Molecular Structures, Reactivity, and NMR Spectroscopic Studies of Cyclic and Non‐cyclic Silyl‐substituted 1, 2‐Dicarba‐closo‐dodecaborane(12) Derivatives

Abstract

Non-cyclic and cyclic silyl-substituted 1,2-dicarba-closo-dodecaborane(12) derivatives were prepared mainly by salt elimination methods. Several known and new compounds were structurally characterized by X-ray analysis in the solid state and by mulinuclear magnetic resonance ( 1 H, 11 B, 13 C, 29 Si, 77 Se, and 125 Te NMR) in solution. This includes the 1,2-bis(trimethylsilyl) and 1,2-bis(chlorodimethylsilyl) derivatives as examples for non-cyclic compounds and a series of 1,1,3,3-tetramethyl-4,5-[1,2-dicarba-closo-dodecaborano(12)]-1,3-1080 disila-2-element-cyclopentanes (element = S, Se, Te). Numerous spinspin coupling constants were determined together with their signs. Molecular gas phase geometries for most compounds studied were optimized by calculations [B3Lyp/6-311+G(d,p)], and NMR parameters were calculated at the same level of theory. The conversion of silylsubstituted ortho-carboranes into their respective 7,8-dicarba-nido-undecaborate(1-) derivatives was explored successfully for several examples.In the presented work, we have set out to prepare the missing heavy homologues of 6a with sulfur (6b), selenium (6c) and tellurium (6d), starting from 4 or 5. Some aspects of the reactivity of 5 was addressed, and first attempts were made to convert some of the silyl-substituted carboranes into the respective 7,8-dicarba-nido-undecaborates(1-). The NMR spectra of 3-6 were measured ( 1 H, 11 B, 13 C, 29 Si, 77 Se, and 125 Te NMR), including the determination of absolute signs of spinspin coupling constants, if possible. The molecular structures of 3, 4, 6c, and 6d were determined by X-ray structural analysis, and the gas phase geometries of 3-6 were optimized [B3LYP/6-311+G(d,p)] followed by calculation of NMR parameters at the same level of theory (except of 6d). Results and Discussion Syntheses and NMR SpectroscopyThe synthetic routes are summarized in Scheme 2. Clearly, the dichloride 4 [11] offers great synthetic potential for further transformations. It proved necessary to use the salt elimination reaction of 4 with dilthium selenide [12] or -telluride [12] to obtain pure samples of 6c and in particular of 6d. In principle, the strained four-membered cycle in 5 [8] appears to be attractive for insertion reactions. [7b] However, it does not react with tellurium, it reacts sluggishly with selenium, and somewhat more readily with sulfur. Surprisingly, 5 does not react with bis(trimethylsilyl)peroxide at room temp., whereas its reaction with trimethylamine N-oxide [13] affords the known disiloxane 6a [9,11,14] quantitatively under mild conditions. Silyl-substituted 1,2-Dicarba-closo-dodecaborane(12) Derivatives Scheme 1. Some known 1,2-silyl-substituted ortho-carborane derivatives, obtained from the reactions of 1,2-dilithio-1,2-dicarba-closo-dodecaborane(12) with various chlorosilanes. Scheme 2. Routes to 1,1,3,3-tetramethyl-4,5-[1,2-dicarba-closo-dodecaborano(12)]-1,3-disila-2-chalkogena-cyclopentanes 6a-6d.The compounds 6b,c,d are crystalline yellowish (6b, 6c) or yellow (6d) solids sensitive to moisture...

Synthesis of Tellurasila Cycles Containing an Annelated Dicarba‐closo‐dodecaborane(12) Unit: Structure of a 1,4‐Ditellura‐2,3‐disilacyclohexane Derivative

“…Again, 8 was formed in the reaction with 1,2‐dichlorotetramethyldisilane, however, not as the major product. Instead 9 , the minor species mentioned above (Scheme ), is now the major product, and one other heterocycle, 10 ,30 could be indentified in the reaction mixture. One may speculate that the Te insertion into the C–Li bonds in 4 was incomplete (see relevant experiments for insertion of Se)18 prior to the following reaction.…”

“…This was investigated in more detail as shown in Scheme (B). Heterocycle 10 has been prepared independently, studied for its NMR spectroscopy parameters and characterised by X‐ray structural analysis 30. Compound 11 has been synthesised previously in another context, and its solid‐state structure has been determined 31.…”

“…A final attempt to prepare 13 , starting from 8 , by exchange with Me 2 SiCl 2 (analogously to experiments in selenium chemistry)4c also failed completely. When we used (Me 2 SiCl) 2 O in the reaction with 7 , hoping that a different ring size might help to stabilise the desired product as a seven‐membered ring, we only identified the tellurium‐free known cyclic disiloxane 15 30…”

“…The resulting mixture thus obtained contained 15 (ca. 80 %)30 and several unidentified side products (from 29 Si NMR spectra).…”