Redox switchable catalysis utilizing a fluorescent dye

Article abstract of DOI:10.1039/c9cc05836b

This report describes the implementation of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye into the ligand framework of a Rh-based catalyst. The redox-active nature of the BODIPY dye is utilized to generate a catalyst that is capable of exhibit

Full text of DOI:10.1039/c9cc05836b

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DOI: 10.1039/C9CC05836B
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Redox Switchable Catalysis Utilizing a Fluorescent Dye
Brena L. Thompson,a Casey R. Simons, b and Zachariah M. Heiden*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
This report describes the implementation of a 4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene (BODIPY) dye into the ligand framework of
a Rh-based catalyst. The redox-active nature of the BODIPY dye is
utilized to generate a catalyst that is capable of exhibiting redox-
switchable catalytic behavior for the hydroboration of alkenes
through a BODIPY-based reduction.
With the advent of more complex chemical products, the
development of switchable catalysts capable of turning a
chemical reaction “on” or “off” with controlled timing is critical
to maximizing the yield of the desired product and minimizing
undesired chemical reactions.1 One of the mechanisms for
switchable reactivity involves the control of oxidation state with
redox-active ligands.1 Although many ligand frameworks have
been utilized in switchable reactivity with redox-active ligands,1-
Figure 1. (left) Molecular structure of BoPNP. Thermal ellipsoids are drawn at
50% probability. Key bond distance and angle: N(3)-C(5) = 1.341(2) Å, dihedral
of C(2)-C(5)-B(1)-C(8) = 162.94(14)°. Pictures of NMR tubes showing BoPNP in
CD2Cl2 under (middle) ambient light and (right) UV light.
2
fluorescent dye molecules represent an exciting new
coworkers have generated BODIPY containing phosphines and
N-heterocyclic carbenes for the investigation of catalyst
intermediates.13-16 BODIPY dyes have also been recently
incorporated into a Pt complex for singlet O2 detection.17-19
Although BODIPY dyes have been incorporated into metal
complexes, the redox character of the BODIPY dye has not been
utilized to control metal center reactivity.
opportunity, as they exhibit reversible electrochemistry in the
absence of a metal,3 can be readily synthesized,4 are biologically
compatible,5 and can even promote electron transfer when
illuminated.6 One dye molecule that has received recent
attention for its ability to exhibit reversible electrochemistry
and tunable redox potentials is the metal-free 4,4-difluoro-4-
bora-3a,4adiaza-s-indacene (BODIPY) dye.7-8
A few groups have implemented BODIPY molecules in metal
complexes for uses beyond medicinal applications.9-11 Higham
and coworkers have shown that BODIPY-containing phosphines
can be used to generate Ag, Au, and Cu complexes, but the
electron deficient nature of the BODIPY fragment generates
phosphine compounds that are weak ligands.12 Plenio and
Rhodium-based complexes have been shown to be capable
catalysts
for
hydrogenations,20
hydrosilylations,21
hydroborations,22 hydroacylations,23 and many other reactions.
The breadth of reactivity of rhodium complexes makes rhodium
an attractive choice as a metal center for the investigation of
switchable reactivity. Wrighton and coworkers have reported
that the catalytic activity of a rhodium complex containing a
cobaltocene center in the ligand framework can switch the
catalyst selectivity between the hydrogenation and
hydrosilylation of alkenes.24 As expected, oxidation of the
cobaltocene center generates a more Lewis acidic rhodium
center, making oxidative addition of H2 at the rhodium center
less favorable.
a. Department of Chemistry, Washington State University, Pullman, WA 99164 .
Email: zachariah.heiden@wsu.edu
b. Department of Chemistry and Biochemistry, Utah State University, Logan, UT
84322.
