Monitoring the stepwise assembly of a 3-D framework from a supramolecular lanthanoid synthon by in situ infrared spectroscopy

Article abstract of DOI:10.1039/c0cc00949k

Construction of a 3-D supramolecular network from [Ln(cpb) ₃(H₂O)₂] (Ln = Nd, Tb; Hcpb = 1-(4-cyanophenyl)-1,3-butanedione) by sequential displacement of a coordinated water by DMF, dimerisation by reciprocal hydrogen bonding in solution and π-π stacking and interdigitation of nitriles has been monitored by in situ fourier transform infrared spectroscopy.

Full text of DOI:10.1039/c0cc00949k

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Monitoring the stepwise assembly of a 3-D framework from a
supramolecular lanthanoid synthon by in situ infrared spectroscopyw
Philip C. Andrews,a William J. Gee,a Peter C. Junk,*a Harald Krautscheidb and
Jonathan G. MacLellana
Received 15th April 2010, Accepted 15th June 2010
DOI: 10.1039/c0cc00949k
Construction of
a
3-D supramolecular network from
asymmetric ligands is taken into account, the number of
potential isomers increases to over 100. Additionally, substitution
of one of the water co-ligands by a different coordinating
solvent molecule further increases the number of isomers,
dependent on location of the exchange. We postulate that
control over this mire of isomerism may be achieved by
implementing hydrogen bonding between pairs of complexes
via coordinated water molecules. The dynamic exchange of
diketonate ligands in solution then acts in our favour, resolving
the isomers to a single motif (synthon). Ligands reorient such
that the less hindered ligand terminus orients inwards, and
the interaction site outwards. Reciprocal hydrogen bonding
necessary for synthon formation requires coordination of only
one aqua ligand per complex.
[Ln(cpb)3(H2O)2] (Ln = Nd, Tb; Hcpb = 1-(4-cyanophenyl)-
1,3-butanedione) by sequential displacement of a coordinated
water by DMF, dimerisation by reciprocal hydrogen bonding in
solution and p–p stacking and interdigitation of nitriles has been
monitored by in situ fourier transform infrared spectroscopy.
Controlling the vast array of intermolecular interactions to
tailor construction of molecular crystals is the elusive goal of
the crystal engineer.1 Although deliberate design of crystalline
achitectures is yet to be achieved,2 the ever increasing number
of tectons3 and synthons4 continues to extend the boundaries
of this field.5 Often the precursor materials are well defined
and the final architecture exquisitely detailed, however, the
means of construction remains a mystery. In situ fourier
transform infrared (FTIR) spectroscopy is a powerful tool for
elucidating reaction mechanisms and identifying intermediates.6
By using this technique the individual steps in the assembly of
a crystalline 3-D framework can be investigated in real time.
Herein we report the stepwise formation of a complex 3-D
framework monitored by in situ FTIR spectroscopy. The
framework generates via formation of a sophisticated new
supramolecular lanthanoid synthon comprising two lanthanoid
diketonate complexes paired by reciprocal hydrogen bonding,
each bearing interaction sites in predetermined orientations.
The synthon interaction sites combine p–p interactions and
bond dipole–bond dipole interactions, promoting the formation
of the 3-D framework.
The asymmetric diketone ligand 1-(4-cyanophenyl)-1,3-
butanedione (Hcpb) was chosen, since the methyl terminus
will reduce steric repulsion between the pairing complexes,
whilst the benzonitrile group promotes both p–p and bond
dipole–bond dipole interactions. Synthesis of the pre-synthon
was by reaction of [LnCl2(H2O)6]Cl (Ln = Nd, Tb) with three
equivalents of Hcpb in a mixture of methanol and triethylamine
at room temperature for 12 hours. In both instances a
pale yellow powder was obtained after removal of solvent
and extraction with toluene. Recrystallisation from DMF
yielded the crystalline 3-D supramolecular network of
[Ln(cpb)3(H2O)(DMF)]ꢀDMF (Ln
= Nd (1), Tb (2))
(Scheme 1). The composition of 1 and 2 was confirmed by
elemental analysis, IR spectroscopy, ESI mass spectrometry
and single crystal X-ray diffraction.z Both compounds are
isostructural, and only the neodymium species 1 is discussed in
detail.
