Kinetics And Mechanism Of Electrophilic Transition Metal Nitrido Complexes In Nitrogen Atom Transfer Reactions

The chemistry of transition metal nitrido complexes（M≡N）has received extensive attention in recent years.Such complexes are key intermediates in chemical and biological nitrogen fixation.They are also useful reagents in nitrogenation of organic compounds The chemistry of the nitrido ligand is interesting because N^3-can act as a nucleophile or electrophile,or both.In this thesis,the electrophilic ruthenium and osmium nitrido complexes bearing Schiff base ligand,[Ru^Ⅵ（N）（salchda）（H₂O）]（PF₆）(Ru^ⅥN)and[Os^Ⅵ（N）（salchda）（H₂O）]（PF₆）(Os^ⅥN)are mainly used to study their reaction towards a variety of nucleophiles.Kinetics and mechanism of the reactionshave been investigated.This thesis is divided into three parts.In part I,the kinetics and mechanism for the reaction of Ru^ⅥN with hypophosphorous acid（H₃PO₂）in aqueous acidic solutions were investigated.The reaction has the following stoichiometry:2[Ru^Ⅵ（N）（salchda）（H₂O）]⁺+3H₃PO₂+H₂O?2[Ru^Ⅲ（salchda）（NH₂P（OH）₂）（H₂O）]⁺+H₃PO₃.The pseudo-first-order rate constant,k_obs,depends linearly on[H₃PO₂],and the second-order rate constant k₂ depends on[H⁺]according to the relationship k₂=k[H⁺]/（[H⁺]+K_a）,where k is the rate constant for the oxidation of H₃PO₂molecule and K_a is the dissociation constant of H₃PO₂.At 298.0 K and I=1.0 M,k=（2.04±0.19）×10^-2 M^?1s^?1 and K_a=（6.38±0.63）×10^?2 M.A kinetic isotope effect（KIE）of 2.92 was obtained when kinetic studies were carried out with D₃PO₂ at p H 1.16,suggesting P-H bond cleavage in the rate-determining step.On the other hand,when the kinetics were done in D₂O,an inverse KIE of 0.21（H₃PO₂ in H₂O vs.H₃PO₂ in D₂O）was found.Based on experimental results and DFT calculations,the proposed mechanism involves an acid-catalyzed tautomerization of P（O）（OH）H₂ to P（OH）₂H;the latter molecule is the reacting species which reacts with Ru^ⅥN via a proton-coupled N-atom transfer step.In part II,we compared the reaction of Ru^ⅥN with hypophosphorous acid（H₃PO₂）in several common organic solvents such as methanol,ethanol,acetonitrile and acetone.Interestingly,we found that different products were produced in different solvents,the product in ethanol was consistent with that in water as[Ru^Ⅲ（salchda）（NH₂P（OH）₂）（Et OH）]⁺,and the product in methanol gave sub-trimethyl phosphate addition product[Ru^Ⅲ（salchda）（NHP（OMe）₃）（Me OH）]⁺.In addition,the product in acetone gave the ruthenium ammonium product[Ru^Ⅲ（salchda）（NH₃）（C₃H₆O）]⁺.In part Ⅲ,the reaction of an osmium nitrido complex Os^ⅥN containing a Schiff base ligand with hydroxide was studied in aqueous solution.Kinetics and mechanism were investigated.The pseudo first order constant k_obs depends linearly on the concentration of OH^-.The rate law is:-d[Os^ⅥN]/dt=k₂[Os^ⅥN][OH^-].According to previous reports,the reaction of osmium nitrido complexes with hydroxides generally produces osmium nitrosyl complexes[Os^IV（NO）（L）].Interestingly,through the analysis of the products,we found that the reaction of Os^ⅥN containing Schiff base ligands with hydroxide does not give osmium nitrosyl complexes,Instead,an osmium peroxo complex[Os^IV（salchda）（O₂）]is formed.