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

Abstract

The structure of cis-[Mo(CO)4(PPh3)2] 1 was determined by Cotton et al. in 1982,1 with the space group P-1. A second polymorph 2 is reported here, with the space group P21/c. The compounds differ in the relation between the conformational chiral triphenylphosphine groups. In 1 there is a pi-pi stacking between adjacent phenyl groups whereas in 2 there are sigma-pi interactions instead. 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 the C(ipso)-P-M-P torsion angles. The majority of compounds fall in the pi-pi stacking data area with torsion angles 10-15° and 55-60°, however for octahedral-coordinated metals the optimum is a sigma-pi interaction at 40° and 40°. This corresponds well to the values in 2: 46°, 40° but for 1 we instead find the torsion angles 11° and 18°. There is indeed a small tendency for these values as well in the data, and it appears that for 1 this conformation is stabilised by weak CO…H-C hydrogen bonds. Density Functional Theory (DFT) calculations indicate that 1 is the more stable polymorph by 72 kJ/mol but 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 sigma-pi interactions (+17 kJ/mol) and pi-pi stacking (+30 kJ/mol).

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Publication details

The article was received on 02 Mar 2018, accepted on 01 Jun 2018 and first published on 08 Jun 2018


Article type: Paper
DOI: 10.1039/C8CE00337H
Citation: CrystEngComm, 2018, Accepted Manuscript
  • Open access: Creative Commons BY-NC license
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    Conformational chiral polymorphism in cis-bis-triphenylphosphine complexes of transition metals

    N. Tshabang, G. P. Makgatle, S. A. Bourne, N. Kann, J. D. Evans, F. Coudert and L. R. Öhrström, CrystEngComm, 2018, Accepted Manuscript , DOI: 10.1039/C8CE00337H

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