Bimetallic Titanocene or Zirconocene/Aluminium Complexes as Active Catalysts in Lactone...

FULL PAPER

Bimetallic Titanocene or Zirconocene/Aluminium Complexes as ActiveCatalysts in Lactone Polymerization Reactions

Perdita Arndt,*[a] Anke Spannenberg,[a] Wolfgang Baumann,[a] Sigurd Becke,[b] andUwe Rosenthal[a]

Keywords: Lactones / Sandwich complexes / Titanium / Zirconium / Aluminum / Ring-opening polymerization

The metallocene alkyne complexes [Cp2M(L)(η2-Me3SiC2R)]react with diisobutylaluminium hydride at room temp. by ad-dition of HAl(iBu)2 to yield the heterobimetallic complexes[{Cp2M}(µ-η1:η2-RCCSiMe3)(µ-H){Al(iBu)2}] [{Cp2M}: (η5-C5H5)2Ti, R = Ph 1, R = SiMe3 2; {Cp2M}: (η5-C5H5)2Zr, R =SiMe3 3; {Cp2M}: (ebthi)Zr, R = SiMe3 4] {ebthi: rac-[1,2-ethylene-1,19-bis(η5-tetrahydroindenyl)]}. Complexes 1−4

Introduction

Group 4 metallocene complexes react with organoalumi-nium compounds to give highly active Ziegler-type catalystsystems.[1] Nevertheless, these catalysts present some disad-vantages, such as the necessity of very large amounts ofMAO. An approach to overcome this problem resides inthe development of new activation methods to extend theseactive systems to other catalytic reactions. The generationof single-component catalysts, or their direct precursors, istherefore of great current interest.[2]

The reaction of metallocene-alkyne and -alkene com-plexes with aluminium or boron organic compounds hasbeen intensively investigated by Erker and Binger[3] andleads to bimetallic systems of the type [{Cp2M1}(µ-η1:η2-RCCR)(µ-X){M2R2

2}] (M1 5 Zr, Hf, X 5 Cl, H, M2 5 Al,Ga; M1 5 Ti, X 5 H, M2 5 B) containing a planar tetraco-ordinate carbon atom bridging between a group 4 transitionmetal and a main group element; [Cp2M(PMe3)(alkyne)]complexes give [{Cp2M}(µ-η1:η2-RCCR)(µ-H){Al(iBu)2}]with diisobutylaluminium hydride.[3f] These compounds areof current interest and have been studied extensively, al-though compounds with a desired activation for catalyticprocesses were not obtained.[4]

We report here the easy synthesis of bimetallic titaniumor zirconium/aluminium compounds, starting from (η2-al-kyne) metallocene complexes (alkyne 5 Me3SiC2R, R 5Ph, SiMe3) and their polymerization capability in the ring-opening polymerization (ROP) of lactones in comparison

[a] Institut für Organische Katalyseforschung an der UniversitätRostock e.V.,Buchbinderstraße 526, 18055 Rostock, GermanyE-mail: perdita.arndt@ifok.uni-rostock.de

[b] Bayer AG, ZSB Zentrale Forschung, Gebäude Q 18,51368 Leverkusen, GermanyFax: (internat.) 149-214/302-4202E-mail: sigurd.becke.sb@bayer-ag.de

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were characterized by NMR spectroscopy and 1, 2 and 4 in-vestigated by X-ray crystal structure analysis. Compounds1−4 were tested as initiators in the ring-opening polymeriz-ation of lactones. The results are discussed in comparison tothe corresponding monometallic titanocene and zirconocenealkyne complexes.

to the corresponding monometallic (η2-alkyne)com-pounds.[11,12]

Results and Discussion

In the reaction of [Cp2M(L)(η2-RC2SiMe3)] {[Cp2M]:(η5-C5H5)2Ti, R 5 Ph, SiMe3, without L; [Cp2M]: (η5-C5H5)2Zr, R 5 SiMe3, L 5 Py; [Cp2M]: rac-(ebthi)Zr, R 5SiMe3, without L} with diisobutylaluminium hydride intoluene (2 h) at ambient temperature, corresponding to theinvestigations by Erker,[3f] the bimetallic complexes[{Cp2M}(µ-η1:η2-RCCSiMe3)(µ-H){Al(iBu)2}] 124 areformed [Equation (1)].

