Supramolecular solvatochromism: Mechanistic insight from crystallography, spectroscopy and theory

Article abstract of DOI:10.1039/c8cc02197j

A solvatochromic dinuclear copper(ii) metallocycle effectively traps tetrahydrofuran, diethyl ether and pentane significantly above their boiling points. X-ray crystallography, EPR and UV-visible spectroscopy were used to delineate an empirical relationsh

Full text of DOI:10.1039/c8cc02197j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C8CC02197J
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Supramolecular solvatochromism: mechanistic insight from
crystallography, spectroscopy and theory
Varvara I. Nikolayenko,a Lisa M. van Wyk,a Orde Q. Munro b* and Leonard J. Barbour a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A solvatochromic dinuclear copper(II) metallocycle effectively traps transformations at atomic resolution, providing a quantitative
tetrahydrofuran, diethyl ether and pentane significantly above molecular understanding of critical host-guest interactions
their boiling points. X-ray crystallography, EPR and UV-visible involved in guest exchange.17,18
spectroscopy were used to delineate an empirical relationship
Here we describe solvation experiments with the dinuclear
), where L is 1,1'-[1,4-
between the guest-induced structural perturbation of the Cu2+ metallocycle [Cu2Cl4L2]•2DMSO (
1
metallocycle, the ligand field splitting parameter Δ (νmax), and the phenylenebis(methylene)]bis(2-methyl-1H-imidazole) (Fig. 1).
EPR g-values of the inclusion compounds, thereby elucidating the The metallocycle coordinates two DMSO molecules, which
solvatochromic mechanism.
block its aperture and render the material non-porous.
Immersion of in an organic solvent induces Cu–O bond
1
In materials science, a significant direction has been the dissociation, DMSO desorption, and solvent exchange.
development of molecular traps and sensors to store or detect
volatile organic compounds in efforts to reduce workforce and
environmental exposure to deleterious vapours. Research on
porous crystalline materials has experienced commensurate
growth, with metal-organic frameworks (MOFs), zeolites,
covalent organic frameworks (COFs) and hydrogen-bonded
organic frameworks (HOFs) leading the search for tractable,
environmentally acceptable solutions.1-10 We have shown that
certain metallocycles exhibit transient porosity, a phenomenon
wherein materials with seemingly nonporous solid-state
structures are highly permeable due to communication
between isolated interstitial lattice voids.11,12 Importantly,
several systems exhibit well defined host-guest interactions
with distinct solvatochromic signatures,13-15 motioning their
potential use as molecular sensors.15,16
For a molecular sensor and/or trap to be successful, it is
critical to delineate the mechanism underpinning the
guest-induced chromatic shift, and the interactions responsible
for guest retention. Single-crystal X-ray diffraction (SCXRD)
methods are uniquely powerful because they permit an
understanding of single-crystal to single-crystal (SC-SC)
Fig. 1
Synthesis
of
[Cu2Cl4L2]•2DMSO
(1)
from
1,1'-
a.Department of Chemistry and Polymer Science, University of Stellenbosch, 7602
Matieland, South Africa. Email: ljb@sun.ac.za
[1,4-phenylenebis(methylene)]bis(2-methyl-1H-imidazole) and copper(II) chloride
dihydrate (top). The center illustration highlights coordination of DMSO to the Cu2+
ions in the X-ray structure of metallocycle 1 and the solvent exchange steps; those
emanating from 2 are SC-SC exchanges (photomicrographs). The visible-region
diffuse reflectance spectra (bottom) quantitatively illustrate the broad-spectrum
(540–624 nm) solvatochromism of 1–6 (Inorm = normalised intensity).
b.School of Chemistry, University of Witwatersrand, 2050 Johannesburg, South
Africa.
Electronic Supplementary Information (ESI) available: Experimental details, SCXRD,
