Structures and hydrogen bonding in the 1:1 and 1:2 complexes of trimethylamine N–oxide with pentachlorophenol

Abstract

The following crystalline complexes of trimethylamine N–oxide (TMAO) with pentachlorophenol (PCP): TMAO·PCP, TMAO·PCP·H 2O and TMAO·2PCP have been prepared and characterized by FTIR spectroscopy, quantum–mechanical calculations with DFT and semiempirical methods and X–ray diffraction (two complexes). The crystals of TMAO·PCP are orthorombic, space group Pccn with a=27.621(6), b=13.642(3), c=7.191(1) Å, V=2709.6(9) Å 3, Z=8. The proton is transferred from PCP to TMAO and both residues are linked by an O–H⋯O hydrogen bond of length 2.464(3) Å and an angle of 168(4)°. The crystals of TMAO·PCP·H 2O are monoclinic, space group C2/c with a=20.388(3), b=10.314(2), c=14.300(2) Å, β=105.43(2)°, V=2898.6(8) Å 3, Z=8. The proton is transferred from PCP to TMAO. Two pentachlorophenolates are bridged by two water molecules to form an eight membered ring with four O–H·O hydrogen bonds of 2.663(3) and 2.647(3) Å, respectively. Each water molecule, in turn, additionally forms a hydrogen bond of 2.497(3) Å to protonated TMAO. BLYP, SAM1 and PM3 calculations have been carried out of the most stable structures. In the case of TMAO·PCP and TMAO·PCP·H 2O a good agreement between the calculated and X–ray data are obtained. FTIR spectra of the investigated complexes are consistent with the O⋯O distances. TMAO·2PCP in acetonitrile solution exists as a mixture of TMAO·PCP and solvated PCP. The observed shifts of the centre of gravity of the broad absorption in TMAO·PCP toward higher wavenumbers relative to those in complexes of pyridine N–oxides with comparable ΔpK a can be explained by the resonance interaction between the N–oxide group and aromatic ring.