Novel amphiphilic diphosphines: synthesis, rhodium complexes, use in hydroformylation and rhodium recycling

Abstract

For the rhodium-catalysed hydroformylation of higher alkenes the novel amphiphilic diphosphines 2,2′-bis[phenyl(3-pyridyl)phosphinomethyl]-1,1′-biphenyl (L¹), 2,2′-bis(diphenylphosphinomethyl)-3,3′-bipyridine (L²), 2,2′-bis[phenyl(3-pyridyl)phosphinomethyl]-3,3′-bipyridine (L³) and 2,2′-bis{[4-(diethylaminomethyl)-phenyl]phenylphosphinomethyl}-1,1′-biphenyl (L⁴) have been synthesised. With oct-1-ene (80 °C, 20 bar CO–H₂, toluene), high normal: branded ratios (up to 51 : 1) were found with 6–8% of isomerised octenes. The diphosphines L¹–L³ gave rhodium catalysts up to twice as active as those derived from 2,2′-bis(diphenylphosphinomethyl)-1,1′-biphenyl (bisbi). The rate of hydroformylation using L¹–L⁴ was first order and approximately first order respectively in the rhodium and oct-1-ene concentration; the order in CO pressure was negative and that in H₂ pressure slightly negative. For L¹ the influence of the L : Rh ratio, temperature and substrate were investigated. Phosphorus-31 and ¹H NMR studies showed that the diphosphines (L–L) form [RhH(CO)(PPh₃)(L–L)] and [RhH(CO)₂(L–L)] complexes, analogously to bisbi. The formation of P–N chelates was not observed. The pH-dependent distribution characteristics of the free diphosphines have been determined; L³ and L⁴ were quantitatively extracted from an Et₂O solution into a H₂SO₄ solution of pH 2. When L⁴ was used, rhodium and the excess of L⁴ were extracted into an acidic aqueous phase at pH 5, allowing separation of the aldehydes, and re-extracted into fresh toluene after neutralisation of the aqueous phase by NaHCO₃. Inductively coupled plasma atomic emission spectroscopy established a rhodium recovery up to 92%. Pressurising the recovered rhodium and excess of phosphine to 20 bar CO–H₂ at 80 °C resulted in regeneration of the original catalytically active species. A retention of catalytic activity of 72% was achieved. Diphosphines L¹–L³ proved inappropriate for rhodium-recycling experiments. Extraction into an acidic aqueous phase was effective, but neutralisation of the acidic phase resulted in the formation of rhodium species which cannot be extracted from the aqueous layer.