Mononuclear [(BP)<inf>2</inf>MX]ⁿ⁺ (M = Cu²⁺, Co ²⁺, Zn²⁺; X = OH<inf>2</inf>, 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 (Tc = 273 K, Δν = 82.8 Hz, J = 8.7 Hz). The absorption spectra of Cu(ii) complexes 2 and 3, in CH 2Cl2 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. © The Royal Society of Chemistry 2009.