Stereospecific Ring-Opening Metathesis Polymerization of Norbornene Catalyzed by Iron Complexes

Article abstract of DOI:10.1002/anie.202011150

Developing well-defined iron-based catalysts for olefin metathesis would be a breakthrough achievement in the field not only to replace existing catalysts by inexpensive metals but also to attain a new reactivity taking advantage of the unique electronic

Full text of DOI:10.1002/anie.202011150

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Accepted Article
Title: Stereospecific Ring-Opening Metathesis Polymerization of
Norbornene Catalyzed by Iron Complexes
Authors: Dmitry S. Belov, Logesh Mathivathanan, Melanie J. Beazley,
William Blake Martin, and Konstantin Bukhryakov
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202011150
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COMMUNICATION
Stereospecific Ring-Opening Metathesis Polymerization of
Norbornene Catalyzed by Iron Complexes
[
a]
[a]
[b]
[c]
Dmitry S. Belov, Logesh Mathivathanan, Melanie J. Beazley, William Blake Martin,
[
a]
and Konstantin V. Bukhryakov*
[a]
Dr. S. Belov, Dr. L. Mathivathanan, Dr. K. V. Bukhryakov
Department of Chemistry and Biochemistry
Florida International University
11200 SW 8th St., Miami, FL 33199, USA
E-mail: kbukhrya@fiu.edu
Dr. M. J. Beazley
Department of Chemistry
University of Central Florida
[
b]
c]
4111 Libra Dr., Orlando, FL 32816, USA
[
W. B. Martin
Department of Macromolecular Science and Engineering
Case Western Reserve University
2100 Adelbert Road, Cleveland, OH 44106, USA
Supporting information for this article is given via a link at the end of the document
Abstract: Developing well-defined iron-based catalysts for olefin
metathesis would be a breakthrough achievement in the field not only
to replace existing catalysts by inexpensive metals but also to attain
a new reactivity taking advantage of the unique electronic structure of
the base metals. Here, we present two-coordinate homoleptic iron
However, this method produces atactic polymers with low
cis/trans selectivity.[17]
Developing a new catalyst for ROMP based on Fe is highly
desirable for four reasons: 1) Fe is the most common transition
metal in the Earth’s crust,[18] and far less expensive than the
precious metals that are currently used; 2) purification, isolation,
and recycling of rare metals consume much energy and generate
2 3 6 2 2 6 3
complex Fe(HMTO) [HMTO = O-2,6-(2,4,6-Me C H ) C H ] capable
of performing ring-opening metathesis polymerization of norbornene
to produce highly stereoregular polynorbornene (99% cis,
a
tremendous amount of waste, leaving an enormous
syndiotactic). The use of heteroleptic Fe(HMTO)(RO) [RO
CH CF CO, CH (CF CO, or Ph(CF CO] prepared in situ,
significantly increases the polymerization rate while preserving
=
environmental footprint. Using Fe would significantly decrease the
energy usage and waste generation making many industrial
processes more environmentally-friendly;[19] 3) Fe is incorporated
in many biological systems and hence has relatively low toxicity,
which is important in pharmaceutical, medicinal, and food
industries;[20] 4) the development of Fe catalysts will diversify our
synthetic tools and may lead to new discoveries in the future.
Fe has attracted much attention since a well-defined Ru
catalyst for olefin metathesis was developed. Grubbs attempted
(
3
)
2
3
3
3
)
2
3 2
)
1
13
selectivity. The resulting polymers were characterized by H and
C
NMR spectroscopy and gel permeation chromatography.
Ring-opening metathesis polymerization (ROMP) of strained
cyclic olefins is a widely applied method for the synthesis of useful
polymers.[1] Starting as
a
mysterious reaction where
3 2 2
the synthesis of Fe complexes analogous to (PCy ) Cl Ru=CHPh
polymerization of norbornene (NBE) catalyzed by ill-defined Ti
complexes that led to an unsaturated polymer seven decades
ago,[2] it is now a well-understood chemical transformation based
on a robust mechanism involving transition metal alkylidene
complexes.[3] The first well-defined catalyst for polymerization of
NBE contained Ti alkylidene was introduced by Grubbs in 1986.[4]
Shortly afterward, Schrock reported the first Ta alkylidene catalyst
for ROMP, in 1987.[5] Nowadays, commonly used homogeneous
catalysts for ROMP of NBE are based on well-defined Ru,[6] Mo,
and W[7-9] alkylidenes, which allow for the synthesis of highly
stereoregular polyNBE.
