Synthesis, characterization and crystal structures of nickel complexes with dissymmetric tetradentate ligands containing a mixed-donor sphere
Introduction
In recent years considerable interest has developed in nickel complexes with mixed-donor ligands as structural models for hydrogenases as well as for CO dehydrogenases [1], [2], [3], [4], [5]. Although it was long thought that the active site of [NiFe]-hydrogenases consisted of a mononuclear nickel atom in a mixed N,O,S environment, FTIR spectroscopy in combination with a crystal structure determination of the enzyme isolated from Desulfovibrio gigas has shown that the active site contains the dinuclear unit [(CysS)2Ni(μ-SCys)2Fe(CO)(CN)2] [6], [7]. For CO dehydrogenase no crystal structure is available as yet; however, a square-planar nickel complex in a mixed N,O,S donor sphere has been proposed, which could possibly be associated with an iron cluster via a bridging ligand [8]. This information prompted us to synthesize nickel complexes with a mixed-donor atom environment [1], [9], [10], [11], [12], [13], incorporating nitrogen, oxygen and sulfur as ligand donors. The present paper presents four new nickel complexes, prepared by facile template reactions as depicted in Scheme 1, and their spectroscopic characterizations.
Section snippets
Materials and instrumentation
All preparations were carried out in reagent grade solvents. All chemicals used in the syntheses were obtained commercially from Acros or Aldrich.
IR spectra were recorded on a Perkin-Elmer FT-IR Paragon 1000 spectrophotometer equipped with a golden gate ATR device, using the reflectance technique (4000–300 cm−1, resolution 4 cm−1). Elemental analyses were carried out at the Microanalytical Laboratory, University College, Dublin, or at Leiden University on a Perkin-Elmer series II CHNS/O analyzer
Synthesis
The complexes in this study were synthesized by template reactions of the appropriate aldehyde and amine in the presence of [Ni(H2O)6](BF4)2, resulting directly in the desired nickel compounds. The pyrazole-containing precursor mempa was developed previously in our group [14], while pyta was synthesized in a satisfactory yield through a known procedure [20]. For the thiolate compounds [Ni(pyzs)]BF4 and [Ni(pyrs)]BF4, the protected o-t-butylthiobenzaldehyde [15] was used to prevent oxidation of
Conclusions
Four new nickel complexes in the mixed-donor environments NSNS and NSNO were synthesized by a facile template reaction in the presence of [Ni(H2O)6](BF4)2. The square-planar complexes have only small tetrahedral distortions. The difference in donor strength of the phenolate oxygen and thiophenolate sulfur ligand is demonstrated in the weakly coordinating solvent methanol, in which the thiophenolate-containing complexes remain square-planar, whereas the phenolate-containing complexes change into
Supplementary material
Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-143914 ([Ni(pyzo)]BF4), CCDC-143915 ([Ni(pyzs)]BF4) and CCDC-143916 ([Ni(pyrs)]BF4). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223/336 033; e-mail: [email protected]).
Acknowledgements
The authors want to thank A. Mathieu and E. van de Wetering for their contribution and S. Gorter for the X-ray data collection. L.P. was supported by the Erasmus student exchange program.
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