Sulfonamide-supported aluminum catalysts for the ring-opening polymerization of rac-lactide

1246 Organometallics 2010, 29, 1246–1260

DOI: 10.1021/om901070g

Sulfonamide-Supported Aluminum Catalysts for the Ring-Opening

Polymerization of rac-Lactide

Andrew D. Schwarz, Zengyong Chu, and Philip Mountford*

Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.

Received December 14, 2009

The synthesis, structures, and ring-opening polymerization (ROP) capability of a wide range

of sulfonamide-supported aluminum alkyl and alkoxide complexes are reported. The synthesis of

the new protio-ligands PhCH₂N(CH₂CH₂NHSO₂R)₂(R = Tol (15, H₂N₂^TsN^Ph) or Me (16,

H₂N₂^MsN^Ph)) is described. These and the previously reported 1,2-C₆H₁₀(NHSO₂R)₂(R = Tol (11,

H₂CyN₂^Ts) or Mes (12, H₂CyN₂^{SO Mes})) and RCH₂N(CH₂CH₂NHSO₂Tol)₂(R = MeOCH₂(13,

2

H₂N₂^TsN^OMe) or 2-NC₅H₄(14, H₂N₂^TsN^py)) reacted with AlEt₃to form Al(CyN₂^Ts)Et(THF) (17),

Al(CyN₂^{SO Mes})Et(THF) (18), and Al(N₂^TsN^R)Et (R = Ph (19), OMe (20), or py (21)), respectively.

2

Subsequent reaction of these ethyl complexes with R⁰OH (R⁰= ⁱPr or Bn) resulted in protonolysis of

the sulfonamide supporting ligands to yield a mixture of products including Al(OR⁰)₃. In contrast,

reaction of Al(OR⁰)Et₂(R⁰= ⁱPr, Bn, CH₂CH₂NH₂, or CH₂CH₂NMe₂) with various protio-ligands

Ts py

i

formed the sulfonamide-supported alkoxides Al(N₂N )(OR⁰) (R⁰= Pr (22) or Bn (23)), Al-

(N₂^MsN^Ph)(OR⁰) (R⁰= ⁱPr (26) or Bn (27)), Al(N₂^TsN^R)(OCH₂CH₂NH₂) (R = Ph (29), OMe (30), or

py (31)), Al(CyN₂^Ts)(OCH₂CH₂NMe₂) (32), and Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (33). Unexpect-

edly, reaction of Al(OⁱPr)Et₂with H₂N₂^TsN^OMeled to O-demethylation of the sulfonamide ligand.

Reaction of AlMe₂Cl with H₂N₂^TsN^Phgave [Al(N^Ts₂N^Ph)Cl]₂(28). X-ray diffraction studies revealed

four- or five-coordinate C_s-symmetric structures for 17-21, a five-coordinate C₂-symmetric sulfon-

amide-bridged dimer for 28, and a five-coordinate C_s-symmetric monomer for 30 stabilized by

intramolecular hydrogen bonding between the sulfonyl oxygens and the amine protons. Compounds 19,

21, 22-27, and 29-33 are all catalysts for the ROP of rac-lactide, with the alkoxide compounds

22-27 and 32 giving well-defined molecular weights and molecular weight distributions. These

compounds were also active in the melt at 130 °C, giving atactic poly(rac-lactide) with moderate to

narrow PDIs and extremely good control of M_nand high activity in the case of 23.

Introduction

chain growth mechanism. It is an area of research that has

been extensively reviewed recently.^8-11Catalyst systems can be

based on many metals such as magnesium,^12-16zinc,^12,14,16-20

Polyesters derived from lactic acid are currently the focus

of intense research due to the biocompatibility and biode-

gradability of the resulting polymers. Derived from 100%

renewable resources including corn and sugar beet, these

polymers can act as replacements for oil-based materials.^1-7

In industry, these materials are synthesized by the metal-

catalyzed ring-opening polymerization (ROP) of the cyclic

ester lactide (LA).³Metal-based catalysts for the ROP of

cyclic esters are generally derived from Lewis acidic metals,

feature one or more alkoxide functional (initiating) groups,

and are supported by a polydentate ancillary ligand (set) that

controls the catalyst nuclearity and coordination-insertion

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*Corresponding author. E-mail: philip.mountford@chem.ox.ac.uk.

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pubs.acs.org/Organometallics

Published on Web 02/10/2010

2010 American Chemical Society

Article

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1248 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

Figure 2. Previously reported examples of aluminum sulfonamide complexes.^74-77

ligands, for which Gibson showed halide substituents on the

phenolate ring enhanced activity.²⁶Therefore, work aimed at

clarifying catalyst structure-activity relationships though

systematic ligand variation and evaluation of catalytic

performance is of high value.

Very recently, we described the use of tetradentate sulfo-

namides (containing -N(R)SO₂R⁰donors) as supporting

ligands for group 4 ROP catalysts.⁶⁷These catalysts effected

the highly controlled ROP of rac-LA and ε-caprolactone,

Figure 3. Previously reported protio-ligands employed in this

study.

giving polymers with narrow PDIs and predictable M_n

values in a living fashion. They gave comparable perfor-

mances to the well-established bis(phenolate) systems exten-

sively studied by Kol and Davidson.^68-72In particular,

zirconium catalysts of the tetradentate ligand N₂^TsN^OMe

(13; see Figure 3) allowed excellent control of the polymer-

ization process in toluene solution at elevated temperature,

albeit with no stereochemical bias, yielding atactic poly-

mer.⁶⁷In these group 4 catalysts, the electron-withdrawing

SO₂R⁰moieties reduce the usual basicity of anionic amide

donors “NR₂” and therefore allowed them to act as pheno-

late mimics. Furthermore, although N(R)SO₂R⁰-based

ligands lack the steric control afforded by bulky ring-sub-

stituted phenolates OAr (e.g., 3-6, Figure 1), the well-

established ability of sulfonamide ligands to adopt κ²(N,O)

coordination modes confers additional stability on otherwise

unsaturated metal centers.⁷³

To date, there have been only two examples of sulfonam-

ide-supported aluminum ROP catalysts, namely, 7 and 8 in

Figure 2.^74,75Compound 7 gave poor control of the polym-

erization process with a much lower than expected M_nand a

moderately broad PDI under melt conditions after 10 h at

130 °C. In contrast, 8 showed excellent control in the

polymerization of ^L-LA, yielding predominantly isotactic

polymers with narrow PDIs and close agreement between

expected M_nvalues and those measured by GPC.⁷⁵In

general a number of aluminum sulfonamide complexes

have been reported (e.g., 9 and 10) and employed for

transformations such as enantioselective catalytic Diels-

Alder reactions,⁷⁶asymmetric aldehyde cycloaddition reac-

tions,^77,78the Meerwein-Ponndorf-Verley reduction of

carbonyl substrates,⁷⁹the enantioselective [2þ2] cycloaddi-

tion of trimethylsilylketene to aldehydes,⁸⁰and the cyclo-

propanation of allylic alcohols.⁸¹

Our success with sulfonamide-supported group 4 ROP

catalysts, and the promising results recently reported for 8,

prompted us to develop and evaluate a series of aluminum-

based ROP catalysts using this type of ligand. Alongside the

tetradentate ligands that we employed previously, we have

also used homologous bidentate and tridentate systems,

since the radius of aluminum is somewhat smaller than those

of the group 4 metals. In this contribution we report the

synthesis, structures, and ring-opening polymerization

(ROP) capability of a wide range of sulfonamide-supported

aluminum alkyl and alkoxide complexes.

Results and Discussion

Synthesis. The previously reported bidentate and tetra-

Ts

dentate protio-ligands (Figure 3) H₂CyN₂(11),⁸²H₂Cy-

N₂^{SO Mes}(12),⁸³H₂N₂^TsN^OMe(13),⁸⁴and H₂N₂^TsN^py(14)⁸⁵

2

were synthesized according to literature methods.

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Article

Organometallics, Vol. 29, No. 5, 2010 1249

structures and ROP properties of the new catalysts, the new

tridentate protio-ligands PhCH₂N(CH₂CH₂NHSO₂R)₂

(R = Tol (15, H₂N₂^TsN^Ph) or Me (16, H₂N₂^MsN^Ph)) were

synthesized. Using a similar synthetic route to that for 13

and 14, reaction of benzyl amine with the appropriate

N-sulfonyl aziridine gave good yields of 15 and 16 (eq 1).

So as to make comparisons with their metal complexes (see

below) and the previously reported 11,^86,8713,⁶⁷and 14,⁸⁵

the X-ray structures of 15 and 16 were determined. The

molecular structures and selected distances and angles are

given in the Supporting Information (SI).

Scheme 1. Synthesis of the New Aluminum Ethyl Complexes

17-21^a

The solid-state molecular structures of 15 and 16 are

comparable, and the geometries represent an arrangement

that is reminiscent of that required to bind to a metal center

(in terms of N(1) lone pair and N-H bond orientations).

Similar arrangements were found previously for 13 and

14.^67,85Unexpectedly, there is no intra- or intermolecular

hydrogen bonding present in either compound. In general

terms, the bond lengths and angles lie within the expected

ranges for compounds of these types.⁸⁸

Initial complexation studies (Scheme 1) focused on alkyl

complexes, which we hoped would act as intermediates en

route to the more desirable alkoxide initiators via proto-

^aAll reactions were carried out in THF.

nolysis reactions with alcohols, according to literature pre-

Ts

(11) or

cedent.⁸⁹Reactions of AlEt₃with H₂CyN₂

Compounds 17 and 19 have approximately tetrahedral

aluminum centers. Each Al is bonded to two tosyl amide

nitrogens and one ethyl ligand, with no significant difference

between the Al-N_Tsand Al-C distances for the two com-

pounds. In 17 the fourth coordination site is occupied by a

THF ligand, whereas in 19 the tertiary nitrogen (N_am) of

N₂^TsN^Phfulfills this role. The aluminum in 20 is also

H₂CyN₂^{SO Mes}(12) in THF gave the four-coordinate Lewis

base adducts Al(CyN₂^Ts)Et(THF) (17) and Al(Cy-

2

N₂^{SO Mes})Et(THF) (18), the latter reaction requiring heating

2

at 80 °C. Reaction with H₂N₂^TsN^Ph(15) at room temperature

in the same solvent gave the four-coordinate, THF-free

Al(N₂^TsN^Ph)Et (19), containing

(sulfonamide)amine ligand. Finally, reaction of AlEt₃with

a

tridentate bis-

approximately tetrahedral, with an additional long Al O

˚

distance of 2.8305(14) A to the pendant arm methoxy donor.

3 3 3

Ts

H₂N₂^TsN^OMe(13) and H₂N₂^TsN^py(14) gave Al(N₂

-

N

^OMe)Et (20) and Al(N₂^TsN^py)Et (21), also as THF-free

The Al O interaction barely perturbs the coordination

3 3 3

1

complexes. The H NMR shift of the 2-position hydrogen

atom of the pyridyl donor of 21 (δ = 9.05 ppm) in CD₂Cl₂is

significantly shifted from that in the free ligand (δ =

8.58 ppm in the same solvent). This is consistent with

coordination to Al, giving the five-coordinate geometry

illustrated in Scheme 1. The isolated yields for 17-21 were

in the range 55-70% after recrystallization. NMR tube scale

monitoring of these reactions in C₆D₆showed quantitative

formation of the complexes.

sphere around Al, as judged by comparison with the

N₂^TsN^Phanalogue 19, bearing a noninteracting CH₂Ph

˚

arm. For example, for compound 19, Al-C = 1.940(3) A,

N_am-Al-C = 120.56(12)^oand for compound 20, Al-C =

1.949(2) A, N_am-Al-C = 126.84(8)^o. In contrast, Al-

˚

(N₂^TsN^py)Et (21), featuring an additional pyridyl donor,

has a distorted trigonal-bipyramidal geometry with an

˚

Al-N_pydistance (2.107(2) A) within the expected range for

a dative bond,⁸⁸consistent with the solution ¹H NMR

data mentioned above. The N_am-Al-C angle has opened

up to 156.74(8)^oto accommodate the additional donor,

The X-ray structures of compounds 17-21 have been

determined and are compared in Figure 4. Selected distances

and angles are listed in Tables 1-4. The solid-state structures

are consistent with those proposed in Scheme 1 on the basis

of solution NMR and other analytical data.

˚

and the Al-C (1.986(2) A) and Al-N_Ts(av 1.932(1) A)

distances are significantly longer than those in 17, 19,

˚

and 20 (range 1.940(3)-1.949(4) and 1.858(2)-1.895(2) A,

respectively).

Only two crystallographically characterized aluminum

tosyl sulfonamide compounds have been reported pre-

viously, namely, 8 (Figure 2)⁷⁵and S,S,S-Al{N(CH-

(Me)CH₂NTs)₃}.⁷³Both have additional intramolecular

(86) Nieger, M.; Josten, W.; Vogtle, F. Private communication, 2004.

(87) Pritchett, S.; Gantzel, P.; Walsh, P. J. Organometallics 1999, 18,

823–831.

(88) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf.

Comput. Sci. 1996, 36, 746 (The United Kingdom Chemical Database

Service).

SdO Al interactions (as shown in Figure 2 for 8) that

3 3 3

(89) Peng, K.-F.; Chen, C.-T. Dalton Trans. 2009, 9800–9806.

are absent in 17-21. There have also been a number of

1250 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

Figure 4. Displacement ellipsoid plots of Al(CyN₂^Ts)Et(THF) (17, top left), Al(N₂^TsN^Ph)Et (19, top right), Al(N₂^TsN^OMe)Et (20,

bottom left), and Al(N₂^TsN^py)Et (21, bottom right). H atoms are omitted for clarity. Ellipsoids are drawn at the 20% probability level.

