Cyanometallates as Halogen Bond Acceptors

Two families of halopyridnium hexacyanometallate salts, (3-XpyMe)(3)[M(CN)(6)] and (3,5-X(2)pyMe)(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)]center dot 2MeCN (2 center dot 2MeCN) and (3,5-Br(2)pyMe)(3)[Cr(CN)(6)]center dot 4H(2)O (10 center dot 4H(2)O), and the solvate (3-IpyMe)(3)[Co(CN)(6)]center dot 2MeCN (3 center dot 2MeCN) has been characterized in addition to the unsolvated 3. All halogens participate in halogen bonding, forming C-X center dot center dot center dot NC(M) halogen bonds and in one case a C-Br center dot center dot center dot O halogen bond (in 10 center dot 4H(2)O). 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 center dot center dot center dot N 2.789(7)-3.116(7), R-IN 0.790-0.883; Br center dot center dot center dot N 2.884(3)-3.166(2), R-BrN 0.848-0.931). Longer halogen bonds are formed in 10 center dot 4H(2)O (Br center dot center dot center dot N 3.041(6)-3.380(6), R-BrN 0.894-0.994) due to competition from O-H center dot center dot center dot N hydrogen bonding. All halogen bonds have interaction geometries at the halogen close to linearity (most have C-X center dot center dot center dot N > 165 degrees; smallest angle is 154.1(3)degrees). The geometry of interaction of the halogen bond donor (C-X) with the cyanide figand either suggests interaction predominantly with the exo lone pair of the nitrogen atom (C N center dot center dot center dot X > 145 degrees) or predominant involvement of the C N pi-bond in the halogen bond (C N center dot center dot center dot X < 105 degrees) acceptor role. The former are shorter interactions than the latter. Halogen bonds become shorter across each isostructural series for Cr > Fe > Co, and this is discussed in the context of metal-to-cyanide pi-back-donation.