This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

The tetrahedral copper(I) diimine complex [Cu(pq)2]BF4 displays high photocatalytic activity for the H2 evolution reaction with a turnover number of 3564, thus representing the first type of a Cu(I) quinoxaline complex capable of catalyzing proton reduction. Electrochemical experiments indicate that molecular mechanisms prevail and DFT calculations provide in-depth insight into the catalytic pathway, suggesting that the coordinating nitrogens play crucial roles in proton exchange and hydrogen formation.

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Two heteroleptic nickel oxothiolate complexes, namely [Ni(bpy)(mp)] (1) and [Ni(dmbpy)(mp)] (2), where mp = 2-hydroxythiophenol, bpy = 2,2′-bipyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine were synthesized and characterized with various physical and spectroscopic methods. Complex 2 was further characterized by single crystal X-ray diffraction data. The complex crystallizes in the monoclinic P 21/c system and in its neutral form. The catalytic properties of both complexes for proton reduction were evaluated with photochemical and electrochemical studies. Two different in their nature photosensitizers, namely fluorescein and CdTe-TGA-coated quantum dots, were tested under various conditions. The role of the electron donating character of the methyl substituents was revealed in the light of the studies. Thus, catalyst 2 performs better than 1, reaching 39.1 TONs vs. 4.63 TONs in 3 h, respectively, in electrochemical experiments. In contrast, complex 1 is more photocatalytically active than 2, achieving a TON of over 6700 in 120 h of irradiation. This observed reverse catalytic activity suggests that HER mechanism follows different pathways in electrocatalysis and photocatalysis.

Η παρούσα διδακτορική διατριβή διενεργήθηκε στο εργαστήριο Ανόργανης Χημείας του Εθνικού και Καποδιστριακού Πανεπιστημίου Αθηνών. Βιβλιογραφικά καλύπτει τα ομογενή φωτοκαταλυτικά και ηλεκτροκαταλυτικά συστήματα παραγωγής υδρογόνου ενώ πειραματικά διενεργήθηκαν: η σύνθεση και ο χαρακτηρισμός νέων θειολικών συμπλόκων και η συγκριτική μελέτη της φωτοκαταλυτικής και ηλεκτροκαταλυτικής δράσης τους ως προς την παραγωγή υδρογόνου. Κατά την διάρκεια της παρούσας διατριβής, μελετήθηκαν πλήρως ως προς την φωτοκαταλυτική και ηλεκτροκαταλυτική τους δράση δεκαπέντε νέα σύμπλοκα του Ni.