control of conformations of piperazidine-bridged bis(phenolato) groups: syntheses and structures of...

Control of Conformations of Piperazidine-Bridged Bis(phenolato)Groups: Syntheses and Structures of Bimetallic and MonometallicLanthanide Amides and Their Application in the Polymerization ofLactidesWenyi Li,† Zhongjian Zhang,† Yingming Yao,*,†,‡ Yong Zhang,† and Qi Shen†

†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering & Materials Science,Dushu Lake Campus, Soochow University, Suzhou, 215123, People’s Republic of China‡The Institute of Low Carbon Economy, Suzhou, 215123, People’s Republic of China

*S Supporting Information

ABSTRACT: A series of bimetallic and monometallic lanthanide amides stabilizedby a piperazidine-bridged bis(phenolato) ligand were successfully prepared, and thefactors controlling the formation of these lanthanide amides were elucidated. Reac-tions of Ln[N(TMS)2]3(μ-Cl)Li(THF)3 (TMS = SiMe3; THF = tetrahydrofuran)with a piperazidine-bridged bis(phenol), H2[ONNO][4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)piperazidine], in a 2:1 molar ratio in THF at 60 °C gave the anionicbimetallic bis(phenolato) lanthanide amido complexes [ONNO]{Ln[N(TMS)2]2(μ-Cl)Li(THF)}2 [Ln = Y (1), Er (2), Eu (3), Sm (4)], whereas the same reactionsconducted at room temperature gave the monometallic bis(phenolato) lanthanideamides [ONNO]LnN(TMS)2(THF) [Ln = Y (5), Sm (6)]. Complex 1 can be trans-formed to a neutral bimetallic bis(phenolato) yttrium amido complex, [ONNO]-{Y[N(TMS)2]2}2 (7), by heating a toluene solution to 80 °C. Complex 7 can alsobe conveniently prepared by the reaction of the yttrium amide Y[N(TMS)2]3 withH2[ONNO] in a 2:1 molar ratio at 60 °C. For neodymium and praseodymium, only the monometallic lanthanide amidocomplexes [ONNO]LnN(TMS)2(THF) [Ln = Nd (8), Pr (9)] were isolated, even when the reactions were conducted at 60 °C.Furthermore, reaction of H2[ONNO] with the less bulky lanthanide amides Ln[N(SiMe2H)2]3(THF)2 in a 2:1 molar ratio at60 °C gave the monometallic lanthanide amido complexes [ONNO]Ln[N(SiMe2H)2](THF) [Ln = Yb (10), Y(11), Nd (12)]as neat products; no bimetallic species were formed. All of these complexes were characterized by IR, elemental analyses, andsingle-crystal X-ray diffraction. Complexes 1, 5, 6, 7, and 11 were further confirmed by NMR spectroscopy. These complexes arehighly efficient initiators for the ring-opening polymerization of L-lactide. In addition, complexes 1, 3, 5, 7, and 11 can initiate rac-lactide polymerization with high activity to give heterotactic-rich polylactides.

■ INTRODUCTIONIn recent years, there has been an increasing focus on theapplication of bridged bis(phenolato) ancillary ligands inorganolanthanide chemistry, and some of these complexeshave been found to be efficient initiators for the ring-openingpolymerization of cyclic esters to produce biodegradable poly-esters.1 These ligand systems have attractive features, such asbeing easily available and tunable, and it is possible to conductsystematic studies of the effects of the steric and electronicproperties of the ligands on the reactivities of the resultingcomplexes.2 To date, a variety of bis(phenolato) ligands con-taining C, N, and S bridges have been used in organolanthanidechemistry.3−5 It has been found that the bridge structure has aprofound influence on the reactivity of the corresponding lanthanidecomplexes, and, in most cases, coordination of electron-donatingheteroatoms to the lanthanide atoms improves the catalytic activitiesof the complexes in the polymerization of cyclic esters.3b,4f

Our current interest is focused on the syntheses and reactivitiesof organometallic complexes stabilized by N-heterocycle-bridged

bis(phenolato) ligands, because the introduction of an N-heterocyclic bridge to the ligand system should result in anincrease in the rigidity of the bridged ligand, and this mightaffect the syntheses and reactivities of the corresponding metalcomplexes.4l−n It was found that the structure of the N-heterocycle affects the syntheses of bridged bis(phenolato)lanthanide complexes. Neutral monomeric lanthanide amidescan be obtained over the full size range of the lanthanide metalsby the reaction of imidazolidine-bridged bis(phenol) withLn[N(TMS)2]3(μ-Cl)Li(THF)3 (TMS = SiMe3; THF =tetrahydrofuran),4l whereas the reactions of ytterbium amideswith piperazidine-bridged bis(phenol) in a 2:1 molar ratio affordthe bimetallic bis(phenolato) ytterbium diamido complexes,which are highly efficient initiators for the ring-opening poly-merization of L-lactide.4m Lanthanide metals have large ionicradii, and it is necessary to meet their high-coordination-number

Received: November 21, 2011Published: April 18, 2012

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requirements to form stable compounds. The controlled syn-theses of stable bimetallic organolanthanide complexes aretherefore still a challenge. The piperazidine ring is flexible, butit possesses two main stable conformations, namely, chair andboat forms, resulting in a wide variety of binding possibilitiesfor creating monometallic and bimetallic species.6 In bimetallicpiperazidine-bridged bis(phenolato) lanthanide complexes, thepiperazidine ring adopts the chair conformation; however,mononuclear piperazidine-bridged bis(phenolato) lanthanidecomplexes are isolated when the piperazidine ring adopts theboat conformation. Indeed, the alkane-elimination reactionsof Ln(CH2SiMe3)3(THF)2 with the same piperazidine-bridgedbis(phenol) in THF gave the mononuclear lanthanide alkylcomplexes in high isolated yields.4n It is therefore interestingto understand how to control the conformation of thepiperazidine ring during the syntheses of piperazidine-bridgedbis(phenolato) lanthanide complexes.To gain insight into the potential factors governing the formation

of the bimetallic lanthanide complexes, we have continued tostudy the reactions of piperazidine-bridged bis(phenol)s withlanthanide amides. It was found that the formation of bimetalliclanthanide amido complexes was significantly influenced byreaction temperature, the size of the lanthanide metal, and thesteric bulkiness of the amido group. For the bulky bis(TMS)-amido group, bimetallic lanthanide bisamido complexes areisolated for the small lanthanide metals (Y, Er, Eu, and Sm),whereas monometallic lanthanide amido complexes are isolatedfor the larger metals (Nd and Pr) when the reaction was con-ducted at 60 °C. In contrast, when the reaction was conductedat room temperature, only the monometallic lanthanide com-plexes were isolated, even for the smaller lanthanide metals.For the less bulky bis(dimethylsily)amido group, only themonometallic lanthanide complexes can be prepared. Theselanthanide amido complexes are highly efficient initiators forthe ring-opening polymerization of lactides. We report theseresults in this paper.

