Single crystal to single crystal transformation and hydrogen-atom transfer upon oxidation of a cerium coordination compound

Article abstract of DOI:10.1021/ic4001973

Trivalent and tetravalent cerium compounds of the octamethyltetraazaannulene (H₂omtaa) ligand have been synthesized. Electrochemical analysis shows a strong thermodynamic preference for the formal cerium(IV) oxidation state. Oxidation of the cerium(III) congener Ce(Homtaa)(omtaa) occurs by hydrogen-atom transfer that includes a single crystal to single crystal transformation upon exposure to an ambient atmosphere.

Full text of DOI:10.1021/ic4001973

Communication
pubs.acs.org/IC
Single Crystal to Single Crystal Transformation and Hydrogen-Atom
Transfer upon Oxidation of a Cerium Coordination Compound
Ursula J. Williams, Brian D. Mahoney, Andrew J. Lewis, Patrick T. DeGregorio, Patrick J. Carroll,
and Eric J. Schelter*
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia,
Pennsylvania 19104, United States
S
* Supporting Information
Of the reported compounds, only the cerium(IV) compound
Ce(tmtaa)2 was structurally characterized.
ABSTRACT: Trivalent and tetravalent cerium com-
pounds of the octamethyltetraazaannulene (H2omtaa)
ligand have been synthesized. Electrochemical analysis
shows a strong thermodynamic preference for the formal
cerium(IV) oxidation state. Oxidation of the cerium(III)
congener Ce(Homtaa)(omtaa) occurs by hydrogen-atom
transfer that includes a single crystal to single crystal
transformation upon exposure to an ambient atmosphere.
In order to better understand the electrochemical and
chemical implications of oxidation from cerium(III) to cerium-
(IV) observed for Ce(Htmtaa)(tmtaa), we have synthesized and
characterized new cerium complexes of the 2,3,6,8,11,12,15,17-
octamethyldibenzotetraaza[14]annulene (octamethyltetraa-
zaannulene, H2omtaa) ligand (Scheme 1). Upon exposure to
an ambient atmosphere, Ce(Homtaa)(omtaa) (1) undergoes a
single crystal to single crystal (SCSC) transformation with only
minor perturbations to the primary coordination environment of
the metal ion. Furthermore, the solution-phase redox behavior of
1 shows that it is a potent reductant that undergoes concomitant
loss of a proton and an electron.
Ce[N(Si(CH3)3)2]3 was synthesized according to a published
procedure9 and reacted with 2 equiv of H2omtaa to produce red-
brown 1 in good yields (Scheme 1).5b The solubility of complex
1 enabled the growth of well-defined single crystals for its
structural determination. Whereas the crystallographic structure
of the reported Ce(Htmtaa)(tmtaa) could not be refined due to
disorder,5b compound 1 provided a good X-ray solution and
refinement. The structure of 1 shows the expected 1:2 sandwich
structure with eclipsed saddle-shaped omtaa2− ligands in an
approximate D2d geometry about the cerium ion (Figure
1).5b,6,7b,10 The hydrogen cations required for charge balance
of the cerium(III) complex 1 could not be specifically located in
the structure in 1. However, the Ce−N bond distances in 1,
ranging from 2.5260(16) to 2.5690(16) Å, are consistent with
the assigned cerium(III) oxidation state.6a Unlike other
structurally characterized complexes that include a trivalent
rare-earth ion in a double-decker tetraazaannulene framework,
the bonds between the cerium ion and the nitrogen atoms in each
of the omtaa2− ligands are equivalent by crystallographically
imposed symmetry. While the crystal data show a very symmetric
environment about the cerium(III) center, 11 resonances were
observed in the 1H NMR analysis of 1 in C6D6, corresponding to
a desymmetrized compound. We attribute this solution behavior
to the coordination of two distinctly different ligands to the
cerium(III) ion: one that is best described as Homtaa− and the
other as omtaa2−. These data suggest that the equivalent bond
lengths between the cerium(III) ion and the nitrogen atoms in
each of the omtaa2− ligands in the solid-state structure result
from disorder of the hydrogen atoms in the lattice of 1.
inary cerium oxides show reversible redox behavior in the
Bsolid state that is important in heterogeneous catalytic
processes, including applications in fuel cells and three-way
catalytic converters.1 This reactivity is predicated on the
oxidation of cerium(III) sesquioxide, Ce2O3, to cerium(IV)
oxide, CeO2, in the presence of molecular oxygen.2 In its
molecular chemistry, cerium is typically most stable in its
trivalent state; cerium(IV) ions show a formal oxidation potential
of +1.3 V versus Fc/Fc+ in HClO4.3 However, recent work has
shown that the cerium(III/IV) couple has a highly tunable redox
potential in response to the electronic nature of its environment.4
In nonaqueous systems, cerium ions can demonstrate dramat-
ically different redox behavior, with reported metal-based waves
shifted to reducing potentials.4a−c,f,g As part of an ongoing study
of cerium redox chemistry, we sought to characterize a series of
related cerium compounds in an electron-donating double-
decker framework in order to harness the reducing capability of
cerium(III) ions on a molecular scale.
