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

<p>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.</p>

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

<p>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.</p>

Examining the Modular Synthesis of [Cp*Rh] Monohydrides Supported by Chelating Diphosphine Ligands

[Cp*Rh] hydride complexes are invoked as intermediates in certain catalytic cycles, but few of these species have been successfully prepared and isolated, contributing to a relative shortage of information on the properties of such species. Here, the synthesis, isolation, and characterization of two new [Cp*Rh] hydrides are reported; the hydrides are supported by the chelating diphosphine ligands bis(diphenylphosphino)methane (dppm) and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos). In both systems, reduction of precursor Rh(III) chloride complexes with Na(Hg) results in clean formation of isolable, formally 18e– Rh(I) species, and subsequent protonation by addition of near-stoichiometric quantities of anilinium triflate to the Rh(I) species returns high yields of the desired monohydride complexes. Single-crystal X-ray diffraction data for these compounds provide evidence of direct Rh–H interactions, confirmed by complementary infrared spectra showing Rh–H stretching frequencies at 1982 cm–1 (for the dppm-supported hydride) and 1936 cm–1 (for the Xantphos-supported hydride). Findings from comprehensive multinuclear NMR experiments reveal the properties of the unique and especially rich spin systems for the dppm-supported hydride; multifrequency NMR studies in concert with spectral simulations enabled full characterization of splitting patterns attributable to couplings involving diastereotopic methylene protons for this complex. Taken together with prior reports of related monohydrides, the results show that the reduction/protonation reaction sequence is modular for preparation of [Cp*Rh] monohydrides supported by diverse diphosphine ligands spanning from four- to eight-membered rhodacycles.