The synthesis of two silica-supported tantalum(V) carbene complexes is reported. The reaction of Ta- (=CHC(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) 3 (1) with a silica dehydroxylated at 500 °C produces a mixture of (≡SiO)Ta(=CHC-(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) 2 (2a) and (≡SiO) 2 Ta(=CHC(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) (2b) and neopentane gas (on average 1.3 equiv/equiv of tantalum). The reaction of 2a/2b with water produces 2.6 mol of neopentane. The reaction of 2a/2b with acetone produces 1 equiv of 2,4,4-trimethyl-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 this reaction (2 deut ) is hydrolyzed with D 2 O 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 the volatile products of these reactions is traced to a mechanism of formation of 2a,b in which the first reaction of surface silanol with 1 is the addition of O—H across the Ta=C double bond.
ASJC Scopus subject areas
- Colloid and Surface Chemistry