RESEARCH & LAB
During the past decade, chemistry of transition metal complexes containing vinylidene ligands has attracted a great deal of attention because of their occurrence as key intermediates in many stoichiometric and catalytic transformations of organic molecules.
Since the method for the preparation of cationic bisubstituted vinylidene complexes via electrophilic attack of metal acetylides was established, the diversity and applications of these metal vinylidene complexes have further expanded.
Organometallic chemistry
Two ruthenium-acetylene complexes [Ru]C≡C(C5H3RN) containing 2-pyridyl group were prepared and studied chemical reactivity. Protonation of the acetylene ruthenium complex 1a with HBF4 occurs at the nitrogen atom and Cβ to give the dianionic pyridinium vinylidene 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 water, both complexes 3a and 4a in solution were transformed into the cationic heterocyclic carbene complex {[Ru]=C(O)CH2(C5H4N→BF2)}BF4 (6a), whose structure is given by X-rays confirmed structural 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 for the exothermic conversion of 5a to 6a is −20.59 kcal/mol. N-alkylation of 1b with two alkyl bromides BrCH2R' (R' = CH=CHCO2Me and CO2Me) yields two pyridinium acetylene complexes {[Ru]C≡C(C5H3MeNCH2R')}Br (7b , R' = CH=CHCO2Me; 7c , R' = CO2Me, respectively). Complex 7c, characterized by X-ray structure determination, was further protonated to give the vinylidene pyridine complex {[Ru]=C=C(H)(C5H4NCH2R')2+ (8c). Interestingly, acetylene complex 7b underwent a C–C coupling reaction of acetyl Cβ with a C=C double bond to give vinylidene complex 9b, which was also characterized by X-ray structure determination.