Conformational chiral polymorphism in cis-bis-triphenylphosphine complexes of transition metals

The structure of cis-[Mo(CO)4(PPh3)2] 1 was determined by F. A. Cotton, D. J. Darensbourg, S. Klein and B. W. S. Kolthammer, Inorg. Chem., 1982, 21, 1651-1655, with the space group P1. A second polymorph 2 is reported here, with the space group P21/c. The compounds differ in the interactions between the conformational chiral triphenylphosphine groups. In 1, there is π-π stacking between adjacent phenyl groups, whereas in 2, there are σ-π interactions instead. A search of the Cambridge Structural Database reveals that this is a relatively frequent occurrence in cis-bis-triphenylphosphine complexes and the phenomenon can be analysed by means of the C(ipso)-P-M-P torsion angles. The majority of compounds fall in the π-π stacking data area with torsion angles of 10-15\ub0 and 55-60\ub0; however, for octahedrally coordinated metals, the optimum is a σ-π interaction at 40\ub0/40\ub0. This corresponds well to the values in 2: 46\ub0/40\ub0, but for 1, we instead find the torsion angles to be 11\ub0/18\ub0. There is indeed a small occurrence of these values as well in the data, and it appears that for 1, this conformation is stabilised by weak COH-C hydrogen bonds. Density functional theory (DFT) calculations indicate that 1 is the more stable polymorph by 72 kJ mol−1but that the strain of the complexes (the difference between a relaxed molecule in the respective conformation and the structure in the crystal) is larger for 1 than for 2, further indicating that a special intermolecular interaction is responsible for the stability of this polymorph. In both polymorphs, the triphenylphosphines have the same conformational chirality, consistent with single-molecule calculations that predict racemic conformations to be substantially higher in energy for both σ-π interactions (+17 kJ mol−1) and π-π stacking (+30 kJ mol−1)