American Chemical Society
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Theoretical Insights into the Ferromagnetic Coupling in Oxalato-Bridged Chromium(III)-Cobalt(II) and Chromium(III)-Manganese(II) Dinuclear Complexes with Aromatic Diimine Ligands

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posted on 2012-03-05, 00:00 authored by Julia Vallejo, Isabel Castro, Mariadel Déniz, Catalina Ruiz-Pérez, Francesc Lloret, Miguel Julve, Rafael Ruiz-García, Joan Cano
Two novel heterobimetallic complexes of formula [Cr­(bpy)­(ox)2Co­(Me2phen)­(H2O)2]­[Cr­(bpy)­(ox)2]·4H2O (1) and [Cr­(phen)­(ox)2Mn­(phen)­(H2O)2]­[Cr­(phen)­(ox)2]·H2O (2) (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, and Me2phen = 2,9-dimethyl-1,10-phenanthroline) have been obtained through the “complex-as-ligand/complex-as-metal” strategy by using Ph4P­[CrL­(ox)2]·H2O (L = bpy and phen) and [ML′(H2O)4]­(NO3)2 (M = Co and Mn; L′ = phen and Me2phen) as precursors. The X-ray crystal structures of 1 and 2 consist of bis­(oxalato)­chromate­(III) mononuclear anions, [CrIIIL­(ox)2], and oxalato-bridged chromium­(III)-cobalt­(II) and chromium­(III)-manganese­(II) dinuclear cations, [CrIIIL­(ox)­(μ-ox)­MIIL′(H2O)2]+ [M = Co, L = bpy, and L′ = Me2phen (1); M = Mn and L = L′ = phen (2)]. These oxalato-bridged CrIIIMII dinuclear cationic entities of 1 and 2 result from the coordination of a [CrIIIL­(ox)2] unit through one of its two oxalato groups toward a [MIIL′(H2O)2]2+ moiety with either a trans- (M = Co) or a cis-diaqua (M = Mn) configuration. The two distinct CrIII ions in 1 and 2 adopt a similar trigonally compressed octahedral geometry, while the high-spin MII ions exhibit an axially (M = Co) or trigonally compressed (M = Mn) octahedral geometry in 1 and 2, respectively. 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 intramolecular ferromagnetic interactions between the CrIII (SCr = 3/2) ion and the high-spin CoII (SCo = 3/2) or MnII (SMn = 5/2) ions across the oxalato bridge within the CrIIIMII dinuclear cationic entities (M = Co and Mn) [J = +2.2 (1) and +1.2 cm–1 (2); H = –J SCr·SM]. Density functional electronic structure calculations for 1 and 2 support the occurrence of S = 3 CrIIICoII and S = 4 CrIIIMnII ground spin states, respectively. A simple molecular orbital analysis of the electron exchange mechanism suggests a subtle competition between individual ferro- and antiferromagnetic contributions through the σ- and/or π-type pathways of the oxalato bridge, mainly involving the dyz(Cr)/dxy(M), dxz(Cr)/dxy(M), dx2y2(Cr)/dxy(M), dyz(Cr)/dxz(M), and dxz(Cr)/dyz(M) pairs of orthogonal magnetic orbitals and the dx2y2(Cr)/dx2y2(M), dxz(Cr)/dxz(M), and dyz(Cr)/dyz(M) pairs of nonorthogonal magnetic orbitals, which would be ultimately responsible for the relative magnitude of the overall ferromagnetic coupling in 1 and 2.