Triphenyl Phosphite TPP: A Candidate for Conformational Polyamorphism

1
Use of Crystal Engineering Aspect for
understanding TPP and its analogues Amorphous
Polymorphs (TPPTriphenylphosphite)
2
Triphenyl Phosphite (TPP) A Candidate for
Conformational Polyamorphism

• Ab initio calculations have shown that it can
  exists in at least 2 conformations
• Modeled Cs to C3v geometry transition to
  determine energy barriers for TPP and TTP
• Gausian98 program
• Hartree Fock Method
• Basis Set
• 6-31g (dashed line)
• 6-31g(d,p) (solid line)

Liquid at RT Mp 29C
3
Two Possible Conformational Conformer

• In case, of TPP, surprisingly Cs conformer is
  debated for the CH-O hydrogen bonding.
• Thus, theory predicts 2 conformers, what about
  Experiments?

4
Experimental Proof for the Behavior of TPP DSC

• Sample isothermally held for
• 120 min (a), 25 min (b) 10 min (c) glass
  transition two crystalization transition
  incomplete formation of glacial phase
• Sample scanned at different rates
• Scan a (heating at 10 K/min) three transitions
  glass, crystalization and melting
• DHcrys DHfus
• Scan b (heating at 1 K/min) four transitions
  glass, 2 crystallizations and melting
• DHcrys(2peaks) Dhfus

This is an experimental proof for the presence of
two possible amorphous conformations, which is in
accordance with NMR, dielectric measurement,
density, viscosity and Neutron data.
5
Problem!!
Are the 2 peaks observed in DSC to do with
Conformational changes, predicted by theory? Or
is it something else is happening!!! If something
else then what it is?..
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Two ways to study this problem

• Understand the behavior of the compound
  thoroughly, by using various techniques

• 2. Synthesize similar compounds to understand the
  underline principlethis might end up in NOVEL
  phenomenona

We are taking 2nd route
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Structure of TPP Closer Look
Single Molecule
Nearest Neighbors
Unit Cell
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Experimental Aim Designing from Understanding
It is evident from the experimental (X-ray,
Neutron and DSC) and theoretical data, that TPP
crystallizes/precipitate in Cs symmetry and
possibly in C3 symmetry due to 1. Steric
hindrance of 3 aryl groups around the
phosphorous 2. Weak hydrogen bonding 3. Weak van
der Waals interaction

• Thus, to understand the behavior of TPP,
  following experiments were investigated
• Substitution of Sulphur and Selenium instead of
  Oxygen
• Introduction of methyl groups to phenyl ring
• 3. Introduction of methoxy group to phenyl
  ring

