Job/Unit: I20415 /KAP1 Date: 29-08-12 15:18:45 Pages: 12 FULL PAPER DOI: 10.1002/ejic.201200415 Structures and Stabilities of Small, Ligated Al n L n 0/2– and Al n L n+2 Clusters (L = H, Cl) – A Theoretical Study Rémi Marchal,* [a,b] Gabriele Manca, [a,b,c] Samia Kahlal, [a,b] Philippe Carbonnière, [d] Claude Pouchan, [d] Jean-François Halet, [a,b] and Jean-Yves Saillard* [a,b] Keywords: Cluster compounds / Aluminum / Structure elucidation / Density functional calculations / Isoelectronic analogues Al n L n 0/2– (n = 4–7, 12), Al n L n+2 (n = 4–7), and Al 4 L 8 clusters were investigated by using the global-search-algorithm-for- minima approach and DFT calculations at the BP86/ LANL2DZ level of theory. These investigations indicate that Introduction Unlike most bare metallic nanoparticles, ligated molecu- lar metal clusters of nanometric size are generally viable, [1] that is, they are stable enough, thermally and kinetically, to be isolated and have a precise composition, structure, and electron count. [2–4] Among this type of nanomolecules, a new chemistry of group-13 clusters, aluminum clusters in particular, has been developed during the last decades. Since the first synthesis of an aluminum cluster [Al 12 - (tBu) 12 ] 2– in 1991 by Hiller et al., [5] several other members of this series, which ranges up to {Al 77 [N(SiMe 3 ) 2 ] 20 } 2– , have been synthesized and structurally characterized mainly by the group of Schnöckel. [2] These high-nuclearity clusters exhibit onionlike shapes and densely packed cores, which are so far only partly understood in terms of their struc- ture, [6] in contrast to borane clusters, which mainly adopt empty, polyhedral structures, which can be rationalized within the framework of the polyhedral skeletal electron pair (PSEP) theory. [7–9] Therefore, one of the most impor- tant challenges connected to these aluminum clusters is to understand their structure and stability. [a] Institut des Sciences Chimiques de Rennes UMR 6226 CNRS – Université de Rennes 1, Avenue de Général Leclerc, 35042 Rennes Cedex, France Fax: +33-2-23236840 E-mail: remi.marchal@univ-rennes1.fr jean-yves.saillard@univ-rennes1.fr [b] Université Européenne de Bretagne, 5 Bd. Laënnec, 35000 Rennes, France [c] Istitudo di Chimica dei Composti Organometalicci, ICCOM- CNR, Via Madonna Del Piano 10, 50019 Sesto Fiorentino, Florence, Italy [d] IPREM/ECP UMR 5254 CNRS – Université de Pau et des Pays de l’Adour, 2 Avenue du Président Angot, 64053 Pau Cedex 09, France Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201200415. Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 most of the computed species are viable and diamagnetic, although there is no clear general relationship between their shapes and their electron counts. Al 4 H 8 and Al 4 Cl 8 appear to be the most stable compounds in the investigated series. The first step towards understanding the structure of larger clusters containing up to a few dozens of atoms prob- ably is to explore that of small clusters. For this purpose, theoretical chemistry is a well-suited tool, because only few experimental data are available on small aluminum clusters. However, this task usually remains nontrivial, even for small clusters, because of the numerous minima that clus- ters can exhibit on their potential energy surface (PES). This is why theoretical studies of such compounds are mainly based on global-optimization methods. In this work, we investigate the structure of small, ligated aluminum clus- ters by using the global-search-algorithm-for-minima (GSAM) exploration approach, which was recently devel- oped by some of us [10] for the global exploration of PES. The efficiency of this approach has been well established in the case of silicon, gallium arsenide, and tin–tellurium clusters. [10,11] After a brief summary of the GSAM approach, we dis- cuss in this paper the structures and stabilities of small alu- minum clusters Al n L n 0/2– (n = 4–7, 12), Al n L n+2 (n = 4–7), and Al 4 H 8 as a function of the nature of the ligands (L = H, Cl), and finally we study their stability in terms of thermodynamics and electronic structures. Method and Computational Details The GSAM Approach Here, we only briefly recall the basics of this method, [10] which are (i) the generation of the initial set of configura- tions, (ii) the selection of the most significant ones, and (iii) the optimization scheme used. Because the topologies of the clusters depend on their compositions, the initial set of configurations has to be as topologically exhaustive as possible. Thus, two main gener- ation schemes are used. The first one consists of the ran-