An interesting feature of elemental boron and boron compounds is the occurrence of highly symmetric icosahedral clusters. The rich chemistry of boron is also dominated by three-dimensional cage structures. Despite its proximity to carbon in the periodic table, elemental boron clusters have been scarcely studied experimentally and their structures and chemical bonding have not been fully elucidated. Here we report experimental and theoretical evidence that small boron clusters prefer planar structures and exhibit aromaticity and antiaromaticity according to the Hückel rules, akin to planar hydrocarbons. Aromatic boron clusters possess more circular shapes whereas antiaromatic boron clusters are elongated, analogous to structural distortions of antiaromatic hydrocarbons. The planar boron clusters are thus the only series of molecules other than the hydrocarbons to exhibit size-dependent aromatic and antiaromatic behaviour and represent a new dimension of boron chemistry. The stable aromatic boron clusters may exhibit similar chemistries to that of benzene, such as forming sandwich-type metal compounds.

[structure: see text] Analysis of the basic pi-aromatic (benzene) and antiaromatic (cyclobutadiene) systems by dissected nucleus-independent chemical shifts (NICS) shows the contrasting diatropic and paratropic effects, but also reveals subtleties and unexpected details.

Experimental and computational simulations revealed that boron clusters, which favor planar (2D) structures up to 18 atoms, prefer 3D structures beginning at 20 atoms. Using global optimization methods, we found that the B20 neutral cluster has a double-ring tubular structure with a diameter of 5.2 Å. For the B 20 ؊ anion, the tubular structure is shown to be isoenergetic to 2D structures, which were observed and confirmed by photoelectron spectroscopy. The 2D-to-3D structural transition observed at B20, reminiscent of the ring-to-fullerene transition at C20 in carbon clusters, suggests it may be considered as the embryo of the thinnest single-walled boron nanotubes.photoelectron spectroscopy ͉ density functional calculation ͉ global minimum search S mall atomic clusters often exhibit structures and properties remarkably different from those of their bulk counterparts. For example, the most stable form of carbon is graphite, consisting of layers of two-dimensional (2D) graphene sheets. Yet small carbon clusters form chains, rings, and fullerenes (1-5). Boron, carbon's lighter neighbor, is also a strongly covalent material consisting of B 12 icosahedral cages (6-8). But small boron clusters were predicted to be planar (9-11), in stark contrast to the bulk three-dimensional (3D) cages. Planar boron clusters have been recently produced in the gas phase and experimentally confirmed up to B 15 (12)(13)(14). However, it is still unclear at what critical size the 2D-to-3D structural transition occurs. We show from concerted photoelectron spectroscopy (PES) and global geometry optimization theoretical studies (15-17) that the transition occurs at the size of 20 atoms. The B 20 neutral cluster is found to overwhelmingly favor a double-ring tubular-type structure over any 2D isomers, whereas in the anion the tubular and several 2D structures are close in energy. The 2D-to-3D transition at B 20 is reminiscent of the ring-to-cage transition at C 20 , which forms the smallest fullerene (5). The tubular B 20 is the smallest stable 3D boron cluster and can be viewed as the embryo of the thinnest boron nanotube, with a diameter of 5.2 Å. Methods PES.The experiments were carried out by using a magnetic-bottle time-of-flight PES apparatus equipped with a laser vaporization supersonic cluster source (15, 17). B n Ϫ cluster anions were produced by laser vaporization of a disk target made of enriched 10 B isotope (99.75%) in the presence of a helium carrier gas and were analyzed with a time-of-flight mass spectrometer. The B 20 Ϫ clusters were mass-selected and decelerated before irradiation by a photodetachment laser beam. Photoelectrons were collected at nearly 100% efficiency by the magnetic bottle and analyzed in a 3.5-m-long electron flight tube. The photoelectron spectra were calibrated by the known spectrum of Rh Ϫ , and the energy resolution of the apparatus was ⌬E k ͞E k ϳ 2.5%, i.e., 25 meV for 1-eV electrons. Effort was devoted to control the cluster temperatures (Fig. 4, which is published as supporting information on th...

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directly onto TEM grids. Ni grids supporting SiO 2 films (approximately 10 nm thick, Ted Pella) were treated by APTES in the same manner as the SiO 2 /Si substrates, followed by Au particle deposition. The grids were imaged by TEM (Philips CM20, operating voltage 200 keV) before the CVD process to characterize the Au particles, and after the CVD process to characterize the grown nanowires and the particle±wire relationship.Patterned growth: Polymethylmethacrylate (PMMA) was first patterned by electron beam lithography (or photolithography) on a SiO 2 /Si substrate to form 5 î 5 mm wells (Figure 4 a). [11] The substrate was treated with APTES and soaked in a Au colloid solution so that Au particles were deposited into the wells (Figure 4 b). Removal of the PMMA in acetone affords Au particles that are confined in square islands (Figure 4 c). The substrate was then subjected to CVD growth.

The reactivity pattern of small (approximately 10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH-, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.

Aromaticity is one of the most important concepts in chemistry and refers to planar cyclic hydrocarbon molecules that exhibit delocalized p-bonds and unusual stability, such as benzene. This concept has been extended to metal-substituted organic molecules, [1,2] as well as main group organometallic complexes. [3][4][5][6] The recent discovery of aromaticity in allmetal clusters, for instance [Al 4 ] 2À in [MAl 4 ] À (M = Cu, Li, Na), [7] has led to a flurry of research activities [8][9][10][11][12][13][14][15][16] and predictions of new aromatic metal clusters, [17][18][19][20][21][22][23] among which is a class of interesting cyclic species containing Cu. [20,23] While main group clusters can give rise to s-and paromaticity, transition-metal-containing clusters can exhibit d-orbital aromaticity or, more interestingly, d-aromaticity due to d bonding interactions. However, d-orbital aromaticity requires significant d-d bonding interactions. Unlike valent s or p orbitals, d orbitals are spatially more contracted, and their tendency to participate in chemical bonding depends strongly on the position of the transition metals in the periodic table and their coordination environments. The predicted aromaticity in the cyclic Cu-containing species, [20,23] for example, has not been verified experimentally. More promisingly, d-orbital aromaticity is expected to be found in early or 4d/5d transition metal systems, where strong d-d interactions are known. Here, we report experimental and theoretical evidence of d-orbital aromaticity in two early 4d and 5d transition metal oxide clusters, namely [M 3 O 9 ] À and [M 3 O 9 ] 2À (M = W, Mo).

Gold is isolobal to hydrogen. Despite the fact that Au and Si do not form stable alloys, Si and Au form stable covalently bounded clusters, as evidenced from anion photoelectron spectroscopy and ab initio calculations. In a series of [SiAun] clusters (n=2–4) that are similar to SiHn (n=2–4) in structure, chemical bonding, and stability, Au atoms are shown to be isolobal to hydrogen atoms (see picture).