Synthesis, electrochemistry, and reactivity of cerium(III/IV) methylene-bis-phenolate complexes

Article abstract of DOI:10.1021/ic400202r

A series of cerium complexes containing a 2,2′-methylenebis(6-tert- butyl-4-methylphenolate) (MBP^2-) ligand framework is described. Electrochemical studies of the compound [Li(THF)₂Ce(MBP) ₂(THF)₂] (1) reveal that the metal based oxidation wave occurs at -0.93 V vs Fc/Fc⁺. This potential demonstrates significant stabilization of the cerium(IV) ion in the MBP^2- framework with a shift of ～2.25 V from the typically reported value for the cerium(III/IV) couple of E°′ = +1.30 V vs Fc/Fc⁺ for Ce(ClO ₄)₃ in HClO₄ solutions. Compound 1 undergoes oxidation to form stable cerium(IV) species in the presence of a variety of common oxidants. The coordination of the redox-active ligands 2,2′-bipyridine and benzophenone to 1 result in complexes in which no apparent metal-to-ligand charge transfer occurs and the cerium ion remains in the +3 oxidation state.

Full text of DOI:10.1021/ic400202r

Article
pubs.acs.org/IC
Synthesis, Electrochemistry, and Reactivity of Cerium(III/IV)
Methylene-Bis-Phenolate Complexes
Brian D. Mahoney, Nicholas A. Piro, Patrick J. Carroll, and Eric J. Schelter*
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104,
United States
S
* Supporting Information
ABSTRACT: A series of cerium complexes containing a 2,2′-
methylenebis(6-tert-butyl-4-methylphenolate) (MBP2−) ligand
framework is described. Electrochemical studies of the
compound [Li(THF)2Ce(MBP)2(THF)2] (1) reveal that the
metal based oxidation wave occurs at −0.93 V vs Fc/Fc+. This
potential demonstrates significant stabilization of the cerium-
(IV) ion in the MBP2− framework with a shift of ∼2.25 V from
the typically reported value for the cerium(III/IV) couple of
E°′ = +1.30 V vs Fc/Fc+ for Ce(ClO4)3 in HClO4 solutions.
Compound 1 undergoes oxidation to form stable cerium(IV)
species in the presence of a variety of common oxidants. The
coordination of the redox-active ligands 2,2′-bipyridine and benzophenone to 1 result in complexes in which no apparent metal-
to-ligand charge transfer occurs and the cerium ion remains in the +3 oxidation state.
the large abundance of cerium in the earth’s crust and its low
cost. Cerium is inexpensive due to its occurrence as a major
byproduct in the separation of light rare earths from bastnaesite
and monazite ores.20 An understanding of cerium(III)
electrochemistry and the chemical oxidation of cerium(III)
complexes is crucial to developing the utility of cerium(III) as a
reductant.10,16−18,21−28
Despite some progress in this area, straightforward one-
electron oxidation reactions to produce cerium(IV) products
remain nontrivial.12 It has been established that electron-
deficient tetravalent cerium ions are most effectively supported
by strongly electron donating ligands.10,18,22,26−28 Our work on
the [M3(THF)x(BINOLate)3CeIII(THF)y] (M  Li, Na, K)
system highlighted the importance of ligand reorganization in
such chemical oxidation reactions of cerium(III). We reasoned
that cerium complexes of methylene-bis-phenolate ligands
(H2MBP, Figure 1) bearing a heterobimetallic Li−Ce−MBP
framework would similarly mitigate the ligand reorganization
component of the oxidation reaction and provide facile access
to cerium(IV) congeners. In addition, the presence of both
−tBu and −Me substituents on the MBP2− framework provide
INTRODUCTION
■
One-electron redox chemistry is an important aspect of the
chemistry of the lanthanides.1−3 Divalent samarium reagents
such as samarium diiodide have found widespread use as
potent, general one-electron reductants, while ceric ammonium
nitrate and related complexes are of interest due to their
application as oxidants.4,5 Whereas the reductive chemistry of
the lanthanides has advanced considerably in recent years,6−9
the oxidative chemistry of cerium has seen relatively less
development.10−13 The unique oxidative utility of cerium
compared with other lanthanides results from the relative
accessibility of its 4f0 tetravalent oxidation state. Due to a large
formal oxidation potential, E°′ = +1.30 V vs Fc/Fc+ for
Ce(ClO4)3 in HClO4 solutions,14 cerium(IV) reagents are
commonly applied in chemical transformations as strong
oxidants. However, the cerium(III/IV) redox couple is highly
sensitive to conditions and ligand environments.