˚

Table 1. Selected Bond Distances (A) and Angles (deg) for

Table 3. Selected Bond Distances (A) and Angles (deg) for

Al(CyN₂^Ts)Et(THF) (17)

Al(N₂^TsN^OMe)Et (20)

Al(1)-N(1)

1.867(3)

1.872(3)

85.95(11)

103.54(13)

124.1(2)

Al(1)-O(5)

1.858(2)

1.949(4)

106.74(12)

123.58(19)

109.47(17)

Al(1)-N(1)

2.035(1)

1.895(2)

1.860(2)

86.06(6)

112.34(7)

112.11(7)

67.38(5)

79.54(6)

Al(1)-C(22)

1.949(2)

2.830(1)

Al(1)-N(2)

Al(1)-C(21)

Al(1)-N(2)

Al(1) O(5)

3 3 3

N(1)-Al(1)-N(2)

N(2)-Al(1)-O(5)

N(2)-Al(1)-C(21)

N(1)-Al(1)-O(5)

N(1)-Al(1)-C(21)

O(5)-Al(1)-C(21)

Al(1)-N(3)

N(1)-Al(1)-N(2)

N(2)-Al(1)-N(3)

N(2)-Al(1)-C(22)

N(1)-Al(1)-N(3)

N(1)-Al(1)-C(22)

N(3)-Al(1)-C(22)

88.00(6)

126.84(8)

124.23(8)

150.95(6)

77.72(6)

N(1)-Al(1) O(5)

N(3)-Al(1) O(5)

N(2)-Al(1) O(5)

3 3 3

˚

O(5) Al(1)-C(22)

Table 2. Selected Bond Distances (A) and Angles (deg) for

Al(N₂^TsN^Ph)Et (19)

3 3 3

˚

Table 4. Selected Bond Distances (A) and Angles (deg) for

Al(N₂^TsN^py)Et (21)

Al(1)-N(1)

Al(1)-N(2)

2.023(2)

1.864(2)

Al(1)-N(3)

Al(1)-C(26)

1.858(2)

1.940(3)

N(1)-Al(1)-N(2)

N(2)-Al(1)-N(3)

N(2)-Al(1)-C(26)

87.15(9)

116.26(10)

114.64(12)

N(1)-Al(1)-N(3)

N(1)-Al(1)-C(26)

N(3)-Al(1)-C(26)

86.37(9)

120.56(12)

122.80(12)

Al(1)-N(1)

1.911(2)

1.952(2)

2.107(2)

111.65(8)

82.92(7)

79.20(7)

118.01(9)

90.47(8)

Al(1)-N(4)

Al(1)-C(25)

2.185(2)

1.986(2)

Al(1)-N(2)

Al(1)-N(3)

N(1)-Al(1)-N(2)

N(1)-Al(1)-N(4)

N(2)-Al(1)-N(4)

N(1)-Al(1)-C(25)

N(3)-Al(1)-C(25)

N(1)-Al(1)-N(3)

N(2)-Al(1)-N(3)

N(3)-Al(1)-N(4)

N(2)-Al(1)-C(25)

N(4)-Al(1)-C(25)

101.95(7)

133.77(8)

74.31(7)

100.26(9)

156.74(8)

structures reported for aluminum complexes containing

-N(R)SO₂CF₃donors, including the four-coordinate alkyl

complex 10 in Figure 2, which has a comparable geometry to

17, 19, and 20.^76,77,79,90

As mentioned, it was hoped that the new aluminum alkyls

would serve as entry points to the corresponding alkoxide

complexes, which are usually superior initiators for ROP.

Unfortunately, reaction of 17-21 with either isopropyl or

benzyl alcohol yielded mixtures of protio-ligand, residual

metal alkyl, and Al(OR)₃species in benzene at room tem-

perature. Performing the reactions at low temperature in

alternative solvents and/or at high dilution gave similar

mixtures. Therefore, a different route to alkoxide complexes

was sought. Drawing upon our previous work on titanium

complexes where sulfonamide ligand protonolysis was

also an issue,⁶⁷we focused on reactions with mixed alkyl-

alkoxide aluminum precursors of the type Al(OR)Et₂

(R = ⁱPr or Bn).^91,92

Ts

Reaction of H₂CyN₂(11) or H₂N₂^TsN^Ph(15) with Al-

i

(OR)Et₂(R = Pr or Bn) led to complicated product

(91) Kunicki, A. R.; Pasynkiewicz, S.; Jankowski, J.; Mank, B. Pol. J.

Chem. 2004, 78, 1857–1864.

(92) Hirabayashi, T.; Sakakibara, T.; Ishii, Y. J. Organomet. Chem.

(90) Corey, E. J.; Sarshar, S.; Bordner, J. J. Am. Chem. Soc. 1992, 114,

7938–7939.

1972, 35, 19–25.

Article

Organometallics, Vol. 29, No. 5, 2010 1251

Scheme 2. Synthesis of the New Aluminum Alkoxide Complexes

22, 23, 26, and 27^a

However, attempted purification by crystallization afforded

only the ligand-demethylated dimer [Al(N₂^TsN^O)]₂(25,

N₂^TsN^O= OCH₂CH₂N(CH₂CH₂NTs)₂) as colorless crys-

1

tals. The H and ¹³C{¹H} NMR data and a preliminary

X-ray structure determination (see Figure S1 in the SI) are

consistent with the structure illustrated in Scheme 3. Com-

pound 25 could be reproducibly synthesized in 49% overall

yield by heating the crude initial mixture containing 24 at

150 °C under reduced pressure for 48 h.

In contrast to the mixtures formed with the tosylated

protio-ligand 15, reaction of H₂N₂^MsN^Ph(16, -NSO₂Me

substituted) with Al(OR)Et₂gave Al(N₂^MsN^Ph)(OⁱPr) (26)

and Al(N₂^MsN^Ph)(OBn) (27) in good yield (Scheme 2). NMR

investigations of analytically pure samples in various sol-

vents showed a mixture of monomeric and dimeric species in

both cases. For example, in C₆D₆the compounds exist as C₂-

symmetric dimers with the PhCH₂N protons appearing as

two mutually coupled doublets, consistent with SdO Al

3 3 3

bridging sulfone groups in a manner analogous to that for

[Al(N₂^TsN^Ph)Cl]₂(28, see below). In CD₂Cl₂, the spectra

show a ca. 1:2 mixture of both dimeric and monomeric

(C_s-symmetric, akin to Al(N₂^TsN^Ph)Et (19)) species, the

latter being suggested by a singlet for the PhCH₂N protons.

Addition of a few drops of pyridine-d₅to the CD₂Cl₂

solution significantly simplified the spectra, which were

consistent with the formation of Lewis base adducts

Al(N₂^MsN^Ph)(OR)(pyridine-d₅).

^aReactions were carried out in toluene.

Scheme 3. Ligand Demethylation Leading to [Al(N₂^TsN^O)]₂(25)

Since reactions between H₂N₂^TsN^Phand Al(OR)Et₂gave

intractable mixtures, alternative routes to the target alkox-

ides via the corresponding chloride complex were investi-

gated. Reaction of AlMe₂Cl with H₂N₂^TsN^Ph(15) gave

[Al(N^Ts₂N^Ph)Cl]₂(28, eq 2) in 58% yield after recrystalliza-

tion. The molecular structure is shown in Figure 5, and

selected bond lengths and angles are given in Table 5.

Compound 28 is an approximately C₂-symmetric, dimeric

species formed though two bridging SdO Al interactions

3 3 3

˚

(Al(1)-O(8) = 1.874(3) A; Al(2)-O(4) = 1.854(2) A), as has

been observed previously,^76,90Each Al has an approximately

trigonal-bipyramidal geometry, with the Cl ligands and

mixtures, and no single product could be identified. The

SO₂Mes

corresponding reaction for H₂CyN₂

(12) also gave a

2

mixture of products, among which was Al(CyN₂^{SO Mes})Et-

(THF) (18), indicative of redistribution reactions. However,

reaction of H₂N₂^TsN^pywith Al(OR)Et₂proceeded smoothly

at 100 °C to give Al(N₂^TsN^py)(OⁱPr) (22) and Al(N₂^TsN^py)-

(OBn) (23) in ca. 65% isolated yield (Scheme 2). The

NMR data for these compounds are in agreement with

five-coordinate, C_s-symmetric aluminum compounds, as

confirmed crystallographically for Al(N₂^TsN^py)Et (21). Sur-

prisingly, reaction of H₂N₂^TsN^OMe(13) with Al(OⁱPr)Et₂

gave a mixture of products, among which appeared to be

the desired alkoxide Al(N₂^TsN^OMe)OⁱPr (24, Scheme 3).

N

_Tsdonors occupying the equatorial positions. The Al-N_Ts

˚

distances in 28 (av 1.904(2) A, range 1.892(3)-1.916(3) A)

lie in general within the ranges mentioned above.

However, the Al-N_Tsdistances associated with the bridging

SO₂Tol groups (av 1.914(2) A) are significantly longer

˚

than those for the nonbridging ones (av 1.894(2) A). Simi-

larly, the SdO Al interactions significantly lengthen

˚

the tosyl SdO bonds (S(2,4)dO = 1.477(2) A for the

˚

bridging oxygens vs av S(2,4)dO = 1.434(2) A for the

terminal ones).

˚

3 3 3

1252 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

Figure 6. Displacement ellipsoid plot of Al(N₂^TsN^OMe)-

(OCH₂CH₂NH₂) (30). C-bound H atoms are omitted for clarity.

H(1) and H(2) are drawn as spheres of arbitrary radius. Ellip-

soids are drawn at the 20% probability level.

Figure 5. Displacement ellipsoid plot of [Al(N^Ts₂N^Ph)Cl]₂(28).

H atoms are omitted for clarity. Ellipsoids are drawn at the 20%

probability level.

Scheme 4. Synthesis of Aluminum Sulfonamide Complexes

Bearing a 2-Aminoethoxide Ligand^a

˚

Table 5. Selected Bond Distances (A) and Angles (deg) for

[Al(N^Ts₂N^Ph)Cl]₂(28)

Al(1)-N(1)

2.085(3)

1.895(3)

1.912(3)

1.874(3)

2.1775(14)

1.435(3)

1.477(2)

Al(2)-N(4)

2.100(3)

1.892(3)

1.916(3)

1.854(2)

2.1814(14)

1.434(2)

1.477(2)

Al(1)-N(2)

Al(2)-N(5)

Al(1)-N(3)

Al(2)-N(6)

Al(1)-O(8)

Al(2)-O(4)

Al(1)-Cl(1)

Al(2)-Cl(2)

S(2)-O(3)

S(4)-O(7)

S(2)-O(4)

S(4)-O(8)

N(1)-Al(1)-N(2)

N(1)-Al(1)-N(3)

N(2)-Al(1)-O(8)

N(1)-Al(1)-Cl(1)

N(3)-Al(1)-Cl(1)

N(4)-Al(2)-N(5)

N(5)-Al(2)-N(6)

N(5)-Al(2)-O(4)

N(4)-Al(2)-Cl(2)

N(6)-Al(2)-Cl(2)

81.68(13)

118.95(13)

99.63(12)

93.08(9)

127.78(10)

82.15(13)

118.79(14)

100.28(13)

92.57(9)

N(1)-Al(1)-N(3)

N(1)-Al(1)-O(8)

N(3)-Al(1)-O(8)

N(2)-Al(1)-Cl(1)

O(8)-Al(1)-Cl(1)

N(4)-Al(2)-N(6)

N(4)-Al(2)-O(4)

N(6)-Al(2)-O(4)

N(5)-Al(2)-Cl(2)

N(6)-Al(2)-Cl(2)

80.12(12)

171.68(12)

92.15(12)

110.91(11)

94.10(9)

80.73(12)

171.47(12)

90.99(12)

110.31(11)

94.21(9)

128.72(10)

It is interesting to contrast the monomeric nature of four-

coordinate Al(N₂^TsN^Ph)Et (19) with the dimeric one found

for [Al(N^Ts₂N^Ph)Cl]₂. The difference can be attributed to the

differences in electronegativity for the different ligands, Et

and Cl. The solution ¹H and ¹³C{¹H} NMR data for 28 are

consistent with this structure being retained in solution, and

the spectra are similar to those of the dimeric forms of the

alkoxide analogues 26 and 27 (Scheme 2). Unfortunately,

attempts to make alkoxide complexes analogous to 26 and 27

from 28 using LiOR (R = ⁱPr or Bn) were unsuccessful, again

giving intractable mixtures of products.

In a further effort to stabilize well-defined monomeric

alkoxide ROP initiators, reactions were undertaken using

Al(OCH₂CH₂NH₂)Et₂.⁹³This contains a potentially chelat-

ing amine functional group, which we hoped would confer

additional stability on the target complexes. During the

course of our studies we structurally characterized this

known compound, and further details are given in the SI

(see Figure S2 and the CIF).

^aAll reactions were carried out in toluene.

As summarized in Scheme 4, heating toluene solutions

of Al(OCH₂CH₂NH₂)Et₂with H₂N₂^TsN^Ph, H₂N₂^TsN^OMe

,

or H₂N₂^TsN^pysuccessfully gave the corresponding 2-amino-

ethoxide complexes Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29),

Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30), and Al(N₂^TsN^py)-

(OCH₂CH₂NH₂) (31), respectively, in isolated yields of

68-79%. The product(s) of the corresponding reaction with

Ts

H₂CyN₂(11) gave broad and complicated spectra, which

we were not able to confidently assign. The solid-state

structure of 30 has been determined and is shown in

Figure 6. Selected bond distances and angles are given in

Table 6.

Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30) has a distorted trigo-

nal-bipyramidal geometry and approximate C_smolecular

symmetry. The anionic sulfonamide nitrogen and alkoxide

oxygen donors occupy the equatorial positions, and the

tertiary N_amof N₂^TsN^OMeand the NH₂of the aminoethoxide

(93) Hurley, T. J.; Robinson, M. A.; Scruggs, J. A.; Trotz, S. I. Inorg.

