Isolation of a TMTAA-Based Radical in Uranium bis-TMTAA Complexes

Article abstract of DOI:10.1002/anie.201810971

We report the synthesis, characterization, and electronic structure studies of a series of thorium(IV) and uranium(IV) bis-tetramethyltetraazaannulene complexes. These sandwich complexes show remarkable stability towards air and moisture, even at elevated temperatures. Electrochemical studies show the uranium complex to be stable in three different oxidation states; isolation of the oxidized species reveals a rare case of a non-innocent tetramethyltetraazaannulene (TMTAA) ligand.

Full text of DOI:10.1002/anie.201810971

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Accepted Article
Title: Heavy and Guilty: Isolation of a TMTAA-Based Radical in
Uranium bis-TMTAA Complexes
Authors: John Arnold, Stephan Hohloch, Mary Garner, Corwin Booth,
Wayne Lukens, Colin Gould, Daniel Lussier, and Laurent
Maron
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810971
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Heavy and Guilty: Isolation of a TMTAA-Based Radical in Uranium
bis-TMTAA Complexes
a,b ╪
a╪
c
c
a
Stephan Hohloch, * Mary E. Garner, Corwin H. Booth, Wayne W. Lukens, Colin A. Gould, Daniel
J. Lussier,a,c Laurent Maron, * and John Arnold *
d
a,c
[
8]
[9]
Abstract: We report the synthesis, characterization, and electronic
only a few porphyrin , phthalocyanine complexes and two
corrole complexes,[ have been reported so far. However, the
synthesis of these macrocycles is often tedious involving multi-
step synthetic and purification procedures, which are very time
consuming and low yielding. An alternative macrocyclic ligand,
which can be prepared in an easy three-step synthesis and on
multi-gram scale is the tetramethyltetraazaannulene (TMTAA)
ligand.[11]. This small-cored macrocyclic ligand has only been
10]
structure studies of a series of thorium(IV) and uranium(IV) bis-
tetramethyltetraazaannulene
complexes.
These
sandwich
complexes show remarkable stability towards air and moisture, even
at elevated temperatures. Electrochemical studies show the uranium
complex to be stable in three different oxidation states; isolation of the
oxidized species revealed a rare case of a non-innocent TMTAA
ligand.
[12a]
used twice in actinide chemistry - by us
During our previous study we encountered a side
reaction leading to the formation of the homoleptic sandwich
and the Hayton
group.[
12b,c]
Coordination chemistry involving actinide elements has
[1]
experienced a renaissance in recent years. Despite this surge
in interest, the study of non-aqueous actinide chemistry is still
relatively unexplored when compared to the s-, p-, and d-block
metals, and lanthanides. Extensive oxidation state accessibility
and potential for f-orbital participation make actinides promising
candidates for small molecule activation,[ atom transfer
complexes 1 and 2. Herein we report the targeted synthesis of
2]
reactions,[ hydrocarbon functionalization and magnetism.
3]
[4]
[1g]
While these goals have been realized with organoactinide
complexes containing the ubiquitous cyclopentadienyl “Cp”
supporting ligand,[ the use of other ligands is still evolving.
5]
[2,6]
Macrocyclic ligands are promising alternatives because, like Cp,
they provide the steric protection necessary to support large
actinide ions. In fact, transition metal macrocycle complexes are
well-known for their kinetic and thermodynamic stability due to the
complexes 1 and 2 and describe their electronic and magnetic
properties.
2 2
Scheme 1. Synthesis of complexes Th(TMTAA) (1) and U(TMTAA) (2)
The reaction between K
2
TMTAA (prepared in situ) and 0.5
or UCl led to the formation
“
macrocyclic effect”.[7] Despite being widely explored with
equiv of ThCl (DME) , UI (Dioxane)
4
2
4
2
4
transition metals and lanthanides, surprisingly little is known
about macrocyclic amine ligands in low-valent actinide chemistry;
of the sandwich complexes 1 (yellow) or 2 (dark red) (Scheme 1).
1H-NMR spectroscopy confirmed successful complexation as the
resonance attributable to the backbone proton of the β-
diketiminate subunit of the TMTAA ligands was shifted from 4.92
[
a]
Jun.-Prof. Dr. Stephan Hohloch, Mary E. Garner, Colin A. Gould,
Daniel J. Lussier, Prof. Dr. John Arnold
Department of Chemistry
University of California, Berkeley
Berkeley, CA, 94720
E-Mail: Arnold@berkeley.edu
Jun.-Prof. Dr. Stephan Hohloch
University of Paderborn
2
ppm in the free ligand (H L) to 4.26 ppm in 1 and 9.31 ppm in 2.
