Carbonate Templated Self-Assembly of an Alkylthiolate Bridged Cadmium Macrocycle

Abstract

In the presence of Cd (ClO₄)₂ and base, a new mixed N,S-donor alkylthiolate ligand supported both carbonate formation from atmospheric CO₂ and self-assembly of a novel bis-capped puckered (CdS)₆ molecular wheel with 3-fold symmetry. The remarkable stability of the complex was demonstrated by slow intermolecular ligand exchange on the ²J(HH) and J(^111/113Cd¹H) time scales at elevated temperature. Both CO₂ and base were required to convert amorphous “CdLClO₄” precipitated in the absence of air to the carbonate complex. The complex shares structural features with the ζ-carbonic anhydrase class associating Cd(II) with the biogeochemical cycling of carbon and is the first structurally characterized carbonate complex of any metal involving an alkylthiolate ligand.

Carbonate is a versatile bridging species found in structurally characterized two-dimensional and mono-, di-, tri-, tetra-, hexa-, ennea- and higher nuclear complexes with at least fourteen different coordination motifs.¹ Several hundred carbonate complexes involving all the physiologically essential metals and a wide range of non-essential transition metals are structurally characterized.² In many cases precursors to these carbonate complexes are able to hydroxylate atmospheric CO₂, forming an intermediary species that spontaneously effects CO₂ fixation through combinatorial self-assembly and selective crystallization.³ Global environmental problems associated with rising greenhouse gas concentration justify continued investigation of these intriguing processes.

In biological systems, hydration of carbon dioxide is catalyzed by carbonic anhydrase, a possibly ubiquitous metalloprotein with five Zn^II-dependent forms across the taxonomic kingdoms sharing no significant similarity in primary sequence or overall structure.⁴ Zinc(II) was viewed as an essential cofactor for this enzyme activity until the seemingly anomolous nutrient-like vertical oceanic profile of cadmium was linked to a highly active Cd^II carbonic anhydrase (CdCA) variant from the marine diatom Thalassiosira weissflogii grown under Zn^II-limiting conditions, thus establishing the ζ form of CA.⁵ All six carbonic anhydrase forms use invariant histidine residues for metal ligation and related aromatic nitrogen donors are quite common in synthetic complexes. Both the the β and ζ forms of these enzymes also have two invariant cysteine residues bound to the metal, yet alkylthiolates have not yet been reported as metal ligands in synthetic carbonate complexes.²

Our interest in multidentate ligands providing simple models of amino acid side chain donors for zinc triad coordination studies led to preparation of LH from ethylene sulfide and N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl-amine.⁶ Herein LH is reported to react with one equivalent of Cd(ClO₄)₂·6H₂O and excess Et₃N in air-saturated acetone to form [(CdL)₆(μ₃-CO₃)₂](ClO₄)₂ (1), the first synthetic carbonate complex with alkylthiolate metal ligation (Figure 1a). The isolated carbonate complex provides a rare example of a bis-capped molecular wheel (Figure 1b).⁷ Alternatively, a noncrystalline “CdLClO₄” (2) precipitate could be isolated from an EtOH/H₂O solution of these reactants in the absence of air. Proton NMR spectroscopy and ESI-MS are used to document the stability of 1 in solution and to show that both base and CO₂ are required for its formation from 2. Furthermore, this is the first study to provide experimental evidence for fixation of CO₂ by a Cd^II complex on a time scale of minutes or less.

Figure 1.

a) Synthesis of 1 showing a stick structure of its dication. Atoms of the (CdS)₆ macrocycle are highlighted as balls. b) Perspective view of carbonate binding to the (CdS)₆ macrocyclic core of 1.

Complex 1 has six crystallographically identical CdL units (Figure 2a). Each Cd^II has a bicapped tetrahedral NN′S₂O₂ metal coordination environment (Figure S1). Each ligand uses an NN′S donor set to form a pair of fused five-membered chelate rings to one metal ion and a thiolate bridge to an adjacent Cd^II. The thioether sulfur of L is clearly pendant in 1, separated from the closest Cd^II by nearly 6 Å. The oxygen donors are derived from asymmetric η²-carbonate coordination with Cd-O distance of 2.230(2) and 2.690(3) Å, well within the sum of the van der Waals radii for cadmium(II) (1.58 Å) and oxygen (1.52 Å).⁸ The asymmetric unit contains one-sixth of a water molecule and charge balance is completed with one-third of a perchlorate ion.

