Mild intramolecular P–C(sp3) bond cleavage in bridging diphosphine complexes of RuII RhIII and IrIII

Three new carboxylic acid functionalised diphosphines, R₂PCH₂N(Ar)CH₂PR₂ [^CyL₁ R = Cy, Ar = (1-CO₂H)(3-OMe)C₆H₃, ^CyL₂R = Cy, Ar = (1-CO₂H)(3-OH)C₆H₃ and ^PhL3 R = Ph, Ar = (1-CO₂H)(5-OMe)C₆H₃] have been prepared from condensation of R₂PCH₂OH and the appropriate aromatic amine in MeOH, and isolated as colourless solids (for ^CyL₁, ^CyL₂) in good yield. Reaction of ^CyL₁, ^CyL₂, or PhL3, along with the previously reported diphosphines ^PhL₁, ^PhL₂, and ^PhL₄, and [RuCl(μ-Cl)(η⁶-Me₂CHC₆H₄Me)]₂ in CH₂Cl₂ affords the P/P-bridging dinuclear ruthenium(II) complexes {RuCl2(η⁶-Me₂CHC₆H₄Me)}₂(μ-^CyL₁−^PhL₄) 1a−f as red/orange solids. Careful monitoring by ³¹P{¹H} NMR spectroscopy of CDCl₃ solutions of 1a−e revealed remarkably clean P−C_sp3 bond cleavage to give Ru^II mononuclear species 2a−e and the known secondary phosphine complexes RuCl₂(η⁶-Me₂CHC₆H₄Me)(PCy₂H) 3 and RuCl₂(η⁶-Me₂CHC₆H₄Me)(PPh₂H) 4. Furthermore, facile P−C_sp³ bond cleavage of ^PhL₁ can be observed using the chloro-bridged dimers [IrCl(μ-Cl)(η⁵-C₅Me₅)]₂ or [RhCl(μ-Cl)(η⁵-C₅Me₅)]₂ instead. Deuterium labelling of ^CyL₁, ^CyL₁, ^PhL₁, and ^PhL₂ enabled the assignment of the methylene protons to be confirmed from ¹H NMR spectroscopy. All new compounds have been characterised using a range of spectroscopic and analytical techniques. Single crystal X-ray structures have been determined for ^CyL₁, 1d·3OEt₂, 1f·2CDCl₃·OEt₂, 2b, 2c, 2d·CDCl₃, 2e·0.5OEt₂ and 6b·1.5CDCl₃. The free phenolic group in ^CyL₁, 1d·3OEt₂, 1f·2CDCl₃·OEt₂, 2b and 2d·CDCl₃ participates in intra- or intermolecular O−H···O hydrogen bonding.