Conformational analysis of triphenylphosphine ligands in stereogenic monometallic complexes: tools for predicting the preferred configuration of the triphenylphosphine rotor

Abstract

The extension of our simple model for predicting the propeller configuration of a triphenylphosphine ligand co-ordinated to achiral metal centres to include stereogenic metal systems is described. By considering nadir energy planes (NEP's) and a series of rigid-body calculations, a model has been developed to reliably predict the configuration of the triphenylphosphine rotor of stereogenic metal complexes. For complexes of the form [M(η⁵-C₅H₅)(PPh₃)(L¹)(L²)], where it is assumed that L¹ is larger than L², the configuration of the triphenylphosphine rotor may be predicted by viewing a Newman projection along the L¹–M bond. In the orientation where the PPh₃ unit is pointing vertically downwards and the orthogonal L² ligand is pointing to the right [i.e., an (R_M)-configured complex, assuming that L² is ranked higher priority than L¹], the conformation of L¹ can be expected to place the most sterically demanding substituent in the top-right quadrant. In cases where ligand L¹ still presents a steric incursion towards the PPh₃ ligand (any part of L¹ other than H proximal to the PPh₃ in the approximate zone −30° to +60° from the M–P bond) an (M)-configured rotor is expected, and when this interaction is not present a (P)-configured propeller is predicted. Without exception, these rules are consistent with all empirical data (>140 known crystal structures).