Trialkylaluminum N-Heterocyclic Olefin (NHO) Adducts as Catalysts for the Polymerization of Michael-Type Monomers

Article abstract of DOI:10.1002/zaac.201900331

The synthesis of new trialkylaluminum adducts with N-heterocyclic olefin (NHO) ligands is described. These well-defined complexes can catalyze the polymerization of various Michael-type monomers, such as 2-vinylpyridine, methylacrylate, and dimethylacryla

Full text of DOI:10.1002/zaac.201900331

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201900331 SHORT COMMUNICATION
Zeitschrift für anorganische und allgemeine Chemie
Trialkylaluminum N-Heterocyclic Olefin (NHO) Adducts as
Catalysts for the Polymerization of Michael-Type Monomers
Ian C. Watson,[a] Yuqiao Zhou,[a] Michael J. Ferguson,[a] Moritz Kränzlein,[b] Bernhard Rieger,*[b] and
Eric Rivard*[a]
Dedicated to Prof. Dr. Manfred Scheer on the Occasion of his 65th Birthday
Abstract. The synthesis of new trialkylaluminum adducts with N-hetero-
cyclic olefin (NHO) ligands is described. These well-defined complexes
can catalyze the polymerization of various Michael-type monomers, such
as 2-vinylpyridine, methylacrylate, and dimethylacrylamide.
son and Masuda, respectively (Figure 1).[8] We also found that
one of our newly prepared NHO·AlR3 complexes is a compe-
tent catalyst for the polymerization of Michael-type monomers
at room temperature.
Introduction
N-Heterocyclic olefins (NHOs), first reported by Kuhn in
the mid 1990s,[1] are an emerging class of ylidic carbon-based
donors that have attracted recent attention due to their ability
to stabilize various reactive main group species.[2] Related to
N-heterocyclic carbenes (NHCs), NHOs feature an exocyclic
alkylidene unit (=CR2) attached to a heterocyclic imidazole
ring, and are good σ-donating ligands but lack the ability to
act as π acceptors.[3]
NHOs have also been explored as organocatalysts/initiators
within the realms or synthetic organic and polymer chemis-
try.[4] For example, Naumann and co-workers showed that
NHOs can be used as initiators in the polymerization of di-
methylacrylamide (DMAA).[4c] Furthermore, Chen and co-
workers demonstrated that NHO·Al(C6F5)3 Lewis pairs are
capable of polymerizing lactones and challenging Michael-
type monomers, such as crotonates;[5] in addition, the Lu group
showed that methylmethacrylate (MMA) can be polymerized
with NHO·Al(C6F5)3 complexes as initiators.[6] In related
work, Chen and co-workers used NHC·AlR3 adducts to poly-
merize methylmethacrylate.[7]
Figure 1. Examples of trimethylaluminum adducts with N-heterocy-
clic donors.
Results and Discussion
The first NHO·AlR3 adduct in our series, MeIPrCH2·AlMe3
(1), was obtained by allowing MeIPrCH2 to combine with one
equivalent of AlMe3 in toluene at room temperature
(Scheme 1). Encouraged by these results, we performed a sim-
ilar reaction between MeIPrCH2 and AlEt3, leading to the for-
mation of the monoadduct MeIPrCH2·AlEt3 (2). Upon binding
of MeIPrCH2 to either AlMe3 or AlEt3, an upfield shift in the
1H NMR signals (in C6D6) is observed relative to the free
NHO. Specifically, the exocyclic CH2 resonance shifts from
2.33 ppm in free MeIPrCH2 to values of 2.01 ppm and
In this communication we report the preparation of
NHO·AlR3 adducts, such as MeIPrCH2·AlMe3 (Figure 1)
[
MeIPrCH2 = (MeCNDipp)2C=CH2; Dipp = 2,6-iPr2C6H3].
