Synthesis and structure of ferrocenylmethylphosphines, their borane adducts, and some related derivatives

Syntheses of the known ferrocenylmethylphosphines FcCH2PH2 (2, Fc = (η5-C5H4)Fe(η5-C5H5)), (FcCH2)2PH (3), and (FcCH2)3P (4) have been reinvestigated. The reaction of [FcCH2NMe3][I] with P(CH2OH)3, generated from [P(CH2OH)4][Cl] and KOH, gave a mixture of the major product (FcCH2)P(CH2OH)2 (1) and over-alkylated (FcCH2)2P(CH2OH) (9). Treatment of pure 9 with Na2S2O5 gave the secondary phosphine 3; slow addition of Na2S2O5 to 1 gave 2 in improved yield. Reaction of 1 with [FcCH2NMe3][I], followed by treatment with NEt3, gave the tertiary phosphine (FcCH2)3P (4), along with the known phosphonium salt [(FcCH2)4P][I] (5), which could be prepared in higher yield by adjusting the stoichiometry. Phosphine 4 oxidized slowly in air to yield (FcCH2)3P(O) (12), was protonated by HBF4(OMe2) to give [(FcCH2)3PH][BF4] (13), and reacted with Pt(COD)Cl2 or PtCl2 to yield a mixture of cis- and trans-Pt(P(CH2Fc)3)2Cl2 (14). Silylation of 2 with n-BuLi/Me3SiCl gave FcCH2P(SiMe3)2 (10); treatment of 1 with Me3SiCl/Et3N gave FcCH2P(CH2OSiMe3)2 (11). The phosphine-borane adducts FcCH2PH2(BH3) (6), (FcCH2)2PH(BH3) (7), (FcCH2)3P(BH3) (8) and (FcCH2)P(CH2OSiMe3)2(BH3) (15) were prepared from the corresponding phosphines and BH3(SMe2). The phosphines 2, 3, and 4, phosphonium salts 5 and 13, phosphine oxide 12, Pt complex trans-14, and phosphine-boranes 6, 7 and 8 were structurally characterized by X-ray crystallography. The solid cone angle of (FcCH2)3P, 139°, in Pt complex 14 showed that 4 was bulkier than PPh3, but less sterically demanding than P(t-Bu)3. The structural changes observed on quaternization of P (shorter P–C bonds and larger angles at P), along with results from NMR and IR spectroscopy and DFT calculations, were consistent with the expected rehybridization at phosphorus. Related observations for analogous methylphosphines suggest that methyl and ferrocenylmethyl phosphorus substituents have similar properties.