Tuning the reactivity of TEMPO by coordination to a lewis acid: Isolation and reactivity of MCl3(η1-TEMPO) (M = Fe, Al)

Article abstract of DOI:10.1021/ja309499h

Addition of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) to MCl₃ (M = Fe, Al) results in the formation of MCl₃(η¹- TEMPO) [M = Fe (1), Al (2)]. Both 1 and 2 oxidize alcohols to generate ketones or aldehydes along with the reduced complexes MCl₃(η¹- TEMPOH) [M = Fe (3), Al (4)]. Complexes 1-4 were fully characterized, including analysis by X-ray crystallography. Additionally, control experiments indicated that neither MCl₃ (M = Al, Fe) nor TEMPO are capable of effecting the oxidation of alcohols independently.

Full text of DOI:10.1021/ja309499h

Communication
pubs.acs.org/JACS
Tuning the Reactivity of TEMPO by Coordination to a Lewis Acid:
1
Isolation and Reactivity of MCl (η ‑TEMPO) (M = Fe, Al)
3
Jeremiah J. Scepaniak, Ashley M. Wright, Richard A. Lewis, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
*
S Supporting Information
2
3
(
TEMPO). In light of the limited information concerning
2
2−25
ABSTRACT: Addition of 2,2,6,6-tetramethylpiperidine-
N-oxyl (TEMPO) to MCl (M = Fe, Al) results in the
Fe−TEMPO coordination chemistry
surrounding the mechanisms of TEMPO-mediated oxidations
more generally, we endeavored to explore the reactivity of
and the uncertainty
3
1
formation of MCl (η -TEMPO) [M = Fe (1), Al (2)].
3
Both 1 and 2 oxidize alcohols to generate ketones or
TEMPO with Fe(III). Herein we report the synthesis and
1
1
aldehydes along with the reduced complexes MCl (η -
3
characterization of the complexes MCl
(η -TEMPO) (M = Fe,
3
TEMPOH) [M = Fe (3), Al (4)]. Complexes 1−4 were
fully characterized, including analysis by X-ray crystallog-
raphy. Additionally, control experiments indicated that
Al) and describe their oxidation of alcohols. These experiments
provide insight into the mechanism of TEMPO-mediated
oxidations and reveal the ability of Lewis acid coordination to
modulate the reactivity of oxyl radicals.
neither MCl (M = Al, Fe) nor TEMPO are capable of
3
effecting the oxidation of alcohols independently.
We began by exploring the possibility of adduct formation
between FeCl , a common Fe(III) synthon, and TEMPO in
3
weakly coordinating solvents. Thus, layering an ethereal
he catalytic aerobic oxidation of primary alcohols using
the Cu/TEMPO (TEMPO = 2,2,6,6-tetramethylpiper-
solution of TEMPO onto an ethereal solution of FeCl
followed by storage at −25 °C resulted in the deposition of
3
T
1
idine-N-oxyl) catalyst system has been the focus of considerable
purple crystalline blocks of FeCl
3
(η -TEMPO) (1) in 73% yield
1
−4
1
interest in recent years.
This protocol exhibits excellent
(eq 1). H NMR spectroscopy of these crystals in C
D
6 6
functional group tolerance and displays high chemoselectivity.
Similar procedures have been developed that utilize other
5
6
7
8
metals, including Ru, Mo, Mn, and Fe, thereby expanding
8
,9
the substrate scope, especially toward 2° alcohols.
Importantly however, there is still considerable debate
concerning the mechanisms of these transformations, despite
significant effort dedicated to understanding this family of
catalysts. For example, in the Cu/TEMPO system, several
research groups favor a mechanism that involves hydride
1
0,11
transfer to nitrogen,
involves H-atom transfer to oxygen.
debate over whether coordination of the alcohol to the metal
while others favor a mechanism that
1
2−15
Moreover, there is
revealed three broad resonances at 45.69, 6.45, and −29.62
ppm, integrating for 4, 12, and 2 H atoms, respectively. These
resonances are assignable to the β, methyl, and γ protons of the
TEMPO moiety. Complex 1 crystallizes in the orthorhombic
space group Pnma. In the solid state, 1 exhibits a
pseudotetrahedral coordination environment about the Fe
1
2,16
occurs during catalysis.