(1)

Complex 1 crystallizes from n-hexane as yellow prisms(m.p. 1502152 °C) and is stable in solution for some dayseven at higher temperatures. The corresponding bis(trime-thylsilyl)-substituted compound 2 (brown-yellow crystals,m.p. 136 °C dec.) is less stable, it regenerates the starting

P. Arndt, A. Spannenberg, W. Baumann, S. Becke, U. RosenthalFULL PAPERcompound [Cp2Ti(η2-Me3SiC2SiMe3)] in solution at highertemperatures (80 °C, 5 h). The stable analogous zirconocenecomplexes 3 (light yellow needles, m.p. 142 °C) and 4 (deepyellow prism, m.p. 1572158 °C) are obtained in a similarreaction. All the above complexes were characterized byNMR spectroscopy and 1, 2 and 4 by X-ray crystal struc-ture analysis.

Spectroscopic Data

The most characteristic proton NMR spectroscopic fea-ture of the compounds 124 is the high-field resonances ofthe bridging hydrido ligands (Table 1). The resonance sig-nals are located in the range expected for a hydrogen atombridging the aluminium and the transition metal center[7]

[broad singlets at: δ 5 23.79 (1), δ 5 24.87 (2), δ 5 22.46(3), δ 5 21.27 (4)].

Table 1. Selected NMR spectroscopic data of complexes 124

1 2 3 4

1H NMR (δ [ppm])Cp 5.15 5.10 5.35H 23.79 24.87 22.46 21.2713C NMR (δ [ppm])Cα, Cβ (alkyne) 230.1 234.3 240.4 243.4

105.0 123.7 138.5 149.5

The methylene groups at aluminium give broad signals inthe 13C NMR spectrum which become sharper upon cool-ing the sample, especially for complex 2. Besides that, nochanges in the NMR spectra were observed. The 13C reson-ances of the planar-tetracoordinated carbon atom are in theexpected olefinic range [δ 5 105.0 (1), δ 5 123.7(2), δ 5138.5 (3), δ 5 149.5 (4)]. The methylene group at the alumi-nium is found in the 13C NMR spectrum as a broad signalat room temp.

These data of complexes 124 are in good agreement withthe zirconocene/aluminium complexes [{Cp2Zr}(µ-η1:η2-RCCR)(µ-H){Al(iBu)2}] from Erker,[3a23f,4] and the titano-cene complexes [{Cp2Ti}(µ-η1:η2-RCCSiMe3)(µ-H)-{Al(iBu)2}] described by Binger[4] and show the same struc-tural features.

X-ray Crystal Structure Analysis of Bimetallic Ti or Zr andAluminium Compounds

It turned out that an unambiguous characterization ofthe compounds 1, 2 and 4 cannot be revealed by NMRspectroscopy as the structural element of the square-planarcarbon coordination geometry does not cause any remark-able features in the NMR spectra. Compounds 1, 2 and 4were therefore crystallized from n-hexane solution by cool-ing to 230 °C.

Eur. J. Inorg. Chem. 2001, 2885228902886

The X-ray crystal structures of the titanium complexesexhibit planar, bicyclic five-membered ring systems con-sisting of the metal atom, the hydridic hydrogen H, Al, Cβ

and Cα (Figure 123 and Table 2). The most remarkablestructural element is the coordination geometry of the car-bon atom Cβ, which is bonded to Cα, M, Al and the Siatom of the SiMe3 group, with all four bonds lying in oneplane (mean deviation from the best plane through Ti, Al,Si, Cα 1: 0.057 A, 2: 0 A). The zirconium complex 4 showsa similar bicyclic ring system as the titanium complexes butin this case the planarity is distorted because of the sterichindrance of the ebthi ligand. The values found for theM2Cβ bond lengths are in the expected range for suchstructural features.[325]

