CO₂ uptake potential of cerium(III) triazolates and tetrazolates

Abstract

Cerous triazolates and tetrazolates were synthesised utilising Ce[N(SiMe₃)₂]₃ as a precursor. The transamination with Htz^Me,Me (tz^Me,Me = 3,5-dimethyl-1,2,4-triazolyl), Htrz^Bz (trz^Bz = 1,2,3-benzotriazolyl) and Htet^Ph (tet^Ph = 5-phenyltetrazolyl) gave insoluble, white solids, while Htz^Ph,Ph (tz^Ph,Ph = 3,5-diphenyl-1,2,4-triazolyl) led to the isolation of the monomeric complex Ce(tz^Ph,Ph)₃(thf)₃. Triazolate [Ce(tz^Me,Me)₃]_n favoured exhaustive CO₂ insertion, while the less basic triazolate [Ce(trz^Bz)₃]_n and tetrazolate [Ce(tet^Ph)₃]_n displayed only partial and no insertion, respectively. Ce(tz^Ph,Ph)₃(thf)₃ reacted with CO₂ in aromatic solvents but the product could not be identified. Addition of the azoles to Ce[N(SiMe₃)₂]₃/Me₃TACN mixtures afforded discrete complexes (Me₃TACN)Ce(az)₃(thf) (Me₃TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane; az = tz^Me,Me, trz^Bz, tet^Ph), which, however, do not insert CO₂ at ambient temperature. Treatment of (Me₃TACN)Ce(trz^Bz)₃(thf) with CO₂ resulted in the separation of Me₃TACN and precipitation of a white powder. The respective pyrazolate (Me₃TACN)Ce(pz^Me,Me)₃ (pz^Me,Me = 3,5-dimethylpyrazolyl) featuring more basic azolato ligands engaged in CO₂ insertion, forming the previously reported complex [Ce₄(pz^Me,Me·CO₂)₁₂].

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Introduction

The anthropogenic emission of greenhouse gases, especially carbon dioxide, is the main cause of global warming and the associated environmental impact and climate change.^1,2 Carbon capture and storage (CCS) and carbon capture and utilisation (CCU) are major strategies to curb the constantly increasing emissions.^3–6 Aqueous amines are currently employed on an industrial scale to chemically capture CO₂ by forming carbamates and/or ammonium bicarbonates.^7–9 This effective uptake process is based on the affinity of the nucleophilic nitrogen atoms to the electrophilic carbon centre of CO₂.^10,11 The oxophilic and Lewis acidic properties of rare-earth metals make the metal cations ideal electrophilic counterparts for the nucleophilic oxygen atoms. Such a synergistic Lewis acid base interaction was successfully targeted for the reversible insertion of CO₂ into homoleptic cerium dimethylpyrazolates, affording Ce^III₄(pz^Me,Me·CO₂)₁₂ and Ce^IV(pz^Me,Me·CO₂)₄ (pz^Me,Me = 3,5-dimethylpyrazolyl).¹² We extended this approach to the light metals magnesium, aluminium, titanium, scandium and yttrium.^13–15

The reversibility of CO₂ insertion into metal–ligand bonds can be assessed using the carboxophilicity criterion, which is strongly affected by the nucleophilicity of the ligand. To further study the impact of azolato bonding at the cerium(III) centre, we envisaged the more Brønsted-acidic triazoles and tetrazoles (pK_a(DMSO) = 19.8 (pyrazole), 14.8 (1,2,4-triazole), 13.9 (1,2,3-triazole), 11.9 (benzotriazole), 8.2 (tetrazole)).¹⁶ Particular attention should be paid to homoleptic cerium tri- and tetrazolates and their capability of taking up CO₂. Caution! Great care must be taken when handling tetrazole derivatives which are classified as energetic materials due to their high nitrogen content.¹⁷

Reports on rare-earth-metal complexes containing anionic triazolato and tetrazolato ligands are very limited. The network compounds [Eu₂(tz^H,H)₅(Htz^H,H)₂]³_∞, [Ho(tz^H,H)₃(Htz^H,H)₂]²_∞(Htz^H,H) and [Yb(tz^H,H)₃]³_∞ were obtained in a melt of parent 1,2,4-triazole (Htz^H,H) with Eu, Ho and Yb metals.^18,19 Additional examples include the monometallic lanthanum complex La(tz^Ph,Ph)₃(thf)₃ ²⁰ and several clusters with 1,2,4-triazolato ligands bearing pyridinyl or pyrazinyl substituents.^21,22 Likewise, the binary system lanthanide metal/1,2,3-triazole (Htrz^H,H) gave the frameworks [Ln(trz^H,H)₃]^2–3_∞ (Ln = La–Lu, except Eu) and [Eu₃(trz^H,H)₆(Htrz^H,H)₂]³_∞, respectively.^23–25 This chemistry was extended to 1,2,3-benzotriazole (Htrz^Bz), yielding [Ln(trz^Bz)₃(Htrz^Bz)]¹_∞ (Ln = Ce, Pr).²⁶ The monometallic rare-earth-metal complex LSc[NH(DIPP)](trz^Ph,SiMe3) (L = [MeC{N(DIPP)}CHC(Me)(NCH₂CH₂N(Me)CH₂CH₂NMe₂)]⁻, DIPP = 2,6-iPr₂C₆H₃) was synthesised by addition of benzonitrile to the nitrilimine LSc[NH(DIPP)](N=N=CSiMe₃).²⁷ Analogously, [Cp*₂Ln{CNN(SiMe₃)}]₂ (Ln = La, Sm; Cp* = pentamethylcyclopentadienyl) reacted with pivalonitrile, forming Cp*₂Ln(trz^SiMe₃,tBu)(NCtBu).²⁸ Recently, our group reported the isolation of the sandwich complexes Cp*₂Ce(tz^Me,Me)(dmap) (dmap = N,N-dimethylpyridin-4-amine), Cp*₂Ce(tz^Ph,Ph)(thf)_x (x = 0, 1) and [Cp*₂Ce(tet^Ph)]₃ (tet^Ph = 5-phenyltetrazolyl) and their reactivity towards CO₂.²⁹ Further rare-earth-metal tetrazolates include LaL₃(H₂O)₃ (L = 5-(2-pyridyl)tetrazolyl), synthesized by reacting LaCl₃, LH and Et₃N in H₂O,³⁰ and Ln₂(bt)₃·14H₂O (Ln = La, Ce; H₂bt = 5,5′-bitetrazole), obtained from a salt metathesis of Ln₂(SO₄)₃ and Ba(bt).³¹

