Latent Ruthenium Benzylidene Phosphite Complexes for Visible-Light-Induced Olefin Metathesis

Article abstract of DOI:10.1021/acscatal.9b05079

Herein we report two ruthenium benzylidene complexes with benzylphosphite ligands for olefin metathesis. Unlike the previously reported benzylidene phosphite complexes, the benzylphosphite complexes adopt a cis-dichloro configuration making them latent at ambient temperatures. Irradiation with visible light (420 nm and blue LED) prompts activation of the complexes and induces catalysis of olefin metathesis reactions. One of the complexes, cis-Ru-1, was found to be especially suitable for 3D printing of multilayered polydicyclopentadiene structures with excellent spatial resolutions. Additionally, complex cis-Ru-2 was designed with a chromatic orthogonal "kill switch" based on the 2-nitrobenzyl chemistry, allowing the destruction of the catalyst upon exposure to UV-C light.

Full text of DOI:10.1021/acscatal.9b05079

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Letter
Latent Ruthenium Benzylidene Phosphite Complexes
for Visible Light Induced Olefin Metathesis
Or Eivgi, Anna Vaisman, Noy B. Nechmad, Mark Baranov, and N. Gabriel Lemcoff
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b05079 • Publication Date (Web): 27 Dec 2019
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Latent Ruthenium Benzylidene Phosphite Complexes for Visible
Light Induced Olefin Metathesis
Or Eivgiƚ‡, Anna Vaismanƚ‡, Noy B. Nechmadƚ, Mark Baranovƚ and N. Gabriel Lemcoffƚǂ*
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ƚDepartment of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 84105
ǂ Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva, Israel,
84105
‡These authors contributed equally
lemcoff@bgu.ac.il
photocatalysis, olefin metathesis, photochemistry, 3D printing, phosphites.
ABSTRACT: Herein we report two ruthenium benzylidene complexes with benzylphosphite ligands for olefin metathesis.
Unlike the previously reported benzylidene phosphite complexes, the benzylphosphite complexes adopt a cis-dichloro
configuration making them latent at ambient temperatures. Irradiation with visible light (420 nm and blue LED) prompts
activation of the complexes and induces catalysis of olefin metathesis reactions. One of the complexes, cis-Ru-1, was found
especially suitable for 3D printing of multilayered polydicyclopentadiene structures with excellent spatial resolutions.
Additionally, complex cis-Ru-2 was designed with a chromatic orthogonal 'kill switch' based on the 2-nitrobenzyl
chemistry, allowing the destruction of the catalyst upon exposure to UVC light.
Olefin metathesis is one of the most important reactions
for the transmutation of carbon-carbon double bonds in
organic synthesis and is routinely utilized in academic
research as well as the pharmaceutical and polymer
industries.1 Extensive research invested in studying the
ligand sphere of the metal complexes yielded a plethora of
specialized complexes for task specific applications; for
example, third generation fast initiating complexes for
precision polymerizations2 or the variety of cyclic alkyl
amino carbene (CAAC) containing complexes to increase
activity, selectivity and reduce double bond isomerization
side reactions.3 Slow initiating, or latent, complexes are a
notable class of complexes, where external stimuli (heat,
pH, ultrasound, etc.) are required to activate an otherwise
dormant catalyst.4 The use of light to initiate a reaction
catalyzed by a latent complex is of particular interest, as
light can be conveniently applied with relatively low costs
and great specificity.5 Additionally, the option to use light
with good spatial resolution and the development of
continuous flow techniques for photochemistry, provides
the opportunity to apply photoinduced olefin metathesis
on an industrial setting and scale.6 In the past decade, our
group has developed a series of sulfur chelated ruthenium
benzylidene complexes that adopt a dormant cis-dichloro
configuration and can isomerize to the active trans form
when irradiated with UV-A light.7 The sulfur chelated
complexes have been tested for their heat and photolatent
ring closing metathesis (RCM) and ring opening
metathesis polymerization (ROMP) activities, including in
sterically demanding reactions8 and even for chromatic
orthogonal applications.9 Originally reported by Cazin et
al. in 2010, commercially available phosphite containing
ruthenium indenylidene complex cis-Caz-1 (Figure 1,
bottom right) also prefers the cis-dichloro configuration
and just as its sulfur-chelated and other phosphine-
chelated counterparts, is latent towards olefin metathesis
reactions at ambient temperatures.10 When heated in
toluene cis-Caz-1 exhibited remarkable activity and
thermal stability, efficiently completing sterically
demanding olefin metathesis reactions with low catalyst
loadings and under air.11 Recently, we reported that cis-
Caz-1 may also be activated by UV-A light (350 nm) at
room temperature.12 Irradiation of cis-Caz-1 with UV-A
light promoted a variety of olefin metathesis reactions
(RCM, CM and ROMP) with high efficiency. Interestingly,
the benzylidene complexes synthesized by Cazin et al.,
trans-Ru-P(OEt)3 and trans-Ru-P(OiPr)3, featured trans-
dichloro geometries and catalyzed olefin metathesis
reactions at room temperature (Figure 1).13 Aiming to
expand the field of photoinduced olefin metathesis, a
chelating phosphite ligand was introduced to produce cis-
PhosRu-1 (Figure 1).12 The chelation effect imposed a cis
configuration, leading to a latent benzylidene phosphite
complex that could be activated with visible light in
toluene; however, the activity of this complex for several
olefin metathesis reactions was disappointingly low,
possibly due to the strong chelation.
