CHEMICAL MODELING OF THE OXYGEN-EVOLVING CENTER IN PLANTS - SYNTHESIS, STRUCTURE, AND ELECTRONIC AND REDOX PROPERTIES OF A NEW MIXED-VALENCE MN-OXO CLUSTER - L-N,N'-BIS(IMIDAZOL-4-YLMETHYL)ETHANE-1,2-DIAMINE) - EPR DETECTION OF AN IMIDAZOLE RADICAL-INDUCED BY UV IRRADIATION AT LOW-TEMPERATURE() (BISIMME(2)EN=N,N')

The compound [(Mn2O2)-O-III.IV(bisimMe(2)en)(2)](ClO4)(3) . H(2)0 (bis imMe(2)en = l-N,N'-bis(imidazol-4-ylmethyl)ethane-1,2-diamine) was syn thesized. It crystallizes in the monoclinic space group C2/C with a = 18.139(8) Angstrom, b = 12.694(5) Angstrom, c = 17.694(8) Angstrom, be ta = 107.5(6)degrees, V = 3885(6) Angstrom(3), and Z = 4. The cation [ (Mn2O2)-O-III.IV(bisimMe(2)en)(2)](3+) contains a di-manganese di-mu-o xo unit. The Mn-Mn axis is a C-2 axis. The Mn-Mn axis is a C-2 imidazo le distances for Mn-III or Mn-IV are distinct: respectively, 2.208(9) and 2.004(8) Angstrom. The ground state is a \S-A=2,S-B=3/2,S=1/2] sta te separated by 420 cm(-1) from the S = 3/2 state. The EPR spectrum ha s been simulated with \A(lxy)\ = 160 10(-4) cm(-1), \A(1z)\ = 1.36.10( -4) cm(-1), \A(2xy)\ = 72.10(-4) cm(-1), \A(2z)\ = 71.10(-4) cm(-1), g (xy) = 1.997, g(z) = 1.993. The ESEEM spectrum is reported. The H-2 nu clei corresponding to the exchangeable hydrogen atoms of imidazole gro ups in D2O have been detected at 2.4 MHz at 3480 G. In cyclic voltamme try, [Mn-2(III.IV) O-2(bisimMe(2)-en)(2)](3+) presents in oxidation tw o reversible waves at 1.04 and 1.40 V (all potentials versus NHE) in s trong contrast with what has been always observed on other Mn2III.IV O -2 units, for which only one wave in oxidation ([III,IV] --> [IV,IV]) is observed. EPR spectroelectrochemistry reveals the disappearance of the 16 line spectrum of the [III,IV] species on oxidation at 1.24 V. U V-vis spectroelectrochemistry at this potential confirms the formation of the [IV,-IV] dimer. The existence of two waves was related to the presence of slow equilibrium between two different [III,IV] forms. UV- irradiation of the starting [Mn-2(III.IV) O-2(bisimMe(2)en)(2)](3+) co mplex in an aqueous berate buffer at pH = 10 at 77 K resulted in the f ormation of an EPR signal that is attributed to an imidazole radical c oupled to the Mn-III-Mn-IV pair. This signal is unsplit and is underst andable in a simple model of two spins S = 1/2 in magnetic interaction . It has some analogy with that observed by Boussac et al. (Nature 199 0, 347, 303-306) on the S-3 State of the Ca2+ and Cl--depleted oxygen- evolving center, although in that case the signal was split. We relate this difference to small differences in the magnetic interaction of t he two spins S = 1/2. ESEEM spectra of the irradiated [(Mn2O2)-O-III.I V(bisimMe(2)en)(2)](3+) complex are reported for magnetic fields corre sponding to the resonance of the radical or the metalduster. These res ults suggest that the formed imidazole radical stays linked to the met al core despite the weak magnetic interaction detected. This rises the possibility that the S-3 radical in the natural system, is a direct l igand to Mn cluster.