Comparison of Ce(IV)/Th(IV)-alkynyl complexes and observation of a trans-influence ligand series for Ce(IV)

Abstract

Organometallic cerium(IV) complexes have been challenging to isolate and characterize due to the strongly oxidizing nature of the cerium(IV) cation. Herein, we report two cerium(IV) alkynyl complexes, [Ce(TriNOx)(C≡C-SiMe₃)] (1-Ce^TMS) and [Ce(TriNOx)(C≡C-Ph)] (1-Ce^Ph) (TriNOx³⁻ = tris(2-tert-butylhydroxylaminato)benzylamine), that include terminal alkyne moieties. The isostructural thorium analogue [Th(TriNOx)(C≡C-SiMe₃)] (1-Th^TMS) was also synthesized and compared with 1-Ce^TMS in bond distance, ¹³C-NMR spectra, vibrational spectra and electronic structure. The Ce–C bond distances were 2.501(3) Å for 1-Ce^Ph and 2.513(5) Å for 1-Ce^TMS on the shorter end of the few reported Ce^IV–C single bonds (2.478(3)–2.705(2) Å), possibly indicating significant Ce 5d- and 4f-orbital involvement. ¹³C-NMR spectroscopy was also consistent with Ce–C covalency, with significantly deshielded resonances ranging from 185–213 ppm. Such ¹³C-NMR shifts demonstrate a strong influence from spin–orbit coupling (SOC) effects, corroborated by computational studies. Raman analysis showed ν_C≡C stretching frequencies of 2000 cm⁻¹ (1-Ce^TMS) and 2052 cm⁻¹ (1-Ce^Ph), indicating the cerium(IV)–alkynyl interaction, compared to the parent HC≡CPh (IR = 2105 cm⁻¹ and Raman = 2104 cm⁻¹). L₃-edge X-ray absorption measurements revealed a predominant Ce(IV) electronic configuration, and magnetic measurements revealed temperature-independent paramagnetism. Electrochemical studies similarly revealed the electron donating ability of the alkynyl ligands, stronger than either fluoride or imido ligands for the Ce(IV)(TriNOx)-framework, with a cerium(IV/III) reduction potential of E_pc = −1.58 to −1.66 V vs. Fc/Fc⁺. Evidence for a trans-influence has been observed by evaluating a series including previously reported [Ce^IV(TriNOx)X]^+/0 complexes with axial ligands X = THF, I⁻, Br⁻, Cl⁻, F⁻, ⁻C≡C-Ph, ⁻C≡C-SiMe₃, ⁻NH(3,5-(CF₃)₂-Ar), ⁻OSiPh₃, ⁻N(M(L))(3,5-(CF₃)₂-Ar) [M(L) = Li(TMEDA), K(DME)₂ or Cs(2,2,2-crypt)]. These data stand in contrast with previous reports of an inverse trans-influence at cerium(IV) and point to differences in involvement of cerium 4f- versus 5d-orbitals in the electronic structures of the complexes.

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Introduction

The bonding interactions between f-block elements and organic fragments are of significant interest due to often complex 4f/5f- and 5d/6d-orbital involvement resulting in variable, partially covalent metal–ligand interactions.^1–5 Modern spectroscopic and computational techniques have contributed fruitful studies on this topic.^6,7 Such studies help delineate the similarities and differences between bonding in lanthanide, actinide, and d-block metal compounds for applications in reactivity, separations chemistry, and others.^4,8–10 A key avenue for exploring f-/d-bond covalency is the study of high oxidation state compounds, where the high charge on the metal cation can promote orbital overlap with varied contributions from 4/5f- and 5/6d-manifolds. Such studies require attaining stable complexes in high oxidation states with spectroscopically active ligands for study.^6,11 In lanthanide chemistry, the most accessible element in this context is cerium(IV). Our group, along with others, has devoted effort to realizing organometallic compounds featuring Ce(IV)–carbon bonds. These studies explored the redox properties and spectroscopic features, together with electronic structure calculations, to model the unique physicochemical signatures and analysis of f-element bond covalency.^1,2,12–18

Organo-cerium(IV) complexes are generally rare,² and their scarcity is likely due to redox chemistry, with carbanions prone to oxidation and cerium(IV) cations prone to reduction. Notable examples in the field include metallocene Ce(IV) complexes, heteroatom-stabilized Ce(IV)–C compounds, and Ce(IV) complexes with monodentate carbanion ligands. Metallocene complexes include cerium(IV) cyclopentadienides,^19,20 cerium(IV) cyclooctatetraenes such as [Ce(COT)₂],^15,21–23 or cerium(IV) bispentalenides, with important examples reported between 1976 and 2010.²⁴ Progress has also been made in heteroatom-stabilized cerium(IV)–carbon complexes. Furthermore, Arnold and coworkers reported the use of N-heterocyclic carbene ligands featuring Ce^IV–C_sp² bonds in 2009,²⁵ and Liddle and coworkers studied the bis(iminophophorano)methandiide (BIPM) ligand in [Ce^IV(BIPM^TMS)₂] featuring a Ce^IV–C_sp² bond in 2013 (Fig. 1).^3,14 Our group reported [Ce^IV(κ²-ortho-oxa)(MBP)₂]⁻ in 2021,² where ortho-oxa = dihydrodimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide and MBP = 2,2′-methylenebis(6-tert-butyl-4-methylphenolate). The complex featured a Ce^IV–aryl (C_sp²) bond supported by a chelating oxazolide group. Recent progress has been made in Ce–C single bond chemistry, without supporting chelating groups using monodentate supporting ligands. Chen, Li, Tamm, et al. recently used an imidazolin-2-iminato (ImN⁻) ligand featuring a Ce^IV–alkynyl (C_sp) bond in [Ce^IV(C≡C-Ph)(ImN)₃] complexes in 2024.²⁶ In the same work, the team also realized compounds featuring Ce^IV–aryl (C_sp²) and Ce^IV–alkyl (C_sp³) bonds. Independently and at the same time, La Pierre and co-workers employed a tri-tert-butyl imidophosphorane ligand on [Ce^IV(Alkyl)(NP(tert-butyl)₃)₃] featuring Ce^IV–alkyl (C_sp³) bonds, where alkyl is neopentyl (Npt) or benzyl (Bn).²⁷ Furthermore, a Ce^IV–cyclopropenyl complex featuring a Ce^IV–C_sp² bond and ring-open isomerization was recently disclosed by us.²⁸

Fig. 1 Recent examples of cerium(IV)–carbon containing complexes, including heteratom-stabilized Ce(IV) compounds, or Ce(IV) complexes with monodentate carbanion ligands.

