1. Photoelectron Spectroscopy Study of Ta 2 B 6 − : A Hexagonal Bipyramidal Cluster Tian Jian, Wei-Li Li, Constantin Romanescu, Lai-Sheng Wang Department of Chemistry, Brown University International Symposium on Molecular Spectroscopy 69TH MEETING JUNE 16-20, 2014 CHAMPAIGN-URBANA, ILLINOIS

2. Outline Motivation Background Planar or quasi planar boron clusters Single metal doped boron clusters Di-metal doped boron clusters Hexagonal bipyramidal Ta 2 B 6 − PES principle and experimental setup PES and ab initio study on Ta 2 B 6 − AdNDP analyses of Ta 2 B 6 − Comparison between Ta 2 B 6 and B 6 motif in solid compounds Summary Acknowledgement

3. Motivation Use clusters as models to understand bulk properties Discover stable atomic clusters as building blocks for clusters assembled nanomaterials The two-dimensional boron clusters have been suggested as potential building blocks or ligands in chemistry Molecular structures of: (Cp*ReH) 2 B 5 Cl 5 and (Cp*Re) 2 B 6 H 4 Cl 2 Le Guennic, B. et al., J. Am. Chem. Soc. 2004, 126, 3203-3217 Optimized structures of Li 6 B 6 H 6 (D 2h, 1 A g ): A, side view; B, front view. Alexandrova, A. N. et al., J. Am. Chem. Soc. 2003, 125, 10786-10787

4. Delocalized bonds Boron: 2s 2 2p 1 Electron deficiency Planar structures Aromaticity, antiaromaticity Planar or quasi-planar boron clusters Zhai, H. J. et al., J. Phys. Chem. A 2003, 107, 9319-9328

5. Bond Types: Two-center-two-electron σ bonds between peripheral boron atoms; Delocalized σ and π bonds between inner and peripheral atoms. Planar or quasi-planar boron clusters Sergeeva, A. P. et al., Acc. Chem. Res. 2014, 47, 1349−1358

6. Single alkali-(earth)-metal-doped boron clusters Combined photoelectron spectroscopy and ab initio calculation show LiB 6 − are charged- transfer complexes. In the complexes of alkali-earth hexaboride (M 2+ B 6 2− ), the B 6 unit were calculated to be similar to the bare clusters. Alexandrova, A. N. et al., J. Chem. Phys. 2005, 122, 054313Li, Q. S. et al., J. Phys. Chem. A 2003, 107, 7869-7873

7. Single aluminum-doped boron clusters Al interacts with boron frames via ionic bonding. Galeev, T. R. et al., J. Chem. Phys. 2011, 135, 104301

8. Single transition-metal-doped boron clusters Romanescu, C. et al., Acc. Chem. Res. 2013, Vol. 46, No. 2 350–358 The single transition-metal boron clusters have been found to prefer to form molecular wheels with octa-, hepta-, and decacoordination numbers.

9. Di-metal doped boron clusters Structure evolution of Ta 2 B x − (x = 2−5). Geometries in the second row are projections viewing along the Ta–Ta axis. Xie, L. et al., J. Chem. Phys. 2013, 138, 034308

10. Photoelectron Spectroscopy Principle M–M– M

11. Experimental Setup Wang, L.S. et al., J. Chem. Phys. 1995, 102, 9480-9493

12. Photoelectron spectra of Ta 2 B 6 ─ at 532nm, 355nm, 266nm and 193nm Photoelectron spectra of Ta 2 B 6 − Simple spectral pattern and sharp peak X. Li, W.L. et al., Angew. Chem. Int. Ed. 2014, 126, 1312 –1316

13. Optimized structures of Ta 2 B 6 − and Ta 2 B 6 Alternative optimized structures for the Ta 2 B 6 − cluster at the BP86/Ta/Stuttgart+2f1g/B/aug-cc-pVTZ level of theory (ΔE of single-point energy at the CCSD(T) level for low-lying isomers areshown in braces. Li, W.L. et al., Angew. Chem. Int. Ed. 2014, 126, 1312 –1316 Alternative optimized structures for the Ta2B6 cluster at the BP86/Ta/Stuttgart+2f1g/B/aug-cc-pVTZ level of theory. Ta blue, B orange.

14. Hexagonal bipyramidal Ta 2 B 6 − Optimized structures of Ta 2 B 6 − and Ta 2 B 6 at the BP86/Ta/Stuttgart+2f1g/B/aug-cc-pVTZ level of theory. Ta blue, B orange. Li, W.L. et al., Angew. Chem. Int. Ed. 2014, 126, 1312 –1316

15. Molecular orbitals of Ta 2 B 6 − Valence conical orbitals of Ta 2 B 6 − at the BP86/Ta/Stuttgart+2f1g/B/aug-cc-pVTZ level of theory. Li, W.L. et al., Angew. Chem. Int. Ed. 2014, 126, 1312 –1316

16. Chemical bonding analyses using AdNDP method Chemical bonding analyses of Ta 2 B 6 using the AdNDP method. ON stands for occupation number. ADNDP method represents the molecular electronic structures in terms of nc-2e bonds. Each cloud represents a bond. Li, W.L. et al., Angew. Chem. Int. Ed. 2014, 126, 1312 –1316

17. Comparison between Ta 2 B 6 and B 6 motif in solid compounds Projection of the structure of Ti 7 Rh 4 Ir 2 B 8 along [001] Two planar Ti 7 wheels with a planar B 6 ring lying between them Ta 2 B 6 Fokwa, B. P. T. et al., Angew. Chem. Int. Ed. 2012, 51, 1702 –1705 Molecular structures of: (Cp*ReH) 2 B 5 Cl 5 and (Cp*Re) 2 B 6 H 4 Cl 2 Le Guennic, B. et al., J. Am. Chem. Soc. 2004, 126, 3203-3217

18. Summary We report the first gas-phase cluster containing a planar B 6 ring coordinated by two Ta atoms in a bipyramidal structure. PES revealed that neutral Ta 2 B 6 is an electronically highly stable system with a closed shell electron configuration and a large HOMO–LUMO gap. Global minimum searches found a bipyramidal Ta 2 B 6 to be the most stable structure. AdNDP analyses revealed the nature of the B 6 and Ta interactions and uncovered strong covalent bonding between B6 and Ta. The D 6h -Ta 2 B 6 gaseous cluster is reminiscent of the structural pattern in the ReB 6 X 6 Re core in the (Cp*Re) 2 B 6 H 4 Cl 2 and the TiB 6 Ti motif in the newly synthesized Ti 7 Rh 4 Ir 2 B 8 solid-state compound. Continued investigations of the transition-metal boron clusters may lead to the discovery of new structural motifs involving pure boron clusters for the design of novel boride materials.

19. Acknowledgement Brown University Prof. Lai-sheng Wang Wei-Li Li, Dr. Constantin Romanescu All Wang group members Prof. Xin Huang Lu xie