Preparation And Crystal Structures Of Nonprecious Metal Complexes Containing Bipyridyl-derived Ligands And Their Catalytic Properties For Visible-light-driven Hydrogen Production

Hydrogenase enzymes, which can efficiently catalyze the reduction of protons, have attracted intensive attention. Since the crystal structures of hydrogenases were revealed, many chemists have put their efforts on the structural and functional biomimic of hydrogenases. In recent years, organometallic chemists have designed and synthesized the complexes which possess the catalytic function alike to that of the hydrogenase active sites in nature, although they do not structurally resemble the hydrogenase active sites.Two bipyridine-derived ligands, L1(2-(2’-pyridyl)-1,8-naphthyridine) and L2(2-(2’-pyridyl)-quinoline), were prepared in this thesis. With these ligands, corresponding Ni, Co, Fe and Cu complexes,[M(L1)2(H2O)2]Y2(M=Ni(1), Co (2), Y=BF4; M=Fe (3), Y=ClO4) and [Cu(L1)2(sol)](sol=MeOH (4), H2O (5)), were synthesized. In addition, a reference complex,[Ni(L2)3](BF4)2(6), was also prepared, which has no internal base and weakly coordinate ligand. The compositions and molecular structures of these complexes were characterized by elemental analysis, MS,1H NMR and IR spectroscopy, as well as single crystal X-ray analyses. The molecular structures show that the general structural characteristics of complexes1-5are:1) having two pyridylnaphthyridine ligands,2) with non-planar configuration,3) containing internal base(s) proximal to the metal centre,4) bearing weakly coordinate ligand(s) such as H2O and MeOH, and5) forming intramolecular hydrogen-bond between internal base and coordinate H2O or MeOH molecule. Formation of intramolecular hydrogen bond helps stabilization of the complexes containing weakly coordinate ligands.The cyclic voltammogramms of complexes1and6show that they display similar redox positions and peak shape. In the presence of p-tolylsulfonic acid, the ligands either coordinate or not, are readily reduced at ca.-0.6V vs. Fc/Fc, the catalytic reduction peak of complex1containing internal bases is150mV more positive than that of6without internal base and easily dissociated ligand.The catalytic properties of complexes1-4and6in photoinduced H2production were studied with fluoresccin as photosensitizer. The results show that nickel complexes1and6are catalytic active for photochemical reduction of water to hydrogen with irradiation of light (λ>450nm). In contrast, cobalt and iron complexes2and3display very low activities and copper complex4is inactive under similar conditions. The results indicate that the metal center of catalyst is a key factor to affect the catalytic activity of hydrogen production. Compared to complex6without internal base and easily disassociating ligand, Complex1displays higher activity and better stability in photocatalytic H2production. The high catalytic activity of1for H2production is presumably ascribed to its internal bases and the coordinated water molecules as the non-coordinated nitrogen atoms may act as proton relays and the coordinated water molecules are capable of constituting a proton transfer channel with the external water molecules through the formation of hydrogen bonds.