Diiminopyrrolide Copper Complexes: Synthesis, Structures, and rac-Lactide Polymerization Activity

Article

pubs.acs.org/Organometallics

Diiminopyrrolide Copper Complexes: Synthesis, Structures, and rac-

Lactide Polymerization Activity

Pargol Daneshmand, Solene Fortun, and Frank Schaper*

̀

́ ́ ́

Centre in Green Chemistry and Catalysis, Department of Chemistry, Universite de Montreal, C. P. 6128 Succ. Centre-Ville, Montreal,

Quebec H3T 3J7, Canada

́

S

* Supporting Information

ABSTRACT: 2,5-Diiminopyrrole ligands with N-xylyl (L3), N-

naphthyl (L4), N-cyclohexyl (L5), N-diphenylmethyl (L6), and N-

p-bromobenzyl (L7), as well as the 5-R-2-iminopyrroles with N-S-

methylbenzyl (L8, R = H; L10, R = Me; L11, R = Cl) and N-

benzyl (L9, R = H) were reacted with Cu(OMe)₂and

pyridylmethanol (R¹OH) or dimethylaminoethanol (R²OH) to

yield the corresponding dinuclear complexes L₂Cu₂(μ-OR)₂.

Reactions in the absence of chelating R¹OH or R²OH only

yielded homoleptic L₂Cu or L₂Cu₂. Complexes with R²OH were

obtained with all ligands but L4, complexes with R¹OH for all

ligands but L3, L4, and L6. All complexes but those with L7

displayed dinuclear crystal structures with pentacoordinated

copper centers and bridging alkoxy groups. The alkoxy group

and the pyrrolide ligand were consistently found in equatorial

positions. Imine, pyridine, or amine occupied the axial position. L7 coordinated both imino groups to copper, yielding a

distorted-octahedral coordination geometry. All complexes containing R¹OH, with the exception of L7, showed an isotactic bias

in the polymerization of rac-lactide, with a maximum P_mvalue of 0.7. Complexes containing R²OH provided atactic PLA or, in

the case of L3 and L6, heterotactic PLA. Reduced stereocontrol in monoiminopyrrolide copper complexes and lack of

stereocontrol with octahedrally coordinated L7 indicate that the catalytic site needs to adapt to chain-end chirality and is

participating in enantiomer selection.

microstructure via the catalyst instead of the monomer.³While

high selectivity toward the heterotactic polymer is obtained

comparatively easily, high selectivity toward isotactic PLA is

diﬃcult and very few, if any, catalyst systems combine high

isoselectivity, high activity, and high catalyst stability.⁴

INTRODUCTION

■

Polylactide or polylactic acid (PLA) is one of the most

successful biodegradable polymers marketed today.¹PLA is

currently obtained on a commercial scale by unselective chain-

growth, ring-opening polymerization of lactide, the dimeric

anhydride of lactic acid (Scheme 1). To achieve the desired

isotactic PLA, enantiopure lactide, obtained by fermentation of

corn starch, is employed.²Given its economic importance and

in analogy to α-oleﬁn polymerization, a large number of studies

have been dedicated to the development of single-site,

coordination−insertion catalysts for the polymerization of

lactide, which ideally would enable control of the polymer

For the most part, catalysts for the coordination−insertion

polymerization of lactide are based on either d⁰or d¹⁰metals,

such as alkali and alkaline-earth metals,^3j,k,p−sgroup 3 and rare-

earth metals,^3f,j,mgroup 4 metals,^3j,m,t,uzinc,^{3b,j−m,p−r}and group

13 metals.^{3f,i−m,p,v−aa}The chemistry of mid-to-late transition

metals with partially ﬁlled d shells, on the other hand, remains

little explored. Of these, iron⁵and copper⁶have received the

most attention. Work on iron-based catalysts by Gibson,

Tolman, and Hillmyer resulted in some highly active

catalysts^5s−vbut was not much pursued further, mostly because

of the lack of stereocontrol. More current work from the Byers

group has concentrated on the possibility of controlling the

polymerization by switching the redox state of the iron

center.^5c−e,m,nCopper-based systems were initially investigated

to replace stannous octanoate in molten-monomer polymer-

izations and were either simple carboxylate salts^6d,l,n,ror

Scheme 1

Received: August 8, 2017

A

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complexes with phenoxyimine or salen ligands.^{6e−g,m,o−q}

Activities were typically moderate, requiring several hours or

days at elevated temperatures or in the melt to reach

completion. Copper catalysts often displayed, however, very

good polymer molecular weight control.^6e−g,l−rKumar and

Maharana reported a surprisingly active copper salen/benzyl

alcohol catalyst, which polymerized lactide in 12−24 h at room

temperature in solution.^6e−gIn the absence of benzyl alcohol,

the complex was inactive. The same salen copper complex had

been used previously without additional alcohol in melt

polymerizations and required 24 h at T > 140 °C for

polymerizations to reach completion.^6oIn 2012, we reported a

discrete copper alkoxide complex carrying a diketiminate

spectator ligand, (nacnac)CuOiPr, which was highly active in

the coordination−insertion polymerization of lactide, complet-

ing polymerization in less than 2 min at ambient temper-

ature.^6i−kAs with all other copper catalysts reported before,

PLA produced with (nacnac)CuOiPr was atactic. Shortly after,

Jeong reported similarly high activities with a diamino copper

catalyst formed in situ from the respective copper chloride

complex and lithium alkoxide.^6a−cIn addition to high activities,

these catalysts showed preference for heterotactic monomer

insertion (P_r= 0.65−0.88), the ﬁrst time notable stereocontrol

was observed for copper-based catalysts.

RESULTS AND DISCUSSION

■

Ligand Synthesis. Known and new diiminopyrroles L1−

L7 and iminopyrroles L9−L11 were prepared by condensation

of diformyl- or monoformylpyrrole with the respective amines,

using techniques described previously for L1,⁷L2,⁷L3,⁸L8,⁹

and L9¹⁰(Scheme 3).

Scheme 3

Attempted Synthesis of Copper Alkoxide Complexes.

Previous work on copper diketiminate complexes revealed

diﬃculties in preparing the desired heteroleptic LCuOR

complexes.^6jWhen saturation of the open coordination site

required for polymerization activity was not possible by

dimerization of the complex, the latter decomposed either by

ligand exchange to the homoleptic copper(II) complex or by β-

H elimination from the isopropoxide ligand (Scheme 4).

In a continuation of our work on copper diketiminate

complexes, we prepared copper diiminopyrrolide complexes 1

and 2.^6hThe complexes showed good polymerization activity

(3−8 h at room temperature to completion). More

importantly, they provided isotactically enriched PLA from

rac-lactide (P_m= 0.7), the ﬁrst copper-based catalysts showing

the desired preference for isotactic insertion (Scheme 2).^6hIn

Scheme 4

Scheme 2

an unexpected dependence of stereocontrol on the initiating

group, the respective complexes 1b and 2b, which contain

dimethylaminoethoxide instead of pyridylmethoxide ligands,

provided atactic PLA. The polymerization mechanism was

investigated in detail and showed that all complexes remain

dinuclear throughout the polymerization reaction.⁷Depending

on the initiating alkoxide, either one or both alkoxides initiate

chain growth, and stereocontrol depends strongly on the

bridging ligand in the dinuclear active species (Scheme 2).⁷

Stereocontrol was independent of ligand chirality, and only

minor diﬀerences were observed between 1 and 2. In the

current study we explore how further variations of the

iminopyrrole spectator ligand aﬀect reactivity and stereocontrol

in rac-lactide polymerization.

Diiminopyrrolide ligands, such as L1−L7, are tridentate ligands,

but formation of two adjacent ﬁve-membered metallacycles is

disfavored for metals with small atomic radii. Consequently,

approximately half of the diiminopyrrolide metal complexes

characterized by X-ray diﬀraction studies show bidentate

coordination of diiminopyrrolide, with one uncoordinated

imino group.¹¹In the case of copper as a central metal, Bailey

et al. reported the synthesis of a homoleptic L₂Cu complex (L₂

= macrocyclic bis(diiminopyrrolide) ligand) with bidentate

coordination of the diiminopyrrolide moieties and of putative

LCu(OR) complexes which were not structurally character-

ized.¹²Babu et al. reported (L^diip)₂Cu(MeOH) (L^diip= 2,5-

bis(diisopropylphenylimino)pyrrolide), which interestingly

showed bidentate coordination of one and tridentate

B

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coordination of the other diiminopyrrolide ligand.¹³Diimino-

pyrrolide complexes have not been used in lactide polymer-

ization, but diiminopyrrolide yttrium and lanthanide complexes

were active in the polymerization of ε-caprolactone.^8,14We

speculated that diiminopyrrolide ligands would be able to

stabilize the open coordination site in a square-planar copper

alkoxide complex, thus avoiding the decomposition reactions

observed for copper diketiminates. In the presence of lactide

monomer they would likely easily isomerize to bidentate

coordination (Scheme 4).

A large number of attempts to prepare heteroleptic LCuOiPr

complexes by reaction of Cu(OiPr)₂with L1−L4 were

unsuccessful. Variation of reaction solvent (toluene, ether,

THF, dichloromethane, acetonitrile), reaction temperature

(room temperature or 50 °C), order of addition, or

stoichiometry (0.45−1.2 equiv of Cu(OR)₂) did not yield the

desired complexes. Several reactions yielded crystalline material

(Table S1 in the Supporting Information), which allowed us to

trace the outcome of these reactions.¹⁵Homoleptic bis-

(diiminopyrrolide) complexes L₂Cu (1c and 4c) were isolated

from reactions of L1 and L4 with Cu(OiPr)₂. The crystal

structure of 1c (Figure 1 and Table 1) shows octahedral

that of the κ³-coordinated diiminopyrrole ligand in L^diip₂Cu-

(MeOH) (Table 1).¹³Complex 1c appeared initially to be one

of the few examples of Cu(II) complexes where Jahn−Teller

distortion results in a compressed octahedral geometry instead

of the more typical elongated geometry.¹⁶Closer inspection of

the thermal displacement parameters, however, reveals that

they contain an unusually large component parallel to the Cu−

imine bond. It thus seems likely that the complex actually

adopts an elongated octahedral geometry, with two trans-

positioned Cu−imine bonds elongated. Disorder of the

elongation direction then complies with crystallographic

symmetry. An apparent compressed octahedral geometry by

disorder of an elongated geometry is an often observed

phenomenon: for example, in Cu(II) bis-terpyridine com-

plexes.¹⁷

The coordination of the pyrrolide ligand in 4c seems to be

intermediate between κ²and κ³coordination (Figure 1): the

diﬀerence in Cu−N_imine,1and Cu−N_pyrroleis reduced to half,

while the two Cu−N_iminedistances and N_imine−Cu−N_pyrrole

angles become notably diﬀerent (Table 1). On the basis of

the Cu−N_imine,2distances of 2.950(2) and 2.998(2) Å, which

are comparable to the sum of the van der Waals (vdW) radii

(2.95 Å), N_imine,2might be considered nonbonding and the

coordination mode κ². The resulting coordination geometry

around copper would be highly distorted, intermediate between

square-planar and tetrahedral coordination (τ₄= 0.5),¹⁸but not

unprecedented for copper. However, N_imine,2in 4c retains the s-

trans conformation of the κ³coordination mode instead of the

sterically favorable s-cis conformation of the κ²coordination,¹³

which would also allow for a less distorted square planar

geometry. Despite the long distances, there thus seem to be

stabilizing interactions of N_imine,2with copper and 4c is best

described as a strongly distorted octahedral coordination

geometry. While pseudo-octahedral coordination geometries

with two elongated cis ligands have been reported for Cu(II),

the distortion is typically much weaker. The only example in

the Cambridge Structural Database of a Cu(II) complex with

four short (<2.4 Å) and two long, cis ligands (>2.8 Å, angle 90

30°) shows a similar geometry but with two large selenium

atoms occupying the weak coordination side.¹⁹Complex 4c

thus seems to be an extreme example of this type of octahedral

distortion.