Figure 1. Molecular structure of complex 1 as an ORTEP plot atthe 30% probability level; selected bond lengths (A) and bondangles (deg):[14] Ti2Al 2.788(1), Ti2H1 1.857, Al2H1 1.628,Ti2C1 2.379(3), Ti2C2 2.111(3), C12C2 1.315(5), Al2C12.098(4); Ti2H12Al 106.1, Al2C12Ti 76.74(11), H12Al2C196.0, Ti2C12Si 172.7(2), C12C22Ti 84.6(2)

Figure 2. Molecular structure of complex 2 as an ORTEP plot atthe 30% probability level; selected bond lengths (A) and bondangles (deg):[14] Ti2Al 2.762, Ti2H 1.765, Al2H 1.624, Ti2C72.341(7), Ti2C6 2.127(7) C72C6 1.311(9), Al2C7 2.145(7);Ti2H2Al 109.1, Al2C72Ti 75.9(2), H2Al2C7 92.4, Ti2C72Si2177.9(4), C72C62Ti 82.1(5)

Titanocene or Zirconocene/Aluminium Complexes as Catalysts in Lactone Polymerization FULL PAPER

Figure 3. ORTEP view of complex 4 (for clarity only atoms re-garded as significant are drawn as 30% probability ellipsoids); se-lected bond lengths (A) and bond angles (deg):[14] Zr2Al 2.866(4),Zr2H2 2.048, Al2H2 1.659, Zr2C15 2.384(15), Zr2C142.221(13), C152C14 1.33(2), Al2C15 2.15(2); Zr2H22Al 100.8,Al2C152Zr 78.3(5), H22Al2C15 96.9, Zr2C152Si1 174.8(9),C152C142Zr 80.1(9)

Table 2. Selected bond lengths [A] and angles [°][14]

Catalytic ROP of Lactones with Complexes 124 asCatalysts

The ROP of ε-caprolactone provides a convenient routeto the synthesis of polylactones which are important formedical applications and as plastic modifiers.[6] These ap-plications require polyesters with predictable molecularweights and low polydispersity indices, which is only pos-sible with initiators having selectivity in terms of initiation,propagation and termination reactions.[7] For example, al-kali metal alkyls or alkoxides give very fast polymerizationof lactones, but inter- and intramolecular side reactions of-

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ten lead to a broadening of the molecular weight distribu-tion of the products. When aluminium alkoxides are usedas initiators, the ROP has been reported to be living andgenerates polyesters with narrow polydispersity indices thatare free of macrocycles.[8a28c] Recently, well characterizedtransition metal and lanthanoid complexes containingcyclopentadienyl ligands (metallocenes) have emerged as anew class of initiators.[9] The polymerization activity (forpolar monomers) was tested using mono(cyclopentadienyl)-titanium,[9] rare earth[7a] and heterobimetalliclanthanocene[7a,10a210c] compounds. Organolanthanidecomplexes in general seem to be highly active and wellsuited as initiators for the ROP of lactones due to theirLewis acidity.[7a]

Recently, we described how (alkyne)zirconocene and tit-anocene complexes are capable of polymerizing ε-caprolac-tone with living characteristics and high yield.[11,12]

(Table 3, runs 1 and 4)The heterobimetallic complexes 124 are of interest in the

context of the activation of catalytic systems. Polymeriz-ation reactions were carried out under the same conditionsas described before;[11,12] for a high conversion it is neces-sary to start with a mixture of ε-caprolactone/catalyst intoluene at room temp. and add a second portion of sub-strate/toluene at 75 °C (Table 3, runs 2, 3 and 5). The poly-ε-caprolactone (PCL) was isolated as a colorless solid andcharacterized by GPC and NMR spectroscopy. The poly-mers formed by 124 are generally of high molecular weightand of moderate polydispersities (Mw/Mn , 2.7). The effici-ency depends on the experimental conditions, with thehighest value being observed for 3.

