Tetranuclear Manganese-Oxo 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 [Mn 4 O 6 (bpea) 4 ](ClO 4 ) 4 ( 1 (ClO 4 ) 4 ) was isolated from a comproportionation reaction of Mn(ClO 4 ) 2 ·6H 2 O and [ n -Bu 4 N][MnO 4 ] with the ligand N,N -bis(2-pyridylmethyl)ethylamine (bpea) in acetonitrile. Characterization by X-ray crystallography reveals that the [Mn 4 O 6 (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 . 1 H 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 (BF 4 ) 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 [Mn 4 O 6 (bpea) 4 ](BF 4 ) 3 ( 2 (BF 4 ) 3 ), rather than the anticipated ligand substituted product. Like 1, the [Mn 4 O 6 (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 1 H 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 (ClO 4 ) 4 to 2 (ClO 4 ) 3 is attendant with a change from net ferromagnetic coupling to overall moderate antiferromagnetic coupling within the manganese-oxo core. Isolation of the {Mn 4 O 6 } 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 {Mn 4 O 6 } 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.