Results and discussion

Cerium triazolates

In accordance with the synthesis of cerous pyrazolate Ce₄(pz^Me,Me)₁₂,³² our initial investigations towards a homoleptic cerium(III) triazolate were based on the protonolysis of Ce[N(SiMe₃)₂]₃ and three equiv. of 3,5-dimethyl-1,2,4-triazole (Htz^Me,Me) in benzene (Scheme 1). Due to the poor solubility of the triazole in non-donating solvents the reaction mixture was heated to 100 °C for three days. The resulting white powder 1 was amorphous and insoluble in aliphatic, aromatic or ethereal solvents including THF. This is most likely due to the formation of a polymeric species, since the 3,5-dimethylpyrazolato derivative forms tetrameric Ce₄(pz^Me,Me)₁₂ in the solid state which is only sparingly soluble in toluene.³² The ¹H NMR MAS (magic-angle spinning) spectrum of 1 shows one broad signal at −1.26 ppm for the methyl groups (Fig. S6). The carbon resonances in the ¹³C HPDEC MAS spectrum (HPDEC = high power proton decoupling) appeared at 181.1 ppm for the carbon atoms in the 3- and 5-positions and at 11.5 ppm for the methyl groups (Fig. S7).

Scheme 1 Reaction of Ce[N(SiMe₃)₂]₃ with three equiv. of Htz^Me,Me, Htrz^Bz and Htet^Ph towards 1, 3 and 4, respectively.

Exposing solid 1 to an atmosphere of 1 bar of CO₂ resulted in a mass gain of 22.3 wt% (5.06 mmol CO₂ g⁻¹) (Scheme 2). This corresponds to around 2.8 molecules of CO₂ per [Ce(tz^Me,Me)₃] unit, hence close to one CO₂ per triazolato ligand. For the implementation of solid-state NMR measurements of the amorphous [Ce(tz^Me,Me·CO₂)₃]_n (1-CO₂), the reaction was repeated employing labelled ¹³CO₂. The ¹H MAS NMR spectrum of [Ce(tz^Me,Me·¹³CO₂)₃]_n (1-¹³CO₂) exhibits two signals at 5.91 ppm and 1.26 ppm, assignable to the methyl groups of the triazolato ligand (Fig. S8). The ¹³C HPDEC MAS spectrum shows two signals (Fig. S9). The signal at 161.0 ppm is ascribed to the inserted ¹³CO₂, overlaying the carbon resonance of the triazolato ring, while the methyl groups appeared at 6.5 ppm.

Scheme 2 Reaction of solid 1, 3 and 4 with excess CO₂.

The CO₂ insertion is further corroborated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of 1-CO₂, which revealed bands at ṽ = 1753 cm⁻¹ and ṽ = 1692 cm⁻¹ assigned to the C=O stretching vibration (Fig. S31). A thermogravimetric analysis (TGA) of 1-CO₂ was performed in the range from 27 °C to 1050 °C (Fig. S39). A mass loss of 6% was observed between 27 °C and 110 °C, followed by another 11% between 110 °C and 370 °C. Then, a mass-loss event of 33% was detected between 370 °C and 550 °C. Unfortunately, none of these events can be assigned to a clear CO₂ loss, but the first two steps possibly include the loss of two molecules of CO₂ and perhaps the remaining solvent. The substantial mass loss in the third step indicates at least partial decomposition of the ligands.

We wondered whether substituting the Me groups with phenyl groups would increase the solubility of the formed complex. Accordingly, addition of three equiv. of Htz^Ph,Ph to Ce[N(SiMe₃)₂]₃ in THF gave a colourless solution (Scheme 3). Cooling a CH₃CN/THF solution to −40 °C yielded colourless crystals of Ce(tz^Ph,Ph)₃(thf)₃ (2). Performing the reaction in a non-donating solvent led to precipitation of a white solid. The ¹H NMR spectrum of 2 in C₆D₆ exhibits five signals, three for the phenyl protons and two for the coordinated THF (Fig. S10).

Scheme 3 Reaction of Ce[N(SiMe₃)₂]₃ with Htz^Ph,Ph in THF to yield Ce(tz^Ph,Ph)₃(thf)₃ (2) and reaction of 2 with CO₂ towards 2-CO₂.

The solid-state structure of 2 is shown in Fig. 1, and is isostructural to the previously reported lanthanum derivative.²⁰ The coordination number of the cerium atom is nine, involving three κ² bonded triazolato ligands. Overall, the cerium centre adopts an octahedral coordination geometry with a facial arrangement of the six ligands. The Ce–N distances range from 2.4650(17) Å to 2.5535(16) Å (avg. 2.504 Å). As expected, they are shorter than those in La(tz^Ph,Ph)₃(thf)₃ (2.498(2) Å to 2.588(2) Å).²⁰ The respective pyrazolato congener Ce(pz^Ph,Ph)₃(thf)₃ (pz^Ph,Ph = 3,5-diphenylpyrazolyl) exhibits a similar average Ce–N distance of 2.507 Å,³³ but in contrast to 2 the ligands are arranged in a meridional fashion. For further comparison, the sandwich complexes Cp*₂Ce(tz^Ph,Ph) and Cp*₂Ce(tz^Ph,Ph)(thf) display Ce–N distances in the range of 2.4611(15) Å to 2.4947(16) Å and 2.501(5) Å to 2.538(5) Å, respectively.²⁹ The Ce–O(thf) distances in 2 range from 2.5680(13) Å to 2.5773(15) Å, which is on average shorter than those in the aforementioned THF adducts.

