Green Chemistry Dynamic Article Links

Cite this: Green Chem., 2011, 13, 2701

www.rsc.org/greenchem COMMUNICATION

Clean and rapid synthesis of double metal cyanide complexes usingmechanochemistry†

Wuyuan Zhang, Lingbin Lu,* Yi Cheng, Nai Xu, Lisha Pan, Qiang Lin* and Yiyun Wang

Received 16th May 2011, Accepted 19th July 2011DOI: 10.1039/c1gc15557a

A novel and green chemistry technique is presented forthe synthesis of double metal cyanide (DMC) complexesthat are further used for the copolymerization of CO2

and propylene oxide. The catalytic activity and selectivityare improved by this strikingly efficient and practicallyapplicable method. The mechanosynthesis of DMC revealsclear merits.

Rapidly increasing energy demands require new strategies,such as economically and environmentally acceptable synthesis,for the production and utilization of energy.1 The interest inusing mechanochemical reactions to perform chemical synthesesis growing.2 Mechanochemical methods, such as solvent-freegrinding, liquid-assisted grinding (LAG) or ion- and liquid-assisted grinding (ILAG), have shown significant potential forthe clean and energy efficient construction of molecules andmaterials.2j–n On the other hand, issues with security of supply,cost and environmental impact have driven the development ofalternative polymer syntheses using renewable resources. CO2

represents an attractive co-monomer and the chemical fixationof it into polymerization processes is of great interest.3 Oneof the most promising reactions in this area is the alternatingcopolymerization of CO2 and epoxides to generate aliphaticpolycarbonates, which provide potentially and hopefully envi-ronmental advantages due to their biodegradability.4 However,these polymerization schemes of CO2 and epoxides that use CO2

have suffered from catalysts that are not sufficiently efficient to beeconomical. Thus, much effort has been devoted to establishingmore efficient catalysts and more convenient methods for cata-lyst syntheses. Among the developed catalyst systems,5 doublemetal cyanide (DMC) complexes are well-known catalysts forthe effective utilization of CO2.6 However, tedious procedures ofDMC preparation in conventional solvent-based methods aretime-, energy- and materials-consuming. Herein, we report agreen, strikingly efficient and practically applicable techniquefor DMC synthesis under mechanochemical conditions: wereport our results on further explorations of DMC synthesis

Department of Materials and Chemical Engineering, Hainan University,Haikou, China. E-mail: lulingbin@126.com, linqiang_1962@163.com† Electronic supplementary information (ESI) available: Detailed char-acterization of catalysts and polymers. See DOI: 10.1039/c1gc15557a

by grinding solid reactants with small amounts of liquid (LAG)in the complete absence of added solvent.

This research was inspired by the activation of chemicalreactions using mechanochemistry.7 Reactions can be readilyactivated without or with minimal solvent in a ball mill. To takeadvantage of this mechanochemical method, we first appliedLAG to synthesize one of the DMCs, Zn3[Fe(CN)6] (complex1), which was prepared by the straightforward grinding ofZnCl2 and K3Fe(CN)6 with a liquid. To find the appropriategrinding conditions for complex 1, grinding time and wearenergy are observed when taken as the control parametersbased on the kinetic model.8 Previous work has indicatedthat the strong solvent-free wear energy by placing two12.70 mm and four 6.35 mm steel balls would contribute a highcatalytic activity to complex 1.6a We explored and confirmedthe wear energy by placing one 12.70 mm and two 6.35 mmsteel balls under LAG conditions. Existing literature shows thatDMCs with a high catalytic activity should be substantiallyamorphous structures.9 Introducing optimized tertiary butylalcohol (tBuOH) complexing agent into DMC is an efficient wayto obtain such an amorphous structure.10 Herein, two pathwayswere attempted to introduce tBuOH into the structure of DMCunder LAG conditions (Fig. 1). Complex 1 was identifiedby comparison of the experimental X-ray powder diffraction(XRPD) patterns with patterns of the material synthesized inthe conventional solvent-based way, respectively.

Fig. 1 Synthesis of DMC using liquid-assisted grinding (LAG). FortBuOH-after, tBuOH was added and stirred for 20 min after LAG hadbeen completed. For tBuOH-with, tBuOH was mixed together with theliquid (1 : 1, v/v) when used in the LAG process.

