Chiral Nanoporosity in silicates

Institute of ChemistryThe Hebrew University of Jerusalem

Nanocenter meeting, Ashkelon, 21.6.10

David Avnir

1 .The Holy Grail

Zeolites:Highly porous, highly symmetric crystalline materials

ZSM-5, a silicate zeolite: NanAlnSi96-nO192•16H2O

Major industrial applications: *Catalysis

*Heterogeneous chemistry *Separation *Adsorbents

Chiral zeolites

Prime importance because of their enantioselective potential applications in:

*Enantioselective catalysis *Enantiomers separation

Known:Zeolite-like, open-pore crystals, MOF’s, etc.Out of over 700 zeolite structures only 5 are recognized as chiral

Desired:Chiral aluminosilicate zeolitesOnly one was reported

We found 21(!) chiral silicate zeoliteswhich 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)

2 .The route to that finding:

.IChiral nanoporosity of amorphous materials

II. The chiral crystal of quartz

Silica

Here is how one can induce chiral porosity in silicates:

*Adsorb on the surface a chiral molecule *Silylate the surface with a chiral silylating agent

*Polymerize a chiral trialkoxysilane *Entrap a chiral molecule using the sol-gel polycondenstion

*Prepare a hybrid of silica with a chiral polymer *Imprint chirally the silica

Synthesis of silica by the sol-gel polycondensation

Si(OCH3)4 + H2O (SiOmHn)p + CH3OH

Variations on this theme:

–the metals, semi-metals and their combinations–the hydrolizable substituent

–the use of non-polymerizable substituents–organic co-polymerizations (Ormosils)

–non-hydrolytic polymerizations

H+ or OH-

Sol Gel XerogelSol Gel Xerogel

sol-particleEntrapped species

monomeroligomer

-

Physical entrapment of molecules within sol-gel matrices

*Small molecules *Polymers

*Proteins *Nanoparticles

Monomers,oligomers

The concept is general and of very wide scope

Sol Gel XerogelSol Gel Xerogel

sol-particleEntrapped species

monomeroligomer

-

Physical entrapment of molecules within sol-gel matrices

*Small molecules *Polymers

*Proteins *Nanoparticles

Monomers,oligomers

The concept is general and of very wide scope

chiral imprinting

CHO

HC

CH3

H

N

CH3

CH2(CH2)10CH3

CH3

+

CH3O

CH C

O

OH

H3C

OCH2CHCH2NHCHCH3

HOH CH3

PO

OH

O

O

Silica imprinted with aggregates of DMB

was capable of separating the enantiomer-pairs of:

BINAPPropranolol

NaproxenPirkle’s alcohol

With S. Fireman, S. Marx, J. Am. Chem. Soc. 127, 2650 (2005)

)1R,2S-(-)-(N-dodecyl-N-methylephedrinium bromide (DMB)

0.93

1.03

1.13

1.23

1.33

Dis

crim

inat

ion

Rat

io

SR

R

General enantioselectivity by chirally imprinted silica

With S. Fierman, S. Marx, Adv. Mater., 19, 2145 (2007)

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

Dis

crim

inat

ion

Rat

io

SR R

0.93

1.03

1.13

1.23

1.33

Dis

crim

inat

ion

Rat

io

SR

R

Comparison of two methods: Doping and imprinting

Before extraction: Chiral dopant (DMB) After extraction: Chiral holes

The recognition handedness changes!

If an amorphous SiO2 material is made chiral by a foreign molecule which either remains there or not, then:#How are the building blocks of the material affected? #Is it possible that an SiO4 tetrahedron which is neighboring to the chiral event, becomes chiral itself?

Conceptual questions :

Nature has already provided an answer –Yes, it is possible a chiral SiO4 tetrahedron!

Quartz

The building blocks of quartz: All are chiral!

SiO4 SiSi4 -O(SiO3)7-Si(OSi)4

D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

Induced circular dichroism of Congo-Red within silica

SO3Na

NH2

N

SO3Na

NH2

NNNCHO

HC

CH3

H

N

CH3

CH2(CH2)10CH3

CH3

+

The chiral inducer: DMB The achiral probe: CR

With S. Fireman, S. Marx, J. Mater. Chem., 17, 536 - 544 (2007)

We shall compare:* Co-doping * Adsorption of CR on silica doped with DMB

CR-DMB@SG (red line) and CR-DMB@OSG (blue line)

The ICD spectra of co-entrapped CR-DMB in hydrophilic and hydrophobic silica sols

S. Fireman

-40

-20

0

20

40

60

80

300 400 500 600

Wavelength (nm)

CD (

mde

g)

CR-DMB in solution (blue line) and CR solution (red line)

Does CR “feel” the chirality of only DMB?

