Effect of Substituents of Cerium Pyrazolates and Pyrrolates on Carbon Dioxide Activation.

Homoleptic ceric pyrazolates (pz) Ce(RR'pz)4 (R = R' = tBu; R = R' = Ph; R = tBu, R' = Me) were synthesized by the protonolysis reaction of Ce[N(SiHMe2)2]4 with the corresponding pyrazole derivative. The resulting complexes were investigated in their reactivity toward CO2, revealing a significant influence of the bulkiness of the substituents on the pyrazolato ligands. The efficiency of the CO2 insertion was found to increase in the order of tBu2pz < Ph2pz < tBuMepz < Me2pz. For comparison, the pyrrole-based ate complexes [Ce2(pyr)6(µ-pyr)2(thf)2][Li(thf)4]2 (pyr = pyrrolato) and [Ce(cbz)4(thf)2][Li(thf)4] (cbz = carbazolato) were obtained via protonolysis of the cerous ate complex Ce[N(SiHMe2)2]4Li(thf) with pyrrole and carbazole, respectively. Treatment of the pyrrolate/carbazolate complexes with CO2 seemed promising, but any reversibility could not be observed.

1. Introduction

Rare-earth–metal complexes are capable of efficiently activating n class="Chemical">carbonylic compounds including carbon dioxide [1,2,3,4,5,6]. Due to its environmental impact, atmospheric CO2 management and, in particular, sustainable solutions for CO2 emission control evolved as a top-priority issue in academic and industrial research [7,8,9,10,11]. On the one hand, this can be realized by capturing and storing carbon dioxide with tailor-made surface-reactive materials [12,13,14,15,16]. On the other hand, the use of CO2 as a cheap, abundant, and nontoxic C1 building block in the synthesis of higher-value chemicals is a main goal in sustainable chemistry [17,18,19,20,21,22,23]. For example, rare-earth metals have successfully been studied as catalysts for the copolymerization of carbon dioxide and epoxides to yield polycarbonates [24,25,26,27,28,29,30]. Various rare-earth-metal-based (pre)catalysts are known to promote the catalytic cycloaddition of carbon dioxide and epoxide-producing cyclic carbonates, which then, in case of propylene carbonate, can be used as electrolyte solvent in lithium-ion batteries [30,31,32,33,34,35,36,37]. However, these complexes often lack the catalytic activity and superb performance of zinc or cobalt-based systems [4].

We have recently described the application of homoleptic ceric pyrazolate [n class="Chemical">Ce(Me2pz)4]2 in the catalytic cycloaddition of carbon dioxide and epoxides as well as the reversible capture of CO2 [38,39]. To study the scope and efficiency of such pyrazolate-promoted CO2 insertion reactions, we extended our study with a broader comparison to differently substituted pyrazole derivatives as ligands for cerium(IV). Since we hypothesized that the basicity of the ligand plays a crucial role in any reversible CO2 uptake, we also envisaged different N-proligands. Pyrroles feature a mono-aza five-membered ring, exhibiting a pKa value that is larger than that of pyrazoles, but close to silylamines (Figure 1) [40]. Metal silylamides, however, were shown to engage in a cascade of reactions with CO2, ultimately affording metal siloxides [41,42,43]. Whereas pyrrole-derived pincer-type ligands are quite popular in rare-earth–metal coordination chemistry [44,45,46,47,48,49,50,51], examples of pure pyrrolyl ligands have remained scarce [52,53,54,55]. Likewise, rare-earth–metal complexes bearing a carbazolyl ligand have been reported [56,57,58,59,60,61,62]. As a pyrrole derivative, carbazole exhibits a pKa value that matches that of pyrazole (Figure 1).

Figure 1

pKa values of the different N-proligands [40,63,64]. a) Determined in DMSO, b) determined in THF.

