Alcohol–alkoxide exchange between Sn(OBut)4and HOButin co-ordinating and non-co-ordinating solvents

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Proton NMR magnetization-transfer experiments have been utilized to measure the kinetic parameters of alcohol interchange between the homoleptic tin(IV) alkoxide Sn(OBut)4 and ButOH in various solvents. The reaction was studied in pyridine with rate constants measured over the temperature range 24–112 °C (k1= 0.22 s–1 at 24 °C to 12 s–1 at 112 °C) from which activation parameters were derived (ΔG‡298= 18.8 kcal mol–1, ΔH‡= 9.5 kcal mol–1 and ΔS‡=–30 cal K–1 mol–1). These data along with variable-temperature 119Sn-{1H} NMR data are consistent with a five-co-ordinate intermediate such as [Sn(OBut)4·HOBut] and suggest that the metal, even in sterically encumbered metal alkoxide compounds such as Sn(OBut)4, is sufficiently co-ordinatively and electronically unsaturated to react with bulky alcohols. In non-co-ordinating solvents such as benzene the exchange rate is faster (k= 1.93 s–1 at 24 °C). Room-temperature solution 119Sn-{1H} NMR spectroscopy of Sn(OBut)4 dissolved in pyridine (py) shows evidence for formation of Sn(OBut)4·py, consistent with an exchange mechanism in which py competes with ButOH for co-ordination sites at tin(IV). Unambiguous evidence for the co-ordination of donor molecules to tin(IV) in homoleptic tin(IV) alkoxide compounds was obtained from the isolation and structural characterization of Sn(OSiPh3)4(NC5H5)2·0.5NC5H5, the first example of a donor adduct of a homoleptic tin(IV) alkoxide. Single-crystal X-ray diffraction showed that this compound is monomeric and approximately octahedral with trans pyridine groups.