μ-CHLOROBIS(DIETHYLALUMINIUM) BIS(DIPHENYLPHOSPHINO)METHANIDE, A COMPLEX WITH WINDSHIELD WIPER FLUXIONALITY

Article abstract of DOI:10.1016/0022-328X(85)87088-1

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 ^- with the difunctional acceptor cation ⁺.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 P2C-Al bond, thereby rendering the PPh2 groups equivalent on the NMR time scale through breaking and re-formation of the Al-P bonds.The process is best regarded as a "windshield wiper" motion, which leaves the AlC bonds intact since complete dissociation of R2MCl units is unlikely in the light of the low activation barriers for the fluxionality.