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The chemistry of some trifluoromethyl-phosphines Beg, Mirza Arshad Ali


One particular aspect of the chemistry of the trifluoromethyl group is its high electron-withdrawing power which reduces the donor properties of normally strong bases. This investigation has been concerned with the chemistry of some phosphines containing this group. For this purpose, substituted phosphines containing methyl or phenyl and trifluoromethyl groups have been prepared. For the study of their donor properties, a series of addition compounds with boron trifluoride, platinum(Il) chloride and nickel(II) salts have been prepared. The reported methods for preparing the methyl-trifluoromethyl-phosphines do not produce a good yield; therefore, an attempt has been made towards a better understanding of the reactions. The phenyl-trifluoromethyl-phosphines have been prepared by reacting trifluoroiodo-methane with a phosphorus compound containing a P-P bond. Thus, a reaction with tetraphenylcyclotetraphosphine gives phenylbistrifluoromethylphosphine and phenyltrifluoromethyl-iodophosphine, and reaction with tetraphenyldiphosphine gives diphenyltrifluoromethylphosphine. The latter has also been prepared by reaction of trifluoroiodomethane with either triphenylphosphine or diphenylchlorophosphine. These new phosphines are colorless liquids (except phenyltrifluoromethyliodophosphine which is reddish-brown) of high boiling point. They are stable in air and cannot be hydrolysed with acid or water, except the iodophosphine C₆H₅CF₃PI, which reacts with water to give phenyltrifluoromethylphosphinic acid, a new oxyacid. Phenylbis-trifluoromethylphosphine can be hydrolysed with aqueous alkali to give fluoroform and phenylphosphonous acid. Diphenyltrifluoromethylphosphine, on the other hand, cannot be hydrolysed by aqueous alkali, but reacts slowly with alcoholic potassium hydroxide to give fluoroform and diphenylphosphinic acid. The phosphines form a further series of new compounds by reaction with halogens. Phenylbistrifluoromethylphosphine reacts with iodine to form trifluoroiodomethane, but forms phenylbistrifluoromethyldibromophosphorane with bromine. This compound also gives phenyltrifluoromethylphosphinic acid on aqueous hydrolysis, as obtained in the case of phenyltrifluoromethyliodophosphine. Besides forming the dibromophosphorane, diphenyltrifluoromethylphosphine is the first trifluoromethyl-phosphine known to form a diiodophosphorane. It is interesting to note that diphenyltrifluoromethylphosphine is difficult to hydrolyse, whereas the phosphoranes can be hydrolysed easily, giving fluoroform and diphenylphosphinic acid. By reaction with methyl iodide, this phosphine also forms a new phosphonium compound, methyldiphenyltrifluoromethylphosphonium iodide, which is readily hydrolysed by cold water with the loss of the trifluoromethyl group. In general, phosphines containing one trifluoromethyl group show similar properties to those of their parent compounds, trimethylphosphine and triphenylphosphine, while those containing two trifluoromethyl groups are very similar in their behaviour to tristrifluoromethylphosphine. The phosphines containing more than one CF₃ group do not form addition compounds with boron trifluoride. The phenyl-trifluoromethyl-phosphines form more stable complexes than the methyl-trifluoromethyl-phosphines. The phosphines containing up to two trifluoromethyl groups form complexes with platinum(II) chloride. A complex with tristrifluoromethylphosphine could not be obtained. Except dimethyltrifluoromethylphosphine, which forms mainly a cis isomer, the other phosphines, CH₃(CF₃)₂P,C₆H₅(CF₃)₂P, and (C₆H₅)₂ CF₃P form mainly trans isomers. The non-occurrence of the tristrifluoromethylphosphine complex and the production of mainly trans isomers of the above-mentioned phosphines has been interpreted in terms of steric phenomenon. The phosphines containing more than one CF₃ group do not form complexes with nickel(II) salts. The nitrato complexes of trimethylphosphine and dimethyltrifluoromethyl-phosphine are paramagnetic, while the dichloro, dibromo, diiodo, and dithiocyanato complexes are diamagnetic. A correlation of the various properties, for example boiling points and heats of vaporization, has shown that the trifluoromethyl substituted phosphines are not anomalous in the general family of phosphines. An attempt has also been made towards a study of the infra-red spectra of the phosphines and their compounds, and towards a correlation with the spectra of other phosphorus compounds. Finally, an approximate estimate of the "electronegativities" of a wide range of substituted phosphines gives values which are in good agreement with the observed order of reactivities of the phosphines studied, and assists in correctly placing the trifluoromethyl-phosphines in such a range of compounds. `

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