Oxidizable Ketones: Persistent Radical Cations from the Single-Electron Oxidation of 2,3-Diaminocyclopropenones.

Article abstract of DOI:10.1002/anie.201902265

Single electron oxidation of 2,3-diaminocyclopropenones is shown to give rise to stable diaminocyclopropenium oxyl (DACO) radical cations. Cyclic voltammetry reveals reversible oxidations in the range of +0.70–1.10 V (vs. SCE). Computational, EPR, and X-r

Full text of DOI:10.1002/anie.201902265

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Accepted Article
Title: Oxidizable Ketones: Persistent Radical Cations from the Single
Electron Oxidation of 2,3-Diaminocyclopropenones.
Authors: Zack M Strater, Michael Rauch, Steffen Jockusch, and Tristan
Hayes Lambert
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902265
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10.1002/anie.201902265
Angewandte Chemie International Edition
COMMUNICATION
Oxidizable Ketones: Persistent Radical Cations from the Single-
Electron Oxidation of 2,3-Diaminocyclopropenones.
Zack M. Strater,[a] Michael Rauch,[a] Steffen Jockusch,[a] and Tristan H. Lambert*[a,b]
Abstract: Single electron oxidation of 2,3-diaminocyclopropenones is
(a) Aminoxyl radical and carbenoxyl radical cations
shown to give rise to stable diaminocyclopropenium oxyl (DACO)
radical cations. Cyclic voltammetry reveals reversible oxidations in the
O
Me
Me
Me
Me
–e–
O
O
+
N
range of +0.70–1.10 V (vs SCE). Computational, EPR, and X-ray
analysis support the view that the oxidized species is best described
as a cyclopropenium ion with spin density located on the heteroatom
substituents, including 23.5% on oxygen. The metal ligand behaviour
of the DACO radical is also described.
R
R
R
R
1
2
3
aminoxyl radical
stable, broadly useful
carbenoxyl radical
(highly reactive)
(b) Diaminocyclopropenone oxidation
Persistent radicals have been used in numerous important
applications across many areas of chemistry. As a prime example,
aminoxyl radicals such as TEMPO (1) have shown widespread
O
O
–e–
R
R
R
R
N
N
R
N
R
N
R
1
3
4
utility as catalysts,[ , 2] radical probes,[ ] spin labels,[ ] biochemical
R
5
6
agents,[ ] and energy storage materials[ ] (Figure 1a). While other
classes of persistent radicals are of great interest, the variety of
functional groups that can undergo single electron oxidation to
form them remains limited. For example, carbonyls (2) generate
high-energy O-centered carbenoxyl radicals (3) upon oxidation,
which tend to be highly unstable and are thus prone to numerous
4
5
diaminocyclopropenone
diaminocyclopropenium
oxyl radical cation
(DACO)
(c) Generation of a DACO radical cation
O
O
–PF6
destructive pathways.
Nevertheless, this type of reactive
NOPF6
character could have significant utility if it could be tamed.
Drawing an analogy with the aminoxyl radicals, carbenoxyl radical
cations would represent a new class of cationic O-centered
persistent radicals that could open new avenues of discovery and
development. Still, the identification of a carbonyl that could be
oxidized to a stable radical cation is not trivial, since even ureas,
with two conjugated amino groups, do not oxidize reversibly.
i-Pr
i-Pr
i-Pr
i-Pr
N
N
N
N
i-Pr
i-Pr
i-Pr
i-Pr
4a
5a
In this regard, we considered the diaminocyclopropenones,
which are highly polarized due to their latent aromatic character
and two conjugated nitrogens (Figure 1b).[ ] This feature, as well
5a (X-ray)
7
as their relation to the readily oxidized trisaminocyclopropenium
ions,[8 ] led us to postulate that 2,3-diaminocyclopropenones 4
might be subject to single-electron oxidation to generate
persistent diaminocyclopropenium oxyl (DACO) radical cations 5.
