ALCOHOL-ALKOXIDE EXCHANGE BETWEEN SN(OBU(T))(4) AND HOBU(T) IN COORDINATING AND NONCOORDINATING SOLVENTS

Proton NMR magnetization-transfer experiments have been utilized to me asure the kinetic parameters of alcohol interchange between the homole ptic tin(IV) alkoxide Sn(OBu(t))(4) and Bu(t)OH in various solvents. T he reaction was studied in pyridine with rate constants measured over the temperature range 24-112 degrees C (k(i) = 0.22 s(-1) at 24 degree s C to 12 s(-1) at 112 degrees C) from which activation parameters wer e derived (Delta G(298)(double dagger) = 18.8 kcal mol(-1), Delta H-do uble dagger = 9.5 kcal mol(-1) and Delta S-double dagger = -30 cal K-1 mol(-1)). These data along with variable-temperature Sn-119-{H-1} 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 e ncumbered metal alkoxide compounds such as Sn(OBu(t))(4), is sufficien tly 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 degrees C). Room-temperature solu tion Sn-119-{H-1} spectroscopy of Sn(OBu(t))(4) dissolved in pyridine (py) shows evidence for formation of Sn(OBu(t))(4) . py, consistent wi th an exchange mechanism in which py competes with Bu(t)OH for co-ordi nation sites at tin(IV). Unambiguous evidence for the co-ordination of donor molecules to tin(IV) in homoleptic tin(IV) alkoxide compounds w as obtained from the isolation and structural characterization of Sn(O SiPh(3))(4)(NC5H5)(2) . 0.5NC(5)H(5), the first example of a donor add uct of a homoleptic tin(IV) alkoxide. Single-crystal X-ray diffraction showed that this compound is monomeric and approximately octahedral w ith trans pyridine groups.