Synthesis and Characterization of a Tripodal Tris(nitroxide) Aluminum Complex and Its Catalytic Activity toward Carbonyl Hydroboration

Article abstract of DOI:10.1021/acs.organomet.8b00933

An aluminum complex of a tripodal tris(nitroxide)-based ligand has been prepared and characterized. The complex has the ability to participate in metal-ligand cooperative catalysis, which has been exploited for the hydroboration of both aldehydes and keto

Full text of DOI:10.1021/acs.organomet.8b00933

Communication
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Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Characterization of a Tripodal Tris(nitroxide)
Aluminum Complex and Its Catalytic Activity toward Carbonyl
Hydroboration
Audra J. Woodside,† Mackinsey A. Smith,† Thomas M. Herb,‡ Brian C. Manor,§ Patrick J. Carroll,§
Paul R. Rablen,† and Christopher R. Graves*,†
†Department of Chemistry & Biochemistry, Swarthmore College, 500 College Avenue, Swarthmore, Pennsylvania 19081, United
States
‡Department of Chemistry & Biochemistry, Albright College, 13th and Bern Streets, Reading, Pennsylvania 19612, United States
§Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States
S
* Supporting Information
ABSTRACT: An aluminum complex of a tripodal tris-
(nitroxide)-based ligand has been prepared and characterized.
The complex has the ability to participate in metal−ligand
cooperative catalysis, which has been exploited for the
hydroboration of both aldehydes and ketones. This represents
new, transition-metal-like reactivity for an earth-abundant
metal.
he development of catalyst systems implementing readily
available resources is an important effort toward more
amines commensurate with transfer of the third OH proton to
the bridgehead nitrogen of the ligand to give the zwitterion
(HTriNOx)AlMe (1) (Scheme 1). Both the Al−CH3 (δ −0.06
ppm) and N−H (δ 11.14 ppm) functional groups are
T
sustainable synthetic practices.1 Aluminum is highly abundant
in the earth’s crust,2 making it an inexpensive and attractive
choice for catalyst development. However, the lack of readily
available redox chemistry has limited the applicability of
conventional aluminum complexes in catalytic oxidative and
reductive chemistries of organic molecules. In recent years,
aluminum complexes of redox-active and/or noninnocent
ligands have offered a platform through which to expand the
utility of aluminum complexes in catalysis by moving away
from traditional solely Lewis acid activation.3
1
assignable in the H NMR spectrum of 1. The formation of
1 is similar to that of the [O3PH]AlCl ([O3PH]2− = [(2-O-3,5-
tBu2C6H2)3PH]2−) complex prepared by Su and Liang in the
reaction between AlCl3 with [(2-HO-3,5-tBu2C6H2)3P]7 and
represents a rare example of a dianionic scorpionate ligand. On
heating a sample of 1 at 100 °C, the third deprotonation
occurs to give the desired (TriNOx3−)Al complex (2), as
evidenced by 1H NMR spectroscopy. The (TriNOx)Al
complex 2 was also prepared through deprotonation of the
ligand precursor with 3 equiv of NaN(SiMe3)2 followed by salt
metathesis with AlCl3. The complex is most conveniently
isolated as the pyridine adduct (TriNOx3−)Al-py (2-py),
which was prepared in 79% yield via the salt metathesis route.
Our efforts in this area have focused on the preparation of
aluminum coordination complexes implementing ligands
containing nitroxide functional groups.4,5 We have previously
prepared complexes of the pyridyl hydroxyl amines
(RpyNO−)2AlCl (RpyNO− = N-tert-butyl-N-(2-(5-R-pyridyl))-
nitroxyl, R = H, CH3, CF3) and explored their electrochemical
behavior. However, in those systems the aluminum is
coordinatively saturated and relatively unreactive. Herein we
report the preparation of the aluminum complex of the tripodal
tris(nitroxide) ligand [{(2-tBuNO)C6H4CH2}3N]3− (Tri-
NOx3−)6 and deduce its electronic structure. We also
demonstrate the catalytic activity of the (TriNOx3−)Al
complex toward the hydroboration of aldehydes and ketones
and propose a mechanism that operates through metal−ligand
cooperative bifunctional catalysis.
1
The complex 2-py was readily characterized by H and 13C
NMR spectroscopy. The complex exhibits one singlet
resonance in the 1H NMR spectra assignable to the tBu
groups, indicating 3-fold symmetry of the tripodal ligand when
it is bound to the metal ion. The protons of the bridgehead
CH2 groups are diasteriotopic, resulting in two doublets (J ≈
1
11 Hz) in the H NMR, both of which integrate to three
protons. The 13C NMR spectra has six unique aromatic
The reaction between TriNOxH3 and AlMe3 at room
temperature results in deprotonation of two of the hydroxyl
Received: December 21, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.8b00933
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Organometallics
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The geometry of the complex (TriNOx3−)Al-py was
optimized using density functional theory. Overall the
computed structure is in good agreement with the solid-state
structure, with the bond distances centered around the
aluminum ion being within ∼0.05 Å (Table S4). Similarly,
all of the bond angles are also in good agreement, with none of
the angles deviating by more than 5°. The HOMO (−4.522
eV), HOMO-1 (−4.626 eV), and HOMO-2 (−4.766 eV) are
totally ligand based, with electron density primarily localized
on the N−O arms.
