Timur R. Galeev

Transition-Metal-Centered Monocyclic Boron Wheel Clusters (M©Bn): A New Class of Aromatic Borometallic Compounds

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 clusters 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.

Aromatic Metal-Centered Monocyclic Boron Rings: Co©B8− and Ru©B9−†

Bulk boron, which is characterized by 3D cage-like structural features, is a refractory material. 2] However, 3D cage structures were suggested to be unstable for small boron clusters, and planar or quasi-planar structures have been proposed instead. Experimental studies combined with high-level calculations have shown that small boron cluster ions are planar up to at least B20 , whereas Bn + ions have been found to be planar up to n = 16. The chemical bonding in the planar boron clusters has been found to be quite remarkable; 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 (4N + 2) H ckel 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 B8 2 and B9 , which each contain a central B atom and a B7 and B8 monocyclic ring, respectively. In the D7h B8 2 and D8h B9 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 M Bn-type clusters. [12]

Observation of the Highest Coordination Number in Planar Species: Decacoordinated Ta©B10− and Nb©B10− Anions

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. 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 D7h B8 neutral or B8 2 as a part of the LiB8 cluster 3] or eight in the D8h B9 molecular wheel. The CB6 2 , C3B4, and CB7 wheel-type structures with hexaand heptacoordinated carbon atom were first considered computationally by Schleyer and co-workers. The high symmetry hypercoordinated structures were found to be local minima because they “fulfill both the electronic and geometrical requirements for good bonding”. 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. 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. 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 AlB6 , AlB7 , AlB8 , AlB9 , AlB10 , and AlB11 systems. Recently, a transition-metal-doped boron cluster, Ru B9 , with the highest coordination number known to date was reported. We developed a chemical bonding model, which allows the design of planar molecules with high coordination numbers. According to the model, 2n electrons in the M Bn 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-of-plane p bonding, and therefore, should satisfy the (4N + 2) H ckel rule separately for s and p aromaticity to attain highly symmetric structures with high electronic stability. In pure wheel-type boron clusters 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. Thus, out of 26 valence electrons in B8 2 or 28 in B9 , 14 or 16 valence electrons form peripheral covalent 2c-2e s bonds, leaving six s and six p electrons (N = 1 for the 4N + 2 rule) for double (s and p) aromaticity. However, pure planar boron clusters cannot go beyond coordination number eight because of the mechanical factor (the small size of the central boron atom). For example, the B10 cluster does not contain a ninecoordinated boron atom, because the boron atom is too small to fit in the central position of a B9 ring. [2] Since the central atom participates only in delocalized bonding, atoms more electronegative than boron such as carbon avoid the central position. 11,19] Transition-metal atoms, on the other hand, are well-suited for the central position in M Bn species. To satisfy the peripheral B B bonding and the s and p H ckel aromaticity for N = 1, the electronic requirement for the central atom in high-symmetry species, such as M Bn k , is x = 12 n k, where x is the valence of the transition-metal atom M. Ru B9 satisfies the formula and is the first example of an [*] T. R. Galeev, Prof. Dr. A. I. Boldyrev Department of Chemistry and Biochemistry Utah State University, Logan, UT 84322 (USA) E-mail: a.i.boldyrev@usu.edu Homepage: http://www.chem.usu.edu/~boldyrev/

Transition-metal-centered nine-membered boron rings: MⓒB9 and MⓒB9(-) (M = Rh, Ir).

We report the observation of two transition-metal-centered nine-atom boron rings, RhⓒB(9)(-) and IrⓒB(9)(-). These two doped-boron clusters are produced in a laser-vaporization supersonic molecular beam and characterized by photoelectron spectroscopy and ab initio calculations. Large HOMO-LUMO gaps are observed in the anion photoelectron spectra, suggesting that neutral RhⓒB(9) and IrⓒB(9) are highly stable, closed shell species. Theoretical calculations show that RhⓒB(9) and IrⓒB(9) are of D(9h) symmetry. Chemical bonding analyses reveal that these complexes are doubly aromatic, each with six completely delocalized π and σ electrons, which describe the bonding between the central metal atom and the boron ring. This work establishes firmly the metal-doped B rings as a new class of novel aromatic molecular wheels.

Diverse human extracellular RNAs are widely detected in human plasma

There is growing appreciation for the importance of non-protein-coding genes in development and disease. Although much is known about microRNAs, limitations in bioinformatic analyses of RNA sequencing have precluded broad assessment of other forms of small-RNAs in humans. By analysing sequencing data from plasma-derived RNA from 40 individuals, here we identified over a thousand human extracellular RNAs including microRNAs, piwi-interacting RNA (piRNA), and small nucleolar RNAs. Using a targeted quantitative PCR with reverse transcription approach in an additional 2,763 individuals, we characterized almost 500 of the most abundant extracellular transcripts including microRNAs, piRNAs and small nucleolar RNAs. The presence in plasma of many non-microRNA small-RNAs was confirmed in an independent cohort. We present comprehensive data to demonstrate the broad and consistent detection of diverse classes of circulating non-cellular small-RNAs from a large population.