Chem. 1967, 3, 1310–1315.

Article

Organometallics, Vol. 29, No. 5, 2010 1253

˚

Table 6. Selected Bond Distances (A) and Angles (deg) for

Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30)

Scheme 5. Synthesis of Aluminum Sulfonamide Complexes

Bearing a OCH₂CH₂NMe₂Ligand^a

Al(1)-N(1)

2.078(4)

1.917(4)

1.915(4)

2.006(5)

1.759(4)

0.94(7)

O(2) H(1)

3 3 3

1.95(7)

2.06(7)

1.448(4)

1.439(4)

1.441(4)

1.443(4)

Al(1)-N(2)

O(4) H(2)

3 3 3

Al(1)-N(3)

S(1)-O(1)

S(1)-O(2)

S(2)-O(3)

S(3)-O(4)

Al(1)-N(4)

Al(1)-O(6)

N(4)-H(1)

N(4)-H(2)

0.81(7)

N(1)-Al(1)-N(2)

N(2)-Al(1)-N(3)

N(2)-Al(1)-N(4)

N(1)-Al(1)-O(6)

N(3)-Al(1)-O(6)

82.68(17)

114.82(19)

95.8(2)

90.71(17)

122.83(19)

N(1)-Al(1)-N(3)

N(1)-Al(1)-N(4)

N(3)-Al(1)-N(4)

N(2)-Al(1)-O(6)

N(4)-Al(1)-O(6)

83.30(17)

175.4(2)

101.3(2)

120.65(19)

86.3(2)

take up the axial positions, the latter having comparable

˚

bond lengths (Al-N_am= 2.078(4); Al-NH₂= 2.006(5) A).

The OMe donor of the pendant arm is not coordinated,

displaced in preference for NH₂coordination (the primary

amine being the more effective Lewis base). In addition,

amine coordination is supplemented by intramolecular hy-

drogen bonding between the NH₂hydrogens and two ad-

^aAll reactions were carried out in toluene.

jacent sulfonamide oxygen atoms. The NH O_Tsdistances

3 3 3

the controlled ROP of rac-LA. To probe these aspects, we

prepared some representative complexes containing a 2-N,

N-dimethylaminoethoxide ligand (Scheme 5), in effect repla-

cing -NH₂with -NMe₂. Reaction of H₂N₂^TsN^Phwith

˚

of 1.95(7) and 2.06(7) A are within the expected ranges for

such interactions.^88,94The IR spectrum of 30 shows ν(N-H)

bands at 3306 and 3268 cm^-1, which are shifted to lower

frequency compared to those in the starting compound

Al(OCH₂CH₂NH₂)Et₂(ν(N-H) = 3356 and 3287 cm^-1),

which has a chelating OCH₂CH₂NH₂ligand but no

93

Al(OCH₂CH₂NMe₂)Et₂gave monomeric Al(N₂^TsN^Ph)-

(OCH₂CH₂NMe₂) (32). The NMR spectra for 32 were

consistent with the five-coordinate structure proposed in

Scheme 5, which is analogous to those of 29, 30, and 31b,

but without the intramolecular hydrogen bonding. The

spectra indicated molecular C_ssymmetry (e.g., equivalent

SO₂Tol substituents and NMe₂methyl groups), and NOE

(nuclear Overhauser effect) experiments confirmed the trans

arrangement of the tertiary N_amand NMe₂donors, together

NH O hydrogen bonds (SI, Figure S2). There is no

3 3 3

significant differences in length between the SdO bonds

engaged in hydrogen bonding (S(1)-O(2), S(2)-O(4)) and

those that are not. The Al-N_Tsdistances (1.917(4) and

1.915(4) A) are slightly shorter than those in Al(N₂^TsN^py)Et

˚

(21, av 1.932(1) A) and very similar to those for

[Al(N^Ts₂N^Ph)Cl]₂(28) (av 1.904(2) A).

˚

with coordination of NMe₂to Al.

Whereas reaction of H₂CyN₂

The IR spectrum of Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29)

has similar ν(N-H) bands (3308 and 3262 cm^-1) to those

of 30, and a similar structure is proposed (Scheme 4). Note

that in the otherwise identical Al(N₂^TsN^Ph)(OCH₂CH₂-

NMe₂) (32, see below), which has a NMe₂donor in place

of NH₂, these bands are absent. The ¹H and ¹³C{¹H} NMR

spectra of 29 and 30 are comparable and relatively simple,

and consistent with the presence of one five-coordinate,

C_s-symmetric isomer in solution in each case.

Ts

with Al(OCH₂CH₂-

NH₂)Et₂gave rather ill-defined materials (see above), reac-

tion with Al(OCH₂CH₂NMe₂)Et₂formed “Al(CyN₂^Ts)-

(OCH₂CH₂NMe₂)” (33) in 85% isolated yield. The

NMR spectra for 33 are more complicated than would be

expected for a C_s-symmetric, four-coordinate monomer

analogous to Al(CyN₂^Ts)Et(THF) and Al(CyN₂^{SO Mes})Et-

2

(THF) (Scheme 1), showing, for example, inequivalent SO₂-

Tol substituents and NMe₂methyl groups. We propose that

33 possesses the dimeric structure shown in Scheme 5. A

number of related dimeric aluminum complexes containing a

OCH₂CH₂NMe₂or related 2-aminoethoxide ligand in the

arrangement shown for 33 have been structurally authenti-

cated,⁸⁸including Al(OCH₂CH₂NH₂)Et₂(SI, Figure S2).

Polymerization Studies: Solution ROP of rac-LA. The

series of complexes Al(N₂^TsN^R)Et (R = Ph (19) or py

In contrast, the IR spectrum of Al(N₂^TsN^py)-

(OCH₂CH₂NH₂) (31) shows at least four ν(N-H) bands in

the range 3320-3160 cm^-1, and the solution ¹H and ¹³C{¹H}

NMR spectra suggest a mixture of two isomers, 31a (major)

and 31b (minor), in a ca. 2:1 ratio. Both isomers have C_s

symmetry and the same composition “Al(N₂^TsN^py)-

(OCH₂CH₂NH₂)” according to integration, consistent with

the satisfactory elemental analysis for bulk samples of 31.

Ts py

Ms

(21)), Al(N₂N )(OR⁰) (R⁰= ⁱPr (22) or Bn (23)), Al(N₂

-

1

N^Ph)(OR⁰) (R⁰= Pr (26) or Bn (27)), Al(N₂^TsN^R)(OCH₂-

CH₂NH₂) (R = Ph (29), OMe (30), or py (31)), Al(N₂^TsN^Ph)-

(OCH₂CH₂NMe₂) (32), and Al(CyN₂^Ts)(OCH₂CH₂NMe₂)

(33) allowed the systematic evaluation of the effects of

sulfonamide ligand coordination number and N-substitu-

ents and initiating group type (Et, “simple” alkoxide, or

2-aminoethoxide). Each was assessed for its ROP perfor-

mance for rac-LA. ROP experiments were performed in

toluene at 70 °C ([rac-LA]₀:[Al] = 100:1), and under these

conditions all the complexes were found to be active. The

progress of each was monitored by regular sampling, and the

results summarized in Table 7 correspond to 72 h reaction

time (see the SI for other data). The molecular weights

i

The H NMR shift of the 2-position hydrogen atom of the

pyridyl donor in the major isomer appears at δ = 8.87 ppm,

consistent with coordination to Al,⁶⁷whereas the shift of δ =

8.52 ppm for the minor isomer is very close to that of the free

H₂N₂^TsN^pyin CD₂Cl₂(δ = 8.58 ppm). We interpret these

data in terms of the structures proposed in Scheme 4, with the

pendant pyridyl and NH₂donors competing for the vacant

coordination site at Al.

As described below, the use of an aminoethoxide moiety in

29-31 appeared to lead to poor performance with regard to

ꢀ

(94) Aullon, G.; Bellamy, D.; Orpen, A. G.; Brammer, L.; Bruton,

E. A. Chem. Commun. 1998, 653–654.

1254 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

Table 7. Solution Polymerization of rac-LA by Al(N₂^TsN^Ph)Et (19), Al(N₂^TsN^py)Et (21), Al(N₂^TsN^py)(OⁱPr) (22), Al(N₂^TsN^py)(OBn) (23),

Al(N₂^MsN^Ph)(OⁱPr) (26), Al(N₂^MsN^Ph)(OBn) (27), Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29), Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30),

Al(N₂^TsN^py)(OCH₂CH₂NH₂) (31), Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (32) and Al(CyN₂^Ts)(OCH₂CH₂NMe₂) (33)^a

entry

catalyst

conversion (%)^b

k_app(h^-1

)

M_n(GPC)^c

M_n(calcd)^d

M_w/M_n

e

1

2

3

4

5

6

7

8

Al(N₂^TsN^Ph)Et (19)

20

48

83

87

81

76

96^f

93^f

82

76

18 320

13 060

19 480

14 270

16 040

16 340

27 150

25 060

24 770

16 960

30 460

2910

8870

1.47

1.73

1.11

1.14

1.12

1.44

1.45

1.44

1.12

1.22

e

Al(N₂^TsN^py)Et (21)

Al(N₂^TsN^py)(OⁱPr) (22)

0.0245(5)

0.0280(8)

0.0230(4)

12 010

12 610

11 690

13 790

13 860

13 400

11 890

10 740

Al(N₂^TsN^py)(OBn) (23)

Al(N₂^MsN^Ph)(OⁱPr) (26)

Al(N₂^MsN^Ph)(OBn) (27)

0.0203(7)

f

Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29)

Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30)

Al(N₂^TsN^py)(OCH₂CH₂NH₂) (31)

Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (32)

Al(CyN₂^Ts)(OCH₂CH₂NMe₂) (33)

f

9

10

11

0.0235(11)

0.0202(8)

^aConditions: [rac-LA]₀:[Al] = 100:1, 4.0 mL of toluene at 70 °C. See Experimental Section for other details. ^b% conversion by NMR at 72 h reaction

time. ^cMolecular weights (g mol^-1) determined by GPC in THF at 30 °C using the appropriateMark-Houwinkcorrections. ^dExpected M_n(g mol^-1) for

1 chain growing per metal center. ^eNot measured due to the poor conversion and sluggish behavior. ^fPolymerization terminated after 72 h, and k_appnot

measured due to gel formation after ca. 18 h.

and PDIs (M_w/M_n) were determined by gel permeation

chromatography (GPC) using the appropriate Mark-

Houwink corrections for PLA.^95-97Polymerization using

these aluminum complexes required extended reaction times

compared to the sulfonamide-supported zirconium analo-

gues we reported previously.⁶⁷For example the zirconium

alkoxide catalyst based upon N₂^TsN^OMeachieved 94% con-

version of 100 equiv of rac-LA after ca. 6 h, while 30 reached

96% conversion only after 72 h. This observation is in

agreement with the literature. For example, bis(phenolate)

complexes of zirconium demonstrate enhanced activity com-

pared to similar aluminum species.^63,65,68This is due to the

smaller radius of aluminum.

The ethyl complexes 19 and 21 (entries 1 and 2, Table 7)

were found to be active but with relatively low conversions

Figure 7. First-order plot for rac-LA consumption using Al-

and poor control. Only ca. 20-50% of the rac-LA had been

polymerized after 72 h, and the polymers formed had broad

(N₂^TsN^py)(OBn) (23). Conditions: [rac-LA₀]:[23] = 100:1, 4 mL

of toluene, 70 °C, 0.1 mL aliquots taken at the given intervals.

=

0.997). See the SI for corresponding plots for 22, 26, 27, 32,

and 33.

PDIs (ca. 1.5-1.7) indicative of a poorly controlled process.

The MALDI-ToF mass spectra of the polymers were com-

plex and showed the presence of both cyclic poly(rac-LA)

and linear chains terminated by EtC(O)CH(Me)O- end

groups, the latter forming as a result of insertion of rac-LA

into the Al-Et of the initiator. The M_nvalues measured by

GPC were substantially higher than predicted on the basis of

See Experimental Section for other details. Linear fit (r²

1

conversion (judged by H NMR analysis of the reaction

mixtures) and assuming each aluminum center is catalyti-

cally active. This deviation is consistent with poor rates of

initiation (LA insertion into the Al-Et bond) compared to

propagation (insertion into the Al-O bond of the so-formed

Al-{OCH(Me)C(O)}_n-Et growing chains), which is a well-

established feature of metal alkyl initiators.^98,99

In contrast to 19 and 21, the alkoxides complexes 22-27

were considerably better behaved (entries 3-6, Table 7). The

polymerization experiments were monitored at 6, 24, 48, and

Figure 8. Plots of M_nand PDI (determined by GPC) vs con-

version for the polymerization of rac-LA using Al(N₂^TsN^py)-

(OBn) (23). Conditions: [rac-LA₀]:[23] = 100:1, 4 mL of

toluene, 70 °C, 0.1 mL aliquots taken at the given intervals.

Hollow diamonds correspond to M_nand hollow circles to PDI.

See the SI for corresponding plots for 22, 26, 27, 29, and 31-33.

(95) Rudin, A.; Hoegy, H. L. W. J. Polym. Sci., Part A: Polym. Chem.

1972, 10, 217–235.

ꢀ ^

ꢀ

(96) Barakat, I.; Jerome, R.; Teyssie, P. h. J. Polym. Sci., Part A:

Polym. Chem. 1993, 31, 505–514.

(97) Dorgan, J. R.; Janzen, J.; Knauss, D. M.; Hait, S. B.; Limoges, B.

R.; Hutchinson, M. H. J. Polym. Sci., Part B: Polym. Phys. 2005, 43,

3100–3111.

