Camouflaged Carborarods Derived from B‐Permethyl‐1,12‐diethynyl‐para‐ and B‐Octamethyl‐1,7‐diethylnyl‐meta‐carborane Modules

Abstract

Rigid camouflaged carborarods constructed from the corresponding C,C'-diethynyl derivatives of B-decamethyl-1,12-dicarbadodecaborane(12) (6) and B-octamethyl-1,7-dicarbadodecaborane(12) (48) have been synthesized by largely conventional organic transformations. These carborarods are the longest discrete rod species available by this method in which B-methylated p-carborane and m-carborane cages are linked through their carbon vertices by using butadiynylene moieties. They exhibit enhanced solubility in common organic solvents relative to all other presently known carborane-based rigid-rod molecules. The oxidative coupling of bis(ethynyl) derivatives of 6 generates oligomers containing, on average, 16 carborane modules. The structural characterization of the corresponding dimeric species revealed that the carborarods possess a sinusoidal chain distortion in the solid state. The stereoelectronic properties of these and related model carborarods were evaluated by using molecular dimensions as a monitor for the comparison of computational and experimental methods. In addition, the effect of exhaustive B-methylation of 12- and 10-vertex para-carborane cages in a series of model C,C'-diethynyl derivatives was similarly investigated by computational and structural studies. As expected, a correlation of intercage C--C bond lengths with cage size was observed and was attributed to hybridization effects. B-Permethylation had no significant structural effect with either 10- or 12-vertex cage derivatives. Relative to unsubstituted compounds, thermal and chemical stabilities of B-permethylated derivatives were increased through the operation of a steric "bumper-car" process, and solubilities in organic solvents were enhanced. The formation of linear, sterically encumbered platina-carborarods using ethynyl derivatives of 6 as precursors is described.

Improved syntheses of bis(ethynyl)-para-carboranes, 1,12-(RCC)2-1,12-C2B10H10 and 1,10-(RCC)2-1,10-C2B8H8 (R=H or Me3Si)

“…The overall yields of 1, 2, 6 and 7 from para-carboranes 3 and 5 are 69%, 67%, 63% and 48%, respectively, which are superior to reported yields of up to 26% from previous single/two-step syntheses [6][7][8]10]. The convenience of a two-step route is also an obvious advantage in effort and cost over the more elaborate multi-step syntheses, which afford the ethynyl carboranes in comparable yield to those described here [9].…”

“…A low-yielding synthesis of 2 has been reported by Kaszynski et al [8] while we have also used 1-bromo-3-methyl-1-butyn-3-ol instead of 4 in a two-step synthesis of 1 in low yields [7]. Herzog et al reported multi-step syntheses of 1 and 2 from 3 in overall yields of 64% [9]. These yields are considerably improved with respect to the simple two-step syntheses from 1 and, whilst elegant, the complete synthetic procedure is somewhat tedious.…”

“…Similar reaction conditions have been previously employed for the coupling of haloaryls with C-copper carborane derivatives (Scheme 2) [5], and a related coupling protocol has been reported for the synthesis of 1,7-(transClCH@CH) 2 -1,7-C 2 B 10 H 10 from meta-carborane, 1,7-C 2 B 10 H 12 [11]. Compound 8 (together with the cis isomer) has been prepared on a previous occasion in three steps from 3 [9].…”

“…However, the expense of the 10-vertex precursor and the low yield of 6 have so far precluded the widespread use of 6 as a precursor of compounds to explore this point. It is noteworthy that not even the 10-vertex analogue of 1, 1,10-(HC"C) 2 -1,10-C 2 B 8 H 8 (7), was known prior to the present study, although calculations of this species have been reported at the HF/6-31G* level of theory [8,9].…”

Dicarba‐closo‐dodecaboranes with One and Two Ethynyl Groups Bonded to Boron

“…Their application in coordination chemistry is especially promising, as suggested by a comparison to related compounds: (i) dicarbacloso-dodecaboranes with ethynyl groups bonded to the cluster carbon atoms have been successfully used as ligands, [2,25,27,29,46] for example, in [{Ru(dppe)Cp*} 2 {µ-1,12-(CϵC) 2 -closo-1,12-C 2 B 10 H 10 }] [27] (dppe = 1,2-bis(diphenylphosphanyl)ethane) and (ii) 9,12-(HCC) 2 -closo-1,2-C 2 B 10 H 10 (1a) as well as 9,10-(HCC) 2 -closo-1,7-C 2 B 10 H 10 (3a) reveal structural similarities to 1,2-diethynylbenzene that has proven to be a versatile bridging ligand, for example, in metallamacrocycles. [47] Furthermore, with the ethynyl-functionalized carba-closo-dodecaborate anions [12-HCC-closo-1-CB 11 -, isoelectronic anionic counterparts [33,48] are available for comparative studies.…”