Published: June 03, 2011

r 2011 American Chemical Society 3313 dx.doi.org/10.1021/cg200379h | Cryst. Growth Des. 2011, 11, 3313–3317

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Structural Chemistry of a New Chiral Anhydrous Phase ofRu(bipy)3(ClO4)2 Established from Powder X-ray Diffraction AnalysisEugene Y. Cheung,† Kotaro Fujii,† Fang Guo,† Kenneth D. M. Harris,*,† Sayoko Hasebe,‡ andReiko Kuroda*,‡,§

†School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, Wales‡Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku,Tokyo 153-8902, Japan§Kuroda Chiromorphology Project, ERATO, Japan Science and Technology Corporation, 4-7-6 Park Building 4F, Komaba,Meguro-ku, Tokyo 153-0041, Japan

bS Supporting Information

The phenomena of chiral symmetry breaking, spontaneouschiral resolution, and chiral amplification have attracted the

interest of scientists for many years, not least because theseprocesses have important implications for understanding thedevelopment of the enantiomeric imbalances that exist innature.1 Although the study of chirality in organic systems hasperhaps attracted the greatest attention,2 the chirality of metalcomplexes is also of considerable interest,3 following the pioneer-ing work of Werner3a�c around a century ago, which recognizedand demonstrated that certain metal complexes with octahedralcoordination, such as cis-CoBr(NH3)(H2NCH2CH2NH2)2

2+,may exist as Δ and Λ enantiomers. In the crystalline state, suchmaterials may exist as enantiomerically pure crystal phases (i.e.,containing either the Λ enantiomer only or the Δ enantiomeronly) and/or as racemic crystal phases (i.e., containing equalamounts of both the Λ and Δ enantiomers within the crystalstructure).

Among the range of metal complexes with octahedral coordi-nation (or approximately octahedral coordination) that are ofrelevance to chirality research, those of the type ML3, where Ldenotes a bidentate ligand, have received significant attention.Examples include Ru(bipy)3

2+ (Scheme 1), which has beenstudied widely not only with regard to issues of chirality4 butalso with regard to applications based on its photophysicaland photochemical properties,5 and Ru(phen)3

2+ (phen =phenanthroline), which has recently been reported to undergoa crystalline state racemization process under conditions of

mechanical grinding.6 In particular, when a polycrystalline sam-ple comprising a 1:1 mixture of the Λ and Δ enantiomorphicforms of the chiral crystalline phase of Ru(phen)3(PF6)2 wassubjected to mechanical grinding followed by annealing at250 �C, a racemic crystalline phase of Ru(phen)3(PF6)2 wasobserved6 to be formed.

The solid state structural properties of a number of salts of thetype [Ru(bipy)3

2+]x(Ay�)2x/y have been investigated previously,

7

encompassing a range of different counteranions Ay�. In thepresent paper, we focus on the material with perchlorate as thecounteranion [i.e., Ru(bipy)3(ClO4)2], for which two solid statestructures have been reported previously: a racemic anhydrousstructure (space group C2/c)7d and a chiral hydrate structure(space group C2).7e

In the present work, a “conglomerate” polycrystalline sample8

comprising a 1:1 mixture of the Λ and Δ enantiomorphic formsof the chiral hydrate phase of Ru(bipy)3(ClO4)2 was subjected tomechanical grinding followed by annealing at 250 �C, analogousto the procedure6 discussed above for the solid state racemizationprocess of Ru(phen)3(PF6)2. In the case of Ru(bipy)3(ClO4)2,visual inspection provided an initial indication that this proce-dure resulted in the formation of a new phase, with the color ofthe powder sample changing from orange (starting material) to

Received: March 24, 2011Revised: June 3, 2011

ABSTRACT: A new anhydrous chiral phase of Ru(bipy)3-(ClO4)2 (bipy = 2,20-bipyridine) has been prepared both bymechanical grinding followed by annealing of the known chiralhydrate phase of this material and by solid-state dehydration ofthe chiral hydrate phase at high temperature. The new phase isobtained from these processes as a microcrystalline powder,thus limiting the opportunity to carry out structural character-ization by single-crystal X-ray diffraction. Instead, our structure determination of the new chiral anhydrous phase has exploited thecapabilities of modern powder X-ray diffraction techniques, employing the direct-space genetic algorithm technique for structuresolution followed by Rietveld refinement. The structural properties of the chiral anhydrous phase are discussed, particularly withregard to assessing the structural relationship to the parent hydrate phase.