(98) Gong, S.; Ma, H. Dalton Trans. 2008, 3345–3357.

(99) Alonso-Moreno, C.; Garcas, A.; Sanchez-Barba, L. F.; Fajardo,

M.; Fernandez-Baeza, J.; Otero, A.; Lara-Sanchez, A.; Antiaolo, A.;

Broomfield, L.; Lapez-Solera, M. I.; Rodraguez, A. M. Organometallics

2008, 27, 1310–1321.

72 h intervals, and representative plots concerning the kinetic

aspects and control of M_nand PDI are given in Figures 7

and 8 (see the SI for other plots). All gave first-order

consumption of rac-LA and a well-controlled polymeriza-

tion process, as evidenced by narrow PDIs and a linear

Article

Organometallics, Vol. 29, No. 5, 2010 1255

Table 8. Solvent-Free (melt) Polymerization of rac-LA by Al(N₂^TsN^Ph)Et (19), Al(N₂^TsN^py)Et (21), Al(N₂^TsN^py)(OⁱPr) (22), Al-

(N₂^TsN^py)(OBn) (23), Al(N₂^MsN^Ph)(OⁱPr) (26), Al(N₂^MsN^Ph)(OBn) (27), Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29), Al(N₂^TsN^OMe)-

(OCH₂CH₂NH₂) (30), Al(N₂^TsN^py)(OCH₂CH₂NH₂) (31), and Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (32)^a

entry

catalyst

yield (%)^b

M_n(GPC)^c

M_n(calcd)^d

M_w/M_n

1

2

3

4

5

6

7

8

9

10

Al(N₂^TsN^Ph)Et (19)

80

75

83

79

94

95

91

81

87

46

41 430

37 390

41 150

34 310

35 480

36 710

36 540

35 000

29 640

16 440

34 600

32 540

35 900

34 160

40 490

40 920

31 100

34 870

29 650

19 700

1.44

1.47

1.21

1.20

1.60

1.53

1.30

1.52

1.45

1.37

Al(N₂^TsN^py)Et (21)

Al(N₂^TsN^py)(OⁱPr) (22)

Al(N₂^TsN^py)(OBn) (23)

Al(N₂^MsN^Ph)(OⁱPr) (26)

Al(N₂^MsN^Ph)(OBn) (27)

Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29)

Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30)

Al(N₂^TsN^py)(OCH₂CH₂NH₂) (31)

Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (32)

^aConditions: [rac-LA]₀:[Al] = 300:1 at 130 °C. See Experimental Section for other details. ^b% conversion by NMR at 2 h reaction time. ^cMolecular

weights (g mol^-1) determined by GPC in THF at 30 °C using the appropriate Mark-Houwink corrections. ^dExpected M_n(g mol^-1) for 1 chain growing

per metal center.

relationship between experimental M_nand % conversion.

The gradients of the M_nversus % conversion plots

were 237(13), 161(8), 208(10), and 220(6) g mol^-1(% con-

version)^-1, respectively, which are higher than that expected

(144.1 g mol^-1(% conversion)^-1for [rac-LA]₀:[Al] = 100:1)

for one poly(rac-LA) chain growing per metal center. This

suggests that not all of the initiating (L)AlOR⁰species have

entered the catalytic cycle, perhaps due to some instability of

the precatalysts (initiators) under the reaction conditions.

Once polymerization is underway, the linear first-order

kinetic plots (e.g., Figure 7) and linear M_nversus conversion

(e.g., Figure 8) all suggest a stable catalyst system. Among

these four catalysts, 23 gave the closest agreement between

experimental M_nand that expected. The PDI of 1.14 and the

linear relationship between M_nand % conversion (Figure 8,

gradient 161(8) g mol^-1(% conversion)^-1) are also indica-

tive of a well-controlled living-type behavior.

The apparent propagation rate constants (k_app) for pairs

of initiators supported by the same ancillary ligand (N₂^TsN^py

for 22 and 23; N₂^MsN^Phfor 26 and 27) had very similar

values, as would be expected since they lead to the same

propagating catalyst. Surprisingly, the values of k_appfor

catalysts based on tetradentate N₂^TsN^py(22 and 23, av

0.0263 h^-1) are slightly larger than those containing the

tridentate N₂^MsN^Ph(26 and 27, av 0.0217 h^-1). A higher

rate might have been expected for the N₂^MsN^Ph-supported

systems since access of rac-LA to the metal should be more

facile. One explanation could be that the tetradentate homo-

logue is more able to support a monomeric (and presumably

more active) active species (L)Al-OR (R = poly(rac-LA)

chain), whereas the tridentate N₂^MsN^Phmight lead to a

higher equilibrium concentration of dormant binuclear com-

plexes with, for example, bridging sulfonyl groups (as in 26,

27, or 28) or a poly(rac-LA) chain (cf. the bridging OR group

in 25 or 33).

Whereas the alkoxides 22-27 led to well-behaved ROP,

analogous experiments using the 2-aminoalkoxide analo-

gues Al(N₂^TsN^R)(OCH₂CH₂NH₂) (29-31; entries 7-9,

Table 7) all gave higher than predicted M_nvalues, larger

PDIs, and, ultimately, gel formation after ca. 18 h reaction

time. In contrast, use of the 2-N,N-dimethylaminoethoxide

Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (32, entry 10, Table 7) once

again gave reasonably well-controlled ROP, as judged by a

better agreement between experimental and predicted M_n, a

narrow PDI, and linear increase of M_nwith conversion (SI,

Figure S20). The k_appfor 32 (0.0235(11) h^-1) was experi-

mentally the same as those of the related Al(N₂^TsN^Ph)(OR⁰)

systems 26 and 27 (0.0230(4) and 0.0203(7) h^-1), as expected

for virtually identical propagating species. The MALDI-

ToF mass spectrum of the polymer obtained using 32

confirmed the presence of the OCH₂CH₂NMe₂end groups.

The spectrum again showed a Δ(m/z) separation of

72 between the polymer chains, consistent with extensive

transesterification.

We propose two explanations for the unusual behavior of

29-31, both based on the presence of the terminal -NH₂

group. First, as seen in the structures (Scheme 4), the

aminoethoxide ligand is strongly chelating to the aluminum

center, augmented by two NH OdS hydrogen bonds.

3 3 3

Conceivably this could lead to sluggish initiation compared

to the “normal” alkoxides 22-27 and 32, the latter having a

less Lewis basic -NMe₂donor and no supporting hydrogen

bonds. Second, the terminal -NH₂group of the growing

polymeryl chain in Al(N₂^TsN^R){poly(rac-LA)}-OCH₂CH₂-

NH₂may also be able to co-initiate the ROP of another rac-

LA through an activated monomer process.¹⁰⁰Such a pro-

cess could lead to cross-linking and gel formation.

Catalyst 33 gives a linear relationship between M_nand

monomer conversion with narrow PDIs (∼1.2) throughout

the polymerization. Disappointingly, it was again observed

that the experimental M_nwas much higher than predicted,

possibly consistent with sluggish initiation compared to

propagation. The dimeric nature proposed for 33, with a

bridged-chelating Al(μ-OCH₂CH₂NMe₂)₂Al core, could

likely account for this.

Polymerization Studies: ROP of rac-LA under Solvent-Free

(Melt) Conditions. Catalysts 19, 21-23, 26, 27, and 29-32

were also assessed for their capabilities toward the ROP of

rac-LA at 130 °C for 2 h ([rac-LA]₀:[Al] = 300:1) under

The ¹H NMR and MALDI-ToF mass spectra con-

firmed the presence of alkoxide-terminated poly(rac-LA).

Disappointingly, all of the polymers produced were

atactic according to the selectively homonuclear-decoupled

NMR spectra. Examination of the MALDI-ToF mass spectra

showed a Δ(m/z) separation of 72 between the polymer chains

(i.e., one-half of a LA unit, 144.1 amu) consistent with

extensive transesterification during the polymerization pro-

cess. This and the high temperatures required for polymeriza-

tion (which would be detrimental to any chain-end control of

monomer insertion) are possible contributors to the lack of

control of polymer tacticity in these systems.

(100) Clark, L.; Cushion, M. G.; Dyer, H. E.; Schwarz, A. D.;

Duchateau, R.; Mountford, P. Chem. Commun. 2010, 46, 273-275.

1256 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

industrially relevant solvent-free (melt) conditions. The

results are summarized in Table 8.

Since most melt ROP experiments are usually carried out

in the range 120-150 °C in very similar ways, this allows a

more precise comparison of our results with those reported

previously. Feijen’s complex 3 (Figure 1) polymerizes 400

equiv of rac-LA to 95% conversion in the melt over 2 days at

130 °C.⁵⁴The κ²-bound sulfonamide complex 7 (Figure 2) is

also active under melt conditions, although still requires

fairly long reaction times (90% conversion after 10 h at

130 °C). The isotactic poly(lactide) formed has modest PDI

values of ca. 1.5, but there is poor control of molecular

weight.⁷⁴Carpentier’s aluminum complexes of salen-like

ligands polymerize 400 equiv of rac-LA at 120 °C to 87%

conversion after 0.5 h. The so-formed isotactic poly(lactide)

had moderate to broad PDIs (1.4-1.9), but poor agreement

was found between found and expected molecular weight.¹⁰¹

Our results (Table 8) compare favorably with these exam-

ples. High conversions of up to 95% within 2 h reaction time

were achieved, along with excellent control in the case of

catalyst 23, as judged by the close agreement between

experimental and calculated M_nand narrow PDI. However,

further iterations of ligand design will be required to find

catalysts capable of achieving stereochemical control

(e.g., high levels of heterotactic enrichment) either under

solution or melt ROP conditions.

The best-performing initiators from the solution-state

studies, namely, the nonchelating alkoxides 22, 23, 26, and

27 (entries 3-6), performed well under the more demanding,

higher temperature melt conditions. Conversions of between

ca. 80% and 95% with good agreements between found and

calculated M_nvalues and reasonable PDIs were found (PDIs

under melt conditions are typically broader than for the

corresponding solution phase studies⁶⁷). In particular the

initiators supported by the tetradentate N₂^TsN^pyligand (22

and 23) gave poly(rac-LA) with a narrow PDI of ca. 1.2. The

agreement between found and calculated M_nwas especially

good for 23, mirroring the excellent performance of this

complex in the solution phase (Table 7). Good M_ncontrol

and a reasonable PDI were also achieved with the 2-N,N-

dimethylaminoethoxide species Al(N₂^TsN^Ph)(OCH₂CH₂-

NMe₂) (32, entry 10).

Surprisingly, both of the ethyl aluminum complexes 19

and 21 (entries 1 and 2) gave significantly improved ROP in

the melt. The extents of conversion and PDIs of the poly(rac-

LA) were comparable to those achieved with the alkoxides

after 2 h, and there was reasonable agreement between the

experimental and calculated M_ndata. Similarly, catalysts

29-31 also gave much-improved performance in terms of

molecular weight control and molecular weight distribu-

tions, with the N₂^TsN^py-supported system 31 providing the

best agreement between measured and calculated M_n. The

origins of this difference are not fully clear. We tentatively

speculate that the more forcing conditions and higher lactide

concentration under melt conditions may help overcome the

kinetic limitations associated with insertion into the

Al-Et bonds of 19 and 21 or opening up the chelating

aminoethoxide-aluminum chelate rings of 29-31. Further

work on other aluminum alkyl catalysts under melt condi-

tions will be required to confirm this hypothesis.

Experimental Section

General Methods and Instrumentation. All manipulations

were carried out using standard Schlenk line or drybox techni-

ques under an atmosphere of argon or dinitrogen. Solvents were

degassed by sparging with dinitrogen and dried by passing

through a column of the appropriate drying agent. Toluene

was refluxed over sodium and distilled. Deuterated solvents

were dried over sodium (C₆H₆) or P₂O₅(CDCl₃and CD₂Cl₂),

distilled under reduced pressure, and stored under dinitrogen in

Teflon valve ampules. NMR samples were prepared under

dinitrogen in 5 mm Wilmad 507-PP tubes fitted with J. Young

Teflon valves. ¹H and ¹³C{¹H} NMR spectra were recorded on

Varian Mercury-VX 300 and Varian Unity Plus 500 spectro-

meters at ambient temperature unless stated otherwise and

referenced internally to residual protio-solvent (¹H) or solvent

(¹³C) resonances, and are reported relative to tetramethylsilane

(δ = 0 ppm). Assignments were confirmed using two-dimen-

sional ¹H-¹H and ¹³C-¹H NMR correlation experiments.

Chemical shifts are quoted in δ (ppm) and coupling constants

in Hz. IR spectra were recorded on a Nicolet Magna 560 ESP

FTIR spectrometer. Samples were prepared in a drybox as

Nujol mulls between NaCl plates, and the data are quoted in

wavenumbers (cm^-1). Elemental analyses were carried out by

the Elemental Analysis Service at the London Metropolitan

University.

