Experimental and theoretical investigations of condensation and disproportionation of Mn(bpy)CL3(H2O) in aqueous solution

Mn-III(bpy)Cl3H2O (Goodwin, H. A.; Sylva, R. N. Aust. J. Chern. 1965, 18, 1 743) crystallizes in the triclinic space group P (1) over bar, with a = 6.6 27(1) Angstrom, b = 9.991(7) Angstrom, c = 10.116(4) Angstrom, alpha = 84.3 8(4)degrees, beta = 73.47(3)degrees, gamma = 78.48(3)degrees, V = 629 Angst rom,(3) and Z = 4. The Mn-III ion has an octahedral surrounding distorted b y a strong Jahn-Teller effect. Two chloride ions and the water molecule an involved into a network of hydrogen bonds. a careful study of aqueous solut ions of (MnLCl3H2O)-L-III (L = bipyridine or phenanthroline) at different P II demonstrated the formation by condensation and disproportionation of [(L 2MnO2MnL2)-O-III-L-IV](ClO4)(3) (Cooper, S. R.; Calvin, M. J. Ain. Chern. S ec. 1977. 99, 6623), [(Mn3O4L4)-O-IV(H2O)(2)](ClO4)(4) (Sarneski, J. E.; et al. J. Am. Chem. Sec. 1990, 112, 7255) and [Mn4O6(bpy)(6)](ClO4)(4) (Philo uze, C.; et al. J. Am. Chem. Soc. 1994, 116, 8557). Condensation of MnLCl3H 2O has been studied theoretically using partial charge models. It was possi ble to identify the mononuclear Mn-III and Mn-IV species which are able to condense through oxolation into polynuclear Mn-IV or mixed-valence Mn-III-M n-IV complexes. It is then shown that direct electron transfer between Mn-I II species is either assisted by the solvent or associated with the transfe r of a bpy ligand from Mn-II toward Mn-IV. Formation of chainlike tetramers is thermodynamically unfavorable and has to be assisted by the solvent to occur. The greater hydrophobicity of phenanthroline in comparison with bipy ridine explains why the chainlike tetramer is not obtained in the case of p henanthroline. Substitution of an oxo bridge by two carboxylato bridges app ears to be the key point for the stabilization of a chainlike tetramer simi lar to that proposed from EXAFS to exist in the plant oxygen evolving compl ex.