TGA, PXRD, EPR, FT-IR, solid-state UV spectroscopy, DFT and molecular mechanics
calculations. CCDC 1818828-1818832, 1819550-1819551.
This journal is © The Royal Society of Chemistry 20xx
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The apohosts, DMSO (
1
) acetonitrile (ACN,
2
), acetone (
3
)
retention.17,22,23 Despite the absence of discerniblVyieswtrAortinclge Ohnolinset
DOI: 10.1039/C8CC02197J
and THF (4) analogues of [Cu2Cl4L2] (obtained through different guest interactions (Fig. S3), crystals of 5 can be reproducibly
methods), have previously been reported.16 The solvatochromic cycled between -173 and 70 °C, with the diethyl ether molecule
nature of these materials was described and SC-SC retaining its site occupancy. Heating beyond 70 °C induces
transformations recorded.16 However, no mechanism was crystal decay. Unit cell determinations above this temperature
proposed to explain the guest-dependent colour changes. Since corroborate the TGA results, indicating that the material
few examples16-18 of such SC-SC structural transformations remains solvated to at least 100 °C. Unusually,
5 retained diethyl
exist, studies to define the molecular mechanism underpinning ether for two weeks in vacuo (Fig. S13).
solvatochromism are crucial.
Solvate
6 contains one pentane molecule per metallocycle,
Bright green rhombic crystals of
1 were grown by diffusion situated on an inversion center (50% site occupancy). The
of the metal salt in ethanol into a DMSO solution of the ligand. disordered pentane molecules are located between
The material crystallises in space group P21/c with the metallocycles, which are best viewed along b (Fig. S2c). This
asymmetric unit comprising one ligand, one Cu2+ metal ion and positioning ensures maximum host–guest interactions in the
one O-bound DMSO molecule (Cu–O = 2.367(3) Å). Cu–O bond form of CH-π (C7H4A2–C4A = 2.847(1) Å; C2H3A1–C3A =
dissociation and exchange in
acetonitrile (ACN), with bright red crystals of
minutes (Fig. 1). From , various other solvent exchanges can be to 112 °C (Fig. S9).
carried out in a SC-SC manner (Table S1). Comparison of the To explain why the metallocycle [Cu2Cl4L2] acts as a
1
are the most rapid for 2.98(3) Å) and van der Waals interactions. TGA analysis of
6
2
attainable in 30 suggests it is a good trap for pentane, which is retained from 55
2
crystal structures implies cooperative motion of the host molecular trap for THF, diethyl ether and pentane, but not
imidazole rings during guest exchange;16 this breathing is acetonitrile, we analysed (Table S6) the inclusion complex
uncommon for such rigid materials.19-21 There are also stabilisation energies using the PBE density functional in
important changes in the N–Cu–N bond angles and coordination Materials Studio (CASTEP module).24 1 and
geometry of the Cu2+ ion (vide infra). Fig. 1 illustrates the high stabilisation energies. This is attributed to the presence of
solvatochromic nature of these exchanges ( = DMSO, = ACN, a metallocycle host–guest Cu–O coordination interaction in
= acetone, = THF, = diethyl ether and = pentane) and the and a CuO electrostatic interaction in . In contrast,
quantitative changes in the diffuse reflectance spectra of the metallocycles and are less stable (by ca 167 kJ mol ) than
powdered crystals. Notably, λmax varies from 540 nm for to and ; their relative energies are, however, similar (within ca
624 nm for , commensurate with a broad spectral response (84 7 kJ mol ). Since
nm). Powder X-ray diffraction (PXRD) and FTIR spectroscopy and , we postulate that an additional energy barrier underpins
provide additional signatures of the solid-state changes guest entrapment. Specifically, the lattice (channel) diffusion
accompanying guest exchange (see Figs. S10-S14, S17-S22). The energy is much larger for and than for . Although diethyl
4 exhibit comparably
1
2
1
3
4
5
6
4

