Cerium–quinone redox couples put under scrutiny

Abstract

Homoleptic cerous complexes Ce[N(SiMe₃)₂]₃, [Ce{OSi(OtBu)₃}₃]₂ and [Ce{OSiⁱPr₃}₃]₂ were employed as thermally robust, weakly nucleophilic precursors to assess their reactivity towards 1,4-quinones in non-aqueous solution. The strongly oxidizing quinones 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or tetrachloro-1,4-benzoquinone (Cl₄BQ) readily form hydroquinolato-bridged ceric complexes of the composition [(Ce^IVL₃)₂(μ₂-O₂C₆R₄)]. Less oxidising quinones like 2,5-di-tert-butyl-1,4-benzoquinone (tBu₂BQ) tend to engage in redox equilibria with the ceric hydroquinolato-bridged form being stable only in the solid state. Even less oxidising quinones such as tetramethyl-1,4-benzoquinone (Me₄BQ) afford cerous semiquinolates of the type [(Ce^IIIL₂(thf)₂)(μ₂-O₂C₆Me₄)]₂. All complexes were characterised by X-ray diffraction, ¹H, ¹³C{¹H} and ²⁹Si NMR spectroscopy, DRIFT spectroscopy, UV-Vis spectroscopy and CV measurements. The species putatively formed during the electrochemical reduction of [Ce^IV{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆H₄) could be mimicked by chemical reduction with Co^IICp₂ yielding [(Ce^III{N(SiMe₃)₂}₃)₂(μ₂-O₂C₆H₄)][Co^IIICp₂]₂.

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Introduction

Quinones are multifunctional organic molecules exhibiting intriguing redox behaviour.^1,2 Of particular note is their importance in biological electron–transfer processes (photosynthesis, respiration)³ and in industrial catalysis (anthraquinone process for hydrogen peroxide production).⁴ Quinones can engage in one or two electron redox processes involving the formation of either semiquinolates or hydroquinolates.⁵ Strikingly, the reduction potential of 1,4-benzoquinones (para-benzoquinones) can easily be modified by introducing electron-withdrawing or donating substituents into the benzene ring.^5,6 As a consequence, tetrachloro-1,4-benzoquinone (chloranil, Cl₄BQ) and even more so 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) emerged as efficient oxidants in organic synthesis.⁷ DDQ has been further successfully applied in photoredox catalysis.⁸ Moreover, anionic η⁴-1,4-benzoquinone manganese tricarbonyl features a quinoid π-complex, broadly used for the fabrication of supramolecular metal–organometallic coordination networks.⁹ Relatedly, deprotonated variants of 2,5-dihydroxy-1,4-benzoquinone (DHBQ) were shown to act as rigid ditopic linkers,¹⁰e.g., to support the formation of pentagonal dodecahedral Ce₂(H₂O)₁₈ cages or in permanently porous aluminium frameworks.¹¹ DHBQ was also probed as a bridging redox-active ligand in bimetallic [LnCl₂(thf)₃]₂(μ-bobq) (Ln = Y, Dy; bobq = 2,5-bisoxide-1,4-benzoquinolato) to build single-molecule magnets.¹² More recently, the related semiquinolato radical-bridged dimeric complexes [LnCl₂(thf)₃(μ-Me₄sq)₂]₂ (Ln = Y, Gd) were obtained by oxidation of the corresponding in situ formed hydroquinolate complexes with FeCl₃.¹³ Semiquinolato-bridged scandium(III) species were reported to promote self-organised electron transfer from d-transition metals (Ir, Fe) to 1,4-quinones.^14,15

Targeted metal-redox chemistry with quinones has been a recurring issue for the rare-earth-metal couples Ln(II)/Ln(III)¹⁶ and Ce(III)/Ce(IV).¹⁷ Especially in the case of molecular cerium chemistry,¹⁷ its unique single-electron-transfer (SET) pathway has recently been extended beyond the traditional application of ceric ammonium nitrate (CAN; redox potential of 1.61 V vs. NHE) in organic synthesis¹⁸ to photoredox catalysis.¹⁹ On the other hand, redox protocols are known to provide efficient access to metalorganic Ce^IV complexes. Typically, such Ce^III → Ce^IV transformations are promoted by halogenating oxidants (e.g. C₂Cl₆, Ph₃CCl, PhICl₂, TeCl₄, FcPF₆, FcBF₄, Ph₃CBF₄, Ph₃CPF₆, I₂),²⁰ silver salts (AgX, X = F, I, BF₄, OTf)²¹ or dioxygen.^20b,22

