Low-Symmetry β-Diketimine Aryloxide Rare-Earth Complexes: Flexible, Reactive, and Selective

Article abstract of DOI:10.1021/acs.inorgchem.0c02170

We report the synthesis, characterization, and reactivity of a new low-symmetry β-diketimine featuring a pendant amino(methyl)phenol donor and its corresponding heteroleptic rare-earth (RE) complexes. This includes the first structurally characterized examples of alcoholysis and insertion from an isolated REIII amide in a β-diketimine framework. The flexible methylene linkage leads to REIII complexes with tunable dynamic solution behavior that defines their stoichiometric and catalytic reactivity. The addition of a strong neutral donor ligand, tricyclohexylphosphine oxide, suppresses a prevalent catalyst degradation pathway (base-promoted elimination) and dramatically enhances the catalyst performance in the stereospecific ring-opening polymerization of rac-β-butyrolactone. Our results further demonstrate the importance of ligand reorganization in the stoichiometric and catalytic activity of REIII ions.

Full text of DOI:10.1021/acs.inorgchem.0c02170

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Low-Symmetry β‑Diketimine Aryloxide Rare-Earth Complexes:
Flexible, Reactive, and Selective
Kerry C. Casey, Jude K. Appiah, and Jerome R. Robinson*
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ABSTRACT: We report the synthesis, characterization, and reactivity of a new
low-symmetry β-diketimine featuring a pendant amino(methyl)phenol donor and
its corresponding heteroleptic rare-earth (RE) complexes. This includes the first
structurally characterized examples of alcoholysis and insertion from an isolated
REIII amide in a β-diketimine framework. The flexible methylene linkage leads to
REIII complexes with tunable dynamic solution behavior that defines their
stoichiometric and catalytic reactivity. The addition of a strong neutral donor
ligand, tricyclohexylphosphine oxide, suppresses a prevalent catalyst degradation
pathway (base-promoted elimination) and dramatically enhances the catalyst
performance in the stereospecific ring-opening polymerization of rac-β-
butyrolactone. Our results further demonstrate the importance of ligand reorganization in the stoichiometric and catalytic activity
of REIII ions.
a
Scheme 1. Ligand and Metal Complex Synthesis
INTRODUCTION
■
β-Diketimines, commonly abbreviated as BDI or NacNac, are
an extremely versatile class of ligands with readily tunable
steric and electronic properties.1 This tunability has enabled
the synthesis of metal complexes across the periodic table with
diverse applications ranging from modeling metalloenzymes,2
materials science,3 and unprecedented stoichiometric4 and
catalytic reactivity.4f,5 The highly Lewis-acidic rare-earth (RE)
ions can display exceptional reactivity in stoichiometric and
catalytic reactions when supported by appropriately designed
β-diketimines that accommodate for their larger ionic radii and
higher charges. For example, bulky diaryl β-diketimines can
support isolable cationic RE alkyl species6 and stereoselective
olefin polymerization catalysts,7 while the introduction of
pendant neutral donors can support rare examples of reactive
a
(i) 1.05 equiv, [Me3O][BF4], CH2Cl2, RT, 1 h; (ii) H2NR, THF,
RT, 1 h; (iii) 1.2 equiv, NEt3, Et2O, RT, 2 h; (iv) RE[N-
(SiHMe2)]3(THF)2, C6H6.
{RE(1L)[N(SiHMe2)2](THF)} (1-RE; RE = LaIII, YIII; THF =
8
9
10
2−
dianionic fragments (e.g., NR2−, PR2−, CR2
)
and diverse
tetrahydrofuran), proceeds in fair to very high yields via
insertion reactivity.11 The inclusion of pendant anionic donor
groups (e.g., amide,12 β-diketimine,13 alkoxide,14 and aryl-
oxide15) could furnish well-defined neutral REIII complexes
featuring a single reactive initiator ideally suited for catalysis,
but their synthesis and reactivity have been limited. Herein, we
report the synthesis, characterization, and reactivity of a novel
low-symmetry β-diketimine featuring a pendant amino-
(methyl)phenol donor and its corresponding heteroleptic
REIII complexes.
protonolysis of H2 1 L with
1 equiv of RE[N-
(SiHMe2)2]3(THF)2 in benzene. The synthesis of 1-La
proceeds in a few hours at room temperature (RT), while
optimized conditions for 1-Y require elevated temperatures
(70 °C), extended reaction times (∼72 h), and small amounts
of coordinating solvent (≥5 equiv of THF).
X-ray crystallography revealed that 1-La and 1-Y are
isostructural monomers with C1 symmetry in the solid state
Received: July 21, 2020
RESULTS AND DISCUSSION
■
1
The novel flexible β-diketimine aryloxide H2 L was synthesized
in three steps from the (2,6-diisopropylphenyl)-β-iminoenol
DippNacAc16 and amino(methyl)phenol17 in 67% yield
(Scheme 1). The preparation of REIII amide complexes,
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A

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Figure 1. Thermal ellipsoid plots displayed at 50% probability for (A) 1-La, (B) 2-La, and (C) 3-La.
(Figure 1A). The geometry of the five-coordinate REIII centers
is best described as distorted trigonal bipyramidal (τ5 = 0.6),
where the silylamide ligand is cis to the bound THF.18 The
equatorial plane is comprised of aryloxide [O(1)], NacNac
[N(2)], and silylamide [N(3)] donors, and the apical positions
are occupied by NacNac [N(1)] and THF [O(2)] donors.
The REIII ions sit tightly within the plane defined by the
NacNac backbone [N(1)−C(17)−C(18)−C(19)−N(2);
REIII−NCCCN plane: 0.022(1)−0.072(1) Å] and contrast
with the significantly displaced REIII ions supported by
NacNac ligands with pendant amide and aminocatecholate
donors [0.645(1)−1.710(1) Å].12,15b,c REIII−OAr and REIII−
N(SiHMe2)2 bond distances fall within ranges similar to those
of other five-coordinate REIII species, which indicates minimal
strain or interligand steric repulsion enforced by 1L.13a,19
Agostic β-H−Si interactions (REIII↼H−Si) are present in the
solid state, as supported by the smaller angle ∠RE(1)−N(3)−
Si(1) compared to ∠RE(1)−N(3)−Si(2) (La, ∼107.1°; Y,
∼110.9°), close RE(1)−Si(1)−H contact [La, 3.3497(13) Å;
Y, 3.3497(13) Å], and lower-energy Si−H stretch in the IR
1
spectrum (agostic, 2011 cm−1; nonagostic, 2075 cm−1; Figures
S3 and S7).20
Figure 2. (a−e) Select variable-temperature H NMR spectra of 1-Y
▲
●
in RT Tol-d8: ( ) SiHMe2; ( ) γ-CH. (f) Interconversion between
C1-symmetric structures, leading to effective Cs symmetry.
