Thermal and photochemical water oxidation catalysed by polypyridyl complexes of ruthenium, cobalt, nickel, and iron

釕, 鈷, 鎳與鐵多吡啶複合物催化水氧化的熱作用和光化學作用

Student thesis: Doctoral Thesis

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Author(s)

  • Siu Mui NG

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date3 Oct 2012

Abstract

The search for efficient and cost-effective methods for solar-driven splitting of water is one of the most challenging tasks facing scientists in this century. Considerable efforts have been made by chemists in recent years to develop efficient transition metal catalysts for water oxidation. Water oxidation provides a scalable source of electrons and protons for the production of fuels such as in the reduction of protons to hydrogen and that of carbon dioxide to methanol. In order to be economically practical, catalysts should be made from earth-abundant materials. So far, however, only a few cobalt, manganese and iron water oxidation catalysts (WOCs) have been developed. In this thesis, a number of transition metal complexes with polypyridyl ligands designed for catalytic water oxidation are reported. Three polypyridyl ligands 2,2':6',2'':6'',2''':6''',2'''':6'''',2'''''-sexipyridine (sexipy), 2,2':6',2'':6'',2'''-quaterpyridine (qpy) and 2-hydroxy-1,10-phenanthroline (PhenOH) have been synthesised and characterised by IR and 1H NMR spectroscopy as well as CHN elemental analysis. Dinuclear ruthenium complexes [Ru2(μ-sexipy)(bpy)2(H2O)2](ClO4)4 (bpy = 2,2’-bipyridine) and [Ru2(μ-sexipy)(clbpy)2(H2O)2](ClO4)4 (clbpy = 4,4’-dichloro-2,2’-bipyridine) have been synthesised and characterised. These compounds catalysed chemical water oxidation using Ce(IV) as the oxidant with a turnover number (TON) of 220. They also catalysed visible light-driven water oxidation using [Ru(bpy)3]2+ as the photosensitiser and Na2S2O8 as the sacrificial oxidant. Three cobalt complexes trans-[Co(qpy)(H2O)2](ClO4)2, cis-[Co(bpy)2(H2O)2](ClO4)2 and cis-[Co(PhenO)2(H2O)2](NO3), a nickel complex cis-[Ni(PhenOH)2Cl2] and an iron complex [(μ3-O)Fe3(μ-OMe)(μ-PhenO)4(Cl)]Cl have been synthesised and characterised by IR spectroscopy, electrospray ionisation mass spectrometry (ESI-MS), 1H NMR spectroscopy, CHN elemental analysis and UV-Vis spectroscopy. Their structures have also been determined by X-ray crystallography. These complexes catalysed water oxidation at pH 8-9 using [Ru(bpy)3]3+ as the chemical oxidant. They also catalysed photochemical oxidation of water using [Ru(bpy)3]2+ as the photosensitiser and Na2S2O8 as the sacrificial oxidant. In addition, a series of ruthenium isocyanide complexes containing 8-quinolinolato ligands with formula [RuQ2(RNC)2] (Q = 8-quinolinolate, R = tert-butyl; 6.1, 4-methoxyphenyl (4-MeOPh); 6.2, 4-chlorophenyl (4-ClPh); 6.3, 2,4,6-tribromophenyl (2,4,6-Br3Ph); 6.4) and trans,trans,trans-[Ru(Tol-Q)2(tBuNC)2] (Tol-Q = 8-hydroxyl-5-tolylquinolinate, 6.6) have been prepared and characterised by IR spectroscopy, ESI-MS, 1H NMR spectroscopy, CHN elemental analysis and UV-Vis spectroscopy. These complexes exhibited intense absorption in the UV region (λmax = 320-390 nm) with molar extinction coefficients (ε) on the order of 104 dm3 mol-1 cm-1 and moderately intense absorption with ε on the order of 103 dm3 mol-1 cm-1 at 400-492 nm. The intense absorption at 320-390 nm has been assigned to the ligand-centred π → π* transitions of the quinolinolate ligands, probably mixed with the π → π* transitions of the isocyanide ligands. The lower-energy absorption at 400-492 nm have been assigned to metal-to-ligand charge-transfer (MLCT) transitions. Upon excitation at λ > 350 nm, complexes 6.1a-6.3a displayed orange-red luminescene in dichloromethane at 298 K. In EtOH/MeOH glass, complexes 6.1-6.4 and 6 showed intense structured emission (593-638 nm).

    Research areas

  • Oxidation, Purification, Metal complexes, Pyridine, Photocatalysis, Water