Synthesis And Derivations Of Half-Sandwich Cobalt-Aryl Complex

Metal aryl complexes are important in organic synthesis and chemical transformations.Cobalt aryl complexes often act as reactive intermediates in catalytic synthesis.Also,the unique Co-CAr bond is able to activate small molecules such as alkynes,alkenes and boranes.However,cobalt aryl complexes are difficult to isolate under most circumstances due to their high reactivity.Therefore,research on the synthesis,structure and reactivity of cobalt aryl complexes is of particular significance in the development of base metal catalysis.The work presented in this thesis is focused on synthesis and derivations of half-sandwich cobalt aryl complexes supported by various aryl ligands.The stoichiometric reactions of these complexes with small molecules(alkynes,alkenes,BH3,amines)afforded a couple of novel cobalt aryl complexes.In particular,cobalt borohydride complexes are synthesized through insertion reaction of BH3 into the Co-CAr units.The oxidized cobalt borohydride complexes are able to activate NR3(NR3=Et2NH;CyNH2;NH3;Ph(NH2)2;Cy(NH2)2)to generate H2.Meanwhile,the B-N structural units are introduced into the half-sandwich cobalt complexes,laying a foundation for the development of cobalt-catalyzed hydrogenation reactions.The results are discussed as follows:1.The reactions of[Cp*Co(μ-Cl)]2 with various aryl lithium reagents(ArLi)are performed.By varying the stoichiometric ratio and the position of the substituents in the aryl ligands,dicobalt complexes with different coordination modes have been obtained.When 1 equiv.of ArLi is added,bridging aryl cobalt complexes[(Cp*Co)2(μ-Cl)(μ-Ar)]are formed.When 2 equiv.of ArLi is added,bridging diaryl cobalt complexes[(Cp*Co)(μ-Ar)]2 are formed.This kind of complexes is unstable and prones to intramolecular Ar-Ar reductive coupling to generate Triple-Decker Cp*Co(Ⅰ)complexes[(Cp*Co)2-μ-(η4:η4-biphenyl)].When there is increased steric hindrance at the o-position of the aryl ligand,the bridging diaryl dicobalt complex[(Cp*Co)2-μ-(o-CH3C6H4)2]is obtained.The reactivity of the triple-decker Cp*Co(Ⅰ)complex towards alkynes,olefins and CO is investigated,especially the 2+2 cycloaddition reaction of alkynes.This work provides more insights into the further development of the catalytic application and derivation of cobalt aryl complexes2.The reaction of[Cp*Co(μ-Cl)]2 with aryl lithium reagent containing phosphine ligand 1,2-Ph2PC6H4Li is investigated.Phosphinoaryl cobalt(Ⅱ)compound[Cp*Co(2-C6H4PPh2)]with a Co-P-C-C four-membered ring is synthesized in one step.This complex can be reversibly oxidized to phosphinoaryl cobalt(Ⅲ)complex[Cp*Co(2-C6H4PPh2)]+.A couple of cobalt arylborohydride complexes[Cp*Co(κ3-H,H,P-H3BC6H4PPh2)]n+(n=0,1)are synthesized by inserting BH3 into the Co-CAryl units of the phosphinoaryl cobalt compound.The reactivity of borohydride(ArBH3)fragments is very dependent on the oxidation state of the cobalt center.The reduced cobalt arylborohydride complex is stable towards amines,whereas the oxidized complex reacts with amines to give the cobalt(Ⅲ)hydride species featuring an arylborane-amine ligand.3.Based on the B-N coupling reaction of the BH3 unit with two N-H bonds in compound[Cp*Co(κ3-H,H,P-H3BC6H4PPh2)]+,the reaction of compound[Cp*Co(κ3-H,H,P-H3B C6H4PPh2)]+with o-phenylenediamine or 1R,2R-cyclohexanediamine is investigated.Complexes[Cp*Co(κ2-H,P-BH(NHRNH)C6H4PPh2)]+(R=phenyl or cyclohexyl)have been synthesized.X-ray single crystal diffraction shows that this type of borane-amino cobalt complex has a typical N-B-N-C-C five-membered ring structure.Notably,it undergoes further dehydrogenation reaction under heating conditions to produce 1 equivalent of H2.We then established catalytic transfer hydrogenation of unsaturated compounds such as ketones,imines and epoxides with the borane-amino complex.