A Rare Thermochromic Zwitterionic Nickel (II) Complex of the Bulky Alpha Ligand 1,2-Bis-(di-tbutylphosphinomethyl)Benzene

The reaction of the bulky ligand 1,2-bis-(di-tert-butylphosphinomethyl)benzene, 1 with [Ni(DME)Cl2], 3, DME = 1,2-dimethoxyethane, at room temperature over extended periods, affords the new blue Zwitterionic complex [2-(C6H4-CH2P(H)tBu2-1-(CH2PtBu2NiCl3)], 4, which contains a phosphonium group and an anionic nickel trichloride. This complex decomposes in alcohols such as methanol and the solution turns yellow. A discussion of the possible mechanism leading to the observed product is presented. Key to this is identification of the source of the phosphonium proton, which we speculated to arise from trace water in the initial nickel complex. To prove that trace water was present in [Ni(DME)Cl2], a sample of this precursor was reacted under similar condition with anhydrous DMF alone. In addition to the known complex [Ni(DMF)6)]2+[NiCl4]2−, 5, we identified the trans-diaqua complex [Ni(Cl)2(H2O)2(DMF)2], 6, which proved the presence of trace water. Interestingly in dimethylformamide, [2-(C6H4-CH2P(H)tBu2-1-(CH2PtBu2NiCl3)] exhibits thermochromic properties: an solution that is pale blue at ambient temperature reversibly changes colour to yellow upon cooling. This behaviour is specific to DMF and is related to the solvato-chromic behaviour exhibited by related DMF–nickel complexes. A discussion of the NMR spectra of compound 4 in a range of solvents is presented. The structures of the previously prepared molybdenum complex, [1,2-(C6H4-CH2PtBu2)2Mo(CO)4] and the bis-(phosphine sulphide) of the ligand, [1,2-(C6H4-H2P(S)tBu2)2], 5, are described for structural comparative purposes.