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 K2 [closo-B11 H11 ] (K2 1). In the first step, K2 1 reacts with KCN in water at elevated temperatures to yield [7-NC-nido-B11 H12 ]2- (5 a). Oxidation of 5 a with PbO2 gives [NC-closo-B11 H10 ]2- (2). In analogous subsequent reactions, dianion 2 was converted with KCN regioselectively to [7,9-(NC)2 -nido-B11 H11 ]2- (6 a), which was further oxidized to [(NC)2 -closo-B11 H9 ]2- (3). The {nido-B11 } dianions 5 a and 6 a were protonated to yield [7-NC-nido-B11 H13 ]- (5 b) and [7,9-(NC)2 -nido-B11 H12 ]- (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 [Ph4 P]2 5 a⋅CH2 Cl2 and [Et4 N]2 6 a. The {closo-B11 } clusters are fluxional in solution. In the crystal of [EMIm]2 2 the BCN vertex is in the 2-position. Two isomers of dianion 3, [2,3-(NC)2 -closo-B11 H9 ]2- and [2,6-(NC)2 -closo-B11 H9 ]2- , were identified in the crystal of its [Et4 N]+ salt. The peak oxidation potentials Epa of anions 1-3, 5 a, 6 a, 5 b, and [nido-B11 H14 ]- (4 b), determined by cyclic voltammetry, increase with increasing number of cyano groups. Increasing Epa in the order 1<2<3 parallels the increasing reactivity toward cyanide anions.