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Reaction progress was monitored by H-NMR, presenting
new benzylidene signals at 15.27 ppm and 15.22 ppm for cis-
Ru-1 and cis-Ru-2 respectively. Moreover, 31P-NMR
experiments showed signals at 130.5 ppm and 133.8 ppm for
cis-Ru-1 and cis-Ru-2, indicating complexation of the
phosphite ligands. The complexes were purified by a series
of precipitations (see detailed protocol in the SI) and
N
N
N
N
1
2
3
4
5
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7
8
Cl
Cl
Ru
Ru
Cl
(EtO)3P
Cl
(i-PrO)3P
13
trans-Ru-P(OEt)3
trans-Ru-P(OiPr)3
obtained as purple powders. C-NMR analysis disclosed a
2JC-P coupling constant between the phosphorous and the
SIMes carbene of 13 Hz for both cis-Ru-1 and cis-Ru-2,
9
supporting
a
cis-dichloro geometry.10a Indeed, this
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N
N
N
P
N
assumption was corroborated in the solid state by single
crystal X-ray crystallography of the target complexes
(Figure 2, bottom).
Cl
Cl
Ph
Ru
Cl
Ru
O
O
(i-PrO)3P
O
Cl
cis-PhosRu-1
cis-Caz-1
Figure 1. Ruthenium indenylidene complex cis-Caz-110
(bottom right) and ruthenium benzylidene complexes bearing
phosphite ligands reported in the literature by Lemcoff12 and
Cazin13 (top) and (bottom, left).
In this work, we report the synthesis of two novel photo-
switchable
non-chelated
ruthenium
benzylidene
complexes, bearing benzylphosphite ligands: cis-Ru-1 and
cis-Ru-2 (Figure 2). Notably, unlike the previously
reported
trans-dichloro
ruthenium
benzylidene
phosphites (Figure 1, top),13 these complexes adopt the
latent cis-dichloro configuration. Upon irradiation with
visible light (420 nm lamps or Blue LEDs), photoinduced
activity for a variety of olefin metathesis reactions was
observed. Additionally, complex cis-Ru-2 was equipped
with a chromatic orthogonal ‘kill switch’14 based on the rich
2-nitrobenzyl photochemistry.15 This feature, installed
directly on the phosphite ligand, enables the destruction of
the catalyst by UV-C light, making it compatible for
chromatic orthogonal applications.9a
Figure 2. (top) Synthesis of ruthenium benzylidene
benzylphosphite complexes cis-Ru-1 and cis-Ru-2. (bottom)
Single crystal X-ray structures of complexes (a) cis-Ru-1 and
(b) cis-Ru-2. Ellipsoids are at 50% probability and the
hydrogens are omitted for clarity.
With the new complexes in hand, their photoactivation
behavior was studied. To determine the optimum
wavelength of activation, a series of experiments using
benchmark RCM substrate diethyl diallylmalonate
(DEDAM, Figure 3) were conducted. Complex cis-Ru-1
showed clear activity under illumination with UV-A and
visible light. As designed, complex cis-Ru-2, bearing the
photosensitive 2-nitrobenzyl phosphite moiety showed
low to negligible activity with UV light. On the other hand,
cis-Ru-2 reported good activity when illuminated with 420
nm light, albeit lower than that obtained with cis-Ru-1.
The synthesis of the required phosphite ligands 1 and 2 was
carried out by alcoholysis reactions of commercially
available
tris(dimethylamino)phosphine
with
the
corresponding benzyl alcohols under neat conditions
(Scheme 1).16 Mixing 1 and 2 with Grubbs 3rd Generation
complex (pyridine) in CH2Cl2 for two hours afforded the
corresponding complexes cis-Ru-1 and cis-Ru-2. (Figure
2).