1H NMR spectra of 1-Y measured in C6D6 or deuterated
toluene (Tol-d8) at RT revealed that C1 symmetry was also
maintained in solution. 1-Y displayed a fully desymmetrized
ligand environment, as evidenced by the two Dipp-CH(CH3)2
(Dipp = 1,2-diisopropylphenyl) and four Dipp-CH(CH3)2
protons and diastereotopic CH2ArO resonances (Figures 2,
S7, and S29). The THF α-CH2 resonances of 1-Y were also
diastereotopic, which reflects tight binding, restricted rotation,
and an asymmetric primary coordination sphere. Increased
temperatures led to coalescence of the CH2ArO, Dipp-
CH(CH3)2, and THF α-CH2 resonances to broad singlets
(Tc = 75 °C; Figure 2). Because rotation about the CAr−NDipp
bond is hindered as a result of the bulky Dipp fragment, the
reduced effective symmetry (from C1 to Cs) is consistent with
the dynamic interconversion of C1 isomers21 through rotation
of the CH2ArO arm on the NMR time scale. Alternative
exchange pathways in 1-RE, such as monomer−dimer
equilibria, were safely excluded by employing diffusion-ordered
NMR spectroscopy (DOSY).22 The hydrodynamic radii (rH)
S1) were in good agreement with those of monomeric
structures.
Quantitative insight into the nature of this dynamic
interconversion was achieved by employing variable-temper-
1
ature H NMR experiments on 1-RE in Tol-d8 and THF-d8.
Exchange rates were determined for 1-RE through dynamic
line-shape analysis of Dipp-CH(CH3)2 (THF-d8) or CH2ArO
(Tol-d8) resonances using DNMR (Topspin 3.6.1; Figures
S21−S40 and Tables S2−S7). In the case of 1-Y in THF-d8,
sufficient spectral resolutions of the Dipp-CH(CH3)2 and
CH2ArO resonances enabled line-shape analysis for both
features. Eyring plots constructed from the obtained exchange
rates enabled determination of the activation parameters for 1-
RE (Table 1). Exchange was sensitive to both the coordinating
solvent (Tol-d8 vs THF-d8) and ionic radius of the central REIII
(Y vs La). ΔG⧧ decreased with increasing ionic radius and
coordinating nature of the solvent (Table 1; 1-Y, Tol-d8 > 1-
La, Tol-d8 > 1-Y, THF-d8 > 1-La, THF-d8). Differences in the
activation parameters obtained for Dipp-CH(CH3)2 and
CH2ArO resonances of 1-Y in THF-d8 were statistically
1
of 1-RE obtained by H DOSY NMR in C6D6 with and
without added THF (10 equiv; Figures S10 and S13 and Table
B
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Table 1. Activation Parameters and Coalescence Temperature Obtained from Variable-Temperature 1H NMR Experiments of
1-RE
a
b
b
b
entry
1-RE
solvent
Tc (°C)
ΔG⧧
(kcal mol−1
)
ΔH⧧ (kcal mol−1
)
ΔS⧧ (cal mol−1 K−1
)
298 K
c
1
2
3
4
5
La
La
Y
Y
Y
Tol-d8
THF-d8
Tol-d8
THF-d8
THF-d8
25
13.7 0.7
10.3 0.6
14.8 0.3
12.9 0.5
13.0 0.6
5.6 0.2
6.4 0.3
12.8 0.2
6.2 0.2
5.9 0.2
−26.9 0.9
−13.1 0.5
−6.70 0.09
−22.8 0.6
−23.9 0.8
d
−80
c
75
c
0
d
0
a
b
Coalescence temperature. Extracted from Eyring plots constructed from exchange rates obtained from dynamic line-shape analysis of the
variable-temperature H NMR data for 1-RE. −CH2ArO. Dipp-CH(CH3)2.
c
d
1
insignificant and confirmed a shared exchange process for the
two fragments of 1L (Table 1, entries 4 and 5). The
significantly negative ΔS⧧ was indicative of increased order
in the transition state and consistent with a nondissociative
process and further ordering of the metal−ligand framework. A
isomerization in 1-RE (vide supra) and further demonstrates
the critical role that ligand reorganization can play in the
reactivity of REIII ions.27
X-ray-quality single crystals of 2-La were obtained from
chilled pentane solutions (−35 °C) and revealed a C2-
symmetric dimer in the solid state (2-La2; Figure 1B). 2-La2
features five-coordinate LaIII centers bridged by aryloxides in a
distorted square-pyramidal geometry (τ5 = 0.27), where the
LaIII ion is significantly displaced from the plane of the NacNac
backbone [1.762(1) Å]. The base of the square pyramid is
comprised of NDipp [N(2)], diphenylmethoxide [O(2)], and
1
variable-temperature H NMR experiment of 1-Y in Tol-d8 in
the presence of 1 equiv of THF (Figure S40) provides
unambiguous evidence for facile exchange between free and
bound THF. Taken together with the significantly negative
ΔS⧧ and lower ΔG⧧ of 1-RE in a coordinating solvent, our
experiments support a Lewis-base-assisted isomerization
process.
bridging aryloxides [O(1)], while NCH ArO [N(1)] is situated in
2
While numerous heteroleptic REIII(NacNac)(NR2) species
have been synthesized,12,23 structurally characterized examples
of alcoholysis or insertion of REIII−NR2 have not been
reported. Clean protonolysis and insertion reactivity are
essential elementary steps in catalytic reactions promoted by
these complexes [e.g., hydroelementation and ring-opening
polymerization (ROP)],12,13,24 although quantitative reactivity
cannot be assumed. The Brønsted basic γ-carbon of the β-
diketiminate backbone can engage in protonation or insertion
reactions, which would dramatically alter the catalyst structure
(and function) during catalysis.5d,6f,25 1-RE undergoes rapid
alcoholysis with diphenylmethanol to form the REIII alkoxide
2-RE and HN(SiHMe2)2 (Scheme 2). 1-La also undergoes
clean insertion of the silylamide with N,N′-dicyclohexylcarbo-
diimide (DCC) to furnish the REIII guanidinate 3-La.26 While
reaction conditions vary slightly between the syntheses of 2-RE
and 3-La, small amounts of coordinating solvent (≥5 equiv of
THF) are necessary to facilitate quick and clean conversion.
