Superatoms. Группа авторов

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СКАЧАТЬ atoms in the metal core as well as the number and type of ligands can be varied independently, one is gifted with considerable flexibility to design ligated metal clusters as superatoms. Consider, for example a metal core consisting of N_C number of core atoms and N_S number of surface atoms. The surface atoms are the ones that bind to the ligands forming either an ionic or a covalent bond. If the number of valence electrons in the core atoms correspond to shell closure, it is likely that such a cluster can gain unusual stability. Hakkinen and coworkers [37, 38] used this approach to explain the unusual stability of some ligated Al and Au clusters. For example, the unusual stability of Al₅₀Cp*₁₂ can be explained by realizing that 12 Cp* ligands withdraw 12 electrons from the Al₅₀ core, thus leaving 50 × 3 − 12 = 138 electrons. This is just enough electrons to close the 1I superatom jellium shell. Similarly, the authors showed that ligand‐protected Au clusters, satisfying electron shell closure at 8, 34, and 58, can indeed be synthesized.

That stable ligated metal clusters consistent with the jellium shell closure rule can be synthesized has been verified by experiments. Some of these clusters include Au₂₅(SR)₁₈ ⁻ [39] and As₇ and As₁₁, with cryptated alkali atoms [40, 41]. However, it has been pointed out that the strength of the ligand interaction and its effect on the geometry on the metal core play an important role in the electron counting [33, 42, 43]. Examples of some ligated clusters consistent with the jellium model are given in Table 2.1.

      2.2.2 Octet Rule

The octet rule was developed in early 1900s to account for the chemistry of low atomic number (<20) elements [8–10]. The noble gas atoms, with their outer electron configuration of ns ² np ⁶, have closed electronic shells and hence, a large HOMO–LUMO gap, high ionization potential, and low electron affinity. Thus, they are very stable and chemically inert. A variety of superatomic clusters containing elements with low atomic number can be designed such that constituent atoms satisfy the octet shell closure. We begin with the description of Group 1 alkali and Group 17 halogen atoms.

Table 2.1 Examples of ligated clusters with effective number (n_e) of valence electrons for jellium shell closure.

ne	Examples
2	Ag14(SR)12(PR)8, Ag16(SR)14(dppe)4, {Au34[Fe(CO)3]6 [Fe(CO)4]8}8−
8	Au11X3(PR3)7, [Au13Cl2(PR3)10]3+, [Au25(PET)18] −, Au28(SR)20, Al4(C5Me5)4, Al4[SiC(CH3)3]4, [Au13Cu2(PR)6(SPy)6]+, [Au13Cu4(PR2Py)4(SR)8]+, [Au13Cu8(SPy)12]+, [Au11(dpdp)6]3+, {Ag21[S2P(O/Pr)2]12}+
18	[Ag44(SR)30]−4, [Ag44(SR)30]−4, [Au12Ag32(SR)30]−4, [Au12+nCu32(SR)30+n]−4 (n = 0, 2, 4, 6), Au24Ag20(SR)4(C2Ph)20Cl2
34	[Au39(PR3)14Cl6]−, Au68(SR)34, [Au67(SR)35] −2, Au39Cl6(PH3)14
40	{Ge9[Si(SiMe3)3]3}−, SiAl14(C5Me5)6
58	Au102(p‐MBA)44, Au102(SMe)44, {GaGa11[GaN(SiMe3)2]11}
138	Al50(C5Me5)12

      2.2.2.1 Superalkalis and Superhalogens

      Alkali atoms, with outer electronic configuration of ns ¹, have one excess electron while halogen atoms, with outer electronic configuration of ns ² np ⁵, need one extra electron to achieve the octet shell closure of their respective ionic cores. As a result, both the atoms are reactive. Gutsev and Boldyrev [44, 45] were the first ones to use the octet rule to design superalkali and superhalogen clusters that not only mimic the chemistry of alkali and halogen atoms, respectively, but also surpass their properties. The ionization potentials of superalkalis are lower than those of the alkali atoms while the electron affinities of superhalogens are higher than those of the halogen atoms. The composition of superalkali is M _{k + 1}X, where M is an alkali atom and X is an atom with valence k. An example of a superalkali is Li₃O whose ionization potential of 3.54 eV [46] is smaller than that of the Li atom, namely, 5.39 eV. The composition of a superhalogen, on the other hand, is MY _{k + 1}, where Y is a halogen atom and M is a metal atom with valence k. A typical example of a superhalogen is LiF₂, which has an electron affinity of 5.45 eV [47]. This is much larger than the electron affinity of F, namely 3.4 eV. The reason for these superior properties is inherent in the nature of the distribution of electrons. Note that the phase space occupied by the outer electrons increases with cluster size. In superalkalis, this makes it easier to remove an electron, hence leading to a lower ionization potential. In a superhalogen, on the other hand, the increased phase space for the electron distribution causes a reduction in electron–electron repulsion; hence, leading to a higher electron affinity. That superhalogens can promote unusual reactions was already realized by Bartlett in 1962, long before Gutsev and Boldyrev coined the word. Bartlett and coworkers showed that O₂ and noble gas atoms such as Xe can be ionized by using PtF₆ and estimated its electron affinity as 6.8 eV [48, 49]. The fact that clusters can mimic the chemistry of alkali and halogen atoms with superior properties provides new opportunities to design supersalts with superalkalis and superhalogens as building blocks [50].

Numerous studies of superalkalis and superhalogens have been carried out over the past 20 years, and Chapter 3 of this book covers the advancements in superhalogen design and synthesis. For the purpose of illustration, we show in Figure 2.12 the electron affinities of fluorinated coinage metal clusters [51]. Note that the electron affinities are higher than that of F once the number of F atoms is greater than 2 and reach a value as high as 8.6 eV in AuF₆.

The octet rule can also be applied to nonmetallic elements to design superalkalis and superhalogens. Consider, for example molecules such as BH₄, BO₂, CN, and NO₃. Each of these molecules needs one extra electron for electronic shell closure. Indeed, with electron affinities of 4.42, 4.32, 3.86, and 4.03 eV, respectively, these molecules are superhalogens. Following the guidelines of Gutsev and Boldyrev, one could imagine that a new class of clusters with electron affinities even higher than those of superhalogens can be designed. Consider, for example, a cluster with composition MZ _{k + 1}, where Z is a superhalogen. This new class of clusters, named hyperhalogens, was discovered by Willis et al. [52] during the study of the interaction of Au with BO₂ molecules. The electron affinities of Au(BO₂) _n СКАЧАТЬ