119Sn SOLID STATE NMR CHARACTERIZATION OF BuSnO(OH)

The Sn MAS NMR characterization of butylstannonic acid, BuSnO(OH), clearly shows that more than 80% of its tin atoms are six-coordinate, the remaining 20% being five-coordinate. Even though a precise structure cannot be proposed, the Sn NMR parameters indicate strong similarities between the sixcoordinate tin atoms of BuSnO(OH) and those constituting the poles of the oxo-cluster {(BuSn)12014(OH)6} INTRODUCTION Butylstannonic acid, BuSnO(OH), is a well known starting material for the synthesis of many organotin oxoclusters.' It is also" used, as a catalyst, in transesterification reactions and appears in several patents dealing with the synthesis of polyesters or the extraction of tocopherol (vitamin E) from natural oils. Its synthesis is based on the basic hydrolysis of BuSnClj, but, because of its amorphous nature, its structure remain unknown, even though a better knowledge, at least of the tin environment, could help to understand its catalytic properties. In the sixties, Luitjen has proposed a polymeric structure based on four-coordinate tin atoms, H0-[BuSn(0H)0]„-H. In the seventies, the Mössbauer results obtained by Davies et al. were found consistent with such a structure containing tetrahedral tin atoms, but the possibility of higher coordinations, resulting from associations, was not ruled out. Indeed, the quadrupolar splitting (AEQ) of 1.52 mm/s reported for BuSnO(OH) also fits in the range reported for octahedral monoorganotin derivatives.' Moreover, if ρ = ΔΕς/δ (6: isomeric shift) is considered as an indication of the tin coordination, BuSnO(OH) exhibits a ρ value of 2.2, which correspond to a coordination greater than four. More recently, Daktemieks et al. have shown that the slow basic hydrolysis of butyltin trichloride yields the oxo-cluster !(BuSn)i20i4(0H)6}Cl2«2H20, which is based on a macrocation containing only fiveand six-coordinate tin atoms in equal amounts. Thus, according to the apparent similarities in the syntheses of this oxo-cluster and of butylstannonic acid, even though\this latter compound is barely soluble, we did naively wonder whether the compound of chemical formula "BuSnO(OH)" could indeed just correspond to 1/12 of {fBuSn)i 20η(0Η)6}(0Η)2·2Η20> a compound in which the anions balancing the positive charge of the macrocation would be hydroxyls instead of chlorides. Such a substitution of anion seems reasonable to consider as the hydrolysis of BuSn(OPr')3 yields the compound {(BuSn)12014(0H)6}(0H)2'4H0Pr. Therefore, according to the well known dependence of the Sn NMR chemical shift with the coordination at tin and to the fairly good sensitivity of Sn MAS NMR to small variations in the tin enironment,' commercial samples of BuSnO(OH) were studied by Sn MAS NMR, in order to prove or reject our structural speculation on butylstannonic acid. MATERIALS ANS METHODS Butylstannonic acid were purchased from Strem Chemicals Inc., Alfa Products or Aldrich. They were used as received. Thermogravimetric analyses were run, under a stream of oxygen and with heating rates of 10°C/min., on a TA instrument STD 2960 analyzer. The Sn MAS (Magic Angle Spinning) NMR experiments were performed on Bruker MSL300 or ASX100 spectrometers (operating at Larmor frequencies of 111.92 and 37.34 MHz for Sn, respectively), both equipped with a 4 mm high-speed Bruker probe (up to 15 kHz). To avoid baseline distortions, which are difficult to correct on spectra containing broad and overlapping resonances, the MAS spectra were acquired using a rotor synchronized Hahn echo sequence (θ τ 2Θ τ acq. ; with T=1/VMAS)· The pulse durations were limited to 2 μβ in order to ensure complete excitation of the spectra. Recycling delays were set to 15 s. The spinning frequencies were stabilized to ±5 Hz. Sn chemical shifts are auoted relative to Me4Sn, using solid tetracyclohexyltin (öiso = -97.35 ppm) as a secondary external reference.