Cyanometallates as Halogen Bond Acceptors

Abstract

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 = <i>N</i>-methyl-3-halopyridinium; 3,5-XpyMe = <i>N</i>-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 (<b>2</b>·2MeCN) and (3,5-Br₂pyMe)₃[Cr­(CN)₆]·4H₂O (<b>10</b>·4H₂O), and the solvate (3-IpyMe)₃[Co­(CN)₆]·2MeCN (<b>3</b>·2MeCN) has been characterized in addition to the unsolvated <b>3</b>. All halogens participate in halogen bonding, forming C–X···NC­(M) halogen bonds and in one case a C–Br···O halogen bond (in <b>10</b>·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), <i>R</i>_IN 0.790–0.883; Br···N 2.884(3)–3.166(2), <i>R</i>_BrN 0.848–0.931). Longer halogen bonds are formed in <b>10</b>·4H₂O (Br···N 3.041(6)–3.380(6), <i>R</i>_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 <i>exo</i> 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