The coordinating solvent increases the rate of ligand
the apical site. X-ray-quality single crystals of 3-La obtained
from standing pentane solutions revealed a monomeric
structure with C1 symmetry (3-La; Figure 1C). The six-
coordinate LaIII center adopts a distorted trigonal-prismatic
geometry, where the faces of the trigonal prism are defined by
OAr−OTHF−NGuan [O(1)−O(2)−N(5)] and NOAr−NDipp
−
NGuan [N(1)−N(2)−N(3)]. LaIII is minimally displaced from
the plane of the NacNac backbone [0.455(1) Å], and the
guanidinate is cis to the bound THF. The bond distances of 2-
La2 and 3-La were comparable to similar fragments28 after
accounting for differences in the ionic radii.29
1H and 13C{1H} NMR spectra of 2-La2 in a noncoordinating
solvent (C6D6) were consistent with the dimeric structure
observed in the solid state. 1L was desymmetrized, and the ipso
aryloxide carbon displayed a significantly upfield 13C signal
(154.1 ppm) that was consistent with a bridging aryloxide.30
1
The nuclearity of 2-La2 was further confirmed by H DOSY
NMR, where the calculated rH was in agreement with that of a
dimer (Figure S11). The addition of a coordinating solvent (10
equiv of THF) fully converted 2-La2 to a THF-solvated
monomer, 2-La. This was supported by the significant
downfield shift of the ipso aryloxide carbon (160.1 ppm;
Figure S4h) and a calculated rH value corresponding to a
monomer (Figure S11 and Table S1). A similar monomer−
dimer equilibrium was absent for 2-Y and 3-La because
monomeric structures were observed in C6D6 with or without
Scheme 2. Ligand and Metal Complex Synthesis
1
added THF. H and 13C{1H} NMR spectra of 2-Y displayed
effective Cs symmetry at RT, indicative of lower barriers for C1
isomer interconversion compared to 1-Y.
REIII β-diketimines can display excellent catalytic reactivity
in the ROP of cyclic esters;24c,31 however, demonstrations of
stereospecific ROP have been limited.24c,32 The ability to
control a polymer’s relative stereochemistry (tacticity) has a
profound impact on its resulting properties,33 and inexpensive
racemic monomers are ideally suited to accomplish this on
scale. Poly(3-hydroxybutyrate) (P3HB) is the most common
member of the large class of naturally occurring and
biodegradable poly(hydroxyalkanoates),34 and its stereospe-
cific synthesis from the racemic four-membered lactone, rac-
C
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Table 2. ROP of rac-BBL Catalyzed by 1-RE, 2-RE, and 3-RE
a
b
c
d
de
,
f
entry
catalyst
time (h)
conv (%)
Mn,calc (kg mol−1
)
Mn,exp (kg mol−1
)
Đ
Pr
1
2
3
4
5
6
7
8
1-La
1-Y
2-La
2-La + 2OPCy3
2-Y
2-Y + 2OPCy3
22
22
22
22
22
22
3
15
4
2.28
0.62
4.26
9.56
3.28
15.49
16.7
1.03
1.24
n.d.
1.19
n.d.
0.45
n.d.
g
25
56
19
90
97
6
2.27
3.89
1.79
8.56
10.11
n.d.
1.14
1.14
1.11
1.13
1.16
n.d.
0.53
0.55
0.58
0.74
0.79
n.d.
h
2-Y + 2OPCy3
3-La
22
a
b
1
Reaction times not optimized. Determined by H NMR integration of the BBL and PHB methine resonances in the crude reaction mixture.
c
d
[BBL]/[RE] × conv × 0.08609 + (MWinitiator × 10−3) kg mol−1. Determined by GPC at 30 °C in THF using polystyrene standards and corrected
by a Mark−Houwink factor of 0.54.43 Mw/Mn. Probability of rac linkages between repeat units. Determined by the integration of P3HB CO
e
f
g
h
resonances using inverse-gated 13C NMR. Not determined. Performed at 0 °C.
BBL, has been particularly challenging.35 Several of the highly
active and stereoselective catalysts for other cyclic esters (e.g.,
rac-lactide), including metal complexes supported by β-
diketimines, display no stereocontrol in the ROP of rac-BBL
(i.e., atactic P3HB).3a,b,36 Given the exceptional reactivity and
selectivity exhibited by REIII catalysts supported by amino-
phenolate ligand fragments30,33e,37 and the well-defined
protonolysis and insertion reactivity displayed by 1-RE, we
explored the reactivity of 1-RE, 2-RE, and 3-La in the
stereospecific ROP of rac-BBL (Table 2).
feature a more congested coordination sphere. Lowering the
reaction temperature (0 °C) led to further improvements of
the 2-Y/2OPCy3 system (entry 8), where the reaction was
essentially complete at 3 h with increased stereoselectivity (Pr
= 0.79) and negligible formation of crotyl end groups. Further
1
end-group analysis using H and 13C{1H} NMR to study
P3HB oligomers generated from the ROP of rac-BBL catalyzed
by 2-Y/OPCy3 at 0 °C ([rac-BBL] = 1 M; 2-Y/OPCy3/rac-
BBL = 1/2/25) unambiguously established the identity of the
polymer end groups (Figure S42). Consistent with a
coordination−insertion ROP mechanism, with REIII-OCHPh2
serving as the initiating group, terminal secondary alcohol and
secondary ester [C(O)OCHPh2] groups were observed
exclusively. Spectroscopic evidence for an anionic ROP
mechanism (e.g., ether and acid end groups) or an initiation
Despite examples of active REIII amide33e,37e,38 and
guanidinate39 initiators, 1-RE and 3-La displayed low
efficiency at RT over 22 h (entries 1, 2, and 8). Metal
alkoxides often display higher efficiencies in the ROP of cyclic
esters because of lower steric profiles and structural similarity
to the propagating chain.3b,37a,d,40 Unfortunately, 2-RE
displayed marginal improvements in reactivity and limited
stereocontrol (entries 3 and 5). One challenge associated with
the ROP of rac-BBL and other β-lactones is that both the
monomer and polymer are susceptible to a variety of
degradation pathways. Polymer chain scission via base-
promoted elimination (Scheme S2) can proceed under mild
reaction conditions and leads to dormant chain ends (crotyl
end group) and catalyst deactivation (metal carboxyla-
te).30,36a,41 In addition to the alcohol and ester end groups
expected for a coordination−insertion mechanism,34b,35a 1-RE,
2-RE, and 3-La formed significant amounts of crotyl end
groups (∼0.5−1.0 equiv/REIII). This suggests base-promoted
elimination as a possible source for the poor catalyst activity.
Recently, our group discovered that strong neutral donor
ligands (e.g., phosphine oxides) could suppress base-promoted
elimination and amplify reactivity and selectivity in the ROP of
rac-BBL with coordinatively unsaturated REIII aminobis-
(phenolate) catalysts.30 Given these findings and the initial
reactivity of 2-RE, we posited that strong neutral donors would
improve the catalyst performance by (i) suppressing
elimination, (ii) increasing steric congestion at the active
site, and (iii) slowing C1 isomer interconversion. Consistent
with this hypothesis, 2 equiv of OPCy3 improved the catalyst
performance (Table 2, entries 3 and 5 vs 4 and 6) and
decreased crotyl end-group formation. Improvements were
much more pronounced with the smaller ion YIII, which would
1
from Bronsted basic fragments of L was notably absent.
