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

The 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(O 2 CPh) 2 ·2H 2 O affords the mononuclear complex [Mn II (O 2 CPh) 2 {(ph)(2‐py)CNOH} 2 ]·1.2H 2 O ( 1· 1.2H 2 O) in high yield. The Mn II 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(O 2 CPh) 2 ·2H 2 O and N n Bu 4 MnO 4 (3:1) in the presence of (ph)(2‐py)CNOH in MeCN/EtOH/CH 2 Cl 2 leads to the isolation of the mixed‐valent cluster [Mn 8 O 2 (OH) 2 (O 2 CPh) 10 {(ph)(2‐py)CNO} 4 ]· 4CH 2 Cl 2 ( 2· 4CH 2 Cl 2 ) in about 55% yield. A second synthetic procedure that leads to pure 2 involves the reaction between the known starting material (N n Bu 4 )[Mn 4 III O 2 (O 2 CPh) 9 (H 2 O)] and four equivalents of the oxime ligand in CH 2 Cl 2 . The centrosymmetric octanuclear molecule contains four Mn II and four Mn III ions held together by two μ 4 ‐O 2− ligands and two μ 3 ‐OH − ions to give the unprecedented [Mn 8 (μ 4 ‐O) 2 (μ 3 ‐OH) 2 ] 14+ core, with peripheral ligation provided by ten PhCO 2 − (two η 1 , four syn , syn η 1 :η 1 :μ 2 and four η 1 :η 2 :μ 3 ) and four η 1 :η 1 :η 1 :μ 2 (ph)(2‐py)CNO − ions. The 1:1 reaction between Mn(O 2 CPh) 2 ·2H 2 O and the ketone (ph)(2‐py)CO affords the trinuclear complex [Mn 3 (O 2 CPh) 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 η 1 :η 2 :μ 2 and two η 1 :η 1 :μ 2 benzoates spanning each pair of metal ions. The terminal Mn II 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 CH 2 Cl 2 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 χ M T 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 [Mn 4 III (μ 3 ‐O) 2 ] 8+ subcore. The results for 3 reveal weak antiferromagnetic coupling, with J = −2.7 cm −1 for adjacent Mn II ions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)