Valence isoelectronic substitution in the B8 − and B9 − molecular wheels by an Al dopant atom: Umbrella-like structures of AlB7 − and AlB8 −

The structures and the electronic properties of two aluminum-doped boron clusters, AlB(7)(-) and AlB(8)(-), were investigated using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of AlB(7)(-) and AlB(8)(-) are both broad, suggesting significant geometry changes between the ground states of the anions and the neutrals. Unbiased global minimum searches were carried out and the calculated vertical electron detachment energies were used to compare with the experimental data. We found that the Al atom does not simply replace a B atom in the parent B(8)(-) and B(9)(-) planar clusters in AlB(7)(-) and AlB(8)(-). Instead, the global minima of the two doped-clusters are of umbrella shapes, featuring an Al atom interacting ionically with a hexagonal and heptagonal pyramidal B(7) (C(6v)) and B(8) (C(7v)) fragment, respectively. These unique umbrella-type structures are understood on the basis of the special stability of the quasi-planar B(7)(3-) and planar B(8)(2-) molecular wheels derived from double aromaticity.

Geometric and electronic factors in the rational design of transition-metal-centered boron molecular wheels

The effects of the electronic and geometric factors on the global minimum structures of MB9(-) (M = V, Nb, Ta) are investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra are obtained for MB9(-) at two photon energies, and similar spectral features are observed for all three species. The structures for all clusters are established by global minima searches and confirmed by comparison of calculated and experimental vertical electron detachment energies. The VB9(-) cluster is shown to have a planar C2v V©B9(-) structure, whereas both NbB9(-) and TaB9(-) are shown to have Cs M©B9(-) type structures with the central metal atom slightly out of plane. Theoretical calculations suggest that the V atom fits perfectly inside the B9 ring forming a planar D(9h) V©B9(2-) structure, while the lower symmetry of V©B9(-) is due to the Jahn-Teller effect. The Nb and Ta atoms are too large to fit in the B9 ring, and they are squeezed out of the plane slightly even in the M©B9(2-) dianions. Thus, even though all three M©B9(2-) dianions fulfill the electronic design principle for the doubly aromatic molecular wheels, the geometric effect lowers the symmetry of the Nb and Ta clusters.

Planarity takes over in the CxHxP6−x (x = 0–6) series at x = 4

In this work we examine a structural transition from non-planar three-dimensional structures to planar benzene-like structures in the C(x)H(x)P(6-x) (x = 0-6) series. The global minima of P(6), CHP(5), and C(2)H(2)P(4) species are benzvalene-like structures. The benzvalene and benzene-like structures of C(3)H(3)P(3) are close in energy with the former being slightly more stable at our best level of theory. The transition occurs at x = 4 (C(4)H(4)P(2)), where the benzene-like structures become significantly more stable than the benzvalene-like structures. We show that the pseudo Jahn-Teller effect, which is responsible for the deformation of planar P(6), CHP(5), and C(2)H(2)P(4) structures, is completely suppressed at x = 3 (benzene-like structures of C(3)H(3)P(3)). We present NICS(zz) values of all the benzene-like isomers in the series.

Aluminum Avoids the Central Position in AlB9– and AlB10–: Photoelectron Spectroscopy and ab Initio Study

The structures and the electronic properties of two Al-doped boron clusters, AlB(9)(-) and AlB(10)(-), were investigated via joint photoelectron spectroscopy and high-level ab initio study. The photoelectron spectra of both anions are relatively broad and have no vibrational structure. The geometrical structures were established by unbiased global minimum searches using the Coalescence Kick method and comparison between the experimental and calculated vertical electron detachment energies. The results show that both clusters have quasi-planar structures and that the Al atom is located at the periphery. Chemical bonding analysis revealed that the global minimum structures of both anions can be described as doubly (σ- and π-) aromatic systems. The nona-coordinated wheel-type structure of AlB(9)(-) was found to be a relatively high-lying isomer, while a similar structure for the neutral AlB(9) cluster was previously shown to be either a global minimum or a low-lying isomer.

Molecular wheel to monocyclic ring transition in boron–carbon mixed clusters C2B6− and C3B5−

In this joint experimental and theoretical work we present a novel type of structural transition occurring in the series of C(x)B(8-x)(-) (x=1-8) mixed clusters upon increase of the carbon content from x=2 to x=3. The wheel to ring transition is surprising because it is rather planar-to-linear type of transition to be expected in the series since B(8), B(8)(-), B(8)(2-) and CB(7)(-) are known to possess wheel-type global minimum structures while C(8) is linear.