Chemical and electrochemical behaviours of a new phenolato-bridged complex [(L)(MnMnII)-Mn-II(L)](2+). Pathways to mononuclear chlorido I(L)(MnCl)-Cl-II/III/IV](0/1/2+) and dinuclear mono-mu-oxido [(L)Mn-III(mu-O)Mn-III/IV(L)](2+/3+) species

摘要

The X-ray structure of a new dinuclear phenolato-bridged Mn-2(II) complex abbreviated as [(L)MnMn(L)](2+) (1), where LH is the [N4O] phenol containing ligand N,N-bis(2-pyridylmethyl)-N'-salicylidene-ethane-1,2-diamine ligand, is reported. A J value of -3.3 cm(-1) (H = -JS (1) over cap center dot S (2) over cap) was determined from the magnetic measurements and the 9.4 GHz EPR spectra of both powder and frozen acetonitrile solution samples were analyzed with temperature. The cyclic voltammetry of 1 displays a reversible anodic wave at E-1/2 = 0.46 V vs. SCE associated with the two-electron oxidation of I yielding the dinuclear Mn-2(III) complex [(L)MnMn(L)](4+) (2). The easy air oxidation of 1 gives the mono-mu-oxido Mn-2(III) complex [(L)-Mn(mu-O)Mn(L)](2) (3). A rational route to the formation of the mixed-valence Mn-2(III,IV) complex [(L)Mn(mu-O)Mn(L)](3+), (4) starting from 1 by bulk electrolysis at E-P = 0.75 V vs. SCE in the presence of one equiv. of base per manganese ion is also briefly reported. Addition of chloride ions to 1 led to the cleavage of the phenolato bridges to give the mononuclear Mn-II complex [(L)MnCl] (5). Cyclic voltammetry of 5 displays two reversible anodic waves at E-1/2 = 0.21 and E-1/2 = 1.15 V vs. SCE, assigned to the two successive one-electron abstractions giving the Mn-III and Mn-IV species [(L)MnCl](+) (6) and [(L)MnCl](2+) (7), respectively. The electronic signatures from UV/Visible and EPR spectroscopy of the electrochemically prepared samples of 6 and 7 confirmed the respective oxidation states. For instance, 7 displays a broad and intense absorption band characteristic of a phenolato to Mn-IV charge-transfer transition at 690 nm (2000 m(-1) cm(-1)) and its 9.4 GHz EPR spectrum shows a strong transition at g = 5.2 consistent with a rhombically distorted S = 3/2 system with a zero-field splitting dominating the Zeeman effect. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).