Archetypical 1,4-benzoquinone (BQ) has been established as a versatile oxidant for the synthesis of homoleptic ceric complexes CeL₄ from cerous ate complexes [CeL₄M(do)_x] via tandem oxidation-ligand redistribution protocols (L = monoanionic ligand, M = alkali metal and do = donor solvent; separation of an alkali-metal hydro-/semiquinolate).²³ In the presence of sterically demanding ligands L, BQ was also shown to form hydroquinolato (hq)–bridged ceric complexes of the general composition [L₃Ce–OC₆H₄O–CeL₃].^20g,24 This very Ce^III → Ce^IV transformation was pioneered by Sen et al. in 1992, resulting in the isolation of [(tBu₃CO)₃Ce(OC₆H₄O)Ce(OCtBu₃)₃] (Chart 1, I).^24a In the same paper, the oxidation of Ce(OCtBu₃)₃ with 2,6-di-tert-butyl-1,4-benzoquinone to the terminal Ce^IV-semiquinolate radical (tBu₃CO)₃Ce(O₂C₆H₂tBu₂) was described as evidenced by ¹H NMR and EPR spectroscopic measurements.^24a More recently, Schelter et al. reported on hq-bridged complex II resulting from the oxidation of cerous Ce(BINOlate)₃(thf)Li₃(thf)₄ with 0.5 equivalents of BQ.^24b Similarly, our group synthesized [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆H₄)^24c (III) and (CeCp^R₃)₂(μ₂-O₂C₆H₄) (Cp^R = C₅H₄Me (IV) and C₅H₄(SiMe₃) (V)).^20g In contrast, the reaction of BQ with [Ce(Me₂pz)₃]_x featuring the sterically less demanding and increasingly nucleophilic 3,5-dimethylpyrazolato ligand (Me₂pz) led in fact to a transient Ce^IV hydroquinolate species (as indicated by the characteristic colour change), which, however, at ambient temperature was converted into the isolable trimetallic Ce^III complex Ce₃(pchd)₂(Me₂pz)₅(thf)₂ (pchd = 1,4-bis(3,5-dimethylpyrazol-1-yl)cyclohex-2,5-diene-1,4-diolato).^23c Apparently, the new pchd ligand formed via 1,4-nucleophilic attack at bq by two adjacent Me₂pz ligands. This nucleophilic reaction pathway could be prevented by using bulky tBu groups on the pz ligand, but homolpetic Ce(tBu₂pz)₄ was formed as the main ceric product via irreversible ligand rearrangement.^23c

Chart 1 Structurally characterised dicerium(IV) hydroquinolate complexes (I–V),^20g,24 obtained via oxidation of cerous precursor with BQ.

As such, cerium–1,4-benzoquinone couples have revealed distinct redox chemistry, we became curious about as to what extent such redox transformations are affected by both the type of 1,4-benzoquinone oxidant and the molecular Ce^III precursor. The present study uncovers some unexpected correlation between Ce^IV–hydroquinolato stabilisation and quinone oxidant strength, as well as a new path to p-semiquinolato–radical-bridged rare-earth-metal complexes.

Results and discussion

Molecular redox precursors

The quinones used in this study comprise 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachloro-1,4-benzoquinone (Cl₄BQ), 1,4-benzoquinone (BQ), tetramethyl-1,4-benzoquinone (Me₄BQ), 2,5-di-tert-butyl-1,4-benzoquinone (tBu₂BQ), 1,4-naphthoquinone (NQ), and 9,10-anthraquinone (AQ). All are commercially available and were selected according to their reduction potentials spanning a E⁰ range of 89 to 887 mV (2e⁻/2H⁺, vs. NHE, cf., Scheme 1).^5,25 The cerous precursors were chosen according to the criteria solubility, weak nucleophilicity, proven access to the tetravalent state, and a stabilizing effect on the latter. Furthermore, the use of sterically bulky ligands was assumed to minimise the occurrence of ligand redistribution reactions. Accordingly, homoleptic Ce[N(SiMe₃)₂]₃ (1) appeared to be an ideal benchmark system.^24c After additional investigations into the respective pyrazolate chemistry, the abovementioned [Ce(R₂pz)₃] (R = Me, tBu) were discarded because of persisting alternative reaction pathways like 1,4-nucleophilic attack of BQ by Me₂pz and ligand redistribution (formation of Ce(tBu₂pz)₄).^23c The new pyrazolate studies clearly confirmed that steric hindrance of both the pyrazolato ligand and the 1,4-benzoquinone can minimise/counteract such undesired reactions, but the formation of product mixtures seems inevitable. Products crystallised from these reactions include minor amounts of ceric [Ce(tBu₂pz)₃(thf)]₂(Me₄hq) or a cerous product of partial pyrazolyl-promoted nucleophilic attack Ce₃(bpad)(pasq)(Me₂pz)₆(thf) (bpad = 1,4-bis(3,5-dimethylpyrazol-1-yl)anthra-1,4-diolato; pasq = 1-(3,5-dimethylpyrazol-1-yl)anthra-1,4-semiquinolato) (84%) mixed with semiquinolate [Ce(Me₂pz)₂(thf)₂(asq)]₂ (asq = anthra-semiquinolato; cf. ESI† for structural details). The use of Ce^III halides was discarded mainly for solubility issues.

Scheme 1 Oxidation of trivalent cerium complexes with 1,4-quinone derivatives under formation of hydroquinolato-bridged ceric complexes. To enable better assessment of the relative oxidation ability 2e⁻/2H⁺ reductions potentials are given in mV versus NHE according to ref. 5. (a) Calculated value.