In pursuit of cerium(IV) organometallic complexes, three primary synthetic strategies have been employed: the use of a supporting ligand with electron-donating groups to stabilize the Ce^IV oxidation state, the use of multidentate ligands with both a carbanion and electronically supporting heteroatoms coordinating to the Ce^IV cation, and tethering of ligands with a steric bulk to kinetically inhibit the homolysis of the Ce^IV–C bond.² In this study, we sought to employ the first strategy to further expand the scope of Ce^IV–C chemistry and realize Ce^IV–alkynyl complexes. The TriNOx³⁻ ligand has been used by us for the stabilization of the Ce(IV) oxidation state and a series of [Ce^IV(TriNOx)X]^+/0 axial complexes, X = THF, I⁻, Br⁻, Cl⁻, F⁻,²⁹ anilide,¹² imide,^6,12,13,30 siloxide,¹³ carbamate,³⁰ and oxo,^6,13 were synthesized previously. For the current work, H–C≡C-SiMe₃ and H–C≡C-Ph have been used as two alkyne precursors. As a result, compounds with Ce^IV–C_sp bonds were obtained without heteroatom stabilization and without steric hindrance provided by bulky substituents. This approach also provided the opportunity for a more direct analysis of the cerium(IV)–carbon bonding interactions without the influence of conjugated heteroatoms.

Herein, we report the isolation of Ce(IV)–C_sp compounds with terminal carbon atom coordination (Fig. 1). The complexes provide opportunities for spectroscopic characterization of the –C≡C– vibrations and electronic structure calculations. The observed ¹³C-NMR chemical shifts of the title compounds are remarkably downfield because of effects originating from the relativistic spin–orbit interaction. The alkynyl moiety is also found to be a strong donor for stabilizing cerium(IV) with metal redox potentials of −1.49 and −1.57 V vs. Fc/Fc⁺ (Fc = Fe(C₅H₅)₂). The information obtained in the present study has been applied to complete a trans influence series for related [Ce^IV(TriNOx)X]^+/0 complexes with axial ligands X including THF, halide, alkynyl, amide, siloxide, and imide ligands. These results provide important opportunities for improving our understanding of the bonding involving lanthanides in high oxidation states.

Results and discussion

Synthesis of cerium(IV) and thorium(IV) alkynyl complexes

The precursor complex [Ce^IV(TriNOx)Cl] (1-Ce^Cl) was obtained by mixing [Ce^III(TriNOx)(THF)] with CPh₃Cl in Et₂O in a modified reaction based on previous reports,²⁹ where the purple solid product precipitates immediately, with a simple workup and high (90%) yield. Addition of a dark purple solution of [Ce(TriNOx)Cl] in CH₂Cl₂ to a colorless solution of Li–C≡C-Ph (1.6 equiv.) in a toluene/Et₂O (1 : 1) mixture resulted in a cloudy maroon colored solution of [Ce(TriNOx)(C≡C-Ph)] (1-Ce^Ph, see Fig. 2A). The use of 1.6 equiv. Li–C≡C-Ph was based on empirical observation and used to ensure complete formation of the product 1-Ce^Ph. The solvent was stripped under reduced pressure and the resulting solid was washed with Et₂O to afford the pure product of 1-Ce^Ph in 80% yield. Maroon X-ray quality crystals were grown from a cooled (−25 °C) solution of 1-Ce^Ph in fluorobenzene with pentane diffusion over 2 days. Complex [Ce(TriNOx)(C≡C-TMS)] (1-Ce^TMS) was obtained in 64% yield, following a similar procedure where pure toluene was used as the solvent instead of a toluene/Et₂O mixture. Dark maroon X-ray quality crystals of 1-Ce^TMS were grown from a cooled (−25 °C) solution of toluene with pentane diffusion over 3 days. 1-Ce^TMS and 1-Ce^Ph exhibit different solubilities; the former is soluble in Et₂O or toluene and the latter is not. A thorium(IV) analogue was synthesized by addition of a HC≡C-SiMe₃ solution in Et₂O to a colorless solution of [Th(CH₂SiMe₃)(TriNOx)] in Et₂O dropwise, resulting in a white cloudy solution (see Fig. 2B). Complex [Th(TriNOx)(C≡C-SiMe₃)] (1-Th^TMS) was obtained in 50% yield following filtration and washing the precipitate with Et₂O. X-ray quality colorless crystals of 1-Th^TMS were similarly grown from a cooled (−25 °C) solution of toluene/dichloromethane with pentane diffusion over 3 days.

Fig. 2 Syntheses of (A) Ce^IV–alkynyl complexes (1-Ce^Ph and 1-Ce^TMS) and (B) Th^IV–alkynyl complex (1-Th^TMS).

X-ray structures of 1-Ce^Ph, 1-Ce^TMS, and 1-Th^TMS

X-ray crystallography experiments confirmed the structures of 1-Ce^Ph and 1-Ce^TMS featuring the –C≡C–R (–R = –Ph or –SiMe₃) moiety in the axial position of the [Ce^IV(TriNOx)]⁺ complex. The Ce–C bond distances were 2.501(3) Å for 1-Ce^Ph and 2.513(5) Å for 1-Ce^TMS (Fig. 3). The slightly shorter bond distance in 1-Ce^Ph is consistent with the computational results, vide infra.

Fig. 3 X-ray crystal structures of 1-Ce^TMS, 1-Ce^Ph and 1-Th^TMS. Thermal ellipsoids were plotted at a 30% probability level. For clarity, hydrogens and solvent molecules in the crystal structure are omitted; tert-butyl groups are displayed in wireframe.

These Ce^IV–C_sp bond distances are shorter than the previously reported Ce^III–C_sp bond distances in cerium–cyanide complexes (2.596(15)–2.736(17) Å),³¹ such as ([NⁿBu₄]₂[(C₅Me₅)₂Ce^III(CN)₃]); alkynyl complexes (2.549(3)–2.71(2) Å),³² such as [(C₅Me₅)₂Ce(CC^tBu)₂Li(THF)]; and a cerium(III) phenolate alkynyl complex (2.652(9) Å),³³ Na[Ce(CCPh)(bdmmp)₃] (bdmmp = 2,6-bis-(dimethylamino)-4-methylphenolate). The shorter distances of the Ce^IV–C_sp bonds in 1-Ce^Ph and 1-Ce^TMS are consistent with the ∼0.1 Å smaller ionic radius of the cerium(IV) ion compared to cerium(III) with similar coordination numbers.³⁴ The interatomic distance is summarized into a table (see Table 1 and the SI).

