Cyanometallates as Halogen Bond Acceptors

Abstract

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.

Organizing Radical Species in the Solid State with Halogen Bonding

“…This work was also recently extended to cyanometallates (CÀXÁ Á ÁNCÀM) as halogen bond donors, in a series of halopyridinium salts of hexacyanometallates [M(CN) 6 ] 3À (M¼Cr, Fe, Co) [68]. The geometry of interaction of the halogen bond donor (CÀX) with the cyanide ligand is either a shorter interaction, predominantly with the exo lone pair of the nitrogen atom (CNÁ Á ÁX > 145 ), or a longer interaction with the CN π-bond (CNÁ Á ÁX < 105 ).…”

Halogen Bonding in Crystal Engineering

“…Ormond-Prout, Smart, and Brammer proposed that halogen bonds can be used to predict and control the process of self-assembly and to fine-tune the electronic properties of cyanometallates. [50] To confirm this assumption they synthesized two types of halopyridium hexacyanometallate salts, (3-XpyMe)3[M(CN)6] and (3, 5-X2pyMe)3[M(CN)6] (X=Cr, Fe, Co). [50] The authors harvested a total of ten crystals, and found out that five out of each family of compounds were isostructural, while other structures were the solvates, (3-IpyMe)3[Fe(CN)6]·2MeCN(2·2MeCN) and (3,5-Br2pyMe)3[Cr(Cr(CN)6]·(10·4H2O).…”

“…[50] To confirm this assumption they synthesized two types of halopyridium hexacyanometallate salts, (3-XpyMe)3[M(CN)6] and (3, 5-X2pyMe)3[M(CN)6] (X=Cr, Fe, Co). [50] The authors harvested a total of ten crystals, and found out that five out of each family of compounds were isostructural, while other structures were the solvates, (3-IpyMe)3[Fe(CN)6]·2MeCN(2·2MeCN) and (3,5-Br2pyMe)3[Cr(Cr(CN)6]·(10·4H2O). The halogen bonding distances in these structures were shorter than the sum of the van der Waals radii.…”

“…In this particular example, the primary reason for the different behavior of the different metals has been attributed to the metalcyanide π-back-donation. [50] …”

Interplay between N∙∙∙H, N∙∙∙X and π∙∙∙X interactions in the complex pairing of pyrazine with hypohalous acids: A NBO and QTAIM (quantum theory of atoms in molecules) analysis

“…Extensive studies have shown that many properties of the halogen bond are analogous to those of the hydrogen bond [12][13][14]. For example, concerning strength, halogen bonds are comparable to hydrogen bonds and thus a competition may occur between them, thus in recent years much attention has been paid to it [9,[15][16][17].…”