Fig. 1 Crystal structure of Ce(tz^Ph,Ph)₃(thf)₃ (2). Ellipsoids are shown at a 50% probability level. Part of 2 is represented as a wireframe model. Hydrogen and disordered atoms are omitted for clarity. Selected interatomic distances/angles are listed in the SI.

Exposing a C₆D₆ or toluene solution of 2 to an atmosphere of 1 bar of CO₂ afforded a slow colour change from colourless to bluish and – depending on the concentration – precipitation of a grey powder (2-CO₂, Scheme 3). The ¹H NMR spectrum of 2-CO₂ displayed seven signals which could not be assigned unequivocally (Fig. S12). A variable temperature (VT) NMR study did not show any re-formation of 2 at temperatures up to 80 °C under a CO₂ atmosphere (Fig. S13 and S14). NMR monitoring did not indicate any reaction of 2 with CO₂ in THF solution (Fig. S15). Consistent with that, the addition of THF to 2-CO₂ resulted in the re-formation of 2. Applying vacuum to a toluene solution of 2-CO₂ or letting the grey powder dry under an argon atmosphere gave a white powder, which proved to be 2 by DRIFT spectroscopy. Exposing solid 2 to an atmosphere of 1 bar of CO₂ did not lead to any mass gain, which further lends evidence to our hypothesis that initial displacement of THF (in non-coordinating solvents) triggers the reaction with CO₂. Therefore, attempts to crystallise 2-CO₂ as well as the isolation of sufficient quantities for IR measurements and elemental analysis failed.

When applying a similar protonolysis protocol, the addition of three equiv. of Htrz^Bz to Ce[N(SiMe₃)₂]₃ in Et₂O gave the white, insoluble powder 3 (Scheme 1). The insolubility possibly originates from the formation of polymeric [Ce(trz^Bz)₃]_n, comparable to [Ce(trz^Bz)₃(Htrz^Bz)]¹_∞ reported by the group of Müller-Buschbaum.²⁶ When exposing solid 3 to an atmosphere of 1 bar of CO₂, a mass gain of 10.9 wt% (2.47 mmol CO₂ g⁻¹) was detected. This matches 1.4 molecules of CO₂ per unit of 3. Compared to that of [Ce(trz^Bz)₃]_n (3), the DRIFT spectrum of [Ce(trz^Bz)_3−x(trz^Bz·CO₂)_x]_n (x = 1.4, 3-CO₂) exhibits an additional band at ṽ = 1691 cm⁻¹ (Fig. S34). The TGA revealed three mass loss events of 6%, 4% and 2% between 25 °C and 205 °C, 205 °C and 310 °C and 310 °C and 395 °C, respectively (Fig. S40). These amount to 12%, which is in the same range as the mass gain observed upon CO₂ uptake. Another mass loss of 13% was detected between 395 °C and 470 °C.

Cerium tetrazolates

To investigate any CO₂ uptake capacity of cerium(III) tetrazolates, 5-phenyltetrazole was chosen as a proligand. The reaction of Ce[N(SiMe₃)₂]₃ with three equiv. of Htet^Ph in Et₂O yielded an insoluble, white powder (4, Scheme 1). In sharp contrast to the CO₂ uptake capability of the cerous triazolate complexes, solid 4 did not show any mass gain when put under a CO₂ atmosphere. The lack of CO₂ insertion was also corroborated by an unchanged IR spectrum. It can be assumed that the nucleophilicity of the tetrazolato ligand is not sufficient to favourably interact with CO₂ at ambient temperature. We would like to point out that heating 4 above 200 °C can lead to an explosive decomposition.

Azacrown-supported discrete cerous azolate complexes

To gain further insight into the cerium(III)–azolate bonding, the formation of monomeric complexes was targeted via the use of chelating donors. The neutral azacrown Me₃TACN (Me₃TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane) seemed a promising candidate since it was successfully employed for the isolation of discrete methyl, halide and alkoxide complexes (Me₃TACN)MX₃ (M = Sc, X = Me, CH₂SiMe₃, Cl, OMe; M = Y, X = Me, CH₂SiMe₃, Cl, I, OSi(OtBu)₃; M = La, X = CH₂C₆H₄-4-Me; M = Tb, X = Cl; M = Yb, X = Me, Cl; M = Lu, X = Me) and several mixed ligand complexes.^34–47 A primary test regarding the interaction of Ce[N(SiMe₃)₂]₃ with Me₃TACN was run on an NMR-scale in C₆D₆ solution, resulting in partial precipitation of a solid. The ¹H NMR spectrum of the filtered solution showed three signals at 2.19, 1.90 and 1.67 ppm with an integral ratio of 9 : 6 : 6 in addition to free Me₃TACN, Ce[N(SiMe₃)₂]₃ and unknown signals (Fig. S16). This clearly indicates an interaction between Ce[N(SiMe₃)₂]₃ and Me₃TACN. Recently, the group of Evans addressed the question of THF coordination to Ce[N(SiMe₃)₂]₃.⁴⁸ Next, one equiv. of Me₃TACN was added to Ce[N(SiMe₃)₂]₃ in THF and stirred for 15 min followed by the addition of three equiv. of Htz^Me,Me (Scheme 4). Crystallisation from THF resulted in colourless crystals of (Me₃TACN)Ce(tz^Me,Me)₃(thf) (5). The ¹H NMR spectrum shows only four signals, of which those at 6.15 ppm and 4.98 ppm can be assigned to the methyl groups of the azacrown and the triazolato ligands, respectively (Fig. S17). The NCH₂ protons of the azacrown appeared at 0.59 ppm and −3.48 ppm. It is plausible that the coordinated THF is displaced under reduced pressure, which is further supported by elemental analysis. Complex 5 crystallised in the space group P2₁/n. The 10-coordinate cerium centre adopts a distorted trigonal bipyramidal geometry (Fig. 2, left). The donor ligands Me₃TACN and THF occupy the axial positions, while the triazolato ligands occupy the equatorial ones. The cerium–nitrogen distances range from 2.5020(9) Å to 2.5985(9) Å for the triazolato ligands and from 2.7714(9) Å to 2.7975(9) Å for the azacrown. The Ce–O(thf) distance is 2.7259(8) Å. For comparison, 9-coordinate complex 2 has shorter Ce–N(triazolate) and C–O(thf) distances. The Ce–N distances of the three-dimensional framework structure of [Ce(trz^H,H)₃]³_∞ featuring 9-cooordinate cerium centres as well are noted in the range of 2.37(3) Å to 2.70(3) Å (derived from Rietveld refinement).²³

Scheme 4 Synthesis of complexes 5, 6, 7 and 8 via Me₃TACN-supported protonolysis of Ce[N(SiMe₃)₂]₃ with the respective proligands.

