Cyanometallates as Halogen Bond Acceptors

Two families of halopyridnium hexacyanometallate salts, (3-XpyMe)3[M­(CN)6] and (3,5-X2pyMe)3[M­(CN)6] (X = I, Br; 3-XpyMe = N-methyl-3-halopyridinium; 3,5-XpyMe = N-methyl-3,5-dihalopyridinium; M = Cr, Fe, Co), have been synthesized and characterized by single crystal X-ray diffraction. Five of the six members of each family are characterized as isostructural compounds, two structures are reported as solvates, (3-IpyMe)3[Fe­(CN)6]·2MeCN (2·2MeCN) and (3,5-Br2pyMe)3[Cr­(CN)6]·4H2O (10·4H2O), and the solvate (3-IpyMe)3[Co­(CN)6]·2MeCN (3·2MeCN) has been characterized in addition to the unsolvated 3. All halogens participate in halogen bonding, forming C–X···NC­(M) halogen bonds and in one case a C–Br···O halogen bond (in 10·4H2O). The halogen bond distances are shorter than the corresponding sum of van der Waals radii, and stronger interactions are formed by iodine than bromine (I···N 2.789(7)–3.116(7), RIN 0.790–0.883; Br···N 2.884(3)–3.166(2), RBrN 0.848–0.931). Longer halogen bonds are formed in 10·4H2O (Br···N 3.041(6)–3.380(6), RBrN 0.894–0.994) due to competition from O–H···N hydrogen bonding. All halogen bonds have interaction geometries at the halogen close to linearity (most have C–X···N > 165°; smallest angle is 154.1(3)°). The geometry of interaction of the halogen bond donor (C–X) with the cyanide ligand either suggests interaction predominantly with the exo lone pair of the nitrogen atom (CN···X > 145°) or predominant involvement of the CN π-bond in the halogen bond (CN···X Fe > Co, and this is discussed in the context of metal-to-cyanide π-back-donation.