On the abundance of chiral crystals (An optimistic lecture for the conclusion of the conference) David Avnir Institute of Chemistry The Hebrew University of Jerusalem, Israel With Chaim Dryzun Department of Chemistry, ETH Zürich Lugano Campus, Switzerland Chirality 2012, Fort Worth, Texas June 10 - June 13, 2012
The unrecognized high abundance of chiral crystals * ~23% of all non-biological crystals are chiral (compared to only ~10% of all non-biological molecules ) * Only ~6% of these are labelled as chiral In numbers: there are out there ~100, 000 crystals the chirality of which has been ignored It means that: The library from which one can select enantioselective catalysts, sensing materials, and chromatographic materials is by far larger than envisaged so far.
Questions to be addressed: # Why was it overlooked? # Why are chiral crystals much more common than chiral molecules? # What are the practical implications of this finding?
What is a chiral crystal? What may be chiral in a crystal? # The molecule # The asymmetric unit # The unit cell # The space-group # The macroscopic habit H. D. Flack, Helv. Chim. Acta, 2003, 86, 905
The classes of space groups Class I: The 165 space groups which contain at least one improperation (inversion, mirror, glide or Sn operations). Always achiral (although the 3 D asymmetric unit is always chiral) Pm
Class II: 22 chiral-helical space groups (11 enantiomeric pairs) Contain at least one screw axis which is not the 2 1 -screw axis. Always chiral even if the AU is achiral
The confusing class III: 43 space groups that contain only proper rotations and the 21 -screw rotation Examples: P 21, P 4, the abundant P 21 21 21. Despite the fact that there are no reflections, inversions etc. , these space groups are achiral Despite the fact that these space groups are achiral, the crystals which pack by them are always chiral How can that be?
Despite the fact that there are no reflections, inversions etc. , these 43 space groups are achiral # P 21 is achiral because reflection of this mathematical entity results in unchanged P 21 # P 61 is chiral because its reflection results in P 65 In general, a crystal may be chiral and yet belong to one of these 43 achiral space groups
Despite the fact that these space groups are achiral, the crystals which pack by them are chiral The reason for: * An AU in 3 D is always chiral. A chiral AU on which only properations are applied, must result in a chiral crystal. * If the AU is achiral (0 D, 1 D, 2 D) – then it will usually pack in a space group which has that achiral operation, coinciding with it.
The Sohncke symmetry space groups Class II and Class III are collectively known as the 65 Sohncke groups II: 22 of the 65 are chiral (helical) III: 43 of the 65 are achiral Bottom line: All of the 65 Sohncke groups - and only these groups represent chiral crystals
Wrong H. D. Flack, Helv. Chim. Acta, 2003, 86, 905
To remove the confusion we suggest: Class I: 165 improper-achiral groups Always an achiral crystal Class II: 22 helical-chiral groups Always a chiral crystal Class III: 43 proper-achiral groups Always a chiral crystal
Simple tests for the chirality of a crystal If the space group contains only properations, the crystal is chiral Properations: rotations, screw-rotations and translations Achiral crystal - improperations (mirror, inversion, S 4, S 6 or glide) Santiago Alvarez’ Criterion: A crystal is chiral if the symbol of its space group is composed only of a capital letter and simple numbers
The numbers Number of reported non-biological crystal structures (CSD, ICSD): 574, 000 Chiral structures: 131, 000 % of all non-biological chiral crystals: 23% Number of structures reported as chiral: 35, 000 (6% only) Number of chiral crystals not recognized as such: ~96, 000
Measuring the degree of chirality
The continuous chirality measure (CCM) G: The achiral symmetry point group which minimizes S(G) Achiral molecule: S(G) = 0 The more chiral the molecule is, the higher is S(G) Mezey, Gilat, Kauzman, Osipov, Mislow, Ruch, Richards, Maruani
The most chiral monodentate complex With S. Alvarez, Europ. J. Inorg, Chem. , 1499 (2001)
The chirality of a unit-cell 516 atoms 1 sec S(C 2)=0. 00 S(chirality)=4. 51 S(Ci)=36. 54 bis((m 2 -phenoxo)-bis(triphenylphosphine)-copper), C 84 H 70 Cu 2 O 2 P 4 (HEZXEP (P 2)); Osakada, K. ; Takizawa, T. ; Tanaka, M. ; Yamamoto, T. J. Organometallic Chem. , 1994, 473, 359 -369.
The optical rotation of quartz: More than 120 years ago Le Chatelier and his contemporaries Le Chatelier, H. Compt. Rend de I'Acad. Sciences. 264 , 109 , 1889
120 years later: an exact match with quantitative chirality changes Chirality, Si. Si 4 Le Chatelier a t/a Si. Si 4 Chirality a t/a 0 Temperature (°K) D. Yogev, Tetrahedron: Asymmetry 18, 2295 (2007)
Examples of publications on chiral crystals where terms such as “Chirality”, “Chiral”, “Optical activity”, etc. , do not appear in the title, abstract and the whole text. All are of class III, the 43 proper-achiral space groups A chemist running a search which has any of these keywords, will simply miss 100, 000 structures!