While chloroform solutions of 1 and 2 decomposed over the
course of several days, benzene solutions showed remarkable
stability: in fact, storing and heating the complexes in benzene
under air for several days did not lead to any signs of
decomposition (see Figures S7 and S9).
[b]
[c]
[d]
Warbuger Straße 100
The solid-state structures of the two complexes were
determined by X-ray crystallography. Both complexes crystallized
from THF/hexane mixtures (2:1) at -40°C. The complexes were
found to be eight-coordinate with a square prismatic coordination
environment (Fig. 1) wherein the two TMTAA ligands are
staggered (offset by about 90° to one another). Similar
33098 Paderborn, Germany
E-Mail: Stephan.Hohloch@upb.de
Dr. Corwin H. Booth, Dr. Wayne W. Lukens, Daniel J. Lussier, Prof.
Dr. John Arnold
Chemical Science Division
Lawrence Berkeley National Laboratory
Berkeley, CA, 94720, United States
Prof. Dr. Laurent Maron
[13]
[14]
conformations have been seen for group IV and lanthanide
bis-TMTAA complexes.
LPCNO, Université de Toulouse, INSA Toulouse,
135 Avenue de Rangueil, 31077 Toulouse, France
E-Mail: laurent.maron@irsamc.ups-tlse.fr
Both these authors contributed equally to this work.
╪
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dramatically upfield from 9.35 ppm in 2 to -0.72 ppm in 2[OTf]. In
contrast to its U(IV) precursor (2), complex 2[OTf] is not air stable
and rapidly underwent decomposition when exposed to aerobic
conditions. Structural investigations by single crystal X-ray
diffraction revealed a shortening of the U-N bond distances in
2[OTf] versus those of 2 by about 0.05 Å while the intra-ligand
distances remained untouched (see Figure 4). Taken together,
these data strongly indicated an oxidation of the uranium(IV)
centre to uranium(V).
Figure 1.
Molecular structure of 1(left) and 2(right). The “apical ligand” of
the sandwich complexes are faded out for clarity. Hydrogen atoms, as well as
additional solvents, have been omitted for clarity. Ellipsoids are shown at a
probability level of 50 %.
The average An-N distances were found to be 2.491(3) Å in 1
and 2.449(4) Å in 2. As a result of the small core size of the
TMTAA ligand, the metal was found to sit 1.559(1) Å above the
plane of TMTAA nitrogen atoms for 1 and 1.502(1) Å for 2.
Inspecting the space filling models of 1 and 2 (see Figure S21)
identified the cause of the complexes’ pronounced stability
towards air and moisture: in both, the actinide metal sits well-
protected between the two staggered TMTAA ligands.
Figure 3.
SQUID measurements on the complexes 2 (red) and 2[OTf]
(blue) plotted as T vs μeff
.
Variable-temperature dc magnetic susceptibility data were
collected for 2 and 2[OTf] (see Figure 3). Complex 2 displays a
room temperature μeff value of approximately 2.81 μ , similar to
the value of 2.8 μ obtained via the Evans method, which
B
B
compares well to previously reported mononuclear U(IV)
complexes.[ The moment for 2 decreases with temperature,
15]
approaching a value of approximately 0.88 μ
typical for a U(IV) ion possessing a poorly-isolated singlet (M
B
at 2 K, which is
=0)
J
ground state arising from crystal field effects and its non-Kramers
Figure 2.
Scan rate 100 mV/s.
Cyclic voltammogram of 2 in 0.1 M NBu4PF6 vs Fc/Fc+ in MeCN.