These complexes are structurally related to the NHC or N-
heterocyclic imine (NHI) adducts made previously by Robin-
* Prof. Dr. B. Rieger
1
E-Mail: rieger@tum.de
1.89 ppm in adducts 1 and 2, respectively. The H NMR reso-
* Prof. Dr. E. Rivard
nances belonging to the AlR3 moieties in 1 and 2 are also
upfield-shifted in comparison to the uncomplexed alanes
AlMe3 and AlEt3; for example, the methyl resonance for the
AlMe3 group in 1 is found at δ = –0.52 ppm in C6D6, while the
corresponding resonance for AlMe3 in C6D6 is –0.37 ppm.[9]
Figure 2 shows the structure of MeIPrCH2·AlMe3 (1), as de-
termined by X-ray crystallography, while the structure of
MeIPrCH2·AlEt3 (2) is found in Figure S3 (Supporting Infor-
E-Mail: erivard@ualberta.ca
[a] Department of Chemistry
University of Alberta
11227 Saskatchewan Drive
Edmonton, Alberta, T6G 2G2, Canada
[b] Catalysis Research Center & WACKER-Chair of Macromolecular
Chemistry
Technical University of Munich
Lichtenbergstrasse 4
85748 Garching, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201900331 or from the au-
thor.
mation).[9] The coordinative CNHO–Al bond in
2.1198(13) Å, and is similar in length as the CNHC–Al distance
1 is
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an alternate coordination mode would be possible when com-
plexes were formed with less hindered Lewis acids, such as
AlMe3. Upon combining MeIPr=CH–CH=CH2 with AlMe3 in
a 1:1 ratio, the corresponding adduct MeIPrCHCHCH2·AlMe3
(3) was obtained [Equation (1)].
Scheme 1. Preparation of MeIPrCH2·AlMe3 (1) and MeIPrCH2·AlEt3 (2).
(1)
found in Robinson’s NHC·AlMe3 adduct [2.124(6) Å] in Fig-
ure 1,[8a] whereas longer than the coordinative NNHI–Al inter-
action in Masuda’s NHI·AlMe3 complex [1.9648(19) Å] (Fig-
ure 1).[8b] The latter observation follows a general trend of
shorter ligand-element bonds with N-heterocyclic imine (NHI)
adducts in comparison to NHO-element bonds.[10] While the
carbene adduct IPr·AlEt3 (IPr = [(HCNDipp)2C:]) has been
previously synthesized by Dagorne in 2017,[11] a crystal struc-
ture has not been reported, obviating the chance to directly
compare its structure with MeIPrCH2·AlEt3 (2).
X-ray crystallography (Figure 3) revealed a similar terminal
NHO-AlMe3 binding mode was present as in our Pd com-
plexes.[12] As with MeIPrCH2·AlMe3 (1), a diagnostic upfield
shift in the methyl resonance for the AlMe3 group in 3 was
found (to a value of –0.42 ppm in C6D6). The formally dative
Al–CNHO distance of 2.1135(13) Å in 3 (Al–C4; Figure 3) is
similar to the corresponding Al–CNHO interaction in
MeIPrCH2·AlMe3 (1) [2.1198(13) Å]. We believe that the ter-
minal olefin coordination mode in 3 is adopted for two
reasons: (1) the steric bulk close to the heterocycle makes
binding to the proximal exocyclic carbon (C2 in Figure 3) dif-
ficult, and (2) Natural Population Analysis (NPA)[12] of
MeIPr=CH–CH=CH2 showed a more negative charge at the ter-
minal carbon compared to the one adjacent to the MeIPr unit
(–0.53 e– vs. –0.46 e–), making the terminal site slightly more
Lewis basic.
Figure 2. Molecular structure of MeIPrCH2·AlMe3 (1) with thermal
ellipsoids shown at a 30% probability level. All hydrogen atoms ex-
cept those on C6 are omitted for clarity. Selected bond lengths /Å and
angles /°: C1–C6 1.4439(17), C6–Al1 2.1198(13), Al1–C8 1.9925(16),
C1–C6–Al1 130.01(9), C7–Al1–C8 109.59(7).