In addition to silylamide 1, the siloxide derivatives [Ce{OSi(OtBu)₃}₃]₂ (2)^21d,26 and [Ce(OSiⁱPr₃)₃]₂ (3) were assessed as suitable cerous precursors. Complexes 2 and 3, with and without intramolecular donor site, respectively, were readily obtained in pure form via protonolysis of 1 with the corresponding silanol.²⁶ The crystal structure of the new complex 3 revealed a dimeric arrangement with two μ₂-bridging and four terminal siloxy groups (Fig. 1), similar to that found for tris(tert-butoxy)siloxy congener 2 or [Ce(OSiPh₃)₃]₂ ²⁷ or [Ce(OCHtBu₂)₃]₂.²⁸ The Ce–O_terminal (2.1659(14) and 2.1671(14) Å) and the Ce–O_μ2 distances (2.3951(12) and 2.4030(12) Å) of 3 are slightly shorter than those in 2 (Ce–O_terminal 2.202(3), 2.186(3) Å; Ce–O_μ2 2.532(2) Å) and [Ce(OSiPh₃)₃]₂ (Ce–O_terminal 2.141(7), 2.185(6) Å; Ce–O_μ2 2.345(6), 2.583(5) Å) reflecting the lower coordination number (CN 4 vs. 5), but slightly longer than in [Ce(OCHtBu₂)₃]₂ (Ce–O_terminal 2.142(2), 2.152(3) Å; Ce–O_μ2 2.363(3) Å).²⁸ The ¹H NMR spectrum of 3 in C₆D₆ shows two singlets at −28.82 and −17.23 ppm for the μ₂-OSiⁱPr₃ groups and two singlets at 6.46 and 9.09 ppm for the terminal siloxy ligands indicating a non-fluxional dimeric species in non-coordinating solvents. When recorded in THF-d₈, only two signals for the OSiⁱPr₃ groups appeared, in accordance with the formation of a monomeric adduct [Ce{OSiⁱPr₃}₃(thf-d₈)_x].

Fig. 1 Crystal structure of [Ce{OSiⁱPr₃}₃]₂ (3). Ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [°]: Ce1–O1 2.3951(12), Ce1–O2 2.1659(14), Ce1–O3 2.1671(14); Ce1–O2–Si2 163.03(9), Ce1–O3–Si3 167.40(10).

Quinone oxidation of Ce[N(SiMe₃)₂]₃ (1)

Treatment of Ce[N(SiMe₃)₂]₃ (1) with each 0.5 equivalents of DDQ, Cl₄BQ, Me₄BQ, tBu₂BQ and NQ, in a mixture of toluene and n-hexane, immediately led to a colour change from yellow to dark brown. Upon recrystallisation from toluene/n-hexane mixtures it was possible to isolate the hydroquinolato-bridged complexes [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆Cl₄) (4^Cl4hq), [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆Cl₂(CN)₂) (4^ddhq), [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆Me₄) (4^Me4hq), [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆tBu₂H₂) (4^tBu2hq) and [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₁₀H₆) (4^nhq) in very good crystalline yields of 71 to 90% (Scheme 1). The crystal structures of the new complexes 4^xhq are isostructural to the previously reported derivative 4^hq24c and only differ in the bridging hq linker (Fig. 2). The Ce1–O1 distances of 2.084(6) to 2.173(2) Å (for a full list of interatomic distances see Table 1) are in the same range as found for other hq-bridged cerium complexes (2.086(10)–2.143(5) Å).^20g,24 Likewise, the Ce1–N bond lengths compare well to other Ce^IV silylamides like [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆H₄) (2.2388(14)–2.2487(14) Å),^24c Ce[N(SiMe₃)₂]₃Cl (2.217(3) Å),^20a and Ce[N(SiHMe₂)₂]₄ (2.2378(11)–2.2574(11) Å).^20d Also, the C–C distances of the hq linker converge as the expected aromatic ring is formed and the C–O distances of 1.318(3) to 1.378(3) Å corroborate the formation of C–O single bonds.

Fig. 2 Crystal structures of [Ce{OSi(OtBu)₃}₃(Me-thf)_2]2(μ₂-O₂C₆H₄) (5^hq·2MeTHF), [Ce{OSiⁱPr₃}₃(thf)]₂(μ₂-O₂C₆Cl₄) (6^Cl4hq), [Ce{N(SiMe₃)_2}3]2(μ₂-O₂C₆Cl₂CN₂) (4^ddq), [Ce{N(SiMe₃)_2}3]2(μ₂-O₂C₆Me₄) (4^Me4hq), and [Ce{OSiiPr_3}3(thf)]₂(μ₂-O₂C₆tBu₂H₂) (6^tBu2hq) (from top down). Ellipsoids are shown at the 50% probability level. Hydrogen atoms, disordered ligands and lattice solvents are omitted for clarity. Selected interatomic distances for 5^hq, 6^Cl4hq, 4^ddq, and 4^Me4hq are given in Tables 1 and 2. Selected bond lengths for 6^tBu2hq [Å]: Ce1–O1 2.109(3), Ce1–O2 2.104(3), Ce1–O3 2.097(3), Ce1–O4 2.107(3), C1–C2 1.391(6), C2–C3 1.391(6), C1–C3′ 1.405(6), C1–O1 1.350(5).

