Not all nitrogen atoms are equal: Contribution of peripheral versus internal amines to the observed reactivity and capture properties of melamine dendrons on SBA-15

Article abstract of DOI:10.1039/c7cc08157j

The role of different nitrogen atoms in melamine dendrons tethered to SBA-15 is elucidated. For the nitroaldol (Henry) reaction all nitrogen atoms of the capping group participate in the reaction. For metal binding, uptake correlates with the diamine cont

Full text of DOI:10.1039/c7cc08157j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Not all nitrogen atoms are equal: Contribution of peripheral
versus internal amines to the observed reactivity and capture
properties of melamine dendrons on SBA-15
Received 00th January 20xx,
Accepted 00th January 20xx
Y. Lou,a and D. F. Shantza,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The role of different nitrogen atoms in melamine dendrons
tethered to SBA-15 is elucidated. For the Nitroaldol (Henry)
reaction both nitrogen atoms of the capping group particpate in
the reaction. For metal binding, uptake correlates with the
diamine content indicating interior amines are accessible. Carbon
dioxide uptake properties are dictated by the presence of the
peripheral amines with little interior contribution. The results
show that different nitrogen atoms contribute to different
processes including catalysis, metal sequestration, and gas
separations.
Organic-inorganic hybrid materials are
a
scientifically
interesting and technologically relevant class of materials that
have found use in areas ranging from biomaterials and drug
delivery to environmental remediation and solar energy
applications.1-5 As the community pushes to ever more
complex materials, a key to advancing the field will be the
ability to understand the function of various structural centers.
Organosilane chemistry has been used extensively to
functionalize silica to give a variety of hybrid materials, which
can be further modified to develop more complicated
hybrids.6-8 As an example, many labs have investigated
complex polyamine brushes on silica supports for carbon
dioxide capture.9-12 Going forward the ability to understand
how various amine groups, i.e. primary versus secondary,
contribute to the mechanism of carbon dioxide capture, and
how the organic architecture can be tuned will be key.13, 14
Here we study the different amine groups in the melamine
dendrons our lab has reported previously.15-17 As shown in
scheme 1 there are multiple types of nitrogen atoms: those in
the aromatic ring of the triazine moiety, tertiary and secondary
amines associated with the diamine spacers, and the amine of
the silane. In this paper, we demonstrate how the different
types of amines in the dendron participate in catalysis, metal
Scheme 1. Molecular structure of Gx-PIP and Gx-CAP (x is dendron
generation) and terms for various types of nitrogen.
sequestration, and gas separations. We couple this with a
model dendron which employs piperazine as the diamine
spacer, but where the dendron is terminated with piperidine,
resulting in a dendron that is structurally identical except that
it does not contain any nitrogen on the external surface of the
dendron, i.e. there are no peripheral amines. As can be seen
in Scheme 1, the difference between the PIP and the CAP
samples is the termination step. The CAP samples have no
peripheral –NH– groups, and half as many nitrogen atoms in
their capping groups. Thermogravimetric analysis (TGA) and
porosimetry of the materials (Figure S1 and Tables S1, S2, ESI)
show that piperidine capping has negligible differences on the
Scheme 2. Nitroaldol (Henry) reaction.
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material properties, i.e. these samples have very similar ligand data has been analysed in numerous ways, including plots of
loadings and the only difference is the presence or absence of nitrobenzaldehyde conversion versus time as well as various
the external –NH– group.
normalized nitrobenzaldehyde consumption rates versus time.
The Nitroaldol reaction was first explored as a material In all cases the selectivity to the nitroalcohol is very high
probe. Figure 1 shows the results of the Nitroaldol reaction (>95%) except G1 PIP (80%).
Figure 1 shows the most interesting normalization, where
the consumption rate is normalized by the amount of nitrogen
in the capping group of the dendrimer, i.e. the last diamine
used to terminate the dendron. For the G1 case on a capping
group nitrogen basis the PIP and CAP samples have the same
reaction rate. This result indicates that for the G1 sample, not
only the external secondary amine in the piperazine but also
the interior tertiary amine of the peripheral PIP and CAP
groups contribute to the observed reactivity. If only the
peripheral nitrogen was responsible for the observed reactivity
the CAP samples would display little or no reactivity. That the
PIP samples fall off relative to the CAP samples at times after
1.5 h is due to the high conversion for the PIP samples at that
point (>80%). In contrast, the initial rates for the G2 and G3
samples are not the same, as the G2 and G3 PIP materials are
more active on any normalization basis. A simple explanation
for this is that as the dendrons grow larger and there is more
mass transfer resistance, the contribution of the peripheral –
NH– group to the observed rate increases. More plots
including conversion versus time on stream are given in the
supporting information (Figure S4, ESI). One last point worth
noting is that a control sample was prepared where the
