Highly active multidentate alkyne metathesis catalysts: Ligand-activity relationship and their applications in efficient synthesis of porphyrin-based aryleneethynylene polymers

Article abstract of DOI:10.1002/adsc.201200243

A series of tris(arylmethyl)ammonium-coordinated molybdenum(VI) propylidyne catalysts was synthesized. Such N-quarternized multidentate catalysts showed high robustness, strong resistance to small alkyne polymerization and significantly enhanced catalytic activity compared to their corresponding tris(arylmethyl)amine-based analogues. The high activity of these new catalysts also enabled the efficient synthesis of ethynylene-bridged porphyrin-based arylene ethynylene polymers via alkyne metathesis, which represents a highly efficient, defect-free, viable approach for the synthesis of this class of intriguing polymers. Copyright

Full text of DOI:10.1002/adsc.201200243

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DOI: 10.1002/adsc.201200243
Highly Active Multidentate Alkyne Metathesis Catalysts: Ligand-
Activity Relationship and Their Applications in Efficient
Synthesis of Porphyrin-Based Aryleneethynylene Polymers
Kuthanapillil Jyothish,a Qi Wang,a and Wei Zhanga,*
a
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA
Fax : (+1)-303-492-5894; e-mail: wei.zhang@colorado.edu
Received: March 22, 2012; Revised: May 2, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200243.
Abstract: A series of tris(arylmethyl)ammonium-
coordinated molybdenum(VI) propylidyne catalysts
was synthesized. Such N-quarternized multidentate
catalysts showed high robustness, strong resistance
to small alkyne polymerization and significantly en-
hanced catalytic activity compared to their corre-
sponding tris(arylmethyl)amine-based analogues.
The high activity of these new catalysts also enabled
the efficient synthesis of ethynylene-bridged por-
phyrin-based arylene ethynylene polymers via
alkyne metathesis, which represents a highly effi-
cient, defect-free, viable approach for the synthesis
of this class of intriguing polymers.
broader substrate scope than the corresponding cata-
lyst containing the analogous monodentate ligand.[5a,b]
It was also observed that the tris(arylmethyl)amine
nitrogen coordinates to the Mo and blocks one of the
two open substrate-binding sites of the Mo-alkylidyne
complex, thus completely inhibiting the undesired
alkyne polymerization side reaction. The multidentate
ligation also has impact on minimizing the non-pro-
ductive substrate binding and enabling the metathesis
of heterocycles containing donor moieties, reactions
in which the analogous monodentate catalyst failed.
Although such a novel multidentate catalyst has
shown great performance, one important question re-
mained unanswered: what is the effect of nitrogen-
molybedenum coordination on the catalyst activity?
It has been reported before that higher electron den-
sity on the metal center would lower the catalyst ac-
Keywords: acyclic diyne metathesis polymerization;
alkyne metathesis; metal-alkylidynes; podand
motif; porphyrin polymers
tivity.[6] Therefore, presumably the N Mo coordina-
À
tion would lower the catalyst activity by increasing
the electron density on the Mo center.
To explore the important nitrogen coordinating
The metal alkylidyne-catalyzed metathesis of alkynes effect of this novel class of multidentate alkyne meta-
has shown great promise over the past two decades thesis catalysts, we designed tri(arylmethyl)ammoni-
for the synthesis of novel ethynylene-bridged poly- um ligands with non-coordinating nitrogen via N-
mers, macrocycles and natural products.[1–4] However, quarternization, studied their structure-activity rela-
there have been many challenging issues that diminish tionship, and compared their activity with that of the
the scope of this powerful metathesis reaction. non-quarternized analogues. In the present study, we
Recent findings, such as those probing the electrophi- demonstrated that quarternization of the triphenola-
licity as well as the substrate binding sites in the cen- mine nitrogen prevents the nitrogen atom from coor-
tral transition metal (Mo/W),[5] indicate that judicious dinating to the Mo center, which increases the elec-
rational design of ligands can give access to highly trophilicity of the metal and thus the catalytic perfor-
active alkyne metathesis catalysts and tackle, to a cer- mance (by facilitating Mo-substrate association). Fur-
À
tain extent, the current problems associated with the thermore, even without N Mo coordination, the
metal-alkylidyne-catalyzed metathesis reactions.
three methylene units can still spatially block one sub-
In this context, in an attempt to study the effect of strate-binding site of the Mo center, thus inhibiting
a multidentate ligand on the catalytic performance of the interfering alkyne polymerization side reaction.
molybdenum alkylidynes, we recently synthesized Such highly active multidentate catalysts were suc-
a Mo(VI) propylidyne catalyst containing a substituted cessfully applied to the efficient synthesis of high mo-
tris(arylmethyl)amine ligand, which showed a signifi- lecular-weight, defect-free porphyrin polymers via
cantly enhanced catalytic activity, longer life time and acyclic diyne metathesis (ADIMET) for the first time.
