Ru???/Ni???—NNN Pincer Complexes Catalyzed Alcohol Dehydrogenation And CO₂ Hydrogenation Reactions

Energy shortage and environmental crises are two major factors that constrain human survival and development.In response to the energy and environmental problems,metal complex catalyzed alcohol dehydrogenation to H2 and CO2 hydrogenantion to formate were developed in the past decades.Pincer complex often have a good performace in a catalytic reaction due to its enhanced stability and adjustable steric,and pincer complex catalyzed alcohol dehydrogenation to H2 and CO2 hydrogenantion to formate have made a major breakthrough recently.However,the reported pincer complexes are normally composed of noble metal centers and phosphine-containing ligands,which are too expensive to be applied in practical.In this paper,phosphine-free Ru(?)-NNN and Ni(?)-NNN type pincer complexes were synthesized and used as catalyst precursors to investigate the alcohol dehydrogenation and CO2 hydrogenation reactions.The mechanism of Ru(?)-NNN pincer complexes catalyzed alcohol dehydrogenation to H2 and carboxylic acid salt was discussed in detail,and the catalytic intermediate was isolated from the catalytic reaction system,which provide a good guidance for design more efficient catalysts in the future.The main results and conclusions are listed as follows.(1)Four Ru(?)-NNN pincer complexes RuCl(L1)(MeCN)2·Cl(1a),Ru(L1)(MeCN)2(OTf)·OTf(1b),RuCl(L2)(MeCN)2·Cl(2)and RuCl(L3)(MeCN)2Cl(3)were synthesized and fully characterized and applied to the catalytic dehydrogenation of alcohols to H2 and carboxylic acid salts.Under the optimized reaction condition,the reaction of complex la catalyzed benzyl alcohol dehydrogenation will be completed in 2.5 h,and benzoic acid was isolated with 94%yield with two equivalents H2 was produced.High yields of carboxylic acids were still achieved with much lower catalyt loading under the same condition,while the catalyst can be reused.These results reveal that the catalytic efficiency of Ru(II)-NNN pincer complexes can be comparable to those phosphine-containing Ru(II)complexes,which give a new thread for design of highly efficient phosphine-free dehydrogenation catalysts.(2)Complexes RuCl(L4)(MeCN)2·Cl(4)and Ru(L1-1)(MeCN)(PPh3)2·OTf(5)were synthesized by the reaction of ligands L1 and L4 with[RuCl2(p-cymene)]2.The single crystal structure of complex 5 show an asymmetric NNN ligand with one imidazoyl "arm" is deprotonated to give an anionic ligand.Complex 4 can catalyze the hydrogenation of CO2 to formate.Further investigations revealed complexes la,1b,4,5 can efficiently catalyze the hydrogenation of sodium bicarbonate to sodium formate,and the catalytic activity follows the order 5>4>1b ? la.Especilly,sodium formate was obtained in good yield(77%)and high TON(1530)when complex 5 was loaded.The present results illustrate the phosphine-free Ru(?)-NNN pincer complex can also efficiently catalyze the hydrogenation of CO2 to formate.(3)Complex Nil was synthesized by the reaction of 2,6-bis(benzimidazolyl-2-yl)pyridine and NiCl2,and the structure of Nil was characterized by X-ray single crystal diffraction.Three Ni(?)-NNN type pincer complexes Nil,Ni2 and Ni3 were used as catalyst precursor to catalyze the acceptorless dehydrogenation of alcohols to carboxylic acids and H2.Complex Ni3 displays the best catalytic reactivity,which catalyze the primary alcohols to the carboxylic acids and H2 in good yields(40%?90%).Further investigation reveals an unexpected alcohol etherification occurs which gives the second oxygen atom for the formation of the carboxylic acid.Our results give a thread for design new nickel complexes without phosphine ligands for efficiently dehydrogenation of alcohols to carbonyl derivatives and H2.(4)The mechanism of Ru(?)-NNN pincer complexes catalyzed dehydrogenation of alcohol to H2 and carboxylate was investigated by experimental methods.Under the same condition,complexes 1-9 were used to catalyze the dehydrogenation of benzyl alcohol,and the benzoic acid was still obtained when complexes 3,7 and 8 were used,respectively.It is noted that there is no N-H bond in the structure of complexes 3,7 and 8.The dehydrogenation rate of complexes la and 3 were tested under the same condition,and the ratio of dehydrogenation rate of la:3 is 3:2,indicating the N-H bond is not necessary in a dehydrogenation catalyst.In the case of la and 3 catalyzed benzyl alcohol dehydrogenation,ln(c(CsOH))show a good linear relationship with the reaction time,which reveal that the dehydrogenation reaction is the first order kinetics,and the reaction rate control step is ?-H elimination.These results illustrate the mechanism of Ru(?)-NNN pincer complex catalyzed alcohol dehydrogenation is "inner-sphere",and the "outer—sphere" dehydrogenation mechanism is not necessary in our system.The catalytic intermediate was isolated from the dehydrogenation reaction,when complex 1c was selected as the catalyst precursor,and the molecular formula of the intermediate was identified as Ru(bbp2-)(CO)(PPh3)2.A possible mechanism of 1c catalyzed alcohol dehydrogenation to H2 and carboxylate was discussed based on the structure of the catalytic intermediate.The mechanism study gives a thread for alcohol dehydrogenation catalyst design,which include:a)the electron-rich donating ligand is necessary to the metal complex;b)the steric bulky moieties of ligands control the stability and reaction selectivity of the complex;c)the auxiliary ligands give the enough vacant coordination sites for substrate coordination.