Novel Nickel Pincer Complexes:Syntheses,Reactivity And Catalytic Application

The world is facing an energy crisis and a series of environmental problems caused by greenhouse gases.As a result,exploring new energy sources and reducing the concentration of carbon dioxide in the atmosphere are among the priorities for scientists.Transition metal complexes have been widely used to catalyze the reduction of carbon dioxide.However there still exist many problems,such as high cost,low efficiency etc.,for this kind of catalysis.Recently,pincer ligated transation metal complexes have received a great deal of attention due to their high thermal stability,well balanced ligand properties and versatile catalytic activity.Developing novel pincer ligated transition metal complexes and investigating their recitivity for different applications,especially for more efficient carbon dioxide reduction,are of great importance.Based on the previous work,a series of novel pincer ligated nickel complexes were synbthesized and fully characterized by multinuclear NMR,FTIR,HRMS,EA and single crystal X-ray analysis.Their reactivity and catalytic application in the reduction of carbon dioxide were investigated.The main work is specified below.A series of new POCOP-Ni complexes[4-Z-2,6-?R₂PO?₂C₆H₂]NiSY?Z=H,CO₂Me;R=^tBu,ⁱPr,Ph;Y=C₆H₅,CH₂C₆H₅,C₆H₄-p-OCH₃,C₆H₄-p-CH₃,C₆H₄-p-CF₃?were synthesized through the modification of the ligand structure.Some of them were used for the catalytic hydroboration of carbon dioxide.Turnover frequency?TOF?up to 2490 h^-1 were obtained.This represents the most efficient homogeneous reduction of carbon dioxide to methanol with transition metal catalysts under mild conditions.The reactivity of[2,6-?^tBu₂PO?₂C₆H₃]NiSCH₂Ph against BH₃·THF was investigated,and borane-mediated C-S activation was observed for the first time.The C-S bond of the thiolato ligand of the metal pincer complex fractured during the reaction to produce[2,6-?^tBu₂PO?₂C₆H₃]NiSH.This result is of significant importance to the activation of the C-S bond of thiols and would initiate further studies on transition metal-catalyzed borane-mediated C-S activation of thiols.A series of nickel pincer complexes with isothiocyanate or azide auxiliary ligands were applied to catalyze the hydroboration of carbon dioxide.It was found that these two types complexes are also active in catalyzing the reduction of carbon dioxide.However,the azide complexes are comparatively better catalysts.By the reactions of[4-Z-2,6-?R₂PX?₂C₆H₂]NiCl?Z=H,CO₂Me;R=^tBu,ⁱPr,Ph;X=O,CH₂?with NaSH,a series of novel pincer nickel mercapto complexes[4-Z-2,6-?R₂PX?₂C₆H₂]NiSH?Z=H,CO₂Me;R=^tBu,ⁱPr,Ph;X=O,CH₂?were synthesized for the first time.The reactivity of these complexes towards BH₃·THF and organic bases was investigated.These complexes were also used for catalytic hydroboration of carbon dioxide,and high TOFs were observed.It was found that the auxiliary mercapto groups in these complexes are hard to be deprotonated by bases.In summary,we have synthesized and characterized series of new nickel pincer complexes based on the previous work,and studied their reactivity and application in the catalytic reduction of carbon dioxide.This work would be of great importance to the development of the chemistry of pincer ligated transition metal complexes.