TETRANUCLEAR MANGANESE-ORE AGGREGATES RELEVANT TO THE PHOTOSYNTHETIC WATER OXIDATION CENTER - CRYSTAL-STRUCTURE, SPECTROSCOPIC PROPERTIES AND REACTIVITY OF ADAMANTANE-SHAPED [MN4O6(BPEA)(4)](4-VALENCE ANALOG [MN4O6(BPEA)(4)](3+)() AND THE REDUCED MIXED)

The tetranuclear Mn-IV adamantane-like complex [MN4O6(bpea)(4)](ClO4)( 4)(1(ClO4)(4)) was isolated from a comproportionation reaction of Mn(C lO4)(2) . 6H(2)O and [n-Bu4N][MnO4] with the ligand N,N-bis(2-pyridylm ethyl)ethylamine (bpea) in acetonitrile. Characterization by X-ray cry stallography reveals that the [Mn4O6(bpea)(4)](4+) cation approaches S -4 point symmetry. There are three distinct types of Mn-N bonds and tw o 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 differe nt protonation isomer and that the isomers are readily distinguished b y their differences in solution structural symmetry and oxo bridge aci dity. In addition, pH-dependent aqueous electrochemical studies show t hat 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+ sto ichiometry. Remarkably, reaction of 1(BF4)(4) with the tridentate nitr ogen 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 substitute d product. Like 1, the [Mn4O6(bpea)(4)](3+) cation also contains the a damantane skeleton. The (Mn-III)(Mn-IV)3 cluster of 2 contains a cryst allographically distinguishable Mn-III ion, as noted by elongation alo ng the N-alkyl-Mn-O-oxo axis. The H-1 NMR solution spectrum of 2 is co nsistent with a valence-delocalized (Mn-III)(Mn-IV)(3) cluster, indica ting fast intramolecular electron transfer on the NMR time scale, and redox titration of 1 to 2 indicates slow intermolecular electron trans fer on the same time scale. Solution magnetic susceptibility measureme nts in acetonitrile show that conversion of 1(ClO4)(4) to 2(ClO4)(3) i s attendant with a change from net ferromagnetic coupling to overall m oderate antiferromagnetic coupling within the manganese-ore core. Isol ation of the {Mn4O6} core in the (IV)(4) and (III)(IV)(3) oxidation st ates is facilitated by the relatively weak donor nature of the bpea li gand, in marked contrast to the strongly basic donor ligands 1,4,7-tri azacyclononane (tacn) and 1,1,1-tris(aminomethyl)ethane (tame), which stabilize the higher oxidation state of the {Mn4O6} cluster, making th e one-electron reduced form less accessible. The novel protonation and electrochemical properties of 1 are discussed in the context of the K ok cycle of photosynthetic water oxidation.