Mononuclear [(BP)_2MX]~(n+)(M =Cu~(2+), Co~(2+), Zn~(2+); X = OH_2, Cl-)complexes with a new biphenyl appended N-bidentate ligand: structural,spectroscopic, solution equilibrium and ligand dynamic studiest

摘要

A new series of five-coordinate [(BP)_2MX]~(n+) complexes, (where X = OH_2,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 andcharacterized by X-ray crystal structure and combined spectroscopic methods. They display uniquetrigonal bipyramidal (TBP) geometry, influenced by the bidentate ligand. The Zn(II) complex 1 revealsligand 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, Δv= 82.8 Hz,J =8.7 Hz). The absorption spectra of Cu(II) complexes2 and 3, in CH_2Cl_2display greatly enhanced d-d bands (800-950 nm, ε>500M~(-1) cm~(-1)). On the otherhand, complex 2 in N,N-dimethylformamide (DMF) showed almost 50% reduction in absorptionintensity as DMF, a coordinating solvent, displaces the weakly-coordinated tertiary amine-nitrogens ofthe ligand and this competitive binding was studied by electronic absorption spectroscopy. When themononuclear 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 chlorocomplex 3 when treated with a base. This mono-dicopper equilibrium and conversion of 2 —> 2a wasmonitored by UV-vis spectroscopy. Copper(II) complexes 2 and 3 displayed "reverse" EPR spectraconsistent with the TBP geometry. Cyclic voltammetry of 2 and 3 in DMF showed an irreversible redoxwave owing to Cu(II)/Cu(I) of five and four-coordinate species. The solution magnetic moment valuesof 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 forthe ligand protons between -30 to +220 ppm. The ESI-mass data complement the data obtained fromX-ray structure.