Hydration Of Nitriles To Amides By Thiolate-bridged Diruthenium Complexes

A series of complexes [Cp*Ru(Cl)(?-SEt)2(RCN)RuCp*][PF6](1[PF6])and [Cp*Ru(?-SEt)(RCN)]2[OTf]2(2[OTf]2)with nitrile ligands were synthesized by the reactions of precursor complex [Cp*Ru(μ-SEt)Cl]2 with various nitriles.Diruthenium centers can realize hydration of nitrile,meanwhile,diruthenium amide intermediate complexes were successfully isolated.Firstly,the complex [Cp*Ru(?-SEt)(RCN)]2[OTf]2(2a[OTf]2,RCN = p-methylbenzonitrile)can be regarded as the functional mimics of NHase to study the mode of coordination activation and hydration process at dimetallic centers.In the presence of Et3 N,the hydration process of coordinated p-methylbenzonitrile ligand occurred to generate deprotonated amide-bridged diruthenium complexes [Cp*Ru(μ-SEt)2(μ-η1:η1-NH(O)CR)RuCp*][OTf](3a[OTf],R = p-methylphenyl).This hydration process of nitriles is achieved by the cooperative effect between the two ruthenium centers.Importantly,amide-bridged complex as intermediate of the hydration of nitrile was successfully isolated.Secondly,we explored the reactivity of amide-bridged intermediate model complex toward proton sources to release corresponding amide,which suggests a complete stoichiometric hydration cycle.Treatment of 3a[OTf] with HBF4·Et2O in the presence of nitriles released corresponding p-methylbenzamide and regenerated the diruthenium complex [2a]2+.During the hydration process,diruthenium core framework still remain stable,which provides the possibility of development of highly efficient catalysis.Finally,investigation on hydration process of complexes [Cp*Ru(Cl)(?-SEt)2(RCN)RuCp*][PF6](1d[PF6],RCN = 2-cyanothiophene)and [Cp*Ru(Cl)(?-SEt)2(H2O)RuCp*][PF6](C17)provides important experimental evidences for the mechanism of the cooperative activation of nitriles in the diruthenium centers.Unlike complex 2a[OTf]2,there is no occurrence of the hydration reaction promoted by complexes 1d [PF6] and C17 in the presence of Et3 N.When 1 equiv.of AgOTf was added to the reaction system,the hydration reaction took place to give the corresponding deprotonated amide complexes [3d][OTf] and [3c][OTf].The above experimental results indicate that the hydration reaction only takes place when the two ruthenium centers simultaneously provide vacant coordination site for nitrile compounds and water.In addition,isotope labeling experiments demonstrated the oxygen atom and hydrogen atom on the nitrogen atom all stem from water.