Additional connections between the catalyst structure and
1
function were provided by H and 31P{1H} NMR spectrosco-
py. Titrating OPCy3 to C6D6 solutions of 2-RE (Figures S5
and S9) clearly supported the formation of (OPCy3)2 adducts
RE(1L)(OCHPh2)(OPCy3)2 [2-RE(OPCy3)2]. Unlike the
previously reported aminobis(phenolate) catalysts,30 1 equiv
of phosphine oxide led to a 50:50 mixture of 2-RE(OPCy3)2
1
and 2-La2 or 2-Y. H NMR spectra of 2-RE with 2 equiv of
OPCy3 revealed complete desymmetrization of 1L at RT,
which was consistent with its effective C1 symmetry and
increased isomerization barriers compared to those of 2-La2 or
2-Y. While increased rates of ligand reorganization and
decreased steric demand at the REIII ion were conducive for
stoichiometric alcoholysis and insertion reactivity (vide supra),
decreased rates of ligand isomerization and increased steric
congestion improved the catalyst performance in stereospecific
ROP. Our spectroscopic studies support that OPCy3 binding
1
rigidifies L and suppresses base-promoted elimination, which
leads to dramatic enhancements in the catalyst reactivity and
selectivity.
CONCLUSION
■
In closing, we have reported the synthesis, characterization,
and reactivity of a novel β-diketimine ligand featuring a
pendant aryloxide donor, 1L, and its heteroleptic REIII
1
complexes, 1-RE, 2-RE, and 3-La. The flexibility of L leads
D
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to REIII complexes with the tunable dynamic solution behavior
that defines their stoichiometric and catalytic reactivity.
Despite implied reactivity during catalysis, we report the first
structurally characterized examples of alcoholysis and insertion
from an isolated REIII−NR2 supported by β-diketimine. The
performance of 2-Y as a catalyst for the stereospecific ROP of
rac-BBL is enhanced by adding 2 equiv of OPCy3, which
of solvent, where maintenance of a dark-blue color for at least 5 min
was sufficient for use.
3,5-Di-tert-butyl-2-hydroxybenzaldehyde (AK Scientific; 98%),
sodium acetate (Alfa Aesar; 99%), sodium bicarbonate (Fisher
Scientific; 99%), hydroxylamine hydrochloride (Acros Organics;
97%), acetylacetone (TCI; 99%), 2,6-diisopropylaniline (Sigma-
Aldrich; 90%), trimethyloxonium tetrafluoroborate (Fisher Scientific;
95%), triethylamine (Alfa-Aesar; 99%), benzhydrol (Alfa-Aesar; 99%),
zinc powder (Strem; 99.9%), potassium hexamethyldisilazide (Sigma-
Aldrich; 95%), 1,1,3,3-tetramethyldisilazane (TCI; 97%), and RECl3
(RE = La, Y; Strem; 99.9%) were purchased and used as received.
Racemic butyrolactone (Sigma-Aldrich; 98%) was freshly distilled
from CaH2 under N2 and degassed by freeze−pump−thaw cycles
prior to use. 4-(2,6-Diisopropylphenyl)aminopent-3-en-2-one,16 2-
(aminomethyl)-4,6-di-tert-butylphenol,17 RE[N(SiMe3)2]3 (RE = La,
Y),44 RE[N(SiHMe2)2]3(THF)2 (RE = La, Y),45 and tricyclohex-
ylphosphine oxide46 were prepared according to reported procedures.
Synthesis of 1-La. A 20 mL scintillation vial was charged with
H2L (520.5 mg, 1.09 mmol, 1 equiv; MW = 476.75 g mol−1),
La[N(SiHMe2)2]3(THF)2 (740.3 mg, 1.09 mmol, 1 equiv; MW =
680.12 g mol−1), benzene (4 mL), THF (100 mg, 2.39 mmol, 1.3
equiv; MW = 71.10 g mol−1), and a Teflon-coated stir bar. The clear
yellow solution was stirred for 16 h at RT, and solvents were removed
under reduced pressure. The crude white solid was triturated with
pentane (3 × 3 mL) to help remove residual amine. The white solid
was dissolved in THF (2 mL) and filtered through a glass pipet
padded with Celite to remove insoluble materials. Volatiles were
removed under reduced pressure, and the white solid was washed with
a cold (−35 °C) solution of THF/pentane (1:20, 4 mL). The solvent
was decanted, the solid was washed with RT pentane (1 mL), and the
sample was dried under reduced pressure to yield 1-La as a white
solid. Yield: 787.0 mg (0.960 mmol, 88%; MW = 818.08 g mol−1).
Crystals suitable for single-crystal X-ray diffraction were obtained by
the slow evaporation of a concentrated solution of diethyl ether at
−35 °C over 7 days.
1
rigidifies L and suppresses a prevalent catalyst deactivation
pathway (base-promoted elimination). While admittedly 2-Y/
2OPCy3 is slower and less selective than the champion
syndioselective catalysts developed by Carpentier,37b adding
neutral achiral donor ligands is a promising and operationally
simple approach to rationally modify the catalyst performance
in stereoselective ROP. Efforts to isolate crystalline phosphine
oxide adducts of 2-RE and investigations into the active
catalyst structure are ongoing.
EXPERIMENTAL SECTION
■
General Methods and Materials. Unless specified, all reactions
were performed under inert conditions (N2) using standard Schlenk
techniques or in a MBraun drybox equipped with a standard catalyst
purifier and solvent trap. Glassware was oven-dried for at least 2 h at
150 °C prior to use. Celite and 3 Å molecular sieves were heated
under reduced pressure at 300 °C for at least 24 h and then cooled
under vacuum prior to use. The following spectrometers were used for
NMR characterization: Bruker Avance III HD Ascend (1H, 600 MHz;
13C, 152 MHz; 31P, 243 MHz) and Bruker DRX (1H, 400 MHz; 13C,
1
101 MHz; 31P, 162 MHz). H and 13C NMR shifts are referenced
relative to the solvent signal (CDCl3: 1H, 7.26 ppm; 13C, 77.16 ppm;
1
C6D6: H, 7.16 ppm; 13C, 128.06 ppm), while 31P NMR shifts are
referenced relative to external solution standards (H3PO4, 0 ppm).
Both instruments were equipped with Z-gradient BBFO probes. Probe
temperatures were calibrated using ethylene glycol and methanol, as
previously described.42 The polymer tacticity (Pr, percentage of rac
diads) was measured using a 13C inverse-gated pulse sequence,
followed by integration of the CO resonances (Figure S43).
Gel permeation chromatography (GPC) measurements were
performed using an Agilent 1260 equipped with two PolyPore
columns heated at 40 °C (7.5 × 300 mm, 5 μm) and UV−vis diode-
array and refractive detectors. The eluent was inhibitor-free THF, and
the system was calibrated with polystyrene standards ranging from
580 to 1,500,000 Da. The reported molecular weights were from
GPC, corrected by a Mark−Houwink factor of 0.54.43 Unless stated
otherwise, all GPC samples were of the quenched crude reaction
mixtures (not precipitated or purified polymers). IR spectra were
recorded on Jasco 4100 Fourier transform infrared spectrometers
using Nujol mulls sandwiched between KBr plates. Elemental analyses
were performed by Robertson Microlit Laboratories (Ledgewood,
NJ) and Midwest Microlab, LLC (Indianapolis, IN), for bench-stable
3
1H NMR (600 MHz, C6D6, 298 K): δ 0.33 (d, J = 3.0 Hz, 12H,
SiHMe2), 1.00 (br s, 12 H, CH(CH3)2, 1.20 (s, 4H, (THF β-CH2),
1.39 (s, 9H, C(CH3)3), 1.56 (s, 3H, C(N)CH3), 1.59 (s, 9H,
C(CH3)3), 2.15 (s, 3H, C(N)CH3), 3.1 (br s, 2H, CH(CH3)2), 3.23
(s, 4H, (THF α-CH2), 4.90 (br s, 2H, CH2ArO), 4.95 (s, 1H,
3
C(N)CH), 5.04 (br quint, J = 3.0 Hz, 2H, Si−H), 6.89−6.99 (m,
4
4
3H, Ar−H), 7.23 (d, J = 2.4 Hz, 1H, CHArO), 7.46 (d, J = 2.4 Hz,
1H, CHArO).
