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# Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts

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Science  14 Jan 2005:
Vol. 307, Issue 5707, pp. 231-235
DOI: 10.1126/science.1105820

## Abstract

Two classes of gas-phase aluminum-iodine clusters have been identified whose stability and reactivity can be understood in terms of the spherical shell jellium model. Experimental reactivity studies show that the Al13I x clusters exhibit pronounced stability for even numbers of I atoms. Theoretical investigations reveal that the enhanced stability is associated with complementary pairs of I atoms occupying the on-top sites on the opposing Al atoms of the Al13 core. We also report the existence of another series, Al14I x, that exhibits stability for odd numbers of I atoms. This series can be described as consisting of an Al14I 3 core upon which the I atoms occupy on-top locations around the Al atoms. The potential synthetic utility of superatom chemistry built upon these motifs is addressed.

The electronic properties and chemical reactivity of small metal clusters can be fundamentally different from bulk metals, which are well described by band theory, and compounds with only a few metal atoms, where bond formation and charge states typically lead to stable atomic shells for each atom. For clusters of free-electron metals, a model is commonly used in which the nuclei and innermost electrons form a positively charged core with an essentially uniform potential. All of the valence electrons from the individual atoms in the cluster are then subjected to this potential, and the jellium electronic shell structure emerges with stable configurations of electrons (2, 8, 18, 20,...) that differ from the atomic series (2, 10, 18,...) (16).

Such clusters could be described as super-atoms, because clusters of a given element can have chemical and electronic properties resembling those of another atom; hence, super-atoms can be regarded as an extension of the periodic table to a third dimension (123). Similar principles have been discussed for the description of the properties of various functional groups in synthetic chemistry, particularly Grimm's hydrogen-displacement theorem (pseudoelements) and Haas' concept of paraelements (24).

We recently demonstrated that an Al13 cluster acts like a superhalogen even when combined with a conventional halogen, namely I, to form an Al13I cluster compound (22, 23). Through reactivity studies of AlnI clusters combined with state-of-the-art ab initio calculations, we find that Al13I is a remarkably stable species and that the extra electron charge in Al13I is mostly localized at the Al13 unit.

Halogens form extended polyhalides in both the gas and condensed phases. For example, I forms polyiodides within the series I2n+1 that consist of I or I3 ions bound to I2 molecules in the form of chains. As an example, the ground state of I 5 is a V-shaped structure with two I2 molecules coordinated to an apical iodide [(I)·2I2]. Given that Al13 can act as a halide ion when coordinated to one or two I atoms, one would predict that interactions with I2 molecules might lead to complex clusters with structures similar to I2n+1 polyiodides.

Herein, we present evidence of the formation of a previously unknown class of polyhalide-like molecules by replacing an I atom in traditional polyiodides with an Al13 cluster. Although it is possible to form Al13Ix clusters for all x, trends in reactivity reveal that members of this series are particularly stable when x is even. Because traditional interhalogen and polyhalide ions require an odd total number of halogen atoms, the Al13Ix clusters appear to resemble these known species. Our ab initio calculations, however, indicate that this apparent similarity is slightly deceptive and that the Al13I2x clusters present an entirely different geometry with subtle differences in chemical behavior. Whereas polyiodides contain I2 molecules, superpolyhalides only contain I atoms. Further, the stability of clusters with even numbers of I atoms has a completely different origin than the stability in conventional polyiodides.

We also show that Al14 can behave as an alkaline earth metal–like superatom. A second series of clusters of the type Al14Ix exhibits pronounced stability when x is odd. The series begins with Al14I3, in which the Al14 core behaves as a dication, and from our theoretical investigations we describe the mechanism by which larger members of the series build upon this core. The synthetic importance of these gas phase findings are underscored by recent results that show that the solution phase “metalloid” cluster compounds (25, 26) synthesized by Schnöckel and co-workers are in fact, at their core, the same cluster species found in the gas phase (27, 28).