At high pH, aqueous solutions of cerium(III) carbonates are
spontaneously oxidized by O2 to form a cerium(IV)−peroxo
complex.15 Nonaqueous electrochemical studies of cerium
complexes have reported formal reversible cerium(III/IV)
couples shifted as far as −1.40 V vs Fc/Fc+.16−18 However,
the ability to use cerium(III) as a reductant in organic chemical
transformations has yet to be demonstrated. We recently
reported the electrochemistry of [Li3(THF)4(BINOLate)3CeIII-
(THF)], which shows a quasi-reversible oxidation with Epa
=
−0.45 V vs Fc/Fc+ in THF.19 The favorable thermodynamics
for electron transfer from [Li3(THF)4(BINOLate)3CeIII-
(THF)] suggest the possibility of applying cerium(III)
complexes as reducing agents. The replacement of samarium-
(II) reagents with cerium(III) compounds is of interest due to
Figure 1. 2,2′-Methylenebis(6-tert-butyl-4-methylphenol) (H2MBP).
Received: January 26, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis and Reactivity of Ce−MBP Complexes
for an electron-rich aryloxide ligand that is conducive to
supporting the electron-deficient cerium(IV) ion. Finally, the
steric demands of the −tBu group contribute to maintaining
open coordination sites at the lanthanide ion, which has also
been implicated as a requirement for straightforward isolation
of cerium(IV) complexes.19 In this context, we sought to
prepare cerium(III) complexes of the MBP2− ligand and to
explore their electrochemical properties and chemical oxidation.
RESULTS AND DISCUSSION
■
Rare earth and thorium complexes of the 2,2′-methylenebis(6-
tert-butyl-4-methylphenolate) (MBP2−) ligand framework have
been reported,29−34 but the related cerium complexes are
unreported to this point. Reaction of Ce(OTf)3 with 2 equiv of
Li2MBP(THF)3 in THF at room temperature afforded
[Li(THF)2CeIII(MBP)2(THF)2] (1) in good yield (Scheme
1). 1H NMR spectroscopy on the product mixture indicated the
formation of a paramagnetic compound with broadened and
Figure 2. Thermal ellipsoid plot of [Li(THF)2Ce(MBP)2(THF)2] (1)
at the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected bond distances (Å): Ce−O(1) 2.3693(10); Ce−O(2)
2.3146(10).
1
shifted H resonances that ranged from −4.36 to +13.99 ppm
in C5D5N. The four ligand resonances indicated the complex is
C2 symmetric in solution and the peaks were assigned based on
integration. 7Li NMR spectroscopy indicated a single peak that
was broadened and shifted to +4.36 ppm. Compound 1 is air-
sensitive, with exposure of solutions of 1 to atmosphere leading
to an intractable mixture of dark purple products with
complex 1 among the most reducing cerium(III) compounds
reported and indicates strong stabilization of the formal
cerium(IV) ion by the MBP2− ligand framework. It is
noteworthy that complex 1 is a more potent reductant than
the Eu(II) ion, as the standard potential for Eu(II) is E°′ =
−0.75 vs Fc/Fc+.6 This is also noteworthy given that
Eu(C5Me5)2 is known to undergo one-electron transfer to
certain α-diimines upon coordination.37−39 Having established
that the metal-based oxidation of 1 is favored thermodynami-
cally, we set out to explore the chemical oxidation of the
complex.
