Cerium(III) Azolate Promoted CO₂ Insertion

Abstract

A series of sandwich cerium azolates (pyrazolates, triazolates, tetrazolates) has been synthesized via salt-metathesis (cerous precursor: [Cp*₂CeCl₂K­(thf)] _n ; Cp* = C₅Me₅) and protonolysis protocols (cerous precursors: Cp*₂Ce­[N­(SiHMe₂)₂] or Cp*₂Ce­[N­(SiMe₃)₂]) and their carboxophilicity has been probed. The sandwich and half-sandwich complexes Cp*₂Ce­(pz^Me,Me) and Cp*Ce­(pz^Me,Me)₂(thf)₂ show exhaustive CO₂ insertion into the pyrazolato moieties, which leads to bis­(carbamato)-bridged dimeric complexes. Complex Cp*₂Ce­(pz^Ph,Ph), featuring the sterically more demanding and less nucleophilic diphenylpyrazolato ligand inserts only one molecule CO₂ per two metal centers, forming the asymmetrically bridged complex Cp*₂Ce­(μ-pz^Ph,Ph·CO₂)­CeCp*₂(pz^Ph,Ph). The triazolato derivatives Cp*₂Ce­(tz^Me,Me)­(dmap) and Cp*₂Ce­(tz^Ph,Ph) indicate CO₂ insertion only for the DMAP-free complex. Utilizing the nitrogen-richer 5-phenyltetrazole resulted in the formation of the trimer [Cp*₂Ce­(tet^Ph)]₃, which does not insert CO₂ at ambient temperature. The absorption and electrochemical properties of the compounds were investigated and their catalytic activity in the cycloaddition of CO₂ and propylene oxide was examined.

Introduction

Carbon capture and storage (CCS) as well as carbon capture and utilization (CCU) display the most commonly pursued strategies to combat the emission of carbon dioxide as the most common greenhouse gas. − Industrial CCS is currently accomplished by using aqueous amines. − The favorable molecular interaction of the nucleophilic amine nitrogen with CO₂ is the basis/key for this technology. More recently, this approach has been also successfully exploited in amine-containing ionic liquids or high surface materials like metal–organic frameworks (MOF) or porous silica. , Rare-earth metals exhibit a great compatibility with CO₂ due to their high oxophilicity and pronounced Lewis acidity of the trivalent metal cations. Recently, we observed the reversible insertion of CO₂ into homoleptic cerium dimethylpyrazolates yielding Ce^III ₄(pz^Me,Me·CO₂)₁₂ and Ce^IV(pz^Me,Me·CO₂)₄ (pz^Me,Me = 3,5-dimethylpyrazolato, max. 25 wt % CO₂). This concept was also applied to the lighter metals magnesium, aluminum and titanium as well as the rare-earth metals scandium and yttrium. −

The present study aimed at a deeper molecular understanding of the carboxophilicity of rare-earth-metal azolato moieties. We stuck to cerium as the most abundant metal center and the feasibility of Ce­(III)/Ce­(IV) redox events. Further, we chose the “inert” sandwich entity [Cp*₂Ce]⁺ (Cp* = pentamethylcyclopentadienyl) ensuring one reactive Ce­(III)-azolato site. The scope of the azole pool should not be limited to differently substituted pyrazoles Hpz^R,R but be extended to triazoles (Htz^R,R = 3,5-R,R-1,2,4-triazole) and 5-phenyltetrazole (Htet^Ph).

The class of molecular rare-earth-metal azolates with three or more nitrogen atoms in the 5-membered ring has not been exhaustively investigated yet. In 2006 the group of Müller-Buschbaum synthesized [Yb­(tz^H,H)₃]_∞ and [Eu₂(tz^H,H)₅(Htz^H,H)₂]_∞ via direct reaction of the metals with Htz^H,H. Another example is the homoleptic complex La­(tz^Ph,Ph)₃(thf)₃ obtained by protonolysis of La­[N­(SiMe₃)₂]₃ with Htz^Ph,Ph. Rare-earth-metal complexes bearing a 1,2,3-triazolato ligand (trz) include [Ln­(trz^H,H)₃]_∞ (Ln = Gd–Lu) , as well as Cp*₂Ln­(trz^SiMe3,tBu)­(NCtBu) (Ln = La, Sm) obtained from [Cp*₂Ln­(CNN­(SiMe₃))]₂ and nitriles. Rare-earth-metal tetrazolates are featured mainly by MOFs or complexes with multidentate tetrazolato ligands. − To access alkali-metal tetrazolate precursors, deprotonation of the proligand with MOH (M = Li, Na, K, Rb, Cs), KH or M₂CO₃ (M = K, Cs) was used. − The group of Winter also reported Nb­(pz ^tBu,tBu)₃(tet^Ph)₂ and Ta­(pz ^tBu,tBu)₃(tet^Ph)Cl as well as Cr­(tBuNC­(CH₃)­NtBu)₂(tet^CF3)­(p-tBuC₅H₄N) via salt metathesis. ,

The Cp*₂Ce-cerocene motif is well established. Already in 1986 the synthesis and structural characterization of Cp*₂CeCl₂Li­(OEt₂)₂ was reported by Rausch et al. The crystal structures of the similar complex [Cp*₂CeCl₂K­(thf)] _n and the first alkyl derivative Cp*₂Ce­[CH­(SiMe₃)₂] were reported two years later. , Many differently substituted amide complexes of the type Cp*₂Ce­(NRR′) (R = R′ = SiMe₃, SiHMe₂, iPr; R = iPr, R′ = C­(CH₃)=CH₂, tBu, SiHMe₂, SiMe₃, SitBuMe₂; R = tBu, R′ = SiMe₃, SitBuMe₂) were synthesized by the group of Andersen and ours and investigated regarding their oxidizability and thermal rearrangement reactions. , Yet the oxidation of Cp*₂Ce­[N­(SiHMe₂)₂] did not result in the formation of a Ce­(IV) complex.

Results and Discussion

Cerous Pyrazolates

Treatment of [Cp*₂CeCl₂K­(thf)] _n with one equivalent of Kpz^Me,Me in n-hexane, toluene or THF and subsequent crystallization from n-hexane produced yellow crystals of Cp*₂Ce­(pz^Me,Me)­(thf) (1 ^thf ) (Scheme ). Complex 1 ^thf is also available via protonolysis of Cp*₂Ce­[N­(SiHMe₂)₂] or Cp*₂Ce­[N­(SiMe₃)₂] with Hpz^Me,Me in THF. Performing the protonolysis in a nondonating solvent results in the formation of Cp*₂Ce­(pz^Me,Me) (1). The ¹H NMR spectrum in C₆D₆ shows two signals for the pz^Me,Me ligand at 7.88 and −3.76 ppm in the ratio 1:6, and one signal for the Cp* ligand at 1.97 ppm (Figure S17).

1. Synthesis of Complexes 1, 1^thf, 3 and 3^thf via Protonolysis and Salt Metathesis from Cp*₂Ce­[N­(SiHMe₂)₂] and [Cp*₂CeCl₂K­(thf)] _n , Respectively, Employing Different Pyrazoles and Potassium Pyrazolates.

The solid-state structure of 1 is shown in Figure /left. The Ce–Ct distance of 2.492 Å (Ct = ring centroid of Cp*) is similar to the known pyrrolate complex Cp*₂Ce­(NC₄Me₄) (2.490–2.492 Å), but shorter compared to most other Cp*₂Ce amide derivatives, e. g. Cp*₂Ce­(NiPr₂) (2.544–2.562 Å) or Cp*₂Ce­(hpp) (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido­[1,2-a]-pyrimidinato) (2.513–2.524 Å). The κ² bonding of the pz^Me,Me ligand results in a larger distance between the cerium and the nitrogen atoms at 2.4334(8) Å than in the κ¹ coordinated nitrogen atoms in Cp*₂Ce­(NC₄Me₄) (2.400(2) Å) and Cp*₂Ce­(NiPr₂) (2.303(4)/2.307(4) Å). The triazolato ligand of the lanthanum sandwich complex Cp*₂La­(N₃C­(tBu)­C­(SiMe₃))­(NCtBu) shows a similar bonding motif, but displays longer Ln–N distances of 2.4719(16) Å and 2.5505(16) Å, respectively.

1.

Crystal structures of Cp*₂Ce­(pz^Me,Me) (1, left), Cp*₂Ce­(pz^Ph,Ph) (3, middle) and Cp*Ce­(pz^Me,Me)₂(thf)₂ (5, right). Ellipsoids are shown at a 50% probability level. A part of 5 is represented by a wireframe model. Hydrogen atoms are omitted for clarity. Selected interatomic distances/angles are listed in the Supporting Information.

Exposing a C₆D₆ solution of 1 ^thf to 1 bar CO₂ atmosphere resulted in the precipitation of a yellow solid. Crystallization from a THF solution at −40 °C yielded yellow crystals of the dimeric complex [Cp*₂Ce­(pz^Me,Me·CO₂)]₂ (2) (Scheme , Figure ). The ¹H NMR spectrum shows more signals than the four expected signals which could not be assigned (Figure S25). This insertion behavior had been also observed for the homoleptic cerium pyrazolates, giving access to complexes Ce^III ₄(pz^Me,Me·CO₂)₁₂ and Ce^IV(pz^Me,Me·CO₂)₄. In complex 2, two Cp*₂Ce entities are bridged by two carbamato pz^Me,Me·CO₂ ligands via one nitrogen and both oxygen atoms. The cerium centers and the two CO₂ form an eight-membered ring whose structure resembles a chair conformation.