Theoretical studies have also failed
This uncertainty is perhaps surprising,
2
,17,18
to find consensus.
considering the advantages of this protocol and also the
widespread interest in proton-coupled electron transfer
1
9,20
1
(
PCET).
In this regard, a better mechanistic understanding
center, comprising three chloride ligands and an η -TEMPO
of TEMPO coordination chemistry and reactivity could allow
for improved substrate scope and the development of new
transformations.
ligand (Figure 1). The Fe1−O1 and N1−O1 bond lengths are
1.8996(12) and 1.2975(17) Å, respectively (Table 1). For
comparison, the N−O bond length in free TEMPO is 1.296(3)
21
1
Our laboratory’s interest in iron coordination chemistry
Å, while that in SiCl
3
(η -TEMPO), which contains the anionic
−
26
brought to our attention the aforementioned Fe/TEMPO
[TEMPO] ligand, is 1.485(2) Å. Additionally, the average
8
−
Fe−Cl bond length is 2.180 Å, identical to that in [FeCl ] (av
Fe−Cl = 2.18 Å, 232 examples). These data suggest that the
TEMPO ligand in 1 should be considered neutral, while the Fe
oxidation protocol. The proposed catalytic cycle involves
4
27
hydride transfer from an alcohol to an Fe(III)−TEMPO
adduct, generating Fe(II)−(TEMPOH) and an aldehyde.
Similarly, MacMillan and co-workers proposed that an
Fe(III)−TEMPO adduct mediates α-oxyamination of alde-
center is best described as Fe(III). For further comparison, the
1
M−O and N−O bond lengths in [CuCl (η -TEMPO)] are
2
2
16
2
2
1
.940(1) and 1.276(2) Å, respectively, while the M−O and
hydes. However, there are no well-defined Fe(III)−TEMPO
complexes in the literature to help evaluate these hypotheses. In
fact, only one structurally characterized Fe−TEMPO complex
Received: September 25, 2012
Published: November 7, 2012
is known, namely, the Fe(II) species [PhB(MesIm) ]Fe-
3
©
2012 American Chemical Society
19350
dx.doi.org/10.1021/ja309499h | J. Am. Chem. Soc. 2012, 134, 19350−19353

Journal of the American Chemical Society
Communication
antiferromagnetic interactions [see the Supporting Information
(
SI)].
To obtain further support for the proposed electronic
structure of 1, we endeavored to synthesize an analogue that
incorporated a redox-inactive metal. Thus, addition of TEMPO
to a toluene slurry of AlCl resulted in the rapid formation of an
3
amber solution. Upon workup, yellow crystalline blocks of
1
33
AlCl (η -TEMPO) (2) were isolated in 72% yield (eq 1).
3
1
The H NMR spectrum of 2 in C D consists of three broad
6
6
resonances at 20.61, −2.96, and −29.49 ppm, integrating for 2,
1
2, and 4 H atoms, respectively. These are assignable to the γ,
methyl, and β protons of the TEMPO ligand. Also present in
the NMR spectrum are signals due to a small amount of a
Figure 1. Solid-state molecular structures of (left) 1 and (right) 3.
Selected H atoms have been omitted for clarity.
1
diamagnetic product, identifiable as AlCl (η -TEMPOH) (4)
3
(
see below). Complex 2 exhibits an effective magnetic moment
Table 1. Comparison of Metrical Parameters of Complexes
of 1.68(2)μB at 300 K in C D , as determined by Evans’
6
6
1
34
1
1
−4, TEMPO, and SiCl (η -TEMPO)
method. This value is consistent with an S = / ground state
2
35,36
3
and is similar to that expected for an oxyl radical.
