Chloride Labilization Resulting from Nucleophilic Addition to cis-Ru(tpy)(CO)₂Cl⁺PF₆^-:  Synthesis and Characterization of New CO₂-Bridged and Formate Complexes of Ruthenium

Reaction of cis-Ru(tpy)(CO)₂Cl⁺PF₆^- (1; tpy = 2,2‘:6‘,2‘‘-terpyridyl) with aqueous Na₂CO₃ in acetonitrile yields the CO₂-bridged complex cis,cis-(CH₃CN)(CO)(tpy)Ru(CO₂)Ru(tpy)(CO)₂²⁺2PF₆^- (3). Reaction of 1 with aqueous NaOCHO yields the corresponding η¹-formato complex 4; reaction of 4 with aqueous Na₂CO₃ also provides 3. Reaction of 3 with CO results in displacement of acetonitrile and formation of 5, cis,cis-(CO)₂(tpy)Ru(CO₂)Ru(tpy)(CO)₂²⁺2PF₆^-. Reaction paths leading to 3 and 4 from 1 are thought to occur by base-assisted trans labilization of chloride resulting from conversion of a π-acceptor ligand (CO) to a σ-donor (COOH). Support for proposed steps in the reaction paths leading to 3 and 4 was provided by a trapping experiment in which 1 reacts with cis-Ru(bpy)₂(CO)COOH⁺PF₆^- in the presence of aqueous Na₂CO₃ to give cis,cis-(CO)(bpy)₂Ru(CO₂)Ru(tpy)(CO)₂²⁺2PF₆^- (7; bpy = 2,2‘-bipyridyl), which has been structurally characterized. Parallels between these reactions and the halide labilizations which occur with ruthenium or rhenium polypyridyl complexes during photochemical or electrochemical reductions of CO₂ are discussed.