nature.[
1a,16]
The magnetometry data for 2[OTf] displays an
expectedly smaller room temperature μeff than 2, with a value of
.54 μ , which is slightly higher than the Evans method
measurement of 2.19 μ , but nearly matching the U(V) free-ion
value of 2.54 μ , supporting the assignment of 2[OTf] being a
Next, we turned our attention to the electrochemical
properties of the complexes. While the thorium complex 1 did not
show any redox processes within the solvent window (1.2 V to -3
V; Figure S22), the uranium complex 2 exhibited two distinct and
reversible oxidations at -0.73 V and 0.21 V vs ferrocene (see
Figure S23). Considering the innocence of the thorium analogue
2
B
B
B
15]
U(V) species.[ No EPR signal could be detected in the solid-
state, even at 2 K, which is surprising since the S symmetry of
the U(V) site allows mixing between m substates with m differing
by ±4. The resulting states, a|7/2+b|-1/2 and c|5/2 +d|-3/2,
where the number in the ket is m , are both EPR active. A weak
EPR signal with g = 3.3 was detected in a frozen 0.01 M solution
of 2[OTf] in CH Cl (Figure S25). The ground state magnetic
4
J
J
(
1), both processes observed for 2 were initially attributed to
metal-centred oxidations corresponding to the uranium(IV/V) and
uranium(V/VI) redox couples; DFT calculations (see supporting
information for more details) also showed the SOMOs of 2 were
located on the metal centre as expected for a uranium(IV) f2
configuration (Figure 5, top).
J
2
2
moment of the species responsible for this signal is at least 1.6 μB.
To better understand the EPR behaviour of 2[OTf] silent, the T
curve obtained from the SQUID measurements was extrapolated
to 0 K (Figure S25) giving a ground state magnetic moment is 1.29
B
μ . This value is much smaller than that predicted by the EPR
spectrum, which indicates that the EPR signal is likely due to a
U(V) impurity rather than to 2[OTf]. The T curve of 2[OTf]
deviates from linearity above 8 K (compare Figure S24), which
indicates that the first excited state is very low in energy, 10-15
cm . The failure to observe an EPR spectrum for 2[OTf] is likely
due to very fast relaxation due to this low-lying state. To better
understand the electronic structure of 2[OTf], computational
studies were performed at the DFT level (see supporting
information). Fixing the uranium to a +V oxidation state, the
Scheme 2. Synthesis of the oxidized uranium bis TMTAA complexes 2[OTf]
and 2[OTf] and their corresponding colours in solution
2
-1
Treatment of 2 with one equivalent of silver(I) triflate afforded
the uranium(V) complex 2[OTf] as a dark brown powder (Scheme
1
2
). The H NMR spectrum of 2[OTf] suggested a successful
oxidation, as all signals from the starting complex 2 were shifted.
The resonance attributable to the TMTAA-CH proton, moved
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calculations reproduced experimental values of the X-ray
observed for 2[OTf]
2
at 2 K. Since these spectroscopic methods
structures well (see SI Table S3). Notably, the SOMO of 2[OTf]
did not unambigously resolve the electronic structure of 2[OTf]
2
,
(
Figure 5, middle) displayed marked ligand character, which
we sought additional insight computationally.
indicates that the unpaired electron is delocalized across the
ligand and the metal centre.
Figure 5.
2
SOMOs of 2 (top), 2[OTf] (middle) and 2[OTf] (bottom).
Performing an energy optimization calculation on 2[OTf]
2
Next, we attempted the double oxidation of 2 using 2.8 equiv
of silver(I) triflate which afforded the teal-coloured complex
2
2[OTf] . The compound was initially suspected to involve
uranium(VI) due to the cyclic voltammetry data. However, NMR
characterization of this new species revealed a paramagnetic
compound with extensive broadening of the resonances. The
resonances were distinctly shifted from those of either 2 or 2[OTf]
(
see Figures S13 + S15). Single crystals suitable for X-ray
diffraction studies were grown from a concentrated DCM solution
at -40 °C. No conformational reorganization took place as a result
of either single or double oxidation of 2; the uranium centre is
always in a square prismatic coordination environment. Analysis
2
of the bond metrics of 2[OTf] also indicated the oxidation
occurred at a TMTAA ligand rather than at uranium centre. While
the U-N distances do not change drastically (as would be
expected for a metal-based oxidation), one of the C-N bond
lengths within the ligand was found to be shortened, to 1.296(9)
Å. The fact that DFT calculations already suggested the non-
innocent character of the TMTAA ligand in 2[OTf], the likelihood
of a second oxidation at one of the TMTAA ligands, rather than
the metal centre, could not be ruled out. Moreover, the uncommon
where the uranium oxidation state was fixed as +VI produced an
optimized structure with U-N bond distances that were drastically
different from those obtained experimentally through X-ray
diffraction studies. In contrast, fixing the oxidation state of the
uranium centre to +V and setting one TMTAA ligand to -1
(corresponding to an oxidized TMTAA mono-anion) reproduced
the experimental bond metrics quite well (see Table S3). Analysis
of the SOMOs of the complex revealed that the unpaired electron
is not located on a single TMTAA, but is instead delocalized over
both of the ligands, which also explains the chemical inertness of
the complex (see Figure 5, top for the SOMOs). Additionally, TD-
2
teal colour of 2[OTf] indicated an unusual electronic structure, as
previously reported homoleptic uranium(VI) amide complexes are
typically dark purple or black.[18] To the best of our knowledge,
there are no reports of structurally characterized stable radical
TMTAA ligands. Instead, when oxidation or reductions are
performed with TMTAA-containing molecules, follow up reactions
at the ligand are typically observed (Scheme S1 and S2).[19]
2
DFT calculation performed on 2[OTf] suggested the band at 580
nm in the UV-Vis spectrum results from both a MLCT and a LLCT.
