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°) or anti values (180°). Analysis of the symmetry of the conformation space defined by the three M–P–X–C (X = CH₂ 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).