Testing the limits of halogen bonding in coordination chemistry

In order to link coordination complexes via halogen bonds and to test the limit of halogen-bond interactions in metal-containing crystalline solids, a series of thirteen new acac complexes of Co II and Ni II with small halogen-bearing aromatic ligands, namely 3-halopyridines, 2-halopirazines and 5-halopyrimidine were prepared. Ligands having a single aromatic ring were used to limit the influence of π-π stacking on the resulting crystal structure, while fine-tuning of the electron density at the halogen atom σ-hole was achieved via two routes: (i) by changing the position of the additional nitrogen heteroatom on the aromatic ring, and (ii) by utilizing different transition metal ions. The moderate 'activation' of the halogen atom produced the desired halogen interaction in a limited number of supramolecular events. Two supramolecular links, C-X O and C-H O, were shown to have a larger influence on the overall supramolecular assembly, and their role could be explained on the basis of calculated molecular electrostatic potential values. Slight electronic changes introduced by metal-cation exchange revealed an additional handle for achieving a switch between different supramolecular interactions and architectures. To test the limit of halogen bonds fine-tuning of electron density was performed via introduction of heteroatoms and metal cations.