New metal-organic polygons involving MM quadruple bonds: M ₈(O₂C^tBu)₄(μ-SC₄H ₂-3,4-{CO₂}₂)₆ (M = Mo, W)

The reactions between M₂(O₂C^tBu) ₄, where M = Mo or W, and thienyl-3,4-dicarboxylic acid (0.5-1.5 equiv) in toluene proceed via a series of detectable intermediates to the compounds M₈(O₂C^tBu)₄(μ-SC ₄H₂-SC₄H₂-3,4-{CO₂} ₂)₆, which are isolated as air-sensitive yellow (M = Mo) or red (M = W) powders and show parent molecular ions in , their mass spectra (MALDI). The structure of the molybdenum complex was determined by single-crystal X-ray crystallography and shown to contain an unusual M ₈ polygon involving four Mo₂ quadruply bonded units linked via the agency of the six 3,4-thienylcarboxylate groups. The structure has crystallographically imposed S₄ symmetry and may be described in terms of a highly distorted tetrahedron of Mo₂ units or a bisphenoid in which two Mo₂ units are linked by a thienyldicarboxylate such that intramolecular Mo₂⋯O bonding is present, while the other thienylcarboxylate bridges merely serve to link these two [Mo ₂]⋯[Mo₂] units together. The color of the compounds arises from intense M₂ δ-to-thienyl π* transitions and, in THF, the complexes are redox-active and show four successive quasi-reversible oxidation waves. The [M₈]⁺ radical cations, generated by one-electron oxidation with AgPF₆, are shown to be valence-trapped (class II) by UV-vis-near-IR and electron paramagnetic resonance spectroscopy. These results are supported by the electronic structure calculations on model compounds M₈(O₂CH) ₄(μ-SC₄H₂-3,4-{CO}₂)₆ employing density functional theory that reveal only a small splitting of the M₂ δ manifold via mixing with the 3,4-thienylcarboxylate π system.