Research paperDoped deltahedral organo-Zintl superalkali cations
Graphical abstract
Introduction
The Group 14 nine atom deltahedral Zintl clusters have been known for a century [1], [2] but they have gained interest in recent years due to their interesting and diverse reactivity and expanding chemistry [3], [4]. It has been observed that they can undergo several reactions leading to alkylated, alkenylated [5], metalated [6], oligomerized [7], and polymerized [8] derivatives besides of functionalization with main-group element, transition-metal organometallic fragments [9], [10], [11], and endohedral transition metals [12]. The reactions of Ge94− with EPh3 for E = P, As, Sb, and Bi were among the very first redox reactions with such ions. Not only reactivity, these clusters have unique bonding and aromaticity pattern. In 2019, Boldyrev et al. described the multiple local σ- aromaticity inside the nonagermanide clusters [13]. These diverse nature of Zintl ions makes them a unique family of molecules with an outstanding structural diversity. Zintl ions can be rationalized within the framework of Wade-Mingos concept [14], which states that closo-polyhedra form stable entities if a valence electron count of 2n + 2, contributing to bonds within the cluster skeleton (with n vertices), is satisfied. Following this idea, closo-clusters with 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, … valence electrons can be regarded as stable and form characteristic closo-structures. Other typical concepts of simple electron counting rules for clusters like the jellium model [15] (2, 8, 20, 40, … electrons), Hirsch’s electron counting rule [16] (2, 8, 18, 32, … electrons), also predict the stability for well-defined structures. On the other hand, there is another special class of molecule named superalkali [17], [18], [19] which belongs to a particular class of molecules able to mimic the chemistry of alkali metals. Coined by Gutsev and Boldyrev in 1982 [19], [20], [21], superalkalis have the general formula Mk+1L where M is an alkali atom and L is an electronegative atom with valence k. Typical examples of super alkalis are Li2F, Li3O, Li4N, etc. Research in the past few years have led to different types of superalkalis such as binuclear- and polynuclear superalkalis, aromatic, organo-Zintl, all metal organic superalkalis [22], [23], [24], [25], [26]. Not only that, superalkali complexes have been found with M@crown ether and cryptands (M = Li, Na, K) which have extremely low ionization energy ranges from 1.52 to 1.70 eV [27]. But there are no such superalkalis, designed by using doped deltahedral organo-Zintl clusters rationalized to date. In this current study, we have designed doped deltahedral organo-Zintl superalkalis by functionalizing doped Zintl ion with suitable organic ligands like –CH3, –C2H5 and –C3H3. It is expected that, as these ligands have electron-releasing character, they will release electrons towards the Zintl core resulting an electron rich core. To gain the stability, the functionalized complex will prone to eject an extra electron to retain its closo-structure. Thus, the removal of this extra electron leads to a low vertical electron affinity (VEA) which is a characteristic feature of superalkali. Herein, we provide a proof-of-concept towards this class of species, in the recently well-developed chemistry of Zintl ions that are readily obtained in the laboratory.
Section snippets
Computational details
Geometry optimizations and subsequent calculations were performed using relativistic DFT methods [28]. We have used Becke, 3-parameter, Lee–Yang–Parr (B3LYP) [29], [30] hybrid functional for exchange correlation and the def2-TZVPP [31] basis set to optimize all the geometries. Frequency calculations were performed using the same level of theory and basis set to ensure the real minima. Natural population analysis (NPA) [32] analysis was performed to determine the partial charges on each atom
Results and discussion
The bare Ge94− cluster is a well-known Zintl ion, where its stability is well explained by Wade-Mingos rules or Jellium shell model. At first, we have optimized Ge94− Zintl ion denoting a tricapped trigonal prism structure, which agrees to earlier calculations [35]. The obtained geometry and NPA charges of Ge94− are shown in Fig. 1, where more negatively charged atoms are located as capped atoms of the trigonal prism. From the figure, it is evident that it has a D3h point group symmetry where
Conclusions
These results suggest the successful designing of doped deltahedral organo-Zintl superalkali by using [Ge7X2]2− (X = P, As, Sb and Bi) Zintl ion as a parent core with suitable aliphatic ligand CH3. The substitution effect on making superalkali cation was also verified. It has been observed that the aliphatic organic ligands like CH3, C2H5 can make better superalkalis compared to cyclic ligand, C3H3. More the electron donating nature, more will be the superalkali nature. The 2c-2e, DOS and PDOS
CRediT authorship contribution statement
G. Naaresh Reddy: Methodology, Software, Data Curation. Rakesh Parida: Methodology, Software, Formal Analysis. Alvaro Muñoz-Castro: Investigation, Formal Analysis. Madhurima Jana: Investigation, Supervision, Writing - Original draft preparation. 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 and DST-SERB project number CRG/2019/001125. Recourses and computational facilities of National Institute of Technology Rourkela are also acknowledged. A.M.-C. acknowledges financial support from ANID via FONDECYT 1180683.
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