A Mild One-Pot Reduction of Phosphine(V) Oxides Affording Phosphines(III) and Their Metal Catalysts

The metal-free reduction of a range of phosphine(V) oxides employing oxalyl chloride as an activating agent and hexachlorodisilane as reducing reagent has been achieved under mild reaction conditions. The method was successfully applied to the reduction of industrial waste byproduct triphenylphosphine(V) oxide, closing the phosphorus cycle to cleanly regenerate triphenylphosphine(III). Mechanistic studies and quantum chemical calculations support the attack of the dissociated chloride anion of intermediated phosphonium salt at the silicon of the disilane as the rate-limiting step for deprotection. The exquisite purity of the resultant phosphine(III) ligands after the simple removal of volatiles under reduced pressure circumvents laborious purification prior to metalation and has permitted the facile formation of important transition metal catalysts.


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A telescoped synthesis of metal complexes from their corresponding phosphine(V) oxides:

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Single-crystal X-ray diffraction analysis report (CPS 15 and Azolium 5) S66 X-ray Reference Preparation of chlorotriphenyl-4 -phosphanes: Dichlorotriphenyl-4 -phosphane, 2a: 1 In a dry Schlenk flask triphenylphosphine(V) oxide (2.78 g, 10.0 mmol, 1.0 equiv.) was sealed under an atmosphere of argon with a septa before being dissolved in methylene chloride (20 mL) with stirring. A gentle flow of argon was established by piecing the septa with a needle attached to an oil bubbler. Oxalyl chloride (1.04 mL, 12.0 mmol, 1.2 equiv.) was added dropwise to the stirred solution from a syringe (Caution: this results in rapid evolution of carbon dioxide and carbon monoxide). The reaction was allowed to stir for 2 hours. The needle (connected to the bubbler) was removed and the tap of the Schlenk flask was closed before being attached to a solvent trap with rubber tubing. The trap was evacuated to 1x10 -3 mbar and purged with argon, this was repeated a further two times, the trap was then placed under the active vacuum and submerged in dewar filled with nitrogen. The vessel was sealed under vacuum and transferred to the glovebox where the chlorophosphonium salt was dissolved in a small volume of methylene chloride and transferred to a vial, the solution was layered with large volume of hexane (3)(4)(5) times that of methylene chloride) and placed in the freezer at −30 °C. Upon complete crystallisation the supernatant was removed before the colourless crystals were dried under high vacuum, affording dichlorotriphenyl-λ5-phosphane (3.06 g, 9.18 mmol, 92% yield) which was stored in the glovebox freezer for future use: 1

