Controlled Redox Chemistry at Cerium within a Tripodal Nitroxide Ligand Framework

Article abstract of DOI:10.1002/chem.201502952

Ligand reorganization has been shown to have a profound effect on the outcome of cerium redox chemistry. Through the use of a tethered, tripodal, trianionic nitroxide ligand, [((2-tBuNOH)C₆H₄CH₂)₃N]^3- (TriNO_x^3-), controlled redox chemistry at cerium was accomplished, and typically reactive complexes of tetravalent cerium were isolated. These included rare cationic complexes [Ce(TriNO_x)thf][BAr^F₄], in which Ar^F=3,5-(CF₃)₂-C₆H₃, and [Ce(TriNO_x)py][OTf]. A rare complete Ce-halide series, Ce(TriNO_x)X, in which X=F^-, Cl^-, Br^-, I^-, was also synthesized. The solution chemistry of these complexes was explored through detailed solution-phase electrochemistry and ¹H NMR experiments and showed a unique shift in the ratio of species with inner- and outer-sphere anions with size of the anionic X^- group. DFT calculations on the series of calculations corroborated the experimental findings. The use of a bulky and strongly donating tethered tripodal nitroxide ligand allowed the controlled redox chemistry at cerium. As a result, rare examples of cationic Ce^IV complexes were synthesized and fully characterized. The full Ce-halide series supported by the tripodal ligand framework is also reported (see scheme).

Full text of DOI:10.1002/chem.201502952

DOI: 10.1002/chem.201502952
Full Paper
&
Electrochemistry
Controlled Redox Chemistry at Cerium within a Tripodal Nitroxide
Ligand Framework
Justin A. Bogart,[a] Connor A. Lippincott,[a] Patrick J. Carroll,[a] Corwin H. Booth,[b] and
Eric J. Schelter*[a]
Abstract: Ligand reorganization has been shown to have
a profound effect on the outcome of cerium redox chemis-
try. Through the use of a tethered, tripodal, trianionic nitro-
xide ligand, [((2-tBuNOH)C6H4CH2)3N]3À (TriNOx3À), controlled
redox chemistry at cerium was accomplished, and typically
reactive complexes of tetravalent cerium were isolated.
These included rare cationic complexes [Ce(TriNOx)thf]
[OTf]. A rare complete Ce–halide series, Ce(TriNOx)X, in
which X=FÀ, ClÀ, BrÀ, IÀ, was also synthesized. The solution
chemistry of these complexes was explored through detailed
solution-phase electrochemistry and 1H NMR experiments
and showed a unique shift in the ratio of species with inner-
and outer-sphere anions with size of the anionic XÀ group.
DFT calculations on the series of calculations corroborated
the experimental findings.
[BArF ], in which ArF =3,5-(CF3)2-C6H3, and [Ce(TriNOx)py]
4
Introduction
A recent survey of the literature by our group showed that the
redox properties of cerium(III) are highly sensitive to ligand
field.[1] For example, exchanging the weakly coordinating ni-
trate ligands in ceric ammonium nitrate (E1/2 =1.21 V vs.
Fc/Fc+) with catecholate ligands in [Ce(O2C6H4)4]4À (E1/2
=
À0.85 V vs. Fc/Fc+) changed the cerium cation from a potent
oxidant to a potent reductant.[2] Understanding of the ability
to tune the redox potential of cerium has broad applications
in energy science,[3] separations chemistry,[4] and organic syn-
thesis.[2b] This has motivated renewed interest by us and others
in studying the coordination chemistry of cerium in the 4+ ox-
idation state.[5]
Figure 1. Published tris-silylamide complexes of cerium.
have been performed on the coordination chemistry of tripo-
dal trianionic ligands to various transition metals, as well as
uranium in the f block (Figure 1).[7]
Few studies have explored the coordination chemistry of
CeIV in tripodal, trianionic ligand environments.[6] In a seminal
study by Scott and co-workers, use of the silyl substituted
The coordination and redox chemistry of cerium within the
related, untethered Ce[N(SiMe3)2]3 system have also been stud-
ied. Unlike the Ce(NN’3) system, no reaction occurred upon ad-
tris(2-aminoethyl)amine
ligand,
[N(CH2CH2N(SiMe2tBu))3]3À
(NN’3), afforded isolation of the CeIV oxidation state, in particu-
lar of the CeI(NN’3) complex. However, in the cases of ClÀ and
BrÀ, the stability of the 4+ oxidation state was not sufficient
to prevent formation of the mixed valent CeIII/IV dimers,
[Ce(NN’3)]2(m-X), X=ClÀ, BrÀ.[6a] In contrast, extensive studies
dition of molecular halogens to CeIII[N(SiMe3)2]3 (CeIII/IV E1/2
=
0.35 V vs. Fc/Fc+).[6a,8] Although CeI(NN’3) could be formed
through oxidation with molecular I2, an analogous reaction in
the formation of CeI[N(SiMe3)2]3 did not occur. In fact, CeI[N-
(SiMe3)2]3 was formed through a halide-transfer reaction from
a cerium(IV) precursor, CeF[N(SiMe3)2]3, using Me3SiÀI rather
than through oxidation of CeIII[N(SiMe3)2]3.[5f] Scott and co-
workers postulated that the increased stability of the CeIV state
in their tethered Ce(NN’3) systems compared to Ce[N(SiMe3)2]3
system was a result of reduced ligand reorganization involved
in accommodating the XÀ ligand in the former. Indeed, ligand
reorganization has been shown to have profound effects on
the redox chemistry at cerium.[5c] As was shown by Anwander
and co-workers, oxidation of Ce[N(SiHMe2)2]3(thf)2 with chlori-
nating agents in THF led to the clean formation of Ce[N-
(SiHMe2)2]4 through ligand redistribution pathways. However,
[a] J. A. Bogart, C. A. Lippincott, Dr. P. J. Carroll, Prof. E. J. Schelter
P. Roy and Diana T. Vagelos Laboratories
Department of Chemistry, University of Pennsylvania
231 S. 34th St., Philadelphia, PA 19104 (USA)
E-mail: schelter@sas.upenn.edu
[b] Dr. C. H. Booth
Chemical Sciences Division
Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502952.
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performing the same reaction in donor-free solvents led to the
formation of the Ce5[N(SiHMe2)2]8Cl7 cluster and other intrac-
table products.[5k] In this context, there is a clear need to
expand the coordination chemistry of cerium using strongly
donating, tethered tripodal systems, in which the reduced
ligand reorganization effects would confer stability to the CeIV
oxidation state.
Recently, we communicated the synthesis of the tripodal tri-
anionic nitroxide, [((2-tBuNOH)C6H4CH2)3N]3À (TriNOx3À), and its
coordination chemistry with the rare-earth elements La, Nd,
Dy, and Y.[9] The h2-(N,O) binding mode of the three ligand
arms provided a single coordination site for substrate binding.