2. Results and Discussion

2.1. Homoleptic Ceric Pyrazolates

Homoleptic cerium di-tert-butyl pyrazolate Ce(tBu2pz)4 (1) was synthesized by treatment of n class="Chemical">Ce[N(SiHMe2)2]4 with four equivalents of tBu2pzH as previously described (Scheme 1) [65]. Likewise, Ce(Ph2pz)4 (2) and Ce(tBuMepz)4 (3) were synthesized using the corresponding pyrazole (Scheme 1). The 1H NMR spectrum of 2 shows a singlet for the proton in the pyrazolato backbone at 7.11 ppm and two multiplets at 6.91 and 7.78 ppm for the aromatic protons (Figure S1, Supporting Information). In contrast to the complexes bearing symmetric pyrazolato ligands, compound 3 with the asymmetric 3-tert-butyl-5-methyl pyrazolato ligand could not be obtained as a crystalline material, but as a dark red sticky solid upon removing the volatiles in vacuo. The general composition of [Ce(tBuMepz)4] was confirmed by 1H NMR spectroscopy showing singlets at 1.24 ppm for the tBu groups, at 2.22 ppm for the methyl groups, and at 6.15 ppm for the C−H proton of the five-membered pyrazole ring (Figure S2, Supporting Information). However, elemental analysis displayed some extent of impurification, indicated by an increased carbon value most likely stemming from retained solvent.

Scheme 1

Synthesis of ceric pyrazolates 1 to 3.

Crystals of complex 2 suitable for an X-ray diffraction (XRD) study could be obtained from a concentrated solution in n class="Chemical">toluene. The crystal structure revealed an eight-coordinate cerium center surrounded by four η2-coordinated diphenylpyrazolato moieties (Figure 2). The Ce1–N distances (2.3381(16) to 2.3790(16) Å) are comparable to other terminal CeIV–N(pz) bonds (1: 2.322(4) to 2.365(4) Å; Ce(Me2pz)4: 2.319(3) to 2.384(2) Å) [65].

Figure 2

Crystal structure of Ce(Ph2pz)4 (2). Ellipsoids are shown at the 50% probability level. Hydrogen atoms and lattice toluene are omitted for clarity. Selected interatomic distances (Å) and angles (°): Ce1−N1 2.3790(16), Ce1−N2 2.3381(16), Ce1−N3 2.3409(15), Ce1−N4 2.3575(16); N1−Ce1−N3 113.41(6), N1−Ce1−N4 118.73(5), N2−Ce1−N3 96.05(6), and N2−Ce1−N4 87.76(5).

2.2. Cerium Pyrrolates and Carbazolates

As the synthesis pathway via protonolysis reaction of Ce[N(SiHMe2)2]4 with n class="Chemical">pyrazole emerged as a feasible route for the synthesis of homoleptic ceric pyrazolates, the envisaged pyrrolates were accessed accordingly. Unexpectedly, treatment of the silylamide with four equivalents of pyrrole did not yield any reaction. Therefore, we reacted the cerous ate complex Ce[N(SiHMe2)2]4Li(thf) [66] with four equivalents of pyrrole (Hpyr) and carbazole (Hcbz), yielding the complexes [Ce2(pyr)6(µ-pyr)2(thf)2][Li(thf)4]2 (4) and [Ce(cbz)4(thf)2][Li(thf)4] (5), respectively (Scheme 2). The 1H NMR spectrum of 4 shows two broadened singlets for the pyrrolato protons at 4.23 and 7.39 ppm (Figure S3, Supporting Information). For complex 5, various broadened proton signals could be detected as well but due to the paramagnetic CeIII center, assignment of the signals was inconclusive (Figure S5, Supporting Information). The persistence of ion-separated intermolecular ate complexes in THF-d8 was evidenced by 7Li NMR spectroscopy (4: δLi = 2.0 ppm; 5: δLi = −0.3 ppm; (Figures S4 and S6, Supporting Information)) [67].