Reaction of Hexachlorodisilane with chlorotriphenyl-4 -phosphane trifluoromethanesulfate, 2b:
In the glovebox Ph3PClOTf (10 mg, 0.03 mmol, 1.0 equiv.) was dissolved in 0.7 mL d2-methylene chloride in an NMR tube, hexachlorodisilane (6 µL, 0.033 mmol, 1.1 equiv.) was added. The tube was capped, shaken and sealed with parafilm before removing from the glovebox and examining by 31 P, 1 H, 19 F and 29 Si NMR, after 10 mins, 1 and 2 days. Reaction is complete after two days. However, a small unidentified impurity (less than 1%) is also represent after this period.  In the glovebox Ph3PClBAr Cl (34.8 mg, 0.03 mmol, 1.0 equiv.) was dissolved in 0.7 mL d2-methylene chloride in an NMR tube, hexachlorodisilane (6 µL, 0.033 mmol, 1.1 equiv.) was added. The tube was capped, shaken and sealed with parafilm before removing from the glovebox and examining by 31 P and 1 H NMR, after 10 mins. No reaction was observed, additional hexachlorodisilane (21 µL, 0.12 mmol, 4.0 equiv.) was added before the reaction was analysis immediately by NMR and after two days, no triphenylphosphine formation was observed.
Figure S11: 31 P NMR immediately after addition of Si2Cl6 to Ph3PClBAr Cl , then at 1 and 2 days with excess Si2Cl6 added: Figure S12: 1 H NMR immediately after addition of Si2Cl6 to Ph3PClBAr Cl , then at 1 and 2 days with excess Si2Cl6 added: One-pot procedure for converting phosphine(V) oxides phosphine(III) via chlorophosphonium salts: General Procedure 1 -In a dry Schlenk flask phosphine(V) oxide (1.0 equiv.) was sealed under an atmosphere of argon with a septa before being dissolved in dry degassed methylene chloride (3-10 mL per 1 mmol of start material) with stirring. A gentle flow of argon was established by piecing the septa with a needle attached to an oil bubbler. Oxalyl chloride (1.01-1.50 equiv. per phosphorus centre)* was added dropwise to the stirred solution from a glass microsyringe, resulting in evolution of carbon dioxide and carbon monoxide, the reaction was allowed to stir for 1 hour. As little as 1.01 equival. of oxalyl chloride may be used* but for expedience we typically employed 1.5 equivalents in the following procedure: 1.05 -5.0 equivalents of oxalyl chloride: The needle (connected to the bubbler) was removed and the tap of the Schlenk flask was closed before being attached to a solvent trap with rubber tubing. The trap/line was evacuated to 1x10 -3 mbar and purged with argon, this was repeated a further two times, the trap was then placed under the active vacuum and submerged in dewar filled with nitrogen. All solvents were removed from the reaction vessel with stirring in vacuo. The Schlenk was then sealed under vacuum and transferred to glovebox ( 31 PNMR may be taken at to ensure complete activation). In the glovebox, the CPS was dissolved in methylene chloride (3-10 mL per 1 mmol of start material) then hexachlorodisilane (1.04 -1.10 equiv. per phosphorus centre) was added dropwise with stirring, ( 31 PNMR may be taken at to ensure complete deprotection). The solvent was evaporated to dryness, the process was repeated twice more to ensure complete removal of residual Si2Cl6 or SiCl4. The solid was dissolved in methylene chloride and filtered through pipette packed with dry cotton and a small pad of celite into a tared flask. The solvent was removed in vacuo to afford spectroscopically pure phosphines(III) product crystalline solid.
1.01-1.05 equivalents of oxalyl chloride: When a small excess of oxalyl chloride* was used hexachlorodisilane (1.04 -1.10 equiv. per phosphorus centre) was added dropwise directly to reaction mixture (on the Schlenk line without prior evaporation) and allowed to stir for 5 minutes. The needle (connected to the bubbler) was removed and the tap of the Schlenk flask was closed before being attached to a solvent trap with rubber tubing. The trap/line was evacuated to 1x10 -3 mbar and purged with argon, this was repeated a further two times, the trap was then placed under the active vacuum and submerged in dewar filled with nitrogen. All solvents were removed from the reaction vessel with stirring in vacuo. The Schlenk was then sealed under vacuum and transferred to glovebox ( 31 PNMR may be taken to ensure complete deprotection). In the glovebox, the phosphine(III) was dissolved in methylene chloride (3-10 mL per 1 mmol of start material) then the solvent was evaporated to dryness, the process was repeated (to ensure complete removal of residual Si2Cl6 or SiCl4). The solid was dissolved in methylene chloride and filtered through pipette packed with dry cotton and a small pad of celite into a tared flask. The solvent was removed in vacuo to afford the desired phosphine(III) product as off-white crystalline solid. * Note: Residual oxalyl chloride appeared to react with hexachlorodisilane vigorously and lead to discoloration of phosphine(III) and even undesired byproducts. Thus, when excess oxalyl chloride (> 1.05 equiv.) was employed all volatiles were first stripped for the intermediate CPS using the vacuum and trap. The solid CPS was then once again dissolved in dry degassed methyl chloride (3 mL) before hexachlorodisilane was added and worked up as per the above procedure.

3-((di-tert-butyldichloro-4 -phosphaneyl)methyl)-1-mesityl-1H-imidazol-3-ium chloride, 15.
In a dry Schlenk flask, 3-((di-tert-butylphosphoryl)methyl)-1-mesityl-1H-imidazol-3-ium 4methylbenzenesulfonate, (2.13 g, 4.0 mmol, 1 equiv.) was sealed under an atmosphere of argon with a septa before being dissolved in anhydrous degassed methylene chloride (20 mL) with stirring. A gentle flow of argon was established by piecing the septa with a needle attached to an oil bubbler. Oxalyl chloride (1.73 mL, 20.0 mmol, 5 equiv.) was added dropwise to the stirred solution, resulting in evolution of carbon dioxide and carbon monoxide. The reaction was allowed to stir for 2 hours. The needle Cl t Bu 2 P N NMes Cl Cl (connected to the bubbler) was removed and the tap of the Schlenk flask was closed before being attached to a solvent trap with rubber tubing. The trap was evacuated to 1x10 -3 mbar and purged with argon, this was repeated a further two times, the trap was then placed under the active vacuum and submerged in dewar filled with nitrogen. All solvents were removed from the crude product, the Schlenk tube containing the solid chlorophosphonium salt was then sealed under vacuum and transferred to the glovebox. The crude was dissolved in the minimum volume of methylene chloride before an equal volume of hexane was added, then all solvents were removed once more. The residue was triturated with neat hexane and the resultant suspension stirred for 1 hour, filtered on a sinter and washed with fresh hexane. The powder was placed back into the Schlenk and suspended in hexane once more, then stirred for a further 30 minutes, filtered and washed with neat hexane. This process was repeated until all of p-toluenesulfonyl chloride was completely removed. The CPS was then immediately deprotected vide infra; 1