On the other hand, the formal Lewis structure representation of 5
Figure 1. (a) The aminoxyl radical TEMPO 1 and the formation of carbenoxyl
radical cations. (b) Diaminocyclopropenone oxidation. (c) Formation of the
DACO radical cation 5a. Thermal ellipsoids are shown at 30% probability.
possesses an O-centered radical attached to
a cationic
cyclopropenium ion, and thus it was by no means clear that such
an arrangement would be stable. In this Communication, we show
that, indeed, 2,3-diaminocyclopropenones[ ] do furnish persistent
DACO radical cations, offering a distinct new entry in this
important area.
First, we found that cyclopropenone 4a could be oxidized with
NOPF6 to furnish the DACO 5a, which proved to be a bench-
stable, crystalline material. We were able to determine the
molecular structure of this salt by X-ray diffraction (Figure 1c). The
cationic character of this ion can clearly be seen by the facially-
associated PF6– counterion. Perhaps the most notable feature of
this structure is the nearly exact equivalence of all three C–C
bond lengths (1.407-1.408 Å) and bond angles (59.97-60.01°) of
the three-membered ring, which reflects the aromatic character
of the cyclopropenium ion.
9
[a]
[b]
Z. M. Strater, M. Rauch, Dr. S. Jockusch, Prof. T. H. Lambert
Department of Chemistry
Columbia University, New York, NY 10027, USA
Prof. T. H. Lambert
Department of Chemistry and Chemical Biology
Cornell University, Ithaca, NY 14853, USA
E-mail: Tristan.lambert@cornell.edu
We also examined the oxidation of this and other
cyclopropenones by cyclic voltammetry (Table 1). For 4a, we
observed a reversible oxidation at 0.74 V (vs. SCE), which is
Supporting information for this article is given via a link at the end of
the document.
remarkably low for a carbonyl compound.[10 For comparison,
]
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10.1002/anie.201902265
Angewandte Chemie International Edition
COMMUNICATION
11
tetramethylurea, a lower homologue[ ] of 4a, was found to oxidize
has only 23.5% (Figure 2b).
The nitrogen-bearing
irreversibly only at ~2.4 V. The oxidation potential was measured
to be approximately 200 mV higher with the additional of either
LiPF6 or HFIP, which reflects a substantial coordination /
hydrogen-bonding to the carbonyl group. To better understand
this interaction, we measured the pKBH+ of cyclopropenone 4a and
found it to be 11.38 in CH3CN, which means 4a is only slightly
less basic than pyridine (pKBH+ = 12.53 in CH3CN).[ 12 ] The
calculated BDE (O-H) of the protonated cyclopropenone 4a (see
Supporting Information) is thus 82.9 kcal mol-1, which is
considerably higher than the BDE (O-H) of TEMPO-H (70.6 kcal
cyclopropenium carbons have a negligible level of spin density
(~3%). This analysis indicates that the formal Lewis depiction of
the DACO radical shown in Figure 1b should be considered only
a partial contributor to the actual structure of this species. The
placement of nearly two-thirds of the radical character on the
nitrogens helps to explain the relative stability of these radicals
and rationalizes why we have found that the more sterically
encumbered derivatives have a longer persistence.
The EPR spectrum of 5a in CH2Cl2 displays a 1:2:3:2:1
quintuplet (Figure 2c, black). This splitting pattern arises from
coupling to the two equivalent nitrogens (A value = 8.11 G),
indicating that the unpaired electron is delocalized as suggested
by the calculated SOMO. The EPR spectrum could be effectively
simulated using these parameters (Figure 2c, red).
,
]
mol-1).[13 14
For other derivatives, modification of the steric demand of the
amino substituents was found to have only a minor impact on
oxidizability (4a-d, entries 1-4).
On the other hand, the
morpholino derivative 4e was more difficult to oxidize at +0.98 V
due to the lower donating capacity of the morpholine nitrogen
(entry 5). Similarly, anilino substituents resulted in higher
oxidation potentials (4f-h, entries 6-8), with the N-methylanilino
compound 4f displaying the highest value at 1.10 V (entry 6). We
also measured the CV of a 2-amino-3-aryl cyclopropenone 6
(entry 9), but found that it had a substantially higher oxidation
potential that was not reversible. We expect that the ability to
modify redox behavior through simple alteration of the amine
substituents will prove to be an advantageous aspect of these
materials.
