Magnetic coupling and spin topology in linear oxalato-bridged tetranuclear chromium(III)–copper(II) complexes with aromatic diimine ligands

A novel heterotetranuclear chromium(III)–copper(II) complex of formula {[CrIII(bpy)(ox)2]2CuII2(bpy)2(ox)}·6H2O (1) has been synthesized by the ligand exchange reaction between Ph4P[CrIII(bpy)(ox)2]·H2O and [CuII(bpy)2(NO3)]NO3·MeOH in methanol (bpy = 2,2′-bipyridine; ox2− = oxalate dianion). The X-ray crystal structure of 1 consists of neutral oxalato-bridged CrIII2Cu2II zigzag entities which are formed by the monodentate coordination of two [CrIII(bpy)(ox)2]− mononuclear anionic units through one of its two oxalato groups toward a [CuII2(bpy)2(ox)]2+ dinuclear cationic moiety featuring relatively long axial bonds at the square pyramidal CuII ions. Variable temperature (2.0–300 K) magnetic susceptibility and variable-field (0–5.0 T) magnetization measurements for 1 have been interpreted on the basis of the linear tetranuclear topology of the CrIII2CuII2 entities [H = –J(S1 · S2 + S3 · S4) – J′ S2 · S3 with S1 = S4 = SCr = 3/2 and S2 = S3 = SCu = 1/2]. A weak antiferromagnetic coupling occurs between the outer CrIII and inner CuII ions through the peripheral bidentate/monodentate(outer) oxalates (J = –8.7 cm−1), while a strong antiferromagnetic coupling is operative between the inner CuII ions across the central bis-bidentate oxalate (J′ = –472.2 cm−1), leading thus to a S = (SCr – SCu) – (SCr – SCu) = 0 ground spin state for the CrIII2CuII2 entity of 1. A simple orbital analysis of the electron exchange interaction in the oxalato-bridged CuII2 and CrIIICuII fragments identify the σ-type pathways involving the d x 2 - y 2 (Cu)/ d x 2 - y 2 (Cu) and dxy(Cr)/ d x 2 - y 2 (Cu) pairs of magnetic orbitals as the two main contributions responsible for the different strength of the intramolecular magnetic coupling parameters for 1. A magneto-structural correlation between the nature and magnitude of the magnetic coupling and the bending angle at the axial carbonyl-oxygen to copper bond has been established for 1 and related oxalato-bridged chromium(III)–copper(II) dinuclear complexes.