Figure 1. X-ray structures of 1c (left) and 4c (right). Hydrogen atoms

are omitted for clarity. Thermal ellipsoids are drawn at the 50%

probability level.

Table 1. Selected Bond Lengths (Å) and Angles (deg) of 1c

and 4c

a

L^diip₂Cu(MeOH)

Alternatively, reactions of L1−L4 with Cu(OiPr)₂sometimes

yielded dinuclear Cu(I) complexes, L₂Cu₂(Table S1 in the

Supporting Information), in yields ranging from traces to 70%.

Figure 2 shows the structures obtained for 2d and 3d. The

coordination chemistry around copper is best described as a

linear coordination to one pyrrolide and one imino group, both

with identical Cu−N distances of 1.88−1.91 Å (Table S2 in the

1c

4c

κ³-L^diip

κ²-L^diip

Cu−N_pyrrole

Cu−N_imine,1

Cu−N_imine,2

1.860(4) 1.882(2), 1.883(2) 1.941(3)

2.537(4) 2.151(2), 2.184(2) 2.738(3)

2.537(4) 2.950(2), 2.998(2) 2.738(3)

73.38(9) 80.55(6), 80.52(6) 70.8

73.38(9) 64.87(6), 64.47(6) 70.8

2.013(3)

2.023(3)

5.1

b

c

N_py−Cu−N_im,1

N_py−Cu−N_im,2

83.1(1)

33.9

a

Data taken from ref 13. L^diip= 2,5-bis(diisopropylphenylimino)-

pyrrolide. 1c, N1−Cu1−N2; 4c, N1−Cu1−N3, N11−Cu2−N13.

1c, N1−Cu1−N2; 4c, N1−Cu1−N2, N11−Cu2−N12.

b

c

coordination of copper by two tridentate diiminopyrrole

ligands. The complex is highly symmetric with the copper

atom on the intersection of three crystallographic C₂axes, and

all Cu−N_iminedistances are thus crystallographically equivalent.

The steric constraints of the diiminopyrrolide ligand result in

some deviations from ideal octahedral geometry. Thus, Cu−

N_iminedistances are signiﬁcantly (≈0.7 Å) longer than Cu−

Figure 2. X-ray structures of 2d and 3d. Thermal ellipsoids are drawn

at 50% probability. Hydrogen atoms, other independent molecules

(3d), and minor components of disordered N substituents (3d) are

omitted for clarity.

N

_pyrroledistances. Due to crystallographic symmetry, all N_imine−

Cu−N_pyrroleangles are equal but deviate from ideal octahedral

symmetry (73.38(9)°). The coordination is highly similar to

C

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Supporting Information). An additional weak coordination of

another imino group with notably longer Cu−N distances

(2.4−2.5 Å) and an intramolecular copper−copper interaction

are likewise present. Cu−Cu distances in 2d and 3d are 2.60 Å

and are in the typical range of cuprophilic interactions in

dinuclear Cu(I) complexes (d_Cu−Cu= 2.55(5) Å, based on 78

structures in the CSD).¹¹

Scheme 6

The combination of these results allowed us to trace the

probable reaction pathway for reactions with copper isoprop-

oxide (Scheme 5). Protonation of one isopropoxide group

Scheme 5

elongated Cu−N distances of 2.2−2.4 Å (Table 2). Deviations

of the τ value from 0 are thus likely caused by the small bite

angle of the ﬁve-membered metallacycles, but the bonding

situation around copper represents square-pyramidal geometry.

The anionic ligands (pyrrolide and bridging oxygen atoms)

occupy an equatorial position in all complexes. Bridging

coordination of an alkoxide ligand is thus preferred over the

coordination of the imino group, which explains the motivation

for the formation of pentacoordinated dimers instead of square-

planar monomers.

likely yields the desired heteroleptic complex LCuOiPr.

Although three-coordinate coordination is observed in 1c and

4c, it either is not present in LCuOiPr or is in thermally

accessible equilibrium with the κ²coordination mode. Similar

to the observed reactivity of diketiminate complexes, (κ²-

L)CuOiPr either allows reaction to the homoleptic complex

(most likely by ligand exchange) or succumbs to β-H

elimination of the isopropoxide ligand. The copper(II) hydrides

thus formed are not stable and undergo homolytic bond

cleavage to the respective Cu(I) complexes.

Reactions with Chelating Alcohols. Since tridentate

coordination of diiminopyrrolide was too weak to stabilize the

desired heteroleptic copper complexes, alcohols carrying a

functional group able to coordinate to copper were employed.

Methyl lactate shows the strongest resemblance to the growing

polymer chain, but none of the numerous attempts to prepare a

LCu(methyl lactate) complex were successful. This is some-

what surprising, given the strong similarity of polymeryloxy and

methyl lactate ligands and the stability of the complexes in this

study under polymerization conditions (vide infra). Likewise

unsuccessful were attempts to prepare LCuOR with ROH

being 2-iminophenol or dipyridylmethanol. Reactions with

Cu(OMe)₂or Cu(OiPr)₂in the presence of pyridylmethanol,

on the other hand, yielded the desired diiminopyrrolide

complexes 1, 2, 5, and 7 and the monoiminopyrrolide

complexes 8−11 (Scheme 6).

All complexes were characterized by X-ray diﬀraction studies

(Figure 3; the synthesis and structures of 1, 2, and 8 were

reported previously).^6h,7Although square-planar coordination

is sterically possible, all complexes crystallize as oxygen-bridged,

pentacoordinated dimers. While τ values²⁰indicate a

coordination geometry intermediate between square pyramidal

and trigonal bipyramidal (Table 2), observed bond distances

are in better agreement with square-pyramidal coordination:

short bond lengths are observed for the two μ-oxy ligands and

for two of the nitrogen ligands (1.9−2.0 Å), while the

remaining nitrogen ligand in the apical position shows

A greater structural ﬂexibility is observed in the general

coordination geometry. As long as the general requirement of

square-pyramidal coordination with anionic pyrrolide and

alkoxy ligands in the equatorial plane is respected, several

possible isomers were observed. Complexes 10 and 11 show

the same geometry as 1: inversion symmetry of the copper

coordination environment with the imine group in an apical

position. Complex 9 shows the same inversion-symmetric

coordination environment (which due to the absence of the

chiral N substituent now coincides with crystallographic

inversion symmetry), but the pyridine ligand is occupying the

axial position. In previously reported 2,⁷the asymmetric unit

contains two molecules, one having the pyridine and the other

the imine in an apical position. In complex 5, imine occupies

the apical position, but the coordination geometry is now C₂

symmetric, with identical chirality of the copper centers and

both pyridine groups on the same side of the Cu₂O₂plane. This

allowed for one, but not both, of the diiminopyrrole ligands in

5 to adopt an s-cis, s-cis conformation and a weak (Cu−N = 3.4

Å) interaction of the second imino group with copper.

Complex 7, which only diﬀers from 2 by the presence of a p-

bromine on the N-benzyl substituent, shows an s-cis, s-cis

conformation and tridentate coordination of both diiminopyr-

role ligands. Although the Cu−N distance of the second imino

group (2.9 Å) is barely below the sum of the vdW radii, the

coordination geometry around copper should now be

considered octahedral. There are no obvious steric reasons

for the observed variations in coordination geometry. Complex

7, for example, is clearly stabilized by triple-decker π stacking

between the N-benzyl substituents and pyridylmethoxide

(angles between planes 7 and 16°, shortest atom−plane

D

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Figure 3. X-ray structures of 5, 7, and 9−11. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms, toluene solvent (11), and a

second independent molecule of similar geometry (10 and 11) are omitted for clarity.

a

Table 2. Selected Geometric Data for Pyridylmethoxide Complexes 1, 5, and 7−11

b

c

1

5

7

8

9

10

11

Cu−N_Pyrrole

Cu−N_imine

Cu−O_short

Cu−O_long

Cu−N_Pyridine

Cu−Cu

1.945(2),

1.964(2)

1.955(2),

1.978(2)

1.936(9)

1.9309(13),

1.9347(13)

1.960(2)

1.923(4),

1.934(4)

1.948(4),

1.934(4)

1.953(4),

1.959(4)

1.940(4),

1.950(4)

2.294(3),

2.242(3)

2.288(3),

2.343(3)

2.484(11),

2.862(11)

2.3170(15),

2.3976(13)

2.047(2)

1.974(2)

1.986(2)

2.173(2)

2.992(1)

2.274(4),

2.390(4)

2.251(4),

2.248(4)

2.286(4),

2.273(4)

2.296(4),

2.415(4)

1.915(2),

1.944(2)

1.941(2),

1.949(2)

1.908(7)

1.940(7)

2.011(8)

3.018(3)

1.9271(11),

1.9306(11)

1.915(3),

1.933(3)

1.922(3),

1.935(3)

1.926(3),

1.926(3)

1.922(3),

1.924(3)

1.960(2),

1.960(2)

1.982(2),

1.966(2)

1.9507(11),

1.9507(11)

1.972(3),

1.936(3)

1.998(3),

2.000(3)

1.981(3),

1.992(3)

1.959(3),

1.937(3)

2.025(3),

1.995(3)

2.024(3),

2.008(2)

