ARTICLE

1. ARTICLE pubs.acs.org/JPCA New Generation of Dialkylsilylenes with Stabilities Comparable to Diaminosilylenes: A Theoretical Study Mohammad R. Momeni, Farnaz A. Shakib,* and Zahra Azizi Department of Chemistry, Shahr_e Qods Branch, Islamic Azad University, Tehran, 37541-13115, Iran Department of Chemistry, Karaj Branch, Islamic Azad University, Karaj, 31485-313, Iran. http://www.kiau.ac.irbS Supporting Information ABSTRACT: A new family of dialkylsilylenes is introduced which enjoys the stabilizing effect of α-cyclopropyl substituents. The singlet and triplet states of acyclic and saturated/ unsaturated cyclic dicyclopropylsilylenes are fully optimized using MP2/6-31G(d) and B3LYP/6-31+G(d) levels. Their higher ΔES-T values compared to the corresponding analogues which possess isopropyl groups instead of cyclopropyl represents the stabilizing interaction of the occupied Walsh orbital of the cyclopropyls with the vacant p-orbital of the silylene center. Appropriate isodesmic reactions clearly show that the stabilizing effect of this interaction on the singlet state is much more considerable than the corresponding triplet state. Cyclopropyls can serve as good sites for bulky substitution and, hence, provide steric protection for silylene. 1. INTRODUCTION The development of N-heterocyclic silylenes is a classic progression started from transient intermediates which turned to matrix-isolated molecules in 1980s.1 Although no one ex- pected to observe these compounds at room temperature, Arduengo’s isolation of the stable carbene I in 19912 encouraged the endeavors toward the synthesis of stable silylenes (Figure 1). In 1994, Denk and his co-workers synthesized II as the first stable silylene,3 which benefits from the similar stabilizing effects enjoyed by I, that is, electronic stabilization by the amino groups, steric protection by the bulky alkyl groups, and additional stabilization of aromatic π-electron delocalization over the ring. Subsequently, several experimental and theoretical investigations appeared on similar silylenes4,5 until Veszpremi et al. found 1H- 2-silapyridine-2-ylidene (III) more stable than its full valence 2-silapyridine isomer.6 They also established the kinetic stability of III through the prohibitive high energy barrier (about 60 kcal/mol) required for its intramolecular isomerization to 2-silapyridine. In a further step, in 1999, Kira et al.7,8 reported the synthesis of the first isolable dialkylsilylene IV, prepared by reductive debromi- nation of the precursor dibromosilane. Interestingly, the stability of this species is derived entirely from steric protection of the divalent silicon atom by two SiMe3 groups attached to each α-carbon. Herein, we are pleased to introduce a new family of dialkylsi- lylenes which benefits from the stabilizing interaction of the occupied Walsh orbital with the vacant p-orbital of the silylene center. Indeed, this interaction was the basis of the spectro- scopic characterization of dicyclopropylcarbene V, generated from the irradiation of dicyclopropyldiazirine in a matrix of N2 at 6 K, by Ammann et al. in 1992.9 Our work is aimed to shed some light on the relative stabilities and reactivities of acyclic and saturated/unsaturated cyclic dicyclopropylsilylenes 1 3 (Figure 2). Figure 1 Figure 2. Schematic representation of the scrutinized silylenes. 2. COMPUTATIONAL DETAILS Full geometry optimizations are accomplished using Møller Plesset MP210 and hybrid functional B3LYP11 13 methods and the 6-31G(d)14,15 and 6-31+G(d)16,17 basis sets, respectively, 18 using the Gaussian 98 code. To obtain more accurate energetic data, single point calculations are performed at MP2/6-311+ +G(d,p) and B3LYP/AUG-cc-pVTZ19 levels of theory based on Received: February 1, 2011 Revised: August 8, 2011 Published: September 08, 2011