Scheme 2

Synthesis of cerous pyrrolate and carbazolate ate complexes 4 and 5.

Crystallization from a concentrated THF solution gave 4 as colorless crystals. The crystal structure of 4 revealed a separated ion pair featuring a n class="Disease">dicerium dianionic entity (Figure 3). Each cerium center is eight-coordinated and surrounded by three terminal pyrrolato moieties (Ce1–N 2.475(2) to 2.515(2) Å), two bridging pyrrolato ligands connecting the cerium centers in an η1 (Ce1–N4′ 2.622(2) Å) and η5 fashion (Ce1–Ct 2.575(16) Å), and one THF donor molecule. The Ce–N distances are slightly elongated compared with other CeIII ate complexes like [Ce{N(SiHMe2)2}4][Li(do)] (do, x = py, 4; tmeda, 2; 12-crown-4, 1 and thf, 4; Ce–N 2.377(6) to 2.438(6) Å), KCe[N(SiHMe2)2]4 (2.3820(12) to 2.4379(12) Å), and [Ce{N(SiMe3)2}4][Na(thf)4(Et2O)] (Ce–N 2.434(6) to 2.448(6) Å), due to the higher coordination number CN of 4 (CN 8 vs. 4) [67,68,69]. The σ,π-bridging motif of the pyrrolato ligands has been detected previously, for example, in complexes [Me2In(µ-pyr)], [Me2Al2(µ-Cl)](µ-Me4pyr)Li, [Mg{µ-C4H2N(2-CH2NHtBu)}{N(SiMe3)2}]2, and (AlMe3)(µ-C4H3N)[2-CH2NH(tBu)Li(tmeda)] [70,71,72,73].

Figure 3

Crystal structure of [Ce2(pyr)6(µ-pyr)2(thf)2][Li(thf)4]2 (4). Ellipsoids are shown at the 50% probability level. Hydrogen atoms and the second [Li(thf)4]+ counter ion are omitted for clarity. Selected interatomic distances (Å) and angles (°): Ce1−N1 2.512(2), Ce1−N2 2.475(2), Ce1−N3 2.515(2), Ce1−N4′ 2.622(2), Ce1−Ct1 2.575(16), Ce1−O1 2.5138(17); N1−Ce1−N2 97.21(7), N1−Ce1-N3 153.53(8), N1−Ce1−N4 86.15(7), N1−Ce1−O1 78.80(7), and N4−Ce1′−Ct1′ 97.8(5).

Complex 5 was recrystallized from a 1:1 mixture of THF and n class="Chemical">Et2O to yield off-white crystals suitable for XRD analysis. The crystal structure of 5 shows a six-coordinate cerium center surrounded by four carbazolyl ligands, two THF donor molecules and a [Li(thf)4]+ counter ion (Figure 4). As seen before, the Ce1–N distances (2.480(4) to 2.531(4) Å) are slightly elongated compared with other separated ion-pair type ate complexes due to the higher coordination number of 6 compared with 4.

Figure 4

Crystal structure of [Ce(cbz)4(thf)2][Li(thf)4] (5). Ellipsoids are shown at the 50% probability level. Hydrogen atoms and disordered atoms are omitted for clarity. Selected interatomic distances (Å) and angles (°): Ce1−N1 2.531(4), Ce1−N2 2.480(4), Ce1−N3 2.497(4), Ce1−N4 2.484(4), Ce1−O1 2.577(4), Ce1−O2 2.583(3); N1−Ce1−N2 128.14(14), N1−Ce1−N3 90.91(13), N1−Ce1−N4 92.69(13), and O1−Ce1−O2 129.06(11).

Targeted oxidations of the ate complexes 4 and 5 with p-benzoquinone led to an immen class="Disease">diate color change to dark green and dark purple, respectively. Although indicative of a redox reaction, the isolation and characterization of a tetravalent cerium pyrrolate or carbazolate was unsuccessful. Dissolving the resulting crude product in THF-d8 resulted in the decolorization of the solution and the obtained 1H NMR spectrum showed only paramagnetically broadened and shifted signals. This play of colors clearly indicated a reduction in the putatively formed tetravalent species. The use of other solvents was not possible due to the insolubility of the cerium pyrrolates and carbazolates in nondonating solvents.

