ALKYLIDYNE(CARBABORANE) COMPLEXES OF THE GROUP-6 METALS .3. FACILE TRANSFORMATIONS OF 12-VERTEX AND 13-VERTEX TUNGSTACARBORANE POLYHEDRA - REVERSIBLE FRAMEWORK REARRANGEMENT AND ACID-INDUCED EJECTION OF A CAGE VERTEX

Reactions of [Y][closo-1,2-Me2-3-(= CR)-3,3-(CO)2-3,1,2-WC2B9H9] (Y = NEt4, N(PPh3)2, PPh4; R = Me, C6H4Me-4) with aqueous HX (X = Cl, I) afforded the salts [Y][closo-1,8-Me2-11-(CH2R)-2-X-2,2,2-(CO)3-2,1,8-WC2B9H8]; the structure of the product with X = Cl, Y = NEt4, and R = C6H4Me-4 has been established by X-ray diffraction. A notable structural feature of the anion is that the carborane CMe groups do not occupy adjacent vertices, unlike the situation in the alkylidyne precursor complex. This observation is particularly remarkable in that the framework rearrangement occurs rapidly at temperatures as low as -78-degrees-C. In addition, one of the boron atoms in the lower pentagonal ring of the cage carries a CH2C6H4Me-4 substituent which evidently results from insertion of an initially formed alkylidene fragment into a cage B-H bond. Reactions between the salts [Y][closo-1,8-Me2-11-(CH2R)-2-X-2,2,2-(CO)3-2,1,8-WC2B9H8] and 1 mol equiv of AgBF4 in the presence of CO gave the compounds [closo-1,8-Me2-11-(CH2R)-2,2,2,2-(CO)4-2,1,8-WC2B9H8] in which the cage CMe groups remain separated. In contrast, an excess of the reagent AgBF4 afforded a 1:1 mixture of the same species and the compounds [closo-1,2-Me2-8-(CH2R)-3,3,3,3-(CO)4-3,1,2-WC2B9H8] containing adjacent CMe groups. These reactions are reversible: Addition of NEt4Cl to tetrahydrofuran solutions of the tetracarbonyl species regenerates [NEt4][closo-1,8-Me2-11-(CH2R)-2-Cl-2,2,2-(CO)3-2,1,8-WC2B9H8]. This is a very rare example of a reversible polytopal rearrangement of CMe groups. Reactions between [Y][closo-1,8-Me2-11-(CH2C6H4Me-4)-2-X-2,2,2-(CO)3-2,1,8-WC2B9H8] and the alkynes PhC = CR' (R' = Ph, Me) in the presence of AgBF4 readily afforded the (alkyne)tungstacarborane complexes [closo-1,8-Me2-11-(CH2C6H4Me-4)-2,2-(CO)2-2-(eta-PhC2R')-2,1,8-WC2B9H8]. An X-ray diffraction study of one of these (R' = Me) confirmed the structure. Protonation of the salt [N(PPh3)2][closo-1,6-Me2-4-(= CC6H4Me-4)-4,4-(CO)2-4,1,6-WC2B10H10] with HX (X = Cl, I) yielded the salts [N(PPh3)2][closo-1,7-Me2-2-X-2,2,2-(CO)3-2,1,7-WC2B9H9], for which an X-ray diffraction study (X = I) revealed that a 13- to 12-vertex cage degradation had occurred, as well as loss of a CC6H4Me-4 fragment. This degradation is remarkable considering the acidic reaction conditions. In addition to the three X-ray diffraction studies, the new compounds were characterized by microanalysis; H-1, C-13, and B-11 NMR spectroscopy; and infrared spectroscopy.