Computational Study on the Thermochemistry of β-Diketones and Their Radicals: O-H vs C-H Homolytic Bond Cleavage

José R B Gomes ,  Manuel Anibal V. Ribeiro da Silva 

Centro de Investigação em Química da Universidade do Porto (CIQUP), Rua do Campo Alegre, 687, Porto P-4169-007, Portugal


The strengths of bonds between transition metals or metallic ions and ligands are of deep interest due to their involvement in important chemical and biological processes and have been widely studied in the past years [1]. Among the large number of ligands appears the family of β-diketones, which in general are bidentate ligands, and that are known to form complexes with almost all metal atoms in the periodic table of elements [2]. The metal complex formation can be described by the following reaction in solution:

Mn+ + n HL → MLn + n H+ [1]

The thermochemistry of the ligands (HL) and of the metal complexes (ML), both in the condensed and gas-phases, was investigated experimentally in the past [3]. However, the determination of metal-ligand enthalpies for the dissociation of the complex in the gas-phase and described by reaction [2]:

ML → M + L [2]

requires the gas-phase enthalpies of formation for the three species included in reaction [2] and, unfortunately, it was not possible to obtain any result for the L radicals, except that for acetylacetone reported very recently [4].

In this communication, a computational study on the thermodynamic properties of thirteen β-diketones, with general formula R1C(-OH)C(-R3)C(=O)R2), will be presented. The semi-empirical B3LYP method was employed to compute the O-H and C-H bond dissociation enthalpies, enthalpy of tautomerization and to estimate standard gas-phase enthalpies of formation for the radicals and for the parent molecules. The gas-phase enthalpies of formation for the neutral molecules are in excellent agreement with available experimental values, supporting the estimates made for the radicals. The latter are very important for the clarification of the thermochemistry of many β-diketonato metal complexes previously reported in the literature. Importantly, when substituents R= -CHR' are attached to the β-diketone's carcass, C-H homolytic bond cleavage is always favored with respect to O-H bond scission [5].

[1] H. Chermette, Coord. Chem. Rev. 1998, 178-180, 699.
[2] R. C. Mehrotra, R. Bohra, D. P. Gaur, Metal β-diketonates and allied derivatives, Academic Press Inc., London, 1978.
[3] M. A. V. Ribeiro da Silva, New J. Chem. 1997, 21, 671.
[4] P. Cabral do Couto, B. J. Costa Cabral, J. A. Martinho Simões, Chem. Phys. Lett. 2006, 419, 486.
[5] J. R. B. Gomes, M. A. V. Ribeiro da Silva, submitted for publication.

Legal notice
  • Legal notice:

    Copyright (c) Pielaszek Research, all rights reserved.
    The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
    In every case, proper references including Author(s) name(s) and URL of this webpage: must be provided.


Related papers
  1. The Dissociation Enthalpy of the (N-O) Bond in Methyl-2-methylquinoxaline-3-carboxylate 1-N-oxide

Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 1, by José R B Gomes
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-17 15:38
Revised:   2009-06-07 00:44
© 1998-2021 pielaszek research, all rights reserved Powered by the Conference Engine