The chemical nature of bis(tributyltin) oxide in Pinus sylvestris sapwood
S J Blunden, R Hill
Tributyltin compounds have been used for many years as wood preservatives. This study has provided, for the first time, an explanation for the previously reported dealkylation and/or volatilisation of the tributyltin species in, and from, Scots pine (Pinus sylvestris) sapwood. Thus, 119 Sn nuclear magnetic resonance studies have shown that, on impregnation into this timber, bis(tributyltin) oxide is rapidly converted to other tributyltin species, Bu3SnOX, and that these subsequently undergo disproportionation to Bu4Sn and Bu2Sn(OX)2 compounds. We have additionally demonstrated that Bu4Sn, so produced, is not substantive in Pinus sylvestris and is lost by volatilisation. Since the rate of disproportionation of the Bu3SnOX species should be dependent upon the nature of the X group, it should be possible to significantly affect, if not stop, this process by the use of alternative tributyltin fungicides, e.g. tris(tributyltin) phosphate or tributyltin methanesulphonate. However, tributyltin fungicides have been used successfully in wood preservation for at least 25 years. Therefore, it must be concluded that, even after disproportionation in timber, in service, sufficient preservative action is retained to prevent decay of wood under the conditions of natural exposure. Nevertheless, it is becoming evident from this work and from previous studies of the compatibility of tributyltin fungicides with synthetic pyrethroid insecticides, that, on chemical grounds, (Bu3Sn)2O should not be the preferred tributyltin preservative.