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Coordination Chemistry Reviews 260 (2014) 21– 36

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

jo ur nal ho me page: www.elsev ier .com/ locate /ccr

eview

he electrochemical behavior of cerium(III/IV) complexes:hermodynamics, kinetics and applications in synthesis

icholas A. Piro, Jerome R. Robinson, Patrick J. Walsh, Eric J. Schelter ∗

. Roy, Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, United States

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222. Aqueous cerium redox chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.1. Acidic media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2. Basic media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.3. Polyoxometalates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3. Non-aqueous electrochemistry of cerium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.1. Measurements of cerium electrochemistry in ionic liquid media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2. Complexes with oxyacids, neutral nitrogen bases, and crown ether ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.3. Complexes with salen, aryl oxide and acetylacetonate ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.4. Complexes of tetrapyrrole and other tetraaza macrocycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.5. Organometallic complexes of cerium and endohedral fullerene complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.6. Cerium amides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4. Preparative chemistry of cerium(IV) complexes by oxidation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.1. Outer-sphere oxidation, H-atom abstraction, and autooxidation reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.2. Oxidative functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.3. Oxidation-induced ligand redistribution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5. The role of kinetics in cerium electrochemistry and oxidation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

a b s t r a c t

r t i c l e i n f o

rticle history:eceived 28 June 2013ccepted 26 August 2013vailable online 7 September 2013

A key characteristic of the element cerium is its reversible redox chemistry between trivalent and tetrava-lent forms, which is central to the application of cerium in synthetic and materials chemistry. Herein wesurvey the general thermodynamic and kinetic characteristics and reported potentials for molecularcerium redox chemistry. The collected data illustrate that the local electronic environment provided bythe coordination sphere around a cerium ion has a great effect on the oxidizing ability of the ion. The

Abbreviations: acac, acetylacetonate; BINOL, 1,1′-bi-2-napthol; Bu, butyl; BQ, benzoquinone; CAN, ceric ammonium nitrate; CV, cyclic voltammetry/voltammagram;MPO, n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide; Cp, cyclopentadienyl; dap, diazaporphyrin; DMSO, dimethylsulfoxide; DPA, dipicolinic acid; DTPA,iethylenetriaminepentaacetic acid; EDTA, ethylenediaminetetraacetic acid; Epa, electrochemical potential at peak anodic current; Epc, electrochemical potential at peakathodic current; Et, ethyl; Fc, ferrocene; H6TrenSal, tris((2-hydroxybenzyl)aminoethyl)amine; HBP, hexadecahydrotetrabenzoporphyrin; HOPO, hydroxypyridinone; IL,onic liquid; iPr, iso-propyl; irr, irreversible; MBP, 2,2′-methylenebis(6-tert-butyl-4-methylphenolate); Me, methyl; Me-3,2-HOPO, 1-methyl-3-hydroxy-2(1H)-pyridinone;BS, N-bromosuccinimide; Nc, naphthalocyanine; NctBu, tetra-tert-butylnapthalocyanine; NHE, normal hydrogen electrode; NTA, nitrilotriacetic acid; OEP, octaethyl-orphyrin; omtaa, 5,14-dihydro-2,3,6,8,11,12,15,17-octamethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine; OPTAP, octapropyltetrazaporphyrin; Pc, phthalocyanine;OM, polyoxometallate; RE, rare earth; salen, N,N′-ethylenebis(salicylimine); salophen, N,N′-(1,2-phenylene)bis(salicylimine); SCE, saturated calomel electrode; TAP,etra(4-methoxyphenyl)porphyrin; TBABF4, tetrabutylammonium tetrafluoroborate; TBAP, tetra-n-butylammonium perchlorate; TBAPF6, tetra-n-butylammonium hex-fluorophosphate; TBPP, tetra(4-tert-butylphenyl)porphyrin; iBu, tert-butyl; TClP, tetra(4-chlorophenyl)porphyrin; TEtAP, tetraethylylammonium perchlorate; THF,etrahydrofuran; Tf, trifluoromethylsulfonyl; tmtaa, dibenzotetramethylaza[14]annulene; TPAB, [N(n-C3H7)4][B(3,5-(CF3)2C6H3)4]; TPP, tetraphenylporphyrin; TPyP,etrapyridylporphyrin; TREN, tris(2-aminoethyl)amine; TrenSal, tris((2-hydroxybenzylidene)aminoethyl)amine; TTP, tetratolylporphyrin; XANES, X-ray absorption near-dge structure.∗ Corresponding author. Tel.: +1 215 898 8633; fax: +1 215 573 2112.

E-mail address: schelter@sas.upenn.edu (E.J. Schelter).

010-8545/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ccr.2013.08.034

22 N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36

Keywords:LanthanidesCeriumElectrochemistryElectron transferOxidation

survey also illustrates the ligand types that most effectively stabilize each oxidation state. We expect thecollection and comparison of these data will facilitate the development of new cerium(IV) chemistry andapplications in oxidation and reduction chemistry.

1

uela[itg[iTir

tcmohcpl[

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Fo

. Introduction

The accessibility of the 4f0 tetravalent oxidation state for molec-lar cerium complexes in solution is unique among the lanthanidelements. The thermodynamic basis for the isolation of molecu-ar cerium(IV) compounds originates from the enhanced stabilityfforded by the [Xe] noble gas electronic configuration for the ion1]. The existence of molecular cerium(IV) compounds and the abil-ty to cycle between the +3 and +4 oxidation states plays a key role inhe separation of cerium from lanthanide-containing ore, and moreenerally in studies of valence-control for nuclear fuel reprocessing2–4]. The facile separation of this element from other lanthanidess a major contributing factor to the relatively low cost of cerium.he interconversion of the +3 and +4 oxidation states also underl-es the utility of cerium oxides as heterogeneous catalysts and inenewable energy applications [5,6].

Traditionally, it is the potent oxidizing ability of cerium(IV)hat is exploited in the redox chemistry of the element, witheric ammonium nitrate (CAN) oxidation reactions being the pri-ary examples [7–10]. While the applications of CAN, Fig. 1, and

ther cerium(IV) reagents in organic synthesis are extensive andave been reviewed previously [7–10], the ability of cerium(III) toarry out reductive transformations has remained relatively unex-lored. This is in contrast to the reductive chemistry of divalent

anthanides, which have formed an area of active investigation7,11,12].

In this review we survey the thermodynamics of molecularerium redox chemistry, primarily through assessment of electro-hemical measurements, with the goal of gaining an understandingf factors that affect cerium reduction potentials. The tuning of

edox potentials through variation of ligand environments for tran-ition metal ions is a widely accepted phenomenon. Perhaps it ishe popular dogma that the bonding in lanthanide complexes is

ig. 1. The structure of the [Ce(NO3)6]2− anion in CAN. The coordinates werebtained from Ref. [13].

© 2013 Elsevier B.V. All rights reserved.

completely ionic that has, in our estimate, prevented a systematicreview of ligand effects on cerium reduction potential. However,we show through this survey that the local electronic environmentprovided by the coordination sphere around a cerium ion can havea great effect on the oxidizing or reducing ability of the ceriumion. The true range of cerium(IV) reduction potentials is remark-ably broad, spanning at least 2.3 V in aqueous solution and 2.9 Vunder non-aqueous conditions (vide infra). Thus, while cerium(IV)is often considered a strong oxidant, variation of the coordinationenvironment can render cerium(III) a strong reductant.

From our survey of reported cerium electrochemistry, it is evi-dent that noticeable differences exist between the redox chemistryof cerium and that of transition metal ions. Cerium in both its +3 and+4 oxidation state remains a hard cation in the Pearson terminol-ogy [14–16]. It is therefore best stabilized by hard anions, whichprovide the greatest stabilization of cerium(IV) complexes. Thus,certain ligands that may be considered electron-rich and often sta-bilize high-valent transition metal ions confer relatively smallerstabilization to tetravalent cerium, including amides and cyclopen-tadienides.