Scheme 1. Synthesis of phosphite ligands 1 and 2
R
N
N
O
P
Neat, Ar
OH
P
N
+
80oC to 100oC
R
O
O
R
1 equiv.
3 equiv.
R
1:
2:
R = H , 45% yield
R = NO2, 30% yield
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light for 0.5 h resulting in an amorphous dark gel. On the
1
2
3
other hand, exposure of the same formulation to 420 nm
yielded solid elastic rubbers (Figure S9, SI).
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Figure 3. Photoactivity of complexes cis-Ru-1 and cis-Ru-2
for the RCM reaction of diethyl diallyl malonate in different
wavelengths. Conversions measured using 1H-NMR.
To probe the scope of the photoinduced metathesis, a
variety of RCM and ROMP reactions were conducted at 420
nm (Table 1). Moreover, given the surprisingly good results
of complex cis-Ru-2 with the blue LED (450-470 nm), an
uncommon light source for direct photoactivation of latent
ruthenium olefin metathesis complexes, the scope of
reactions with complex cis-Ru-2 was extended to blue LED
activation as well (Table 2). The findings indicate that cis-
Ru-1 can efficiently catalyze RCM and ROMP reactions
using visible light at low catalyst loadings in ambient
temperature and under air. In addition, the activity of cis-
Ru-1 was compared to the state-of-the-art catalyst cis-Caz-
1 for di and tri-substituted RCM reactions, highlighting the
superior activity of cis-Ru-1 at 420 nm (Figure S7, SI).
Complex cis-Ru-2 also showed good activity at similar
conditions; however, due to the slow decomposition of the
complex occurring at 420 nm, the catalyst loadings had to
be doubled to reach good reaction conversions.
Figure 4. NMR study of the “kill switch” efficiency of complex
cis-Ru-2
for
RCM
reaction
of
N,N-diallyl-4-
methylbenzenesulfonamide. (a) Control experiment, sample
covered with aluminum foil during irradiation at 254 nm and
then exposed to 420 nm for 6h (86% conversion). (b) Sample
exposed 45 minutes to 254 nm light and then 6h to 420 nm
light (0% conversion).
The lower activity of complex cis-Ru-2 compared to cis-
Ru-1 at 420 nm could probably be related to an accelerated
decomposition of the complex. We hypothesized that the
competition between two distinct photochemical
reactions – the activation of the complex at 420 nm and the
photo removal of 2-nitrobenzyl moiety (that may still
occur slowly at this wavelength) – could be the cause for
the lower activity. To test our hypothesis, molecular UV
filters were used.9b Thus, a dichloromethane solution
containing DEDAM and cis-Ru-2 was irradiated with 420
nm light using different concentrations of an external
solution of 1-pyrene carboxaldehyde. Indeed, the
conversion of the reaction was significantly increased from
49% to 77% simply by protecting the reaction with a 10 mM
of external solution of pyrene carboxaldehyde as a
“molecular UV filter” (Figure S10 and Figure S11, SI).
The efficiency of the UV-C kill switch embedded in
complex cis-Ru-2, was then tested by a series of
experiments. First, the RCM reaction of N,N-diallyl-4-
methylbenzenesulfonamide was studied (Figure 4).
Reaction mixtures that were exposed to UV-C light,
followed by 420 nm showed no conversion at all; while,
control experiments that were covered with aluminum foil
when irradiated by UV-C, showed almost complete
conversion upon exposure to 420 nm light; demonstrating
the efficacy of the method. Furthermore, ROMP of cis-1,5-
cyclooctadiene (COD) was also investigated (Figure S8, SI).
Exposure of a 0.1 M COD solution in toluene to 254 nm for
45 minutes resulted in 18% conversion. Further exposure
to 420 nm for 1 h did not lead to additional monomer
consumption, indicating the destruction of the catalyst.
