Heteroallene Insertions into Tin(II) Alkoxide Bonds.

A series of iso-carbamate complexes have been synthesized by the reaction of [Sn(OPr)] or [Sn(OBu)] with either aryl or alkyl isocyanates, ONC-R (R = 2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (Dipp), isopropyl (Pr), cyclohexyl (Cy) and -butyl (Bu)). In the case of aryl isocyanates, mono-insertion occurs to form structurally characterized complexes [Sn{κ--R-NC(OPr)O}(μ-OPr)] (: R = Mes, : R = Dipp) and [Sn{κ--R-NC(OBu)O}(μ-OBu)] (: R = Mes, : R = Dipp). The complicated solution-state chemistry of these species has been explored using H DOSY experiments. In contrast, reactions of tin(II) alkoxides with alkyl isocyanates result in the formation of bis-insertion products [Sn{κ--R-NC(OPr)O}] (: R = Pr, and : R = Cy) and [Sn{κ--R-NC(OBu)O}] (: R = Pr, : R = Cy), of which complexes - have also been structurally characterized. H NMR studies show that the reaction of Bu-NCO with either [Sn(OPr)] or [Sn(OBu)] results in a reversible mono-insertion. Variable-temperature 2D H-H exchange spectroscopy (VT-2D-EXSY) was used to determine the rate of exchange between free Bu-NCO and the coordinated Bu-iso-carbamate ligand for the {OPr} alkoxide complex, as well as the activation energy ( = 92.2 ± 0.8 kJ mol), enthalpy (Δ = 89.4 ± 0.8 kJ mol), and entropy (Δ = 12.6 ± 2.9 J mol K) for the process [Sn(OPr)] + Bu-NCO ↔ [Sn{κ-N,O-Bu-NC(OPr)O}(OPr)]. Attempts to form Sn(II) alkyl carbonates by the insertion of CO into either [Sn(OPr)] or [Sn(OBu)] proved unsuccessful. However, Sn{H} NMR spectroscopy of the reaction of excess CO with [Sn(OPr)] reveals the presence of a new Sn(II) species, i.e., [(PrO)Sn(OCOPr)], VT-2D-EXSY (H) of which confirms the reversible alkyl carbonate formation ( = 70.3 ± 13.0 kJ mol; Δ = 68.0 ± 1.3 kJ mol and Δ = -8.07 ± 2.8 J mol K).