Metal-Bound Nitrate Anion as an Acceptor for Halogen Bonds in Mono-Halopyridine-Copper(II) Nitrate Complexes

Fifteen n-halopyridine-Cu(NO3)2 complexes (n = 2, 3, 4) obtained from two different solvents, acetonitrile and ethanol, are investigated for C–X···O–N halogen bonds (XBs) in the solid state by single and powder X-ray diffraction. The nitrate anions bind copper(II) via anisobidentate modes and one of three oxygens act as an XB acceptor to halogens on the core pyridine rings. The N-metal coordination activates the electron-deficient π-system and triggers even C2- and C4-chlorines in the corresponding [Cu(2-chloropyridine)2(NO3)2] and [Cu(4-chloropyridine)2(NO3)2(ACN)] complexes to form short C–Cl2/Cl4···O–N halogen bonds. Notably, the C2–Cl2···O–N XBs with a normalized XB distance parameter (RXB) of 0.89 is close to C2–I2···O–N XBs [RXB = 0.88] in the [Cu(2-iodopyridine)2(NO3)2] complex. In overall, the C–X···O–N halogen bonds in the studied complexes range from moderately short to roughly the van der Waals contact distance of the respective X···O atoms (RXB = 0.88–0.99) and have a varying significance in governing the molecular packing respective of the complex. Among the studied complexes, two main coordination modes were observed—distorted octahedral and distorted pentagonal bipyramid—of which the latter results from the coordination of acetonitrile to the Cu(II) ion. The crystal structures showed that the steric bulk of C2-halogens in 2-halopyridines prevent this, while in similar conditions the 3- and 4-halopyridine ligands yield acetonitrile bound Cu(II)-complexes.