Hydrophosphination of formaldehyde catalysed by tris-(hydroxymethyl)phosphine complexes of platinum, palladium or nickel

Abstract

The addition of PH₃ to formaldehyde to give P(CH₂OH)₃ is catalysed by a range of platinum-(IV), -(II) and -(0) compounds including Na₂[PtCl₆], K₂[PtCl₄], [PtCl₂{P(CH₂OH)₃}₂]2 and [Pt{P(CH₂OH)₃}₄]·H₂O 3. A large ratio of P(CH₂OH)₃ to Pt is present under the conditions in which the catalysis is operated and the hypothesis that a common platinum–P(CH₂OH)₃ complex is present under these conditions has been tested. It is shown that Na₂[PtCl₆] is reduced by 3 equivalents of P(CH₂OH)₃ to give [PtCl₂{P(CH₂OH)₃}₂]2, the same species that is formed upon addition of 2 equivalents of P(CH₂OH)₃ to K₂[PtCl₄]. Addition of 1 equivalent of P(CH₂OH)₃ to 2 gives [PtCl{P(CH₂OH)₃}]₃Cl 4a while addition of 2 equivalents of P(CH₂OH)₃ to 2 gives an unstable species, tentatively assigned the five-co-ordinate structure [PtCl{P(CH₂OH)₃}₄]Cl 5a. In the presence of further P(CH₂OH)₃, 5a decomposes smoothly to give a mixture of [PtH{P(CH₂OH)₃}₄]Cl 3b, [P(CH₂OH)₄]Cl and the unusual bis(chelate) salts trans- and cis-[Pt{(HOCH₂)₂PCH₂OP(CH₂OH)₂}₂]Cl₂6a and 6b. A mechanism rationalising this reaction is presented. The pH of the reaction medium during the catalysis is ca. 9–10 and it is therefore concluded that [Pt{P(CH₂OH)₃}₄]3a is the main platinum-containing species present during all the platinum catalysed reactions. It is shown that [Ni{P(CH₂OH)₃}₄]18 and [Pd{P(CH₂OH)₃}₄]19 are also catalysts for the hydrophosphination of formaldehyde and a general mechanism is suggested.