The chirality of the SiO4 building block in materials

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The chirality of the SiO4 building block in
materials David Avnir Institute of
Chemistry, The Hebrew University, Jerusalem
Special Symposium on Chemistry Honoring Santiago
Alvarez on the Occasion of His 65th Birthday
Barcelona, June 18, 2015
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By Andrea Carter, Vocalist Jaimsy Kennedy
http//www.songlegacy.com/audio/65thBirthdayWomanE
xcerpt.mp3
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Motivation
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The abundance of elements in Earths crust
The silicates
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The most common mineral in Earths crust
Quartz 59.7 (weight) Feldspar
15.4Haematite 2.6 MgO 4.4
Quartz is chiral
(https//answers.yahoo.com/question/index?qid2012
1205130239AAPOhoq)
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Quartz is chiral on all scales From the
macroscopic crystal habit to the molecular
building blocks
Space groups AP3121 BP3221
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
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Let us change that a little!
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Thanks Dina Yogev Chaim Dryzun Michael
Ottolenghi Sharon Fireman Sharon Marx Yitzhak
Mastai Hagit Zabrodsky
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Our focus
Amorphous and crystalline materials based on SiO4
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Step 1 Amorphous silica
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Amorphous silica
How is it possible to induce chirality in this
amorphous material?
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How is it possible to induce chirality in silica?
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
Key question to keep in mind All of these
methods induce chiral functionality, but does the
material itself become chiral?
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• 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-
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The chiral sol-gel polymerization approach
Fibers widths 2 to 5 nm
Michel Wong-Chi-Man et al, J. Am. Chem. Soc,
2001, 123, 1509-1510
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The sol-gel doping approach
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The sol-gel chiral imprinting approach
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Imprinting silica with a chiral surfactant
DMB The imprinting molecule
The sol-gel monomers p-p 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)
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Silica (partially phenylated) imprinted with
aggregates of DMB
was capable of separating the enantiomer-pairs of
BINAP Propranolol
Naproxen
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General enantioselectivity of imprinted silica
With S. Fireman, S. Marx
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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?
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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?
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Adsorption before and after extraction of the
imprinting molecule
Before extraction Chiral dopant (DMB)
After extraction Chiral holes
The recognition handedness changes!
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2nd proof that the building blocks near a chiral
event become chiral Induced circular dichroism
of Congo-red within silica
The chiral inducer DMB The
achiral probe CR
We shall compare Co-doping Adsorption of CR
on silica doped with DMB
With S. Fireman, S. Marx
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The ICD spectra of co-entrapped CR-DMB in
hydrophilic and hydrophobic silicas
CR-DMB_at_Silica (red line) and CR-DMB_at_Octylated
silica (blue line)
Has the silica matrix become chiral?
S. Fireman
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The ICD signal of CR adsorbed on DMB_at_silica
Co-dopingCR/DMB_at_silica
CR adsorbed on DMB_at_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_at_silica blue
DMB_at_C8-silica
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Step 2 Quartz and chiral silicate-zeolites
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All of the building blocks of quartz are chiral!
32- Left Helix
31- Right Helix
C2-symmetry, not exact Td
SiO4 Si(OSi)4
SiSi4
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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?
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Revisiting the aluminosilicate zeolites
ZSM-5, NanAlnSi96nO19216H2O
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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) Editors Choice, Science, 323, 1266 (2009)
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The 21 re-discovered chiral silicate zeolites
ZSM 23 (MTT) Laumontite (LAU) Goosecreekite (GOO)
GUS 1 (GON) Edingtonite 10 (EDI) Nabesite (NAB)
LTQ (BPH) RUB 23 Bikitaite (BIK)
LTA (LTA) SSZ-55 (ATS) Gismondine (GIS)
ZYT 6 (CHA) H-ZSM-5 (MFI) Franzinite (FRA)
ERS 12 Zeolite N (EDI) Epistilbite (EPI)
RUB 10 (RUT) Zeolite F (EDI) Amicite (GIS)
The chirality of these x-ray analyzed zeolites is
not mentioned in the original reports!
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The building blocks of zeolites we analyzed
TO4
TT4
T(OT)4
The secondary building unit (SBU)
The asymmetric unit T, Si, Al, O
Goosecreekite
The unit cell
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The isothermal titration calorimetry (ITC)
experiment on Goosecreekite
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
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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
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The continuous chirality measure Major
contributions by Santiago Alvarez
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By how much is one molecule more chiral than the
other?
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G The nearest achiral symmetry point group
Achiral molecule S(G) 0 The more chiral the
molecule is, the higher is S(G)
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The most chiral monodentate complex
S. Alvarez, Europ. J. Inorg, Chem., 1499 (2001)
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Example in focus Goosecreekite (GOO)
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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) a-Quartz
TT4 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
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The varying degree of chirality of quartz in
Nature
SiO4
Dina Yogev-Einot
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Phase diagram of the SiO2 family
Stishovite
Cristobalite
Coesite
Low-Quartz
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Pressure-chirality correlations in quartz
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Temperature and pressure effects Unified picture
T
P
A dAmour H (1979), B Jorgensen J D (1978) , C
Hazen R M (1989), D Glinneman J (1992), T
Kihara (1990).
D. Yogev-Einot
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The molecular distortion leading to the chirality
changes
The chirality measure as a single structural
parameter
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Quartz-germania (GeO2), quatz-silica Unified
picture
Chirality-pressure correlation
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The optical rotation of quartz 126 years ago Le
Chatelier and his contemporaries
Le Chatelier, H. Com. Rend Acad Sci 1889, 109,
264 .
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126 years later an exact match with quantitative
chirality changes
SiSi4
D. Yogev, Tetrahedron Asymmetry 18, 2295 (2007)
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Step 4 What is a left-handed SiO4
tetrahedron?
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Reminder of the CIP rules logic

• Rank the 4 substituents purplegtredgtbluegtgreen
• 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
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To answer the question what is a left-handed
SiO4 tetrahedron? one has to invent a
convention of handedness for chiral AB4
species. Lets do it!
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A method to assign handedness to AB4 species The
Triangle-Method

• The steps
• 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 5.774 2 4.913 3 4.369
D. Yogev
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Chiral zeolite Goosecreekite is left-handed
(Al(1)Si4)
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Yes, but if the definition is arbitrary why this
and not another one? Indeed, let us try another
one!
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The edge-torsion approach

• 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)

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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 rulesright handed by the
triangles rules
SiO4 Low-Cristobalite P41212 (no. 92) D. Peacor
(1973)
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Conclusion Where does the arbitrariness of
handedness labeling leave us? You must be very
careful
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because when you encounter your enantiomer,
she/he may claim to be the real thing!
Me and my enantiomer
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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)
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What does it mean for amorphous silica?

• Each tetrahedron has a unique distortion
• - therefore its enantiomer tetrahedron is
  statistically similar

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Kelvins 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
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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.