Electronic structure of triangular trigold(I) complexes. A theoretical study L.E. Sansores * , R. Salcedo, A. Martı ´nez, N. Mireles Instituto de Investigaciones en Materiales, Universidad Nacional Auto ´noma de Me ´xico. Apartado Postal 70-360, Me ´xico DF 04510, Me ´xico Received 28 November 2005; received in revised form 15 December 2005; accepted 15 December 2005 Available online 24 March 2006 Abstract Triangular trigold(I) complexes have been synthesized and characterized by several authors. These molecules have linear two-coordinated gold(I) centers connected by bridging ligands to form nine-membered rings. Although this type of molecules have been known for more than 20 years, some remarkable properties have attracted new interest. Au 3 (CH 3 NaCOCH 3 ) 3 shows solvoluminescence, supramolecular association and some (m-pz-N,N) 3 Au 3 form mesophases with hexagonal columnar structures with weak dimer formation. In this work we study the electronic structure of these molecules. We also include [Au(m-N 3 ,C 2 -bzim)] 3 and [2-pyridyl-Au] 3 . Calculations were done at B3LYP level with 6-31CCG basis for all atoms except Au where LAN2LDZ was used. Geometry optimization shows that all molecules are flat and highly symmetric. For molecules 1 and 4 the LUMO is mainly located on the rings and the HOMO main contributions are from the Au atoms. In molecules 2 and 3 the LUMO main contributions are from the gold’s. An electronic delocalization similar to aromaticity is found in the nine-member ring of all compounds. This effect is studied with the NICS index. q 2006 Elsevier B.V. All rights reserved. Keywords: electronic structure; aromaticity; gold; auriophilicity 1. Introduction The chemistry of pyrazole and pyrazolate compounds is quite extensive and has been studied for more than 20 years. In particular, trinuclear gold compounds have attracted renewed attention due to recent reports of novel properties [1–18] such as: solvoluminescence, supra molecular association and columnar mesophases. Four good examples are those shown in Fig. 1, that is the matter of this report. The trimeric organometallic complex [Au 3 (CH 3 - NaCOCH 3 ) 3 ] (Compound 1) was first synthesized and characterized in 1974 [1,2]. Balch et al. have carried several studies and found the interesting effect of solvoluminescence [3–7]. Crystals of 1 were irradiated with near-UV light; after irradiation, a long-lived photoluminescence was found. Addition of a solvent to the irradiated crystals produces a bright burst of yellow light. Their X-Ray studies show that in solid state this compound has a columnar structure. This complex undergoes oxidative addition of halogens to form mixed valence complexes of the type Au I 2 Au III I 2 L 3 , Au I Au III 2 - I 4 L 3 and Au III 3 I 6 L 3 [4,5]. The electronic structure and a possible explanation for the luminescence of compound 1 has been reported previously by our group [8]. To explain the luminescence, theoretical calculations of the complex in gas phase and in solution were done (the simulated solvent was chloroform). We concluded that the yellow light emission corresponds to a triplet state that in the solid is an excitonic state. The solvoluminescence emission is due to the recombi- nation of the exciton triggered by the solvent. Pyrazolate complexes of the type 2 have attracted attention since they form columnar mesophases at room temperature [9,10]. For this type of compounds, Bovio et al. [11] reported the synthesis and X-Ray characterization of tris[m-3,5- bis(trifluoromethyl)pyrazolato-N,N 0 ]trigold(I) and later Mur- ray et al. [12] reported similar characterization for tris[m-3,5- diphenylpyrazolato-N,N 0 ]trigold(I). The first found a non- planar ring and no crystallographic symmetry, while the later, found that the molecule belongs to the D 3h point group and has a rigorously planar nine member ring. Complexes with longer side branches, which show the mesophases, have also been characterized [9,10]. In other words the side chains are very important for the structure and properties of these compounds. With respect to the chemical reactivity, complex 2 only form the Au I 2 Au III Cl 2 L 3 compound [13]. In 2005, Omary et al. [14] reported the X-Ray structure of a similar fluorinated derivative including a discussion concerning the luminescence behavior. They concluded that luminescence Journal of Molecular Structure: THEOCHEM 763 (2006) 7–11 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.12.021 * Corresponding author. Tel.: C52 555 6224500; fax: C52 555 6160754. E-mail address: sansores@servidor.unam.mx (L.E. Sansores).