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

Abstract

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.