1
diamagnetic H resonances. Characterization of 1 by single-
crystal X-ray diffraction revealed Ce−OAr bonds that ranged
from 2.3146(10)−2.3693(10) Å (Figure 2). The bond
distances agreed with the reported isostructural complex
[Na(THF)2Nd(MBP)2(THF)2] when accounting for the
0.027 Å difference in size between the metal ions.35 The
Nd−OAr bonds in [Na(THF)2Nd(MBP)2(THF)2] ranged
from 2.278(2)−2.315(2) Å.31,35
Electrochemical characterization of 1 in THF solution
revealed a reversible oxidation wave centered at E1/2 = −0.93
V versus Fc/Fc+ (Figure 3). The electron-rich MBP2− ligand
field decreases the reduction potential of the cerium(III/IV)
couple by ∼2.25 V relative to the standard reported oxidation
potential for cerium(III).36 This shift of potential places
Stirring solutions of 1 in THF or toluene over CuCl2 resulted
in an immediate color change to a dark purple solution that
1
exhibited sharp H NMR resonances ranging from +1.36 to
+7.36 ppm, consistent with the formation of a closed-shell
complex. The oxidant byproduct was removed by filtration, and
the cerium products were crystallized by layering a concen-
trated THF solution with pentane. This resulted in
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Figure 3. Cyclic voltammograms of compound 1 displaying the scan
rate dependence of the cerium(III) oxidation and return reduction in
THF. Potentials are referenced to a Fc/Fc+ internal standard with 100
Figure 4. Electronic absorption spectra showing a ligand-π to Ce-4f
charge transfer absorption for the CeIV complex 2 (purple trace) in the
visible range (Emax = 19 050 cm−1; λmax = 525 nm, ε = ∼4800 M−1
cm−1) in comparison with complex 1 (red trace) and complex 7
(green dashed trace).
mM [nPr4N][BArF ] supporting electrolyte and ∼1 mM analyte
4
concentration.
cocrystallization of Ce(MBP)2(THF)2 (2) and the −ate
complex [Li(THF)3CeCl(MBP)2(THF)] (3) in ∼4:1 ratio.
Diffraction studies on compounds 2−5 revealed that the
products were similarly six-coordinate cerium ions (Figure 5).
The Ce−OAr bond lengths in 2−5 shorten to 2.113(2)−
2.204(3) Å, in accordance with the decrease in the cerium(III/
IV) ionic radius from 1.01 to 0.87 Å.35 For complex 2, the two
THF ligands are retained at the cerium(IV) ion in cis-
coordination sites following oxidation. Complex 3 exhibits a
chloride ion loosely coordinated to the cerium(IV) ion with a
CeIV−Cl distance of 2.7641(8) Å. A four-coordinate lithium
cation is also associated with the cerium-bound chloride ligand
with a Li−Cl −ate complex interaction of 2.337(6) Å. Three
THF molecules complete the coordination sphere of the
tetrahedral lithium cation. Electrochemical studies on mixtures
of 2 and 3 show a reversible reduction occurring at the same
potential as the oxidation of 1 (Figure S1, Supporting
Information).
In order to improve the conversion to an isolable cerium(IV)
product, 2,2′-bipyridine (bpy) was used as a chelating ligand for
the cerium ion to promote elimination of the LiX salt. Reaction
of 1 with 2,2′-bipyridine and subsequent crystallization from
THF/hexane led to isolation of [Li(THF)2Ce(MBP)2(bpy)]
(6) in good yield. Structural determination of 6 revealed a six-
coordinate CeIII ion where the bpy ligand fits into the cleft
formed by the two MBP2− ligands (Figure 6). Two
independently refined complexes are present in the asymmetric
unit of this complex. The Ce−Nbpy bond distances were
unsymmetric between the two Ce−Nbpy sites with Ce−N(1)
2.684(6) or 2.680(6) Å and Ce−N(2) 2.748(6) or 2.747(6) Å.
The Ce−O bond distances ranged from 2.208(5) to 2.415(6)
Å, with the longer Ce−O bond lengths due to coordination of
the Li cation to two of the oxygen atoms. Oxidation of 6 by
CuX2 (X = Cl− or Br−) in THF solutions produced a single
product, which was assigned as Ce(MBP)2(bpy) (7) based
upon 1H and 7Li NMR spectroscopy and subsequently
confirmed by elemental analysis. Complex 7 could also be
obtained by reactions of 2−5 with 2,2′-bipyridine, as verified by
NMR spectroscopy (Scheme 1). The electronic absorption
spectrum of 7 shows the same low-energy charge transfer band
as observed for 2, in addition to increased UV absorbance that
is assigned to transitions within the bipyridine ligand (Figure
4).
1
The product distribution was approximated using H NMR
spectroscopy on the isolated crystals. Analogous reactions with
CuBr2 or I2 as the oxidants afforded mixtures of 2 and
[Li(THF)3CeBr(MBP)2(THF)] (4) or [Li(THF)CeI-
(MBP)2(THF)] (5), respectively. Similar mixtures of products
were obtained by reaction with the copper(I) reagents CuX (X
 Cl, Br, I) in toluene, though these reactions proceed more
slowly. On exposure to atmosphere compounds 2−5
decompose over the course of days due to protonation of the
MBP2− ligand by moisture, as evident by the appearance of
H2MBP in the NMR spectra of the products.
