Atomic clusters have intermediate properties between that of individual atoms and bulk solids, which provide fertile ground for the discovery of new molecules and novel chemical bonding. In addition, the study of small clusters can help researchers design better nanosystems with specific physical and chemical properties. From recent experimental and computational studies, we know that small boron clusters possess planar structures stabilized by electron delocalization both in the σ and π frameworks. An interesting boron cluster is B(9)(-), which has a D(8h) molecular wheel structure with a single boron atom in the center of a B(8) ring. This ring in the D(8h)-B(9)(-) cluster is connected by eight classical two-center, two-electron bonds. In contrast, the cluster's central boron atom is bonded to the peripheral ring through three delocalized σ and three delocalized π bonds. This bonding structure gives the molecular wheel double aromaticity and high electronic stability. The unprecedented structure and bonding pattern in B(9)(-) and other planar boron clusters have inspired the designs of similar molecular wheel-type structures. But these mimics instead substitute a heteroatom for the central boron. Through recent experiments in cluster beams, chemists have demonstrated that transition metals can be doped into the center of the planar boron clusters. These new metal-centered monocyclic boron rings have variable ring sizes, M©B(n) and M©B(n)(-) with n = 8-10. Using size-selected anion photoelectron spectroscopy and ab initio calculations, researchers have characterized these novel borometallic molecules. Chemists have proposed a design principle based on σ and π double aromaticity for electronically stable borometallic cluster compounds, featuring a highly coordinated transition metal atom centered inside monocyclic boron rings. The central metal atom is coordinatively unsaturated in the direction perpendicular to the molecular plane. Thus, chemists may design appropriate ligands to synthesize the molecular wheels in the bulk. In this Account, we discuss these recent experimental and theoretical advances of this new class of aromatic borometallic compounds, which contain a highly coordinated central transition metal atom inside a monocyclic boron ring. Through these examples, we show that atomic clusters can facilitate the discovery of new structures, new chemical bonding, and possibly new nanostructures with specific, advantageous properties.
Bulk boron, which is characterized by 3D cage-like structural features, is a refractory material. [1,2] However, 3D cage structures were suggested to be unstable for small boron clusters, and planar or quasi-planar structures have been proposed instead. [3][4][5] Experimental studies combined with high-level calculations have shown that small boron cluster ions are planar up to at least B 20 À , [6][7][8][9][10] whereas B n + ions have been found to be planar up to n = 16.[11] The chemical bonding in the planar boron clusters has been found to be quite remarkable; [6][7][8][9] in addition to the strong and localized bonding in the circumferences, there are two types of delocalized bonding-the in-plane s and the out-of-plane p bonding, each of which follows the (4 N + 2) Hückel rule for aromaticity. In particular, systems with six s and six p electrons (N = 1) are doubly aromatic, and give rise to highly symmetric planar clusters, such as B 8 2À and B 9 À , which each contain a central B atom and a B 7 and B 8 monocyclic ring, respectively.[6] In the D 7h B 8 2À and D 8h B 9 À molecular wheels, each B atom in the circumference contributes two electrons to the B-B peripheral covalent bonds and one electron to the delocalized bonds, whereas the central B atom contributes all its valence electrons to the delocalized bonds. These novel bonding situations suggest that other atoms with appropriate numbers of valence electrons and sizes may be able to replace the central boron atom to produce MB n -type clusters. [12] Hexagonal, heptagonal, and octagonal CB n -type clusters have been proposed from theoretical calculations as examples of hexa-, hepta-, and octacoordinate planar carbons. [13][14][15] However, photoelectron spectroscopy (PES) studies showed that carbon occupies the peripheral position in such clusters rather than the center, [16,17] because C is more electronegative than B and thus prefers to participate in localized two-center-two-electron (2 c-2 e) s bonding, which is possible only at the circumference of the wheel structure. Transition-metal atoms are better suited for the central position in the MB n clusters, as these metals favor participation in delocalized bonding at the center over localized bonding at the periphery. For an MB n cluster, the electronic requirement for the central atom is x = 12 À n or x = 12 À n À k for an MB n kÀ anion, where x is the valence of the transitionmetal atom M, in order to satisfy the peripheral BÀB s bonding and the s and p Hückel aromaticity for N = 1. Indeed, all 3d transition-metal atoms have been tested computationally for the MB n -type hypercoordinate complexes. [18][19][20][21] Two complexes, namely CoB 8 À and FeB 9 À , in which the Co and Fe atoms are trivalent and divalent, respectively, were found to be closed-shell global minima, in agreement with our electronic design principle.We have focused our experimental efforts on transitionmetal-doped boron clusters that involve Group 8 (Fe, Ru, Os) and 9 (Co, Rh, Ir) elements. The experiments were carried out usi...
Coordination number is one of the most fundamental characteristics of molecular structures. Molecules with high coordination numbers often violate the octet and the 18 electron rules and push the boundary of our understanding of chemical bonding and structures. We have been searching for the highest possible coordination number in a planar species with equal distances between the central atom and all peripheral atoms. To successfully design planar chemical species with such high coordination one must take into account both mechanical and electronic factors. The mechanical factor requires the right size of the central atom to fit into the cavity of a monocyclic ring. The electronic factor requires the right number of valence electrons to achieve electronic stability of the high-symmetry structure. Boron is known to form highly symmetric planar structures owing to its ability to participate simultaneously in localized and delocalized bonding. [1][2][3][4][5][6][7] The planar boron clusters consist of a peripheral ring featuring strong two-center-two-electron (2c-2e) B-B s bonds and one or more central atoms bonded to the outer ring through delocalized s and p bonds. The starting point for the present work is that the bare eight-atom and nine-atom planar boron clusters were found to reach coordination number seven in the D 7h B 8 neutral or B 8 2À as a part of the LiB 8 À cluster [1,3] or eight in the D 8h B 9 À molecular wheel.[1]The CB 6 2À , C 3 B 4 , and CB 7 À wheel-type structures with hexa-and heptacoordinated carbon atom were first considered computationally by Schleyer and co-workers. [8,9] The high symmetry hypercoordinated structures were found to be local minima because they "fulfill both the electronic and geometrical requirements for good bonding". [8,9] In particular, Schleyer and co-workers pointed out that the wheel structures are p aromatic with 6 p electrons. In joint photoelectron spectroscopy (PES) and theoretical studies it was shown that carbon occupies the peripheral position in such clusters rather than the center, because C is more electronegative than B and thus prefers to participate in localized 2c-2e s bonding, which is possible only at the circumference of the wheel structures. [10,11] A series of planar wheel-type boron rings with a main group atom in the center and coordination numbers 6-10 have been probed theoretically. [12][13][14] So far the joint PES and ab initio studies of aluminum-doped boron clusters showed that the aluminum atom avoids the central position in the AlB 6 À , AlB 7 À , AlB 8 À , AlB 9 À , AlB 10 À , and AlB 11 À systems.[ [15][16][17] Recently, a transition-metal-doped boron cluster, RuB 9 À , with the highest coordination number known to date was reported.[18] We developed a chemical bonding model, which allows the design of planar molecules with high coordination numbers.[18] According to the model, 2n electrons in the MB n species form n 2c-2e peripheral B-B s bonds. The remaining valence electrons form two types of delocalized bonding, in-plane s and out-o...
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