■ EXPERIMENTAL SECTIONGeneral Procedures. All of the manipulations were performed

under an argon atmosphere using the standard Schlenk techniquesand a glovebox. HN(TMS)2 (TMS = SiMe3), HN(HSiMe2)2,

nBuLi,L-lactide, and rac-lactide are commercially available. THF, toluene, andhexane were distilled from sodium benzophenone ketyl before use.HN(TMS)2 was dried over CaH2 for 4 days and distilled before use.Lactides were sublimated and then recrystallized from hot anhydroustoluene twice. Deuterated solvents (C6D6, THF-d8, and CDCl3) wereobtained from CIL; [ONNO]H2 {H2[ONNO] = 1,4-bis(2-hydroxy-3,5-di-tertbutylbenzyl)piperazidine},7 Ln[N(TMS)2]3(μ-Cl)Li(THF)3(Ln = Pr, Nd, Sm, Eu, Er, Y),8 Y[N(TMS)2]3,

9 and Ln[N-(SiMe2H)2]3(THF)2 (Ln = Y, Yb, Nd)10 were prepared according tothe published methods.Samples of organolanthanide complexes for NMR spectroscopic

measurements were prepared in the glovebox using a J. Young valveNMR tube. NMR (1H, 13C) spectra were recorded on a Varian Unityspectrometer at 25 °C. Lanthanide analyses were performed by EDTAtitration with a xylenol orange indicator and a hexamine buffer.11

Carbon, hydrogen, and nitrogen analyses were performed by directcombustion with a Carlo-Erba EA-1110 instrument. The IR spectrawere recorded with a Nicolet-550 FTIR spectrometer as KBr pellets.Molecular weight and molecular weight distribution (PDI) were deter-mined against a polystyrene standard by gel permeation chromatog-raphy (GPC) on a PL 50 apparatus, and THF was used as an eluent ata flow rate of 1.0 mL/min at 40 °C.Synthesis of [ONNO]{Y[N(TMS)2]2(μ-Cl)Li(THF)}2 (1). A hot

(about 60 °C) THF solution (15 mL) containing [ONNO]H2 (0.50 g,

0.96 mmol) was added slowly to a THF solution of Y[N(TMS)2]3(μ-Cl)Li(THF)3 (1.58 g, 1.92 mmol, 30 mL) in an oil bath (60 °C). Thereaction mixture was stirred overnight at 60 °C, and then the solventwas removed under vacuum. Toluene (about 15 mL) was added to theresidue, and the precipitate formed was removed by centrifugation.The solution was concentrated to about 3 mL, and then hexane wasadded (1:5, v/v). Colorless crystals were obtained at 0 °C in severaldays (0.63 g, 42%). 1H NMR (THF-d8, 300 MHz, ppm): δ 7.08 (m,2H, Ar-H), 6.69 (m, 2H, Ar-H), 3.55 (br, 8H, THF-α-CH2), 2.26 (s,4H, ArCH2), 1.69 (br, 8H, THF-β-CH2), 1.42 (s, 18H, C(CH3)3),1.38 (br, 4H, N(CH2CH2)2N), 1.19 (s, 18H, C(CH3)3), 1.16 (br, 4H,N(CH2CH2)2N), 0.13 (s, 72H, TMS). 13C NMR (THF-d8, 75 MHz,ppm): δ 160.48, 136.94, 136.52, 125.05, 123.60, 122.62 (Ar-C), 68.03(THF-α-CH2), 61.85 (ArCH2N), 51.6, 47.60 (N(CH2)2N), 35.67,34.19, 32.08, 31.00 (C(CH3)3), 26.17 (THF-β-CH2), 6.49 (TMS).Anal. Calcd for C66H140Cl2Li2N6O4Si8Y2: C, 50.52; H, 8.99; N, 5.35; Y,11.33. Found: C, 50.83; H, 8.93; N, 5.23; Y, 10.92. IR (KBr, cm−1):2957(s), 2870(m), 2825(m), 1608(s), 1480(s), 1457(s), 1438(s),1358(s), 1315(m), 1256(s), 1242(s), 1203(m), 1125(s), 1036(m),998(s), 931(w), 881(s), 828(s).

Synthesis of [ONNO]{Er[N(TMS)2]2(μ-Cl)Li(THF)}2 (2). Thesynthesis of complex 2 was carried out in the same way as thatdescribed for complex 1, but [ONNO]H2 (0.64 g, 1.22 mmol) andEr[N(TMS)2]3(μ-Cl)Li(THF)3 (2.66 g, 2.44 mmol) were used. Pinkcrystals were obtained from a mixture of toluene and hexane solu-tion (1:3 v/v) at 0 °C in several days (0.84 g, 40%). Anal. Calcd forC66H140Cl2Li2N6O4Si8Er2: C, 45.93; H, 8.18; N, 4.87; Er, 19.38.Found: C, 45.52; H, 8.33; N, 4.57; Er, 19.04. IR (KBr, cm−1): 2959(s),2872(m), 2827(m), 1607(s), 1557(s), 1539(s), 1457(s), 1417(s),1362(s), 1342(m), 1316(m), 1260(s), 1169(s), 1124(m), 1051(m),1006(s), 972(s), 932(w), 880(s), 832(s).

Synthesis of [ONNO]{Eu[N(TMS)2]2(μ-Cl)Li(THF)}2 (3). Thesynthesis of complex 3 was carried out in the same way as thatdescribed for complex 1, but [ONNO]H2 (0.69 g, 1.33 mmol) andEu[N(TMS)2]3(μ-Cl)Li(THF)3 (2.86 g, 2.66 mmol) were used.Orange crystals were obtained from a mixture of toluene and hexanesolution (1:2 v/v) at 0 °C in several days (0.97 g, 43%). Anal. Calcdfor C66H140Cl2Li2N6O4Si8Eu2: C, 46.76; H, 8.32; N, 4.96; Eu, 17.93.Found: C, 46.98; H, 8.64; N, 4.53; Eu, 17.55. IR (KBr, cm−1): 2956(s),2871(m), 2827(m), 1639(s), 1522(s), 1480(s), 1458(s), 1436(s),1419(s), 1366(s), 1342(m), 1316(m), 1259(s), 1203(w), 1172(s),1124(m), 1064(s), 1007(s), 970(s), 932(w), 880(s), 829(s).

Synthesis of [ONNO]{Sm[N(TMS)2]2(μ-Cl)Li(THF)}2 (4). Thesynthesis of complex 4 was carried out in the same way as thatdescribed for complex 1, but [ONNO]H2 (0.78 g, 1.50 mmol) andSm[N(TMS)2]3(μ-Cl)Li(THF)3 (3.23 g, 3.00 mmol) were used.Colorless crystals were obtained from a mixture of toluene and hexanesolution (1:2 v/v) at 0 °C in several days (0.79 g, 31%). Anal. Calcdfor C66H140Cl2Li2N6O4Si8Sm2: C, 46.85; H, 8.34; N, 4.96; Sm, 17.77.Found: C, 46.43; H, 7.96; N, 4.67; Sm, 17.94. IR (KBr, cm−1):2955(s), 2871(m), 2828(m), 1636(s), 1524(s), 1482(s), 1457(s),1435(s), 1419(s), 1367(s), 1342(m), 1315(m), 1257(s), 1202(w),1172(s), 1125(m), 1066(s), 1008(s), 970(s), 933(w), 880(s), 828(s).

Synthesis of [ONNO]YN(TMS)2(THF) (5). A THF solution(15 mL) containing [ONNO]H2 (0.50 g, 0.96 mmol) was slowlyadded to a THF solution of Y[N(TMS)2]3(μ-Cl)Li(THF)3 (1.58 g,1.92 mmol, 30 mL) at room temperature. The reaction mixture wasstirred overnight, and then the solvent was removed under vacuum.The residue was washed with cold hexane twice (2 × 10 mL), and theresultant solid was dissolved in hot hexane (about 10 mL). The lesssoluble Y[N(TMS)2]3(μ-Cl)Li(THF)3 and LiCl were removed bycentrifugation as the precipitate, and the solution was cooled slowly toroom temperature. Colorless crystals were obtained in several days(0.55 g, 68%). 1H NMR (THF-d8, 400 MHz, ppm): 7.09 (d, 4J(H,H) = 2.4 Hz, 2H, Ar-H), 6.71 (d, 4J(H, H) = 2.4 Hz, 2H, Ar-H), 4.59(d, 2J(H, H) = 13.6 Hz, 2H, ArCH2), 3.78 (m, 2H, NCH2CH2N), 3.54(br, 4H, α-CH2 THF), 3.10 (d,

2J(H, H) = 13.6 Hz, 2H, ArCH2), 2.99(m, 2H, NCH2CH2N), 2.52 (m, 2H, NCH2CH2N), 1.96 (m, 2H,NCH2CH2N), 1.69 (br, 4H, β-CH2 THF), 1.38 (s, 18H, C(CH3)3),1.18 (s, 18H, C(CH3)3), 0.12 (s, 18H, TMS). 13C NMR (C6D6,

