Oxidative Stretching of Metal-Metal Bonds to Their Limits

Oxidation of quadruply bonded Cr-2(dpa)(4), Mo-2(dpa)(4), MoW(dpa)(4), and W-2(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 [Cr-2(dpa)(4)](2+) (1), [Mo-2(dpa)(4)](2+) (2), [MoW(dpa)(4)](2+) (3), and [W-2(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 center dot center dot center dot 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 angstrom) 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/k(B) = -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.