2.3. Reactivity toward CO2

Preliminary studies on the reactivity of the n class="Chemical">silylamide Ce[N(SiHMe2)2]4 (red color) [74] as well as Ce(NiPr2)4 (purple) [75] and Ce(NiPr2)4Li(thf) (orange) [75], bearing the very basic diisopropylamido ligand (Figure 1), with CO2 resulted in an instant decolorization of the solutions and the formation of colorless precipitates. 1H NMR spectroscopic measurements of the reaction mixtures provided inconclusive results. As noted in the Introduction, for rare-earth–metal silylamides, the formation of siloxide species and the elimination of isocyanates via silyl migration has been previously described [41,42,43]. Therefore, we focused on cerium pyrazolates and pyrrolates and their reactivity toward CO2. The possible insertion of CO2 into the Ce−N(pyrazolato) bond was examined via in situ IR spectroscopic measurements in toluene. Because compounds 1 and 2 did not display any insertion of CO2 at ambient temperature, the reactions were conducted at −20 °C. It was shown previously for dimethyl pyrazolate [Ce(Me2pz)4]2 that the CO2 insertion is more efficient at lower temperatures [38]. These experiments revealed no (1), slow (2) and fast insertion (3), as indicated by an increasing intensity of the characteristic C−O vibrations at around 1600 to 1750 cm−1 in the DRIFT (diffuse reflectance infrared Fourier transform) spectra (Figure 5). However, the insertion processes were slower and less efficient than observed previously for the dimethyl pyrazolate [Ce(Me2pz)4]2 [38]. 1H NMR spectroscopic investigations revealed only minor insertion of CO2 into the Ce−N(pyrazolato) bond of complex 1 (Figure S7, Supporting Information). Notwithstanding, a new signal set appeared assignable to a tBu2pzCO2 ligand (Figure 6), displaying two singlets at 1.02 and 1.50 ppm for the tBu-groups and one singlet at 5.97 ppm for the C−H proton of the pyrazole backbone. For the reaction of compound 2 with CO2, the respective 1H NMR spectrum showed a shift of all signals, indicating complete conversion (Figure S8, Supporting Information). However, unlike what was expected for an asymmetrically CO2-inserted pyrazolato ligand, a splitting of the phenyl signals was not detected. For the reaction of compound 3 with CO2, the formation of two signal sets with a ratio of 9:1 was observed (Figure S9, Supporting Information). Both sets displayed singlets for the tBu and Me groups and one for the pyrazole C−H. This signal pattern might have resulted from a favorable insertion of CO2 into the methyl site of the pyrazolato ligand due to the lower steric hindrance (Figure 6). These results and previous findings clearly show that the steric demand of the substituents on the pyrazolato ligands is the key factor in the CO2-insertion reaction [38]. Therefore, the efficiency of the CO2 insertion with complexes [Ce(RR’pz)4] increases in the order of tBu2pz < Ph2pz < tBuMepz < Me2pz. Unfortunately, any crystallization and structural elucidation of a CO2-inserted product was not successful.

Figure 5

In situ IR spectroscopic measurements of the reaction of CO2 and (a) Ce(tBu2pz)4 (1), (b) Ce(Ph2pz)4 (2), and (c) Ce(tBuMepz)4 (3) at −20 °C. The wavenumber of the C=O band is marked with a red box.

Figure 6

Putative insertion products of the reaction of complexes 1 to 5 with CO2.

