Heteroallene Insertions into Tin(II) Alkoxide Bonds

A series of iso-carbamate complexes have been synthesized by the reaction of [Sn^II(OPr)₂] or [Sn^II(OBu)₂] with either aryl or alkyl isocyanates, ONC-R (R = 2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (Dipp), isopropyl (ⁱPr), cyclohexyl (Cy) and tert-butyl (^tBu)). In the case of aryl isocyanates, mono-insertion occurs to form structurally characterized complexes [Sn{κ²-N,O-R-NC(OPr)O}(μ-OPr)]₂ (1: R = Mes, 2: R = Dipp) and [Sn{κ²-N,O-R-NC(OBu)O}(μ-OBu)]₂ (3: R = Mes, 4: R = Dipp). The complicated solution-state chemistry of these species has been explored using ¹H DOSY experiments. In contrast, reactions of tin(II) alkoxides with alkyl isocyanates result in the formation of bis-insertion products [Sn{κ²-N,O-R-NC(OⁱPr)O}₂] (5: R = Pr, and 6: R = Cy) and [Sn{κ²-N,O-R-NC(O^tBu)O}₂] (7: R = Pr, 8: R = Cy), of which complexes 6-8 have also been structurally characterized. ¹H NMR studies show that the reaction of Bu-NCO with either [Sn(OPr)₂] or [Sn(OBu)₂] results in a reversible mono-insertion. Variable-temperature 2D ¹H-¹H 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 (E_a = 92.2 ± 0.8 kJ mol^-1), enthalpy (ΔH^‡ = 89.4 ± 0.8 kJ mol^-1), and entropy (ΔS^‡ = 12.6 ± 2.9 J mol^-1 K^-1) for the process [Sn(OPr)₂] + Bu-NCO ↔ [Sn{κ²-N,O-Bu-NC(OPr)O}(OPr)]. Attempts to form Sn(II) alkyl carbonates by the insertion of CO₂ into either [Sn(OPr)₂] or [Sn(OBu)₂] proved unsuccessful. However, ¹¹⁹Sn{¹H} NMR spectroscopy of the reaction of excess CO₂ with [Sn(OPr)₂] reveals the presence of a new Sn(II) species, i.e., [(PrO)Sn(O₂COPr)], VT-2D-EXSY (¹H) of which confirms the reversible alkyl carbonate formation (E_a = 70.3 ± 13.0 kJ mol^-1; ΔH^‡ = 68.0 ± 1.3 kJ mol^-1 and ΔS^‡ = -8.07 ± 2.8 J mol^-1 K^-1).