Synthesis, Structure, and Characterization of Tris(1-ethyl-4-isopropyl-imidazolyl-κN)phosphine Nickel(II) Complexes

In this work we report the synthesis of five new nickel(II) complexes all coordinated to the tripodal ligand tris(1-ethyl-4- Pr-imidazolyl)phosphine (TlEt4iPrIP). 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 1-3 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.