R₃E⁺ Carborane Salts of the Heavier Group 14 Elements: E = Ge, Sn, and Pb

Abstract

The synthetic and structural chemistry of catalytically useful trialkylsilylium salts with weakly coordinating carborane anions, R 3 Si(CHB 11 X 11 ), has been extended to the heavier group 14 elements. Et 3 Ge(CHB 11 H 5 Br 5 ) was prepared from Et 3 GeH and trityl CHB 11 H 5 Br 6 -. Its X-ray crystal structure shows ion-like character very similar to its Si congener. The heavier element analogues, Et 3 E(CHB 11 H 5 Br 6 ) (E = Sn, Pb), were prepared by chloride ion abstraction from Et 3 SnCl and Et 3 PbCl, respectively, using Et 3 Si(CHB 11 H 5 Br 6 ). Their crystal structures differ from those of the four-coordinate lighter elements by adopting five-coordinate trigonal-bipyramidal stereochemistries, reflecting the periodic table transition of these elements from semi-metals to metals. The carborane anions are weak, bridging, axial ligands connecting trigonal Et 3 E þ cation-like moieties in polymeric chain structures. When the less coordinating CHB 11 Cl 11 -anion is used, excess germane competes with the carborane anion for coordination to Ge and a salt of the new hydride-bridged cation [Et 3 Ge-H-GeEt 3 ] þ can be isolated. It has a distinctive νGeHGe IR band at ca. 1740 cm -1 analogous to the 1875 cm -1 band of the [Et 3 Si-H-Si-Et 3 ] þ cation. Comparable chemistry is not observed for the Sn and Pb congeners.

Reactive p-block cations stabilized by weakly coordinating anions

“… 337 Examples for lead are still missing and the only formally [R 3 Pb] + containing ion-like substance Et 3 Pb(HCB 11 H 5 Br 6 ) has like its Si, Ge and Sn analogs stronger interactions between the ions. 338 …”

Carboranes in the chemist's toolbox

“…5,6 By this means, isolable C 6 H 7 + , n-C 6 H 13 + , and CMe 3 + salts of HCB 11 F 11 − have been generated from benzene, n-hexane, and (remarkably) n-butane, respectively. Among these are (n-C 4 H 9 ) 3 Sn + , 8 protonated porphyrins, 9 imidazolium salts, 10,11 C 59 N + (azafullerene), 12,13 silyl-stabilized allyl cations, 14 chloroalkanes, 15 ClR 2 + (chloronium ions, R = Me or Et), 16 oxonium ions, 17 R 3 E + ions where E is Ge, Sn, or Pb, 18 [19][20][21] Also noteworthy is the protonation of fullerenes, with the isolation of HC 60 + CB 11 H 5 Cl 6 − as a stable salt and its structural characterization via 13 C CPMAS spectroscopy, revealing rapid movement of the proton that renders all 60 carbon atoms equivalent on the NMR time scale. 7 The capability of protonating hydrocarbons under ambient conditions affords a powerful tool for investigation in several areas such as hydrocarbon reformation on zeolites, a process which normally occurs at high temperature and has been difficult to study.…”

Carba‐closo‐dodecaborates – Synthesis, Structure, and Energetics

“…Moreover, the fluorine derivatives also have larger negative NICS values compared to the less electronegative species (e.g., Cl‐, Br‐, I‐, or H‐substituted species). The literature reveals that the outstanding formal [R 3 Si] + ‐transfer properties of [R 3 Si][CHB 11 H 5 Br 6 ], and [R 3 Si] 3 , , , are already being used unconsciously.…”

“…In recent years a lot of work was carried out with such cage molecules, which are now appreciated for their low basicity, low nucleophilicity, as well as their (electro)chemical and thermal robustness . Borane‐based cage molecules have found a wide range of applications in medicine,, catalysis,, and as weakly coordinating anions (WCAs) for the stabilization of highly reactive cations in solution and the solid state if none of the well‐known WCAs work (e.g., stable [Cat][CHB 11 Cl 11 ] vs. unstable [Cat][A]; [Cat] + = H + , [Alk 3 E] + ; Alk = Me, Et, t Bu; E = group 14 element; [A] – = [BF 4 ] – , [PF 6 ] – , [B(C 6 F 5 ) 4 ] – , [Al(OR F ) 4 ] – ) . In reviews of carba‐ closo ‐dodecaboranes Michl et al summarized the development of such chemistry in the last decades , .…”