Previous studies by our group showed that nitroxide ligand
fields provided significant stabilization to the CeIV state
through increased electron donation into the empty metal 4f
orbitals from the well energy matched NÀO p* orbitals.[5b,10]
We hypothesized that the environment of TriNOx3À would
impart an improved stabilization to the CeIV state compared to
the tren framework while providing a single coordination site
to elucidate controlled redox chemistry.
Scheme 1. Schematic of the synthesis of Ce(TriNOx)thf (1), top, and the self-
association equilibrium of 1 to form [Ce(TriNOx)]2 (2), bottom.
Herein, we report the synthesis of the monomeric
CeIII(TriNOx)thf (1) and dimeric [CeIII(TriNOx)]2 (2), analogues of
the previously published M(TriNOx)thf, M=La, Nd, Dy, and Y,
and [M(TriNOx)]2, M=La and Nd, complexes. CeIV complexes,
[Ce(TriNOx)thf][BArF ] (3-BArF ) and [Ce(TriNOx)pyr][OTf] (3-
4
4
OTf), rare examples of crystallographically characterized cation-
ic CeIV complexes, were accessed from 1. The complete halide
series, Ce(TriNOx)X, in which X=FÀ (3-F), ClÀ (3-Cl), BrÀ (3-Br),
and IÀ (3-I), are also reported.
Detailed characterization of the cerium complexes to under-
stand their coordination chemistries in solution are disclosed.
These include solution-phase electrochemistry and 1H NMR
spectroscopy experiments. DFT calculations corroborated the
experimental findings and provided evidence for 4f orbital
mixing in the CeIV complexes.
Results and Discussion
Synthesis and characterization
The shortage of tridentate frameworks in the literature that
stabilize the CeIV oxidation state prompted us to study the
redox chemistry of cerium using the TriNOx3À framework. The
synthesis of the CeIII(TriNOx)thf complex (1) was achieved
through an analogous route to the previously reported
MIII(TriNOx)thf complexes, M=La, Nd, Dy, and Y, in which a hex-
anes solution of Ce[N(SiMe3)2]3 was layered onto a THF solution
of H3TriNOx (Scheme 1, top).[9] X-ray crystallography confirmed
1 was isostructural with the M(TriNOx)thf complexes, with re-
tention of the h2-(N,O) bonding mode of the three nitroxide
arms and the coordination of a THF molecule to the central
cerium cation (Figure 2, top).
Figure 2. Thermal ellipsoid plot and space-fill diagram of 1 (top) and thermal
ellipsoid plot of 2 (bottom). Hydrogen atoms omitted for clarity. Selected
bond lengths [Š] for 1: Ce(1)ÀO(1) 2.2921(18), Ce(1)ÀN(1) 2.581(2), Ce(1)À
O(2) 2.577(6); for 2: Ce(1)ÀO(1) 2.297(3); Ce(1)ÀO(3) 2.273(3); Ce(1)ÀO(2)
2.364(3); Ce(1)ÀN(1) 2.639(3); Ce(1)ÀN(3) 2.606(3); Ce(1)ÀN(2) 2.689(3);
Ce(1)ÀO(5) 2.523(3).
The coordination chemistry of 1 in solution was analogous
1
to the early M cations, M=La and Nd, as indicated by H NMR
pressure (Scheme 1, bottom). Crystals of 2 suitable for X-ray
diffraction analysis were formed by cooling a saturated Et2O
solution of 1 to À258C, which confirmed its dimeric structure
(Figure 2, bottom).
spectroscopy (see the Supporting Information). The [Ce-
(TriNOx)]2 complex (2) could be synthesized by dissolving 1 in
toluene solution and removing the volatiles under reduced
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To gain insight into the chemically accessible redox states of
1, electrochemistry experiments were performed in dichloro-
methane solution. As shown in Figure S19 in the Supporting
Information, the cyclic voltammogram of 1 showed only one
wave centered at À0.96 V versus Fc/Fc+ (Epa =À0.88 V, Epc =
À1.04 V). Based on the open circuit potential of À1.14 V, this
wave was assigned as the metal-based CeIII/IV redox couple,
which was later confirmed chemically through the isolation of
a series of CeIV complexes (see below). The measured potential
was compared to that of the CeIII/IV redox couple for Ce[N-
ligand were oxidized, was chemically unfeasible. This result
was surprising given the ease at which free H3TriNOx was oxi-
dized (E1/2 =À0.55 V vs. Fc/Fc+, Figure S18 in the Supporting
Information) and indicated that coordination to the highly
Lewis acidic CeIV center stabilized the potential of the TriNOx3À
redox couple by over 1.0 V.
Based on the measured redox potential of cerium within the
TriNOx3À ligand framework, ferrocenium salts were selected as
oxidants for controlled oxidation chemistry at the cerium
cation. The quasi-reversibility of the CeIII/IV redox wave in the
CV of 1 suggested that a cationic [Ce(TriNOx)thf]+ complex
(SiMe3)2]3, which showed a quasi-reversible CeIII/IV wave at E1/2
=
0.35 V versus Fc/Fc+.[8] This 1.31 V shift in CeIII/IV redox potential
between 1 and Ce[N(SiMe3)2]3 indicated the stability to the 4+
oxidation state imposed by the TriNOx3À framework relative to
the [N(SiMe3)2]À framework. Furthermore, the peak separation,
DE, of 0.16 V in 1 compared to approximately 0.5 V in Ce[N-
(SiMe3)2]3 was suggestive of a relatively small ligand reorganiza-
tion involved in the oxidation of 1 compared to that in the oxi-
dation of Ce[N(SiMe3)2]3 (Table 1).
could be isolated. In fact, reaction of 1 with Fc[BArF ] in tolu-
4
ene led to the immediate formation of a sparingly soluble dark
red brown solid. Crystallization of this complex through vapor
diffusion of pentane into a saturated THF solution induced for-
mation of crystals of [Ce(TriNOx)thf][BArF ] (3-BArF ), ArF =3,5-
4
4
(CF3)2-C6H3, that were suitable for X-ray diffraction analysis.
Similarly, reaction of 1 with Fc[OTf] in toluene led to the forma-
tion of an insoluble dark red brown solid. Crystallization of this
complex by vapor diffusion of Et2O into a saturated pyridine
solution gave crystals of [Ce(TriNOx)pyr][OTf] (3-OTf) suitable
for X-ray diffraction analysis (Scheme 2).
Table 1. Redox potentials of the CeIII/IV couple (in V vs. Fc/Fc+) for cerium
in salient ligand frameworks.