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Scheme 1. Mo alkylidynes used in the present study.
The synthesis of the multidentate ligands 314.8 ppm for catalyst 1 (Supporting Information, Fig-
(Scheme 1, L1–L4) for catalysts 1–4 was accomplished ure S6), which also supported the succesful genera-
in good yields starting from the corresponding tion of a triol-coordinated Mo(VI) complex.
methyl-protected salicylaldehyde (Supporting Infor-
The metathesis activity of 1 was examined by using
mation, Scheme S1 and Scheme S2). Reductive ami- 4-propynylanisole as the substrate. Interestingly, no
nation and deprotection yielded the multidentate metathesis reaction was observed even at elevated
ligand L1, while for the ligands L2–L4 the nitrogen is temperature (708C) and with higher catalyst loading
À
quarternized with methyl iodide, before the deprotec- (up to 15 mol%), presumably due to the N Mo coor-
tion. For the ligand L4, nitration using dilute HNO3 dination that significantly increased the Mo electron
was conducted after deprotection to provide the final density and makes the metal less electrophilic and
ligand. The catalysts 1–4 were generated in situ from thereby shuts down its catalytic activity. In great con-
a 1:1 mixture of the Mo(VI) propylidyne precursor[7] trast, complex 2, the N-quarternized analogue of 1,
and the respective ligands in various solvents. The in showed high catalytic activity towards most of the
situ generation of catalyst 1 and 3 is confirmed by substrates tested and even those challenging ones
1H NMR experiments using 1,4-dimethoxybenzene as such as pyridine and benzaldehyde substrates
an internal standard, which showed a quantitative dis- (Table 1). The X-ray photoelectron spectroscropy
placement of the precursor ligands by tris(arylme- (XPS) also showed an increased electron binding
thyl)amine or -ammonium ligands to form the multi- energy of Mo 3d (2.7 eV, Supporting Information, Fig-
dentate metal complexes (Supporting Information, ure S7) of catalyst 2 vs. catalyst 1, which is consistent
Figure S5). In addition, the 13C NMR analysis of the with the observed higher activity of 2. The dramatic
trisamido Mo(VI) propylidyne precursor, before and difference in the activity between complexes 1 and 2
after mixing with the ligands showed a significant de- prompted us to study the structure-activity relation-
shielding effect; the chemical shift of the carbyne ship of this novel class of N-quarternized multidentate
carbon bonded to the Mo moved from 302.6 ppm[7] to metal alkylidyne complexes.
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Highly Active Multidentate Alkyne Metathesis Catalysts: Ligand-Activity Relationship
Table 1. Homodimerization, RCAM and cyclo-oligomerization reactions of propynyl substrates.[a,b]
Entry
Substrate
Product
Yield [%]
1
2
3
4
5
94
87
80
43
84
6
79
7
95
8
94[c]
[a]
Reaction conditions: 2–3 mol% catalyst 2 loading for all entries; reaction temperature: 408C for entries 1–5, 7, 8 and
708C for entry 6; reaction times: 4–8 h for entries 1, 2, 5, 6, 7 and 9–13 h for entries 3 and 4.
In a closed system (solvent CCl4), with the reaction solution exposed to vacuum for 6–8 times to remove the metathesis
by-product 2-butyne.
[b]
[c]
In a closed system, under precipitation-driven conditions, the reaction was completed in 20 min at room temperature.