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bright red (product). Powder X-ray diffraction (Figure 1) con-firmed that the product phase was different from the startingmaterial (i.e., the chiral hydrate structure7e) and was alsodifferent from the racemic anhydrous structure of Ru(bipy)3-(ClO4)2 reported previously.7d

In common with most procedures for preparing newmaterialsby mechanical grinding, the new phase of Ru(bipy)3(ClO4)2obtained by the procedure described above was a microcrystal-line powder, which did not contain single crystals of suitable sizeto allow structural characterization by single-crystal X-ray dif-fraction. Under such circumstances, structure determinationmust be tackled instead from powder X-ray diffraction data,although, in general, structure determination from powder X-raydiffraction data is a considerably more challenging task thanstructure determination from single-crystal X-ray diffraction data,particularly in the case of molecular solids. Fortunately, however,the opportunities for carrying out complete structure determina-tion of molecular solids directly from powder X-ray diffractiondata have advanced significantly in recent years,9 particularlythrough the development of the direct-space strategy for struc-ture solution. In the present paper, we apply these modernpowder X-ray diffraction techniques to establish the structuralproperties of the new phase of Ru(bipy)3(ClO4)2 prepared asdescribed above.

For structure determination of the new phase of Ru(bipy)3-(ClO4)2, high-quality powder X-ray diffraction data were re-corded at ambient temperature.10 The powder X-ray diffraction

data were indexed using the program DICVOL,11 giving thefollowing unit cell with orthorhombic metric symmetry: a =10.14 Å, b = 13.28 Å, c = 23.75 Å. Unit cell and profile refinementwere carried out using the Le Bail fitting procedure,12 giving goodagreement between experimental and calculated powder X-raydiffraction profiles for this unit cell (Figure 3a; Rwp = 2.63%, Rp =1.77%). On the basis of systematic absences, the space groupwas assigned as P212121. Density considerations suggest thatthere are four formula units Ru(bipy)3(ClO4)2 in the unit cell(calculated density, 1.60 g cm�3), and hence (as Z = 4 for spacegroup P212121), the asymmetric unit comprises one formula unitRu(bipy)3(ClO4)2.

Structure solution was carried out directly from the powderX-ray diffraction data using the direct-space genetic algorithm(GA) technique13 implemented in the programEAGER.14 As theasymmetric unit comprises one formula unit Ru(bipy)3(ClO4)2,the direct-space search involved three rigid fragments [oneRu(bipy)3

2+ cation and two ClO4� anions] represented by a

total of 18 structural variables [with three variables (x, y, z)required to define the position and three variables (θ, j, ψ)required to define the orientation for each rigid fragment within theasymmetric unit]. TheGA structure solution calculationwas carriedout for 100 generations with a population size of 100 structuresand with 25 mating operations and 10 mutation operationscarried out per generation. The best structure obtained in theGA structure solution calculation (i.e., the structure correspond-ing to the best agreement with the experimental powder X-raydiffraction pattern) was used as the starting structural modelfor Rietveld refinement,15 which was carried out using the GSAS

Scheme 1

Figure 1. Powder X-ray diffraction patterns of (a) the new chiralanhydrous phase (experimental data), (b) the racemic anhydrous phase(simulated data from the reported structure7d), and (c) the chiralhydrate phase (simulated data from the reported structure7e).

Figure 2. Results from (a) the Le Bail refinement and (b) the finalRietveld refinement for the chiral anhydrous phase. In each case, theexperimental (red, + marks), calculated (green, solid line), and differ-ence (purple, lower line) powder X-ray diffraction profiles are shown.Tick marks indicate peak positions.