Complexes 3-BArF and 3-OTf are rare examples of structur-
4
ally characterized cationic cerium complexes. To date, only one
other crystallographically characterized cationic CeIV complex,
[(TRENDSAL)Ce][BPh4], where TRENDSAL3À = {N[CH2CH2N=
CH(C6H2tBu2-3,5-O-2]3}3À, has been reported.[13] Similar to
TriNOx3À, the Schiff base framework in [(TRENDSAL)Ce][BPh4]
was sufficiently bulky and electron rich to mitigate the strong
Lewis acidity of the CeIV cation and prevent unwanted reactivi-
E1/2 CeIII/IV
DE1/2 CeIII/IV
Ipa/ipc
Ref.
[11]
[12]
[5c]
[10]
[nBu4N]2[Ce(NO3)6]
Ce[N(SiMe3)2]3
CeLiB[a]
Ce[2-(tBuNO)py]4
Ce(TriNOx)THF
0.62
0.35
À0.76
À1.80
À0.96
0.075
ꢀ0.5
0.64
0.19
0.16
–
ꢀ2
1.94
0.90
1.08
this work
[a] CeLiB=[M3(THF)4][(binolate)3Ce(THF)].
ty. The structural metrics of 3-BArF and 3-OTf were consistent
4
with the central cerium cation being in the 4+ oxidation state.
The NÀO bond lengths ranged from 1.418(8) to 1.436(7) Š in
The absence of a second oxidation feature to 1.0 V suggest-
ed that the formation of a [CeIV(TriNOx2À)]2+ complex, in which
both the metal center and one of the nitroxide arms of the
3-BArF and 1.431(4) to 1.435(4) Š in 3-OTf, which fell in the
4
range of typical anionic nitroxide bond lengths and are similar
Scheme 2. Schematic of the redox chemistry performed on 1 to form the series of 3-X complexes, in which X=BArF , OTf, F, Cl, Br, and I.
4
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to that of 1.433(2) Š for the NÀO bonds found in 1.[5b,14] The
average CeÀOnitroxide and CeÀNnitroxide bond lengths of 2.156(5)
Table 2. Bond metrics for 1, 3-BArF4, 3-OTf, and 3-Cl.
and 2.493(6) Š, respectively, in 3-BArF and 2.177(3) and
1
3-BArF
3-OTf
3-Cl
4
4
2.489(3) Š in 3-OTf were approximately 0.10–0.14 Š shorter
than those in 1 consistent with the smaller ionic radius of CeIV
compared to CeIII.[15] Lastly, the CeÀOthf bond length of
NÀO
1.433(2)
1.418(8)–
1.436(7)
2.156(5)
2.493(6)
2.507(4)
1.431(4)–
1.435(4)
2.177(3)
2.489(3)
2.645(3)
1.425(3)–
1.436(3)
2.169(2)
2.531(3)
2.7436(8)
(CeÀOnitroxide
)
2.2921(18)
2.581(2)
2.577(6)
av
(CeÀNnitroxide
)
av
2.507(4) Š in 3-BArF and the CeÀNpyr bond length of
4
CeÀX[a]
2.645(3) Š in 3-OTf were typical of neutral oxygen and nitro-
gen donors bound to a CeIV cation.[5a,g]
[a] X=Othf for 1 and 3-BArF4, Npyr for 3-OTf, and ClÀ for 3-Cl.
A space-fill diagram of 1 with the THF molecule removed in-
dicated that the TriNOx3À framework limited the central cerium
cation to one open coordination site that we postulated would
allow the formation of stable 1:1 adducts with anionic ligands
upon oxidation. To test this hypothesis, we sought to synthe-
size the halide series, Ce(TriNOx)X, in which X=FÀ (3-F), ClÀ (3-
Cl), BrÀ (3-Br), and IÀ (3-I), using AgF, Ph3CCl, Ph3CBr, and I2 as
oxidants, respectively. Indeed, these reactions proceeded
cleanly and in moderate to good yields to dark red brown
products in pyridine solutions for 3-F, 3-Cl, and 3-Br, and tolu-
ene solution for 3-I (Scheme 2). To the best of our knowledge,
this is only the second complete halide series reported for CeIV
within a conserved ligand framework. The other reported
halide series for CeIV is the CeX[N(SiMe)3] system completed by
us with the isolation of CeF[N(SiMe)3].[12] The low solubility of
these complexes in solution made growing diffraction quality
crystals difficult.[16] However, crystals of 3-Cl suitable for X-ray
diffraction analysis could be isolated by layering Et2O onto a sa-
turated pyridine solution of the complex.
Cl)], in which NN’3 =[N(CH2CH2N(SiMe2tBu]3À, due to the bridg-
ing mode of the chloride ligand between the CeIII/IV sites in the
latter.[6a]
The 4+ oxidation state of cerium in 3-Cl was confirmed by
Ce LIII-edge X-ray absorption spectroscopy. Figure 4 shows the
Figure 3 shows the thermal ellipsoid plot of 3-Cl. Again, the
structural metrics were consistent with a central CeIV cation
Figure 4. Overlay of the Ce LIII-edge XAS spectra of 1 and 3-Cl.
near-edge regions of the XAS spectra of 1 and 3-Cl. The spec-
trum of 3-Cl showed the two features indicative of the core-
hole excitation from a central CeIV cation to final states
1
1
0
1
¯
¯
2p¯4f L5d and 2p¯4f 5d , in which L indicates a ligand hole. The
data were compared to the spectrum of 1, which showed only
one feature indicative of the core-hole excitation from a central
III
1
1 [18]
¯
Ce cation to final state 2p¯4f L5d .
Interestingly, the 1H NMR spectrum of 3-Br in deuterated
pyridine showed two CeIV species in solution in a 4:1 ratio by
integration. To gain insight into the identities of these two spe-
cies, 3-Br was synthesized by an alternate route starting from
the reported CeIV complex, CeBr[N(SiMe3)2]3,[5f] and protonated
1
H3TriNOx. The H NMR spectrum of the isolated dark red brown
powder from this protonolysis reaction was identical to that of
3-Br synthesized by using Ph3CBr, with the same two CeIV spe-
cies in solution again in a 4:1 ratio by integration. These data
indicated that the presence of two CeIV species was character-
istic of the solution chemistry of the 3-Br product and not due
to different reaction pathways occurring during the oxidation
process. Based on these results, the two CeIV species were as-
signed as Ce(TriNOx)Br and [Ce(TriNOx)]Br, with inner- and
outer-sphere bromide ions, respectively. This assignment was
supported by electrochemistry experiments and corroborated
Figure 3. Thermal ellipsoid plot of 3-Cl. Hydrogen atoms are omitted for
clarity. Selected bond lengths [Š]: Ce(1)ÀO(1) 2.163(2), Ce(1)ÀO(2) 2.171(2),
Ce(1)ÀO(3) 2.174(2), Ce(1)ÀN(1) 2.548(3), Ce(1)ÀN(2) 2.527(3), Ce(1)ÀN(3)
2.519(3), Ce(1)ÀCl(1) 2.7436(8).
