Me₂AlCH₂PMe₂: A New, Bifunctional Cocatalyst for the Ni(II)-Catalyzed Oligomerization of PhSiH₃

Abstract

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.

Design principles in frustrated Lewis pair catalysis for the functionalization of carbon dioxide and heterocycles

“…While investigating the role of methylaluminoxanes (MAO) in the homologation of hydrosilanes with Zargarian, we were able to demonstrate that species (Me2AlCH2PMe2)2 (1) could react with nickel(II) precursors to generate an extremely active catalyst for the oligomerization of phenylsilane (Scheme 2A) [13].…”

“…Consequently, no 13 C incorporation into methoxy products was observed when using the H2 13 C=O adduct of the catalyst 7 (8*). Similarly, 8* is not generated from the unlabelled catalyst 8 during the catalytic reduction of 13 CO2.…”

Ambiphilic Diphosphine–Borane Ligands: Metal→Borane Interactions within Isoelectronic Complexes of Rhodium, Platinum and Palladium

“…In addition, the Lewis acidic component of ambiphilic ligands has been shown to eventually interact with other basic centres located within the coordination sphere (such as a chloride ligand [12b,d, 15b] or the oxygen atom of a coordinated dibenzylideneacetone [15a] ), a situation that opens interesting perspectives in organometallic catalysis. [16,17] The coordination of preformed ambiphilic ligands affords access to a broad variety of TM complexes featuring coordinated Lewis acids, thereby offering a unique opportunity to gain a better understanding of such unusual M!ER 3 interactions. In this context, we report here the synthesis and complete characterization of the [RhCl(CO)A C H T U N G T R E N N U N G (dpb)] complex, [18] providing spectroscopic and structural evidence for i) trans coordination of the DPB ligand, ii) the existence of two isomeric structures differing only in their relative orientations around the rhodium and boron centres, iii) the noticeable influence of the CO co-ligand on the Rh!B interaction, and iv) the quantification of the electron-withdrawing effect of the s-accepting borane moiety through analysis of the ñ(CO) frequency.…”

Coordination Chemistry of Neutral (Ln)–Z Amphoteric and Ambiphilic Ligands

“…[52,53] Substitution of the NEt 3 by more nucleophilic quinuclidine gives 46·quinuclidine, where the highly donating amine binds the Lewis acid irreversibly. The resulting complex was found to be no more active than (1-MeInd)Ni(PMe 3 )Me, where no ambiphilic ligand is present.…”

“…The resulting complex was found to be no more active than (1-MeInd)Ni(PMe 3 )Me, where no ambiphilic ligand is present. [52] Although the exact role of the ambiphilic ligand is not known, it was proposed that the constrained geometry imposed by the tether was favouring the formation of zwitterionic species such as 47, whereas the usual Lewis acids, such as AlMe 3 , instead formed adducts in the style of 48, in which the sites where the incoming substrates usually bind are occupied. Indeed, the addition of AlMe 3 to (1-MeInd)Ni(PMe 3 )Me was found to inhibit phenylsilane dehydropolymerization.…”