Electronic Supplementary Information (ESI) available: Synthetic methods, catalytic
conditions and results, photographs of the catalytic reactions, multinuclear NMR
spectra of BoPNP and [(COD)Rh(BoPNP)]BF4, details of molecular structure analysis,
cyclic voltammograms, normalized absorbance and emission spectra, sample input
file for Gaussian09, free energies for the hydroboration of alkenes, gas phase
energies, molecular orbitals, and 3D coordinates of all computed structures. See
DOI: 10.1039/x0xx00000x
To generate a bidentate phosphine containing a BODIPY
dye, bis[2-(diphenylphosphino)ethyl]ammonium chloride was
treated with 8-methylthio-BODIPY (BoSMe) in the presence of
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one equivalent of triethylamine generating 8-(bis[2- Information), which are only slightly shifted (2-3 nm
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(diphenylphosphino)ethyl]amine)-BODIPY (BoPNP) in a 68 % bathochromic shift) from the free ligand (DλOabIs: 1=0.3104309a/Cn9dC4C20858n3m6B,
yield (Equation 1). When illuminated with UV light, the resulting
red colored solid fluoresced yellow in the solid-state and green
when dissolved in methylene chloride (Figure 1). The presence
of a singlet at -20.8 ppm in the 31P{1H} NMR spectrum, a 1:1:1:1
1
quartet at -149.1 ppm with a JBF coupling of 23 Hz in the 19F
1
NMR spectrum, and a triplet at 0.09 ppm with a JBF = 23 Hz
coupling constant in the 11B NMR spectrum verified the addition
of
a BODIPY fragment to the amine center of bis[2-
(diphenylphosphino)ethyl]amine.
Figure 2. (left) Molecular structure for [(COD)Rh(BoPNP)]+. Thermal ellipsoids
are drawn at 50% probability. The BF4− anion and CH2Cl2 solvent molecules are
omitted for clarity. (middle) NMR tubes containing [(COD)Rh(BoPNP)]BF4 in THF
(right) under ambient light, and (right) under UV light. Key bond distances and
angles: Rh-P(1) = 2.3124(8) Å, Rh-P(2) = 2.3500(8) Å. P(1)-Rh-P(2) = 96.48(3)°.
Dihedral of C(2)-C(5)-B(1)-C(8) = 172.1(5)°.
To examine the nature of a BODIPY dye incorporated into
the BoPNP ligand, x-ray quality crystals of BoPNP were grown
by vapor diffusion of pentanes into a methylene chloride
solution of BoPNP. X-ray crystallographic analysis revealed a
BODIPY dye that deviated about 17° from planarity (C(2)-C(5)-
B(1)-C(8) dihedral angle). The deviation of the pyrrole rings is an
important property as the planarity of the BODIPY core and the
extended π-system are predominantly responsible for the
optical properties of 8-amino-BODIPYs.25 BoPNP was found to
be soluble in most organic solvents, air stable in the solid state,
and moderately air stable in solution.
λemission
= 536 nm). Although the quantum yield of
[(COD)Rh(BoPNP)]BF4 was low at 0.072, it was about two times
greater than free BoPNP (Φ = 0.042) in CH2Cl2. The excited state
lifetimes (τ = 4.4 and 5.5 ns in CH2Cl2) of [(COD)Rh(BoPNP)]BF4
were almost identical to free BoPNP (τ = 2.6 and 5.7 ns). Due to
the similarity between the photophysical properties of
[(COD)Rh(BoPNP)]BF4 and free BoPNP, the photophysical
properties of [(COD)Rh(BoPNP)]BF4 are attributed to the
BODIPY dye, as opposed to the Rh center.
The generation of [(COD)Rh(BoPNP)]BF4 was further verified
by crystallographic analysis. X-ray quality crystals were grown
by the diffusion of pentanes into a dichloromethane solution at
To generate a Rh complex from BoPNP, 1.2 equivalents of
BoPNP was treated with one equivalent of [(COD)2Rh]BF4,
where
COD
=
cis-1,5-cyclooctadiene,
generating
−
30
square planar rhodium center containing a bidentate BoPNP
ligand with a bite angle of 96.48(3)
and an η4-COD ligand with
the average Rh-C distance of 2.234 Å. The BODIPY fragment of
°C. Crystallographic analysis revealed the presence of a
[(COD)Rh(BoPNP)]BF4 in 63 % yield (Equation 2). The orange
colored [(COD)Rh(BoPNP)]BF4 fluoresced yellow green under
UV light in THF (Figure 2). The generation of the desired
rhodium complex was demonstrated by the presence of a triplet
at 0.17 ppm in the 11B{1H} NMR spectrum, a quartet at -147.2
ppm in the 19F NMR spectrum, and a broad doublet at 26.5 ppm
(1JRhP = 146 Hz) in the 31P{1H} NMR spectrum, which is
characteristic of coordination of the BoPNP to a Rh center.
