Tebbe-like and Phosphonioalkylidene and -alkylidyne Complexes of Scandium.

Abstract

The bonding between scandium and carbon in a series of alkylidene- and alkylidyne-like moieties is compared. The Tebbe analogue complex (PNP)Sc(μ2-CHSiMe3)(μ2-CH3)[Al(CH3)(CH2SiMe3)] (2) (PNP- = N[2-PiPr2-4-methylphenyl]2) could be formed by adding AlMe3 to (PNP)Sc(CH2SiMe3)2 (1). The fluxional behavior of 2 is studied by a combination of 2D 13C-1H HMQC, HMBC, and other heteronuclear NMR spectroscopic experiments. The phosphonioalkylidene complex (PNP)Sc(CHPPh3)(CH3) (3) could be prepared from 2 by treatment with 2 equiv of the ylide H2CPPh3 or by methane elimination from (PNP)Sc(CH3)2 and 1 equiv of H2CPPh3. The reactivity of the alkylidene in 2 was further explored with N3Ad, which gave insertion at the Sc-C bond, yielding (PNP)Sc(CH3)[η2-N3AdCHSiMe3Al(CH3)(CH2SiMe3)] (4), while DMAP provided C-H activation across the alkylidene with loss of the Al-C bond to form (PNP)Sc(η2-NC5H3NMe2)(CH2SiMe3) (5). Utilizing the same approach that yielded 2, methane elimination in 3 could further be promoted with Al(CH3)3 to furnish the first example of a scandium phosphonioalkylidyne complex, (PNP)Sc(μ2-CPh3)(μ2-CH3)Al(CH3)2 (6). Experimental and theoretical studies were combined to compare the bonding in 2, 3, and 6, in order to understand the legitimacy of Sc-C multiple bond character.