2. Reaction of Cp*₂Ce­(pz^Me,Me)­(thf) (1^thf) with CO₂ in THF.

2.

Crystal structure of [Cp*₂Ce­(pz^Me,Me·CO₂)]₂ (2). Ellipsoids are shown at a 50% probability level. Except for the cerium atom the second half of the molecule is represented by a wireframe model. Hydrogen atoms and cocrystallized C₆D₆ are omitted for clarity. Selected interatomic distances/angles are listed in the Supporting Information.

Compared to 1 the Ce–Ct distances of 2.540 Å and 2.573 Å are elongated, but only slightly longer than in 1 ^thf . The Ce–N1 distance in 2 amounts to 2.708(3) Å, which suggests a Ce–N donor interaction. The Ce–O distances of 2.469(3) Å and 2.532(2) Å lie within the range of those detected for the cerium­(III) pyrazolate [Ce₄(pz^Me,Me·CO₂)₁₂] (2.364(6)–2.771(6) Å). Similar to the bridging pz^Me,Me·CO₂ moieties in [Ce₄(pz^Me,Me·CO₂)₁₂], the C–O distances in 2 are in the same range (1.242(4) Å and 1.247(4) Å) indicating delocalization of the C–O double bond. For further comparison, the first structural authentication of a CO₂-inserted organorare-earth-metal complex was reported for scandocene Cp*₂Sc­(O₂C)­C₆H₄CH₃. Treatment of the cerocene hydride [Cp*₂Ce­(μ-H)]₂ with CO₂ afforded the carbonate species Cp*₂Ce­(μ-CO₃)­CeCp*₂ (ref ) while Cp*₃Sm was converted to carboxylate Cp*₂Sm­(O₂CC₅Me₅).

Stirring a n-pentane solution of 1 ^thf under 1 bar CO₂ atmosphere resulted first in a color change from green to purple followed by precipitation of yellow 2. It can be hypothesized that the monomeric complex Cp*₂Ce­(pz^Me,Me·CO₂)­(thf) is formed initially which then loses THF and dimerizes to 2 accompanied by precipitation. The DRIFT spectrum of 2 shows strong absorption bands in the region between 1600 and 1750 cm^–1 and 1250 and 1400 cm^–1 for the C–O stretching vibrations (Figure , Figure S70).

3.

VT DRIFTS of 2 in the range of ṽ = 1730–1650 cm^–1.

A variable-temperature (VT) NMR study of 2 in THF-d ₈ was carried out to determine the reversibility of the CO₂ insertion (cf. Figure S50). Already at 40 °C distinct signals of the starting material were visible which increased with further heating. Back-cooling to ambient temperature fully reformed complex 2. Heating the THF-d ₈ solution under argon atmosphere at 60 °C for 3 h and measuring a ¹H NMR spectrum at ambient temperature still gave an unchanged spectrum. However, stirring the THF-d ₈ solution of 2 inside the glovebox in an open vial at 40 °C for 30 min yielded 1 ^thf , although some decomposition was observed. Retreatment of 1 ^thf with CO₂ restores 2 (cf. Figure S51). After three cycles, most of the starting material had decomposed. Furthermore, exposing 2 to reduced pressure did not yield complex 1. Performing a thermogravimetric analysis (TGA) showed a 4% mass loss between 60 and 160 °C and another mass loss of 4% between 160 and 200 °C which would be in accordance with the loss of one molecule CO₂ (4%) each (Figure S81). This is further corroborated by conducting a VT-IR experiment. Upon replacing the argon atmosphere with CO₂, instant formation of 2 was observed. After renewing the argon atmosphere, the sample was heated in 10 °C increments from 30 to 120 °C (Figure ). The absorption of the band at 1691 cm^–1 decreases visibly in the measured temperature range. It appears that the THF donor plays a crucial role in lowering the deinsertion temperature to 40 °C. This behavior resembles the reported complexes [Cp*₂Sm­(μ-EPh)]₂ (E = S, Se) which react with CO₂ to form [Cp*₂Sm­(μ-CO₂EPh)]₂. Adding THF to [Cp*₂Sm­(μ-CO₂SePh)]₂ leads to decarboxylation and the THF adduct Cp*₂Sm­(SePh)­(thf).

To probe the steric demand of the pyrazolato ligand, the synthesis of the diphenylpyrazolate complex was envisaged. Performing a salt metathesis of [Cp*₂CeCl₂K­(thf)] _n and Kpz^Ph,Ph in n-hexane at 40 °C and subsequent crystallization yielded yellow and blue crystals. Similar to 1, the yellow crystals were identified as the THF adduct Cp*₂Ce­(pz^Ph,Ph)­(thf) (3 ^thf ) and the blue ones as THF-free Cp*₂Ce­(pz^Ph,Ph) (3). By applying high vacuum (10^–4 mbar) and 100 °C, 3 ^thf could be converted to 3. The ¹H NMR spectrum of 3 in C₆D₆ shows the expected signals. Complex 3 crystallizes in the tetragonal space group P4₁2₁2 (Figure /middle). The Ce–Ct distance of 2.486 Å is slightly shorter than in 1, while the Ce–N distance of 2.444(3) Å is in the same range. The previously reported isostructural ytterbium congener Cp*₂Yb­(pz^Ph,Ph) exhibits, due to the significantly smaller rare-earth metal center, shorter Yb–Ct and Yb–N distances of 2.304 Å and of 2.248(18) Å, respectively.

Treatment of 3 with CO₂ (1 bar pressure) in toluene-d ₈ led to a color change from blue to reddish. Addition of n-pentane and cooling to −40 °C produced a few red crystals. A single-crystal X-ray diffraction (SCXRD) analysis revealed the formation of the bimetallic insertion product Cp*₂Ce­(μ-pz^Ph,Ph·CO₂)­CeCp*₂(pz^Ph,Ph) (4) (Scheme , Figure ). Similar to 2 the ¹H NMR spectra in C₆D₆, toluene-d ₈ or THF-d ₈ were not conclusive (Figures S32–S34). In THF-d ₈ only partial insertion was observed at ambient temperature.

3. Reaction of Cp*₂Ce­(pz^Ph,Ph) (3) and Excess CO₂ to Form Mono-Inserted Bimetallic 4.

4.

Crystal structure of Cp*₂Ce­(μ-pz^Ph,Ph·CO₂)­CeCp*₂(pz^Ph,Ph) (4). Ellipsoids are set a 50% probability level. The Cp* ligands are represented by a wireframe model. Hydrogen atoms are omitted for clarity. Selected interatomic distances/angles are listed in the Supporting Information.

The bridging pyrazolato derivative shows a μ-carbamato-κ² N,O/κO′ binding motif. Noteworthily, both pyrazolato ligands are located on the same side of the complex. The average Ce–Ct distance amounts to 2.535 Å. Both the terminal and the bridging nitrogen atoms exhibit larger distances to the cerium metal centers compared to 3 (2.498(3)–2.563(3) Å). Compared to 2 the C–O distances of the inserted CO₂ differ far more (1.230(4) Å and 1.254(4) Å) which indicates less delocalization. This is also reflected in increasingly distinct Ce–O distances of 2.429(2) Å and 2.588(2) Å.

Unlike 1, insertion of CO₂ occurred into only one pyrazolato ligand of 3, probably due to the higher steric demand of the phenyl groups compared to the methyl groups. Contrary to 2, which starts to release CO₂ at 60 °C in the solid state, 4 is stable under CO₂ atmosphere but partially releases CO₂ under argon atmosphere at ambient temperature. This hampered the isolation of a sufficient quantity of complex 4 for FTIR and TGA measurements.

Similarly to 1, Cp*CeI₂(thf)₃ was treated with two equivalents of Kpz^Me,Me in n-hexane aiming at the corresponding half-sandwich complex (Scheme ). Slow evaporation at ambient temperature yielded colorless crystals of Cp*Ce­(pz^Me,Me)₂(thf)₂ (5, Figure /right). The ¹H NMR spectrum in C₆D₆ shows five signals seemingly assignable to 5 (Figure S35), however, slow ligand scrambling is indicated by the appearance of peaks ascribed to 1 ^thf . The ligands in 5 are arranged in a distorted trigonal-byramidal fashion. The Cp* and one THF ligand occupy the axial positions while the remaining THF as well as the two pyrazolatos bend away from the Cp* in the equatorial positions. Compared to 1 ^thf the Ce–Ct distance of 5 is shorter (2.505 Å vs 2.535 Å). The Ce–N distances involving the κ²-bonded pyrazolato ligands range from 2.432(2) Å to 2.500(3) Å. The equatorial THF is closer (Ce–O 2.591(3) Å) to the metal center than the axial one (Ce–O 2.6195(18) Å).

