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)(3)-2,4-C2B4H5 AND RELATED CARBORANES

The reaction of gaseous HCl with either the disodium or dilithium comp ound of the [nido-2,4-(SiMe3)(2),-2,4-C2B4H4](2-) dianion(I) in 1:1 st oichiometry in THF produced the monoprotonated species 1-Na(THF)(2)-2, 4-(SiMe3)(2)-2,4-C2B4H5 (II) or 1-Li(THF)(2)-2,4-(SiMe3)(2)-2,4-C2B4H5 (III), in 81% and 80% yields, respectively. This method proved superi or to that involving the direct reduction of the closo-C2B4 carborane by metal hydrides. II and III were characterized by elemental analysis , H-1, B-11 and C-13 NMR and IR spectra. Compound ZI was recrystallize d from a mixture THF, hexane and TMEDA (1:2:1) to isolate colorless cr ystals of the mixed solvated species, 1-(THF)-1-(TMEDA)-1-Na-2,4-(SiMe 3)(2)-2,4-C2B4H5 (IV), which were subsequently used for X-ray diffract ion studies. The structure of IV showed that the capping metal occupie d the apical position above the open C,B, face of the carborane and th at a hydrogen atom was bridging the two adjacent boron atoms on that f ace. The B-11 and C-13 NMR spectra calculated by GIAO (gauge independe nt atomic orbital) methods at the 6-311G* level on the B3LYP/6-31G* o ptimized geometries of I-III, and a number of related nido- and closo- carboranes, gave excellent agreement with experiment, even in compound s where electron correlation effects are known to be important. (C) 19 98 Elsevier Science S.A.