Synthesis, coordination behavior, and catalytic properties of dppf congeners with an inserted carbonyl moiety

Ferrocene phosphines have been extensively studied as ligands for coordination chemistry and catalysis. Among the vast number of compounds reported to date, 1,1-bis(diphenylphosphino)ferrocene (dppf) still holds a prominent place due to its versatile coordination behavior and wide catalytic applications. This paper describes the synthesis of four semihomologous dppf congeners with a carbonyl spacer inserted between the ferrocene unit and one of the phosphine moieties, R 2 PfcC(O)PR 2 (R/R = Ph/Ph, Ph/Cy, Cy/Ph and Cy/Cy; Cy = cyclohexyl and fc = ferrocene-1,1-diyl). These compounds were prepared from the respective borane-protected phosphinocarboxylic acids R 2 PfcCO 2 HBH 3 by chlorination with oxalyl chloride, reaction with HPR 2 /NEt 3 and subsequent removal of the protecting group. The compounds were further converted to selenides R 2 P(Se)fcC(O)P(Se)R′ 2 to evaluate their electronic properties through the 1 J PSe coupling constants. In addition, two series of palladium complexes were prepared, viz. the Pd( ii ) chelate complexes cis -[PdCl 2 (R 2 PfcC(O)PR′ 2 -κ 2 P , P ′)] and the Pd(0) complexes [Pd(η 2 -mi)(R 2 PfcC(O)PR′ 2 -κ 2 P , P ′)] (mi = N -methylmaleimide). The latter compounds were applied as catalysts for the Stille cross-coupling reaction of 4-fluorobenzoyl chloride with phenyl-tributylstannane to produce 4-fluorobiphenyl. The best catalytic results were obtained with the [Pd(η 2 -mi)(Ph 2 PfcC(O)PPh 2 -κ 2 P , P ′)] complex that contained the least electron-rich ligand. Dppf-type ligands, desymmetrized by an inserted carbonyl group coordinate Pd( ii ) and Pd(0). Among the air-stable Pd(0) complexes, the compound containing the least electron-donating ligand gives the best catalytic results in the Stille reaction.