The XRPD analyses demonstrate that complex 1 can bereadily synthesized within 12 min using LAG (Figs. S1

This journal is © The Royal Society of Chemistry 2011 Green Chem., 2011, 13, 2701–2703 | 2701

Dow

nloa

ded

by U

nive

rsity

of

Ten

ness

ee a

t Kno

xvill

e on

15/

05/2

013

07:0

7:08

. Pu

blis

hed

on 2

5 A

ugus

t 201

1 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1GC

1555

7AView Article Online / Journal Homepage / Table of Contents for this issue

Table 1 Propylene oxide and CO2 copolymerization resultsa

Entry Complexb Grinding time/min TOFc Selectivityd (% PPC) Mne (¥103) PDIe Mw/Mn wCO2

f

1g ,h 1 / 10.8 89 71 2.33 0.372h 1 14 19.3 91 98 1.93 0.403 1-DFP 10 22.9 92 83.2 1.87 0.404 1-DMSO 10 20.0 90 72.0 2.12 0.395 1-DMF 12 15.3 88 61.9 2.36 0.366 1-ACN 12 16.6 84 64.2 1.91 0.347i 2 / 21.1 NRj 1.1 3.9 0.228g 2 / 24.6 87 5.2 2.53 0.399 2-DFP 9 69.4 >95 23.5 2.16 0.4210 2-DMSO 9 61.6 >95 16.1 1.95 0.4211 2-MeOH 9 17.8 71 3.9 4.50 0.2312 2-DMF 9 52.2 91 11.2 2.37 0.3713 2-ACN 9 54.9 89 9.0 2.12 0.3414 2-Acetone 9 42.5 85 7.4 2.73 0.3715 2-CHCl3 9 27.6 85 9.9 3.35 0.32

a The copolymerization reaction was carried out with neat PO (100 mL) by employing 1 (0.6 g) at 55 ◦C for 40 h and 2 (0.2 g) at 70 ◦C for20 h. PCO2

(25 ◦C) = 20 bar. TOF < 15 is shown in Table S1.† b Complexes 1 and 2 synthesized using LAG by the tBuOH-with introduction method.Liquids: ACN = acetonitrile, CHCl3 = chloroform. c Turnover frequency in gpolymer gZn

-1 h-1. d Determined by 1H NMR spectroscopy. e Determinedby gel permeation chromatography relative to polystyrene standards in tetrahydrofuran. f Mass fraction of CO2 in the copolymer, as determined by1H NMR spectroscopy. g DMC prepared in the conventional solvent-based way. h Ref. 6a, complex 1 synthesized by solvent-free grinding. i Ref. 6b.PCO2

(25 ◦C) = 9.6 bar, reaction time = 24 h, T = 50 ◦C. j Not reported.

and S2†). Complex 1 made using DFP (1,1,1,2,3,4,4,5,5,5-decafluoropentane), DMSO (dimethyl sulfoxide) or MeOH(methanol) as the grinding liquid was poorly crystalline. Itis worthy to note that when tBuOH-with way was used,the resulting XRPD patterns displayed very broad peaks notassociated with any sharp peaks between 2q values of 13.5 and22.5◦, and 2q = 23.6◦, which means that an amorphous structureis formed. Forming amorphous materials is advantageousfor the copolymerization of epoxides and CO2, as is widelyacknowledged9,11 and according to observation (Table 1).

To demonstrate that this green method of DMC synthesisis generally applicable, we further applied LAG to synthesizeZn3[Co(CN)6] (complex 2).12 Based on the synthesis of 1,we attempted to synthesize complex 2 using the tBuOH-withmethod with 12 min of grinding. Unfortunately, the obtained2 showed no catalytic activity in CO2 and propylene oxide(PO) copolymerization. For this reason, the grinding conditionswere explored. Given 5 or 7 min grinding of the reactantsand the mixed together DFP-tBuOH (250 mL), complex 2 wasobtained with very low catalytic activity; shorter or longergrinding times had no effect on the activity, even though 2 isamorphous (Fig. S3†). We conclude that these surprising resultsare due to incomplete transformation resulting from a shortageof mechanical energy during the shorter grinding time and theactivity loss caused by the higher temperature during longergrindings.13 Finally, an intermediate grinding method was found:reactants were ground with the liquid for 3 min and then pausedfor 7 min; such a process repeated three times promoted thetransformation and avoided the high temperature. Thus, activecomplex 2 was successfully obtained with a total grinding 9 minunder mechanochemical conditions (Figs. S4 and S5†).

All of the DMCs showed a cubic lattice. The XRPD patternsof complexes 1 and 2 employing varied liquids could be dividedinto crystallized and amorphous. Complexes 1 and 2 usingDMSO or MeOH are amorphous, compared with those ofneat grinding and conventional synthesis. This might be an

indication that easier framework collapse results from thestronger polarity of the liquid under LAG conditions. Basically,the crystallinity decreases with increasing liquid polarity. Thus,liquid polarity is deduced to play a positive role in formingan amorphous structure. The hypothesis is confirmed by theXRPD patterns of 1 and 2 employing varying liquids. It canbe concluded that DMCs with extremely polar liquids, such asDMSO, DMF (N,N-dimethylformamide) and MeOH, have avery low crystallinity in contrast to those employing relativelyweak polarity cyclohexane, ethanol and chloroform, whateveramount of tBuOH is introduced, as shown in the XRPD patterns.

