Halogen-Bonded Co-Crystals of Aromatic N-oxides: Polydentate Acceptors for Halogen and Hydrogen Bonds

Seventeen new halogen-bonded co-crystals characterized by single crystal X-ray analysis are presented from 8 x 4 combinations using methyl-substituted pyridine N-oxides and 1, omega-diiodoperfluoroalkanes. The N-O group in six of 17 co-crystals is monodentate and 11 have mu-O,O bidentate halogen bond acceptor modes. Remarkably, the N-O group in co-crystals of 3-methyl-,4-methyl-and 3,4-dimethylpyridine N-oxides with octafluoro-1,4-diiodobutane acted as a mu-O, O, O, O halogen and hydrogen bond acceptor, while acting as a mu-O, O, O acceptor in the co-crystal of 2,5-dimethylpyridine N-oxide and tetrafluoro-1,2-diiodoethane. The C-H center dot center dot center dot O-N hydrogen bonds demonstrated the polydentate cooperativity of the N-O group as a mixed halogen-hydrogen bond acceptor. The co-crystal of 2,4,6-trimethylpyridine N-oxide and dodecafluoro-1,6-diiodohexane exhibited C-I center dot center dot center dot O--N+ halogen bonds with RXB value 0.76, the shortest of its kind compared to previously reported structures. The R-XB values between 0.76 and 0.83 suggested that the C-I center dot center dot center dot O--N+ halogen bonds are moderately strong compared to our previously studied N--I+center dot center dot center dot O--N+ system, with RXB in the order 0.66.