MeIPr=CH–CH=CH2, an allyl-appended NHO with two po-
tential sites to accommodate a Lewis acid, has been reported
by our group (Scheme 2).[12] In our previously described palla-
dium complexes with this ligand, only evidence of coordina-
tion via the terminal exocyclic carbon atom was found, pre-
sumably due to the steric crowding imparted by the flanking
Dipp groups in MeIPr=CH–CH=CH2.[12] Thus we wondered if
Figure 3. Molecular structure of MeIPrCHCHCH2·AlMe3 (3) with ther-
mal ellipsoids shown at a 30% probability level. Co-crystallized tolu-
ene solvate and all hydrogen atoms besides those on C2, C3 and C4
are omitted for clarity. Selected bond lengths /Å and angles /°: C1–C2
1.4205(16), C2–C3 1.3665(17), C3–C4 1.4203(17), Al1–C4
2.1135(13); C1–C2–C3 128.34(11), C3–C4–Al1 116.12(9).
The new NHO·AlR3 adducts 1–3 are only sparingly soluble
in C6D6, which made the acquisition of 13C{1H} NMR spectra
a challenge. When 1–3 were dissolved in [D8]THF (to possibly
obtain more intense 13C{1H} NMR resonances), the dissoci-
ation of these adducts into free NHO and AlR3 was found.[9,13]
Motivated by prior work[4c,6,7,14] involving the use of Frus-
trated Lewis pairs (FLPs) as initiators, we decided to investi-
Scheme 2. Important resonance forms associated with MeIPr=CH–
CH=CH2, illustrating two potential sites of coordination.
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gate if 1 could act as a viable polymerization catalyst for diethylvinylphosphonate (DEVP) failed to yield any poly-
Michael-type monomers (Scheme 3).
mer.[17]
The polymerization of 2-vinylpyridine (2VP) with catalytic
1 affords very high molecular weight polymer, with Mn values
exceeding 800 kDa (Table 1). 2VP has also been polymerized
with NHC/Al(C6F5)3 systems (excess Lewis acid) to yield
polymer with molecular weights (Mn) in the 10–80 kDa
range.[18]
Chen has demonstrated that an NHO·AlMe3 Lewis pair can
promote the slow polymerization of methylmethacrylate
(MMA) (38% yield over 12 h),[16] however no reactions were
tried with the less hindered monomer methyl acrylate (MA).[16]
Combining 0.5 mol% of 1 with MA in THF only gave a small
amount of isolated poly(methylacrylate) (ca. 5% yield) with a
low Mn value of 10 kDa (PDI = 2.08; Table 1). To the best of
our knowledge, Lewis pair polymerization has not been used
to polymerize MA prior to our work. Notably, poly(methyacyl-
ate) with a narrow PDI (1.03) and high I* (ca. 80%) was ob-
tained from MA via group transfer polymerization with 1-tri-
isopropylsiloxy-1-methoxy-2-methyl-1-propene (MTSiPr) as an
initiator and C6F5CHTf2 as a catalyst (Tf = SO2CF3).[19] Indus-
trially, the polymerization of acrylic monomers is typically
achieved using radical initiators,[20] however these methods
often yield high PDI values,[20c] making Frustrated Lewis pair
catalysis attractive when smaller PDI values are desired.
We also attempted polymerization trials with MA and 2VP
monomer using MeIPrCHCHCH2·AlMe3 (3) as a catalyst.
However these trials gave disappointingly low yields of poly-
mer (3 and 15% for pMA and p2VP, respectively); thus we
did not explore this catalyst further. The ability of
MeIPrCH2·AlEt3 (2) to act as a catalyst was not investigated
due to the constant presence of a minor amount of unidentified
Scheme 3. (top) The Michael-type monomers investigated in this
study; (bottom) polymerization conditions.
