Synthesis, characterization and crystal structures of nickel complexes with dissymmetric tetradentate ligands containing a mixed-donor sphere

doi:10.1016/S0020-1693(00)00285-1

Inorganica Chimica Acta

Volume 310, Issue 2, 15 December 2000, Pages 183-190

https://doi.org/10.1016/S0020-1693(00)00285-1 Get rights and content

Abstract

Four new nickel(II) complexes of dissymmetric tetradentate ligands, containing mixed-ligand donor sets of NSNS or NSNO, have been synthesized. These complexes were prepared by facile 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 nickel compounds were characterized by analytical, spectroscopic and electrochemical methods. The structures of [Ni(pyzs)]BF₄, [Ni(pyrs)]BF₄ and [Ni(pyzo)]BF₄ (see Scheme 1) have been determined by single-crystal X-ray crystallography, showing the geometry of the nickel ion to be square-planar. Vis–NIR spectra show that the phenolate-containing complexes [Ni(pyzo)]BF₄ and [Ni(pyro)]BF₄ (see Scheme 1) are essentially square-planar in nitromethane, but tetragonal octahedral in methanol, whereas the thiophenolate-containing compounds [Ni(pyzs)]BF₄ and [Ni(pyrs)]BF₄ remain square-planar in both solvents. Titration of the thiophenolate-containing complexes with 1-methylimidazole results in diamagnetic five-coordinated complexes. Electrochemistry shows quasi-reversible reductions to Ni(I) to occur for [Ni(pyzo)]BF₄, [Ni(pyrs)]BF₄ and [Ni(pyro)]BF₄.

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 [(CysS)₂Ni(μ-SCys)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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Inorganica Chimica Acta

Abstract

Introduction

Section snippets

Materials and instrumentation

Synthesis

Conclusions

Supplementary material

Acknowledgements

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Inorg. Chim. Acta

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