Probing the Influence of Phosphine Substituents on the Donor and Catalytic Properties of Phosphinoferrocene Carboxamides: A Combined Experimental and Theoretical Study
Abstract
The stereoelectronic influence of phosphine substituents on the coordination and catalytic properties of phosphinoferrocene carbox-amides was studied for the model compounds R2PfcCONHMe (1a-d), where fc = ferrocene-1,1'-diyl and R = i-Pr (a), t-Bu (b), cyclohexyl (Cy; c), Ph (d), using experimental and DFT-computed parameters. The electronic parameters were examined via 1J(Se,P) coupling constants determined for R2P(Se)fcCONHMe (6a-d) and CO stretching frequencies of the Rh(I) complexes trans-[RhCl(CO)(1-kappa P)2] (7a-d); the steric properties of 1a-d were assessed through Tolman's ligand cone angles (theta) and solid angles (Omega).
Generally, a very good agreement between the calculated and experimental values was observed. Whereas the donor ability of the amidophosphines was found to increase from 1d through 1a,c to 1b, the trends in steric demand suggested by the two parameters differed, reflecting the different spatial properties of the phosphine substituents.
In situ NMR studies and catalytic tests on the Suzuki-Miyaura cross-coupling of 4-bromoanisole with a bicyclic 4-tolylborate to give 4-methyl-4'-methoxybiphenyl using [Pd(eta(2):eta(2)-cod)(eta(2)-ma)] (cod = cycloocta-1,5-diene, ma = maleic anhydride) as a Pd(0) precursor revealed that different Pd-1 species (precatalysts) were formed from different ligands and participated in the reaction. Specifically, the bulky and electron-rich donor 1b favored the formation of [Pd(1b)(ma)], while the remaining ligands provided the corresponding bis-phosphine complexes [Pd(1)2(ma)].
The best results in terms of catalyst longevity and efficacy were observed for ligands 1a,c.