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, Mn-IV(mu-O)Mn-IV(mu-O)MnIV(u-O)Mn-IV, is found in the complex [(MnO4)-O-IV(EtOterpy)(4)(OH)(2)(OH2)(2)](ClO4)(6)(.)8H(2)O (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 Mn-IV 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 Mn(IV)4 oxidation state assignment, The di-mu-oxo- and mono-y-oxo-bridged Mn-Mn distances are 2.80 and 3.51 angstrom, 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 (H) over cap HDvV = -J(1)(S) over cap (2)(S) over cap (1) - J(2)(S) over cap (1)(S) over cap (1A) - J(1)(S) over cap (1A)(S) over cap (2A) (S-1 = S-1A = S-2 = S-2A = 3/2) with J(1) = -432 cm(-1) and J(2) = -164 cm(-1) (where J(1) and J(2) are exchange constants through the mono-mu-oxo and the di-mu-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.