1
2
,
5
6
4
1
4

1
2
2 is unable to retain its guest, in contrast to 5
6
5
6
2
structures of
5
and
6
represent novel inclusion compounds.
(green), (orange) and
ether and pentane are positioned differently within the host
(red channel, their stabilisation energies are similar. From the
Solvated metallocycles of
4
5
6
brown) are particularly interesting as guest inclusion causes the calculated gyration radii (Table S7) of the guests (Et2O, 1.856 Å;
host to act as a molecular trap. effectively traps THF, with pentane, 1.974 Å; ACN, 1.182 Å), the size and shape of the
SCXRD showing two full-occupancy THF molecules per guests in and clearly inhibit diffusion through the corrugated
metallocycle (Fig. S2a). A discrete Cu O coordination bond is channels of the host. Acetonitrile in has the highest mobility
absent in ; the interaction (CuO = 2.843(9) Å) lies well outside through the crystal due to its low gyration radius.
4
5
6

2
4
the range observed for Cu–O(THF) bonds in the CSD (1.9–2.8 Å;
mean = 2.2(2) Å) and is electrostatic in nature.
Since the host metallocycle contains two paramagnetic Cu2+
ions and the diffuse reflectance spectra (Fig. 1) point to
Thermogravimetric analysis (TGA) of
4 (Fig. S7) shows an guest-induced chromatic shifts, electron paramagnetic
initial mass loss occurring between ca 65 and 118 °C, which resonance (EPR) spectroscopy was used (Fig. 2) to probe the
exceeds the boiling point of THF (65 °C). Remarkably,
THF under ambient conditions for two weeks (Fig. S11). spectra of
Although the metallocycle retains THF to ca 118 °C, high ground state with gx ranging from 2.189 in
temperature (> 70 °C) SCXRD data collection was thwarted by
, the spin on each Cu2+ ion behaves independently, affording
crystal decay. and are analogous systems as they are of standard doublet state rhombic EPR spectra. The maximum
similar size and shape (despite the heteroatom of ). SCXRD absorption (integrated EPR spectrum) is well-resolved (gy
revealed that contains one diethyl ether molecule per ranges from 2.137 in to 2.192 in ). The location of gz was less
metallocycle, located on an inversion centre in the cavity of the certain and required spectral simulation with PIP26 (Table S2) to
metallocycle (50% site occupancy; Fig. S2b). This configuration estimate its range (2.090 in to 2.106 in ). Triplet states due
is attributed to electronic interactions between the heteroatom to ferromagnetic ordering of spins on the Cu2+ ions within each
of the guest and the π electrons of the host. Subsequent TGA metallocycle are thermally inaccessible at 298 K in all cases.
showed that retains diethyl ether above its boiling point The g-values for the metallocycles are relatively insensitive
4
retains basis of solvatochromism in
1
–
6
. At 298 K, the powder EPR
2
1–6 reflect a rhombic g-tensor (Table S2) and dz
1
to 2.239 in 5.25 For
1–6
5
6
5
5
6
5
4
5

5
(34.6 °C). The first mass loss (approx. one diethyl ether molecule to the identity of the guest (Fig. 2). However, careful analysis of
per metallocycle) occurs between ca 55 and 119 °C (Fig. S8), the most intense EPR absorption band (gy) using spectral
rivalling other examples in the literature for diethyl ether deconvolution (Fig. S15) reveals a linear increase in the ligand
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Fig. 2 a) Ambient temperature EPR spectra of powdered samples of 1–6. Photomicrographs of representative crystals of the inclusion complexes and coordination
geometries (SCXRD) of the Cu2+ ions are shown. b) 3D graph of the variation in the optical electronic absorption energy (max) of the inclusion complex, the empirical
rhombicity parameter xy (measured from the deconvoluted EPR x- and y- absorption peaks in units of g), and the parameter  for metallocycles 2–6. The plane
represents the bivariate linear least-squares fit of the data (max = 21866(428) cm–1 – 1.11(7)  105 (xy) + 2.04(17) (); R2 = 0.994). The parameter  measures the guest-
induced structural perturbation and is defined as the cage ratio, R, multiplied by the van der Waals volume (VvdW) of the guest:  = R x VvdW, where R = (N–Cu–N bond
angle)/(CuCu distance). The correlation shows that max (a spectral measure of the ligand field strength) for the complex increases linearly with decreasing in-plane
(xy) anisotropy of the g-tensor and increasing . The guest-induced perturbation from a rhombic Cu2+ coordination geometry to a slightly flatter, partly more tetragonal
geometry evidently accounts for the color change from red in 2 to green in 4.
field splitting (measured by max, Fig. 1) with increasing gy, Cu2+ ions, the
-symmetry 3p orbitals of the chloride ions, and
suggesting that the EPR parameters can sense the solvation of the imidazole N atom lone pairs. Although the DFT-calculated
the host (Fig. S16). In practice, a suitable parameter to measure EPR g-values are from 3 to 47% larger than the experimental
the guest-induced structural perturbation of the host is the values (the deviation is largest for gx), they correctly predict (i)
product of the cage ratio R, where R = (N–Cu–N bond that the EPR spectrum will be of the rhombic type (gx
angle)/(CuCu distance), and the van der Waals volume of the and (ii) that xy (i.e., gx – gy) will be smallest for and largest for
guest (VvdW). We have called this parameter . From Fig. 2b, max , consistent with experiments.
The calculations for and 4 (Fig. 3) provide a theoretical
 gy  gz)

