Magnetic, spectroscopic and X-ray crystallographic structural studies on copper(II) complexes of tridentate NNS Schiff base ligands formed from 2-acetylpyrazine and S-methyl- and S-benzyldithiocarbazates

Abstract

New copper(II) complexes of general empirical formula, [Cu( NNS)X] ( NNS = anionic forms of the 2-acetylpyrazine Schiff bases of S-methyl- and S-benzyldithiocarbazate, Hapsme and Hapsbz) and X = Cl −, Br −, NCS − and NO 3 − have been synthesized and characterized. X-ray crystal structures of the free ligand, Hapsbz and the complexes, [Cu(apsbz)(NO 3)] ∞, [Cu(apsme)(NCS)] 2 and [Cu(apsme)Cl] 2 have been determined. In the solid state, the Schiff base, Hapsbz remains in its thione tautomeric form with the thione sulfur atom trans to the azomethine nitrogen atom. X-ray diffraction shows that the [Cu(apsbz)(NO 3)] ∞ complex is a novel coordination polymer in which one of the nitrogen atoms of the pyrazine ring bridges two adjacent copper(II) ions. The Schiff base is coordinated to the copper(II) ion in its iminothiolate form via the thiolate sulfur atom, the azomethine nitrogen atom and one of the pyrazine nitrogen atoms, the overall geometry of each copper atom in the polymer being close to a square-pyramid. The complexes, [Cu(apsme)X] 2 (X = NCS −, Cl −) are dimers in which each copper atom adopts a five-coordinate near square-pyramidal geometry with an N 3S 2 coordination environment. The Schiff base coordinates as a uninegatively charged tridentate ligand chelating via the pyridine and azomethine nitrogen atoms and the thiolate sulfur atoms. A nitrogen atom of a unidentate thiocayanate or chloride ligand and a bridging sulfur atom from a second ligand completes the coordination sphere. Room temperature μ eff values for the complexes in the solid state are in the range 1.70–2.0 μ B typical of uncoupled or weakly coupled Cu(II) centres. Variable temperature susceptibility studies show that the chain complex displays weak ferromagnetic coupling across the pyrazine bridges, while the S-bridged dinuclear compounds display either weak ferromagnetic or weak antiferromagnetic coupling that relates to subtle bridging geometry differences. EPR studies of frozen DMF solutions give rather similar g and A Cu values for all compounds indicative of Cu ( d x 2 - y 2 ) ground state orbitals on the Cu centers.