Ferromagnetic coupling and magnetic anisotropy in oxalato-bridged trinuclear chromium(iii)-cobalt(ii) complexes with aromatic diimine ligands

Abstract

Two novel heterotrinuclear chromium(III)-cobalt(II) complexes of formula {[Cr^III(bpy)(ox)₂]₂Co^II(Me₂bpy)}·2H₂O (1) and {[Cr^III(phen)(ox)₂]₂Co^II(Me₂bpy)}·1.5H₂O (2) [ox = oxalato, bpy = 2,2′-bipyridine, Me₂bpy = 6,6′-dimethyl-2,2′-bipyridine, and phen = 1,10-phenanthroline] have been synthesized using the “complex-as-ligand/complex-as-metal” strategy. The X-ray crystal structure of 2 consists of neutral oxalato-bridged Cr^III₂Co^II bent entities formed by the coordination of two anionic [Cr^III(phen)(ox)₂]⁻ complexes through one of their oxalato groups toward a cationic cis-[Co^II(Me₂bpy)]²⁺ complex. The three tris(chelated), six-coordinated metal atoms possess alternating propeller chiralities leading thus to a racemic mixture of heterochiral (Λ,Δ,Λ)- and (Δ,Λ,Δ)-Cr^IIICo^IICr^III triads, whereby the two peripheral chromium(III) ions adopt a trigonal distorted trapezoidal bipyramidal geometry and the central high-spin cobalt(II) ion exhibits a compressed rectangular bipyramidal one. The intermolecular π–π stacking interactions between the enantiomeric pairs of heterochiral Cr^III₂Co^II entities through the aromatic diimine terminal ligands lead to a unique two-dimensional supramolecular network. Variable temperature (2.0–300 K) magnetic susceptibility and variable-field (0–5.0 T) magnetization measurements for 1 and 2 reveal the presence of weak but non-negligible intermolecular antiferromagnetic interactions [zj = −0.012 (2a) and −0.08 cm⁻¹ (2b)] between the Cr^III₂Co^II molecules possessing a moderately anisotropic S = 9/2 ground state. This results from the moderately weak intramolecular ferromagnetic coupling [J = +2.43 (1) and +2.34 cm⁻¹ (2)] between the two peripheral Cr^III (S_Cr = 3/2) and the central high-spin Co^II (S_Co = 3/2) ions across the oxalato bridge as well as the appreciable single-ion axial magnetic anisotropy of the central high-spin Co^II (S_Co = 3/2) ion [D_Co = −2.29 (1) and −2.15 cm⁻¹ (2)]. A simple molecular orbital analysis of the exchange interaction in 1 and 2 identifies the σ- and π-type pathways involving the dx²_–y²(Cr)/d_xy(Co) and d_xz(Cr)/d_yz(Co) pairs of orthogonal magnetic orbitals, respectively, as the two main individual contributions responsible for the overall ferromagnetic coupling observed.