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

The novel ionic pentacoordinate Ni<sup>II</sup>−π-allyl complexes [(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Ni(PMe<sub>3</sub>)<sub>3</sub>]Y (<b>1a</b>−<b>d</b>) and [(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Ni{P(OMe)<sub>3</sub>}<sub>3</sub>]Y (<b>2a</b>−<b>d</b>) (Y = SO<sub>3</sub>CF<sub>3</sub> (<b>a</b>), PF<sub>6</sub> (<b>b</b>), Br (<b>c</b>), I (<b>d</b>)) have been synthesized and investigated by DSC, solid-state NMR, and X-ray single-crystal and powder diffractometry. Apart from <b>1a</b>, all complexes are polymorphic. The PMe<sub>3</sub> complexes <b>1b</b>−<b>d</b> undergo monoclinic-monoclinic phase transitions. The solid-state structures of <b>1a</b> and <b>1b-II</b> are rigid at low temperature but fluxional at ambient temperature, whereas those of <b>1c-II</b> and <b>1d-II</b> are almost rigid, while somewhat faster dynamics occur for <b>1c-I</b> and <b>1d-I</b>. Similarly, the structures of <b>2c-II</b> and <b>2d-II</b> are fully or almost fully rigid at ambient temperature. The properties of the P(OMe)<sub>3</sub>-ligated OTf and PF<sub>6</sub> salts <b>2a</b>,<b>b</b> are completely different. <b>2a</b> crystallizes in the triclinic phase <b>2a-II</b>, which transforms at −21 °C into the highly mobile, plastically crystalline mesophase <b>2a-I</b> with a cubic primitive cell. <b>2b</b> crystallizes from solution in the monoclinic phase <b>2b-III</b>, having a rigid structure, and transforms at ambient temperature reversibly into the similarly crystalline phase <b>2b-II</b>, which undergoes slow dynamics. Heating either phase to 53 °C gives rise to the plastically crystalline mesophase <b>2b-I</b>. For complexes <b>1a</b>−<b>d</b> (all phases) and the phases <b>2a-II</b> and <b>2b-II</b> the solid-state dynamics start with turnstile rotation of the three phosphorus ligands. For the mesophases <b>2a-I</b> and <b>2b-I</b> additional π-allyl ligand mobility has been established and anion mobility must also be assumed. DFT MO calculations on the cations <b>1</b> and <b>2</b> and the parent PH<sub>3</sub> complex <b>3</b> gave low rotational barriers of up to 3.5 kcal/mol in the gas phase. Complexes <b>2a</b>,<b>b</b> are rare examples of metallorganic plastic crystals (dynamically disordered mesophases).