We began our polymerization trials by combining dimethyl-
acrylamide (DMAA) with 0.5 mol-% of 1 in THF. Upon add-
ing 0.5 mol-% of 1 to a stirring solution of DMAA in THF,
we noted a rapid increase in temperature, as is typical for this
type of polymerization. After stirring for 1 h, the solution was
quenched by the addition of ethanol, and the resulting polymer
was isolated and purified via precipitation from a concentrated
solution of the polymer in CH2Cl2 into cold (–30 °C) pentane.
According to gel permeation chromatography (GPC) on the
isolated sample in THF/H2O (with 9 g·L–1 [nBu4N]Br added
to increase the ionic strength), a number average molecular
weight (Mn) of 150 kDa and a polydispersity index (PDI) of
1.18 was found (Table 1). For comparison, the use of 1 as a
catalyst afforded higher molecular weight polymer vs. Nau-
mann’s NHO-only polymerization, with (MeCNMe)2C=CMe2
as an initiator (67% conversion, 2 h, 0.5 mol-% NHO).[4c] Of
note, the expected molecular weight if 1 fully instigated the
living polymerization of DMAA would be ca. 20 kDa (i.e. a
degree of polymerization, DP, of 200); however the higher mo-
lecular weight obtained (150 kDa), while keeping a low PDI,
is consistent with a low effective initiator efficiency (I*).[15]
Another notable system is the polymerization of DMAA by
the NHO-alane adduct [(DippNC(H)C(H)NiPr)C]·Al(C6F5)3,
which transpired in only 4 min, significantly faster than 1, with
a low PDI (ca. 1.05) and an Mn of 170 kDa.[7] This is consis-
tent with the observation that the polymerization of Michael-
type monomers occurs quickly in the presence of strong Lewis
acids.[16]
1
NHO-containing impurity (according to H NMR analysis[9]),
which we were unable to remove via washing or recrystalli-
zation.
To verify that the FLP pair of 1 in THF was performing the
polymerizations, we re-examined the interaction of 2-vinylpyr-
idine (2VP) with both MeIPrCH2 and AlMe3 individually.
When MeIPrCH2 was combined with 2VP under the same con-
ditions outlined in Table 1, no polymerization was detected in
situ by 1H NMR spectroscopy. Likewise treatment of 2VP with
AlMe3 gave no evidence of polymerization by in situ 1H NMR
analysis of the mixture in [D8]THF. Interestingly, when the
mixture of AlMe3 and 2VP was quenched with methanol, we
observed a violent exothermic reaction, as expected upon the
reaction of AlMe3 with alcohol; however somewhat to our sur-
prise, a small amount of poly(2-vinylpyridine) was observed
(6% isolated yield). Knowing that unstabilized 2VP can auto-
polymerize at –20 °C over the course of a week,[20b] we hy-
pothesize that the reflux induced by quenching the mixture
with MeOH is responsible for some polymerization of 2VP.
Table 1. Polymerization of various Michael-type monomers using
MeIPrCH2·AlMe3 0.5 mol-% as an initiator in THF (1 h).
Monomer
Isolated yield /%
Mn /103 kDa
Mw/Mn
DMAA
2VP
MA
Ͼ 99
98
5
150
840
10
1.18
1.35
2.08
DEVP
no polymerization
Knowing that DMAA is polymerized by 1, we then at-
tempted the polymerization of several other monomers
(Scheme 3). Methyl acrylate (MA), 2-vinylpyridine (2VP), and
diethylvinylphosphonate (DEVP) were each combined with 1
Conclusions
We have described the synthesis of the new NHO-trialkylal-
under the same conditions used for DMAA (vide supra). While uminum complexes (1–3) and found that MeIPrCH2·AlMe3 (1)
the polymerizations MA and 2VP were successful (Table 1), showed FLP-type behavior in THF, leading to the polymeriza-
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Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-1968013, CCDC-1968014, and CCDC-1968015
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
tion of several Michael-type monomers under very mild condi-
tions. Future work will involve targeting the synthesis of more
electron deficient and lower coordinate NHO-aluminum spe-
cies bearing anionic NHOs[21] as supporting ligands.