Table 1 Selected analytical data of complexes 4^hq, 4^Cl4hq, 4^ddhq, 4^Me4hq, 4^tBu2hq, 4^nhq. Interatomic distances are given in [Å], angles in [°], chemical shifts in [ppm], μ_eff in [BM], UV/Vis absorption band in [nm], and E_pc/E_pa in [V vs. Fc/Fc⁺]

Complex	4^hq ^24c	4^Cl4hq	4^ddhq	4^Me4hq	4^t^Bu2hq	4^nhq
a NMR spectra recorded in C6D6. b UV-Vis spectra recorded in toluene. c Determined in THF using c(analyte) = 2 mM and c(electrolyte) = 0.1 M and a scan rate of 50 mV s^−1. d Determined in toluene-d8 at 0 °C.
Ce1–O1	2.0895(13)	2.149(4)	2.1731(15)	2.082(6)	2.117(2)	—
Ce1–N1	2.2388(14)	2.265(5)	2.2206(18)	2.280(7)	2.235(2)	—
Ce1–N2	2.2398(15)	2.230(5)	2.2112(18)	2.229(7)	2.246(2)	—
Ce1–N3	2.2487(14)	2.244(5)	2.2467(19)	2.238(7)	2.259(2)	—
C–Carom	1.387(3)–1.399(2)	1.383(8)–1.392(8)	1.390(4)–1.407(3)	1.390(11)–1.408(12)	1.388(4)–1.405(4)	—
C1–O1	1.356(2)	1.324(7)	1.318(3)	1.366(10)	1.378(3)	—
Ce1–O1–C1	173.13(11)	156.1(4)	161.02(15)	162.4(6)	146.79(18)	—
^1H NMR^a	0.43	0.45	0.45	0.43^d	0.49	0.44
^13C{^1H} NMR^a	—	—	5.6	—	—	5.6
^29Si{^1H} NMR^a	—	−8.1	−7.3	−8.8	—	−8.1
μ eff	0.67	0.68	0.59	0.89	—	1.19
UV-Vis absorption bands^b	485	319/518	384/511	362/411/681	—	339/425/474/677
E pc	−0.699/−0.966	−0.415/−0.624	−0.546	—	—	—
E pa	−0.558	−0.290	−0.1685	—	—	—
E 1/2	−0.76	−0.46	−0.36
ΔE	0.408	0.334	0.377	—	—	—

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The ¹H NMR spectra of compounds 4^xhq in C₆D₆ show singlets for the trimethylsilyl (TMS) groups at 0.43 to 0.45 ppm along with signals for the bridging hydroquinolato moieties. Further, the ¹³C{¹H} NMR spectra of 4^ddhq and 4^nq display a singlet for the TMS groups at 5.6 ppm and signals in the aromatic region for the different hydroquinolates, indicative of a successful reduction of the respective benzoquinone derivatives. The characterisation of 4^tBu2hq in solution (C₆D₆) was not feasible, due to the prevailing equilibrium shown in Scheme 2, and ready back-formation of 1 and 2,5-di-tert-butyl-1,4-benzoquinone.

Scheme 2 Equilibrium between 4^tBu2hq and reformation of reactants in solution and solid state.

While the ¹H NMR spectrum of 4^tBu2hq primarily shows signals for the starting materials and only minor product signals, its DRIFT spectrum indicated the absence of any strong C=O absorption band, and therefore the stability of 4^tBu2hq in the solid state (see Fig. S10 and S45 in ESI†). In contrast, complexes 4^hq, 4^Cl4hq and 4^ddhq derived from the stronger oxidizing quinones are very stable in the solid state and in solution. This fits again well with the already pronounced instability of 4^Me4hq and 4^nhq which slowly decompose in n-hexane and toluene at ambient temperature and rapidly undergo decomposition in THF. Tracking of the progress of the decomposition by ¹H NMR spectroscopy revealed the formation of 1 and other paramagnetic Ce^III species which, however, could not be identified. The progressing decomposition can also be seen in the ligand-to-metal charge transfers observed in the UV-Vis spectra (Fig. S68, ESI†). As the spectra of 4^hq, 4^Cl4hq and 4^ddhq show mainly one strong absorption band at around 500 nm (ε > 5060 L mol⁻¹ cm⁻¹), the spectra of 4^Me4hq and 4^nhq show several absorption bands with significantly lower intensities (ε < 4400 L mol⁻¹ cm⁻¹) indicative of Ce^III species and therefore redox decomposition of the compounds.

All attempts to isolate putative 4^ahq, derived from the weakest oxidising quinone under study, namely 9,10-anthraquinone (E⁰ = 89 mV; 2e⁻/2H⁺, vs. NHE),⁵ were unsuccessful with the reaction mixtures showing no colour change immediately after addition of the anthraquinone. However, a colour change from yellow to green occurred after two days and the respective ¹H NMR spectrum showed multiple paramagnetic signals.

Quinone oxidation of siloxides [Ce{OSi(OtBu)₃}₃]₂ (2) and [Ce{OSiⁱPr₃}₃]₂ (3)

Reacting cerous siloxides 2 and 3 with the selected quinones in THF immediately gave a colour change of the reaction mixtures (from colourless to: dark purple (BQ), dark red (Cl₄BQ), dark yellow/orange (DDQ), pale purple (tBu₂BQ), pale blue (Me₄BQ), pale green (NQ)). The ceric compounds [CeL₃(thf)]₂(μ₂-O₂C₆H₄) (5^hq, 6^hq), [CeL₃(thf)]₂(μ₂-O₂C₆Cl₄) (5^Cl4hq, 6^Cl4hq), [CeL₃(thf)]₂(μ₂-O₂C₆Cl₂(CN)₂) (5^ddq, 6^ddq), with L = OSi(OtBu)₃ or OSiⁱPr₃ derived from quinones with a relatively strong oxidising effect were successfully isolated from these reactions (Scheme 1).

