Complexes of adamantine-based group 13 Lewis acids and superacids: bonding analysis and thermodynamics of hydrogen splitting

The electronic structure and chemical bonding in donor-acceptor complexes formed by group 13 element adamantine and perfluorinated adamantine derivatives EC9Rʹ15 (E = B, Al; R´= H, F) with Lewis bases XR3 and XC9H15 (X=N, P; R= H, CH3) have been studied using energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. Larger stability of complexes with perfluorinated adamantine derivatives is mainly due to better electrostatic and orbital interactions. Deformation energies of the fragments and Pauli repulsion are of less importance, with exception for the boron-phosphorus complexes. The MO analysis reveals that LUMO energies of EC9Rʹ15 significantly decrease upon fluorination (by 4.7 and 3.6 eV for E = B and Al, respectively) which results in an increase of orbital interaction energies by 27-38 (B) and 15-26 (Al) kcal mol-1. HOMO energies of XR3 increase in order PH3 < NH3 < PMe3 < PC9H15 < NMe3 < NC9H15. For the studied complexes, there is a linear correlation between the dissociation energy of the complex and the energy difference between HOMO of the donor and the LUMO of the acceptor molecules. The fluorination of the Lewis acid significantly reduces standard enthalpies of the heterolytic hydrogen splitting H2 + D + A = [HD]+ + [HA]-. Analysis of the several types of the [HD]+ ··[HA]- ion pair formation reveals that orientation with additional H···F interactions is the most favorable energetically. Taking into account the ion pair formation, hydrogen splitting is predicted to be highly exothermic in case of the perfluorinated derivatives. Thus, fluorinated adamantine-based Lewis superacids are attractive synthetic targets and good candidates for the construction of the donor-acceptor cryptandsThis work was financially supported by St. Petersburg State University research grant 12.50.1194.2014. Excellent service of the Centre de Serveis Científiics i Acadèmmics de Catalunya (CESCA) and computer cluster at St. Petersburg State University is gratefully acknowledged. J.P. thanks the Netherlands Organization for Scientific Research (NWO-CW, NWO-EW, NWO-ALW) for financial support. M. S. thanks the following organizations for financial support: the Spanish government (MINECO, project number CTQ2014-54306-P), the Generalitat de Catalunya (project number 2014SGR931, ICREA Academia 2014 prize for excellence in research, and Xarxa de Referència en Química Teòrica i Computacional), and the FEDER fund (European Fund for Regional Development) for the grant UNGI10-4E-80