Interestingly, complex 2 has been previously generated in situ
av. M−Cl ∑(E−N−
M−O (Å)
N−O (Å)
(Å)
E)
3
3,37
and studied by EPR spectroscopy.
1
1.8996(12)
1.8073(7)
1.8840(8)
1.7745(8)
1.2975(17)
1.3129(9)
1.4006(11)
1.4014(10)
1.296(3)
2.180
2.121
2.189
2.129
356.9
356.2
339.2
338.7
356.3
331.3
Complex 2 is isomorphous with its Fe analogue in the solid
state. It exhibits Al1−O1 and N1−O1 bond lengths of
2
3
4
1
.8073(7) and 1.3129(9) Å, respectively (Table 1). The
shorter M−O bond in 2 in comparison with that in 1 is
a
TEMPO
−
−
2.024
3+
38
consistent with the smaller ionic radius of Al , while the
similarity of its N−O bond length with that in 1 further
confirms the M(III)−(TEMPO·) electronic structures pro-
posed for these complexes.
1
SiCl (η -
1.619(1)
1.485(2)
3
a
TEMPO)
a
Data taken from ref 26.
1
With complexes 1 and 2 in hand, we endeavored to explore
their reactivity with alcohols. Addition of 0.5 equiv of benzyl
alcohol to a deep-purple solution of 1 in C D instantaneously
N−O bond lengths in Cu(hfac) (η -TEMPO) (hfac =
2
−
[
OC(CF )CH(CF )CO] ) are 1.920(5) and 1.269(7) Å,
3
3
2
8
6
6
respectively. These parameters are similar to those exhibited
by 1, and like 1, both Cu complexes feature a neutral TEMPO
ligand.
1
produced a bright-yellow solution. H NMR spectroscopy of
this mixture revealed the consumption of 1 and formation of a
1
new paramagnetic product, FeCl (η -TEMPOH) (3) (Scheme
3
Complex 1 exhibits an effective magnetic moment of 4.72μB
at 300 K, as determined by SQUID magnetometry (Figure 2).
1
), as revealed by diagnostic resonances at 50.86 and 15.26
Scheme 1
Figure 2. Solid-state magnetic susceptibility data for 1 and 3 from 4 to
ppm (Figure S8 in the SI). Filtration of this solution through
basic alumina removed the paramagnetic product, allowing for
the observation of benzaldehyde, as indicated by a sharp
resonance at 9.66 ppm. Similarly, addition of 0.5 equiv of 1-
phenylethanol or cyclohexanol to a C D solution of 1 resulted
in the formation of acetophenone or cyclohexanone,
respectively (see the SI). Complex 1 could also oxidize 1-
octanol, as evidenced by the formation of 3 (Figure S11);
however, this reaction generated a complex mixture of organic
products. The oxidation of 9,10-dihydroanthracene (DHA) was
also possible, resulting in the clean formation of anthracene, but
the reaction was substantially slower than those with alcohols.
300 K.
This value is consistent with an S = 2 ground state and can be
explained by invoking strong antiferromagnetic coupling
6
6
5
between the TEMPO ligand and the S = / spin state of the
2
high-spin Fe(III) center. Similar strong antiferromagnetic
coupling in TEMPO complexes has been observed pre-
1
6,28−31
viously.
The magnetic data were modeled in julX
−1
32
using S = 2, g = 1.95, and D = 7.0 cm . Interestingly, this
1
1
model exhibited somewhat poor agreement with the exper-
imental data below 30 K, likely because of intermolecular
1
9351
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Journal of the American Chemical Society
Communication
3
9
Fortuitously, X-ray-quality crystals of the iron-containing
product 3 precipitated from the DHA reaction mixture upon
standing for 24 h at room temperature. An X-ray diffraction
study revealed that complex 3 is isomorphous with complexes 1
and 2, exhibiting a pseudotetrahedral coordination environment
about the Fe center; however, the metrical parameters of the
TEMPO ligand indicate a change in TEMPO oxidation state. In
particular, the N1−O1 bond length in 3 is 1.4006(11) Å and
the sum of the angles around N1 is 339.2° (Table 1), consistent
ppm is assignable to the NH proton. Overall, these data
confirm the TEMPOH binding mode found in the solid state.
