Binuclear Copper Complexes Based on the 6,6′-Bis[[bis(2-pyridylmethyl)amino]methyl]-2,2′-bipyridine Ligand

The tripodal tetradentate ligand TMPA (tris(2-pyridylmethyl)amine) has provided a wealth of valuable new Cu(I) and Cu(II) chemistry, in particular associated with copper(I)/dioxygen reactivity studies (Karlin, K. D.; Kaderli, S.; Zuberbühler, A. D. Acc. Chem. Res. 1997, 30, 139-147). Dinucleating analogues have also been recently investigated, and here we describe new copper complexes of the 6,6′-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2′-bipyridine ligand (bTMPA). The synthesis and X-ray crystallographic characterization of [(bTMPA)Cu^II(CH₃-CN)₂(ClO₄) ₂]²⁺ (1, as ClO₄^- salt) and [(bTMPA)Cu₂^II(N₃)₂(ClO ₄)₂] (2) are provided. [1: space group C2\c; a = 15.907(4) Å, b = 29.268(7) Å, c = 13.941(2) Å; β= 97.79(2)°; Z = 4; volume = 6431(2) ų. 2: space group P2₁/c; a = 8.118(5) Å, b = 29.743(8) Å, c = 9.120(6) Å; β= 114.00(5)°; Z = 2; volume = 2012(2) ų.] Both solid state structures possess six-coordinate copper(II) ions, and in neither case does the 2,2′-bipyridine (bipy) moiety within bTMPA chelate to a single metal ion. Dissociation of bound perchlorate and the presence of pentacoordinate solution structures are suggested by spectroscopic (UV-vis with two d-d absorptions; axial EPR spectra) along with conductivity data (1, 1:4 electrolyte; 2, 1:2 electrolyte). Electrochemical measurements by cyclic volammetry have been carried out, and for a dicopper(I) analogue, [(bTMPA)(Cu₂^IClO₄ (3(ClO₄)₂), a single quasireversible redox wave is observed; E_1/2 = +199 mV (versus Ag/AgCl in dimethylformamide), which is ∼280 mV more positive than that observed for the simple "parent" compound [Cu^I(TMPA)(CH₃CN)]-(ClO₄). Unlike [Cu^I(TMPA)(CH₃CN)](ClO₄), 3 does not readily form dioxygen adducts.