MALDI-ToF mass spectra were measured using a Waters

MALDI micro equipped with a 337 nm nitrogen laser. An

accelerating voltage of 25 kV was applied. The polymer samples

were dissolved in THF at a concentration of 1 mg mL^-1. The

cationization agent used was potassium trifluoroacetate (Fluka,

> 99%) dissolved in THF at a concentration of 5 mg mL^-1. The

matrix used was trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-pro-

penylidene]malononitrile (DCTB) (Fluka) and was dissolved in

THF at a concentration of 40 mg mL^-1. Solutions of matrix,

salt, and polymer were mixed in a volume ratio of 4:1:4,

respectively. The mixed solution was hand-spotted on a stainless

steel MALDI target and left to dry. The spectra were recorded in

the refectron mode. Polymer molecular weights (M_n, M_w) were

Conclusion

We have reported the first comprehensive study of the

synthesis, structures, and rac-LA ROP capability of sulfon-

amide-supported aluminum complexes. Both previously re-

ported and new protio-ligands reported herein reacted

readily with AlEt₃to form Al(CyN₂^R)Et(THF) (R = Ts or

SO₂Mes) and Al(N₂^TsN^R)Et (R = Ph, OMe, or py). Corre-

sponding reactions with Al(OR⁰)Et₂(R⁰

=

ⁱPr, Bn,

Ts py

CH₂CH₂NH₂, or CH₂CH₂NMe₂) gave Al(N₂N )(OR⁰),

Al(N₂^MsN^Ph)(OR⁰), Al(N₂^TsN^R)(OCH₂CH₂NH₂) (R = Ph,

OMe, or py), Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂), and Al(Cy-

N₂^Ts)(OCH₂CH₂NMe₂). In general all of the above synth-

eses were straightforward, although reaction of Al(OⁱPr)Et₂

with H₂N₂^TsN^OMeled to O-demethylation of the sulfonam-

ide ligand. The X-ray structures of eight sulfonamide com-

plexes showed a variety of four- and five-coordinate stru-

ctures depending on the sulfonamide ligand and/or addi-

tional ligand (ethyl, chloride, 2-aminoethoxide).

Detailed comparison of solution ROP results with those in

the literature should be undertaken with caution because of

differences in experimental conditions such as temperature,

solvent type, monomer:initiator ratio, and concentration.

Generally speaking, however, the activity and molecular

weight control achieved by the sulfonamide-supported cat-

alysts compare well with literature results in the solution

phase.⁶³

(101) Bouyahyi, M.; Grunova, E.; Marquet, N.; Kirillov, E.;

Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2008,

27, 5815–5825.

Article

Organometallics, Vol. 29, No. 5, 2010 1257

determined by GPC using a Polymer Laboratories Plgel Mixed-

D column (300 mm length, 7.5 mm diameter) and a Polymer

Laboratories PL-GPC50 Plus instrument equipped with a re-

fractive index detector. THF (HPLC grade) was used as an

eluent at 30 °C with a rate of 1 mL min^-1. Linear polystyrenes

were used as primary calibration standards, and Mark-

Houwink corrections for poly(rac-LA) in THF were applied

for the experimental samples.^95-97

solid. Yield: 1.80 g (70%). Diffraction-quality crystals were

grown from a saturated solution of THF and hexanes at room

temperature. ¹H NMR (C₆D₆, 299.9 MHz, 303 K): δ 7.94

3

(4H, d, J = 8.1 Hz, 2-C₆H₄Me), 6.84 (4H, d, J = 8.1 Hz,

3-C₆H₄Me), 3.56 (4H, m, THF), 3.34 (2H, d, ³J = 8.7 Hz, 1,6-

C₆H₁₀N₂), 2.36 (2H, m, C₆H₁₀N₂), 1.92 (6H, s, C₆H₄Me), 1.78

(3H, t, ³J = 8.1 Hz, AlCH₂Me), 1.42 (4H, m, THF), 1.15 (6H, br

m, overlapping C₆H₁₀N₂, CH₂Me, and C₆H₁₀N₂), 0.82 (2H, br

m, C₆H₁₀N₂) ppm. A satisfactory ¹³C NMR spectrum could not

be obtained due to the insolubility and fluxional nature of the

compound. IR (NaCl plates, Nujol mull, cm^-1): 1597 (w), 1495

(w), 1209 (w), 1147 (m), 1108 (m), 1088 (m), 1050 (m), 1021 (w),

983 (m), 958 (m), 903 (m), 865 (m), 842 (m), 824 (m), 816 (w), 798

(w), 733 (m), 720 (w), 678 (m), 670 (m), 615 (m), 583 (m), 553 (s).

Anal. Found (calcd for C₂₆H₃₇AlN₂O₅S₂): C, 56.82 (56.91); H,

6.84 (6.80); N, 5.07 (5.11).

Starting Materials. H₂CyN₂(11),⁸²H₂CyN₂

(12),⁸³

Ts

SO₂Mes

H₂N₂^TsN^OMe(13),⁸⁴H₂N₂^TsN^py(14),⁸⁵N-mesylaziridine,¹⁰²

Al(OⁱPr)Et₂,^91,92Al(OBn)Et₂,^91,92Al(OCH₂CH₂NH₂)Et₂,⁹³

and Al(OCH₂CH₂NMe₂)Et₂were synthesized according to

93

published procedures. rac-LA was recrystallized twice from

toluene and then sublimed twice prior to use. Other reagents

were purchased from Sigma-Aldrich and used without further

purification.

H₂N₂^TsN^Ph(15). To a slurry of tosylaziridine (4.93 g, 0.025

mol) in EtOH (250 mL) was added dropwise benzylamine (1.4

mL, 0.013 mol). The reaction mixture was stirred for 24 h at

35 °C, after which time volatiles of the resulting brown mixture

were removed under reduced pressure, yielding a thick brown

oil. The oil was purified by column chromatography (silica gel,

eluent EtOAc and Et₃N (100:1)) to give 15 as a light brown oil,

which crystallized on standing. Yield: 4.31 g (69%). Light brown

single crystals suitable for X-ray diffraction were obtained from

Al(CyN₂^{SO Mes})Et(THF) (18). To a solution of H₂Cy-

2

SO₂

N₂Mes (1.0 g, 2.04 mmol) in THF (10 mL) was added a

solution of AlEt₃in hexanes (1.0 M, 2.24 mL, 2.24 mmol). The

mixture was heated up to 80 °C for 16 h. The volatiles of the

resulting orange solution were removed under reduced pressure,

and the white solid was dissolved in a minimum volume of THF

and layered with hexanes (40 mL). The resulting white solid was

filtered, washed with hexanes (3 ꢀ 15 mL), and dried in vacuo,

giving 18 as a white solid. Yield: 0.81 g (66%). ¹H NMR (C₆D₆,

299.9 MHz, 303 K): δ 6.67 (4H, s, C₆H₂Me₃), 3.54 (4H, m,

THF), 3.40 (2H, br s, 1,6-C₆H₁₀N₂), 2.70 (12H, br s, 2,6-

NC₆H₂Me₃), 1.94 (6H, s, 4-NC₆H₂Me₃), 1.94 (2H, m,

1

a saturated diethyl ether solution at 4 °C. H NMR (CDCl₃,

299.9 MHz): δ 7.78 (4H, d, ³J = 9.0 Hz, 2-C₆H₄Me), 7.33 (4H, d,

³J = 9.0 Hz, 3-C₆H₄Me), 7.20-7.00 (5H, m, Ph), 5.43 (2H, app.

t, ³J = 6.2 Hz, NH), 3.46 (2H, s, CH₂Ph), 2.94 (4H, app. q, ³J =

6.2 Hz, CH₂NHS), 2.54 (4H, t, ³J = 3.8 Hz, CH₂CH₂NHS), 2.45

(6H, s, C₆H₄Me). ¹³C{¹H} (CDCl₃, 75.4 MHz): δ 143.3 (1-

C₆H₄), 137.7 (4-C₆H₄), 136.6 (1-C₆H₅), 129.7 (2-C₆H₄), 128.9 (2-

C₆H₅), 128.5 (3-C₆H₅), 127.1 (3-C₆H₄), 58.4 (CH₂Ph), 53.1

(CH₂NHS), 40.6 (CH₂NHS), 21.5 (C₆H₄Me). ES-MS: [M þ

H]^þ502.1834 (calcd for C₂₅H₃₂N₃O₄S₂, 502.1834). IR (NaCl

plates, Nujol mull, cm^-1): 3283 (m), 1924 (w), 1812 (w), 1494 (s),

1261 (s), 1093 (s), 1028 (s), 1020 (m) 997 (m), 847 (w), 769 (w).

Anal. Found (calcd for C₂₅H₃₁N₃O₄S₂): C, 59.41 (59.85); H,

6.22 (6.23); N, 8.04 (8.38).

3

C₆H₁₀N₂), 1.71 (3H, t, J = 8.1 Hz, CH₂Me), 1.42 (4H, m,

THF), 1.07 (4H, br m, overlapping C₆H₁₀N₂and CH₂Me), 0.97

(4H, br m, C₆H₁₀N₂) ppm. A satisfactory ¹³C NMR spectrum

could not be obtained due to the fluxional nature of the

compound. IR (NaCl plates, Nujol mull, cm^-1): 1602 (w),

1565 (w), 1402 (w), 1292 (m), 1253 (m), 1211 (w), 1132 (m),

1115 (m), 1098 (m), 1074 (m), 1054 (m), 982 (m), 932 (w), 898

(m), 865 (w), 847 (w), 833 (m), 796 (w), 732 (w), 703 (m), 689 (m),

674 (m), 660 (m), 619 (m), 591 (m). Anal. Found (calcd for

C₃₀H₄₅AlN₂O₅S₂): C, 59.69 (59.58); H, 7.39 (7.50); N, 4.69

(4.63). ES-MS: m/z 503 [M - Et, THF þ H]^þ25%.

H₂N₂^MsN^Ph(16). To a solution of 1-(methylsulfonyl)aziridine

(2.77 g, 0.022 mol) in EtOH (100 mL) was added benzylamine

(1.14 mL, 0.010 mol) dropwise. After stirring at 40 °C for 16 h

the resulting yellow solution was concentrated under reduced

pressure and purified using column chromatography (eluent

ethyl acetate/pentane/NH₄OH, 50:10:1) to give 16 as a light

yellow powder. Yield: 3.30 g (95%). Diffraction-quality crystals

Al(N₂^TsN^Ph)Et (19). AlEt₃(2.2 mL, 1 M in hexanes) was

added to a solution of a solution of H₂N₂^TsN^Ph(1.00 g, 2.0

mmol) in THF (30 mL) at 0 °C. The mixture was stirred at RT

for 4 h, resulting in a colorless solution. Removal of the volatiles

under reduced pressure yielded 19 as a white solid, which was

washed with pentane (3 ꢀ 20 mL) and dried in vacuo. Yield: 0.70

g (63%). Diffraction-quality crystals were grown from a con-

1

were grown from a concentrated benzene solution. H NMR

centrated dichloromethane solution layered with hexanes. H

3

(CDCl₃, 299.9 MHz): δ 7.25-7.00 (5H, m, Ph), 5.20 (2H, br t,

³J = 5.54 Hz, NH), 3.56 (2H, s, CH₂Ph), 3.10 (4H, q, ³J = 5.49

Hz, ⁴J = 4.93 Hz, CH₂NHS), 2.84 (6H, s, SO₂Me), 2.62 (4H, t,

³J = 5.21 Hz, CH₂CH₂NHS). ¹³C{¹H} (CDCl₃, 75.4 MHz): δ

138.1 (i-C₆H₅), 129.9 (o-C₆H₅), 129.2 (m-C₆H₅), 128.2 (p-C₆H₅),

59.1 (CH₂Ph), 53.7 (CH₂NH), 40.9 (CH₂CH₂NH), 39.4 (SMe).

ES-MS: [M þ 1]^þ350.1195 (calcd for C₁₃H₂₄N₃O₄S₂, 350.1208).

IR (NaCl plates, Nujol mull, cm^-1): 3277 (m), 1496 (s), 1466 (s),

1378 (s), 1332 (s), 1279 (w), 1233 (w), 1161 (s), 1136 (m), 1089

(m), 1048 (w), 993 (m), 978 (m), 940 (vw), 926 (w), 907 (w), 820

(m), 783 (s), 750 (s), 731 (m), 700 (m), 668 (w), 615 (w), 600 (w),

524 (w). Anal. Found (calcd for C₁₃H₂₃N₃O₄S₂): C, 44.18

(44.68); H, 6.68 (6.63); N, 11.30 (12.02).

NMR (CD₂Cl₂, 299.9 MHz): δ 7.83 (4H, d, J = 8.0 Hz, 2-

C₆H₄Me), 7.41 (3H, m, overlapping 3- and 4-C₆H₅), 7.28 (4H, d,

³J = 8.0 Hz, 3-C₆H₄Me), 7.41 (2H, m, 2-C₆H₅), 3.97 (2H, s,

CH₂C₆H₅), 3.23 (2H, m, TsNCH₂CH₂), 3.00 (2H, m,

TsNCH₂CH₂), 2.83 (2H, m, TsNCH₂CH₂), 2.62 (2H, m,

TsNCH₂CH₂), 2.42 (6H, s, C₆H₄Me), 1.26 (3H, t, ³J = 8.0

3

Hz, AlCH₂Me), 0.42 (2H, q, J = 8.0 Hz, AlCH₂Me) ppm.

¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 142.7 (1-C₆H₄Me), 137.6

(4-C₆H₄Me), 131.3 (4-C₆H₅), 129.7 (1-C₆H₅), 129.4 (3-

C₆H₄Me), 129.3 (2-C₆H₅), 128.3 (3- C₆H₅), 127.3 (2-C₆H₄Me),

55.0 (NCH₂C₆H₅), 48.9 (TsNCH₂CH₂), 39.7 (TsNCH₂CH₂),

21.2 (C₆H₄Me), 8.7 (AlCH₂Me), -1.5 (AlCH₂Me). IR (NaCl

plates, Nujol mull, cm^-1): 3066 (w), 1495 (w), 1310 (s), 1303 (s),

1286 (w), 1267 (w), 1244 (w), 1152 (s), 1126 (m), 1097 (m), 1053

(w), 1043 (w), 1022 (w), 998 (m), 970 (w), 940 (w), 889 (w), 845

(w), 815 (w), 799 (w), 742 (w), 711 (w), 702 (w), 683 (w), 652

(w),630 (w). Anal. Found (calcd for C₂₇H₃₄AlN₃O₄S₂): C, 58.30

(58.36); H, 6.28 (6.17); N, 7.56 (7.46).

