Metalloid Aluminum and Gallium Clusters: Element Modifications on the Molecular Scale?

Abstract

As members of the same group in the periodic table, the industrially significant elements aluminum and gallium exhibit strong similarities in the majority of their compounds. In contrast there are significant differences in the structures of the two elemental forms: Aluminum forms a typical closest-packed metallic structure whereas gallium demonstrates a diversity of molecular bonding principles in its seven structural modifications. It can therefore be expected that differences between Al and Ga compounds will arise when, as for the elemental forms, many metal-metal bonds are formed. To synthesize such cluster compounds, we have developed the following synthesis procedure: Starting from gaseous monohalides at around 1000 degrees C, metastable solutions are generated from which the elements ultimately precipitate by means of a disproportionation reaction at room temperature. On the way to the elemental forms, molecular Al and Ga cluster compounds can be obtained by selection of suitable ligands (protecting groups), in which a core of Al or Ga atoms are protected from the formation of the solid element by a ligand shell. Since the arrangement of atoms in such clusters corresponds to that in the elements, we have designated these clusters as metalloid or elementoid. In accordance with the Greek word [see text] (ideal, prototype), the atomic arrangement in metalloid clusters represents the prototypic or ideal atomic arrangement in the elements at the molecular level. The largest clusters of this type contain 77 Al or 84 Ga atoms and have diameters of up to two nanometers. They hold the world record with respect to the naked metal-atom core for structurally characterized metalloid clusters.

Structure, Thermodynamics, and Energy Content of Aluminum–Cyclopentadienyl Clusters

“…In our work we observe significant relaxation of the ligands in the Al 50 Cp 12 complex, and it is possible that this is the origin of our disagreement with the previous report, which gives no information on the calculated structure or thermodynamics of Al 50 Cp 12 . As a check, we have also calculated enthalpies of formation using BP86 with an SVP basis set, similar to the methodology used in previous work.…”

“…The structure of our energy diagram is mirrored after that proposed by Huber et 12 (g) + AlCp 3 (g). In contrast with previous work, the Al 50 structures are "barrier" states in both the methylated and unmethylated systems, though we do find that this barrier is lower for the Cp clusters than in the Cp*.…”

“…The first is the AlCp 3 and AlCp 3 * systems, which contain trivalent aluminum and exhibit significant steric interaction between the ligands (see Figure 2). The second is the Al 50 Cp 12 cluster with its unmethylated ligands, in which 8 of the 12 binding ligands shift to an η 1 position (Figure 3). In systems with mixed hapticities, all bonding configurations are listed in Table 2.…”

“…8,13 Our calculated structure for this compound is given in Figure 3, along with the theoretical geometry of the unmethylated (and experimentally unobserved) analog Al 50 Cp 12 . Both compounds are divided into the following pieces: an inner Al 8 shell, an exterior Al 12 shell that is shown in teal and consists of the Al atoms bound directly to ligands, and a final Al 30 shell between these. As noted above, 8 of the 12 ligands in the Al 50 Cp 12 system change The Journal of Physical Chemistry A ARTICLE hapticity (shown in Figure 4).…”

“…The presence of this barrier is suggested to "trap" the tetramer complexes on their way to metallization. In ref 21, these authors present an energy level diagram in which the energy change for an idealized reaction taking Al 4 Cp 4 to Al 50 Cp 12 and AlCp 3 is slightly negative, whereas the analogous reaction in the methylated system is strongly positive. There is not sufficient information to reproduce the energy calculations given in ref 21, as no details are given on the calculated structures, enthalpies of formation, or if the energies are corrected from the bare DFT SCF energy to account for thermal effects.…”