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.
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