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

Abstract

Proton NMR magnetization-transfer experiments have been utilized to measure the kinetic parameters of alcohol interchange between the homoleptic tin(IV) alkoxide Sn(OBu^t)₄ and Bu^tOH 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(OBu^t)₄·HOBu^t] and suggest that the metal, even in sterically encumbered metal alkoxide compounds such as Sn(OBu^t)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(OBu^t)₄ dissolved in pyridine (py) shows evidence for formation of Sn(OBu^t)₄·py, consistent with an exchange mechanism in which py competes with Bu^tOH 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.