1H NMR (400 MHz, THF-d8, 298 K): δ 0.054 (d, J = 3.0 Hz,
3
12H, SiHMe2), 1.15 (d, 6H, 3J = 6.7 Hz, CH(CH3)2), 1.17 (d, 6H, 3J
= 6.7 Hz, CH(CH3)2), 1.26 (s, 9H, C(CH3)3), 1.35 (s, 9H,
C(CH3)3), 1.53 (s, 3H, C(N)CH3), 1.77 (s, 4H, (THF β-CH2), 2.14
(s, 3H, C(N)CH3), 3.08 (quint, 3J = 6.7 Hz, 2H, CH(CH3)2), 3.61 (s,
3
4H, (THF α-CH2), 4.60 (s, 2H, CH2ArO), 4.68 (quint, J = 3.0 Hz,
2H, Si−H), 4.76 (s, 1H, C(N)CH), 7.02 (d, 4J = 2.4 Hz, 1H, CHArO
)
4
3
7.04 (d, J = 2.4 Hz, 1H, CHArO), 7.17 (t, J = 7.67 Hz, 1H, Ar−H),
3
7.24 (d, J = 7.67 Hz, 2H, Ar−H).
1
Note: The CH2ArO and CH(CH3)2 peaks are extremely broad at
RT in C6D6, indicative of exchange on the NMR time scale.
13C{1H} NMR (100 MHz, C6D6, 298 K): δ 3.3 (SiHMe2), 22.1
(C(N)CH3), 24.0 (CH(CH3)2) 24.4 (C(N)CH3), 26.0 (THF β-
CH2), 27.90 (CH(CH3)2), 30.50 (C(CH3)3), 32.32 (C(CH3)3), 34.29
(C(CH3)3), 35.42 (C(CH3)3), 52.34 (CH2ArO), 69.46 (THF α-
CH2), 100.01 (C(N)CH), 122.49 (COAr), 124.65 (COAr), 124.79 (m-
CAr), 125.95 (p-CAr), 128.86 (COAr), 134.28 (CAr), 137.20 (CAr),
141.62 (CAr), 145.40 (CAr), 160.9 (CAr−O, terminal), 163.04 (C(N)),
165.20 (C(N)).
(H2 L and [H3L][BF4]) and air-sensitive (1-RE, 2-RE, and 3-La)
compounds, respectively. Samples were shipped in vacuum-sealed
plastic bags within a sealed 2 mL vial placed in a sealed 20 mL
scintillation vial.
Tetrahydrofuran (THF; inhibitor-free), diethyl ether (inhibitor-
free), toluene, hexanes, and pentane were purchased from Fisher
Scientific. Solvents were sparged for 20 min with dry Ar and dried
using a commercial two-column solvent purification system (LC
Technologies). Solvents were further dried by storing them over 3 Å
molecular sieves for at least 48 h prior to use. Benzene (DriSolv) was
purchased from Sigma-Aldrich, degassed with three freeze−pump−
thaw cycles, and stored over 3 Å molecular sieves for at least 48 h
prior to use. Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc. C6D6 was degassed with three freeze−
pump−thaw cycles and stored over 3 Å molecular sieves for at least 48
h prior to use. A qualitative assessment of the moisture content in
these solvents was performed by adding 1 drop of a concentrated
solution of a sodium benzophenone radical anion (purple) to 10 mL
Note: Assignment for the bridging versus terminal phenolate in the
13C NMR data was made based on a comparison of the dinuclear 2-
La. The bridging phenolate CAr−O is shifted significantly upfield
(154.1 ppm) in comparison to the corresponding terminal CAr−O (1-
Y, 161.7 ppm; 1-La, 160.9 ppm).
IR (Nujol, cm−1): 2081 [m, ν(Si−H)], 2024 [s, ν(La↼H−Si)],
1575 [s, ν(CN)], 1516 [s, ν(CN)], 1301 (s), 1263 (s), 1199
(w), 1176 (w), 1129 (w), 1101 (w), 1055 (s), 1023 (s), 950 (s), 889
E
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Featured Article
(s), 832 (s), 792 (s), 760 (s), 704 (m), 680 (m), 618 (m), 529 (m),
440 (m).
Elem anal. Calcd for C40H68LaN3O2Si2: C, 58.73; H, 8.38; N, 5.14.
Found: C, 58.83; H, 8.31; N, 4.99.
solid. The crude solid was triturated with pentane (2 × 2 mL) to
remove residual THF, dissolved in toluene (2 mL), and then filtered
through a glass-pipet padded with Celite to remove insoluble
materials. Volatiles were removed under reduced pressure, the residue
was dissolved in pentane (2 mL), and colorless crystals formed after
the solution sat undisturbed for 12 h at RT. The mother liquor was
decanted, and the volatiles were removed under reduced pressure to
yield 3-La as a white solid. Yield: 156.0 mg (0.164 mmol, 67%; MW =
952.30 g mol−1).
Synthesis of 2-La2. A 20 mL scintillation vial was charged with 1-
La (350 mg, 0.43 mmol, 1 equiv; MW = 818.08 g mol−1), a Teflon-
coated stir bar, and THF (1 mL). To the stirring colorless solution
was added dropwise over the course of 1 min benzhydrol (78.8 mg,
0.43 mmol, 1 equiv; MW = 184.24 g mol−1) as a THF solution (1
mL). The solution remained colorless, and after 30 min of stirring at
RT, the volatiles were removed under reduced pressure to yield a
white foam. The crude product was triturated with pentane (2 × 2
mL) to remove residual amine, dissolved in THF (2 mL), and filtered
through a glass-pipet padded with Celite to remove insoluble
materials. Volatiles were removed under reduced pressure, and the
resulting white foam was dissolved in pentane (2 mL). Colorless
blocks formed after the solution sat undisturbed for 6 h at −35 °C.
The mother liquor was decanted, and volatiles were removed under
reduced pressure to afford 2-La2 as a white solid. Yield: 451 mg (0.40
mmol, 94%; MW = 796.87 g mol−1).