(Table 2). The CeÀCl bond length of 2.7436(8) Š was significant-
ly longer than that of 2.597(2) Š found in CeCl[N(SiMe3)2]3 indi-
cative of the larger steric demand of the TriNOx3À ligand.[17]
However, the CeÀCl bond length in 3-Cl was significantly
shorter than that of 3.0080(3) Š in the known [{Ce(NN’3)}2(m-
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by more detailed analysis of the 1H NMR spectroscopic data
(see below).
It is noteworthy that the cyclic voltammogram of 3-Br exhib-
ited two separate reduction features consistent with the
1H NMR spectrum of the complex, which showed two CeIV spe-
cies in solution. The first reduction feature occurred with an Epc
of approximately À1.04 V, which was similar to those observed
An analogous route to the formation of 3-F from 1 was not
readily available. However, a titanium fluoride complex sup-
ported by the TriNOx3À ligand framework was recently reported
by us through reaction of [Ti(TriNOx)]Cl with AgF.[19] Reduction
of Ag+ by 1 to Ag0 (E1/2 Ag+/0 = 0.65 V vs. Fc/Fc+ in CH2Cl2)
would also be thermodynamically favorable.[20] Therefore, AgF
was expected to act as both an oxidant and fluoride-transfer
reagent toward 1. Indeed, reaction of 1 with AgF in pyridine
led to the clean formation of Ag0 and 3-F.
The final complex of the series, 3-I, was synthesized from
1 in an analogous manner as 3-F using AgI as the oxidant.
However, our preferred method was the synthesis of 3-I in tol-
uene using 0.5 equivalents of I2 as the oxidant. This led to the
precipitation of 3-I, which was easily isolated as an analytically
pure dark brown powder.
in 1, 3-BArF , and 3-OTf, supporting the assignment of this fea-
4
ture to the reduction of the [Ce(TriNOx)]Br species with an
outer-sphere bromide ion. The second reduction feature oc-
curred with an Epc =À1.16 V, which was assigned as the reduc-
tion of the Ce(TriNOx)Br species, with an inner sphere bromide
ion. Despite the presence of two species in solutions of 3-Br,
only one return metal-based oxidation feature with Epa =
À0.85 V was observed in the cyclic voltammogram. Given the
similarity of this feature to the return oxidation waves for 1, 3-
BArF , and 3-OTf, this suggested that upon reduction to CeIII,
4
the bromide ion dissociated from the [CeIII(TriNOx)Br]À species
on the electrochemical timescale.
The cyclic voltammogram of 3-Cl was similar to that of 3-Br
except that only one reduction feature with Epc =À1.26 V was
observed. This observation was consistent with effectively all
of the chloride ligand being bound to the cerium cation in so-
lution. The shift of 100 mV towards more negative potentials
between the Epc of 3-Cl and that of 3-Br was a result of in-
creased stabilization of the 4+ oxidation state of the central-
metal cation by ClÀ compared to BrÀ (Figure 5). However, the
return oxidation feature occurred with Epa =À0.87 V, which in-
dicated that upon reduction of the metal center, the chloride
ion dissociated from the central CeIII cation similar to the case
of the [CeIII(TriNOx)Br]À species in solutions of 3-Br.
Solution electrochemistry
Solution electrochemistry experiments were performed on the
series of 3-X, in which X=BArF , OTf, F, Cl, Br, and I, in 0.10m
4
[nPr4N][BArF ] dichloromethane solutions. Figure 5 shows the
4
metal-based CeIII/IV redox couples for the series of complexes.
The cyclic voltammograms of 3-BArF and 3-OTf exhibited
4
metal-based features with Epa =À0.86 V and Epc =À1.04 V
versus Fc/Fc+, which were at similar potentials to the CeIII/IV
feature in 1, consistent with their solid-state structures of
having non-coordinating, outer-sphere anions.
Unlike in the cyclic voltammograms of 3-Cl and 3-Br, the
metal-based feature in the cyclic voltammogram of 3-F was
more reversible (Epa =À1.31 V, Epc =À1.40 V vs. Fc/Fc+). The
140 mV shift towards more negative potentials between the
Epc of 3-F and that of 3-Cl indicated that FÀ more effectively
stabilized the 4+ oxidation state of Ce than ClÀ. However, the
large shift in potential of 440 mV towards more negative po-
tentials between the Epa of 3-F and that of 3-Cl, and the rever-
sibility of the wave for 3-F, indicated that the fluoride ion re-
mained bound to the reduced [CeIII(TriNOx)F]À species. Based
on the similarity of the metal-based wave in the CV of 3-I to
that of 1, 3-BArF , and 3-OTf, the solution structure of 3-I was
4
assigned as [Ce(TriNOx)]I with completely outer-sphere iodide.
These results were corroborated through analysis of the
1H NMR spectroscopic data for 3-F, 3-Cl, 3-Br, and 3-I (see
below).
A similar trend in the potential of the CeIII/IV redox couple
was observed in the electrochemistry of the related CeX[N(-
SiMe3)2]3 halide series. There, the addition of a BrÀ ligand to
the central cerium cation shifted the measured reduction po-
tential by 0.66 V to more negative potentials, whereas replac-
ing the BrÀ with a ClÀ had no effect on the position of the of
the measured redox potential. This potential was shifted by
a further 0.25 V through the coordination of an FÀ ligand.
However, the peak separations of the CeBr[N(SiMe3)2]3 and
CeCl[N(SiMe3)2]3 complexes were significantly smaller than the
related 3-Br and 3-Cl complexes due to the dissociation of
halide ligands upon reduction of the latter. The peak separa-
Figure 5. Metal-based CeIV/II redox regions of the cyclic voltammograms for
1 and 3-X collected at a scan rate of 100 mVsÀ1
.
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tion of the 3-F complex, in which fluoride coordination was
conserved during the redox cycling, was significantly smaller
than that of the CeF[N(SiMe3)2]3 complex, suggesting fast elec-
tron-transfer (ET) kinetics in the redox cycling of 3-F.
by integration. The major species had an aromatic resonance
at 7.95 ppm, diastereotopic benzylic proton resonances with
a DdbenzylH of 0.4 ppm, and a tert-butyl resonance at 0.72 ppm.
The minor species exhibited aromatic resonances between 7–
7.5 ppm, diastereotopic benzylic proton resonances with
a DdbenzylH of 1.3 ppm, and a tert-butyl resonance at 0.94 ppm.
These spectral characteristics supported the assignment of the
major species as [Ce(TriNOx)]Br with an outer-sphere bromide
and the minor species as Ce(TriNOx)Br with an inner-sphere
bromide ligand.
