Phenyl 2‐Pyridyl Ketone and Its Oxime in Manganese Carboxylate Chemistry: Synthesis, Characterisation, X‐ray Studies and Magnetic Properties of Mononuclear, Trinuclear and Octanuclear Complexes

Abstract

AbstractThe use of phenyl 2‐pyridyl ketone [(ph)(2‐py)CO] and its oxime [(ph)(2‐py)CNOH] in manganese benzoate chemistry has been investigated. The reaction of an excess of (ph)(2‐py)CNOH with Mn(O2CPh)2·2H2O affords the mononuclear complex [MnII(O2CPh)2{(ph)(2‐py)CNOH}2]·1.2H2O (1·1.2H2O) in high yield. The MnII ion is coordinated by two monodentate benzoates and two N,N′‐bidentate chelating (ph)(2‐py)CNOH molecules in a cis‐cis‐trans fashion. The comproportionation reaction between Mn(O2CPh)2·2H2O and NnBu4MnO4 (3:1) in the presence of (ph)(2‐py)CNOH in MeCN/EtOH/CH2Cl2 leads to the isolation of the mixed‐valent cluster [Mn8O2(OH)2(O2CPh)10{(ph)(2‐py)CNO}4]· 4CH2Cl2 (2·4CH2Cl2) in about 55% yield. A second synthetic procedure that leads to pure 2 involves the reaction between the known starting material (NnBu4)[Mn4IIIO2(O2CPh)9(H2O)] and four equivalents of the oxime ligand in CH2Cl2. The centrosymmetric octanuclear molecule contains four MnII and four MnIII ions held together by two μ4‐O2− ligands and two μ3‐OH− ions to give the unprecedented [Mn8(μ4‐O)2(μ3‐OH)2]14+ core, with peripheral ligation provided by ten PhCO2− (two η1, four syn,syn η1:η1:μ2 and four η1:η2:μ3) and four η1:η1:η1:μ2 (ph)(2‐py)CNO− ions. The 1:1 reaction between Mn(O2CPh)2·2H2O and the ketone (ph)(2‐py)CO affords the trinuclear complex [Mn3(O2CPh)6{(ph)(2‐py)CO}2] (3) in more than 80% yield. As judged from single‐crystal X‐ray crystallography, the complex adopts a linear structure with one η1:η2:μ2 and two η1:η1:μ2 benzoates spanning each pair of metal ions. The terminal MnII ions are capped by bidentate chelating (ph)(2‐py)CO ligands. The three complexes have been characterised by IR spectroscopy. The CV study of complex 2 in CH2Cl2 reveals irreversible reduction and oxidation processes. The magnetic properties of 2 and 3 have been studied by variable‐temperature dc magnetic‐susceptibility techniques. As the temperature approaches zero, the value of the χMT product for 2 approaches zero and, thus, the octanuclear complex has an S = 0 ground state. This S = 0 ground state is explained in terms of the magnetic behaviour of the central, butterfly‐like [Mn4III(μ3‐O)2]8+ subcore. The results for 3 reveal weak antiferromagnetic coupling, with J = −2.7 cm−1 for adjacent MnII ions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)