Rare-earth ions typically form 1:2 sandwich complexes with
anions of the 6,8,15,17-tetramethyldibenzotetraaza[14]annulene
(tetramethyltetraazaannulene, H2tmtaa) ligand, although 2:3
“triple-decker” compounds have also been reported.5 Double-
decker sandwich structures of this type include a central trivalent
rare-earth cation stabilized by 2 equiv of the tetradentate
dianionic ligand as well as 1 equiv of an alkali-metal cation.6 Rare-
earth double-decker compounds have also been reported with
their charges stabilized by one fully deprotonated dianionic
tmtaa2− ligand and one singly protonated monoanionic Htmtaa−
ligand.5b,7 Recently, the cerium(III) compound Ce(Htmtaa)-
(tmtaa) was reported and was shown to undergo facile
conversion to the cerium(IV) compound Ce(tmtaa)2 in the
presence of mild oxidants including O2, benzoquinone, and
[Cp2Fe][PF6].5b The oxidation state of Ce(tmtaa)2 was
confirmed as cerium(IV) using X-ray absorption spectroscopy.8
Received: January 25, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic4001973 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Scheme 1. Syntheses of 1 and 2, Where [ox] = TEMPO or O2
Table 1. Crystal Data Showing Similarity of the Structural
Data in Compounds 1 and 2, Which Facilitates SCSC
Transformation
1
2
space group
a (Å)
C2/c
C2/c
18.524(4)
18.559(3)
16.309(3)
94.435(9)
5590.0(18)
4
18.620(3)
18.510(3)
16.014(2)
98.146(7)
5463.6(14)
4
b (Å)
c (Å)
β (deg)
V (Å3)
Z
Figure 1. Thirty percent probability thermal ellipsoid plot of compound
1. Hydrogen atoms have been removed for clarity. Selected bond
distances (Å): Ce(1)−N(1) 2.5517(17), Ce(1)−N(2) 2.5690(16);
Ce(1)−N(3) 2.5570(17); Ce(1)−N(4) 2.5260(16).
T (K)
143(1)
143(1)
Ce−N (Å)
2.5260(16)−2.5690(16)
2.4367(15)−2.4600(16)
or exposure to UV light.11 However, the redox-driven SCSC
transformation described here is unprecedented in reported
lanthanide coordination chemistry.
Exposure of crystalline, red-brown samples of 1 to an ambient
atmosphere yielded the cerium(IV) complex Ce(omtaa)2 (2)
quantitatively over a period of 30 min, concomitant with a color
change of the crystals to dark green. The 1H NMR spectrum of 2,
recorded in benzene-d6, revealed changes that support oxidation
from a paramagnetic cerium(III) center to a closed-shell formally
The facile oxidation of compound 1 in the solid state
prompted exploration of the solution chemistry of 1 and 2.
Electrochemical analysis of 2 shows a reversible metal-based
wave centered at −1.7 V versus Fc/Fc+ in THF with 0.1 M
[nPr4N][BArF ] (Figure 2). Assignment of this wave as the metal-
1
cerium(IV) complex. The H NMR spectrum of 2 shows four
4
sharp resonances from +7.24 to +1.86 ppm. While the
transformation of 1 to form 2 necessarily proceeds through the
loss of a proton and an electron, attempts to spectroscopically
observe the byproducts of the reaction of 1 with O2 were
unsuccessful. Exposure of single crystals of 1 to an ambient
atmosphere or O2 followed by dissolution in C6D6 showed only
resonances corresponding to 2 by 1H NMR. Similarly, exposure
of crystals of the deuterated analogue of 1, Ce(Domtaa)(omtaa)
(1D), to an ambient atmosphere or O2 followed by dissolution in
benzene showed no resonances by 2H NMR.
To study the nature of the oxidation reaction in the solid state,
single-crystal X-ray diffraction data were collected on a dark-red-
brown sample of 1 that had been covered in Paratone oil.
