Activation of SO₂ by [Zn(Cp*)₂] and [(Cp*)Zn^I–Zn^I(Cp*)]

Abstract

Interesting reactivity was observed in reactions of SO₂ with [Zn(Cp*)₂] and [(Cp*)Zn^I–Zn^I(Cp*)]. These reactions proceeded with insertion of SO₂ into the Zn–C bonds. Spectacularly, the lability of the C–S bond in the O₂SCp* ligands led to the thermal decomposition of [Zn(O₂SCp*)₂(tmeda)] to afford [Zn₂(μ-SO₃)(μ-S₂O₄)(tmeda)₂].

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The reduction of SO₂ to elemental sulfur (the Claus process) or sodium dithionite are important industrial reactions, but the primary use of SO₂ is in the manufacture of sulfuric acid via the contact process. Befitting its industrial importance, there has been renewed academic interest in the activation of SO₂ by molecular species spanning much of the periodic table. Reactions of low-valence complexes with SO₂ can form dithionite complexes¹ but insertion of SO₂ into M–O² or M–C bonds,^1b,c,3 affording sulfite and sulfinate complexes, respectively, is also known, along with other reactions.⁴ However, despite these recent advances, reactions of SO₂ with zinc complexes remain poorly studied, with the few reports thus far only detailing insertion reactions.^3,5 Herein, we report the reactions between SO₂ and the zinc complexes, [Zn(Cp*)₂] and [(Cp*)Zn^I–Zn^I(Cp*)] (Cp* = C₅Me₅⁻).

When two equivalents of SO₂ were condensed onto a stirring solution of [Zn(Cp*)₂] in thf, a white solid precipitated immediately. The white solid that formed did not dissolve to any appreciable extent, even with heating. However, when the same reaction was performed in the presence of tmeda (tmeda = N,N′-tetramethylethylenediamine), a clear solution was obtained. From this solution, the SO₂ insertion product, [Zn(O₂SCp*)₂(tmeda)] (1), was obtained in good yield (Scheme 1).

Scheme 1 Syntheses of [Zn(O₂SCp*)₂(tmeda)] (1) and [Zn₄(O₂SCp*)₆O] (2).

Complex 1 crystallised from thf in the monoclinic space group, P2/c. The molecular structure and selected bond lengths are listed in Fig. 1. The six-coordinate zinc atom features a distorted octahedral coordination geometry environment, reminiscent of related carboxylate complexes, e.g. [Zn{O₂C(CH=CH)CH₃}₂(tmeda)]⁶ and [Zn(O₂CCH₃)₂(tmeda)].⁷ The Zn–O bond lengths (2.1139(12) and 2.2340(12) Å) are unequal in length, but this is also observed in the aforementioned carboxylate complexes, and it is presumably a result of the hexacoordination of the zinc atom in complex 1. It is a rare example of a Zn sulfinate complex, although other examples have been reported.^3,5

Fig. 1 Molecular structure of [Zn(O₂SCp*)₂(tmeda)] (1). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Zn(1)–O(1) = 2.1139(12), Zn(1)–O(2) = 2.2340(12), Zn(1)–N(1) = 2.1665(14). Half of the molecule is generated by symmetry.