Al(CyN₂^Ts)Et(THF) (17). To a solution of H₂CyN₂^Ts(2.00 g,

4.73 mmol) in THF (30 mL) cooled to 0 °C was added dropwise a

solution of AlEt₃in hexanes (1.0 M, 5.68 mL, 5.68 mmol). The

mixture was allowed to warm to room temperature and stirred

for 16 h. The orange solution was concentrated to half its

volume under reduced pressure, and hexanes (30 mL) was

added. The resulting white solid was filtered, washed with

hexanes (3 ꢀ 15 mL), and dried in vacuo, giving 17 as a white

Al(N₂^TsN^OMe)Et (20). AlEt₃(2.34 mL, 1 M in hexanes) was

added to a solution of H₂N₂^TsN^OMe(1.00 g, 2.13 mmol) in THF

(30 mL) at 0 °C. The mixture was stirred at RT for 3 h, resulting

in a colorless solution. Removal of the volatiles under reduced

pressure yielded a white solid, which was recrystallized from a

(102) Herbert, B. Jus. Liebigs. Ann. Chem. 1950, 566, 210–244.

1258 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

concentrated toluene solution at 100 °C to yield 20 as a white

solid, which was washed with pentane (3 ꢀ 20 mL) and dried in

vacuo. Yield: 0.60 g (54%). Diffraction-quality crystals were

grown from a concentrated dichloromethane solution layered

Found (calcd for C₂₇H₃₅AlN₄O₅S₂): C, 55.10 (55.27); H, 6.05

(6.01); N, 9.60 (9.55).

Al(N₂^TsN^py)(OBn) (23). A solution of Al(OBn)Et₂(0.19 g, 1.0

mmol) and H₂N₂^TsN^py(0.5 g, 1.00 mmol) in toluene (30 mL) was

heated at 100 °C for 16 h. Removal of the volatiles under

reduced pressure yielded a brown solid, which was recrystallized

from a concentrated THF and pentane (1:1) solution to yield 23

as a white solid, which was washed with pentane (3 ꢀ 20 mL) and

1

with hexanes. H NMR (CD₂Cl₂, 299.9 MHz): δ 7.78 (4H, d,

³J = 6.0 Hz, 2-C₆H₄Me), 7.26 (4H, d, ³J = 6.0 Hz, 3-C₆H₄Me),

3.57 (6H, app. t, ³J = 6.0 Hz, OCH₂CH₂), 3.25 (3H, s, OMe),

3.01-2.92 (10H, m, overlapping CH₂), 2.40 (6H, s, C₆H₄Me),

1.15 (3H, t, ³J = 9.0 Hz, AlCH₂Me), 0.28 (2H, q, ³J = 9.0 Hz,

AlCH₂Me) ppm . ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 142.9

(1-C₆H₄Me), 138.0 (4-C₆H₄Me), 129.6 (3-C₆H₄Me), 127.6

(2-C₆H₄Me), 67.4 (MeO), 58.7 (OCH₂CH₂), 51.7 (TsNCH₂-

CH₂), 51.0 (TsNCH₂CH₂), 40.3 (OCH₂CH₂), 21.5 (C₆H₄Me),

8.9 (AlCH₂Me), -2.1 (AlCH₂Me). IR (NaCl plates, Nujol mull,

cm^-1): 1598 (w), 1350 (w), 1304 (s), 1285 (m), 1245 (m), 1227 (w),

1203 (w), 1146 (s), 1095 (s), 1067 (w), 1035 (m), 1023 (w), 991 (s),

931 (w), 912 (w), 899 (w), 846 (m), 818 (m), 800 (w), 748 (w), 728

(w), 712 (w), 679 (m), 652 (w), 630 (w), 618 (w). Anal. Found

(calcd for C₂₃H₃₄AlN₃O₅S₂): C, 52.59 (52.75); H, 6.40 (6.54); N,

7.86 (8.02).

1

dried in vacuo. Yield: 0.38 g (61%). H NMR (CD₂Cl₂, 499.9

MHz): δ 9.35 (1H, d, ³J = 4.5 Hz, 2-NC₅H₄), 7.96 (1H, dt, ³J =

7.8 Hz, J = 1.2 Hz, 4-NC₅H₄), 7.80 (4H, d, J = 8.1 Hz, 2-

4

3

C₆H₄Me), 7.39 (4H, d, J = 7.5 Hz, overlapping 2-C₆H₅, 3-

NC₅H₄and 5-NC₅H₄), 7.28 (2H, t, ³J = 7.5 Hz, 3-C₆H₅) 7.15

(5H, d, ³J = 8.1 Hz, overlapping 3-C₆H₄Me and 4-C₆H₅), 5.18

(2H, s, OCH₂C₆H₅), 4.00 (2H, s, pyCH₂N), 3.25 (2H, m,

TsNCH₂CH₂N), 3.12 (2H, m, TsNCH₂CH₂N), 2.86 (2H, m,

TsNCH₂CH₂N), 2.61 (2H, m, TsNCH₂CH₂N), 2.36 (6H, s,

C₆H₄Me) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 155.1

(6-NC₅H₄), 152.1 (2-NC₅H₄), 147.0 (1-C₆H₅), 142.7 (4-NC₅H₄),

141.7 (1-C₆H₄Me), 141.0 (4-C₆H₄Me), 129.2 (3-C₆H₄Me), 128.0

(3-C₆H₅), 127.4 (2-C₆H₅), 127.0 (2-C₆H₄Me), 125.8 (4-C₆H₅),

124.9 (3- NC₅H₄), 123.9 (5-C₆H₅), 66.2 (OCH₂C₆H₅), 55.8

(pyCH₂N), 52.7 (TsNCH₂CH₂N), 42.4 (TsNCH₂CH₂N), 21.4

(C₆H₄Me) ppm. IR (NaCl plates, Nujol mull, cm^-1): 1559 (w),

1334 (m), 1261 (s), 1183 (m), 1104 (s), 1012 (s), 801 (m), 722 (m),

689 (w). Anal. Found (calcd for C₃₁H₃₅AlN₄O₅S₂): C, 58.75

(58.66); H, 5.61 (5.56); N, 8.81 (8.83).

Al(N₂^TsN^py)Et (21). AlEt₃(2.2 mL, 1 M in hexanes) was added

to a solution of H₂N₂^TsN^py(1.00 g, 2.00 mmol) in THF (30 mL)

at 0 °C. The mixture was stirred at RT for 16 h, resulting in a

colorless solution with a white precipitate. Removal of the

volatiles under reduced pressure yielded a white solid, which

was recrystallized from a concentrated THF and dichloro-

methane (1:1) solution to yield 21 as a white solid, which was

washed with pentane (3 ꢀ 20 mL) and dried in vacuo. Yield: 0.69

g (63%). Diffraction-quality crystals were grown from a con-

[Al(N₂^TsN^O)]₂(25). A mixture of Al(OⁱPr)Et₂(0.31 g, 2.13

mmol) and H₂N₂^TsN^OMe(1.00 g, 2.13 mmol) in toluene (30 mL)

was heated at 100 °C for 16 h. Removal of the volatiles under

reduced pressure yielded crude Al(N₂^TsN^OMe)(OⁱPr) a light

yellow solid. This was heated at 150 °C for 48 h under a dynamic

vacuum (1 mbar). The resulting white solid (25) was washed with

pentane (3 ꢀ 20 mL) and dried in vacuo. Yield: 0.75 g (73%).

Diffraction-quality crystals were grown from a saturated solu-

tion of dichloromethane at room temperature. ¹H NMR

(CD₂Cl₂, 499.9 MHz): δ 7.91 (4H, d, ³J = 8.2 Hz, 2-C₆H₄Me),

7.37 (4H, d, ³J = 8.2 Hz, 3-C₆H₄Me), 4.14 (2H, t, ³J = 5.7 Hz,

OCH₂), 3.29 (2H, m, TsNCH₂CH₂N), 3.15 (2H, m,

1

centrated dichloromethane solution layered with hexanes. H

NMR (CD₂Cl₂, 299.9 MHz): δ 9.05 (1H, dd, ³J = 7.6 Hz, ⁴J =

3

1.5 Hz, 2-NC₅H₄), 8.04 (1H, dt, J = 7.6 Hz, J = 7.0 Hz,

3-NC₅H₄), 7.71 (4H, d, ³J = 8.2 Hz, 2-C₆H₄Me), 7.63 (1H, dd,

³J = 7.0 Hz, ³J = 7.6 Hz, 4-NC₅H₄), 7.43 (1H, d, ³J = 7.0 Hz, 5-

3

NC₅H₄), 7.19 (4H, d, J = 8.2 Hz, 3-C₆H₄Me), 4.06 (2H, s,

pyCH₂N), 3.09 (4H, m, overlapping TsNCH₂CH₂N),

2.75-2.45 (4H, m, TsNCH₂CH₂N), 2.37 (6H, s, C₆H₄Me),

3

0.91 (3H, t, J = 7.6 Hz, AlCH₂Me), 0.65 (2H, q, J = 7.6

Hz, AlCH₂Me) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ

154.5 (6-NC₅H₄), 150.1 (1-C₆H₄Me), 142.0 (2-NC₅H₄), 140.9

(4-C₆H₄Me), 140 (3-NC₅H₄), 129.5 (3-C₆H₄Me), 127.5 (2-C₆H₄-

Me), 125.5 (4-NC₅H₄), 124.1 (5-NC₅H₄), 68.3 (pyCH₂N), 57.11

(TsNCH₂CH₂N), 42.4 (TsNCH₂CH₂N), 26.1 (AlCH₂Me),

21.6 (C₆H₄Me), 10.1 (AlCH₂Me) ppm. IR (NaCl plates, Nujol

mull, cm^-1): 1653 (w), 1306 (m), 1281 (m), 1281 (m), 1261 (m),

1162 (w), 1141 (m), 1095 (w), 1022 (w), 980 (m), 957 (w), 813 (w),

799 (w), 723 (w), 672 (w). Anal. Found (calcd for

C₂₆H₃₃AlN₄O₄S₂): C, 55.98 (56.10); H, 5.86 (5.98); N, 9.98

(10.06).

3

TsNCH₂CH₂N), 2.83 (2H, t, J = 5.7 Hz, OCH₂CH₂), 2.66

(2H, m, TsNCH₂CH₂N), 2.62 (2H, m, TsNCH₂CH₂N), 2.42

(6H, s, C₆H₄Me) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ

142.7 (1-C₆H₄Me), 140.5 (4-C₆H₄Me), 130.2 (3-C₆H₄Me), 127.4

(2-C₆H₄Me), 60.2 (OCH₂), 53.6 (TsNCH₂CH₂N), 52.9

(OCH₂CH₂), 42.2 (TsNCH₂CH₂N), 21.8 (C₆H₄Me) ppm. IR

(NaCl plates, Nujol mull, cm^-1): 3029 (w), 1559 (w), 1281 (s),

1158 (w), 1140 (m), 1104 (m), 1066 (w), 1048 (w), 1034 (w), 967

(m), 911 (w), 869 (w), 829 (m), 742 (m), 722 (m), 694 (w), 667 (w).

Anal. Found (calcd for C₄₀H₅₂Al₂N₆O₁₀S₄): C, 50.14 (50.09); H,

5.56 (5.46); N, 8.88 (8.76).

Al(N₂^TsN^py)(OⁱPr) (22). A solution of Al(OⁱPr)Et₂(0.29 g, 2.0

mmol), H₂N₂^TsN^py(1.0 g, 2.00 mmol), and toluene (30 mL) was

heated at 100 °C for 16 h. Removal of the volatiles under

reduced pressure yielded a brown solid, which was washed with

Al(N₂^TmN^Ph)(OⁱPr) (26). A mixture of Al(OⁱPr)Et₂(0.12 g,

0.86 mmol) and H₂N₂^TmN^Ph(0.30 g, 0.86 mmol) in toluene (30

mL) was heated at 100 °C for 48 h, resulting in a colorless

solution with a white precipitate. This was filtered and washed

with pentane (3 ꢀ 20 mL) and dried in vacuo to give 26 as a light

brown solid. Yield: 0.19 g (51%). ¹H NMR (CD₂Cl₂þ NC₅D₅,

299.9 MHz): δ 7.20-7.05 (5H, m, overlapping NCH₂(2-C₆H₅),

1

Et₂O (3 ꢀ 20 mL) and dried in vacuo. Yield: 0.77 g (66%). H

3

NMR (CD₂Cl₂, 499.9 MHz): δ 9.73 (1H, d, J = 6.3 Hz, 2-

NC₅H₄), 8.04 (1H, dt, ³J = 7.8 Hz, ⁴J = 1.5 Hz, 4-NC₅H₄), 7.78

3

(4H, d, J = 7.8 Hz, 2-C₆H₄Me), 7.63 (1H, t, J = 6.3 Hz, 3-

NC₅H₄), 7.41 (1H, d, ³J = 7.8 Hz, 5-NC₅H₄), 7.20 (4H, d, ³J =

7.8 Hz, 3-C₆H₄Me), 4.50 (1H, sept., ³J = 6.0 Hz, CHMe₂), 4.06

(2H, s, pyCH₂N), 3.16 (2H, m, TsNCH₂CH₂N), 3.08 (2H, m,

TsNCH₂CH₂N), 2.79 (2H, m, TsNCH₂CH₂N), 2.60 (2H, m,

3

NCH₂(3-C₆H₅), and NCH₂(4-C₆H₅)), 4.31 (1H, sept., J = 6

Hz, OCHMe₂), 4.16 (2H, s, NCH₂C₆H₅), 3.43 (2H, m, TsN-

CH₂CH₂N), 3.26 (2H, m, TsNCH₂CH₂N), 2.95 (2H, m, TsN-

CH₂CH₂N), 2.68 (2H, m, TsNCH₂CH₂N), 2.57 (6H, s,

SO₂Me), 1.28 (6H, d, ³J = 6 Hz, OCHMe₂) ppm. ¹³C{¹H}

NMR (CD₂Cl₂þ NC₅D₅, 75.4 MHz): δ 131.8 (1-C₆H₅), 131.5

(3-C₆H₅), 128.6 (2-C₆H₅), 128.5 (4-C₆H₅), 63.8 (OCHMe₂), 57.4

(NCH₂C₆H₅), 50.9 (MsNCH₂CH₂N), 42.4 (MsNCH₂CH₂N), 39.6

(SO₂Me), 27.6 (OCHMe₂) ppm. IR (NaCl plates, Nujol mull,

cm^-1): 1496 (w), 1328 (w), 1291 (s), 1264 (s), 1230 (w), 1180 (m),

1120 (s), 1104 (m), 1079 (w), 1064 (w), 1019 (s), 982 (m), 898 (w),

776 (m), 738 (m), 706 (m), 669 (w), 652 (w). Anal. Found (calcd for

C₁₆H₂₈AlN₃O₅S₂): C, 44.36 (44.33); H, 6.46 (6.51); N, 9.69 (9.66).

