Post-assembly guest oxidation in a metallo-supramolecular host and structural rearrangement to a coordination polymer

Article abstract of DOI:10.1039/c4cc05422a

PTA hosted in a copper metallo-supramolecular triangle undergoes post-assembly oxidation to form PTAO in aerated solutions. The oxidation is triggered by selected co-solvents that also govern the formation of the final crystalline product leading to a discrete host-guest triangle {PTAOat[Cu(o-L)]₃} or to a 1D coordination polymer {(PTAO)₂at[Cu₈][Cu₂]}_∞containing a {Cu₈} ring with a double hosting pocket.

Full text of DOI:10.1039/c4cc05422a

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Post-assembly guest oxidation in a
metallo-supramolecular host and structural
rearrangement to a coordination polymer†
Marzio Rancan,*ab Jacopo Tessarolo,a Silvio Quicic and Lidia Armelaoab
Cite this: Chem. Commun., 2014,
50, 13761
Received 14th July 2014,
Accepted 18th September 2014
DOI: 10.1039/c4cc05422a
www.rsc.org/chemcomm
PTA hosted in a copper metallo-supramolecular triangle undergoes of a constitutional dynamic library (CDL) of metallo-supramolecular
post-assembly oxidation to form PTAO in aerated solutions. The polygons where a dimeric rhomboid [Cu(o-L)]2 and a trimeric
oxidation is triggered by selected co-solvents that also govern triangle [Cu(o-L)]3 are both accessible and in equilibrium.4 Selection
the formation of the final crystalline product leading to a discrete toward the triangular species is thermodynamically driven in the
host–guest triangle {PTAO@[Cu(o-L)]3} or to a 1D coordination presence of well-suited molecules acting as guests to form the
polymer {(PTAO)2@[Cu8][Cu2]}N containing a {Cu8} ring with a double {G@[Cu(o-L)]3} host–guest architectures,4a,c according to a molecular
hosting pocket.
recognition mechanism with two possible paths (Fig. 1). Among
the possible guests, PTA and its oxide PTAO are included. Both
Coordination driven metallo-supramolecular architectures the host–guest species {PTA@[Cu(o-L)]3} and {PTAO@[Cu(o-L)]3}
where metal ions and polytopic ligands self-assemble to form are accessible: they revealed to have quite strong and compar-
discrete boxes and capsules represent intriguing host systems.1 able host–guest association constants4c (log K HPTGA = 3.39 (Æ2) and
PTAO
HG
Impressive examples of their potentiality have been reported.2 log K
= 3.11 (Æ1) at 298 K) and a robust supramolecular
These metallo-supramolecular capsules can be used to safely motif. To the best of our knowledge, {PTA@[Cu(o-L)]3} is one of
store unstable and reactive compounds,2a for space confined the rare examples5 of a m3-PTA molecule with three coordinating
reactivity2b or ‘‘enzyme-like’’ catalysis.2c Moreover, they can be N atoms and a P atom not bonded to a metal centre. Therefore, the
employed as nano-sized reactors by permitting in their cavities phosphorus atom is a free and available site to explore a series of
the synthesis of metal oxide nanoparticles with remarkable well-known PTA derivatizations of practical interest.6 Firstly, we
control of the nanoparticle size.2d As recently demonstrated,3 the investigated the use of the [Cu(o-L)]3 host as a scaffold for the
functional properties of such systems can be further tailored through post-assembly oxidation of the PTA molecule. PTA is an air-stable
post-functionalization of the metallo-supramolecular architecture. and water soluble phosphine, commonly used as a neutral P-donor
This approach is challenging and can be successfully adopted only ligand.6 It is usually oxidized with strong oxidation agents such as
in the presence of robust self-assembled architectures.