2.0222(13),

2.0261(13)

2.028(4),

1.989(4)

2.056(4),

1.993(5)

2.040(4),

2.028(4)

2.029(4),

1.989(4)

3.025(5)

2.9199(6)

3.0628(5),

3.0395(4)

3.0267(10)

3.035(1)

3.019(1)

3.018(1)

τ

0.6, 0.4

C, A

0.6, 0.4

C, C

0.4, 0.4

0.6, 0.6

A, C

0.6, 0.4

C, A

0.7, 0.5

C, A

0.7, 0.5

C, A

0.6, 0.3

C, A

chirality at Cu

C, A/C, A

imine, imine

atom in apical

position

imine, imine imine, imine

pyridine,

pyridine

imine, imine imine, imine imine, imine imine, imine

a

The second values cited refers to the second copper center of the dimer, if crystallographically inequivalent. Two columns are used for molecules

b

c

having two independent molecules in the asymmetric unit. Taken from ref 6h for comparison. Taken from ref 7 for comparison.

distance 3.6 Å), but it is unclear why such stacking interactions

should be more favorable for bromine-substituted 7 than for

unsubstituted 2.

complexes above. Unsurprisingly, coordination of the second

imino group leads to further elongation of the Cu−N_imino

distance, but the eﬀects seems to be subtle: the structure of 6e

contains three independent molecules. One shows diﬀerent

Cu−N_iminodistances of 2.6 and 3.1 Å and is best described as

square pyramidal with an additional weak interaction. On the

other extreme, one of the molecules shows equal Cu−N_imino

distances of 2.7 and 2.8 Å and should be considered a Jahn−

Teller-distorted octahedral coordination. The presence of such

a wide range of distances in the same asymmetric unit indicates

that interactions of Cu with the ligands in the apical positions

are very weak. The copper center containing the chloride ligand

shows a square-planar coordination geometry and is unaﬀected

by the observed variation in the coordination mode of

diiminopyrrolide.

Pyridylmethoxide complexes could not be prepared for

sterically demanding ligands L3, L4, and L6. In the case of L6,

the mixed dinuclear complex LCu(μ-OCH₂Py)₂CuCl (6e) was

obtained (Figure 4 and Table S3 in the Supporting

Information). The absence of a diiminopyrrole ligand on the

other copper center allows a weak coordination of the second

imino group to the copper center (Cu−N = 2.6−3.1 Å) and the

ligand adopts the s-cis, s-cis conformation observed in 7,

instead of the s-cis, s-trans conformation in the other

L₂Cu(OR)₂complexes, which orients the second imino group

away from the metal. Tridentate coordination is further

stabilized by weak hydrogen bonding between the aromatic

protons of the diphenylmethyl substituent with the heter-

oatoms in the equatorial plane. However, similar weak

hydrogen bonds were observed also in the L₂Cu(OR)₂

A complex similar to 6e, 1e, has been obtained in early

synthetic attempts from reactions of chloride-contaminated

Cu(OiPr)₂. The structure of 1e is similar to that of 6e. Again,

E

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Reactivity similar to that for pyridylmethanol was observed

for reactions of Cu(OMe)₂with L1−L11 in the presence of

dimethylaminoethanol (Scheme 6). The latter seems to be

sterically less demanding, and dinuclear L₂Cu₂(μ-

OC₂H₄NMe₂)₂complexes 1b−11b were obtained for all

ligands but L4. In the latter case, the respective mixed complex

(L4)Cu(μ-OR)₂CuCl (4e) was obtained instead after recrystal-

lization from dichloromethane, the crystal structure of which is

similar to that of 1e and 6e (Figure 4 and Table S3 in the

Supporting Information). Analogously to 6e (Scheme 7), we

suspect that the inability to form dimeric 4b led to partial

decomposition of (L4)Cu(OC₂H₄NMe₂).

Since aminoethoxide complexes were shown to be more

easily accessible, we attempted to prepare 3 and 6 by reaction

of 3b and 6b with pyridylmethanol. However, none of these

attempts (nor attempts to obtain 1 from 1b) were successful. It

thus does not seem possible to replace the chelating alkoxide

ligand once the dinuclear complex is formed.

Complexes 1b−3b and 5b−11b yield very similar dinuclear

structures (Figure 5 and Table 3; synthesis and structures of 1b

and 2b have been reported previously).^6h,7In comparison to

the respective pyridylmethoxide complexes, the amino group

has a slightly higher tendency to coordinate to the apical

position, e.g. 2 (pyridyl or imine in the apical position) vs 2b

(amine) and 5 (imine) vs 5b (amine). The weaker

coordination of the amino group to copper in comparison to

pyridyl indicated by the X-ray structures is in line with lactide

reactivity (vide infra), in which pyridylmethoxide complexes in

general show longer induction periods. Bond distances of the

equatorial ligands, Cu−N_pyrroleand Cu−O, in 1b−11b diﬀer by

less than 5 pm from those of the respective pyridylmethanol

complexes 1−11 (Table 3). Variation of the pyrrole substituent

in 8, 10, and 11 or 8b, 10b and 11b among H, Me, and Cl

likewise does not seem to have an eﬀect on structural features:

bond distances to Cu diﬀer by less than 3 pm for Cu−N_pyrrole

and Cu−O and there is no evident correlation with electronic

or steric factors. The nature of the imino substituent also shows

little eﬀect on Cu−X bond lengths: Cu−N_pyrrole(1.92−1.98 Å),

C−O_short(1.90−1.97 Å), and Cu−O_long(1.94−2.01 Å) diﬀer by

only 6−7 pm for all dinuclear complexes regardless of the

nature of the imino substituent or the complex geometry, and

again without evident correlation to steric or electronic factors.

Changing the nature of the bridge, variation of the imino N

substituent, or even complete replacement of the imino group

by methyl or chloride thus does not have any signiﬁcant eﬀect

on equatorial Cu−X bond distances.

Figure 4. Crystal structures of 1e, 4e, and 6e. Hydrogen atoms and

two of the three independent molecules in 6e were omitted for clarity.

Thermal ellipsoids were drawn at the 50% probability level.

the all s-cis conformation of the ligand allows a weak (Cu−N =

3.1 Å) coordination of the second imino group (Figure 4 and

Table S3 in the Supporting Information). In the case of 6e, the

Cu(OMe)₂starting material was analytically pure and chloride

impurities cannot explain the obtained crystallized yield of 29%.

Most likely, the inability to form the stabilized dimer 6 resulted

in (partial) decomposition to a Cu(I) complex, similar to

reactions in the absence of pyridylmethanol. Upon recrystal-

lization of the oily reaction mixture in dichloromethane,

copper(I) species were reoxidized by abstraction of a chlorine

from the solvent and 6e was formed by reaction with

pyridylmethanol (or copperbispyridylmethoxide) present in

the reaction mixture (Scheme 7). Complexes 1e and 6e did not

initiate lactide polymerization at room temperature (no

conversion even after 48 h) and will not be discussed further.

The only exception is again L7, where the seemingly minor

variation of bromination in the p-benzyl position led to a

notably diﬀerent structure. Complex 7b has a τ value of 0.8.

More importantly, the imine and the amine ligand in 7b are

found with essentially identical Cu−N bond distances of

2.162(7) and 2.130(7) Å, while 0.2−0.4 Å diﬀerences are

observed for the other aminoethoxide complexes. The

coordination geometry in 7b is thus trigonal bipyramidal

instead of the square-pyramidal geometry observed in all other

cases.

Scheme 7

Given the wide variety of coordination geometries, the lack

of obvious correlation with steric or electronic factors, and

essentially invariant Cu−X bond distances, the diﬀerent

coordination isomers are probably very close in energy, so

that even small eﬀects can lead to the observed geometries. In

solution, the coordination geometry likely adapts easily to even

small changes in the complex environment.

F

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Organometallics

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Figure 5. X-ray structures of 3b and 5b−11b. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and disorder of the N

substituent (3b, 8b) are omitted for clarity.

a

Table 3. Selected Geometric Data for Dimethylaminoethoxide Complexes 3b and 5b−11b

b

1b

3b

5b

6b

7b

8b

9b

10b

11b

Cu−N_pyrrole

Cu−N_imine

Cu−O_short

Cu−O_long

Cu−N_amine

1.924(12),

1.924(12)

1.9617(12)

1.9609(15)

1.9621(15)

1.954(7)

1.9507(12)

1.9639(9)

1.964(4),

1.961(4)

1.969(3),

1.944(3)

2.327(11),

2.244(10)

2.535(8)

2.0184(14)

1.9607(12)

1.9817(12)

2.3100(17)

2.0697(15)

1.9510(12)

2.0138(13)

2.2409(16)

2.163(7)

1.934(6)

1.995(6)

2.124(7)

2.3361(13)

1.9206(11)

1.9628(11)

2.0731(14)

2.0444(8)

1.9615(7)

1.9847(7)

2.2628(9)

2.382(4),

2.343(4)

2.339(3),

2.458(3)

1.896(9),

1.896(8)

1.9295(11)

1.9542(11)

2.0785(13)

1.930(3),

1.943(3)

1.923(3),

1.922(3)

1.965(8),

1.993(9)

1.954(3),

1.975(3)

1.956(3),

1.958(3)

2.048(12),

2.056(14)

2.094(4),

2.082(4)

2.062(3),

2.088(3)

Cu−Cu

τ

3.014(2)

0.5, 0.5

3.0770(8)

0.4, 0.4

C, A

2.9753(5)

0.4, 0.4

C, A

2.9824(5)

0.5, 0.5

A, C

2.952(2)

0.8, 0.8

A, C

3.0325(4)

0.3, 0.3

C, A

2.9579(3)

0.4, 0.4

C, A

3.0219(8)

0.6, 0.5

3.0026(7)

0.4, 0.5

chirality at Cu

A, C

C, A

A, C

atom in apical

position

imine, imine

imine, imine amine,

amine

amine,

amine

tbp

geometry

imine, imine amine,

imine, imine

amine

a

b

The second values cited refers to the second copper center of the dimer, if crystallographically inequivalent. Taken from ref 6h for comparison.

rac-Lactide Polymerization Activity. Complexes 5, 7−

multiaddition experiments showed signiﬁcantly reduced poly-

mer molecular weights in comparison to theory and broadened

polydispersities (Table S4, entries 4, 7, 16, 33, and 44). P_m

values show a steady increase in these long-term reactions,

likewise an indication for transesteriﬁcation side-reactions (rr

triads formed in transesteriﬁcation overlap with mmm, thus

artiﬁcially leading to an increased P_mvalue).²¹For copper

pyrrolide complexes, transesteriﬁcation side reactions thus

become noticeable at these long reaction times, while the

related copper diketiminate complexes did not show evidence

for transesteriﬁcation even over >1000 half-lives.^6k

Table 4 compares the activities of complexes 1−11 and 1b−

11b. More detailed kinetic analyses and complete polymer-

ization results are available in Table S4 and Figure S1 −S17 in

the Supporting Information. Mechanistic investigations on the

polymerization of rac-lactide with 1/1b and 2/2b have shown

that complexes remain dinuclear during polymerization

11, and 3b−11b were tested for activity in rac-lactide

polymerization and compared to previously studied 1/1b and

2/2b to determine the inﬂuence of the ligand substitution

pattern on reactivity and selectivity. All complexes tested were

active for the polymerization of lactide at room temperature in

benzene solution with catalyst concentrations of 1−3 mM.