Table 1 Cerium–carbon distance (Å), including literature values and present systems

Bond type	Bond distance (Å)	Reference
Ce^III–Ccyanide	2.596(15)–2.736(17)	31
Ce^III–Calkynyl	2.549(3)–2.71(2)	32
Ce^III–Calkynyl	2.652(9)	33
Ce^IV–Ccarbene	2.652(7)–2.705(2)	25
Ce^IV–Caryl	2.571(7)–2.5806(19)	2
Ce^IV–Caryl	2.539(3)	26
Ce^IV–Calkyl	2.515(4)	26
Ce^IV–Calkyl	2.508(2)–2.562(2)	27
Ce^IV–Calkynyl	2.478(3)–2.523(10)	26
Ce^IV–Calkynyl	2.513(5)	1-Ce^Ph
Ce^IV–Calkynyl	2.501(3)	1-Ce^TMS
Ce^IV–Cheter-atom supported multiple bond	2.385(2)–2.441(5)	14

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These Ce^IV–C_sp bond distances are also shorter than those of previously reported single Ce^IV–C bonds in Ce^IV N-heterocyclic carbene complexes (2.652(7)–2.705(2) Å),²⁵ such as Ce^IV[OCMe₂CH₂(1-C{NCHCHNⁱPr})]₄; Ce^IV oxazolide aryl complexes (2.571(7)–2.5806(19) Å),² such as [Li(DME)₃][Ce^IV{κ²-dihydrodimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide}](2,2′-methylenebis(6-tert-butyl-4-methylphenolate)₂); Ce^IV tris(imidazoline-2-iminato) alkyl complexes (2.515(4) Å) and aryl complexes (2.539(3) Å);²⁶ Ce^IV imidophosphorane alkyl complexes (2.508(2)–2.562(2) Å),²⁷ such as Ce^IV benzyl tris(tri-tert-butyl imidophosphorane). However, our Ce^IV–C_sp bond distances are similar to the Ce^IV–C bond distances in Ce^IV tris(imidazoline-2-iminato) alkynyl complexes (2.478(3)–2.523(10) Å)²⁶ and are longer than the Ce^IV and carbon bond distances in heteroatom-supported bis(iminophosphorano)methandiide complexes (2.385(2)-2.441(5) Å),¹⁴ such as Ce{C[PPh₂N(SiMe₃)]}[O(2,6-diisopropylphenyl)]₂ (Fig. 1). The short Ce^IV–C_sp distances possibly indicate appreciable cerium(IV)–carbon interactions, especially considering that the coordination of the alkynyl moieties to the cerium is unsupported in the case of the compounds reported here.

X-ray crystallography experiments confirmed the molecular structure of related 1-Th^TMS was isostructural to 1-Ce^TMS. There are two independent molecules of 1-Th^TMS in the asymmetric unit with minor differences, with Th–C_sp bond distances of 2.593(14) and 2.605 (14) Å. Thorium(IV)–alkynyl complexes were reported previously,^35–39 where the reported Th^IV–C bond lengths ranged from 2.450(7) to 2.642(5) Å. The Th^IV–C bond distance in 1-Th^TMS is similar to or slightly longer than these literature values, likely due to the electron-donating TriNOx³⁻ ligand. This bond length in 1-Th^TMS is also 0.086 Å longer than the Ce–C distance in 1-Ce^TMS, consistent with a slightly larger ionic radius of thorium(IV) of 1.05 Å than cerium(IV) of 0.97 Å, with an oxidation state of +4 and a coordination number of 8.³⁴

NMR spectroscopy and computational studies

Consistent with our previous work,²⁹ the solution ¹H-NMR spectra of the complexes studied here indicated C_3v symmetry of the TriNOx³⁻ ligand framework with diastereotopic benzylic protons (CH₂). Complex 1-Ce^Ph exhibits –CH₂ resonances at 4.62–4.59 and 3.08–3.05 ppm and a ^tBu resonance at 0.93 ppm, while 1-Ce^TMS exhibits –CH₂ resonances at 4.57–4.54 and 3.03–3.00 ppm and a ^tBu resonance at 0.97 ppm. The overall chemical shifts were consistent with closed shell complexes for both 1-Ce^TMS and 1-Ce^Ph. Based on the chemical shifts in the ¹H-NMR spectra and cerium–carbon bond distances, we assigned 1-Ce^TMS and 1-Ce^Ph as Ce^IV complexes.

We also observed previously that the ¹H-NMR chemical shifts of the diastereotopic benzylic resonances (CH₂) and tert-butyl (^tBu) resonances in the TriNOx³⁻ arms were sensitive to the axial ligand identity. The –CH₂ and –^tBu proton resonances were used to differentiate inner- or outer-sphere coordination to the [Ce(TriNOx)]⁺ fragment in the solution phase.²⁹ For example, [Ce(TriNOx)]I exhibits ¹H-NMR CH₂ resonances between 4.73–4.67 and 4.07–4.03 ppm and a ^tBu resonance at 0.72 ppm, features that were confirmed to correspond to an outer-sphere iodide anion, whereas the [Ce(TriNOx)Cl] complex has an inner-sphere chloride axial ligand in solution with –CH₂ proton resonances spanning 4.66–4.62 and 3.16–3.12 ppm and characteristic ^tBu resonance at 0.94 ppm. In particular, the larger differences in the diastereotopic –CH₂ chemical shift ranges were correlated with the geometry difference between inner- and outer axial ligand coordination. In the current work, the chemical shifts of 1-Ce^Ph and 1-Ce^TMS resemble those of the inner-sphere [Ce(TriNOx)Cl] at around 3.1 and 0.9 ppm (Fig. 4A). Thus, the chemical shift indicates, as expected, that the alkynyl is coordinated to the cerium(IV) cation in solution, consistent with the result from cyclic voltammetry, vide infra. The thorium(IV) alkynyl complex 1-Th^TMS resembles the cerium analogue 1-Ce^TMS in C₆D₆ (see the SI) and was also assigned as an inner-sphere alkynyl complex.

Fig. 4 (A) ¹H-NMR spectra of 1-Ce^Cl, 1-Ce^Ph and 1-Ce^TMS recorded in pyridine-d₅. The spectrum of 1-Ce^TMS includes peaks for residual toluene after crystallization and drying. (B) ¹³C-NMR spectra of 1-Ce^Ph, 1-Ce^TMS and 1-Th^TMS in CD₂Cl₂.

