molecules Article Effect of Substituents of Cerium Pyrazolates and Pyrrolates on Carbon Dioxide Activation Uwe Bayer, Adrian Jenner, Jonas Riedmaier, Cäcilia Maichle-Mössmer and Reiner Anwander * Institute of Inorganic Chemistry, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany; [email protected] (U.B.); [email protected] (A.J.); [email protected] (J.R.); [email protected] (C.M.-M.) * Correspondence: [email protected] Abstract: 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(m-pyr)2(thf)2][Li(thf)4]2 (pyr = pyrro- lato) 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. Keywords: cerium; pyrazoles; pyrroles; carbazoles; carbon dioxide Citation: Bayer, U.; Jenner, A.; Riedmaier, J.; Maichle-Mössmer, C.; Anwander, R. Effect of Substituents of 1. Introduction Cerium Pyrazolates and Pyrrolates on Rare-earth–metal complexes are capable of efficiently activating carbonylic com- Carbon Dioxide Activation. Molecules pounds including carbon dioxide [1–6]. Due to its environmental impact, atmospheric CO2 2021, 26, 1957. https://doi.org/ management and, in particular, sustainable solutions for CO2 emission control evolved 10.3390/molecules26071957 as a top-priority issue in academic and industrial research [7–11]. On the one hand, this can be realized by capturing and storing carbon dioxide with tailor-made surface-reactive Academic Editor: Elena V. Grachova materials [12–16]. 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 chem- Received: 12 March 2021 istry [17–23]. For example, rare-earth metals have successfully been studied as catalysts Accepted: 24 March 2021 Published: 31 March 2021 for the copolymerization of carbon dioxide and epoxides to yield polycarbonates [24–30]. Various rare-earth-metal-based (pre)catalysts are known to promote the catalytic cycload- dition of carbon dioxide and epoxide-producing cyclic carbonates, which then, in case of Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in propylene carbonate, can be used as electrolyte solvent in lithium-ion batteries [30–37]. published maps and institutional affil- However, these complexes often lack the catalytic activity and superb performance of zinc iations. or cobalt-based systems [4]. We have recently described the application of homoleptic ceric pyrazolate [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 Copyright: © 2021 by the authors. pyrazole derivatives as ligands for cerium(IV). Since we hypothesized that the basicity of Licensee MDPI, Basel, Switzerland. This article is an open access article the ligand plays a crucial role in any reversible CO2 uptake, we also envisaged different distributed under the terms and N-proligands. Pyrroles feature a mono-aza five-membered ring, exhibiting a pKa value that conditions of the Creative Commons is larger than that of pyrazoles, but close to silylamines (Figure1) [ 40]. Metal silylamides, Attribution (CC BY) license (https:// however, were shown to engage in a cascade of reactions with CO2, ultimately affording creativecommons.org/licenses/by/ metal siloxides [41–43]. Whereas pyrrole-derived pincer-type ligands are quite popular in 4.0/). rare-earth–metal coordination chemistry [44–51], examples of pure pyrrolyl ligands have Molecules 2021, 26, 1957. https://doi.org/10.3390/molecules26071957 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 13 cascade of reactions with CO2, ultimately affording metal siloxides [41–43]. Whereas Molecules 2021, 26, 1957 pyrrole-derived pincer-type ligands are quite popular in rare-earth–metal2 of 12 coordination chemistry [44–51], examples of pure pyrrolyl ligands have remained scarce [52–55]. Likewise, rare-earth–metal complexes bearing a carbazolyl ligand have been reported [56– remained scarce [52–55]. Likewise, rare-earth–metal complexes bearing a carbazolyl ligand Molecules 2021, 26, x FOR PEER REVIEW62]. As a pyrrole derivative, carbazole exhibits a pKa value2 of that 13 matches that of pyrazole have been reported [56–62]. As a pyrrole derivative, carbazole exhibits a pKa value that matches(Figure that 1). of pyrazole (Figure1). cascade of reactions with CO2, ultimately affording metal siloxides [41–43]. Whereas a) a) b) b) pyrrole-derived19.8 pincer-type23.0 ligands25.8 are quite popular36.0 in rare-earth–metal coordination chemistry [44–51], examples of pure pyrrolyl ligands have remained scarce [52–55]. HN(SiMe ) HNiPr Likewise, rare-earth–metalNH NHcomplexes bearing3 2 a carbazolyl2 ligand have been reported [56– 62]. As aN pyrrole derivative, carbazole exhibits a pKa value that matches that of pyrazole (Figure 1). 