°
[(COD)Rh(BoPNP)]BF4 deviates about 8
about 10 less than the free ligand.
Electrochemical analysis of
[(COD)Rh(BoPNP)]BF4 showed
°
from planarity, which is
°
a
THF solution of
a
quasireversible reduction
at -1.65 V vs. Cp2Fe/Cp2Fe+ (+122 mV from BoPNP). The
reduction potential of BoPNP (-1.77 V) is about 200 mV more
negative than the average reduction potential (-1.53 V) for a
BODIPY dye.7 The single reversible wave near the reduction of
the free ligand likely suggests that the reduction wave occurs at
the BODIPY center of the Rh complex. Reduction of the BODIPY
dye instead of the Rh center suggests that the BODIPY-based
redox event could be utilized to alter the reactivity at the Rh
center.
To further verify that a redox event would result in a
BODIPY-based reduction, [(COD)Rh(BoPNP)]BF4 was treated
with 1.1 equiv. KC8 in THF (Equation 3), resulting in a red orange
(
λabs = 338 and 422 nm) colored solution that did not fluoresce
Investigation of the photophysical properties of
[(COD)Rh(BoPNP)]BF4 yielded two absorptions of 344 and 431
nm and an emission at 538 nm in CH2Cl2 (see Supporting
under UV light. EPR analysis of the reaction product in THF at
90K revealed a g-value of 2.005 (Figure 3). Due to the absence
of hyperfine splitting, at RT or 90K, the EPR results suggest the
presence of a carbon-based (BODIPY-based) radical. We have
2 | J. Name., 2012, 00, 1-3
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Table 1. Catalytic hydroboration of (0.115 mmol) styrene with [(COD)Rh(BoPNP)]BF4 with
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also observed a carbon-based radical in the reduction of a
tetramethylguanidine-appended BODIPY dye.8 If a rhodium–
0.83 equivalents of pinacolborane in THF at 23 °C. Conversion was determined by NMR
DOI: 10.1039/C9CC05836B
spectroscopy.
Catalyst Concentration
Time
(hours)
24
Conversion
(mol %)
0.1
0%
0.5
24
20%
1.0
3.0
5.0
10
4
0.5
100%
100%
100%
To probe the substrate scope for the catalytic hydroboration
of alkenes with [(COD)Rh(BoPNP)]BF4, the hydroboration of five
alkenes was examined (Table 2). The hydroboration of 1-hexene
occurred twice as fast as styrene. No conversion for the
based radical as opposed to an organic-based radical had
formed from the reduction of [(COD)Rh(BoPNP)]BF4 a doublet
and a g-value shifted away from 2.006 would have been
observed.26-27 Based on these results the reduction product of
[(COD)Rh(BoPNP)]BF4 is assigned as the zwitterionic complex,
[(COD)Rh(BoPNP)] (Equation 3). Crystallization attempts of
[(COD)Rh(BoPNP)] resulted in crystals that did not diffract.
hydroboration of (+)-limonene,
α-pinene, or β-pinene with
pinacolborane was observed. Under the same reaction
conditions, the hydroboration of (+)-limonene with 0.83
equivalents of catecholborane resulted in the hydroboration of
only the terminal alkene. Addition of a second equivalent of
catecholborane did not result in the hydroboration of the
internal alkene of (+)-limonene. The catalytic hydroboration of
α
-pinene or
β
-pinene with catecholborane led to the complete
-pinene and no conversion of -pinene over
hydroboration of
β
α
24 hours. Hydrolysis of the hydroboration products of β-pinene
resulted in a 1:1 ratio of the myrtanol diastereomers. The
increased reactivity for the hydroboration of sterically bulky
alkenes with catecholborane, as opposed to pinacolborane, is
attributed to the decreased steric bulk of catecholborane when
compared to pinacolborane.
Table 2. Substrate scope for the hydroboration of (0.088–0.115 mmol) alkenes with 3
mol
% [(COD)Rh(BoPNP)]BF4 with 0.83 equivalents of pinacolborane (HBpin) or
Figure 3. EPR spectrum of a THF solution of [(COD)Rh(BoPNP)] at 90K.
catecholborane (HBcat) in THF at 25 °C. Conversion was determined by NMR
spectroscopy.