The competition between LiX salt elimination to form
Ce(MBP)2(THF)2 (2) and −ate complex formation to form 3,
4, or 5 was found to be influenced by reaction solvent and
crystallization conditions. However, dissolving the mixtures of
solids in either coordinating or noncoordinating solvents,
followed by filtration and crystallization, did not lead to
straightforward purification of the products. Crystallization at
either room temperature or −30 °C similarly resulted in
cocrystallization of the product mixture in varying ratios. Thus,
in our hands a reliable route to isolate either 2 or 3−5
independent of the other could not be determined. Oxidation
reactions of 1 using N-bromosuccinimide, [Fc][PF6], and
Ph3CCl also produced mixtures and attempts to crystallize
single products from these reactions were similarly unsuccess-
ful.
On an occasion where a small amount of relatively pure
crystalline 2 was isolated, it was characterized by UV−vis
spectroscopy, Figure 4. The spectrum of 2 reveals a broad band
at ∼19 000 cm−1 that corresponds to a ligand-π to vacant Ce-4f
transition, which is the electronic transition underlying the
compound’s dark purple color. A broad, intense transition such
as this is characteristic of Ce(IV) complexes.40,41 The spectrum
of the colorless complex 1 is shown for comparison and
displays only transitions in the UV region associated with
ligand-centered transitions; the Ce(III) f−f transition is
typically very low in energy (<3000 cm−1) and hence not
observed in our experimental spectral range.42
Despite the difficulties in obtaining analytically pure
cerium(IV) complexes by direct oxidation of 1, it was possible
to obtain X-ray structural information on crystals of 2−5.
In addition to providing an isolable cerium(IV) complex of
this system, the isolation of the bpy complexes 6 and 7
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Figure 5. Thermal ellipsoid plots of Ce(MBP)2(THF)2 (2, left) and [Li(THF)3CeCl(MBP)2(THF)] (3, right) at the 50% probability level.
Hydrogen atoms and interstitial solvent molecules have been omitted for clarity. Selected bond distances (Å): (2) Ce−O(1) 2.124(2), Ce−O(2)
2.152(2), Ce−O(3) 2.145(2), Ce−O(4) 2.113(2); (3) Ce−O(1) 2.1376(18), Ce−O(2) 2.1540(18), Ce−O(6) 2.1602(17), Ce−O(7) 2.1265(18),
Ce−Cl 2.7641(8).
the first reduction wave of 2,2′-bipyridine in THF was
measured at E1/2 = −2.60 V vs Fc/Fc+. This potential is 1.67
V more reducing than the observed oxidation of 1 at E1/2
=
−0.93 V versus Fc/Fc+. However coordination of terpyridine to
lanthanide cations is known to shift its reduction potential by
up to +0.94 V,45 while coordination of carbonyls can lead to
shifts of up to +1.3 V.46 These results suggested that the
reduction of 2,2′-bipyridine by 1 could be thermodynamically
feasible upon coordination despite the large difference in formal
potential between the constituent metal oxidation and ligand
reduction potentials. These observations prompted us to
evaluate possible metal−ligand charge separation in complex 6.
The solution electrochemistry of 6 measured in THF shows
the cerium(III) oxidation and 2,2′-bipyridine reduction
processes are unchanged compared with the measurement of
those constituents alone under the same conditions (Figure S3,
Supporting Information).47 This is in agreement with the
observation that the cerium complex and 2,2′-bipyridine are not
1
associated in THF solutions based on H NMR and electronic
Figure 6. Thermal ellipsoid plot for one independent molecule of
[Li(THF)2Ce(MBP)2(bpy)] (6) at the 50% probability level.
Hydrogen atoms, the second independent molecule, and interstitial
solvents have been omitted for clarity. Selected bond distances (Å):
Ce−O(1) 2.390(5), 2.421(5); Ce−O(2) 2.258(6), 2.208(5); Ce−
O(3) 2.415(6), 2.408(6); Ce−O(4) 2.258(5), 2.236(5); Ce−N(1)
2.684(6), 2.680(6); Ce−N(2) 2.748(6), 2.747(6).
absorption spectroscopy. Thus solution studies of 6 dissolved
in THF demonstrated that the bipyridine ligand is displaced by
THF in solution, and we therefore relied on assignment of the
electronic structure based on solid-state characterization.