The Al cluster anions react readily with I2 (Fig. 1 and fig. S1), leading to a distribution of clusters of the type AlnIx (29). Despite the addition of I2, x need not be even. The addition of single I atoms might be partially attributed to the equilibrium of I2 with atomic I in the vapor phase, but, as we describe in more detail below, etching reactions that lead to AlI or AlI3 formation are likely the primary source of AlnI x clusters. At very low concentrations of I2, a broad distribution of peaks, which includes the Al13I peak, is evident (fig. S1). Even in this concentration regime, however, Al13I 2 is more abundant. At higher concentrations (Fig. 1B), a distinct pattern emerges, featuring a pair of peaks spaced 27 atomic mass units (amu) from each other and repeating in intervals of 127 amu. These correspond to Al13I x and Al14I x. The Al14I x series begins at x = 3, and the Al14I 3 cluster was actually observed previously as a product of the reaction between Al n and HI (22, 23). In the present experiments, the more intense peak in each 13-14 pair alternates at each value of x. When x is even, the Al13I x clusters dominate, and when x is odd the Al14I x clusters dominate. This evidence alone suggests that the chemical stability of the two-cluster series is dependent on the odd-even character of x. Most importantly, however, this trend is confirmed when the iodized clusters undergo oxygen etching (Fig. 1C): The minor peak in each pair is substantially diminished, and the major peak persists. Al13I x clusters are only stable when x is even, whereas Al14I x clusters require that x is odd.

In our previous report on Al13's super-halogen character (22), the stability of Al13I was explained in terms of the ability of the halogen-like Al13 cluster moiety to retain its preferred geometric, electronic, and charge properties. In the present case, the same principles apply, but surprising intricacies emerge. If the I atoms were simply a source of electrons, then we would expect the stability of Al13I x to increase with x without any dependence on odd or even values. The oxygen-etching experiments indicate that this is not the case. Simply providing the Al13 cluster with electron-rich neighbors proves an inadequate chemical scenario for the emergence of special stability.

To further probe the origin of the stability of these complexes and the nature of electronic interactions, we carried out theoretical calculations with the use of a first-principles molecular orbital approach wherein the cluster wave function is expressed as a linear combination of atomic orbitals centered at the atomic sites (3034). The exchange correlation corrections were included within a gradient-corrected density functional (34). The ground state of Al13 is an icosahedral structure with an Al-Al bond length of 2.80Å between the surface sites. For a single I, the ground state of Al13I corresponds to an almost-perfect icosahedral Al13 moiety with I occupying an on-top site (Fig. 2). An analysis of the charge density of the highest occupied molecular orbital (HOMO) (Fig. 2) reveals that most of the charge of the additional electron is localized at the Al13 moiety, thus suggesting super-halogen character and the possibility of polyhalide generation. The occupation of the on-top site by I generates charge localization on the Al vertex opposite from the I atom. These regions of localized charge are termed active centers.

We recently examined structures for Al13I 2 in which an I2 molecule was placed in several orientations around a surface Al site or where a second, dissociated I occupied various Al sites different from the initial I occupation on Al13 (23). In each case, the geometry was optimized by moving atoms in the direction of forces until the forces dropped below a threshold value of 0.001 atomic units. After an investigation of these geometries and possible spin multiplicities, we found that the ground state (Fig. 2) corresponds to an I atom occupying the on-top site opposite from the first I atom, where the active center on Al13I was observed. In terms of polyhalide-like clusters, Al13 and Al13I 2 can be considered as analogous to the Br and BrI 2 building blocks discussed previously (22). We extended the studies to all Al13I x (x values from 3 to 12) by successively adding an I atom and examining all possible positions (including structures more closely related to the branching chains found in polyhalides, which were found to be less energetically stable) and spin multiplicities. In Fig. 2, we show the ground-state geometries and the charge density of the corresponding HOMO levels. All of the Al13I x clusters with odd x are marked by the presence of active centers, whereas in the clusters with even x a pair of I atoms occupy opposing sites, thus filling the active site generated by the previous (odd) I. A careful look at the HOMO charge density reveals another interesting feature. In the clusters with odd x, the bonding to Al13 is reminiscent of a σ bond, whereas in those with even x the bonds look π-like, indicating a change in bonding between a single I or a pair to an orbital of Al13. In all of the Al13I x clusters, the Al moiety has an almost perfect icosahedral geometry, just as in Al13.

Our theoretical studies can account for the greater stability of clusters with even numbers of I atoms as shown in the plot of the change in energy (δEx) $Math$(1) which represents the gain in energy as successive I are added (Fig. 2B). Here, E(Al13I x) is the total energy of a cluster of 13 Al and x I atoms. The δEx for clusters with odd x is around 0.9 eV (20.8 kcal mol-1) less than the value for clusters with even x, which indicates that the clusters with even x are particularly stable.