In this communication, we study the oxidation of PTA (1,3,5-
triaza-7-phosphaadamantane) to PTAO (1,3,5-triaza-7-phospha-
adamantane-7-oxide) in the environment of a coordination
driven host. The reaction is performed in a post-assembled
host–guest ensemble where PTA plays the role of a templating
guest for the metallo-supramolecular host. Recently, we have
reported that the self-assembly of Cu2+ ions with the ortho bis-
(3-acetylacetone)benzene ligand (hereafter, o-LH2) leads to formation
a Department of Chemical Sciences and INSTM, University of Padova,
via Marzolo 1, 35131 Padova, Italy. E-mail: marzio.rancan@unipd.it
b IENI–CNR, via Marzolo 1, 35131 Padova, Italy
c ISTM–CNR, via C. Golgi, 19, 20133 Milano, Italy
† Electronic supplementary information (ESI) available: Experimental details,
Fig. 1 Representation of the two possible molecular recognition selection
processes to give the {G@[Cu(o-L)]3} host–guest architecture developed in
our previous studies.4
supplementary FT-IR spectra, PXRD patterns and SCD data. CCDC 1013528. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc05422a
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30% H2O2,7a nitrogen tetroxide7b or with diluted H2O2 solutions in
the presence of a rhenium based catalyst.7c Examples are also
reported where PTA is partially oxidized in the presence of reduced
metal ions (Ni(II),8a Co(II)8b,c and Cu(II)8d). Herein, we report that
oxidation can be successfully carried out under mild conditions
using aerated solutions at room temperature, starting from the
{PTA@[Cu(o-L)]3} species. The reaction is triggered by selected
co-solvents. Several co-solvents were tested such as methanol,
ethanol, acetonitrile, ethyl acetate, tetrahydrofuran, 1,4-dioxane,
acetone, n-hexane, cyclo-hexane and n-heptane. In three cases
we succeeded in obtaining single crystals showing that the
co-solvent also strongly affects the final crystalline product leading
to: the starting host–guest motif {PTA@[Cu(o-L)]3} from a CHCl3–
n-hexane solution, the target oxidized species {PTAO@[Cu(o-L)]3}
from a CHCl3–CH3CN solution, or an unpredictable oxidized species,
a 1D coordination polymer, {[(PTAO)2(H2O)[Cu(o-L)]8][Cu(o-L)]2}N
(hereafter, {(PTAO)2@[Cu8][Cu2]}N) from a CHCl3–EtOH solution.
The PTA host–guest species, {PTA@[Cu(o-L)]3}, is very stable in air
in the solid state as well as when dissolved in dichloromethane
or chloroform solutions. Upon heating the supramolecular
structure is not affected and the PTA guest is stable against
oxidation. Interestingly, when {PTA@[Cu(o-L)]3} is dissolved in a
chloroform–acetonitrile solution and left in a closed vial, PTA
spontaneously oxidizes (Fig. 2), as revealed from FT-IR spectra by
monitoring the PQO stretching band at ca. 1160 cmÀ1. Fig. 3 shows
that after 6 hours the diagnostic PQO signal is clearly visible. The
band intensity increases with time. Moreover, after one week, the
formation of dark green cube shaped crystals of {PTAO@Cu3} was
observed and, after three weeks, a quantitative crop of single crystals
was obtained from the CHCl3–CH3CN solution.
Fig. 4 Normalized FT-IR spectra of {PTA@[Cu(o-L)]3} in a CHCl3–EtOH
solution. The increase in the PQO band intensity with time.
Results obtained from FT-IR analysis showed a similar trend
for {PTA@[Cu(o-L)]3} dissolved in a chloroform–ethanol solution
(Fig. 4). However, in this case a crystalline blue precipitate was
obtained. Single crystal diffraction (SCD) and powder X-ray
diffraction (PXRD) analyses revealed that this compound is a
coordination polymer with a structure composed of two alternating
subunits: a {Cu2} dimer and a {Cu8} ring (Fig. 5). The structure of
the first subunit corresponds to the [Cu(o-L)]2 dimer as it has been
obtained from the CDL solution by constitutional crystallization4b
(see the ESI† for comparison). The main difference is that in the
{Cu2} subunit the Cu2+ ion is coordinated, in an apical position, by
a nitrogen atom of a PTAO molecule.
Fig. 2 Oxidation of {PTA@[Cu(o-L)]3} to {PTAO@[Cu(o-L)]3} in a CHCl3–
CH3CN solution. Colour code: Cu purple, P orange, N blue, O red, C grey,
H atoms are omitted for clarity.
Fig. 5 (a) Capped stick view of the crystal structure of {(PTAO)2@[Cu8][Cu2]}N
:
a polymeric chain of alternating {Cu2} and {Cu8} units; (b) and (c) side and top
views of the double arrow {Cu8} ring. Colour code: Cu purple, P orange, N blue,
O red, C grey, H atoms are omitted for clarity.
Fig. 3 Normalized FT-IR spectra of {PTA@[Cu(o-L)]3} in a CHCl3–CH3CN
solution. The increase in the PQO band intensity with time.
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Fig. 7 UV-Vis spectroscopy. (i) PTA is oxidized by bubbling air in a
{PTA@[Cu(o-L)]3} CHCl3–CH3CN solution. (ii) PTAO is recovered after
guest exchange with HMT.
Fig. 6 Structural correlation among the two CDL constituents and the
{Cu8} ring. PTAO guests in the {Cu8} ring have been omitted for clarity.
