Coordination, Agostic Stabilization, and C−H Bond Activation of N-Alkyl Heterocyclic Carbenes by Coordinatively Unsaturated Ruthenium Hydride Chloride Complexes

Abstract

The products formed upon reaction of Ru(PPh3)3HCl and [Ru(PiPr3)2HCl]2 with the N-heterocyclic carbenes 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr2Me2, 1) and 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (IEt2Me2, 2) proved to be dependent on the phosphine and the N-substituent of the carbene. Thus, Ru(PPh3)3HCl reacts with both 1 and 2 at room temperature in CH2Cl2 to give a mixture of products consisting of cis-/trans-PPh3 isomers of the agostic complexes Ru(NHC)(PPh3)2HCl (NHC = IiPr2Me2, 3a/3b; IEt2Me2, 8a/8b), the anagostic species Ru(NHC)(PPh3)2HCl (NHC = IiPr2Me2, 4; IEt2Me2, 9), and in the case of IiPr2Me2 the C−H activated complex Ru(IiPr2Me2)′(PPh3)2Cl (5). Addition of 1 atm of C2H4 to the mixture of 3a/3b, 4, and 5 leads to complete conversion to 5. [Ru(PiPr3)2HCl]2 reacts with both 1 and 2 to yield only the anagostic complexes Ru(NHC)(PiPr3)2HCl (NHC = IiPr2Me2, 6; IEt2Me2, 10), which on the basis of NMR evidence react with C2H4 to give the doubly C−H activated alkenyl-NHC complexes Ru{η2-C(NiPr)CMeCMeN(CMe═CH)}(PiPr3)2Cl (7) and Ru{η2-C(NEt)CMeCMeN(CH═CH)}(PiPr3)2Cl (11). Addition of 1 to Ru(PiPr3)2(CO)HCl affords the agostic stabilized monocarbene complex Ru(IiPr2Me2)(PiPr3)(CO)HCl (12) and the tris-carbene species Ru(IiPr2Me2)3(CO)HCl (13). Complexes 3a, 4, 5, 6, 8a, 10, 12, and 13 have been structurally characterized.