DOI: 10.1002/cphc.200500216 Aromaticity Analysis of Lithium Cation/ p Complexes of Aromatic Systems Mireia Güell, [a] Jordi Poater, [b] Josep M. Luis, [a] Otilia Mó, [c] Manuel YµÇez, [c] and Miquel Solà* [a] 1. Introduction In the last two decades, extensive experimental and theoretical investigations have been carried out for alkali metal cation/p complexes involving aromatic rings to investigate the partici- pation of these complexes in important processes related to chemical and biological recognition. [1–20] With few excep- tions, [4,16] all of these works have concentrated mainly on gas- phase basicities of alkali metal cations. Only few of them [18,19] have concisely analyzed the change in aromaticity of aromatic rings on complexation of alkali metal cations by using nucleus- independent chemical shifts (NICS) [21] as indicators of aromatic- ity. Garau et al. [18,19] found that cation/p complexes have more positive NICS that indicate a reduction in aromaticity on com- plexation of the cation. The analysis of these authors was re- stricted to species derived from binding of alkali metal cations to monocyclic aromatic systems such as benzene, 1,3,5-trifluor- obenzene, s-triazine, and 2,5-dichloropyrazine. Systems having more than one aromatic ring are particularly interesting be- cause it is possible to discuss whether the strength of the binding to a given aromatic ring is related to its aromatic char- acter or to the change in aromaticity on association with Li + . Thus, it seems pertinent to extend this analysis to fused aro- matic species. Moreover, Garau et al. [18,19] used only the NICS as a descriptor of aromaticity. However, as found by Katritzky, Kry- gowski et al. by means of principal component analyses [22–26] and also as pointed out by other authors, [27–29] aromaticity is a multidimensional property. This means that the different quan- titative measures of aromaticity based on different physical phenomena (energetic, magnetic, geometric, or electron-deloc- alization properties) are not necessarily correlated. Indeed, some of these criteria are even considered to be orthogonal. [25] In addition, several studies have confirmed that different indi- ces of aromaticity can afford divergent answers. [29–32] In this sense, it has been proposed that fully aromatic systems are those cyclic p compounds that fulfill most aromatic criteria. Those that fulfill only some should be considered only as partly aromatic. [26] As a consequence of this multidimensional character of aromaticity, aromatic compounds cannot be fully characterized by a unique index. Thus, to make reliable com- parisons restricted to groups of relatively similar compounds it is usually recommended to employ a set of aromaticity de- scriptors. [26,29–32] For this reason, it seems also convenient to an- alyze the changes in local aromaticity on complexation of alkali metal cations by using a series of indicators of aromatici- ty. We have analyzed the changes in local aromaticity on Li + as- sociation in a series of extended fused aromatic compounds. The aim is to discuss whether the strength of binding of the lithium cation to a given aromatic ring in a molecule is related [a] M. Güell, Dr. J. M. Luis, Prof. Dr. M. Solà Institut de Química Computacional and Departament de Química Universitat de Girona, Campus de Montilivi 17071 Girona, Catalonia (Spain) Fax: (+ 34)972-418356 E-mail: miquel.sola@udg.es [b] Dr. J. Poater Afdeling Theoretische Chemie Scheikundig Laboratorium der Vrije Universiteit De Boelelaan 1083, NL-1081 HV Amsterdam (The Netherlands) [c] Prof. Dr. O. Mó, Prof. Dr. M. YµÇez Departamento de Química, C-9, Universidad Autónoma de Madrid Cantoblanco, 28049-Madrid (Spain) Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. The changes in the local aromaticity of aromatic rings on com- plexation with lithium cation are discussed for a series of poly- cyclic aromatic hydrocarbons by using, as probes of aromaticity, the para-delocalization index (PDI), the aromatic fluctuation index (FLU), the harmonic oscillator model of aromaticity index (HOMA), and the nucleus-independent chemical shift (NICS). In most cases, changes in the electronically based PDI and FLU indi- ces are almost irrelevant. For this reason, the analysis is carried out mainly with the more sensitive HOMA and NICS descriptors. These indices indicate that changes in the local aromatic charac- ter of the different rings are small. In general, Li + interacts more favorably with the ring having the largest p charge, but there is no correlation between these magnitudes. The ring directly inter- acting with the lithium cation suffers a slight but significant re- duction of aromaticity, while the reduction of the local aromatici- ty of the adjacent rings is minor. For those rings located further away from the ring directly connected to Li + , we found generally a small increase in aromaticity. Although there is no clear corre- lation between aromaticity and Li + binding enthalpies, there is a rough correlation between the latter and the change in the aro- maticity of the ring directly interacting with the metal cation. 2552 # 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemPhysChem 2005, 6, 2552 – 2561