The isolation and characterization of 3 and 4 unambiguously
reveals the destination of the H atom upon abstraction by 1 and
2, respectively. In addition, the observation that complex 2,
which contains redox-inactive Al, is a competent oxidant
demonstrates that a redox change at the metal is not necessary
for substrate oxidation. This latter point is perhaps the most
important, as almost every mechanistic scheme invokes redox
−
26
8,12,13,16
with the presence of [TEMPO] . Additionally, a hydrogen
atom was located in the difference map and successfully refined
on N2. This binding mode of TEMPOH has been observed in
cycling of the metal center.
Our experiments allow us to
make other mechanistic inferences as well. In particular, the
oxidation of DHA by 1 and 2 (eq 2) demonstrates that
coordination of the substrate to the metal center is not required
for H-atom transfer, as DHA is unlikely to coordinate to the
metal. However, the stark difference in the reaction times for
DHA and alcohols suggests that two different mechanisms may
be operative and that in the case of alcohols, a metal−alcohol
adduct may be formed during the transformation. Finally,
2
4,39,40
only a few instances.
Finally, the average Fe−Cl (2.189
Å) and Fe−O [1.8840(8) Å] bond lengths are comparable to
those in 1, suggesting the presence of an Fe3+ center in 3.
Complex 3 was isolated in 81% yield as a pale-pink crystalline
solid by the addition of 0.5 equiv of DHA to 1 in C H (eq 2).
6
6
control experiments indicated that neither MCl (M = Al, Fe)
3
nor TEMPO are capable of effecting the oxidation of benzyl
alcohol independently (see the SI), demonstrating the
synergistic role of MCl and TEMPO in the transformation.
3
The observed selectivity of 1 and 2 is also consistent with
previous systems. Liang and co-workers reported that the Fe/
TEMPO protocol is competent for oxidation of both 1° and 2°
8,9
alcohols, while most Cu/TEMPO protocols are unable to
1
,16,42,43
Complex 3 could also be isolated in 53% yield by addition of
efficiently oxidize 2° alcohols such as cyclohexanol.
The
1
benzyl alcohol to 1 in C H . The H NMR spectrum of 3 in
6
6
difference in selectivity may be related to the binding mode of
C D exhibits two broad resonances at 14.97 and 5.75 ppm,
6
6
TEMPO in these two systems. Previously, Rey and co-workers
assignable to two unique methyl environments. The
inequivalence of the methyl groups is consistent with the
TEMPOH binding mode found in the solid state. A resonance
assignable to the NH proton could not be definitively identified
by NMR spectroscopy; however, a sharp NH stretch was
2
16
reported that CuBr (η -TEMPO), in which the TEMPO
2
ligand is bound through both nitrogen and oxygen, could
oxidize 1-octanol but not 3-octanol at room temperature,
suggesting a steric component to the selectivity.
We suggest that the transformation proceeds via a
mechanism involving concerted proton-coupled electron trans-
−1
observed at 3079 cm in its IR spectrum. Finally, complex 3
exhibits an effective magnetic moment of 5.74μ at 300 K, as
B
fer (CPET) from the C−H bond of the substrate to the
determined by SQUID magnetometry (Figure 2). This value is
19,20
nitrogen atom of 1 or 2.
generating complex 3 or 4,
5
consistent with the S = / ground state expected for a high-
2
4
1
5
respectively, and a ketyl radical. The latter is then quickly
oxidized by a second equivalent of 1 or 2 to form the aldehyde
or ketone. Alternately, H-atom transfer could occur at the
oxygen of TEMPO, with subsequent isomerization to the N-
bound form observed in 3 and 4. Interestingly, Hoffman and
co-workers showed that the spin density on the nitrogen atom
of TEMPO increases upon Lewis acid coordination, suggesting
spin Fe(III) center and can be modeled using S = / , g =
2
1
−1
32
2
.037, and D = 12.0 cm .
