The chirality of the SiO4 building block in materials

David Avnir

Institute of Chemistry, The Hebrew University, Jerusalem

Special Symposium on ChemistryHonoring Santiago Alvarez

on the Occasion of His 65th Birthday Barcelona, June 18, 2015

By Andrea Carter, Vocalist: Jaimsy Kennedy

http://www.songlegacy.com/audio/65thBirthdayWomanExcerpt.mp3

Motivation

The abundance of elements in Earth’s crust

The silicates

The most common mineral in Earth’s crust

(https://answers.yahoo.com/question/index?qid=20121205130239AAPOhoq)

Quartz = 59.7 (%weight) Feldspar = 15.4

Haematite = 2.6 MgO = 4.4

Quartz is chiral

Space groups: A:P3121 & B:P3221

Quartz is chiral on all scales: From the macroscopic crystal habit to the molecular building blocks

There are by far more Si species which are chiral than chiral C species which are chiral on planet Earth

Si: 28.1%, C: 0.18%, Si/C = 160

But only ~0.1% of the chirality papers are on Si

Let us change that a little!

Thanks

Dina Yogev

Chaim Dryzun

Michael Ottolenghi

Sharon Fireman

Sharon Marx

Yitzhak Mastai

Hagit Zabrodsky

Our focus:

Amorphous and crystalline materials based on

SiO4

Step 1: Amorphous silica

Amorphous silica

How is it possible to induce chirality in this amorphous material?

The classical approach: Use of auxiliaries

* Adsorb on the surface a chiral molecule

* Covalently silylate the surface with a chiral silylating agent

* Polymerize a chiral trialkoxysilane

* Entrap physically a chiral molecule

* Hybridize the material a with a chiral polymer

* Imprint the material with a chiral template

How is it possible to induce chirality in silica?

Key question to keep in mind:

All of these methods induce chiral functionality, but does the material itself become chiral?

The sol-gel polycondensation reaction

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-

NH

NH

NH

NH

O

O

SiO1.5

SiO1.5

(R,R)n

NH

NH

NH

NH

O

O

SiO1.5

SiO1.5

(S,S)n

Fibers widths

2 to 5 nmMichel Wong-Chi-Man et al, J. Am. Chem. Soc, 2001, 123, 1509-1510

The chiral sol-gel polymerization approach

The sol-gel doping approach

The sol-gel chiral imprinting approach

Imprinting silica with a chiral surfactant

CHO

HC

CH3

H

N

CH3

CH2(CH2)10CH3

CH3

+

DMB: The imprinting molecule

The sol-gel monomers

# interactions (with Si-Ph)# Hydrogen bonding (with Si-OH and Si-O-Si)

# Ionic interaction (with Si-O-)# Hydrophobic interactions (with Si-Ph, Si-O-Si, Si-OEt)

CHO

HC

CH3

H

N

CH3

CH2(CH2)10CH3

CH3

+

CH3O

CH C

O

OH

H3COCH2CHCH2NHCHCH3

HOH CH3

PO

OH

O

O

Silica (partially phenylated) imprinted with aggregates of DMB

was capable of separating the enantiomer-pairs of:

BINAP Propranolol Naproxen

0.93

1.03

1.13

1.23

1.33

Dis

crim

inat

ion

Rat

io

SR

R

General enantioselectivity of imprinted silica

With S. Fireman, S. Marx

If an SiO2 material is made chiral by a foreign molecule, 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?

# Is it possible that the material becomes chiral farther from the chiral event?

Before and after imprinting

After imprinting, enantioselective imprinting occurs in the imprinted hole, and non-selective adsorption occurs in the other pores.

If the imprinted molecule remains inside, adsorption is still possible in the other pores – have some of them become enantioselective?

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

Dis

crim

inat

ion

Rat

io

S

R R

0.93

1.03

1.13

1.23

1.33

Dis

crim

inat

ion

Rat

io

SR

R

Adsorption before and after extraction of the imprinting molecule

Before extraction: Chiral dopant (DMB)

After extraction:

Chiral holes

The recognition handedness changes!