To determine the electronic structure of 6 in the solid state,
the redox inactive lanthanum analogue, [Li(THF)2La-
(MBP)2(bpy)] (6-La), was synthesized. The electronic
structure of this compound is unambiguous because metal-to-
ligand charge transfer cannot occur from the 4f05d0 lanthanum-
(III) cation. A comparison of the bond metrics based on X-ray
diffraction showed no significant differences between 6 and 6-
La within the 2,2′-bipyridine ligand (Table 1).48,49 Compar-
isons with representative lanthanide−bpy adducts from the
literature demonstrate that the bipyridine bond lengths are in
warranted investigation of their electronic structures. The use
of 2,2′-bipyridine as a ligand allowed the incorporation of a
redox-active group with the potential to act as an electron
acceptor. In reported lanthanide and actinide chemistry,
coordination of an electron-accepting redox-active ligand has
resulted in charge-separated compounds that exhibit unique
electronic structure properties and reactivity.43,44 In our hands,
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Table 1. Comparison of 2,2′-Bipyridine Bond Lengths of [Li(THF)2La(MBP)2(bpy)] (6-La), [Li(THF)2Ce(MBP)2(bpy)] (6),
48,49
La(NO3)3(bpy)2,48,49 and SmIII(Cp*)2(bpy)•−
A
B
C
D
E
F
G
[Li(THF)2La(MBP)2(bpy)]
[Li(THF)2Ce(MBP)2(bpy)]
1.484(10)
1.512(10)
1.315(9)
1.344(8)
1.346(8)
1.372(9)
1.304(11)
1.341(10)
1.343(10)
1.369(11)
1.326(15)
1.363 (16)
1.378(4)
1.388(4)
1.319(9)
1.357(10)
1.374(10)
1.381(10)
1.387(11)
1.348(12)
1.369(12)
1.379(12)
1.390(13)
1.426 (20)
1.409 (20)
1.365 (5)
1.372(5)
1.343(12)
1.355(12)
1.361(13)
1.392(12)
1.334(14)
1.340(15)
1.360(15)
1.412(15)
1.395 (24)
1.411 (21)
1.397(7)
1.335(13)
1.354(12)
1.385(12)
1.416(13)
1.331(15)
1.345(15)
1.388(14)
1.420(15)
1.392(23)
1.394 (22)
1.335(7)
1.369(9)
1.394(9)
1.397(9)
1.399(9)
1.374(12)
1.386(11)
1.394(12)
1.404(10)
1.405(20)
1.412(17)
1.416(4)
1.421(4)
1.325(10)
1.331(10)
1.372(10)
1.324(12)
1.327(12)
1.332(12)
1.379(12)
1.328(16)
1.340 (20)
1.353(4)
1.476(12)
1.520(11)
La(NO3)3(bpy)2
1.450(17)
1.429(4)
SmIII(Cp*)2(bpy)•−
1.359(4)
1.414(7)
1.339(6)
close agreement with compounds bearing a neutral coordinated
2,2′-bipyridine. On the basis of this data, we assigned the
electronic structure of 6 as cerium(III)−(2,2′-bipyridine)0.
