Photoinduced Hydrogen Production With Homogeneous Systems Based On Iron And Cobalt Molecular Catalysts

Hydrogen is known as a clean and efficient energy, which is considered as a promising alternative to fossil fuels. Conversion of solar energy into molecular hydrogen has attracted more and more attention in recent years. Designing and constructing cheap and efficient homogeneous systems for photocatalytic hydrogen production is an effective way. Two types of proton-reduction catalysts based on inexpensive metals were studied in this dissertation: [2Fe2S] model complexes and cobaloximes inspired by the Fe-Fe hydrogenase active site and the structure of the cobalt complex in vitamin B12, respectively, can mimic the function of the active site of hydrogenase. Up to now, the systems using such non-precious metal complexes as catalyst were limited by two factors:the low effiency and the requirement of expensive noble-metal-containing sensitizers. On the basis of this context, efficient and cheap photochemical hydrogen production systems were constructed by using [Ir(ppy)2(bpy)](PF6) (ppy=2-phenylpyridine; bpy=2,2'-bipyridine) and Rose Bengal as photosensitizers. On the other hand, three noble-metal-free molecular devices were designed and synthesized, the effeciency of these devices was apparently improved by the assembly of the electron donor triethylamine and photosensitizer porphyrin unit.In this dissertation, new photochemical hydrogen production systems based on [2Fe2S] model complexes ([{(μ-SCH2)2NCH2C6H5}{{Fe(CO)2L}{Fe(CO)3}], L=P(Pyr)3, F1;L= CO, F2;[{μ-SCH2)2CH2}{{Fe2(CO)6}], F3;[{(μ-SCH2)2O}{{Fe2(CO)6}], F4) were designed and constructed using [Ir(ppy)2(bpy)](PF6) as photosensitizer. Under optimal conditions, the turnover numbers (TON) were up to 466 based on the catalyst Fl after 8 h irradiation, which was the highest turnover number of photochemical hydrogen production systems based on inexpensive metal complexes until then. The comparative experiments indicate that the deactivation of the systems was mainly due to the decomposition of the iridium photosensitizer.The application of a new generation of cobaloxime catalysts, [Co(DO)(DOH)pnBr2] (C1) and [Co((DO)2BF2)pnBr2] (C2) ({(DOH)(DOH)pn}=N2,N2'-propanediylbis (2,3-butanedione 2-imine-3-oxime)), for photochemical hydrogen production was studied in this dissertation in combination with [Ir(ppy)2(bpy)](PF6) photosensitizer. Fine tuning of the coordination sphere of the catalyst by addition of PPh3 ligand successfully improved the stability of the photochemical system, resulting in improvement of the TON of hydrogen production to 696 in a 10 h irradiation. The formation of a reduced CoⅠintermediate was evidenced by UV-vis spectroscopic monitoring of the reaction.Hydrogen evolution was observed from the noble-metal-free catalyst systems, comprising Rose Bengal, BFx-bridged cobaloximes, and triethylamine (TEA), in an aqueous solution under irradiation of visible light. Two types of BFx-bridged cobaloximes, namely, the annulated cobaloximes [Co(dmgBF2)2(H2O)2] (C3, dmgBF2= (difluoroboryl)dimethylglyoximate anion) and [Co(dpgBF2)2(H2O)2] (C4, dpgBF2= (difluoroboryl)diphenylglyoximate anion), and the clathrochelated cobaloximes [Co(dmg(BF)2/3)3](BF4) (C5) and [Co(dpg(BF)2/3)3](BF4) (C6), were used as catalysts. Among the four cobalt complexes, complex C3 displayed the highest hydrogen-evolving efficiency, with turnovers up to 327. Complexes C4 and C6 that bear the diphenylglyoximate ligands exhibited much lower efficiencies as compared to their analogues C3 and C5 that have the dimethylglyoximate ligands. The hydrogen-evolving efficiency of the annulated CoⅡcomplex C3 that contains two labile axial ligands is more than three times as high as that of the encapsulated CoⅢcomplex C5 that has a single macrobicyclic ligand. The different pathways for formation of the CoⅠspecies in these two types of cobaloximes are proposed on the basis of the results obtained from fluorescence and laser flash photolysis spectroscopic studies.Five noble-metal-free molecular devices, [{Co(dmgH)2Cl}{Zn(PyTBPP)}] (PyTBPP= 5-(4-pyridyl)-10,15,20-tri-p-t-butylphenylporphyrin;D1), [{Co(dmgH)2Cl} {Mg(PyTBPP)}](D2), [{Co(dmgH)2Cl}(PyTBPP)](D3), [{Co(dmgH)2Cl}{Zn(PyTPP)}] (PyTPP=5-(4-pyridyl)-10,15,20-triphenylporphyrin;D4) and [{Co(dmgH)2Cl} {Zn(apPyTPP)}] (apPyTPP=5-[4-(isonicotinamidyl)phenyl]-10,15,20-triphenylporphyrin; D5) were prepared and characterized by 1H NMR, MS, elemental analyses and UV-vis spectroscopy. The high efficiency of D1, D4 and D5 was presumably attributed to the weak coordination interaction between electron donor TEA and the zinc porphyrin part of the molecular devices to form a TEA-[ZnPor]-[Co] triad in solution, which was confirmed by UV-vis spectra. The photoinduced hydrogen production efficiency of D4 up to 46 turnover numbers under the optimal conditions, is higher than Dl (TON=22) with butyl groups in the zinc porphyrin part and D5 (TON=35) with a longer bridge. To the best of our knowledge, the TON of hydrogen evolution by D4 is the highest one for the noble-metal-free molecular devices ever reported.