New metal-organic polygons involving MM quadruple bonds:: M8(O2CtBu)4(μ-SC4H2-3,4-{CO2}2)6 (M = Mo, W)

The reactions between M-2(O(2)0C(t)Bu)(4), where M = Mo or W, and thienyl-3,4-dicarboxylic acid (0.5-1.5 equiv) in luene proceed via a series of detectable intermediates to the compounds M-8((O2CBu)-Bu-t)(4)(mu-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 S-4 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 (MO2O)-O-... bonding is present, while the other thienylcarboxylate bridges merely serve to link these two [MO2](...)[MO2] units together. The color of the compounds arises from intense M-2 delta-to-thienyl pi* transitions and, in THF, the complexes are redox-active and show four successive quasi-reversible oxidation waves. The [M-8](+) 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 M-8(O2CH)(4)(mu SC4H2-3,4-{CO}(2))(6) employing density functional theory that reveal only a small splitting of the M-2 delta manifold via mixing with the 3,4-thienylcarboxylate pi system.