Gaseous Cyclodextrin-closo-Dodecaborate Complexes ?CD·[B12X12]2- (?= a, ß, ?; X = F, Cl, Br, I): Electronic Structures and Intramolecular Interactions
A fundamental understanding of cyclodextrin-closo-dodecaborate inclusion complexes has become of great interest in supramolecular chemistry. Herein, we report a systematic investigation on the electronic structure and intramolecular interactions of perhalogenated closo-dodecaborate dianions B12X122- (X= F, Cl, Br and I) binding to a-, ß-, and ?-cyclodextrins (CDs) in the gas phase using combined negative ion photoelectron spectroscopy (NIPES) and density functional theory (DFT) calculations. The vertical detachment energy (VDE) of each complex and electronic stabilization of each dianion due to the CD binding (?VDE, relative to the corresponding isolated B12X122-) are determined from the experiments along a-, ß-, ?-CD in the form of VDE (?VDE): 4.00 (2.10), 4.33 (2.43), 4.30 (2.40) eV in X = F; 4.09 (1.14), 4.64 (1.69), 4.69 (1.74) eV in X = Cl; 4.11 (0.91), 4.58 (1.38), 4.70 (1.50) eV in X = Br; and 3.54 (0.74), 3.88 (1.08), 4.05 (1.25) eV in X = I, respectively. All complexes have significant higher VDEs than the corresponding isolated dodecaborate dianions with the ?VDE spanning from 0.74 eV at (a, I) to 2.43 eV at (ß, F), sensitive to both host CD size and guest substituent X. DFT optimized complex structures exhibit that all B12X122- prefer binding to the wide openings of CDs with the insertion depth and binding motif strongly dependent on the CD size and halogen X. Dodecaborate anions with heavy halogens, i.e. X = Cl, Br, I, are found outside of a-CD, while B12F122- is completely wrapped by ?-CD. Partial embedment of B12X122- into CDs are observed for the other complexes via multi-pronged B-X···H-O/C interlocking patterns. The simulated spectra based on density of states agree well with the experiments and the calculated VDEs well reproduce the experimental trends. Molecular orbital analyses suggest that the spectral features at low binding energies originated from electrons detached from the dodecaborate dianion, while those at higher binding energies derived from electron detachment from CDs. Energy decomposition analyses reveal the electrostatic interaction plays a dominating role in contributing to the host-guest interactions for the X = F series partially due to the formation of O/C-H···X-B hydrogen bonding network, and the dispersion forces gradually become important with the increase of halogen size.