Estimating Effective Steric and Electronic Impacts of Bulky Aryl Groups in Organophosphine Ligands

The steric and electronic impacts of five bulky aryl groups a–e (a: –C6H3-3,5-(CF3)2, b: –C6H3-3,5-Me2, c: –C6H2-3,5-Me2-4-OMe, d: –C6H3-3,5-tBu2, e: –C6H2-3,5-tBu2-4-OMe) in organophosphines were quantified. The Tolman electronic parameters (TEPs) of homoleptic PAr3 (1a–e) were determined by IR spectroscopy on their respective complexes Ni(CO)3(PAr3), revealing that 1a is less donating than P(OMe)3 and more donating than P(OPh)3. On the other hand, the electronic properties of b and c are slightly more donating than p-C6H4OMe and of d and e are similar to that of the methyl group. The νCO values for RhCl(CO)(P–P), which are with Ar2P(CH2)2PAr2 (2a–e), were also utilized in determining the electronic properties of the P-bound aryl groups in 2a–e. The steric parameters of 1a–e in (Ar3P)AuCl (5a–e), namely the Tolman cone angles (TCA) and the percent buried volumes (%Vbur), were evaluated using the X-ray structures of 5a–e. The cone angles of 1a–e are regulated by the meta substituents in the aryl groups, and all of them exhibit larger cone angles than triphenylphosphine. The cone angles of 1a and 1d/1e are larger than those of 1b/1c. The %Vbur values of 5a–e using the 5.0 Å spherical radii showed a linear correlation with the TCA values.