4. Salt-Metathesis Reaction of Cp*CeI₂(thf)₃ and Kpz^Me,Me to Yield Cp*Ce­(pz^Me,Me)₂(thf)₂ (5)­ .

^a Treatment of 5 with CO₂ resulted in the formation of dimeric [Cp*Ce­(pz^Me,Me·CO₂)₂(thf)]₂ (6).

Overall, the coordination geometry of 5 is distinct from the fluorenyl half-sandwich complex FluCe­(pz^Me,Me)₂(thf)₂ which adopts a slightly bent pseudo square pyramidal coordination geometry with trans-positioned κ²-pyrazolato ligands. The Ce–N distances in FluCe­(pz^Me,Me)₂(thf)₂ average 2.480 Å.

Exposing a solution of 5 in C₆D₆ to 1 bar of CO₂ led to the precipitation of a yellow powder. Crystallization from a THF solution at −40 °C yielded yellow crystals of the dimeric complex [Cp*Ce­(pz^Me,Me·CO₂)­(μ-pz^Me,Me·CO₂)­(thf)]₂ (6) (Scheme , Figure ). The ¹H NMR spectrum of 6 features several broad peaks which could not be assigned (Figure S37).

5.

Crystal structure of [Cp*Ce­(pz^Me,Me·CO₂)₂(thf)]₂ (6). Ellipsoids are set a 50% probability level. Part of the molecule is represented by a wireframe model for better visibility. Hydrogen atoms are omitted for clarity. Selected interatomic distances/angles are listed in the Supporting Information.

The cerium centers in 6 are coordinated axially by one Cp* ligand and one THF donor and equatorially by one terminal pz^Me,Me·CO₂ carbamato ligand. Furthermore, two pz^Me,Me·CO₂ ligands bridge the two cerium centers, in a similar manner as observed for 2. The Ce–Ct distance is 2.522 Å and thus elongated compared to precursor 5. The Ce1–N2/N4 distances amount to 2.743(4) Å and 2.727(4) Å and are typical for a metal–donor interaction. The O–C–O distances of the terminal pz^Me,Me·CO₂ ligands imply a localized double bond (C12–O3 1.263(6) Å, C12–O4 1.222(5) Å) and delocalization for the bridging ligands (C6–O1 1.246(5) Å, C6–O2 1.250(5) Å), fully in line with the Ce–O distances which are shorter for the terminal pz^Me,Me·CO₂ ligand (Ce1–O3 2.389(3) Å) compared to the bridging ones (Ce1–O1 2.479(3) Å, Ce1̀'–O2 2.564(3) Å). This is further supported by DRIFTS measurements. The IR spectrum of 6 shows strong absorption bands at 1728 and 1668 cm^–1 ascribed to the stretching vibration of the C–O double bond. In the area between 1470 and 1290 cm^–1 there are several strong bands for the stretching of the C–O single bond as well as the delocalized C–O bonds. The TGA of complex 6 revealed no clear loss of CO₂. Instead, a mass loss of 60% was observed in the range from 85 to 480 °C, showing two steps of 45% between 85 and 300 °C and 15% between 300 and 480 °C (Figure S82).

Cerous Triazolates and Tetrazolates

To examine the effect of less nucleophilic azolato ligands, the synthesis of sandwich cerium triazolates and tetrazolates was pursued. However, both the salt metathesis of [Cp*₂CeCl₂K­(thf)] _n and potassium triazolate Ktz^Me,Me as well as the protonolysis of Cp*₂Ce­[N­(SiHMe₂)₂] with triazole Htz^Me,Me yielded only slightly soluble yellow solids. This can be interpreted by the formation of oligomeric structures. For example, Zhang et al. reported that the protonolysis of Cp′₃Yb (Cp′ = C₅H₄Me) with Htz^H,H in THF yielded tetrameric [Cp′₂Yb­(tz^H,H)]₄. We found that addition of DMAP to the protonolysis reaction mixture and subsequent crystallization from Et₂O gave colorless needles of Cp*₂Ce­(tz^Me,Me)­(dmap) (7) (Scheme , Figure /left). The ¹H NMR spectrum in C₆D₆ shows the expected five signals (Figure S38).

5. Synthesis of Cp*₂Ce­(tz^Me,Me)­(dmap) (7), Cp*₂Ce­(tz^Ph,Ph) (8) and Cp*₂Ce­(tz^Ph,Ph)­(thf) (8 ^thf) via Protonolysis and Salt Metathesis, Respectively.

6.

Crystal structures of Cp*₂Ce­(tz^Me,Me)­(dmap) (7, left), Cp*₂Ce­(tz^Ph,Ph) (8, middle) and Cp*₂Ce­(tz^Ph,Ph)­(thf) (8 ^thf , right). Complex 7 has two molecules in the asymmetric unit. One molecule is severely disordered and omitted for clarity. Ellipsoids are set at a 50% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances/angles are listed in the Supporting Information.

The Ce–N distances of 7 involving the triazolato ligand amount to 2.458(4) Å and 2.537(4) Å and are similar to the C–N­(pyrazolato) distances in 1 ^thf . The DMAP ligand coordinates with the pyridine nitrogen atom at a distance of 2.592(4) Å. Contrary to the pyrazolates, complex 7 does not react with CO₂ at ambient temperature. This could be due to the presence of the stronger donor ligand DMAP compared to THF, the less nucleophilic triazolato ligand or both.

Analogously to the pyrazolates the synthesis of the diphenyl triazolate derivative was carried out next. Once more, both the donor-free Cp*₂Ce­(tz^Ph,Ph) (8) as well as the THF-containing sandwich complex Cp*₂Ce­(tz^Ph,Ph)­(thf) (8 ^thf ) could be isolated via protonolysis or salt metathesis, individually (Scheme , Figure /middle, right).

The ¹H NMR spectra in C₆D₆ revealed four and six signals, respectively (Figures S40 and S42). For 8, the signal of the Cp* methyl protons appeared at 2.22 ppm, the signals for the para and meta protons at 4.64 and 3.05 ppm. The ortho protons are detected at −5.08 ppm. Compared to 8, the signals of 8 ^thf are shifted to lower field, except the two THF signals which appear at −4.31 and −13.63 ppm.

There are two independent units in the crystal structure of 8. The triazolato ligands each coordinate in κ²-fashion to the cerium center. The Ce–N distances range from 2.4611(15) Å to 2.4947(16) Å being longer than in the corresponding pyrazolate 3. The average Ce–Ct distance of 2.473 Å is slightly shorter than in 3. This can be attributed to the lower nucleophilicity of the triazolato compared to the pyrazolato ligand. Adding CO₂ to 8 in C₆D₆ or n-pentane resulted in a color change from blue to dark green and precipitation of an off-white solid. The solid shows an IR absorption band around 1677 cm^–1 ascribed to the stretching vibration of a C–O double bond. Numerous strong bands are detected between 1467 cm^–1 and 1355 cm^–1 corresponding to the stretching of C–O single bonds and delocalized C–O bonds. This suggests a successful reaction of 8 with CO₂, however, in THF-d ₈ no CO₂ insertion was detected via NMR spectroscopy. Due to the low solubility in aromatic solvents and the rapid deinsertion in THF, no crystalline material could be obtained to date.

Turning to the even less nucleophilic tetrazolates, the sandwich complex [Cp*₂Ce­(tet^Ph)]₃ (9-Ce) could be obtained via salt metathesis from [Cp*₂CeCl₂K­(thf)] _n and Ktet^Ph (Scheme ). The ¹H NMR spectrum shows only three signals due to overlap of the meta protons of the phenyl ring with Cp* methyls (Figure S44). Contrary to the pyrazolates and triazolates 9-Ce constitutes a trimeric structure (Figure ). Two Cp* ligands coordinate to each metal center while the tetrazolato ligands bridge the cerium atoms via a μ-κ² N,N′/κ² N″,N‴ binding motif.

6. Synthesis of [Cp*₂Ce­(tet^Ph)]₃ (9-Ce) via Salt Metathesis and Protonolysis.

7.

Crystal structure of [Cp*₂Ce­(tet^Ph)]₃ (9-Ce). Ellipsoids are set at a 50% probability level. Part of the molecule is represented by a wireframe model for better visibility. The lanthanum congener 9-La is isostructural. Hydrogen atoms are omitted for clarity. Selected interatomic distances/angles are listed in the Supporting Information.

The cerium centers and the phenyltetrazolato ligands form a slightly distorted plane. The cerium nitrogen distances range from 2.596(3) Å to 2.682(3) Å for the inner (N2/N3, N6/N7, N10/N11) and from 2.620(3) Å to 2.830(4) Å for the outer nitrogen atoms (N1/N4, N5/N8, N9/N12). Complex 9-Ce does not insert CO₂ at 1 bar pressure in C₆D₆, toluene-d ₈ or THF-d ₈. Despite the larger size of the La­(III) center, no CO₂ insertion was observed for the corresponding [Cp*₂La­(tet^Ph)]₃ (9-La). Isostructural complex 9-La was synthesized according to the salt metathesis shown in Scheme .