(a)
(b)
O
23.5%
C
0%
N
32.8%
N
32.6%
C
C
3.2%
3.2%
(c)
Table 1. CV E1/2 potentials of aminocyclopropenones.[a]
O
(1)
(4)
(7)
(2)
(5)
(8)
(3)
(6)
(9)
O
O
i-Pr
i-Pr
Cy
Cy
N
N
N
N
N
N
i-Pr
i-Pr
Cy
Cy
4a
4c
4b
+0.74 V
+0.83 V
+0.70 V
+0.93 V (LiPF6)
+0.96 V (HFIP)
O
O
O
Ph
Ph
N
N
N
N
N
N
Me
Me
O
O
4e
4f
4d
Figure 2. (a) Calculated SOMO of radical cation 5a. (b) Calculated spin density
plot of 5a. (c) Ambient temperature X-band EPR spectrum of 5a in CH2Cl2, both
experimental (black) and simulated (red).
+1.10 V
+0.78 V
+0.98 V
O
O
O
Cy
Ph
Ph
N
Ph
We also attempted to oxidize 4a with ceric ammonium nitrate
(CAN), expecting to form a Ce(III) species and radical cation 5a.
N
N
N
N
Cy
Cy
Cy
6
4g
4h
1 16
[ ,
]
Eox = +1.9 V
(irreversible)
+0.91 V
+0.96 V
Instead, we observed the formation of a dark blue solution,
from which we were able to isolated complex 7 following anion
17
]
exchange (Figure 3a).[
Analysis of this complex by X-ray
[a] CV in 0.25M TBAPF6 in CH3CN (vs. SCE).
crystallography revealed a dicationic, octahedral Ce(IV) complex
with four cyclopropenone ligands and two nitrate ligands
counterbalanced by two tetraarylborate anions (Figure 3b).
To address the question of whether these oxidized species
should be properly considered O-centered radicals, we calculated
the SOMO of the radical cation 5a (Figure 2a),[15] which revealed
orbital density on the cyclopropenium ring and the three
heteroatoms. However, in terms of spin density, the greatest
share resides on the two nitrogens (33% each), while the oxygen
The crystal structure of 7 suggests a Ce(IV) complex with
neutral L-type cyclopropenone ligands; however, we expected
18
that a Ce(IV) metal center (e.g. CAN = 1.37 V vs SCE)[ ] should
be capable of oxiding the cyclopropenones. We thus wondered
whether 7 should be considered to have a Ce(IV) center with
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10.1002/anie.201902265
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COMMUNICATION
(a)
N(i-Pr)2
neutral cyclopropenone ligands, a Ce(III) center with an oxidized
ligand shell, or whether some type of dynamic process was
occurring. Because 7 should not be EPR active, we were
surprised to discover that it did produce an EPR signal in solution,
identical to that of the free DACO radical 5a. With the
experiments described below, we found that complex 7 is in
equilibrium with a dissociated DACO radical 5a and, by inference,
the Ce(III) complex 8 (Figure 4a).
NO3
–
B(C6F5)4
(i-Pr)2N
(i-Pr)2N
O
O
2+
N(i-Pr)2
N(i-Pr)2
+
Ce
O
N(i-Pr)2
NO3
NO3
(i-Pr)2N
(i-Pr)2N
N(i-Pr)2
N(i-Pr)2
O
O
O
Ce
(i-Pr)2N
N(i-Pr)2
O
8
NO3
O
(i-Pr)2N
N(i-Pr)2
2 B(C6F5)4
5a
N(i-Pr)2
+
7
–
(i-Pr)2N
B(C6F5)4
–
(a)
(b)
2+
N(i-Pr)2
N(i-Pr)2
NO3
O
(i-Pr)2N
(i-Pr)2N
N(i-Pr)2
N(i-Pr)2
O
O
O
1. Ce(NH4)2(NO3)6
2. LiB(C6F5)4
Ce
i-Pr
i-Pr
O
N
N
NO3
i-Pr
i-Pr
(i-Pr)2N
N(i-Pr)2
2 B(C6F5)4
4a
7
–
(deep blue)
(b)
Ce(IV)
7
Figure 4. (a) Dissociation of complex 7. (b) Temperature-dependent EPR of
complex 7 in CH2Cl2.
