By variation of the four substituents on the phosphonium cation along with the available anions, we can obtain an enormous number of possible salts. Among them, the salts that melt below the normal boiling point of water, which are known as “ionic liquids (ILs)”, have attracted extensive attention. In some applications where they are used as phase transfer catalysts in chemical synthesis, electrolytes in energy storage systems, including lithium ion batteries, solar cells, actuators, and supercapacitors, and additives in medicaments and water treatment, quaternary phosphonium based ILs show superior performance as compared to quaternary ammonium based ILs. However, quaternary phosphonium based ILs have been much less studied. The slow progress on quaternary phosphonium based ILs can be attributed to the difficulty in synthesizing their starting materials, for example phosphine derivatives, and further the process for preparation of them.
Generally, quaternary phosphonium salts are synthesized by reacting tertiary phosphines with alkyl halides, followed by an anion-exchange process if the anion other than halide ions (X−) is desired.R1R2R3P+R4X→[R1R2R3R4P]+X−  (1)[R1R2R3R4P]+X−+M+A−→[R1R2R3R4P]+A−+M+X−  (2)[R1R2R3R4P]+X−+H+A−→[R1R2R3R4P]+A−+H+X−  (3)
Alternatively, it is done by the addition of a metal salt M+[A]− with precipitation of M+X− or by displacement of the original ion by a strong acid MAI with release of H+X−. It deserves attention that those salts obtained by anion-exchange process are possibly contaminated with a small amount of X− ions unless the exchange reaction is fully completed.
As a synthesis process for a quaternary phosphonium salt with the anion of SO42−, for example, a quaternary phosphonium chloride has to be prepared first as shown in equation (1), and further a reaction of the quaternary phosphonium chloride with sulfuric acid as shown in equation (3) is required. During the anion-exchange process, the reaction equilibrium is shifted to the right side by continuous removal of volatile hydrochloric acid. In another case of preparing a quaternary phosphonium salt with the anion of BF4−, a quaternary phosphonium chloride is prepared as shown in equation (1), and followed by a reaction of the quaternary phosphonium chloride with NaBF4 in organic solvent such as acetone as shown in equation (2). When a silver salt is used instead of the alkali metal salt, the anion exchange reaction proceeds fast. In despite of the high cost, however, it is still difficult to obtain the desired quaternary phosphonium salt of high purity wherein the content of halide ion is very low.
On the other hand, the variation in properties between the quaternary phosphonium salts, even those with a same cation but different anions, is dramatic. The salts with anions such as NO3−, CO32+, PF6−, BF4−, C2O42−, Al2Cl7−, CH3COO−, CF3SO3−, C4H9SO3−, CF3COO−, N(CF3SO2)2−, N(C2F5SO2)2−, N(C4F9SO2)2−, N[(CF3SO2)(C4F9SO2)]−, and C(CF3SO2)3−, etc., may exhibit lower melting point, higher electrical conductivity, lower viscosity and/or stronger hydrophobicity as compared to the quaternary phosphonium halides. This sort of variation in physical and/or chemical properties gives rise to wider application in green chemistry and as novel electrochemical materials in various electrochemical energy storage systems. It is of great significance, therefore, to develop a new process of preparing high pure quaternary phosphonium salts with different anions.
U.S. Pat. No. 4,892,944 discloses a process of preparing a quaternary phosphonium salt by reacting a tertiary phosphine with a carbonic acid diester to form a corresponding quaternary phosphonium carbonate and further mixing it with an acid to perform decarboxylaiton, as shown in equations below.R1R2R3P+Me2CO3→[R1R2R3PMe]+MeCO3−  (4)[R1R2R3PMe]+MeCO3−+H+A−→[R1R2R3PMe]+A−+MeOH+CO2  (5)
According to the process, numerous quaternary phosphonium salts having various anions can be efficiently produced. To some extent the contamination of the anions could be avoided because in the presence of acid the alkyl carbonate ion is readily transformed to alcohol and CO2, which can be removed by simple method. However, this methodology is only applicable when tertiary phosphines are used as substrates. More readily available phosphine (PH3) and other phophine derivatives, including primary phosphines (RPH2) and secondary phosphines (R1R2PH), can not be alkylated by carbonic acid diester to give quaternary phosphonium compounds as shown in equation (4).