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Perhydrocarbyl Re-VII complexes: Comparison of molecular and surface complexes

The molecular complex [Re(equivalent toC(t)Bu)(=(CHBu)-Bu-t)((CH2Bu)-Bu-t)2] (1) reacts with a silica partially dehydroxylated at 700 degreesC to give syn-2, [(equivalent toSiO)Re(equivalent toC(t)Bu)(=(CHBu)-Bu-t)((CH2Bu)-Bu-t)], as a single isomer according to mass-balance analysis, IR, and solid-state NMR spectroscopy. 1 D and 2D solid-state NMR.(HETCOR and long-range HETCOR) on a C-13-labeled-2 has allowed us to observe the chemical shifts of all carbons (including those that are not labeled) and ascertain their assignments. Moreover, EXAFS data are consistent with the presence of two carbons at a relatively short distance (1.79 Angstrom), which cannot be cleconvoluted, but which are consistent with the presence of alkylidene and alkylidyne carbons along with two other first neighbors at a longer distance (2.01 Angstrom), the alkyl carbon and the 0 atom by which the Re is attached to the surface. Moreover, the data also suggest the presence of a siloxane bridge of the silica surface at 2.4 Angstrom in the coordination sphere of the Re center. Thermal and photochemical treatment allow us to observe the anti isomer, which was also fully characterized by 1D and 2D solid-state NMR. This behavior parallels the reactivity of molecular Re complexes, and their respective H-1 and C-13 chemical shifts match those of the corresponding molecular analogues syn- and anti-2m and n. Finally, the grafting of 1 onto silica involves the reaction of both the alkyl and the alkylidene ligand with an equiprobability, leaving the alkylidyne as a spectator ligand. Noteworthy is the formation of 2 [(equivalent toSiO)Re(equivalent toC(t)Bu)(=(CHBu)-Bu-t)((CH2Bu)-Bu-t)], rather than the corresponding trisneopentyl-neopentylidyne Re complex, monografted on silica, [(equivalent toSiO)Re(equivalent toC(t)Bu)((CH2Bu)-Bu-t)(3)], which would have been expected from the reactivity of I with various molecular Bronsted acids and which also suggests that a proximal siloxane bridge forces the alpha-H abstraction process, leading to syn-2a.

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