Homoleptic copper(II) and copper(I) complexes of 5,6-dihydro-5,6-epoxy-1,10-phenanthroline. Six-coordinate copper(I) in solution

Abstract

Reaction of 5,6-dihydro-5,6-epoxy-1,10-phenanthroline (L) with Cu(ClO4)2·6H2O in methanol in 3:1M ratio at room temperature yields light green [CuL3](ClO4)2·H2O (1). The X-ray crystal structure of the hemi acetonitrile solvate [CuL3](ClO4)2·0.5CH3CN has been determined which shows Jahn–Teller distortion in the CuN6 core present in the cation [CuL3]2+. Complex 1 gives an axial EPR spectrum in acetonitrile–toluene glass with g||=2.262 (A||=169×10−4cm−1) and g⊥=2.069. The Cu(II/I) potential in 1 in CH2Cl2 at a glassy carbon electrode is 0.32V versus NHE. This potential does not change with the addition of extra L in the medium implicating generation of a six-coordinate copper(I) species [CuL3]+ in solution. B3LYP/LanL2DZ calculations show that the six Cu–N bond distances in [CuL3]+ are 2.33, 2.25, 2.32, 2.25, 2.28 and 2.25Å while the ideal Cu(I)–N bond length in a symmetric Cu(I)N6 moiety is estimated as 2.25Å. Reaction of L with Cu(CH3CN)4ClO4 in dehydrated methanol at room temperature even in 4:1M proportion yields [CuL2]ClO4 (2). Its 1H NMR spectrum indicates that the metal in [CuL2]+ is tetrahedral. The Cu(II/I) potential in 2 is found to be 0.68V versus NHE in CH2Cl2 at a glassy carbon electrode. In presence of excess L, 2 yields the cyclic voltammogram of 1. From 1H NMR titration, the free energy of binding of L to [CuL2]+ to produce [CuL3]+ in CD2Cl2 at 298K is estimated as −11.7 (±0.2)kJmol−1.