Taken together, these data further support the second oxidation
occurring at the TMTAA ligand (see SI Figure S26). To obtain
further information on the electronic structure of this unique
TMTAA radical, we performed CASSCF calculations wherein the
two unpaired electrons were distributed across four orbitals (two
uranium f-orbitals and two *-orbitals of the ligand). We found that
the ground state is an open-shell triplet with the associated open-
shell singlet configuration almost degenerate, being only 0.009 eV
higher in energy and the closed-shell singlet, which would
correspond to a potential U(VI) configuration being 2.71 eV above
the the open shell triplet state. We believe that the line broadening
in the NMR results from the population of excited magnetic states
at room temperature. In sum, these calculations, X-ray data, and
Figure 4.
2
Structure of 2[OTf] (left); Table compares the relevant
bond distances of the three complexes. Further structural information can be
found in the SI, Tables S1 and S2.
The UV-Vis spectrum of 2[OTf]
80 nm that points towards ligand oxidation. In 2011, Wieghardt
and Khusniyarov reported an oxidized β-diketiminate nickel
complex of similar colour whose UV-Vis spectrum showed a
similar band at 591 nm. The authors attributed the band to metal
to ligand charge transfer (MLCT).[21] Attempts to record the
2
showed a diagnostic band at
5
2
UV-Vis data strongly indicate the electronic structure of 2[OTf] is
best described as U(V)-TMTAA(1.5-)-TMTAA(1.5-) akin to the
previously described multi-configurational cerocene, which also
possess a singlet ground state.20
2
magnetic susceptibility of 2[OTf] were unsuccessful. The sample
is non-magnetic (neither paramagnetic nor diamagnetic) at low
temperature due to cancellation of the inherent diagmanetism of
the ligand by weak paramagnetism of the U centre. This result is
consistent with a singlet ground state, either U(VI) and two
TMTAA2- ligands or U(V) strongly coupled to a ligand-based
In conclusion, we have presented the first air- and moisture-
stable actinide(IV) bis-TMTAA sandwich complexes. The uranium
complex 2 was found to undergo two reversible oxidations. The
first oxidation is metal-centred, while the second oxidation leads
radical forming an open shell singlet in analogy to COT
Consistent with a singlet ground state, no EPR signal was
2
Ce.[20]
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to a unique example of an oxidized, non-innocent TMTAA ligand.
Garner, T.D. Lohrey, S. Hohloch, J. Arnold, J. Organomet. Chem. 2018,
857, 10–15.
Notably, 2[OTf]
2
represents the first structurally characterized
[
7]
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TMTAA complex, where the TMTAA ligand undergoes oxidation
[
1
9,22
and no further chemical transformations.
We believe that this
exciting and unprecedented chemistry can be applied to
transuranic metals including neptunium and we are interested in
exploring the reactivity of the radical TMTAA complex 2[OTf]
towards small molecules.
2
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Acknowledgements
[
[
2
S.H. acknowledges the German Academic Exchange Service
DAAD) for a postdoctoral scholarship. M.E.G. acknowledges the
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(
NSF-GRFP (CHE-0840505) for a graduate research fellowship.
This work was supported by the Director, Office of Science, Office
of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences Heavy Element Chemistry
Program of the U.S. Department of Energy (DOE) at LBNL under
Contract No. DE-AC02-05CH11231. We also acknowledge Prof.
Dr. Jeffrey Long for the use of his SQUID magnetometer,
supported by NSF grant CHE-1800252.
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Maron, R.A. Andersen, Inorg. Chem., 2018, 57, 7290.
3
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2
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Booth, J.B. Love, J. Arnold, Dalton Trans., 2017, 46, 11615. j) M.E.
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