1H NMR spectroscopy
The solution structures of 3-F, 3-Cl, 3-Br, and 3-I were studied
in deuterated pyridine by using 1H NMR spectroscopy. As
shown in Figure 6, all the complexes displayed characteristic
DFT calculations
Cerium pyridyl nitroxide systems reported recently by us
showed strong stability of the 4+ oxidation state of Ce due to
symmetry allowed donation from the NÀO p* orbitals into the
Ce 4fzðx2
as a result of the D2d complex symmetry,[10] and Ko-
Þ
Ày2
zimor and co-workers recently describe metal–ligand covalency
for a CeIV chloride complex using chlorine K-edge XAS spec-
troscopy.[21] We hypothesized that symmetry allowed donation
from TriNOx3À into the 4fyð3x2
orbital of Ce as a result of the
Þ
Ày2
C3 complex symmetry was similarly lending structural stability
to these complexes.
The frontier molecular orbitals of [Ce(TriNOx)thf]+ ([1]+), the
cationic portion of 3-BArF , were compared with those of 1.
4
Indeed, the in-phase interaction between the oxygen 2p orbi-
tals of TriNOx3À and the Ce 4fyð3x2
orbital was observed in
Þ
Figure 6. 1H NMR (in [D5]pyr) spectral overlay.
Ày2
the HOMOÀ12 of [1]+. The corresponding out-of-phase inter-
action was observed in the LUMO+6. The main interaction be-
tween the orbitals of TriNOx3À and those of the central cerium
cation in [1]+, however, was observed in the HOMO with
head-on overlap of the NÀO p* orbitals with a linear combina-
1
diamagnetic H NMR spectra with resonances appearing in the
d=0–10 ppm range. The presence of diastereotopic benzylic
resonances between 2.5–5.0 ppm was indicative of coordina-
tion of TriNOx3À to the central Ce cation. However, subtle differ-
tion of the Ce 4fz3 and Ce 4fxðx2
orbitals. The corresponding
Þ
À3y2
1
ences in the H NMR spectra were observed. Both 3-F and 3-Cl
antibonding interaction of these orbitals was observed in the
LUMO+3 (Figure 7). In contrast, the occupied frontier MOs of
1 contained less Ce 4f character, whereas the frontier unoccu-
pied MOs of 1 contained less ligand character.
To corroborate these findings, population analyses on 1 and
[1]+ were performed. Upon oxidation of the central metal
cation from CeIII to CeIV, there was a marginal increase in the
natural charge on cerium, qCe, from 1.78 to 1.90, as was expect-
ed for an increase in formal oxidation state. However, the natu-
ral charge to formal charge ratio decreased from 0.593 in 1 to
0.475 in [1]+. As shown in Table 3, this decrease was a result of
increased donation of the ligand electron density into the un-
filled 4f, 5d, and 6s orbitals on the cerium cation.
have aromatic resonances between d=7.5 and 7.3 ppm, dia-
stereotopic benzylic proton resonances at approximately 4.5
and 3 ppm for both species (DdbenzylH >1 ppm), and tert-butyl
resonances at around 0.90 ppm. Based on solid-state structural
determination of 3-Cl and the solution electrochemical data
across the series, these spectral signatures were attributed to
species with coordinated halides.
In contrast, the aromatic resonances of 3-I were more diffuse
and appeared as far up field as 7.9 ppm. Furthermore, the dia-
stereotopic benzylic proton resonances were closer together at
4.7 and 4.0 ppm (DdbenzylH =0.7 ppm), and the tert-butyl reso-
nance was shifted upfield by 0.2 to 0.7 ppm. These spectral
signatures were assigned to the species with outer-sphere hal-
ides. The NMR spectroscopy characteristics are similar to what
was observed in the case of Ti(TriNOx)F, in which a coordinated
fluoride ligand compared to an outer-sphere chloride ligand
resulted in the appearance of an aromatic resonance downfield
at 7.89 ppm, as well as diastereotopic benzylic resonances with
Population and Mayer bond order (MBO) analyses were also
performed on geometry-optimized structures of 3-F, 3-Cl, 3-Br,
and 3-I to probe the ionicity and strength of the metal–halide
bond. The metrics suggested that the ionicity of the Ce–halide
bond generally decreased upon traversing the series towards
the heavier halides, as was indicated by the smaller positive
natural charge on cerium and the smaller negative natural
charge on the X ligand. However, the metal–iodide bond was
calculated to be more ionic than the metal–bromide bond and
comparable to that of the metal–chloride bond. Similarly, there
was a general decrease in the strength of the CeÀX bond
a
smaller shift: DdbenzylH (DdbenzylH =1.56 for Ti(TriNOx)F;
DdbenzylH =0.48 for [Ti(TriNOx)]Cl).[19]
As mentioned above, 3-Br was unique in that both species
were present in solution, as indicated by the presence of two
sets of diastereotopic benzylic proton resonances in a 4:1 ratio
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These results are in contrast with those obtained from calcu-
lations on the CeX[N(SiMe3)2]3 system. Similar to the 3-X series,
the ionicity of the metal–halide bonds in the CeX[N(SiMe3)2]3
series generally decreased with coordination of the heavier hal-
ides. However, unlike in the 3-X series, the calculated MBO of
the CeÀX bond in the CeX[N(SiMe3)2]3 system trended upward
with coordination of the heavier halides.
Conclusion
The CeIII precursor 1 was successfully synthesized and used as
a starting material for controlled redox chemistry at the central
cerium cation. Detailed solution electrochemistry experiments
showed that the tripodal nitroxide environment of TriNOx3À
provided significant stabilization of the CeIV oxidation state, as
was indicated by the measured CeIII/IV redox potential of
À0.96 V versus Fc/Fc+. The electron-rich and sterically protect-
ed environment imposed by the TriNOx3À framework allowed
the isolation and characterization of the stable cationic CeIV
complexes, 3-BArF and 3-OTf. Due to the strong Lewis acidity
4
of the CeIV cation, these types of complexes are quite rare.
However, the TriNOx3À framework was sufficiently bulky and
electron rich to mitigate the high Lewis acidity of CeIV.
Figure 7. HOMOÀ12 (top left), LUMO+6 (top right), HOMO (bottom left),
and LUMO+3 (bottom right) of [Ce(TriNOx)thf]+ ([1]+), the cationic portion
of 3-BArF .
4
The complete halide series, 3-X, in which X=F, Cl, Br, and I,
was also synthesized. Herein, the bulky TriNOx3À framework
caused diverse solution behavior within the series. When the
size of the XÀ ligand increased, we observed increased concen-
trations of [Ce(TriNOx)]X species, with outer-sphere halides in
solution. We are interested in exploring the use of CeIV sup-
ported by TriNOx3À-type frameworks in Lewis acid catalysis,
which would require binding and activation of substrate
through polarization of the Ce–substrate bond. Further modifi-
cation of the nitrogen R groups is, therefore, warranted to ac-
commodate larger anionic ligands in the cleft and to prevent
the formation of outer-sphere anions.