in 2001 using diazoalkanes, but instead of the formation of iron
alkylidene, phosphazine complexes were formed.[21] To the best
2 2
of our knowledge, no direct Fe analog of L Cl Ru=CRR’ is known,
where R = H, Alk, or Ar and L = phosphine or N-heterocyclic
carbene (NHC). This can be explained by a weak iron-carbene
bond; a theoretical calculation has indicated that the bond
dissociation energy (BDE) of Fe=CH
approximately one-half of the BDE of Ru=CH
NHC)(PH )Cl Ru=CH
2
in (NHC)(PH
3
)Cl
2
Fe=CH
2
is
in
2
(44.5 vs. 87.7 kcal/mol).[22] Additionally, it
(
3
2
2
has been shown that the formation of metallacyclobutane, the
crucial intermediate in olefin metathesis, in the case of
Homogeneous complexes of other transition metals are also
capable of performing ROMP of NBE, including third-row
transition metals, such as Re[10] and Os;[11] second-row Zr[12] and
Nb;[13] and first-row V.[14]Attempts to use complexes based on
other first-row transition metals, such as Fe, Co, and Ni, in the
reaction with NBE lead to the formation of saturated polymers
through addition polymerization.[15] An alternative approach to
obtain polyNBE is metal-free photoredox mediated ROMP, which
uses an organic redox mediator and a vinyl ether initiator.[16]
(
(
NHC)(PH
3
)Cl
)Cl
2
2
Fe=CH is an endothermic process, unlike for
.[22]
NHC)(PH
3
2
Ru=CH
2
Furthermore, other calculations
Cl Fe=CRR’ should mainly favor
indicate that complexes L
2
2
alkene cyclopropanation over olefin metathesis, both
thermodynamically and kinetically.[23] Another theoretical work
2
has shown that (NHC)Cl Fe=CHPh has an open-shell triplet
ground state at the metal center and could be prone to radical
chemistry rather than to olefin metathesis.[24] However, other
1
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Angewandte Chemie International Edition
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COMMUNICATION
calculations suggest that complexes L
olefin metathesis.[25]
2
Cl
2
Fe=CRR’ can promote
Nine carbene precursors (2-10, Table 1) have been tested in
the presence of 1 and NBE in benzene at room temperature:
Despite the extensive theoretical[22-27] and experimental
studies[15, 21, 28, 29] to the best of our knowledge, there are no
successful examples of iron-catalyzed olefin metathesis. Here,
we describe a Fe-based catalytic system capable of ROMP of
NBE to produce highly stereoregular polyNBE (99% cis,
syndiotactic).
diazocompounds
precursor),[36] 5 (:CBr
(vinylidene precursor),[37] cyclopropene 7 (alkylidene precursor for
Ru),[38] phosphorus ylide 8 (alkylidene precursor for W),[39]
cycloheptatriene 9 (carbene precursor for Au),[40] and norbornene
oxide 10 (alkylidene precursor for Ru).[41]
2
and 3, zwitterion
4
(:CF
carbene
2
2
carbene precursor), terminal acetylene 6
As a starting point, we tested the possibility of in situ formation
of (HMTO)
the presence of NBE. ROMP of NBE would indicate the formation
of catalytically active alkylidene. Fe(HMTO) was chosen for three
2 2
Fe=CRR’ from Fe(HMTO) and carbene precursors in
Table 1. Test of carbene precursors for ROMP of NBE.
10 mol % 1
2
reasons: 1) bulky 2,6-terphenoxide ligands allow the synthesis of
low-coordinate Fe(II) complexes[30, 31] without neutral ꢀ-donor
ligands, to avoid complications in the alkylidene formation;[21, 28] 2)
10 mol % additive
C6D6, 22 ºC, 24 h
n
NBE
additives:
polyNBE
TMS
monodentate ligands facilitate the required geometry for
N
2
N2
COO
F2C
CHBr +
LiN(SiMe3)2
3
intermediates. It has been shown for Mo and W that rigid
bidentate[32] and tridentate[33] ligands prevent access to trigonal
EtOOC
Ph
Ph
PPh
3
2
3
4
5
6
bipyramidal metallacyclobutane complex, a crucial intermediate
for the cycloreversion step. 3) Mo and W-based alkylidene
complexes containing 2,6-terphenoxide ligands have been
extensively used in olefin metathesis for the past few years[34] and
exhibit unprecedented reactivity and selectivity.[35]
Ph Ph
PPh3
O
O
7
8
O
9
10
entry
additive
polyNBE, NMR conversion, %
Fe(HMTO)
2
(1) was synthesized from Fe[N(SiMe
3
)
2
]
2
and two
equivalents of HMTO-H in good yield (Scheme 1).
