Concomitant Carboxylate and Oxalate Formation From the Activation of CO2 by a Thorium(III) Complex.

Improving our comprehension of diverse CO2 activation pathways is of vital importance for the widespread future utilization of this abundant greenhouse gas. CO2 activation by uranium(III) complexes is now relatively well understood, with oxo/carbonate formation predominating as CO2 is readily reduced to CO, but isolated thorium(III) CO2 activation is unprecedented. We show that the thorium(III) complex, [Th(Cp'')3 ] (1, Cp''={C5 H3 (SiMe3 )2 -1,3}), reacts with CO2 to give the mixed oxalate-carboxylate thorium(IV) complex [{Th(Cp'')2 [κ2 -O2 C{C5 H3 -3,3'-(SiMe3 )2 }]}2 (μ-κ2 :κ2 -C2 O4 )] (3). The concomitant formation of oxalate and carboxylate is unique for CO2 activation, as in previous examples either reduction or insertion is favored to yield a single product. Therefore, thorium(III) CO2 activation can differ from better understood uranium(III) chemistry.

There has been an international drive to reduce emissions of CO2 through cleaner energy generation since its identification as a key contributor to global warming.1 In tandem the employment of CO2 as a C1 feedstock for fine chemical (by direct insertion into organic molecules)2 and liquid fuel (via reduction to CO for Fischer–Tropsch processes)3 synthesis have rapidly expanded to complement the optimized photosynthetic pathways employed by nature.4 Early d‐transition metal complexes have received most attention for CO2 activation as their inherent oxophilicity is advantageous in overcoming the considerable thermodynamic and kinetic barriers in this process.5 Similarly, actinides are highly oxophilic, so CO2 activation by UIII complexes is also developing rapidly6 and proof of concept catalytic processes have been disclosed.7 The mapping of UIII‐mediated CO2 activation by DFT calculations has provided key insights into possible mechanistic pathways.8 In contrast, Cloke reported the only example of CO2 activation by a putative ThIII intermediate9 as ThIII small molecule activation is in its infancy.10, 11 Herein we report the first reaction of an isolated ThIII complex with CO2, and CS2 for comparative studies.

[Th(Cp′′)3] (1, Cp′′={C5H3(SiMe3)2‐1,3}) reacts with 0.5 to 10 equivalents of CS2 to give [{Th(Cp′′3)}2(μ‐κ1:κ2‐CS2)] (2) as the only isolable product in 45 % yield (Scheme 1; see the Supporting Information for full details). This reaction is consistent with UIII chemistry as the double reduction of CS2 by [U(Cp′)3] (Cp′=C5H4SiMe3) yields [{U(Cp′3)}2(μ‐κ1:κ2‐CS2)].12 However, 1 reacts with excess CO2 to give [{Th(Cp′′)2[η2‐O2C{C5H3‐3,3′‐(SiMe3)2}]}2(μ‐κ2:κ2‐C2O4)] (3) in 65 % yield (Scheme 1), in contrast to the UIII reduction of CO2 by [U(Cp′)3] to afford [{U(Cp′)3}2(μ‐O)] and CO.13 The FTIR spectrum of 3 has absorptions at 1653 cm−1 and 1560 cm−1 that can be attributed to asymmetric C−O stretches of the oxalate and carboxylate groups respectively.14 The reaction of 2 with CO2 gave a mixture of products including carboxylate (see the Supporting Information).

Scheme 1

Synthesis of 2 and 3 from 1.

The solid state structures of 2 and 3⋅2C7H8 were determined by single crystal XRD (Figure 1. A polymorph 2 b⋅2C6H14 was also obtained; see the Supporting Information). The (μ‐CS2)2− unit in 2 was disordered over two positions so only the major component is discussed here. This fragment binds in an asymmetrical μ‐κ1:κ2‐fashion [S−C: 1.644(11) and 1.717(10) Å; S‐C‐S: 124.4(7)°], in common with the motif seen for [{U(Cp′3)}2(μ‐κ1:κ2‐CS2)] [S−C: 1.464(19) and 1.831(19) Å; S‐C‐S: 131.7(13)°]12 and similar to that seen for [{U[OSi(OtBu)3]3}2(μ‐κ2:κ2‐CS2)] [S−C: 1.594(12) and 1.748(11) Å; S‐C‐S: 131.6(8)°].15 The oxalate of 3 has similar metrical parameters to those seen in [{Th(COTTIPS2)(Cp*)}2(μ‐κ2:κ2‐C2O4)].9 The carboxylate ligand exhibits both C−C and C=C lengths in the C5 ring and a geminal 3,3′‐disilane. The C−Ocarboxylate [1.284(5) and 1.263(5) Å] lengths evidence delocalization about the carboxylate framework, although the binding is asymmetric due to sterics [Th−Ocarboxylate 2.400(3) and 2.484(2) Å]. The electronic structures of 2 and 3 were characterized at the DFT level, employing the B3LYP exchange‐correlation functional and a polarized split‐valence basis set for structural optimizations. Structural parameters of 3 were in good agreement with experiment (see the Supporting Information for full details).

