Tetrafluorophosphate Anion

Abstract

The elusive POF4- anion has been characterized for the first time. It is formed from N(CH3)4F and POF3 in CHF3 solution at −140 °C and can be observed at this temperature by 19F and 31P NMR spectroscopy. Between −140 and −100 °C it reacts with POF3 forming the OF2P−O−PF5- anion, which, at higher temperatures, reacts with F- anions to give PO2F2- and PF6-. This reaction sequence provides a low activation energy barrier pathway for the highly exothermic dismutation of pseudo-trigonal-bipyramidal POF4- to tetrahedral PO2F2- and octahedral PF6- and explains the previous failures to isolate POF4- salts. In addition to POF4-, OF2POPF5-, and PO2F2-, the protonated form of OF2POPF5-, i.e., HOF2POPF5, was also identified as a byproduct by NMR spectroscopy. The structure, vibrational spectra, force field, NMR parameters, and dismutation energy of POF4- were calculated by ab initio electronic structure methods using, where required, the closely related and well-known PF4- ion and POF3, PF3, SOF4, and SF4 molecules for the determination of scaling factors. Although the structures and dismutation energies of isoelectronic POF4- and SOF4 are very similar, their dismutation behavior is strikingly different. While POF4- dismutates rapidly at low temperatures, SOF4 is kinetically stable toward dismutation to SO2F2 and SF6. This fact is attributed to the lack of a low activation energy barrier pathway for SOF4. Furthermore, the dismutation energy calculations for POF4-, SOF4, and ClOF3 revealed very large errors in the previously published thermodynamic data for the heats of formations of SO2F2 and SOF4 and the dismutation reaction energy of POF4-. Energy barriers and the C4v transition states for the Berry-style pseudorotational exchange of equatorial and axial fluorines in POF4-, SOF4, PF4-, and SF4 were also calculated and can account for the observation that on the NMR time scale the exchange in PF4- and SF4 can be frozen out at about −40 °C, while in POF4- and SOF4 it is still rapid at −140 and −150 °C, respectively.