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.

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