When [(Cp*)Zn^I–Zn^I(Cp*)] was treated with SO₂, complete oxidation of Zn^I to Zn^II was observed, and the zinc oxo-cluster, [Zn₄(O₂SCp*)₆O] (2), was obtained in moderate yield (Scheme 1). This reaction was performed multiple times and 2 is the only product that has been isolated to date. Complex 2 evidently arose from insertion of SO₂ into the Zn–Cp* bonds, analogously to 1, and possibly reduction of SO₂ or an in situ formed derivative by the Zn–Zn bond to abstract an oxygen atom, but other pathways cannot be ruled out.⁸ We do not think that water is a likely source of the oxide ion since we used high purity SO₂ and complex 2 was isolated multiple times from different batches of [(Cp*)Zn^I–Zn^I(Cp*)].⁹ Furthermore, we did not ever observe the formation of Zn metal, which would accompany disproportionation, and thus we believe that the Zn–Zn unit is oxidised by SO₂ or an in situ formed derivative, e.g. the O₂SCp* ligands. It is difficult to rule out other sources, e.g. thf cleavage, and attempts to do so have so far been inconclusive. Nonetheless, our case quite possibly represents a rare case of deoxygenation of SO₂ by a low-valent metal complex. The reductive cleavage of SO₂ into SO and O²⁻ is generally unfavourable due to the instability of SO, and this highlights the impressive reducing ability of Zn^I in the current case, when even trivalent uranium^2b or divalent lanthanide^1b,c systems, which feature highly reducing and oxophilic metal centres, have proven incapable of abstracting an oxygen atom from SO₂. It is worth noting that it has been reported that CO₂ does not react with [(Cp*)Zn^I–Zn^I(Cp*)].¹⁰ We repeated this reaction and found the same result.

Complex 2·1.5(C₅H₁₂) crystallised from pentane in the triclinic space group, P1̄. The complex has a pentanuclear [Zn₄O] core featuring four zinc atoms coordinated tetrahedrally around a central oxide ion. Each Zn atom is further ligated by one oxygen atom of three separate O₂SCp* ligands, with six such ligands in total. Such a [Zn₄O] core is a common feature in zinc-oxo complexes, and it is also a very popular node in MOF chemistry.¹¹ The zinc atoms are all four coordinate, in contrast to the six-coordinate zinc atom in complex 1. The distorted tetrahedral coordination sphere of each zinc atom is completed by coordination to one oxygen atom of three separate bridging O₂SCp* ligands, giving six such ligands in the complex (Fig. 2).

Fig. 2 Molecular structure of [Zn₄(O₂SCp*)₆O]·1.5(C₅H₁₂) (2·1.5(C₅H₁₂)). Hydrogen atoms and lattice solvent have been omitted for clarity. One of the O₂SCp* moieties (corresponding to S(6)) is disordered over two positions, so only one of the conformations is shown. Selected bond lengths (Å): Zn(1)–O(1) = 1.939(5), Zn(1)–O(6) = 1.943(5), Zn(1)–O(9) = 1.969(5), Zn(1)–O(13) = 1.958(6).

Complex 2 is the sulfinate analogue of [Zn₄(O₂CCp*)₆O].^11c In contrast to complex 1, the Zn–O(sulfinate) bond lengths are shorter (1.90(2)–1.99(2) Å), and this is presumably a function of the lower coordination number in complex 2. The lower coordination in complex 2 also accounts for the shorter Zn–O bond lengths. The Zn–O(oxide) bond lengths also fall in the same range as the Zn–O(sulfinate) bond lengths. Unsurprisingly, the Zn–O bond lengths in 2 are very similar to those in [Zn₄(O₂CCp*)₆O].

Both 1 and 2 were characterised by NMR (vide infra) and IR spectroscopy, and satisfactory microanalyses were obtained for both complexes.

¹H NMR studies of 1 and 2 revealed that the complexes are thermally unstable and that they slowly decompose in solution to form multiple products. An interesting feature of 1 is that the O₂SCp* ligands only give rise to two broad signals, in contrast to the three sharp singlets in a 2 : 2 : 1 ratio observed for O₂CCp* ^11c or S₂CCp* ¹² ligands bound to Zn cations. The broadness of the signals for the O₂SCp* ligands is indicative of rapid exchange of the Cp* ring carbon atoms and the sulfur atom of the SO₂ moiety. This hinted that the instability of the complexes was due to the lability of the C–S bond in the O₂SCp* ligands. The ¹H NMR spectrum of complex 2 is much more complex than that of 1, and no obvious assignment has yet proved possible, even after obtaining spectra at low temperatures. It seems likely that there is dissociation of the complex into multiple species upon dissolution.