Another unique aspect of cerium redox chemistry is the majorrole of ligand reorganization in oxidation reactions. The changefrom f1 to f0 configuration upon oxidation from the trivalent totetravalent state results in a change in ionic radius from 1.01 A to0.87 A [17]. In contrast to the d-block transition metals, covalentbonding plays only a negligible role in shaping the coordinationsphere of cerium complexes and maintaining the coordinationsphere throughout an oxidation reaction. Thus, large inner spherereorganization energies are expected and thermodynamic consid-erations alone do not necessarily confer predictive power in ceriumoxidation reactions [18].

Herein we document these effects with a brief review of aque-ous chemistry followed by a focus on cerium complexes that havebeen characterized electrochemically in non-aqueous conditions.We also discuss the synthetic chemistry of cerium(III/IV) transfor-mations and the kinetics of electron transfer reactions. For ease ofcomparison in this review we have converted all electrochemicalpotentials to a common reference, SCE. This was done by per-forming the following adjustments to the published values whenreported versus the following references: Fc/Fc+ in THF, +0.56 V;Fc/Fc+ in MeCN, +0.40 V; Fc/Fc+ in H2O, +0.16 V; Ag/AgCl (sat KCl),−0.045 V; NHE, −0.24 V [19–23].

2. Aqueous cerium redox chemistry

2.1. Acidic media

In acidic, aqueous media Ce(IV) is a potent one-electron oxidant.This was recognized as early as 1931, when a series of papersdocumented the cerium(III/IV) reduction potential under a varietyof conditions. Kunz, Noyes and Garner, and Smith and Getz eachpublished reference values for cerium reduction potentials in

acidic conditions ranging from +1.04 to +1.63 V, demonstrating adependence on acid identity and concentration [24–26]. Anion-and concentration-dependent reduction potentials under severalof these conditions are presented in Table 1. The standard potential

N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36 23

Table 1Redox potentials of cerium ions under various aqueous conditions.

Complexes Potential (V)vs. SCE

Reported potential(V)

Conditions Ref.

Ce(ClO4)3 1.63 1.87 vs. NHE 8 M HClO4 [26]Ce(ClO4)3 1.47 1.71 vs. NHE 4.83 M HClO4 [28]Ce(ClO4)3 1.46 1.70 vs. NHE 1 M HClO4 [26]Ce(ClO4)3 1.39 1.63 vs. NHE 0.1 M HClO4 [28]Ce(NO3)3 1.37 1.61 vs. NHE 1 M HNO3 [25,26]CeCl3 1.22 1.46 vs. NHE 1 M HCl, 0.2 M NaClO4 [24,26,29]Ce2(SO4)3 1.20 1.44 vs. NHE 1 M H2SO4 [24,26]CeCl3 1.04 1.28 vs. NHE 1 M HCl [26]Ce(EDTA)−a

(1:1/[Ce]:[EDTA])a0.805 0.85 vs. Ag/Ag+ 0.2 M KCl [30]

Ce(DTPA)2−a

(1:2.5/[Ce]:[DTPA])a0.546 0.581 vs. Ag/Ag+ 1 M KNO3, pH 4.19 [31]

Ce(NTA)2a 0.464 0.509 vs. Ag/Ag+ (0.002 M

Ce(NO3)3 + 0.05 M NTA)1 M NaNO3, pH 6.0)

[32]

[Ce(DPA)3][(C(NH2)3)+]a 0.44 – 0.16 M NaOAc pH 4.66 [33]

Ce(HOPO-2)2a 0.26 0.503 vs. NHE 0.1 M KClb with DMF [34]

Ce(HOPO-3)2a 0.24 0.475 vs. NHE 0.1 M KClb [34]

Ce(HOPO-4)a 0.13 0.175 vs. Ag/Ag+ 1.0 M SO42− , pH 8.5 [35]

[Ce(CO3)5]6− −0.21 – 5 M [CO32−], pH 12.7 [36]

[Ce(CO3)5]6− −0.25 – 5 M [CO32−], pH 13.6 [36]

[Ce(tironate)4]12−c −0.49 – 5 M NaOH/1 M tironc [37][Ce(catecholate)4]4− −0.69 – 5 M NaOH/1 M catechol [38]

2.

fffacrmhl

2

tttoMprw

caotooowtlioprc

a Structures of EDTA, DTPA, NTA, DPA, HOPO-2, and HOPO-3 are provided in Fig.

b Measured using static potentiometry.c Tironate = 4,5-dihydroxy-1,3-benzenedisulfonate.

or the ion, at +1.46 V versus SCE (+1.70 V versus NHE), is takenrom the oxidation of Ce(ClO4)3 in 1 M HClO4 [26]. The potentialsor cerium oxidation increase markedly as the acid’s coordinatingbility decreases: changing from 1 M HCl to 1 M HClO4 shifts theerium ion potential by over 400 mV. Furthermore, it has beenecognized that addition of fluoride to solutions of Ce(IV) givesixtures that are incapable of oxidizing iodide, further reflecting

ow the Ce(IV) redox potential varies widely with the identity ofigands and counter ions in aqueous solution [27].

.2. Basic media

The observations from acidic media, that lower acid concentra-ion and coordinating ligands stabilize the cerium(IV) ion, suggesthat basic conditions would yield stabilization of cerium(IV) reduc-ion potentials. This is true and is attributed to strong coordinationf the oxophilic cerium ion by oxygen-donor ligands in solution.orris and Hobart investigated the redox chemistry of cerium(IV),

repared by air-oxidation of CeCl3, in strongly basic and carbonate-ich solutions [36]. Under these conditions the cerium(III/IV) coupleas observed at −250 mV versus SCE.

Polyanionic chelating ligands impart stability to high oxidationompounds and also significantly reduce the cerium potential inqueous solution, Table 1. This was demonstrated by the workf Raymond and co-workers, where catecholates in basic solu-ion depress the reduction potential of Ce(IV) to −0.69 V, a shift ofver 2 V relative to the formal Ce(III/IV) couple measured in aque-us perchloric acid [37,38]. The stabilized cerium(III/IV) oxidationbserved in the reversible electrochemistry of cerium complexesith ligands related to Me-3,2-HOPO, Fig. 2, was included in a

hermodynamic cycle to assess binding strengths of catechol-typeigands. The authors concluded that cerium(IV) electrochemistryn this family of complexes served as models for the behavior

f plutonium(IV) complexes. However, while the cerium(IV) andlutonium(IV) ions have similar ionic radii, at 0.87 A and 0.86 A,espectively [17], subsequent work has shown that the electro-hemical behavior of cerium(IV) in other ligand frameworks more

closely resembles that of berkelium(IV) [39–42]. Recent work fromAbergel et al. has investigated the solution thermodynamic proper-ties of an octadentate hydroxypyridinonate, HOPO-4, Fig. 2, whichis a promising candidate as a therapeutic for the removal of Lnand An ions [35]. The incorporation of four hydroxypyridinonateunits in HOPO-4 results in a 100 mV shift of the Ce(III/IV) coupleto more negative values compared with HOPO-3. Additionally, theHOPO ligands listed in Table 1 and shown in Fig. 2 demonstrateremarkable solution thermodynamic stability, having log (ˇ) valuesranging from 40 to 42 [35].

Multidentate carboxylic acids also significantly shift theCe(III/IV) couple to less oxidizing potentials. Electrochemical inves-tigations of cerium complexes from ethylenediaminetetracetic acid(EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriaceticacid (NTA), and dipicolinate (DPA), Fig. 2, demonstrate that theCe(III/IV) couple can be shifted by 825–1190 mV to less oxidiz-ing potentials from those observed at low pH. The E1/2 valuesfor the cerium carboxylates also shift to less oxidizing potentialswhen more carboxylate groups are coordinated to the Ce(III) center,Table 1.