Organometallics Article

dx.doi.org/10.1021/om201164t | Organometallics 2012, 31, 3499−35113500

75 MHz, ppm): δ 159.83, 137.54, 136.21, 124.71, 124.30, 121.62(Ar-C), 69.30 (THF-α-CH2), 60.74 (ArCH2N), 50.06, 46.02 (N(CH2)2N),35.69, 34.18, 32.11, 30.80 (C(CH3)3), 25.43 (THF-β-CH2), 6.19 (TMS).Anal. Calcd for C44H78N3O3Si2Y: C, 62.75; H, 9.33; N, 4.99; Y, 10.56.Found: C, 62.55; H, 9.54; N, 4.76; Y, 10.23. IR (KBr, cm−1): 2957(s),2906(s), 2869(s), 1607(m), 1466(s), 1441(s), 1413(s), 1362(s),1305(s), 1237(s), 1202(m), 1164(m), 1134(m), 1028(w), 935(s),879(s), 833(s).Synthesis of [ONNO]SmN(TMS)2(THF) (6). The synthesis of

complex 6 was carried out in the same way as that described forcomplex 5, but [ONNO]H2 (0.87 g, 1.66 mmol) and Sm[N-(TMS)2]3(μ-Cl)Li(THF)3 (3.08 g, 3.32 mmol, 30 mL) were used.Colorless crystals were obtained from a concentrated hexane solution(about 20 mL) at room temperature in several days (0.97 g, 65%). 1HNMR (C6D6, 400 MHz, ppm): 13.24 (br, 1H), 9.55 (br, 1H), 8.56 (d,4J(H, H) = 2.4 Hz, 2H), 8.15 (m, 2H), 7.91 (d, 4J(H, H) = 2.4 Hz,2H), 3.37 (br, 4H), 2.86 (m, 2H), 1.96 (s, 18H), 1.82 (m, 2H), 1.57(br, 1H), 1.32 (br, 4H), 1.25 (br, 1H), 1.20 (m, 2H), 0.55 (s, 18H,),−0.74 (s, 18H). Anal. Calcd for C44H78N3O3Si2Sm: C, 58.49; H, 8.70;N, 4.65; Sm, 16.64. Found: C, 58.85; H, 8.33; N, 4.92; Sm, 16.30. IR(KBr, cm−1): 2955(s), 2907(s), 2867(s), 1605(m), 1467(s), 1442(s),1411(s), 1363(s), 1305(s), 1237(s), 1202(m), 1165(m), 1132(m),1028(w), 935(s), 880(s), 834(s).Synthesis of [ONNO]{Y[N(TMS)2]2}2 (7). Method A: The

synthesis of complex 7 was carried out in the same way as thatdescribed for complex 1, but [ONNO]H2 (1.00 g, 1.92 mmol) andY[N(TMS)2]3 (2.19 g, 3.84 mmol, 30 mL) were used. Colorlesscrystals were obtained from the concentrated hexane solution (about15 mL) at 5 °C in several days (1.08 g, 42%). 1H NMR (C6D6, 300MHz, ppm): 7.64 (s, 2H, Ar-H), 7.25 (s, 2H, Ar-H), 4.35 (s, 4H,ArCH2), 3.73 (m, 4H, NCH2CH2N), 2.79 (m, 4H, NCH2CH2N), 1.67(s, 18H, C(CH3)3), 1.39 (s, 18H, C(CH3)3), 0.36 (s, 72H, TMS). 13CNMR (C6D6 + THF-d8, 75 MHz, ppm): δ 159.78, 138.99, 136.49,124.91, 124.31, 122.85 (Ar-C), 56.33 (ArCH2N), 46.87 (N(CH2)2N),35.23, 34.26, 31.90, 30.36 (C(CH3)3), 5.21 (Si-C). Anal. Calcd forC58H124N6O4Si8Y2: C, 51.77; H, 9.11; N, 6.12; Y, 12.96. Found: C,51.52; H, 9.27; N, 6.24; Y, 12.84. IR (KBr, cm−1): 2956(s), 2871(m),2827(m), 1607(s), 1479(s), 1458(s), 1443(s), 1418(s), 1362(s), 1343(m),1310(s), 1239(s), 1202(s), 1164(m), 1125(s), 1006(s), 982(s), 933(w),880(s), 832(s).Method B: Complex 1 (1.22 g, 0.78 mmol) was dissolvedin toluene (30 mL), and the solution was stirred at 80 °C in an oil bathfor 1 day. The precipitate formed was removed by centrifugation fromthe solution, and then the solvent was evaporated completely underreduced pressure. Hexane (20 mL) was added to the residue, and thenthe precipate formed was removed by centrifugation again. Colorlessmicrocrystals were obtained from the concentrated hexane solution(about 15 mL) at 5 °C in several days (0.65 g, 61%).Synthesis of [ONNO]NdN(TMS)2(THF) (8). Method A: A

hot (about 60 °C) THF solution (15 mL) containing [ONNO]H2(0.77 g, 1.47 mmol) was added slowly to a THF solution ofNd[N(TMS)2]3(μ-Cl)Li(THF)3 (2.59 g, 2.94 mmol, 20 mL) in an oilbath (60 °C). The reaction mixture was stirred overnight at 60 °C,and then the solvent was removed under vacuum. The residue waswashed with cold hexane twice (2 × 10 mL), and the resultant solidwas dissolved in hot hexane (about 15 mL). The less solubleNd[N(TMS)2]3(μ-Cl)Li(THF)3 and LiCl were removed by centrifu-gation as the precipitate, and the solution was cooled slowly to roomtemperature. Blue crystals were obtained in several days (0.94 g, 71%).Anal. Calcd for C44H78N3O3Si2Nd: C, 58.89; H, 8.76; N, 4.68; Nd,16.07. Found: C, 58.50; H, 8.68; N, 4.59; Nd, 16.35. IR (KBr, cm−1):2957(s), 2905(s), 2869(s), 1606(m), 1467(s), 1441(s), 1413(s),1361(s), 1303(s), 1237(s), 1202(m), 1166(m), 1134(m), 1026(w),934(s), 879(s), 834(s). Method B: A THF solution (15 mL) con-taining [ONNO]H2 (1.53 g, 2.94 mmol) was added to a THF solutionof Nd[N(TMS)2]3(μ-Cl)Li(THF)3 (2.59 g, 2.94 mmol) at room tem-perature, and then the solvent was removed under vacuum. Theresidue was washed with cold hexane twice (2 × 10 mL), and theresultant solid was dissolved in hot hexane (about 25 mL). The lesssoluble LiCl was removed by centrifugation as the precipitate, and the

solution was cooled slowly to room temperature. Blue crystals wereobtained at room temperature in several days (2.10 g, 80%).

Synthesis of [ONNO]PrN(TMS)2(THF) (9). The synthesis ofcomplex 9 was carried out in the same way as that described forcomplex 8 (method B), but [ONNO]H2 (1.57 g, 3.00 mmol) andPr[N(TMS)2]3(μ-Cl)Li(THF)3 (2.64 g, 3.00 mmol) were used.Colorless crystals were obtained from a concentrated hexane solution(about 20 mL) at room temperature in several days (2.04 g, 76%).Anal. Calcd for C44H78N3O3Si2Pr: C, 59.10; H, 8.79; N, 4.70; Pr,15.76. Found: C, 59.25; H, 8.53; N, 4.75; Pr, 15.41. IR (KBr, cm−1):2955(s), 2905(s), 2869(s), 1606(m), 1475(s), 1441(s), 1413(s),1361(s), 1301(s), 1237(s), 1202(m), 1165(m), 1134(m), 1025(w),934(s), 880(s), 832(s).

Synthesis of [ONNO]Yb[N(SiMe2H)2](THF) (10). Method A:The synthesis of complex 10 was carried out in the same way as thatdescribed for complex 8 (method A), but [ONNO]H2 (0.61 g, 1.16mmol) and Yb[N(SiMe2H)2]3(THF)2 (1.52 g, 2.32 mmol) were used.Yellow crystals were obtained from a mixture of THF/hexane (1:5v/v) solution at 5 °C in several days (0.63 g, 61%). Anal. Calcd forC42H72N3O3Si2Yb: C, 56.28; H, 8.10; N, 4.69; Yb, 19.31. Found: C,56.32; H, 8.13; N, 4.76; Yb, 19.21. IR (KBr, cm−1): 2956(s), 2866(s),2117(s), 1605(s), 1474(s), 1458(s), 1440(s), 1413(s), 1361(s),1307(s), 1238(s), 1203(s), 1166(m), 1135(s), 1004(s), 908(s),879(s), 835(s).

