Hydrogen and Halogen Bond Mediated Coordination Polymers of Chloro-Substituted Pyrazin-2-Amine Copper(I) Bromide Complexes

A new class of six mono- (1; 3-Cl-, 2; 5-Cl-, 3; 6-Cl-) and di-(4; 3,6-Cl, 5; 5,6-Cl-, 6; 3,5-Cl-) chloro-substituted pyrazin-2-amine ligands (1-6) form complexes with copper (I) bromide, to give 1D and 2D coordination polymers through a combination of halogen and hydrogen bonding that were characterized by X-ray diffraction analysis. These Cu(I) complexes were prepared indirectly from the ligands and CuBr2 via an in situ redox process in moderate to high yields. Four of the pyrazine ligands, 1, 4-6 were found to favor a monodentate mode of coordination to one Cu-I ion. The absence of a C6-chloro substituent in ligands 1, 2 and 6 supported N1-Cu coordination over the alternative N4-Cu coordination mode evidenced for ligands 4 and 5. These monodentate systems afforded predominantly hydrogen bond (HB) networks containing a catenated (mu(3)-bromo)-Cu-I 'staircase' motif, with a network of 'cooperative' halogen bonds (XB), leading to infinite polymeric structures. Alternatively, ligands 2 and 3 preferred a mu(2)-N,N' bridging mode leading to three different polymeric structures. These adopt the (mu(3)-bromo)-Cu-I 'staircase' motif observed in the monodentate ligands, a unique single (mu(2)-bromo)-Cu-I chain, or a discrete Cu2Br2 rhomboid (mu(2)-bromo)-Cu-I dimer. Two main HB patterns afforded by self-complimentary dimerization of the amino pyrazines described by the graph set notation R-2(2)(8) and non-cyclic intermolecular N-H center dot center dot center dot N' or N-H center dot center dot center dot Br-Cu leading to infinite polymeric structures are discussed. The cooperative halogen bonding between C-Cl center dot center dot center dot Cl-C and the C-Cl center dot center dot center dot Br-Cu XB contacts are less than the sum of the van der Waals radii of participating atoms, with the latter ranging from 3.4178(14) to 3.582(15) angstrom. In all cases, the mode of coordination and pyrazine ring substituents affect the pattern of HBs and XBs in these supramolecular structures.