However, H2O, of extreme polarity, formed 1 and 2 withreally high crystallinities. This might be a result of the goodsolubility of both ZnCl2 and K3M(CN)6 (M = Fe, Co) in H2O.Good solubility will cause conventional formation of the DMCbefore mechano-activation. Moreover, DFP, with a relativelyweak polarity, formed largely amorphous DMCs. The reasonfor this surprising solvent effect remains unresolved at present.

The synthesis of 2 showed the same simplicity as that of 1,implying that LAG can be used as a convenient way to syn-thesize DMCs for manipulation. Therefore, mechanochemicalreactions, which exploit the synthesis of DMCs under grindingconditions, can extend the scope of grinding-based syntheses.

Although requisite tBuOH was not demonstrated to be anideal liquid when separately used in LAG conditions becauseof the resulting high crystallization, all of the XRPD analysesindicate that tBuOH-with was a better introduction route thantBuOH-after for the LAG synthesis of DMCs. At this point,a one-step method was found to synthesize active DMCs.This one-step method completely discards conventional time,energy and material consumption, and tedious the operation ofintroducing a complexing agent into DMC structures.14

A key observation is that the selectivity of DMCs was en-hanced by this novel method. The alternating copolymerizationof CO2 and PO with the obtained ground DMC catalystsanticipates poly(propylene carbonate) (PPC) (Figs. S6–S8†). 1H

2702 | Green Chem., 2011, 13, 2701–2703 This journal is © The Royal Society of Chemistry 2011

Dow

nloa

ded

by U

nive

rsity

of

Ten

ness

ee a

t Kno

xvill

e on

15/

05/2

013

07:0

7:08

. Pu

blis

hed

on 2

5 A

ugus

t 201

1 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1GC

1555

7A

View Article Online

NMR spectroscopy demonstrated that the content of undesiredether linkages varied in the PPC owing to kinetically controlledhomopolymerization (Fig. S9†).† The PO and CO2 copolymer-ization results showed that 1 and 2 with DFP had the highestselectivities and catalytic activities (Table 1). Strikingly, theground DMC showed a higher catalytic activity and the obtainedPPC showed higher Mn values, narrower polydispersity indicesand greater CO2 contents than those produced by solvent-basedDMCs (Table 1, entries 1, 7 and 8). These results are maybeanother indication of the significant impact of liquid polarityon the active Zn2+ centre in the formation of DMCs,6c which willaffect the copolymerization further.

In summary, for the first time, we report that DMCs canbe readily synthesized using a liquid-assisted grinding method.Merit is shown by (1) clean and rapid room temperaturesyntheses within minutes and (2) evidently improved catalyticactivities and selectivities. This more efficient and green methodreveals a clear shortcut in DMC synthesis compared withconventional solvent-based methods. Overall, these findingsimprove our insight into the possibilities offered by grinding-induced transformations and extend the application of grindingas a convenient solvent-free or minimal-solvent method. Futurework will be focused on mechanistic investigations of complexreconstruction under grinding conditions, as well as furtherimproving activities via optimization of the DMC and co-catalyst structure.

Acknowledgements

This work was financially supported by the Key Scientificand Technical Project of Hainan Province (no. 090502) andthe Project of Scientific and Technical Personnel Server forEnterprises (no. 2009GJE20014).

Notes and references1 N. Armaroli and V. Balzani, Angew. Chem., Int. Ed., 2007, 46, 52.2 (a) A. Stolle, T. Szuppa, S. E. S. Leonhardt and B. Ondruschka,

Chem. Soc. Rev., 2011, 40, 2317; (b) T. Friscic, J. Mater. Chem.,2010, 20, 7599; (c) A. Lazuen-Garay, A. Pichon and S. L. James,Chem. Soc. Rev., 2007, 36, 846; (d) B. Rodrıguez, A. Bruckmann, T.Rantanen and C. Bolm, Adv. Synth. Catal., 2007, 349, 2213; (e) T.Friscic and W. Jones, Cryst. Growth Des., 2009, 9, 1621; (f) R. Kuroda,J. Yoshida, A. Nakamura and S. Nishikiori, CrystEngComm, 2009,

11, 427; (g) G. Kaupp, Top. Curr. Chem., 2005, 254, 95; (h) K. Tanakaand F. Toda, Chem. Rev., 2000, 100, 1025; (i) K. Tanaka, Solvent-FreeOrganic Synthesis, Wiley-VCH, Weinheim, 2003; (j) D. A. Fulmer,W. C. Shearouse, S. T. Medonza and J. Mack, Green Chem., 2009, 11,1821; (k) W. Yuan, T. Friscic, D. Apperley and S. L. James, Angew.Chem., Int. Ed., 2010, 49, 3916; (l) P. J. Beldon, L. Fabian, R. S. Stein,A. Thirumurugan, A. K. Cheetham and T. Friscic, Angew. Chem.,Int. Ed., 2010, 49, 9640; (m) T. Friscic, A. V. Trask, W. Jones andW. D. S. Motherwell, Angew. Chem., Int. Ed., 2006, 45, 7546; (n) T.Friscic and L. Fabian, CrystEngComm, 2009, 11, 743.

