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Tebbe-like and Phosphonioalkylidene and -alkylidyne Complexes of Scandium

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posted on 22.05.2020, 21:05 by Pavel Zatsepin, Eunji Lee, Jun Gu, Michael R. Gau, Patrick J. Carroll, Mu-Hyun Baik, Daniel J. Mindiola
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.

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