Halogen-bonded Co-crystals of aromatic N-oxides: Polydentate acceptors for halogen and hydrogen bonds

Abstract

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