Structural Properties and Dynamics of Five-Coordinate Nickel(II)−π-Allyl Complexes Containing Monodentate Phosphorus Ligands

The novel ionic pentacoordinate Ni^II−π-allyl complexes [(η³-C₃H₅)Ni(PMe₃)₃]Y (1a−d) and [(η³-C₃H₅)Ni{P(OMe)₃}₃]Y (2a−d) (Y = SO₃CF₃ (a), PF₆ (b), Br (c), I (d)) have been synthesized and investigated by DSC, solid-state NMR, and X-ray single-crystal and powder diffractometry. Apart from 1a, all complexes are polymorphic. The PMe₃ complexes 1b−d undergo monoclinic-monoclinic phase transitions. The solid-state structures of 1a and 1b-II are rigid at low temperature but fluxional at ambient temperature, whereas those of 1c-II and 1d-II are almost rigid, while somewhat faster dynamics occur for 1c-I and 1d-I. Similarly, the structures of 2c-II and 2d-II are fully or almost fully rigid at ambient temperature. The properties of the P(OMe)₃-ligated OTf and PF₆ salts 2a,b are completely different. 2a crystallizes in the triclinic phase 2a-II, which transforms at −21 °C into the highly mobile, plastically crystalline mesophase 2a-I with a cubic primitive cell. 2b crystallizes from solution in the monoclinic phase 2b-III, having a rigid structure, and transforms at ambient temperature reversibly into the similarly crystalline phase 2b-II, which undergoes slow dynamics. Heating either phase to 53 °C gives rise to the plastically crystalline mesophase 2b-I. For complexes 1a−d (all phases) and the phases 2a-II and 2b-II the solid-state dynamics start with turnstile rotation of the three phosphorus ligands. For the mesophases 2a-I and 2b-I additional π-allyl ligand mobility has been established and anion mobility must also be assumed. DFT MO calculations on the cations 1 and 2 and the parent PH₃ complex 3 gave low rotational barriers of up to 3.5 kcal/mol in the gas phase. Complexes 2a,b are rare examples of metallorganic plastic crystals (dynamically disordered mesophases).