Synthesis, structure, and characterization of tris(1-ethyl-4-isopropyl-imidazolyl-қN)phosphine nickel(II) complexes

[Display omitted] •The structures of five nickel(II) tris-imidazolylphosphine complexes are reported.•The nickel complexes can be interconverted by altering the solvent or reagents.•The electronic structures of these complexes have been characterized by DFT. In this work we report the synthesis of five new nickel(II) complexes all coordinated to the tripodal ligand tris(1-ethyl-4-iPr-imidazolyl)phosphine (T1Et4iPrIP). They are [Ni(T1Et4iPrIP)(CH3CN)2(OTf)](OTf) (1), [Ni(T1Et4iPrIP)(OTf)2] (2), [Ni(T1Et4iPrIP)(H2O)(OTf)](OTf) (3), [Ni(T1Et4iPrIP)Cl](OTf) (4), and [Ni(T1Et4iPrIP)Cl2] (5). The complexes serve as bioinorganic structural model complexes for histidine-coordinated nickel proteins. The X-ray structures have been determine for all complexes which feature coordination numbers 4–6. We investigated the spectroscopic interconversions for these compound in dichloromethane solution and demonstrate interconversion between 1 and 3 via 2 and conversion of 2 to 4. Complex 5 can be spectroscopically converted to the cation of 4 by dissolving it in dichloromethane. Fits of variable temperature magnetic susceptibility data yielded the following parameters: g = 1.944, D = −0.327 cm−1, E/D = 3.706 for 1; g = 2.280, D = −0.365 cm−1, E/D = 22.178 for 2; g = 2.000, D = −7.402 cm−1, E/D = −0.272 for 3; g = 2.176, D = −0.128 cm−1, E/D = −0.783 for 4; g = 2.258, D = 14.288 cm−1, E/D = 0.095 for 5. DFT structure optimizations afforded HOMO and LUMO energies indicating that complex 1 is the most stable.