Synthesis, characterisation and molecular structures of the closo and pseudocloso heptamethylindenyl carbarhodaboranes 1-Ph-3-(η-C9Me7)-3,1,2-closo-RhC2B9H10 and 1,2-Ph2-3-(η-C9Me7)-3,1,2-pseudocloso-RhC2B9H9. Experimental assignment of the 11B NMR spectrum of a pseudocloso carbametallaborane

Abstract

The reaction between [{RhCl₂(η-C₉Me₇)}₂] and Tl₂[7-Ph-8-R-7,8-nido-C₂B₉H₉](R = H or Ph) in CH₂Cl₂ afforded the heptamethylindenyl carbarhodaboranes 1-Ph-3-(η-C₉Me₇)-3,1,2-RhC₂B₉H₁₀1 and 1,2-Ph₂-3-(η-C₉Me₇)-3,1,2-RhC₂B₉H₉2, respectively, in good yields. The ¹¹B NMR chemical shifts of 1 and 2 showed that they posses closo and pseudocloso structures, respectively. Compound 1 was found to be fluctional in solution at room temperature via rotation of the Ph substituent, but cooling below ca. 230 K arrested this process, presumably by locking the indenly ligand into its preferred cisoid conformation and preventing phenyl rotation. An X-ray diffraction study of a single crystal of 1 confirmed both the closo nature of the cage and the cisoid conformation, but additionally showed that, of two possible cisoid conformations, the one adopted places the cage phenyl substituent underneath the six-membered ring of the indenyl group. Compound 2 was also subjected to crystallographic analysis, which confirmed the pseudocloso nature of the cage. Again, the relative conformation of the two η-bonded ligands is cisoid. An ¹¹B–¹¹B COSY spectrum of compound 2, the first to be recorded from a pseudocloso species, allowed experimental assignment of the ¹¹B chemical shifts which were in excellent agreement with those previously obtained theoretically.