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program.16 Inspection of difference Fourier maps generated inthe Rietveld refinement suggested that the structural modelcorrectly represented all significant electron density in thematerial. Furthermore, the structure did not contain any sig-nificant voids that could arise, for example, if water molecules hadbeen erroneously omitted from the model. These facts supportour assignment of the new phase as an anhydrous phase of Ru-(bipy)3(ClO4)2. As shown in Figure 2b, the Rietveld refinement17

gave good agreement between experimental and calculated powderX-ray diffraction patterns, with the following data obtained in thefinal refinement: a = 10.1535(4) Å, b = 13.2333(6) Å, c =23.7320(13) Å; V = 3188.7(4) Å3; Rwp = 3.88%, Rp = 2.82%(2θ range, 5.00�68.97�; 3826 profile points; 256 refined variables).

The fact that the new phase of Ru(bipy)3(ClO4)2 has a chiralcrystal structure (with space group P212121) implies that theproduct anhydrous phase is a conglomerate polycrystallinesample (we recall that the sample of the parent chiral hydratephase from which it was prepared was also a conglomerate).Thus, the preparation procedure, involving mechanical grindingfollowed by annealing of a polycrystalline sample comprising a1:1 mixture of the Λ and Δ enantiomorphic forms of the chiralhydrate phase of Ru(bipy)3(ClO4)2, did not lead to any solid-state racemization process analogous to that observed previously6

for Ru(phen)3(PF6)2. Instead, this procedure yields a new phaseof a chiral anhydrous structure of Ru(bipy)3(ClO4)2.

To further confirm our assignment of the material as a chiralphase, the same preparation procedure was repeated startingfrom an enantiomerically pure sample comprising only the Δ

enantiomorphic form of the chiral hydrate phase of Ru(bipy)3-(ClO4)2. The powder X-ray diffraction pattern of the productobtained in this case was identical to that of the productobtained from the preparation (discussed above) starting fromthe conglomerate polycrystalline sample comprising a 1:1mixture of the Λ and Δ enantiomorphic forms of the chiralhydrate phase.

In the crystal structure of the new chiral anhydrous phase(Figure 3a and b), the Ru(bipy)3

2+ cations are arranged along theb-axis in one-dimensional columns (indicated by yellow shadingin Figure 3a and b). Within each column, adjacent cations arerelated to each other by the 21 screw axis. The ClO4

� anions arearranged in the region of space between the columns of Ru-(bipy)3

2+ cations. One of the two independent ClO4� anions

also forms a column-like arrangement parallel to the b-axis(vertical in Figure 3b), whereas the other ClO4

� anion lies closerto the periphery of the columns of Ru(bipy)3

2+ cations.For comparison, the crystal structure of the parent chiral

hydrate phase7e is shown in Figure 3c and d. This structure(space group C2) may also be described in terms of columns ofRu(bipy)3

2+ cations parallel to the b-axis (the unique axis of themonoclinic system), with the two independent Ru(bipy)3

2+

cations in this structure forming separate columns parallel to thisaxis (indicated by yellow and orange shading in Figure 3c and d).The water molecules and ClO4

� anions are arranged in theregion of space between the columns of Ru(bipy)3

2+ cations.Within this region, a scheme of O�H 3 3 3O hydrogen bonds isformed, involvingO�Hbonds of thewatermolecules as hydrogen

Figure 3. Crystal structure of the chiral anhydrous phase viewed (a) along the b-axis and (b) along the c-axis (in part b, only the range 0e z e 0.5 isshown). For comparison, the structure of the chiral hydrate phase7e is shown viewed (c) along the b-axis and (d) along the a-axis (in part d, only the range0 e x e 0.5 is shown). Columns of Ru(bipy)3

2+ cations are indicated by yellow and orange shading, the bipy ligands and ClO4� anions are shown in

capped-stick format, and the Ru atoms and oxygen atoms of water molecules are shown as blue and red circles, respectively. Hydrogen atoms are omittedfor clarity. In part c, the blue dashed lines indicate (approximately) the unit cell of the chiral anhydrous phase superimposed on the structure of the chiralhydrate phase, illustrating the relation between the anhydrous and hydrate structures discussed in endnote 18. The relative orientations of the structuresplotted in parts a and c have been chosen such that the blue dashed lines in part c match the orientation of the unit cell of the anhydrous structure shownin part a.