3
1H NMR (400 MHz, C6D6, 298 K): δ 0.30 (d, J = 3.0 Hz, 12H,
SiHMe2), 0.93 (br s, 6H, (CH(CH3)2), 1.22 (br s, 6H, (s,
CH(CH3)2), 1.16−1.41 (br s, 6H, CH(CH3)2), 1.16−1.56 (br m,
12H, Cy-CH2), 1.33 (quint, 4H, THF β-CH2), 1.39 (s, 9H,
C(CH3)3), 1.62 (s, 3H, C(N)CH3), 1.68 (s, 9H, C(CH3)3), 1.77
(br d, 3J = 9.9 Hz, 8H, Cy−CH2) 2.19 (s, 3H, C(N)CH3), 2.95 (br s,
1H, CH(CH3)2), 3.37 (br quint, 2H, Cy−CH), 3.47 (br quint, 4H,
THF α-CH2), 3.51 (br s, 1H, CH(CH3)2), 4.53 (br s, 2H, CH2ArO),
4
4.97 (s, 1H, C(N)CH), 6.95−7.08 (m, 3H, Ar−H), 7.29 (d, J = 2.4
4
Hz, 1H, CHArO), 7.52 (d, J = 2.4 Hz, 1H, CHArO).
13C{1H} NMR (100 MHz, C6D6, 298 K): δ −0.11 (d, 3J = 3.0 Hz,
12H, SiHMe2), 22.1 (C(N)CH3), 25.0 (C(N)CH3), 25.2 (β-CTHF),
26.2 (br CH(CH3)2), 26.6 (Cy−CH2), 26.7 (Cy−CH2), 28.0
(CH(CH3)2), 30.7 (C(CH3)3), 32.3 (C(CH3)3), 34.2 (C(CH3)3),
35.5 (C(CH3)3), 36.81 (br Cy−CH2), 37.4 (br Cy−CH2), 51.6
(CH2ArO), 56.1 (Cy−CH), 69.7 (α-CTHF), 98.80 (γ-CH), 122.4
(COAr), 124.3 (COAr), 124.4 (m-CAr), 124.9 (p-CAr), 129.0, 134.4,
136.7,145.0 (CAr), 161.8 (CAr−O, terminal), 163.6 (C(N)), 163.82
(C(N)), 166.43 (C(N)).
3
1H NMR (400 MHz, C6D6, 298 K): δ 0.36 (d, J = 7.0 Hz, 3H,
3
3
CH(CH3)2), 0.79 (d, J = 7.0 Hz, 3H, CH(CH3)2), 1.08 (d, J = 7.0
3
Hz, 3H, CH(CH3)2), 1.40 (d, J = 7.0 Hz, 3H, CH(CH3)2), 1.41 (s,
9H, C(CH3)3), 1.56 (s, 9H, C(CH3)3), 1.70 (s, 3H, (C(N)CH3), 1.96
3
(s, 3H, (C(N)CH3)), 2.57 (quint, J = 7 Hz, 1H, CH(CH3)2), 3.03
(quint, 3J = 7 Hz, 1H, CH(CH3)2), 3.72 (d, 2J = 14 Hz, 1H, ArCH2),
4.36 (d, 2J = 14 Hz, 1H, ArCH2), 4.77 (s, 1H, C(N)CH), 5.75 (s, 1H,
OCHPh2), 6.64 (d, 4J = 2.4 Hz, 1H, CHArO), 6.79 (d, 3J = 7.9 Hz, 1H,
Ar−H), 6.85−6.97 (m, 6H, Ar−H), 7.01 (t, 3J = 7.9 Hz, 2H, Ar−H),
7.07 (t, 3J = 7.9 Hz, 1H, Ar−H) 7.14−7.21 (m, 2H, Ar−H), 7.26 (d,
IR (Nujol, cm−1): 2134 [m, ν(Si−H)], 2100 [m, ν(Si−H)], 1573
[s, ν(CN)], 1513 [s, ν(CN)], 1301 (s), 1259 (s), 1201 (w),
1171 (s), 1135 (s), 1068 (w), 1028 (s), 1002 (s), 966 (s), 911 (s),
875 (s), 806 (s), 788 (m), 774 (m), 661 (m), 613 (w), 524 (w), 483
(w), 447 (s).
4
1H, J = 2.4 Hz, CHArO).
1H NMR (600 MHz, C6D6 + 10 equiv of THF, 298 K): δ 1.02 (d,
3
3J = 7.0 Hz, 6H, CH(CH3)2), 1.10 (d, J = 7.0 Hz, 6H, CH(CH3)2),
Elem anal. Calcd for C53H90LaN5O2Si2: C, 62.14; H, 8.86; N, 6.84.
Found: C, 62.38; H, 9.02; N, 6.57.
1.42 (s, 9H, C(CH3)3), 1.66 (s, 9H, C(CH3)3), 1.67 (s, 3H,
(C(N)CH3), 2.16 (s, 3H, (C(N)CH3)), 3.23 (quint, J = 7 Hz, 2H,
3
Synthesis of 1-Y. A 20 mL scintillation vial was charged with
CH(CH3)2), 4.57 (s, 2H, ArCH2), 4.91 (s, 1H, C(N)CH), 5.98 (s,
1
H2 L (1.09 g, 2.29 mmol, 1 equiv; MW = 476.75 g mol−1),
1H, OCHPh2), 6.64 (d, 4J = 2.4 Hz, 1H, CHArO), 7.02−7.05 (m, 3H,
3
3
Y[N(SiHMe2)2]3(THF)2 (1.44 g, 2.29 mmol, 1 equiv; MW = 630.12
g mol−1), THF (1.32 g, 18.3 mmol, 8 equiv; MW = 78.11 g mol−1),
benzene (2 mL), and a Teflon-coated stir bar. The resulting yellow
solution was heated to 70 °C for 2.5 days. The volatiles were removed
under reduced pressure, forming a yellow foam. The crude product
was dissolved in pentane (3 mL) and THF (2 mL) and filtered
through a glass-pipet padded with Celite to remove insoluble
materials. The volatiles were removed under reduced pressure, and
the resulting yellow solid was triturated with pentane (3 × 2 mL) to
remove residual amine and THF. The crude product was then
dissolved in pentane (3 mL), and colorless crystals formed after that
solution sat undisturbed for 1 h at RT. The mother liquor was
decanted, the crystals were washed with pentane (3 × 3 mL), and
volatiles were removed under reduced pressure to yield 1-Y as a white
solid. Yield: 700.0 mg (0.911 mmol, 40%; MW = 768.08 g mol−1).
Ar−H), 7.08 (d, J = 7.9 Hz, 2H, Ar−H), 7.18 (t, J = 7.9 Hz, 4H,
4
4
Ar−H), 7.26 (d, 1H, J = 2.4 Hz, CHArO), 7.26 (d, 1H, J = 2.4 Hz,
3
CHArO), 7.53 (t, J = 7.9 Hz, 4H, Ar−H).