4

2
2
increases linearly with increasing and decreasing separation

description of their electronic structures that mirrors the ideas
of ligand field theory. Specifically, the magnitude of the ligand
between the x and y absorption maxima in the EPR spectrum,
xy (Table S4). In the limit of tetragonal symmetry for the
g-tensor, xy = 0. True tetragonal symmetry (D4h) cannot exist
for the present compounds. However, xy clearly maps a shift
field splitting parameter,
of will exhibit blue-shifted bands relative to
). Although TD-DFT spectral simulations for

, suggests that the optical spectrum
(~20% smaller
and (Fig. 3d)
4
2
2

4
from a more bent coordination geometry (e.g.
2
with N–Cu–N =
are not quantitatively accurate (transition energies deviated
from experiment by ca +500 nm, Figs. S28 and S29), they
145
157


) to one that is considerably flatter (e.g.
), or more tetragonal (x = y
4 with N–Cu–N =
z), in the metallocycle.

correctly predict that the switch from
2 to 4 will be
The correlation in Fig. 2b shows that small guests (e.g. ACN)
induce the smallest host framework perturbation, preserving a
more rhombic ligand field symmetry (larger xy) with smaller
ligand field d-orbital splittings ( , Fig. 3). This accounts for the
accompanied by a blue shift ( ca 115 nm) in max, mirroring
the experimental band shift to higher energy ( ca 84 nm, Fig.
3dz2 (HOMO-28; –9.41 eV

3dz2 (HOMO-1; –6.09 eV

1). For


2
, the transitions 3dxz(
LUMO; –3.15 eV) and 3dxz(
LUMO), and their symmetry counterparts (Table S8), account


-Cl)
-Cl)


red-shifted max values for such solvates. The converse is true
for larger guests (more spherical shapes, e.g. THF) whose
sorption by the host expands the metallocycle structure,
for ca 60% of the intensity of the lowest-energy band,
confirming that a host framework ligand field perturbation
mechanism underpins the observed solvatochromism in this
inducing larger tetragonal ligand field splittings (
1) for the Cu2+
system
.
ions. A caveat is that the correlation is limited to
non-coordinating guests. Guests such as DMSO switch the
geometry of the Cu2+ ions from distorted Td to trigonal
bipyramidal, thus altering the point group symmetry and d-
orbital energy levels.
The electronic structures of 2–6 were confirmed using DFT
simulations (X-ray coordinates). Fig. 3 highlights the ligand field
splittings (d-orbital energies), EPR parameters, and unpaired
In summary, solvatochromism in a series of metallocyclic
inclusion compounds (1–6) was studied using electronic, IR and
EPR spectroscopy, as well as SCXRD analysis and DFT
simulations. Three of the inclusion compounds exhibit solvent
trapping. In the case of 4, THF is retained due to host-guest
(CuO) electrostatic interactions. Solvates 5 and 6 are
promising, robust diethyl ether and pentane traps because the
guest size and shape effectively limit lattice diffusion.
Collectively, solid-state EPR and electronic spectroscopy,
combined with an analysis of the guest-induced structural
spin densities for
earlier, the unpaired spins are localized in the dz orbitals of the
2
and
4
(i.e., the extrema in Fig. 2b). As noted
2
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perturbation of the host (X-ray crystallography), have allowed
us to delineate the mechanism of solvatochromism. The
possibility that such materials might find utility as
solvent-trapping molecular sensors thus exists.
Conflicts of interest
There are no conflicts to declare
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DOI: 10.1039/C8CC02197J
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