1
2
none
16
5
2
3
OH
Mes
Fe
3
10
15
5
O
4
4
O
HMTO-H
Mes
Fe[N(SiMe ) ]
Mes
3
2 2
toluene, 110 ºC, 16 h
Mes
5
5
1
, 81%
6
6
10
11
<1
15
5
Scheme 1. Synthesis of Fe(HMTO)
2
(1).
7
7
8
8
Compound 1 is a monomeric two-coordinate Fe complex, as
determined by single-crystal X-ray diffraction (Figure 1). The bent
structure of 1 (O1-Fe1-O2, 142.8(4)°) is in sharp contrast to the
9
9
10
10
linear structure of Fe(TIPTO)
Pr
] where O1-Fe1-O2 is 180°.[30] The bond distances
Fe1-O1 (1.849(9) Å), Fe1-O2 (1.855(9) Å) and angles Fe1-O1-C1
130.7(8)°), Fe1-O2-C25 (124.8(7)°) in 1 are similar to those in
Fe(TIPTO) [1.847(1) Å, 129.33(8)°]. The Fe1-C17, 2.569(9) Å,
2
[TIPTO
=
O-2,6-(2,4-i-
We were somewhat surprised to find that 1 can catalyze
ROMP of NBE. Additives do not improve polymerization, which
suggests the formation of active species in the presence of NBE
2
6 3 2 6 3
C H ) C H
(
only. Furthermore, the resulting polymer (entry 1, Table 1) is 99%
cis, syndiotactic.[8]
2
and Fe1-C31, 2.507(7) Å, distances indicate weak secondary
interactions between Fe and carbon atoms in mesityl rings.
Complex 1 has been analyzed by ICP-MS to eliminate the
possibility that trace quantities of adventitious metals catalyze
ROMP. The content of Ru (< 1 ppt), Mo (0.008 ppm), W (0.01
ppm), and V (0.88 ppm) is insignificant to affect polymerization
Magnetic measurements showed that 1 has a ꢁeff of 5.32 ꢁ
B
,
6
corresponding to the high-spin d configuration.
(
see SI for details and additional control experiments).
The reaction of 1 with NBE was tested in the presence of
alcohols to examine how the formation of mixed Fe alkoxides in
situ can affect ROMP. The results are summarized in Table 2.
Mixed Fe alkoxides that contain HMTO and fluorinated alkoxides
(
3 2 3 3 3 2 3 2
CH ) CF CO, CH (CF ) CO, or Ph(CF ) CO improve the
reaction rate with constant selectivity (entry 3, 4, and 9, Table 2).
Interestingly, the use of fluorinated alcohol with hydrogen at ꢂ-
carbon atomꢃ(CF
, Table 2). The most efficient catalyst is the mixed alkoxide
Fe(HMTO)(Ph(CF CO) (entry 9, Table 2), which was confirmed
by H NMR, thus, when 1 reacted with 1 equiv. of Ph(CF COH
3 2
) CHOH leads to a decrease of selectivity (entry
8
3 2
)
1
Figure 1. Molecular structure of 1 with the displacement ellipsoids drawn at 30%
probability level. Hydrogen atoms are not shown for clarity.
3 2
)
2
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COMMUNICATION
in C
6
D
6
at 22 °C, the proton signals of a new complex along with
polymer with a conversion comparable to the reaction in benzene
free HMTO-H signals immediately appeared (see SI for details).
The resulting polymers have high molecular weights (MW)
and polydispersity indexes (PDI) based on gel permeation
chromatography (GPC), which suggests that the rate of the
propagation is higher than that of initiation. The formation of the
active species in the presence of NBE is a relatively slow process,
as a result, most of the Fe complex stays intact during the ROMP,
which was confirmed by 1H NMR. The reaction of
3 2
(entry 12, Table 2). Surprisingly, Fe(HMTO)(Ph(CF ) CO) can
catalyze ROMP of NBE at −35 °C in toluene. The reaction is
slower but gives a higher MW compare to the reaction at 22 °C.
