AQUEOUS CHEMISTRY OF HIGH-VALENT MANGANESE - STRUCTURE, MAGNETIC, ANDREDOX PROPERTIES OF A NEW-TYPE OF MN-OXO CLUSTER, [(MN-IV-O-4(6)(BPY)(6)](4- RELEVANCE TO THE OXYGEN-EVOLVING CENTER IN PLANTS() )

The tetranuclear species [(Mn4O6)-O-IV(bpy)(6)] (ClO4)(4).H2O was isol ated from an aqueous nitric acid solution (pH = 2) of Mn-III(bpy)Cl-3( H2O) upon addition of NaClO4. It crystallizes in the triclinic space g roup <P(1)over bar> with a = 20.213(7) Angstrom, b = 13.533(7) Angstro m, c = 13.411(8) Angstrom, alpha = 112.01(8)degrees beta = 96.72(11)de grees, gamma = 100.34(12)degrees, V = 3276.9 Angstrom(3), and Z = 2. T he cation has a nonrectilinear chain structure [(bpy)(2)Mn-IV aO2MnIV (b)(bpy)O2MnIV (c)(bpy)O2MnIV (d)(bpy)(2)](4+). The metal-metal distan ces are Mn-a-Mn-b = 2.746(5) Angstrom, Mn-b-M(c) = 2.760(5) Angstrom, Mn-c-Mn-d = 2.735(4) Angstrom, Mn-a-Mn-c = 4.899(10) Angstrom, Mn-b-Mn -d = 4.897(6) Angstrom, and Mn-a-Mn-d = 6.419(11) Angstrom. The variab le temperature magnetic susceptibility data for [Mn-IV O-4(6)(bpy)(6)] (ClO4)(4).H2O in the range 12-294 K were fit using the spin Hamiltonia n H-S = -J(ab)S(a)S(b)J(bc)S(b)S(c) - J(cd)S(c)S(d), with J(ab) = J(cd ) = -176 cm(-1) and J(bc) = -268 cm(-1). The lowest spin states are or ganized similarly to those for a dinuclear Mn-IV-Mn-IV unit. This is d ue to the stronger antiferromagnetic coupling between the two central Mn-IV ions relative to the coupling between a and b on one hand and c anc d on the other hand. This compound is EPR inactive at 4 K but show s a spectrum around 100 K which was attributed to the triplet and quin tet excited states. Electrochemically, the cation [Mn-IV O-4(6)(bpy)(6 )](4+) is irreversibly reduced to Mn-II at +0.5 V/SCE. Its lower oxidi zing power than those of [Mn-IV (2)O(2)L(4)](4+) (L = bpy or phen) was attributed to the dispersion of the positive charge and the surroundi ng of Mn-IV central ions by oxo groups. The [Mn-IV O-4(6)(bpy)(6)](4+) differs by the nature of the central bridge from the proposal by Klei n et al. (Science 1993, 260, 675-679) for the structure of the oxygen evolving center (OEC) in plants. In the hypothesis that in the S-2 sta te, the oxidation state of the OEC is Mn-IV Mn-3(III), the cation bb[M n-IV O-4(6)-(bpy)(6)](4+) would correspond at least formally to the S- 2 state. Both [Mn-IV O-4(6)(bpy)(6)](4+) and Klein's model have the sa me spin coupling chain topology. We thus applied our results on spin c oupling in [Mn-IV O-4(6)(bpy)(6)](4+) to the spin coupling properties of the S-2 state of the OEC. We showed that in order to interpret the ground state observed for the pre-S-2 and S-2 states, Klein's model ha s to be adapted in either of the two following possibilities. (i) The ''short-short-long'' model, which has the metal-metal bond distances r eordered as Mn-a-Mn-b = 2.7 Angstrom, Mn-b-Mn-c = 2.7 Angstrom, Mn-c-M n-d = 3.3 Angstrom. The distribution of oxidation states along abcd is then (IV,III,IV,IV). Depending on the sign of J(cd), this model has a S = 1/2 (J(cd) antiferro) or a S = 5/2 ground state (J(cd) ferro). Th is model could be dismissed if it would be proven that the g = 4.1 sig nal originates from a S = 3/2 ground state. (ii) The ''ring'' model. B y analyzing the exact solutions of the ring spin coupling problem H-S = -J(ab)S(a)S(b) - J(bc)S(b)S(c) - J(cd)S(c)S(d) - J(da)S(d)S(a), We s howed that such a ring model with J(ab) = J(cd) < 0, J(bc) < 0, and J( da) > 0 can have a ground state with S = 1/2, 3/2, or 5/2, depending o n J(da)/J(ab) and J(bc)/J(ab). For instance, with S-a = 2, S-b = S-c = S-d = 3/2, and J(bc)/J(ab) = 1, a ratio J(da)/J(ab) = -0.35 is necess ary to get S = 3/2, which implies the existence of an efficient coupli ng group between the two terminal manganese atoms.