Heteroallene Insertions into Tin(II) Alkoxide Bonds

TL;DR

This paper reports the synthesis and study of tin(II) complexes formed by inserting isocyanates and CO2 into tin-alkoxide bonds.

Contribution

The paper introduces new iso-carbamate Sn(II) complexes and investigates their insertion mechanisms and thermodynamic properties.

Findings

Mono-insertion of aryl isocyanates forms [Sn{κ2-N,O-R-NC(OiPr)O}(μ-OiPr)]2 complexes.

Bis-insertion occurs with alkyl isocyanates, forming [Sn{κ2-N,O-R-NC(OiPr)O}2] complexes.

CO2 insertion into [Sn(OiPr)2] forms a reversible alkyl carbonate with measurable activation energy.

Abstract

A series of iso-carbamate complexes have been synthesized
by the
reaction of [SnII(OiPr)2] or [SnII(OtBu)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(OiPr)O}(μ-OiPr)]2 (1: R = Mes, 2: R = Dipp)
and [Sn{κ2-N,O-R-NC(OtBu)O}(μ-OtBu)]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 = iPr, and 6: R = Cy) and [Sn{κ2-N,O-R-NC(OtBu)O}2] (7: R = iPr, 8: R = Cy), of
which complexes 6–8 have also been
structurally characterized. 1H NMR studies show that the
reaction of tBu-NCO with either [Sn(OiPr)2] or [Sn(OtBu)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 tBu-NCO and the coordinated tBu-iso-carbamate ligand for the {OiPr} alkoxide complex, as well as the activation energy (Ea = 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(OiPr)2] + tBu-NCO ↔ [Sn{κ2-N,O-tBu-NC(OiPr)O}(OiPr)]. Attempts to form
Sn(II) alkyl carbonates by the insertion of CO2 into either
[Sn(OiPr)2] or [Sn(OtBu)2] proved unsuccessful. However, 119Sn{1H} NMR spectroscopy of the reaction of excess
CO2 with [Sn(OiPr)2] reveals the presence of a new Sn(II) species, i.e., [(iPrO)Sn(O2COiPr)], VT-2D-EXSY (1H) of which confirms the reversible
alkyl carbonate formation (Ea = 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).

A series of mono- or bis-iso-carbamate Sn(II) complexes
have been synthesized by the reaction of [SnII(OiPr)2] or [SnII(OtBu)2] with heteroallenes, i.e., aryl and
alkyl isocyanates, ONC-R (R = 2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl
(Dipp), isopropyl (iPr), cyclohexyl (Cy), tert-butyl (tBu)) and CO2. The
degree to which the parent alkoxides undergo either mono- or bis-insertion
into the heteroallene group has been explored both experimentally
and computationally. The mechanism of aryl and alkyl isocyanate insertion
and CO2 insertion into {Sn–OR} bonds are discussed.

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