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. 2024 Jun 17;63(24):10967-10979.
doi: 10.1021/acs.inorgchem.3c04551. Epub 2024 Jun 4.

Heteroallene Insertions into Tin(II) Alkoxide Bonds

Aidan T Ryan  1 , Andrew Brookes  1   2 , Andrew J Straiton  1 , Thomas Wildsmith  1   2 , John P Lowe  3 , Kieran C Molloy  1 , Michael S Hill  1 , Andrew L Johnson  1
Affiliations

Heteroallene Insertions into Tin(II) Alkoxide Bonds

Aidan T Ryan et al. Inorg Chem. .

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).

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Possible insertion mechanisms of heteroallenes O=C=E (E = O or NR’) into M–NR2 or M–OR bonds. The formation of carbamate, alkyl carbonate, iso-ureide, and iso-carbamate systems from the reaction of metal amides and metal alkoxides with CO2 and isocyanates.
Scheme 1
Scheme 1. Synthesis of Sn(II) Iso-carbamate Complexes 1–8 via the Mono-insertion of Aryl Isocyanates (1–4) or the Bis-insertion of Alkyl Isocyanates (5–8) into Sn–OR Bonds
Figure 2
Figure 2
Solid-state molecular structures of complexes 1 (a), 2 (b), 3 (c), and 4 (d). In all cases, hydrogen atoms have been omitted for clarity, and thermal ellipsoids are shown at 50% probability. For complex 4, solvent of crystallization (toluene) has been omitted for clarity. For complexes 2 and 4 (b, d), only the molecules based around the Sn(1) atoms are shown. The second molecule in each asymmetric unit cell is identical, within experimental error, and omitted for clarity. Symmetry transformations are used to generate equivalent atoms (#); 1: 1 – X, 1 – Y, 1 – Z; 2: −X, 2 – Y, −Z; 3: −X, −Y, −Z; 4; 2 – X, 1 – Y, 1 – Z.
Scheme 2
Scheme 2. Scheme Highlighting the Proposed Identities (and Associated 119Sn NMR Resonances) of the Postulated Species Observed in the Solution State at 298 and 350 K
Figure 3
Figure 3
Molecular structures of compounds 6 (a), 7 (b), and 8 (c) showing the labeling of the asymmetric unit; ellipsoids are shown at 50% probability, and hydrogen atoms are omitted for clarity. Symmetry transformations are used to generate equivalent atoms 7(#); 1 – X, Y, and 1/2 – Z.
Scheme 3
Scheme 3. Reaction of the Sn(II) Alkoxides [Sn(OiPr)2] and [Sn(OtBu)2] with tBu-isocyanate
Figure 4
Figure 4
(a) Proposed reaction between [Sn(OiPr)2] and tBu-isocyanate. (b) 2D EXSY NMR spectrum (at 298 K in d8-Tol) of the reaction between [Sn(OiPr)2] and tBu-isocyanate showing the presence of both free tBu-isocyanate (blue) and tBu-iso-carbamate (orange).
Figure 5
Figure 5
(a) Proposed reaction between [Sn(OiPr)2] and CO2. (b) 2D EXSY NMR spectrum (at 268 K in d8-Tol) of the reaction between [Sn(OiPr)2] and CO2, showing the methine protons of both the isopropoxide group,{Sn-OCHMe2}, (blue) and the isopropoxy carbonate group, {Sn–O2COCHMe2}, (orange), showing exchange.
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