Synthesis, Properties, and Coordination Chemistry of t-Bu-Xantphos Ligands

Abstract

This thesis provides an account of research into a group of diphosphine ligands with a rigid xanthene backbone and tert -butyl substituents on the phosphorus atoms. The three ligands have different groups in the bridgehead position of the backbone (CMe₂, SiMe₂, or S) which change the natural (calculated) bite-angle of the ligand. The coordination chemistry of these t -Bu-xantphos ligands with late-transition metals has been investigated with a focus on metal complexes that may form in catalytic reactions.

The three t -Bu-xantphos ligands were synthesised by lithiation of the backbone using sec -butyllithium/TMEDA and treatment with PtBu₂Cl. The natural biteangles of the Ph-xantphos (111.89–114.18°) and t -Bu-xantphos (126.80–127.56°) ligands were calculated using DFT. The bite-angle of the t -Bu-xantphos ligands is larger due to the increased steric bulk of the tert -butyl substituents. The electronic properties of the t -Bu-xantphos ligandswere also investigated by synthesis of their phosphine selenides. The values of ¹J PSe (689.1–698.5Hz) indicate that the t -Bu-xantphos ligands have a higher basicity than Ph-xantphos between PPh₂Me and PMe₃.

The silver complexes, [Ag(t -Bu-xantphos)Cl] and [Ag(t -Bu-xantphos)]BF₄ were synthesised with the t -Bu-xantphos ligands. In contrast to systems with phenyl phosphines, all species were monomeric. [Rh(t -Bu-xantphos)Cl] complexes were synthesised, which reacted with H₂, forming [Rh(t -Bu-xantphos-ĸP,O,P ’)Cl(H)₂] complexes, and with CO, forming [Rh(t -Bu-xantphos)(CO)₂Cl] complexes. The [Rh(t -Bu-xantphos)Cl] species are air-sensitive readily forming [Rh(t -Bu-xantphos)Cl(ƞ²-O₂)] complexes. The crystal structure of [Rh(t -Bu-xantphos)Cl(ƞ²-O₂)], contained 15% of the dioxygen sites replaced with an oxo ligand. This is the first crystallographic evidence of a rhodium(III) oxo complex, and only the third rhodium oxo species reported.

The coordination chemistry of the ligands with platinum(0) and palladium(0) showed some differences. [Pt(t -Bu-xantphos)(C₂H₄)] complexes were synthesised for all three ligands. However, reaction with [Pt(nb)₃] produced a mixture of [Pt(t -Bu-xantphos)] and [Pt(t -Bu-xantphos)(nb)] for t -Bu-sixantphos and t -Buthixantphos. Although few examples of isolable [Pt(PP)] complexes with diphosphines have been reported [Pt(t -Bu-thixantphos)] was isolated by removal of the norbornene. t -Bu-Xantphos formed small amounts of [Pt(t -Bu-xantphos)] initially, which progressed to [Pt(t -Bu-xantphos)H]X. The analogous reactions with [Pd(nb)₃] gave [Pd(t -Bu-xantphos)] and [Pd(t -Bu-xantphos)(nb)] complexes in all cases. [Pt(t -Bu-thixantphos)(C₂H₄)] and [M(t -Bu-thixantphos)] (M = Pd, Pt) react with oxygen forming [Pt(t -Bu-thixantphos)(ƞ²-O₂)], which reacts with CO to give [Pt(t -Bu-thixantphos-H-ĸ-C,P,P ’)OH] through a series of intermediates.

[M(t -Bu-xantphos)Cl₂] (M = Pd, Pt) complexes were synthesised, showing exclusive trans coordination of the diphosphine ligands. The X-ray crystal structure of [Pt(t -Bu-thixantphos)Cl₂] has a bite-angle of 151.722(15)°. This is the first [PtCl₂(PP)] complex with a bite-angle between 114 and 171°. In polar solvents a chloride ligand dissociates from the [Pt(t -Bu-xantphos)Cl₂] complexes producing [Pt(t -Bu-xantphos-ĸP,O,P ’)Cl]⁺. The analogous [Pd(t -Bu-xantphos-ĸP,O,P ’)Cl]⁺ complexes were formed by reaction of the dichlorides complexes with NH₄PF₆. The [Pt(t -Bu-xantphos-ĸP,O,P ’)Me]⁺ pincer complexes were the only product from reaction with [Pt(C₆H₁₀)ClMe], with the stronger trans influence of the methyl ligand promoting loss of the chloride. The formation of the pincer complexes was further explored using DFT.

The values of J PtC for the methyl carbons in the [Pt(t -Bu-xantphos-ĸP,O,P ’)Me]⁺ complexes, and J RhH for the hydride trans to the oxygen atom in the [Rh(t -Buxantphos-ĸP,O,P ’)Cl(H)₂] complexes were largest for t -Bu-sixantphos, then t -Buthixantphos, then t -Bu-xantphos. The trans influence of the t -Bu-xantphos oxygen donor follows the trend t -Bu-sixantphos < t -Bu-thixantphos < t -Bu-xantphos.