The heterobimetallic complexes show an increase of mo-lecular weights (runs 2 and 3, Ti) by a factor of 10 for tita-nium compared to their monometallic derivatives and aredependent on the metal in the metallocene unit. The loweryield of run 2 might be explained by the instability of com-plex 2: as mentioned above, it forms the monometallic com-pound [Cp2Ti(η2-Me3SiC2SiMe3)] in solution at highertemperatures (80 °C, 5 h).

The results of NMR studies of the ROP with titanoceneinitiators were disappointing. All subsequent steps of thecatalytic cycles such as initiation, propagation and chaintermination are very fast on the NMR time scale, and couldnot be detected. This is consistent with the results we ob-tained in the case of (alkyne)zirconocene catalysts (for pro-pagation and termination)[12] and those obtained by othergroups in monitoring lactone polymerization by NMRspectroscopy.[10b] Furthermore, no end-group signals werefound.[12] This has been explained in other reports bythe formation of a cyclic polymer as the terminationstep.[10a,10d] The corresponding heterobimetallic zircono-cene(alkyne)/aluminium derivatives (3, 4) (Table 3, runs 5,7) show no significant changes in comparison to theirmonometallic example in the ROP. The NMR investigationof the ROP of ε-caprolactone with initiator 4 shows theformation of a five-membered metallacyclic spiro-zirconadihydrofuran complex, which was already describedas the first step in the ROP of lactones by the monometallic

P. Arndt, A. Spannenberg, W. Baumann, S. Becke, U. RosenthalFULL PAPER

Table 3. Polymerization of ε-caprolactone (CL) in toluene at 75 °C by complexes 124 in comparison to the corresponding monometallic(η2-alkyne) compounds (75 °C, toluene)

Run Initiator CL/initiator Time Yield Mn(calcd.) Mw(GPC) Mn(GPC) Mw/Mn Efficiency[a]

[h] [%]

1 Cp2Ti(Me3SiC2SiMe3)[11] 5 000 1 86 490 000 110 000 68 500 1.6 7.22 Cp2Ti(Me3SiC2SiMe3)-(µ-H)(AliBu)2 (2) 10 000 1 67 764 000 942 900 390 000 2.4 1.93 Cp2Ti(Me3SiC2Ph)-(µ-H)(AliBu)2 (1) 50 000 1 97 5 535 000 1 072 000 410 000 2.6 13.54 Cp2Zr(Py)(Me3SiC2SiMe3)[11] 5 000 1 80 450 000 80 000 44 400 1.8 10.35 Cp2Zr(Me3SiC2SiMe3)-(µ-H)(AliBu)2 (3) 5 000 1 82 460 000 67 570 48 170 1.4 9.76 (ebthi)Zr(Me3SiC2SiMe3)[12] 5 100 1 34 190 000 2 2 2 27 (ebthi)Zr(Me3SiC2SiMe3)-(µ-H)(AliBu)2 (4) 5 000 1 10 57 000 52 760 39 680 1.3 1.4

[a] Efficiency is defined as the fraction of active initiators, calculated from the observed number of polymer chains relative to the theoreticalones, assuming that each metallocene initiates one polymer chain.

(zirconocene) alkyne complexes;[12] it is followed by a rapidpolymerization. No influence of the HAl(iBu)2 was found.A control experiment with HAl(iBu)2 as catalyst gave nopolymerization. This is in agreement with the fact that Al-(iBu)3 has no catalytic activity for the ROP of ε-capro-lactone.[13b]The catalytic activity of the titanocene hetero-bimetallic compounds can be explained by an interactionbetween the active species and the monomer by coordina-tion of the exo- or endocyclic oxygen of ε-caprolactone todifferent metal units (Al, Ti). This could cause the increasedrate of polymerization and high molecular weights of theresulting polylactones. Such an activation was demon-strated by Sheng[13a,13b] for rare-earth coordination initi-ators obtained in situ by adding trialkylaluminium to Lncompounds. Also, heterobimetallic lanthanide/lithium com-plexes have been found to be active in the polymerizationof ε-caprolactone, as reported by Okuda.[10a] In this case,however, another activation mechanism involving an amidenitrogen atom was proposed.