Once again, control experiments where the reaction
mixture was covered at 254 nm and exposed to 420 nm for
1h led to almost complete conversion (92%). The kill switch
was also tested for bulk polymerization. A formulation
containing 1 g of cyclooctene (COE) and 0.005% mol of cis-
Ru-2 was placed in a steel mold and exposed to 254 nm
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Table 1. Catalytic activity of complexes cis-Ru-1 and cis-Ru-2 with 420 nm irradiation.a
1
2
3
4
5
loading
(% mol)
% conversion
(dark control)b
Mn (g/mol)
(Mw/Mn)
time
(h)
entry
1
substrate
product
catalyst
EtO2C CO2Et
EtO2C CO2Et
8
8
1
2
cis-Ru-1
cis-Ru-2
92 (4)
80 (14)
-
6
7
8
9
Ts
N
Ts
N
8
8
1
2
cis-Ru-1
cis-Ru-2
92 (0)
86 (0)
2
3
-
-
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EtO2C
CO2Et
EtO2C CO2Et
8
8
1
2
cis-Ru-1
cis-Ru-2
73 (6)
46 (3)
Ts
N
Ts
8
8
1
2
cis-Ru-1
cis-Ru-2
92(6)
80 (4)
N
4
5
-
1.49*104 (1.99)
1.67*104 (2.01)
1
1
0.1
0.05
cis-Ru-1
cis-Ru-2
100 (4)
94 (4)
n
n
1
1
0.05
0.05
cis-Ru-1
cis-Ru-2
94 (5)
97 (14)
1.76*105 (1.52)
1.65*105(2.05)
6
7
8
0.1
0.2
cis-Ru-1
cis-Ru-2
100 (40)
100 (16)
9.89*105 (1.57)
6.62*105 (1.66)
CO2tBu
n
1
1
1
1
CO2tBu
0.1
0.2
cis-Ru-1
cis-Ru-2
95(0)
94 (3)
n.dc
n.dc
n
OAc
AcO
aReaction conditions: All reactions were carried out in a Luzchem LZC-org photoreactor equipped with 10 LZC-420 lamps. RCM:
In an NMR tube under air, 0.1M of substrate dissolved in 0.45 mL tol-d8 and 0.05 mL of catalyst stock solution in dichloromethane-
d2 were irradiated for 8 hours. Reaction conversions were monitored using H-NMR spectroscopy. ROMP: 0.5M of monomer in
0.45 mL tol-d8 and 0.05mL of catalyst stock solution in dichloromethane-d2 were irradiated for 1 hour. Reaction conversions were
monitored using 1H-NMR using 10 µL of mesitylene as internal standard. b Dark control experiments are covered with aluminum
foil to prevent exposure to light c not determined due to insolubility of the obtained polymer.
1
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Table 2. Catalytic photoactivity of cis-Ru-2 with blue LED (450 – 470 nm) irradiationa
1
2
3
4
5
6
7
8
9
loading
(% mol)
% conversion
(dark control)b
Mn (g/mol)
(Mw/Mn)
entry
1
substrate
product
time (h)
8
EtO2C CO2Et
EtO2C CO2Et
2
2
2
70 (11)
-
-
-
Ts
N
Ts
N
2
3
8
80 (1)
65 (2)
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EtO2C
CO2Et
EtO2C CO2Et
20
Ts
N
Ts
N
4
8
2
79 (5)
-
5
6
7
8
1
3.5
2
0.2
0.05
0.2
100 (15)
92 (16)
100 (9)
98 (4)
2.24*104 (1.64)
1.62*105 (1.70)
8.30*105 (1.99)
n.dc
n
n
CO2tBu
n
CO2tBu
n
2
0.2
OAc
AcO
aReaction conditions: All reactions were carried out in a Luzchem LZC-org photoreactor equipped with 10 LZC-Blue LED lamps.
In an NMR tube under air, solutions as specified in Table 1 were irradiated for the time specified. Reaction conversions were
monitored using 1H-NMR using 10 µL of mesitylene as internal standard. b Dark control experiments are covered with aluminum
foil to prevent exposure to light. c Not determined due to insolubility of the obtained polymer.
with DCPD (mixture of endo- and exo- isomers). The mixture
Finally, to assess the efficiency of the phosphite complexes for
was drop casted on a microscope slide and different patterns
industrial applications, the possibility to print 3D materials by
were irradiated with a programmable LED based light source
stereolithography was perused.17 The patterning of polymeric
(DLP projector, 385 nm) for short exposure times (approx. 30-
materials using photoinduced olefin metathesis was first
60 seconds per layer, see SI for details). This process was
explored by Grubbs and co-workers who developed a
repeated for the desired number of layers and the excess of the
photolithographic olefin metathesis polymerization (PLOMP)
non-polymerized formulation was rinsed with acetone to
method based on a negative photoresist consisting of COD
afford the printed structures (Figure 5).
and
a curable bifunctional 5-ethylidene 2-norbornene
monomer.18 Other recent approaches to photoinduced ROMP
include the work by Rovis et al. who used a photoredox
activated bis-NHC catalyst to polymerize dicyclopentadiene
(DCPD) using a mask5c and Chemtob et. al. that generate the
NHC by using light in the presence of ([RuCl2(p-cymene)]2) to
initiate the reaction.19 We are also interested in printing
polyDCPD based materials, as this polymer is known for its
excellent thermal and mechanical properties.20 Moore and co-
workers have shown that phosphite additives can be used to
modulate the reactivity of a Grubbs 2nd generation catalyst to
prepare bench stable mixtures of catalyst and DCPD.21 Thus,
we anticipated that cis-Ru-1 could form a processable
formulation with a relatively long pot life when mixed with
DCPD. Thus, cis-Ru-1 (200 ppm) in a small amount of
dichloromethane (35 µl per 1 g of bulk monomer) was mixed
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version of the manuscript. ‡These authors contributed
equally.