2.3. Polyoxometalates

The concept of stabilizing tetravalent cerium by introduction ofpolyanions to the coordination sphere has been explored throughthe use of polyoxometalates (POMs). These species can act as large,polyanionic ligands that coordinate cerium and can serve to sta-bilize the tetravalent state, and have applications in radiologicalfuel separations science [4]. POMs ranging in charge from −10 to−20 have been used to form complexes with cerium. The shifts incerium redox potentials created by the use of these polyanions areshown in Table 2.

Detailed studies of the Ce-POMs have revealed that the charge

of the POM is not the only consideration for the observedcerium redox potential. Instead, the local coordination environ-ment available to cerium upon complexation to the oxometallatescaffold greatly affects the cerium(III/IV) couple. For example,

24 N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36

s stud

ttabcadofbfl

TR

Fig. 2. Polydentate pro-ligand

he Preyssler anion, [Mn+P5W30O110]n−15, carries a large nega-ive formal charge; however, the large, rigid coordination sitesre not able to effectively approach and donate charge to sta-ilize a cerium(IV) ion [43–45]. Surprisingly, oxidation of theerium(III) ion is more difficult when complexed by the Preysslernion than the cerium(III) cation in acidic conditions, and oxi-ation to cerium(IV) did not reportedly occur even at potentials

f +1.9 V versus Ag/Ag+. In contrast, the flexible Wells-Dawsonramework, [�-2-P2W17O61]6−, can contract around the cerium iony ca. 0.25 A upon oxidation from cerium(III) to cerium(IV). Theexibility of the coordination environment facilitates the redox

able 2eduction potentials of cerium ions coordinated to polyoxometallate complexes.

Complex Potential vs. SCE (V) Reported po

[Ce(W5O18)2]6−a 0.86 1.10 vs. NH[Ce(PW11O39)2]10− 0.64 0.88 vs. NH[Ce(SiW11O39)2]12− 0.67 0.91 vs. NH[Ce(�-2-P2Wl7O61)2]16− 0.54 0.78 vs. NH[Ce(PW11O39)2]10− 0.38 0.424 vs. Ag[Ce(�-2-P2Wl7O61)2]16− 0.32 0.365 vs. Ag[Ce(SiW11O39)2]12− 0.28 0.323 vs. Ag

a In the original report the authors had listed the formulation as Ce[W10O35]6− , howev

ied in cerium binding [30–35].

change and affords reversible electrochemistry at a modest +0.32 V[43].

3. Non-aqueous electrochemistry of cerium complexes

Studies of the coordination chemistry of cerium ions under non-aqueous conditions allow for a wide range of ligand environments

that are not available under aqueous conditions. Also, low dielec-tric solvents serve to destabilize charged compounds, which in thepresence of polyanionic ligands can stabilize cerium(IV) complexesrelative to cerium(III). Here we will survey the redox potentials

tential (V) Conditions Ref.

E 0.1 M KCl [44]E 0.1 M KCl [44]E 0.1 M KCl [44]E 0.1 M KCl [44]/AgCl pH 4.5 0.1 M Na2SO4 [45]/AgCl pH 4.5, pH 5.7, 0.1 M Na2SO4 [45], [43]/AgCl pH 4.5, 0.1 M Na2SO4 [45]

er, the correct formulation is [Ce(W5O18)2]6− [4].

N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36 25

Table 3Reduction potentials of cerium complexes in ionic liquids.

Complex Potential (V) vs. SCE Reported potential (V) Conditions Ref.

[CeCl6]3-/2− 0.64 vs Al0.37 vs Fc/Fc+a

AlCl3:MeEtimCl (2:1)40 ◦C

[55]

[CeCl6]3−/2− – −0.519 vs Pd bmimCl (70 ◦C) [55,56][CeCl ]3−/2− 0.308 vs Ag/Ag+

.25 vs[BuMePyr+][NTf −] [54]

fss

3m

puisnmnm[nibc

ircagpcsatcg

3e

o

6

0

a Fc/Fc+ vs. Al = 0.270 V in AlCl3:MeEtimCl, 2:1[53,59,20,60].

or reported cerium(III) and cerium(IV) complexes that have beentudied in non-aqueous condition, principally in MeCN, DCM or THFolvents.

.1. Measurements of cerium electrochemistry in ionic liquidedia

Due to desirable physical properties, such as low vaporressures, high ion conductivity, and non-flammability, ionic liq-ids (ILs) have found many applications in chemistry and engineer-

ng. Applications related to cerium include synthetic chemistry,eparations chemistry, and studies toward the processing of spentuclear fuel [46–52]. Advantages to performing electrochemicaleasurements in ILs include the large measurement window, the

on-coordinating nature of the electrolyte, and mild measure-ent conditions [53]. For example, measurement of [CeCl6]3− in

BuMePyr+][NTf2−] showed no Cl− oxidation events and exhibited

o stability issues. In contrast, Ce(IV) ions are otherwise unstablen alkali metal chloride molten salts [54]. Despite these potentialenefits, there are only limited reports on the electrochemistry oferium complexes in ILs (Table 3).

In the case of CeCl3, the nature of the analyte in the IL is crit-cal for applications. In [BuMePyr+][NTf2

−], adjusting the Ce:Cl−

atio to 1:6 results in the clean formation and effective electro-hemical measurement of the complex ion, [CeCl6]3−. However, at

ratio of 1:3, CeCl3 rapidly precipitates, which could be advanta-eous for separations applications [54]. Generally, comparison ofotentials between different ILs is complicated by the absence ofommon voltammetric reference standards [53]. However, mea-urement of [CeCl6]3− in two ILs [54,55], AlCl3:MeEtimCl (2:1)nd [BuMePyr+][NTf2

−], showed ∼100 mV variation and suggestedhat the electrochemical behavior of the complex ion does nothange dramatically between the two ILs. ILs undoubtedly holdreat promise for future applications in cerium redox chemistry.

.2. Complexes with oxyacids, neutral nitrogen bases, and crown

ther ligands

Complexes of neutral ligands or oxyanions form a large portionf the known cerium coordination chemistry [16]. Despite this body

Fig. 3. Assorted ligands for non-aque

Fc+2

(50 ◦C, 0.095 M CeIII)

of work, electrochemical characterization of the ion in well-definedcomplexes with such ligands is limited. Complexes of cerium(III)with the three mixed donor ligands shown in Fig. 3 have been char-acterized electrochemically. The Schiff base ligand L1 was treatedwith Ce(NO3)3 hydrate and Ce(L1)(NO3)3 was isolated. This com-plex displays an irreversible oxidation at +0.77 V in MeCN, whichis shifted by only a small amount relative to Ce(NO3)3 hydrateunder similar conditions (+0.83 V). A cerium complex of the formyl-derived ligand L2, Ce(L2)(NO3)3, also shows irreversible redoxbehavior, at +0.85 V in DMSO [57,58]. The phosphonate/aminemacrocycle L3 has more reversible electrochemistry, but also showsevidence of ligand decomposition upon oxidation in MeCN [59]. Thepotentials for each of these complexes, Table 4, are not significantlyshifted versus that for cerium(III) nitrate under similar conditions,though they are shifted from the ‘-ate’ complex [nBu4N]2[Ce(NO3)6]which has been measured at 1.02 V in MeCN at −40 ◦C.

Phosphine oxides complexes of cerium have also been char-acterized electrochemically. A CMPO complex was formed byaddition of CMPO to cerium(III) nitrate in MeCN and this mixturecharacterized electrochemically. Also, (nBuO)3PO has been usedas an extractant for partitioning cerium ions from aqueous solu-tions and the extracts similarly characterized [60,61]. Each of theseligands forms a complex with cerium that displays reversible elec-trochemistry, however, the potentials for these cerium(III/IV) redoxevents remain high at ca. +1.1 V.