Synthesis of [ONNO]Y[N(SiMe2H)2](THF) (11). Method A: Thesynthesis of complex 11 was carried out in the same way as thatdescribed for complex 8 (method A), but [ONNO]H2 (0.52 g, 1.00mmol) and Y[N(SiMe2H)2]3(THF)2 (1.26 g, 2.00 mmol, 20 mL)were used. Colorless crystals were obtained from a mixture of THF/hexane (1:5 v/v) solution at 5 °C in several days (0.54 g, 67%). 1HNMR (C6D6, 300 MHz, ppm): 7.53 (d, 4J(H, H) = 2.1 Hz, 2H, Ar-H),6.89 (d, 4J(H, H) = 2.1 Hz, 2H, Ar-H), 5.09 (m, 2H, Si-H), 4.65 (d,2J(H, H) = 13.2 Hz, 2H, ArCH2), 3.95 (br, 4H, α-CH2 THF), 3.52 (m,2H, NCH2CH2N), 2.86 (m, 2H, NCH2CH2N), 2.78 (d, 2J(H, H) =13.2 Hz, 2H, ArCH2), 1.77 (m, 2H, NCH2CH2N), 1.65 (s, 18H,C(CH3)3), 1.45 (m, 2H, NCH2CH2N, overlap with THF signal), 1.42(br, 4H, β-CH2 THF), 1.40 (s, 18H, C(CH3)3), 0.32 (d, J = 2.7, 12H,Si-CH3).

13C NMR (C6D6, 75 MHz, ppm): δ 160.05, 136.91, 136.68,125.02, 124.11, 122.35 (Ar-C), 70.31 (THF-α-CH2), 60.16 (ArCH2N),51.62, 45.28 (N(CH2)2N), 35.55, 34.19, 32.18, 30.75 (C(CH3)3),25.39 (THF-β-CH2), 5.06 (Si-C). Anal. Calcd for C42H72N3O3Si2Y: C,62.12; H, 8.94; N, 5.17; Y, 10.95. Found: C, 62.28; H, 9.01; N, 5.29; Y,10.51. IR (KBr, cm−1): 2957(s), 2867(m), 2119(s), 1605(s), 1474(s),1458(s), 1440(s), 1390(s), 1361(s), 1306(s), 1254(s), 1203(s),1166(m), 1136(s), 1004(s), 908(s), 879(s), 835(s). Method B: Thesynthesis of complex 11 was carried out in the same way as thatdescribed for complex 8 (method B), but [ONNO]H2 (0.52 g, 1.03mmol) and Y[N(SiMe2H)2]3(THF)2 (0.65 g, 1.03 mmol) were used.Colorless crystals were obtained fron a mixture of THF/hexane (1:5v/v) solution at 5 °C over several days (0.62 g, 75%).

Synthesis of [ONNO]Nd[N(SiMe2H)2](THF) (12). The synthesisof complex 12 was carried out in the same way as that described forcomplex 8 (method A), but [ONNO]H2 (1.04 g, 1.99 mmol) andNd[N(SiMe2H)2]3(THF)2 (2.62 g, 3.98 mmol) were used. Blue crystalswere obtained from a mixture of THF/hexane (1:4 v/v) solution at5 °C in several days (1.10 g, 64%). Anal. Calcd for C42H72N3O3Si2Nd:C, 58.15; H, 8.37; N, 4.84; Nd, 16.63. Found: C, 58.24; H, 8.23; N,4.91; Nd, 16.32. IR (KBr, cm−1): 2957(s), 2866(s), 2117(s), 1604(s),1474(s), 1458(s), 1441(s), 1412(s), 1361(s), 1307(s), 1237(s),1203(s), 1165(m), 1135(s), 1004(s), 908(s), 878(s), 835(s).

A Typical Polymerization Procedure. The procedures for thepolymerization of lactide initiated by complexes 1−12 were similar,and a typical polymerization procedure is given below. A 50 mLSchlenk flask, equipped with a magnetic stirring bar, was charged withthe desired amount of L-lactide and toluene. Then a toluene solutionof the initiator was added in a glovebox at room temperature. Thereaction mixture was quickly placed into an oil bath preheated at60 °C, and the solution was stirred for the prescribed time, duringwhich time an increase in viscosity was observed. The reaction mixturewas quenched by the addition of 1 M HCl/ethanol solution and then



poured into methanol to precipitate the polymer, which was driedunder vacuum and weighed.X-ray Crystallography. Suitable single crystals of complexes 1−8

and 10−12 were sealed in a thin-walled glass capillary for deter-mination of the single-crystal structure. Intensity data were collectedwith a Rigaku Mercury CCD area detector in ω scan mode using MoKα radiation (λ = 0.71070 Å). The diffracted intensities were correctedfor Lorentz polarization effects and empirical absorption corrections.Details of the intensity data collection and crystal data are given inTables 1−3.The structures were solved by direct methods and refined by full-

matrix least-squares procedures based on |F|2. All of the non-hydrogenatoms were refined anisotropically. All of the hydrogen atoms wereheld stationary and included in the structure factor calculation in thefinal stage of full-matrix least-squares refinement. The structureswere solved and refined using SHELXL-97 programs. Because of lossof the solvent molecules, the potential solvent area of 202 Å3 per unitcell (20.9% of the total cell volume calculated by the Platon program12) incomplex 10 has not been located.

■ RESULTS AND DISCUSSION

Syntheses and Characterizations of LanthanideAmido Complexes. We previously reported that the amine-elimination reaction of Yb[N(TMS)2]3(μ-Cl)Li(THF)3 withthe proligand H2[ONNO] gave a bimetallic lanthanide amidocomplex.4m Initially, we intended to explore the effect of ionicradii on this reaction. The reaction of Y[N(TMS)2]3(μ-Cl)-Li(THF)3 with the proligand H2[ONNO] was thereforeconducted, because ytterbium and yttrium have similar ionicradii. To our surprise, the 1H NMR monitoring reactionrevealed that the reaction of Y[N(TMS)2]3(μ-Cl)Li(THF)3with 0.5 equiv of H2[ONNO] in C6D6 at room temperaturetook place rapidly and after a few minutes afforded the mono-metallic yttrium amido complex [ONNO]Y[N(TMS)2](THF)as a neat product, with the elimination of HN(TMS)2.

No bimetallic species was formed. On a preparative scale, ahot THF solution of H2[ONNO], because of its poor solubilityeven in THF, was added to a THF solution of Y[N(TMS)2]3-(μ-Cl)Li(THF)3 in a 1:2 molar ratio. After workup, the bime-tallic species [ONNO]{Y[N(TMS)2]2(μ-Cl)Li(THF)}2 (1) wasisolated and was confirmed by X-ray structure determina-tion. These results indicate that the reaction temperature playsa crucial role in controlling the formation of bi- and mono-metallic lanthanide amido complexes. A series of bimetalliclanthanide bisamido complexes [ONNO]{Ln[N(TMS)2]2(μ-Cl)Li(THF)}2 [Ln = Er (2), Eu (3), Sm (4)] can be syn-thesized as colorless (for Sm), pink (for Er), and orange (forEu) microcrystals in moderate isolated yields (31−43%) by thereactions of H2[ONNO] in a hot THF solution (about 60 °C)with Ln[N(TMS)2]3(μ-Cl)Li(THF)3 in a 1:2 molar ratio, asshown in Scheme 1. However, the same reactions conducted atroom temperature gave the neutral monometallic bis(phenolate)lanthanide amido complexes [ONNO]Ln[N(TMS)2](THF) [Ln =Y (5), Sm (6)] in good isolated yields, as shown in Scheme 2. It isworth noting that preheating the THF solution of the proligandis crucial. If a cold solution of the proligand is added to a hotsolution of Ln[N(TMS)2]3(μ-Cl)Li(THF)3, only the mono-metallic lanthanide amido complex can be isolated, because theexchange reaction is very rapid. There is no equilibrium be-tween the bimetallic and monometallic lanthanide amido com-plexes. The monometallic complexes cannot be transformedinto bimetallic complexes in the presence of Ln[N(TMS)2]3(μ-Cl)Li(THF)3 at 60 °C, which is different from those observedin the aluminum systems.13 The neutral bimetallic yttrium amidocomplex [ONNO]{Y[N(TMS)2]2}2 (7) was isolated when atoluene solution of the anionic bimetallic yttrium amido com-plex 1 was heated to 80 °C for 1 day. Complex 7 can also beprepared by the reaction of H2[ONNO] with Y[N(TMS)2]3 in

