Stepwise Introduction of Cyano Groups into nido- and closo-Undecaborate Clusters.

Eleven-vertex closo and nido boron clusters with one or two exo-cyano groups were obtained by a series of consecutive cage-opening and cage-closure reactions starting from K [closo-B H ] (K 1). In the first step, K 1 reacts with KCN in water at elevated temperatures to yield [7-NC-nido-B H ] (5 a). Oxidation of 5 a with PbO gives [NC-closo-B H ] (2). In analogous subsequent reactions, dianion 2 was converted with KCN regioselectively to [7,9-(NC) -nido-B H ] (6 a), which was further oxidized to [(NC) -closo-B H ] (3). The {nido-B } dianions 5 a and 6 a were protonated to yield [7-NC-nido-B H ] (5 b) and [7,9-(NC) -nido-B H ] (6 b). All anions were studied by NMR spectroscopy and, except for 6 b, salts of all anions were characterized by IR and Raman spectroscopy and by elemental analysis. The position of the cyano group(s) in 5 a and 6 a were confirmed by NMR data and single-crystal X-ray diffraction studies on [Ph P] 5 a⋅CH Cl and [Et N] 6 a. The {closo-B } clusters are fluxional in solution. In the crystal of [EMIm] 2 the BCN vertex is in the 2-position. Two isomers of dianion 3, [2,3-(NC) -closo-B H ] and [2,6-(NC) -closo-B H ] , were identified in the crystal of its [Et N] salt. The peak oxidation potentials E of anions 1-3, 5 a, 6 a, 5 b, and [nido-B H ] (4 b), determined by cyclic voltammetry, increase with increasing number of cyano groups. Increasing E in the order 1<2<3 parallels the increasing reactivity toward cyanide anions.