Considering the ¹H-NMR resonances of 1-Ce^Ph and 1-Ce^TMS it is noteworthy that the upfield chemical shift of the –CH₂ signals corresponded to a relatively stronger bonding interaction of the axial ligand with the cerium(IV) cation. The trend was noted by us previously for [Ce(TriNOx)Br] (4.71–4.62, 3.34–3.30 ppm), [Ce(TriNOx)Cl] (4.66–4.62, 3.16–3.12 ppm), and [Ce(TriNOx)F] (4.64–4.60, 3.08–3.04 ppm) in pyridine-d₅.²⁹ In comparison, the –CH₂ ¹H-NMR resonances for 1-Ce^Ph (4.62–4.59, 3.08–3.05 ppm) are similar to those for [Ce(TriNOx)F], while the –CH₂ resonance for 1-Ce^TMS (4.57–4.54 and 3.03–3.00 ppm) is further upfield. This result indicates the stronger electron donating ability of –C≡C-SiMe₃ than –C≡C-Ph for the Ce^IV cation, consistent with the cerium(III/IV) electrochemistry results, the ¹³C-NMR spectroscopy results, and the observations of a trans-influence for the system (vide infra).

After confirming the chemical structures and the ¹H-NMR spectra of 1-Ce^TMS, 1-Ce^Ph and 1-Th^TMS, we turned to investigations of ¹³C-NMR spectra. Atoms directly bound to closed-shell heavy metal centers with formally empty d- and/or f-shells, especially actinides, are known to have a characteristic and often large downfield shift.^40,41 This deshielding effect has been correlated with the degree of covalency in the metal–ligand bond.^41–43 The effect has been found, for example, for C_sp³, C_sp² and hydrogen atoms in diamagnetic Th(IV) and U(VI) complexes.^7,44–48 For thorium, the largest ¹³C-NMR spectroscopy chemical shift δ 230.8 ppm was observed for Th(2-C₆H₄CH₂NMe₂)₄,⁴⁸ while the largest ¹H-NMR chemical shift was estimated using DFT calculations at 20.3 ppm for ThH[N(SiMe₃)₂]₃.⁴⁹ Recent progress in obtaining isolable cerium(IV)–carbon bonded complexes has demonstrated similar downfield shifts to those in actinide compounds,^2,3,14,25 where the largest ¹³C chemical shift of δ 343.5 ppm was observed for a Ce^IV[C(Ph₂PNSiMe₃)₂]₂ complex.³ Notably, this cerium(IV) complex had a Ce–C bond supported by heteroatom coordination and conjugation, including two phosphorus atoms bound at the central carbon, qualifying a direct comparison.

In the current cases (Fig. 4B), the ¹³C chemical shifts are 184.7 ppm for 1-Ce^Ph in CD₂Cl₂ and 207.0 ppm for 1-Ce^TMS in CD₂Cl₂ (and 213.0 ppm in C₆D₆), well outside the typical range for alkynyl resonances (65–90 ppm).⁵⁰ Thus, the strong downfield shift indicates potentially a large spin–orbit coupling contribution to the chemical shift, as with other cerium(IV)–carbon moieties.⁷ This observation also reflects the strong cerium–alkynyl bonding interactions. [Ce(TriNOx)(C≡C-Ph)] has larger ¹³C-NMR shifts at 184.7 ppm than those reported in the Ce^IV–C_sp complex [Ce^IV(ImN)₃(C≡C-Ph)] at 176.4 ppm by Chen and Li.²⁶ It should be noted that these ¹³C-NMR shifts are smaller than those reported in the Ce^IV–C_aryl complex (255.6 ppm for a cerium oxazolide complex) by us previously.² This is presumably because the ¹³C_sp²-NMR chemical shift (100–170 ppm for arenes)² is intrinsically larger than the ¹³C_sp-NMR chemical shift (60–100 ppm for alkynes).

To understand the Ce^IV/Th^IV–C bonding interactions and the influence of spin–orbit coupling, computational analyses were carried out, including calculations of the carbon NMR chemical shifts. Density functional theory (DFT) calculations were initially performed for 1-Ce^TMS, 1-Ce^Ph, and 1-Th^TMS using the B3LYP hybrid functional. Complete computational details are provided in the SI. B3LYP is frequently used for molecular structure optimizations and bonding studies and therefore we used it for these purposes in the present investigation.^51,52 Selected optimized bond distances of 1-Ce^TMS, 1-Ce^Ph, and 1-Th^TMS are compared to the experimental crystal structure data in Table S3. The optimized M–C bond metrics align closely with the experimental data, with deviations of only 0.001 Å and 0.03 Å for the Ce–C and Th–C distances, respectively, confirming the reliability of the chosen computational model.

To explore the nature of the M–C interactions, we employed natural localized molecular orbital (NLMO) analyses to assess the covalency resulting from the σ donation in 1-Ce^TMS, 1-Ce^Ph, and 1-Th^TMS. Relevant orbitals are depicted in Fig. 5, and atomic hybrid contributions in the NLMOs and bond order analysis representing the important bonding interactions are listed in Tables S4 and S5. Given the similarity in the bonding characteristic of 1-Ce^TMS and 1-Ce^Ph, we mainly focused on 1-Ce^TMS versus 1-Th^TMS. For 1-Ce^TMS, the NLMO analysis reveals a σ(Ce–C_α) bond with 17% total Ce density weight (18% 6s; 62% 5d; 20% 4f), along with two orthogonal π(C_α–C_β) bonds showing negligible Ce weights (1% each). In other words, there is a Ce—C_α σ bond which is polarized toward carbon. The 17% density weight on Ce means that electron density corresponding to about 0.34 electrons is donated to Ce (the NLMO is doubly occupied). In contrast, the σ(Th–C_α) NLMO for 1-Th^TMS exhibits 14% (60% 6d; 15% 5f) Th weight, i.e., lower than that of the Ce analogue. The calculations therefore indicate that the extent of metal–ligand covalent bonding with the ligands studied herein is somewhat more pronounced for Ce(IV)- than for Th(IV)-carbon bonds, which is in line with reported data for thorium(IV)-imido versus cerium(IV)-imido complexes from our group.⁵³ These results and a growing body of evidence suggest that the notion that lanthanide bonding is not covalent is inappropriate.