Table 1. Crystallographic Data for Complexes 1−4

1 2·2toluene 3·2toluene 4·2toluene

formula C66H140Cl2Li2N6O4Si8Y2 C80H156Cl2Er2Li2N6O4Si8 C80H156Cl2Eu2Li2N6O4Si8 C80H156Cl2Li2N6O4Si8Sm2

fw 1569.16 1910.13 1879.53 1876.31T (K) 223(2) 223(2) 223(2) 223(2)cryst syst triclinic monoclinic monoclinic monoclinicspace group P1 ̅ P21/n P21/n P21/ncryst size (mm3) 0.58 × 0.36 × 0.30 0.40 × 0.30 × 0.15 0.23 × 0.21 × 0.20 0.60 × 0.58 × 0.32a (Å) 11.6295(11) 16.669(3) 16.779(3) 16.8041(10)b (Å) 12.8313(7) 12.1728(19) 12.1559(19) 12.1432(7)c (Å) 16.4147(12) 25.639(4) 25.691(4) 25.6713(16)α (deg) 75.220(7)β (deg) 88.127(8) 99.955(3) 99.650(3) 99.651(2)γ (deg) 79.826(7)V (Å3) 2330.9(3) 5124.0(14) 5165.9(15) 5164.2(5)Z 1 2 2 2Dcalcd (g cm−3) 1.118 1.238 1.208 1.207μ (mm−1) 1.439 1.815 1.390 1.313F(000) 840 1996 1976 1972θmax (deg) 25.50 25.50 27.46 25.50collected reflns 18 961 28 408 28 779 25 848unique reflns 8596 9475 11766 9580obsd reflns [I > 2.0σ(I)] 6528 8403 9097 8472no. of variables 408 468 468 463GOF 1.068 1.095 1.168 1.135R 0.0618 0.0589 0.0859 0.0481wR 0.1326 0.1308 0.1418 0.1065largest diff peak, hole (e Å−3) 0.738, −0.557 0.821, −0.730 0.696, −0.844 0.873, −0.932



a 1:2 molar ratio at 60 °C. Similarly, complex 5 can be obtainedby conducting the same reaction at room temperature, as shownin Scheme 3.However, for the larger lanthanide metals, neodymium

and praseodymium, the characterized products were the

monometallic lanthanide amido complexes [ONNO]LnN-(TMS)2(THF) [Ln = Nd (8), Pr (9)], even when the reac-tions were conducted at 60 °C, as shown in Scheme 4, whichindicates that the ionic radii of the lanthanide metals also have asignificant effect on the outcome of this reaction.


10 11 12·0.5THF

formula C42H74N3O3Si2Yb C42H74N3O3Si2Y C44H78N3O3.5Si2Ndfw 898.26 814.13 905.51T (K) 223(2) 223(2) 223(2)cryst syst monoclinic monoclinic orthorhombicspace group P21/n P21/n Pbcncryst size (mm3) 0.50 × 0.40 × 0.20 0.60 × 0.35 × 0.14 0.45 × 0.28 × 0.15a (Å) 15.2615(11) 15.3128(14) 22.5843(10)b (Å) 14.3842(11) 14.4084(12) 15.7763(7)c (Å) 21.9011(15) 21.982(2) 27.2441(13)β (deg) 95.566(2) 95.892(3)V (Å3) 4785.2(6) 4824.3(8) 9707.0(8)Z 4 4 8Dcalcd (g cm−3) 1.247 1.121 1.239μ (mm−1) 2.040 1.294 1.158F(000) 1876 1752 3832θmax (deg) 25.50 25.50 25.50collected reflns 24 106 24 350 42 612unique reflns 8864 8934 8992obsd reflns [I > 2.0σ(I)] 8070 6527 7959no. of variables 448 448 484GOF 1.140 1.099 1.258R 0.0504 0.0918 0.0901wR 0.1049 0.2081 0.1844largest diff peak, hole (e Å−3) 0.949, −0.965 0.864, −0.651 1.004, −0.921


5 6 7 8

formula C44H78N3O3Si2Y C44H78N3O3Si2Sm C70H152N6O4Si8Y2 C44H78N3O3Si2Ndfw 842.18 903.62 1512.52 897.51T (K) 223(2) 223(2) 223(2) 223(2)cryst system monoclinic monoclinic triclinic monoclinicspace group P21/c P21/c P1̅ P21/ccryst size (mm3) 0.50 × 0.40 × 0.20 0.60 × 0.40 × 0.30 1.00 × 0.80 × 0.50 0.60 × 0.40 × 0.20a (Å) 14.9649(15) 15.1517(18) 12.0450(19) 15.2427(12)b (Å) 28.896(3) 28.820(4) 14.6328(15) 28.7880(19)c (Å) 11.8985(12) 11.9193(15) 15.158(2) 11.9569(10)α (deg) 67.361(15)β (deg) 110.514(2) 110.833(4) 79.056(17) 111.109(2)γ (deg) 68.957(14)V (Å3) 4818.9(8) 4864.6(10) 2297.2(6) 4894.7(6)Z 4 4 1 4Dcalcd (g cm−3) 1.161 1.234 1.093 1.218μ (mm−1) 1.297 1.294 1.400 1.147F(000) 1816 1908 820 1900θmax (deg) 25.50 25.50 25.50 25.50collected reflns 23 804 24 993 19 547 24 661unique reflns 8938 9034 8473 9094obsd reflns [I > 2.0σ(I)] 6637 7637 5276 7313no. of variables 467 455 381 473GOF 1.098 1.164 0.998 1.154R 0.0766 0.0686 0.0780 0.0641wR 0.1536 0.1312 0.2028 0.1217largest diff peak, hole (e Å−3) 1.193, −1.050 1.726, −0.963 1.582, −0.868 0.695, −0.886



On the basis of the effects of ionic radii on this reaction,we thought that it might be possible to drive the reactionto bimetallic lanthanide amido complexes using the lessbulky amides Ln[N(SiMe2H)2]3(THF)2, instead of the bulkyLn[N(TMS)2]3, as the starting material. The reaction ofYb[N(SiMe2H)2]3(THF)2 with H2[ONNO] in a 2:1 molarratio at 60 °C in THF was therefore carried out. However,only the monometallic lanthanide amido complex [ONNO]-Yb[N(SiMe2H)2](THF) (10) was isolated, which was con-firmed by X-ray structure determination. This result revealedthat less bulky amido groups cannot stabilize the bimetallic

lanthanide amido complexes. As expected, for yttrium andneodymium, only the monometallic lanthanide amido com-plexes [ONNO]Ln[N(SiMe2H)2](THF) [Ln = Y (11), Nd(12)] were isolated in good yields (61−75%), from a THF/hexane (1:5 v/v) mixed solution, as shown in Scheme 5.These complexes gave satisfactory elemental analyses, and

their IR spectra showed the characteristic absorptions of thebis(phenolato) ligands. Complexes 1, 5, 6, 7, and 11 were alsocharacterized by NMR spectroscopy. For the bimetalliclanthanide amido complexes 1 and 7, the 1H NMR spectrashow one set of single resonances for the TMS protons and the