Following data collection, the crystal was warmed to room
temperature under an ambient atmosphere and was left on the
goniometer head for 24 h to induce complete conversion to 2. A
data set was then collected on the same crystal, which had turned
dark green. The X-ray data confirmed the SCSC transformation
of 1→ 2 upon exposure to the atmosphere.
Figure 2. Isolated Ce(III/IV) couple in the cyclic voltammogram of 2
centered at −1.7 V vs Fc/Fc+ in THF with 0.1 M [nPr4N][BArF ]; [2] =
4
ca. 1 mM; ν = 500 mV/s.
based feature was made by comparison of the cyclic voltammo-
grams of 2 and the H2omtaa ligand (Figure S4 in the SI). On the
basis of this electrochemical analysis, compound 2 represents the
most highly reducing, reversible cerium(III/IV) wave that has
been described. To the best of our knowledge, two cerium-
containing compounds with more reducing potentials than 2
have been reported; both compounds show irreversible electro-
chemical behavior.4f,g
The crystal structure of 2 (Figure S2 in the Supporting
Information, SI) shows Ce−N bond distances that are
contracted in comparison to the structure of 1; the Ce−N
bond distances in 2 range from 2.4367(15) to 2.4600(16) Å and
are consistent with the assigned cerium(IV) oxidation state.5b
A
comparison of the unit cell dimensions of 1 and 2 shows
conservation of molecular packing (Table 1). The system retains
the monoclinic space group C2/c during oxidation and exhibits
only small changes to the unit cell parameters, as shown in Table
1. SCSC transformations have been observed in rare-earth
complexes concomitant with changes in solvation, temperature,
In order to understand the reaction pathway involved in the
oxidation of 1 to 2, chemical oxidation of 1 was explored. In
solution, 1 is oxidized in the presence of the stable organic radical
1
TEMPO to yield TEMPO-H and 2, as detected by H NMR
B
dx.doi.org/10.1021/ic4001973 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
spectroscopy (Scheme 2). The production of TEMPO-H results
from hydrogen-atom transfer from 1 to TEMPO and is
AUTHOR INFORMATION
Corresponding Author
*E-mail: schelter@sas.upenn.edu.
Notes
■
Scheme 2. Reaction of 1 with TEMPO (Top) and Reduction
of 2 in the Presence of Potassium and 18-Crown-6 (Bottom)
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Pennsylvania for financial support
and the NSF for support of the X-ray diffractometer (Grant
CHE-0840438).
REFERENCES
■
(1) Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Imperial
College Press: London, 2002.
(2) Morss, L. R. In Handbook on the Physics and Chemistry of Rare
Earths; Gschneidner, K. A., Jr., Eyring, L., Choppin, G. R., Lander, G. H.,
Eds.; Elsevier: New York, 1994; Vol. 18, p 239.
(3) (a) Smith, G. F.; Getz, C. A. Ind. Eng. Chem., Anal. Ed. 1938, 10,
191. (b) Wadsworth, E.; Duke, F. R.; Goetz, C. A. Anal. Chem. 1957, 29,
1824.
(4) (a) Wester, D. W.; Palenik, G. J.; Palenik, R. C. Inorg. Chem. 1985,
24, 4435. (b) Bian, Y.; Jiang, J.; Tao, Y.; Choi, M. T. M.; Li, R.; Ng, A. C.
H.; Zhu, P.; Pan, N.; Sun, X.; Arnold, D. P.; Zhou, Z.-Y.; Li, H.-W.; Mak,
T. C. W.; Ng, D. K. P. J. Am. Chem. Soc. 2003, 125, 12257.
(c) Streitwieser, A.; Kinsley, S. A.; Jenson, C. H.; Rigsbee, J. T.
Organometallics 2004, 23, 5169. (d) Yu, P.; O’Keefe, T. J. J. Electrochem.
Soc. 2006, 153, C80. (e) Binnemans, K. In Handbook on the Physics and
Chemistry of the Rare Earths; Gschneidner, K. A., Jr., Bunzli, J.-C. G.,
Pecharsky, V. K., Eds.; Elsevier: New York, 2006; Vol. 36, p 281.
(f) Broderick, E. M.; Thuy-Boun, P. S.; Guo, N.; Vogel, C. S.; Sutter, J.
R.; Miller, J. T.; Meyer, K.; Diaconescu, P. L. Inorg. Chem. 2011, 50,
2870. (g) Robinson, J. R.; Carroll, P. J.; Walsh, P. J.; Schelter, E. J. Angew.
Chem., Int. Ed. 2012, 51, 10159.