As a further probe of the active center hypothesis, we performed experiments whereby AlnI x clusters were first formed by reaction with I2 and then reacted with methyl iodide (CH3I). Previously, we found that Al13 was unreactive toward CH3I (23, 35). In the present experiments, Al13I x clusters with odd x were quite reactive toward CH3I, but those with even x were substantially less reactive (36). Recall that the polyhalides also exhibit enhanced stability for I n with odd n. There, the stability is associated with the presence of I2 molecules, because the unit can be expressed as X·(I2)n (X indicates I or I 3 for polyiodides or, for example, Br for an interhalogen). In the case of replacement of an I atom by the Al13 superatom, the odd-even trend in stability has a completely different electronic origin. These interhalogen molecules represent a new class of superatom-containing molecules with distinctive chemical properties.

As in many cluster systems, the addition or removal of one I atom has a profound effect on the stability of Al13I x clusters. The present experiments show that, as we would expect, the system is also extremely sensitive to the number of Al atoms. We now address the emergence of the prominent Al14I x series. The previous observation of Al14I 3 as a product in the HI etching experiments (22, 23), and the fact that this cluster is the first member of the Al14I x series in the present study, strongly indicate that it represents the core upon which the larger clusters are built. The behavior of this core is also explainable in terms of the spherical shell model and the superatom concept. Because Al14 has a lower electron affinity than I, one could imagine that each I added to the cluster will lead to the withdrawal of one electron. Because Al14 has 43 valence electrons, three I atoms will be needed to recreate a 40-electron core that could serve as the foundation upon which larger clusters are built. Theoretical studies are critical to the confirmation of this conjecture. For the superatom description to provide an accurate representation of this system, one would expect the Al14 core in Al14I x clusters to mimic a free Al14 in Al14I, a free Al14+ in Al14I2, and a free Al142+ in Al14I3.

Credence is given to the assignment of an Al142+ core to Al14I x by considering similarities between the ground-state geometries of the Al14Ix series and free Al14, Al14+, and Al142+ (Fig. 3). The structure of Al14 can be regarded as an extra Al atom bound to the hollow site of three Al atoms occupying a triangular face of the Al13 icosahedron. Bond lengths between the extra (on-top) Al and Al atoms of the triangular face, as well as bond lengths between cotriangular-face Al atoms, follow the same bond length trends in both the iodized and free cluster series. Bond lengths between the Al atoms of the triangular face increase continuously with the oxidation of Al14 as with the addition of I to Al14I. The Al-Al bonds are ultimately broken, comparably, in Al142+ and in Al14I3. Furthermore, the broken Al-Al bond length is larger in free Al142+ than in Al14I3, as expected because of reduction of the Coulomb repulsion with the addition of I to the free cluster. Additionally, the overall shape of the Al14 unit becomes increasingly spherical as it is charged. These systemic trends support the description of Al14I x clusters within the spherical shell jellium model.

To further confirm the resemblance of the Al14 core of Al14I3 to free Al142+, we first superimposed the charge density of a free Al142+ cluster and three free neutral I atoms positioned to conform to the geometric configuration of Al14I3. This superimposed charge density was then subtracted from the charge density of Al14I3 to determine the localization of the Al14I3 cluster's three additional electrons (two from Al142+ and one making the cluster anionic). By this method, most of the electronic charge was found to be localized at the I sites, reconfirming that the Al14 core indeed resembles Al142+.

The theoretical studies show that the progression of larger clusters upon the Al14I3 core proceeds in an interesting manner. In Al14I 4, the additional I occupies the site (marked A in Fig. 3A) of the Al14I3 base. The structure of Al14I 5, however, corresponds to an Al14I3 core and the two I atoms occupying a pair of sites opposite to each other in the Al14 portion, leaving site A unoccupied. Thus, whereas site A is occupied in all of the clusters with an even number of I atoms, the clusters with an odd number of I atoms feature pairs of I atoms (not bound as I2) at the Al sites opposite to each other, much in the same way as in the Al13I x series. However, the I at site A is not as strongly bound, so the clusters with an odd number of I atoms are more stable than those with an even number of I atoms. The final manifestation of such a model comes in the variation of the energy δEx (Fig. 3). Note the odd-even oscillations in energy after three I atoms.