To investigate the catalytic activity of [(COD)Rh(BoPNP)]BF4,
the catalytic hydrogenation (4 atm of H2) of limonene, the
hydrosilylation of limonene with Ph2SiH2, the hydroboration of
styrene with pinacolborane, and the hydroacylation of 2,2-
dimethyl-4-pentenal were undertaken with 1.5 mol % catalyst
for 24 hours at 23 °C. Only the hydroboration reaction resulted
in complete conversion. Attempting the same catalytic
reactions with [(COD)Rh(BoPNP)], as opposed to
[(COD)Rh(BoPNP)]BF4, showed no conversion in any of the
catalytic reactions, suggesting that the addition of an electron
to the BODIPY dye can be utilized to turn “off” the catalytic
activity of [(COD)Rh(BoPNP)]BF4.
The catalytic activity of [(COD)Rh(BoPNP)]BF4, for the
hydroboration of styrene, was further examined (Table 1). THF
and CH2Cl2 were superior solvents to acetone and acetonitrile
(see Supporting Information). Although similar catalytic results
for the hydroboration of styrene were obtained with
catecholborane or pinacolborane, pinacolborane was chosen
for further investigations. Hydrolysis of the hydroboration
Alkene
Time
Borane Conversion
Styrene
1-hexene
(+)-limonene 24 hours
(+)-limonene 24 hours
4 hours
2 hours
HBpin
HBpin
HBpin
HBcat
100%
100%
0%
100%
α-pinene
α-pinene
β-pinene
β-pinene
24 hours
24 hours
24 hours
24 hours
HBpin
HBcat
HBpin
HBcat
0%
0%
0%
100%
Since [(COD)Rh(BoPNP)]BF4 was catalytically active for the
hydroboration of styrene with pinacolborane and
[(COD)Rh(BoPNP)] was not, the sequential addition of a
reductant and an oxidant was hypothesized to be able to toggle
between an active and inactive catalyst (Equation 4). The
catalytic hydroboration of styrene was allowed to proceed for 1
hour, resulting in a 35% conversion. Then, 0.95 equiv of Cp*2Co
was added, resulting in [(COD)Rh(BoPNP)], and an additional 2%
conversion in the hydroboration of styrene occurred over the
next hour (Figure 4). 0.95 equiv [Cp2Co]PF6 was then added,
products resulted in
phenylethanol.
a 7:1 ratio of 1-phenylethanol:2-
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J. Name., 2013, 00, 1-3 | 3
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regenerating [(COD)Rh(BoPNP)]+, and an additional 40% of CoBRE NIGMS P20GM103546) and the X-ray diffractometer was
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DOI: 10.1039/C9CC05836B
conversion for the hydroboration of styrene resulted. Repeating purchased in part through support from National Science
Foundation (NSF)-MRI (CHE - 1337908).
Addition of
1 equiv.
100
[Cp2Co]+
Addition of
1 equiv.
Cp*2Co
80
Conflicts of interest
There are no conflicts to declare.
Addition of
1 equiv.
60
40
20
0
Addition of
1 equiv.
Cp*2Co
[Cp2Co]+
Notes and references
1. Blanco, V.; Leigh, D. A.; Marcos, V. Chem. Soc. Rev. 2015, 44,
5341-5370.
0
50
100
150
200
2. Wei, J.; Diaconescu, P. L. Acc. Chem. Res. 2019, 52, 415-424.
3. Nepomnyashchii, A. B.; Bard, A. J. Acc. Chem. Res. 2012, 45,
1844-1853.
4. Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891-4932.
5. Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.;
Burgess, K. Chem. Soc. Rev. 2013, 42, 77-88.
Time (Minutes)
Figure 4. Plot showing the conversion of the hydroboration of styrene with 3
mol % [(COD)Rh(BoPNP)]BF4 when exposed to sequential additions of Cp*2Co
(reductant) and [Cp2Co]PF6 (oxidant).
6. Magagnano, G.; Gualandi, A.; Marchini, M.; Mengozzi, L.;
Ceroni, P.; Cozzi, P. G. Chem. Commun. 2017, 53, 1591-1594.
7. Thompson, B. L.; Heiden, Z. M., Redox Chemistry of BODIPY
Dyes. In BODIPY Dyes - A Priviledge Molecular Scaffold with
Tunable Properties, Bañuelos-Prieto, J.; Llano, R. S., Eds.
IntechOpen: 2018; pp 45-64.