Similarly, pyrrole-derived complexes 4 and 5 were treated with n class="Chemical">CO2 resulting in a shift of the signals in the 1H NMR spectra (Figures S10 and S11, Supporting Information). For pyrrolate 4, two broadened singlets were observed at 6.82 and 9.99 ppm, indicating the formation of a carbamato-like ligand CO2∙pyr (Figure 6). Similar ligand formation was already found in the reaction of nBuLi, pyrrole, and carbon dioxide [76]. The 1H NMR spectrum of the reaction of carbazolate 5 with CO2 shows various paramagnetically broadened and shifted signals ruling out any conclusive interpretation. Nevertheless, the DRIFT spectra of the crude products, after removal of all volatiles under reduced pressure, clearly revealed an intense band for the C−O vibrations at 1600 to 1750 cm−1 (Figure 7). This finding also indicated that, unlike [Ce(Me2pz)4]2 [38], the CO2 insertion into the Ce−N(pyrrolato) bond is not reversible at ambient temperature under reduced pressure. Regrettably, a structural elucidation of the reaction products was not feasible.

Figure 7

Stacked DRIFT (diffuse reflectance infrared Fourier transform) spectra of the reactions of 4 before (green trace) and after treatment with CO2 (orange trace) as well as 5 before (yellow) and after treatment with CO2 (blue) from bottom to top. The region for C−O vibrations is highlighted in blue.

2.4. Difference between Pyrazolates and Pyrrolates/Carbazolates

Based on our findings, it can be hypothesized that n class="Chemical">CO2 insertion into the Ce−N(RR’pz) bond is mainly affected by the sterics of the pyrazole substituents. This is clearly indicated by the performance of [Ce(Me2pz)4]2 being far superior to that of Ce(tBu2pz)4. Accordingly, any significant electronic effect caused by differently alkyl-substituted pyrazolato ligands can be ruled out. However, our results suggest that the reversibility of the CO2 insertion is most likely driven by the two adjacent N-donor atoms of the pyrazolato ligand, preserving a Ce–N coordination also in the Ce−pz∙CO2 fragments (Scheme 3). Apparently, the formation of a five-membered heterocyclic ring with κ2(N,O) coordination mode is outperforming a four-membered ring with terminal κ2(O,O) carboxylato coordination. The latter κ2(O,O) coordination would result from the insertion of CO2 into the Ce−N(pyr) or Ce−N(cbz) bonds (Scheme 3). Note that any reversible CO2 insertion has not yet been evidenced for such simple carbamates.

Scheme 3

Reversible and irreversible CO2 insertion into Ce−N(pz) and Ce−N(pyr)/Ce−N(cbz) bonds, respectively.

3. Materials and Methods

3.1. General Procedures

All manipulations were performed under an inert atmosphere (Ar) using a glovebox (MBraun 200B; <0.1 ppm O2, <0.1 ppm n class="Chemical">H2O), or according to standard Schlenk techniques in oven-dried glassware. The solvents were purified with Grubbs-type columns (MBraun SPS, solvent purification system) and stored in a glovebox. Ce[N(SiHMe2)2]4Li(thf), Ce[N(SiHMe2)2]4, and Ce(tBu2pz)4 (1) were synthesized according to published procedures [65,66,67]. HMe2pz, HtBu2pz, pyrrole, and carbazole were purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received. HPh2pz and HtBuMepz were synthesized according to the literature [77]. C6D6, toluene-d8, and THF-d8 were purchased from Euriso-top (Saint-Aubin, France) and pre-dried over NaK alloy and filtered off prior use; THF-d8 was re-condensed. NMR spectra were recorded at 26 °C on a Bruker AVII+400 (1H: 400.13 MHz) or a Bruker AVIIIHD-300 (1H: 300.13 MHz, 7Li 116.64 MHz) using J. Young-valved NMR tubes. 1H NMR shifts are referenced to a solvent resonance and reported in parts per million (ppm) relative to tetramethylsilane. 7Li NMR spectra are reported relative to LiCl. Analyses of NMR spectra were performed with ACD/NMR Processor Academic Edition (product version: 12.01). Infrared spectra were recorded on a ThermoFisher Scientific (Waltham, MA, USA) NICOLET 6700 FTIR ( = 4000–400 cm−1) spectrometer using a DRIFTS chamber with dry KBr/sample mixtures and KBr windows. Elemental analysis (C, H, and N) was performed on an Elementar vario MICRO cube. In situ IR spectra were recorded on a METTLER TOLEDO (Columbus, OH, USA) ReactIR 15. For details on XRD analyses and crystallographic data, see Supporting information.

