Mononuclear [(BP)₂MX]ⁿ⁺ (M = Cu²⁺, Co²⁺, Zn²⁺; X = OH₂, Cl⁻) complexes with a new biphenyl appended N-bidentate ligand: structural, spectroscopic, solution equilibrium and ligand dynamic studies

A new series of five-coordinate [(BP)₂MX]ⁿ⁺ complexes, (where X = OH₂, M = Zn(II) (1), Cu(II) (2); X = Cl⁻, M = Cu(II) (3), Co(II) (4)) with a new bidentate chelating ligand [{N,N(1,1′-biphenyl-2,2′-dimethylene)-N(2-pyridyl methyl)} amine] with a biphenyl group (BP), have been synthesized and characterized by X-ray crystal structure and combined spectroscopic methods. They display unique trigonal bipyramidal (TBP) geometry, influenced by the bidentate ligand. The Zn(II) complex 1 reveals ligand dynamics due to an atropisomeric biphenyl moiety as indicated by variable temperature (VT) proton NMR spectroscopy. The calculated free energy for the inversion of the bridged biphenyl is 13.08 kcal mol⁻¹ (T_c = 273 K, Δν = 82.8 Hz, J = 8.7 Hz). The absorption spectra of Cu(II) complexes 2 and 3, in CH₂Cl₂ display greatly enhanced d–d bands (800–950 nm, ε >500 M⁻¹ cm⁻¹). On the other hand, complex 2 in N,N-dimethylformamide (DMF) showed almost 50% reduction in absorption intensity as DMF, a coordinating solvent, displaces the weakly-coordinated tertiary amine-nitrogens of the ligand and this competitive binding was studied by electronic absorption spectroscopy. When the mononuclear copper aqua complex 2 was treated with a base, a dicopper dihydroxide complex, [{(BP)Cu}₂(μ-OH)₂]²⁺, (2a) was obtained. The same phenomenon was also observed with chloro complex 3 when treated with a base. This mono-dicopper equilibrium and conversion of 2→2a was monitored by UV-vis spectroscopy. Copper(II) complexes 2 and 3 displayed “reverse” EPR spectra consistent with the TBP geometry. Cyclic voltammetry of 2 and 3 in DMF showed an irreversible redox wave owing to Cu(II)/Cu(I) of five and four-coordinate species. The solution magnetic moment values of 1.76, 1.81 and 4.47 μB for 2, 3 and 4, respectively, are in agreement with Cu(II) (S = 1/2) and Co(II) (S = 3/2) high-spin configurations. The ¹H NMR of 4 displays sharp but hyperfine shifted signals for the ligand protons between −30 to +220 ppm. The ESI-mass data complement the data obtained from X-ray structure.