Fig. 2 Crystal structures of (Me₃TACN)Ce(tz^Me,Me)₃(thf) (5, left), (Me₃TACN)Ce(trz^Bz)₃(thf) (6, middle left), (Me₃TACN)Ce(tet^Ph)₃(thf) (7, middle right) and (Me₃TACN)Ce(pz^Me,Me)₃ (8, right). Ellipsoids are shown at a 50% probability level. Disordered atoms for 6 and 7 are omitted for clarity. Hydrogen atoms are omitted for clarity. Selected interatomic distances/angles are listed in the SI.

The benzotriazolate complex (Me₃TACN)Ce(trz^Bz)₃(thf) (6) can be accessed using the same protocol as applied for 5 (Scheme 4). However, the formation of large amounts of insoluble white powder was observed. Consequently, only a small number of crystals could be obtained from a THF solution. The ¹H NMR spectrum of 6 shows seven signals including the two peaks from THF (Fig. S19). The protons of the benzotriazolato ligand appear at 11.17 ppm and 8.63 ppm, respectively. Again, the NCH₃ protons appear at a higher field at 5.74 ppm in comparison with the NCH₂ protons which resonate at 0.85 ppm and −1.98 ppm. The ¹H NMR spectrum also revealed the presence of small signals of separated Me₃TACN, which increased significantly over several hours (Fig. S21), accompanied by the formation of a white solid. This indicates that 6 slowly decomposes to polymeric [Ce(trz^Bz)₃(thf)_x]_n species in THF, which also explains the low yield of the crystalline material.

Similar to the methyl-substituted triazolate derivative 5, complex 6 crystallised in the space group P2₁/c (Fig. 2, middle left). The ligand arrangement matches that of 5, with the benzotriazolato ligands coordinating in a κ²-fashion. The Ce–N(triazolato) distances are comparable to those detected for 5 and range from 2.529(2) Å to 2.587(8) Å. For further comparison, the coordination polymer [Ce(trz^Bz)₃(Htrz^Bz)]¹_∞ exhibits Ce–N distances ranging from 2.521(9) Å to 2.653(8) Å.²⁶ The Ce–N(azacrown) distances (2.733(2)–2.765(2) Å) of complex 6 are in the range of those observed for 5, but the Ce–O(thf) distance is significantly shorter (2.6693(19) Å).

In contrast to triazolate complex 6 the phenyltetrazolato congener (Me₃TACN)Ce(tet^Ph)₃(thf) (7) could be obtained in high crystalline yield when employing the same reaction conditions (Scheme 4). Its ¹H NMR spectrum shows the phenyltetrazolato protons at 10.39 ppm, 8.06 ppm and 7.83 ppm (Fig. S22). The signal of the NCH₃ protons of the azacrown ligand overlaps with the THF signal, while the NCH₂ protons appear at 1.86 ppm and 1.65 ppm. The crystal structure of 7 is shown in Fig. 2 (middle right). Once again the five ligands form a distorted trigonal bipyramid with a κ²-coordination of the azolato ligands. Interestingly, one phenyltetrazolato coordinates symmetrically via the nitrogen atoms in the 2- and 3-positions, while the other two coordinate asymmetrically involving the nitrogen atoms in the 1- and 2-positions. However, the respective Ce–N distances of 2.543(2)/2.599(2) Å and 2.5422(19) Å to 2.599(2) Å, respectively, are in the same range. When compared to the azolato metrics of the triazolate complexes 5 and 6, the shortest Ce–N distance of 7 is longer, while the longest one is similar. The Ce–N(azacrown) distances of 7 range from 2.710(2) Å to 2.722(2) Å and the Ce–O(thf) distance amounts to 2.6322(18) Å.

For further comparison the Ce–N distance in the 8-coordinate azobis(tetrazolide) carbonate complex [Ce₂(tetN=Ntet)₂(CO₃)(H₂O)₁₀]·2 H₂O was noted as 2.662(2) Å.⁴⁹ Unfortunately, none of the azacrown-supported complexes 5, 6 and 7 engages in CO₂ insertion at 1 bar pressure.

We also wondered whether the azacrown would promote the formation of a monomeric 3,5-dimethylpyrazolate complex.¹² Therefore, by applying the same synthesis protocol, three equiv. of Hpz^Me,Me were added to a preformed mixture of Ce[N(SiMe₃)₂]₃/Me₃TACN (Scheme 4). Crystallisation from THF yielded crystals of the composition (Me₃TACN)Ce(pz^Me,Me)₃ (8). Differing from the aforementioned complexes, THF did not coordinate to the metal centre. The ¹H NMR spectrum in C₆D₆ exhibited the expected set of five signals (Fig. S24). The resonances at 9.16 ppm and 3.78 ppm displayed an integral ratio of 3 : 18, consistent with the ring protons and methyl groups of the pz^Me,Me moieties. The NCH₂ groups appeared at 2.02 ppm and 0.75 ppm, while the NCH₃ groups of the azacrown were detected at 1.58 ppm. Noteworthily, addition of Me₃TACN to a solution of tetrameric [Ce₄(pz^Me,Me)₁₂] in THF-d₈ also gave access to monomeric complex 8 (Fig. S27). Complex 8 crystallised in the trigonal space group R3 featuring a 9-coordinate cerium centre (Fig. 2, right). The Ce–N(pyrazolato) distances were noted at 2.4675(18) Å and 2.5379(19) Å, which are on average shorter than those in 10-coordinate 5. For further comparison, the donor-free [Ce₄(pz^Me,Me)₁₂], featuring two 10-, one 9- and one 8-coordinate cerium centre, exhibits Ce–N(pyrazolato) distances in the range from 2.368(4) Å to 3.077(4) Å.³² The Ce–N(azacrown) distances of 8 are 2.751(2) Å.