This observation confirms that the stability of the active species
decreases at higher temperatures (entry 11-13, Table 2). Notably,
the nature of the active species remains the same since the highly
stereoregular polymers are obtained in all cases. The reaction of
3 2
Fe(HMTO)(Ph(CF ) CO) with NBE in benzene at 22 °C at a
Fe(HMTO)(Ph(CF
3
)
2
CO) with NBE in benzene at 70 °C produces
shorter reaction time (4 and 14 hours, entry 14 and 15, Table 2)
produces a larger polymer with lower PDI. We assume that the
formed active species have a short lifetime and, presumably,
decompose after termination. Newly formed active species
produce smaller polymer due to decreasing of the concentration
of NBE, leading to growing PDI over time.
a smaller polymer with lower yield (entry 10, Table 2) as
compared to the reaction at 22 °C suggesting decomposition of
the active species at a higher temperature. The stability of the
active species on the growing polymer chain depends on the
solvent. Thus, the reaction in toluene at 22 °C produces a smaller
Table 2. Test of additives, solvents, and temperature for ROMP of NBE.
1
0 mol % 1
0 mol % additive
C D , 22 ºC, 24 h
1
n
6
6
NBE
cis,syndiotactic polyNBE
entry
additive
none
temp., ºC
conv., %[a]
cis/trans
99:1
-
tacticity
M
n
(g/mol) [b]
w n
M /M
1
2
3
4
5
6
7
8
9
22
22
22
22
22
22
22
22
22
70
−35
22
100
22
22
22
22
22
22
17
syndiotactic
262800
2.33
-
(CH
(CH
CH
(CF
allyl(CF
(CH
(CF
Ph(CF
Ph(CF
3
)
3
COH
0
42
48
20
3
-
-
663400
470600
625000
-
3
)
2
CF
3
2
COH
COH
>99:1
>99:1
>99:1
-
syndiotactic
2.05
2.29
2.11
-
3
(CF
3
)
syndiotactic
3
)
3
COH
COH
syndiotactic
3
)
2
-
3
)
2
CHOH
0
-
-
-
-
3
)
2
CHOH
42
84 (81) [c]
28
43
75
10
62
74
15
0
91:9
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
-
syndiotactic
syndiotactic
syndiotactic
syndiotactic
syndiotactic
syndiotactic
syndiotactic
syndiotactic
-
590000
930700
362100
565700
416000
82300
1141800
1038700
-
2.22
1.97
2.33
2.36
2.66
3.31
1.57
1.65
-
3
)
2
COH
10
11
12
13
14
15
16
17
18
19
3
)
2
2
2
2
2
COH
Ph(CF
Ph(CF
Ph(CF
Ph(CF
3
)
)
)
)
COH[d]
COH[d]
COH[d]
COH[e]
3
3
3
3 2
Ph(CF )
COH[f]
3,4-dihydro-2H-pyran
benzaldehyde
-
-
-
-
acetophenone
7
-
-
-
-
benzophenone
12
-
-
-
-
1
[
a] by H NMR. [b] GPC data in THF vs. polystyrene standards. [c] Isolated yield. [d] Reaction in toluene-d
8
. [e] 4h. [f] 14h.
We have attempted to elucidate the mechanism of Fe
The first considered mechanism involves the formation of Fe
alkylidene in the presence of NBE in situ. According to
computational studies,[23, 26] Fe alkylidene and metallacyclobutane
of the active catalyst are low spin complexes. However, the
formation of low spin, diamagnetic species, was not detected by
catalyzed ROMP of NBE. Three possible mechanisms were
considered: involving Fe alkylidene, photoredox mediated ROMP,
and carbonyl-olefin metathesis.
3
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COMMUNICATION
NMR during the catalysis in our case. The addition of cyclooctene
or octa-1,7-diene (see SI for the full list of tested substrates)
during the reaction does not lead to the formation of
polycyclooctene or cyclohexene, which one can expect to
observe if the reaction of 1 and NBE produced an active Fe
alkylidene. The origin of the tacticity of polyNBE is an essential
question in the Fe catalyzed ROMP involving alkylidenes. In the
case of Mo and W catalysts, the formation of cis, syndiotactic
polymers is achieved through stereogenic metal control, where
[1]
a) L. Delaude, A. F. Noels, Kirk‐Othmer Encyclopedia of Chemical
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[
2]
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W. L. Truett, D. R. Johnson, I. M. Robinson, B. A. Montague, J. Am.
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[
a) Y. Chauvin, Angew. Chem. Int. Ed. 2006, 45, 3740-3747; b) R. H.
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[4]
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[6]
L. R. Gilliom, R. H. Grubbs, J. Am. Chem. Soc. 1986, 108, 733-742.
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J. Am. Chem. Soc. 2012, 134, 2040-2043.
