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

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, [Cr^IIIL­(ox)₂]⁻, and oxalato-bridged chromium­(III)-cobalt­(II) and chromium­(III)-manganese­(II) dinuclear cations, [Cr^IIIL­(ox)­(μ-ox)­M^IIL′(H₂O)₂]⁺ [M = Co, L = bpy, and L′ = Me₂phen (1); M = Mn and L = L′ = phen (2)]. These oxalato-bridged Cr^IIIM^II dinuclear cationic entities of 1 and 2 result from the coordination of a [Cr^IIIL­(ox)₂]⁻ unit through one of its two oxalato groups toward a [M^IIL′(H₂O)₂]²⁺ moiety with either a trans- (M = Co) or a cis-diaqua (M = Mn) configuration. The two distinct Cr^III ions in 1 and 2 adopt a similar trigonally compressed octahedral geometry, while the high-spin M^II 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 Cr^III (S_Cr = 3/2) ion and the high-spin Co^II (S_Co = 3/2) or Mn^II (S_Mn = 5/2) ions across the oxalato bridge within the Cr^IIIM^II dinuclear cationic entities (M = Co and Mn) [J = +2.2 (1) and +1.2 cm^–1 (2); H = –J S_Cr·S_M]. Density functional electronic structure calculations for 1 and 2 support the occurrence of S = 3 Cr^IIICo^II and S = 4 Cr^IIIMn^II 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 d_yz(Cr)/d_xy(M), d_xz(Cr)/d_xy(M), d_x²–y²(Cr)/d_xy(M), d_yz(Cr)/d_xz(M), and d_xz(Cr)/d_yz(M) pairs of orthogonal magnetic orbitals and the d_x²–y²(Cr)/d_x²–y²(M), d_xz(Cr)/d_xz(M), and d_yz(Cr)/d_yz(M) pairs of nonorthogonal magnetic orbitals, which would be ultimately responsible for the relative magnitude of the overall ferromagnetic coupling in 1 and 2.