Lastly, we consider the results of our Mulliken charge density analysis. The directionality of the cluster-I bonds switches from polar toward the Al cluster in Al13-based species to polar toward the I atoms in the Al14-based clusters. As described above, in the Al13 series the Al13 superhalogen cluster pulls electronic charge from the I atoms in order to create what is essentially an Al13 core. In Al14I3, the cluster-I bonds are polar in the other direction so that the majority of the anionic cluster's charge resides with the I atoms. This piece of information, combined with the preceding analyses, shows that the description of this system as a molecule containing an alkaline earth–like superatom is valid. That is, although bare Al14 does not lend itself to description by the spherical shell model (37), it can be structurally and electronically manipulated to do so in an appropriate chemical environment. Significantly, similar arguments of charge-withdrawing or -donating ligands on clusters have been advanced by other researchers, in particular, the work of Fagerquist et al. on AgxIy+/- clusters (38, 39) and the work of Wang and co-workers on “multiply charged” metal clusters (40).

We can use our theoretical treatment to understand the thermochemistry involved in our experiments. On the basis of values reported in our previous publication (23), we describe several of the key reaction pathways involved in the formation of Al13I x clusters: $Math$(2) $Math$(3) $Math$(4) According to our calculations, the reaction in Eq. 2 is energetically favorable by 3.63 eV so that, although this is the most favored reaction between Al13 and I2, the reaction in Eq. 3, which is energetically favorable by 0.31 eV, is also possible. Moreover, from a kinetic point of view, this requires a third body for stabilization. These mechanisms are similar to those described in the reactions with HI (22, 23). Equation 4 shows an example of I2 acting as an etchant; the particular etching reaction shown here is energetically favorable by 2.25 eV. Other etching pathways may also be possible at various values of n, as shown in Eqs. 5 to 7. $Math$(5) $Math$(6) $Math$(7)

Although Eqs. 5 and 6 could be applied to the x = 0 case, Eq. 7 could not. The Al14I x series must form via one or more of the etching mechanisms in Eqs. 4 to 6 from clusters with n > 14. The energetic treatment of Eq. 4 shows that, in contrast to the Al13I x series, the Al14I x series cannot arise directly via attachment of I atoms to the bare Al cluster anion. The energetics of the reactions between AlnI x clusters and oxygen are less easily analyzed, but for various values of n and x, the oxygen etching reaction yields different Al oxides, I oxides, and AlIO2 molecules. The relative resistance of some clusters to reaction with oxygen seems to be kinetically, rather than thermodynamically, mediated.

In a previous study (22), we had compared Al13I and Al13I2 to BrI and BrI2 by considering electron affinity (EA) the defining characteristic of a halogen. It seems apparent from the structural and electronic properties of the larger Al13Ix clusters that perhaps the defining characteristic in these interhalogens is the size, rather than the EA, of the “halogen.” If EA were more important, branching chain structures more reminiscent of bromoiodides would emerge; instead, fluorohalide-like structures are found in which the smaller halogen atoms decorate a central larger halogen atom.

In the Al14Ix series, we have shown that the Al14 moiety approximates the structural and electronic character of an Al142+ ion. It is therefore concluded that Al14I3 serves as the fundamental core upon which larger Al14I x clusters build because of the presence of a closed shell (40-electron) Al14 alkaline earth–like superatom. We have shown that, in the proper chemical environment, Al14 can be described by the spherical shell model. On the basis of our analysis, we predict that the stability of the Al14 series should follow similar trends for the other halogens, because their EA values allow them to be even more efficient as electron-withdrawing ligands. In both the Al14Ix and the Al13Ix series, the sequential addition of I atoms to the superatom core leads to the emergence and quenching of active centers. These centers should provide many opportunities to explore new cluster chemistry.

The implementation of the spherical shell jellium model allows for a sound understanding of these halogenated aluminum clusters. By extension of the superatom concept, it is possible to understand that the chemical manipulations highlighted here should apply to other metal cluster systems. Although the type of halogen chemistry that is possible with Al13 may not be completely universal to all superatoms with halogen-like electronic structure (the superhalogen nature of Al13 is quite distinctive, but Al23, Al37, and Al55 also have anomalously high electron affinities), the observation of superatom character for Al14-based clusters, which were not previously thought to be amenable to a jellium description, suggests that the synthetic potential of superatom chemistry may completely exceed prior expectations.

Supporting Online Material

Fig. S1

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