However, the weakly oxidizing quinones Me₄BQ and NQ did not lead to tetravalent cerium species, as indicated by the detection of only paramagnetic signals in the ¹H NMR spectra (for an example of such a ¹H NMR spectrum, see Fig. S34 in the ESI;† formation of semiquinolates, vide infra). The accessible complexes 5 and 6 were obtained in moderate to good crystalline yields of 42 to 71% upon recrystallisation from THF or THF/Et₂O mixtures. Crystals suitable for XRD analysis were obtained for complexes 5^hq, 5^Cl4hq, 6^Cl4hq, 6^ddhq and 6^tBu2hq, revealing the same structural motif as complexes 4, that is two CeL₃ moieties connected via a hydroquinolato linker (Fig. 2).

Strikingly, the ¹H NMR spectrum of 6^tBu2hq indicated the existence of an equilibrium similar to that of ceric 4^tBu2hq (cf.Scheme 2). However, along with the reactants 3 and tBu₂BQ additional signals assignable to distinct dia- and paramagnetic decomposition products were detected. Further, the crystal structures of complexes 5 and 6 show that the cerium atoms are additionally coordinated by THF donor molecules. The Ce1–O_siloxide distances of 2.066(2) to 2.1534(10) (see Table 2 for a complete list of interatomic distances) compare well to other ceric siloxides like Ce{OSi(OtBu)₃}₄ (2.089(2)–2.157(2) Å ²⁶ and 2.084–2.160 Å ^21d) or Ce{OSiPh₃}₄(dme) (2.098(1)–2.133(1) Å).²⁹ Also, as seen for the silylamides 4, the Ce1–O_hq distances of 2.1244(10) to 2.2325(16), as well as the C–C and C–O distances underline the formation of an aromatic hq linker.^20g,241H NMR spectroscopic measurements also validate the formation of Ce^IV species, showing a sharp singlet for the tert-butyl groups and a doublet plus a septet for the iso-propyl groups depending on the siloxy co-ligand.

Table 2 Selected analytical data of complexes 5^hq, 5^Cl4hq, 5^ddq, 6^hq, 6^Cl4hq, 6^ddq. Interatomic distances are given in [Å], angles in [°], chemical shifts in [ppm], μ_eff in [BM], UV/Vis absorption band in [nm], and E_pc/E_pa in [V vs. Fc/Fc⁺]

Complex	5^hq	5^Cl4hq	5^ddq	6^hq	6^Cl4hq	6^ddq
a NMR spectra recorded in THF-d8. b Spectra recorded in toluene. c Spectra recorded in THF. d Determined in THF using c(analyte) = 2 mM and c(electrolyte) = 0.1 M and a scan rate of 50 mV s^−1.
Ce1–O1	2.1244(10)	2.184(3)	—	—	2.207(2)	2.2325(16)
Ce1–O2	2.1534(10)	2.091(3)	—	—	2.095(2)	2.0841(16)
Ce1–O3	2.1334(11)	2.094(3)	—	—	2.066(2)	2.0710(17)
Ce1–O4	2.1396(11)	2.104(3)	—	—	2.080(2)	2.091(2)
C–Carom	1.390(2)–1.395(2)	1.373(8)–1.406(5)	—	—	1.380(4)–1.407(4)	1.379(5)–1.415(3)
C1–O1	1.3540(17)	1.326(4)	—	—	1.322(3)	1.316(3)
Ce1–O1–C1	151.76(10)	140.0(2)	—	—	138.54(18)	144.85(14)
^1H NMR^a	1.36	1.36	1.36		1.12/1.05	1.12/1.05
^13C{^1H} NMR^a	72.5/32.6	72.8/32.4	73.0/32.5		18.0/14.0	19.1/15.1
^29Si{^1H} NMR^a	−103.2	−104.6	−105.3	7.0	9.6	10.7
μ eff	0.82	0.54	0.60	0.68	0.50	0.66
UV-Vis absorption band^b	369/622	493	384/450	526^c	511	381/470
E pc	−1.7855	−1.414	−1.353	−1.116/−1.816	−1.580	−1.149/−1.484
E pa	−0.3625	−0.521	0.130	−1.273/−0.817	−0.729	−0.751
ΔE	1.416	0.893	1.483	0.999	0.851	0.733

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Electrochemical investigation of complexes 4^xhq, 5^xhq and 6^xhq

Cyclic voltammetry (CV) measurements of complexes 4^xhq, 5^xhq and 6^xhq have been conducted at ambient temperature in 0.2 mM solutions in THF and 0.1 M [nPr₄N][B(C₆H₃(CF₃)₂-3,5)₄] as a support electrolyte, and referenced vs. Fc/Fc⁺. Due to the low stability of compounds 4^Me4hq, 4^tBu2hq and 4^nhq in polar solvents CV measurements of these complexes were not feasible. Most of the CV measurements revealed successive quasireversible (4) or irreversible (5/6) Ce^IV → Ce^III reduction steps, but badly resolved (for E_pc values see Tables 1 and 2). The detection of two closely adjacent redox events (E_pc values) in some cyclic voltammograms may correspond to a successive reduction/oxidation of the cerium centres. Similar features were also described for the hq-bridged Ce(IV)–BINOLate complex II.^24b All complexes under study display redox processes with a large separation of E_pc and E_pa (ΔE ≈ 0.6 V for 4; 1.5 V for 5 and 1.0 V for 6).

