STRUCTURAL SYSTEMATICS .4. CONFORMATIONS OF THE DIPHOSPHINE LIGANDS IN M2(MU-PH2PCH2PPH2) AND M(PH2PCH2CH2PPH2) COMPLEXES

Data were retrieved from the Cambridge Structural Database for 405 crystal structures containing suitable geometric data for either M2(mu-dppm) 1 (dppm = Ph2PCH2PPh2) fragments (409 located) or M(dppe) 2 (M = transitional metal, dppe = Ph2PCH2CH2PPh2) fragments (274 such located). These data were analysed to examine the conformational preferences for the five-membered MP2C2 (or M2P2C) rings and the attached phenyl groups. The technique of principal component analysis was used to reduce the dimensionality of the torsion angle data set and to aid in identification of favoured conformations. The M(dppe) fragments show a preference for a twist (C2) conformation of the MP2C2 five-membered ring, in which the P-C-C-P torsion angle is far from zero (typically ca. +/- 50-degrees). In contrast the M2P2C five-membered rings of the M2(mu-dppm) fragments are predominantly of the envelope (C(s)) conformation, with the methylene carbon out of the plane of the near-planar M2P2 unit, ie. having the P-M-M-P torsion angle close to zero and the M-P-C-P torsion angles ca. +/- 45-degrees. The distribution of structures indicates that in both cases the ring conformations interconvert by a pseudo-rotation pathway, similar to those observed for other five-membered ring systems, in which planar intermediates are avoided. In both 1 and 2 the two phenyl groups on each phosphorus adopt conformations typical of one- and two-ring flip mechanisms of rotation. For M2(mu-dppm) there is considerable constraint of phenyl group orientation due to clashes between phenyl groups in axial sites on the envelope M2P2C ring. In contrast, although weak preferences may be seen in the phenyl group conformations for the M(dppe) fragments, these arise primarily because of contacts between the phenyl groups and the ethylene hydrogens of the MP2C2 ring and not due to transannular Ph ... Ph contacts. The implications of these observations for the design of new diphosphine ligands are discussed.