13C{1H} NMR (100 MHz, C6D6, 298 K): δ 21.8 (C(N)CH3), 23.2
(CH(CH3)2), 24.1 (CH(CH3)2), 24.7 (C(N)CH3), 25.3 (CH-
(CH3)2), 25.4 (CH(CH3)2), 28.4 (CH(CH3)2), 29.1 (CH(CH3)2),
32.1 (C(CH3)3), 32.1 (C(CH3)3), 34.3 (C(CH3)3), 35.6 (C(CH3)3),
55.1 (CH2ArO), 84.14 (OCHPh2), 91.9 (γ-CH), 123.68, 124.03,
124.74, 125.24, 125.59, 126.16, 127.16, 128.19 (CHAr), 132.09,
138.27, 140.10, 141.22, 141.83, 145.82, 149.07, 149.84 (CAr), 154.11
(CAr−O, bridging), 162.56 (C(N)), 163.31 (C(N)).
13C{1H} NMR (152 MHz, C6D6 + 10 equiv of THF, 298 K): δ
21.8 (C(N)CH3), 24.3 (CH(CH3)2), 24.5 (C(N)CH3), 25.3
(CH(CH3)2), 28.0 (CH(CH3)2), 30.4 (C(CH3)3), 32.3 (C(CH3)3),
34.2 (C(CH3)3), 35.5 (C(CH3)3), 55.2 (CH2ArO), 83.8 (OCHPh2),
98.0 (γ-CH), 122.3, 124.3, 124.6, 124.9, 126.0, 127.1, 128.15 (CHAr),
128.3, 129.1, 134.5, 125.8, 144.5, 150.6 (CAr), 161.1 (CAr−O,
bridging), 163.3 (C(N)), 164.1 (C(N)).
3
1H NMR (600 MHz, C6D6, 298 K): δ 0.33 (d, J = 3.0 Hz, 12H,
SiHMe2), 0.79 (d, 3J = 7.0 Hz, 3H, CH(CH3)2), 0.93 (d, 3J = 7.0 Hz,
3
3H, CH(CH3)2), 1.22 (d, J = 7.0 Hz, 3H, CH(CH3)2), 1.22 (s, 4H,
3
THF β-CH2), 1.33 (d, J = 7.0 Hz, 3H, CH(CH3)2), 1.41 (s, 9H,
Note: Only three of the four unique signals for o- and m-
diphenylmethanoxide, 31, 32, 35, and 36 ppm in Figure S4b,e, were
observed in C6D6, which we attribute to the accidental equivalence of
one of the peaks with the signal of C6D6.
C(CH3)3), 1.58 (s, 3H, C(N)CH3), 1.65 (s, 9H, C(CH3)3), 2.08 (s,
3H, C(N)CH3), 2.84 (br s, 1H CH(CH3)2), 3.37 (br s, 1H,
2
CH(CH3)2), 3.51 (br s, 4H, THF α-CH2), 4.57 (d, J = 14 Hz, 1H,
IR (Nujol, cm−1): 1531 [s, ν(CN)], 1234 (s), 1201 (m), 1197
(s), 1166 (m), 906 (w), 976 (w), 934 (w), 878 (w), 802 (w), 775
(m), 758 (m), 671 (w), 604 (w), 592 (w).
ArCH2), 4.97 (s, 1H, C(N)CH), 5.03 (quint, 3J = 3.0 Hz, 1H,
SiHMe2), 5.17 (d, 2J = 14 Hz, 1H, ArCH2), 6.94 (d, 3J = 7.0 Hz, 1H
3
3
m- Ar−H), 6.98 (t, J = 7.5 Hz, 1H, p-Ar−H) 7.05 (d, J = 7.0 Hz,
1H, m-Ar−H), 7.19 (d, 4J = 2.4 Hz, 1H, CHArO), 7.52 (d, 4J = 2.4 Hz,
1H, CHArO).
Elem anal. Calcd for C45H57LaN2O2: C, 67.83; H, 7.21; N, 3.52.
Found: C, 67.97; H, 7.32; N, 3.34.
Synthesis of 3-La. A 20 mL scintillation vial was charged with 1-
La (200 mg, 0.25 mmol, 1 equiv; MW = 818.08 g mol−1), DCC (50.4
mg, 0.25 mmol, 1 equiv; MW = 206.13 g mol−1), THF (90 mg, 1.22
mmol, 5 equiv; MW = 71.10 g mol−1), benzene (2 mL), and a Teflon-
coated stir bar. The light-yellow solution was stirred for 3 h at RT.
Volatiles were removed under reduced pressure and formed a white
13C{1H} NMR (152 MHz, C6D6, 298 K): δ 3.4 (SiHMe2), 21.6 (α-
CH3), 23.8 (CH(CH3)2), 24.7 (CH(CH3)2), 24.80 (α-CH3),
24.83(CH(CH3)2), 25.1 (β-CTHF), 26.2 (CH(CH3)2), 28.1 (CH-
(CH3)2), 28.9 (CH(CH3)2), 30.6 (C(CH3)3), 32.3 (C(CH3)3), 34.3
(C(CH3)3), 35.5 (C(CH3)3), 71.0 (α-CTHF), 99.6 (1H, γ-CH), 122.6
(COAr), 124.0 (m-CAr), 124.2 (COAr), 124.5 (m-CAr), 125.7 (p-CAr),
F
https://dx.doi.org/10.1021/acs.inorgchem.0c02170
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Featured Article
128.35, 129.0, 134.9, 137.5, 144.3, 144.7, 144.9, 161.73 (CAr−O,
terminal), 163.8 (C(N)), 165.6 (C(N)).
End-Group Determination. In a glovebox, a 4 mL vial was
charged with 2-Y (42.30 mg, 0.05165 mmol, 1.0 equiv; MW = 818.98
g mol−1), tricyclohexylphosphine oxide [28.14 mg, 0.1033 mmol, 2.0
equiv; MW = 296.13 g mol−1; 1.185 mL of a 50:50 (v/v) solution of
CH2Cl2/C6H6], and a Teflon-coated stir bar. After approximately 10
min in the freezer (−35 °C), rac-BBL (111.16 mg, 1.29 mmol, 25
equiv; MW = 86.09 g mol−1) was added to the catalyst solution at 0
°C. After 45 min at 0 °C, the reaction was quenched by 5 drops of
acetic acid, and all volatiles were removed under reduced pressure.
The residual oily solid was dissolved in CH2Cl2 (2 mL) and filtered
through a glass pipet padded with silica gel to remove metal
impurities. The CH2Cl2 solution was added dropwise to a stirring
solution of a 70:30 (v/v) solution of pentane/isopropanol (15 mL) to
precipitate the polymer. The solvent was decanted off and all volatiles
were removed under reduced pressure, yielding a white solid that was
used for NMR and GPC characterization.