Scheme 2

Scheme 3

Scheme 4

Scheme 1



http://pubs.acs.org/action/showImage?doi=10.1021/om201164t&iName=master.img-001.png&w=340&h=82




benzylic methylene units, consistent with a flexible bimetallicstructure. For the monometallic lanthanide amido complexes 5and 11, the 1H NMR spectra in d8-THF (for 5) and C6D6(for 11) show that the complexes have Cs symmetry, with thephenoxy group in the trans configuration. This is indicated bythe symmetry-related phenolate rings, as well as the 1H AB spinsystems for the two benzylic methylene units (δ 3.10 and 4.59ppm for 5, 2.78 and 4.65 ppm for 11) and the single resonancefor the silylamido groups (δ 0.12 ppm for 5 and 0.32 ppm for11). The splitting peaks for the piperazidine ring group werefound at δ 3.78, 2.99, 2.52, and 1.96 ppm for 5 and δ 3.52, 2.86,1.77, and 1.45 ppm for 11, which are different from thosein the bimetallic lanthanide amido complexes 1 and 7. Theseresults are in accordance with the relatively rigid structures ofthe monometallic lanthanide complexes and confirm that thesix-coordinated geometry observed in the solid state is retainedin solution. In addition, the resonances of the coordinatedTHF were found at δ 3.54 and 1.69 ppm for 5 and δ 3.95 and1.42 ppm for 11. The 1H NMR spectrum of complex 4 is com-plicated, and complexes 2, 3, 8, 9, 10, and 12 did not provideany resolvable 1H NMR spectra, because of the strong para-magnetism of the lanthanide ions. The definitive molecularstructures of these complexes were confirmed by single-crystalstructure determination.14 These complexes are extremely sen-sitive to air and moisture. The crystals decompose in a fewminutes when they are exposed to air, but neither the crystalsnor their solutions showed any sign of decomposition afterseveral months when stored under argon.Complexes 1−4 are isomorphous, but they crystallize in dif-

ferent crystal systems. Complex 1 crystallizes in the triclinicsystem, whereas complexes 2−4 crystallize in the monoclinicsystem. These complexes have solvated centrosymmetriclanthanide−lithium heterotetranuclear structures. The over-all molecular structure is similar to that of [ONNO]{Yb[N-(TMS)2]2(μ-Cl)Li(THF)}2.

4m Therefore, only the ORTEPdiagram of complex 1 is shown in Figure 1. As expected, thepiperazidine ring adopts the chair conformation in these com-plexes. Each of the lanthanide atoms is coordinated with twonitrogen atoms from two amido groups, one oxygen atomfrom an aryloxo group, and one chlorine atom to form a four-coordinated distorted tetrahedral geometry. The lanthanideatom and the lithium atom are connected by an aryloxo oxygenatom and a chlorine atom, and the lithium atom is furthercoordinated with one nitrogen atom from the bis(phenolato)ligand and one oxygen atom from a THF molecule. TheLn−O(Ar) bond lengths, 2.140(3), 2.124(4), 2.199(6), and2.206(3) Å in complexes 1−4, respectively, are comparablewith the corresponding bond lengths in bridged bis(phenolato)lanthanide complexes.15 It is worth noting that there are πinteractions between the carbon atoms of the arene rings of thearyloxo groups and the lithium atoms, but not the lanthanideatoms, in complexes 1−4. There are relatively strong interac-tions between Li1 and C1. The bond lengths of Li1−C1 in

these complexes range from 2.504(13) to 2.564(8) Å, which areshorter than the range of π interactions of lithium ions to arenerings reported in the literature (2.31−2.57 Å).16 Additionalweaker contacts between Li1 and C6 are also observed. TheLi1−C6 bond lengths, ranging from 2.745(13) to 2.769(9) Å,are comparable with the values in LiCu(C6H3Mes2-2,6)2(2.704(10) and 2.780(12) Å).16a

Complexes 5, 6, 8, and 9 are isomorphous and have asolvated monomeric structure. Only the ORTEP diagram ofcomplex 5 is therefore shown in Figure 2. The overall molec-ular structures of complexes 5, 6, 8, and 9 are similar to those ofthe piperazidine-bridged bis(phenolato) lanthanide alkylcomplexes.4n The boat conformation of the piperazidine ringin these complexes, which is different from that in the bimetalliccomplexes 1−4, results from the formation of monometalliclanthanide amido complexes. Two nitrogen atoms from thepiperazidine ring were found to bind to the same metal centerin the solid state. The central lanthanide metal atom is six-coordinated by two oxygen atoms, two nitrogen atoms from thebis(phenolato) ligand, one nitrogen atom from the amidogroup, and one oxygen atom from the THF molecule. Thecoordination geometry around the central metal can best bedescribed as a distorted trigonal bipyramid, if the η2-coordinated piperazidine ring is considered to occupy onecoordination site. The average Ln−O(Ar) bond lengths rangefrom 2.223(9) (for Nd) to 2.148(8) Å, which reflects the usuallanthanide contraction from Nd3+ to Y3+.17 The Ln−N(TMS)2bond lengths range from 2.322(4) (for Y) to 2.407(4) Å (forNd), which are also in accordance with those of the bridged

Figure 1. ORTEP diagram of complex 1 showing the atom-numberingscheme. Thermal ellipsoids are drawn at the 20% probability level, andhydrogen atoms and methyl groups on Si atoms are omitted for clarity.Complexes 2−4 are isomorphous with complex 1.

Scheme 5





bis(phenolato) lanthanide amido complexes mentioned above,when the difference in ionic radii is considered.3c,4l

The ORTEP diagram of complex 7 is shown in Figure 3.Complex 7 has a centrosymmetric dinuclear structure, and the

overall molecular structure is similar to that of the previouslyreported [ONNO]{Yb[N(TMS)2]2}2.

4m Each yttrium atom iscoordinated with one oxygen atom and one nitrogen atom fromthe bridged bis(phenolato) ligand and with two nitrogen atomsfrom two amido groups to form a distorted tetrahedral geometry.The Y−O(Ar), Y−N(piperazidine), and average Y−N(amido)bond lengths are 2.076(3), 2.531(4), and 2.244(9) Å, respec-tively, which are similar to those in complex 1.Complexes 10−12 are isomorphous, but they crystallize in

different crystal systems. Complex 12 crystallizes in the ortho-rhombic system, whereas complexes 10 and 11 crystallize in themonoclinic system. Only the ORTEP diagram of complex 11 istherefore shown in Figure 4. The overall molecular structuresof complexes 10−12 are similar to those of the monometalliccomplexes 5, 6, 8, and 9. The Ln−N(HSiMe)2 bond lengths

range from 2.256(5) (for Yb) to 2.395(6) Å (for Nd), whichare also comparable to those in the monometallic complexes,when the difference in ionic radii is considered.

Ring-Opening Polymerization of L-Lactide by Com-plexes 1−12. Biodegradable polymers have recently receivedgreat attention, because of their biodegradability/biocompati-bility and their practical applications. Among the new bio-degradable polymers that have been developed during the pastdecade, poly(ε-caprolactone) and poly(lactide) are the moststudied and desirable.18 The major method used to synthesizethese polymers is the ring-opening polymerization of lactones/lactides and functionally related compounds. Many kinds oforganometallic and coordination complexes have been reportedto be efficient initiators for the ring-opening polymerization ofthese monomers, to give the polymers with both high molec-ular weights and narrow molecular weight distributions.19 Withthe organolanthanide amido complexes in hand, their catalyticbehaviors for the ring-opening polymerization of L-lactide wereexamined.It was found that all of these lanthanide amido complexes can

effectively initiate L-lactide polymerization, and the results aresummarized in Table 4. The polymerization medium played animportant role in influencing the polymerization, and toluenewas a better solvent than THF. When complex 3 was used asthe initiator, the yield in toluene reached 98% in less than30 min at 60 °C and gave a polymer with a high molecularweight (entry 7), whereas the yield was 83% in THF under thesame polymerization conditions and the resultant polymer hada relatively low molecular weight (entry 5). Thus, all of thepolymerizations were carried out in toluene. The solvent effectin our case is in contrast to those observed in dithiaalkanediyl-bridged bis(phenolato)5a and alkoxyamino-bridged bis-(phenolato) rare-earth metal amido systems.4e

The ionic radius of the central metal ion has a significanteffect on the catalytic activity for L-lactide polymerization. Theobserved orders of decreasing activities for bimetallic lantha-nide amides, Sm > Eu > Er ≈ Y, and monometallic lanthanideamides, Pr > Nd > Sm > Y > Yb, are in agreement with theorder of the ionic radii, which are also consistent with those ob-served for amine-bridged bis(phenolato) lanthanide systems.4l,n,o

These results may be attributed to the larger ionic radii, resultingin greater opening of the metal coordination sphere in thevicinity of the σ ligand, making the coordination and insertionof L-lactide into the Ln−N bonds easier.

