Identification of the Dinuclear and Tetranuclear Air‐Oxidized Products Derived from Labile Phenolate‐Bridged Dimanganese(II) Pyridyl‐Chelate Compounds

Abstract

AbstractDioxygen‐sensitive dinuclear manganese complexes of the phenoxo‐hinged dinucleating ligand 2,6‐bis{[N,N′‐bis(2‐picolyl)amino]methyl}‐4‐tert‐butylphenolato (bpbp–) containing exogenous labile THF, water and perchlorato ligands are described. The manganese(II) complexes [Mn2(bpbp)(ClO4)2(THF)]+ (1) and [Mn2(bpbp)(ClO4)(H2O)2]2+ (2) have been isolated as the salts 1·ClO4·THF·3H2O, 1·B(C6H5)4·4THF and 2·(ClO4)2·H2O. Complexes 1 and 2 are spontaneously oxidised in air in solution and the solid state. The reaction products of the air oxidation in THF, water and methanol solutions are labile dinuclear MnII–MnIII, MnIII2 and MnIII–MnIV complexes containing water‐ and methanol‐derived exogenous ligands. In addition, a Mn4 complex has been isolated. Magnetic susceptibility data confirm the MnII–MnIII oxidation state assignment with an S = 2/S = 5/2 model with weak antiferromagnetic coupling (J = –3.7 cm–1) in [Mn2(bpbp)(CH3O)2(H2O)2](ClO4)2 [3·(ClO4)2]. A tetranuclear complex, [Mn4(O)4–n(OH)n(bpbp)2](ClO4)4 [n = 1 or 2; 7·(ClO4)4], recovered from THF shows a Mn4O6 adamantane‐type core with the O bridges furnished by the two phenolato groups and four hydroxide/oxide bridges. We have arrived at two feasible formulations for the core metal oxidation states and oxo‐bridge protonation states, namely [MnIII4(O)2(OH)2(bpbp)2]4+ and [MnIII3MnIV(O)3(OH)(bpbp)2]4+, for 7, on the basis of a bond valence sum analysis of the crystal structure, elemental analysis and XANES. Thus, complex 7 is at least two oxidation state levels lower than known complexes with the Mn4O6 adamantane core structure. The magnetism of 7 was fitted well to an MnIII4 three‐J model. Complex cations related to 3 by homology, and to 7 by hydration/solvation, have been identified by ESI mass spectrometry. The [Mn2(bpbp)(OH)2(H2O)2]2+ ion (4) present in aqueous solutions on dissolution of 1·ClO4·THF·3H2O in air or by simple dissolution of 3 in water‐containing solvent is isoelectronic to 3. In the presence of significant amounts of water the MnII–MnIII complexes 3 and 4 are susceptible to further metal oxidation and concomitant aquo ligand deprotonation to give ions assignable to [MnIII/IV2(bpbp)O(OCH3)2(H2O)]2+ (5) and [MnIII2(bpbp)(OH)3(H2O)]2+ (6). ESI mass spectra of water or methanol solutions of 1, 2, 3 and 7 show predominantly an ion assignable to the oxide [Mn2(bpbp)(O)]2+ (8). Cation 8 is most likely not present in solution. Using mild source conditions and MS‐MS techniques, the gas‐phase fragmentation pathways to generate 8 have been mapped. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)