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-.

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}2Cl(2)].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)2CO2(2-), 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 6 in 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 mu-oxygen atoms from the two 3.2211 (py)2CO2(2-) ligands to give the unprecedented [MnII(mu-OR)MnIII(mu-OR)2MnIII(mu-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 5 of 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 Mn(II) 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 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 J(Mn(II)Mn((III))=-1.7 (2), -1.5 (4) cm(-1) and J(Mn(III)Mn(III))=-3.0 (2, 4) cm(-1).