Figure 2. ORTEP diagram of complex 5 showing the atom-numberingscheme. Thermal ellipsoids are drawn at the 20% probability level, andhydrogen atoms are omitted for clarity. Complexes 6 and 8 areisomorphous with complex 5.

Figure 3. ORTEP diagram of complex 7 showing the atom-numberingscheme. Thermal ellipsoids are drawn at the 20% probability level, andhydrogen atoms are omitted for clarity.

Figure 4. ORTEP diagram of complex 11 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 20%probability level, and hydrogen atoms are omitted for clarity.Complexes 10 and 12 are isomorphous with complex 11.






The activities of the neutral bimetallic piperazidine-bridgedbis(phenolato) lanthanide amido complexes are much higherthan those of the anionic complexes. For example, using com-plex 1 as the initiator, a 62% yield can be obtained within 2 h at60 °C when the molar ratio of monomer to metal is 4000:2,whereas using complex 7 as the initiator, the yield is as highas 84% after 1 h under the same polymerization conditions(Table 4, entries 2 and 13). These results can be attributed tothe less steric congestion around the metal center in the neutralbimetallic lanthanide amide.On the other hand, the bimetallic lanthanide amido com-

plexes display significantly higher activities than the corre-sponding monometallic lanthanide amides. The reason for theapparent enhancement in activity for the bimetallic lanthanideamides is still unclear. We postulate that the difference ori-ginates from the lower steric crowding around the lanthanidemetals in the bimetallic systems. It was also found that theactivities of the less bulky monometallic lanthanide amidocomplexes [ONNO]Ln[N(SiMe2H)2](THF) [Ln = Yb (10), Y(11), Nd (12)] were higher than those of [ONNO]LnN-(TMS)2(THF) [Ln = Y (5), Sm (6), Nd (8), Pr (9)]. For

example, using complex 12 as the initiator, 99% yield can beobtained within 10 min at 60 °C when the molar ratio ofmonomer to metal is 3000:1, whereas the yield is only 85%after 3 h using complex 8 as the initiator, even when the molarratio of monomer to initiator decreases to 2000:1 (Table 4,entries 21 and 31).Kinetic studies using complex 5 as initiator have been

investigated at various concentrations of complex 5 (3, 5,10 mM, respectively) with respect to a constant concentrationof [LA] (1 M) in toluene at 60 °C ([LA]0/[Y]0 = 100, 200,300, respectively). The semilogarithmic plots are shown inFigure 5a. In each case, the plot of ln{[LA]0/[LA]t} versus time

(s) is linear, indicating that the polymerization proceeds withfirst-order dependence on monomer concentration. Thus, therate of polymerization may be written as −d[LA]/dt = kapp[LA],where kapp = kp[Y]0

x, in which kp is the propagation rate constant.To determine the order in lanthanide complex (x), ln kappversus ln[Y]0 was plotted (Figure 5b). The linear relationshipwith a slope of ca. 0.507 ± 0.01 reveals that the order in theinitiator is 0.5. Therefore, the polymerization rate constant, kp,is 8.67 L mol−1 s−1 when [LA]0/[Y]0 is 300:1.The analysis of the kinetics for L-lactide polymerization initiated

by the bimetallic lanthanide amide complex 1 was also studiedin toluene under dilute conditions because of its high activity([LA]0 = 0.5 M, [1]0 = 0.5−0.83 mM, [LA]0/[Y]0 = 300−500)

Table 4. Polymerization of L-Lactide by Complexes 1−12a

entry initiator [LA]:[Ln] tyield(%)b

Mcc

(×104)Mn

d

(×104) PDId

1 1 2000:2 30 min 98 7.06 20.38 1.662 1 4000:2 2 h 62 8.94 23.62 1.683 2 2000:2 30 min 96 6.92 19.99 1.714 2 4000:2 2 h 68 9.79 16.07 1.655e 3 1000:2 30 min 83 3.00 8.70 1.786f 3 1000:2 2 h 54 1.96 4.72 1.427 3 1000:2 30 min 98 3.54 9.55 1.528 3 4000:2 30 min 98 14.12 22.09 1.669 3 8000:2 2 h 80 23.05 23.74 1.4610 4 8000:2 1 h 97 27.94 37.84 1.6911 4 10 000:2 2 h 78 28.09 34.61 1.6212 7 3000:2 1 h 88 9.51 24.22 1.5813 7 4000:2 1 h 84 12.11 27.92 1.5714 7 5000:2 13 h 40 7.21 17.11 1.5415 5 300:1 30 min 95 4.11 4.98 1.5216 5 600:1 3 h 74 6.40 5.35 1.4917 6 800:1 1 h 98 11.30 10.75 1.4618 6 1000:1 3 h 87 12.54 10.84 1.4819 8 500:1 30 min 98 7.07 7.86 1.5920 8 1000:1 1 h 96 13.83 12.30 1.6221 8 2000:1 3 h 85 24.49 13.53 1.5522 9 2000:1 1 h 99 28.52 20.86 1.5123 9 2500:1 3 h 79 28.45 18.95 1.4724 10 500:1 4 min 92 6.63 15.97 1.3525f 10 500:1 2 min 97 6.99 13.04 1.6526 10 500:1 2 h 88 6.35 8.97 1.3027g 10 1500:1 10 min 97 20.96 24.91 1.6728 10 2000:1 3 h 46 13.26 16.23 1.5829 11 2000:1 10 min 98 28.23 36.90 1.6630 11 3000:1 3 h 54 23.34 41.38 1.7531 12 3000:1 10 min 99 42.78 29.53 1.5332 12 4000:1 20 min 76 43.79 33.91 1.64

aGeneral polymerization conditions: in toluene, [L-LA] = 1 mol/L,60 °C. bYield: weight of polymer obtained/weight of monomer used.cMc = (144.13) × [LA]/[silylamido group] × (polymer yield) (%).dMeasured by GPC in THF calibrated with standard polystyrenesamples and corrected using the Mark−Houwink factor of 0.58. eInTHF, 60 °C. fIn toluene, 25 °C. gIn THF, 25 °C.

Figure 5. (a) Linear plots of ln{[LA]0/[LA]t} versus time in toluene at60 °C using complex 5 as initiator, [LA]0 = 1 M, [LA]0/[Y]0 = 100(■), 200 (●), 300 (▲). For plot I, kapp = 0.881 s−1 (linear fit, R =0.999); plot II, kapp = 0.628 s−1 (linear fit, R = 0.998); and plot III, kapp =0.475 s−1 (linear fit, R = 0.999). (b) Linear plot of ln kapp versusln[Y]0 for the polymerization of L-lactide with complex 5 as initiator(toluene, 60 °C, [LA]0 = 1 M).




