Selective borane reduction of phosphinoferrocene carbaldehydes to phosphinoalcohol–borane adducts. The coordination behaviour of 1-(diphenylphosphino)- 1′-(methoxymethyl)ferrocene, a new ferrocene O,P-hybrid donor prepared from such an adduct

Abstract

The reduction of ferrocene phosphino-aldehydes, R₂PfcCHO (R = Ph, 2; Cy, 3; fc = ferrocene-1,1′-diyl, Cy = cyclohexyl) and (S_p)-[Fe(η⁵-C₅H₃-1-CHO-2-PPh₂)(η⁵-C₅H₅)] ((S_p)-4), with BH₃·THF or BH₃·SMe₂ in THF at 0 °C selectively afforded the corresponding phosphinoalcohol–borane adducts, R₂PfcCH₂OH·BH₃ (R = Ph, 5; Cy, 6) and (S_p)-[Fe(η⁵-C₅H₃-1-CH₂OH-2-PPh₂)(η⁵-C₅H₅)]·BH₃ ((S_p)-7), in quantitative yields. In contrast, the reactions performed at elevated temperatures favoured the formation of methyl derivatives (e.g., Ph₂PfcCH₃·BH₃ (8)) resulting from overreduction (deoxygenation). The crystal structures of 3, 5, (S_p)-7, 8 and Cy₂PfcBr (9) have been determined by single-crystal X-ray diffraction analysis. The crystal assemblies of adducts 5 and (S_p)-7 are built up by means of C–H⋯O contacts, O–H⋯HB dihydrogen bonds and other soft interactions but, surprisingly, not via the conventional O–H⋯O hydrogen bonds. Adduct 5 was smoothly deprotected to give the corresponding free phosphine, Ph₂PfcCH₂OH (1), and was further used for the preparation of a hybrid phosphinoether ligand, Ph₂PfcCH₂OMe (11). The latter compound was studied as a donor for Group 8–10 metal ions and for Cu(I), whereupon the following complexes were isolated and structurally characterised: [(η⁶-p-cymene)RuCl₂(11-κP)] (12*), [(η⁶-p-cymene)RuCl(11-κP)(MeCN)][SbF₆] (13*), [RhCl(cod)(11-κP)] (cod = η²:η²-cycloocta-1,5-diene; 14), trans-[PdCl₂(11-κP)₂] (trans-15*), [PdCl(μ-Cl)(11-κP)]₂ (16*), cis- and trans-[PtCl₂(11-κP)₂] (cis-17 and trans-17*), and [Cu(CF₃SO₃-κO)(11-κP)(H₂O)] (18) (the asterisk indicates that the crystal structure was determined). In all these compounds, ligand 11 behaves as a P-monodentate donor while its ether group remains uncoordinated. This probably reflects structural flexibility of 11 resulting from the presence of the methylene linker and also distinguishes 11 from its known, non-spaced analogue Ph₂PfcOMe.