We also probed the protonolysis of the pyrazolato ligand of 1 ^thf with the more acidic Htz^Me,Me in THF-d ₈ and Htet^Ph in toluene. However, instead of the azolate sandwich complexes no product could be isolated. Among other compounds the ¹H NMR spectra show the formation of HCp* (Figures S52 and S53).

Electronic Absorption Spectra

The pyrazolate complexes 1 and 1 ^thf display similar absorption spectra (in n-hexane), featuring a slight shoulder at around 268 nm and growing absorption until the cutoff wavelength of the solvent (Figure S83). The half-sandwich congener 5 exhibits a shoulder at 275 nm but has a similar increase of the absorption toward the cutoff. The global absorption maximum for the diphenyl derivative 3 is detected at 257 nm, while the tz^Ph,Ph complex 8 displays two local maxima at 262 and 235 nm. The global absorption maximum of trimetallic 9-Ce appears at 242 nm. The complexes 1 ^thf , 2, 3 ^thf , 6, 7, and 8 ^thf were measured in THF solution (Figure S84). The CO₂-inserted complex 2 exhibits two absorption maxima at 266 and 225 nm, while none was observed for the half-sandwich CO₂ complex 6 in the absorption range from 300 to 220 nm. A local maximum of 7 is detected at 254 nm. Overall, the THF adducts show similar spectra to their donor-free congeners. Due to poor solubility in n-hexane and partial deinsertion of CO₂ in THF and CH₃CN complex 4 was measured in toluene (Figure S91), ruling out a measurement at shorter wavelengths. The absorption ranges from about 400 to 300 nm.

Electrochemical Properties

The redox behavior of the bis­(alkoxy) cerium­(IV) metallocene complexes Cp*₂Ce­(OR)₂ (R = Et, iPr, tBu, CH₂ tBu, SiMe₃, SiPh₃) was recently investigated by cyclic voltammetry. Naturally, we probed the electrochemical properties of the compounds under study to determine the accessibility of the Ce­(IV) oxidation state and if sandwich cerium­(IV) pyrazolates are feasible and isolable. The respective electrochemical measurements were performed in a glovebox (argon atmosphere) at ambient temperature with a glassy-carbon (GC) working electrode in THF solvent with 0.1 M [nPr₄N]­[B­(C₆H₃(CF₃)₂-3,5)₄] supporting electrolyte.

All potentials are referenced versus the Fc/Fc⁺ redox couple. Without exception, the surveyed complexes revealed qualitatively irreversible oxidation waves (Figures and S98–S107). The isolation of single waves did not result in reversible redox events. The cyclic voltammogram of 1 ^thf exhibits three oxidation waves at −0.46 V, −0.24 and +0.37 V, respectively. Similar oxidation events are visible for 3 ^thf at −0.24 V and +0.41 V, with an additional event at +0.74 V. Overall, the cyclic voltammogram is reminiscent of that of the chloride precursor [Cp*₂CeCl₂K­(thf)] _n whose Ce­(III/IV) redox couple at E _ox = −0.57 V was found irreversible as well. The waves of the diphenyl-substituted triazolato derivative 8 ^thf appeared at more negative potentials of −0.29 V, +0.36 V and +0.60 V indicating an easier oxidation compared to the corresponding pyrazolate complex. The trimeric complex 9-Ce exhibits redox events at −0.16 V, +0.41 V and +0.70 V.

8.

(a) Cyclic voltammograms of Cp*₂Ce­(pz^Me,Me)­(thf) (1 ^thf , black) and Cp*₂Ce­(pz^Ph,Ph)­(thf) (3 ^thf , red) vs Fc/Fc⁺ in THF at a glassy-carbon electrode obtained at a scan rate of 50 mV/s. The arrow indicates the scan direction. The analyte concentration was 1 mM, and the electrolyte concentration was 100 mM [nPr₄N]­[B­(C₆H₃(CF₃)₂-3,5)₄]. (b) Cyclic voltammograms of Cp*₂Ce­(pz^Ph,Ph)­(thf) (3 ^thf , black), Cp*₂Ce­(tz^Ph,Ph)­(thf) (8 ^thf , red) and [Cp*₂Ce­(tet^Ph)]₃ (9-Ce, blue) vs Fc/Fc⁺ in THF at a glassy-carbon electrode. The arrow indicates the scan direction. The analyte concentration was 1 mM, and the electrolyte concentration was 100 mM [nPr₄N]­[B­(C₆H₃(CF₃)₂-3,5)₄]. The scan rate was 50 mV/s for 3 ^thf and 8 ^thf and 250 mV/s for 9-Ce.

The corresponding lanthanum complex Cp*₂La­(pz^Me,Me)­(thf) (1 ^thf -La) was synthesized to compare the electrochemical behavior of 1 ^thf with a redox-innocent metal (Figures and S108). The lanthanum derivative features oxidation events at 0.00 V, +0.30 V, +0.51 V and +0.70 V. Compared to the cerium complex 1 ^thf the oxidation events are shifted to a higher potential and suggest that the first oxidation event detected for 1 ^thf at E _ox = −0.46 V belong to the Ce­(III/IV) redox couple. At higher potential, the cyclic voltammogram looks similar to those of the cerium congeners, hinting at ligand-based oxidation processes. The attempted chemical oxidation of 1 ^thf with C₂Cl₆ in C₆D₆ did not result in the formation of a Ce­(IV) complex (Scheme , Figure S55). The corresponding ¹H NMR spectrum of the blue solution revealed signals assigned to Cp*₂, alongside with a paramagnetic Ce­(III) species. A crystallization attempt from n-hexane resulted in a few colorless crystals which could be identified as the Ce­(III) cluster Cp*₄Ce₄(μ₂-Cl)₃(μ₃-Cl)₂(μ₄-Cl)­(μ₂-pz^Me,Me)­(pz^Me,Me)­(thf) (10). Complex 10 features a Ce₄Cl₆-cluster core. Each cerium center is capped by one Cp* ligand. One pyrazolato ligand coordinates κ ^₂ in a terminal fashion while the other bridges two metal centers in a μ-1κ²(N,N′):2κ­(N) fashion. The Cp*₄Ln₄Cl₆ motif resembles the structures of the reported rare-earth-metal complexes Cp*₅Nd₅(AlMe₄)­Cl₉ (ref ) and Cp*₆Sm₅Cl₉. The Ce–Cl distances range from 2.7529(16) to 3.1121(15) Å. For comparison, the Ce–Cl distances in the dodecanuclear cluster [(C₅H₄SiMe₃)­CeCl₂]₁₂ are between 2.845(2) and 3.164(3) Å. Apparently, any transient Ce­(IV) decomposes under formation of a Ce­(III) species and Cp*₂ or the ligand is oxidized directly. This is in contrast to the sandwich alkoxide complexes which can be isolated, albeit slow conversion to Ce­(IV) half-sandwich derivatives is noted.

9.

Cyclic voltammograms of Cp*₂Ce­(pz^Me,Me)­(thf) (1 ^thf , black) and Cp*₂La­(pz^Me,Me)­(thf) (1 ^thf -La, red) vs Fc/Fc⁺ in THF at a glassy-carbon electrode obtained at a scan rate of 50 mV/s. The arrow indicates the scan direction. The analyte concentration was 1 mM and the electrolyte concentration was 100 mM [nPr₄N]­[B­(C₆H₃(CF₃)₂-3,5)₄].

7. Oxidation of Cp*₂Ce­(pz^Me,Me)­(thf) (1 ^thf) with C₂Cl₆ Yielding Cp*₂, Cp*₄Ce₄Cl₆(pz^Me,Me)₂(thf) (10) and Other Products (Left): Crystal Structure of 10 (Right).

For further comparison, the alkoxide complexes Cp*₂Ce­(OR)₂ (R = Et, iPr, tBu, CH₂ tBu) display fully reversible oxidation waves of the Ce­(III/IV) redox couple at E _ox = −1.5 V. Conceivably, the pyrazolato ligand is not a strong enough donor to support the Ce­(IV) oxidation state within the metallocene scaffold. The addition of C₂Cl₆ to the lanthanum congener 1 ^thf -La in C₆D₆ was also investigated. The ¹H NMR spectrum after 10 min shows mostly starting material 1 ^thf -La and small peaks belonging to Cp*₂. The integral ratio of the pz-CH proton compared to the six proton signals of Cp*₂ at 1.15 ppm accounted for 1:0.03 and increased to 1:0.21 after 18 h (Figures S56 and S57). The proton NMR spectrum of the oxidation with 1 ^thf did not show any signals of the starting material left after 10 min. This hints at a possible Ce­(IV) intermediate followed by decomposition rather than direct ligand oxidation. It is noteworthy that the reaction of KCp* with C₂Cl₆ led also to the formation of Cp*₂ corroborating that Cp*^– can be directly oxidized by C₂Cl₆.

