Probing the Influence of Phosphine Substituents on the Donor and Catalytic Properties of Phosphinoferrocene Carboxamides: A Combined Experimental and Theoretical Study

The stereoelectronic
influence of phosphine substituents on the
coordination and catalytic properties of phosphinoferrocene carboxamides
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 1JSeP coupling constants determined for R2P­(Se)­fcCONHMe (6a–d) and CO stretching frequencies
of the Rh­(I) complexes trans-[RhCl­(CO)­(1-κP)2] (7a–d); the steric properties of 1a–d were assessed through Tolman’s ligand cone angles
(θ) and solid angles (Ω). 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(η2:η2-cod)­(η2-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.