Fig. 5 Isosurfaces (±0.03 a.u.) of σ(Ce–C_α) bonding NLMOs in 1-Ce^TMS, 1-Ce^Ph, and 1-Th^TMS, along with total weight-% metal character and 6d vs. 5f contributions at the metal. (Color code: Ce yellow, Th sky blue, Si wheat, C gray, O red, N blue, and H white).

The ¹³C-NMR chemical shifts for complexes 1-Ce^TMS and 1-Ce^Ph were calculated using a functional denoted here as PBE0(40) (a PBE-based hybrid with 40% exact exchange), without and with inclusion of the SO interaction. This functional was previously shown to accurately predict ¹³C-NMR chemical shifts in organometallic cerium complexes.^2,54,55 A summary of calculated shielding constants and chemical shifts is listed in Table S6. The calculated and experimental ¹³C chemical shifts are in excellent agreement. For example, the calculated C_α shift for 1-Ce^TMS is 207.3 ppm (expt. = 207.0 ppm), which includes a 19.4 ppm deshielding contribution due to SO effects that is primarily attributed to the strong SO coupling within the Ce 4f (and 5d) shell and its involvement in the chemical bond with C_α.

For comparison, the calculated C_α shift for 1-Th^TMS is 195.5 ppm (expt. = 193.2 ppm), with a 25.4 ppm shift due to SO coupling. This is consistent with a weaker covalency of thorium compared with cerium but stronger SO coupling for Th due to the higher nuclear charge. In another comparison, the calculated C_α shift for 1-Ce^Ph is 186.8 ppm (expt. = 184.7 ppm), which includes a 20.4 ppm deshielding from the SO interaction. The similar SOC deshielding in 1-Ce^Ph relative to 1-Ce^TMS is commensurate with its similar Ce–C_α bond covalency.

Vibrational spectroscopy and analysis

Bonding interactions of alkynyl moieties with various cationic moieties were studied by vibrational spectroscopies previously.^56–65 Upon bonding to a metal cation, the metal–alkynyl (M–C) interaction might be expected to influence the vibrational frequency of the –C≡C– moiety due to metal-alkynyl σ- and π-bonding interactions.^56,65 For the present study, we have only considered the impact of σ-bonding interactions.

No vibrational spectra of cerium(IV)–alkynyl complexes have previously been reported. Here, we collected the infrared (IR) absorption and Raman spectra of 1-Ce^TMS, 1-Ce^Ph, and 1-Th^TMS to identify the vibrational frequencies of the –C≡C– moiety (Fig. 6 and the SI) and compared them with those of other metal–alkynyl complexes. For the M–C≡C-TMS complexes, the observed ν_C≡C Raman stretching frequencies were 2000 and 2003 cm⁻¹ for 1-Ce^TMS and 1-Th^TMS respectively. The notably weak Raman signal for 1-Ce^TMS was intrinsic and observed over multiple samples/measurements. The reasons for the lower Raman signal strength for 1-Ce^TMS compared to 1-Th^TMS are, as yet, unclear. For M–C≡CPh, 1-Ce^Ph (Raman = 2052 cm⁻¹) was shown to have a ν_C≡C stretching frequency that was lower than that in HC≡CPh (IR = 2105 cm⁻¹ and Raman = 2104 cm⁻¹). Vibrational spectra of related metal–acetylide complexes reported in the literature include Yb(C≡CPh)(C₅H₅)₂ (IR = 2040 cm⁻¹)⁶³ and Zr(C≡CPh)₂(C₅Me₅)₂ (IR = 2073 cm⁻¹).⁵⁷ Based on these reported results, the vibrational frequency of the alkynyl moiety reflects the metal–alkynyl σ-bond interactions (Fig. S48). We contend that the ν_C≡C stretching frequencies for 1-Ce^TMS, 1-Ce^Ph, and 1-Th^TMS are approximately intermediate between those observed for more purely ionic f-block (low frequency) and relatively more covalent d-block (high frequency) metal congeners.^1,16,21

Fig. 6 Raman spectra of 1-Ce^TMS, 1-Ce^Ph, and 1-Th^Ph, and comparison with a lithium–acetylide complex and protonated alkyne.

Magnetometry, L₃-edge X-ray absorption spectroscopy and UV-vis absorption spectroscopy

The short Ce–C bonds, downfield ¹³C chemical shifts, negative reduction potentials and characteristic ν_C≡C stretching frequencies highlighted the interactions between cerium metal and alkynyl ligands in 1-Ce^TMS and 1-Ce^Ph. These complexes present an opportunity for further study, as the unfilled low-lying 4f-orbitals in cerium(IV) can result in multi-configurational ground states in some cases.^6,66

The Ce L₃-edge (Ce 2p → 5d) X-ray absorption near edge structure (XANES) spectra of 1-Ce^TMS and 1-Ce^Ph are shown in Fig. S56–S57. A characteristic double-peaked structure is observed, which is attributed to excitations of the core hole electron (2p) to 4f¹L̄5d¹ and 4f⁰5d¹ final states, where L̄ indicates a hole (vacancy) at the ligand.^6,17 The intensities of both peaks were evaluated using established curve-fitting methods, which indicated that the weight of the 4f⁰5d¹ configuration had statistically equivalent values for both complexes, with relative weights of 0.43(3) for 1-Ce^TMS and 0.46(3) for 1-Ce^Ph. These configuration weights are comparable to those of other cerium(IV) complexes, such as cerium(IV) TriNOx³⁻ imido complexes (0.37(3)–0.45(3))⁶ and cerium(IV) TriNOx³⁻ oxo complexes (0.39(3)–0.41(3)).⁶ Although fitting was unable to quantify any difference, visual inspection of the normalized spectra shows that the feature at higher energy is more intense for 1-Ce^Ph. We reach the qualitative determination that the 4f electron density at Ce is lower for 1-Ce^Ph relative to 1-Ce^TMS.