Propylene Carbonate Formation

Previous investigations by our group explored the catalytic activity of metal pyrazolates in the cycloaddition of epoxides and CO₂. − Naturally, we wanted to determine the effect of different azolates on the catalytic performance. The reaction was conducted in propylene oxide as the solvent with 0.5 mol% catalyst, 1 mol% TBAB (TBAB = tetra-n-butylammonium bromide) as cocatalyst and 1 bar CO₂ pressure at ambient temperature for 24 h (Table ). The conversion was determined via ¹H NMR spectroscopy. The donor-free dimethylpyrazolate 1 showed a slightly better performance than the THF adduct 1 ^thf (97% versus 95% conversion, entries 1 and 2). The sterically more demanding diphenylpyrazolate 3 displayed a slightly lower conversion of 92% (entry 3). Using the THF adduct 3 ^thf significantly decreased the carbonate formation to 75% (entry 4). Albeit having two azolate moieties, the half-sandwich congener 5 shows a catalytic activity similar to 3 ^thf (entry 5), i. e. lower than 1 ^thf . Complex 7 exhibits a significantly lower conversion of 63% compared to 1 ^thf (entry 6), probably due to the coordination of the stronger DMAP donor. The diphenyltriazolato derivatives 8 and 8 ^thf gave comparable results indicating a slightly better performance of the THF adduct (entries 7 and 8). Overall, the exchange of the pyrazolato for the triazolato ligand has only minor influence. The best performance with only traces of starting material left displayed the trimeric tetrazolato complex 9-Ce (entry 9). This is in accordance with the catalytic performance of magnesium pyrazolates which exhibited the highest activity for the fluorinated derivative Mg₂(pz^CF3,CF3)₄(thf)₃ which like complex 9-Ce did not form an isolable carbamate complex.

1. Formation of Propylene Carbonate From Propylene Oxide and CO₂, Catalyzed by the Sandwich Cerium Azolates Under Study.

entry	catalyst	conversion [%]	TON
1	1	97	194
2	1 thf	95	190
3	3	92	184
4	3 thf	75	150
5	5	74	148
6	7	63	126
7	8	87	174
8	8 thf	92	184
9	9-Ce	>99	199

^aReaction conditions: 1 bar CO₂, 0.5 mol% catalyst or 0.167 mol% for 9-Ce (≡0.5 mol% Ce centers), 1 mol% TBAB at ambient temperature for 24 h in neat epoxide.

^bDetermined via ¹H NMR by comparison of the proton integrals in α-position of the propylene oxide and the propylene carbonate.

^c((1/[Ce])/100)·conversion.

Compared to the systems described in literature the catalysts in this study only exhibit moderate catalytic activity. A heteroscorpionate lanthanum complex by the group of Otero revealed a TOF of 15,000 h^–1 and a TON value of 306,667.

Conclusion

Differently substituted sandwich cerium­(III) azolates were synthesized and their reactivity toward CO₂ examined. The pyrazolate complex Cp*₂Ce­(pz^Me,Me) inserts CO₂ forming dimeric [Cp*₂Ce­(pz^Me,Me·CO₂)]₂. Extending the steric demand from dimethyl to the diphenylpyrazolato derivative gave a bimetallic complex with only one inserted CO₂. The half-sandwich Cp*Ce­(pz^Me,Me)₂(thf)₂ exhibits exhaustive CO₂ insertion and forms a dimer with bridging and terminal pz^Me,Me·CO₂ ligands. The triazolate Cp*₂Ce­(tz^Me,Me)­(dmap) could only be isolated by utilizing the strong donor DMAP, which however thwarted the insertion of CO₂. In contrast, the donor-free triazolate Cp*₂Ce­(tz^Ph,Ph) gave CO₂ insertion in nondonating solvents. Overall, the less nucleophilic and less basic triazolato ligands seem to have a lower affinity for CO₂ insertion than the pyrazolatos. The decreasing carboxophilicity is ultimately revealed by trimeric tetrazolate complex [Cp*₂Ce­(tet^Ph)]₃ which gave no detectable insertion of CO₂. Electrochemical measurements showed quantitively irreversible oxidation events, which are accompanied by the formation of Cp*₂ as the possible oxidation product. Cp*₂ is also produced in chemical oxidations of Cp*₂Ce­(pz^Me,Me)­(thf) or KCp* with C₂Cl₆. All cerium­(III) compounds promote the cyclization reaction of propylene oxide and CO₂ to propylene carbonate although only with moderate catalytic activity.

Experimental Section

General Procedures

No uncommon hazards are noted. All reactions were performed under an inert atmosphere (Ar) by using a glovebox (MBraun UNIlab pro; <0.1 ppm of O₂, <0.1 ppm of 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. 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, toluene-d ₈ and THF-d ₈ were purchased from Sigma-Aldrich and dried over NaK alloy. C₆D₆, toluene-d ₈, and THF-d ₈ were filtered and stored in a glovebox. THF-d ₈ was also stored over molecular sieves (3 Å). 3,5-Dimethyltriazole and 5-phenyltetrazole were purchased from TCI. DMAP, potassium bis­(trimethylsilyl)­amide, 3,5-dimethylpyrazole and C₂Cl₆ were purchased from Sigma-Aldrich. Lithium bis­(dimethylsilyl)­amide was synthesized from bis­(dimethylsilyl)­amine (abcr) using nBuLi. 3,5-Diphenyltriazole, KCp*, [Cp*₂CeCl₂K­(thf)] _n , Cp*CeI₂(thf)₃ (ref ) and Cp*₂Ce­[N­(SiHMe₂)₂] were synthesized according to published procedures. {Cp*₂LaCl₂K­(thf)} was prepared in the same way as [Cp*₂CeCl₂K­(thf)] _n . Kpz^Me,Me, Kpz^Ph,Ph, Ktz^Ph,Ph, and Ktet^Ph were synthesized from the respective azoles with KN­(SiMe₃)₂ in toluene.

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, ¹³C: 125.76 MHz) at 26 °C using J. Young-valved NMR tubes. 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 ACD/NMR Processor Academic Edition (Product Version: 12.01). Multiplicities of signals are given as singulet (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. VT IR spectra were recorded using a praying mantis unit. Elemental analysis (C, H, N) was performed on a Elementar vario MICRO cube. Absorption measurements were performed on a PerkinElmer Lambda 35 spectrometer. Cyclic voltammetry (CV) experiments were performed with a Nordic Electrochemistry ECi-200 workstation applying the IR-compensation mode. The data were recorded using Nordic Electrochemistry EC4 DAQ software (version 4.1.90.1) and processed with EC-4 VIEW software (version 1.2.36.1). The CV experiments were performed in a glovebox under argon atmosphere at ambient temperature. The setup comprised a 4 mL vial, equipped with a CHI 104 glassy carbon disc working electrode (CH Instruments, Inc.), a platinum wire counter electrode, and a Ag/AgCl quasi-reference electrode. The surface of the working electrode was polished prior to the measurement. Solutions containing ∼1 mM analyte and 100 mM [nPr₄N]­[B­(C₆H₃(CF₃)₂-3,5)₄] supporting electrolyte were used for the electrochemical analysis. The potentials are reported in volts versus the Fc/Fc⁺ couple, which was used as the internal standard for cell calibration, and determined the end of each measurement.

Crystals for X-ray crystallography were handpicked in a glovebox, coated with Parabar 10,312 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 COSMO employing φ and ω scans. Raw data were processed using APEX3 and SAINT. Corrections for absorption effects were applied using SADABS. The structures were solved by direct methods and refined against F². Disorder models are calculated using DSR, a program included in ShelXle. For compound 4 and 9-Ce the serious disorder was treated using Platon/Squeeze.

Cp*₂Ce­(pz^Me,Me) (1)

Pyrazole Hpz^Me,Me (14.4 mg, 150 μmol, 1.00 equiv) in toluene (4 mL) was slowly added to a solution of Cp*₂Ce­[N­(SiHMe₂)₂] (81.4 mg, 150 μmol, 1.00 equiv) in toluene (1 mL) and stirred for 2 h. A color change from red to blue was observed. The reaction mixture was evaporated to dryness. Crystallization from n-hexane yielded blue crystals of 1 (62.4 mg, 123 μmol, 82%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 7.88 (1H, s, pz–CH), 1.97 (30H, s, Cp–CH ₃), −3.76 (6H, s, pz–CH ₃) ppm. ¹³C­{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 208.9 (Cp-CCH₃), 158.3 (pz-NC), 122.4 (pz-CH), 8.1 (pz-CH₃, Cp-CH₃) ppm. DRIFT: ṽ = 3288 (vw), 3099 (w), 2959 (s), 2910 (vs), 2858 (vs), 2725 (w), 2459 (vw), 1514 (s), 1433 (vs), 1382 (m), 1327 (w), 1145 (vw), 1049 (w), 1001 (m), 953 (m), 779 (m), 729 (w), 587 (w), 488 (vw), 434 (w) cm^–1. Elemental analysis (%) calcd. for C₂₅H₃₇CeN₂ (505.70 g/mol): C 59.38, H 7.38, N 5.54; found, C 59.82, H 7.24, N 5.70.