To further explore the possibility of synthesizing charge-
separated cerium complexes, the reactivity of 1 with
benzophenone, Ph2CO, was explored. A recent report of
coordination of Ph2CO to Ce[N(TMS)2]3 and Ce(C5H5)3
found that in both cases cerium to ligand charge transfer does
not occur.46 However, the electrochemistry of 1 demonstrates
that the cerium(III/IV) couple in the MBP2− framework occurs
at substantially more reducing potentials than in the silylamide
or cyclopentadienide ligand frameworks.19,50 The reduction
potential of Ph2CO was measured at −2.31 V vs Fc/Fc+ in
THF, demonstrating that it is easier to reduce than 2,2′-
bipyridine. The addition of Ph2CO to 1 in toluene results in a
color change from colorless to red. The product was crystallized
in the presence of 3 equiv of Ph2CO to produce [Li(Ph2CO)-
Ce(MBP)2(Ph2CO)2] (8) in good yield (Figure 7). Infrared
spectroscopy shows carbonyl stretching frequencies of 1629
and 1619 cm−1 for 8 compared with 1654 cm−1 for free
benzophenone. This shift in stretching frequency is consistent
with coordination to a Lewis acid rather than reduction of the
benzophenone with reported examples of coordinated, reduced
benzophenone showing a shift of the carbonyl stretch of ∼100
cm−1.51 Additionally, the CO bond distances in 8 ranged
from 1.230(3) to 1.236(4) Å compared with 1.223(2) Å for
free benzophenone,52 while reduction of Ph2CO coordinated to
a lanthanide typically results in a lengthening of this bond by
over 0.1 Å.53 As such, we assign the electronic structure of the
Figure 7. Thermal ellipsoid plot of [Li(Ph2CO)Ce(MBP)2(Ph2CO)2]
(8) at the 50% probability level. Hydrogen atoms and interstitial
solvents have been omitted for clarity. Selected bond distances (Å):
Ce−O(1) 2.3316(19), Ce−O(2) 2.2714(18), Ce−O(3) 2.2800(18),
Ce−O(4) 2.3371(18), Ce−O(6) 2.530(2), Ce−O(7) 2.513(2),
O(5)−C(47) 1.230(3), O(6)−C(60) 1.236(4), O(7)−C(73)
1.233(3).
0
benzophenone complex 8 as CeIII−(benzophenone)3 . Based
on these results, it is evident that more easily reduced electron-
accepting groups or more strongly electron-donating support-
ing ligands are required to isolate charge-separated cerium
complexes and to develop the potential use of cerium(III) as a
one-electron reductant.
classified thermodynamically as a mild reductant.36 In the
isolation of cerium(IV)−MBP complexes, the products of one-
electron oxidation reactions were dependent upon the
competition between salt elimination and −ate complex
formation. The resulting cerium(IV) compounds were stable
toward O2. We reported rare electrochemical data for the
cerium(III/IV) couple in a nonaqueous system. The coordina-
tion of 2,2′-bipyridine and Ph2CO as redox-active electron
acceptors did not induce electron transfer from the metal
center. Efforts are currently underway to develop more
electron-donating ligand frameworks that provide more
strongly reducing cerium(III) compounds as well as to
CONCLUSIONS
■
The coordination of electron-rich MBP2− donors and the use of
nonaqueous conditions served to reduce the reduction
potential of the cerium(III/IV) couple by ∼2.25 V versus the
standard reported potential for aqueous cerium(III) and
provided an isolable cerium(III) compound that is best
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2
during refinements. The weighting scheme used was w = 1/[σ2(Fo ) +
incorporate substrates with more accessible redox couples into
the Ce−MBP framework.
(aP)2 + bP] where P = (Fo + 2Fc2)/3 and a and b are refined. Non-
2
hydrogen atoms were refined anisotropically, and hydrogen atoms
were refined using a riding model. In the cases where difference
Fourier peaks suggested that disordered solvent was present in the
lattice but for which a reliable model could not be built, the data were
treated using PLATON-SQUEEZE.60
Li2MBP(THF)3. A 20 mL scintillation vial was charged with H2MBP
(1.98 g, 5.82 mmol) dissolved in 15 mL of THF. Solid Li[N(TMS)2]
(2.16 g, 12.90 mmol, 2.2 equiv) was added, causing a color change to
dark green-brown. After 3 h, the solution had turned tan, and the
product precipitated as a white solid. The reaction mixture was dried
under reduced pressure, and the product was washed with pentane and
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise indicated all reactions
and manipulations were performed under an inert atmosphere (N2)
using standard Schlenk techniques or in a Vacuum Atmospheres, Inc.,
Nexus II drybox equipped with a molecular sieves 13X/Q5 Cu-0226S
catalyst purifier system. Glassware was oven-dried overnight at 150 °C
prior to use. 1H NMR spectra were obtained on a Bruker DMX-300 or
on a Bruker AM-500 Fourier transform NMR spectrometer at 300 or
400 MHz, respectively. 7Li{1H}-NMR were recorded on a Bruker AM-
500 Fourier transform NMR spectrometer at 194 MHz. Proton
chemical shifts were recorded in units of parts per million downfield
from TMS by referencing to residual proteo solvent peaks. The
7Li{1H} spectra were referenced to external solution standards of LiCl
in H2O (at zero ppm). Elemental analyses were performed at the
University of California, Berkeley, Microanalytical Facility using a
Perkin-Elmer series II 2400 CHNS analyzer. Solution UV−vis/near-IR
spectra were collected on a PerkinElmer Lambda 950 UV/vis/NIR
spectrometer at concentrations of 0.1 to 5 mM. One millimeter path
length screw cap quartz cells were used with a blank measured before
each run. The infrared spectra were obtained from 400 to 4000 cm−1
using a Perkin-Elmer 1600 series infrared spectrometer.