Ensuring formation of the supramolecular lanthanoid
synthon poses a considerable challenge owing to difficulty in
controlling both motif and ligand orientation. Both propeller7
and square antiprism8 motifs are known, each possessing
several isomers determined by orientation of the chelated
asymmetric diketonate ligands. To further complicate matters,
diketonate ligands can be labile in solution and have the ability
to bridge metals, generating polynuclear cluster species.9 An
eight coordinate square antiprism neodymium diketonate
complex can be expected to have nineteen major isomers, nine
of which are enantiomeric pairs. When directionality of the
The asymmetric unit of
1
contains
a
complete
[Nd(cpb)3(H2O)(DMF)] structural unit and a single molecule
of DMF in the lattice. The metal is eight coordinate, bound
solely by oxygen atoms, six of which are derived from three
a School of Chemistry, Monash University, Clayton, Melbourne,
Vic 3800, Australia. E-mail: Peter.Junk@sci.monash.edu.au;
Fax: +61 (0)3-9905 4597; Tel: +61 (0)3-99054570
b Institut fur Anorganische Chemie, Universitat Leipzig,
¨
¨
04103 Leipzig, Germany
w Electronic supplementary information (ESI) available: Detailed
description of all synthetic protocols and dipole interactions, additional
crystallographic data and in situ FTIR data. CCDC 765987 and
767170. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0cc00949k
Scheme 1 Synthetic scheme for the synthesis of the lanthanoid
supramolecular synthon 2{[Ln(cpb)3(H2O)(DMF)]ꢀDMF}.
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have been shown to affect the orientation of neighbouring
molecules.11 The three nitrile groups present in the asymmetric
unit are within 5 A of nitrile groups from six neighbouring
complexes, indicative of bond dipole–bond dipole interactions.
Two dipole interactions are short (3.505 A) indicating strong
interaction complementary to the p–p stacking of adjacent
complexes mentioned previously. The nine long nitrile bond
dipole–bond dipole interactions (>3.6 A) can be expected to
give weaker interactions, however cumulatively their effect on
lattice formation is expected to be significant. Pairs of nitriles
exhibiting antiparallel alignment are of particular significance,
representing optimal geometry for bond dipole–bond dipole
overlap. An examination of the interactions between parallel
p–p stacking chains of 1 identifies two sites per neodymium
complex where antiparallel bond dipole–bond dipole inter-
actions occur, the stronger of which is 3.616 A (see above).
The interactions are repeated on both sides of the chain,
strengthened by numerous weaker dipole–dipole interactions
and serving to bind the chains into sheets containing columns
of close proximity nitriles. The third antiparallel bond
dipole–bond dipole interaction with a distance of 4.069 A
links the sheets together, completing the 3-D molecular frame-
work. The stepwise construction of the molecular framework
is summarised in Fig. 3.
Fig.
1
Molecular structure of the supramolecular synthon
2{[Ln(cpb)3(H2O)(DMF)]ꢀDMF} 2{1} showing 50% thermal ellipsoids
with lattice DMF omitted for clarity. Broken red lines represent
hydrogen bonding. Symmetry transformations: ꢂx, ꢂy, 1ꢂz.
chelating bidentate cpb ligands and one each from coordinated
water and DMF. Protons from the aqua ligand of
[Nd(cpb)3(H2O)(DMF)]ꢀDMF form hydrogen bonds to O(1)0
and O(5)0 of a symmetry generated (ꢂx, ꢂy, 1ꢂz) molecule.
The symmetry generated molecule in turn hydrogen bonds in
an identical manner with the parent molecule generating the
synthon (Fig. 1). The donor–acceptor distances are 2.80 and
2.77 A, respectively. Hydrogen bonding interactions between
paired complexes facilitate orientation of the less sterically
hindering methyl termini inwards towards the neighbouring
complex. Consequently, the benzonitrile functionalities protrude
outwards aligned with the c-axis, analogous to the prongs of a
trident. Two of the three ligands participate in p–p stacking
with ligands of symmetry generated molecules (by translation
xꢂ1, y, z and 1+x, y, z). These interactions result in the
synthon generating a 1-D chain motif propagating along the
a-axis. Moderate planar aromatic overlap is observed, with
centroid distances of 3.72 A and 3.39 A (perpendicular
distance). Propagation of the chain results in alignment of
the nitrile groups at the termini of the synthons into channels.
This results in adjacent chains interdigitating in a zipper-like
fashion along both the a-axis and b-axis directions (Fig. 2).
There is controversy in the literature regarding the impor-
tance of bond dipole–bond dipole interactions to crystal
engineering.10 Nitriles possess one of the largest bond dipole
moments of 3.6 D, which, at short distances (o3.6 A),
Stepwise assembly of the molecular framework was visualised
using in situ FTIR spectroscopy scanning across the region
4000 to 650 cmꢂ1. This allowed observation of changes in
band frequency arising from intermolecular interactions
between species en route to the final 3-D framework. The
crude product, [Nd(cpb)3(H2O)2], does not exist as a hydrogen
bonded dimer in solution based on the lack of an O–H
stretching vibration in a dilute solution of chloroform.12
Dissolving the crude product in DMF resulted in rapid
displacement of a single coordinated water molecule giving
[Nd(cpb)3(H2O)(DMF)]ꢀDMF, seen as the red trace (Fig. 4).