2nd proof that the building blocks near a chiral event become chiral:

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

We shall compare:

* Co-doping

* Adsorption of CR on silica doped with DMB

CR-DMB@Silica (red line) and CR-DMB@Octylated silica (blue line)

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

S. Fireman

-40

-20

0

20

40

60

80

300 400 500 600

Wavelength (nm)

CD

(m

deg)

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

Has the silica matrix become chiral?

-6

-5

-4

-3

-2

-1

0

1

2

300 400 500 600

Wavelength (nm)C

D (

mde

g)

The ICD signal of CR adsorbed on DMB@silica

Co-doping:CR/DMB@silica

CR adsorbed on DMB@silica

What do we see:

Reversal of the ICD signal indicates that the chirality-inducer is different in the two cases. The only possibility is that chiral skeletal porosity was induced by the doped DMB

Red: Reference silica; black: DMB@silica; blue: DMB@C8-silica

Step 2: Quartz and chiral silicate-zeolites

31- Right Helix31- Right Helix

SiO4 Si(OSi)4 SiSi4

All of the building blocks of quartz are chiral!

32- Left Helix

C2-symmetry,

not exact Td

If chiral SiO4 is a stable solution in Nature and in amorphous silicas, could it be that it is much more common than previously thought?

Revisiting the aluminosilicate zeolites

ZSM-5, NanAlnSi96–nO192·16H2O

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

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)

Goosecreekite (GOO) Laumontite (LAU) ZSM – 23 (MTT)

Nabesite (NAB) Edingtonite 10 (EDI) GUS 1 (GON)

Bikitaite (BIK) RUB 23 LTQ (BPH)

Gismondine (GIS) SSZ-55 (ATS) LTA (LTA)

Franzinite (FRA) H-ZSM-5 (MFI) ZYT 6 (CHA)

Epistilbite (EPI) Zeolite N (EDI) ERS 12

Amicite (GIS) Zeolite F (EDI) RUB 10 (RUT)

The 21 “re-discovered” chiral silicate zeolites

The chirality of these x-ray analyzed zeolites is not mentioned in the original reports!

The building blocks of zeolites we analyzed

TO4TT’4T(OT’)4

The asymmetric unit

T, Si, Al, O

The secondary building unit (SBU)

The unit cellGoosecreekite

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 on Goosecreekite

L-histidine

With Y. Mastai and A. Shvalb

In all of the examples of Steps 1 and 2, the Si building blocks have been chirally distorted to different levels:

Is it possible to evaluate quantitatively the degree of the chirality of the various building blocks?

Step 3: Evaluation of the chiral distortion

The continuous chirality measure

Major contributions by Santiago Alvarez

“By how much is one molecule more chiral than the other?”

Calculating the degree of symmetry and 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)

S(TP)

[Ta(CCSitBu3)6]- [Ti2(-SMe)3(SMe)6]2-[Zr(SC6H4-4-OMe)6]2-

1.88

18.8°

1.67

8.27

5.51

1.34

33.3°

4.45

3.94

2.16

30.4°

5.09

S(chir)

S(Oh)

The most chiral monodentate complex

S. Alvarez, Europ. J. Inorg, Chem., 1499 (2001)

Example in focus: Goosecreekite (GOO)

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

SiO4

0

0.01

0.02

0.03

0.04

0.05

Te

tra

he

dric

ity

0

0.003

0.006

0.009

Ch

ira

lity

Tetrahedricity

Chirality

The varying degree of chirality of quartz in Nature

Dina Yogev-Einot

Phase diagram of the SiO2 family

Cristobalite

Low-Quartz

Stishovite

Coesite

a SiSi4

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0 3 6 9 12Pressure (GPa)

Ch

ira

lity

a SiO4

0

0.02

0.04

0.06

0.08

0 3 6 9 12Pressure (GPa)

Ch

ira

lity

Pressure-chirality correlations in quartz

Temperature and pressure effects: Unified picture

A: d’Amour H (1979), B: Jorgensen J D (1978) , C: Hazen R M (1989), D: Glinneman J (1992), T: Kihara (1990).