Experimental Section
Synthesis of MeIPrCH2·AlMe3 (1): A solution of AlMe3 (0.210 mL,
2.0 m solution in toluene, 0.42 mmol) was gently layered atop of a
solution of MeIPrCH2 (0.1810 g, 0.420 mmol) in 1.5 mL of toluene.
After allowing the mixture to remain undisturbed for 4 h, colorless
X-ray quality crystals formed. The supernatant was decanted away and
the crystals of MeIPrCH2·AlMe3 (1) were washed with 3ϫ2 mL of
Materials and Instrumentation: All reactions were performed using
standard Schlenk line techniques in an atmosphere of nitrogen or in
an inert atmosphere glovebox (Innovative Technology Inc.). Solvents
were dried using a Grubbs-type solvent purification system manufac-
tured by Innovative Technology Inc., and stored under an atmosphere
of nitrogen over 4 Å molecular sieves prior to use. MeIPrCH2 and
[3]
1
cold (–30 °C) toluene, and dried in vacuo. (0.1428 g, 68%). H NMR
[12]
MeIPr = CH-CH=CH2
were prepared according to literature pro-
3
(500 MHz, C6D6): δ = 7.21 (t, 2 H, JHH = 7.6 Hz, ArH), 7.17 (d, 4
cedures. Trimethylaluminum (2.0 m solution in toluene) and trieth-
ylaluminum (1.0 m solution in hexanes) were purchased from Sigma–
Aldrich and used as received. Dimethylacrylamide (DMMA) and 2-
vinylpyridine (2VP) were purchased from Sigma–Aldrich, distilled
over calcium hydride and freeze-thaw degassed before use. Methyl
acrylate (MA) was purchased from Sigma–Aldrich, washed with a sat-
urated NaOH solution, distilled over calcium hydride, and freeze-thaw
degassed before use. [D8]THF was purchased from Sigma–Aldrich and
distilled from sodium benzophenone, then stored over Na/K before
use. 1H and 13C{1H} NMR spectra were recorded on 400 MHz,
500 MHz, and 700 MHz Varian Inova spectrometers and referenced
externally to SiMe4 (1H, 13C{1H}). Elemental analyses were performed
by the Analytical and Instrumentation Laboratory at the University of
Alberta. Melting points were measured in sealed glass capillaries under
nitrogen using a MelTemp melting point apparatus and are uncor-
rected. GPC measurements for p2VP and pMA were performed at
40 °C using THF as the eluent at a flow rate of 0.5 mL per minute. A
Viscotek VE 2001 autosampler, one Viscotek T6000M column, GPC
270 Max dual detector, and Viscotek VE 3580 refractive index detector
were used for sample analysis and data collection. Multidetector cali-
bration was done using refractive index (RI) detection in conjunction
with low angle light scattering (LALS) and right angle light scattering
(RALS), using 99 kDa polystyrene to create the calibration method
and 235 kDa polystyrene to verify the calibration. GPC measurements
for pDMAA were performed using two PL Polargel columns in
THF:H2O [1:1; v:v] [with 272 mg·L–1 3,5-di-tert-butyl-4-hydroxytolu-
ene (BHT) and 9 g·L–1 tetrabutylammonium bromide (TBAB)] as the
eluent. The determination of the absolute molecular weights was per-
formed with multi-angle light scattering on a Wyatt Dawn Heleos II
instrument equipped with an Wyatt Optilab rEX 536 RI detector for
concentration determination. The dn/dc value for the absolute molecu-
lar weight measurements was determined to be 0.1282 mL·g–1.