Table 3. Natural charges (qCe and qX), natural populations (6s, 5d, and 4f),
Mayer bond orders (MBOs), and theoretical formal shortness ratios (FSR)
for 1, [1]+, 3-F, 3-Cl, 3-Br, and 3-I.
Ce
6s
MBO
CeÀX
FSR[a]
qCe
qX
5d
4f
1
1.78
1.90
1.89
1.71
1.66
1.68
À0.60
À0.61
À0.53
À0.48
À0.44
À0.49
0.11
0.13
0.12
0.15
0.17
0.18
0.83
1.01
1.04
1.17
1.20
1.18
0.17
0.87
0.85
0.88
0.88
0.88
0.202
0.255
1.044
0.973
0.940
0.870
[1]+
3-F
3-Cl
3-Br
3-I
0.892
0.946
0.948
0.988
[a] FSR=(calculated bond lengthCeÀX)/(ionic radiusCeIV +ionic radiusXÀ).
Experimental Section
General methods
across the series, as was indicated by the decrease in MBO
from 1.044 for 3-F to 0.870 for 3-I. This is consistent with the
solution chemistry of these species in coordinating solvents,
such as pyridine, in which solvent molecules competed with
the heavier halides for metal ligation.
Unless otherwise noted, all reactions and manipulations were per-
formed under an inert atmosphere (N2) using standard Schlenk
techniques or in a drybox equipped with a molecular sieves 13X/
Q5 Cu-0226S catalyst purifier system. Glassware was oven dried for
at least three hours at 1508C prior to use. 1H and 19F{1H} NMR spec-
tra were obtained on a Bruker DMX-300 Fourier transform NMR
spectrometer at 300 MHz and 282.2 MHz, respectively. 13C(1H} NMR
spectra were obtained on a Bruker DRX-500 Fourier transform NMR
spectrometer or a Bruker AVIII 500 Fourier transform NMR spec-
trometer equipped with a cryogenic probe at 125.7 MHz. Chemical
shifts were recorded in units of parts per million downfield from
Another useful metric for determining the strength of the
metal–halide interaction is the theoretical formal shortness
ratio (FSR) of the metal–halide bond. These were calculated for
the series of Ce–halide complexes using the determined CeÀX
bond length from the gas-phase optimized structures and the
tabulated Shannon radii for Ce4+ and XÀ ions. Values signifi-
cantly less than one indicate stronger interactions between
metal and halide ligand. As shown in Table 3, the theoretical
FSR of the CeÀX bond increased significantly from 0.892 in 3-F
to 0.988 in 3-I. This trend is consistent with experiment, as well
as the findings from the MBO analysis.
1
residual proteo solvent for H NMR, characteristic solvent peaks for
13C NMR, or relative to an external CFCl3 reference (0 ppm). Elemen-
tal analyses were performed either at the University of California,
Berkeley, Microanalytical Facility using a PerkinElmer Series II 2400
CHNS analyzer or at Complete Analysis Laboratories, Inc. using
a Carlo Erba EA 1108 analyzer.
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packed into the slots of a machined aluminum sample holder in an
N2 atmosphere drybox. Aluminized mylar was affixed to the holder
with an indium-wire seal. After packaging, the samples were trans-
ported in dry, nitrogen-filled containers to the beamline. Sample
holders were quickly transferred to the vacuum chamber, exposing
the sealed holders to air for less than thirty seconds before pump-
ing out the chamber and collecting the data under vacuum. Com-
pound 1 is extreme air sensitive and has easily identifiable spectral
changes upon exposure. This sample served as a “canary” sample
and was monitored to check for sample-holder integrity. Following
measurement, no significant changes in the sample were ob-
served.
Materials
Tetrahydrofuran, dimethoxyethane, diethyl ether, dichloromethane,
toluene, hexanes, and pentane were purchased from Fisher Scien-
tific. All solvents were sparged for 20 min with dry N2 and dried
using a commercial two-column solvent-purification system com-
prising columns packed with Q5 reactant and neutral alumina re-
spectively (for hexanes and pentane), or two columns of neutral
alumina (for THF, Et2O, and CH2Cl2). Deuterated solvents were pur-
chased from Cambridge Isotope Laboratories, Inc. and stored over
4 Š molecular sieves prior to use. Cerium chloride (Strem), potassi-
um bis(trimethylsilyl)amide (Sigma), silver triflate, trityl chloride,
and trityl bromide were used as received. Iodine was sublimed
prior to use. Fc[BArF ],[22] Fc[OTf],[23] Ce[N(SiMe3)2]3,[24] and [nPr4N]
[B(3,5-(CF3)2-C6H3)4][25] were synthesized according to literature pro-
4
Computational details
cedures.
Gaussian 09, Revision D.01, was used in electronic structure calcula-
tions.[32] The B3LYP hybrid DFT method was employed, with a 28-
electron small core pseudopotential on cerium with published seg-
mented natural orbital basis set incorporating quasi-relativistic ef-
fects,[33] and the 6–31G* basis set on all other atoms.[34] No re-
straints were imposed other than spin. Frequency calculations
were preformed to confirm the geometry was a minimum (no neg-
ative frequencies). NBO calculations were run using the NBO6
package.[35] Fragment orbital analysis was performed using the
AOMix software.[36] Molecular orbitals were rendered with the
ChemCraft v1.6 program.[37]
Electrochemistry
All experiments were performed under an inert atmosphere (N2) in
a drybox with electrochemical cells that consisted of a 4 mL vial,
glassy carbon disk (3 mm diameter) working electrode, a platinum-
wire counterelectrode, and a silver wire plated with AgCl as
a quasi-reference electrode. The working electrode surfaces were
polished prior to each set of experiments. Potentials recorded in
CH2Cl2 were referenced versus ferrocene, which was added as an
internal standard for calibration at the end of each run. Solutions
employed during CV studies were approximately 3 mm in analyte
Synthetic details and characterization
and 100 mm in [nPr4N][BArF ]. All data were collected in a positive-
4
Ce(TriNOx)THF (1): Ce(TriNOx)THF was synthesized by modifying
a previously published procedure.[9] A THF solution of H3TriNOx
(0.26 g, 0.47 mmol, 1 equiv) was layered on a hexanes solution of
Ce[N(SiMe3)2]3 (0.30 g, 0.47 mmol, 1 equiv). The reaction was al-
lowed to sit, undisturbed, at room temperature for 48 h. The result-
ing red orange crystals of 1 were collected, washed with THF, and
dried under reduced pressure. Yield: 0.30 g (84%). 1H NMR
(500 MHz, [D5]pyr): d=13.04 (d, J=6.5 Hz, 3H) 8.39 (dd, J=6.7,
6.5 Hz, 3H), 7.15 (dd, J=7.2, 6.7 Hz, 3H), 4.58 (d, J=7.2 Hz, 3H),
3.66 (m, 4H), 2.95 (s, 27H), À3.28 (s, 3H), À3.58 ppm (s, 3H); ele-
mental analysis calcd for C37H53N4O4Ce (Mw = 757.97 gmolÀ1): C
58.63, H 7.05, N 7.39; found: C 58.88, H 7.05, N 7.56.
feedback IR compensation mode. The CH2Cl2 solution cell resistan-
ces were measured prior to each run to insure resistances
ꢁꢀ500 W.[25] Scan-rate dependences of 50–1000 mVsÀ1 were per-
formed to determine electrochemical reversibility.
