Mixed ligand Cu(II) complexes: Square pyramidal vs trigonal bipyramidal with the pyrrole-based dipodal ligand having hydrogen bond acceptors

Abstract

The condensation between N,N-di(α-formylpyrrolyl-α-methyl)-N-methylamine LH2 and ethanolamine or picolylamine afforded the [1 + 2] acyclic Schiff base in an excellent yield. The reaction between LH2 and [Cu(OAc)2·H2O] yielded a bis(aqua)-bridged binuclear copper(II) complex [Cu{MeN(CH2–C4H2N–CHO)2-κN,N,N)}(μ-H2O)]21. The same product was obtained from the reaction between acyclic, cyclic or bicyclic Schiff base of LH2 and [Cu(OAc)2·H2O] or [CuCl2·2H2O] via hydrolysis of the Schiff-bases. Complex 1 readily reacts with diimines such as 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (dmp) to give mixed ligand mononuclear Cu(II) complexes [Cu{MeN(CH2–C4H2N–CHO)2-κN,N,N)}(bipy)] 2, [Cu{MeN(CH2–C4H2N–CHO)2-κN,N,N)}(phen)] 3, and [Cu{MeN(CH2–C4H2N–CHO)2-κN,N,N)}(dmp)] 4 in very high yields. In all structures, L2− retains its original coordination mode and the geometry around the copper atom in 1, 2 or 3 is best described as a distorted square pyramidal (sp); on the other hand, the copper atom assumes a distorted trigonal bipyramidal (tbp) geometry with dmp in 4 owing to the steric bulk of methyl groups. DFT calculations gave the energy difference between sp and tbp geometries of different copper(II) complexes containing bipy and phen ligands bearing sterically encumbered substituents. As the steric bulk increases, tbp becomes the preferred geometry.