J Mol Model (2015) 21: 28 DOI 10.1007/s00894-014-2552-6 ORIGINAL PAPER s-Block metallabenzene: aromaticity and hydrogen adsorption Rafał Roszak & Szczepan Roszak Received: 30 September 2014 /Accepted: 30 November 2014 /Published online: 29 January 2015 # The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Second group metal dimers can replace the carbon Nowadays, this term is also used to describe other compounds atom in benzene to form metallabenzene (C5H6M2) compounds. including heterocyclic [1] and metallacyclic compounds [2–4] These complexes possess some aromatic character and promis- as well as metal clusters [5, 6]. Since the term “aromaticity” is ing hydrogen adsorption properties. In this study, we investigat- used for many different types of resonance stabilization phe- ed the aromatic character of these compounds using aromaticity nomena, the classification of compounds as aromatic or non- indices and molecular orbital analysis. To determine the nature of aromatic is not always obvious. The quantitative determina- interactions between hydrogen and the metallic center, variation- tion of this property is possible via introduced aromaticity perturbational decomposition of interaction energy was applied indices (AI). Several AI have been proposed; however, all of together with ETS-NOCVanalysis. The results obtained suggest them are based on one of the following ring criteria: geomet- that the aromatic character comes from three π orbitals located rical, energetic, magnetic or electronic properties. The geo- − mainly on the C5H5 fragment. The high hydrogen adsorption metrical aspect of aromaticity is measured predominantly by energy (up to 6.5 kcal mol−1) results from two types of interac- the HOMA (harmonic oscillator model of aromaticity) index tion. In C5H6Be2, adsorption is controlled by interactions be- proposed by Kruszewski and Krygowski [7]. HOMA is the tween the empty metal orbital and the σ orbital of the hydrogen most commonly used AI; however, this index depends on molecule (Kubas interaction) together with corresponding back- empirical parameters that are defined for organic compounds donation interactions. Other C5H6M2 compounds adsorb H2 due only. Therefore HOMA cannot be applied to organometallic to Kubas interactions enhanced by H2–π interactions. compounds. NICS (nucleus-independent chemical shift) rep- resents a series of indices [8] that allow evaluation of aroma- Keywords Metallabenzene . Metallaromaticity . Kubas ticity on the basis of the magnetic properties of the ring. NICS interactions . Hydrogen adsorption . Hydrogen storage uses a negative value of absolute shielding in the given posi- tion. NICS indices do not depend on empirical parameters and thus may be applied easily to any aromatic system. A negative Introduction value of an index indicates that the compound is aromatic whereas positive values states that the investigated compound The concept of aromaticity was defined in the nineteenth is antiaromatic. There are three commonly used NICS indices: century to explain the unusual properties of benzene. NICS(0), defined as negative value of NICS in the ring center; NICS(1), which represents negative value of NICS measured This paper belongs to Topical Collection 6th conference on Modeling & 1 Å above the center of a ring; and NICS(1)zz, which corre- Design of Molecular Materials in Kudowa Zdrój (MDMM 2014) sponds to anisotropic part of NICS(1) in the direction perpen- Electronic supplementary material The online version of this article dicular to the ring. The ring center is usually defined as a (doi:10.1007/s00894-014-2552-6) contains supplementary material, geometric center; however, it can also be defined as a ring which is available to authorized users. : critical point (RCP) based on the atoms in molecules (AIM) R. Roszak S. Roszak (*) theory introduced by Bader [9]. Institute of Physical and Theoretical Chemistry, Wroclaw University The key feature of aromatic compounds is a delocalization of Technology, Wybrzeze Wyspianskiego 27, π 50-370 Wroclaw, Poland of electrons, which leads to additional stabilization of a e-mail: [email protected] structure. The stabilization energy is called the resonance 28 Page 2 of 18 J Mol Model (2015) 21: 28 energy or aromatic stabilization energy (ASE). In general, groups are replaced by transition metals with ligands, so called ASE represents the energy difference between an aromatic metallabenzenes [19, 20]. Compounds of this type containing compound and its non-aromatic unsaturated isomer. ASE can transition metal atoms have been obtained experimentally and be calculated in the isodesmic reaction [10], where substrates reveal aromatic properties (e.g., undergo aromatic substitu- and products have the same