Polymerizations reached completion for all complexes but 6b,

11, and 11b (Table S4 in the Supporting Information),

although polymerization kinetics indicated some catalyst

decomposition for 5b (Figures S13 and S14 in the Supporting

Information). General catalyst stability under polymerization

conditions was tested for 8/8b and 9/9b (Figure 6). All four

complexes retained suﬃcient activity to enable complete

monomer conversion even after 3−4 days: i.e., 50−150 half-

lives of the reaction. Similar stability at prolonged reaction

times had been observed for 1.⁷GPC analysis of these

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From the pairs with notable diﬀerences (1 < 1b, 5 > 5b, and 10

< 10b), there is no conclusion as to whether pyridylmethoxide

or dimethylaminoethoxide provides the more active catalyst.

With regard to the steric bulk of the imino substituent,

complexes with secondary alkyl imines (1, 1b, 5, 5b, 8, or 8b)

react as fast as or even faster than those with benzyl imines (2,

2b, 7, 9, or 9b). The highest activity is observed for cyclohexyl-

substituted 5, which might indicate that π stacking of benzyl or

methylbenzyl substituents with pyridylmethoxide (Figure 3)

might be detrimental to activity. However, complex 7, for which

π stacking was most evident in its crystal structure, does not

show any reduced activity. π-Stacking interactions are thus

unlikely to be a structural element important to activity. Larger

diﬀerences are observed for complexes 3b and 6b, carrying

sterically very demanding ligands, whose activities are about 1

order of magnitude lower than those of 1/1b.

Monoiminopyrrolide complexes 8/8b and 9/9b, lacking the

second, dangling imino substituent, show slightly higher

activities in comparison to the respective diiminopyrrolide

complexes 1/1b and 2/2b. The respective 5-methyl-substituted

monoiminopyrrolide complexes 10/10b, on the other hand,

show slightly lower activities. Relative activities of 8 > 1 > 10

and 8b > 1b > 10b correlate with the steric but not the

electronic nature of the pyrrole substituent.²²Chloride-

substituted monoiminopyrrolide complexes 11/11b were

investigated to gain closer insight into potential electronic

eﬀects, but they proved to be barely active and reactions did not

reach completion even after several days. Such a large eﬀect on

reactivity is unlikely, considering the minor variations between

1/1b, 8/8b and 10/10b, and the low activities are probably due

to complex decomposition rather than due to an electronic

inﬂuence of the chloride substituent. In agreement with this,

complex 11 did not show any stereocontrol, while a preference

for isotactic enchainment was observed in 8−10 (Table 5 and

Table S4 in the Supporting Information).

In summary, the activity of iminopyrrolide complexes seems

insensitive toward most changes of the substitution pattern of

the pyrrole ring, substitution of the imino position, removal of

the pendant imino substituent, or nature of the bridging

alkoxide. Apparent ﬁrst-order rate constants of pyridylmeth-

oxide complexes 1, 2, 5, and 7−10 ranged from 0.4 to 1.2 h⁻¹

and those of the respective complexes 1b, 2b, 5b and 8b−10b

from 0.6 to 1.4 h⁻¹(Table 4). Only large N substituents in L3

and L6 showed a more notable eﬀect on activities, indicative of

increased steric crowding of the metal center.

Polymer Molecular Weight Control. Polymer molecular

weights in 1/1b and 2/2b were indicative of the polymerization

mechanism and stereocontrol: generation of one chain per

catalyst dimer in 1 and 2 indicated the continued presence of a

pyridylmethoxide in the active species, which was essential for

isotactic stereocontrol. Initiation of both alkoxide groups in 1b

and 2b led to active species containing a less rigid polymeryl

alkoxy bridge and loss of stereocontrol (Scheme 2). The same

correlation is complicated in 5/5b and 8−11/8b−11b by

depression of polymer molecular weight by transesteriﬁcation

reactions. For example, in immortal polymerizations with 8/8b,

complex 8 consistently provides higher polymer molecular

weights in comparison to 8b, in agreement with one

pyridylmethoxide ligand in 8 not initiating polymerization.

Polymer molecular weights for both catalysts are, however,

depressed from their theoretical values (Figure 7 and Table S4

in the Supporting Information, entries 6, 8−12, 32, and 34−

38). Complex 8 provides 1.1−1.5 chains per catalyst dimer,

Figure 6. Conversion/time plots for addition of several batches of

monomer over several days. Black arrows indicate time of monomer

addition: (top left, blue circles) 8, [8] = 1 mM in C₆D₆, addition of

200, 200, and 25 equiv of rac-lactide; (top right, blue circles) 8, 3 mM,

300, 200, 100 equiv; (middle, red circles) 8b, 3 mM, 300, 200, 100

equiv; (bottom left, blue triangles) 9, 3 mM, 300, 100, 100, 200 equiv;

(bottom right, blue triangles) 9b, 3 mM, 300, 100, 200 equiv. Solid

lines are based on pseudo-ﬁrst-order rate constants determined in the

ﬁrst addition.

Table 4. Apparent First-Order Rate Constants for rac-

Lactide Polymerization with Complexes 1−11 and 1b−11b

a

OR = OCH₂Py

OR = OC₂H₄NMe₂

N-R/N-R,R′

k_obs(h⁻¹

)

k_obs(h⁻¹

)

b

NCH(Me)Ph

1

2

0.6 0.1

0.7 0.1

1b

2b

3b

5b

6b

1.4 0.2

0.6 0.1

b

NCH₂Ph

c

N-xylyl

N-c-C₆H₁₁

NCHPh₂

0.05

c

5

1.8 0.2

0.8

c

0.07

c

NCH₂C₆H₄Br

NCH(Me)Ph, H

NCH₂Ph, H

7

8

1.0

b

1.2 0.3

8b

1.4 0.1

c

9

1.1

9b

0.8

NCH(Me)Ph, Me

NCH(Me)Ph, H

10

11

0.4 0.3

10b

11b

0.9 0.1

c

0.004

0.003

a

Conditions: [catalyst] = 2 mM, C₆D₆, ambient temperature,

lactide:catalyst = (100:1)−(300:1). The provided error is the largest

b

deviation from the average in multiple experiments. Data for 1, 1b, 2,

c

2b, and 8 from ref 7. Single experiment.

(Scheme 2).⁷Complexes 1 and 2 retain one pyridylmethoxide

as a spectator ligand in the active species, while both alkoxides

initiate in 1b and 2b. Although both complexes thus form

diﬀerent active species with potentially diﬀerent kinetics, the

eﬀect of the bridging alkoxide on polymerization activity is very

subtle. Pseudo-ﬁrst-order rate constants for 1/1b, 2/2b, 5/5b,

and 8−11/8b−11b diﬀer by a factor of 2 at the maximum and

are identical within the margin of error in half of the cases.

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While transesteriﬁcation leads to a calculated number of

polymer chains per catalyst dimer higher than the theoretical

value, pyridylmethoxide complexes consistently provide higher

molecular weight than the respective aminoethoxide complexes

(Table 5). They also consistently show an isotactic bias, while

aminoethoxide complexes do not. The same correlation of only

one initiating alkoxide and isotactic stereocontrol in pyridyl-

methoxide complexes and two initiating alkoxides without

stereocontrol in aminoethoxide complexes is thus likewise

observed for 5−10 and 5b−10b, and they follow the same

stereocontrol mechanism as 1/1b and 2/2b.

Chloride-substituted 11 displays, in addition to drastically

reduced activity (3 orders of magnitude) and lack of

stereocontrol, also a large number of chains produced per

catalyst dimer (Table 5), reinforcing the interpretation that the

active species in this catalyst diﬀers from those in 1−10: i.e.,

catalyst decomposition.

Stereocontrol. Increasing the steric demand of a spectator

ligand is a textbook tool in catalyst optimization, since it

typically leads to tightening of the catalytic pocket, stronger

diﬀerentiation of transition states, and thus increased

selectivities. Unfortunately, the isoselective pyridylmethoxide

complexes were synthetically not accessible for sterically very

demanding ligands L3, L4, and L6. The high activities of

cyclohexyl-substituted 5 (Table 4) already indicated that its

steric demand is not notably higher than in 1 or 2, and

consequently 5 provided the same isotactic stereocontrol

(Table 5).