mono(1-((di-tert-butylchloro-λ 4 -phosphaneyl)methyl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3ium) dichloride.
In a dry Schlenk flask, 1-((di-tert-butylphosphoryl)methyl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium 4-methylbenzenesulfonate, (1.01 g, 1.75 mmol, 1.0 equiv.) was sealed under an atmosphere of argon with a septa before being dissolved in anhydrous degassed methylene chloride (10 mL) with stirring. A gentle flow of argon was established by piecing the septa with a needle attached to an oil bubbler. Oxalyl chloride (0.76 mL, 8.75 mmol, 5.0 equiv.) was added dropwise to the stirred solution, resulting in evolution of carbon dioxide and carbon monoxide. The reaction was allowed to stir for 2 hours. The needle (connected to the bubbler) was removed and the tap of the Schlenk flask was closed before being attached to a solvent trap with rubber tubing. The trap was evacuated to 1x10 -3 mbar and purged with argon, this was repeated a further two times, the trap was then placed under the active vacuum and submerged in dewar filled with nitrogen. All solvents were removed from the crude product, the Schlenk tube containing the solid chlorophosphonium salt was then sealed under vacuum and transferred to the glovebox. The crude was dissolved in the minimum volume of methylene chloride before an equal volume of hexane was added, then all solvents were removed once more. The residue was triturated with neat hexane and the resultant suspension stirred for 1 hour, filtered on a sinter and washed with fresh hexane. The powder was placed back into the Schlenk and suspended in hexane once more, then stirred for a further 30 minutes, filtered and washed with neat hexane. This process was repeated until all of p-toluenesulfonyl chloride was completely removed. The CPS was then immediately deprotected; 1

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In the glovebox 1-((di-tert-butylphosphaneyl)methyl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium chloride from the previous step was dissolved in methylene chloride (5 mL) with stirring in a Schlenk tube, hexachlorodisilane (0.48 mL, 2.8 mmol, 1.6 equiv.) was added dropwise from a syringe and the reaction allowed to stir for 5 minutes. All solvents were removed in vacuo, the residue was dissolved in the minimum methylene chloride before an equal volume of hexane was added and the solvent evaporated to give solid that was dried for a further 10-30 mins. The process was repeat twice more before the solid or foam was dissolved in minimum volume of methylene chloride and filtering through a pad of celite into a tared flask. The solvent was removed in vacuo, to a foam which dried for 30 mins under high vacuum before it was triturated with a small volume of ether to induce precipitation. The pale brown powder was then dried under high vaccum to afford the desired phosphine(III), 3-((di-tertbutylphosphaneyl)methyl)-1-mesityl-1H-imidazol-3-ium chloride, (0.75 g, 2.8 mmol, 92%) in accordance to reference compound; 1  A telescoped synthesis of metal complexes from their corresponding phosphine(V) oxides General Procedure 2: The selected phosphine(V) oxide was converted to phosphine(III) using general procedure 1. In the glovebox, the phosphine(III) and the metal precursor where stirred in methylene chloride for the appropriate period of time, the reaction mixture was then filtered through a celite plug, washing with methylene chloride. The solvent was evaporated to dryness and the solid triturated and then stirred in a small volume of hexane before filtering and drying to afford the final desired metal complex, recrystallising where necessary.

Computation:
Quantum chemical calculations using the TURBOMOLE program were carried out to study the thermodynamics and kinetics of the reaction. Using the harmonic oscillator and rigid rotator approximation with a reference pressure of 1 bar, Gibbs free energies are given at the PBE0-D3/def2-TZVPP//PBE-D3/dhf-SV(P) level of theory. 17 Our calculations show that the formation of phosphines by direct liberation of CL2 is uphill in free energy by 94 kJ/mol. Formation of (unstabilized) SiCl2 by disproportionation of Si2Cl6 is also expected to be very unfavorable (DG = 107 kJ/mol). Formation of the free phosphine with Si2Cl6 releasing two SiCl4 molecules, however is thermodynamically favorable (DG = -246 kJ/mol).

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Investigated compounds are crystallizing in the monoclinic P21/c (CPS 15) and P21/n (Azolium 5) space group. In case of CPS 15 the asymmetric unit of the crystal lattice contains one cation, two chloride anions and the molecule of hydrochloride (Fig. 2S), whereas in azolium 5 independent part of the unit cell consists two ionic pairs of compound (Fig. 3S). The crystallographic data are summarized in the in Table 1S. The values of bond lengths, valence and torsion angles are given in Tables