Tris(3,5-di-tert-butylcatecholato)molybdenum(VI): Lewis Acidity and Nonclassical Oxygen Atom Transfer Reactions

In the solid state, tris­(3,5-di-tert-butylcatecholato)­molybdenum­(VI) forms a dimer with seven-coordinate molybdenum and bridging catecholates. NMR spectroscopy indicates that the dimeric structure is retained in solution. The molybdenum center has a high affinity for Lewis bases such as pyridine or pyridine-N-oxide, forming seven-coordinate monomers with a capped octahedral geometry, as illustrated by the solid-state structure of (3,5-tBu2Cat)3Mo­(py). Structural data indicate that the complexes are best considered as Mo­(VI) with substantial π donation from the nonbridging catecholates to molybdenum. Both the dimeric and the monomeric tris­(catecholates) react rapidly with water to form free catechol and oxomolybdenum bis­(catecholate) complexes. Monooxomolybdenum complexes are also obtained, more slowly, on reaction with dioxygen, with organic products consisting mostly of 3,5-di-tert-butyl-1,2-benzoquinone with minor amounts of the extradiol oxidation product 4,6-di-tert-butyl-1-oxacyclohepta-4,6-diene-2,3-dione. The pyridine-N-oxide complex reacts on heating (with excess pyO) to form initially (3,5-tBu2Cat)2MoO­(Opy) and ultimately MoO3(Opy), with quinone and free pyridine as the only organic products. The decay of (3,5-tBu2Cat)3Mo­(Opy) shows an accelerated, autocatalytic profile because the oxidation of its product, (3,5-tBu2Cat)2MoO­(Opy), produces an oxo-rich, catecholate-poor intermediate which rapidly conproportionates with (3,5-tBu2Cat)3Mo­(Opy), providing an additional pathway for its conversion to the mono-oxo product. The tris­(catecholate) fragment Mo­(3,5-tBu2Cat)3 deoxygenates Opy in this nonclassical oxygen atom transfer reaction slightly less rapidly than does its oxidized product, MoO­(3,5-tBu2Cat)2.