X-ray crystallography
X-ray intensity data were collected on a Bruker APEXII CCD area
detector employing graphite-monochromated MoKa radiation (l=
0.71073 Š) at a temperature of 143(1) K. In all cases, rotation
frames were integrated using SAINT,[26] producing a listing of un-
averaged F2 and s(F2) values, which were then passed to the
SHELXTL[27] program package for further processing and structure
solution. The intensity data were corrected for Lorentz and polari-
zation effects and for absorption using TWINABS[28] or SADABS.[29]
The structures were solved by direct methods (SHELXS-97).[30] Re-
finement was done by full-matrix least squares based on F2 using
SHELXL-97.[30] All reflections were used during refinements. Non-hy-
drogen atoms were refined anisotropically and hydrogen atoms
were refined using a riding model.
[Ce(TriNOx)]2 (2): [Ce(TriNOx)]2 was synthesized by modifying a pre-
viously published procedure.[9] Isolated 1 (0.088 g, 0.12 mmol,
1 equiv) was dissolved in toluene, and solvent was removed under
reduced pressure. Yield: 0.083 g (92%). 1H NMR (500 MHz, C6D6):
d=30.31 (a.s., a.s. = apparent singlet, 2H), 16.76 (a.s., 2H), 13.87
(a.s., 2H), 10.65 (a.t., J=7.2 Hz, 2H), 10.29 (a.s., 2H), 9.22 (d, J=
7.2 Hz, 2H), 6.76 (a.t., J=7.2 Hz, 2H), 4.71 (br s, 18H), 2.06 (s, 2H),
1.38 (d, J=5.5 Hz, 2H), 1.18 (s, 2H), 1.03 (s, 2H), 0.94 (a.t, J=6.8 Hz,
2H), À0.08 (s, 18H), À0.79 (s, 2H), À2.46 (d, J=7.2 Hz, 2H), À3.21
(a.s., 2H), À4.79 (s, 18H), À5.18 (s, 2H), À10.15 (a.s., 2H),
À13.56 ppm (a.s., 2H); elemental analysis calcd for
C66H90N8O6Ce2·Et2O (Mw = 1445.85 gmolÀ1): C 58.15, H 6.97, N 7.75;
found: C 58.13, H 6.89, N 7.77.
X-ray absorption spectroscopy
Ce LIII-edge XANES data were collected at the Stanford Synchrotron
Radiation Lightsource, beamline 11–2, using a Si 220 (f=0) double
monochromator that was detuned to 20% to reduce harmonic
contamination. The resulting data have an energy resolution limit-
ed by the broadening due to the 2p3/2 core-hole lifetime of 3.2 eV.
Data were collected in transmission, using a CeO2 reference to cali-
brate the energy scale, setting the first inflection point of the CeO2
absorption to 5723 eV. A linear pre-edge background was subtract-
ed, and the data were subsequently normalized at 5800 eV.
Ce(TriNOx)F (3-F): To a pyridine solution of 1 (0.10 g, 0.13 mmol,
1 equiv) was added solid AgF (0.018 g, 0.15 mmol, 1.1 equiv), and
the reaction was stirred in the dark for 6 h. The Ag0 by-product
was removed by filtration through a coarse porosity fritted filter
and the pyridine filtrate was collected. Volatiles were removed
under reduced pressure, and the resulting dark red brown powder
was washed with minimal diethyl either and dried. Isolated yield:
0.026 g (28%). 1H NMR (300 MHz, [D5]pyr): d=7.56–7.47 (overlap
6H), 7.43 (ddd, J=7.7, 7.5, 1.8 Hz, 3H), 7.34 (ddd, J=7.6, 7.5,
1.5 Hz, 3H), 4.62 (d, J=11.9 Hz, 3H), 3.06 (d, J=11.9 Hz, 3H),
The samples were prepared for these experiments using proce-
dures outlined previously.[31] In particular, each sample was ground
into a powder, mixed with dry boron nitride as a diluent, and then
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0.091 ppm (s, 27H); 13C NMR (125.7 MHz, [D5]pyr): d=149.6, 133.3,
133.0, 130.1, 129.6, 127.9, 65.2, 62.1, 26.5; 19F NMR (282.2 MHz,
[D5]pyr): d=219.1 ppm; elemental analysis calcd for C33H45N4O3CeF
(Mw = 704.86 gmolÀ1): C 56.23, H 6.44, N 7.95; found: C 55.99, H
6.54, N 8.09.
12.5 Hz, 3H), 1.63 (m, 4H), 0.74 ppm (s, 27H); 13C NMR (125.7 MHz,
1
[D5]pyr): d=163.1 (q, J(11B,13C)=49 Hz), 146.9, 133.9, 132.1, 131.1,
130.8, 130.3 (qq, 2J(19F,13C)=31, 4J(19F,13C)=3 Hz), 129.8, 125.5 (q,
1J(19F,13C)=273 Hz), 118.7 (sept, 3J(19F,13C)=4 Hz), 68.3, 67.1, 60.9,
26.8, 26.3 ppm; 19F NMR (282.2 MHz, [D5]pyr): d=À62.1; elemental
analysis calcd for C69H65N4O4F24BCe (Mw = 1621.19 gmolÀ1): C
51.12, H 4.04, N 3.46; found: C 50.87, H 4.14, N 3.24.
Ce(TriNOx)Cl (3-Cl): To a pyridine solution of 1 (0.10 g, 0.13 mmol,
1 equiv) was added solid Ph3CCl (0.056 g, 0.20 mmol, 1.5 equiv)
and the reaction was stirred for 3 h. Volatiles were removed under
reduced pressure, and the resulting dark red brown solid was
washed with Et2O and dried. X-ray quality crystals were obtained
by vapor diffusion of diethyl ether into a saturated pyridine solu-
[Ce(TriNOx)pyr][OTf] (3-OTf): To a toluene solution of 1 (0.057 g,
0.075 mmol, 1 equiv) was added solid FcOTf (0.025, 0.075 mmol,
1 equiv), and the reaction was stirred for 5 h. The resulting dark
red brown powder was isolated on a medium-porosity fritted filter
and rinsed with Et2O. Crystals of 3-OTf suitable for X-ray diffraction
analysis were grown for vapor diffusion of Et2O into a saturated
1
tion of 3-Cl. Yield: 0.079 g (84%). H NMR (300 MHz, [D5]pyr): d=
7.56–7.42 (overlap, 9H), 7.36 (a.td., a.td. = apparent triplet of dou-
blets, J=7.1, 2.1 Hz, 3H), 4.64 (d, J=12.0 Hz, 3H), 3.14 (d, J=
12.0 Hz, 3H), 0.94 ppm (s, 27H); 13C NMR (125.7 MHz, [D5]pyr): d=
133.1, 133.0, 130.3, 129.9, 128.4, 66.3, 62.0, 26.8; elemental analysis
calcd for C33H45N4O3CeCl (Mw = 721.31 gmolÀ1): C 54.95, H 6.29, N
7.77; found: C 54.57, H 6.57, N 7.76.
