent susceptibility (25 deg C, μ_eff = 1.93μ_B) and a singlet ground state, either ¹E_g, ¹A_1g (D_3d) or one arising from a Jahn-Teller distortion of the ¹E_g level.Treatment of 2 with CO (ca. 1 atm) at -78 deg C, followed by warming to 25 deg C, results in an uptake of 0.97 equiv. of CO and the production of an inseparable mixture of 3 and a monomeric ketenylidene, (silox)3Ta=C=C=O (6).Diamagnetic 6 (ν(C=O) = 2076 cm^-1, ¹J_CC = 100 Hz), a precursor to 5, is linear according to EHMO calculations.Mechanistic investigations concerning formation of 5 and 6, utilizing labeling experiments, kinetics, and chemical models, support the following sequence of reactions: (1) (silox)3Ta (2) binds CO to form unstable (silox)3TaCO (2-CO). (2) In donor solvents, 2-CO is trapped and stabilized by the solvent (solvent = S) (-78 deg C) as (silox)3STaCO (S-2-CO). (3) Aggregation of S-2-CO and equilibrium amounts of 2-CO, dimerization of 2-CO, disproportionation of monocarbonyl species to (silox)3Ta(CO)2 (2-(CO)2) and 2, which quickly recombine, generates (-78 to -50 deg C) red precipitate <(silox)3TaCO>_n (<2-CO>_n, n is probably 2). (4) In nondonor solvents 2-CO either dimerizes or disproportionates to ultimately give <2-CO>_n. (5) Degradation of <2-CO>_n (ca. 5 deg C), produces ketenylidene (silox)3Ta=C=C=O (6) and oxo (silox)3Ta=O (3). (6) Another (silox)3Ta (2) deoxygenates ketenylidene 6 (ca. 0 deg C), possibly via intermediate (6.2), to afford oxo 3 and a transient vinylidene, (silox)3Ta=C=C: (2-C₂), that electronically resembles CO, as substantiated by EHMO arguments. (7) A final (silox)3Ta (2) unit scavenges the vinylidene (2-C₂), resulting in <(silox)3Ta>₂(μ-C₂) (5).Alternatives to the heterogenous dissociative adsorption of CO, the first step in the Fischer-Tropsch process, are suggested on the basis of this work.