Tetranuclear manganese-ore aggregates relevant to the photosynthetic water oxidation center.: Crystal structure, spectroscopic properties and reactivity of adamantane-shaped [Mn4O6(bpea)4]4+ and the reduced mixed-valence analog [Mn4O6(bpea)4]3+

The tetranuclear Mn-IV adamantane-like complex [MN4O6(bpea)(4)](ClO4)(4)(1(ClO4)(4)) was isolated from a comproportionation reaction of Mn(ClO4)(2) . 6H(2)O and [n-Bu4N][MnO4] with the ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) in acetonitrile. Characterization by X-ray crystallography reveals that the [Mn4O6(bpea)(4)](4+) cation approaches S-4 point symmetry. There are three distinct types of Mn-N bonds and two types of bridging oxo ligands in 1. H-1 NMR protonation studies of 1 in acetonitrile reveal that each type of oxo bridge renders a different protonation isomer and that the isomers are readily distinguished by their differences in solution structural symmetry and oxo bridge acidity. In addition, pH-dependent aqueous electrochemical studies show that the proton-coupled electron-transfer behavior of 1 is significant because it is the first example of a Mn-oxo aggregate exhibiting a e(-)/2H(+) stoichiometry and because it displays pH-dependent e(-)/H+ stoichiometry. Remarkably, reaction of 1(BF4)(4) with the tridentate nitrogen donor ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me(3)tacn) in acetonitrile affords the one-electron reduced complex [Mn4O6(bpea)(4)](BF4)(3) (2(BF4)(3)), rather than the anticipated ligand substituted product. Like 1, the [Mn4O6(bpea)(4)](3+) cation also contains the adamantane skeleton. The (Mn-III)(Mn-IV)3 cluster of 2 contains a crystallographically distinguishable Mn-III ion, as noted by elongation along the N-alkyl-Mn-O-oxo axis. The H-1 NMR solution spectrum of 2 is consistent with a valence-delocalized (Mn-III)(Mn-IV)(3) cluster, indicating fast intramolecular electron transfer on the NMR time scale, and redox titration of 1 to 2 indicates slow intermolecular electron transfer on the same time scale. Solution magnetic susceptibility measurements in acetonitrile show that conversion of 1(ClO4)(4) to 2(ClO4)(3) is attendant with a change from net ferromagnetic coupling to overall moderate antiferromagnetic coupling within the manganese-ore core. Isolation of the {Mn4O6} core in the (IV)(4) and (III)(IV)(3) oxidation states is facilitated by the relatively weak donor nature of the bpea ligand, in marked contrast to the strongly basic donor ligands 1,4,7-triazacyclononane (tacn) and 1,1,1-tris(aminomethyl)ethane (tame), which stabilize the higher oxidation state of the {Mn4O6} cluster, making the one-electron reduced form less accessible. The novel protonation and electrochemical properties of 1 are discussed in the context of the Kok cycle of photosynthetic water oxidation.