Representatively, the cyclic voltammograms of the DDQ-functionalized Ce^III/Ce^IV redox couples are depicted in Fig. 3 (top graphic). The silylamide complexes 4 gave reduction potentials similar to those reported for halogenido-functionalised ceric complexes Ce[N(SiMe₃)₂]₃X (E_1/2 = −0.56 (X = F), −0.30 (X = Cl), −0.31 (X = Br)) with E_1/2 values of −0.46 V for 4^Cl4hq and −0.36 V for 4^ddhq.³⁰ Only 4^hq with E_1/2 = −0.76 V gave a significantly higher stabilisation by 0.20 V. The extra-large separation of the reduction/oxidation events observed for the siloxide complexes 5 and 6 had been noticed previously for rare-earth-metal siloxides and was assigned to oxidation-state-dependent ligand reorganisation processes.³¹

Fig. 3 Top: Stacked cyclic voltammograms of complexes 4^ddq (red), 5^ddq (blue), 6^ddq (black) in THF (v = 50 mV s⁻¹; c(analyte) = 2 mM; c([nPr₄N][B(C₆H₃(CF₃)₂-3,5)₄]) = 0.1 M). Bottom: comparison of the second reduction potentials of complexes 4 (red), 5 (black), and 6 (blue) in THF vs. Fc/Fc⁺. Squares: complexes with bridging 1,4-hydroquinolates; cycles: complexes with bridging tetrachloro-1,4-hydroquinolates; triangles: complexes with bridging 2,3-dichloro-5,6-dicyano-1,4-hydroquinolates.

Stabilisation of the tetravalent oxidation state of cerium in complexes 4, 5, and 6 increases in the order of N(SiMe₃)₂ < OSi(OtBu₃)₃ < OSiⁱPr₃ as co-ligand (Fig. 3/bottom) which is in accordance with previous findings.^{20f,26,30,31c} Surprisingly, the stabilisation of Ce^IV proceeds in reverse order of the oxidation potential of the 1,4-quinones under study giving the most stable complexes for the hydroquinolato-bridged complexes and the least stable compounds for its 2,3-dichloro-5,6-dicyano-hydroquinolato congeners. A reason for this trend could be the increasingly electron-deficient nature of the aromatic hydroquinolato linkers due to the large −I effect of the substituents. The Ce^IV oxidation state can be stabilised by increasing donor strength of the ligands.³² Based on this, it seems surprising that isolable complexes 4^Me4hq and 4^nhq, derived from weakly oxidizing quinones, are not stable in solution at ambient temperature. This might be a result of another reaction pathway preferred after formation of the hydroquinolato-bridged Ce^IV complexes (like following up redox processes and the formation of Ce^III semiquinolates, cf. vide infra).

Reduction of silylamide 4^hq with cobaltocene

Having investigated the electrochemical reduction of compounds 4, 5 and 6, the chemical reduction with cobaltocene (CoCp₂) (−1.31 V vs. Fc/Fc⁺ in DME)^2a was attempted, as it has already been shown to engage in such reductions.^31a,33 Accordingly, treatment of a solution of 4^hq in THF with two equivalents of CoCp₂ resulted in a colour change from dark brown to pale yellow (Scheme 3). The ¹H NMR spectrum of the reaction mixture showed complete consumption of CoCp₂ and only broadened signals indicating the formation of a paramagnetic Ce^III species. Crystallisation from a concentrated THF-d₈ solution at −40 °C gave light brown crystals of the composition [(Ce{N(SiMe₃)₂}₃)₂(μ₂-O₂C₆H₄)][CoCp₂]₂ (7) (Fig. 4).

Scheme 3 Reduction of 4^bq with two equivalents of CoCp₂.

Fig. 4 Crystal structure of [(Ce{N(SiMe₃)₂}₃)₂(μ₂-O₂C₆H₄)][CoCp₂]₂ (7). Ellipsoids are shown at the 50% probability level. Hydrogen atoms, disordering of the cyclopentadienyl ligands and lattice THF are omitted for clarity. Selected interatomic distances [Å]: Ce1–N1 2.420(2), Ce1–N2 2.402(2), Ce1–N3 2.418(2), Ce1–O1 2.202(2), C1–O1 1.344(4), C1–C2 1.391(4), C1–C3 1.392(4), C2–C3′ 1.392(4).

Complex 7 shows the same structural motif as 4^hq but is flanked by two cobaltocenium cations. Compared to 4^hq, the Ce–N and Ce1–O1 distances are elongated by approximately 0.19 Å as expected for the larger Ce^III ion size.³⁴ On the contrary, the bonding parameters within the bridging hydroquinolato linker did not change, further corroborating a cerium-borne redox chemistry. Reacting 4^hq with one equivalent of CoCp₂ did not lead to a mixed Ce^III/IV complex but gave a mixture of 50% of 7 and 50% of unreacted starting material.

