High-valent bis(oxo)-bridged dinuclear manganese oxamates: Synthesis, crystal structures, magnetic properties, and electronic structure calculations of bis(μ-oxo)dimanganese(IV) complexes with a binucleating o-phenylenedioxamate ligand

Two novel bis(oxo)-bridged dinuclear manganese(IV) complexes with the binucleating ligand o-phenylenebis(oxamate) (opba), formulated as (Me4N)4[Mn2O2(opba)2] (1a) and (Me4N)2(Ph4P)2[Mn2O2(opba)2] · 8H2O (1b), have been synthesized and characterized structurally and magnetically. Like the parent complex (Ph4P)4[Mn2O2(opba)2] · 4H2O (1c), they possess unique Mn2(μ-O)2 bridging cores with two additional o-phenylenediamidate bridges which lead to exceptionally short Mn–Mn distances (2.63–2.67 Å) and fairly bent Mn–O–Mn angles (93.8–95.5°). Complexes 1a–c show a moderate to strong antiferromagnetic coupling between the two high-spin MnIV ions through the bis(oxo)bis(o-phenylenediamidate) quadruple bridge (−J = 70–164 cm−1; H = −JS1 · S2). Along this series, the −J values increase with the shortening of the Mn–Mn distance and/or the lessening of the Mn–O–Mn angle. Electronic structure calculations on model complexes reproduce the observed dependence of −J   with the Mn–O–Mn angle at short intermetallic distances, and reveal that the magnetic coupling is dominated by the in-plane 3dx2-y23dx2-y2 type superexchange pathway via the oxo bridges with a small but nonnegligible contribution from direct 3dx2-y23dx2-y2 type Mn–Mn σ-bond.

Graphical abstractA new magneto-structural correlation between the antiferromagnetic coupling (−J) and the Mn–O–Mn bridging angle (α) in a novel series of dinuclear manganese(IV) oxamate complexes possessing an unexpected Mn2(μ-O)2(μ-N2C6H4)2 bridging core is reported, which raises the question on the role of superexchange interaction through the oxo bridges versus direct metal–metal bonding in the modulation of the magnetic coupling for high-valent bis(oxo)-bridged manganese dimersFigure optionsDownload full-size imageDownload as PowerPoint slide