Reactivities of Metallocarbohedrenes:  Nb₈C₁₂⁺

Abstract

A compact supersonic source is employed with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to generate and investigate the metallocarbohedrene, Nb 8 C 12 + . Nb 8 C 12 + reacts with water and alcohols via sequential attachments and dehydrogenations leading to the initial truncation products Nb 8 C 12 (OR) 4 + (R = H, CH 3, C 2 H 5, C 4 H 9 ). Also, Nb 8 C 12 + reacts with NH 3 via sequential attachments to produce the initial truncation product Nb 8 C 12 (NH 3 ) 4 + . Nb 8 C 12 L 5 - 8 + peaks for ammonia and alcohols are also observed, but they grow in slowly. A measurement of the relative rate constants for the sequential addition reactions of NH 3 confirms that addition of the fifth NH 3 is at least an order of magnitude slower than the first four attachments. In reactions with acetonitrile and benzene, a maximum of four attachments of these ligands is observed. Sequential halide abstractions with CH 3 X yield Nb 8 C 12 X n + truncating at n = 4 for X = Cl and n = 5 for X = Br and I. The “titration” results and the relative rate constants for the sequential addition reactions of NH 3 provide supporting evidence that the geometric structure of Nb 8 C 12 + is the theoretically more stable T d or D 2 d symmetry having two sets of four equivalent metal sites, as opposed to the T h symmetry with all eight equivalent metals. Finally, the results for Nb 8 C 12 + are compared to those previously obtained for V 8 C 12 + and Ti 8 C 12 + .