The structure of potassium and tetramethylammonium salts of the cyclic organohydroborate anion [H2BC8H14]- with different solvent Ligands

Solvated 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[(mu-H)(2)BC8H14]}(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 alpha-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 mu(4)-coordination arrangement, and the other one is connected to two potassium atoms and one boron atom in a mu(3)-coordination arrangement. There are four mu(4)-hydrogen arrangements in the structure. They are located inside of the K4B4 cube. The four mu(3)-hydrogen arrangements are located outside of the distorted K4B4 cube. The coordination number of potassium is 8 with the three mu(-)(4), two mu(3)-bridging hydrogen atoms, two a-C-H hydrogen atoms, and one oxygen atom of the solvent, L (there is a weak pi-eta(6) 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 mu(3)-bridging hydrogen atoms, two apparent agostic alpha-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. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).