HYDROGEN-BONDS IN 1 2-COMPLEXES OF SUBSTITUTED PYRIDINE N-OXIDES WITHPENTACHLOROPHENOL/

Pyridine N-oxides form two types of crystalline complexes with pentach lorophenol, with 1:1 and 1:2 base-to-acid ratios. The 1:2 complex of 2 ,6-dimethylpyridine N-oxide with pentachlorophenol crystallizes in spa ce group P (1) over bar with a = 7.335(1) Angstrom; b = 11.324(2) Angs trom, c = 15.824(2) Angstrom, alpha = 100.38(1)degrees, beta = 94.63(1 )degrees, gamma = 106.60(1)degrees, V = 1226.7(6) Angstrom(3) and Z = 2. The structure has been refined to R = 0.046 for 3408 observed Mo K alpha reflections. The oxygen atom of the N-oxide group accepts hydrog en bonds from two molecules of pentachlorophenol, with O...O distances of 2.639(5) and 2.642(5) Angstrom and OHO angles of 141.2 degrees and 157.6 degrees, respectively; Both N-O...H-O bridges are formed in, or near, the directions of the electron lone-pairs of the N-oxygen atom. The two pentachlorophenol rings (A and B) are nearly parallel to each other and they are almost perpendicular to the pyridine ring. FTIR sp ectra of eleven 1:2 complexes in the solid state are similar and indep endent of the proton acceptor properties of the N-oxides. Five lines i n the Cl-35 NQR spectra of the 1:2 complexes provide evidence that bot h molecules of pentachlorophenol are equivalent. In CHCl3 solution, al l the 1:2 complexes exist as a mixture of the 1:1 complex and pentachl orophenol. (C) 1997 Elsevier Science B.V.