1
collected (2.84 g, 4.99 mmol, 86%). H NMR (C6D6, 400 MHz, 298
K): δ 7.40 (s, 2H, ArH), 7.15 (s, 2H, ArH), 4.22 (bs, 2H, CH2), 3.31
(t, 12H, C4H8O), 2.31 (s, 6H, CH3), 1.58 (s, 18H, C(CH3)3), 1.27 (t,
12H, C4H8O).
[Li(THF)2Ce(MBP)2(THF)2] (1). A 20 mL scintillation vial was
charged with Li2MBP(THF)3 (1.65 g, 2.91 mmol) dissolved in 15 mL
of THF, and Ce(OTf)3 (0.81 g, 1.38 mmol) was added. After 3 h, all
of the solids had dissolved to produce a pale orange solution. The
reaction mixture was stripped of volatiles under reduced pressure, and
the residue was suspended in 60 mL of toluene. The toluene solution
was filtered through a Celite-packed coarse porosity fritted filter, and
the volatiles were removed to produce a white residue. The white solid
was washed with cold pentane, dissolved in 8 mL of THF, and filtered
through a Celite-packed Pasteur pipet. Crystalline colorless solids
suitable for X-ray crystallography and elemental analysis were grown
overnight by layering the concentrated THF solution with 20 mL of
pentane (999 mg, 0.89 mmol, 65%). 1H NMR (C5D5N, 300 MHz, 298
K): δ 13.99 (bs Δν1/2 = 20 Hz, 4H, ArH2), 9.75 (bs Δν1/2 = 25 Hz, 4H
ArH), 5.03 (s Δν1/2 = 7 Hz, 12H, CH3), 3.63 (m, 16H, C4H8O), 1.60
Electrochemistry. Voltammetry experiments were performed
using a CH Instruments 620D electrochemical analyzer/workstation,
and the data were processed using CHI software v 9.24. All
experiments were performed in a N2 atmosphere drybox using
electrochemical cells that consisted of a 4 mL vial, glassy carbon (3
mm diameter) working electrode, a platinum wire counter electrode,
and a silver wire plated with AgCl as a quasi-reference electrode. The
working electrode surfaces were polished prior to each set of
experiments and were periodically replaced to prevent the buildup
of oxidized or reduced products on the electrode surfaces. Solutions
employed during CV studies were ∼1 mM in analyte and 100 mM in
[nPr4N][B(3,5-(CF3)2-C6H3)4]. Potentials were reported versus Fc/
Fc+, which was added as an internal standard for calibration at the end
of each run. All data were collected in a positive-feedback IR
compensation mode to minimize uncompensated resistance in the
solution cells. The THF solution cell resistances were measured prior
to each run to ensure resistances ∼1500 Ω or less.
7
(m, 16H, C4H8O), −4.36 (bs Δν1/2 = 90 Hz, 18H, C(CH3)3). Li
NMR (C5D5N, 400 MHz, 298 K): δ 4.36. Anal. Calcd for
C62H92CeLiO8: C, 66.94; H, 8.34. Found: C, 66.57; H, 8.17.
General Procedure for the Oxidations of 1 To Form
Compounds 2−5. To a solution of 1 (0.86 g, 0.76 mmol) in 10
mL of toluene was added 1 equiv of CuCl2, CuBr2, or I2, causing an
immediate color change to dark purple. After 1 h, the solution was
filtered through a Celite-packed Pasteur pipet and stripped of volatiles
under reduced pressure. The solids were dissolved in 4 mL of THF,
and the solution was filtered through a Celite-packed Pasteur pipet and
layered with 15 mL of pentane. Crystals were grown over 7 days at
−30 °C and were found to contain mixtures of 2 with 3, 4, or 5 by 1H
NMR spectroscopy.