3 (a) W. Leitner, Coord. Chem. Rev., 1996, 153, 257; (b) D. Walther, M.Ruben and S. Rau, Coord. Chem. Rev., 1999, 182, 67.

4 (a) E. J. Beckman, Science, 1999, 283, 946; (b) Y. Qin and X. Wang,Biotechnol. J., 2010, 5, 1164; (c) M. Okada, Prog. Polym. Sci., 2002,27, 87.

5 For reviews on epoxide/CO2 copolymerizations, see: (a) M. R.Kember, A. Buchard and C. K. Williams, Chem. Commun., 2011,47, 141; (b) G. W. Coates and D. R. Moore, Angew. Chem., Int. Ed.,2004, 43, 6618; (c) W. Kuran, Prog. Polym. Sci., 1998, 23, 919; (d) M.S. Super and E. Beckman, Trends Polym. Sci., 1997, 5, 236.

6 (a) W. Zhang, Q. Lin, Y. Cheng, L. Lu, B. Lin, L. Pan and N. Xu,J. Appl. Polym. Sci., 2011, DOI: 10.1002/app.34544; (b) I. Kim, M.J. Yi, K. J. Lee, D.-W. Park, B. U. Kim and C.-S. Ha, Catal. Today,2006, 111, 292; (c) I. K. Lee, J. Y. Ha, C. Cao, D.-W. Park, C.-S.Ha and I. Kim, Catal. Today, 2009, 148, 389; (d) J. Kuyper, V. S.Struykenkamp, U.S. Pat. 4,472,560, 1984.

7 M. K. Beyer and H. Clausen-Schaumann, Chem. Rev., 2005, 105,2921.

8 (a) F. Kh. Urakaev and V. V. Boldyrev, Powder Technol., 2000, 107,93; (b) F. Kh. Urakaev and V. V. Boldyrev, Powder Technol., 2000,107, 197.

9 (a) S. Chen, G.-R. Qi, Z.-J. Hua and H.-Q. Yan, J. Polym. Sci., PartA: Polym. Chem., 2004, 42, 5284; (b) R. J. Herold and R. A. Livigni,Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 1972, 13, 545;(c) R. J. Herold and R. A. Livigni, Adv. Chem., 1973, 128, 208; (d) R. J.Herold, Macromol. Synth., 1974, 5, 9; (e) J. Kuyper and G. Boxhoon,J. Catal., 1987, 105, 163.

10 (a) B. Le-Khac, U.S. Pat. 5,693,584, 1997; (b) H. Jorg, G. Pramod, P.Harald, O. Pieter, S. Walter, E.P. Pat. 0,892,002, 1999; (c) Y. Huang,G. Qi and L. Chen, Appl. Catal., A, 2003, 240, 263; (d) H. Liu, X.Wang, Y. Gu and W. Guo, Molecules, 2003, 8, 67.

11 (a) M. M. Dharman, J.-Y. Ahn, M.-K. Lee, H.-L. Shim, K.-H. Kim,I. Kim and D.-W. Park, Green Chem., 2008, 10, 678.

12 The conventional solvent-based synthesis of 2 is very sensitive toreaction temperature, stirring speed and charging rate. Therefore,the synthesis of 2 is much harder than 1 under grinding conditions,and solvent-free grinding cannot produce active 2. Therefore, thesuccessful synthesis of 2 demonstrates the general applicability ofthe LAG method for DMC synthesis.

13 B. Rodrıguez, A. Bruckmann, T. Rantanen and C. Bolm, Adv. Synth.Catal., 2007, 349, 2213 and references therein.

14 A. Soltani-Ahmadi, B. Le-Khac and G. A. Bullano, U.S. Pat.,5,900,384, 1999; Ref. 6d and references therein.

This journal is © The Royal Society of Chemistry 2011 Green Chem., 2011, 13, 2701–2703 | 2703

Dow

nloa

ded

by U

nive

rsity

of

Ten

ness

ee a

t Kno

xvill

e on

15/

05/2

013

07:0

7:08

. Pu

blis

hed

on 2

5 A

ugus

t 201

1 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1GC

1555

7A

View Article Online