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(mu(2)-CHSIMe3)(mu(2)-CH3)[Al-(CH3)(CH2SiMe3)] (2) (PNP- = N [2-(PPr2)-Pr-i-4-methylphenyl](2)) could be formed by adding AIMe(3) to (PNP)Sc(CH2SiMe3)(2) (1). The fluxional behavior of 2 is studied by a combination of 2D C-13-H-1 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 N(3)Ad, which gave insertion at the Sc-C bond, yielding (PNP)Sc(CH3)-[eta(2)-N(3)AdCHSiMe(3)Al(CH3)(CH2SiMe3)] (4), while DMAP provided C-H activation across the alkylidene with loss of the Al-C bond to form (PNP)Sc(eta(2)-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(mu(2)-CPh3)(mu(2)-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.