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.
Highlights
The halogen bond (XB), analogous to the hydrogen bond (HB), has been defined [1] as a net attractive interaction between an electrophilic region of a halogen atom bound to a molecular entity and a nucleophilic site in another moiety, e.g., a nitrogen, oxygen or sulphur atom [2]
Halogen bonds between N−O and C−I groups were explored as the driving force propagating 1-D polymers with alternate acceptors and donors
The ability of N-oxide oxygen to act as a μ2, μ3 - and μ4 -acceptor is a complex process
Summary
The halogen bond (XB), analogous to the hydrogen bond (HB), has been defined [1] as a net attractive interaction between an electrophilic region of a halogen atom bound to a molecular entity and a nucleophilic site in another moiety, e.g., a nitrogen, oxygen or sulphur atom [2]. This region of positive electrostatic potential, called the “sigma hole” [3,4], is characteristic for halogen atoms attached to an organic backbone, with its magnitude generally decreasing in the order I > Br >.
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