Figure 3. (a) Formation of Ce(IV) complex 7. (b) Molecular structure of complex
7. Thermal ellipsoids are shown at 30% probability.
(a)
DACO radical (5a)
To probe this equilibrium, we found that the EPR signal of
7 was quite sensitive to temperature; cooling the sample from 300
K to 270 K resulted in a decrease of over half of the signal intensity
(Figure 4b). The DACO radical 5a was also measured as a
control, but its signal intensity was significantly less affected by
the same drop in temperature. We thus attribute the majority of
the temperature dependence to the dissociation of complex 7 to
form 8 and 5a.
Ce(IV) complex (7)
Our investigation of the effect of concentration by UV-VIS
spectroscopy clearly revealed the equilibrium behavior (Figure
5a). Thus, successive dilution of 7 led to a diminution of the
650 nm band of 7 and an appearance of the DACO radical 5a
peaks at 475 and 515 nm. Furthermore, we prepared the Ce(III)
complex Ce(NO3)3(4a)3 (9, see supporting information for X-ray
structure) as a model of complex 8 and found that addition of 9 to
the DACO radical 5a (red line, Figure 5b) attenuated these same
peak intensities and led to the appearance of the broad peak at
~650 nm (yellow line). With 4 equiv of 9 added, the 5a peaks
were almost completely gone (blue line), and the sample turned
the characteristic blue of 7.
(b)
O
–PF6
(i-Pr)2N
N(i-Pr)2
5a
5a + 4 eq 9
5a + 2 eq 9
Figure 5. (a) UV-Vis of 7 under successive dilutions (dark blue to light blue)
showing the emergence of DACO 4a. (b) UV-Vis spectrum of 5a (red), 5a plus
2 equiv Ce(NO3)3(4a)3 9 (yellow), and 5a plus 4 equiv 9 (blue). These
measurements were taken in CH2Cl2 solvent.
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COMMUNICATION
These experiments strongly support the proposal that the
Ce(IV) complex 7 is in equilibrium with a Ce(III) species and the
free DACO radical 4a. Metal-oxygen homolytic ligand
possibilities for the synthesis of metal complexes with interesting
dynamic behavior. Because the 2,3-diaminocyclopropenones can
be readily synthesized on scale and are easily diversified, these
compounds may offer new possibilities as redox mediators, redox
19
dissociation has also been seen in aminoxyl complexes;[
]
20
however, we believe this is the first example of the reversible
homolytic dissociation of a neutral oxygen ligand to a free radical
cation. This unusual behavior can be attributed to the unique
structure of the cyclopropenone, which is a proficient L-type donor
but with a stable one-electron oxidation state that can be
accessed by strongly oxidizing metals such as Ce(IV).
active ligands, and catalysts.
Acknowledgements
Financial support for this work was provided by NIGMS (R01
GM102611). M. R. acknowledges the National Science Foundation for
a Graduate Research Fellowship under Grant No. DGE-16-44869.
In conclusion, we have shown that 2,3-dialkylamino-
cyclopropenones undergo single electron oxidation to produce
persistent radical cations. These “DACO” radical cations have
appreciable spin-density on the oxygen atom and represent a
structurally distinct addition to the category of persistent radicals.
The ligand behavior of the DACO radicals appears to offer unique
Keywords: radical cation • O-centered radical • cyclopropenium
• oxidation • ligand dissociation
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10.1002/anie.201902265
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COMMUNICATION
COMMUNICATION
Stable radical cations result from
the single-electron oxidation of
2,3-diaminocyclopropenones.
These “DACO” radicals have
approximately 25% radical
Zack M. Strater, Michael
Rauch, Steffen Jockusch,
Tristan H. Lambert
O
O
–e–
+e–
R
R
R
R
N
R
N
R
N
R
N
R
Page No. – Page No.
DACO
radical cation
character on the oxygen atom,
with the remainder primarily
residing on the nitrogens.
Title
Characterization of these species
by X-ray crystallography, EPR
spectroscopy and computational
analysis is described. In addition,
the equilibrium behaviour of a
DACO-metal complex is reported.
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