Fourier transform infrared spectroscopic characterization of olefin complexation by silver salts in solution

Complex formation between 1-hexene and silver salts in chloroform solution has been characterized using Fourier transform infrared spectroscopy. Silver tetrafluoroborate, AgBF₄, and silver triflate, AgCF₃SO₃, form 1:2 Ag⁺:1-hexene complexes in solution. The olefinic carbon carbon vibrational stretching frequency at 1640 cm^-1 in 1-hexene shifts to approximately 1580 cm^-1 upon complexation with these silver salts. Silver trifluoroacetate, AgCF₃CO₂, exhibits limited complexation and silver nitrate, AgNO₃, exhibits no measurable complexation. Salts with the larger BF₄^- and CF₃SO₃^- anions have a much stronger tendency to form complexes than those with the smaller NO₃^- and CF₃CO₂^- anions. The tendency of these salts to form complexes in solution was most strongly correlated with anion size and was not well-correlated with measures of anion-donor electron density or softness parameter. The effect of ether-containing compounds, such as triethylene glycol (TEG) and poly(ethylene glycol) (PEG), on the silver-olefin interaction in chloroform solution was also studied. The formation of a silver ion-ether oxygen-olefin complex is consistent with the data. For TEG added to a solution of AgBF₄ and 1-hexene in chloroform, there are approximately 2.5 oxygens per silver ion in the complex. For PEG added to a solution of silver triflate and 1-hexene in chloroform, there are approximately 1.8 oxygens per silver ion.