Scheme 1. Synthesis of the Complex (TriNOx3−)Al-py (2-
py) through Deprotonation and Salt Metathesis and Its
Reactivity with MeOTf
The complex (TriNOx3−)Al-py has the potential to exist
over multiple redox states owing to the three nitroxide groups
of the ligand. The cyclic voltammogram of 2-py was collected
(Figure S11) and shown to exhibit two reversible features,
which we assign to the sequential one-electron oxidation
processes of the ligand to form the [(TriNOx2−)Al]+ and
[(TriNOx−)Al]2+ species. Optimization of the singly
([(TriNOx)Al]+) and doubly reduced ([(TriNOx)Al]2+)
compounds and visualization of their frontier orbitals support
these assignments (see the Supporting Information): The
SOMO of the [(TriNOx)Al]+ complex is ligand-based and
primarily localized on one N−O group and the attached
aromatic ring. The LUMO of the [(TriNOx)Al]2+ complex is
also ligand-based, with the hole being delocalized over all three
of the ArN−O moieties.
The (TriNOx3−)Al-py complex is able to act as both a Lewis
acid through the aluminum ion and as a Lewis base through
the nitrogen atoms of the nitroxide arms. For example, reaction
of 2-py with excess methyl triflate results in N-methylation of
two of the N−O arms of the ligand along with coordination of
one triflate anion to the aluminum ion. Increasing the amount
of MeOTf does not result in methylation of the third nitroxide
nitrogen. The [(Me2TriNOx)AlOTf]OTf product 3 was
resonances assignable to the TriNOx3− ligand along with
signatures for both the tBu substituents and methylene
carbons. Single crystals of the (TriNOx3−)Al-py complex
were grown from a concentrated pyridine solution at −5 °C,
allowing for X-ray crystallography to corroborate the
formulation of 2-py. The molecule crystallizes in the centric
space group C2/c. The asymmetric unit is disordered by having
both enantiomers superimposed in unequal proportions.
Figure 1 shows a representation of the major disorder
1
isolated in 89% yield (Scheme 1). The H NMR spectra of
3 shows three unique tBu resonances and six unique
resonances assignable to the C−Hs of the bridgehead
methylene units, indicating desymmetrization of the ligand.
There is only one signal in the 19F{1H} NMR spectrum (δ
−79.35 ppm), suggesting rapid exchange of the inner- and
outer-sphere triflate anions in solution. The dual Lewis basic/
Lewis acidic nature of the (TriNOx3−)Al complex opens the
possibility for metal−ligand cooperative bifunctional catalysis.
With this in mind, we investigated the hydroboration of
ketones with pinacolborane (HBPin) catalyzed by the
(TriNOx)Al complex. With 10 mol % loading of 2-py and
1.2 equiv of HBPin, the reduction of acetophenone was
accomplished in 54% yield at room temperature after 20 h, in
comparison to no product without the catalyst. Increasing the
reaction temperature to 75 °C resulted in 70% product under
otherwise identical reaction conditions. The activity of the
catalyst was significantly improved by removal of the pyridine:
use of 10 mol% of the free-base complex 2 and 1.2 equiv of
HBPin gave the borate ester i in quantitative yield after 20 h at
room temperature (Scheme 2). These optimized conditions
were applicable to a broader scope of ketones. Interestingly,
incorporation of either electron-withdrawing or electron-
donating groups significantly reduced the yield of borate
ester product (ii and iii). The reduction of benzophenone was
more difficult, with the reduced product iv being generated in
only 18% yield. Increasing the reaction temperature to 75 °C
did give product iv in 53% yield. The reduction of 4-phenyl-3-
buten-2-one proceeded cleanly to give v in 60% yield. In this
case, heating the sample to 75 °C resulted in a reduced yield of
Figure 1. Solid-state structure of the complex (TriNOx3−)Al-py (2-
py). Ellipsoids are projected at the 50% probability level, and H atoms
are omitted for clarity. tert-Butyl groups are depicted using a
wireframe model.
component, and details of the refinement are given in the
Supporting Information. The aluminum ion is pentacoordinate
with a distorted-trigonal-bipyramidal geometry (τ5 = 0.88).8
The average N−O distance in 2-py is 1.44 Å, which compares
well with analogous parameters for the series of rare-earth
(TriNOx3−)RE complexes prepared by the Schelter group
(N−Oave = 1.44 Å)6,9,10 and suggests a fully reduced TriNOx3−
ligand.
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DOI: 10.1021/acs.organomet.8b00933
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Communication
a
(TriNOx3−)Al system does not have an Al−H, and we propose
an alternative mechanism that involves metal−ligand cooper-
ative bifunctional catalysis (Scheme S1). The aluminum ion in
(TriNOx3−)Al is Lewis acidic, as gauged by its measured AN
value of 75,14−16 which suggests interaction with and activation
of the carbonyl substrate. This is supported by the observation
that pyridine inhibits the reaction, presumably through
competitive binding with the substrate at aluminum.