Experimental Section

General: All operations were carried out under argon with standardSchlenk techniques. Solvents were freshly distilled prior to use fromsodium tetraethylaluminate and stored under argon. Deuteratedsolvents were treated with sodium or sodium tetraethylaluminate,distilled and stored under argon. ε-Caprolactone (Aldrich) wasdried over molecular sieves and degassed before use. 2 The follow-ing spectrometers were used: Mass spectra: AMD 402. 2 NMRspectra: Bruker ARX 400. Chemical shifts referenced to signals ofthe used solvents: [D6]benzene (δH 5 7.16, δC 5 128.0). The spectrawere assigned with the help of DEPT experiments. 2 Meltingpoints: sealed capillaries, Büchi 535 apparatus. 2 Elemental ana-lyses: Leco CHNS-932 elemental analyzer. 2 Gel permeation chro-matographic (GPC) analyses of the polymers were performed usinga Liquid Chromatograph 1090 HP [column: SDV 104 A 1 103 A1100 A (Polymer Standard Service), eluent: THF]. Molecularweights were corrected by universal calibration relative to polysty-rene standards.

Preparation of 1: 1.2 mL (1.2 mmol) of a 1.0 solution ofAlH(iBu)2 in n-heptane was added to a solution of [Cp2Ti(η2-

Eur. J. Inorg. Chem. 2001, 2885228902888

PhC2SiMe3)] (375 mg, 1.18 mmol) in 10 mL of toluene. The mix-ture was stirred for 2 h at room temp. and the color changed tobrown. After evaporation of the solvents to dryness, the residue waswashed twice with 2 mL of n-pentane and dissolved in n-hexane (60°C). The solution was filtered, and after 12 h at room temp. yellowcrystals formed. The crystals were washed with cold n-pentane anddried in vacuo to give 435 mg (75%) of 1. M.p. 150 2152 °C. 21H NMR (C6D6, 297 K): δ 5 23.79 (br., 1 H, µ-H), 0.11 (2 s, 9H, SiMe3), 0.47, 0.62 (m, 4 H, CH2), 1.34, 1.36 (2 d, 3J 5 5.9/5.8 Hz, 12 H, CH3), 2.35 (m, 2 H, CH), 5.13 (d, 2 H, Ph), 5.15 (s,10 H, Cp), 6.83 (d, 2 H, Ph), 6.98 (t, 1 H, Ph). 2 13C{1H} NMR(C6D6, 297 K): δ 5 3.8 (SiMe3), 26.8 (br., CH2), 27.3 (CH), 28.8,29.0 (CH3), 104.9 (Cp), 123.8, 125.1, 128.0 (CH, Ph), 150.4 (ipso-C, Ph), 105.0(C-SiMe3), 230.1 (C-Ph). 2 MS (70 eV): m/z (%) 5

352 [Cp2Ti(η2-PhC2SiMe3)]1, 178 [Cp2Ti1], 173 [PhC2SiMe3]1,141 [Al(iBu)2]1. 2 C29H43AlSiTi (494.61): calcd. C 70.42, H 8.76;found C 70.10, H 8.59.

