The Sixteen CB 11 H n Me 12 - 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: 12.11). 11/2007; 129(43):12960-80. DOI: 10.1021/ja066247z
Source: PubMed


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.

Download full-text


Available from: Zdenek Havlas,
  • Article: Boron
    [Show abstract] [Hide abstract]
    ABSTRACT: This chapter reviews the literature reported in 2007 concerning the chemistry of the boron, and focuses on those articles and publications that have appeared in 'high impact' journals. Because of the breadth of research into the area of boron chemistry, boron is again handled separate from the other elements of Group 13. The text excludes much material or technological chemistry unless original, fundamental or notable for other reasons. Due to space limitations this review is not exhaustive and the author apologises in advance that not all published work can be covered to the same extent.
  • [Show abstract] [Hide abstract]
    ABSTRACT: After a brief introduction to the electronic structure of the three-dimensionally aromatic icosahedral closo-monocarbadodecaborate anion CB11H12-, some recent results for its permethylated version, CB11Me12- and three highly reactive electroneutral analogs are presented and discussed. These are the radical CB11Me12 center dot, the boronium ylide CB11Me11 with a naked boron vertex, and the isomeric carbonium ylide with a naked carbon vertex. These ylides are probably better viewed as unusual types of singlet borylene and carbene, respectively.
    Pure and Applied Chemistry 03/2008; 80(3). DOI:10.1351/pac200880030429 · 2.49 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The rate of interconversion of the two inequivalent edges of the pyridine rings in the trigonal prism 3c, self-assembled from 3 equiv of the star connector, tetrakis[4-(4-pyridylethynyl)phenyl]cyclobutadienecyclopentadienylcobalt, and 6 equiv of a platinum linker, cis-(Me3P)2Pt(2+) 2 TfO(-), was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy DeltaH(double dagger) is approximately 12 kcal/mol and an activation entropy DeltaS(double dagger) ranges from approximately -15 to approximately 0 cal/mol x K as a function of the nature and concentration of the anions present. The reaction is attributed to hindered rotation of the pyridine rings about the Pt-N bond, facilitated by a tight pairing with a counterion. Above a counterion-dependent limiting temperature, DeltaH(double dagger) and DeltaS(double dagger) change abruptly to approximately 35 kcal/mol and approximately 60 cal/mol x K, respectively. The changes largely compensate, such that the reactions have comparable rates in the two regimes, both amenable to DNMR measurement, but their mechanisms clearly differ. Several kinetic models for the involvement of ion pairing equilibria fit the observed data nearly equally well, and they all contain a reaction step with high DeltaH(double dagger) and DeltaS(double dagger) values in the high-temperature regime. Its mechanism is proposed to involve a counterion-assisted reversible dissociation of one or two adjacent Pt-N bonds, followed by nearly free rotation of the terminal pyridine ring or rings and subsequent bond reclosure, which is similar to the last presumed step in the initial prism assembly. An interpretation of the very high DeltaS(double dagger) value is suggested by molecular dynamics calculations: at equilibrium, there is a bubble of gaseous nitromethane solvent inside the prism, and it collapses when the prism opens as the transition state is reached. A simple calculation of the entropy of cavitation provides quantitative support for this tentative proposal. The presence of such voids might be generally important for the formation and properties of self-assembled cages.
    Journal of the American Chemical Society 07/2008; 130(24):7629-38. DOI:10.1021/ja801341m · 12.11 Impact Factor
Show more