Having evaluated the amount of charge transfer for 1-Ce^TMS and 1-Ce^Ph, we next turned to temperature dependent magnetic studies of the compounds. Van Vleck temperature-independent paramagnetism (TIP) has been observed for molecules, formally cerium(IV) complexes,^6,66–68 arising from small energy differences between the open-shell singlet ground state and admixed, low-lying triplet excited states.⁶⁷ Magnetometry studies were carried out on 1-Ce^Ph. The variable field magnetization was collected for 1-Ce^Ph at 2 K, which shows the presence of a negligible magnetic impurity that saturates below M < 0.01 μB (Fig. S52), indicating the +IV oxidation state of cerium in 1-Ce^Ph. A variable temperature magnetic susceptibility plot for 1-Ce^Ph was collected at a 1.0T applied field (Fig. S53 and S54). The susceptibility plot of χT vs. T shows a linear decrease from high (300 K) to low temperature (2 K). After accounting for the intrinsic diamagnetic contribution using Pascal's constants, the TIP value for this cerium(IV) complex was determined to be 6.34(1) × 10⁻⁴ emu mol⁻¹. This value is similar to, or larger than, those of reported cerium(IV) complexes such as cerocene (1.4(2) × 10⁻⁴ emu mol⁻¹),⁶⁸ [NEt₄]₂[CeCl₆] (1.6(2) × 10⁻⁴ emu mol⁻¹),⁶⁶ and [K(DME)₂][Ce(TriNOx)(=NAr^F)] (2.2(2) × 10⁻⁴ emu mol⁻¹).⁶ UV-vis absorption spectra were recorded for both 1-Ce^TMS and 1-Ce^Ph, where 1-Ce^TMS showed a peak at 358 nm and 1-Ce^Ph showed a peak at 382 nm, and both were attributed to LMCT (Fig. S50). Overall, our experimental data confirm pure cerium(IV) oxidation states and reveal the electronic structure and configuration for the cerium–alkynyl complex.

Electrochemical studies

To better understand the impact of the alkynyl group on the stabilization of the cerium(IV) ion, electrochemical studies were performed on 1-Ce^Ph and 1-Ce^TMS to measure the potential of the Ce(III/IV) redox couple. The electrochemical properties of [Ce^IV(TriNOx)X] were studied previously to identify the solution speciation and compare the redox potentials, where X⁻ = F⁻, Cl⁻, Br⁻, I⁻, OTf⁻ (OTf⁻ = CF₃SO₂O⁻), or [NAr^F][M(L)_x]⁻ (Ar^F = 3,5-(CF₃)₂-C₆H₃; M = Li, K, Rb, or Cs; L = THF, TMEDA, DME, or 2.2.2-cryptand, x = 1 or 2). The previously reported redox potentials of the [Ce^IV(TriNOx)]⁺ framework were in accord with the electron-donating character of the axially coordinated moieties. For example, the potentials of the reduction waves versus Fc/Fc⁺ of the Ce(IV) compounds with axial ligands ranging from weakly donating to strongly donating ligands are [Ce(TriNOx)][OTf] (E_pc = −1.04 V), [Ce(TriNOx)Br] (E_pc = −1.04/−1.16 V), [Ce(TriNOx)Cl] (E_pc = −1.26 V), [Ce(TriNOx)F] (E_pc = −1.40 V) and [M(L)_x][Ce(TriNOx) = N–Ar] (–Ar = –(3,5-(CF₃)₂C₆H₃), E_pc = −1.39 to −1.45 V) at a 100 mV s⁻¹ scan rate (Fig. 7).⁶⁹ We performed cyclic voltammetry studies of 1-Ce^TMS and 1-Ce^Ph under similar conditions (Fig. 7A). Reduction features were observed for 1-Ce^TMS at E_pc = −1.66 V vs. Fc/Fc⁺ and 1-Ce^Ph at E_pc = −1.58 V. The E_pc values compared with that of [Ce(TriNOx)][OTf] (E_pc = −1.04 V) with an outer-sphere OTf⁻ ligand indicate inner-sphere coordination of the cerium alkynyl in solution in both cases.

Fig. 7 (A) Cyclic voltammograms (CVs) of 1-Ce^TMS and 1-Ce^Ph (3 mM) in CH₂Cl₂ using [ⁿPr₄N][BAr^F₄] (100 mM) as the supporting electrolyte. Scan rate: 100 mV s⁻¹. All sweeps were performed in the oxidative direction. The trace shows reduction features for 1-Ce^TMS (E_pc = −1.66 V) and 1-Ce^Ph (E_pc = −1.58 V), followed by an oxidation feature at E_pa = −0.82/−0.81 V, the latter of which does not appear in the first scan. (B) Differential Pulse Voltammagrams (DPVs) of 1-Ce^TMS and 1-Ce^Ph.

The relatively negative E_pc values of the cerium(IV) alkynyl complexes indicate the strong electron donating character of the alkynyl ligand, notably stronger than that of the fluoride ligand, with a ∼0.2 V more negative redox potential. It should be noted that although the cerium(IV) imido complexes have a E_pc values less negative than that of the alkynyl analogues: [M(L)_x][Ce(TriNOx) = N–Ar] (–Ar = –(3,5-(CF₃)₂C₆H₃), E_pc = −1.39 to −1.45 V), this observation is likely due to the reduction of the imido ligand instead of a cerium(IV) cation. Indeed, the imido ligand is a better electron-donating ligand, as indicated in the trans-influence section, vide infra. In comparison, they have a similar reduction potential to [Li(THF)₄][Ce(κ²-ortho-oxa)(MBP)₂],²⁶ while [Ce^IV(Alkyl)(NP(tert-butyl)₃)₃] has a much more negative reduction potential (−2.55 V to −2.92 V) due to the strong electron donating –N=P(tert-butyl) moiety in addition to an alkyl ligand.²⁷ In general, these electrochemical data provide supporting evidence for a strong interaction between the Ce(IV) cation and –C≡C–R moieties (–R = –SiMe₃ or –Ph).

Reduction waves (E_pc) for 1-Ce^TMS and 1-Ce^Ph demonstrate the reduction of the Ce(IV) alkynyl complexes to corresponding Ce(III) moieties (Fig. 7A and Table 2). However, this process is largely irreversible due to an accompanying chemical process. In this process, the reduced species, [Ce^III(C≡C–R)(TriNOx)]⁻, is not stable and the alkynyl ligand presumably dissociates under the electrochemical conditions employed, similar to reported results for [Ce^IIICl(TriNOx)]⁻.²⁹ Following the cathodic features of 1-Ce^TMS and 1-Ce^Ph, the subsequent anodic sweeps (E_pa) indicate the oxidation of the newly generated Ce(III) species, likely “[Ce^III(TriNOx)]”, based on the observed oxidation potential and comparison with our previous results.²⁹

Table 2 Cerium (III/IV) redox potential. [Ce^IV(TriNOx)]⁺ derivatives, several common Ce^IV complexes, and several Ce^IV–C complexes from the literature are shown for comparison

