Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

The reaction of Mo(triple bond; length of mdashN)(CH2t Bu)3 (1) and SiO2-(700) generates (triple bond; length of mdashSiO)Mo(double bond; length as m-dashNH)(double bond; length as m-dashCHt Bu)(CH2t Bu) (2) when performed in C6H6 (material [1/SiO2-(700)]C6H6[1/SiO2-(700)]C6H6). The grafting occurs presumably by protonation of the nitrido ligand to form an intermediate (triple bond; length of mdashSiO)Mo(double bond; length as m-dashNH)(CH2t Bu)3 (3), a pentacoordinated complex, which decomposes into 2 and 2,2-dimethylpropane. While [1/SiO2-(700)]C6H6[1/SiO2-(700)]C6H6 is highly active in olefin metathesis, [1/SiO2-(700)]CH2Cl2[1/SiO2-(700)]CH2Cl2 and [1/SiO2-(700)]THF are poorly active or inactive catalysts respectively. In contrast, when Mo(triple bond; length of mdashN)(CH2tBu)3 reacts with a molecular silanol derivative, a soluble model of the surface of SiO2-(700), it yields a very stable complex, (c-C5H9)7Si7O12SiO–Mo(double bond; length as m-dashNH)(CH2tBu)3 (3m), which does not spontaneously generate 2,2-dimethylpropane and an alkylidene complex in contrast to the surface complex. Moreover, 3m does not catalyse olefin metathesis at room temperature as it does not already contain the initiating carbene ligand, and it is necessary to heat up the reaction mixture to 110 °C to obtain low catalytic activity. Nevertheless, the complex 3m generates well-defined metallocarbenes when heated in the presence of PMe3: (c-C5H9)7Si7O12SiO–Mo(triple bond; length of mdashN)(double bond; length as m-dashCHtBu)(P(CH3)3)2 (4m) as a 10:1 mixture of its syn and anti rotamers with the loss of 2 equiv. of 2,2-dimethylpropane.