Metal-Metal Redox Exchange to Produce Heterometallic Manganese-Cobalt Oxo Cubanes via a "Dangler" Intermediate.

Pendent metals bound to heterocubanes are components of well-known active sites in enzymes that mediate difficult chemical transformations. Investigations into the specific role of these metal ions, sometimes referred to as "danglers", have been hindered by a paucity of rational synthetic routes to appropriate model structures. To generate pendent metal ions bonded to an oxo cubane through a carboxylate bridge, the cubane Co 4 (μ 3 -O) 4 (OAc) 4 ( t -Bupy) 4 (OAc = acetate, t -Bupy = 4- tert -butylpyridine) was exposed to various metal acetate complexes. Reaction with Cu(OAc) 2 gave the structurally characterized (by X-ray diffraction) dicopper dangler Cu 2 Co 4 (μ 4 -O) 2 (μ 3 -O) 2 (OAc) 6 (Cl) 2 ( t -Bupy) 4 . In contrast, the analogous reaction with Mn(OAc) 2 produced the Mn IV -containing cubane cation [MnCo 3 (μ 3 -O) 4 (OAc) 4 ( t -Bupy) 4 ] + by way of a metal-metal exchange that gives Co(OAc) 2 and [Co III (μ-OH)(OAc)] n oligomers as byproducts. Additionally, reaction of the formally Co IV cubane complex [Co 4 (μ 3 -O) 4 (OAc) 4 ( t -Bupy) 4 ][PF 6 ] with Mn(OAc) 2 gave the corresponding Mn-containing cubane in 80% yield. A mechanistic examination of the related metal-metal exchange reaction between Co 4 (μ 3 -O) 4 (OBz) 4 (py) 4 (OBz = benzoate) and [Mn(acac) 2 (py) 2 ][PF 6 ] by ultraviolet-visible (UV-vis) spectroscopy provided support for a process involving rate-determining association of the reactants and electron transfer through a μ-oxo bridge in the adduct intermediate. The rates of exchange correlate with the donor strength of the cubane pyridine and benzoate ligand substituents; more electron-donating pyridine ligands accelerate metal-metal exchange, while both electron-donating and -withdrawing benzoate ligands can accelerate exchange. These experiments suggest that the basicity of the cubane oxo ligands promotes metal-metal exchange reactivity. The redox potentials of the Mn and cubane starting materials and isotopic labeling studies suggest an inner-sphere electron-transfer mechanism in a dangler intermediate.