Binding Four Iodine Atoms to a Single Pnictogen Atom in Halogen-Bonded Assemblies of Nitroprusside [Fe(CN)5NO]2− and C2I2

The realization of high-order multifurcated halogen bonds (X-bonds) at a single atomic acceptor remains a major challenge in supramolecular chemistry. Here, we report the first example of a tetrafurcated nitrogen X-bond acceptor achieved by utilizing the nitroprusside anion, [Fe(CN)5(NO)]2−. Through co-crystallization with the ditopic donor diiodoacetylene (C2I2), two distinct frameworks were isolated: ([PPh4]+)2[Fe(CN)5(NO)]2−·2C2I2·6CH2Cl2 (1) and ([PNP]+)2[Fe(CN)5(NO)]2−·5C2I2·5.7CH2Cl2 (2). The use of voluminous cations and low temperatures proved essential to access these assemblies. While 1 exhibits only short, monofurcated X-bonds (d(N···I) = 2.771−2.816 Å/RNI = 0.78−0.80), the nitroprusside node in 2 is involved in ten X-bonds. Remarkably, four of these contacts occur at a single cyanide nitrogen, yielding the first example of a nitrogen atom binding to four iodine atoms simultaneously. Despite the steric demands of this motif, all four distances remain well below the sum of the van der Waals radii (d(N···I) = 3.021−3.276 Å/RNI = 0.86−0.93). Density functional theory calculations, combined with QTAIM, NCIplot, and NBO analyses, rationalize the formation of this unprecedented assembly, revealing that cooperative primary charge-transfer contacts and secondary dispersive interactions overcome the energetic penalty of geometric bending and local electron depletion at the nitrogen center.