SURFACE ORGANOMETALLIC CHEMISTRY OF INORGANIC OXIDES - THE SYNTHESIS AND CHARACTERIZATION OF (SIO)TA(=CHC(CH3)(3))(CH2C(CH3)(3))(2) AND (SIO)(2)TA(=CHC(CH3)(3))(CH2C(CH3)(3))

The synthesis of two silica-supported tantalum(V) carbene complexes is reported. The reaction of Ta(=CHC(CH3)(3))(CH2C(CH3)(3))(3) (1) With a silica dehydroxylated at 500 degrees C produces a mixture of (=SiO)T a(=CHC(CH3)(3))(CH2C(CH3)(3))(2) (2a) and (=SiO)(2)Ta(=CHC(CH3)(3))(CH 2C(CH3)(3)) (2b) and neopentane gas (on average 1.3 equiv/equiv of tan talum). The reaction of 2a/2b with water produces 2.6 mol of neopentan e. The reaction of 2a/2b with acetone produces 1 equiv of 2,4,4-trimet hyl-2-pentene per grafted tantalum, indicating that 2a,b each bear one carbene ligand. When 1 is reacted with fully (>90%) deuterated silica , the product neopentane is only 38% monodeuterated. The product of th is reaction (2(deut)) is hydrolyzed with D2O to produce 2.6 equiv (per Ta) of neopentane as a mixture of neopentane-d(0), -d(1), -d(2), and -d(3) (3.3:54.4:36.7:5.5). The presence of neopentane-d(3) is evidence for the incorporation of deuterium into the tantalum surface complex during the synthesis of 2a/2b. The volatile product of the reaction of 2(deut) with acetone is 1 equiv of 2,4,4-trimethyl-2-pentene which is 15% monodeuterated, demonstrating some incorporation of deuterium in the carbenic positions of 2(deut). The isotopomeric distribution in th e volatile products of these reactions is traced to a mechanism of for mation of 2a,b in which the first reaction of surface silanol with 1 i s the addition of O-H across the Ta=C double bond.