Reactions of ruthenium acetylide and vinylidene complexes containing a 2-pyridyl group

Article abstract of DOI:10.1021/om800253e

Two ruthenium acetylide complexes [Ru]C≡C(C₅H ₃RN) (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 Css, giving the dicationic pyridiniumvinylidene complex {[Ru]=C=C(H)(C₅H ₄NH)}(BF₄)₂ (3a). Addition of BF₃ to la yields the Lewis acid/base adduct [Ru]OC(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)(C₅H ₄N←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 Css with the C=C double bond to give the vinylidene complex 9b, characterized also by X-ray structure determination.