Figure 2.

a) ORTEP diagram of Cd^II coordination in 1 with atomic numbering scheme and thermal ellipsoids at 50% level. b) Space filling diagram of the 1²⁺ cation looking down the C₃ axis.

The novel obloid core of 1 is comprised of two trinuclear [(CdL)₃(μ₃-CO₃)] units with 3-fold molecular symmetry (Figure 1b). Two trinuclear units with opposite chirality at N2 (all R vs. all S) and joined to each other by six bridging thiolato-sulfur atoms form a puckered bis-capped 12-membered (CdS)₆ macrocycle. The distances between the symmetry-equivalent Cd atoms in individual Cd₃(μ₃-CO₃) units are 4.882(4) Å. Sulfur-bridged Cd^II are separated by 3.679(3) Å. The asymmetric η² carbonate oxygen atoms are bound 0.2628(17) Å above the coordinated plane of three alternate macrocycle Cd^II (Figure S2a). The carbonate carbon separation of 2.906(8) Å, less than the sum of the van der Waals radii (3.40 Å)⁸ (Figure S2b), suggests assembly stabilization by π-π interaction.

The bis-capped (MetalS)₆ macrocycle is unique to 1. The dodecanuclear complex ([(cyclam)Mn^IV(μ-O)₂Mn^III(H₂O)(μ-OH)]₆(mu;₃-CO₃)₂)Cl₈·24H₂O⁹ provides the only precedent for CO₂ fixation in a puckered bis-capped (MetalX)₆ macrocycle.² To the best of our knowledge, 1,3,5-triazine is the only other established three-fold symmetric templating species for a puckered (MetalX)₆ macrocycle.¹⁰ One hexanuclear dodecacarbonate of Cd^II is known,¹¹ but no bis-carbonates of just Zn^II or Cd^II.2 The five reported Cd^II carbonate complexes have varied metal coordination environments, including N₄O₂, N₂O₆, and N₄(SR₂)₂O₂ (Figure S3).¹² Features shared between the published Cd^II carbonate complexes and 1 are perchlorate counterions and ligands with multiple nitrogen donor groups. Both μ₂- and μ₃-binding modes are observed for carbonate in these Cd^II complexes, with hapticities ranging from nearly symmetrical η² to highly asymmetrical η¹. As observed for 1, each Cd center in the μ₃-carbonate complexes^12b-d has two carbonate oxygens within the sum of the van der Waals radii for Cd^II and oxygen.⁸ Although there is considerable ligand diversity amongst known metal carbonato complexes, the sulphur donors have been limited to a handful of thiolates bound to sp² hybridized carbons (Figure S4) and thioethers.²

In the context of extensive metal-thiolate coordination studies, the paucity of synthetic carbonate complexes with alkylthiolate ligands is intriguing given their importance to biological carbon dioxide chemistry. Adventitious crystallization of multinuclear carbonate complexes requires kinetically efficient CO₂ (< 0.04% of air by volume) hydroxylation and deprotonation, as well as thermodynamic redistribution of the molecular assembly to exceed the saturation limit. Structural features likely to enhance carbonate-driven dynamic combinatorial self-assembly and prevent intermolecular interactions are evident. The staggered carbonates are close enough for π-π stacking (Figure S2b) and the sterically demanding pyridyl rings circumscribing the macrocycle faces have approximate edge-to-face orientations severely limiting carbonate solvent exposure (Figure 1b). Thermodynamic and kinetics aspects of the formation of 1 were further investigated by ¹H NMR and ESI-MS.

