The Structure of Potassium and Tetramethylammonium Salts of the Cyclic Organohydroborate Anion [H2BC8H14]– with Different Solvent Ligands

Abstract

AbstractSolvated potassium salts of the 9‐BBN hydroborate anion have been prepared and their structures have been determined by single‐crystal X‐ray diffraction analyses. A tetrameric complex {LK[(μ‐H)2BC8H14]}4 is formed when L is toluene, MeTHF, toluene/THF, THF/Et2O mixtures. The basic framework consists of a distorted K4B4 cube in which potassium and boron are at alternate corners of the cube linked by bridging hydrogen atoms and α‐C–H hydrogen atoms from the 9‐BBN hydroborate that appear to form agostic interactions with potassium. There are two types of bridging hydrogen atoms: one is connected to three potassium atoms and one boron atom in a μ4‐coordination arrangement, and the other one is connected to two potassium atoms and one boron atom in a μ3‐coordination arrangement. There are four μ4‐hydrogen arrangements in the structure. They are located inside of the K4B4 cube. The four μ3‐hydrogen arrangements are located outside of the distorted K4B4 cube. The coordination number of potassium is 8 with the three μ4‐, two μ3‐bridging hydrogen atoms, two α‐C–H hydrogen atoms, and one oxygen atom of the solvent, L (there is a weak π‐η6 interaction when the solvent is toluene). On the other hand, in the chelating solvent DME, a dimeric structure is formed in which the inner part is planar with potassium and boron located at alternate corners. The potassium is connected to four μ3‐bridging hydrogen atoms, two apparent agostic α‐C–H bonding hydrogen atoms, and four oxygen atoms of two DME molecules. Its coordination number is 10 including the agostic interactions. The structure of the uncoordinated [H2BC8H14]– anion is observed in the tetramethylammonium salt. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)