Cp*₂Ce­(pz^Me,Me)­(thf) (1^thf)

(a) Salt-metathesis route: [Cp*₂CeCl₂K­(thf)] _n (119 mg, 200 μmol, 1.00 equiv) and Kpz^Me,Me (26.8 mg, 200 μmol, 1.00 equiv) were suspended in THF (5 mL) and stirred for 2 d. All volatiles were removed, the yellow solid was extracted with n-hexane (2 × 5 mL) and filtered. The resulting green solution was evaporated to dryness producing a yellow powder. Crystallization from n-hexane yielded green crystals of 1 ^thf (106 mg, 183 μmol, 92%). Performing the reaction in aliphatic or aromatic solvents resulted in lower yields (62%–76%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 7.09 (1H, s, pz–CH), 3.39 (30H, s, Cp–CH ₃), −2.04 (6H, s, pz–CH ₃), −4.72 (4H, s, OCH₂CH ₂), −13.59 (4H, s, OCH ₂) ppm. ¹³C­{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 181.7 (Cp-CCH₃), 157.0 (pz-NC), 119.6 (pz-CH), 10.1 (pz-CH₃), 8.0 (Cp-CH₃) ppm. The THF signals in C₆D₆ are only visible in the 2D NMR spectra. ¹H NMR (THF-d ₈, 400.1 MHz, 26 °C): δ = 6.63 (1H, s, pz–CH), 3.54 (4H, s, OCH ₂), 3.29 (30H, s, Cp–CH ₃), 1.78 (4H, m, OCH₂CH ₂), −2.03 (6H, s, pz–CH ₃) ppm. ¹³C­{¹H} NMR (THF-d ₈, 100.6 MHz, 26 °C): δ = 178.3 (Cp-CCH₃), 156.8 (pz-NC), 119.0 (pz-CH), 68.0 (OCH₂), 26.3 (OCH₂ CH₂), 10.0 (pz-CH₃), 7.7 (Cp-CH₃) ppm. DRIFT: ṽ = 2968 (s), 2904 (vs), 2856 (vs), 2721 (w), 1910 (vw), 1841 (vw), 1764 (vw), 1519 (m), 1428 (m), 1376 (w), 1028 (m), 1007 (m), 955 (w), 872 (m), 780 (m), 726 (w) cm^–1. Elemental analysis (%) calcd. for C₂₉H₄₅CeN₂O (577.81 g/mol): C 60.28, H 7.85, N 4.85; found: C 60.01, H 7.92, N 4.82. (b) Protonolysis route: Hpz^Me,Me (14.4 mg, 150 μmol, 1.00 equiv) in THF (4 mL) was slowly added to a solution of Cp*₂Ce­[N­(SiHMe₂)₂] (81.4 mg, 150 μmol, 1.00 equiv) in THF (2 mL) and stirred for 2 h. A color change from red to yellow was observed. The reaction mixture was evaporated to dryness. Crystallization from n-hexane yielded 1 ^thf (62.4 mg, 123 μmol, 82%).

[Cp*₂Ce­(pz^Me,Me·CO₂)]₂ (2)

Cp*₂Ce­(pz^Me,Me) (55.4 mg, 110 μmol, 1.00 equiv) was dissolved in n-pentane (5 mL) and the solution stirred under 1 bar CO₂ for 60 min. The yellowish solution first turned purple followed by a precipitation of a yellow solid (43.2 mg, 39.3 μmol, 71%). Performing the reaction in C₆D₆ followed by addition of THF to the suspension and storing at −40 °C yielded yellow crystals of 2. DRIFT: ṽ = 3087 (w), 2969 (s), 2906 (s), 2858 (s), 2723 (w), 1729 (m), 1691 (vs), 1640 (vs), 1604 (vs), 1469 (m), 1378 (s), 1320 (vs), 1104 (m), 1035 (m), 979 (m), 858 (m), 803 (m), 755 (m), 599 (w), 478 (w), 413 (w) cm^–1. Elemental analysis (%) calcd. for C₅₈H₇₄Ce₂N₄O₄ (1099.42 g/mol): C 56.81, H 6.78, N 5.10; found, C 56.85, H 7.21, N 5.08.

Cp*₂Ce­(pz^Ph,Ph) (3)

(a) Salt-metathesis route: [Cp*₂CeCl₂K­(thf)] _n (119 mg, 200 μmol, 1.00 equiv) and Kpz^Ph,Ph (51.7 mg, 200 μmol, 1.00 equiv) were suspended in n-hexane (5 mL) and stirred at 40 °C for 1 d. The white precipitate was filtered off and extracted with n-hexane (5 mL). The resulting blue solution was evaporated to dryness producing a yellow powder. Crystallization from n-hexane yielded yellow crystals of 3 ^thf and blue crystals of 3. ¹H NMR (3, C₆D₆, 400.1 MHz, 26 °C): δ = 7.60 (1H, s, pz–CH), 4.60 (2H, t, para–CH), 2.95 (4H, t, meta–CH), 2.11 (30H, s, Cp–CH ₃), −5.64 (4H, d, ortho–CH) ppm. ¹H NMR (3, toluene-d ₈, 400.1 MHz, 26 °C): δ = 7.56 (1H, s, pz–CH), 4.58 (2H, t, para–CH), 2.92 (4H, t, meta–CH), 2.09 (30H, s, Cp–CH ₃, overlap with solvent signal), −5.61 (4H, d, ortho–CH) ppm. ¹³C­{¹H} NMR (3, C₆D₆, 100.6 MHz, 26 °C): δ = 219.7 (Cp-CCH₃), 159.6 (pz-NC), 124.7 (meta-C), 123.1 (para-C), 115.7 (ipso-C or ortho-C), 115.6 (ipso-C or ortho-C), 114.8 (pz-CH), 9.4 (Cp-CH₃) ppm. ¹³C­{¹H} NMR (3, toluene-d ₈, 100.6 MHz, 26 °C): δ = 219.0 (Cp-CCH₃), 159.7 (pz-NC), 124.6 (meta-C), 123.1 (para-C), 115.8 (ortho-C), 114.8 (pz-CH), 9.2 (Cp-CH₃) ppm. The signal of the ipso carbon atom overlaps with the solvent signals. DRIFT (3): ṽ = 3063 (m), 3040 (m), 2964 (m), 2898 (s), 2855 (s), 2727 (w), 1605 (m), 1467 (vs), 1444 (m), 1404 (m), 1378 (m), 1178 (vw), 1069 (w), 1047 (m), 1025 (m), 968 (s), 903 (vw), 754 (vs), 702 (s), 680 (s), 537 (m), 477 (w), 427 (m) cm^–1. Elemental analysis (%) calcd. for C₃₅H₄₁CeN₂ (629.84 g/mol): C 66.74, H 6.56, N 4.45; found, C 66.94, H 6.58, N 4.61. (b) Protonolysis route: Hpz^Ph,Ph (44.1 mg, 200 μmol, 1.00 equiv) in toluene (5 mL) was slowly added to a solution of Cp*₂Ce­[N­(SiHMe₂)₂] (109 mg, 200 μmol, 1.00 equiv) in toluene (1 mL) and stirred at 40 °C for 1 h. A color change from red to blue was observed. The reaction mixture was evaporated to dryness. Crystallization from n-hexane yielded 3 (106 mg, 168 μmol, 84%).

Cp*₂Ce­(pz^Ph,Ph)­(thf) (3^thf)

[Cp*₂CeCl₂K­(thf)] _n (119 mg, 200 μmol, 1.00 equiv) and Kpz^Ph,Ph (51.7 mg, 200 μmol, 1.00 equiv) were suspended in THF (5 mL) and stirred for 2 d. The reaction mixture was evaporated to dryness. The resulting yellow powder was extracted with n-hexane (2 × 5 mL) and then again evaporated to dryness. Crystallization from a mixture of THF and n-hexane yielded 3 ^thf as colorless crystals (96.1 mg, 137 μmol, 68%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 7.54 (1H, s, pz–CH), 5.21 (2H, t, para–CH), 4.08 (4H, t, meta–CH), 2.98 (30H, s, Cp–CH ₃), −1.31 (4H, s, ortho–CH), −1.78 (4H, s, OCH₂CH ₂), −5.48 (4H, s, OCH ₂) ppm. ¹³C­{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 203.3 (Cp-CCH₃), 160.0 (pz-NC), 130.1 (ipso-C), 125.6 (meta-C), 123.8 (para-C), 118.7 (ortho-C), 115.1 (pz-CH), 20.4 (OCH₂ CH₂), 9.3 (Cp-CH₃) ppm. According to 2D NMR spectra the THF-OCH₂ signal appears around 52 ppm, but it is not visible in recorded ¹³C spectra. DRIFT: ṽ = 3065 (w), 2960 (m), 1897 (s), 2857 (s), 2720 (w), 1606 (m), 1466 (s), 1446 (m), 1400 (m), 1222 (w), 1156 (w), 1047 (m), 1024 (s), 970 (s), 910 (m), 876 (s), 757 (vs), 696 (s), 594 (vw), 541 (w), 496 (w), 413 (m) cm^–1. Elemental analysis (%) calcd. for C₃₉H₄₉CeN₂O (701.95 g/mol): C 66.73, H 7.04, N 3.99; found, C 67.10, H 7.13, N 4.02.