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bond donors and oxygen atoms of both the ClO4� anions and

water molecules as hydrogen bond acceptors.Although the crystal structures of the chiral anhydrous and

chiral hydrate phases clearly differ in terms of the absence orpresence of water molecules, respectively, and consequently interms of the existence of O�H 3 3 3O hydrogen bonding in thelatter case, these structures nevertheless share several commonfeatures concerning the spatial arrangements of the Ru(bipy)3

2+

cations and ClO4� anions, with only relatively small differences

in the relative positions and orientations of these components inthe two structures. Indeed, this structural similarity is reflected ina comparison of the unit cell parameters of the chiral hydrate andchiral anhydrous phases.18 As a consequence of these structuralsimilarities, interconversion between the hydrate and anhydrousphases by means of hydration and dehydration processes re-quires comparatively little reorganization of the crystal structure,and such processes may be expected to be relatively facile.Indeed, we have observed (and confirmed by powder X-raydiffraction analysis) that, when the chiral anhydrous phase is leftunder normal atmospheric conditions at ambient temperatureover a period of time, it transforms to the chiral hydrate phase,presumably as a result of taking up water from the atmosphere.

To explore the dehydration of the chiral hydrate phase inmoredetail, we have carried out combined powder X-ray diffractionand differential scanning calorimetry (PXRD-DSC) experimentson heating the sample from ambient temperature to 170 �C.19Starting from the chiral hydrate phase at ambient temperature(Figure 4a), the powder X-ray diffraction data and DSC dataindicate that an endothermic transformation occurs at ca. 95 �C.At higher temperatures (by ca. 140 �C), the product from thistransformation is identified from the powder X-ray diffractiondata (Figure 4c) as a monophasic sample of the chiral anhydrousphase. Thus, it is clear from the PXRD-DSC experiment that thechiral hydrate phase does indeed undergo a dehydration processwithin the temperature range investigated. In the “intermediate”temperature region from ca. 95 �C to ca. 140 �C, the powderX-ray diffraction pattern recorded in our PXRD-DSC experiment(Figure 4b) differs from those characteristic of the chiral hydrateand the chiral anhydrous phases discussed above, although itdoes bear some significant resemblance to that of the originalchiral hydrate phase (Figure 4a). This observation may suggest

that, in this temperature region, the sample is a partially dehy-drated material that is still structurally similar to the originalhydrate phase, rather than representing a new transient structuretype. However, more detailed experimental studies would berequired in order to establish the structural properties of thematerial present in this temperature region, including the mea-surement of powder X-ray diffraction data of significantly higherquality than that obtained in the present combined PXRD-DSC study.

Finally, we note that the chiral anhydrous phase discovered inthe present work has lower density (1.60 g cm�3) than theracemic anhydrous phase (1.69 g cm�3) reported previously(at 20 �C),7d confirming that Ru(bipy)3(ClO4)2 conforms toWallach’s Rule.20

’ASSOCIATED CONTENT

bS Supporting Information. CIF file containing crystallo-graphic information for the chiral anhydrous phase of Ru(bipy)3-(ClO4)2. This material is available free of charge via the Internetat http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail addresses: HarrisKDM@cardiff.ac.uk; ckuroda@mail.ecc.u-tokyo.ac.jp.

’ACKNOWLEDGMENT

We are grateful to EPSRC, Cardiff University, and JapanScience Technology Agency for financial support.

’REFERENCES

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Figure 4. Powder X-ray diffraction and DSC data recorded in the combined PXRD-DSC experiment: (a) the initial chiral hydrate phase (data recordedin the temperature range 36.1�41.2 �C); (b) the intermediate phase obtained directly on dehydration of the chiral hydrate phase (data recorded in thetemperature range 103.6�108.4 �C); and (c) the final chiral anhydrous phase (data recorded in the temperature range 150.6�155.2 �C). We note that,as the powder X-ray diffraction data were recorded (by scanning from low 2θ to high 2θ) while increasing the temperature of the sample continuously,the temperature of the sample is different at the start and at the end of the measurement of each powder X-ray diffraction pattern.