3.3. Complexes with salen, aryl oxide and acetylacetonate ligands

Complexes of cerium with a variety of anionic oxygen-based lig-ands – such as alkoxides, aryl oxides (including salen-type ligands),and ˇ-diketonates – have been prepared and studied electrochemi-cally. This body of work demonstrates that such ligands are capableof depressing the cerium(III/IV) potential considerably relative tothe standard cerium potential in aqueous solution. Data for thecomplexes described in this section are compiled in Table 5.

The electrochemistry of a series of 8-coordinate bis(salen) com-

plexes with a central cerium(IV) ion has been reported [63]. Thepotential of the parent compound Ce(salen)2 was reported at−0.676 V in MeCN. The data reveal that even within a series ofisostructural ligands, Fig. 4, potentials vary by 250 mV as the

ous cerium complexes [57–60].

26 N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36

Table 4Reduction potentials of non-aqueous cerium nitrates supported by ligands.

Complex Potential (V) vs. SCE Reported potential (V) Conditionsa Ref.

Ce(CMPO)3(NO3)3 1.1 0.74 vs Fc0.80 vs Ag/Ag+

1.29 vs NHE

0.1 M TBAP MeCN [60]

[nBu4N]2[Ce(NO)6] 1.02 0.62 vs Fc/Fc+ MeCN at−40 ◦C

[62]

Ce(L2)(NO3)3 0.85 (irr) 0.42vs Fc/Fc+

0.1 M TBAP in DMSO [58]

Ce(NO3)3·6H2O 0.83 – MeCN [57]Ce(L1)(NO3)3 0.77 (irr) – 0.1 M TEtAP in MeCN [57]Ce(L3)− 0.60 – 0.05 M TBAPF6 in MeCN [59]

a TBAP = [NBu4][ClO4], TBAPF6 = [NBu4][PF6], TEtAP = [NEt4][ClO4].

Table 5Reduction potentials of cerium ion supported by anionic oxygen ligands.

Complex Potential (V)vs. SCE

Reportedpotential (V)

Conditionsa Ref.

Ce(acac)4 −0.02 0.22 ± 0.02vs. NHE

0.1 M TBAPF6 inMeCN/acetone

[70]

Ce(HOPO-2)2 −0.16 –0.11vs. Ag/AgCl

0.1 M TBAPF6 in MeCN [34]

Ce(HOPO-1)2 −0.18 –0.13 vsAg/AgCl

0.1 M TBAPF6 in MeCN [34]

Ce(HOPO-3)2 −0.21 –0.16vs. Ag/AgCl

0.1 M TBAPF6 in MeCN [34]

[Li(THF)2][Ce(MBP)2(THF)2] −0.37 –0.93vs. Fc/Fc+

0.1 M TPAB in THF [66]

Ce(5,5′-Br2-Salen)2 −0.52 — 0.1 M TBAP in MeCN [63][Li3(THF)4][(BINOLate)3Ce(THF)] Epc = −0.53

Epa = −0.12Epc = –1.085Epa = –0.445vs. Fc/Fc+

0.1 M TPAB in THF [67]

Ce(salophen)2 −0.53 — 0.1 M TBAP in MeCN [63]Ce(salen)2 −0.68 — 0.1 M TBAP in MeCN [63][Na3(THF)6][(BINOLate)3Ce] Epc = −0.74

Epa = −0.34Epc = –1.295Epa = –0.895vs. Fc/Fc+

0.1 M TPAB in THF [67]

Ce(5,5′-(OMe)2-Salen)2 −0.78 — 0.1 M TBAP in MeCN [63][K3(THF)6][(BINOLate)3Ce] Epc = −0.83

Epa = −0.49Epc = −1.385Epa = −1.045vs. Fc/Fc+

0.1 M TPAB in THF [67]

Ce(L4)(OtBu)2 Epc = −1.51Epa = −0.45

Epc = −2.07Epa = −1.01vs. Fc/Fc+

0.5 M TPAB in THF [65]

Ce(L5)(OtBu)2 Epc = −1.83Epa = −1.14

Epc = −2.39Epa = −1.70

0.5 M TPAB in THF [65]

(3,5-(C

eiaareoIc

a TBAP = [NBu4][ClO4], TBAPF6 = [NBu4][PF6], [NEt4][ClO4], TPAB = [N(n-C3H7)4][B

lectron-donating ability of the ligand is tuned through substitut-ons on the aromatic ring. The more electron-rich 5-methoxynalog adjusts the reversible cerium(III/IV) potential to −0.776 V,nd the less-donating 5-bromosalen and salophen ligands haveeversible couples at −0.521 and −0.529 V, respectively. An inter-

sting comparison can be drawn with the electrochemical behaviorf 1:1 Cu(II) complexes of the identical series of salen ligands.n the case of the Ce(III) complexes, the shift in potentials uponhanging from methoxy to bromo is 255 mV, more than twice

Fig. 4. 5,5′-X2-salen and salophen

vs. Fc/Fc+

F3)2C6H3)4].

the difference observed for the same ligands when they areused to form 1:1 complexes with Cu(II). In the case of theCu(II) complexes, the Cu(I/II) redox couple potential shifts by112 mV upon changing the salen 5,5′-substitution from methoxyto bromo [64]. Thus, the Ce(III/IV) couple is more sensitive to

substituent effects in its 2:1 salen complexes than the Cu(I/II)couple in the 1:1 copper complexes. A comparison of the poten-tial ranges for the cerium and copper complexes are depicted inFig. 5.

ligands, X = –H, –OMe, –Br.

N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36 27

Fig. 5. Ranges in metal ion potentials for series of Cu and Ce 5,5′-substituted salenc

llpsanoatTCeptsa

b2acScls[t(Sret[

3.4. Complexes of tetrapyrrole and other tetraaza macrocycles

omplexes. Values taken from Refs. [63,64].

A complex of cerium(IV) with a ferrocene-bridged, salen-likeigand has also been prepared with two tert-butoxide ancillaryigands, Fig. 6 [65]. Although the ligand features a redox-active com-onent, XANES measurements supported a cerium(IV) oxidationtate assignment while Mössbauer measurements corroborated

divalent ferrocene backbone. This complex has an extraordi-ary reduction potential in THF of −1.51 V; however, the shapef the voltammetric wave and large peak separation indicate

significant kinetic barrier and overpotential associated withhe electrochemical event, which biases the measured potential.he related ferrocene-linked phosphinimido ligands adjusts thee(IV) to Ce(III) reduction event to −1.83 V with similar signs oflectrochemical irreversibility, despite the measurements beingerformed in higher concentrations of weakly-coordinating elec-rolyte, TPAB. Even with the associated overpotentials, the dramatichifts in potential suggest that alkoxide and aryloxide ligands have

potent effect on stabilization of cerium(IV) relative to cerium(III).A methylene bisphenolate ligand, 2,2′-methylenebis(6-tert-

utyl-4-methylphenolate), has recently been employed to provide:1 complexes with cerium that are easily oxidized chemicallynd electrochemically to give cerium(IV) complexes. The reversibleerium(III/IV) potential for Ce(MBP)2(THF)2 lies at −0.37 V versusCE [66]. A series of heterobimetallic cerium–alkali metal BINOLateomplexes have been synthesized that demonstrated the effect ofigand reorganization on the redox events of a central cerium ion. Ahift of 450 mV is realized from [Li3(THF)n][(BINOLate)3Ce(THF)] toK3(THF)n][(BINOLate)3Ce], where the alkali metal ions are integralo the structure of the complexes and coordination environmentincluding coordination number) at the cerium cation (Fig. 6).econdary coordination sphere effects have large impacts on theeduction potentials of transition metal ions as demonstrated, for

xample, in proteins. It is remarkable, however, that an effect ofhis magnitude is also found for a lanthanide ion (see Section 5)68].