Materials. Tetrahydrofuran, diethyl ether, dichloromethane,
fluorobenzene, hexanes, and pentane were purchased from Fisher
Scientific. The solvents were sparged for 20 min with dry N2 and dried
using a commercial two-column solvent purification system compris-
ing columns packed with Q5 reactant and neutral alumina (for hexanes
and pentane) or two columns of neutral alumina (for THF, Et2O, and
toluene). Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc., and stored over potassium mirror overnight
prior to use. Cerium(III) triflate (Strem Chemicals Inc.) was heated at
150 °C for 12 h at ∼100 mTorr prior to use. 2,2′-Bipyridine was
purchased from Acros Organics and purified by sublimation prior to
use. 2,2′-Methylenebis(6-tert-butyl-4-methylphenolate) was purchased
from Sigma Aldrich and used without further purification. Li[N-
(TMS)2] was purchased from Acros Organics and recrystallized from
toluene prior to use. The supporting electrolyte, [nPr4N][B(3,5-
[Li(THF)2Ce(MBP)2(bpy)] (6). A 20 mL scintillation vial was
charged with 1 (619 mg, 0.556 mmol) dissolved in 10 mL of toluene.
Solid 2,2′-bipyridine (89 mg, 0.57 mmol) was added, causing an
immediate color change to dark red. After 2 h, the cloudy orange
solution was stripped of volatiles under reduced pressure. The orange
solid residue was dissolved in 3 mL of THF then filtered through a
Celite-packed Pasteur pipet. Dark red crystalline solids suitable for X-
ray crystallography and elemental analysis were grown over 48 h by
layering of the concentrated THF solution with 15 mL of hexane (575
mg, 0.511 mmol, 92%). 1H NMR (C5D5N, 400 MHz, 298 K): δ 14.11
(bs, 4H, ArH2), 9.80 (bs, 4H ArH2), 8.69 (t, 2H, py-H), 7.74 (t, 2H,
bpy-H), 5.08 (s, 12H, CH3), 3.65 (s, 8H, C4H8O), 1.61 (s, 8H,
C4H8O), −4.44 (bs, 36H, C(CH3)3). Li NMR (C5D5N, 400 MHz,
298 K): δ 2.88 (s). Anal. Calcd for C64H84CeLiN2O6: C, 68.36; H,
7.53; N, 2.49. Found: C, 68.16; H, 7.80; N, 2.32.
[Li(THF)2La(MBP)2(THF)2]. The lanthanum analog of 1 was
prepared by analogous reaction of La(OTf)3 with Li2MBP to afford
[Li(THF)2La(MBP)2(THF)2] (287 mg, 0.258 mmol, 51%). 1H NMR
(C5D5N, 400 MHz, 298 K): δ 7.54 (d, J = 1.6 Hz, 4H, ArH2), 7.10 (d,
J = 2.0 Hz, 4H, ArH2), 5.56 (d, J = 13.2 Hz, 2H, CH2), 3.76 (d, J =
13.2 Hz, 2H, CH2), 3.67 (m, 12 H, C4H8O), 2.33 (s, 12H, CH3), 1.63
(CF3)2-C6H3)4] ([nPr4N][BArF ]), was prepared according to
4
7
literature procedures.54
X-ray Crystallography. X-ray reflection intensity data were
collected on a Bruker APEXII CCD area detector employing
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at a
temperature of 143(1) K. In all cases, rotation frames were integrated
using SAINT,55 producing a listing of unaveraged F2 and σ(F2) values,
which were then passed to the SHELXTL56 program package for
further processing and structure solution. The intensity data were
corrected for Lorentz and polarization effects and for absorption using
TWINABS57 or SADABS.58 The structures were solved by direct
methods (SHELXS-97) and refinement was by full-matrix least-
squares based on F2 using SHELXL-97.59 All reflections were used
7
(m, 12 H, C4H8O), 1.53 (s, 36H, C(CH3)3) Li NMR (C5D5N, 400
MHz, 298 K): δ 4.64 (s). Anal. Calcd for C62H92LaLiO8: C, 67.01; H,
8.34. Found: C, 66.91; H, 8.16.
F
dx.doi.org/10.1021/ic400202r | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[Li(THF)2La(MBP)2(bpy)] (6-La). Compound 6-La was prepared
by reaction of [Li(THF)2La(MBP)2(THF)2] with 2,2′-bipyridine
under identical conditions used for the synthesis of 6 (67 mg, 0.060
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ASSOCIATED CONTENT
* Supporting Information
■
S
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crystallographic data, and electrochemical data. This material is
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