Ab Initio, DFT and semi-empirical studies on interactions of phosphoryl, carbonyl, imino and thiocarbonyl ligands with the li+ cation: affinity and associated parameters

ARTICLE   Ab Initio, DFT and semi-empirical studies on interactions of phosphoryl, carbonyl, imino and thiocarbonyl ligands with the li+ cation: affinity and associated parameters     Leonardo M. da CostaI; Lílian W. C. PaesII; José Walkimar de M. Carneiro*,I IInstituto de Química, Universidade Federal Fluminense, Outeiro de São João Batista, s/n, 24020-141 Niterói-RJ, Brazil IIDepartamento de Ciências Exatas, Escola de Engenharia Industrial e Metalurgia de Volta Redonda, Universidade Federal Fluminense, Av. Dos Trabalhadores,420, Vila Santa Cecília, 27255-125 Volta Redonda-RJ, Brazil     ABSTRACT The affinity of the Li+ cation for a set of para-substituted phosphoryl, carbonyl, imino and thiocarbonyl ligands was calculated with the ab initio MP2/6-311+G(d,p), DFT B3LYP/6-311+G(d,p) and semi-empirical PM6 methods. Each set of ligand is constituted by the following para-substituted groups: NH2, OCH3, CH3, H, Cl, CN and NO2. The interaction enthalpy was calculated to quantify the affinity of the ligands for the Li+ cation. Geometric and electronic parameters were correlated with the strength of the metal-ligand interaction. The electronic nature of the para-substituted group is the main parameter that modulates the intensity of the metal-ligand binding energy. Electron donor groups make the interaction enthalpy more exothermic, whereas electron acceptor groups make the interaction enthalpy less exothermic. The energy decomposition analysis shows that the para-substituted groups modulate the intensity of the electrostatic component of the interaction without affecting the covalent component. Keywords: lithium cation, substituent effect, interaction enthalpy, EDA, ligand interaction RESUMO A afinidade do cátion Li+ para uma série de ligantes fosforila, carbonila, imino e tiocarbonila substituídos na posição para de um anel aromático foi calculada com os métodos ab initio MP2/6-311+G(d,p), DFT B3LYP/6-311+G(d,p) e semi-empírico PM6. Cada série de ligante é constituída pelos seguintes grupos substituídos na posição para: NH2, OCH3, CH3, H, Cl, CN and NO2. A entalpia de interação foi calculada para quantificar a afinidade dos ligantes para o cátion Li+. Parâmetros geométricos e eletrônicos foram correlacionados com a força da interação metal-ligante. A natureza eletrônica dos grupos substituintes é o principal parâmetro que modula a intensidade da ligação metal-ligante. Grupos doadores de elétrons tornam a entalpia de interação mais exotérmica, enquanto grupos aceptores de elétrons tornam a entalpia de interação menos exotérmica. Análise da decomposição da energia mostra que os grupos substituintes modulam a intensidade da componente eletrostática da interação sem afetar a componente covalente.     Introduction The interaction between metal cations and neutral bases is a subject of continuous interest in biochemistry, because of their relevant functions in many biological processes.1,2 Alkaline metal cations are indispensable for the human body, playing an important role in DNA syntheses, hormonal regulation, muscle contraction and in the maintenance of blood pressure.3,4 More specifically, the Li+ cation can also inhibit multiple enzymes, such as cytochrome P450 and glutathione S-transferase, stabilize the structure of nucleotides, stimulate glycogen synthesis and interact with various types of neurotransmitters.5 In all these processes the Li+ cation interacts with a wide range of biological molecules with several different organic functional groups. The most common types of interactions are with oxygen atoms of alcohols,6 ketones,7 carboxylic acids8,9 and phosphates,10 nitrogen atoms of amines,9,11 amides12 and imides13 and sulfur atoms of thiols.14 In the biological media the Li+ cation is surrounded by different types of atoms,5 forming either electrostatic or polar interactions, due to its small size and high nuclear effective charge.15 The bond between the Li+ cation and ligands can be described as an acid-base interaction. According to Pearson's acid base theory the Li+ cation is classified as a hard acid and forms stable compounds when interacting with hard bases.16 Interactions between the Li+ cation and oxygen donor ligands (hard base) are stronger than those with ligands having nitrogen or sulfur atoms (less harder bases) due to its small van der Waals radius and strong polarization power. Nevertheless, the neighborhood of the atom interacting with the Li+ cation is also important to define the strength of the interaction. Inductive and resonance effects of neighbor pendant groups can modulate the electron releasing or withdrawing ability of the ligand by rearranging the charges through the molecule.17,18 Computational studies have extensively contributed to the understanding of the strength of the interactions between the Li+ cation and ligands by calculating the lithium cation affinity (LCA) and basicity (LCB). The LCA is defined as the negative of the enthalpy change (DH) of equation 1, whereas the LCB is the DG298 associated with the corresponding thermodynamic equilibrium.19     The widest LCA and LCB series were proposed by Burk et al.,19 based on Taft's work,20 and consist of an experimental and ab initio study of 205 neutral organic and inorganic ligands with a 29.6 kcal mol-1 LCB range. This scale has been widely used to convert relative basicities into absolute ones and to evaluate the strength of the binding energy between the Li+ cation and ligands.19 Monofunctional sulfuryl and phosphoryl ligands were evaluated by Buncel et al.21 and Borrajo et al.21 showing, as a general trend, that the Li+ cation interaction is stronger with harder (Pearson) bases. A detailed DFT study of the influence of electron donor and electron acceptor groups in a set of para-substituted acetophenones on the lithium affinity was reported by Senapati et al.22 It demonstrates that pendant aromatic groups can modulate the strength of the lithium interaction and that some specific electronics and geometrical parameters are strongly correlated to the affinity. Similar correlations were found by Palusiak23 in para-substituted Cr(CO)5-pyridine complexes, by Ma in silver complexes with carbonyl, nitrogenous, thio and aromatic ligands24 and by Gal et al.25 in lithium complexes with substituted phenyl rings. High level electronic structure calculations of lithium affinities show high degree of correlation between ab initio and DFT results.19,24,26 Continuing our previous investigations27 about the effect of substituents on the interaction between oxo ligands and the alkaline earth cations, in the present work we quantify the intensity of the binding energy of several ligands, with different functional groups, to the Li+ cation. As model ligands we studied some constitutive blocks of organic compounds, with C=O (carbonyl), C=N (imino), C=S (thiocarbonyl) and P=O (phosphoryl) groups, as shown in Figure 1. To mimic electronic effe (...truncated)