Fig. 6. Structures of selected alkoxide- and aryloxide-supported

Fig. 7. Structure of Ce(acac)4. Coordinates from Ref. [70].

Cerium tetrakis(acetylacetonate), Fig. 7, known since 1913,has been studied with regard to the thermodynamics of thecerium redox event, as well as the kinetics of electron transfer[69–71]. The 8-coordinate, tetranionic coordination environmentdepresses the cerium potential by 800–1000 mV relative to thenon-aqueous cerium nitrates. This stabilization of ceruim(IV) is notas extensive as that provided by the harder alkoxide ligands, whichdisplay greater localization of charge. These data suggest that thecerium(IV) ion is stabilized more effectively by ligands with donoratoms of high charge-density.

The hydroxypyridinonate ligands, HOPO-1, HOPO-2 and HOPO-3, Fig. 2, studied by Raymond and co-workers under aqueousconditions have also been analyzed in acetontrile. Under the non-aqueous conditions the order in which the ligands modulate theCe(III/IV) potential is different from aqueous solvent and this isattributed to variations in solvation energies between the Ce(III)and Ce(IV) complexes of the three ligands [34].

The cerium(IV) ion is well-accommodated in the eight coor-dinate environment provided by a sandwich of two equivalents

cerium complexes with known electrochemistry [65–67].

28 N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36

azo m

oatlmevsine

Fig. 8. Tetra

f tetrapyrrole macrocycles such as porphyrins, azaporphyrins,nd phthalocyanines, Fig. 8. In these compounds, assigning a dis-inct Ce(III/IV) redox event is often complicated by the presenceigand redox activity and hole delocalization, in which the for-

ally Ce(IV) complexes have partial Ce(III) character with a radicallectron hole in the ligand � system, and exist in intermediate-alent ground states [72]. For the formally tetravalent, neutral

andwich compounds a more accurate description of the valences [CeX(L(n−2))2], where X is a valence between III and IV, and

represents the fraction of the electron-hole delocalized overach ligand’s � system. The intermediate valence phenomenon

acrocycles.

cannot be assigned on the basis of electrochemistry alone andis typically modeled on spectroscopy data including cerium LIIIedge X-ray absorption or photoelectron spectroscopies. In com-pounds of this type, the first reduction event is most oftenascribed to a formally cerium redox event by comparison tothe electrochemical properties of non-redox active lanthanides[73].

The bisporphyrin compounds are regarded as correspondingto true cerium(IV) complexes with minimal oxidation of the por-phyrin ligand in the ground state of the neutral compounds. Thisassignment is supported by the absence of spectroscopic signatures

N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36 29

Table 6Reduction potentials of cerium tetranitrogen macrocycle complexes.

Complex Potential (V)vs. SCE

Reported potential (V) Conditionsa Ref.

Ce2(OEP)3 0.76, 0.91 0.85, 0.90 vs Ag/AgCl 0.1 M TBAPF6 in DCM [84]Ce(Pc)(TPyP)(X = 3.68)

0.10 – 0.1 M TBAP in DCM [73]

Ce(TPP)Pc −0.05 – TBAP in DCM [78]Ce(Nc)(TBPP) −0.07 – 0.1 M TBAP in DCM [73]Ce(TAP)(Pc) −0.10 – TBAP in DCM [78]Ce(NctBu)2

(trivalent)−0.11 – 0.1 M TBAP in DCM [73]

Ce(TClP) 2 −0.11 – TBAPF6 in DCM [75]Ce(TPP)(Pc-1) −0.145 – TBAP in DCM [78]Ce(Pc-5)2

(X = 3.59)−0.18 – 0.1 M TBAP in DCM [73]

Ce(Nc)(OEP)(X = 3.68)

−0.19 – 0.1 M TBAP in DCM [73]

Ce(Pc-12)2 −0.20 – 0.1 M TBAP in DCM [73]Ce(TPP)2 −0.21 −0.16 vs. Ag/AgCl, sat

KCl0.1 M TBAP in DCM [74]

Ce(Pc-8)2 −0.21 – 0.1 M TBAP in DCM [73]Ce(OEP)(Pc) −0.26 – TBAP in DCM [78]Ce(TPP)2 −0.27 – TBAPF6 in DCM [75]Ce(TTP)2 −0.29 – TBAPF6 in DCM [75]Ce(TAP)2 −0.30 – TBAPF6 in DCM [75]Ce(Por-Por) −0.30 to −0.31 – TBAP in DCM [76]Ce(OPTAP)2 Epc = −0.68

Epa = −0.18−1.24, −0.74 vs. Fc/Fc+ 0.1 M TBAPF6 in DCM [79]

Ce(OEP)(TPP) −0.44 – TBAPF6 in DCM [75]Ce(dap)(OEP) −0.51 (1st)b

−0.87 (2nd)b– 0.1 M TBAP in DCM [77]

Ce(HBP)2 −0.52 – TBAPF6 in DCM [75]Ce(OEP)2 −0.56 −0.51 vs. Ag/AgCl, sat

KCl0.1 M TBAP in DCM [74]

Ce(OEP)2 −0.58 – TBAPF6 in DCM [75]Ce(omtaa)2 −1.1 −1.7 vs. Fc/Fc+ 0.1 M TPAB in THF [87]

B(3,5-ved re

t

fdaptr[i

a TBAP = [NBu4][ClO4], TBAPF6 = [NBu4][PF6], = [NEt4][ClO4], TPAB = [N(n-C3H7)4][b The authors of assign the second reduction event to cerium, but the first obser

his table.

or the porphyrin radical monoanion [74–76]. Electrochemicalata for the one-electron reduction of several of these complexesre presented in Table 6. Mixed porphyrin/diazaporphyrin, por-hyrin/phthalocyanine, and bisporphyrazine complexes are also

ypically regarded as containing Ce(IV) ions, and the one-electroneduction events of these complexes are also included in Table 677–79]. From the data in Table 6 it is evident that the tetran-onic nitrogen-rich coordination environments do not provide

Fig. 9. Structure of Ce(Pc)2. Co

(CF3)2C6H3)4].duction is more consistent with potentials observed for the related compounds in

comparable stabilization of cerium(IV) to the basic anionic oxygendonors of Section 3.3.

In contrast to the porphyrin sandwich complexes, bisphthalo-cyanine and naphthalocyanine complexes, Fig. 9, are described as

having appreciable cerium(III) character [73,77,80–83]. These com-plex are nevertheless included in the Table 6, and a value for thevalence, X, is given in cases where it was determined experimen-tally by XANES spectroscopy [73].

ordinates from Ref. [86].

3 hemistry Reviews 260 (2014) 21– 36

ic+oTctotasp

d2ocacTtoe

3f

oeaotaaaTtdfa

osCtajwcsii−

TR

0 N.A. Piro et al. / Coordination C

The triple decker sandwich Ce2(OEP)3 has also been character-zed electrochemically, and in comparison to the europium analog,erium oxidation events for each ion were identified at +0.76 and0.91 V [84]. Related cerium triple-decker complexes could also bexidized, but the metal centered events were not identified [85].he presence of only 1.5 porphyrin rings per metal ion in theseomplexes and the associated increase in charge at the metal cen-ers have a significant effect on the ease with which the metal isxidized, as these potentials are greater than 1.3 V more oxidizinghan that for Ce(OEP)2. The triple decker sandwiches Ce2(TPP)(Pc)2nd Ce2Pc(TPP)2 have also been studied electrochemically andhow behavior that is complicated by phthalocyanine oxidationrocesses [78].