3
H, JHH = 7.6 Hz, ArH), 2.80 [m, 4 H, CH(CH3)2], 2.01 (s, 2 H,
3
CCH2AlMe3), 1.38 (s, 6 H, CN–CH3), 1.38 [d, 12 H, JHH = 6.8 Hz,
CH(CH3)2], 0.98 [d, 12 H, 3JHH = 6.8 Hz, CH(CH3)2], –0.52 [s, 9 H, –
Al(CH3)3]. 13C{1H} NMR (125 MHz, C6D6): δ = –4.4 [–Al(CH3)3],
9.6 (H3CCN), 24.5 [CH(CH3)2], 24.6 [CH(CH3)2], 28.8 [CH(CH3)2],
125.3 (ArC), 128.0 (ArC), 128.2 (ArC), 128.4 (ArC), 131.0 (ArC),
145.5 (NCN), 146.9 (CCH2-AlEt3); one of the ArC resonances could
not be observed. C33H55AlN2: calcd. C, 78.84; H, 10.23; N, 5.57%;
found: C, 78.69; H, 10.23; N, 5.46%. Mp (°C): 229 (dec.).
Synthesis of MeIPrCH2·AlEt3 (2): A solution of AlEt3 (1.0 m solution
in hexanes, 1.476 mL, 1.5 mmol) was carefully layered atop of a solu-
tion of MeIPrCH2 (0.6357 g, 1.476 mmol) in 1.5 mL of toluene. After
allowing the mixture to remain undisturbed for 4 h, colorless crystals
of MeIPrCH2·AlEt3 (2) were deposited. The supernatant was then de-
canted away, the remaining crystals washed with 3ϫ2 mL of cold (–
30 °C) hexanes and dried in vacuo (0.6115 g, 75%). 1H NMR
3
(700 MHz, C6D6): δ = 7.22 (t, 2 H, JHH = 7.8 Hz, ArH), 7.12 (d, 4
3
3
H, JHH = 7.8 Hz, ArH), 2.70 [sept, 4 H, JHH = 6.8 Hz, CH(CH3)2],
3
1.89 (s, 2 H, CCH2-AlEt3), 1.37 [d, 12 H, JHH = 6.8 Hz, CH(CH3)2],
1.34 (t, 9 H, JHH = 8.1 Hz, AlCH2CH3), 1.33 (s, 6 H, H3CCN), 0.95
[d, 12 H, JHH = 6.9 Hz, CH(CH3)2], 0.01 (q, 6 H, JHH = 8.1 Hz,
AlCH2CH3). 13C{1H} NMR (175 MHz, C6D6):
3
3
3
δ
=
3.0
[–Al(CH2CH3)3], 9.5 (H3CCN), 11.4 [Al(CH2CH3)3], 24.3
[CH(CH3)2], 24.4 [CH(CH3)2], 28.8 [CH(CH3)2], 125.3 (ArC), 125.7
(ArC), 128.3 (ArC), 128.4 (ArC), 131.0 (ArC), 131.8 (ArC), 145.8
(NCN), 146.6 (CCH2–AlEt3). C36H57AlN2: calcd. C, 79.36; H, 10.55;
N, 5.14%; found: C, 78.82; H, 10.47; N, 4.99%. Mp (°C): 132 (dec).