Cp*Ce­(pz^Me,Me)₂(thf)₂ (5)

Cp*CeI₂(thf)₃ (149 mg, 200 μmol, 1.00 equiv) and Kpz^Me,Me (53.7 mg, 400 μmol, 2.00 equiv) were suspended in n-hexane (5 mL) and stirred at 40 °C for 2 d resulting in a yellow suspension. The white precipitate was filtered off and extracted with n-hexane (2 × 4 mL). Crystallization from n-hexane by evaporation at ambient temperature yielded colorless crystals of 5. In solution 5 slowly decomposes to 1 ^thf and unknown side products. ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 13.06 (2H, s, pz–CH), 4.98 (15H, s, Cp–CH ₃), 2.11 (18H, s, pz–CH ₃ or THF), −1.10 (18H, s, pz–CH ₃ or THF), −2.46 (14H, s, THF) ppm. ¹H NMR (THF-d ₈, 400.1 MHz, 26 °C): δ = 12.26 (2H, s, pz–CH), 3.87 (15H, s, Cp–CH ₃), 3.62 (4H, m, OCH ₂), 1.79 (20H, m, pz–CH ₃ and OCH₂CH ₂) ppm. The signal at 3.62 ppm should have an integral of 8. There are also signals of the decomposition product 1 ^thf visible. Due to the impurities some signals as well as the ¹³C NMR spectrum could not be assigned. DRIFT: ṽ = 3090 (w), 2975 (s), 2901 (vs), 2858 (s), 2719 (w), 1513 (s), 1428 (vs), 1036 (s), 1007 (s), 959 (m), 877 (m), 774 (m), 731 (m), 433 (w) cm^–1. Elemental analysis (%) calcd. for C₂₈H₄₅CeN₄O₂ (609.81 g/mol): C 55.15, H 7.44, N 9.19; found, C 55.36, H 7.37, N 9.22.

[Cp*Ce­(pz^Me,Me·CO₂)­(thf)­(μ-pz^Me,Me·CO₂)]₂ (6)

Cp*Ce­(pz^Me,Me)₂(thf)₂ (61.0 mg, 100 μmol, 1.00 equiv) was dissolved in n-pentane (5 mL) and then stirred under 1 bar of CO₂ atmosphere for 30 min resulting in a yellowish precipitate. Yield: 49.0 mg (39.2 μmol, 78%). Crystals suitable for X-ray diffraction were grown from a THF solution at −40 °C. DRIFT: ṽ = 3097 (vw), 2967 (w), 2927 (w), 2856 (w), 1728 (s), 1668 (vs), 1572 (w), 1468 (m), 1417 (s), 1383 (s), 1360 (s), 1335 (vs), 1290 (s), 1208 (w), 1128 (m), 1038 (m), 981 (w), 841 (w), 795 (m), 759 (m), 928 (vw), 581 (vw), 476 (vw), 416 (vw) cm^–1. Elemental analysis (%) calcd. for C₅₂H₇₄Ce₂N₈O₁₀ (1251.44 g/mol): C 49.91, H 5.96, N 8.95; found, C 47.86, H 5.76, N 9.75. The deviation between theoretical and experimental microanalytical data can be attributed to removal of THF from the complex under vacuum.

Cp*₂Ce­(tz^Me,Me)­(dmap) (7)

DMAP (20.8 mg, 170 μmol, 1.00 equiv) in THF (3 mL) was added to Cp*₂Ce­[N­(SiHMe₂)₂] (92.2 mg, 170 μmol, 1.00 equiv) and stirred for 15 min. Then Htz^Me,Me (16.5 mg, 170 μmol, 1.00 equiv) in THF (3 mL) was added and stirred for 2 h. The resulting yellow solution was evaporated to dryness producing a pale-yellow powder. Crystallization from THF yielded yellow crystals of 7 (64.9 mg, 103 μmol, 61%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 3.53 (30H, s, Cp–CH ₃), 1.61 (2H, s, DMAP), 0.25 (6H, s, N­(CH ₃)₂), −1.96 (6H, s, tz–CH ₃), −12.54 (2H, s, DMAP) ppm. ¹³C­{¹H} NMR (C₆D₆, 125.8 MHz, 26 °C): δ = 180.8 (Cp-CCH₃), 147.5 (tz-NC), 101.1 (DMAP), 36.5 (N­(CH₃)₂), 10.0 (tz-CH₃), 8.3 (Cp-CH₃) ppm. The other signals of the DMAP ligand were not detected. DRIFT: ṽ = 2967 (m), 2903 (s), 2858 (s), 1613 (vs), 1536 (m), 1494 (m), 1453 (s), 1393 (s), 1307 (w), 1233 (s), 1112 (w), 1066 (w), 1001 (s), 952 (w), 872 (vw), 811 (s), 742 (w), 700 (w), 534 (w) cm^–1. Elemental analysis (%) calcd. for C₃₁H₄₆CeN₅ (628.86 g/mol): C 59.21, H 7.37, N 11.14; found, C 59.48, H 7.52, N 10.97.

Cp*₂Ce­(tz^Ph,Ph) (8)

Triazole Htz^Ph,Ph (44.3 mg, 200 μmol, 1.00 equiv) in toluene (4 mL) was slowly added to a solution of Cp*₂Ce­[N­(SiHMe₂)₂] (109 mg, 200 μmol, 1.00 equiv) in toluene (1 mL) and stirred for 2 h. A color change from red to blue violet was observed. The reaction mixture was evaporated to dryness. Crystallization from n-hexane yielded blue crystals of 8 (89.9 mg, 143 μmol, 71%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 4.64 (2H, t, para–CH), 3.05 (4H, t, meta–CH), 2.22 (30H, s, Cp–CH ₃), −5.08 (4H, s, ortho–CH) ppm. ¹³C­{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 233.0 (Cp-CCH₃), 164.9 (tz-NC), 124.8 (para-C), 124.6 (meta-C), 124.2 (ipso-C), 116.5 (ortho-C), 10.6 (Cp-CH₃) ppm. DRIFT: ṽ = 3063 (w), 2958 (m), 2909 (s), 2857 (s), 2729 (w), 1605 (vw), 1468 (vs), 1426 (vs), 1407 (m), 1384 (w), 1285 (w), 1177 (w), 1070 (w), 1023 (w), 991 (m), 919 (vw), 790 (w), 728 (s), 694 (s), 488 (vw), 426 (m) cm^–1. Elemental analysis (%) calcd. for C₃₄H₄₀CeN₃ (630.83 g/mol): C 64.74, H 6.39, N 6.66; found, C 65.58, H 6.32, N 6.53. The carbon result is outside the range of analytical purity, but no better elemental analysis could be obtained to date, possibly because of cocrystallized n-hexane.

Addition of CO₂ (1 bar) to a n-pentane solution of 8 resulted in the precipitation of an off-white solid. The solid was centrifuged and dried under argon atmosphere. DRIFT: ṽ = 3067 (vw), 2971 (w), 2909 (w), 2858 (w), 1677 (w), 1584 (m), 1529 (m), 1467 (vs), 1427 (vs), 1401 (s), 1355 (s), 1070 (w), 992 (vw), 844 (w), 788 (w), 729 (s), 694 (s), 425 (vw) cm^–1.

Cp*₂Ce­(tz^Ph,Ph)­(thf) (8^thf)

[Cp*₂CeCl₂K­(thf)] _n (119 mg, 200 μmol, 1.00 equiv) and Ktz^Ph,Ph (51.9 mg, 200 μmol, 1.00 equiv) were suspended in a mixture of n-hexane (5 mL) and THF (0.5 mL) and stirred at 40 °C for 2 d. The reaction mixture was evaporated to dryness resulting in a yellow solid. Crystallization from n-hexane yielded yellow crystals of 8 ^thf (54.5 mg, 77.5 μmol, 39%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 5.62 (2H, t, para–CH), 4.80 (4H, s, meta–CH), 3.83 (30H, s, Cp–CH ₃), 1.51 (4H, s, ortho–CH), −4.31 (4H, s, OCH₂CH ₂), −13.63 (4H, s, OCH ₂) ppm. ¹³C­{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 202.7 (Cp-CCH₃), 165.7 (tz-NC), 126.0 (meta-C), 125.7 (para-C), 120.9 (ortho-C), 16.1 (OCH₂ CH₂), 10.3 (Cp-CH₃) ppm. The signals of the ipso-C and the OCH₂ were not visible in the spectrum. The ¹H–¹³C HSQC spectrum shows a weak signal for the THF-OCH₂ at around 40 ppm. DRIFT: ṽ = 3065 (m), 2973 (vs), 2905 (vs), 2859 (s), 2723 (w), 1606 (m), 1465 (s), 1424 (vs), 1402 (m), 1284 (vw), 1175 (w), 1069 (m), 1017 (m), 992 (m), 921 (w), 866 (m), 789 (m), 730 (vs), 698 (vs), 593 (vw), 553 (w), 481 (vw), 428 (m) cm^–1. Elemental analysis (%) calcd. for C₃₈H₄₈CeN₃O (702.94 g/mol): C 64.93, H 6.88, N 5.98; found, C 64.98, H 6.48, N 5.87.