Table 5. Selected Results for rac-Lactide Polymerization with

5−11 and 3b−11b

a

d

conversn (%),

M_n

M_w/

M_n

b

c

e

time

k_obs(h⁻¹

)

P_m

(kDa)

n_PLA/n_cat.

f

1

99, 24 h

99, 28 h

72−89, 28 h

95, 75 h

99, 23 h

88, 28 h

81, 23 h

96, 6 h

0.6 0.1

1.4 0.2

0.65 0.1

0.6 0.1

0.046(1)

1.75(2)

0.72

0.49

0.67

0.47

0.33

0.71

0.46

0.43

0.53

0.64

0.46

0.59

0.46

0.59

0.44

0.43

13

1.3

1.2

1.4

1.2

1.4

1.5

1.2

1.7

2.2

1.3

2

1.1

f

1b

5

2.5

f

2

7−28

4−7

8.6

0.5−2

1.6−3.3

1.6

2b

3b

5

17.3

5.6

0.8

5b

6b

7

0.82(6)

2.3

0.074(2)

1.02(2)

6.5

1.8

6.1

2.3

8

99, 10 h

99, 34 h

99, 32 h

99, 27 h

97, 5 h

1.29(1)

16.9

3.2

0.8

8b

9

1.41(1)

4.5

1.06(4)

7.0

2.1

9b

10

0.80(2)

3.9

1.3

1.5

1.3

1.2

1.4

3.7

0.52(3)

12.5

7.1

1.1

10b

11

1.00(3)

2

76, 16 days

64, 16 days

0.0021(2)

0.0027(3)

1.6

6.9

11b

3.5

2.7

a

Conditions: [cat.] = 2 mM, lactide:catalyst = 100:1, C₆D₆, ambient

temperature. For a full list of polymerization results, see Table S4 in

b

the Supporting Information. Time at which the reaction was

quenched. This time is not indicative of activity (see k_obsand Table

c

4). Determined from decoupled ¹H NMR spectra (see the

d

Experimental Section). Determined by GPC (see the Experimental

e

Section). Number of polymer chains per catalyst dimer calculated

f

from n_PLA/n_cat= ([lactide]/[cat.]·M_lact.·conversion + M_ROH)/M_n. Data

Aminoethoxide complexes 3b−10b did not provide isotactic

PLA, in agreement with the polymerization mechanism

proposed for 1/1b (Table 5 and Scheme 2). The sterically

crowded complex 3b aﬀorded a heterotactic bias of P_m= 0.33,

while all other complexes were essentially atactic. This indicates

that bulky N substituents can indeed increase the steric

pressure in the transition state suﬃciently to increase

selectivity. Pyridylmethoxide complexes with ligands L3 and

L6 could not be prepared from the respective aminoethoxide

complexes (vide supra), but the polymerylalkoxide ligand in the

active species should be less stabilized than aminoethoxide. We

thus investigated whether a pyridylmethoxide spectator ligand

could be introduced in situ by protonation of the active

bis(polymerylalkoxide) species (Scheme 8). Polymerizations

with 1b in the presence of pyridylmethanol showed indeed the

induction period normally associated with polymerizations with

1 and provided isotactically enriched PLA, indicative of

incorporation of a pyridylmethoxide ligand into the active

species. Reaction with pyridylmethanol might involve either

from ref 7. Data are averages of several experiments.

Figure 7. Polymer molecular weights in rac-lactide polymerizations

with 8 (blue circles) and 8b (red squares) in the presence of benzyl

alcohol. Polymer molecular weights were corrected for yield (M_n,corr

=

M_n/yield) to allow comparison with the theoretical values. The blue

solid line indicates polymer molecular weight expected for one

alkoxide per catalyst dimer initiating and the red dashed line that for

both alkoxides initiating. Hollow circles and squares show the

respective polydispersities (right axis).

Scheme 8

while 2.9−7.5 chains per dimer are observed for 8b,

accompanied by slightly increased polydispersities. MALDI

spectra of 8 show the presence of peaks with m/z n·72 +

M(Na⁺), in agreement with intramolecular transesteriﬁcation

reactions (Figure S18 in the Supporting Information).

Monoiminopyrrolide complexes 8/8b seem thus more prone

to transesteriﬁcation than their respective diiminopyrrolide

analogues 1/1b. Unsurprisingly, transesteriﬁcation side reac-

tions thus increase when the steric demand is reduced.

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direct formation of 1*, the active species of 1, by reaction with

1 equiv of pyridylmethanol, or formation of 1 by reaction with

2 equiv of alcohol (Scheme 8). In the former case,

polymerizations of 1b/PyCH₂OH (1/1) are indistinguishable

from polymerizations with 1; in the latter case we obtain

polymerization from a 1/1 mixture of 1 and 1b and a polymer

of intermediate characteristics.²³Obtained P_mvalues and

polymer molecular weights in polymerizations of rac-lactide

with 1b in the presence of varying amounts of pyridylmethanol

indicate that reactions of 1b* with pyridylmethanol provide 1

rather than 1* (Figure 8). This agrees with the observed

such as transesteriﬁcation, might be responsible for the reduced

stereocontrol. From the available data there is thus no evidence

for the formation of pyridylmethoxide complexes with ligands

L3 or L6 even by in situ reaction of the bisalkoxide species with

pyridylmethanol.

Mechanism of Stereocontrol. Stereocontrol in 1 has been

shown to follow a chain-end control mechanism.⁷This is

further supported by the fact that stereocontrol was not aﬀected

by enforced chain transfer in immortal polymerizations, neither

for 1 and 2⁷nor for 8−10 (Table S4 in the Supporting

Information). Monoiminopyrrolide complexes 8−10 showed

the expected isotactic bias for polymerization, but the obtained

P_mvalues are notably lower than those obtained with the

respective diiminopyrrolide complexes 1 and 2 (Table 5).

Stereocontrol was lower for methyl-substituted 10 than for

hydrogen-substituted 8, indicating that steric factors are not

primarily responsible for the reduced stereocontrol. The

mechanism proposed for 1/1a involves adaptation of the chiral

catalytic site to chain-end chirality and the chirality of the

catalytic site participating in or even dominating enantiomer

selection for the next insertion.^6h,7Such a “catalytic-site

mediated chain-end control mechanism” was initially proposed

by Okuda for heterotactic control in scandium complexes²⁴but

was also observed by Jones for zirconium.²⁵On the basis of the

high ﬂexibility of the coordination environment observed in the

crystal structures of 1/1a−11/11b, the chiral coordination

environment is likely to adapt to the chirality of the chain end.

Adaption of the catalytic site to chain-end chirality (i.e.,

catalytic site epimerization) is probably fast in 1, 2, or 5, since it

requires only coordination of the pending imino substituent,

possibly passing through a tridentate intermediate similar to

complex 7 via an associative mechanism (Scheme 9, top).

Monoiminopyrrolide complexes 8−10, on the other hand,

require dissociation and rotation around the Cu−pyrrole bond

or Berry pseudorotation of a trigonal-bipyramidal intermediate,

both likely to be more diﬃcult considering the ligands involved.

The presence of the 5-methyl substituent in 10 slows these

isomerizations even more. Hindered catalytic site epimerization

and thus the possibility of mismatches between chain end and

catalytic site are thus probably responsible for the reduced

stereocontrol in 8−10 (Scheme 9, middle).

The involvement of the catalytic site in monomer selection is

further supported by polymerizations with 7. The bromobenzyl

complex 7 surprisingly provided only atactic PLA (P_m= 0.53,

Table 5). In this it deviates from the otherwise strictly followed

observation that pyridylmethoxide-containing catalysts provide

isotactic PLA. In comparison to other changes in the ligand

substitution pattern in 1−10, which led to minor variations in

the P_mvalue (0.59−0.71), the electronic and steric eﬀect of

replacing hydrogen by bromine in the para position of the

iminobenzyl substituent should be negligible. The atypical

polymerization behavior correlates with its atypical crystal

structure: complex 7 is the only example in which a tridentate

coordination of the diiminopyrrolide ligand was observed. The

catalytic site in 7 contains octahedral copper centers with an

imino group coordinated above and below the equatorial plane

(Figure 3). Regardless of chain-end chirality, the catalytic site is

thus achiral and will not prefer one enantiomer over the other

(Scheme 9, bottom). The fact that the only catalyst with an

achiral coordination environment is also the only pyridylmeth-

oxide complex not showing preference for isotactic monomer

insertion strongly supports that the catalytic site is responsible

Figure 8. Isotacticities (P_m, left axis, black circles) and polymer

molecular weights (M_n, right axis, blue squares) obtained for

polymerizations of rac-lactide with 1b in the presence of varying

amounts of pyridylmethanol. The solid lines represent values expected

if 1 is formed in the reaction and the dashed lines values expected for

the direct formation of the active species 1*.

induction periods in 1b/PyCH₂OH systems, which should be

absent if 1* is formed directly. Reactions are not well

controlled, and the full isotacticity observed for 1 could not

be restored even by addition of more than 2 equiv of

pyridylmethanol to 1b.

Table 6 compares isotacticities of aminoethoxide complexes

obtained in the presence of additional pyridylmethanol. For 1b,

Table 6. Stereocontrol by in Situ Formation of

Pyridylmethoxide Species

PyCH₂OH added

a

b

catalyst

0 equiv

1 equiv

P_m(X/Xb)

P_m(X)

c

1b

2b

3b

5b

6b

0.48

0.59

0.58

0.45

0.59

0.46

0.60

0.57

0.72

c

0.47

0.67

0.33

0.46

0.43

0.59

0.71

a

b

P_mvalue expected for a 1:1 mixture of X and Xb. P_mvalue of X.

c

Taken from ref 7.

2b, and 5b isotacticities obtained in polymerizations in the

presence of 1 equiv of pyridylmethanol correspond well with

the isotacticities expected from a 1/1 mixture of the respective

aminoethoxide and pyridylmethoxide complexes. Addition of

pyridylmethanol to 3b or 6b, however, did not provide polymer

with an isotactic bias. In the case of 3b, the polymer became

notably more atactic, but polymerizations with 3b/PyCH₂OH

required 3 days to reach >80% conversion and other reasons,

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Scheme 9

pyrrole-2-methyl-5-carbaldehyde,²⁹1H-pyrrole-2-chloro-5-carbalde-

hyde,²⁹L3,⁸L8,⁹and L9¹⁰were prepared according to the literature.

Solvents were dried by passage through activated aluminum oxide

(MBraun SPS), deoxygenated by repeated extraction with nitrogen,

and stored over molecular sieves. C₆D₆was dried over molecular

sieves. rac-Lactide (98%) was purchased from Sigma−Aldrich, puriﬁed

by 3× recrystallization from dry ethyl acetate, and kept at −30 ^◦C. All

other chemicals were purchased from common commercial suppliers

for monomer selection even if the chiral information is derived

from the chain end.²⁶

CONCLUSIONS

■

The reaction mechanism proposed for isotactic selectivity in

iminopyrrolide complexes is supported by the results obtained

in varying the ligand substitution pattern. As found for 1 and 2,

the nature of the bridging alkoxide ligand in the dinuclear active

species is of utmost importance for stereocontrol: a

pyridylmethoxide ligand in the active species enables stereo-

control, while the more ﬂexible/less coordinating polymer-

ylalkoxide group does not. Reduced stereocontrol in the

absence of a pendant imine and loss of stereocontrol in

complex 7 displaying an achiral catalytic site support the

participation of the catalytic site in monomer stereoselection

(catalytic site mediated chain-end control). In combination,

these results deﬁne the essential requirements to obtain

isotactic selectivity in dinuclear copper (and eventually other

metal) complexes: a bridging ligand of suﬃcient rigidity and

stability as well as a ﬂexible, chiral catalytic site.

