Synthesis and reactivity of heterodinuclear Fe-Ni complexes with a bridging alkoxysilyl ligand.: Crystal structure of [(OC)3Fe{μ-Si(OMe)2(OMe)}(μ-dppm)NiMe]

The new heterobimetallic complex [(OC)(3)Fe{mu -Si(OMe)(2)(OMe)}(mu -dppm)NiCl] (1) was obtained in 95% yield by reaction of [NiCl2(PPh3)(2)]in THF with K[Fe{Si(OMe)(3)}(CO)(3)(dppm-P)] at -78 degreesC. The analogous bromo and iodo complexes were also obtained; the latter is, however, less stable and could not be isolated pure. They display the first examples of a bridging alkoxysilyl ligand between Fe and Ni, and the mu (2)-eta (2)-SiO bridge is also present in the methyl complex [(OC)(3)Fe{mu -Si(OMe)(2)(OMe)}(mu -dppm)NiMe] (4) and its phenyl analogue 5. The presence of the Fe-Si-O --> Ni four-membered rings was confirmed by a crystal structure determination of 4. Treatment of 1 with excess (allyl)MgCl led to the expected bimetallic allyl complex [(OC)(3){(MeO)(3)Si}Fe(mu -dppm)Ni(eta (3)-C3H5)] (6). The rapid eta (3)-allyl --> eta (1)-allyl --> eta (3)-allyl rearrangement is potentially assisted through stabilization of the coordinatively unsaturated Ni center by a SiO --> Ni interaction. The bimetallic benzyl derivative 7 was also isolated. Purging a THF or benzene solution of 4 at room temperature with CO yielded after a few seconds the acyl complex [(OC)(3)Fe{mu -Si(OMe)(2)(OMe)}(mu -dppm)-NiC(O)Me] (8), which readily decarbonylates. Its reaction with norbornadiene leads to the insertion product, which is thought to exist as an isomeric mixture with terminal or chelating acyl group (11 reversible arrow 11'). When complex 4 was reacted with (BuNC)-Bu-t, rapid insertion occurred and further coordination of a terminal (BuNC)-Bu-t ligand to Ni led to the iminoacyl complex [(OC)(3){(MeO)(3)Si}Fe(mu -dppm)Ni{(C((NBu)-Bu-t)Me}((CNBu)-Bu-t)] (12). Complex 1 proved to be a more efficient catalyst (TON = 4100) for the dehydrogenative coupling of Ph3SnH than its mononuclear counterpart [NiCl2(PPh3)(2)] (TON = 1050). The maximum turnover frequency (TOF) was ca. 9800 h(-1).