Synthesis of LowDimensional Antiferromagnet Ladders and Cu4OBr62Aminopyridine4

1
Synthesis of Low-Dimensional Antiferromagnet
Ladders and Cu4OBr6(2-Aminopyridine)4
Matthew Phillips 06 (sponsored by Professors M.
Turnbull and C. Landee)Clark University 950 Main
Street Worcester, MA 01610
Our research is aimed at developing a
synthetic pathway which would provide control
over the number of rails in low-dimensional
antiferromagnet ladder chain. This synthetic
pathway is hypothesized to work with complexes
made from both 2-aminopyridine and
2-aminopyrimidine with copper halides. By varying
ratios of each, we hope to control the number of
rails. Overall, synthetic techniques used to
construct pure complexes has produced ligand
impurities. Further modifications to the
syntheses will be conducted. Through this
research, an adamantanoid complex has been
synthesized with the potential to exhibit spin
frustration.
This value for copper is roughly 0.4. The value
actually measured was close to 0.6 which suggests
that the formula used to calculate the processed
data is incorrect. This can be juxtaposed with
(figure 9) where the Curie constant is about .35
and is continuing to rise closer to 0.4. Other
plots constructed showed strong interactions
within the compounds. When the data for
Cu(2-APM)2Cl2 was processed with the structure
Cu(2-APM)Cl2 the Curie constant measured was much
closer to the expected 0.4. This suggested that
the reaction used to create the 21 compound was
not successful. This was confirmed with the CHN
analysis data. Data collected from CHN
analysis suggests that all the compounds except
for the pyrimidine bromide complexes contain
between ten to twenty percent extra ligand. This
again confirms the conclusions from the previous
analytical procedures and suggests that new
synthetic means are required to achieve the
desired pure products.
Introduction
Magnetism is a characteristic of materials
based on the its number of electrons as well as
the alignment of their magnetic moments (an
electrons property of acting like a small
magnet). All compounds are either diamagnetic or
paramagnetic. Paramagnetic materials have at
least one unpaired electron, which causes the
magnetic moment to remain unbalanced. Paramagnet
compounds can be further subdivided into
antiferromagnets and ferrimagnets. Both are
compounds whose magnetic moments align opposite
to each other in decreasing temperatures
however, ferrimagnets have moments that are
stronger in one direction than the other.
Cu4OBr6(2-AP)4
During a reaction between 2-Aminopyridine
and Copper (II) Bromide black single crystals
were formed. Crystal data was analyzed and
interpreted to be a compound analogous to
adamantane (Figure 10).
Our research involves the study and
synthesis of low-dimensional antiferromagnet
ladder chains. Our focus is to study the magnetic
character of these metal complexes as the number
of rails changes. Research in this field has led
to the discovery of roughly a dozen different
two-linked chain compounds (Figure 1) and only a
few 3-linked chain compounds (Figure 2). In
addition, the three-linked chains all require an
impractical amount of magnetic field strength to
collect any data points.
Initially, both reaction 1 and 2 were
performed with water as a solvent. This led to a
copper hydroxide complex for a product. This side
reaction had been seen before with basic
solutions as the ligands acted as a Lewis base
with their free lone pair electrons. This forced
the use of absolute ethanol as the solvent.
Initially, it was necessary to confirm whether a
reaction had taken place. This was accomplished
with both visual observations and infrared
spectroscopy. All of the six reactions performed
produced a significant color change from the
starting materials, which is characteristic of
complex formations. The pyridine compounds both
produced similar spectra (Figure 7) in comparison
to the spectrum of 2-aminopyridine (figure 6).
Most of the change that occurred in the 3000 to
3500 cm-1 range. These pecks represent the amino
(-NH2) functional group in the compound. This
change is appropriate as complex structure allows
for hydrogen bonding between the amino group and
halides attached to the copper. This creates a
six-member ring (figure 5) which is very stable.
Other pecks in the 1300 to 1800 cm-1 range
represent the aromatic bonds of the compound.
Most exhibited a similar shift in energy also
suggesting that something has bonded to the
ligand.
It is thought that this was only a minor
product from the reaction as the cell constants
measured did not match a powder diffraction
analysis when the remaining product was analyzed.
Additional syntheses have been performed
to try to synthesize this specific molecule as
its structure has yet to be published. Emphasis
is being placed on first forming the multicyclic
copper base before addition of the the pyridine
ligand.

This past research has led to our current
goal of developing a

synthetic pathway
to allow a simplistic method of controlling the
number of rails on a metal complex. Our plan to
achieve this is to use two compounds. The first
is a is a two-dimensional sheet made from
2-aminopyrmidine and CuX2 (XCl,Br) (Figure 3).
The second is a one-dimensional chain made out of
a complex of 2-aminopyridine and CuX2 (XCl,Br)
(Figure 4).
From each of these systems one can see that
the pyrmidine compound acts as a bridge molecule
by being able to bind in two directions because
of the lone pairs on each of its ring nitrogens.
The pyridine compound can be classified as a
cap molecule because it can only bind once with
its single ring nitrogen. By taking advantage of
each of these two distinct types of ligands our
hope is to be able to vary the stoichiometric
amount of each, to
(Figure 10)
obtain a ladder molecule with a distinct number
of rails.
One of the interesting aspects of the
crystal structure of the isolated adamantanoid
complex is its potential to exhibit
characteristics of a spin frustrated system.
Frustration occurs in magnetic compounds that are
unable to exist in one clearly defined ground
state. This can be exemplified by a dipole system
that is positioned in the shape of an equilateral
triangle (figure 11). Two of the vectors can be
positioned anti-parallel to one another, however
the direction of the third then becomes an issue.
The geometry of the oxygen in
Cu4OBr6(2-AP)4 is extremely close to that of
tetrahedral. Research is currently being
performed to produce enough Cu4OBr6(2-AP)4 to run
magnetic susceptibility tests.
To achieve this it is necessary to obtain
magnetic and crystallography data of pure samples
of both the pyrimidine and pyridine complexes.
This can then be later used to juxtapose a
mixture to determine if a new complex structure
was formed.


Overall, it is important to study such
compounds because they have an application in
quantum mechanics theory. Another application of
studying these magnetic ladder chains is their
application in super conductors. Ladder molecules
can act as superconductors due to their low
dimensionality.
These trends also appear in the IRs of the
pyrimidine compounds. One other aspect of the IRs
worth noting was the fact that the spectrum of
the Cu(2-APM)2Cl2 was nearly identical to that of
Cu(2-APM)Cl2. This suggested that these were the
same compounds.
(Figure 11)
To further analyze the compounds, magnetic
data was collected in a superconducting quantum
interference device (SQUID) magnetometer. All of
the compounds collected displayed strong
interactions as observed from the Chi, ChiT, and
1/Chi data collected. The ChiT vs. T graph
(figure 8) can also be used to help identify the
structure of the compound. The chart reaches a
horizontal maxima which represents the Curie
Constant, unique for each metal.
The next step in this research project is
to modify the methods used to synthesize the
complexes. It is necessary to determine the
reason behind the inability to produce a ligand
to metal complex in a two-to-one ratio. One
possible way to achieve this, is to modify the
ratio of ligand and metal used in the reaction.
This would also help determine whether or not the
extra ligand found in the products is unreacted
reagent or a combination of different complexes
formed in the reaction. In addition, a
large effort has been placed on reproducing the
synthesis of the adamantanoid compound. Further
crystal data analysis is underway as well as
carbon, hydrogen, and nitrogen analysis. This
data will provide a better means of understanding
the reactions performed and help lead to a more
efficient synthesis.
(Figure 8)