3.2. Synthesis of Ce(Ph2pz)4∙tol ()

Ce[N(SiHMe2)2]4 (0.40 g, 0.60 mmol) in n class="Chemical">n-hexane (2 mL) was added to a suspension of HPh2pz (0.33 g, 2.4 mmol) in toluene (2 mL). After 1 h, all volatiles were removed under reduced pressure. Crystallization from concentrated toluene solutions produced 2 as dark purple crystals. Yield: 0.52 g (0.47 mmol, 79%). 1H NMR (C6D6, 400.13 MHz, 26 °C): = 7.78 (m, 16 H, C−H Ph), 7.11 (s, 4 H, C−H pz), 6.91 (m, 24 H, C−H Ph) ppm; IR (DRIFT): = 3064 (w), 3036 (w), 2916 (w), 1944 (w), 1883 (w), 1807 (w), 1755 (w), 1682 (w), 1602 (w), 1562 (w), 1489 (w), 1467 (s), 1421 (m), 1401 (m), 1335 (w), 1282 (w), 1254 (w), 1151 (w), 1103 (w), 1072 (w), 1018 (m), 999 (m), 965 (m), 914 (w), 837 (w), 809 (w), 760 (s), 731 (s), 700 (s), 692 (m), 678 (m), 666 (w), 537 (w), 481 (w), 465 (w), 429 (m), 422 (m), 405 (w) cm−1; elemental analysis (%) calcd. for C67H52CeN8 (1109.33): C 72.54, H 4.73, N 10.10; found: C 72.40, H 4.52, N 10.68.

3.3. Synthesis of [Ce(tBuMepz)4]n ()

Ce[N(SiHMe2)2]4 (0.40 g, 0.60 mmol) in n class="Chemical">toluene (2 mL) was added to a suspension of HtBuMepz (0.53 g, 2.4 mmol) in toluene (2 mL). After 2 h, all volatiles were removed in vacuo, producing 3 as a dark red sticky solid. Yield of the crude product: 0.41 g. 1H NMR (C6D6, 400.13 MHz, 26 °C): = 6.15 (s, 4 H, C−H pz), 2.22 (s, 12 H, Me), 1.24 (s, 36 H, tBu) ppm; IR (DRIFT): = 3105 (w), 2961 (vs), 2924 (s), 2901 (s), 2862 (m), 1558 (w), 1508 (s), 1474 (m), 1458 (m), 1425 (s), 1387 (m), 1362 (m), 1291 (w), 1237 (s), 1210 (w), 1124 (w), 1084 (w), 1032 (w), 1020 (w), 994 (m), 962 (w), 819 (w), 797 (m), 718 (w), 700 (w), 686 (w), 567 (w), 507 (m) cm−1; elemental analysis (%) calcd. for C32H52CeN8 (688.94): C 55.79, H 7.61, N 16.26; found: C 57.17, H 7.49, N 16.29. The slightly increased carbon value might be a result of retained solvent.