Cerium complexes of other nitrogen macrocycles such asibenzotetramethylaza[14]annulene (tmtaa) and 5,14-dihydro-,3,6,8,11,12,15,17-octamethyldibenzo[b,i][1,4,8,11]tetraazacycl-tetradecine (omtaa) have also been isolated. The bis(omtaa)omplex, Ce(omtaa)2, has been characterized electrochemicallynd displays, to our knowledge, the most negative reversibleerium(III/IV) redox event reported to date, at −1.1 V versus SCE.he omtaa ligand with its anionic charges more localized than inhe tetrapyrrole complexes (due to the saddle-shaped geometryf the non-planar omtaa macrocycle) is evidently a very effectivenvironment for stabilizing the cerium(IV) ion.

.5. Organometallic complexes of cerium and endohedralullerene complexes

The cyclopentadienide ligand plays a prominent role inrgano-transition metal chemistry, where it is considered anlectron-rich donor to a variety of metals. Cyclopentadienidelso supports f-block metals, where more than two of the unitsften coordinate to the large, highly charged cations. The sodiumetrakis(cyclopentadienyl)cerium(III) ate-complex is reportedlyccessible, but shows irregular electrochemistry [88]. The ceriumlkoxide complex Cp3Ce(OiPr) has been prepared and fully char-cterized and shows a reversible reduction event at +0.31 V inHF/TBABF4, Table 7 [89]. This result is particularly telling abouthe electronics of the Cp ligand within the Ce framework; the moreelocalized charge across the Cp ring seems to less efficiently trans-er charge to the cerium center, and results in Cp3Ce(OiPr) behavings a mild oxidant with potentials comparable to that of Fc+.

Cerocene, Ce(C8H8)2, Fig. 10, is a particularly interestingrganocerium molecule. Cerocene has been studied extensivelyince its first reliable report in 1976 [94–97]. While formally ae(IV) ion sandwiched between two cyclooctatetraene dianions,he true electronic configuration is complex and best describeds an intermediate valent cerium ion: an electron hole is sharedointly by the cerium ion and the two C8H8 rings. The ground state

avefunction of cerocene is best described as an admixture of aerium(III) and cerium(IV) electronic configurations [72,93,98]. As

uch, any description of a Ce(III/IV) redox couple for this molecules particularly imprecise. Nevertheless, reversible redox chemistrys observed for cerocene; the first reduction event is observed at0.8 V, which comes at a significantly negative potential relative

able 7eduction potentials of organometallic cerium complexes.

Complex Potential (V) vs. SCE Reported po

CeSc2N@C80 0.89 0.33 vs. Fc/FcCeLu2N@C80 0.57 0.01 vs. Fc/FcCeY2N@C80 0.49 −0.07 vs. Fc/Cp3Ce(OiPr) 0.31 0.32 vs. SCE0Ce(C8Me6)2 −0.27 −0.83 vs. Fc/Ce(C8H8)2 −0.8 −1.4 vs. Fc/F

a TBABF4 = [NBu4][BF4], TBAPF6 = [NBu4][PF6].

Fig. 10. Structure of Ce(C8H8)2. Coordinates from Ref. [100].

to the formal Ce(III/IV) couple. An electronically related com-plex, bis(permethylpentalene)cerium is also best described withan intermediate valent ground state, however, the reversible elec-trochemical reduction occurs more readily by 530 mV comparedwith cerocene, Table 6 [92,99].

Endohedral fullerene complexes that contain cerium ions havealso been prepared and thoroughly characterized electrochemi-cally. Interestingly, a series of trimetallic compounds, M2CeN@C80(M = Sc, Y, Lu), displayed reversible redox events that were assignedas Ce(III/IV) oxidations (Table 7) [90,91]. The trimetallic nitridecluster shows a unique strain-driven tunability of the redox cou-ple; the identity of the redox-inactive metal results in a 400 mVpotential range of the cerium oxidation and trends with theirionic radii. Larger M cations show greater pyramidalization of theCeM2N unit and shorter Ce N bond distances, which results ina more electron-rich cerium center and more negative oxidationpotentials. Notably, other cerium containing endohedral fullerenes,including Ce3N@C80, only display cage oxidation events up to thesolvent window and do not show assignable Ce(III/IV) redox events[101–106].

3.6. Cerium amides

Cerium amides, in particular Ce[N(SiMe3)2]3, have a well-developed synthetic chemistry, but studies of their electrochem-

istry are rare [107–110]. To our knowledge, the reduction potentialof only one amide-supported cerium complex has been reported,CeCl[N(SiMe3)2]3. This lone data point, presented in Table 8, sug-gests that amides are not as well-suited to stabilizing cerium(IV) as

tential (V) Conditionsa Ref.

+ TBABF4 in o-DCB [90]+ TBABF4 in o-DCB [90]Fc+ TBABF4 in o-DCB [91].01 vs Fc/Fc+ 0.25 M TBABF4 in THF [89]Fc+ unspecified [92]c+ 0.1 M TBAPF6 in THF [93]

N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36 31

Table 8Reduction potentials of an amide supported cerium complex.

Complex Potential (V) vs. SCE Reported potential (V) Conditionsa Ref.

0.30 v

aoa

4o

atctaddh[otssaorofr

4a

npimpoccttpct

CeCl[N(SiMe3)2]3 0.26 −

a TPAB = [N(n-C3H7)4][B(3,5-(CF3)2C6H3)4].

re alkoxide or aryloxide ligands, although studies with complexesf directly comparable coordination numbers at the cerium cationre currently lacking.

. Preparative chemistry of cerium(IV) complexes byxidation reactions

The development of the chemistry of cerium(IV) is an activerea of research [27,110]. Two synthetic strategies have been usedo access complexes of cerium(IV). The first is to start with aerium(IV) precursor, such as CeO2, [CeCl6]2−or CAN, and proceedhrough ligand substitution reactions. This strategy has beenpplied successfully, however, preservation of the tetravalent oxi-ation state can be challenging due to the reducing nature of manyeprotonated ligands [89,111,112]. The successes of this approachave been reviewed recently and will not be discussed in detail27]. A second method is to access cerium(IV) complexes throughne-electron oxidation of cerium(III) complexes. As illustrated overhe previous sections, the cerium(III/IV) potentials are highly ligandet dependent; the strategy of oxidizing cerium(III) complexes totable cerium(IV) complexes becomes applicable with ligands thatre good donors and depress the cerium potential into the rangef common synthetic laboratory oxidants. Below, we survey theeported cerium(III) oxidations by reaction type: (1) outer-spherexidation, H-atom abstraction, and autooxidation, (2) oxidativeunctionalization, and (3) oxidation-induced ligand redistributioneactions.

.1. Outer-sphere oxidation, H-atom abstraction, andutooxidation reactions

The cerium(III) to cerium(IV) transformation can occur sponta-eously with oxidants as simple as dioxygen. From a preparativeerspective, this oxidation occurs during the introduction of ligands

n order to isolate cerium(IV) products from cerium(III) startingaterials. A derivative of TrenSal is oxidized to cerium(IV) by sim-

le stirring in the presence of O2, Scheme 1 [113]. A Zn complexf the fully hydrogenated ligand H6TrenSal, when treated witherium(III) nitrate in the presence of air affords a bimetallic ceriumomplex with a bridging �2;�2, �2-O2 unit. It has been proposedhat this complex is mixed-valent and contains a superoxo bridge,

hough this has not been confirmed [114]. One peculiar exam-le involves a silesquioxane ligand, which spontaneously yieldederium(IV) products from cerium(III) precursors. The authors con-est that adventitious O2 or peroxides from ethereal solvents do not

Scheme 1. Synthesis of CeCl(TrenSal

s. Fc/Fc+ 0.1 M TPAB in THF [67]

serve as the oxidant, although, the identity of the oxidant remainsunclear [115].

Many of the formally cerium(IV) complexes of porphyrinand porphyrin-like ligands of Section 3.4 are synthesized fromcerium(III) acetylacetonate complexes by treatment with theligand, its dianion, or its condensation precursors, and air. Thisstrategy is effective for isolation of numerous compounds of thisclass [116].

