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The reaction of Mo( N)(CH2tBu)3 (1) and SiO2-(700) generates ( SiO)Mo( NH)( CHtBu)(CH2tBu) (2) when performed in C6H6 (material [ 1 / SiO 2 - ( 700 ) ] C 6 H 6 ). The grafting occurs presumably by protonation of the nitrido ligand to form an intermediate ( SiO)Mo( NH)(CH2tBu)3 (3), a pentacoordinated complex, which decomposes into 2 and 2,2-dimethylpropane. While [ 1 / SiO 2 - ( 700 ) ] C 6 H 6 is highly active in olefin metathesis, [ 1 / SiO 2 - ( 700 ) ] CH 2 Cl 2 and [1/SiO2-(700)]THF are poorly active or inactive catalysts respectively. In contrast, when Mo( N)(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( NH)(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( N)( CHtBu)(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. |
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