Reacting pyrazolate complex 8 with CO₂ in C₆D₆ or toluene-d₈ resulted in the disappearance of the NMR signals of the starting material (Fig. S29). The ¹H NMR spectra revealed the release of Me₃TACN as well as peaks assigned to [Ce₄(pz^Me,Me·CO₂)₁₂].¹² Consequently, there is exhaustive insertion of CO₂ into 8, affording the same insertion product as for the homoleptic pyrazolate complex [Ce₄(pz^Me,Me)₁₂]. Moreover, the CO₂ insertion seems to outperform azacrown coordination at cerium pyrazolate complexes.

Electronic absorption spectra

In THF solution, the triazolate complex 2 shows two absorption maxima at 236 nm and 266 nm (Fig. S41). The azacrown-supported triazolato and pyrazolato complexes 5 and 8 feature growing absorption until the cutoff wavelength of the solvent (Fig. S42 and S44). The global absorption maximum of 7 is detected at 243 nm (Fig. S43).

Conclusions

Homoleptic cerium(III) tri- and tetrazolates are easily accessible via protonolysis of Ce[N(SiMe₃)₂]₃ with the respective azoles, affording white powders insoluble in common organic solvents. The solids can chemisorb CO₂ at atmospheric pressure depending on the basicity of the azolato ligand. The 1,2,4-triazolate [Ce(tz^Me,Me)₃]_n showed near exhaustive CO₂ insertion, while the 1,2,3-triazolate [Ce(trz^Bz)₃]_n inserted 1.4 molecules of CO₂ per monomeric unit. Solid tetrazolate [Ce(tet^Ph)₃]_n featuring the least basic azolato ligand did not react with CO₂ under the prevailing conditions. The Htz^Ph,Ph proligand also gave access to soluble triazolate complex Ce(tz^Ph,Ph)₃(thf)₃, which revealed a CO₂ reaction in aromatic solvents.

Utilising the tridentate azacrown Me₃TACN in the protonolysis mixtures favoured the isolation of the monomeric complexes (Me₃TACN)Ce(tz^Me,Me)₃(thf), (Me₃TACN)Ce(trz^Bz)₃(thf) and (Me₃TACN)Ce(tet^Ph)₃(thf). While none of these complexes showed CO₂ insertion under ambient conditions, the corresponding pyrazolato complex, (Me₃TACN)Ce(pz^Me,Me)₃, did. Treatment of this complex with CO₂ resulted in the loss of the Me₃TACN donor ligand and the formation of tetrameric [Ce₄(pz^Me,Me·CO₂)₁₂]. This behaviour corroborates our hypothesis that the most basic cerous azolates favour CO₂ insertion into the Ce–N(azolato) bond, even in the presence of a chelating donor such as azacrowns. The respective complexes with less nucleophilic tri- and tetrazolato ligands do not react with CO₂ under ambient conditions. The present study might also give some valuable molecular insights into rare-earth-metal–azolate framework structures which are currently employed for CCS and CCU applications.^50–53

Experimental

General considerations

Caution: 5-Phenyltetrazole is an energetic compound with sensitivity towards heat. Although at temperatures below 200 °C we did not face any problems, 5-phenyltetrazole and derivatives thereof should be handled with care, especially when larger quantities are required. All reactions were performed under an inert atmosphere (Ar) by using a glovebox (MBraun UNIlab^pro; <0.1 ppm O₂, <0.1 ppm H₂O) or according to standard Schlenk techniques in oven-dried glassware. Unless otherwise stated, the solvents were purified with Grubbs-type columns (MBraun SPS, solvent purification system) and stored in a glovebox. Benzene was dried by refluxing with Na followed by distillation. CO₂ was purchased from Westfalen AG. Anhydrous cerium(III) chloride (99.5%) was purchased from abcr and activated via Soxhlet extraction with THF, giving CeCl₃(thf). C₆D₆ and THF-d₈ were purchased from Sigma Aldrich and dried over NaK alloy. C₆D₆ and THF-d₈ were filtered and stored in a glovebox. THF-d₈ was also stored over molecular sieves (3 Å). 3,5-Dimethyltriazole and 5-phenyltetrazole were purchased from TCI. Benzotriazole and potassium bis(trimethylsilyl)amide were purchased from Sigma Aldrich. 1,4,7-Trimethyl-1,4,7-triazacyclononane was purchased from abcr. Ce[N(SiMe₃)₂]₃ was synthesised according to a modified published procedure utilising KN(SiMe₃)₂.⁵⁴

NMR spectra were recorded on a Bruker AVII+400 (¹H: 400.11 MHz, ¹³C: 100.61 MHz) or a Bruker AVII+500 (¹H: 500.13 MHz) at 26 °C using J. Young-valved NMR tubes. Solid-state NMR spectra were recorded on a Bruker AVIIIHD-300WB (¹H: 300.13 MHz, ¹³C: 75.47 MHz) using a ZrO₂ rotor (4 mm diameter). ¹³C MAS spectra were recorded utilizing high power proton decoupling (HPDEC). ¹H and ¹³C NMR chemical shifts are referenced to a solvent resonance and reported in parts per million (ppm) relative to tetramethylsilane. Analysis of the NMR spectra was performed with TopSpin 3.6.0 and ACD/NMR Processor Academic Edition (product version: 12.01). Multiplicities of signals are given as singlet (s), doublet (d), triplet (t) and multiplet (m). DRIFT spectra were recorded on a Bruker INVENIO R spectrometer and converted using the Kubelka–Munk refinement. The samples were mixed with KBr and measured in a cell with KBr windows. Elemental analysis (C, H, N) was performed on a Elementar vario MICRO cube. Absorption measurements were performed on a PerkinElmer Lambda 35 spectrometer. Crystals for X-ray crystallography were handpicked in a glovebox, coated with Parabar 10312 and stored on microscope slides. Crystallographic data were collected on a Bruker APEX II DUO diffractometer by using QUAZAR optics and Mo K_α radiation (λ = 0.71073 Å). The data collection strategy was determined using COSMO⁵⁵ employing φ and ω scans. Raw data were processed using APEX3 ⁵⁶ and SAINT.⁵⁷ Corrections for absorption effects were applied using SADABS.⁵⁸ The structures were solved using direct methods and refined against F² using SHELXTL^59,60 and Shelxl.⁶¹ Disorder models were calculated using DSR,⁶¹ a program included in ShelXle.⁶² All graphics were produced employing Mercury⁶³ and POV-Ray.⁶⁴