Synthesis of MeIPrCHCHCH2·AlMe3 (3): A solution of AlMe3
(2.0 m solution in toluene, 0.193 mL, 0.39 mmol) was gently layered
atop of a solution of MeIPr=CH–CH=CH2 (0.1760 g, 0.3854 mmol)
in 1.5 mL of toluene. The mixture was then left undisturbed for
16 h, resulting in the formation of colorless crystals of
X-ray Crystallography: Crystals of appropriate quality for X-ray dif-
fraction studies[9] were removed from either a Schlenk tube under a MeIPrCHCHCH2·AlMe3 (3). The supernatant was decanted away, and
stream of nitrogen, or from a vial (glove box) and immediately covered
with a thin layer of hydrocarbon oil (Paratone-N). A suitable crystal
was then selected, attached to a glass fiber, and quickly placed in a δ = 7.22 (t, 2 H, JHH = 7.7 Hz, ArH), 7.08 (d, 4 H, JHH = 7.8 Hz,
the remaining crystals were washed with 3 mL of cold (–30 °C) tolu-
1
ene and dried in vacuo (0.1271 g, 62%). H NMR (700 MHz, C6D6):
3
3
low-temperature stream of nitrogen. All data were collected using a
Bruker APEX II CCD detector/D8 diffractometer using Mo-Kα or Cu-
Kα radiation, with the crystal cooled to –100 °C or –80 °C, respec-
tively. The data were corrected for absorption through Gaussian inte-
gration from indexing of the crystal faces. Structures were solved using
ArH), 6.57 (dt, 1 H, 3JHH = 14.3, 3JHH = 10.7 Hz, CHCHCH2–AlMe3),
3
3
4.67 (d, 1 H, JHH = 14.3 Hz, CHCHCH2–AlMe3), 2.79 (d, 2 H, JHH
3
= 10.7 Hz, CHCHCH2–AlMe3), 2.60 [sept, 4 H, JHH = 6.9 Hz,
CH(CH3)2], 1.30 (s, 6 H, H3C-CN), 1.29 [d, 12 H, JHH = 7.4 Hz,
CH(CH3)2], 1.01 [d, 12 H, JHH = 6.9 Hz, CH(CH3)2], –0.42 [s, 9 H,
3
3
the direct methods programs SHELXT-2014,[22] and refinements were -Al(CH3)3]. 13C{1H} NMR (175 MHz, C6D6): δ = - 6.1 [–Al(CH3)3],
completed using the program SHELXL-2014.[23] Hydrogen atoms
were assigned positions based on the sp2- or sp3-hybridization geome-
8.7 (H3CCN), 23.6 [CH(CH3)2], 24.2 [CH(CH3)2], 29.1 [CH(CH3)2],
58.6 (CH–CHCH2–AlMe3), 85.3 (CHCHCH2–AlMe3), 121.4 (ArC),
tries of their attached carbon atoms, and were given thermal parameters 125.3 (ArC), 127.9 (ArC), 128.4 (ArC), 130.4 (ArC), 131.4 (ArC),
20% greater than those of their parent atoms.
146.7 (H3CCN), 147.5 (NCN), 162.2 (CHCHCH2–AlMe3).
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[9] For full crystallographic details and copies of the NMR and GPC
data for this study, see the Supporting Information.
[10] a) C. Hering-Junghans, P. Andreiuk, M. J. Ferguson, R. McDonald,
E. Rivard, Angew. Chem. Int. Ed. 2017, 56, 6272–6275; b) M. Lui,
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Chem. 2015, 54, 2040–2049; for a review on NHI complexes, see:
c) T. Ochiai, D. Franz, S. Inoue, Chem. Soc. Rev. 2016, 45, 6327–
6344.
H, 9.91; N, 5.12%. Mp (°C): 218 (dec).
Polymerization of Michael-type Monomers (General Procedure):
MeIPrCH2·AlMe3 (0.025 g, 0.050 mmol) was added to a solution of
monomer (10 mmol) in 5 mL of THF. After 1 h of stirring the reaction
mixture was quenched with ca. 0.5 mL of ethanol, and the volatiles were
removed in vacuo. The resulting solid was dissolved in ca. 5 mL of
dichloromethane and precipitated into 100 mL of pentane at –30 °C. The
resulting polymer was dried under high vacuum while heated at 50 °C.
