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Fixation of Ammoniacal Copper Preservatives: Reaction of Vanillin, a Lignin Model Compound with Ammoniacal Copper Sulphate Solution

A model for the fixation of ammoniacal copper preservatives in wood was examined through reaction of vanillin, a lignin model compound, with ammoniacal copper sulphate solution. The compound produced on evaporation of the aqueous ammoniacal solution has been characterized spectroscopically as di(amine)bis(vanillinato)copper(II) ; [Cu(vanillin) 2 (NH 3 ) 2 ]. Single crystal X-ray crystallographic studies showed that the crystals of [Cu(vanillin) 2 (NH 3 ) 2 ] are monoclinic with space group P21 with a = 5.4172(9), b = 15.452(2), c = 10.4595(9)A, Z = 2. The ligand arrangement around the Cu(II) is a distorted octahedron. Two Cu-N [2.014(5) and 2.034(5) A] and two Cu-O (hydroxyl) [1.969(4) and 1.972(4) A] bonds form a plane with Cu-O (methoxyl) [2.371(3) and 2.388(3) A] bonds in an axial direction at an angle 75° to the plane. The copper-nitrogen bonding is trans oriented in the plane. Electron spin resonance (ESR) was used to determine the g-tensor and hyperfine tensor elements from both a polycrystalline sample and a DMSO solution. The result suggests that ammoniacal copper wood preservatives can form stable copper-nitrogen-lignin complexes through reaction with guaiacyl units in lignin.