The cerium(III) complex Ce(Htmtaa)(tmtaa) undergoes oxi-dation with O2, benzoquinone, or ferrocenium with loss of aproton to afford the Ce(tmtaa)2 complex [117]. In the caseof the related Ce(Homtaa)(omtaa) complex, the reaction withO2 occurs in a single-crystal to single-crystal transformationto afford the cerium(IV) product [87]. In solution, TEMPO alsooxidizes Ce(Homtaa)(omtaa) through a formal H-atom transferreaction.

Net outer-sphere oxidations of reducing cerium(III) com-pounds will also generate cerium(IV) complexes. Cerocene andits derivatives are often accessed from the oxidation of thecerium(III) bis(cyclooctatetraene) anions with oxidants suchas AgI or allyl bromide [93,118,119]. The phenolate complex[Li(THF)2CeIII(MBP)2(THF)2] is oxidized with a range of oxidants– CuCl2, CuBr2, CuCl, CuI, I2, NBS, Fc+ – forming a mix-ture of the neutral Ce(MBP)2(THF)2 and the ‘-ate’ complexes[Li(THF)CeX(MBP)2(THF)], X = Cl, Br, I [66].

4.2. Oxidative functionalization

Oxidative functionalization is the simultaneous oxidation of ametal center and introduction of new ligands to the coordinationsphere. In the case of cerium(III) complexes, oxidative function-alization is another method of preparing cerium(IV) complexes.For example, Ce[N(SiMe3)2]3 reacts with chlorine atom donorssuch as TeCl4, PhICl2, or Ph3CCl to afford ClCe[N(SiMe3)2]3, Fig. 11[109,120,121,115,126–128]. Inner-sphere oxidations that form astrong Ce X bond concomitant with electron-transfer, and do notrequire either release of high-energy radicals (e.g. Cl•) or termolec-ular reactions, are crucial to successful synthetic schemes. That isto say that the Ce X bond should form during the initial oxidation,and that the electron be deposited into a low energy intermediate.The use of PhICl2, which fits these requirements, was shown to be

a general reagent for the synthesis of a variety of Ce(IV) complexes,Fig. 12 [109].

By conformationally constraining cerium(III) employinga TREN-based ligand, Ce[N(C2H4NSiMe2

tBu)3], Ce(III) can be

). Scheme adapted from [113].

32 N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36

ative h

ocadAbbtiitm[c[bff

tactldiuclMos[

Fig. 11. A comparison of oxidants for the oxid

xidized by I2 to afford the only known Ce(IV) iodide complex. Inontrast, bulky monodentate Ce(III) amides such as Ce[N(SiMe3)2]3nd Ce[TMP]3 (TMP = 2,2,6,6-tetramethylpiperidinide) are not oxi-ized with iodine [122,123]. Additionally, neither Cl2, Br2, I2 norg+ is capable of oxidizing Ce[N(SiMe3)2]3, despite several of theseeing thermodynamically better oxidants than reagents that haveeen successfully applied. These results suggest a kinetic barriero accessing Ce(IV) complexes and are discussed in further detailn Section 5. Interestingly, oxidation of Ce[N(C2H4NSiMe2

tBu)3]n pentane with either Cl2 or Br2 in place of I2 does not facilitatehe isolation of monomeric [CeIV]–X complexes, instead the

ixed-valent species [CeIV] − X → [CeIII] were obtained, whereCe] = CeN(C2H4NSiMe2

tBu)3 and X = Cl−, Br–[122]. Scott andoworkers proposed that disproportionation of [CeIV] − X toCeIV] − X → [CeIII] and ½ X2 was favored for the harder Lewisases Cl−and Br−, and suggested that stronger dative interactionsor CeIV − X → CeIII offset the loss of bond enthalpy associated withormation of CeIV − X.

Recently, trityl chloride has been reported as an effec-ive chloride group transfer oxidant, with amide, alkoxide,nd aryloxide supporting ligand frameworks [67,120]. In theseases, homoleptic and heteroleptic complexes are oxidizedo their corresponding Ce(IV) chloro complexes without anyigand redistribution being observed. Typically, different oxi-ants have been investigated in cerium(III) oxidation chemistry

n order to change reactivity preferences; however, three ofs have recently reported that the choice of alkali metalan result in different oxidation reactivity within a conservedigand framework [67]. Oxidation of [M3(THF)n][(BINOLate)3Ce],

= Li, Na, K, with trityl chloride yields either the product ofxidative functionalization [Li3(THF)5 ][(BINOLate)3Ce–Cl] or thealt-eliminated products [M2(THF)n][(BINOLate)3Ce], M = Na, K67].

Fig. 12. Use of PhICl2 for the formation of Ce(IV)

alogenation of Ce[N(SiMe3)2]3 [109,120,121].

Oxidants that form Ce O bonds are also amenable to the syn-thesis of well-defined Ce(IV) products. Ce(OCtBu3)3 is oxidizedby a variety of organic peroxides as well as quinones to formtetravalent products with new Ce O bonds, Fig. 13 [124]. Ben-zoquinone, BQ, results in the formation of a dimeric cerium(IV)product, whereas use of a bulky quinone results in monomericCe(IV) semiquinolates. The use of BQ as an oxidant for Ce(III) issurprising due to Ce(III) being a mild reductant and benzoquinonea mild oxidant. However, coordination of a Lewis basic oxidantto a strong Lewis acid can result in large Lewis Acid PromotedPotential Shifts (LAPPS). Recently, BQ has been used as an oxidantfor [Li3(THF)5 ][(BINOLate)3Ce(THF)], which forms an authenticatedcerium(IV) dimer bridged by the doubly reduced hydroquinolate[125]. The presence of the fully reduced hydroquinone representsan overall shift of BQ’s oxidizing potential by ∼1.6 V, demonstrat-ing the potential utility of Lewis acid coordination in RE oxidationchemistry.

4.3. Oxidation-induced ligand redistribution reactions

Under oxidizing conditions, cerium complexes will oftenundergo ligand redistribution reactions to afford rearrangedproducts. ˇ-Diketonate complexes of cerium have been syn-thesized by metallating with cerium(III) precursors to givetris(ligand) complexes, which under aerobic conditions redis-tribute to afford cerium(IV) products [126–128]. That theseconditions afford the cerium(IV) products in good yields is atestament to the stability afforded the cerium(IV) ion by theˇ-diketonate ligand field. The aerobic oxidation of Ce(NCy2)3(THF)

is also used to afford homoleptic Ce(NCy2)4 in 35% yield ina ligand redistribution [129]. The homoleptic Ce(IV) complexCe[N(SiHMe2)2]4 is similarly formed through a redistribution pro-cess in the reaction of Ce[N(SiHMe2)2]3(THF)x (x = 0, 2) with PhICl2,

Cl bonds. Scheme adapted from Ref. [109].

N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36 33

Fig. 13. Oxygen–cerium bond-formin

Fig. 14. Oxidation–rearrangement reactions to afford homoleptic amide complexes[

tpct

cgta

octc

modynamic predictions and observed reactivity suggests a kineticaspect, such as ligand reorganization, might be a mitigating fac-tor in oxidation reactions of cerium(III) complexes. This is furthersupported by the observation that a conformationally-constrained

108,129,130].

rityl chloride, or hexachloroethane [108]. A similar redistributionrocess is observed in the oxidation of the tris(dithiocarbamate)omplex Ce(�2-S2CNEt2)3 with O2, which afforded the cerium(IV)etrakis(dithiocarbamate) complex, Fig. 14 [130].

Benzoquinone can also act as a net outer-sphere oxidant. Theerium tris(alkoxide-carbene) complex shown in Fig. 15 under-oes oxidation with benzoquinone, XeF2, or ferrocenium to affordhe ligand distributed product [131]. Notably, this oxidation is notfforded by TeCl4, PBr2Ph3 or I2.