Preparation of 2: 1.2 mL (1.2 mmol) of a 1.0 solution ofAlH(iBu)2 in n-heptane was added to a solution of [Cp2Ti(η2-Me3SiC2SiMe3)] (405 mg, 1.16 mmol) in 10 mL of toluene. Themixture was stirred for 2 h at room temp. and the color changedto green-brown. After evaporation of the solvents to dryness, theoil was washed twice with 1 mL of cold n-pentane and dissolvedin a small amount of n-hexane. After filtration the solvent was re-moved in vacuo until the first yellow-brown crystals formed. Uponcooling to 278 °C for 12 h a brown precipitate formed which wasfiltered off, washed with cold n-pentane and dried in vacuo to give400 mg (76%) of 2. M.p.: 136 °C (dec.). 2 1H NMR (C6D6, 297 K):δ 5 24.87 (br., 1 H, µ-H), 0.29, 0.33 (2 s, 18 H, SiMe3), 0.46, 0.60(m, 4 H, CH2), 1.30, 1.35 (2 d, 3J 5 6.5 Hz, 12 H, CH3), 2.31 (m,2 H, CH), 5.10 (s, 10 H, Cp). 2 13C{1H} NMR (C6D6, 297 K):δ 5 2.0, 3.6 (SiMe3), 25.8 (br, CH2), 25.7 (CH), 27.4, 27.5 (CH3),102.5 (Cp), 123.7, 234.3 (C-SiMe3). 2 1H NMR ([D8]toluene,192 K): δ 5 25.05 (br., 1 H, µ-H), 0.18, 0.18 (2 s, 18 H, SiMe3),0.43, 0.56 (br., 4 H, CH2), 1.32, 1.40 (2 d, 12 H, CH3), 2.30 (m, 2H, CH), 5.10 (s, 10 H, Cp). 2 13C{1H} NMR ([D8]toluene, 192 K):δ 5 2.0, 3.6 (SiMe3), 26.5 (br., CH2), 27.0 (CH), 28.8, 28.9 (CH3),103.5 (Cp), 123.7, 234.3 (C-SiMe3). 2 MS (70 eV): m/z (%) 5348[Cp2Ti(η2-Me3SiC2SiMe3)]1, 178 [Cp2Ti1], 173 [Me3SiC2SiMe3]1,141 [Al(iBu)2]1. 2 C26H47AlSi2Ti (490.69): calcd. C 63.64, H 9.65;found C 63.09, H 9.23.

Preparation of 3: 1.1 mL (1.1 mmol) of a 1.0 solution ofAlH(iBu)2 in n-heptane was added to a solution of [Cp2Zr(Py)(η2-Me3SiC2SiMe3)] (503 mg, 1.07 mmol) in 10 mL of toluene. The

Titanocene or Zirconocene/Aluminium Complexes as Catalysts in Lactone Polymerization FULL PAPERmixture was stirred for 2 h at 40 °C and the color changed to lightgreen. After evaporation of the solvents to dryness. the residue wassolved in a small amount of n-hexane. Upon cooling to 278 °C for12 h light yellow-green crystals were formed which were filtered off,washed with cold n-pentane and dried in vacuo to give 410 mg(71%) of 3. M.p.: 142 °C. 2 1H NMR (C6D6, 297 K): δ 5 2 2.46(br., 1 H, µ-H), 0.34, 0.35 (2 s, 18 H, SiMe3), 0.39, 0.60 (m, 4 H,CH2), 1.29, 1.34 (2 d, 3J 5 6.4 Hz, 12 H, CH3), 2.28 (m, 2 H, CH),5.35 (s, 10 H, Cp). 2 13C{1H} NMR (C6D6, 297 K): δ 5 3.5, 4.7(SiMe3), 26.8 (br., CH2), 27.2 (CH), 28.6, 28.9 (CH3), 104.6 (Cp),138.5, 240.4 (C-SiMe3). 2 MS (70 eV): m/z (%) 5 534 [M1], 220[Cp2Zr1], 170 [Me3SiC2SiMe3]1. 2 C26H47AlSi2Zr (534.03): calcd.C 58.48, H 8.87; found C 58.23, H 8.60.

