HB9X9- and H2B9X9 (X = Cl, Br, I):: Neutral closo-nonaboranes in the novel series BnHn+1, and BnHn+2 -: Syntheses, ab initio calculations, and electronic structures

Chemical reduction of B9X9 (X = Cl, Br, I) with gaseous HI proceeds stepwise to give the neutral paramagnetic clusters HB9X9., and the corresponding diamagnetic clusters H2B9X9. Together they comprise the first neutral derivatives in the series BnHn+1 and BnHn+2 with n = 9. The EPR spectra of the paramagnetic HB9X9. (X = Cl, Br, I) in glassy frozen CH2Cl2 solutions showed increasing g anisotropy for the heavier halogen derivatives, illustrating significant halogen participation at the singly occupied MO due to the larger spin-orbit coupling contributions. Temperature dependent H-1 NMR spectra of H2B9X9 (in CD3CN, X = Cl, Br) indicate the presence of H2B9X9, [HB9X9](-), and [CD3CNH](+) with H2B9X9 acting as a Bronsted acid. The corresponding B-11 NMR spectra (in CD3CN) show the presence of the dianions [B9X9](2-) as a result of the protonation of CD3CN. The B-11 resonances of the species H2B9X9 and [HB9X9](-) are obscured by superimposition of the two resonance lines of the dianions [B9X9](2-). Temperature dependent B-11{H-1}) MAS-NMR spectra of H2B9Br9 show coalescence at 410 K and hence dynamic behaviour of the neutral B-9-cluster in the solid. Cyclic voltammetry experiments of H2B9Br9 in CH3CN solvent) are compatible with the redox sequence [B9Br9](2)-[B9Br9](.-)-B9Br9. Quantum chemical calculations with the electron localization function (ELF) are described.