Tebbe-like and Phosphonioalkylidene and -alkylidyne Complexes of Scandium

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.