8. Kieffer, I. A.; Allen, R. J.; Fernandez, J. L.; Deobald, J. L.;
Thompson, B. L.; Wimpenny, J. D.; Heiden, Z. M. Angew. Chem.,
Int. Ed. Engl. 2018, 57, 3377 –3380.
this procedure for an additional cycle lead to the complete
hydroboration of styrene over 3.5 hours. The use of an oxidant
stronger than [Cp2Co]+ resulted in the oxidation of
pinacolborane as opposed to [(COD)Rh(BoPNP)]. This fact
suggests that catalysts that are capable of redox-switchable
hydroborations require reduction potentials similar to [Cp2Co]+
to favor oxidation of the catalyst as opposed to the borane.
9. Lifschitz, A. M.; Shade, C. M.; Spokoyny, A. M.; Mendez-Arroyo,
J.; Stern, C. L.; Sarjeant, A. A.; Mirkin, C. A. Inorg. Chem. 2013,
52, 5484-5492.
10. Lifschitz, A. M.; Young, R. M.; Mendez-Arroyo, J.; Roznyatovskiy,
V. V.; McGuirk, C. M.; Wasielewski, M. R.; Mirkin, C. A. Chem.
Commun. 2014, 50, 6850-6852.
11. Bertrand, B.; Passador, K.; Goze, C.; Denat, F.; Bodio, E.;
Salmain, M. Coord. Chem. Rev. 2018, 358, 108-124.
12. Davies, L. H.; Harrington, R. W.; Clegg, W.; Higham, L. J. Dalton
Trans. 2014, 43, 13485-13499.
13. Vasiuta, R.; Plenio, H. Chem. - Eur. J. 2016, 22, 6353-6360.
14. Sashuk, V.; Schoeps, D.; Plenio, H. Chem. Commun. 2009, 770-
772.
15. Kos, P.; Plenio, H. Angew. Chem., Int. Ed. 2015, 54, 13293-
13296.
16. Kos, P.; Plenio, H. Chem. - Eur. J. 2015, 21, 1088-1095.
17. Geist, F.; Jackel, A.; Winter, R. F. Inorg. Chem. 2015, 54, 10946-
10957.
18. Irmler, P.; Winter, R. F. Dalton Trans. 2016, 45, 10420-10434.
19. Irmler, P.; Winter, R. F. Organometallics 2018, 37, 235-253.
20. Cui, X.; Burgess, K. Chem. Rev. 2005, 105, 3272-3296.
21. Brunner, H. Angew. Chem. 1983, 95, 921-31.
22. Pereira, S.; Srebnik, M. Tetrahedron Lett. 1996, 37, 3283-3286.
23. Willis, M. C. Chem. Rev. 2010, 110, 725-748.
24. Lorkovic, I. M.; Duff, R. R.; Wrighton, M. S. J. Am. Chem. Soc.
1995, 117, 3617-3618.
These results show that the reduction of a BODIPY fragment
can “turn off” the catalytic activity of an appended metal center,
with the catalytic activity returning upon exposure to an
oxidant. Although BODIPY fragments are capable of promoting
chemical reactions when photolyzed, the BODIPY dye was not
close enough to the Rh center to influence the catalytic activity
when illuminated. The reported redox switchable behavior
presents
a new approach towards controlling catalytic
reactions. The incorporation of BODIPY dyes into molecular
scaffolds to influence catalytic reactivity is a topic of interest of
our research group.
This research was supported by Washington State University
and the Washington State University Seed Grant Program. We 25. Osorio-Martinez, C. A.; Urias-Benavides, A.; Gomez-Duran, C. F.
A.; Banuelos, J.; Esnal, I.; Lopez Arbeloa, I.; Pena-Cabrera, E. J.
Org. Chem. 2012, 77, 5434-5438.
26. de Bruin, B.; Russcher, J. C.; Gruetzmacher, H. J. Organomet.
Chem. 2007, 692, 3167-3173.
27. Orsini, J.; Geiger, W. E. J. Electroanal. Chem. 1995, 380, 83-90.
would also like to acknowledge the EPR facility at Utah State
University. EPR spectra were collected using a Bruker EMXplus
EPR spectrometer, principally supported by NSF MRI-0722849.
We acknowledge the single crystal X-ray diffraction facility at
the University of Montana (UM). X-ray diffraction studies were
supported by the CBSD at UM (National Institutes of Health,
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