The solvent compatibility of catalyst 2 was tested, cules, (iii) the ring closing alkyne metathesis (RCAM)
with 4-propynylanisole as the substrate, in a series of of diynes to cycloalkyne, and (iv) the precipitation
solvents (carbon tetrachloride, dichloroethane, chloro- driven cyclooligomerization reaction of the carbazole
form, toluene, chlorobenzene, 1,2-dichlorobenzene, substrate (entry 8).[9] Interestingly, catalyst 2 was
1,2,4-trichlorobenzene and THF, in closed system). found to be compatible with all the different sub-
Although the catalyst is metathesis active in all the strates tested, even those challenging ones containing
above solvents, the highest activity is observed in nitro and aldehyde functional groups. All the meta-
carbon tetrachloride. We also observed that a small thesis products were obtained in good to excellent
percentage of CCl4 (25 vol%) can significantly en- yields except p-nitropropynylbenzene, under ambient
hance the reaction conversion in all the above sol- conditions. Half-lives of less than 1 hour were gener-
vents, and in those mixed solvents containing di- ally observed for these model reactions, even with
chloroethane, chloroform or chlorobenzene, the con- catalyst loading as low as 2–3 mol% (based on Mo).
version is comparable to that observed in pure CCl4.
In addition, the efficient inhibition of small alkyne
Table 1 summarizes some model experiments using polymerization by the multidentate-ligand[5a] structur-
the in situ generated catalyst 2, in a closed system al feature was tested with 2-butyne as the substrate,
with periodic exposure of the reaction solution to which is the metathesis by-product of propynyl sub-
vacuum to remove the 2-butyne by-product.[8] The strates. Indeed, as hypothesized, even in the presence
scope of the metathesis activity was probed with vari- of a large excess of 2-butyne (>100 equiv.), catalyst 2
ous substrates: (i) containing electron-donating/elec- did not show any polymerization even after 24 h.
tron-withdrawing substituents, (ii) heterocyclic mole- However, the catalyst generated from phenol, the cor-
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responding monodentate analogue, showed significant
alkyne polymerization that occurred within 1 h after
exposure to 2-butyne. This result is consistent with
our previous observation,[5a] and clearly shows block-
À
ing of N Mo coordination through N-quarternization
can significantly enhance the catalyst activity of these
Mo-triphenolamine complexes without sacrificing
their capability of resisting the alkyne polymerization
side reaction.
The observed significant difference in the catalytic
activity of complex 1 vs. 2 prompted us to systemati-
cally vary the electrophilicity of the catalyst by substi-
Figure 1. Generic structure of porphyrin substrates used for
alkyne metathesis.
tuting the ligand with different electron withdrawing date, most literature reports utilizing the palladium-
substitutents (e.g., catalysts 3 and 4). The activities of catalyzed cross-coupling for the polymer synthesis.
catalysts 2, 3 and 4 were compared side-by-side However, it is well known that a common side reac-
through a kinetic study (Supporting Information, Fig- tion in poly(aryleneethynylene) (PAE) synthesis via
ure S8) by using 4-nitropropynylbenzene as the sub- the cross-coupling approach is the formation of buta-
strate, which is one of the most challenging substrates diyne defect sites in the growing polymer chain when
for alkyne metathesis. It turned out that catalysts 3 two terminal acetylenes are coupled.[12] In this con-
and 4 gave much higher conversions, 61% and 64%, text, alkyne metathesis offers a very efficient and
respectively, than catalyst 2 (43%) after 12.6 h. The defect-free synthesis of ethynylene-bridged polymer-
similar trend was also observed for the metathesis of s.[2a]
4-formylpropynylbenzene (Supporting Information,
As a proof-of-concept, a few different porphyrin
Figure S9) and 3-propynylpyridine (Supporting Infor- monomers (5–7) were prepared and subjected to
mation, Figure S10). One-step cyclooligomerization alkyne metathesis (Figure 1, Table 2). For the meta-
of diacetylenecarbazole (Table 1, entry 8) to form the thesis of 5, relatively low conversion was observed
2-D macrocycle was used as another model study to even under high temperature (708C), presumably due
compare the activity of catalysts 2, 3 and 4, which to lower electron density of the propynes directly con-
also showed the superior performance of the nitro- nected to the porphyrin backbone. Metathesis of mo-
substituted tris(arylmethyl)ammonium catalyst (4, nomer 6 gave only short oligomers of <5 repeating
72% yield compared to 59% and 68% for 2 and 3 units (observed in the soluble fraction), and most of
after 10 min reaction, Supporting Information, the reaction mixtures precipitated out during the re-
Table S1). These kinetic studies demonstrate the im- action, due to their insufficient solubility. In great
portance of electrophilicity of the metal center (or contrast, when monomer 7 that contains six long solu-
electron-withdrawing group substitution on the bilizing chains was used, the solution stayed homoge-
ligand) to the enhancement of the catalyst activity.