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Commun. 1979, 849. (b) Rillema, D. P.; Jones, D. S.; Woods, C.; Levy,H. A. Inorg. Chem. 1992, 31, 2935. (c) Biner, M.; Burgi, H.-B.; Ludi, A.;Rohr, C. J. Am. Chem. Soc. 1992, 114, 5197. (d) Harrowfield, J. M.;Sobolev, A. N. Aust. J. Chem. 1994, 47, 763. (e) Krausz, E.; Riesen, H.;Rae, A. D. Aust. J. Chem. 1995, 48, 929. (f) Breu, J.; Domel, H.; Stoll, A.Eur. J. Inorg. Chem. 2000, 2401. (g) Wang, W.-Z.; Liu, X.; Liao, D.-Z.;Jiang, Z.-H.; Yan, S.-P.; Yao, X.-K.; Wang, G.-L. Inorg. Chem. Commun.2001, 4, 416.(8) A “conglomerate” polycrystalline sample is a physical mixture of

two types of homochiral crystals, the structures of which have anenantiomorphic relation to each other. Each individual crystal is homo-chiral, and in the present case, each individual crystal contains either theΔ form only or theΛ form only. Clearly, it is obligatory that such crystalshave a chiral space group. As our sample comprises equal amounts of theΔ andΛ forms of the Ru(bipy)3

2+ cation, the sample is overall racemic,but each individual crystal within the polycrystalline sample ishomochiral.(9) (a) Lightfoot, P.; Tremayne, M.; Harris, K. D. M.; Bruce, P. G.

J. Chem. Soc., Chem. Commun. 1992, 1012. (b) Harris, K. D.M.; Tremayne,M.; Lightfoot, P.; Bruce, P. G. J. Am. Chem. Soc. 1994, 116, 3543.(c) Kariuki, B. M.; Zin, D. M. S.; Tremayne, M.; Harris, K. D. M. Chem.Mater. 1996, 8, 565. (d) Chernyshev, V. V.Russ. Chem. Bull. 2001, 50, 2273.(e) David, W. I. F., Shankland, K., McCusker, L. B., Baerlocher, C., Eds.;Structure Determination from Powder Diffraction Data, OUP/IUCr, 2002.(f) Huq, A.; Stephens, P. W. J. Pharm. Sci. 2003, 92, 244. (g) Brunelli, M.;Wright, J. P.; Vaughan, G. R. M.; Mora, A. J.; Fitch, A. N. Angew. Chem., Int.Ed. 2003, 42, 2029. (h) Harris, K. D. M. Cryst. Growth Des. 2003, 3, 887.(i) Harris, K. D. M.; Cheung, E. Y. Chem. Soc. Rev. 2004, 33, 526.(j) Tremayne, M. Philos. Trans. R. Soc. 2004, 362, 2691. (k) Favre-Nicolin,V.; �Cern�y, R. Z. Kristallogr. 2004, 219, 847. (l) Brodski, V.; Peschar, R.;Schenk, H. J. Appl. Crystallogr. 2005, 38, 688. (m) Tsue, H.; Horiguchi, M.;Tamura, R.; Fujii, K.; Uekusa, H. J. Synth. Org. Chem. Jpn. 2007, 65, 1203.(n) David, W. I. F.; Shankland, K. Acta Crystallogr., Sect. A 2008, 64, 52.

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(10) The powder X-ray diffraction data were recorded on a BrukerD8 diffractometer, operating in transmission mode with Ge-monochro-mated Cu KR1 radiation (λ = 1.5406 Å) and a linear position-sensitivedetector covering 12� in 2θ. The total 2θ range was 5� to 70�, measuredin steps of 0.017� over 12 h.

