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 < 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.