Preparation of 4: 0.7 mL (0.7 mmol) of a 1.0 solution ofAlH(iBu)2 in n-heptane was added to a solution of [(ebthi)Zr(η2-Me3SiC2SiMe3)] (378 mg, 0.72 mmol) in 10 mL of toluene wereadded of AlH(iBu)2 as a solution in n-heptane (1.0 ). The lightyellow solution was stirred for 2 h at room temp. After evaporationto dryness. the residue was dissolved in 5 mL of n-hexane. Afterfiltration and cooling to 240 °C for 12 h crystals formed, whichwere filtered off and dried in vacuo to give 234 mg (48%) of 4.M.p.: 1572158 °C. 2 1H NMR (C6D6, 297 K): δ 5 21.27 (br., 1H, µ-H), 0.48, 0.50 (2 s, 18 H, SiMe3), 0.6320.72 (m, 4 H, CH2),1.20, 1.29, 1.38, 1.43 (4 d, 3J 5 6.4 Hz, 12 H, CH3), 2.23 (m, 2 H,CH), 4.38, 5.51, 5.54, 6.35 (4 d, J 5 2.5 Hz, 4 H, CH ebthi). 213C{1H} NMR (C6D6, 297 K): δ 5 3.5, 4.7 (SiMe3), 26.8 (br.,CH2), 27.1 (CH), 28.6, 28.9 (CH3), 99.2, 100.46, 103.0, 104.8 (CH,ebthi), 116.7, 119.2, 122.1, 124.0, 126.4, 126.2 (q-C, ebthi) 149.5,243.4 (C-SiMe3). 2 C36H61AlSi2Zr (668.25): calcd. C 64.71, H 9.20;found C 63.99, H 9.02.

Polymerization of ε-Caprolactone: A stirred 5 solution of themonomer in toluene (5 mL) was heated to 75 °C and an appropri-ate amount of the initiator complex dissolved in toluene was added.

Table 4. Crystal data, data collection and structure refinement forcompounds 1, 2 and 4

1 2 4

Cryst. color yellow yellow yellowCryst. system orthorhombic monoclinic monoclinicSpace group Pbca C2/m P21

Lattice constantsa [A] 9.867(2) 20.713(4) 9.448(2)b [A] 20.212(4) 14.630(3) 18.131(4)c [A] 28.261(6) 9.979(2) 11.581(2)α [°] 90.00 90.00 90.00β [°] 90.00 105.19(3) 108.46(3)γ [°] 90.00 90.00 90.00Z 8 4 2

F(000) [e] 2128 1064 716

µ [mm21] 0.392 0.416 0.401Temp. [K] 293(2) 293(2) 293(2)No. of rflns. (meas.) 10532 2957 5651No. of rflns. (indep.) 2982 1562 2951No. of rflns. (obsd.) 2290 1092 1182No. of parameters 290 148 361R1 [I $ 2σ(I)] 0.042 0.056 0.049wR2 (all data) 0.110 0.152 0.110

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Ten minutes later a further amount of lactone dissolved in toluenewas added. After one hour the reaction mixture was worked up bydropwise addition of a tenfold volume of methanol. The poly(ε-caprolactone)s were filtered off and dried in vacuo until the weightremained constant.

PCL: 1H NMR (C6D6, 297 K): δ 5 1.5, 1.41, 1.17 (3 m, 6 H, CH2);2.11, 3.98 (2 t, 4 H, CH2).

X-ray Crystallographic Study of Complexes 1, 2 and 4: Diffractiondata were collected on a STOE-IPDS diffractometer using graph-ite-monochromated Mo-Kα radiation. Crystal data, details of thedata collections and structure refinement are compiled in Table 4.The structures were solved by direct methods (SHELXS-86: G. M.Sheldrick, Acta Crystallogr., Sect. A 1990, 46, 467) and refined byfull-matrix least-squares techniques against F2 (SHELXL-93: G.M. Sheldrick, University of Göttingen, Germany, 1993).Crystallographic data (excluding structure factors) for the struc-tures reported in this paper have been deposited with the Cam-bridge Crystallographic Data Centre as supplementary publicationnos. CCDC-157428 (1), -157429 (2), -157430 (4). Copies of the datacan be obtained free of charge on application to CCDC, 12 UnionRoad, Cambridge CB2 1EZ, UK [Fax (internat.): 144-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].

AcknowledgmentsWe thank the Deutsche Forschungsgemeinschaft (AR 321/1-1) andthe Fonds der Chemischen Industrie for financial support.

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[14] Values without standard deviations were determined by theprogram XP (Siemens Analytical X-ray Instruments, Inc.).

Received March 2, 2001[I01078]