| Summary: | This thesis reports the synthesis and the characterisation of functional analogues of the active site of [NiFe] hydrogenase and their application in the light- driven production of H2. Chapter 1 introduces the different types of hydrogenase enzymes and particular attention is given to the functional models of the active sites of [NiFe] and [FeFe] hydrogenases, respectively. The concept of proton relay groups is introduced and an overview on the progress towards the conversion of light into H2 by bioinspired complexes is discussed. Chapter 2 describes the synthesis and the characterisation of [NiFe2] complexes 2 - 13 which incorporate various functional groups such as CN, N02 and NH2 (Figure A.l). The catalytically active species [2 - 13r were studied by electrochemical and spectroscopic techniques. Compounds 3a, 4a, 4b and 4c feature N02 groups and they are not active towards the production of H2. The insertion of NH2 groups in 3b improved the catalytic activity by up to four times when compared with 1, previously reported by our group. The reduced forms [5 - Sr decomposed, due to the fragile amido linker. The insertion of functional groups in 1,3-position of the propane linker on the ligand reduced the stability of [9 - Ll]", such that these compounds decomposed to [Fe2(/l-tdt)(CO)6] (tdtH2 = 3,4-toluenedithiol) in ea. 2 minutes. Moreover, 13(+) and 13(-) are reported which are the first [NiFe2] complexes optically active, incorporating the group ~(+ )-glucose-pentaacetate Chapter 3 discusses the attempts to shift the electrocatalytic potential of analogues of 1 towards more anodic values. Complexes 15 - 19 incorporate groups with electron-donating / withdrawing power, they were characterised and tested for catalytic activity. The insertion of CF3 groups in 19 resulted in the production of H2 at a potential 140 m V more anodic compared to 1, although the catalytic activity of 19 decreased by 50 % compared to 1. Complex 18 was not active towards H2 production due to irreversible reaction with H+. The catalytically active forms [15 - 17 and 19r showed high stability during catalytic turnover. Chapter 4 focuses on [NiRu] analogues of the active site of [NiFe] hydrogenase. The synthesis and characterisation of [NiRu] complexes 21 - 26 is reported (Figure A.2). The catalytic activity of 21 - 26 towards H2 production was studied in different conditions {CH2Cb / 0.4 M [NnB14][BF4] with TFA and DMF / 0.1 M [NnB14][BF 4] with [NHEt3]CI} to investigate the mechanism of H+ reduction and for comparison with other [NiFe2] catalysts reported. Complexes 21 - 26 show catalytic activity 10 % greater than 3b, at a potential ea. 150 m V more cathodic. Chapter 5 describes the use of [NiFe2] models as catalyst for light-driven H2 production. The system ReCl(bpy)(CO) (bpy = 2,2'-bipyridine) 11/N(CH2CH20H)3 I [NHEt3][BF4] in CH3CN upon irradiation with visible light produces H2 with TON of up to 10 h-I. The photochemistry and the mechanism of this catalysis was studied by time-resolved IR spectroscopy. The synthesis and characterisation of the first [NiFe2Re] dyad 27 (Figure A.3), where a photosensitizer is covalently linked to a [NiFe2] model, is reported. Furthermore, upon irradiation with visible light a mixture of 27 I NEt3 I benzoic acid in CH3CN evolves H2 and the photochemistry of this system was studied by time-resolved IR spectroscopy. These studies showed that the efficiency of electron transfer from the Re moiety to the [NiFe2] core is lower in the intramolecular system compared to the intermolecular mixture of ReCl(bpy)(CO) 1/N(CH2CH2OH3) / [NHEt3][BF4] in CH3CN. This was due to the rigidity of the [NiFe2Re] structure of 27 and due to the short lifetime of the excited state of the.
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