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ACKNOWLEDGMENT
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The Israel Science Foundation and the Kamin program from
the Israel Innovation Authority are acknowledged for funding.
We thank Zvi Kotler, Gil Bernstein Toker and Asaf Levy from
the Orbotech Company for valuable discussions on material
requirements for target 3D applications and technical
assistance in carrying out the 3D-printed structures.
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ASSOCIATED CONTENT
Supporting Information. General information, synthetic
procedures, and supplemental data. This material is available
free of charge via the Internet at http://pubs.acs.org.”
REFERENCES
1. (a) Grela, K.; Preface In Olefin Metathesis, ; Grela, K. (Ed.)
John Wiley & Sons, Inc.:Hobiken, NJ, 2014; pp i-xiv; (b) Wenzel,
A. G.; O’Leary, D. J.; Khosravi, E.; Grubbs, R. H. Front Matter,
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Fogg, D. E., Olefin Metathesis at the Dawn of Implementation in
Pharmaceutical and Specialty-Chemicals Manufacturing. Angew.
Chem. Int. Ed. 2016, 55, 3552-3565.
Figure 5. 3D DCPD structures printed in layers using complex
cis-Ru-1. a) Pyramids consisting of four layers of squares in
decreasing size, b) zoom in on a single square c) Ben-Gurion
University logo printed in two layers d) 3D image of River
Plate club symbol.
2. (a) Choi, T.-L.; Grubbs, R. H., Controlled Living Ring-
Opening-Metathesis Polymerization by
a
Fast-Initiating
Ruthenium Catalyst. Angew. Chem. Int. Ed. 2003, 42, 1743-1746;
(b) Yasir, M.; Liu, P.; Tennie, I. K.; Kilbinger, A. F. M., Catalytic
living ring-opening metathesis polymerization with Grubbs’
second- and third-generation catalysts. Nat. Chem. 2019, 11, 488-
494.
In conclusion, the preparation, characterization and
photoactivity of two new ruthenium benzylidene complexes
bearing phosphite ligands, cis-Ru-1 and cis-Ru-2 is
presented. Both benzylidene complexes adopt the cis dichloro
configuration and are latent at ambient temperatures. Visible
light (420 nm and blue LED) activates these complexes and
promotes photoinduced catalysis of several olefin metathesis
reactions. Moreover, cis-Ru-1 was found suitable for 3D
printing applications of metathesis-based polymers allowing
the preparation of a stable multilayered poly-DCPD designs
using a programmable light patterning projector. Given the
exceptional physical properties of poly-DCPD this application
should have wide-ranging practical implications. In addition,
complex cis-Ru-2 was equipped with a chromatic orthogonal
‘self-destruct’ switch based on the photoreactive 2-
nitrobenzyl protecting group and could be decomposed under
irradiation of shortwave UV light, expanding the capabilities
for one pot divergent photochemical processes.
3. (a) Marx, V. M.; Sullivan, A. H.; Melaimi, M.; Virgil, S. C.;
Keitz, B. K.; Weinberger, D. S.; Bertrand, G.; Grubbs, R. H., Cyclic
Alkyl Amino Carbene (CAAC) Ruthenium Complexes as
Remarkably Active Catalysts for Ethenolysis. Angew. Chem. Int.
Ed. 2015, 54, 1919-1923; (b) Gawin, R.; Tracz, A.; Chwalba, M.;
Kozakiewicz, A.; Trzaskowski, B.; Skowerski, K., Cyclic Alkyl
Amino Ruthenium Complexes—Efficient Catalysts for
Macrocyclization and Acrylonitrile Cross Metathesis. ACS Catal.
2017, 7, 5443-5449; (c) Butilkov, D.; Frenklah, A.; Rozenberg, I.;
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AUTHOR INFORMATION
Corresponding Author
* lemcoff@bgu.ac.il.
Author Contributions
OE and AV synthesized the complexes and conducted all the
experiments, NBN assisted with the 3D printing experiments.
MB solved the crystal structures. The manuscript was written
by OE and NGL. All authors have given approval to the final
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