Example 1: C 25 H 18 O 2 CSD: ABUCOP, space group: P 2 21 21 (#18), CCM-UC = 11. 15 S. Apel, S. Nitsche, K. Beketov, W. Seichter, J. Seidel, E. Weber, J. Chem. Soc. , Perkin Trans. 2, 2001, 7, 1212
Example 1: C 25 H 18 O 2 CCM of one molecule = 2. 82
Example 2: C 12 H 40 Cs 4 N 4 Si 4 CSD: JUFWUK, space group: P 3 2 (#195), CCM-UC = 0. 47 Tesh, K. F. ; Jones, B. D. ; Hanusa, T. P. ; Huffman, J. C. J. Am. Chem. Soc. 1992, 114, 6590.
Example 2: C 12 H 40 Cs 4 N 4 Si 4 CCM of one molecule = 0. 47
Example 3: C 16 H 12 N 2 O 2 CSD: BIXLOJ, the most common proper-achiral group: P 2 1 21 21, (#19) CCM of the UC = 2. 01
Example 3: C 16 H 12 N 2 O 2 (CSD code: BIXLOJ) Space group: P 21 21 21 (#19) CCM of one molecule inside the crystal = 0. 19
Example 4: NH 3, Ammonia Space group: P 21 3 (#198), UC-CCM = 1. 89, CCM one molecule = 0 The terms “chirality”, “optical activity” etc’ do not appear in ANY of the publications on ammonia crystals ! Boese, R. ; Niederpruem, N. ; Blaeser, D. ; Maulitz, A. H. ; Antipin, M. ; Yu. ; Mallinson, P. R. J. Phys. Chem. B, 1997, 101, 5794– 5799.
Example 5: Crystallization of a racemate leads to a P 21 chiral crystal The pair of enantiomers in the AU are related by pseudo-inversion: the phenyl rings, which are twisted differently (±)-(1 RS, 3 SR, 4 RS)-1 Phenyl-cis-3, 4 -butano 3, 4, 5, 6 -tetrahydro-1 H 2, 5 - benzoxazocine hydrochloride Steinberg, A. , Ergaz, I. , Toscano, R. A. , Glaser, R. 2011. Cryst. Growth Des. 11, 1262 -1270.
Why are chiral crystals much more common than chiral molecules? % of all non-biological chiral crystals: 23% % of all non-biological molecules: ~10% # Solution-achiral molecules need not crystallize in their equilibrium achiral structure # They provide a very rich library of chiral conformers, which is the source of the abundance of chiral crystals
Why was it overlooked? * The confusion, even in text books, of what is a chiral crystal. * For a crystallographer the chirality maybe obvious from the space-group. The cost: Chemists searching “chiral” will miss it. * Crystallization from a racemic mixture results in a mixture of right- and left-handed crystals which needs to be separated
Practical aspects: Chiral Silicate Zeolites Most silicate-zolites are highly symmetric ZSM-5, a silicate zeolite: Nan. Aln. Si 96 -n. O 192 • 16 H 2 O
Chiral zeolites Prime importance: * Enantioselective catalysis * Enantiomers separation * Enantioselective sensing Known: Zeolite-like, open-pore crystals, MOF’s, etc. Out of over 700 zeolite structures only 5 are recognized as chiral Desired: Chiral aluminosilicate zeolites Only one was reported
We found 21(!) chiral silicate zeolites which have been under the nose all the time! a. Goosecreekite. b. Bikitaite. c. The two enantiomeric forms of Nabesite Ch. Dryzun et al, J. Mater. Chem. , 19, 2062 (2009) Editor’s Choice, Science, 323, 1266 (2009)
Out of 120 classical silicate zeolites, we found 21 chiral zeolites, that were not recognized as such That is very close to the 23% general abundance we found All belong to the non-helical Sohncke space groups
The chirality values are comparable or larger than the chirality values of the known chiral zeotypes and of quartz Goosecreekite (GOO) Chiral zincophosphate I (CZP) α-Quartz TT’ 4 2. 05 2. 94 0. 55 SBU 0. 86 0. 37 ------ A. U. 14. 76 1. 28 0. 00 Unit cell 4. 90 8. 91 1. 28
The isothermal titration calorimetry (ITC) experiment L-histidine Adsorption of D-histidine (the lower curve) or L-histidine (the higher curve) on Goosecreekite (GOO): The heat flow per injection With Y. Mastai and A. Shvalb, Bar-Ilan
Conclusion There are some 100, 000 unrecognized chiral crystals out there, waiting to be utilized for enantioselective catalysis, sensing, and separation. C. Dryzun and D. Avnir, Chem. Commun. , 2012, 48, 5874– 5876, Special Chirality web themed issue
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