Research paper
Unique magnetic shielding and bonding in Pnicogen nortricyclane Zintl clusters

https://doi.org/10.1016/j.cplett.2020.137414Get rights and content

Highlights

  • Covalent bonding in E73− (E = P, As, Sb, Bi) Zintl clusters decreases down the group.

  • E73− (E = P, As, Sb, Bi) Zintl clusters exhibiting σ-aromaticity.

  • High NICS value at the base triangle of E73− (E = P, As, Sb, Bi) Zintl clusters.

  • Spherical aromaticity is noted in E73− clusters.

Abstract

Using first principle calculations, in-depth bonding and aromaticity pattern of bare anionic nortricyclane, E73− (E = P, As, Sb, and Bi) Zintl clusters have been explored. A detailed topological analysis reveals that every cluster comprises of nine 2c-2e σ-bond with an occupation number of 1.96–1.99 |e|. We find an impressive covalence in the E73− cluster which decreases down the group from P73− to Bi73−. The nucleus independent chemical shift (NICS) foretell about the aromatic property of the Zintl cluster which is also decreasing along the group. In addition, the response with respect to external magnetic field of the nucleus independent shielding tensor was obtained to explore the possible formation of the shielding cone behavior.

Introduction

Zintl ions are generally formed by Group 13, 14, and 15 elements and are multiply charged anionic clusters. These anions are the building blocks of the Zintl [1], [2], [3], [4], [5] phase. They are found to possess polyhedral geometries like boranes or metal carbonyl clusters. During the past decade, an outstanding development in Zintl chemistry has been carried out by the synthetic chemistry research groups and theoretical chemist as well [6], [7], [8]. Wade-Mingos rule and Jellium shell model are utilized to categorize and predict the shape, stability, and reactivity of Zintl type clusters. Such clusters have more importance in industrial applications [9], [10]. One of the most important features of these clusters has been their use as building blocks of materials with tailored properties [11], [12], [13]. Giri et al. [14] showed how P7R4 (R = Me, CH2Me, CH(Me)2, and C(Me)3) clusters can be used as superalkali [15], [16], [17], [18]. There are also evidences of superhalogens having Zintl cluster as core. On the other hand, Konvir et al. [19] reported the synthesis as well as crystal and electronic structure of Ba2P7X (X = Cl, Br, and I) Zintl salts. It has been reported that heptapnictide trianions [E7]3− (E = P, As, Sb, and Bi) including homoatomic cage like Zintl ion can only formed by the Group 15 elements [6]. Due to their unique structure and intriguing chemical bonding, many studies have been going to forth insight into the structural and bonding properties.

The findings on E73− Zintl cluster can open a new era in the chemistry of nortricyclane type charged anionic cluster. Detailed knowledge on the bonding pattern of these types of species can trigger the advancement of research in this particular field. So far, many investigations have been done to establish the bonding pattern of these clusters by means of XRD, XPES, DOS, PDOS, and UV analysis. Although, in 2018 Zhai et al. [20] have shown the nature of bonding in P73− cluster and its relevant alloy, [P7ZnP7]4− via canonical molecular orbital (CMO), but their study was limited to only phosphorus in the pnictogen group.

In this letter, we are reporting a detail bonding pattern analysis, which can deliver new information about E73− Zintl clusters. Our study includes topological analysis, Wiberg bond indices (WBI) analysis, Laplacian of electron density analysis, and adaptive natural density partitioning (AdNDP) [21] analysis. Moreover, we have done nucleus independent chemical shift (NICS) [22] as well as nucleus independent shielding tensor calculation to get information about the aromaticity of the Zintl cluster.

Basically, critical points are giving the idea of existence of bonds, nucleus, rings, and cages in a particular studied system. WBI can give a direct measure of bond strength. Laplacian of electron density analysis helps in ascertaining the nature of bonding, and particularly in this study it would eliminate the discrepancy between covalent bond and ionic bond in E73− Zintl clusters. 2c-2e Bonds and 3c-2e bonds are very important for the constitution of the studied pnictogen cluster anions, which reflects clearly in our AdNDP analysis. Aromaticity not only helps to understand the structure, stability, and reactivity of many organic compounds, but also found significant in variety of molecules including inorganic compounds. Delocalization of π or σ electron in a molecular framework can be learned simply by computing NICS. As the magnetic property of molecular clusters is being used in the preparation of nanoelectronic devices and high density magnetic devices, we have made an effort to evaluate the magnetic property in line with the NICS of our studied cluster. The orbital magnetic moment and spin magnetic moment of electrons contribute towards the total magnetic moment.

Section snippets

Computational details

All geometries of E73− (E = P, As, Sb, and Bi) were fully optimized employing Gausian 09 [23] software at B3LYP/def2TZVPP [24], [25] level of theory. In every case, absence of imaginary frequency indicates that the studied systems are local minima on their respective potential energy surface. Frontier molecular orbital (FMO) and natural bond orbital analysis (NBO) [26] were used to analyse the bonding in the E73− (E = P, As, Sb, and Bi) cluster. For more details of bonding pattern, we performed

Result and discussion

Earlier it has been proposed that [14], [34] the stability and reactivity of E73− Zintl ion can be explained by octet rule. As E7 is a 35-electron system, it requires 3 more electrons to fulfil the Octet rule and thereby becomes stable. But in this letter, we have successfully shown a concerted bonding pattern in E73− cluster beyond octet rule. Which can also explain the stability and reactivity of E73− cluster. The topology analysis can give us a concrete idea about nature of bonding and

Conclusions

From the bonding analysis of E73− (E = P, As, Sb, and Bi) bare clusters, it has been found that all the complexes possess a nortricyclane like structure with C3v point group. All the clusters having nine 2c-2e bonds with an occupation number of 1.96–1.99 |e| which suggest that the bonds between the atoms are strong enough. The critical point analysis confirms the same. The Laplacian of electron density analysis divulges the covalent character of the studied cluster, which decreases from P73− to

CRediT authorship contribution statement

Rakesh Parida: Methodology, Software. G. Naresh Reddy: Methodology. Edison Osorio: Investigation. Alvaro Muñoz-Castro: Investigation. Sukanta Mondal: Investigation, Data analysis, Writing - original draft, reviewing, and editing. Santanab Giri: Supervision, Investigation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by Department of Science and Technology INSPIRE award no. IFA14-CH-151, Government of India. Resources and computational facilities of National Institute of Technology Rourkela and DST SERB grant no: SB/FT/CS-002/2014 are also greatly acknowledged. SM thanks University Grants Commission, New Delhi for UGC-BSR Research Start-Up-Grant (No. F.30-458/2019(BSR)). AMC thanks to FONDECYT 1180683 for financial support.

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