The reaction of Si(IV) or P(V) fragments with HMPA will be shown to produce cationic species. Nonmetallic, cationic elements at higher valency are known as onium ions. These onium ions are often highly reactive intermediates. Therefore, we studied them to better understand the synthetic routes in which they are suspected to play a role in. The use of a metal-assisted route will be explored for the formation of siliconium and phosphonium species.
The reaction of M(CO)5HMPA with SiR3H in benzene produces a cationic species stabilized by a weakly coordination anion in a liquid clathrate matrix when M is a group six transition metal. The reaction utilizes a one-pot synthesis. The hydrido silanes have been used in this reaction to form stable siliconium cations [SiR3(HMPA)+] or [SiR3(HMPA)2+] that have been proposed intermediates in several organic syntheses. This general synthesis produces a liquid clathrate through the presence of the weakly coordination anion [(CO)5MHM(CO)5-]. This anion’s specific geometry seems to be crucial to the formation of the liquid clathrate and therefore the stabilization of the cations. In order to obtain X-ray crystallographic data of the cation an ion exchange reaction was conducted between [SiEt3(HMPA)+][H(W(CO)5)2-] and K+[B(C6F5)4)-]. X-ray crystallographic data was obtained for the [SiEt3(HMPA)+][B(C6F5)4+] complex. Mass spectrometry was showed the ions [SiEt3(HMPA)+]
an [HW2(CO)8-] for [SiEt3(HMPA)+][H(W(CO)5)2-]. However, mass spectrometric characterization of [SiH2Ph(HMPA)2+][H(W(CO)5)2-] showed presence of the [SiPhH(HMPA)Na+] cation due to the sensitivity of the compound. The decreased stability of the [SiH2Ph(HMPA)2+] cation is due to the high reactivity of the silicon-hydrogen bond as well as the tendency for five-coordinate silicon compounds to be highly reactive. The generality of this synthesis was investigated and the products were characterized by multi nuclear NMR, ESI-MS and X-ray crystallography.
This same metal assisted route was unsuccessful in producing the desired phosphonium cation [P3N3Cl5(HMPA)+] from [PCl2N]3. This cation is similar to a proposed intermediate in the ring-opening polymerization of [PCl2N]3. However, the reaction of HMPA with a variety of different cyclic chlorophosphazenes was found to form a different phosphonium cation, [P(NMe2)3Cl+]. A general oxygen/chlorine transfer reaction was observed between the chlorophosphazenes and HMPA resulting in oxygen containing anions. A reaction with minimal byproducts was established for [PCl2N]3 and [PCl2N]4 with HMPA resulting in the cationic species [P(NMe2)3Cl+][P3N3Cl5O-] and [P(NMe2)3Cl+][P4N4Cl7O-] respectively. The generality of the reaction of chlorophosphazenes with HMPA was investigated. The products were characterized by multinuclear NMR, ESI-MS and X-ray crystallography.