Complex	Epc	E1/2
a Ep stands for redox potential from differential pulse voltammetry (DPV) instead of E1/2.b The scan rate was unknown, while other Epc values were measured at a 100 mV s^−1 scan rate.
[Ce(TriNOx)](OTf)	−1.04 V	−0.95 V
[Ce(TriNOx)]I	−1.04 V	—
[Ce(TriNOx)Br]	−1.16 V (−1.04 V)	—
[Ce(TriNOx)Cl]	−1.26 V	—
[Ce(TriNOx)F]	−1.40 V	−1.36 V
[M][Ce(TriNOx)=NAr]	−1.39 V to −1.45 V	—
[Ce(TriNOx)(C≡CPh)]	−1.58 V	−1.49 V (Ep)^a
[Ce(TriNOx)(C≡CTMS)]	−1.66 V	−1.57 V (Ep)^a
[N^nBu4]2[Ce(NO3)6]	—	0.62 V
[CeCl6]^2−	—	0.03 V
[Ce[N(SiMe3)2]3Cl]	—	−0.30 V
[Ce(COT)2]	—	−1.40 V
[Ce(BIPM^TMS)2]	—	−1.63 V
[Li(THF)4][Ce(κ^2-ortho-oxa)(MBP)2]	−1.67 V	—
[Ce^IV(Alkyl)(NP(tert-butyl)3)3]	^b−2.55 V to −2.92 V	—

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Differential pulse voltammetry (DPV) experiments⁷⁰ were also used to determine potential values for the redox processes of 1-Ce^TMS and 1-Ce^Ph, where E_p = −1.57 V for 1-Ce^TMS and −1.49 V for 1-Ce^Ph vs. Fc/Fc⁺ (Fig. 7B), for comparison with the measured E_1/2 values (Table 2).^29,71 These observed E_p values are more negative than those of some commonly observed cerium(IV) complexes and are comparable with those of other organo-cerium(IV) complexes such as [Ce(COT)₂] or [Ce^IV(BIPM^TMS)₂].^2,3,14

Observation of a trans-influence in the [Ce^IVX(TriNOx)] series

The trans-influence is a structural phenomenon where a ligand bonded to a metal center weakens the bond of the metal to a second ligand located in the trans position. The structural trans-influence is well established for transition metal complexes,^72,73 especially for heavy, late transition metals. Some reports, typically structural ones, were also presented on the trans-influence in lanthanide(III) complexes.^74–77 Conversely, the inverse-trans-influence has been described in high oxidation state actinide complexes,^{3,42,43,78–83} where a ligand in a complex strengthens, rather than weakens, the bond of the ligand trans to it. Our group has studied this phenomenon in uranium(V, VI) chemistry previously.^42,43,81 However, there are open questions on the appearance of a trans-influence or inverse-trans-influence in cerium(IV) complexes, where few studies have been conducted. The Liddle group discussed the possibility of an inverse-trans-influence in the tetravalent cerium(IV) phosphoimido complexes,³ but the influence(s) at cerium(IV) requires additional study with a broad series of compounds (Table 3).

Table 3 Data relevant to computational studies on trans-influence. The data include Ce–N(4) computed distance, Ce–X and Ce–N(4) Mayer bond orders, and LUMO energy

Axial ligand X to [Ce(TriNOx)]^+	Computed distance (Å)	Bond order		Energy
	Ce–N(4)	Ce–X	Ce–N(4)	ELUMO (Hartree)
THF	2.826	0.2564	0.2590	−0.1884
I^−	3.082	0.7353	0.1821	−0.0873
Br^−	3.111	0.9373	0.1712	−0.0820
Cl^−	3.129	0.9726	0.1695	−0.0821
F^−	3.147	1.0430	0.1559	−0.0767
^−C≡C-Ph	3.082	0.8552	0.1660	−0.0710
^−C≡C-TMS	3.069	0.8506	0.1691	−0.0772
^−NHAr	3.071	0.7999	0.1711	−0.0831
^−OSiPh3	3.187	0.8972	0.1485	−0.0758
=NAr Li(TMEDA)	3.228	1.1970	0.1359	−0.0520
=NAr K(DME)2	3.335	1.5158	0.1151	−0.0383
=NAr [Cs(Cryptand)]	3.510	1.7020	0.0774	0.0550

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In the current work, a trans-position has been studied for the current and previously reported, structurally conserved [Ce^IV(TriNOx)X] complexes, considering the axial nitrogen atom N(4) from the tridentate TriNOx³⁻ ligand and terminal X⁻ (or L) ligand (Fig. 8). For example, X⁻ in 1-Ce^TMS/1-Ce^Ph indicates the alkynyl ligands. The angles of X–Ce–N(4)_TriNOx of the complexes range from 173.8(3)° to 179.34(8)° in the crystal structures and range from 172.84° to 179.98° in the DFT optimized structures, indicating the validity for the trans-position of X⁻ and N(4) atoms on the cerium atom. A series of X⁻ (or L) ligands for the complexes have been compared from weakly coordinating to strongly coordinating: -THF, -halide (iodide, bromide, chloride, and fluoride), -alkynyl, -anilide, -siloxide, and -imide (Li-capped, K-capped and uncapped imides) (see the SI). The Ce(IV)–N(4) bonding interaction is understood to be weak but nevertheless an important and diagnostic metric on axial bonding in this system. It should be noted that, in the solution phase, the iodide ligand is an outer-sphere and bromide ligand is partially outer-sphere (in chemical equilibrium between the inner- and outer-sphere). But these two ligands, I⁻ and Br⁻, were considered inner spheres for the structures considered in this study. The complexes were ranked by increasing Ce–N(4) distance, excepting the halides. Among halide complexes [Ce^IV(TriNOx)X, X = F⁻, Cl⁻, Br⁻, I⁻], only the Cl⁻ complex was previously characterized by crystallography, while the rest were characterized by elemental analysis and NMR.²⁹ Here, all the halide complexes were used as model complexes for computational studies. It should also be noted that these computational studies on the trans-influence used methods, with the B3LYP hybrid functional, that were chosen for consistency with previous work on the [Ce^IV(TriNOx)X] system.²⁹

Fig. 8 Trans-influence on [Ce(TriNOx)(X)] complexes. (A) Experimental and computed Ce–N(4) distance (Cl was excluded as an outlier). (B) Computed Ce–N(4) distance and Mayer bond order. (C) Computed Ce–X bond order and Ce–N(4) bond order. (D) Computed Ce–N(4) bond distance and LUMO energy correlation.

