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 = −JS 1 · S 2). 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.