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Organo-Zintl Clusters [PR]: A New Class of Superalkalis Santanab Giri, G. Naaresh Reddy, and Puru Jena J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02892 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 18, 2016
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Organo-Zintl Clusters [P7R4]: A New Class of Superalkalis Santanab Giri*,$, G.N. Reddy$ and Puru Jena*,# $
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Department of chemistry, National Institute of Technology Rourkela, INDIA
Department of Physics, Virginia Commonwealth University, Richmond, USA
Corresponding Authors:
[email protected] ;
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ABSTRACT
Zintl ions composed of Group 13, 14 and 15 elements are multiply charged cluster anions that form the building blocks of the Zintl phase. Superalkalis, on the other hand, are cationic clusters that mimic the chemistry of the alkali atoms. It is, therefore, counter intuitive to expect that Zintl anions can be used as a core to construct superalkalis. In this paper, using density functional theory, we show that this is indeed possible. The results are compared with calculations at the MP2 level of theory. A systematic study of a P73- Zintl core decorated with organic ligands [R= Me, CH2Me, CH(Me)2 and C(Me)3] shows that the ionization energies of some of the P7R4 species are smaller than those of the alkali atoms and hence can be classified as superalkalis. This opens the door to the design and synthesis of a new class of superalkali moieties apart from the traditional ones composed of only inorganic elements.
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Atomic clusters composed of a few to a few thousand atoms have emerged as a new phase of matter whose properties are not only size and composition-specific but also depend on the geometrical arrangements of their atoms1. One of the major goals of cluster science has been to use clusters as building blocks of materials with tailored properties2,3. To this end it is necessary that atomic clusters retain their structure and properties once they are assembled into a bulk material. While clusters composed of metal atoms have a tendency to coalesce, those that are stable either as cations or anions tend to retain both their structure and stability. Two particular sets of clusters that belong to this category are superalkalis and super halogens. Discovered about 30 years ago, superalkalis4 and superhalogens5-8 mimic the chemistry of alkali and halogen atoms, respectively, but the ionization energies of superalkali species are less than those of the alkali atoms while the electron affinity of the superhalogen species are larger than those of the halogen atoms. Superalkalis (superhalogens) act as reducing (oxidizing) agents and play an important role in chemical industries. Because of their large size and unique electronic properties superalkalis and superhalogens can combine to make supersalts9 which can be regarded as super alkali-halide crystals. The potential applications of these supersalts in energy storage and energy conversion materials such as hydrogen storage materials, Li-ion batteries, and hybrid perovskite solar cells have motivated the design and synthesis of new superion moieties10. Superhalogens were initially designed by decorating a metal atom with halogen atoms whose number exceeds the maximal valence of the metal atom by one. They have the formula MXk+1 where M is a metal atom with maximal valence k and X is a halogen atom. Research in the past 10 years has greatly expanded the scope of superhalogens by suitably choosing the core as well as the ligand atoms. For example, the core can consist of transition metal atoms or a compound cluster and the ligands can consist of oxygen, pseudo halogens such as CN, and
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superhalogens such as BO211-13. Clusters containing neither metal nor halogen atoms as well as organic molecules have also been shown to exhibit superhalogen properties14. Superalkalis15,16, on the other hand, belong to a class of molecules that mimic the chemistry of alkali atoms. Coined by Gutsev and Boldyrev in 198217-19 superalkalis have the 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 dinuclear- and polynuclear superalkalis, nonmetallic superalkalis, and aromatic superalkalis20-24. In this paper we show that a new class of superalkalis composed of a Zintl ion core and ligated with organic molecules can yield organo-Zintl superalkalis. Zintl ions25-30 are a class of multiply charged anions consisting of group 13, 14 and 15 elements in the periodic table. Generally they exist in the form of Zintl phase compounds where the charge on the negative ion is counterbalanced by the positive charge on the alkali or alkaline earth metal. The chemical bonding in such systems can be rationalized following the Zintl-Klemm concept31, which assumes that the electropositive elements act as electron donors while the electronegative counter parts form covalent bonds to obtain closed shell electronic configuration. Due to its wide range of reactivity several organic and inorganic complexes are available in the literature based on the Zintl ions. At a first glance Zintl ions and superalkalis appear to be two entirely different classes of molecules where the former has a large negative charge while the latter releases an electron quickly to gain a stable electronic configuration. Looking at this difference it is apparently a challenging task if we want to create a superalkali complex out of Zintl ions. Here we address this challenge and show that superalkali complexes can indeed be created from a well-known Zintl ion, P73-
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The stability and reactivity of P73- Zintl ion is well understood. Its stability comes from the octet rule33. As P7 is a 35 electron system, it needs 3 extra electrons to gain a stable 38 electronic shell configuration. Consequently, P73- forms the Zintl phase with K resulting in K3P7. P73- belongs to the group 15 Zintl ion E73- (E=P, As, Sb)34-41 and have a nortricyclane-like structure with C3V point group. Early studies on P73- reveal that it can be systematically functionalized to create complexes like R3P7 (R= H, nBu, SnMe3 etc.)42-52. Controlled amount of alkylation on P73- also leads to the formation of monoanions like R2P7- (R= Me, Et, nBu, Bz etc.)53-55 where the charge of the Zintl ions is reduced allowing the formation of an exo bond with substituent R. This bond can be explained as a two-center two-electron bond. Over benzylation on P73- also leads to [P7R6]3+ (R= Bz)56. Existence of these types of complexes prompted us to wonder if it may be also possible to create mono cationic complexes [P7R4]+ in a controlled way. The reason behind creating [P7R4]+ is that it would be a 38 electron system. So the neutral [P7R4] being a 39 electron system can gain stability by releasing one electron. As a result, [P7R4] may behave like a superalkali. To study this possibility we have chosen the substituents R [CHO, H, Me, CH2Me, CH(Me)2 and C(Me)3] in a systematic way depending on their electron releasing capacity. The electron releasing power of the above mentioned groups as per inductive effect57 follow the order, CHO
