Differences between the N·H·O and O·H·O hydrogen bonds in complexes of 2,6-dichloro-4-nitrophenol with pyridines and pyridine N-oxides

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Abstract Complexes of five pyridines and nine pyridine N-oxides with 2,6-dichloro-4-nitrophenol (DCNP) in solution and the solid state were studied by Fourier transform IR and UV spectroscopy, by quantum-mechanical calculations with the semiempirical parametric method 3 (PM3) and by X-ray analysis. The crystals of the 1 : 1 complex of 4-methoxy-2,6-dimethylpyridine N-oxide with DCNP are monoclinic, space group P 2 1 n , a = 4.5936(5) A , b = 21.953(3) A , c = 15.664(2) A , β = 92.87(1)°, V = 1577.6(8) A 3 , Z = 4. The molecules of the complex are joined together by an N+OH⋯O− hydrogen bond with an O⋯O distance of 2.425(3) A, a CO− distance of 1.286(3) A and a (N+O)H⋯O− angle of 152.9°. The PM3 method predicts for all the investigated complexes two minima, the deeper one for B⋯HA complexes and the shallower one for the B+H⋯A− forms. For the 4-methylpyridine complex the N+H⋯O− distance is reproduced correctly but for the 4-methoxy-2,6-dimethylpyridine N-oxide complex the N+H⋯O− distance is too long. The predicted hydrogen-bond angles differ from the experimental values by more than 10°. In solid state complexes of pyridines the N⋯O distances and the broad absorption due to a protic vibration are not directly related to ΔpKa. This is due to the crystal packing forces. In solution the broad absorption varies with ΔpKa. A band in the 3500 cm−1 region due to the solvated phenol is present in all investigated complexes in solution. Absorption in the 3000−2000 cm−1 region of pyridine complexes is more intense than that of the pyridine N-oxides, in agreement with the difference in N⋯O and NO⋯O distances. The broad absorption in the spectra of pyridine complexes is more influenced by solvent effects than in the pyridine N-oxide complexes. The UV spectra of the pyridine complexes show two bands due to B⋯HA (305–315 nm) and B+H⋯A− (382–395 nm) forms. The UV spectra of complexes of pyridine N-oxides of intermediate strengths in CH2Cl2 are not combinations of the spectra of phenol and phenolate. The band in the intermediate position denotes that neither species close to phenol nor to phenoxide ion is present. In these complexes the proton is probably localized in a single minimum and the minimum moves from the donor to the acceptor or, what is more probable, reorganization of the solvent molecules around the complex is faster than the time range of UV spectroscopy. In acetonitrile the situation is quite different as two bands are present, in agreement with a prototropic equilibrium. Effects of solvent, concentration and stoichiometry on interactions of DCNP with pyridines and pyridine N-oxides are compared and discussed. An extended mechanism of the proton-transfer reaction is proposed.