Oxidative Stretching of Metal–Metal Bonds to Their Limits

Oxidation of quadruply bonded Cr2(dpa)4, Mo2(dpa)4, MoW­(dpa)4, and W2(dpa)4 (dpa = 2,2′-dipyridylamido) with 2 equiv of silver­(I) triflate or ferrocenium triflate results in the formation of the two-electron-oxidized products [Cr2(dpa)4]2+ (1), [Mo2(dpa)4]2+ (2), [MoW­(dpa)4]2+ (3), and [W2(dpa)4]2+ (4). Additional two-electron oxidation and oxygen atom transfer by m-chloroperoxybenzoic acid results in the formation of the corresponding metal–oxo compounds [Mo2O­(dpa)4]2+ (5), [WMoO­(dpa)4]2+ (6), and [W2O­(dpa)4]2+ (7), which feature an unusual linear M···MO structure. Crystallographic studies of the two-electron-oxidized products 2, 3, and 4, which have the appropriate number of orbitals and electrons to form metal–metal triple bonds, show bond distances much longer (by >0.5 Å) than those in established triply bonded compounds, but these compounds are nonetheless diamagnetic. In contrast, the Cr–Cr bond is completely severed in 1, and the resulting two isolated Cr3+ magnetic centers couple antiferromagnetically with J/kB= −108(3) K [−75(2) cm–1], as determined by modeling of the temperature dependence of the magnetic susceptibility. Density functional theory (DFT) and multiconfigurational methods (CASSCF/CASPT2) provide support for “stretched” and weak metal–metal triple bonds in 2, 3, and 4. The metal–metal distances in the metal–oxo compounds 5, 6, and 7 are elongated beyond the single-bond covalent radii of the metal atoms. DFT and CASSCF/CASPT2 calculations suggest that the metal atoms have minimal interaction; the electronic structure of these complexes is used to rationalize their multielectron redox reactivity.