Kristallstrukturen und Phasenumwandlungen von Kalium‐ Tetrakis(trifluormethyl)borat, K[B(CF₃)₄]   

Abstract

Below room temperature K[B(CF3)4] was found to be trimorphic according to DSC measurements, single‐crystal X‐ray diffraction, and vibrational spectroscopy. In its modification below –65 °C K[B(CF3)4] crystallises in the orthorhombic space group P212121 (Nr. 19) with a = 9.2593(14), b = 9.7527(15), c = 10.2170(16) Å and four formula units in the unit cell. At this temperature the anion exhibit nearly T‐symmetry. Between –65 °C and –50 °C K[B(CF3)4] becomes trigonal in the space group P3221 (Nr. 154) / P3121 (Nr. 152) with a = 6.9064(7), c = 17.1509(18) Å, and three formula units per unit cell. In this temperature range the CF3 groups rotate hindered. At room temperature the salt transforms into a plastic phase in the cubic space group Fm\bar{3}m (Nr. 225) with a = 9.9145(28) Å, and four formula units per unit cell, and the anions rotate completely. The unusual properties of K[B(CF3)4] are in part due to the low coordinating ability of the spherical big anions. In comparison to other anions [B(CF3)4]– is very weakly coordinating as demonstrated by the high ν(N–H) vibrational frequency (3234 cm–1) of the ion pair (n‐C8H17)3N+H···[B(CF3)4]– in CCl4 solution.

Cyanoborates

“…Detailed studies on the structural, spectroscopic, and thermal properties of K[BF(CN) 3 ] have been published ,. [20a], In Figure the 19 F, 10/11 B, and 13 C NMR spectra of K[ 11 BF( 13 CN) 3 ] and K[ 10 BF( 13 CN) 3 ] are depicted. [23a] The isotopically labeled salts have been prepared from isotopically enriched starting compounds according to the synthesis shown in Scheme , A .…”

“…The properties of both anions and their salts have been studied in detail and many crystal structures, i.e . of K[BF 2 (CN) 2 ] and K[BF 3 (CN)], have been determined …”

The Trifluoromethyl Group in Transition Metal Chemistry

“…This feature has been widely exploited in the synthesis of a number of organic superconductors with general formula (ET) 2 M(CF 3 ) 4 · (solv), in which solv = halogenated organic solvent and ET = bis(ethylenedithio)tetrathiafulvalene 157. These square‐planar [M III (CF 3 ) 4 ] – anions provide a nice complement to the tetrahedral isoleptic species [B(CF 3 ) 4 ] – 40b,158. The choice can thus be made depending on the more appropriate shape of the weakly coordinating anion.…”

Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low‐Viscosity Ionic Liquids

“…Perfluoroalkylborate anions are another important class of anions that are of interest for catalysis, materials science and organic synthesis. The tetrakis(trifluoromethyl)borate anion [B(CF 3 ) 4 ] − ( II , Figure ) is a very weakly coordinating anion that was used for the stabilisation of reactive cations, such as [Ag(CO) 4 ] + , [Au(NCMe) 2 ] + , [Co(CO) 5 ] + , N 5 + , NO +[31] and [H(OEt 2 ) 2 ] + . Furthermore, the [B(CF 3 ) 4 ] − anion is electrochemically very stable and solutions of its Li + salt reveal a high Li‐ion conductivity .…”