3.4. Synthesis of [Ce2(pyr)6(µ-pyr)2(thf)2][Li(thf)4]2 ()

Pyrrole (141 mg, 2.11 mmol) in n class="Chemical">n-hexane (3 mL) was added to a stirred solution of Ce[N(SiHMe2)2]4Li(thf) (394 mg, 0.526 mmol) in n-hexane (5 mL). Immediately, a colorless precipitate was formed. After 1 h, all volatiles were removed under reduced pressure. The resulting solid was recrystallized from THF, producing 4 as colorless crystals. Yield: 189 mg (0.122 mmol, 46%). 1H NMR (THF-d; 400.11 MHz, 26 °C) δ = 7.39 (bs), 4.23 (bs) ppm; 7Li NMR (THF-d, 116.64 MHz, 25 °C) δ = 2.0 ppm. DRIFTS: = 3085 (w), 1441 (w), 1365 (w), 1213 (vw), 1200 (vw), 1140 (w), 1078 (m), 1048 (w), 1021 (s), 894 (w), 858 (w), 789 (vs), 751 (m), 670 (w), 660 (m), 416 (w) cm−1; elemental analysis (%) calcd. for C72H112Ce2Li2N8O10 (1543.85): C 56.02, H 7.31, N 7.36; found: C 46.96, H 3.90, N 13.74. Calculated without THF donor ligands for C42H54Ce2N8O2 (983.18): C 51.31, H 5.54, N 11.40. On multiple attempts, no better elemental analysis could be obtained.

3.5. Synthesis of [Ce(cbz)4(thf)2][Li(thf)4] ()

Ce[N(SiHMe2)2]4Li(thf) (406 mg, 0.557 mmol) in n class="Chemical">n-hexane (5 mL) was added to a stirred suspension of carbazole (363 mg, 2.23 mmol) in n-hexane (5 mL). After 1 h, a yellow precipitate formed, and volatiles were removed under reduced pressure. The resulting solid was recrystallized from THF/Et2O (1:1), giving 5 as off-white crystals. Yield: 486 mg (0.415 mmol, 74%). 1H NMR (THF-d; 400.11 MHz, 26 °C) δ = 8.64 (d), 7.90 (bs), 7.70 (d), 7.32 (d), 7.02 (bs), 6.70 (bs) 6.28 (s), 6.04 (s), 5.30 (s), 4.22 (d), 3.62 (s, α-CH(thf)), 1.77 (s, β-CH(thf)), 0.33 (bs), −10.16 (bs) ppm; 7Li NMR (THF-d, 116.64 MHz, 25 °C) δ: −0.3 ppm. DRIFTS: = 3418 (vw), 3054 (w), 2979 (w), 2878 (w), 1635 (m), 1576 (m), 1541 (m), 1489 (m), 1479 (m), 1447 (s), 1395 (vs), 1329 (vs), 1312 (s), 1287 (m), 1216 (m), 1151 (w), 1121 (vw), 1043 (w), 934 (vw), 889 (vw), 826 (m), 797 (m), 753 (s), 725 (m), 684 (m), 658 (w), 617 (vw), 567 (vw), 526 (vw), 458 (w), 425 (m) cm−1; elemental analysis (%) calcd. for C72H80CeLiN4O6 (1244.51): C 69.49, H 6.48, N 4.50; found: C 68.42, H 6.42, N 3.97.

4. Conclusions

The feasibility of CO2 insertion reactions was further substantiated for homoleptic n class="Chemical">ceric pyrazolates [Ce(RR’pz)4]. Crucially, with increasing bulkiness of the substituents R/R’ on the pyrazolato ligand, the insertion of carbon dioxide becomes increasingly hindered; therefore, the least bulky dimethyl-substituted pyrazolato ligand performed best in CO2 insertions. En route to homoleptic CeIV pyrrolates and carbazolates, we were able to isolate cerous ate complexes [Ce2(pyr)6(µ-pyr)2(thf)2][Li(thf)4]2 and [Ce(cbz)4(thf)2][Li(thf)4]. Such Ce(III) pyrrolates and carbazolates are also capable of CO2 activation, but most likely in an irreversible manner. Oxidation attempts to form ceric pyrrolate or carbazolate compounds were conducted; however, the structural elucidation of any CeIV species was not feasible.