μ-Chlorobis(diethylaluminium)bis(diphenylphosphino)methanide, a complex with windshield wiper fluxionality

Lithiated bis(diphenylphosphino)methane reacts with two equivalents of a dialkylchloro-alane or -gallane (R₂MCl; R = CH₃, C₂H₅; M = Al, Ga;) to form coordination compounds (3-6) which have peculiar structure and show unusual fluxional behaviour. A single crystal X-ray diffraction study of the compound ((C₆H₅)₂P]₂CH[(C₂H₅)₂Al]₂Cl (4) reveals that these complexes can be described as the unsymmetrical adducts of the potential six-electron donor anion [Ph₂PCHPPh₂]^- with the difunctional acceptor cation [Et₂AlClAlEt₂]⁺. Through P→ Al and C→ Al donor/acceptor bonds a five-membered ring PCAlClAl is generated, which has an envelope conformation typical for this ring size and a center of chirality at the central carbon atom of the anion. Variable temperature ¹H, ¹³C, and ³¹P NMR spectroscopy (in toluene) indicates that the acceptor portion R₂MClMR₂ appears to undergo a rotation around the central P₂CAl bond, thereby rendering the PPh₂ groups equivalent on the NMR time scale through breaking and re-formation of the AlP bonds. The process is best regarded as a "windshield wiper" motion, which leaves the AIC bonds intact since complete dissociation of R₂MCl units is unlikely in the light of the low activation barriers for the fluxionality.