Conformational analysis of PEt3 and P(OMe)(3) in metal complexes

The conformations of the archetypal acyclic phosphorus ligands PEt3 and P(OMe)(3) are classified on the basis of the observation that torsions about the P-C (or P-O) bonds show favoured conformations lying close to gauche (+/- 60 degrees) or anti values (180 degrees). Analysis of the symmetry of the conformation space defined by the three M-P-X-C (X = CII2 or O) torsion angles (t(1-3)) implies the existence of seven unique conformer types (A (aaa), B (g(+)g(+)g(+)), C (ag(+)g(+)), D (aag(+)), E (g(-)ag(+)), F (ag(-)g(+)), G (g(-)g(+)g(+)) and their symmetry equivalents) arising from the combinations of g(-), g(+) and a torsions. These conformers are observed in 1972 M-PEt3 and 735 M-P(OMe)(3) fragments from crystal structures of metal complexes in the CSD following the popularity sequence: F > C >> D > B > G > E >> A for M-PEt3; and: C > D > F >> E >> A, B, G. for M-P(OMe)(3). Pathways for low-energy interconversion of these conformers, dominated by single chain flip routes, are readily inferred for M-P(OMe)(3). The conformers of M-PEt3 are apparently less readily interconverted. The popularity of conformations is only loosely related to the energies of these conformations as calculated by DFT or MM methods for two-, four-(square planar) and six-coordinate metal complexes of these ligands (and free PEt3 and P(OMe)(3)). It would appear that the conformational preferences observed are determined by a balance between intra-ligand effects (repulsion between chains and avoidance of syn-pentane-like); inter-ligand effects (repulsions between gauche substituents at P and cis co-ligands notably when the coordination number at the metal is high); and residual anomeric effects (weakly favouring anti conformations in P(OMe)(3) species).