Reactions of Ruthenium Acetylide and Vinylidene Complexes Containing a 2-Pyridyl Group

Abstract

Two ruthenium acetylide complexes [Ru]C≡C(C5H3RN) (1a, R = H; 1b, R = Me; [Ru] = Cp(PPh3)2Ru) containing 2-pyridyl groups are prepared and their chemical reactivities are explored. Protonation of the ruthenium acetylide complex 1a with HBF4 takes place at both the nitrogen atom and Cβ, giving the dicationic pyridiniumvinylidene complex {[Ru]═C═C(H)(C5H4NH)}(BF4)2 (3a). Addition of BF3 to 1a yields the Lewis acid/base adduct [Ru]C≡C(C5H4N→BF3) (4a). In the presence of moisture both complexes 3a and 4a in solution transform into the cationic heterocyclic carbene complex {[Ru]═C(O)CH2(C5H4N→BF2)}BF4 (6a), for which the structure is confirmed by X-ray structure determination. The formation of 6a involves the intermediate {[Ru]═C═C(H)(C5H4N→BF2OH)}BF4 (5a), characterized by spectroscopic methods. DFT calculations show that the Gibbs free energy change of the exothermic transformation of 5a to 6a is −20.59 kcal/mol. N-Alkylation reactions of 1b with two alkyl bromides BrCH2R′ (R′ = CH═CHCO2Me and CO2Me) yield two pyridiniumacetylide complexes {[Ru]C≡C(C5H3MeNCH2R′)}Br (7b, R′ = CH═CHCO2Me; 7c, R′ = CO2Me, respectively). Complex 7c, characterized by X-ray structure determination, undergoes further protonation to give the pyridiniumvinylidene complex {[Ru]═C═C(H)(C5H4NCH2R′)2+ (8c). Interestingly, the acetylide complex 7b undergoes a C−C coupling reaction of the acetylic Cβ with the C═C double bond to give the vinylidene complex 9b, characterized also by X-ray structure determination.