3

TsNCH₂CH₂N), 2.37 (6H, s, C₆H₄Me), 1.13 (6H, d, J = 6.0

Hz, CHMe₂) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 155.4

(6-NC₅H₄), 152.1(2-NC₅H₄), 142.4 (4-NC₅H₄), 141.7 (1-

C₆H₄Me), 141.1 (4-C₆H₄Me), 129.2 (3-C₆H₄Me), 127.2 (2-

C₆H₄Me), 124.8 (3-NC₅H₄), 123.5 (5-NC₅H₄), 63.8 (CHMe₂)

57.1 (pyCH₂N), 54.6 (TsNCH₂CH₂N), 42.6 (TsNCH₂CH₂N),

27.6 (CHMe₂), 21.4 (C₆H₄Me) ppm. IR (NaCl plates, Nujol

mull, cm^-1): 3050 (w), 1734 (w), 1559 (w), 1298 (s), 1144 (m),

1093 (m), 1025 (w), 971 (w), 816 (w), 722 (w), 668 (w). Anal.

Article

Al(N₂^TmN^Ph)(OBn) (27). A mixture of Al(OBn)Et₂(0.17 g,

Organometallics, Vol. 29, No. 5, 2010 1259

(3-C₆H₄Me), 128.9 (4-C₆H₅), 128.8 (3-C₆H₅), 127.4 (2-

C₆H₄Me), 60.6 (OCH₂CH₂NMe₂), 55.2 (NCH₂C₆H₅), 44.8

(TsNCH₂CH₂N), 42.6 (OCH₂), 41.7 (TsNCH₂CH₂N), 21.5

(C₆H₄Me) ppm. IR (NaCl plates, Nujol mull, cm^-1): 3308

(m), 3262 (w), 1598 (w), 1299 (s), 1260 (s), 1154 (s), 1091 (s),

1020 (m), 974 (m), 908 (w), 851 (w), 803 (w), 722 (m), 668 (w).

Anal. Found (calcd for C₂₇H₃₅AlN₄O₅S₂): C, 55.42 (55.27); H,

6.11 (6.01); N, 9.57 (9.55).

0.86 mmol) and H₂N₂^TmN^Ph(0.30 g, 0.86 mmol) in toluene

(30 mL) was heated at 100 °C for 48 h, resulting in a colorless

solution with a white precipitate. This was then filtered and

washed with pentane (3 ꢀ 20 mL) and dried in vacuo to give 27 as

a cream solid. Yield: 0.27 g (65%). ¹H NMR (CD₂Cl₂þ NC₅D₅,

299.9 MHz): δ 7.53 (2H, d, ³J = 8.3 Hz, OCH₂(2-C₆H₅)), 7.20-

7.08 (8H, m, overlapping OCH₂(3-C₆H₅), OCH₂(4-C₆H₅),

NCH₂(2-C₆H₅), NCH₂(3-C₆H₅), and NCH₂(4-C₆H₅)), 5.11

(2H, s, OCH₂C₆H₅), 3.97 (2H, s, NCH₂C₆H₅), 3.57 (2H, m,

TsNCH₂CH₂N), 3.31 (2H, m, TsNCH₂CH₂N), 2.95 (2H, m,

TsNCH₂CH₂N), 2.71 (2H, m, TsNCH₂CH₂N), 2.59 (6H, s,

SO₂Me) ppm. ¹³C{¹H} NMR (CD₂Cl₂þ NC₅D₅, 75.4 MHz):

Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30).

A mixture of Al-

(OCH₂CH₂NH₂)Et₂(0.15 g, 1.00 mmol) and H₂N₂^TsN^OMe

(0.50 g, 1.03 mmol) in toluene (30 mL) was heated at 100 °C

for 16 h. Removal of the volatiles under reduced pressure yielded

30 as a cream solid, which was washed with pentane (3 ꢀ 20 mL)

and dried in vacuo. Yield: 0.45 g (79%). Diffraction-quality

crystals were grown from a saturated solution of dichloro-

methane layered with hexanes at RT. ¹H NMR (CD₂Cl₂,

499.9 MHz): δ 7.69 (4H, d, ³J = 8.0 Hz, 2-C₆H₄Me), 7.25

δ

145.5 (OCH₂(1-C₆H₅)), 132.9 (NCH₂(1-C₆H₅)), 131.5

(OCH₂(2-C₆H₅)), 128.9 (NCH₂(6-C₆H₅)), 128.8 (OCH₂(3-

C₆H₅)), 128.6 (NCH₂(3-C₆H₅)), 126.9 (OCH₂(4-C₆H₅)), 126.5

(NCH₂(4-C₆H₅)), 65.6 (OCH₂C₆H₅), 56.9 (NCH₂C₆H₅), 50.8

(SO₂Me), 42.7 (TsNCH₂CH₂N), 39.9 (TsNCH₂CH₂N) ppm. IR

(NaCl plates, Nujol mull, cm^-1): 3039 (w), 1296 (s), 1273 (m),

1226 (w), 1166 (m), 1149 (w), 1133 (m), 1098 (m), 1080 (w), 1024

(m), 983 (m), 800 (w), 772 (m), 742 (w), 704 (w), 668 (w), 656 (w).

Anal. Found (calcd for C₂₀H₂₈AlN₃O₅S₂): C, 49.92 (49.88); H,

5.85 (5.86); N, 8.70 (8.73).

3

(4H, d, ³J = 8.0 Hz, 3-C₆H₄Me), 4.75 (2H, br t, J = 7.5 Hz,

NH₂), 3.78 (2H, t, ³J = 7.5 Hz, OCH₂), 3.43 (2H, t, ³J = 7.5 Hz,

H₂NCH₂), 3.16 (3H, s, OMe), 3.05-2.95 (4H, m, TsNCH₂), 2.72

(2H, m, TsNCH₂CH₂N), 2.43 (2H, m, TsNCH₂CH₂N), 2.20

(6H, s, C₆H₄Me) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ

142.7 (1-C₆H₄Me), 138.4 (4-C₆H₄Me), 129.6 (3-C₆H₄Me), 127.4

(2-C₆H₄Me), 67.9 (OCH₂CH₂NH₂), 60.5 (MeOCH₂), 58.7

(OMe), 50.8 (TsNCH₂CH₂), 46.2 (TsNCH₂), 42.5 (OCH₂-

CH₂N), 41.9 (MeOCH₂CH₂N), 21.5 (C₆H₄Me) ppm. IR

(NaCl plates, Nujol mull, cm^-1): 3306 (m), 3268 (m), 2688 (w),

1636 (w), 1307 (w), 1296 (w), 1280 (s), 1258 (m), 1193 (w), 1156

(s), 1145 (m), 1087 (s), 1054 (m), 1018 (m), 976 (m), 906 (w), 808

(w), 736 (w), 667 (w). Anal. Found (calcd for C₂₃H₃₅AlN₄O₆S₂):

C, 49.77 (49.80); H, 6.36 (6.26); N, 10.02 (10.10).

[Al(N₂^TsN^Ph)Cl]₂(28). AlMe₂Cl (2.0 mL, 1 M in hexanes) was

added dropwise to a solution of H₂N₂^TsN^ph(1.00 g, 2.00 mmol)

in C₆H₆(30 mL). The mixture was stirred at RT for 16 h,

resulting in a yellow solution. Removal of the volatiles under

reduced pressure yielded a light yellow solid, which was washed

with ether (3 ꢀ 15 mL) and recrystallized from a concentrated

dichloromethane solution (10 mL) layered with hexanes (30 mL)

to yield 28 as a light yellow solid. This was washed with pentane

(3 ꢀ 20 mL) and dried in vacuo. Yield: 0.65 g (58%). Diffraction-

quality crystals were grown from a saturated solution of di-

Al(N₂^TsN^py)(OCH₂CH₂NH₂) (31). A mixture of Al(OCH₂-

CH₂NH₂)Et₂(0.14 g, 1.0 mmol) and H₂N₂^TsN^py(0.50 g, 1.00

mmol) in toluene (30 mL) was heated at 100 °C for 48 h.

Removal of the volatiles under reduced pressure yielded a brown

solid, which was recrystallized from a concentrated THF (10

mL) solution layered with pentane (30 mL). The resulting light

brown solid (31) was washed with pentane (3 ꢀ 20 mL) and dried

in vacuo. Yield: 0.44 g (75%).

1

chloromethane layered with hexanes at room temperature. H

NMR (C₆D₆, 299.9 MHz): δ 8.72 (2H, d, ³J = 8.1 Hz, bridging

2-C₆H₄Me), 8.31 (2H, d, ³J = 8.7 Hz, terminal 2-C₆H₄Me), 7.18

(2H, d, ³J = 8.1 Hz, bridging 3-C₆H₄Me), 7.00 (1H, m, 4-C₆H₅),

3

6.90 (2H, t, J = 7.5 Hz, 3-C₆H₅), 6.78 (2H, d, J = 8.7 Hz,

terminal 3-C₆H₄Me), 6.90 (2H, d, J = 7.5 Hz, 2-C₆H₅), 4.55

3

1

2

Major isomer 31a: H NMR (CD₂Cl₂, 499.9 MHz): δ 8.87

(1H, d, J = 15 Hz, NCH₂C₆H₅), 4.44 (1H, d, J = 15 Hz,

NCH₂C₆H₅), 3.95 (1H, dt, ²J = 6.9 Hz, ³J = 4.8 Hz, TsNCH₂-

CH₂N), 3.26 (1H, m, TsNCH₂CH₂N), 2.95-2.55 (4H, m,

TsNCH₂CH₂N), 1.96 (3H, s, bridging C₆H₄Me), 1.83 (5H, s,

overlapping C₆H₄Me and TsNCH₂CH₂N) ppm. ¹³C{¹H} NMR

(C₆D₆, 75.4 MHz): δ 142.8 (1-C₆H₄Me), 141.6 (1-C₆H₄Me),

140.5 (4-C₆H₄Me), 136.0 (4-C₆H₄Me), 132.0 (2-C₆H₅), 131.4 (1-

C₆H₅), 129.4 (3-C₆H₄Me), 129.1 (3-C₆H₄Me), 128.7 (2-

C₆H₄Me), 128.6 (2-C₆H₄Me), 128.5 (4-C₆H₅), 128.4 (4-C₆H₅),

55.6 (NCH₂C₆H₅) 47.4 (TsNCH₂CH₂N), 42.2 (TsNCH₂-

CH₂N), 41.8 (overlapping TsNCH₂CH₂N), 26.1 (AlCH₂Me),

21.6 (C₆H₄Me), 10.1 (AlCH₂Me) ppm. IR (NaCl plates, Nujol

mull, cm^-1): 2726 (w), 1506 (w), 1304 (m), 1262 (s), 1155 (m),

1062 (s), 1021 (s), 1003 (w), 807 (w), 722 (s), 668 (w). Anal.

Found (calcd for C₅₈H₅₈Al₂Cl₂N₆O₈S₄): C, 53.47 (53.42); H,

5.32 (5.20); N, 7.44 (7.44).

(1H, d, ³J = 5.1 Hz, 2-NC₅H₄), 7.86 (1H, dt, ³J = 7.7 Hz, ⁴J =

1.3 Hz, 3-NC₅H₄), 7.72 (5H, d, J = 8.2 Hz, overlapping 2-

3

C₆H₄Me and 4-NC₅H₄), 7.45 (1H, d, ³J = 7.7 Hz, 5- NC₅H₄),

7.20 (4H, d, ³J = 8.2 Hz, 3-C₆H₄Me), 4.98 (2H, br t, ³J = 5.5

Hz, NH₂), 4.17 (2H, s, pyCH₂N), 3.56 (2H, t, J = 5.5 Hz,

3

OCH₂), 3.25-2.75 (10H, m, overlapping TsNCH₂CH₂N and

CH₂NH₂), 2.37 (6H, s, C₆H₄Me) ppm. ¹³C{¹H} NMR (CD₂-

Cl₂, 75.4 MHz): δ 152.7 (6-NC₅H₄), 147.6 (2-NC₅H₄), 141.9

(1-C₆H₄Me), 140.7 (4-NC₅H₄), 139.1 (4-C₆H₄Me), 130.1

(5-NC₅H₄), 129.6 (3-C₆H₄Me), 127.7 (2-C₆H₄Me), 127.1 (3-

NC₅H₄), 61.8 (pyCH₂N), 59.9 (CH₂O) 57.3 (TsNCH₂CH₂N),

43.7 (CH₂NH₂), 43.2 (TsNCH₂CH₂N), 21.6 (C₆H₄Me) ppm.