Much to our surprise, monitoring the decomposition of 1 by ¹H NMR spectroscopy showed that one of the decomposition products was (C₅Me₅)₂. The origin of this species is the coupling of two Cp*˙ radicals, which form from the loss of one electron per Cp* anion (eqn (1)). Evans has demonstrated that [Ln(Cp*)₃] complexes are highly reducing despite having the lanthanide ions in their highest accessible oxidation state, and the area has been termed sterically-induced reduction.¹³ Similar Cp*-based reductions have recently been observed in Zn chemistry,¹⁴ but base-induced reduction from a functionalised Cp* ligand is, to the best of our knowledge, unprecedented.

2(C₅Me₅)⁻ → (C₅Me₅)₂ + 2e⁻ (1)

In an effort to study these intriguing decomposition reactions, complex 1 was generated in situ and then heated at 70 °C for several hours. Although the decomposition reaction is complex, we isolated single crystals of [Zn₂(μ-SO₃)(μ-S₂O₄)(tmeda)₂] (3) in modest yield (Scheme 2). Importantly, the reaction is repeatable. Complex 3 is highly insoluble, thus preventing its characterisation by NMR spectroscopy, but it was characterised by X-ray crystallography, IR spectroscopy and microanalysis.

Scheme 2 Formation of [Zn₂(μ-SO₃)(μ-S₂O₄)(tmeda)₂] (3) from the thermal decomposition of [Zn(O₂SCp*)₂(tmeda)] (1).

Complex 3·thf crystallised from thf in the monoclinic space group, P2₁/c. The molecular structure is shown in Fig. 3, along with selected bond lengths. The dinuclear complex features two six-coordinate zinc atoms that are each bound to one tmeda ligand, one bridging dithionite ligand and one bridging sulfite ligand. The binding of the sulfite and dithionite ligands to the two zinc atoms is slightly asymmetric. Surprisingly, to the best of our knowledge the only other Zn complex containing dithionite ligands is the simple Zn salt, [Zn(S₂O₄)(NC₅H₅)].¹⁵ On the other hand, there are multiple examples of Zn sulfite complexes, but they remain a fairly limited class, and most examples feature polymeric arrays. However, related bimetallic complexes such as [Zn(μ-SO₃)(phen)]₂¹⁶ (phen = 1,10-phenanthroline) and [Zn(μ-SO₃)(bipy)]₂¹⁷ (bipy = 2,2′-bipyridine) are known.

Fig. 3 Molecular structure of [Zn₂(μ-SO₃)(μ-S₂O₄)(tmeda)₂]·thf (3·thf). Hydrogen atoms and lattice solvent have been omitted for clarity. Selected bond lengths (Å): Zn(1)–O(1) = 2.142(2), Zn(1)–O(3) = 2.162(2), Zn(1)–O(4) = 2.100(2), Zn(1)–O(6) = 2.143(2), Zn(1)–N(1) = 2.203(3), Zn(2)–O(1) = 2.096(2), Zn(2)–O(2) = 2.347(3), Zn(2)–O(5) = 2.103(2), Zn(1)–O(7) = 2.099 (2), Zn(2)–N(3) = 2.189(3).

Whilst it is difficult to state exactly what occurs during this decomposition reaction, the presence of the sulfite ligand (SO₃²⁻) can be rationalised by a combination of deoxygenation of an SO₂ moiety to yield a Zn oxo species, and subsequent insertion of another molecule of SO₂ into the Zn–O bond to form the sulfite ligand. Recently, we showed that SO₂ can insert into a metal oxide bridge (M–O–M) to give the corresponding sulfite complex, (M–SO₃–M).^2a The dithionite ligand (S₂O₄²⁻) presumably arises from homolytic cleavage of the C–S bond in O₂SCp*, formally giving the two radical species, SO₂˙⁻ and Cp*˙. Both of these species can couple to form S₂O₄²⁻ and (C₅Me₅)₂, respectively, the latter of which was detected in solution. Subtle changes in reaction conditions presumably affect the outcome of these reactions but it is a spectacular demonstration of the reactivity of these complexes. The decomposition of 1 to afford 3 was deemed to be too complicated to model computationally but we modelled the insertion of SO₂ into [Zn(Cp*)₂] to yield 1 (Fig. 4).

