2-Pyridinealdoxime [(py)CHNOH] in manganese(II) carboxylate chemistry: mononuclear, dinuclear, tetranuclear and polymeric complexes, and partial transformation of (py)CHNOH to picolinate(−1)

Abstract

The preparation and crystal structures of four Mn II carboxylate complexes containing 2-pyridinealdoxime, (py)CHNOH, are reported. The 1:1 reaction between Mn(O 2CPh) 2 · 2H 2O and (py)CHNOH in MeCN, followed by evaporation of the resulting solution, redissolution of the residue in CHCl 3 and precipitation with Et 2O leads to isolation of [Mn 4(O 2CPh) 6{(py)CO 2} 2{(py)CHNOH} 2] · 0.6CHCl 3 ( 1 · 0.6CHCl 3). The most interesting synthetic feature is the in situ formation of the picolinate(−1) ligand, (py)CO 2 − . The centrosymmetric tetranuclear cluster consists of an exactly planar zig-zag array of Mn II ions and is held together by four syn, syn η 1:η 2:μ 2 and two η 1:η 2:μ 3 PhCO 2 − groups, two η 1:η 2:μ 2 (py)CO 2 − ligands and two N, N ′-bidentate chelating (py)CHNOH molecules. The 1:4:7 [Mn 3 II,III, III O(O 2CPh) 6(pyr) 2(H 2O)]/Me 3SiCl/(py)CHNOH reaction mixture in MeCN (pyr=pyridine) yields the 1D coordination polymer [Mn(O 2CPh){(py)CO 2}{(py)CHNOH}] n ( 2), in which the partial (py)CHNOH → (py)CO 2 − transformation has again occurred. In each chain the Mn II ions are bridged by syn, anti carboxylate groups of the η 1:η 1:η 1:μ 2 (py)CO 2 − ligands, while the monodentate PhCO 2 − group and the N, N ′-bidentate chelating (py)CHNOH molecule complete the octahedral coordination at each metal ion. The 1:3 reaction between Mn(O 2CMe) 2 · 4H 2O and (py)CHNOH in EtOH leads to the isolation of the dinuclear complex [Mn 2(O 2CMe) 2{(py)CO 2} 2{(py)CHNOH} 2] ( 3); crystallography reveals again the partial (py)CHNOH → (py)CO 2 − transformation. The dinuclear molecule is located at a centre of symmetry; the Mn II ions are bridged by two oxygen atoms from the two symmetry-related η 1:η 1:μ 2 (py)CO 2 − ligands, whose nitrogen atoms are ligated to different metal ions. Each Mn II ion is further coordinated by a monodentate MeCO 2 − group and a N, N ′-bidentate chelating (py)CHNOH molecule. Reaction of Mn(hfac) 2 · 3H 2O (hfacH=hexafluoroacetylacetone) with 1 equivalent of (py)CHNOH in CH 2Cl 2 yields complex [Mn(O 2CCF 3) 2{(py)CHNOH} 2] ( 4); the CF 3CO 2 − ligand is one of the decomposition products of the hfac − ligand. The Mn II ion in 4 is coordinated by two monodentate CF 3CO 2 − groups and two N, N ′-bidentate chelating (py)CHNOH ligands in a cis–cis–trans fashion. The four complexes have been characterised by IR spectroscopy; characteristic bands are discussed in terms of the known structures and the coordination modes of the ligands. Variable-temperature magnetic susceptibility and EPR studies indicate weak antiferromagnetic exchange interactions in 1.