(5) (a) Magull, J.; Simon, A. New Compounds with Gadolinium and
Samarium. http://www2.fkf.mpg.de/simon/publications/jb91/magull.
html (accessed Jan 15, 2013). (b) Walter, M. D.; Fandos, R.; Andersen,
R. A. New J. Chem. 2006, 30, 1065.
(6) (a) Magull, J.; Simon, A. Z. Anorg. Allg. Chem. 1992, 615, 77.
(b) Williams, U. J.; Mahoney, B. D.; DeGregorio, P. T.; Carroll, P. J.;
Nakamaru-Ogiso, E.; Kikkawa, J. M.; Schelter, E. J. Chem. Commun.
2012, 48, 5593.
promoted by oxidation of the cerium center. Deuterium-labeling
experiments were used to show that compound 1 is the source of
the hydrogen atom donated to TEMPO. Reaction of the
deuterated compound 1D with 1 equiv of TEMPO yielded
compound 2 and TEMPO-D, as evidenced by a peak in the 2H
NMR spectrum at 3.68 ppm, recorded in proteobenzene (Figure
S15 in the SI).
Chemical reduction of the cerium(IV) center in compound 2
was accomplished using potassium metal in the presence of 18-
crown-6, yielding [K(DME)(18-crown-6)][Ce(omtaa)2] (3;
DME = 1,2-dimethoxyethane; Scheme 2). Crystallization of 3
from a mixture of DME and toluene allowed for isolation of pure
compound 3, as confirmed by CHN combustion analysis.
Because of the very air-sensitive nature of the compound as well
as the high surface area of the platelike crystals, fully refined
structural data were not obtained for 3. However, a preliminary
solution collected on 3 did confirm the expected connectivity of
the atoms with two disordered omtaa2− ligands sandwiching the
cerium(III) center and 1 equiv of 18-crown-6 and DME bound to
the potassium ion (Figure S5 and Table S1 in the SI). This
structural motif is common to analogous reported compounds,
which include a trivalent rare-earth ion sandwiched by two
tmtaa2− ligands.6b
(7) (a) Wang, Z.; Sakata, K.; Hashimoto, M. Polyhedron 1998, 17,
4451. (b) Wang, Z.; Hu, N.; Sakata, K.; Hashimoto, M. J. Chem. Soc.,
Dalton Trans. 1999, 1695.
(8) Booth, C. H.; Walter, M. D.; Daniel, M.; Lukens, W. W.; Andersen,
R. A. Phys. Rev. Lett. 2005, 95, 267202.
In summary, the oxidation of compound 1 has been explored
in both the solid-state and solution phases. In the solid state,
oxidation of the cerium(III) center proceeds through a SCSC
transformation upon exposure to an ambient atmosphere. In the
solution phase, we have shown reversible redox behavior of 2 at a
highly reducing potential and have taken advantage of the facile
oxidation of a cerium(III) ion in the omtaa2− ligand framework to
promote hydrogen-atom donation to an organic substrate.
Although the transformation of 1 to 2 is as yet irreversible, these
results illustrate the possibility of manipulating small-molecule
equivalents with redox changes, reminiscent of the behavior of
CeO2, using a molecular material in a conserved framework.
Further work is underway to explore molecular cerium materials
in this context.
(9) Bradley, D. C.; Ghotra, J. S.; Hart, F. A. J. Chem. Soc., Dalton Trans.
1973, 10, 1021.
(10) Magull, J.; Simon, A. Z. Anorg. Allg. Chem. 1992, 615, 81.
(11) (a) Zhang, L.-Z.; Gu, W.; Liu, X.; Dong, Z.; Li, B. CrystEngComm
2008, 10, 652. (b) Mishra, S.; Jeanneau, E.; Daniele, S.; Hubert-
Pfalzgraf, L. G. CrystEngComm 2008, 10, 814. (c) Zhang, L.-Z.; Gu, W.;
Dong, Z.; Liu, X.; Li, B. CrystEngComm 2008, 10, 1318. (d) Song, Y.-M.;
Luo, F.; Luo, M.-B.; Liao, Z.-W.; Sun, G.-M.; Tian, X.-Z.; Zhu, Y.; Yuan,
Z.-J.; Liu, S.-J.; Xu, W.-Y.; Feng, X.-F. Chem. Commun. 2012, 48, 1006.
(e) Liu, J.; Zhang, X.-P.; Wu, T.; Ma, B.-B.; Wang, T.-W.; Li, C.-H.; Li,
Y.-Z.; You, X.-Z. Inorg. Chem. 2012, 51, 8649.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic files (CIFs), full experimental details,
cyclic voltammograms, and NMR data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
C
dx.doi.org/10.1021/ic4001973 | Inorg. Chem. XXXX, XXX, XXX−XXX