neous and porphyrin polymers with high molecular
The multidentate catalyst 2 also showed much weight (Mn =12,100, PDI=1.50) were obtained. The
higher stability than its corresponding monodentate 13C NMR spectrum showed only a single type of
analogue. The metathesis activity of these two cata- alkyne signal (Figure 2) that corresponds to the car-
lysts at different time intervals after their in situ gen- bons in the ethynylene repeating units, thus support-
eration (in the absence of substrates) was compared, ing the defect-free structure of the porphyrin poly-
with 4-propynylanisole as the substrate. Catalyst 2 mer. This result opens many new possibilities for the
showed a comparable activity (<10% decrease) even synthesis and study of porphyrin-based aryleneethy-
after 24 h while the catalyst generated from simple nylene polymers.
phenol ligands showed activity only within the first
few hours, which is consistent with our previous ob- tris(arylmethyl)amine ligand could dramatically en-
servations.[5a]
hance the activity of this novel class of multidentate
In summary, quarternization of the nitrogen in the
Given the high metathesis activity, strong resistivity alkyne metathesis catalysts, due to the decrease of the
to alkyne polymerization, and great functional group electron density on the metal center. These N-quar-
tolerance of these new N-quarternized multidentate ternized catalysts also show high robustness and
Mo(VI) catalysts, we further studied their potential in strong resistence to the alkyne polymerization side re-
the synthesis of porphyrin-based aryleneethynylene action. The high functional group tolerance, fast reac-
polymers, in view of their potential electrical and op- tion rate and high stability represent great advantages
toelectronic applications.[10] The ethynylene-bridged of these newly developed multidentate alkyne meta-
porphyrin polymers are good candidates for molecu- thesis catalysts. In addition, the high activity of these
lar electronics due to their high efficiency for trans- new catalysts has enabled, for the first time, the syn-
porting charge over long distances.[11] However, to thesis of ethynylene-bridged porphyrin-based aryle-
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Highly Active Multidentate Alkyne Metathesis Catalysts: Ligand-Activity Relationship
Table 2. Mo(VI)-catalyzed metathesis of porphyrin substrates.[a,b]
Entry
Substrate
Product
Yield [%]
40
1
monomer 5
monomer 6
polymer 8
polymer 9
2
90
63
3
monomer 7
polymer 10
[a]
Reaction conditions: 408C and 5 mol% per site catalyst 4 loading for all entries, reaction times: 4–18 h.
In a closed system (solvent CCl4) with the reaction solution exposed to vacuum for 6–8 times to remove the metathesis
[b]
by-product 2-butyne.
Experimental Section
General Procedure for Metathesis Experiments
The ligand and the precursor were premixed in dry carbon
tetrachloride for 2–4 min to initiate the in situ generation of
the catalyst. Subsequently, the substrate was added and the
stirring was continued with regular monitoring of the reac-
tion by NMR. During the reaction, the solution was exposed
to vacuum (~6–8 times, 20 sec each time) to remove the
metathesis by-product 2-butyne. The loss of solvent during
the application of vaccum was compensated by adding fresh
solvent each time. For purification of the metathesis reac-
tion products of entries 1–7, the solvent was removed by
rotory evaporation and the residue obtained was subjected
to column chromatography over silica gel. For the precipita-
Figure 2. 13C NMR in C6D6 of monomer 7 and polymer 10.
tion-driven metathesis reaction of entry 8, the reaction mix-
ture was filtered to remove the by-product diarylalkyne and
the filtrate was concentrated and subjected to column chro-
matography over silica gel. For characterization of all
known metathesis substrates and products, the analytical
and spectroscopic data were compared with those of litera-
ture reported values.
neethynylene polymers via alkyne metathesis, which
represents a highly efficient, defect-free, viable ap-
proach for the synthesis of this class of intriguing
polymers. Currently, development of novel alkyne
metathesis catalysts containing non-tris(arylmethyl)a-
mine-based, multidentate ligands, and their applica-
tions in construction of well-defined 2D and 3D mo-
lecular architectures are being pursued in our lab, and
will be reported in due course.
Acknowledgements
The authors acknowledge National Science Foundation
(DMR-1055705) for the funding support and Dr. Haishen
Yang for the help with the kinetic studies.
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Highly Active Multidentate Alkyne Metathesis Catalysts:
Ligand-Activity Relationship and Their Applications in
Efficient Synthesis of Porphyrin-Based Aryleneethynylene
Polymers
7
Adv. Synth. Catal. 2012, 354, 1 – 7
Kuthanapillil Jyothish, Qi Wang, Wei Zhang*
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