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 Co₄(μ₃-O)₄(OAc)₄(<i>t</i>-Bupy)₄ (OAc = acetate, <i>t</i>-Bupy = 4-<i>tert</i>-butylpyridine) was exposed to various metal acetate complexes. Reaction with Cu(OAc)₂ gave the structurally characterized (by X-ray diffraction) dicopper dangler Cu₂Co₄(μ₄-O)₂(μ₃-O)₂(OAc)₆(Cl)₂(<i>t</i>-Bupy)₄. In contrast, the analogous reaction with Mn(OAc)₂ produced the Mn^IV-containing cubane cation [MnCo₃(μ₃-O)₄(OAc)₄(<i>t</i>-Bupy)₄]⁺ by way of a metal-metal exchange that gives Co(OAc)₂ and [Co^III(μ-OH)(OAc)]<i>_n</i> oligomers as byproducts. Additionally, reaction of the formally Co^IV cubane complex [Co₄(μ₃-O)₄(OAc)₄(<i>t</i>-Bupy)₄][PF₆] with Mn(OAc)₂ gave the corresponding Mn-containing cubane in 80% yield. A mechanistic examination of the related metal-metal exchange reaction between Co₄(μ₃-O)₄(OBz)₄(py)₄ (OBz = benzoate) and [Mn(acac)₂(py)₂][PF₆] 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.