Simultaneously, the borane can coordinate to one of the
nitroxide nitrogens of the ligand, creating an adduct that
polarizes the B−H bond and increases its hydricity. Similar
activation was reported in the NaOH-catalyzed hydroboration
system developed by Wu et al.17 This is also supported by the
11B NMR spectra of a mixture of HBPin and (TriNOx3−)Al,
which shows a slight upfield shift relative to free HBPin.
Together, these dual activations result in delivery of the
hydride to the carbonyl with installation of the B−O bond to
turn over the catalyst. The reaction between acetophenone and
HBPin catalyzed by AlCl3 or TriNOxH3 gives product in only
12% and 9% yields, respectively, suggesting that both activation
parameters are necessary for productive catalysis.
In conclusion, we have prepared an aluminum complex
implementing a multidentate ligand incorporating multiple
nitroxide functional groups. In addition to allowing for
multiple redox states for the complex, the N−O groups also
add Lewis basic sites that can be coupled with the Lewis acidic
aluminum ion to enable new activation parameters that result
in transition-metal-like catalysis. This chemistry around an
earth-abundant metal makes for an economically attractive lead
for future research. We are currently investigating other
reduction chemistries for which the (TriNOx3−)Al complex is
catalytically viable and also deducing how changes in the ligand
framework are manifested in changes in reactivity of the
complex.
Scheme 2. Scope of Hydroboration of Ketone Substrates
a
Reaction conditions: ketone (0.5 mmol), HBPin (0.6 mmol), 2
(0.05 mmol), CDCl3 (1.0 mL). Yields were determined by H NMR
spectroscopy using hexamethylcyclotrisiloxane (0.083 mmol) as an
internal standard.
1
product, which we hypothesize results from competitive
reactivity with the alkene under the reaction conditions. The
reduction of aliphatic ketones was also successful, with both 2-
octanone and cyclohexanone being reduced to their borate
esters vi and vii, respectively, in excellent yields. Our
hydroboration protocol could also be applied to aldehydes.
As expected, aldehyde substrates are more easily reduced
relative to their ketone counterparts and excellent yields of
borate esters could be obtained using only 5 mol% loading of 2
in only 4 h at room temperature (Scheme 3). Using these
a
Scheme 3. Scope of Hydroboration of Aldehyde Substrates
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00933.
Experimental, crystallographic, and computational de-
tails, NMR spectra, visualization of molecular orbitals,
and proposed catalytic cycle for the hydroboration
reaction (PDF)
a
Reaction conditions: aldehyde (0.5 mmol), HBPin (0.6 mmol), 2
(0.025 mmol), CDCl3 (1.0 mL). Yields were determined by 1H NMR
spectroscopy using hexamethylcyclotrisiloxane (0.083 mmol) as an
internal standard.
Cartesian coordinates for DFT-optimized structures
(MOL)
Accession Codes
CCDC 1862705−1862707 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
conditions, benzaldehyde was reduced to boronate ester viii in
92% yield. In contrast to the ketone system, both electron-
donating (ix) and -withdrawing (x) substituents were tolerated
without a significant decrease in yield. Aliphatic aldehydes
were also readily reduced (xi and xii).
The application of aluminum complexes as catalysts for the
reduction of organic substrates has gained traction in recent
years,11 including systems for the hydroboration of carbon-
yls.12,13 In comparison to those systems, our catalyst is less
active and higher catalyst loadings are required to obtain
similar yields. We attribute this at least partially to a different
mechanism for our catalyst. The previous systems implement
complexes that have an Al−H moiety which is proposed to
insert into the carbonyl in a key mechanistic step. Our
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for C.R.G.: cgraves1@swarthmore.edu.
ORCID
Paul R. Rablen: 0000-0002-1300-1999
Christopher R. Graves: 0000-0001-5853-2446
C
DOI: 10.1021/acs.organomet.8b00933
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Organometallics
Communication
(16) The acceptor number (AN) was calculated by the Gutmann−
Beckett method using the formula AN = 2.21 × (31LA·Et3PO − 41).
(17) Wu, Y.; Shan, C.; Ying, J.; Su, J.; Zhu, J.; Liu, L. L.; Zhao, Y.
Catalytic hydroboration of aldehydes, ketones, alkynes and alkenes
initiated by NaOH. Green Chem. 2017, 19, 4169−4175.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.R.G. thanks Swarthmore College and the National Science
Foundation (CHE-1664902) for financial support of this work.
A.J.W. was supported through a James H. Scheuer Summer
Internship in Environmental Studies. M.A.S. was supported
through a Frances Velay Women’s Science Research Fellow-
ship. We thank the NSF (CHE-1337494) for funding toward
the NMR spectrometer used in this work. We thank Bren E.
Cole (University of Pennsylvania) for assistance in collecting
the X-ray diffraction data.
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DOI: 10.1021/acs.organomet.8b00933
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