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Final Activity Report Summary - METCOMP (Metal Complexes for Hydrogen Activation)

The research aims of this project were to prepare new, designed thiolate ligands that would bind to metal centres, especially nickel (Ni), to afford stable and useable complexes that might react further to afford binuclear and polynuclear mixed metal [NiFe] complexes. Such compounds were of interest since they would mimic the structure of [NiFe] hydrogenase enzymes that were known to catalyse the reversible oxidation and production of hydrogen in vivo.

Our aims were to prepare low-molecular weight analogues of the active site of hydrogenase enzymes so as to not only mimic structural aspects of this biosite, but also to develop new functional catalysts for hydrogen chemistry. A series of S-rich thiolate ligands were designed and synthesised. This was a highly challenging and delicate synthesis with the products being air-sensitive and highly smelly. These products were fully characterised by elemental analysis, Infrared (IR), proton Nuclear magnetic resonance spectroscopy (1H NMR) and carbon-13 (13C) NMR spectroscopy. Eight such ligands were identified and brought forward for subsequent complexation reactions and a series of Ni complexes were synthesised.

We also combined the use of phosphorus (P) and sulfur (S) containing co-ligands in order to stabilise the metal complex units. The nickel complexes containing binaphthalene group were less stable than the other nickel complexes, which was probably due to the steric hindrance of the aryl group and the rigid conformation of the bulky organic ligand. The reaction of nickel complexes containing phosphine P-ligands with 1,2-ethanedithiol or 1,3-dithiolpropane gave binuclear nickel complexes. In the case of the ligands 2, 2'(ethylenedithio)bis(benzyl thiol) and 2, 2' 1, 3-propanediylbis(thio)bis(benzylthiol), the introduction of a methylene group between the benzene ring and the terminal thiol group gave additional flexibility in the ligands. Thus, the combination of aryl and alkyl thiolates and their effects on metal complexation were monitored. In addition, this unique feature provided a range of ligand conformations upon complexation with the metal ion. The yield of the mononuclear nickel complexes with these ligands was only moderate and so scale-up and obtaining sufficient materials remained an issue. Mononuclear nickel complexes with other ligands were successfully obtained in a good yield.

All complexes were fully characterised by elemental analysis, IR, 1H NMR and 13C NMR spectroscopy, and, where appropriate, by single crystal X-ray diffraction. The reaction of NiL5 with (BDA)Fe(CO)3 in toluene yielded the highly novel complex NiL5'(CO)Fe(CO)3 in which desulfurisation of the aromatic ring and the formation of a Ni-C(aryl) bond was observed. To the best of our knowledge, NiL5'(CO)Fe(CO)3 was the first example where nickel and iron centres were bridged by thiolate and carbonyl ligand in a heterobimetallic model for the active site of (NiFe) hydrogenase. In addition, this complex represented an important model for the desulfurization of aromatic thiols, which was an important process in the clean-up of fossil fuels.

Current work sought to develop this modelling chemistry further. Most notably, we also reported, shortly after the project completion, the first example of the catalytic production of hydrogen from protons using one of our NiFe model complexes.

Reported by

University of Nottingham
University Park
NG7 2RD Nottingham
United Kingdom
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