19.8a) 23.0a) 25.8b) 36.0b) 20 24 36 pKa HN(SiMe ) HNiPr NH NH 3 2 2 N H HN(SiHMe2)2 N 22.8b) 20 24 36 pKa H HN(SiHMe2)2 N 22.8b) 19.9a) a) b) Figure19.9 1. a)pKa values of the different N-proligands [40,63,64]. Determined in DMSO,a) determined Figure 1. pKa values of the different N-proligands [40,63,64]. Determined in DMSO, inb) THF. Figure determined 1. pKa values inof theTHF. different N-proligands [40,63,64]. a) Determined in DMSO, 2.b) determined Results and in THF. Discussion 2.1. Homoleptic Ceric Pyrazolates 2. Results Results and Discussionand Discussion 2.1. HomolepticHomoleptic Ceric cerium Pyrazolates di-tert -butyl pyrazolate Ce(tBu2pz)4 (1) was synthesized by treat- ment2.1. Homoleptic of Ce[N(SiHMe Ceric2)2]4 Pyrazolateswith four equivalents of tBu2pzH as previously described Homoleptic cerium di-tert-butyl pyrazolate Ce(tBu2pz)4 (1) was synthesized by (Scheme1)[ 65]. Likewise, Ce(Ph pz) (2) and Ce(tBuMepz) (3) were synthesized us- treatment of Ce[N(SiHMe2)2]4 with four2 equivalents4 of tBu2pzH as previously4 described2 4 Homoleptic cerium di-tert-butyl1 pyrazolate Ce(tBu pz) (1) was synthesized by ing(Scheme the corresponding 1) [65]. Likewise, pyrazole Ce(Ph2pz) (Scheme4 (2) and1 ).Ce( ThetBuMepz)H NMR4 (3) spectrumwere synthesized of 2 shows using a singlet treatment of Ce[N(SiHMe2)2]4 with four equivalents of tBu2pzH as previously described forthe thecorresponding proton in pyrazole the pyrazolato (Scheme backbone 1). The 1H atNMR 7.11 spectrum ppm and of 2 two shows multiplets a singlet atfor 6.91 and 7.78the(Scheme proton ppm forin 1) the the [65]. pyrazolato aromatic Likewise, backbone protons Ce(Ph (Figureat 7.11 ppm2 S1,pz) Supportingand4 ( 2two) and multiplets Information).Ce( tatBuMepz) 6.91 and In 7.78 contrast4 (3) were to synthesized using theppmthe complexes correspondingfor the aromatic bearing protons symmetric pyrazole (Figure pyrazolato S1, (Scheme Supporting ligands, 1). Information). The compound 1H NMRIn 3contrastwith spectrum the to asymmetricthe of 2 shows a singlet for 3-complexestert-butyl-5-methyl bearing symmetric pyrazolato pyrazolato ligand ligands, could not compound be obtained 3 with as athe crystalline asymmetric material, 3- but astertthe a-butyl-5-methyl dark proton red sticky in pyrazolatothe solid pyrazolato upon ligand removing could backbone thenot be volatiles obtained at in asvacuo.7.11 a crystalline ppm The general material,and compositiontwo but multiplets of at 6.91 and 7.78 as a dark red sticky solid upon removing1 the volatiles in vacuo. The general composition [Ce(ppmtBuMepz) for the4]n wasaromatic confirmed protons by H NMR (Figure spectroscopy S1, showingSupporting singlets Information). at 1.24 ppm for In contrast to the of [Ce(tBuMepz)4]n was confirmed by 1H NMR spectroscopy showing singlets at 1.24 ppm thecomplexestBu groups, bearing at 2.22 ppm symmetric for the methyl pyrazolato groups, and at ligands, 6.15 ppm forcompound the C−H proton 3 with of the asymmetric 3- thefor the five-membered tBu groups, at pyrazole 2.22 ppm ringfor the (Figure methyl S2, groups, Supporting and at 6.15 Information). ppm for the However,C−H proton elemental of the five-membered pyrazole ring (Figure S2, Supporting Information). However, analysistert-butyl-5-methyl displayed someextent pyrazolato of impurification, ligand indicatedcould not by anbe increased obtained carbon as a value crystalline material, but elemental analysis displayed some extent of impurification, indicated by an increased mostcarbonas a likelydark value stemming mostred likelysticky from stemming solid retained from upon solvent. retained removing solvent. the volatiles in vacuo. The general composition of [Ce(tBuMepz)4]n 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. Scheme 1. Synthesis of ceric pyrazolates 1 to 3.