[
the inversion of the configuration at the chiral four-coordinated
metal center proceeds with each insertion.[7] The same principle
does not apply to the mononuclear Fe alkylidene formed from
NBE and 1 containing two identical HMTO groups. The formation
of polynuclear iron clusters that contain chiral metal center seems
plausible explanation.
[
7]
8]
R. R. Schrock, Acc. of Chem. Res. 2014, 47, 2457-2466.
B. Autenrieth, R. R. Schrock, Macromolecules 2015, 48, 2493-2503.
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3152; b) R. R. Schrock, Dalton Trans. 2011, 40, 7484-7495.
[
[9]
The second mechanism is the redox-initiated ROMP
mentioned previously.[16] This mechanism involves oxidation of a
vinyl ether to form radical cation to initiate ROMP in the presence
of a photoredox mediator. The addition of 3,4-dihydro-2H-pyran
does not improve the reaction rate (entry 16, Table 2).
Furthermore, redox-initiated ROMP produces atactic polymers
due to the radical intermediates, which is not the case in our
system. It is noteworthy, we have never observed any evidence
of radical reactions, e.g., alkene cyclopropanation or radical olefin
polymerization. Additionally, the presence of light does not affect
the ROMP of NBE with 1.
[10] F. Christian Manfred, B. Olivier, B. Heinz, Pure Appl. Chem. 2006, 78,
877-1887.
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[
1
Sanctis, J. Organomet. Chem. 2001, 622, 43-46.
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[
The third mechanism is based on the recently developed
carbonyl-olefin metathesis (COM),[42] where Fe complex acts as a
Lewis acid. Although Fe-mediated COM has not been applied for
ROMP of NBE yet,[43] we tested the possibility of this mechanism
by adding carbonyl compounds to the reaction (entry 17-19, Table
[14] a) K. Nomura, A. Sagara, Y. Imanishi, Macromolecules 2002, 35, 1583-
1590; b) K. Nomura, K. Suzuki, S. Katao, Y. Matsumoto, Organometallics
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2
). However, no improvement of ROMP was observed,
[15] M. C. Sacchi, M. Sonzogni, S. Losio, F. Forlini, P. Locatelli, I. Tritto, M.
suggesting that this mechanism is implausible.
Licchelli, Macromol. Chem. Phys. 2001, 202, 2052-2058.
[
16] K. A. Ogawa, A. E. Goetz, A. J. Boydston, J. Am. Chem. Soc. 2015, 137,
400-1403.
Our studies demonstrate that Fe-based alkoxide complexes
can catalyze ROMP of NBE to produce highly stereoregular
polyNBE (>99% cis, syndiotactic). The use of heteroleptic
1
[17] L. M. M. Pascual, A. E. Goetz, A. M. Roehrich, A. J. Boydston, Macromol.
Rapid Commun. 2017, 38, 1600766.
Fe(HMTO)(RO), where RO is (CH
Ph(CF CO prepared in situ, significantly increases the rate of
the reaction compared to homoleptic Fe(HMTO) while preserving
3 2 3 3 3 2
) CF CO, CH (CF ) CO, or
[
18] W. F. McDonough, S. s. Sun, Chemical Geology 1995, 120, 223-253.
19] P. Chirik, R. Morris, Acc. of Chem. Res. 2015, 48, 2495-2495.
3 2
)
[
2
[20] I. Bauer, H.-J. Knölker, Chemical Reviews 2015, 115, 3170-3387.
[21] J. Louie, R. H. Grubbs, Organometallics 2001, 20, 481-484.
[22] M. Vasiliu, A. J. Arduengo, D. A. Dixon, J. Phys. Chem. C 2014, 118,
stereoselectivity. The formation of polyNBE with high MW and PDI
indicates that the rate of propagation is higher than the rate of
initiation. The stability of active species strongly depends on the
solvent and temperature. However, the nature of the active
catalyst remains unclear. The reaction may involve polynuclear
iron clusters and/or a yet unknown mechanism, and requires
further investigations.
13563-13577.
[
[
[
23] É. de Brito Sá, L. Rodríguez-Santiago, M. Sodupe, X. Solans-Monfort,
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[
26] a) É. de Brito Sá, L. Rodríguez-Santiago, M. Sodupe, X. Solans-Monfort,
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Entry for the Table of Contents
Fe(HMTO)2
benzene 22 ºC, 24 h
n
cis,syndiotactic polyNBE
NBE
Fe(HMTO)2
Homoleptic iron complex catalyzes ring-opening metathesis polymerization of norbornene to yield highly stereoregular
polynorbornene (99% cis, syndiotactic). The influence of solvent, temperature, and additives was examined.
6
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