Me2AlCH2PMe2:: A new, bifunctional cocatalyst for the Ni(II)-catalyzed oligomerization of PhSiH3

The role of methylaluminoxane (MAO) in the Ni-catalyzed dehydrogenative homologation of PhSiH3 has been investigated with a view to designing new cocatalysts possessing well-defined chemical compositions and structures. These studies show that species such as the bifunctional reagent (Me2PCH2AlMe2)(2), 3, should act as co-catalyst for the Si-Si bond formation reactions. Thus, it was found that the combination of (1-Me-indenyl)Ni(PPh3)Me, 2a, and 3 (Ni/Al ratio of 1:1) converts PhSiH3 to cyclic oligomers (PhSiH)(n) with a turnover frequency (TOF) of >500 h(-1), 50 times faster than with 2a alone. Detailed NMR studies have indicated that this acceleration is due to the formation of the intermediate (1-Me-indenyl)Ni(Me)(Me2PCH2AlMe2), 4. Coordination of the PMe2 moiety in this complex to the Ni center allows the tethered AlMe2 moiety to interact with the Ni-Me moiety in such a way that promotes fairly slow Al-Me/Ni-CD3 exchange (t(1/2) ca. 12 h) but accelerates the Si-H bond activation and Si-Si bond formation reactions. The catalysis promoted by 2a/3 proceeds even faster in the presence of NEt3 or THF (TOF > 1600 h(-1)), because these Lewis bases favor the monomeric form of 3, which in turn favors the formation of 4. On the other hand, the much more nucleophilic base quinuclidine suppresses the catalysis (TOF < 300 h(-1)) by hindering the Ni...R...Al interactions. These observations point to an emerging strategy for using bifunctional reagents such as 3 to place geometrically constrained Lewis acid moieties adjacent to metal centers, thereby activating certain metal-ligand bonds.