Proton NMR comparisons of 1 and 2 in CD₃CN supported thermodynamic stability of 1 in solution (Figure 3). The ¹H NMR spectrum of 1 had a single set of ligand resonance from −40 to 80 °C and was stable to extended periods at elevated temperature. Strong geminal coupling between all the methylene and ethylene ¹H at elevated temperature is consistent with a well-defined structure. In addition, one of the methylene H_e had 9 Hz J(^111/113Cd¹H) satellites, which is to our knowledge the first observation of this interaction in a thiolate ligated Cd^II complex (Figure 3a) and comparable in magnitude to couplings observed for related complexes. In contrast, the proton NMR of 2 had three major ligand environments in a 1:1:1 ratio and two minor ligand environments in a 1:1 ratio at −40 °C suggesting at least two components (Figure 3c). A single exchange-averaged ligand environment was observed for 2 at 80 °C (Figure 3b) with geminal coupling of the methylene protons but not the ethylene protons. There was no evidence for any of the components of 2 in the ¹H NMR spectra of 1.

Figure 3.

¹H NMR spectra of a) 1 at 80 °C and 2 at b) 80 °C and c) −40 °C (CD₃CN; * CH₂ resonance of co-precipitated HNEt₃ClO₄).

Similarly, acetonitrile solutions of 1 and 2 had distinct ESI-MS speciation. The base peak for 1 was centered on m/z 1121 (Figure 4e) and corresponded to a combination of [Cd₃L₃CO₃]⁺ and [Cd₆L₆(CO₃)]²⁺ (Figure S5a). In the high mass spectrum (200–4000 m/z), a peak centered on m/z 2343 was observed for [Cd₆L₆(CO₃)₂ClO₄]⁺ (Figure S5b). A modest number of additional ions with low relative abundance were observed. None of the carbonate-containing ions were detected in the ESI-MS spectra for dilute acetonitrile solutions of 2 prepared using stringent air-free conditions (Figure 4a). The base peak for 2 was [CdL]⁺ (Figure 4a) The other major component was [Cd₂L₂ClO₄]⁺ (m/z 807). Interestingly, the ESI-MS speciation of 2 was fairly limited compared to that observed for Hg(ClO₄)₂ with N-(2-pyridylmethyl)-N-(2-(ethylthiolato)amine.¹³ The thoiether group found pendant in 1 may help limit the oligomeric speciation in the absence of carbonate.

Figure 4.

ESI-MS spectra for 0.2 mg/mL CH₃CN solutions of 2 (bottom to top) a) originally, b) bubbled with CO₂ for 5 minutes, c) 1 μL NEt₃ added, d) 1 μL NEt₃ added then bubbled with CO₂ for 5 minutes (qualitatively similar to reverse order of addition) and e) 1. Selected assignments with m/z of isotope distribution maximum are ❶ 355 [CdL]⁺, ❷ 725 [Cd₂L₂OH]⁺ ❸ 807 [Cd₂L₂(ClO₄)]⁺ ❹ unassigned (atypical isotope distribution), ❺ 1121 [Cd₃L₃(CO₃)]⁺ & [Cd₆L₆(CO₃)₂]²⁺ ❻ 1887 [Cd₅L₅(CO₃)₂]⁺.

Finally, in situ generation of 1 by addition of CO₂ and NEt₃ to 2 in acetonitrile was explored. Independently neither of these additives produced solutions with ESI-MS (Figure 4b,c) or ¹H NMR (Figure S6b,c) peaks matching those for 1. Treatment of 2 with both additives resulted in ESI-MS ion patterns (Figure 4d) and ¹H NMR (Figure S6d) spectra that were qualitatively similar to those of samples prepared directly from 1. Furthermore, this transformation was complete within minutes, providing the first documentation of rapid CO₂ fixation by a Cd^II complex.¹²

In summary, in the presence of Cd(ClO₄)₂ and base the new potentially tetradentate thiol ligand LH was found to limit complex oligomerization, support efficient carbonate formation from atmospheric CO₂ and effectively sequester carbonate by assembly of a novel puckered (CdS)₆ macrocycle. Parallels between 1 and metal binding sites of two carbonic anhydrase forms, including the recently discovered CdCA, suggest additional mixed NS donor alkylthiolate ligands may provide favorable electronics and sterics for carbonate formation and capture by Cd^II as well as possibly by Zn^II. We are actively investigating this possibility.