We posited that the most telling case for the trans influence in the current system would be made with multiple lines of experimental and computational metrics that incorporate X-ray structural and computational electronic structural aspects concurrently (Fig. 8).⁴³ The computational methods undertaken here are different from those described in previous sections of the current work and a description of the trans influence computational study is also provided in the SI. As described before,²⁹ the accuracy between the computed distance of Ce–O_TriNOx and experimental result indicates an acceptable computational model performance. The Ce–X distances exhibit a good correlation between the experimental and computed results, with differences less than 0.08 Å, except for the chloride complex (0.12 Å). The Ce–N(4)_TriNOx distances also demonstrated reasonable agreement between experimental and computed values, except for chloride (Fig. 8A). We also observed a negative correlation between computed Ce–N(4) bond distance and Mayer bond order (Fig. 8B), consistent with a stronger bond with a shorter distance.

Mayer bond order has been used to compare the halide derivatives of [Ce^IV(TriNOx)X] complexes.²⁹ Based on the computational results in Fig. 8C, it is evident that a larger Mayer bond order of Ce–X (or L) exhibits a trans-influence and results in a smaller Mayer bond order of Ce^IV–N(4), indicating the stronger donating ability of the X/L ligand on cerium resulting in a weaker bond between Ce and N(4). The LUMO of these complexes primarily consists of 4f character and the LUMO energies are correlated with the electron donating ability of the ligands.⁸⁴ We observe a similar trend between the LUMO energy and Ce–N(4) distance, where a stronger Ce–X bond and longer but weaker Ce–N(4) bond result in a higher LUMO energy (Fig. 8D), similar to previous observation.²⁹

To better illustrate the trans-influence, this phenomenon can be interpreted using the Ce–N(4) experimental bond distances (Table S1). Weakly coordinated THF resulted in a Ce–N(4) bond length of 2.717(9)–2.787(8) Å and the chloride (Cl⁻) ligand resulted in a Ce–N(4) bond length of 2.825(3) Å. The relatively strong sigma-donating alkynyl ligands yielded a Ce–N(4) bond length of 2.906(2) Å for [Ce(TriNOx)(C≡C-Ph)] and 2.932 Å for [Ce(TriNOx)(C≡C-TMS)], whereas the anilide ligand resulted in a bond length of 2.934(3) Å for [Ce(TriNOx)(NHAr)] (Ar = (3,5-(CF₃)₂C₆H₃)). Thus, notably, the alkynyl ligand exhibits a strong donating ability comparable to that of the anilide ligand, indicating a stronger bonding interaction between cerium and alkynyl moieties (see the SI). By way of comparison, the extraordinarily strongly coordinating uncapped imido ligand gives a bond distance of 3.335 Å for Ce–N(4) of [Cs(2,2,2-cryptand)][Ce(TriNOx)(NAr)]. To conclude, the bond distance and bond order well reflect the bonding interactions of the Ce–X and Ce–N(4) moieties and illustrate a structural trans-influence is operative in this series.

Conclusions and outlook

The synthesis and characterization of cerium(IV)/thorium(IV) alkynyl complexes, 1-Ce^Ph, 1-Ce^TMS, and 1-Th^TMS, have expanded the understanding of cerium/throium-carbon bonding by providing insights into their structural, spectroscopic, and electrochemical properties. These studies reveal the unique covalent contributions of Ce 5d- and 4f-orbitals in M–C bonding interactions, as evidenced by X-ray crystallography, Raman spectroscopy, and ¹³C-NMR spectroscopy chemical shifts. Comparative analyses with thorium(IV) analogues and other cerium(IV) complexes highlight the strong electron-donating nature of alkynyl ligands, resulting in notable redox properties. Furthermore, these findings challenge traditional notions of f-element bonding, by emphasizing the significant covalency in Ce(IV)–carbon bonds and the influence of spin–orbit coupling effects. The observed trans-influence in the [Ce^IV(TriNOx)X] framework enriches our understanding of ligand-field effects in high oxidation state lanthanide complexes and offers a foundation for future studies on bonding and possibly unique reactivity in f-block chemistry.

Author contributions

Q. Y. proposed and synthesized 1-Ce^TMS and 1-Ce^Ph, conducted NMR spectroscopy, electrochemistry, UV-vis spectroscopy, and trans-influence studies and drafted an initial manuscript draft. Q. Y. and E. L. synthesized 1-Th^TMS. X. Y. and J. A. conducted the computational analysis for 1-Ce^TMS, 1-Ce^Ph and 1-Th^TMS. The trans-influence computational study was carried out by Q. Y. with input from X. Y. and J. A. aspects of the work. P. P. and Q. Y. conducted the Raman and IR studies. P. P. conducted elemental analysis of the three complexes reported. P. W. S. and S. G. M. conducted the XAS studies. H. G. conducted the magnetism studies with Q. Y. taking part. Q. Y., E. L., X. Y., J. A. and E. J. S. wrote the manuscript. All co-authors participated in the revisions. M. R. G. and P. J. C. resolved the crystal structures. E. J. S. supervised all aspects of the project.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Crystallographic data for 1-Ce^TMS, 1-Ce^Ph and 1-Th^TMS have been deposited at the CCDC under 2448118–2448120 and can be obtained from https://www.ccdc.cam.ac.uk/structures/.

CCDC 2448118–2448120 contain the supplementary crystallographic data for this paper.^85–87

The supplementary information includes NMR spectroscopic, X-ray crystallographic, electrochemical, vibrational spectroscopic, UV-vis spectroscopic, magnetometric, X-ray absorption spectroscopic, and computational data. See DOI: https://doi.org/10.1039/d5sc03222a.

Acknowledgements

Q. Y. thanks Dr Bren E. Cole for his mentorship, support and assistance on [Ce(TriNOx)Cl] synthesis. E. J. S. thanks the U. S. National Science Foundation (CHE-2247668) for financial support of this work. E. J. S. also thanks the University of Pennsylvania for support. This work was supported at Lawrence Berkeley National Laboratory by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences Heavy Element Chemistry (HEC) program of the United States Department of Energy (DOE) under contract DE-AC02-05CH11231. We also thank JASCO for use of Raman instrumentation in the JASCO facility for Spectroscopic Excellence in the Chemistry Department at the University of Pennsylvania. X. Y. and J. A. thank the center for computational research at U. Buffalo (https://hdl.handle.net/10477/79221) for providing computational resources.

Notes and references

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Footnotes

† Current address: 9700 S. Cass Avenue, The Center of Nanoscale Materials, Argonne National Lab, IL 60439 (USA).

‡ Current address: 410 Forest Ave, Sheboygan Falls, Ereztech Labs, WI 53085.

§ Current address: Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.

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