1
pyridine solution. Yield: 0.040 g (58%). H NMR (300 MHz, [D5]pyr):
d=7.79 (dd, J=7.4, 1.7 Hz, 3H), 7.57 (ddd, J=8.1, 7.4, 1.7 Hz, 3H),
7.48 (ddd, J=7.4, 7.4, 1.4 Hz, 3H), 7.33 (dd, J=8.1, 1.4 Hz, 3H), 4.73
(d, J=12.5 Hz, 3H), 3.69 (d, J=12.5 Hz, 3H), 0.72 ppm (s, 27H);
13C NMR (125.7 MHz, [D5]pyr): d=146.8, 134.1, 132.4, 130.9, 130.7,
129.8, 67.0, 60.4, 26.9 ppm; 19F NMR (282.2 MHz, [D5]pyr): d=
À77.2 ppm; elemental analysis calcd for C39H50N5O6F3SCe·0.5pyr
(Mw = 953.58 gmolÀ1): C 52.27, H 5.55, N 8.08; found: C 52.22, H
5.62, N 7.96.
Ce(TriNOx)Br (3-Br): Method A: Ce(TriNOx)Br was synthesized in
a similar manner to Ce(TriNOx)Cl, except Ph3CBr was used as the
oxidant. To a pyridine solution of 1 (0.074 g, 0.098 mmol, 1 equiv)
was added solid Ph3CBr (0.047 g, 0.15 mmol, 1.5 equiv), and the re-
action was stirred for 3 h. The reaction mixture was filtered, and
volatiles were removed under reduced pressure. The resulting dark
red brown solid was washed with Et2O and dried. Yield: 0.059 g
Acknowledgements
(79%). Method B: To
a THF solution of H3TriNOx (0.060 g,
E.J.S. acknowledges the U.S. Department of Energy, Office of
Science, Early Career Research Program (Grant DE-SC0006518),
the Research Corporation for Science Advancement (Cottrell
Scholar Award to E.J.S.), and the University of Pennsylvania for
financial support of this work. This work used the Extreme Sci-
ence and Engineering Discovery Environment (XSEDE), which is
supported by the National Science Foundation Grant ACI-
1053575. Portions of this work were supported by the Director,
Office of Science (OS), Office of Basic Energy Sciences, of the
U.S. Department of Energy (DOE) under Contract No. DE-AC-
02-05CH11231 and were carried out at SSRL, a Directorate of
SLAC National Accelerator Laboratory and an OS user facility
operated for the DOE OS by Stanford University.
0.11 mmol, 1 equiv) was then added a THF solution of freshly pre-
pared CeBr[N(SiMe3)2]3 (0.077 g, 0.11 mmol, 1 equiv), and the reac-
tion was allowed to react for 3 h, after which a dark red brown
solid precipitated. This solid was isolated on a medium-porosity
fritted filter, washed with Et2O, and dried under reduced pressure.
1
Yield: 0.030 g (36%). H NMR (300 MHz, [D5]pyr): d (major species,
80%)=7.95 (d, J=7.6 Hz, 3H), 7.59–7.27 (overlap, 9H), 4.69 (d, J=
12.4 Hz, 3H), 4.29 (d, J=12.4 Hz, 3H), 0.72 ppm (s, 27H); (minor
species, 20%) 7.59–7.27 (overlap, 12H), 4.64 (d, J=12.1 Hz, 3H),
3.32 (d, J=12.1 Hz, 3H), 0.94 ppm (s, 27H); 13C NMR (125.7 MHz,
[D5]pyr): d (major species)=146.9, 134.4, 132.9, 130.7, 130.5, 129.6,
66.9, 59.7, 26.9 ppm; (minor species) 148.7, 133.3, 133.1, 130.3,
130.0, 128.5, 66.6, 61.6, 26.9 ppm (overlap); elemental analysis
calcd for C33H45N4O3CeBr (Mw = 765.77 gmolÀ1): C 51.76, H 5.92, N
7.32; found C 51.55, H 5.82, N 7.15.
Ce(TriNOx)I (3-I): To a toluene solution of 1 (0.20 g, 0.26 mmol,
1 equiv) was added solid I2 (0.040 g, 0.13 mmol, 0.6 equiv) causing
the immediate precipitation of a dark red brown powder. The reac-
tion was stirred for 6 h. The dark red brown powder was isolated
on a medium-porosity fritted filter, washed with Et2O, and dried
under reduced pressure. Yield: 0.17 g (79%). 1H NMR (300 MHz,
[D5]pyr): d=7.88 (dd, J=7.5, 1.7 Hz, 3H), 7.56 (ddd, J=8.0, 7.7,
1.7 Hz, 3H), 7.46 (ddd, J=7.7, 7.5, 1.4 Hz, 3H), 7.32 (dd, J=8.0,
1.4 Hz), 4.71 (d, J=12.5 Hz, 3H), 4.05 (d, J=12.5 Hz), 0.72 ppm (s,
27H); 13C NMR (125.7 MHz, [D5]pyr): d=146.9, 134.2, 132.6, 130.9,
130.6, 129.7, 67.0, 60.0, 26.9 ppm; elemental analysis calcd for
C33H45N4O3CeI (Mw = 812.77 gmolÀ1): C 48.77, H 5.58, N 6.89;
found: C 48.63, H 5.37, N 6.79.
Keywords: cerium
electrochemistry · nitroxides · redox chemistry
·
density functional calculations
·
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[Ce(TriNOx)THF][BArF ] (3-BArF ): To a toluene solution of 1 (0.30 g,
4
4
0.39 mmol, 1 equiv) was added Fc[BArF ] (0.41 g, 0.39 mmol,
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moved under reduced pressure, and the resulting dark red brown
powder was rinsed with hexanes and recrystallized from vapor dif-
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1
(61%). H NMR (300 MHz, [D5]pyr): d=8.43 (m, 8H), 7.84 (br s, 4H),
7.69–7.57 (overlap, 6H), 7.49 (a.td., J=7.4, 1.2 Hz, 3H), 7.37 (dd, J=
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Chem. Eur. J. 2015, 21, 17850 – 17859
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Full Paper
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Published online on October 27, 2015
Chem. Eur. J. 2015, 21, 17850 – 17859
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