New Linear High-Valent Tetranuclear Manganese-Oxo Cluster Relevant to the Oxygen-Evolving Complex of Photosystem II with Oxo, Hydroxo, and Aqua Coordinated to a Single Mn(IV)

An unprecedented atom connectivity, MnIV(μ-O)MnIV(μ-O)2MnIV(μ-O)MnIV, is found in the complex [MnIV4O4(EtO-terpy)4(OH)2(OH2)2](ClO4)6·8H2O (EtO-terpy = 4‘-ethoxyl-2,2‘:6‘,2‘ ‘-terpyridine), which has been characterized by
X-ray crystallography, X-ray powder diffraction, EPR spectroscopy, and magnetic studies. This complex is the first
example of a compound where a MnIV ion is coordinated by all three types of water-derived ligands:  oxo, hydroxo,
and aqua. Bond distances and angles for this complex are consistent with a MnIV4 oxidation state assignment. The
di-μ-oxo- and mono-μ-oxo-bridged Mn−Mn distances are 2.80 and 3.51 Å, respectively. The variable-temperature
magnetic susceptibility data for this complex, in the range of 10−300 K, are consistent with an S = 0 ground state
and were fit using the spin Hamiltonian ĤHDvV = −J1Ŝ2Ŝ1 − J2Ŝ1Ŝ1A − J1Ŝ1AŜ2A (S1 = S1A = S2 = S2A = 3/2) with
J1 = −432 cm-1 and J2 = −164 cm-1 (where J1 and J2 are exchange constants through the mono-μ-oxo and the
di-μ-oxo bridges, respectively). The first excited spin state of this tetramer is a spin triplet state at 279 cm-1 above
the diamagnetic ground state. The next spin states are the S = 1 and S = 2 levels at about 700 and 820 cm-1
above the S = 0 ground state, respectively. These large energy gaps are consistent with the absence of an EPR
signal for this complex, even at high temperature.