1
Interestingly, addition of 0.5 equiv of benzyl alcohol,
cyclohexanol, or 1-phenylethanol to a C D solution containing
6
6
2
resulted in immediate oxidation of the alcohol and formation
1
of a new Al-containing product, AlCl (η -TEMPOH) (4)
3
(
Scheme 1). 1-Octanol was also oxidized under these
3
3,37,44
greater radical character at that site,
an observation which
conditions, but as in the iron reaction, a complex mixture of
products was generated (Figure S18). Conveniently, in the
reaction of DHA with 2, X-ray-quality crystals of 4 precipitated
from the reaction mixture. Complex 4 is isostructural with
complex 3 and exhibits similar metrical parameters for the
TEMPOH ligand. For example, the N1−O1 bond length in 4 is
would appear to support the initial transfer to nitrogen.
On the basis of bond strength considerations, TEMPO
[
TEMPOH bond dissociation free energy (BDFE) = 65.2 kcal
−1
19
mol , C H ] should not be able to effect the oxidation of
DHA (BDFE = 76.0 kcal mol , DMSO)
(
6
6
−1
19,45
or cyclohexanol
−1
46
bond dissociation energy = 92.4 kcal mol ). Thus, it
1
.4014(10) Å and the sum of the angles around N1 is 338.7°
26
appears that the coordination of TEMPO to a Lewis acid
increases the BDFE of TEMPOH by making TEMPO either a
(
Table 1). As observed for 3, a hydrogen atom was located
and successfully refined on N1, while the Al−Cl (av 2.129 Å)
and Al−O [1.7745(8) Å] bond lengths are comparable to those
in 2.
19
better oxidant or a better base. We prefer the former
rationale, as coordination of an alcohol (in this case
•
+
47
On a preparative scale, addition of 1 equiv of DHA to 2 in
[TEMPOH] ) to a metal generally decreases its pK
a
.
It is
C H resulted in the rapid formation of a colorless solution and
also possible that by employing even stronger Lewis acids we
may be able to activate a broader range of C−H bonds. Erker
and co-workers recently reported that frustrated Lewis pair-
6
6
deposition of colorless crystalline blocks of 4 in 79% yield (eq
). Complex 4 could also be isolated in 61% yield by addition
2
1
48,49
of benzyl alcohol to 2 in C H . The H NMR spectrum of 4 in
derived oxyl radicals are potent H-atom abstractors,
6
6
C D revealed two resonances at 1.20 and 0.70 ppm assignable
demonstrating that the reactivity of oxyl radicals can be greatly
6
6
to two different methyl environments, while a singlet at 6.90
modified by changing the substituents attached to the nitrogen.
1
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Journal of the American Chemical Society
Communication
Our work shows that coordination to a metal ion is another
way to modify the reactivity of an oxyl radical.
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In conclusion, addition of TEMPO to MCl (M = Fe, Al)
3
1
9392.
results in the formation of the isolable adducts MCl (η -
3
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6
961.
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9
,10-dihydroanthracene. Importantly, the latter reaction dem-
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1
3
TEMPO) to oxidize alcohols indicates that metal-based redox
changes are not necessary either. These findings may have
mechanistic implications for other metal-catalyzed TEMPO
oxidation protocols. In future work, we plan to interrogate
further the mechanism of H-atom transfer in this system and
will attempt to expand the substrate scope beyond alcohols.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures, crystallographic details (CIF),
spectral data for 1−4, alcohol oxidations, and control reactions.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
*
S
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AUTHOR INFORMATION
Corresponding Author
hayton@chem.ucsb.edu
■
18, 2337.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Science Foundation (CHE 1059097)
and the Alfred P. Sloan Foundation for financial support.
■
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(
(
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dx.doi.org/10.1021/ja309499h | J. Am. Chem. Soc. 2012, 134, 19350−19353