Total conversion of [Nd(cpb)3(H2O)2], shown as black, was
Fig. 2 The interlocking mesh of 1-D chains of 1. Broken red lines
Fig. 3 The intermolecular forces driving formation of the molecular
represent hydrogen bonding.
framework.
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of potential new crystalline architectures by modifying the
donor/acceptor properties on the exterior of the synthon.
Notes and references
z Single-crystal studies:w Data for 1 were collected at 180 K using a
STOE IPDS 2T diffractometer with MoKa radiation (l = 0.71073 A).
Structural solution and refinement was carried out using
SHELXL-9715 utilising the graphical interface X-Seed.16 Numerical
absorption corrections were applied.17 Data for 2 were collected at
123(2) K on the MX1 beam-line at the Australian Synchrotron,
Victoria, Australia. The data collection and integration were
performed within Blu-Ice18 and XDS19 software programs. Crystal
data for 1ꢀDMF: NdC39H40N5O9, M = 867.00, 0.20 ꢃ 0.15 ꢃ 0.10
ꢀ
mm, triclinic, space group P1, a = 9.8275(6), b = 10.7284(7),
c = 19.2871(12), a = 91.021(5), b = 102.172(5), g = 95.652(5) A,
V = 1976.6(2) A3, Z = 2, 11 341 reflections collected, 6505 unique
(Rint = 0.0414). Final GooF = 1.003, R1 = 0.0275, wR2 = 0.0629 for
all data, 504 parameters. Crystal data for 2ꢀDMF: TbC39H40N5O9,
Fig. 4 Trend curves corresponding to discrete species observed
during the crystallisation process.
ꢀ
M = 881.68, 0.03 ꢃ 0.02 ꢃ 0.02 mm, triclinic, space group P1,
a = 9.774(2), b = 10.682(2), c = 19.281(4), a = 91.16(3), b =
101.99(3), g = 95.68(3) A, V = 1957.7(7) A3, Z = 2, 19 623 reflections
collected, 5250 unique (Rint = 0.0705). Final GooF = 1.064,
R1 = 0.0371, wR2 = 0.0967 for all data, 499 parameters.
evident after 90 minutes. The liberation of water during the
formation of [Nd(cpb)3(H2O)(DMF)]ꢀDMF generates a strong
absorption band at 3451 cmꢂ1, whilst the region between
1700 and 1350 cmꢂ1 was populated with broad overlapping
bands, suggestive of multiple isomers in solution. The CH3
rocking vibration at 1109 cmꢂ1 was used to identify the
newly coordinated DMF, as the more intense n(CQO)
bands were obscured by carbonyl and phenyl stretches within
1.13 The concentration of [Nd(cpb)3(H2O)(DMF)]ꢀDMF
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reaches
a
maximum after approximately six hours.
During this time a new species, shown in blue, increases in
concentration. The new species, identified as the synthon
2{[Ln(cpb)3(H2O)(DMF)]ꢀDMF} and step 2 of the 3-D
framework formation, exhibits greater resolution of bands
suggesting the hydrogen bonding has resolved the exchange
of isomers to the configuration shown in Fig. 3. An O–H
stretch suggesting reciprocal hydrogen bonding of coordinated
water molecules was observed at 3292 cmꢂ1. Well resolved
bands corresponding to conjugated n(CQC) and n(CQO)
were observed at 1599 and 1519 cmꢂ1, respectively.14 Less
intense aromatic n(CQC) bands were also evident in the region
of 1560 to 1370 cmꢂ1. The concentration of the hydrogen
bonded dimer plateaus after 13 hours as it converts to a final
species, shown in green, at a rate approximately equal to its
rate of formation from monomeric [Nd(cpb)3(H2O)(DMF)]ꢀ
DMF. The new species is the 3-D supramolecular network,
formed by concerted p–p stacking assisted by bond dipole–
bond dipole interactions shown in Fig. 3 as step 3. A comparison
of the solution IR spectrum of the framework with the solid-
state spectrum obtained from single crystals of 1 displays
excellent correlation between bands.
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In conclusion, the stepwise formation of a 3-D network by
displacement of a coordinated water by DMF, dimerisation in
solution by reciprocal hydrogen bonding and finally network
formation by cooperative bond dipole–bond dipole and p–p
interactions has been viewed using in situ FTIR. The solution
stability of the dimeric lanthanoid synthon opens a wide range
19 W. Kabsch, J. Appl. Crystallogr., 1993, 26, 795.
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This journal is The Royal Society of Chemistry 2010
5950 | Chem. Commun., 2010, 46, 5948–5950