1

and used without further puriﬁcation. H and ¹³C NMR spectra were

acquired on Bruker Advance 300 and 400 spectrometers. Chemical

shifts were referenced to the residual signals of the deuterated solvents

1

(CDCl₃: H, δ 7.26 ppm; ¹³C, δ 77.16). Some ¹³C NMR chemical

shifts were determined using HSQC spectra. Elemental analyses were

performed by the Laboratoire d’analyse el

́ ́ ́

ementaire (Universite de

Montreal). Molecular weight analyses were performed on a Waters

́

1525 gel permeation chromatograph equipped with three Phenomenex

columns and a refractive index detector at 35 °C. THF was used as the

eluent at a ﬂow rate of 1.0 mL min⁻¹, and polystyrene standards

(Sigma−Aldrich, 1.5 mg mL⁻¹, prepared and ﬁltered (0.2 mm) directly

prior to injection) were used for calibration. Obtained molecular

weights were corrected by a Mark−Houwink factor of 0.58.³⁰All UV−

vis measurements were done in degassed and anhydrous toluene or

CH₂Cl₂at ambient temperature in a sealed quartz cell on a Cary 500i

UV−vis−NIR spectrophotometer.

The results presented herein also show that further

optimization of diiminopyrrolide copper complexes is limited:

smaller steric variations do not show notable improvement in

stereocontrol, while bulkier ligand systems prevent the

introduction of the pyridylmethoxide group required for

stereocontrol. It seems thus unlikely that further modiﬁcations

of this ligand system will provide signiﬁcantly improved

catalysts and future work in our group will aim at incorporating

the above requirements into new catalytic systems.

2,5-Bis((1-naphthyl)aldimino)pyrrole (L4). 1H-Pyrrole-2,5-di-

carbaldehyde (0.5 g, 4.1 mmol) was dissolved in ethanol (25 mL). 1-

Naphthylamine (1.2 g, 8.2 mmol) was added, and the reaction mixture

was heated to reﬂux overnight. Cooling to ambient temperature

resulted in a yellow precipitate which was ﬁltered oﬀ, washed with cold

ethanol, and dried under vacuum (1.22 g, 80%).

¹H NMR (CDCl_3,300 MHz): δ 10.51 (s, 1H, NH), 8.51−8.37 (m,

4H, (NC)H and Ar), 7.92−7.83 (m, 2H, Ar), 7.75 (d, J = 8 Hz, 2H,

Ar), 7.58−7.51 (m, 4H, Ar), 7.48 (dd, J = 8, 7 Hz, 2H, Ar), 7.10 (d, J =

7 Hz, 2H, Ar), 6.81 (s, 2H, pyrrole). ¹³C{¹H} NMR (CDCl_3,75

MHz): δ 149.4 ((N)CH), 149.0 (1-naphthyl), 134.5 (2/5-pyrrole),

134.16 (5-naphthyl), 129.0 (naphthyl), 127.8 (naphthyl), 126.6

(naphthyl), 126.2 (naphthyl), 126.1 (naphtyl), 126.0 (naphthyl),

EXPERIMENTAL SECTION

General Considerations. All reactions were carried out using

■

Schlenk or glovebox techniques under a nitrogen atmosphere.

27

Cu(OiPr)_2,Cu(OMe)₂,²⁷1H-pyrrole-2,5-dicarbaldehyde,²⁸1H-

K

DOI: 10.1021/acs.organomet.7b00609

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Article

124.1 (naphthyl), 117.1 (2-naphthyl), 113.0 (3/4-pyrrole). ESI-HRMS

(m/z): [M + H]⁺(C₂₆H₂₀N₃) calcd 374.1652; found 374.1659.

2,5-Bis(cyclohexylaldimino)pyrrole (L5). 1H-Pyrrole-2,5-dicar-

baldehyde (0.50 g, 4.1 mmol) was dissolved in ethanol (25 mL). A

catalytic amount of formic acid and cyclohexylamine (0.81 g, 8.2

mmol) were added, and the reaction mixture was stirred overnight at

ambient temperature. The solvent was removed from the brown

solution under vacuum, resulting in a red oil (0.35 g, 30%).

¹H NMR (CDCl₃, 400 MHz): δ 8.07 (s, 2H, (NC)H), 6.41 (s,

2H, pyrrole), 3.10 (tt, J = 10, 4 Hz, 2H, NCH), 1.86−1.76 (m, 4H,

CH₂), 1.75−1.61 (m, 6H, CH₂), 1.49 (ddd, J = 15, 12, 3 Hz, 4H,

CH₂), 1.41−1.19 (m, 6H, CH₂). ¹³C{¹H} NMR (CDCl₃, 101 MHz):

δ 148.8 ((N)C), 133.0 (2,5-pyrrole), 114.0 (3,4-pyrrole), 69.7

(NCH), 34.7 (CH₂), 25.8 (CH₂), 25.0 (CH₂). ESI-HRMS (m/z): [M

+ H]⁺(C₁₈H₂₈N₃) calcd 286.2277; found 286.2272.

2,5-Bis((diphenyl)aldimino)pyrrole (L6). 1H-Pyrrole-2,5-dicar-

baldehyde (0.50 g, 4.1 mmol) was dissolved in dry ether (25 mL) and

treated with 2.5 g of K₂CO₃and diphenylamine (1.5 g, 8.2 mmol).

The reaction mixture was stirred overnight at ambient temperature.

The brown suspension was ﬁltered and the solvent removed under

vacuum. The residue was treated with hexane (20 mL), resulting in an

orange crystalline powder, which was separated by ﬁltration and dried

under vacuum (0.28 g, 15%).

145.4 (ipso-Ph), 142.4 (2-pyrrole) (HSQC), 128.5 (o-Ph), 126.9 (m-

Ph), 126.7(p-Ph), 115.1 (5-pyrrole), 110.5 (2-pyrrole), 108.3 (3-

pyrrole), 68.7 (CH(Me)Ph), 24.5 (Me), 13.3 (CH₃-pyrrole). ESI-

HRMS (m/z): [M + H]⁺(C₁₄H₁₇N₂) calcd 213.1386; found

213.1386.

2-Chloro-5-((^L-(−)-α-methylbenzyl)aldimino)pyrrole (L11).

The procedure was analogous to that for L7, from 1H-pyrrole-2-

chloro-5-carbaldehyde (1.0 g, 7.7 mmol), dry toluene (25 mL), 5 g of

MgSO₄, a catalytic amount of Amberlyst 15, and L-(−)-α-

methylbenzylamine (0.93 g, 7.7 mmol) to yield a light yellow powder

(0.52 g, 29%).

¹H NMR (CDCl_3,400 MHz): δ 7.80 (s, 1H, (NC)H), 7.38−7.31

(m, 4H, Ph), 7.30−7.23 (m, 1H, Ph), 7.04 (br s, 1H, NH), 6.49 (d, J =

3 Hz, 1H, pyrrole), 6.10 (d, J = 3 Hz, 1H, pyrrole), 4.50 (q, J = 6 Hz,

1H, CH(Me)Ph), 1.57 (d, J = 6 Hz, 3H, Me). ¹³C{¹H} NMR (CDCl_3,

101 MHz): δ 148.4 ((N)C), 144.2 (ipso-Ph), 130.0 (2-pyrrole),

128.8 (o-Ph), 127.4 (m-Ph), 126.7 (p-Ph), 125.4 (5-pyrrole), 118.3

(4-pyrrole), 110.0 (3-pyrrole), 66.5 (CH(Me)Ph), 24.2 (Me). ESI-

HRMS (m/z): [M + H]⁺(C₁₃H₁₄ClN₂) calcd 233.0840; found

233.0849.

(L1)₂Cu (1c). To a suspension of copper diisopropoxide (105 mg,

0.58 mmol) in dichloromethane (15 mL) was added L1 (420 mg, 1.28

mmol) with stirring. Stirring was continued for 1 day, after which the

suspension was ﬁltered. The ﬁltrate was concentrated and layered with

hexane to yield crystalline 1c after several days (112 mg, 25%).

UV−vis (CH₂Cl₂, 9.9 × 10⁻⁵M or 9.9 × 10⁻⁶M) [λ_max, nm (ε,

mol⁻¹cm²)]: 364 (27100). Anal. Calcd for C₄₄H₄₄CuN₆: C, 73.36; H,

6.16; N, 11.67. Found: C, 72.88; H, 6.28; N, 11.39.

¹H NMR (CDCl_3,300 MHz): δ 8.19 (s, 2H, (NC)H), 7.43−7.15

(m, 20H, Ph), 6.48 (s, 2H, pyrrole), 5.57 (s, 2H, NCH), 5.22 (s, 1H,

NH). ¹³C{¹H} NMR (75 MHz, CDCl₃): δ 151.3 ((N)C), 145.8

(ipso-Ph), 143.6 (ipso-Ph), 133.0 (2,5-pyrrole), 128.6 (o-Ph), 128.0

(o-Ph), 127.7 (m-Ph), 127.2 (m-Ph), 127.1 (p-Ph), 127.0 (p-Ph),

115.4 (3,4-pyrrole), 59.9 (NCH). ESI-HRMS (m/z): [M + H]⁺

(C₃₂H₂₈N₃) calcd 454.2277; found 454.2286.

[(L1)ClCu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (1e). Complex 1e was obtained

as an undesired side product in the preparation of 1 using chloride-

contaminated Cu(OR)₂starting material (0.006 g, 6%).

Anal. Calcd for C₃₀H₄₂ClCu₂N₅O₂: C, 54.00; H, 6.34; N, 10.50.

Found: C, 54.72; H, 6.39; N, 10.30.

2,5-Bis((4-bromobenzyl)aldimino)pyrrole (L7). 1H-Pyrrole-

2,5-dicarbaldehyde (0.5 g, 4.1 mmol) was dissolved in dry toluene

(25 mL). MgSO₄(5 g), a catalytic amount of Amberlyst 15, and 4-

bromobenzylamine (1.5 g, 8.2 mmol) were added. The reaction

mixture was stirred overnight at ambient temperature. The light yellow

suspension was ﬁltered and the solvent removed under vacuum. The

residue was treated with hexane (20 mL), resulting in a dark brown oil.

The oil was separated by decantation and dried under vacuum to give

0.43 g (23%) of a 1:1 mixture of L7 and 2-carboxaldehyde-5-((4-

bromobenzyl)aldimino)pyrrole. Puriﬁcation attempts were unsuccess-

ful, and the mixture was used without puriﬁcation in further synthesis.

Data for 2,5-bis((4-bromobenzyl)aldimino)pyrrole are as follows.

¹H NMR (CDCl_3,400 MHz): δ 8.14 (s, 2H, (NC)H), 7.45 (d, J = 8

Hz, 4H, m-Ph), 7.17 (d, J = 8 Hz, 4H, o-Ph), 6.53 (s, 2H, pyrrole),

4.67 (s, 4H, CH₂). ¹³C{¹H} NMR (CDCl_3,101 MHz): δ 152.6

((N)CH), 138.5 (2,5-pyrrole), 138.1 (ipso-Ph), 132.0 (m-Ph),

130.1 (o-Ph), 121.4 (C−Br), 115.7 (3,4-pyrrole), 64.1 (CH₂). ESI-

HRMS (m/z): [M + H]⁺(C₂₀H₁₈Br₂N₃) calcd 457.9862; found

457.9860.