15.4

15.6

15.8

16

16.2

16.4

16.6

16.8

90 95 100 105 110 115 120

Ch

iral

ity

A

B

C

D

T

Unit Cell Volume

P

T

D. Yogev-Einot

Pressure (GPa)

0.0001

6.8

Temperature (K)

298

838

2

1

43

0

2

1

43

0

The molecular distortion leading to the chirality changes

The chirality measure as a single structural parameter

0 5 10 15

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15

Pressure (GPa)

Chi

ralit

y

GeGe4

SiSi4

GeO4

SiO4

a

b

20 SiO2

GeO2

SiO2

GeO2

20

GeGe4

SiSi4

0 5 10 15

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15

Pressure (GPa)

Chi

ralit

y

GeGe4

SiSi4

GeO4

SiO4

GeO4

SiO4

GeO4

SiO4

a

b

20 SiO2

GeO2

20 SiO2

GeO2

SiO2

GeO2

20 SiO2

GeO2

20 SiO2

GeO2

20 SiO2

GeO2

20

GeGe4

SiSi4

Quartz-germania (GeO2), quatz-silica: Unified picture

Chirality-pressure correlation

Le Chatelier, H. Com. Rend Acad Sci 1889, 109, 264 .

The optical rotation of quartz: 126 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

teli

er

t

Ch

irality, SiSi4

Chirality t

126 years later: an exact match with quantitative chirality changes

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

SiSi4

Step 4:

What is a left-handed SiO4 tetrahedron?

Reminder of the CIP rules logic

1. Rank the 4 substituents: purple>red>blue>green

2. Look from the green to the black; two different purple-to-blue rotations are seen: Left handed and right handed.

But there is no hierarchy in the 4 oxygen atoms of SiO4

To answer the question

“what is a left-handed SiO4 tetrahedron?”

one has to invent a convention of handedness for chiral AB4 species.

Let’s do it!

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

A method to assign handedness to AB4 species

The Triangle-Method

Chiral zeolite Goosecreekite is left-handed (Al(1)Si4)

Yes, but if the definition is arbitrary why this and not another one?

Indeed, let us try another one!

1. Project one edge onto the other - three angles form.

2. Select the smallest angle from the three.

3. Check the angle direction from top to bottom and assign the helix notation

(Right handedness is shown)

The edge-torsion approach:

Could it be that the same object is right-handed by one definition and left-handed by the other?

Yes.

Example: SiO4 of Low-Cristobalite:

Left handed by the torsion rules;right handed by the triangles rules

SiO4 Low-Cristobalite P41212 (no. 92)

D. Peacor (1973)

Conclusion:

Where does the arbitrariness of handedness labeling leave us?

You must be very careful…

… because when you encounter your enantiomer, she/he may claim to be the real thing!

Me and my enantiomer

What does it mean for amorphous silica?

# The tetrahedra are chiral because the environment of each is non-isotropic, and because the chance that the distortion retains a reflection mirror, is small.

#Silica is a racemic mixture of chiral SiO4 tetrahedra:

- Half comprise a homochiral left-handed set, and half a right-handed set

- This is true for ANY handedness definition(

What does it mean for amorphous silica?

# Each tetrahedron has a unique distortion;

- therefore its enantiomer tetrahedron is statistically similar

Kelvin’s definition of chirality

“I call any geometrical figure, or any group of points, chiral, and say it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself." – Lord Kelvin

What does it mean for amorphous silica?

# Each tetrahedron has a unique distortion;

- therefore its enantiomer tetrahedron is statistically similar

# Induction of chirality by any of the auxiliary methods, will enrich the chiral population of SiO4 tetrahedra with one type of handedness.