Figure 1

Molecular structure of a) 2 and b) 3.2C7H8 with selected atom labelling and displacement ellipsoids set to 30 % probability level. Hydrogen atoms, minor disorder components and lattice solvent omitted for clarity.

We postulated that 3 forms via a [{Th(Cp′′)3}2(μ‐CO2)] intermediate that is analogous to 2. The bulky Cp′’ ligands hinder the elimination of CO and the formation of [{Th(Cp′′)3}2(μ‐O)],16 so a second molecule of CO2 reacts with the (μ‐CO2)2− fragment to give an oxalate. There are many examples of sterically demanding ligands promoting oxalate formation over a μ‐oxo or carbonate in f‐block CO2 activation.8, 17 Subsequent insertion of CO2 into a Th–Cp′’ moiety and silyl/proton migration yields 3. The insertion of CO2 into lanthanide‐CpR bonds to form carboxylates has been postulated not to require steric strain to proceed.18 Additional experiments were performed to probe the mechanism of formation of 3 (see the Supporting Information for full details). A Toepler pump was used to react 1 with 1 or 2 equivalents of CO2 or 13CO2 at −78 °C, and 3/3‐ was the only identifiable product by 1H and 13C{1H} NMR spectroscopy in all cases. The reaction of 1 with supercritical CO2 was monitored by 1H NMR spectroscopy, and comparison with an authentic sample showed the formation of 3. Minor products in all reaction mixtures could not be identified. In situ FTIR spectroscopy was used to monitor the conversion of 1 to 3 at −78 °C in methylcyclohexane with stoichiometric CO2. No intermediates could be detected but the CO2(g) absorption at 2338 cm−1 diminished on slow warming to room temperature, coincident with the ingress of an oxalate absorption at 1653 cm−1 that is seen in the FTIR spectrum of crystalline 3. The experiment was repeated with 13CO2 and the oxalate absorbance of 3‐ was observed at 1609 cm−1, consistent with reduced mass considerations.

Given that no intermediates could be detected experimentally, we performed DFT studies to rationalize this unusual mechanism. Figure 2 shows the calculated enthalpy reaction profile for the formation of 3, with the double reduction of CO2 to give a μ‐κ1:κ2‐CO2 dinuclear ThIV complex the proposed first step based on the analogous CS2 reaction as well as CO2 reactivity reported with other actinide complexes.8, 12, 15, 17 The oxalate formation invokes nucleophilic attack of a CO2 molecule by a dimetalloxycarbene intermediate [{Th(Cp′′3)}2(μ‐κ1:κ1‐CO2)] (C2) in a carbenic fashion, which has previously only been seen in d‐block CO2 activation for TiIV.19 No pre‐interaction is required between the ThIV centers and the second CO2 molecule, which is in contrast with all previous examples of SmII[17a–c] and UIII[17d] oxalate formation. The resultant cis‐μ‐κ1:κ1‐C2O4 transition state (TS1) is one of several possible conformers that have Δr H° values within the estimated error of the calculation (ca. 3 kcal mol−1) of each other, thus we do not comment on this further. Rearrangement of the oxalate to a trans‐μ‐κ1:κ1‐binding mode increases the steric demands about the ThIV centers (C3). The potential energy surface for these rearrangements is very flat and despite our efforts it was not possible to locate a transition state. This leads to insertion of CO2 at a single position of a Th–Cp′’ moiety at each ThIV center (TS2) as the silicon centers stabilize negative charge at the beta position, allowing the best overlap with the empty orbital of CO2. These insertions are accompanied by the rearrangement of the oxalate to a μ‐κ2:κ2‐binding mode. Subsequent proton and silyl group migrations in the dearomatized Cp′′ rings give the observed product 3 at an energetic minimum.

Figure 2

Computed enthalpy reaction profile for the formation of 3.

To conclude we have shown that although CS2 activation by [Th(Cp′′)3] is analogous to that seen for a similar UIII system, the mechanism by which it reacts with CO2 to form a mixed oxalate/carboxylate product has no precedent in UIII chemistry, in which CO2 reduction (and subsequent carbonate formation depending on the supporting ligands) predominates. We probed this reaction to show that the oxalate is generated by a mechanism only seen previously in d‐block chemistry, whereas the carboxylate forms via a route only seen before in f‐element chemistry for lanthanide complexes. This shows that ThIII small molecule activation can furnish results that complement and contrast with uranium, lanthanide and d‐transition metals. Future studies will target heteroallene activation by ThIII complexes supported by different ligand systems to test the generality or divergence of these processes.

Experimental Section

Full synthetic details, characterization data and computational data for 2 and 3 is available in the Supporting Information. Additional research data supporting this publication are available from The University of Manchester eScholar repository at DOI: 10.15127/1.302780.

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Supplementary

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