Fig. 4 Computed enthalpy profile (kcal mol⁻¹) for the reaction of SO₂ with [Zn(Cp*)₂] at room temperature.

The reaction begins with the coordination of a tmeda molecule to the zinc atom, inducing a haptotropic shift of the originally η⁵-Cp* ligand, which then becomes sigma-bonded through one carbon atom. This shift was found to be endothermic by 10.1 kcal mol⁻¹ but it is crucial for the subsequent reactivity with SO₂. Indeed, from this complex, the approach of SO₂ leads to the partial decoordination of a Cp* ligand, which is then able to nucleophilically attack the incoming SO₂ molecule. Starting from the tmeda-coordinated complex, the barrier for such an attack is very low (4.9 kcal mol⁻¹ or 15.0 kcal mol⁻¹ with respect to the tmeda-free decamethylzinconcene). This reaction yields an intermediate that appears to be quite stable (−33.7 kcal mol⁻¹ with respect to the entrance channel). In this intermediate, the inserted SO₂ molecule only interacts with the Zn centre through one oxygen atom in order to maintain the four-fold coordination around the metal. The approach of a second SO₂ molecule leads to a similar process as described before. Indeed, the second Cp* ligand gets decoordinated and nucleophilically attacks the coordinated SO₂. The barrier for this second insertion is negligible (a few tenths of a kcal mol⁻¹), in line with a facile process. As for the first insertion, this second one is strongly exothermic by 29.7 kcal mol⁻¹, yielding the formation of complex 1. The Cp* ligand, often considered an innocent ligand, reacts like an alkyl group. Such reactivity appears to be enhanced by the sigma-coordination of the Cp* ligand in the complex (favoured by the coordination of tmeda), and this has been observed before in zinc Cp* chemistry.^11c,12,14b

In summary, the reactions of the zinc complexes, [Zn(Cp*)₂] and [(Cp*)Zn^I–Zn^I(Cp*)], with SO₂ yielded the sulfinate complex, [Zn(O₂SCp*)₂(tmeda)] (1), and the sulfinate/oxo cluster, [Zn₄(O₂SCp*)₆O] (2), respectively. In both cases, nucleophilic attack of the Cp* ligand on the incoming SO₂ molecule was observed, and in the reaction with [(Cp*)Zn^I–Zn^I(Cp*)], complete oxidation of the Zn^I–Zn^I bond was observed. DFT calculations clearly show the decoordination of the Cp* ligand, followed by nucleophilic attack on the SO₂ molecule, and finally, coordination of the newly formed O₂SCp*⁻ sulfinate anion. Somewhat surprisingly, these complexes are thermally unstable due to the lability of the C–S bond of the sulfinate ligands. In the case of complex 1, thermally-induced decomposition allowed us to isolate the mixed dithionite/sulfite complex, [Zn₂(μ-SO₃)(μ-S₂O₄)(tmeda)₂] (3), in modest yield. This multi-step activation of SO₂ is noteworthy since activation of SO₂ by molecular systems tends to stop after insertion or coupling.

This work was supported by the DFG-funded transregional collaborative research center SFB/TRR 88 “3MET”. LM would like to thank the Alexander-von-Humboldt Foundation, Chinese Academy of Science and ANR for financial support. CalMip is also gratefully acknowledged for a generous grant of computing time.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Full experimental procedures and data, computational data and crystallographic data. CCDC 1490513–1490514. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc05638e

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