Synthesis and Coordination Chemistry of Iminophosphanes

Azophosphines: Synthesis, Structure and Coordination Chemistry

The conceptual replacement of nitrogen with phosphorus in common organic functional groups unlocks new properties and reactivity. The phosphorus-containing analogues of triazenes are underexplored but offer great potential as flexible and small bite-angle ligands. This manuscript explores the synthesis and characterisation of a family of air-stable azophosphine-borane complexes, and their subsequent deprotection to the free azophosphines. These compounds are structurally characterised, both experimentally and computationally, and highlight the availability of the phosphorus lone pair for coordination. This is confirmed by demonstrating that neutral azophosphines can act as ligands in Ru complexes, and can coordinate as monodentate or bidentate ligands in a controlled manner, in contrast to their nitrogen analogues.

Atypical and Asymmetric 1,3-P,N Ligands: Synthesis, Coordination and Catalytic Performance of Cycloiminophosphanes

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These "cycloiminophosphanes" possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ1 - and P,N-κ2 -tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl2 ]2 demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ1 and κ2 species; treatment with AgOTf gives full conversion to the κ2 complex. The potential for catalysis was shown in the RuII -catalyzed, solvent-free hydration of benzonitrile and the RuII - and IrI -catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l-menthone, as well as their P,N-κ2 -RhIII and a P-κ1 -RuII complexes.

Synthesis and coordination of a neutral phosphaguanidine and comparison of its basicity with a guanidine

A phosphaguanidine [Me2NC(PPh2)=N i Pr], the analogous guanidine [Me2NC(NPh2)=N i Pr], and their hydrochloride (HCl) salts were prepared to study the influence of substituting a phosphorus atom for a nitrogen atom on the basicity of the two compounds and the bonding in their conjugate acids. The pK a values of both HCl salts were measured in acetonitrile by NMR titration. Surprisingly, the substitution of P for N has essentially no effect on basicity even though the geometry at that atom is changed. The presence of phenyl substituents in the protonated guanidine reduces the resonance in the CN3 core, whereas poor orbital overlap between P and C reduces the resonance in the N2CP core of the protonated phosphaguanidine. The neutral phosphaguanidine binds to a Cu(I) halide through both the Nimine and the P, which suggested that the basic N atom on the bound ligand may have little utility as a Brønsted base. Fortunately, however, a Cu(I) halide complex of the protonated phosphaguanidine is stable. Thus, the tendency of the basic N to bind to metals does not proscribe it serving as a metal-proximate Brønsted base.

Rh(i) and Ru(ii) phosphaamidine and phosphaguanidine (1,3-P,N) complexes and their activity for CO2 hydrogenation

Phosphaamidine metal complexes Rh₂Cl₂[Ph₂PC(Ph)[double bond, length as m-dash]NPh]₂μ-CO (1), RuCl₂[Ph₂PC(Ph)[double bond, length as m-dash]N(Ph)]₂ (2), [Rh{ⁱPr₂PC(Ph)[double bond, length as m-dash]NⁱPr}(COD)]BF₄ (3), and RuCl₂[ⁱPr₂PC(Ph)[double bond, length as m-dash]NⁱPr](DMSO)₂ (4) are prepared by combining phosphaamidines Ph₂P-C(Ph)[double bond, length as m-dash]NPh and ⁱPr₂P-C(Ph)[double bond, length as m-dash]NⁱPr (1,3-P,N) with their corresponding metal ions. Complexes 1 and 2 are stable in air while 3 and 4 are stable under inert conditions. For further comparison of structure and stability, a Ru(ii) phosphaguanidine complex, RuCl₂[Me₂NC(PPh₂)[double bond, length as m-dash]NⁱPr](DMSO)₂ (6) was prepared. Complex 6 is stable in air and in the presence of water. The structures of the phosphaamidine and phosphaguanidine complexes, determined using single crystal X-ray diffraction, revealed P,N bidentate coordination. While all five complexes have some activity as precatalysts, complex 6 was the most active.

Teaching c-phosphanylimines the titanaaziridine coordination mode

The employment of a c-phosphanylimine for the synthesis of a novel titanaaziridine with an intramolecular phosphine donor side is reported, thus teaching the c-phosphanylimine a new coordination mode.

Iminophosphanes: Synthesis, Rhodium Complexes, and Ruthenium(II)-Catalyzed Hydration of Nitriles

Highly stable iminophosphanes, obtained from alkylating nitriles and reaction of the resulting nitrilium ions with secondary phosphanes, were explored as tunable P-monodentate and 1,3-P,N bidentate ligands in rhodium complexes. X-ray crystal structures are reported for both κ¹ and κ² complexes with the counterion in one of them being an unusual anionic coordination polymer of silver triflate. The iminophosphane-based ruthenium(II)-catalyzed hydration of benzonitrile in 1,2-dimethoxyethane (180 °C, 3 h) and water (100 °C, 24 h) and under solvent free conditions (180 °C, 3 h) results in all cases in the selective formation of benzamide with yields of up to 96%, thereby outperforming by far the reactions in which the common 2-pyridyldiphenylphosphane is used as the 1,3-P,N ligand.

Nitrilium ions – synthesis and applications

Access to both stable and transient nitrilium ions and the use of these synthons in the organic synthesis of imines and a large spectrum of heterocycles and transition metal complexes are succinctly summarized.

Reactivity of the geminal phosphinoborane tBu2PCH2BPh2 towards alkynes, nitriles, and nitrilium triflates

The reactivity of the geminal phosphinoborane tBu₂PCH₂BPh₂ towards terminal alkynes, nitriles and nitrilium salts is investigated.