number of atoms, bonds, and tion). Fernandez and Frenking [21] have proved that bond types. The standard example constitutes the hypothetical metallabenzenes containing Os, Ru, Ir, Rh, Pt, and Pd possess reaction of three cyclohexenes that form benzene and two five π orbitals and can be considered as 10 electron Hückel cyclohexanes. The other reaction suited to calculating ASE aromatic systems. is a double-bond migration isomerization reaction [11]. In this Recently, we reported that the Be2 fragment, which is reaction, the product constitutes an aromatic compound with a isoelectronic with the carbon atom, can act as C(sp2)[22]. methyl substituent in the meta position whereas the substrate The Be2 center can replace the carbon atom in aromatic has a methylidene substituent and an additional hydrogen in hydrocarbons and form compounds characterized by structure the para position. and molecular orbitals analogous to that in corresponding A number of indices are based on electronic properties. The hydrocarbons. The aim of this study was to characterize the aromaticity of a molecule can be characterized by properties bonding scheme and aromaticity of compounds hosting Be2 of electron density calculated in RCP. Palusiak et al. [12]and and other M2 fragments (with M being second group metals; Ebrahimi et al. [13] proved that the value of electron density, Table 1). laplacian of electron density, and energies of electron density, Compounds including a Be2 center constitute promising including potential (V), kinetic (G) and total (H) electron materials for hydrogen storage, and can adsorb hydrogen energy, are strongly correlated with a HOMA value. Besides with energy up to 6 kcal mol−1. Hence, the second aim of electronic properties in bond critical point (BCP), several this work was to characterize interactions of the M2 moiety other indices are defined within the AIM framework. Howev- with hydrogen molecule(s). Here, we are concentrating on er, most common indexes [14], e.g., PDI (para-delocalization metallabenzene (C5M2H6). Previous studies on Be2-contain- index) [15], ΔDI [15], FLU [16]andFLUπ [16], cannot be ing compounds have shown that C5Be2H6 preserves all applied to C5M2H6 compounds. The reasons are as follows features of higher aromatic hydrocarbons. [14]: by definition, the PDI index can be used only for 6- membered rings; ΔDI requires a clearly defined Lewis struc- ture; FLU, like HOMA, needs empirical parameters for each Computational details bond type whereas FLUπ can be used only for planar com- pounds. One of few AIM-based indices that can be applied in If not mentioned otherwise, molecular structures and com- the studied case is the Shannon aromaticity (SA) index [17] plexes with hydrogen molecule(s) were minimized at MP2 based on the Shannon entropy [18]. The formula describing level of approximation [23] applying the aug-cc-pvdz SA is presented below: atomic basis set [24]. Harmonic vibration frequencies re- vealed no imaginary frequencies indicating that structures XN correspond to true minima. NICS values were calculated SA ¼ lnðÞN − pilnpi ð1Þ i using the gauge-independent atomic orbital (GIAO) meth- od [25] applied at the same level of theory. Calculations mentioned above were performed in the Gaussian 09 soft- ware suite [26]. where In order to investigate the nature of interactions between ρ BCPi the M2 center and molecular hydrogen, the total MP2 pi ¼ ð2Þ XN interaction energy was decomposed according to the hy- ρBCPj brid variational-perturbational scheme [27]. In adopted j¼1 scheme the total interaction energy is partitioned into Hartree-Fock (HF) (ΔEHF) and second order perturbation (EMP2)terms: with N representing the overall number of bonds in the ring ΔEMP2 ¼ ΔEHF þ EMP2 ¼ Eel þ Edel þ Eex þ EMP2 ð3Þ and ρBCP being electron density in the BCP. The value 0 corresponds to the homonuclear aromatic ring. SA increases when aromaticity decreases. Several metalloaromatic compounds have been reported in The HF interaction energy is further decomposed into first literature and the topic is covered by several reviews [2–4]. order electrostatic (Eel), Heitler-London exchange (Eex)and Most of these compounds are benzene analogues where CH higher order delocalization (Edel) terms. All energy J Mol Model (2015) 21: 28 Page 3 of 18 28 Table 1 Aromaticity of C5H6M2 [electron density properties at ring critical point (RCP)]. NBO Natural bond orbital, NICS nucleus-independent chemical shift, SA Shannon aromaticity, ASE aromatic stabilization energy, H total electron energy, G kinetic electron energy, V potential electron energy a Compound C5H6Be2 C5H6Mg2 C5H6Ca2 C5H6BeMg C5H6BeCa C5H6MgCa NBO charge (e) Be: 0.84 Mg: 1.162 Caup: 1.327 Be: 0.540 Be: 0.573 Mg: 1.110 Cadown: 1.311 Mg: 1.220