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Electride Characteristics of M2( 5-E5)

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Electride Characteristics of M2( 5-E5) Electride Characteristics of M2(h5-E5)2 (M= Be, Mg; E= Sb5-) Prasenjit Das IIT Kharagpur: Indian Institute of Technology Kharagpur Pratim Kumar Chattaraj ( [email protected] ) IIT Kharagpur Research Article Keywords: Electride, Binuclear Sandwich Compound, Non-nuclear attractor (NNA), Electron localization function (ELF) basin, NLO properties Posted Date: February 24th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-232917/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Version of Record: A version of this preprint was published at Structural Chemistry on April 23rd, 2021. See the published version at https://doi.org/10.1007/s11224-021-01783-1. 5 - Electride Characteristics of M2( -E5)2 (M= Be, Mg; E= Sb5 ) Prasenjit Das,a Pratim Kumar Chattaraja,b,* a Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India b Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, 400076, India *E-mail: [email protected] (PKC) Abstract 5 Ab initio computation is performed on the binuclear sandwich complexes, M2( -Sb5)2. Eclipsed 5 - and staggered conformations are generated due to the mode of binding by Sb5 ligand with the alkaline earth metals (Be and Mg metals). The complexes are thermodynamically stable at room temperature. The electron density descriptors and the natural bond orbital (NBO) analysis 5 5 confirmed the covalent nature of the M-M bond. Both Be2( -Sb5)2 and Mg2( -Sb5)2 complexes have one non-nuclear attractor (NNA) at the center of the M-M bond which is predicted and confirmed by the electron density analysis. At the NNA the values of the Laplacian of electron density are negative and an electron localization function basin (ELF) is present at the center of the M-M bond for localized electrons. Both the complexes show large values of nonlinear optical 5 5 properties (NLO). Both the designed binuclear sandwich complexes Be2( -Sb5)2 and Mg2( - Sb5)2 behave as electride. Keywords: Electride; Binuclear Sandwich Compound; Non-nuclear attractor (NNA); Electron localization function (ELF) basin; NLO properties Introduction 1 Electrides are those kinds of interesting ionic systems in which cavity trapped electrons served as anions [1-,2,3,4]. In recent years electride properties of materials have generated great attention in experiments as well as in theoretical studies. The history of the electride systems begins with the introduction of solvated electrons in alkali metal ammonia solution [5,6]. Dye et al. provided valuable studies on electride materials [1,7,8,9,-10,11,12]. The trapped electrons in electride systems are not associated with any specific atom. These trapped electrons are confined at cavities and the interstitial position in cryptands and solid crystals, respectively [3,13]. The electron density analysis confirmed the presence of confined electrons in the cavities of solid alkali metals + – [14,15]. In 1983, the first stable crystalline organic electride, Cs (18-crown-6)2e was synthesized by Ellaboudy et al. [16] and in 2003, Matsuishi et al. synthesized the first stable 4+ – inorganic electride, [Ca24Al28O64] ·(4e ) [17]. In both cases, the excess electrons are confined in the void spaces. Further, six temperature and air-stable electride systems were experimentally synthesized [18,19,20,21-,22,23,24]. In these systems, the cryptand ligands or crown ethers are complexed with alkali metals. Electride materials are very sensitive to temperature and air [25]. So, it is very challenging to synthesize and characterize air and temperature stable electride materials. The presence of cavity trapped loose electrons causes a lowering of the work function of electrides so that they can be used as an electron donor in chemical reactions. Electride materials are very interesting for their remarkable potential applications such as emitting diodes for organic light [26], reversible hydrogen storage materials [27], catalyst for the CO2 activation [28], splitting of N2 molecule [29,30], powerful reducing agents [31,32,33], and superconductivity [34]. The experimental identification of the position of localized trapped electrons is very difficult because of the low density of these localized electrons. So, experimentalist used indirect evidence for its experimental characterization [11,35]. Therefore, computational studies can be helpful for the 2 identification of electride materials. For that purpose, people used different computational tools to characterize electride materials. Computationally, the required criteria for characterizing electride material are, (i) presence of non-nuclear attractors (NNA) of the electron density; [1,36,37] (ii) the Laplacian of electron density ( 2 ) should be negative at the position where NNA exists; (iii) high NLO properties; [38] (iv) existence of electron localization