Syntheses

[Ce(tz^Me,Me)₃]_n (1). Ce[N(SiMe₃)₂]₃ (621 mg, 1.00 mmol, 1.00 equiv.) in benzene (10 mL) was added to Htz^Me,Me (291 mg, 3.00 mmol, 3.00 equiv.) in benzene (5 mL) in a pressure tube and heated to 100 °C for 3 days, resulting in a white suspension. The mixture was centrifuged and washed with benzene (2 × 5 mL) and the residual solvent was removed under reduced pressure. An off-white powder was obtained (381 mg, 0.889 mmol, 89%). ¹H MAS NMR (300.1 MHz, 26 °C, MAS at 8 kHz): δ = −1.26 (tz-CH₃) ppm. ¹³C MAS NMR (75.5 MHz, 24 °C, MAS at 8 kHz): δ = 181.1 (tz-NC), 11.5 (tz-CH₃) ppm. DRIFT: ṽ = 2968 (w), 2931 (w), 1513 (s), 1479 (s), 1412 (vs), 1365 (m), 1308 (m), 1042 (vw), 1007 (vw), 976 (w), 747 (m), 699 (w) 617 (w), 454 (vw) cm⁻¹. Elemental analysis (%) calcd for C₁₂H₁₈CeN₉ (428.46 g mol⁻¹): C 33.64, H 4.23, N 29.42; found: C 34.95, H 4.26, N 28.13. Although the results are outside the range viewed as establishing analytical purity (C +1.31%, N −1.29%), they are provided to illustrate the best values obtained to date.

[Ce(tz^Me,Me·CO₂)₃]_n (1-CO₂). A Schlenk tube was loaded with solid [Ce(tz^Me,Me)₃]_n (107 mg, 250 µmol), and the atmosphere was exchanged with 1 bar CO₂, which was maintained during the reaction time. After 18 h 1-CO₂ could be collected as a white powder (138 mg). ¹H MAS NMR (300.1 MHz, 24 °C, MAS at 8 kHz): δ = 5.91 (tz-CH₃), 1.26 (tz-CH₃) ppm. ¹³C MAS NMR (75.5 MHz, 25 °C, MAS at 8 kHz): δ = 161.0 (NCO₂, tz-NC), 6.5 (tz-CH₃) ppm. DRIFT: ṽ = 2970 (w), 2932 (w), 1753 (m), 1692 (s), 1631 (vw), 1586 (w), 1513 (s), 1479 (s), 1444 (s), 1412 (vs), 1365 (s), 1308 (vs), 1147 (w), 1044 (w), 1008 (m), 976 (m), 841 (w), 790 (w), 766 (m), 748 (m), 699 (m), 617 (m), 500 (vw), 452 (w) cm⁻¹. Elemental analysis calcd for C_14.8H₁₈CeN₉O_5.6 (551.68 g mol⁻¹): C 32.22, H 3.29, N 22.85; found: C 33.62, H 4.21, N 24.72. The results are outside the range viewed as establishing analytical purity (C +1.40%, H +0.92%, N +1.87%). This could be due to residual solvent and/or loss of CO₂ under an argon atmosphere.

Ce(tz^Ph,Ph)₃(thf)₃ (2). Ce[N(SiMe₃)₂]₃ (124 mg, 200 µmol, 1.00 equiv.) in THF (3 mL) was added dropwise to Htz^Ph,Ph (133 mg, 600 µmol, 3.00 equiv.) in THF (2 mL) and stirred for 1 h. Then all volatile compounds were removed, giving an off-white powder. Crystallisation from n-hexane/THF at −40 °C yielded colourless crystals of 2 (173 mg, 170 µmol, 85%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 12.32 (12H, s, ortho-CH), 8.16 (12H, s, meta-CH), 7.84 (6H, s, para-CH), −1.32 (12H, s, thf-CH₂), −2.15 (12H, s, thf-OCH₂) ppm. ¹³C{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 183.7 (tz-NC), 137.3 (ipso-C), 130.6 (meta-C), 129.3 (para-C), 58.1 (thf-OCH₂), 22.0 (thf-CH₂) ppm. The signal of the ortho carbon could be detected, possibly due to overlap with the solvent signal. DRIFT: ṽ = 3062 (m), 2982 (m), 2888 (m), 1605 (w), 1467 (vs), 1426 (vs), 1402 (s), 1285 (m), 1175 (m), 1069 (m), 1025 (s), 992 (m), 922 (m), 869 (s), 790 (m), 730 (vs), 696 (vs), 553 (vw), 490 (vw), 422 (m) cm⁻¹. Elemental analysis (%) calcd for C₅₄H₅₄CeN₉O₃ (1017.20 g mol⁻¹): C 63.76, H 5.35, N 12.39; found: C 63.62, H 5.55, N 11.85. Although the results are outside the range viewed as establishing analytical purity (N −0.54%), they are provided to illustrate the best values obtained to date.