1

Minor isomer 31b: H NMR (CD₂Cl₂, 499.9 MHz): δ 8.52

(1H, d, ³J = 5.5 Hz, 2-NC₅H₄), 7.98 (1H, dt, ³J = 7.9 Hz, ⁴J =

3

1.3 Hz, 3-NC₅H₄), 7.59 (1H, d, J = 7.9 Hz, 5-NC₅H₄), 7.28

(4H, d, ³J = 7.8 Hz, 2-C₆H₄Me), 7.23 (1H, d, ³J = 7.9 Hz, 5-

NC₅H₄), 7.13 (4H, d, ³J = 8.2 Hz, 3-C₆H₄Me), 5.14 (2H, br t,

³J = 5.7 Hz, NH₂), 3.95 (2H, s, pyCH₂N), 3.88 (2H, t, ³J = 5.7

Hz, OCH₂), 3.11 (2H, t, ³J = 5.7 Hz, NH₂CH₂), 3.25-2.75 (8H,

m, overlapping TsNCH₂CH₂N), 2.29 (6H, s, C₆H₄Me) ppm.

¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 152.3 (6-NC₅H₄), 145.3

(2-NC₅H₄), 141.6 (1-C₆H₄Me), 140.6 (4-NC₅H₄), 140.1 (4-

C₆H₄Me), 137.5 (5-NC₅H₄), 129.5 (3-C₆H₄Me), 127.5 (2-

C₆H₄Me), 124.2 (3-NC₅H₄), 59.7 (CH₂O), 66.9 (pyCH₂N),

54.5 (TsNCH₂CH₂N), 43.8 (CH₂NH₂), 42.4 (TsNCH₂CH₂N),

21.4 (C₆H₄Me) ppm. IR (NaCl plates, Nujol mull, cm^-1): 3318 (w),

3310 (w), 3202 (w), 3159 (w), 1612 (m), 1266 (s), 1134 (s), 1103 (s),

1085 (s), 1029 (m), 996 (m), 974 (m), 933 (w), 879 (w), 801 (w), 725

(w), 672 (w), 662 (w), 626 (w). Anal. Found (calcd for

C₂₆H₃₄AlN₅O₅S₂): C, 53.23 (53.14); H, 5.90 (5.83); N, 11.82 (11.92).

Al(N₂^TsN^Ph)(OCH₂CH₂NH₂) (29).

A mixture of Al-

(OCH₂CH₂NH₂)Et₂(0.14 g, 1.0 mmol) and H₂N₂^TsN^Ph(0.50

g, 1.00 mmol) in toluene (30 mL) was heated at 100 °C for 16 h.

Removal of the volatiles under reduced pressure yielded 29 as a

cream solid, which was washed with pentane (3 ꢀ 20 mL) and

1

dried in vacuo. Yield: 0.40 g (68%). H NMR (CD₂Cl₂, 299.9

3

MHz): δ 7.71 (4H, d, J = 7.8 Hz, 2-C₆H₄Me), 7.29 (3H, m,

overlapping 2-C₆H₅and 4-C₆H₅), 7.23 (4H, d, ³J = 7.8 Hz, 3-

3

C₆H₄Me), 7.04 (2H, m, 3-C₆H₅), 4.81 (2H, br t, J = 6.0 Hz,

NH₂), 3.87 (2H, s, NCH₂C₆H₅), 3.87 (2H, t, ³J = 6.0 Hz,

CH₂O), 3.20 (2H, m, TsNCH₂CH₂N), 3.01 (2H, m, CH₂NH₂),

2.73 (2H, m, TsNCH₂CH₂N), 2.56 (2H, m, TsNCH₂CH₂N),

2.38 (6H, s, C₆H₄Me), 2.15 (2H, m, TsNCH₂CH₂N) ppm.

¹³C{¹H} NMR (C₆D₆, 75.4 MHz): δ 142.8 (1-C₆H₄Me),

138.1 (4-C₆H₄Me), 131.8 (2-C₆H₅), 131.7 (1-C₆H₅), 129.6

1260 Organometallics, Vol. 29, No. 5, 2010

Schwarz et al.

Al(N₂^TsN^Ph)(OCH₂CH₂NMe₂) (32). A mixture of Al(OC-

H₂CH₂NMe₂)Et₂(0.17 g, 1.0 mmol) and H₂N₂^TsN^py(0.50 g,

1.00 mmol) in toluene (30 mL) was heated at 100 °C for 16 h.

Removal of the volatiles under reduced pressure yielded a cream

solid. Recrystallization from a concentrated dichloromethane

solution layered with pentane (30 mL) gave 32 as a cream solid,

which was washed with pentane (3 ꢀ 20 mL) and dried in vacuo.

Yield: 0.52 g (85%). ¹H NMR (C₆D₆, 299.9 MHz): δ 8.10 (4H, d,

³J = 8.1 Hz, 2-C₆H₄Me), 7.03 (3H, m, overlapping 3-C₆H₅and

at the respective time. Upon completion of the reaction, wet

THF (10 mL) was added and the solution evaporated to dryness

to give the poly(rac-LA). Conversions were determined by H

NMR integration of the OCHMe resonance relative intensities

of the residual rac-LA and poly(rac-LA).

General Procedure for Solvent-Free (Melt) Polymerization of

rac-LA. A Schlenk flask was charged with catalyst and rac-LA

at the desired ratio and heated to 130 °C for 30 min with stirring.

The mixture was cooled to RT, wet THF (10 mL) was then

added, and the resulting solution was evaporated to dryness to

1

3

4-C₆H₅), 7.15 (4H, d, J = 8.1 Hz, 3-C₆H₄Me), 6.83 (2H, dd,

³J = 7.8 Hz, ⁴J = 2.1 Hz, 2-C₆H₅), 4.08 (2H, s, NCH₂C₆H₅),

3.77 (2H, t, ³J = 6.0 Hz, OCH₂), 3.15-3.10 (4H, m, TsNCH₂-

CH₂N), 2.98 (2H, br t, ³J = 6 Hz, OCH₂CH₂NMe₂), 2.69 (6H, s,

OCH₂CH₂NMe₂), 2.65 (4H, m, TsNCH₂CH₂N), 1.92 (6H, s,

C₆H₄Me) ppm. ¹³C{¹H} NMR (C₆D₆, 75.4 MHz): δ 141.5 (1-

C₆H₄Me), 140.9 (4-C₆H₄Me), 134.3 (1-C₆H₅), 131.8 (2-C₆H₅),

129.5 (3-C₆H₄Me), 128.5 (3-C₆H₅), 128.4 (4-C₆H₅), 127.5

(2-C₆H₄Me), 62.6 (OCH₂CH₂NMe₂), 58.8 (NCH₂C₆H₅), 57.8

(OCH₂), 51.3 (TsNCH₂CH₂N), 46.3 (OCH₂CH₂NMe₂), 43.0

(TsNCH₂CH₂N), 21.0 (C₆H₄Me) ppm. IR (NaCl plates, Nujol

mull, cm^-1): 2730 (w), 1700 (w), 1301 (m), 1261 (m), 1143 (m),

1092 (s), 1019 (s), 800 (s), 723 (s), 667 (s). Anal. Found (calcd for

C₂₉H₃₉AlN₄O₅S₂): C, 56.62 (56.66); H, 6.39 (6.39); N, 9.10

(9.11).

give the crude polymer.

Crystal Structure Determinations of H₂N₂^TsN^Ph(15), H₂N₂

-

Ms

N^Ph(16), Al(CyN₂^Ts)Et(THF) (17), Al(N₂^TsN^Ph)Et (19), Al-

(N₂^TsN^OMe)Et (20), Al(N₂^TsN^py)Et (21), [Al(N^Ts₂N^Ph)Cl]₂(28),

and Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30). X-ray data collection

and processing parameters are given in Table S3 of the SI.

Crystals were mounted on glass fibers using perfluoropolyether

oil and cooled rapidly in a stream of cold N₂using an Oxford

Cryosystems Cryostream unit. Diffraction data were measured

using an Enraf-Nonius KappaCCD diffractometer. As appro-

priate, absorption and decay corrections were applied to the

data and equivalent reflections merged.¹⁰³The structures were

solved with SIR92¹⁰⁴or SHELXS-97,¹⁰⁵and further refine-

ments and all other crystallographic calculations were per-

formed using either the CRYSTALS program suite¹⁰⁶or

SHELXS-97.¹⁰⁷Other details of the structure solution and

refinements are given in the SI (CIF data). A full listing of

atomic coordinates, bond lengths and angles, and displacement

parameters for all the structures have been deposited at the

Cambridge Crystallographic Data Centre. See Notice to

Authors, Issue No. 1.

Ts

Al(CyN₂^Ts)(OCH₂CH₂NMe₂) (33). A mixture of H₂CyN₂

(1.00 g, 2.37 mmol) and Al(OCH₂CH₂NMe₂)Et₂(0.450 g, 2.60

mmol) in toluene (30 mL) was heated to 100 °C and stirred at this

temperature for 16 h. The volatiles were removed under reduced

pressure to give white solid. The solid was redissolved in

minimal volume of dichloromethane and layered 4-fold with

hexanes to give needle-like crystals. The crystals was filtered,

washed with hexanes (3 ꢀ 15 mL), and dried under vacuum to

give 33 as a white solid. Yield: 0.99 g (78%). Diffraction-quality

crystals were grown from a saturated solution of dichloro-

methane and hexanes at room temperature. ¹H NMR

(CD₂Cl₂, 299.9 MHz, 303 K): δ 7.78 (4H, d, ³J = 8.1 Hz,

2-C₆H₄Me), 7.73 (4H, d, ³J = 8.1 Hz, 2-C₆H₄Me), 7.24 (4H, d,

³J = 8.1 Hz, 3-C₆H₄Me), 6.84 (4H, d, ³J = 8.1 Hz, 3-C₆H₄Me),

4.91 (2H, m, C₆H₁₀N₂), 4.30 (2H, m, C₆H₁₀N₂), 4.00 (2H, t, ³J =

8.4 Hz, OCH₂CH₂NMe₂), 3.19 (2H, m, OCH₂CH₂NMe₂), 3.00

(3H, s, OCH₂CH₂NMe₂), 2.74 (3H, s, OCH₂CH₂NMe₂), 2.59

(2H, m, C₆H₁₀N₂), 2.38 (3H, s, C₆H₄Me), 2.14 (3H, s, C₆H₄Me),

1.40 (2H, m, C₆H₁₀N₂), 1.00 (4H, m, C₆H₁₀N₂), 0.84 (2H, m,

C₆H₁₀N₂) ppm. ¹³C{¹H} NMR (C₆D₆, 75.4 MHz): δ 144.2

(4-C₆H₄Me), 142.9 (4-C₆H₄Me), 141.8 (1-C₆H₄Me), 141.1

(1-C₆H₄Me), 129.3 (3-C₆H₄Me), 128.4 (3-C₆H₄Me), 127.4

(2-C₆H₄Me), 126.1 (2-C₆H₄Me), 62.3 (C₆H₁₀N₂), 61.9 (OCH₂-

CH₂NMe₂), 58.6 (C₆H₁₀N₂), 55.7 (OCH₂CH₂NMe₂), 47.3

(OCH₂CH₂NMe₂), 47.0 (OCH₂CH₂NMe₂), 32.1 (C₆H₁₀N₂),

31.9 (C₆H₁₀N₂), 25.2 (C₆H₁₀N₂), 21.4 (C₆H₄Me), 21.2

(C₆H₄Me) ppm. IR (NaCl plates, Nujol mull, cm^-1): 1598 (w),

1277 (m), 1245 (w), 1212 (w), 1144 (m), 1088 (m), 1070 (m), 1052

(m), 1019 (w), 983 (m), 950.2 (m), 902.2 (m), 838 (m), 817 (w), 799

(w), 730 (m), 676 (m), 588 (m), 558 (m). Anal. Found (calcd for

C₂₄H₃₄AlN₃O₅S₂): C, 53.68 (53.81); H, 6.36 (6.40); N, 7.74 (7.84).

General Procedure for Solution Polymerization of rac-LA.

rac-LA (6.00 mmol) and catalyst (0.06 mmol) were added to a

Schlenk flask and heated to 70 °C. To this was added hot (70 °C)

toluene (6.0 mL), rapidly dissolving both solids. The resultant

solution was heated at 70 °C and aliquots were taken via syringe

Acknowledgment. We thank the EPSRC and Chinese

Scholarship Council for support, and the Chemical Crys-

tallography Laboratory, Oxford, for access to the KCCD

diffractometers. We also thank Dr. A. L. Thompson for

advice with regard to the crystallography and Miss

K. Herbert for assistance with the synthesis of 16.

Supporting Information Available: X-ray crystallographic

data in CIF format for the structure determinations of

H₂N₂^TsN^Ph(15), H₂N₂^MsN^Ph(16), Al(CyN₂^Ts)Et(THF) (17),

Al(N₂^TsN^Ph)Et (19), Al(N₂^TsN^OMe)Et (20), Al(N₂^TsN^py)Et (21),

[Al(N^Ts₂N^Ph)Cl]₂(28), Al(N₂^TsN^OMe)(OCH₂CH₂NH₂) (30),

and Al(OCH₂CH₂NH₂)Et₂; preliminary structure determina-

tion of [Al(N₂^TsN^O)]₂(25); displacement ellipsoid plot of Al-

(OCH₂CH₂NH₂)Et₂; additional data concerning the ROP

catalysis; X-ray data collection and processing parameters. This

information is available free of charge via the Internet at http://

pubs.acs.org.

(103) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction

Data Collected in Oscillation Mode; Academic Press: New York, 1997.

(104) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.;

Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,

435.

(105) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.

(106) Betteridge, P. W.; Cooper, J. R.; Cooper, R. I.; Prout, K.;

Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.

(107) Sheldrick, G. M.; Schneider, T. R. Methods Enzymol. 1997, 277,

319–343.

Article Doi

Sulfonamide-supported aluminum catalysts for the ring-opening polymerization of rac-lactide

DOI: 10.1021/om901070g

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