-6

-5

-4

-3

-2

-1

0

1

2

300 400 500 600

Wavelength (nm)C

D (m

deg)

The ICD signal of CR adsorbed on DMB@silica

The only possibility is chiral skeletal porosity induced by the doped DMB

Co-doping: CR/DMB@silica

CR adsorbed on DMB@silica

Reversal of the ICD signal indicates that the chirality-inducer is different in the two cases.

Silica is a racemic mixture of distorted tetrahedra and of chiral pores

2 .The route to that finding:

II. The measurement of chirality

The building blocks of quartz: All are chiral!

SiO4 SiSi4 -O(SiO3)7-Si(OSi)4

D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

SiSi4 is much more chiral than SiO4

A useful tool: Quantitative measure of chirality

Various degrees of chirality:

Calculating the degree of chirality

1001)(2

12

n

kkk NP

nDGS

G: The nearest achiral symmetry point group

Achiral molecule: S(G) = 0

The more chiral the molecule is, the higher is S(G)

H. Zabrodski Hel-Or, J. Am. Chem. Soc., 117, 462(1995); 120, 6152 (1998); 126 , 1755 (2004). A. Zayit et al, Chirality, in press (2010)

Le Chatelier, H. Com. Rend de I'Acad Sciences 1889, 109, 264.

The optical rotation of quartz: 120 years ago

Le Chatelier and his contemporaries

0.97

1.02

1.07

1.12

1.17

98 298 498 698 898 1098

Temperature ( K)

0.54

0.56

0.58

0.6

0.62

0.64

Temperature (°K)

Le

Cha

telie

r

t

Chirality, SiSi4

Chirality t

More than 120 years later :An exact match with quantitative chirality changes

D. Yogev, Tetrahedron: Asymmetry 18, 2295 (2007)

SiSi4

3 .Back to the zeolites

The symmetry space groups of quartz

-O(SiO3)7-

D. Yogev-Einot, Chem. Mater. 15, 464 (2003)

A helical chiral space group: P3121 or P3221

#The relevant space group symmetries which are indicative of a chiral crystal, are the

65 space groups which lack any improper symmetry element )reflection, inversion, glide or roto-inversion ,(collectively known as

Sohncke space groups

#Surprisingly, not all are chiral

#22 of the 65 are chiral (helical)

Systematic search for Sohncke space groups in zeolites

#43 of the 65 are non-helical

#They are achiral space groups, despite the fact that they do not contain improper symmetries!

The non-helical Sohncke space groups

#They provide chiral crystals if the asymmetric unit is chiral

All of the chiral zeolites we found belong to that category

Example in focusGoosecreekite (GOO)

Si, Al, O

The main finding: Out of 120 classical silicate zeolites, we found 21 that must be chiral, but were not recognized as such

Ch. Dryzun et al, J. Mater. Chem., 19, 2062 (2009)Editor’s Choice, Science, 323, 1266 (2009)

The chiral TT’4 building blocks

Goosecreekite (GOO)

Chiral zincophosphate I

(CZP)α-Quartz

TT’42.052.940.55

SBU0.860.37------

A.U.14.761.280.00

Unit cell4.908.911.28

The chirality values are comparable or larger than the chirality values of the known chiral zeotypes and of quartz

Adsorption of D-histidine (the lower curve) or L-histidine (the higher curve) on Goosecreekite (GOO): The heat flow per injection

The isothermal titration calorimetry (ITC) experiment

L-histidine

With Y. Mastai and A. Shvalb, Bar-Ilan

Why have they been overlooked?

Conclusion

Always look under the lamp – it might be there!

The building blocks of quartz and of chiral zeolites

SiO4 SiSi4

So, what is a left-handed SiO4 tetrahedron?

CIP rules distinguish between the enantiomers of A(bcde) molecules:

F

Cl

Br

I

F

Cl

Br

I

Si(OSi)4

The steps:

1. Find the triangle with the maximal perimeter.

2. Check the direction from the

longest edge to the shortest one, facing the triangle.

3. Clockwise rotation (shown) is a right handed tetrahedron.

(The CIP logic of hierarchy)

1

2

3

R*

1 :5.774

2 :4.913

3 :4.369

D. Yogev et al Tetrahedron: Asymmetry 18, 2295 (2007)

A method to assign handedness to AB4 speciesThe Triangle-Method

The analyzed Goosecreekite (GOO) is a left-handed (So), as determined from the handedness of its most chiral TT`4 unit, Al(1)Si4