Water-soluble hydroxyalkylated phosphines: examples of their differing behaviour toward ruthenium and rhodium

Abstract

The reaction of P(CH₂OH)₃ (I) and P(C₆H₅)(CH₂OH)₂ (II) with RuCl₃ in methanol eliminates two equivalents of formaldehyde to yield the mixed tertiary and secondary phosphine complexes all-trans-[RuCl₂(P(CH₂OH)₃)₂(P(CH₂OH)₂H)₂] (1) and [RuCl₂(P(C₆H₅)(CH₂OH)₂)₂(P(C₆H₅)(CH₂OH)H)₂] (2), respectively. There is a high degree of hydrogen-bonding interactions between the hydroxymethyl groups in 1 and 2, although the phenyl groups of the latter reduce the extent of the network compared to 1. The generation of these mixed secondary and tertiary phosphine complexes is unprecedented. Under the same reaction conditions, the tris(hydroxypropyl)phosphine III formed no ruthenium complex. The reaction of P(CH₂OH)₃, P(C₆H₅)(CH₂OH)₂ and P{(CH₂)₃OH}₃ with [RhCl(1,5-cod)]₂ in an aqueous/dichloromethane biphasic medium yielded [RhH₂(P(CH₂OH)₃)₄]⁺ (3), [RhH₂(P(C₆H₅)(CH₂OH)₂)₄]⁺ (4) and [Rh(P(C₆H₅)(CH₂OH)₂)₄]⁺ (5) and [Rh(P{(CH₂)₃OH}₃)₄]⁺ (6), respectively. Treating 5 with dihydrogen rapidly gave 4. The hydroxypropyl compound 6 formed the corresponding dihydride much more slowly in aqueous solution, although [RhH₂(P{(CH₂)₃OH}₃)₄]⁺ (7) was readily formed by reaction with dihydrogen. Two separate reaction pathways are therefore involved; for P(CH₂OH)₃ and to a lesser extent P(C₆H₅)(CH₂OH)₂, the hydride source in the product is likely to be the aqueous solvent or the hydroxyl protons, whilst for P{(CH₂)₃OH}₃ an oxidative addition of H₂ is favoured. The protic nature of 3 and 4 was illustrated by the H–D exchange observed in d₂-water. Dihydrides 3 and 4 reacted with carbon monoxide to yield the dicarbonyl cations [Rh(CO)₂(P(CH₂OH)₃)₃]⁺ (8) and [Rh(CO)₂(P(C₆H₅)(CH₂OH)₂)₃]⁺ (9). The analogous experiment with [RhH₂(P{(CH₂)₃OH}₃)₄]⁺ resulted in phosphine exchange, although our experimental evidence points to the possibility of more than one fluxional process in solution.