Effect of trimethylsilyl substitution on the chemical properties of triarylphosphines and their corresponding metal-complexes: solubilising effect in supercritical carbon dioxide

Abstract

The donor strengths of the following triarylphosphine ligands P(Ar)₂(Ar′) (Ar = Ar′ = 4-Me₃SiC₆H₄, 1b; 4-Me₃CC₆H₄, 1d; 4-F₃CC₆H₄, 1e; Ar = C₆H₅, Ar′ = 4-Me₃SiC₆H₄, 1c) have been evaluated experimentally and theoretically. The measurements of the J(P–Se) coupling constants of the corresponding synthesised selenides SeP(Ar)₂(Ar′), 2b,c and the DFT calculation of the energies of the phosphine lone-pair (HOMO) reveal insignificant influence on the electronic properties of the substituted phosphines when the trimethylsilyl group is attached to the aryl ring, in marked contrast to the strong electronic effect of the trifluoromethyl group. These triarylphosphine ligands P(Ar)₂(Ar′) reacted with (η⁵-C₅H₅)Co(CO)₂, (η⁵-C₅H₅)Co(CO)I₂ or PdCl₂ to yield the new compounds (η⁵-C₅H₅)Co(CO)[P(Ar)₂(Ar′)], 3b,d; (η⁵-C₅H₅)CoI₂[P(Ar)₂(Ar′)], 4b–e; and PdCl₂[P(Ar)₂(Ar′)]₂, 5b,c, respectively. These complexes have been characterized and their spectroscopic properties compared with those reported for the known triphenylphosphine complexes. Again, the contrast of the ³¹P NMR and ¹³C NMR chemical shifts or C–O or M–Cl stretching frequencies, when applied, does not show an important electronic effect on the metal complex of the trimethylsilyl substituted phosphines with respect to P(C₆H₅)₃ derivatives. Solubility measurements of complexes 3a and 3b in scCO₂ were performed. We conclude that Me₃Si groups on the triarylphosphine improve the solubility of the corresponding metal complex in scCO₂.