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TPP Analogues
Substituting Oxygen in TPP to Sulphur and
Selenium, results in white colored crystalline
compounds. These compounds crystallize in C3
symmetric structures and have melting points in
the range from 340 - 370K. On the basis of DSC,
both compound, didnt show any structural change.
There structures are analogues to
tri(meta-methoxyphenyl) phosphite. By
introduction of methyl groups, -ortho, -meta,
and/or -para gave a colourless liquid. These
compounds show increase in glass transition as
compared to TPP, in the order of -meta, -para and
-ortho methyl phenyl phosphites. But like TPP,
these liquids didnt show any obvious phase
transition in calorimetric analysis.
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Zusammen Hydrogen and Steric Interactions
Tris(ortho-Methoxy phenyl) phosphite White
Solid Tris(meta-methoxy phenyl) phosphite yellow
sticky solid Tris(para-methoxy phenyl)phosphite
colourless sticky solid
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Synthesis Strategies
General Method of synthesis of Phosphites/TPP
PCl3 3 Phenol
P(Phenol)3 3HCl
New Method of Synthesis..
PCl3
OºC
P(Ome-Phenol)3
Methoxy-phenol
nBuLi
High yield Less biproduct/clean synthesis Time
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Crystal Structure of Phosphite1
In this process we were the first to solve the
Single crystal structure of simple aryl
phosphites at Room Temperature. It is a rare
example of a triaryl phosphite, which is solid at
ambient temperature.
Crystals suitable for X-ray crystallography were
readily grown from toluene. The molecule has
3-fold rotational symmetry and crystallizes in
the cubic, non-centro symmetric space group P213.
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Crystal Structure of Phosphite2
Single Molecule
Nearest Neighbor
Unit Cell
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Characterization NMR
All the aryl phosphites show a single peak in 31P
NMR , around 120 ppm. NMR spectra shows all the
possible peaks. Interestingly 13C NMR and 1H NMR
shows metaH or meta-C in the ring is more
deshielded than ortho and para
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Fluorescent Molecular Glasses
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Chiral Molecular Glasses!
The chirality of the compound was assigned on the
basis of an ? refinement The compound is of
course a racemic mixture as prepared, the crystal
studied contains a single enatiomeric conformer,
of the form R, assigned because the methoxy
groups are oriented in a clockwise fashion.
Circular Dichroism experiments do not show any
chirality.
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onclusion
Theory predicts, TPP can possibly crystallize in
at least two conformers, CS being the more stable
one than C3 conformer. Steric factors in C3
conformation is ruled out, because its analogues,
crystallize in C3 conformer only. This means in
TPP hydrogen bonding might be a driving force for
the stable CS conformer. More efforts are
required to understand this phenomenon, in less
anisotropic small molecules. We were the first
solve the Crystal Structure of simple Aryl
Phosphite, at Room Temperature Synthesis might
help in developing NOVEL molecular glasses, like
chiral, or luminescent.
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Acknowledgement
Prof. Jeff Yarger All Yarger group members for
healthy interactions Prof. Anderson and his
group for patience!!
RAM RAM
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Concept of Polyamorphismby Physicists
When Viscous liquids are supercooled, then
sometimes transform into metastable phases that
are different from the liquid, the glass and the
normal crystal. These phases are apparently
amorphous in the sense that their X-ray or
Neutron scattering patterns do not show Bragg
Peaks, and the phenomenon is often referred to as
POLYAMORPHISM. Most of the examples of
polyamorphism concern tetrahedrally bonded
systems such as water, silica and the change is
usually obtained by applying a high pressure.
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Concept of Polyamorphismby Chemists
Similar to well known Polymorphism
The phenomenon that a system can exists in at
least two liquid or amorphous phases having the
same chemical composition but distinct physical
behavior like, density, structure has been coined
POLYAMORPHISM.
USE?
Commonly observed for molecules with LARGE
ANISOTROPY like liquid crystals, biomolecules,
or polymers.
It looks like TPP shows similar behavior!!!
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NMRHH
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NMR 13C-1H
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Highlights of TPP Crystal Structure Neutron and
X-ray data
1. Crystallizes with hexagonal symmetry, in the
space group R3. The unit cell contains 18
molecules, with cell volume 7156 Å3. 2. Two
phenyl rings of a TPP molecule point in upward
direction towards the phosphorous atom, whereas
the third phenyl ring is oriented in the opposite
direction (Figure 1). The former phenyl rings are
twisted by roughly 15 from the ecliptical
position. 3. Neighboring molecules are arranged
parallel towards each other forming one
dimensional infinite rods, which are aligned
parallel to the c-axis of the unit cell (Figure
2). The dipole moments of the molecules in one
rod are aligned parallel. Thus, each rod has a
net dipolar moment. In order to minimize the
repulsive forces between neighboring rods in the
hexagonal packing, the rods are aligned
anti-parallel. Consequently, the 6-fold rotation
axis present in the hexagonal rod packing is
reduced to 3-fold rotoinversion symmetry. 4.
Figure 3 shows part of such a rod containing two
molecules. The shortest interatomic distances are
in the range of 2.8 - 3.4 Å demonstrating that
the dominating attractive interaction between TPP
molecules is based on van der Waals forces.
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Analogy of crystal structures
In contrast TPP, molecule by Neutron data, as
well as Powder X-ray data, showed no symmetry.
Two OPO angles are with 102.3, and 101.4 ,
larger than 96.6 found in meta-methoxy phenyl
phosphite, whereas the third OPO angle is
smaller, 91.4 . The benzyl rings are connected
to O2 and O3 are twisted by roughly 15 against
each other, while in case of 2 they are eclipsed.