Synthesis, characterization and rearrangements of C1 ruthenium(II) polypyridyl complexes relevant to electrocatalytic reductions of carbon dioxide.

摘要

High yield syntheses of the isomeric cations cis-[Ru(bpy)(eta 2-tpy)(CO)2](PF6)2 (1a and 2a: bpy=2,2'-bipyridine, tpy=2,2':6',2"-terpyridine) have been developed. Compound 1a was produced from reactions of cis-[Ru(eta3-tpy)(CO)2(CH3CN)](PF 6)2 (6) with bpy. The linkage isomer 2a is produced by thermolysis of 1a. Procedures are described for the sodium borohydride reductions of 1a, 2a to the corresponding formyl (1b,2b) and hydroxymethyl (1c,2c) derivatives. Solutions of 1b decomposed readily at room temperature, and 2b isomerized to 1b in solution. Compound 2c isomerized to 1c in solution and also produced formaldehyde. Reaction of 2c with acid produced the metallacycle 2f. Acetylation of 1c and 2c produced acetoxymethyl derivatives 1d and 2d. Compound 2d isomerized to 1d in solution. Compounds 1a, 2a, 1c, 2d and 2f have been characterized by X-ray crystallography. The possible intermediacy of 1b, 2b, 1c and 2c in catalytic reductions of CO2 leading to glyoxylic or glycolic acid is discussed. There was no evidence of a CO inserted product or a hydrolysis product from any formyl or hydroxymethyl complex however, their lability led us to make their methoxymethyl analogues for additional study. Three isomeric methoxymethyl complexes [Ru(bpy)(eta2-tpy)(CO)(CH 2OCH3)]PF6 (1e, 2e and 3e) were synthesized and structurally characterized. Also, the precursor complexes, cis-[Ru(tpy)(CO)(S)CH2OCH3]PF 6(4, 5 S=MeCN, MeOH) were synthesized and structurally characterized. Packing diagrams indicate extensive pi-pi stacking interactions for 1e, 2e, and 3e. Compounds 2e and 3e are linkage isomers while 1e has distinct stereochemistry 2e and 3e establish equilibrium in solution and slowly convert to 1e. Independent studies beginning with pure 2e or 3e did not establish whether one, or both, can isomerize to 1e. Non-dissociative trigonal twist mechanisms, in some cases followed by linkage isomerization, can rationalize the isomerizations. Alternatively, isomerization of 2e to 1e may occur by a "conrotatory twist" pathway followed by linkage isomerization. Although the linkage isomer of 1e would have the required stereochemistry for migratory insertion, no evidence for CO insertion was observed.