Heteroallene Insertions into Tin(II) Alkoxide Bonds.

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