(L2)₂Cu₂(2d). To a suspension of copper diisopropoxide (136 mg,

0.75 mmol) in dichloromethane (15 mL) was added L2 (495 mg, 1.64

mmol) with stirring. Stirring was continued at ambient temperature for

1 day, after which the solution was ﬁltered. The ﬁltrate was

concentrated and layered with hexane to yield 210 mg (77%) of 2d.

¹H NMR (CDCl₃, 400 MHz, 298 K): δ 7.97 (s, 4H, NCH), 7.15

(m, 12H, Ar), 7.05 (m, 8H, Ar), 6.60 (s, 4H, 3,4-pyrrole), 4.39 (s, 8H,

NCH₂). ¹³C NMR (CDCl₃, 400 MHz, 298 K): δ 159.3 (NCH).

141.0 (ipso-Ar), 138.8 (2,5-pyrrole), 128.3 (m-Ar), 128.0 (o-Ar), 127.0

(p-Ar), 118.0 (3,4-pyrrole), 63.9 (N−CH₂). UV−vis (CH₂Cl₂, 3.4

× 10⁻⁵M or 6.9 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 318 (20200),

332 (20100), 374 (23000), 500 (sh). Anal. Calcd for C₄₀H₃₆Cu₂N₆: C,

66.01; H, 4.99; N, 11.55. Found: C, 66.65; H, 4.82; N, 11.63.

[(L3)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (3b). Cu(OMe)₂(38 mg, 0.30

mmol) was suspended in toluene (3 mL). Dimethylaminoethanol (60

μL, 0.60 mmol) was added to the blue suspension, which was stirred

for 45 min. A freshly prepared orange solution of L3 (100 mg, 0.30

mmol) in toluene (2 mL) was added dropwise, resulting in a dark

green solution. The reaction mixture was stirred 24 h at room

temperature, ﬁltered to remove trace impurities, concentrated to one-

third of the volume, diluted with hexane (18 mL), and kept at −30 °C

for 4 h, resulting in green crystals. The crystals were separated by

decantation and washed with hexane (3 × 10 mL) (0.024 g, 17%).

UV−vis (toluene, 2.3 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 296

(sh), 382 (48400). Anal. Calcd for C₅₂H₆₄Cu₂N₈O₂: C, 65.04; H, 6.72;

N, 11.67. Found: C, 65.22; H, 6.91; N, 11.84.

Data for 2-carbaldehyde-5-((4-bromobenzyl)aldimino)pyrrole are

1

as follows. H NMR (CDCl_3,400 MHz): δ 9.60 (s, 1H, COH), 8.22

(s, 1H, (NC)H), 7.45 (d, J = 8 Hz, 2H, m-Ph), 7.17 (d, J = 8, 2H, o-

Ph), 6.94 (d, J = 3 Hz, 1H, pyrrole), 6.58 (d, J = 3 Hz, 1H, pyrrole),

4.72 (s, 2H, CH₂). ¹³C{¹H} NMR (CDCl_3,101 MHz): δ 180.40

(COH), 152.3 ((N)CH), 138.5 (2,5-pyrrole), 138.1 (ipso-Ph),

132.1 (2,5-pyrrole), 132.00 (o-Ph), 130.1 (m-Ph), 121.4 (C−Br),

120.89 (3,4-pyrrole), 115.2 (3,4-pyrrole), 64.3 (CH₂). ESI-HRMS (m/

z): [M + H]⁺(C₁₃H₁₂BrN₂O) calcd 291.0128; found 291.0141.

2-Methyl-5-((^L-(−)-α-methylbenzyl)aldimino)pyrrole (L10).

The procedure was analogous to that for L7, from 1H-pyrrole-2-

methyl-5-carbaldehyde (1.0 g, 9.2 mmol), dry toluene (25 mL), 5 g of

MgSO₄, a catalytic amount, of Amberlyst 15 and L-(−)-α-

methylbenzylamine (1.1 g, 9.2 mmol) to yield a light yellow oil

which was puriﬁed by silica gel chromatography (0−30% EtOAc in

hexane) (0.47 g, 25%).

(L3)₂Cu₂(3d). To a stirred green suspension of copper

diisopropoxide (436 mg, 2.4 mmol) in THF (40 mL) at 50 °C was

added a THF solution of L3 (658 mg, 2 mmol). The color changed to

dark brown, and heating was continued for 74 h. After it was cooled to

room temperature, the suspension was ﬁltered. The residue was

extracted with dichloromethane and layered with hexane to yield 258

mg (33%) of 3d.

¹H NMR (CDCl_3,400 MHz): δ 8.06 (s, 1H, (NC)H), 7.42−7.30

(m, 4H, Ph), 7.27−7.21 (m, 1H, Ph), 6.39 (d, J = 3 Hz, 1H, 3-

pyrrole), 5.93 (d, J = 3 Hz, 1H, 2-pyrrole), 4.45 (q, J = 6 Hz, 1H,

CH(Me)), 2.27 (s, 3H, pyrrole-CH₃), 1.57 (d, J = 6 Hz, 3H,

CH(CH₃). ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 149.9 ((N)C),

¹H NMR (CDCl₃, 400 MHz, 298 K): δ 7.87 (s, 4H, NCH), 6.91

(s, 12H, Ar), 6.69 (s, 4H, 3,4-pyrrole), 1.82 (s, 24H, Me). ¹³C NMR

(CDCl₃, 101 MHz, 298 K): δ 159.7 (NCH), 149.5 (ipso-Ar), 141.9

(2.5-pyrrole), 129.6 (m-Ar), 128.1 (o-Ar), 124.3 (p-Ar), 119.2 (3,4-

L

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Article

pyrrole), 18.2 (Me). UV−vis (CH₂Cl₂, 6.2 × 10⁻⁵M or 6.2 × 10⁻⁶M)

[λ_max, nm (ε, mol⁻¹cm²)]: 386 (59000), 550 (sh, < 1500). Anal. Calcd

for C₄₄H₄₄N₆Cu₂: C, 67.41; H, 5.66; N, 10.72. Found: C, 67.10; H,

5.58; N, 10.57.

[(L7)₂Cu₂(μ-O,κ_N-OCH₂Py)₂] (7). The procedure was analogous to

that for 3b, from Cu(OMe)₂(28 mg, 0.22 mmol) in toluene (3 mL),

2-pyridinemethanol (42 μL, 0.44 mmol), and L7 (100 mg, 0.22 mmol)

in toluene (2 mL). The ﬁltered and concentrated (one-third of the

volume) green solution was diluted with hexane (18 mL) and kept at

−30 °C for 4 h. Decantation and washing with hexane (3 × 10 mL)

aﬀorded 22 mg (16%) of green X-ray-quality crystals.

(L4)₂Cu (4c). To a suspension of copper diisopropoxide (111 mg,

0.61 mmol) in toluene (16 mL) was added L4 (500 mg, 1.34 mmol).

After it was stirred for 3 days, the suspension was ﬁltered oﬀ and the

light golden brown residue extracted with dichloromethane. The

solvent was removed and the residue recrystallized from diﬀusion of

hexane into dichloromethane (195 mg, 44%).

UV−vis (toluene, 2 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 301

(30200), 349 (15500), 373 (sh), 395 (sh). Anal. Calcd for

C₅₂H₄₄Br₄Cu₂N₈O₂: C, 49.58; H, 3.52; N, 8.90. Found: C, 49.31; H,

3.57; N, 8.78.

UV−vis (CH₂Cl₂, 6.2 × 10⁻⁵M or 6.2 × 10⁻⁶M) [λ_max, nm (ε,

mol⁻¹cm²)]: 376 (111500). Anal. Calcd for C₅₂H₃₆CuN₆: C, 77.26;

H, 4.49; N, 10.40. Found: C, 76.67; H, 4.63; N, 9.98.

[(L7)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (7b). The procedure was analo-

gous to that for 3b, from Cu(OMe)₂(28 mg, 0.22 mmol) in toluene

(3 mL), dimethylaminoethanol (44 μL, 0.44 mmol), and L7 (100 mg,

0.22 mmol) in toluene (2 mL). The ﬁltered and concentrated (one-

third of the volume) green solution was diluted with hexane (18 mL)

and kept at −30 °C for 4 h. Decantation and washing with hexane (3

× 10 mL) aﬀorded very few green X-ray-quality crystals.

[(L8)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (8b). The procedure was analo-

gous to that for 3b, from Cu(OMe)₂(63 mg, 0.50 mmol) in toluene

(3 mL), dimethylaminoethanol l (100 μL, 1.00 mmol), L8 (100 mg,

0.50 mmol) in toluene (2 mL). The ﬁltered and concentrated (one-

third of the volume) green solution was diluted with hexane (18 mL)

and kept at −30 °C for 4 h. Decantation and washing with hexane (3

× 10 mL) aﬀorded 76 mg (44%) of green X-ray-quality crystals.

UV−vis (toluene, 2.3 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 344

(12900); 427 (1960), 481 (sh). Anal. Calcd for C₃₄H₄₆Cu₂N₆O₂: C,

58.52; H, 6.64; N, 12.04. Found: C, 58.19; H, 6.98; N, 12.05.

[(L9)₂Cu₂(μ-O,κ_N-OCH₂Py)₂] (9). The procedure was analogous to

that for 3b, from Cu(OMe)₂(68 mg, 0.54 mmol) in toluene (3 mL),

2-pyridinemethanol (105 μL, 1.10 mmol), and L9 (100 mg, 0.54

mmol) in toluene (2 mL). The ﬁltered and concentrated (one-third of

the volume) green solution was diluted with hexane (18 mL) and kept

at −30 °C for 4 h. Decantation and washing with hexane (3 × 10 mL)

aﬀorded 74 mg (38%) of green X-ray-quality crystals.

[(L4)ClCu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (4e). The procedure was

analogous to that for 3b, from Cu(OMe)₂(34 mg, 0.27 mmol) in

toluene (3 mL), 2-pyridinemethanol (52 μL, 0.54 mmol), and L4 (100

mg, 0.27 mmol) in toluene (2 mL). The ﬁltered and concentrated

(one-third of the volume) green solution was diluted with hexane (18

mL) and kept at −30 °C for 4 h. The oily residue was dissolved in

DCM and layered with hexane (1/3), yielding green X-ray-quality

crystals (0.026 g, 29%).

