news of the week JULY 21, 2014 ED ITED BY MANNY I. FOX MORONE & CRAIG BETTENHAUSEN
BORON ARRIVES AT THE FULLERENE BALL MATERIALS: First-observed all-
boron molecular cage displays unusual bonding and structure
T
HE DISCOVERY OF soccer-ball-shaped buck-
minsterfullerene in 1985 inspired researchers to lock atoms of a single element together into novel shapes, such as nanotubes, for applications in drug delivery and electronics. But the search for one alluring nanostructure has vexed researchers for decades. A team is now reporting the first molecular cage made entirely of boron (Nat. Chem. 2014, DOI: 10.1038/nchem.1999). Unlike a C60 buckyball’s regular patchwork of pentagons and hexagons, the newly discovered boron molecule, B40–, looks a bit like a rounded box with two hexagonal lids, four heptagonal sides, and 48 triangles filling out its form. The boron-boron bonds in the “borospherene” are also exotic. Delocalized
CAGE MATCH Comparing the polyhedral structures of B40 and C60
B40
C60
“Borospherene”
Buckminsterfullerene
48
12
2
20
4
SOURCES: S. D. LI, J. LI & L. S. WANG; ISTOCK
π- and σ-bonds hold the boron fullerene together, with electrons shared among three, five, six, or seven atoms. This unique bonding situation alone makes the discovery noteworthy, says computational chemist Anastassia N. Alexandrova of UCLA, who was not part of the study. But the work also appears to solve a longstanding problem: Boron analogs to carbon cages have long eluded chemical synthesis. Since the advent of carbon fullerenes, scientists have been trying to make an analogous boron cage. They were hopeful because boron neighbors carbon on the periodic table and because nanoengineered boron might be a boon for a range of applications. Researchers at Brown University and three universities in China have now identified and characterized borospherene by combining computational chemistry and spectroscopic techniques. Using supercomputers, the team screened more than 10,000 structures made of 40 boron atoms. Two energetically favorable types of candidates emerged: all-boron fullerenes and quasi-planar sheets. The scientists then calculated electron binding energies for both types of structures to produce simulated photoelectron spectra, or what team leader LaiSheng Wang of Brown calls “electronic fingerprints.” The team blasted a boron target with a laser to produce a vapor of atoms that condensed into boron clusters as it was cooled by helium gas. A mass spectrometer selected out only boron clusters with 40 atoms to be sent to the photoelectron spectroscopy stage. The team found that only a mixture of the spectra from simulated planar and fullerene structures could produce the experimental spectrum. A distinctive peak at low energies was key to the conclusion. “That low binding energy feature can only be produced by a three-dimensional cage,” says Wang, who has studied all-boron molecules, from B3 to B36 since 2001. All of those clusters were two-dimensional or quasi-planar. “We’ve never observed a boron cluster with such a low binding energy.” The researchers also predicted based on their calculations that uncharged B40 fullerenes were stable and thus could be formed. But without a charge, they cannot be sorted by mass spectrometry and measured experimentally at this time. Narayan S. Hosmane, a boron researcher at Northern Illinois University, says the report isn’t necessarily surprising. His mentor, Nobel Laureate William N. Lipscomb Jr., predicted the structure of buckyballlike boranes (BxHy) in the 1970s. Hosmane believes borospherene and boron-based heteroatom fullerenes could be useful in medicine and novel electronics. As for witnessing the culmination of a decades-long search, Hosmane says, “This is something exciting.”— MATT DAVENPORT
CEN.ACS.ORG
4
JULY 21, 2014
