New copper(ii)-centered complexes with organometallic donor-acceptor substituted unsymmetrical Schiff base ligands

The synthesis and full characterization using various spectroscopic techniques and single-crystal X-ray diffraction analysis of a family of three robust copper(ii)-centered complexes with organometallic donor-acceptor substituted unsymmetrical Schiff base ligands, namely, the binuclear derivatives Cu[Fc-C(O)CHC(CH₃)N-X-NCH-(2-O,5-Br-C₆H₃)] (3: X = o-C₆H₄, 4: X = CH₂CH₂), and the corresponding ionic trinuclear counterpart of 4, [Cu{Fc-C(O)CHC(CH ₃)N-CH₂CH₂-NCH-(η⁶-2-O,5-Br- C₆H₃)RuCp*}][PF₆] (5) (Fc = (η⁵-C₅H₅)Fe(η⁵-C ₅H₄), Cp* = η⁵-C₅Me ₅), have been explored. Single crystal X-ray diffraction analysis of complex 3 indicates a bowed structure of the unsymmetrical Schiff base scaffold. The Cu²⁺ ion is tetracoordinated in a square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The EPR spectra of solid trinuclear complexes 5 and 7 (unsubstituted salicylidene ring and X = o-C ₆H₄) at low temperature (68 K) consisted of fine-structure transitions (ΔMs = ±1) with zero-field splitting (ZFS) values of 0.0458 and 0.0415 cm^-1, respectively, and a half-field signal (ΔMs = ±2) at ca. 1600 G, suggesting the formation of dimeric species (S = 1). In those dimers, Cu-Cu distances of 3.97 and 4.14 Å, respectively, are computed from ZFS terms. The variable-temperature (2-300 K) magnetic susceptibilities of powdered samples of 5 and 7 have been measured, and an intermolecular antiferromagnetic interaction with exchange couplings of J = -1.05 and -0.65 cm^-1, respectively, were found. The electrochemical behavior of both the neutral binuclear and ionic trinuclear compounds was investigated by cyclic voltammetry. Notably, the ability of the copper Schiff base core to transmit electronic effects between the two organometallic termini has been demonstrated by the observed anodic shift of the reversible voltammetric signal ascribed to the Fe^II/Fe^III couple of the ferrocenyl donor fragment.