Synthesis,Structure And Intramolecular Electron Transfer Of Square Planar Ni(?) Complexes Supported By Redox-Active Ligands

A series of sterically hindered iminopyridine ligands containing benzhydryl(CHPh2)groups and their corresponding nickel complexes,[OO2-]Ni?[NN0]([OO2-]=9,10-Phenanthrenequinone(pdiol2-)or Benzil(BZ2-),[NN]=iminopyridine(L1-L10,L12)or iminoquinoline(L11),Ni?[NNL11·-]2,were synthesized.All organic ligands were characterized by proton nuclear magnetic resonance spectroscopy(1HNMR).The composition,structure and intramolecular electron transfer properties of these nickel complexes were characterized by 1HNMR,13CNMR,single crystal X-ray diffraction,UV-vis-NIR spectroscopy and cyclic voltammetry(CV)methods.NMR characterization shows that these nickel complexes are diamagnetic.Crystallographic studies have shown that the nickel center is four-coordinated with two nitrogen atoms of the iminopyridine ligands and two oxygen atoms from the[OO]ligands(9,10-Phenanthrenequinone or Benzil)in a square planar coordination geometry,Investigation on the bond lengths of organic ligand backbones in the obtained crystal structure reveals that the[OO]ligands bear two negative charge in the nickel complexes,while the[NN]ligands remain neutral.Thus,the nickel centers all have+2 oxidation state.An aluminum complex[NNL3]AlCl3 was also synthesized.The electronic spectra of these compounds are featured with strong absorption within near-infrared(NIR)region,with the wavelength of maximum absorbance(?max)located 835?1192 nm,This is ascribed to ligand-to-ligand charge transfer(LL'CT)from electron donor([OO2-]ligands)to electron acceptor(iminopyridine ligands).The excited state energies(ELL'CT)of these complexes were estimated from the spectroscopic data,to be in the range of 0.822 to 1.070 eV,which can be regulated through the type of[OO]ligands and modification of the iminopyridine ligands.The absorption maxima is blue-shifted when the polarity of the organic solvents increases,further confirming the more polar ground state of these nickel compounds([OO2-]Ni?[NN0]).The cyclic voltammograms of all Ni(II)compounds show four consecutive reversible(or quasi-reversible)single-electron redox processes,which is consistent with the three possible oxidation states(0,-1 or-2)for each redox-active ligands.The redox potentials can be effectively regulated by modification of the two types of ligands.The chemical oxidation and reduction reactions of some Ni(?)complexes were investigate by using silver triflate(AgOTf)as an oxidant and decamethylcobaltocene as a reductant.The crystal structures of the products indicated that the oxidation occurs at[OO2-]ligands while the reduction occurs at[NN]ligands,with the nickel centers remain at+2 oxidation state through the reaction.The excited-state redox potentials of these Ni(?)compounds can be estimated by combining the electrochemical and the spectroscopic data.It is found that these complexes are strong excited-state reductants.The good photophysical and photochemical properties of these nickel compounds may find potential applications in related research field such as photosensitizers.