cyanuric chloride was hydrolysed, i.e. G-0.5 was made. This
sample shows no reactivity, indicating the triazine group does
not contribute to the observed reactivity.
Palladium metal binding was used to probe how the
different nitrogen atoms participate in metal chelation (Figure
2). As expected, with increasing dendron generation more
metal is captured. However, on a total nitrogen basis the metal
uptake goes down with increasing dendron generation.
Possible explanations for this observation include that for
larger dendrons as the available void space in the pores
decreases that there is a decrease in metal uptake. Also, given
that not all amines are likely participating in metal chelation,
the normalization on a total nitrogen basis is not the most
meaningful one. Consistent with this, the most interesting
observation is that on a unit diamine nitrogen basis the two
materials PIP and CAP bind the same amount of metal, which
is shown in Figure 2 (middle). This and the fact that the G1-
CAP binds half as much Pd as G1-PIP leads to the conclusion
that both nitrogen atoms in the diamine bind metal but a
smaller fraction is accessible as one goes to larger generation
dendrons. This last observation is consistent with the rate of
decrease of metal uptake in Figure 2 (middle) being similar.
Finally, Figure 2 (bottom) where one normalizes by the capping
groups further indicates that the diamine nitrogen is the key
factor for binding Pd. The metal capture is the same at G1, but
Figure 1. Henry reaction comparison results of dendrimer with and
different at G2 and G3, perhaps due to more surface
without capping for (top to bottom), G1, G2, and G3. Aldehyde
peripheral amines in PIP interacting with the surface silanols
consumption rate is normalized by nitrogen in the capping group.
groups as the dendrons get larger and span the pores. The
results in Figure 2 show Pd+2 can penetrate into the dendron.
1
over the G1-G3 PIP and CAP samples (raw H NMR data and
conversion plots are given in Figure S3 and Figure S4, ESI). The
This is in contrast to what is observed for the Henry reaction.
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Figure 3. (a) TEM image and (b) histogram image of 1%Pd/G2-CAP.
the only difference in the parent materials is the terminating
group on the dendron. The conclusion from this is that the
absence of the nitrogen atoms on the periphery leads to larger
nanoparticles. This result shows that by changing the dendron
microstructure it is possible to manipulate nanoparticle size.
Figure 4 shows the carbon dioxide uptake from a gas
stream of pure carbon dioxide on a unit silica basis (top). The
two most significant observations are that the carbon dioxide
uptake only marginally increases (~15%) with each increasing
generation, and that by contrast capping with piperidine
versus piperazine leads to much larger differences in the
amount of CO2 adsorbed at a given generation. The CO2 uptake
of CAP samples is approximately half of the uptake of PIP
samples, consistent with a decrease of a factor of two in the
number of capping amine groups. To further test our
hypothesis that the CO2 uptake is mostly due to the peripheral
amines, the CO2 uptake was plotted versus the number of
capping amines for each sample, as shown in Figure 4 (middle).
It shows a linear relationship between CO2 uptake and number
of amine capping groups. This finding is consistent with our
hypothesis that the capping amine groups largely determine
carbon dioxide uptake, and that the tertiary interior amines
are much less important for carbon dioxide capture. This is
consistent with prior literature that indicates tertiary amines
Figure 2. Pd uptake results of dendrimer with and without capping (top)
per ligand, (middle) per diamine N, (bottom) per capping group N.
These findings provide insights into the size of probes that can
access the dendron interior.
Interestingly, these two very similar dendrons clearly bind
the palladium differently. As evidence of this, 1 wt% Pd was
loaded in the G2-CAP sample, following with reduction to Pd0.
STEM images of this material and the Pd particle size
histogram are shown in Figure 3. The Pd particles are
distributed uniformly in the sample with an average particle
size of 2.8 nm. In our previous report,18 1 wt% Pd/G2-PIP was
successfully prepared with an average size of 1.8 nm (Figure
S6, ESI). From the direct comparison of those two samples,
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dendrons. The Nitroaldol reaction findings indicate that the
amine sites in the capping group of the dendron contribute
most to the reactivity, whereas for palladium binding all the
non-aromatic amines contribute. Finally, for carbon dioxide
capture the peripheral amine groups are the major contributor
to the uptake properties. These types of molecular insights
are key to developing next-generation complex hybrid
materials for a range of emerging applications.
Acknowledgement
Y. L. acknowledges partial financial support from Schlumberger
via a Schlumberger Scholarship. D. F. S. acknowledges partial
financial support from Entergy and Tulane University.
Conflicts of interest
There are no conflicts to declare.
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Figure 4. CO2 uptake results for G1 to G3 (top), CO2 uptake plotted as
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Conclusions
The current work, through using well-controlled model
dendrons and a variety of probe molecule binding tools, shows
the role of the different nitrogen groups in melamine
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