Investigations of Electronic Interactions Between closo-Boranes and Triple-Bonded Substituents

Abstract

Intramolecular electronic interactions were investigated in five series of 12-, 10-, and 6-vertex closo-boranes and hydrocarbons (benzene, acetylene, bicyclo[2.2.2]octane, and cubane) substituted with triple-bonded groups Y≡Z (C≡CR, C≡N, C≡O, and N≡N). Structural data (single crystal X-ray crystallography), spectroscopic properties (UV, IR, and NMR), chemical behavior (dediazoniation reactions), and electronic structure calculations (hybridization and π bond order) are all in agreement that the degree of electronic conjugation between the cluster and the Y≡Z substituent is lowest for the 12-vertex closo-borane and highest for the 6-vertex analog.

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 protected acetylenes 2 and 6 were likewise obtained from sequential reaction of 8 and 10 with methyllithium and chlorotrimethylsilane. 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].…”

“…As previously noted for other pairs of 10-and 12-vertex cage derivatives [8][9][10]12], the single bond C1-C2 between the ethene and cage is longer in the 12-vertex 8 than in the 10-vertex 10 with values of 1.495(2) and 1.479(3) Å for 8 and 10, respectively. The influence of the substituents on the cage geometry of 1,12-disubstituted-para-carboranes has been discussed in detail elsewhere [7].…”

“…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].…”

“…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].…”

Boron Clusters Come of Age

“…Kaszynski and coworkers investigated the electronic interactions between closo-boron clusters of 6, 10, and 12 vertices (see Figure 2) linked by alkynyl and other triple-bonded groups, using UV, IR, and NMR spectroscopic data and theoretical calculations, and concluded that the electronic conjugation between the clusters and the linkers is greatest for the 6-vertex cages (92). Similar studies demonstrated that the 1,12-C 2 B 10 H 12 ( pcarborane) polyhedron transmits electronic effects, although the mechanism was not established (93).…”