Oxygen has also been used as an oxidant to give various cerium-xo clusters originating from tris(amide) complexes [132]. In theselusters, oxygen incorporation is coupled with ligand redistributiono give a variety of products that have been identified by X-rayrystallography.

Fig. 15. Oxidative ligand redistribution of an alkoxide/carbene complex. [

g oxidation reactions [124,125].

5. The role of kinetics in cerium electrochemistry andoxidation reactions

The existence and stability of strongly oxidizing, molecularcerium(IV) complexes such as ceric ammonium nitrate (CAN) in thepresence of moisture has been ascribed not only to the large over-potential for water oxidation but also to kinetic inertness of thecerium(IV) compounds in the solid state [133]. For example, CANfeatures a saturated CeIV coordination sphere and highly hydrogenbonded secondary coordination sphere, which leads to a kineticallystable and robust reagent. Ceric ammonium fluorides also demon-strate a unique kinetic inertness; (NH4)2CeF6 is stable to heat andmoisture, whereas M2CeF6 and M3CeF7 are only isolable in theabsence of water [133].

Slow electron transfer rates have also been implicated as adetriment to the preparation of cerium(IV) compounds [131].An interesting example is the oxidation chemistry of theCe[N(SiMe3)2]3 (Fig. 16). Oxidation reactions of this homolepticamide have proven challenging; oxidants that might be predicted tooxidize the amide on the basis of thermodynamics have otherwiseshown no reaction [121,122]. This disagreement between ther-

Ox] = benzoquinone, XeF2, or [Fc][OTf]. Scheme adapted from [131].

34 N.A. Piro et al. / Coordination Chemistry Reviews 260 (2014) 21– 36

F vorable and unfavorable couples relative to that observed for ClCe[N(SiMe3)2]3. Oxidantsw

co

lc[tiwcTttfvaptrsusia(

re

FM

ig. 16. Oxidants used to oxidize Ce[N(SiMe3)2]3 grouped by thermodynamically faith accessible potentials that are unreactive are highlighted (red boxes).

erium tris(amide) can be oxidized by halogens, whereas the mon-dentate tris(amides) are not [122,123].

Investigations of the kinetics of the CeIII/IV couple have focusedargely on CeIV in acidic media, due to the application of cerium(IV)ompounds in batteries, fuel cells, and separations chemistry134–144]. In non-aqueous media there have been few studies ofhe kinetics of the CeIII/IV couple [67,71]. Matsumoto and coworkersnvestigated the electrochemical and chemical kinetics associated

ith the CeIII/IV couple for Ce(acac)40/−by cyclic voltammetry and

ross reactions of outer sphere electron transfer (ET) reagents.hese studies determined the CeIII/IV self-exchange rate (kex) usinghe Marcus cross relation. As expected, values of kex depend onhe driving force (�G◦) of the cross-reaction partner, where largerree energy differences resulted in closer agreement to calculatedalues of kex. Similar behavior has been observed for EuII/III underqueous conditions, and has been attributed to poor electronic cou-ling (HAB) between donor and acceptor for ET [145]. Physically,he origin for this poor electronic coupling is the shielding of theadially contracted 4f orbitals by outer shell orbitals, resulting inlower rates of electron transfer. The authors suggest that contrib-tions from the product excited state with larger driving forceserve to overcome the poor electronic coupling; values for kex aren agreement with those calculated using the Marcus cross relation,nd suggest a change from a concerted to a directional ET processFig. 17).

More recently, three of us have investigated the role of ligandeorganization in CeIII oxidation chemistry [67]. We studied thelectrochemical, Fig. 18, and chemical oxidation behavior for a

ig. 17. Depiction of non-adiabatic direct ET and directional ET processes followingarcus theory. Scheme adapted from Ref. [71].

Fig. 18. Normalized cyclic voltammograms of [M3(THF)n][(BINOLate)3Ce], taken at100 mV/s in 0.1 M [N(n-Pr)4][BArF

4]THF.

Adapted from Ref. [67].

heterobimetallic framework; we found that the rates of heteroge-neous electron transfer (ks) run counter to rates of the chemicaloxidations with trityl chloride (kobs). An increase in the drivingforce, �G◦, and associated increase in the rate constant ks wereobserved as the Lewis acidity of the secondary cation M+ decreased(K+ > Na+ > Li+), whereas the rate of chemical oxidation kobs fol-lowed an opposite trend corresponding to the accessibility of theCeIII ion to Lewis base coordination (Li+ > > Na+ > K+), Table 9. Theopposing trends in ks and kobs support different mechanisms forthe electrochemical and chemical processes, and indicate an innersphere mechanism for the chemical oxidation of the complexeswith trityl chloride. These findings highlight the importance of

ligand reorganization; the Ce/Li framework readily reorganizesto allow access to both six and seven-coordinate geometries,which are essential for coordination of the oxidant and oxidationreaction. When reorganization between six- and seven-coordinate

Table 9Kinetics Data for [M3(THF)n][(BINOLate)3Ce] (M = Li, Na, K) [67].

Entry Alkali metal, M ks [× 10−4 cm s−1]a kobs [× 10−4 s−1]b

1 K 9.45 0.2162 Na 5.97 1.933 Li 4.84 30.0

a Obtained by cyclic voltammetry measurements in THF using 0.1 M TPAB at100 mV/s.

b Obtained under pseudo-first order conditions using UV–vis absorptionspectroscopy;[Ce]/[Ph3CCl] = 1:10.

hemis

goFabcc

6

rtwaibdfiarIinibaeecith

A

Ofnts

R

N.A. Piro et al. / Coordination C

eometries becomes high in energy, sluggish reaction kinetics arebserved and alternate lower energy pathways become available.or heterobimetallic or ‘-ate’ complexes, salt elimination facilitates

non-destructive pathway for oxidation reactions; however, inulky or non-chelating ligand frameworks, ligand redistributionan occur leading to unpredictable products and/or low yields oferium(IV) compounds of interest.

. Conclusions.

As evident from the data reported in this review, the Ce(III/IV)edox couple is thermodynamically and kinetically highly sensi-ive to its ligand environment. Under aqueous, acidic conditionsith poor donor ligands the cerium(III) form is strongly favored,

n aqueous oxidation potential of +1.63 V was reported for Ce(IV)ons in 8 M HClO4. In general, the cerium(IV) state is favored byasic conditions with higher coordination numbers of effectiveonors, especially anionic oxygen donor ligands. This is exempli-ed by Ce(L5)(OtBu)2, Fig. 6, a cerium(IV) compound that is reduced,lbeit irreversibly, at −1.83 V in non-aqueous conditions. Thus, theeported cerium redox events span a nominal range of ∼3.5 V.mportantly, ligands that are traditionally associated with stabiliz-ng higher oxidation state transition metals and actinides are notecessarily as well suited for stabilizing cerium(IV); the predom-

nantly ionic bonding eliminates significant contributions from �onding or other covalent interactions. As such, cyclopentadienylnd amide frameworks do not transfer charge to the cerium ion asffectively as the more localized charge of alkoxide ligands. Wexpect these data will be useful for the expansion of molecularhemistry of cerium(IV), the development of a new reductive chem-stry of cerium(III), and the rational design of cerium complexes forailored redox applications. It is clear that cerium redox chemistryas great “potential” for new fundamental and applied chemistry.

cknowledgments

E.J.S. gratefully acknowledges the U.S. Department of Energy,ffice of Science, Early Career Research Program (DE-SC0006518)

or support. E.J.S. and P.J.W. also thank the University of Pennsylva-ia and the NSF (CHE-1026553) for support. We are also grateful tohe reviewers for their highly constructive assessment and helpfuluggestions for this review.

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