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

Article abstract of DOI:10.1016/j.poly.2003.09.009

The preparation and crystal structures of four Mn^II carboxylate complexes containing 2-pyridinealdoxime, (py)CHNOH, are reported. The 1:1 reaction between Mn(O₂CPh)₂·2H₂O and (py)CHNOH in MeCN, followed by evaporation of the resulting solution, redissolution of the residue in CHCl₃ and precipitation with Et ₂O leads to isolation of [Mn₄(O₂CPh) ₆{(py)CO₂}₂{(py)CHNOH}₂]·0. 6CHCl₃ (1·0.6CHCl₃). The most interesting synthetic feature is the in situ formation of the picolinate(-1) ligand, (py)CO₂^-. The centrosymmetric tetranuclear cluster consists of an exactly planar zig-zag array of Mn^II ions and is held together by four syn,syn η¹:η²:μ₂ and two η¹:η²:μ₃ PhCO ₂^- groups, two η¹:η²: μ₂ (py)CO₂^- ligands and two N,N′-bidentate chelating (py)CHNOH molecules. The 1:4:7 [Mn ₃^{II,III, III}O(O₂CPh)₆(pyr) ₂(H₂O)]/Me₃SiCl/(py)CHNOH reaction mixture in MeCN (pyr=pyridine) yields the 1D coordination polymer [Mn(O₂CPh) {(py)CO₂}{(py)CHNOH}]_n (2), in which the partial (py)CHNOH→(py)CO₂^- transformation has again occurred. In each chain the Mn^II ions are bridged by syn,anti carboxylate groups of the η¹:η¹:η¹:μ ₂ (py)CO₂^- ligands, while the monodentate PhCO₂^- group and the N,N′-bidentate chelating (py)CHNOH molecule complete the octahedral coordination at each metal ion. The 1:3 reaction between Mn(O₂CMe)₂·4H₂O and (py)CHNOH in EtOH leads to the isolation of the dinuclear complex [Mn ₂(O₂CMe)₂{(py)CO₂} ₂{(py)CHNOH}₂] (3); crystallography reveals again the partial (py)CHNOH→(py)CO₂^- 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:η¹:μ₂ (py)CO₂^- ligands, whose nitrogen atoms are ligated to different metal ions. Each Mn ^II ion is further coordinated by a monodentate MeCO₂ ^- group and a N,N′-bidentate chelating (py)CHNOH molecule. Reaction of Mn(hfac)₂·3H₂O (hfacH= hexafluoroacetylacetone) with 1 equivalent of (py)CHNOH in CH₂Cl ₂ yields complex [Mn(O₂CCF₃) ₂{(py)CHNOH}₂] (4); the CF₃CO₂ ^- ligand is one of the decomposition products of the hfac ^- ligand. The Mn^II ion in 4 is coordinated by two monodentate CF₃CO₂^- 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.