Sandwich compounds of second-row elements: A quantum chemical study

Abstract

The structures and stabilities of a number of neutral and charged sandwich type boron, carbon, and nitrogen compounds designed based on the cyclophane cage and obeying the "electron octet" rule were studied by the B3LYP/6 311+G** density functional method. The possibility of targeted modification of the electronic structures of such compounds by varying the basal or bridging atomic groups was investigated. Key words: hypercoordinate boron, hypercoordinate carbon, hypercoordinate nitrogen, sandwich systems, quantum chemical calculations.Search for and studies of compounds containing hypercoordinate main group element atoms belong to the most intensively developing avenues of theoretical and experimental research. 1-3 An important problem in this field consists in elucidation of methods for targeted de sign of novel structural types of molecular systems that allow formation and stabilization of hypercoordinate cen ters. 1 In this connection, sandwich systems seem to be very attractive because their architectures allow the na ture and coordination number of the hypercoordinate cen ters to be varied over a rather wide range. The electronic factors responsible for stability of the sandwich compounds of main group elements have been studied in detail; they are summarized in the form of the "electron octet" rule 4,5 substantiated by the discovery of various sandwich com pounds of alkali 6 and alkali earth 7,8 metals and some of the third to sixth row elements (Si, Ge, Sn, Pb, P, As, Bi, etc.). 8,9 At the same time there are only a few ex amples of stable sandwich derivatives of second row ele ments available at the moment.For the simplest carbocenes 1 (X = C) the "electron octet" rule implies the formation of systems bearing rather large positive charges (m = 2 for 1, n = 3 and m = 4 for 1, n = 4); stabilization of such spe cies is precluded by electrostatic factors. 10 Additional stability can be achieved by replacing the car bon centers X by electropositive groups that favor stabilization of nonclassical configurations of the carbon atom. 3,4 For instance, B3LYP/6 311++G** calcula tions predicted that silicon containing analog of carbo cene 1 (X = Si, n = 3, m = 2) should be a stable com pound. 11 At the same time this method of solving the stabilization problem of sandwich carbon containing de rivatives is still of limited use, being confirmed by a single successful example as yet. A much more efficient is the approach based on the replacement of the carbon centers in carbocenes by boron atoms (X = B) and additional stabilization of anionic systems thus formed by lithium counterions or bridging hydrogen atoms. 10,12According to MP2 and DFT calculations with the 6 311+G* basis set, the initially unstable anionic system 2 can be stabilized on going to neutral systems 3 or 4 that contain counterions. 10,12 This approach is fruitful, 12 be ing at present a unique efficient strategy of targeted design of sandwich derivatives of the second row elements.In this work we propose a novel method for design of hydrocarbon and he...

Octacoordinate Carbons Encaged Inside Carborane Clusters: A Density Functional Theory Investigation

“…The Becke−Lee−Yang−Parr three-parameter (B3LYP) – functional was employed for exchange and correlation. All the computations were carried out with the 6-311+G(d,p) basis set. ,,,,,– Once the fully optimized geometry was obtained, vibration frequency analyses were performed at the same level of theory to characterize the stationary points and to evaluate the zero-point energy correction (ZPE). Topological analyses of the electron density were done based on Bader’s atoms in molecules (AIM) theory – by using the AIM2000 program , in order to verify the bond critical points (BCPs).…”

“…26 Among the carboranes studied here (2a, 2b, 3a-3j, and 4a-4m, see Figure 2), the compounds 3a and 3i have been proposed in our previous work, 26 and 2a, 2b, and 3a have also been computationally studied recently. 48,49 The structural and bonding characteristics of these novel carborane compounds are extensively described. On the basis of the calculated energies, thermodynamic stabilities are discussed and several stability rules are proposed.…”