Synthesis and Structural Studies of Subicosahedral Adjacent-Carbon Carboranes

Abstract

Synthesis and structural studies, employing combined NMR, X-ray crystallographic, and ab initio/IGLO/NMR methods, of a variety of new subicosahedral carboranes with adjacent cage carbons are reported. Acetonitrile-induced cage degradation of arachno-4,5-C2B7H12 - gave nido-4,5-C2B6H9 - (1 - ) in nearly quantitative yield, which can then be protonated to give the neutral carborane nido-4,5-C2B6H10 (1) in good yield. Both of these nido electron-count clusters are shown to have an arachno-type geometry, i.e. a six-membered open face. The nido-4,5-C2B6H10 (1) hydroborated alkenes or alkynes which following deprotonation gave nido-7-R-4,5-C2B6H8 - (2a - − c - ) ions. Both nido-4,5-C2B6H9 - (1 - ) and nido-4,5-C2B6H10 (1) serve as useful precursors to other adjacent cage-carbon clusters. Thus, nido-4,5-C2B6H9 - (1 - ) reacted with BH3·THF to give arachno-5,6-C2B7H12 - (3 - ) which a single-crystal X-ray diffraction study showed is the first carborane to adopt the n-B9H15 cage geometry. Thermal or chemical degradation of nido-4,5-C2B6H10 (1) gave closo-2,3-C2B5H7 (5) in good to moderate yields. The nido-4,5-C2B6H9 - (1 - ) was also found prone to lose a cage boron as evidenced by its reactions with (η-C5H5)Co(CO)I2 and (η6-C6Me6)2Ru2Cl4 which gave closo-3,1,2-(η-C5H5)CoC2B5H7 (6) and closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7), respectively. NMR studies showed the nido-4,5-C2B6H10 (1) was converted to arachno-4,5-C2B6H11 - by reaction with LiEt3BH, and an alkyl derivative, arachno-7-CH3-4,5-C2B6H10 - (4 - ), was formed by reacting MeLi with nido-4,5-C2B6H9 - (1 - ) followed by protonation. The closo-2,3-C2B5H7 (5) was also converted in high yields to the smaller nido carborane, nido-2,3-C2B4H8, via reaction with TMEDA/H2O, and to nido-3,4-C2B5H8 - (8 - ) by reaction with LiEt3BH.

Solution and Solid-State Structure of the Anion [Ag2{closo-CB11H12}4]2-

“…Calculations. In the past decade, the reliable “ab initio/GIAO/NMR” method has been used to determine the molecular structures of carboranes in solution. − However, there are only two reports of the method being used to determine the likely geometries of metal salts of carborane anions in solution . Given the marked differences observed between the 11 B NMR spectra of Ag[ closo -CB 11 H 12 ] when measured in CD 2 Cl 2 versus acetone, we sought to use the ab initio/GIAO/NMR method to elucidate the structures of the species present in these two solvents, in addition to the structure of complex 1 in solution.…”

Computational Studies of Nido-8-Vertex Boranes, Carboranes, Heteroboranes, and the Lewis Base Adduct nido-B8H10L

“…Two isomers of C 2 B 6 H 10 are known experimentally ( 49 and 52 ), and each has been structurally characterized computationally by employing the ab initio/IGLO/NMR method. , The most stable isomer is the known carborane 49 , which has two nonadjacent peripheral 4 k carbons and two 55 -bridge hydrogens. The next most stable isomer is 50 , which could potentially be synthesized via protonation of the known C 2 B 6 H 9 - carborane 41 . The other experimentally known C 2 B 6 H 10 carborane, 52 , has adjacent 3 k peripheral carbons and is calculated to be 22.5 kcal/mol higher in energy than 49 .…”

Density Functional Theory/Finite Perturbation Theory Calculations of Nuclear Spin−Spin Coupling Constants for Polyhedral Carboranes and Boron Hydrides

“…The complexity of polyhedral boron-containing compounds often makes structural assignments difficult. For this purpose, the accurate predictions of 11 B NMR chemical shifts using ab initio molecular orbital methods (such as IGLO and GIAO) provide powerful new structural methods. − In contrast, there are few theoretical studies of nuclear spin − spin coupling in boron compounds. − In view of the extensive applications of coupling constants in other molecular systems, this may seem surprising. Perhaps the most important factor is the extreme difficulty of obtaining accurate coupling constants in polyboranes since even the smaller compounds such as diborane exhibit exceedingly complex NMR spectra. ,− The occurrence of numerous chemically equivalent but magnetically nonequivalent nuclei can lead to spectra requiring spectral analyses even in the absence of second-order features in the spectra. , For boron-containing compounds the situation is more difficult because the two boron isotopomers (80:20 ratio for 11 B: 10 B) produce a superposition of 2 N spectra, where N denotes the number of boron atoms in the molecule.…”