Di-2-pyridyl ketone oxime [(py)2CNOH] in manganese carboxylate chemistry: mononuclear, dinuclear and tetranuclear complexes, and partial transformation of (py)2CNOH to the gem-diolate(2?) derivative of di-2-pyridyl ketone leading to the formation of NO3??

Dalton Transactions Pub Date: 2005-01-05 DOI: 10.1039/B412355G

Abstract

The use of di-2-pyridyl ketone oxime, (py)2CNOH, in manganese carboxylate chemistry has been investigated. Using a variety of synthetic routes complexes [Mn(O2CPh)2{(py)2CNOH}2]·0.25H2O (1·0.25H2O), Mn4(O2CPh)2{(py)2CO2}2{(py)2CNO}2Br2]·MeCN (2·MeCN), [Mn4(O2CPh)2{(py)2CO2}2{(py)2CNO}2Cl2]·2MeCN (3·2MeCN), [Mn4(O2CMe)2{(py)2CO2}2{(py)2CNO}2Br2]·2MeCN (4·2MeCN), [Mn4(O2CMe)2{(py)2CO2}2{(py)2CNO}2(NO3)2]·MeCN·H2O (5·MeCN·H2O) and [Mn2(O2CCF3)2(hfac)2{(py)2CNOH}2] (6) have been isolated in good yields. Remarkable features of the reactions are the in situ transformation of an amount of (py)2CNOH to yield the coordination dianion, (py)2CO22?, of the gem-diol derivative of di-2-pyridyl ketone in 2–5, the coordination of nitrate ligands in 5 although the starting materials are nitrate-free and the incorporation of CF3CO2? ligands in 6 which was prepared from Mn(hfac)2·3H2O (hfac? = hexafluoroacetylacetonate). Complexes 2–4 have completely analogous molecular structures. The centrosymmetric tetranuclear molecule contains two MnII and two MnIII six-coordinate ions held together by four μ-oxygen atoms from the two 3.2211 (py)2CO22? ligands to give the unprecedented [MnII(μ-OR)MnIII(μ-OR)2MnIII(μ-OR)MnII]6+ core consisting of a planar zig-zag array of the four metal ions. Peripheral ligation is provided by two 2.111 (py)2CNO?, two 2.11 PhCO2? and two terminal Br? ligands. The overall molecular structure of 5 is very similar to that of 2–4 except for the X? being chelating NO3?. A tentative reaction scheme was proposed that explains the observed oxime transformation and nitrate generation. The CF3CO2? ligand is one of the decomposition products of the hfac? ligand. The two MnII ions are bridged by two neutral (py)2CNOH ligands which adopt the 2.0111 coordination mode. A chelating hfac? ligand and a terminal CF3CO2? ion complete a distorted octahedral geometry at each metal ion. The CV of complex 3 reveals irreversible reduction and oxidation processes. Variable-temperature magnetic susceptibility studies in the 2–300 K range for the representative tetranuclear clusters 2 and 4 reveal weak antiferromagnetic exchange interactions, leading to non-magnetic ST = 0 ground states. Best-fit parameters obtained by means of the program CLUMAG and applying the appropriate Hamiltonian are JMn(II)?Mn((III) = ?1.7 (2), ?1.5 (4) cm?1 and JMn(III)?Mn(III) = ?3.0 (2, 4) cm?1.

Graphical abstract: Di-2-pyridyl ketone oxime [(py)2CNOH] in manganese carboxylate chemistry: mononuclear, dinuclear and tetranuclear complexes, and partial transformation of (py)2CNOH to the gem-diolate(2?) derivative of di-2-pyridyl ketone leading to the formation of NO3?
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