The reactions of CoCp₂ with other complexes 4 to 6 in THF-d₈ showed immediate decolourisation of the solution while the ¹H NMR spectra of the reaction mixtures displayed only paramagnetic signals (for an example, see Fig. S33 in the ESI†). However, the isolation of additional reduced species similar to 7 was not successful.

Cerium semiquinolates

A closer look at the reactions of cerous siloxides 2 and 3 with the weakly oxidising quinone Me₄BQ (which did not produce any tetravalent cerium species; vide supra) revealed another important detail of the cerium–quinone redox system. Treatment of 2 or 3 with 0.5 equivalents of Me₄BQ led to a colour change from colourless to light blue. Upon recrystallisation from THF dark blue crystals suitable for X-ray diffraction could be grown and were identified as cerous semiquinolates [CeL₂(thf)₂]₂(μ₂-O₂C₆Me₄)₂ (with L = OSi(OtBu)₃ (8) or OSiⁱPr₃ (9)) (Scheme 4).

Scheme 4 Formation of cerous semiquinolates 8 and 9 from the reaction of 2 or 3 with Me₄BQ.

Examining the reaction mixtures by ¹H NMR spectroscopy in THF-d₈ showed, besides paramagnetic signals for 8 and 9, a sharp singlet at 1.39 ppm (for 8) or a doublet plus a septet at 1.13 and 1.04 ppm (for 9), indicating the formation of homoleptic Ce[OSi(OtBu)₃]₄ or [Ce(OSiⁱPr₃)₄], respectively, as a result of the one-electron reduction of Me₄bq followed by ligand redistribution. Crucially, such a reaction pathway seems unfeasible for complexes 4, since putative homoleptic “Ce[N(SiMe₃)₂]₄” is unknown.^20a Emergent kinetic constraints in the case of ceric complexes 4 were also suggested by the redox behaviour of [Ce{N(SiHMe₂)₂}₃]₂ derived from a less bulky silylamido ligand. Accordingly, the cerous bis(dimethylsilyl)amide complex was treated with one equivalent of both BQ and Me₄BQ in THF-d₈ and C₆D₆ (see Fig. S38–S41, ESI†). The ¹H NMR spectra of these reactions suggest the formation of a tetravalent species of the composition “[Ce{N(SiHMe₂)₂}₃]₂(μ₂-O₂C₆R₄)”. However, the ceric products appear to be unstable in solution at ambient temperature. In C₆D₆, the formation of Ce[N(SiHMe₂)₂]₄ was observed in the reaction with BQ as well as other insoluble products. In THF-d₈, the resulting product seemed more stable but after 24 h in solution also traces of decomposition products were found. The Me₄BQ reaction in C₆D₆ also indicated successful oxidation, however, after 24 h the ¹H NMR spectrum revealed signals for trivalent decomposition products as well as traces of Ce[N(SiHMe₂)₂]₄. In THF-d₈, the putatively formed hydroquinolate complex was even less stable, showing signals for trivalent by-products directly after addition of Me₄BQ. In addition, the stability of 4^bq was investigated in THF-d₈ showing small amounts of decomposition products like Ce[N(SiMe₃)₂]₃ after 24 h (Fig. S42, ESI†).

Regrettably, purification of complexes 8 and 9 was impeded by co-crystallisation with the ceric by-products CeL₄. The crystal structures of 8 and 9 revealed two six-coordinate cerium atoms surrounded by two siloxy ligands, two THF donor molecules and two bridging tetramethyl semiquinolato moieties (Fig. 5). The Ce1–O_silanolato distances are elongated by about 0.1 Å compared to the respective tetravalent compounds 5 and 6 and in accordance with other Ce^III siloxides like 3, Ce{OSi(OtBu)_3]3(thf)₃ (2.243(2)–2.249(2) Å), [Ce{OSi(OtBu)₃}₃]₂ (Ce–O_term 2.186(3)–2.202(3) Å),²⁶ Ce{OSiPh₃}₃(thf)₃ (Ce–O_avg 2.222(4) Å)³⁵ and [Ce{OSiPh₃}₃]₂ (Ce–O_term 2.141(7)–2.184(6) Å).²⁷ As expected for semiquinolato ligands the six-membered rings display two shortened C–C and four elongated C–C bonds. Additionally, the six-membered rings are slightly bent in comparison to the flat aromatic hydroquinolato linkers in complexes 4, 5 and 6 resulting in an angle of 170.34° for 8 and 170.29° for 9, respectively (see Fig. 5, bottom). Notwithstanding, the bridging radicals engage in significant π-stacking as indicated by close semiquinolato–semiquinolato distances of 3.112 Å for 8 and 3.156 Å for 9. Overall, complexes 8 and 9 display the same arrangement of the semiquinolato radical bridges as observed in complexes [LnCl₂(THF)₃(μ-Me₄sq)₂]₂ (Ln = Y, Gd) (Ct⋯Ct 3.097 Å; Ct = centroid of benzene rings).¹³

Fig. 5 Top: Crystal structure of [Ce{OSiⁱPr₃}₂(thf)₂]₂(μ₂-O₂C₆Me₄)₂ (9). Ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances [Å]: Ce1–O1 2.446(7), Ce1–O2 2.422(7), Ce1–O3 2.219(9), Ce1–O4 2.237(6), Ce1–O5 2.648(7), Ce1–O6 2.644 (7), C1–O5 1.306(5), C4–O6 1.307(5), C1–C2 1.458(6), C2–C3 1.401(6), C3–C4 1.449(8), C4–C5 1.463(6), C5–C6 1.388(6), C1–C6 1.463(8). Bottom: side view of [(Ce{OSiⁱPr₃}₂(thf)₂)₂(μ₂-O₂C₆Me₄)₂] (9).