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(14) (a) Tedesco, E.; Turner, G. W.; Harris, K. D. M.; Johnston,R. L.; Kariuki, B. M. Angew. Chem., Int. Ed. 2000, 39, 4488. (b) Tedesco,E.; Della Sala, F.; Favaretto, L.; Barbarella, G.; Albesa-Jov�e, D.;Pisignano, D.; Gigli, G.; Cingolani, R.; Harris, K. D. M. J. Am. Chem.Soc. 2003, 125, 12277. (c) Cheung, E. Y.; Kitchin, S. J.; Harris, K. D. M.;Imai, Y.; Tajima, N.; Kuroda, R. J. Am. Chem. Soc. 2003, 125, 14658.(d) Albesa-Jov�e, D.; Kariuki, B. M.; Kitchin, S. J.; Grice, L.; Cheung,E. Y.; Harris, K. D. M. ChemPhysChem 2004, 5, 414. (e) Guo, F.; Harris,K. D. M. J. Am. Chem. Soc. 2005, 127, 7314. (f) Hirano, S.; Toyota, S.;Toda, F.; Fujii, K.; Uekusa, H. Angew. Chem., Int. Ed. 2005, 45, 6013.(g) Pan, Z.; Xu, M.; Cheung, E. Y.; Harris, K. D. M.; Constable, E. C.;Housecroft,C. E. J. Phys. Chem. B2006,110, 11620. (h)Guo, F.;Mart�i-Rujas,J.; Pan, Z.; Hughes, C. E.; Harris, K. D.M. J. Phys. Chem. C 2008, 112, 19793.(i) Fujii, K.; Lazuen Garay, A.; Hill, J.; Sbircea, E.; Pan, Z.; Xu, M.; Apperley,D. C.; James, S. L.; Harris, K. D. M. Chem. Commun. 2010, 46, 7572.

(15) (a) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65.(b) Young,R. A., Ed. The Rietveld Method; International Union of Crystallography:Oxford, 1993. (c)McCusker, L. B.; Von Dreele, R. B.; Cox, D. E.; Lou€er,D.; Scardi, P. J. Appl. Crystallogr. 1999, 32, 36.

(16) Larson, A. C.; Von Dreele, R. B. GSAS; Los Alamos LaboratoryReport No. LA-UR-86-748; Los Alamos National Laboratory: LosAlamos, NM, 1987.

(17) In the Rietveld refinement, the atomic positions were refinedsubject to standard geometric restraints on bond lengths and bondangles, which were relaxed gradually as the refinement progressed.Common isotropic displacement parameters were refined for the atomsof the Ru(bipy)3

2+ cation [Uiso = 0.168(3) Å2] and for the atoms of thetwo ClO4

� anions [Uiso = 0.174(5) Å2]. In the final stages of refinement,

hydrogen atoms were added according to standard geometries, with theisotropic displacement parameter taken as 1.25 times the refined valuefor the Ru(bipy)3

2+ cation [i.e., Uiso = 0.210(4) Å2]. We note that therefined isotropic displacement parameters are somewhat larger thanusual for structures determined at ambient temperature, suggesting thatthere may be some degree of orientational disorder (particularly for theClO4

� anions) in this structure.(18) As evident from comparison of parts a and c of Figure 3, the

a-axis of the anhydrous phase (10.15 Å) is close to half the c-axis of thehydrate phase (10.95 Å), the b-axis of the anhydrous phase (13.23 Å) isclose to the b-axis of the hydrate phase (14.27 Å), and the c-axis of theanhydrous phase (23.73 Å) is relatively close to a sin β for the hydratephase (20.85 Å).

(19) The PXRD-DSC experiment was carried out on a Rigaku XRD-DSC II instrument, using Cu KR radiation (50 kV, 300 mA). The rate oftemperature increase for the DSC measurement was 2 �C/min.

(20) (a) Wallach, O. Liebigs Ann. Chem. 1895, 286, 90. (b) Brock,C. P.; Schweizer, W. B.; Dunitz, J. D. J. Am. Chem. Soc. 1991, 113, 9811.