STRUCTURAL AND MAGNETIC STUDIES OF MANGANESE(II) COMPLEXES OF THE IMIDAZOLE-CONTAINING LIGAND 5-NO2-SALIMH =4-(2-((5-NITROSALICYLIDENE)AMINO)ETHYL)IMIDAZOLE] WITH VARYING NUCLEARITY

Manganese(II) forms structurally diverse complexes of varying nucleari ty with the imidazole-containing Schiff base ligand 5-NO2-salimH(2) [5 -NO2-salimH(2) 4-(2-((5-nitrosalicylidene)amino)ethyl)imidazole]. The type of complex formed is determined by the type and stoichiometry of carboxylate added to the reaction mixture. The structures of the compl exes Mn(5-NO2-salimH)2, 1, and [Mn(5-NO2-salimH)(mu-HCO2)(CH3OH)](n), 2, have been characterized by X-ray crystallography and are compared t o the structure of the linear trinuclear Mn(II) complex Mn-3(5-NO2-sal imH)(2)(mu u-CH3CO2)(4), 3, which we reported previously [Baldwin, M. J.; Kampf, J. W.; Pecoraro, V. L. J. Chem. Sec., Chem. Commun. 1993, 1 741-1743]. Complex 1 is a mononuclear complex with two meridionally bo und ligands. It was crystallized in the orthorhombic space group P2(1) 2(1)2(1), with Z = 8, R = 0.0605, and R(w) 0.0590. The unit cell param eters are a 17.170(4) Angstrom, b = 17.811(4) Angstrom, c 18.305(4) An gstrom, and alpha = beta = gamma = 90.000 degrees. Complex 2 is a poly meric chain with equatorial-to-axial formate bridges in the anti-anti configuration, a meridionally bound 5-NO2-salimH in the other equatori al positions, and a methanol ligating in the other axial position. It was crystallized in the orthorhombic space group Pcab (the alternative setting to Pbca), with Z = 8, R = 0.0404, and R(w) = 0.0433. The unit cell parameters are a = 8.070(2) Angstrom, b = 11.895(3) Angstrom, c 34.29(1) Angstrom and alpha = beta = gamma = 90.000 degrees. Complex 3 is a linear trimer with 5-NO2-salimH bound meridionally to each termi nal Mn and three bridges from each terminal Mn to the central Mn consi sting of the phenolate oxygen of the tridentate ligand, a triatomicall y bridging acetate, and a monatomically bridging acetate producing a u nique -phenolato)(mu-carboxylato-O)(mu-carboxylato-O,O') bridging set which may be of relevance to the carboxylate shift concept. Comparison of the manganese-imidazole bond lengths in these complexes to those o f the few 6-coordinate Mn(II)imidazole complexes in the literature pro vides a range of ''normal'' Mn(II)-imidazole distances for comparison to structurally characterized biological systems and indicates that co mplexes in which there are multiple bridges to other manganese ions te nd to have shorter Mn-N(imidazole) bonds than mononuclear complexes. T he variable temperature and variable field magnetic behavior of 2 and 3 were also studied. 2 displays an intrachain exchange interaction J = -0.25 cm(-1). The magnetic behavior of 3 is well modeled with an exch ange interaction of J = -1.7 cm(-1) between each terminal manganese an d the central manganese, with no coupling between the two terminal man ganese ions. The zero-field splitting parameter for 3 is measured from the 4.8 K EPR spectrum as \D\ = 0.19 cm(-1) for the trimeric unit.