function (ELF) basin [14] at the NNA region. Some molecules which do not possess confined electrons in electronic structure can show one or more of the above-mentioned properties. Therefore, none of these criteria by itself can be used to characterize electride systems, unambiguously. Some previous studies reported some molecules as electride materials based on large NLOPs are not considered to be materials with electride properties on these days. By simultaneous confirmation of these four criteria, we can say that a cavity-trapped electron is present within the structure of a molecule and it constitutes a real electride material. Most recently one theoretical work has - shown that binuclear sandwich complexes of Be and Mg atoms bonded with isoelectronic C5H5 , - - - N5 , P5 , As5 ligands obeyed all these above-mentioned criteria to behave as electride materials [39]. In these sandwich complexes, the Be and Mg atoms are in +1 oxidation state. In this article, our main objective is to introduce molecules with electride property. Here - 5 we have taken Sb5 ligand and the complexes which are formed by the ligand are M2( -Sb5)2 (M= Be and Mg). We have used ab initio method for the study of the structure, stability, and nature of bonding in these complexes. Then the NLO properties of both these systems have been studied. Finally, the electride characteristic of these two complexes has been analyzed. Computational Details 3 We have used the MP2 method in conjunction with correlation consistent double zeta quality basis set for geometry optimization and subsequent frequency calculations [40,41]. For Be and Mg atoms cc-pVDZ basis set and for Sb atom cc-pVDZ-PP basis set has been used along with relativistic effective core potential (RECPs) [42,43,44]. The real harmonic frequency values indicate that these are the energy minimum structures on their respective potential energy surfaces. We have used Gaussian 16 program package for all the computations [45]. We have carried out the natural bond orbital (NBO) analysis to know the charge distribution on each atom. The computation for this analysis has been performed at the MP2 method in conjunction with correlation consistent triple zeta quality basis set. For Be and Mg atoms cc- pVTZ basis set and for Sb atoms cc-pVTZ-PP basis set has been used along with relativistic effective core potential (RECPs). For this computation, we have used NBO 3.1 [46,47] as implemented in Gaussian 16. Multiwfn program package [48] has been used for atoms-in-molecule analysis (AIM) [49] of electron density. We have used the MP2 method for this analysis and various bond critical points (BCP) have been generated. Both AIM and ELF basin populations have been analyzed. We have used B3LYP/cc-pVDZ/cc-pVDZ-PP method to compute the average polarizability ( ), first hyperpolarizability ( ), and second hyperpolarizability ( ), where, cc-pVDZ basis set is used for Be and Mg atoms and cc-pVDZ-PP basis set is used for Sb atoms. For this computation, the optimized structures of the complexes have been taken from the MP2 method. The equations which are used to calculate the , and values are as follows [50], 4 1 ii 3 i x,y,z 1 1 ()2 2 () i , where i ijj jij jji i x,y,z 3 j x,y,z 1 ()iijj ijij ijji 15 i,j x,y,z RESULTS AND DISCUSSION Geometries and Energetics - The optimized geometries of the ligand (Sb5 ) and the complexes considered for this work are given in Figure 1. The negative charge on the ligand is delocalized through the whole cyclic ring and makes all the Sb-Sb bond lengths are equal. The ligand has D5h point group of symmetry. The Sb-Sb bond length in the ligand is 2.724 Å. This cyclic ligand can bind the metal atoms through 1, 2, 3, 4, 5 mode of bonding, and these modes of binding are shown in Scheme 1. For the studied complexes the 1, 2, 3, 4 modes of binding with Be and Mg metals by the ligand are unstable but the 5 mode of binding is stable. For this reason, we have taken only 5 mode of bonding throughout our manuscript. Due to this 5 mode of bonding two conformation of the complexes has been observed and that are, (i) eclipsed conformation; (ii) staggered conformation. When two cyclic rings are cis to each other eclipsed conformation has been observed with D5h point group of symmetry. But when the rings are anti to each other staggered conformation resulted and the point group for this conformation is D5d. For both the complexes, the energy difference between eclipsed and staggered conformation is almost zero. Throughout 5 the manuscript, we have considered only the staggered conformation of all the complexes. The 5 eclipsed conformation provides identical results. In Be2( -Sb5)2 complex the Be-Be bond length 5 is 2.045 Å, the Be-Sb bond length is 2.714 Å and the Sb-Sb bond length is 2.744 Å. In Mg2( - Sb5)2 complex the Mg-Mg bond length is 2.789 Å, the Mg-Sb bond length is 3.138 Å and the Sb- 2+ Sb bond length is 2.755 Å.
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