The sixteen CB11HnMe12-n- anions with fivefold substitution symmetry: Anodic oxidation and electronic structure

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA.
Journal of the American Chemical Society (Impact Factor: 11.44). 11/2007; 129(43):12960-80. DOI: 10.1021/ja066247z
Source: PubMed

ABSTRACT The 15 symmetrically methylated derivatives of the CB11H12(-) anion (1a) have been synthesized and found to vary greatly in ease of oxidation. Cyclic voltammetry in liquid SO2 yielded fully reversible oxidation potentials for five of those that have no adjacent unsubstituted vertices in positions 7-12; three others showed some indication of reversibility. The anions 1a-16a and the Jahn-Teller distorted neutral radicals 1r-16r have been characterized by ab initio and density functional theory calculations. In the state average CASSCF(13,12)/6-31+G* approximation, the ground state potential energy surface of 1r contains five symmetry-related pairs of minima. The computational results account for the reversible redox potentials very well when the solvent is included explicitly (RI-DFT(BP)/TZVP, COSMO). For display and for a semiquantitative understanding of methyl substituent effects in terms of perturbation theory, the molecular orbitals of 1a have been expressed in the symmetry-adapted cluster basis. The results serve as an underpinning for a set of additive empirical increments for redox potential prediction. Relative to the usual hydrogen standard, a single methyl group facilitates oxidation by approximately 50, 70, 70, and 10 mV in positions 1, 2, 7, and 12, respectively. This electron donor effect on the redox potential is due to a contribution, whereas those of (inductive and direct field) type are negligible.

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    ABSTRACT: The trideuteriomethylation of BH vertices in CB11H12− and its derivatives with CD3OTf (OTf=triflate, trifluoromethanesulfonate) yields a mixture of BCD3 and BCHD2 substitution products, thus demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed if CD3OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σ adduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square-pyramidal methane attached axially to a “naked” boron vertex. The subsequent proton or deuteron loss is presently not understood in detail. A general comparison of electrophilic substitution on closo-boranes and arenes is provided and similarities as well as differences are discussed. A recalculation of the optimized geometry of the CB11Me12. radical produced a second Jahn–Teller distorted minimum and resulted in a somewhat improved agreement between calculated and measured proton hyperfine coupling constants.
    ChemPlusChem 09/2013; 78(9). DOI:10.1002/cplu.201300219 · 3.24 Impact Factor
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    ABSTRACT: Disubstituted carba-closo-dodecaborate anions with one functional group bonded to the cluster carbon atom and one ethynyl group bonded to the antipodal boron atom were synthesized from easily accessible {closo-1-CB11} clusters. [Et4N][1-NC-12-HC≡C-closo-1-CB11H10] ([Et4N]4b) was prepared starting from Cs[12-Et3SiC≡C-closo-1-CB11H11] (Cs1c) via salts of the anions [1-HO(O)C-12-HC≡C-closo-1-CB11H10](-) (2b) and [1-H2N(O)C-12-HC≡C-closo-1-CB11H10](-) (3b). In a similar reaction sequence [Et4N][1-CN-12-HC≡C-closo-1-CB11H10] ([Et4N]7b) was obtained from Cs[1-H2N-12-HC≡C-closo-1-CB11H10] (Cs5b) by formamidation to yield [Et4N][1-H(O)CHN-12-HC≡C-closo-1-CB11H10] ([Et4N]6b) and successive dehydration. In addition, the synthesis of the isonitrile [Et4N][1-CN-closo-1-CB11H11] ([Et4N]7a) is presented. The {closo-1-CB11} derivatives were characterized by multinuclear NMR as well as vibrational spectroscopy, mass spectrometry, and elemental analysis. The crystal structures of [Et4N][1-HO(O)C-12-HC≡C-closo-1-CB11H10] ([Et4N]2b), [Et4N][1-H2N(O)C-12-HC≡C-closo-1-CB11H10] ([Et4N]3b), [Et4N][1-NC-12-HC≡C-closo-1-CB11H10] ([Et4N]4b), [Et4N][1-H(O)CHN-12-HC≡C-closo-1-CB11H10] ([Et4N]6b), [Et4N][1-CN-12-HC≡C-closo-1-CB11H10] ([Et4N]7b), and K[1-H(O)CHN-closo-1-CB11H11] ([Et4N]6a) were determined. The transmission of electronic effects through the carba-closo-dodecaboron cage was studied based on (13)C NMR spectroscopic data, by results derived from density functional theory calculations, and by a comparison to the data of related benzene and bicyclo[2.2.2]octane derivatives.
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    ABSTRACT: The inner salts 12-H3N-closo-1-CB11H11 (H1) and 7-H3N-12-Hal-closo-1-CB11H10 [Hal = F (H3), Cl (H4)] were synthesized by a microwave-assisted Buchwald–Hartwig amination starting from the corresponding monoiodinated {closo-1-CB11} clusters and LiN(SiMe3)2. H1 was methylated to result in the zwitterion 12-Me3N-closo-1-CB11H11 (2). All new compounds were characterized by multinuclear NMR spectroscopy, elemental analysis, and mass spectrometry. The crystal structures of H1·Me2CO and 2 were determined. The pKa value of H1 in aqueous solution was estimated by potentiometric titration and by 11B{1H} NMR spectroscopic studies. The stepwise deprotonation of H1 with nBuLi in [D8]THF was studied by multinuclear NMR spectroscopy. In a first step, the amine 1 is obtained and then the amide [12-HN-closo-1-CB11H11]2– and finally the trianion [12-HN-closo-1-CB11H10]3– or more probably lithiated species thereof are formed. The experimental spectroscopic and structural data, as well as the pKa value of H1 are compared to values derived from DFT and ab initio calculations.
    Berichte der deutschen chemischen Gesellschaft 01/2013; 2013(1). DOI:10.1002/ejic.201200969

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