[Ce(trz^Bz)₃]_n (3). Htrz^Bz (179 mg, 1.50 mmol, 3.00 equiv.) in Et₂O (7 mL) was added dropwise to Ce[N(SiMe₃)₂]₃ (311 mg, 0.500 mmol, 1.00 equiv.) in Et₂O (3 mL), resulting in the precipitation of a white solid. The resulting slurry was stirred at 40 °C for 1 d. The reaction mixture was centrifuged and washed with Et₂O (5 mL) and the residual solvent was removed under reduced pressure. An off-white powder was obtained (228 mg, 461 µmol, 92%). DRIFT: ṽ = 3052 (w), 1574 (w), 1485 (w), 1443 (m), 1390 (vw), 1283 (s), 1259 (m), 1174 (m), 1149 (m), 989 (w), 939 (vw), 914 (m), 848 (w), 782 (vs), 745 (vs), 694 (m), 633 (s), 551 (s), 433 (w) cm⁻¹. Elemental analysis (%) calcd for C₁₈H₁₂CeN₉ (494.47 g mol⁻¹): C 43.72, H 2.45, N 25.49; found: C 44.72, H 2.48, N 25.09. Although the results are outside the range viewed as establishing analytical purity (C +1.00%), they are provided to illustrate the best values obtained to date.

[Ce(trz^Bz·CO₂)₃]_n (3-CO₂). A Schlenk tube was loaded with solid 3 (124 mg, 250 µmol), and the atmosphere was exchanged with 1 bar CO₂, which was maintained during the reaction time. After 18 h 3-CO₂ could be collected as a white powder (139 mg). DRIFT: ṽ = 3052 (w), 1691 (w), 1574 (w), 1486 (m), 1444 (m), 1390 (w), 1284 (s), 1145 (s), 989 (w), 914 (s), 845 (w), 782 (vs), 746 (vs), 694 (m), 633 (s), 551 (s), 434 (m) cm⁻¹. Elemental analysis calcd for C_19.4H₁₂CeN₉O_2.8 (556.09 g mol⁻¹): C 41.90, H 2.18, N 22.67; found: C 43.81, H 2.64, N 23.94. Although the results are outside the range viewed as establishing analytical purity (C +1.91%, N +1.27%), they are provided to illustrate the best values obtained to date.

[Ce(tet^Ph)₃]_n (4). Htet^Ph (219 mg, 1.50 mmol, 3.00 equiv.) in Et₂O (7 mL) was added dropwise to Ce[N(SiMe₃)₂]₃ (311 mg, 0.500 mmol, 1.00 equiv.) in Et₂O (3 mL). The reaction mixture was stirred for 1 d, resulting in a white suspension. The mixture was centrifuged and washed with Et₂O (5 mL) and the residual solvent was removed under reduced pressure. An off-white powder was obtained (271 mg, 0.471 mmol, 94%). DRIFT: ṽ = 3065 (w), 1525 (w), 1449 (vs), 1358 (m), 1281 (w), 1161 (w), 1143 (w), 1073 (w), 1010 (m), 922 (w), 846 (vw), 786 (m), 730 (vs), 693 (vs), 509 (w), 462 (w) cm⁻¹. Elemental analysis (%) calcd for C₂₁H₁₅CeN₁₂ (575.55 g mol⁻¹): C 43.82, H 2.63, N 29.20; found: C 43.92, H 2.71, N 28.59. Although the results are outside the range viewed as establishing analytical purity (N −0.61%), they are provided to illustrate the best values obtained to date.

Reaction of [Ce(tet^Ph)₃]_n with CO₂. A Schlenk tube was loaded with solid 4, and the atmosphere was exchanged with 1 bar CO₂, which was maintained during the reaction time. After 18 h a white powder could be collected, which proved to be 4.

Reaction of Ce[N(SiMe₃)₂]₃ with Me₃TACN. Ce[N(SiMe₃)₂]₃ (18.6 mg, 30.0 µmol, 1.00 equiv.) in C₆D₆ (0.3 mL) and Me₃TACN (5.1 mg, 30 µmol, 1.0 equiv.) in C₆D₆ (0.2 mL) were placed in a J. Young NMR tube, resulting in the slow precipitation of a solid. The solution was filtered before NMR measurement (Fig. S16).

(Me₃TACN)Ce(tz^Me,Me)₃(thf) (5). Me₃TACN (34.3 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) was added dropwise to Ce[N(SiMe₃)₂]₃ (124 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) and stirred for 15 min. Then, Htz^Me,Me (58.3 mg, 600 µmol, 3.00 equiv.) in THF (4 mL) was added dropwise whereby the solution became slightly cloudy. After stirring the mixture for 1 h, all volatile compounds were removed, resulting in a white solid. Crystallisation from THF at −40 °C yielded colourless crystals of 5 (75.1 mg, 125 µmol, 63% yield without coordinated THF). ¹H NMR (THF-d₈, 400.1 MHz, 26 °C): δ = 6.15 (9H, s, NCH₃), 4.98 (18H, s, tz-CH₃), 0.59 (6H, s, NCH₂), −3.48 (6H, s, NCH₂) ppm. ¹³C{¹H} NMR (THF-d₈, 100.6 MHz, 26 °C): δ = 176.4 (tz-NC), 42.5 (NCH₃), 38.9 (NCH₂), 17.9 (tz-CH₃) ppm. DRIFT: ṽ = 2982 (m), 2922 (m), 2862 (m), 2817 (m), 1483 (s), 1461 (vs), 1416 (vs), 1365 (m), 1301 (s), 1207 (w), 1150 (w), 1076 (s), 1043 (w), 1008 (s), 889 (w), 766 (m), 745 (s), 619 (m), 576 (w), 423 (m) cm⁻¹. Elemental analysis (%) calcd for C₂₁H₃₉CeN₁₂ (599.74 g mol⁻¹) (without THF): C 42.06, H 6.55, N 28.03; found: C 42.18, H 6.35, N 28.05.

