The First Fully Sandwiched Lithiacarborane Complex: A Synthetic and Structural Investigation

摘要

It is generally found that the monoanionic cyclopentadienides form stable sandwich complexes with metals in their lowest oxidation states, while the dianionic carborane (Cb~(2-)) ligands stabilize the higher oxidation states of the metals. Consequently, the neutral Si(II) sandwiched complex is known only in the Cp (Cp = eta~5-C_5R_5, R = hydrogen and an alkyl substituent) system, but the corresponding Si(IV) species exists exclusively in the Cb systems; a number of other examples of these oxidation state preferences are in the literature. Therefore, the recent report of the formation of group 1 metal complexes in which the metal was sandwiched between pi-electron Cp ligands to give the anionic full-sandwich (Cp_2M)~- (M = Li, Na) complexes raised the question as to whether lithium or sodium could form the analogous full-sandwich complex with the carborane ligands. If such complexes are stable, what synthetic method should be applied in their preparation? Our previous experience with the reactions of neutral and monoanionic nido-carboranes with group 1 bases, plus our recent work on the magnesacarboranes in which it was found that the use of a monoalkyl Mg reagent produced the half-sandwich magnesacarborane while the synthesis of a full-sandwich Mg-(carborane)) complex was possible only with magnesium dialkyls, indicated that the reactions of the monoalkyl lithiums with the neutral or monoanionic carborane ligands would not be profitable avenues to the synthesis of the full-sandwich group 1 metallacarborane species. Here, we report a new method for the synthesis of an anionic full-sandwich lithiacarborane complex, whose solid state structure closely resembles that of the (Cp_2Li)~- complex. To our knowledge, this is the first example of a carborane species in which a group 1 metal is fully sandwiched between two carborane ligands.