UV−vis (toluene, 3.4 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 317

(13600), 403 (24000). Anal. Calcd for C₃₄H₃₈ClCu₂N₅O₂: C, 57.42;

H, 5.39; N, 9.85. Found: C, 61.13; H, 5.56; N, 10.03. The elemental

analysis does not agree. No puriﬁcation was attempted.

[(L5)₂Cu₂(μ-O,κ_N-OCH₂Py)₂] (5). The procedure was analogous to

that for 3b, from Cu(OMe)₂(44 mg, 0.35 mmol) in toluene (3 mL),

2-pyridinemethanol (67 μL, 0.70 mmol), and L5 (100 mg, 0.35 mmol)

in toluene (2 mL). The ﬁltered and concentrated (one-third of the

volume) green solution was diluted with hexane (18 mL) and kept at

−30 °C for 4 h. Decantation and washing with hexane (3 × 10 mL)

aﬀorded 36 mg (22%) of green X-ray-quality crystals.

UV−vis (toluene, 3.4 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 315

(45500), 368 (41500). Anal. Calcd for C₄₈H₆₄Cu₂N₈O₂·C₆H₁₄: C,

64.97; H, 7.88; N, 11.22. Found: C, 64.31; H, 7.67; N, 11.28. (One

molecule of hexane was found in the X-ray structure.)

UV−vis (toluene, 2.3 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 346

(8170), 423 (1100), 492 (sh). Anal. Calcd for C₃₆H₃₄Cu₂N₆O₂: C,

60.92; H, 4.83; N, 11.84. Found: C, 60.62; H, 4.65; N, 12.00.

[(L9)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (9b). The procedure was analo-

gous to that for 3b, from Cu(OMe)₂(68 mg, 0.54 mmol) in toluene

(3 mL), dimethylaminoethanol (109 μL, 1.10 mmol), and L9 (100 mg,

0.54 mmol) in toluene (2 mL). The ﬁltered and concentrated (one-

third of the volume) green solution was diluted with hexane (18 mL)

and kept at −30 °C for 4 h. Decantation and washing with hexane (3

× 10 mL) aﬀorded 75 mg (42%) of green X-ray-quality crystals.

UV−vis (toluene, 2.3 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 349

(11400), 427 (1470), 474 (sh). Anal. Calcd for C₃₂H₄₂Cu₂N₆O₂: C,

57.38; H, 6.32; N, 12.55. Found: C, 57.17; H, 6.57; N, 12.44.

[(L10)₂Cu₂(μ-O,κ_N-OCH₂Py)₂] (10). The procedure was analogous

to that for 3b, from Cu(OMe)₂(59 mg, 0.47 mmol) in toluene (3

mL), 2-pyridinemethanol (91 μL, 0.94 mmol), and L10 (100 mg, 0.47

mmol) in toluene (2 mL). The ﬁltered and concentrated (one-third of

the volume) green solution was diluted with hexane (18 mL) and kept

at −30 °C for 4 h. Decantation and washing with hexane (3 × 10 mL)

aﬀorded 41 mg (22%) of green X-ray-quality crystals.

[(L5)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (5b). The procedure was analo-

gous to that for 3b, from Cu(OMe)₂(44 mg, 0.35 mmol) in toluene

(3 mL), dimethylaminoethanol (70 μL, 0.70 mmol), and L5 (100 mg,

0.35 mmol) in toluene (2 mL). The ﬁltered and concentrated (one-

third of the volume) green solution was diluted with hexane (18 mL)

and kept at −30 °C for 4 h. Decantation and washing with hexane (3

× 10 mL) aﬀorded 29 mg (19%) of green X-ray-quality crystals.

UV−vis (toluene, 2 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 341 (sh),

357 (25200), 374 (sh). Anal. Calcd for C₄₄H₇₂Cu₂N₈O₂·C₆H₁₄: C,

62.66; H, 9.05; N, 11.69. Found: C, 62.04; H, 9.55; N, 11.45. One

molecule of hexane was found in the X-ray structure.

[(L6)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (6b). The procedure was analo-

gous to that for 3b, from Cu(OMe)₂(28 mg, 0.22 mmol) in toluene

(3 mL), dimethylaminoethanol (44 μL, 0.44 mmol), and L6 (100 mg,

0.22 mmol) in toluene (2 mL). The ﬁltered and concentrated (one-

third of the volume) green solution was diluted with hexane (18 mL)

and kept at −30 °C for 4 h. Decantation and washing with hexane (3

× 10 mL) aﬀorded 46 mg (35%) of green X-ray-quality crystals.

UV−vis (toluene, 2.3 × 10⁻⁵M) [λ_max, nm (ε, mol⁻¹cm²)]: 295

(sh), 352 (sh), 363 (27100), 392 (sh). Anal. Calcd for

C₇₂H₇₂Cu₂N₈O₂: C, 71.56; H, 6.01; N, 9.27. Found: C, 71.43; H,

6.29 N, 9.19.

UV−vis (toluene, 2 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 347

(61600), 375 (sh), 433 (sh).Anal. Calcd for C₄₀H₄₂Cu₂N₆O₂: C,

62.73; H, 5.53; N, 10.97. Found: C, 62.62; H, 5.71; N, 11.27.

[(L10)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (10b). The procedure was

analogous to that for 3b, from Cu(OMe)₂(59 mg, 0.47 mmol) in

toluene (3 mL), dimethylaminoethanol (94 μL, 0.94 mmol), and L10

(100 mg, 0.47 mmol) in toluene (2 mL). The ﬁltered and

concentrated (one-third of the volume) green solution was diluted

with hexane (18 mL) and kept at −30 °C for 4 h. Decantation and

washing with hexane (3 × 10 mL) aﬀorded 58 mg (34%) of green X-

ray-quality crystals.

[(L6)ClCu₂(μ-O,κ_N-OCH₂Py)₂] (6e). The procedure was analogous

to that for 3b, from Cu(OMe)₂(28 mg, 0.22 mmol) in toluene (3

mL), 2-pyridinemethanol (42 μL, 0.44 mmol), and L6 (100 mg, 0.22

mmol) in toluene (2 mL). The ﬁltered and concentrated (one-third of

the volume) green solution was diluted with hexane (18 mL) and kept

at −30 °C for 4 h. The oily residue was dissolved in DCM and layered

with hexane (1/3), yielding green X-ray-quality crystals (0.026 g,

29%).

UV−vis (toluene, 2 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 323

(10750), 338 (sh), 381 (27100), 394 (45700). Anal. Calcd for

C₄₄H₃₈ClCu₂N₅O₂: C, 63.57; H, 4.61; N, 8.42. Found: C, 63.54; H,

5.05; N, 8.33.

UV−vis (toluene, 2 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 334 (sh),

375 (109600), 387 (sh). Anal. Calcd for C₃₆H₅₀Cu₂N₆O₂: C, 59.56; H,

6.94; N, 11.58. Found: C, 59.30; H, 7.00; N, 11.33.

M

DOI: 10.1021/acs.organomet.7b00609

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

N

DOI: 10.1021/acs.organomet.7b00609

Organometallics XXXX, XXX, XXX−XXX

Organometallics

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DOI: 10.1021/acs.organomet.7b00609

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

[(L11)₂Cu₂(μ-O,κ_N-OCH₂Py)₂] (11). The procedure was analogous

to that for 3b, from Cu(OMe)₂(54 mg, 0.43 mmol) in toluene (3

mL), 2-pyridinemethanol (83 μL, 0.86 mmol), and L11 (100 mg, 0.43

mmol) in toluene (2 mL). The ﬁltered and concentrated (one-third of

the volume) green solution was diluted with hexane (18 mL) and kept

at −30 °C for 4 h. Decantation and washing with hexane (3 × 10 mL)

aﬀorded 33 mg (20%) of green X-ray-quality crystals.

AUTHOR INFORMATION

Corresponding Author

*E-mail for F.S.: Frank.Schaper@umontreal.ca.

ORCID

Frank Schaper: 0000-0002-3621-8920

Notes

The authors declare no competing ﬁnancial interest.

■

UV−vis (toluene, 2 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 350

(69200). Anal. Calcd for C₃₈H₃₆Cl₂Cu₂N₆O₂: C, 56.58; H, 4.50; N,

10.42. Found: C, 56.27; H, 4.45; N, 10.38.

ACKNOWLEDGMENTS

[(L11)₂Cu₂(μ-O,κ_N-OC₂H₄NMe₂)₂] (11b). The procedure was

analogous to that for 3b, from Cu(OMe)₂(54 mg, 0.43 mmol) in

toluene (3 mL), dimethylaminoethanol (87 μL, 0.86 mmol), and L11

(100 mg, 0.43 mmol) in toluene (2 mL). The ﬁltered and

concentrated (one-third of the volume) green solution was diluted

with hexane (18 mL) and kept at −30 °C for 4 h. Decantation and

washing with hexane (3 × 10 mL) aﬀorded 47 mg (28%) of green X-

ray-quality crystals.

■

Funding was supplied by the NSERC discovery program

(RGPIN-2016-04953) and the Centre for Green Chemistry

and Catalysis (FQRNT). We thank D. Kapoor and C. Barcha

for participation in the preparation of 8−11b during their

internship. We thank Marie-Christine Tang and Dr. Alexandra

Furtos for support with MALDI-MS and Francine Belanger and

́

Dr. Todd J. J. Whitehorne for support with X-ray

crystallography.

UV−vis (toluene, 5 × 10⁻⁶M) [λ_max, nm (ε, mol⁻¹cm²)]: 349

(37400). Anal. Calcd for C₃₄H₄₄Cl₂Cu₂N₆O₂: C, 53.26; H, 5.78; N,

10.96. Found: C, 53.18; H, 6.00; N, 10.96.

rac-Lactide Polymerization. In a glovebox, the desired amount of

rac-lactide was placed into a J. Young tube together with C₆D₆. A stock

solution of an additive (benzyl alcohol or pyridylmethanol) was added,

if required, followed by a stock solution of the catalyst (∼20 mM in

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ASSOCIATED CONTENT

■

S

* Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.organo-

met.7b00609.

Additional reaction details, and crystallographic, charac-

terization, and polymerization data (PDF)

Accession Codes

CCDC 1567039−1567058 contain the supplementary crystal-

lographic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-

ing 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.

P

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DOI: 10.1021/acs.organomet.7b00609

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DOI: 10.1021/acs.organomet.7b00609

Organometallics XXXX, XXX, XXX−XXX

Article Doi

Diiminopyrrolide Copper Complexes: Synthesis, Structures, and rac-Lactide Polymerization Activity

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