Tetranuclear manganese-oxo aggregates relevant to the photosynthetic water oxidation center. Crystal structure, spectroscopic properties and reactivity

Abstract

The tetranuclear Mn(IV) adamantane-like complex [Mn4O6(bpea)4](ClO4)4(1(ClO4)4) was isolated from a comproportionation reaction of Mn(ClO4)2·6H2O 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 S4 point symmetry. There are three distinct types of Mn-N bonds and two types of bridging oxo ligands in 1. 1H 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 (Me3tacn) 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 1H 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-oxo 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.