SYNTHESES, CARBONYLATIONS, AND DIHYDROGEN EXCHANGE STUDIES OF MONOMERIC AND DIMERIC SILOX ((T)BU3SIO-) HYDRIDES OF TANTALUM - STRUCTURE OF [(SILOX)2TAH2]2

Reduction of (silox)3TaCl2 (1) (silox = tBu3SiO-) with Na/Hg in THF under H-2 afforded (silox)3TaH2 (2, 43%); 2 thermally cyclometalated to (silox)2HTaOSitBu2CMe2CH2 (4), but was reconstituted with H-2 (6 days, 3 atm). Treatment of 2 with C2H4, neat CCl4, and CH3I in Et2O generated (silox)3HTaEt (5, 63%), (silox)3HTaCl (6-Cl, 63%), and (silox)3HTaI (6-I, 62%). Reduction of (silox)3HTaI (6-I) with Na/Hg in THF produced a ring-opened THF compound, [(silox)3TaH]2(mu:eta1,eta1-CH2(CH2)3O) (7, 58%). Photolysis of (silox)2Cl2TaCH2Ph (8) under 3 atm of H-2 gave [(silox)2TaCl)]2(mu-H)2 (10), C7H8, and a trace of bibenzyl. Reduction of (silox)2TaCl3 (9) with Na/Hg under 1 atm of H-2 (approximately 15 days) yielded an unbridged D2d dimer [(silox)2TaH2]2 (11, 83%), which possessed a 2.720(4) angstrom Ta-Ta bond. Crystal data for 11: cubic, I43dBAR, a = 28.125(6) angstrom, Z = 12, T = 23-degrees-C, 1190 reflections (F > 3.0sigma(F)), R = 0.079, and R(w) = 0.050. Exposure of 11 to 2 equiv of HCl, 1.0 equiv of O2, and 1.0 equiv of Me3NO provided 10 (78%), [(silox)2TaH]2(mu-O)2 (12, 95%), and [(silox)2TaH]2(mu-H)2(mu-O) (14,67%); derivatization of 12 with C2H4 gave [(silox)2TaCH2CH3]2(mu-O)2 (13, 39%). Mu-oxo dimer 14 exists as two C2 isomers; the hydrides of 14a exchanged with DELTAG(double dagger) almost-equal-to 8 kcal/mol, while those of 14b exchanged coincidently with interconversion of the isomers (DELTAG(double dagger) almost-equal-to 11 kcal/mol; 14a half arrow right over half arrow left 14b, DELTAH = -1.1(3) kcal/mol, DELTAS = -4.6(9) eu). Isotopomers (silox)4Ta2DnH4-n (11-d(n)) were distinguished by a +0.022 ppm/D (23-degrees-C) NMR isotope shift. Dihydride 2 undergoes sigma-bond metathesis with D2, initially forming (silox)3TaHD (2-d1), and the exchange of 11 with H-2 was directly measured by spin saturation transfer H-1 NMR techniques (50-degrees-C, k = 9.2(3) x 10(2) m-1 s-1; DELTAH(double dagger) = 6.2(1) kcal/mol, DELTAS(double dagger) = -26(3) eu). Carbonylation of 2 and 5 afforded eta2-aldehydes (silox)3Ta(eta2-OCHR) (R = H, 15, 77%; Et, 16, 41%), alternatively prepared from (silox)3Ta (3) and CH2O and EtCHO, respectively. Only 15 and 15-d2 were generated from 2 and 2-d2 (5 atm). Spectroscopic, C-13-labeling, and protic quenching studies confirmed the constitutions of various dimeric carbonylation products. Treatment of 10 with CO yielded [(silox)2TaCl]2(mu-H)(mu:eta2,eta2-CHO) (18, 56%), while exposure of 11 to approximately 1.0 equiv of CO afforded [(silox)2TaH]2(mu-CH2)(mu-O) (19, 67%). Reformation of the C-O bond occurred when 19 was carbonylated. [(silox)2TaH](mu:eta2,eta2-CHO)(mu:eta1,eta2-CH2O)[Ta(silox)2] (20) was isolated in 55% yield and converted (1 h, 60-degrees-C) to[(silox)2Ta]2(mu-O)2(mu-CHMe)(21,61%). The sequence 11 + CO --> 19 --> 20 --> 21 exhibits the critical C-O bond-breaking and the C-H and C-C bond-making events of the Fischer-Tropsch (F-T) process. Extended exposure of 11 or 20 to 1 atm of CO provided [(silox)2Ta]2(mu:eta1,eta2-CH=CHO)(mu:eta1,eta2-CH2O)(mu-O) (22, 50%). Carbonylation of 12 generated [(silox)2Ta]2(mu-O)2(mu-CH2O) (23) as the major product (70-90%), while treatment of 14 with CO yielded first [(silox)2HTa](mu-O)2[TaMe(silox)2] (24, 90%) and then [(silox)2Ta]2(mu-O)2(mu-MeCHO) (25, approximately 90%). (CO)-C-13-labeling studies were used to follow the 19 --> 21 and 22 conversions, providing the basis of a mechanistic assessment. Dimeric structures allow oxygenated fragments to remain coordinated to two tantalums throughout the sequence. Insertion into Ta-H bonds may initiate each carbonylation process. Stereochemical consequences of silox ligation are discussed in relation to the structures and dynamics of the binuclears, while the electrophilic tantalum centers are important in H/D exchange and carbonylation chemistry. The carbonylation chemistry underscores three critical points regarding the F-T process: (1) hydride transfer to CO is a reasonable alternative to CO dissociation; (2) adsorbed hydrocarbyl and oxygenate fragments are related by reversible C-O bond-breaking and bond-making events; and (3) oxygenate and hydrocarbyl adsorbates can be removed protolytically, akin to hydrogenation.