Pyridine derived N-heterocyclic germylenes: A density functional perspective M.Z. Kassaee * , M.R. Momeni, F.A. Shakib, M. Ghambarian Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran article info Article history: Received 17 November 2009 Received in revised form 10 December 2009 Accepted 14 December 2009 Available online 21 December 2009 Keywords: N-heterocyclic germylene Germanimine Nucleophilicity Electrophilicity DFT abstract Three novel germanimines, 2-, 3-, and 4-germapyridines (1, 2, and 3, respectively) along with their iso- meric germylenes, are compared and contrasted at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G * level of theory. From a thermodynamic viewpoint, two germylenes out of a total of eight singlet minima, 1H-2-germa- pyridine-2-ylidene (1a) and 1H-4-germapyridine-4-ylidene (3a), are found 29.2 and 15.4 kcal/mol more stable than their corresponding aromatic germapyridine isomers, respectively. Indeed, 1a is the global minimum on the potential energy surface of cyclic C 4 NGeH 5 with a singlet–triplet energy gap larger than that of Herrmann’s germylene, i.e. 57.4 vs. 49.7 kcal/mol. From a kinetic viewpoint, the calculated energy barrier for 1,2-H shift of 1a to 1 is 70.8 kcal/mol compared to more prohibitive 92.5 kcal/mol for 1,4-H shift of 3a to 3. No Ge@Ge doubly bonded minimum structure is found as dimer for 1a. The doubly bonded dimer of 3a is 11.2 kcal/mol less stable than its two separate monomers. This study signifies the thermodynamic and kinetic stabilities of divalent 1a and 3a hoping to prompt the experimental attentions toward them. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction In 1960s, organometallic chemistry encountered the start point of a series of revolutionary studies when the old idea of the unsta- ble double bonds between heavier main group elements was bro- ken down [1–4]. In this way, the valuable studies directed by Satgé introduced first transient intermediates having germanium doubly bonded to nitrogen (germanimines) in 1978 [5,6]. On the other hand, following the pioneering work of Märkl in 1980s [7] and the theoretical calculations of Kiprof and Brown [8], Tokitoh et al. reported the synthesis of the first stable germabenzene in 2002 [9]. Combining these two apparently different subjects, germani- mine and germabenzene, one may reaches at germapyridines whose identification opens the gates toward the heavier congeners of our interested pyridine-derived N-heterocyclic carbenes [10] and silylenes [11]. Here, we have considered three ortho, meta, and para germapyridines none of which has been previously under experimental or theoretical investigations. Following the previous reports of our laboratory, now, we are looking for a new generation of N-heterocyclic germylenes. The study of N-heterocyclic germyl- enes backs to 1987 when Veith succeeded to synthesize a series of metallylenes, including germylene (I), with a four-membered structure stabilized by two adjacent nitrogens (Scheme 1) [12]. Some years later and following the synthesis of the first stable car- bene by Arduengo in 1991 [13] Hermann’s group reported the syn- thesis of the five-membered N-heterocyclic germylene congener (II) in 1992 [14]. After four and five-membered N-heterocyclic germylenes it was time for the synthesis of six-membered ones which was accomplished in 1998 in a push-pull structure (III) [15] and in 2006 in a dicoordinated one (IV) [16] and will possibly go on with germapyridine-derived ones in future. Now, we are pleased to report the first theoretical study on the cyclic germapyridines with an especial focus on the corresponding germylenes and show how these novel N-heterocyclic germylenes seem thermodynamically and kinetically more stable than the pre- viously synthesized ones. 2. Computational details Full geometry optimizations were accomplished without any symmetry constraints by means of hybrid functional B3LYP [17– 19] and the 6-31+G * basis set, employing the GAUSSIAN 98 code [20]. The applied basis set was comprised of Pople’s well known 6-31G * basis set [21,22] and an extra plus due to the importance of diffuse functions [23,24]. The reliability of the optimized struc- tures was confirmed through altering of the basis set (B3LYP/ LANL2DZ) and method (MP2/6-31+G * [25]). To obtain more accu- rate energetic data, single point calculations were performed at B3LYP/AUG-cc-pVTZ [26] based on the B3LYP/6-31+G * geometries. The transition states (TSs) linking the initial and final structures, were found using the reactants-products quasi-synchronous transit (QST2) algorithm [20]. The frequency calculations were applied to characterize the structures as minimum or TS [27]. Nucleus independent chemical shift (NICS) calculations [28] were 0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2009.12.013 * Corresponding author. Tel.: +98 21 82883441; mobile: +98 912 1000392; fax: +98 21 88006544. E-mail address: Kassaeem@Modares.ac.ir (M.Z. Kassaee). Journal of Organometallic Chemistry 695 (2010) 760–765 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem