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, [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), dx2–y2(Cr)/dxy(M), dyz(Cr)/dxz(M), and dxz(Cr)/dyz(M) pairs of orthogonal magnetic
orbitals and the dx2–y2(Cr)/dx2–y2(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.

通过“配体/金属复合体”策略，利用Ph4P[CrL(ox)2]·H2O（L = bpy 和 phen）以及[ML′(H2O)4](NO3)2（M = Co 和 Mn；L′ = phen 和 Me2phen）作为前驱体，成功合成了两例新型的异金属双核配合物，其化学式分别为[Cr(bpy)(ox)2Co(Me2phen)(H2O)2]­[Cr(bpy)(ox)2]·4H2O（1）和[Cr(phen)(ox)2Mn(phen)(H2O)2]­[Cr(phen)(ox)2]·H2O（2）（bpy = 2,2′-联吡啶，phen = 1,10-菲咯啉，Me2phen = 2,9-二甲基-1,10-菲咯啉）。1 和 2 的X射线晶体结构由双(草酸根)铬酸根单核阴离子[CrIII(L(ox)2)]−和草酸根桥连的铬(III)-钴(II)以及铬(III)-锰(II)双核阳离子[CrIII(L(ox)­(μ-ox)­MIIL′(H2O)2]+ [M = Co, L = bpy, L′ = Me2phen (1)；M = Mn, L = L′ = phen (2)]组成。1 和 2 中的草酸根桥连的 CrIII-MII 双核阳离子实体是由[CrIII(L(ox)2)]−单元通过其两个草酸根之一与[MIIL′(H2O)2]2+基团配位而形成的，该基团具有反式（M = Co）或顺式二水合（M = Mn）的构型。1 和 2 中的两个不同的 CrIII 离子采用相似的三角压缩八面体几何构型，而高自旋的 MII 离子在 1 和 2 中分别表现出轴向（M = Co）或三角压缩（M = Mn）的八面体几何构型。对 1 和 2 进行的变温（2.0–300 K）磁化率和变场（0–5.0 T）磁化测量揭示了 CrIII (SCr = 3/2) 离子与高自旋 CoII (SCo = 3/2) 或 MnII (SMn = 5/2) 离子之间在 CrIII-MII 双核阳离子实体内部的草酸根桥连处存在微弱的分子内铁磁性相互作用（M = Co 和 Mn，J = +2.2 (1) 和 +1.2 cm–1 (2)；H = –J SCr·SM）。1 和 2 的密度泛函电子结构计算支持了 S = 3 CrIII-CoII 和 S = 4 CrIII-MnII 的基态自旋态。对电子交换机制的简单分子轨道分析表明，σ-和/或π型途径中个体铁磁和反铁磁贡献之间存在微妙的竞争，主要涉及dyz(Cr)/dxy(M)、dxz(Cr)/dxy(M)、dx2–y2(Cr)/dxy(M)、dyz(Cr)/dxz(M)和dxz(Cr)/dyz(M)等正交磁轨道对以及dx2–y2(Cr)/dx2–y2(M)、dxz(Cr)/dxz(M)和dyz(Cr)/dyz(M)等非正交磁轨道对，这最终将决定 1 和 2 中整体铁磁性耦合的相对大小。