Supporting Information (see footnote on the first page of this article):
NMR spectra of NHO-AlR3 complexes and AlR3 starting materials, mo-
lecular structure of 2, full X-ray crystallographic details, and GPC traces
of p2VP, pMA, and pDMAA.
[11] G. Schnee, A. Bolley, F. Hild, D. Specklin, S. Dagorne, Catal. To-
day 2017, 289, 204–210.
[12] I. C. Watson, A. Schumann, H. Yu, E. C. Davy, R. McDonald, M. J.
Ferguson, C. Hering-Junghans, E. Rivard, Chem. Eur. J. 2019, 25,
9678–9690.
Acknowledgements
This work was supported by the Canadian Foundation for Innovation,
the Faculty of Science at the University of Alberta, and the Natural Sci-
ences and Engineering Research Council (NSERC) of Canada (Discov-
ery and Accelerator Supplement grants to E.R.; CREATE fellowship to
I.W.). B.R. and E.R. acknowledge funding from DFG IRTG (2022) and
NSERC CREATE (CREATE 4639900–2015) program for the Alberta/
TU München International Graduate School. I.W. thanks B.R. for host-
ing him as a researcher at TUM. I.W. thanks Bruno Luppi for his assist-
ance with GPC measurements and plots. I.W. thanks Alvaro Omaña Mo-
reno for assistance with GPC measurements. E.R. is also grateful to the
Alexander von Humboldt Foundation for support, and to Prof. Dr.
Manfred Scheer for his friendship and hospitality during E.R.’s excellent
stays in Regensburg.
[13] Variable temperature 1H NMR analysis of 1:1 mixtures of
MeIPrCH2/AlMe3, MeIPrCH2/AlEt3, and MeIPrCH2CHCH2/AlMe3
in [D8]toluene were conducted in the temperature range of –60 to
+80 °C. None of these Lewis pair adducts (1–3) showed any evi-
dence of separating into their respective free Lewis acid and base.
See Figures S18, S19, and S20 (Supporting Information) for the
spectra.
[14] M. Weger, R. K. Grötsch, M. G. Knaus, M. M. Giuman, D. C.
Mayer, P. J. Altmann, E. Mossou, B. Dittrich, A. Pöthig, B. Rieger,
Angew. Chem. Int. Ed. 2019, 58, 9797–9801.
[15] a) Z. Zhao, Q. Wang, J. He, Y. Zhang, Polym. Chem. 2019, 10,
4328–4335; b) T. Xu, E. Y.-X. Chen, J. Am. Chem. Soc. 2014, 136,
1774–1777.
[16] Q. Wang, W. Zhao, S. Zhang, J. He, Y. Zhang, E. Y.-X. Chen, ACS
Catal. 2018, 8, 3571–3578.
[17] For the polymerization of DEVP, see: M. Weger, P. Pahl, F.
Schmidt, B. S. Soller, P. J. Altmann, A. Pöthig, G. Gemmecker, W.
Eisenreich, B. Rieger, Macromolecules 2019, 52, 7073–7080 and
references cited therein.
Keywords: N-Heterocyclic olefins; Aluminum; Polymeriza-
tion; Michael-type monomers; Frustrated Lewis pairs
[18] J. He, Y. Zhang, E. Y.-X. Chen, Synlett 2014, 25, 1534–1538.
[19] K. Takada, K. Fuchise, Y. Chen, T. Satoh, T. Kakuchi, J. Polym. Sci.
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Received: December 11, 2019
Published Online: ᭿
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© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
SHORT COMMUNICATION
Zeitschrift für anorganische und allgemeine Chemie
I. C. Watson, Y. Zhou, M. J. Ferguson, M. Kränzlein,
B. Rieger,* E. Rivard* ...................................................... 1–6
Trialkylaluminum N-Heterocyclic Olefin (NHO) Adducts as
Catalysts for the Polymerization of Michael-Type Monomers
Z. Anorg. Allg. Chem. 2020, 1–6
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