1H NMR (600 MHz, THF-d8, 298 K): δ 0.07 (d, 3J = 3.0 Hz, 12H,
SiHMe2), 1.08 (br s, 6H, CH(CH3)2), 1.22 (d, 6H, CH(CH3)2), 1.27
(s, 9H, C(CH3)3), 1.39 (s, 9H, C(CH3)3), 1.53 (s, 3H, C(N)CH3),
2.13 (s, 3H, C(N)CH3), 3.09 (br s, 2H CH(CH3)2), 4.65 (br s, 2H,
ArCH2), 4.69 (quint, 3J = 3.0 Hz, 1H, SiHMe2), 4.82 (s, 1H,
C(N)CH), 7.01 (d, 4J = 2.4 Hz, 1H, CHArO), 7.07 (d, 4J = 2.4 Hz, 1H,
CHArO), 7.15 (t, 3J = 7.67 Hz, 1H, Ar−H), 7.20 (d, 3J = 7.67 Hz, 2H,
Ar−H).
IR (Nujol, cm−1): 2095 [br m, ν(Si−H)], 2032 [br m, ν(Y↼H−
Si)], 1574 [s, ν(CN)], 1518 [s, ν(CN)], 1393 (s), 1313 (s),
1239 (s), 1201 (s), 1177 (w), 1131 (w), 1102 (w), 1028 (s), 941 (s),
898 (s), 834 (s), 795 (m), 761 (m), 681 (m), 622 (w), 536 (w), 460
(w).
Elem anal. Calcd for C40H68N3O2Si2Y: C, 62.55; H, 8.92; N, 5.47.
Found: C, 62.26; H, 8.81; N, 5.16.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02170.
Synthesis of 2-Y. A 20 mL scintillation vial was charged with 1-Y
(300 mg, 0.39 mmol, 1 equiv; MW = 768.08 g mol−1), THF (140 mg,
1.95 mmol, 5 equiv; MW = 78.11 g mol−1), benzene (1 mL), and a
Teflon- coated stir bar. The resulting colorless solution was frozen in a
glovebox freezer (mp ∼5.5 °C), along with a benzene (1 mL) solution
of benzhydrol (72 mg, 1.95 mmol, 1 equiv; MW = 184.24 g mol−1).
The solutions were removed from the freezer, and the benzhydrol
solution was added dropwise (∼1 min) to the rapidly stirring solution
of 1-Y immediately upon thawing. After 30 min at RT, volatiles were
removed under reduced pressure to afford a white foam. The crude
product was dissolved in pentane (2 mL) and filtered through a glass-
pipet padded with Celite to remove insoluble materials. Volatiles were
removed under reduced pressure, and the resulting white foam was
dissolved in pentane (1.5 mL) and THF (0.1 mL). A white
microcrystalline material formed upon standing for 2 h at −35 °C.
The mother liquor was decanted, the crystals were washed with
pentane (0.5 mL), and the volatiles were removed under reduced
pressure to yield 2-Y as a white solid. Yield: 300 mg (0.337 mmol,
94%; MW = 818.98 g mol−1).
■
sı
Additional discussion of experimental details (X-ray
crystallography, DOSY NMR, and variable-temperature
1
NMR), synthetic description of the ligand (H2 L), and
spectra (IR and NMR), along with additional figures,
tables, and references (PDF)
Accession Codes
CCDC 2017875, 2017878, 2017879, 2017888, and 2017890
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, or by emailing data_request@ccdc.
cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033.
3
1H NMR (600 MHz, C6D6, 298 K): δ 1.07 (d, J = 7.0 Hz, 6H,
CH(CH3)2), 1.11 (br d, 3J = 7.0 Hz, 3H, CH(CH3)2), 1.21 (br s, 8H,
THF β-CH2), 1.43 (s, 9H, C(CH3)3), 1.60 (s, 3H, C(N)CH3), 1.68
(s, 9H, C(CH3)3), 2.03 (s, 3H, C(N)CH3), 3.22 (br s, 2H,
CH(CH3)2), 3.42 (br s, 8H, THF α-CH2), 4.60 (br s, 2H, ArCH2),
4.84 (s, 1H, C(N)CH), 6.07 (s, 1H, OCHPh2), 7.00−7.07 (m, 6H,
Ar−CH), 7.14−7.18 (m, 4H, Ar−CH), 7.52−7.54 (m, 5H, Ar−CH).
13C{1H} NMR (152 MHz, C6D6, 298 K): δ 21.6 (C(N)CH3), 24.1
(C(N)CH3), 24.3 (CH(CH3)2), 25.0 (CH(CH3)2), 25.4 (β-CTHF),
28.3 (CH(CH3)2), 30.5 (C(CH3)3), 32.3 (C(CH3)3), 52.8 (ArCH2),
69.3 (α-CTHF), 82.1 (OCHPh2), 98.8 (C(N)CH), 122.5, 124.3, 124.4,
126.4, 127.2, 128.2, 128.4, (Ar−CH), 129.15, 135.15, 136.9, 144.1,
144.8, 149.9, 162.03 (CAr−O, terminal), 162.92 (C(N)), 166.0
(C(N)).
AUTHOR INFORMATION
Corresponding Author
■
Jerome R. Robinson − Department of Chemistry, Brown
University, Providence, Rhode Island 02912, United States;
orcid.org/0000-0002-9111-3822;
Email: Jerome_robinson@brown.edu
Authors
Kerry C. Casey − Department of Chemistry, Brown University,
Providence, Rhode Island 02912, United States; orcid.org/
0000-0003-3962-3873
Jude K. Appiah − Department of Chemistry, Brown University,
Providence, Rhode Island 02912, United States
IR (Nujol, cm−1): 1548 [s, ν(CN)], 1515 [s, ν(CN)], 1308
(s), 1231 (m), 1186 (m), 1135 (s), 1069 (m), 1025 (s), 919 (w), 873
(m), 833 (m), 791 (m), 767 (m), 702 (w), 621 (w), 605 (w), 531
(w), 471 (w), 448 (w).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02170
Elem anal. Calcd for C49H65N2O3Y: C, 71.86; H, 8.00; N, 3.42.
Found: C, 71.28; H, 8.02; N, 4.04.
Notes
Polymerization of rac-BBL. Reactions at 0 °C. In a glovebox, a J.
Young NMR tube was charged with a 2-Y (4.75 mg, 0.0058 mmol, 1.0
equiv; MW = 818.98 g mol−1) from a 50:50 (v/v) solution of
CH2Cl2/C6H6 (0.244 mL total). Tricyclohexylphosphine [OPCy3;
2% (w/w) in a 50:50 (v/v) solution of CH2Cl2/C6H6, 0.162 mL, ρ =
1.06 g mL−1; 3.44 mg, 0.0116 mmol, 2.0 equiv; MW = 296.13 g
mol−1] was then added to the clear colorless solution. The total
volume was brought up to 0.485 mL by an additional volume of the
CH2Cl2/C6H6 solvent mixture (0.079 mL). After approximately 10
min in the freezer (−35 °C), rac-BBL (100 mg, 1.16 mmol, 200 equiv;
MW = 86.09 g mol−1) was added to the catalyst solution at 0 °C.
After 3 h at 0 °C, the reaction was quenched by 2 drops of acetic acid,
and all volatiles were removed under reduced pressure.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Brown University for support of this research and
Audra J. Woodside for helpful discussions.
■
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