Four-Coordinate Silyl Cobalt Complexes:Synthesis,Characterization,and Reactivity

Silyl ligands have application in the synthesis of novel complexes,small molecule activation and catalysis conversion because of its strong electron donating ability and trans effect.The research of cobalt silyl complexes mainly focuses on the saturated high coordination numbers(five and six)complexes.The reported cobalt silyl complexes with low coordination numbers(three and four)are rare,and their reactivity is poorly understood.This dissertation describes the study on the synthesis,characterization and reactivity of four-coordinate cobalt silyl complexes with NHC ligation.1.Synthesis and characterization of four-coordinate cobalt silyl complexes.The cobalt(0)complex[(ICy)2Co(vtms)]can react with hydrosilanes(MesSiH3,PhSiH3,CySiH3,Ph2SiH2,Et2SiH2)to form four-coordinate cobalt silyl complexes[(ICy)2Co(SiHRR’)2](R=H,R’=Mes,1;R=H,R’=Ph,2;R=H,R’=Cy,3;R=R’=Ph,4;R=R’=Et,5).Cobalt silyl complex 2 with smaller steric hindrance can also been formed from the reactions of 1 and 3-5 with PhSiH3.Complexes 1-5 have been characterized by 1H NMR,elemental analysis,solution magnetic susceptibility measurement,UV-Vis-NIR and infrared spectroscopies.The structures of complexes 1,2 and 4 were also confirmed by single crystal X-ray diffraction studies.Complexes 1-5 are low spin(S=1/2)cobalt(Ⅱ)complexes.2.Reactivity of four-coordinate cobalt silyl complexes.Cobalt silyl complex 4 can react with potassium in THF to form polymeric cobalt(Ⅲ)bis(silyl)bis(hydride)complex {[K(THF)][(ICy)2Co(H)2(SiHPh2)2]}n(6).When 18-Crown-6 is added to the reaction system,the cobalt(Ⅲ)bis(silyl)bis(hydride)complex[K(18-Crown-6)][(ICy)2Co(H)2(SiHPh2)2](7)is yielded from the reaction of 4 with KC8 in THF.The formation of 6 and 7 may involve the ring opening of THF.The reaction of 4 with AgBF4 leads to the formation of the cobalt(Ⅱ)silyl complex[(ICy)2Co(THF)(SiFPh2)][BF4](8).Complex 4 can also react with terminal alkynes with different substituents(CyC≡CH,Me3SiC≡CH and 4-CF3C6H4C≡CH)to give cobalt alkynyl complexes[(ICy)2Co(C=CCy)2](9)、[(ICy)2Co(C≡CSiMe3)((SiMe3)C=CH2)](10)and[(ICy)2Co(C≡CAr)((Ar)C=CH(SiHPh2)C=CHAr)](Ar=4-CF3C6H4)(11).Different electronic nature of the substituents may have effect on the reaction pathways.The reaction of 4 with 2-hydroxypyridine gives cobalt(Ⅲ)complex[(ICy)Co(H)(2-OPySiPh2)2](12)bearing pyridine-silyl ligand along with the elimination of ICy ligand.