Regarding the shape of the {Cu8} subunit, it can be described
as a ring folded in a double arrow structure. This conformation is undergo oxidation when the CHCl3–co-solvent solutions were
held up by the presence of two PTAO molecules hosted in two stored under argon.
triangular cavities. The pockets are reminiscent of the [Cu(o-L)]3
Air bubbling experiments were also used to quantitatively
metallo-supramolecular triangular cavity. In fact, the {Cu8} ring oxidize and then recover the PTAO molecule. In this case, the
can be described to be composed of ‘‘open triangles’’ (Fig. 6, reaction was followed by UV-Vis spectroscopy. After bubbling
blue part) connected by ‘‘open dimers’’ (Fig. 6, red part). SCD air overnight in a {PTA@[Cu(o-L)]3} CHCl3–CH3CN solution, the
analysis indicates the existence of strong structural correlations {PTA@[Cu(o-L)]3} spectrum (red line, Fig. 7) changes to the
between the {Cu8} ring and the CDL constituents. It can be characteristic {PTAO@[Cu(o-L)]3} one (green line, Fig. 7).10
hypothesized that {Cu8} analogues are involved as transitory Subsequently, PTAO can be quantitatively recovered after
intermediates in the CDL self-assembly and in the equilibrium guest-exchange with hexamethylenetetramine (HMT) to form
of the two constituents (3[Cu(o-L)]2 " 2[Cu(o-L)]3). At variance the more stable {HMT@[Cu(o-L)]3} species (log KHHGMT = 6.28 (Æ3)
with the {PTAO@[Cu(o-L)]3} species, in the coordination polymer at 298 K).4c The same experiment was repeated with a CHCl3–
PTAO interacts with the {Cu8} ring also through the PQO moiety. THF solution, in this case a quantitative oxidation was achieved
CCDC database research did not reveal any other example of after six hours (ESI†) of air bubbling.
PQOÁ Á ÁCu interaction involving the PTAO ligand (PQOÁ Á ÁCu
In conclusion, we have reported that a metallo-supramolecular
2.362(2) Å). In {(PTAO)2@[Cu8][Cu2]}N there are two m4-PTAO host can be used as a suitable environment for PTA oxidation
molecules with two nitrogen atoms coordinated to two Cu atoms under mild conditions. The oxidation is triggered by several
of the {Cu8} ring, while the third nitrogen atom connects the co-solvents, while it does not occur in pure chloroform or
alternating {Cu2} and {Cu8} subunits (see the ESI† for structural chloroform–alkane solutions. We found that the co-solvents
details). It is interesting to note that {(PTAO)2@[Cu8][Cu2]}N is strongly affect the final metallo-supramolecular crystalline
obtained only as a consequence of PTA oxidation from chloroform– product. In one case (CH3CN), the desired {PTAO@[Cu(o-L)]3}
ethanol solutions of {PTA@[Cu(o-L)]3}. In fact, crystallization of host–guest architecture was crystallized. In the other one
the {PTAO@[Cu(o-L)]3}9 species from the same solvents led to (EtOH), the metallo-supramolecular motif underwent a deep
single crystals of the host–guest {PTAO@[Cu(o-L)]3} supramolecular structural rearrangement to a coordination polymer characterized
triangle with no evidence of the structural conversion to the by the presence of a {Cu8} ring with two strained triangular pockets
{(PTAO)2@[Cu8][Cu2]}N coordination polymer.
where two PTAO molecules are hosted. The experimental results
We found that the PTA oxidation rate can be significantly clearly suggest that molecular oxygen is the oxidizing species, but
speeded up by bubbling air in CHCl3–co-solvent solutions the co-solvent trigger effect is still under investigation. Collected
(co-solvents = methanol, ethanol, acetonitrile, ethyl acetate, data evidence that apolar solvents, such as alkanes, do not promote
tetrahydrofuran, 1,4-dioxane, acetone, n-hexane, cyclo-hexane and PTA oxidation. On the other hand, a clear scale among the active
n-heptane). In particular, THF, MeOH and EtOAc co-solvents gave co-solvents cannot be found when we consider their polarity11 and
the best oxidation results. FT-IR evidenced a significant increment their trigger ability in PTA oxidation. Another aspect to be taken
of the PQO stretching band after few hours (ESI†). In contrast, into account is O2 solubility12 in the CHCl3–co-solvent system, but
chloroform–alkane solutions did not evidence any PTA oxidation. in this case also there is no correlation between the gas solubility
In particular, CHCl3–n-hexane solutions yielded {PTA@[Cu(o-L)]3}4c and PTA oxidation. As a matter of fact, O2 is much more soluble in
single crystals. Finally, we found that {PTA@[Cu(o-L)]3} did not apolar solvents than polar ones. For these reasons, further studies
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This work was supported by the Italian MIUR through FIRB
RBAP114AMK ‘‘RINAME’’. J. T. thanks the CaRiPaRo Foundation
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