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⁺ (<b>1</b>), [Mo₂(dpa)₄]²⁺ (<b>2</b>), [MoW­(dpa)₄]²⁺ (<b>3</b>), and [W₂(dpa)₄]²⁺ (<b>4</b>). Additional two-electron oxidation and oxygen atom transfer by <i>m</i>-chloroperoxybenzoic acid results in the formation of the corresponding metal–oxo compounds [Mo₂O­(dpa)₄]²⁺ (<b>5</b>), [WMoO­(dpa)₄]²⁺ (<b>6</b>), and [W₂O­(dpa)₄]²⁺ (<b>7</b>), which feature an unusual linear M···MO structure. Crystallographic studies of the two-electron-oxidized products <b>2</b>, <b>3</b>, and <b>4</b>, 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 <b>1</b>, and the resulting two isolated Cr³⁺ magnetic centers couple antiferromagnetically with <i>J</i>/<i>k</i>_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 <b>2</b>, <b>3</b>, and <b>4</b>. The metal–metal distances in the metal–oxo compounds <b>5</b>, <b>6</b>, and <b>7</b> 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