(Figure 6). The semilogarithmic plots for several polymer-izations are shown in Figure 6a. In each case, first-order kinetics

in monomer concentration was observed. Thus, the rate ofpolymerization may be written as −d[LA]/dt = kapp[LA]. Fromthe plot of ln kapp versus ln[Y]0, a slope of 0.94 ± 0.31 wasobtained (Figure 6b). Therefore, the polymerization of L-lactideby complex 1 follows a first-order dependence on yttrium con-centration with kp = 404.79 L mol−1 min−1 (6.75 L mol−1 s−1)when [LA]0/[Y]0 is 300:1.To gain some insights into the polymerization mechanism

with these lanthanide amido complexes, a 1H NMR monitoringreaction of complex 5 with L-LA was carried out. Treatment ofcomplex 5 with 20 equiv of L-lactide in C6D6 at 60 °C revealedthat, after 30 min, the signal of the −N(SiMe3)2 group at 0.34ppm in 5 disappeared and a set of signals in the region 0.08−0.10 ppm was observed, which can be assigned to the endgroup of −CO−N(SiMe3)2 (Figure 7). These result revealedthat the silylamido group acted as the initiating group in thepolymerization of L-lactide with the monometallic lanthanideamido complex as initiator. For the bimetallic lanthanide amidocomplex 1, a similar 1H NMR monitoring reaction in d8-THFat room temperation revealed that the signal of −N(SiMe3)2groups at 0.13 ppm in complex 1 disappeared upon addition of20 equiv of L-LA and a set of signals in the region of 0.05−0.10ppm was observed, which can also be assigned to the end groupof −CO−N(SiMe3)2 (Figure 8). On the basis of this result, it is

reasonable to postulate that all of the amido groups participatedin the initiation reaction during the polymerization of L-LA. Themolecular weights determined by GPC are apparently superiorto the calculated ones for the bimetallic systems, which can be

Figure 6. (a) Linear plots of ln{[LA]0/[LA]t} versus time in toluene at60 °C using complex 1 as initiator, [LA]0 = 0.5 M, [LA]0/[Y]0 = 300(■), 400 (●), 500 (▲). For plot I, kapp = 0.676 min−1 (linear fit, R =0.997); plot II, kapp = 0.586 min−1 (linear fit, R = 0.998); and plot III,kapp = 0.409 min−1 (linear fit, R = 0.999). (b) Linear plot of ln kappversus ln[Y]0 for the polymerization of L-lactide with complex 1 asinitiator (toluene, 60 °C, [LA]0 = 0.5 M).

Figure 7. 1H NMR trace spectra (400 MHz, C6D6) of reactionbetween complex 5 and L-LA ([LA]/[5] = 20, at 60 °C).

Figure 8. 1H NMR trace spectra (400 MHz, d8-THF) of reactionbetween complex 1 and L-LA ([LA]/[1] = 20, at 25 °C).






attributed to the lower initiator efficiency. These observationsare consistent with those in the ε-caprolactone polymerizationinitiated by the group 4 metal alkoxides.20

Ring-Opening Polymerization of rac-Lactide byComplexes 1, 3, 5, 7, and 11. Stereoselective rac-lactidepolymerization has received a significant amount of interest inboth academic and industrial research in recent years. Thepolymerization is extremely sensitive to the catalyst structure,because the environment of the last unit of the propagatingactive species must be sterically appropriate for incorporation ofthe configurationally opposite enantiomer.4g It has been foundthat amine-bridged bis(phenolato) lanthanide derivatives,including alkyl,4g,n amide,4a,r,5b alkoxide,4e aryloxide,4o andborohydride,4c,q can initiate the heteroselective polymerizationof rac-lactide, giving polymers with medium to very hightacticity, depending on the ancillary ligands. To furtherelucidate the effects of the structures of the ancillary ligandsof lanthanide-based catalysts on the polymerization stereo-selectivity, the catalytic behaviors of complexes 1, 3, 5, 7, and11 in the ring-opening polymerization of rac-lactide were alsotested. The results are summarized in Table 5.It can be seen that these complexes are efficient initiators for

the ring-opening polymerization of rac-lactide. The polymer-izations proceed smoothly in THF at 25 °C and gave polymerswith high molecular weights and relatively narrow molecularweight distributions (Mw/Mn = 1.42−1.82). The size of thecentral metal ion has a significant effect on the catalytic activityin rac-lactide polymerization. The increasing order of theobserved activities is in agreement with the order of the ionicradii and is also consistent with the orders observed in theL-lactide polymerizations mentioned above. However, all of the

complexes showed moderate stereoselectivity for rac-lactidepolymerization. The Pr values of the resultant polymers rangedfrom 0.51 to 0.68. The stereocontrollabilities of these lantha-nide amides are similar to those of piperazidine-bridged bis-(phenolato) yttrium alkyl complexes,4n but inferior to those ofamine-bridged bis(phenolato) lanthanide complexes.4g,o Wenoticed that Cui and her co-workers reported that the co-ordination geometry around the metal center plays a criticalrole in governing the stereoselectivity of rac-lactide polymer-ization.4g However, in our cases, differences in stereo-selectivities among the bimetallic and monometallic lanthanidecomplexes as the initiators were not obvious, although theircoordination environments around the lanthanide metals arequite different. The polymerization medium has an apparenteffect on the stereoselectivity for rac-lactide polymerization. Forinstance, using complex 3 as the initiator, the Pr value of theresultant polymer is 0.63 in THF, whereas the value is 0.49 intoluene under the same polymerization conditions (Table 5,entries 6 and 7). This solvent effect is consistent with thosereported in the literature.21−25

■ CONCLUSION

In conclusion, a series of bimetallic and monometallic lanth-anide amido complexes stabilized by piperazidine-bridgedbis(phenolato) ligands were synthesized via controlled amine-elimination reactions, and the factors controlling the con-formations of the piperazidine ring in these reactions wereexplored. The reaction temperature plays a crucial role in theformation of bimetallic and monometallic lanthanide com-plexes, and the ionic radii of the lanthanide metals and thebulkiness of the amido groups also have significant effects on

Table 5. Polymerization of rac-Lactide by Complexes 1, 3, 5, 7, and 11a

entry initiator [LA]:[Ln] t yield (%)b Mcc (×104) Mn

d (×104) PDId Pre

1 1 500:2 2 h 97 1.75 3.99 1.73 0.582 1 1000:2 3 h 93 3.35 8.25 1.67 0.633 1 1500:2 3 h 93 5.02 12.68 1.60 0.624 1 2000:2 3 h 53 3.82 9.35 1.75 0.595 1 3000:2 5 h 46 4.97 11.73 1.73 0.566 3 500:2 2 h 93 1.52 7.46 1.70 0.637f 3 500:2 2 h 37 0.67 3.96 1.81 0.498 3 800:2 2 h 93 2.68 11.66 1.82 0.639 3 1500:2 3 h 100 5.40 19.98 1.72 0.6110 3 2000:2 3 h 90 6.48 21.10 1.72 0.6311 3 3000:2 3 h 97 10.48 25.02 1.76 0.6612 3 5000:2 3 h 96 17.28 33.89 1.67 0.6413 3 7000:2 3 h 24 6.0514 7 1000:2 3 h 99 3.57 6.57 1.80 0.5115 7 2000:2 13 h 68 4.90 10.50 1.81 0.5316f 5 300:1 1 h 18 0.7817 5 300:1 1 h 98 4.23 6.40 1.52 0.6518 5 500:1 2 h 90 6.48 10.35 1.42 0.6619 5 800:1 2 h 65 7.49 7.69 1.59 0.6620 5 1000:1 3 h 41 5.91 6.96 1.70 0.6821 11 500:1 2 h 100 7.21 8.31 1.70 0.5322f 11 500:1 1 h 97 6.99 6.36 1.58 0.4023 11 1000:1 3 h 85 12.25 8.55 1.57 0.5524 11 1500:1 13 h 25 5.40 3.13 1.69 0.58

aGeneral polymerization conditions: THF as the solvent, [rac-LA] = 1 mol L−1, 25 °C. bYield: weight of polymer obtained/weight ofmonomer used. cMc = (144.13) × [LA]/[silylamido group] × (polymer yield) (%). dMeasured by GPC calibrated with standard polystyrenesamples and corrected using the Mark−Houwink factor of 0.58. eMeasured by homodecoupling 1H NMR spectroscopy at 20 °C in CDCl3.fIn toluene, 25 °C.



the reaction outcomes. These lanthanide amides are highly effi-cient initiators for the ring-opening polymerization of lactides.NMR monitoring reactions revealed that the ring-opneingpolymerization of lactides proceeded via a coordination−insertion mechanism, and all of the four amido groups in thebimetallic systems participated in the initiation reaction duringlactide polymerization. These results provided new insights intothe design and synthesis of highly efficient lanthanide-basedcatalysts for homogeneous catalysis.

■ ASSOCIATED CONTENT*S Supporting InformationThis material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Fax: (86)512-65880305. Tel: (86)512-65882806. E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support from the National Natural Science Founda-tion of China (Grants 20972108, 21174095, and 21132002),PAPD, and the Qing Lan Project is gratefully acknowledged.

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control of conformations of piperazidine-bridged bis(phenolato) groups: syntheses and structures of...

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