To investigate the electronic behaviour of the bridging semiquinolates, X-band EPR spectra of compounds 8 and 9 were recorded from a crystal powder sample at 123 K (Fig. 6). For both complexes cw-EPR spectra are composed of two distinct sets of resonances, one that results from the transition within the Kramers doublet corresponding to m_j = ±1/2 and one at half-field, H ≈ 160 mT. The transition for the m_j = ±1/2 state associates with an axial g tensor with principal components g_II = 2.094 and g_⊥ = 2.032 for 8 and g_II = 2.088 and g_⊥ = 2.032 for 9, respectively. The transition locating at half-field gives rise to a very broad dispersion line with g_II ≈ 4.359 for 8 and g_II ≈ 4.351 for 9, respectively. Notably, an identical line pattern derives from a frozen 2-Me-THF solution at 77 K; cf. Fig. S67, ESI,† for pertinent details. The cw-EPR spectra corroborate the presence of Ce³⁺, and agree with early work on mononuclear complexes of Ce³⁺.³⁶ This indicates a radical–radical π-bonding, as it was recently shown for yttrium and gadolinium semiquinolates [LnCl₂(thf)₂]₂(μ₂-O₂C₆Me₄)₂ (Ln = Y or Gd).¹³ Additionally and also similar to the yttrium semiquinolate, complex 9 shows a signal with a g value of 1.999, which most likely results from a non-coupled radical impurity.

Note that the reactions of cerium siloxides 2 and 3 with 1,4-naphthoquinone resulted also in the formation of homoleptic ceric siloxides as well as paramagnetic by-products (cf. Fig. S36, ESI†) indicating a similar reactivity as observed for Me₄BQ. Unfortunately, any putative semiquinolate complexes could not be isolated.

Fig. 6 X-Band cw-EPR spectra of crystalline [Ce{OSi(OtBu)₃}₂(thf)₂(μ₂-O₂C₆Me₄)]₂ (8) (black trace) and [Ce(OSiⁱPr₃)₂(thf)₂)(μ₂-O₂C₆Me₄)]₂ (9) (red trace).

Conclusions

Cerium(III) silylamides and siloxides are suitable reagents for assessing the oxidising power/reducibility of differently substituted 1,4-quinones in non-aqueous solutions. The cerium–quinone redox matching is revealed by the ease of formation of Ce^IV hydroquinolates [CeL₃]₂(μ₂-O₂C₆R₄), in the case of the parent 1,4-benzoquinone (BQ) or when R represents electron-withdrawing groups (Cl, CN). Depending on their reduction potential, alkyl-substituted BQs engage in redox equilibria, with the Ce^IV hydroquinolate species being preferentially stable in the solid state, but also afford semiquinolates via redox ligand redistribution. The structurally characterized siloxide semiquinolate complexes [(CeL₂(thf)₂)(μ₂-O₂C₆Me₄)]₂ (L = OSi(OtBu)₃, OSiⁱPr₃) exhibit a molecular arrangement, recently detected for [LnCl₂(THF)₃(μ-Me₄sq)₂]₂ (Ln = Y, Gd).¹³ The stabilisation of the tetravalent oxidation state in hydroquinolato-bridged complexes [Ce^IVL₃]₂(μ₂-O₂C₆R₄) was examined by electrochemical measurements, as well as NMR and UV/Vis spectroscopies. In accordance with previous findings,^16,18–20 the stability of the ceric complexes increases in the order of N(SiMe₃)₂ < OSi(OtBu₃)₃ < OSiⁱPr₃ as supporting ligand, but surprisingly drops in reverse order of the oxidation potential of the 1,4-quinones, being the least stable for the 2,3-dichloro-5,6-dicyano-hydroquinolato congener. The preferred formation of hydroquinolato-bridged silylamides [Ce{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆R₄) seems kinetically favoured. Finally, the electrochemical reduction of the hydroquinolato-bridged ceric complexes [Ce^IVL₃]₂(μ₂-O₂C₆R₄) can be mimicked by chemical reduction with cobaltocene, as shown for the isolation and structural characterisation of cerous [(Ce{N(SiMe₃)₂}₃)₂(μ₂-O₂C₆H₄)][CoCp₂]₂. The cerium–quinone redox matching and tuning might be used as a role-model in tetravalent praseodymium and terbium chemistry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Institute of Organic Chemistry at EKUT for EPR measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2022461, 2022462, 2022463, 2022464, 2022465, 2022466, 2022467, 2022468, 2022469, 2022470, 2022471, 2022472, 2022473, 2022474, 2022475 and 2022476. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc04489j

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