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 ]·2MeCN ( 2 ·2MeCN) and (3,5-Br 2 pyMe) 3 [Cr(CN) 6 ]·4H 2 O ( 10 ·4H 2 O), 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 ·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···N 2.789(7)–3.116(7), R IN 0.790–0.883; Br···N 2.884(3)–3.166(2), R BrN 0.848–0.931). Longer halogen bonds are formed in 10 ·4H 2 O (Br···N 3.041(6)–3.380(6), R BrN 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 < 105°) 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 π-back-donation.