[Cp*₂Ce­(tet^Ph)]₃ (9-Ce)

(a) Salt-metathesis route: [Cp*₂CeCl₂K­(thf)] _n (119 mg, 200 μmol, 1.00 equiv) and Ktet^Ph (36.9 mg, 200 μmol, 1.00 equiv) were suspended in toluene (7 mL) and stirred for 5 d. The white precipitate was filtered off and extracted with toluene (3 mL). The resulting yellow solution was evaporated to dryness giving a yellow powder. Crystallization from n-hexane yielded yellow crystals of 9-Ce (86.3 mg, 51.8 μmol, 78%). ¹H NMR (toluene-d ₈, 400.1 MHz, 26 °C): δ = 4.11 (3H, t, para–CH) 2.64 (96H, m, meta–CH and Cp–CH ₃), −5.64 (6H, d, ortho–CH) ppm. ¹³C­{¹H} NMR (toluene-d ₈, 100.6 MHz, 26 °C): δ = 192.5 (Cp-CCH₃), 143.9 (tet-NC), 126.2 (para-CH), 123.5 (meta-CH), 116.6 (ortho-CH), 114.3 (ipso-CH), 8.6 (Cp-CH₃) ppm. DRIFT: ṽ = 3061 (m), 3024 (m), 2977 (s), 2945 (vs), 2898 (vs), 2859 (vs), 2724 (m), 2529 (vw), 2449 (vw), 2022 (w), 1952 (w), 1864 (vw), 1810 (w), 1603 (m), 1520 (m), 1494 (m), 1444 (vs), 1378 (m), 1358 (s), 1279 (m), 1176 (m), 1120 (m), 1073 (m), 1012 (m), 921 (w), 785 (m), 728 (vs), 695 (s), 594 (w), 551 (vw), 507 (m), 466 (m) cm^–1. Elemental analysis (%) calcd. for C₈₁H₁₀₅Ce₃N₁₂ (1667.16 g/mol): C 58.36, H 6.35, N 10.08; found, C 58.39, H 6.37, N 10.11. (b) Protonolysis route: tetrazole Htet^Ph (29.2 mg, 200 μmol, 1.00 equiv) was suspended in toluene (7 mL) and slowly added to a solution of Cp*₂Ce­[N­(SiHMe₂)₂] (109 mg, 200 μmol, 1.00 equiv) in THF (1 mL). The color changed from red to orange. After being stirred for 3 d the reaction mixture was evaporated to dryness to yield 9 as an orange solid (109 mg, 65.4 μmol, 98%).

[Cp*₂La­(tet^Ph)]₃ (9-La)

Cp*₂LaCl₂K­(thf) (118 mg, 200 μmol, 1.00 equiv) and Kpz^Me,Me (36.9 mg, 200 μmol, 1.00 equiv) were suspended in toluene (10 mL) and stirred for 1 day. The solvent was removed and the resulting white solid was extracted with n-hexane (2 × 5 mL). The resulting colorless solution was evaporated to dryness giving a white powder. Crystallization from n-hexane yielded colorless crystals of 9-La (92.1 mg, 55.4 μmol, 84%). ¹H NMR (toluene-d ₈, 400.1 MHz, 26 °C): δ = 8.70 (6H, d, ortho–CH), 7.48 (6H, t, meta-CH), 7.28 (3H, t, para–CH), 1.98 (90H, s, Cp–CH ₃) ppm. ¹³C­{¹H} NMR (toluene-d ₈, 100.6 MHz, 26 °C): δ = 161.1 (tet-NC), 131.2 (para-CH), 129.1 (meta-CH, overlap with solvent signal), 128.4 (ortho-CH), 127.1 (ipso-C), 120.5 (Cp-CCH₃), 11.7 (Cp-CH₃) ppm. Elemental analysis (%) calcd. for C₈₁H₁₀₅La₃N₁₂ (1663.53 g/mol): C 58.48, H 6.36, N 10.10; found, C 58.33, H 6.26, N 9.96.

Cp*₂La­(pz^Me,Me)­(thf) (1^thf-La)

Cp*₂LaCl₂K­(thf) (150 mg, 254 μmol, 1.00 equiv) and Kpz^Me,Me (34.0 mg, 254 μmol, 1.00 equiv) were suspended in THF (5 mL) and stirred for 3 d. The reaction mixture was evaporated to dryness. The resulting solid was extracted with n-hexane (2 × 5 mL) followed by evaporation of the solvent. Crystallization from n-hexane yielded colorless crystals of 1 ^thf -La (102 mg, 177 μmol, 70%). ¹H NMR (C₆D₆, 400.1 MHz, 26 °C): δ = 6.19 (1H, s, pz–CH), 3.73 (4H, m, OCH ₂), 2.34 (6H, s, pz–CH ₃), 1.96 (30H, s, Cp–CH ₃), 1.35 (4H, m, OCH₂CH ₂) ppm. ¹³C­{¹H} NMR (C₆D₆, 100.6 MHz, 26 °C): δ = 143.8 (pz-NC), 118.3 (Cp-CCH₃), 110.1 (pz-CH), 70.8 (OCH₂), 25.4 (OCH₂ CH₂), 14.0 (pz-CH₃), 11.0 (Cp-CH₃) ppm. DRIFT: ṽ = 3110 (vw), 2966 (s), 2903 (vs), 2857 (vs), 2722 (w), 1519 (m), 1427 (s), 1377 (m), 1314 (w), 1245 (vw), 1177 (vw), 1089 (w), 1027 (s), 1007 (m), 955 (m), 871 (m), 780 (m), 726 (m), 672 (vw), 589 (vw), 431 (w) cm^–1. Elemental analysis (%) calcd. for C₂₉H₄₅LaN₂O (576.60 g/mol): C 60.41, H 7.87, N 4.86; found, C 60.55, H 7.44, N 4.92.

Reaction of 1^thf with Htz^Me,Me

Cp*₂Ce­(pz^Me,Me)­(thf) (14.4 mg, 25 μmol, 1.00 equiv) in THF-d ₈ (0.2 mL) was slowly added to Htz^Me,Me (2.4 mg, 25 μmol, 1.00 equiv) in THF-d ₈ (0.3 mL) and stirred for 30 min resulting in a white suspension. The ¹H NMR spectrum indicated the formation of HCp* among other unidentified products.

Reaction of 1^thf with Htet^Ph

Tetrazole Htet^Ph (2.9 mg, 20 μmol, 1.00 equiv) in toluene (2 mL) was slowly added to Cp*₂Ce­(pz^Me,Me)­(thf) (11.6 mg, 20 μmol, 1.00 equiv) in toluene (2 mL) and stirred for 30 min. The reaction mixture was evaporated to dryness. The ¹H NMR spectrum indicated the formation of HCp* among other unidentified products.

Oxidation of 1^thf with C₂Cl₆

C₂Cl₆ (2.4 mg, 10 μmol, 0.50 equiv) in C₆D₆ (0.25 mL) was added to Cp*₂Ce­(pz^Me,Me)­(thf) (11.6 mg, 20.0 μmol, 1.00 equiv) in C₆D₆ (0.25 mL) in a J. Young valved NMR tube resulting in a dark blue solution. The ¹H NMR spectrum indicated the formation of Cp*₂. Cooling a n-hexane solution of the reaction mixture to −40 °C resulted in the formation of a few colorless crystals of Cp*₄Ce₄Cl₆(pz^Me,Me)₂(thf) (10).

Oxidation of 1^thf-La with C₂Cl₆

C₂Cl₆ (2.4 mg, 10 μmol, 0.50 equiv) in C₆D₆ (0.25 mL) was added to Cp*₂La­(pz^Me,Me)­(thf) (11.5 mg, 20.0 μmol, 1.00 equiv) in C₆D₆ (0.25 mL) in a J. Young valved NMR tube resulting in a colorless solution. The ¹H NMR spectrum after 10 min showed primarily signals of the starting material and small signals of Cp*₂. The ratio of the Cp*₂ signals compared to the starting material increased over time.

Oxidation of KCp* with C₂Cl₆

C₂Cl₆ (3.6 mg, 15 μmol, 0.50 equiv) in C₆D₆ (0.5 mL) was added to KCp* (5.2 mg, 30 μmol, 1.00 equiv) and stirred for 2 h. The ¹H NMR spectrum indicated the formation of Cp*₂.

3,5-Diphenylpyrazole

Hydrazine hydrate (3.00 g, 60.0 mmol, 1.20 equiv) in EtOH (50 mL) was added to 1,3-diphenylpropane-1,3-dione (11.2 g, 50.0 mmol, 1.00 equiv) in EtOH (100 mL) and heated for 3 h under reflux. Then, saturated NaHCO₃ solution (100 mL) was added resulting in the precipitation of a white solid. The mixture was concentrated under reduced pressure and subsequently extracted with Et₂O (4 × 100 mL). The solution was dried over Na₂SO₄, filtered and the solvent was removed under reduced pressure. The yield was not determined. The ¹H NMR data were comparable to the data reported in the literature.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c03187.

• Supporting figures, detailed crystallographic data, spectroscopic data (NMR), and analytical details (PDF)

The authors are grateful to the VECTOR foundation (grant P2021-0099) for generous support.

The authors declare no competing financial interest.