(Me₃TACN)Ce(trz^Bz)₃(thf) (6). Me₃TACN (34.3 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) was added dropwise to Ce[N(SiMe₃)₂]₃ (124 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) and stirred for 15 min. Then, Htrz^Bz (71.5 mg, 600 µmol, 3.00 equiv.) in THF (4 mL) was added dropwise. After stirring the mixture for 1 h, all volatile compounds were removed, resulting in a white solid. Crystallisation from THF at −40 °C yielded colourless crystals of 6 (5.8 mg, 8.7 µmol, 4%). ¹H NMR (THF-d₈, 400.1 MHz, 26 °C): δ = 11.17 (6H, s, trz-CH), 8.63 (6H, s, trz-CH), 5.74 (9H, s, NCH₃), 3.62 (4H, m, thf-OCH₂), 1.78 (4H, m, thf-CH₂), 0.85 (6H, s, NCH₂), −1.98 (6H, s, NCH₂) ppm. ¹³C{¹H} NMR (THF-d₈, 100.6 MHz, 26 °C): δ = 163.7 (trz-NC), 125.4 (trz-C), 121.9 (trz-C), 68.0 (thf-OCH₂), 43.8 (NCH₃), 40.7 (NCH₂), 26.2 (thf-CH₂) ppm.

(Me₃TACN)Ce(tet^Ph)₃(thf) (7). Me₃TACN (34.3 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) was added dropwise to Ce[N(SiMe₃)₂]₃ (124 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) and stirred for 15 min. Then, Htet^Ph (87.7 mg, 600 µmol, 3.00 equiv.) in THF (4 mL) was added dropwise whereby the solution became slightly cloudy. After stirring the mixture for 1 h, all volatile compounds were removed, resulting in a white solid. Crystallisation from THF at −40 °C yielded colourless crystals of 7 (145 mg, 177 µmol, 88%). ¹H NMR (THF-d₈, 400.1 MHz, 26 °C): δ = 10.39 (6H, d, ortho-CH), 8.06 (6H, t, meta-CH), 7.83 (3H, t, para-CH), 3.62 (13H, m, NCH₃ and thf-OCH₂), 1.86 (6H, s, NCH₂), 1.78 (4H, m, thf-CH₂), 1.65 (6H, s, NCH₂) ppm. ¹³C{¹H} NMR (THF-d₈, 100.6 MHz, 26 °C): δ = 174.8 (tet-NC), 132.4 (ipso-C), 130.3 (meta-C), 130.0 (para-C), 128.9 (ortho-C), 68.0 (thf-OCH₂), 45.2 (NCH₂), 42.8 (NCH₃), 26.2 (thf-CH₂) ppm. DRIFT: ṽ = 3063 (w), 2981 (m), 2869 (m), 1494 (w), 1444 (s), 1358 (m), 1279 (w), 1204 (w), 1151 (m), 1114 (m), 1073 (s), 1029 (m), 1005 (s), 924 (w), 883 (m), 790 (m), 769 (m), 735 (vs), 698 (s), 579 (w), 511 (w), 461 (w), 425 (w) cm⁻¹. Elemental analysis (%) calcd for C₂₅H₄₇CeN₁₂O (671.85 g mol⁻¹): C 44.69, H 7.05, N 25.02; found: C 49.78, H 5.29, N 24.43. Although the results are outside the range viewed as establishing analytical purity (C +5.09%, H −1.76%, N −0.59%), they are provided to illustrate the best values obtained to date. This could be due to incomplete combustion.

(Me₃TACN)Ce(pz^Me,Me)₃ (8). Me₃TACN (34.3 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) was added dropwise to Ce[N(SiMe₃)₂]₃ (124 mg, 200 µmol, 1.00 equiv.) in THF (1 mL) and stirred for 15 min. Then, Hpz^Me,Me (57.7 mg, 600 µmol, 3.00 equiv.) in THF (4 mL) was added dropwise and the mixture was stirred for 2 h. After removal of all volatile compounds a white solid was obtained. Crystallisation from THF at −40 °C yielded colourless crystals of 8 (56.8 mg, 95.2 µmol, 48%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 9.16 (3H, s, pz-CH), 3.78 (18H, s, pz-CH₃), 2.02 (6H, s, NCH₂), 1.58 (9H, s, NCH₃), 0.75 (6H, s, NCH₂) ppm. ¹H NMR (toluene-d₈, 400.1 MHz, 26 °C): δ = 9.20 (3H, s, pz-CH), 3.79 (18H, s, pz-CH₃), 2.35 (9H, s, NCH₃), 1.76 (6H, s, NCH₂), −0.23 (6H, s, NCH₂) ppm. ¹³C{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 161.9 (pz-NC), 119.7 (pz-NCCH), 46.6 (NCH₂), 34.0 (NCH₃), 16.6 (pz-CH₃) ppm. DRIFT: ṽ = 3092 (m), 2969 (s), 2914 (vs), 2872 (s), 2821 (s), 2720 (w), 1512 (s), 1463 (s), 1415 (s), 1368 (m), 1297 (m), 1208 (w), 1152 (w), 1127 (w), 1074 (m), 1043 (m), 1009 (vs), 965 (w), 888 (w), 768 (s), 731 (m), 653 (vw), 571 (w), 499 (vw), 423 (w) cm⁻¹. Elemental analysis (%) calcd for C₂₄H₄₂CeN₉ (596.78 g mol⁻¹): C 48.30, H 7.09, N 21.12; found: C 48.51, H 6.96, N 21.22.

Author contributions

J. R.: synthesis and characterisation of compounds, writing and editing the original draft; C. M.-M.: crystallography, editing the original draft; R. A.: conceptualisation, supervision, writing and editing the original draft, project administration, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data supporting this article have been uploaded as part of the supplementary information (SI). Supplementary information: experimental, spectroscopic and structural data. See DOI: https://doi.org/10.1039/d6dt00453a.

CCDC 2530846–2530850 contain the supplementary crystallographic data for this paper.^65a–e

Acknowledgements

We are grateful to the VECTOR Foundation (grant P2021-0099) for generous support. We thank Dr Markus Ströbele for performing the TGA experiments, Dr Klaus Eichele for the solid-state NMR measurements and Philipp Wetzel for the VT-NMR experiments.

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