EXPERIMENTAL AND THEORETICAL STUDIES ON GROUP 1 METALLACARBORANES : SYNTHESIS, STRUCTURE AND AB INITIO CALCULATIONS OF THE NMR CHEMICAL SHIFTS OF THE 1-(THF)-1-(TMEDA)-1-NA-2,4-(SIME3)2-2,4-C2B4H5 AND RELATED CARBORANES

Abstract The reaction of gaseous HCl with either the disodium or dilithium compound of the [ nido -2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 ] 2− dianion ( I ) in 1:1 stoichiometry in THF produced the monoprotonated species 1-Na(THF) 2 -2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 5 ( II ) or 1-Li(THF) 2 -2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 5 ( III ), in 81% and 80% yields, respectively. This method proved superior to that involving the direct reduction of the closo -C 2 B 4 carborane by metal hydrides. II and III were characterized by elemental analysis, 1 H, 11 B and 13 C NMR and IR spectra. Compound II was recrystallized from a mixture THF, hexane and TMEDA (1:2:1) to isolate colorless crystals of the mixed solvated species, 1-(THF)-1-(TMEDA)-1-Na-2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 5 ( IV ), which were subsequently used for X-ray diffraction studies. The structure of IV showed that the capping metal occupied the apical position above the open C 2 B 3 face of the carborane and that a hydrogen atom was bridging the two adjacent boron atoms on that face. The 11 B and 13 C NMR spectra calculated by GIAO (gauge independent atomic orbital) methods at the 6-311G ** level on the B3LYP/6-31G * optimized geometries of I – III , and a number of related nido - and closo -carboranes, gave excellent agreement with experiment, even in compounds where electron correlation effects are known to be important.