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Inorganic Chemistry
Article
were loaded into quartz capillary tubes (4 mm diameter). Specific
parameters of the dark scan are as follows: receiver gain, 1.00 × 105;
modulation frequency, 100 kHz; modulation amplitude, 1.00 G; sweep
width, 50 G; resolution, 1024 points; conversion, 40.960 ms; sweep
time, 41.943 s. Specific parameters of the irradiated sample scan are as
follows: receiver gain, 5.02 × 104; modulation frequency, 100 kHz;
modulation amplitude, 5.00 G; sweep width, 40 G; resolution, 1024
points; conversion, 81.920 ms; sweep time, 83.886 s. Photoirradiation
was performed by directing a broad-spectrum white light through the
sample cavity grating (approximately 30% transmittance) for 10 min
and collecting single scans to follow the signal decay with time.
Solid-State Spectroelectrochemistry (SEC). Visible−near-infra-
red (vis−near-IR) data were collected using a CARY5000
spectrometer equipped with a Harrick Omni Diff Probe attachment.
Electrochemical experiments were performed in a Teflon SEC cell
comprising two side arms separately accommodating a platinum-wire
auxiliary electrode and an Ag/Ag+ wire quasi-reference electrode.7
These side arms connect at a central compartment that harbors a 0.1
M n-Bu4NPF6/CH3CN electrolyte. The sample was affixed to an
indium−tin oxide (ITO) coated quartz slide (working electrode) by a
strip of Teflon tape, and the circuit was completed with conductive
copper adhesive tape. This slide was inverted over the central
compartment to enable contact between the sample and electrolyte,
and was finally fixed with adhesive tape. A baseline scan of the Teflon
tape was collected from 5000 to 25000 cm−1. Potentials were
controlled using an eDAQ e-corder 410 potentiostat. Continuous
scans of the sample were undertaken at a potential of 0 V until spectral
equilibration was achieved. A cathodic potential was then manually
applied incrementally during a cycled collection where time was given
for spectral equilibration before a more cathodic potential was applied.
Gas Adsorption Measurements. Pure gas adsorption measure-
ments were conducted using a Micromeritics 2020 Accelerated Surface
Area and Porosimetry System (ASAP). The solid (66.8 mg) was
degassed via a temperature program of 2 °C min−1 to 80 °C followed
by 1 °C min−1 to 140 °C, where it was held for at least 6 h prior to
analysis. Surface area measurements were obtained at 77 K via
incremental dosing of N2 from 0 to 1 bar, and the Brunauer−
Emmett−Teller (BET) surface area was determined through the use of
the ASAP 2020 V4.01 software.
Computational Methods. Standard density functional theory
(DFT) calculations were carried out with Gaussian 09.16 The
geometry of the model was extracted from the crystal structure of
the coordination polymer, primarily without modification. The
carboxylates in the NDC moiety were capped with Li+ ions (at the
location of the Zn2+ atoms) for the purpose of charge balance. This
choice of capping is based on the similar sizes of Li (1.45 Å) and Zn
(1.35 Å). For the napthalenediimide moiety, the terminal N−H bond
lengths were optimized at the B3-LYP/6-31G(d) level.
These models were embedded in a continuum to approximate the
effect of crystal packing in the solid, which is similar to the approach
used by McCarthy et al.2 to model a related metal−organic framework.
The SMD continuum model17 was used in conjunction with M05-2X/
6-31G(d),18 which is a method recommended for use with the SMD
model. The parameters for benzene were employed in the SMD
calculations to reflect the solid-state environment that is rich in
aromatic moieties. We note that the dielectric constant of benzene is
2.27, which is close to the value of 2 employed by McCarthy et al.
The TD-BMK/6-31G(d) procedure19 was employed to compute
the vis−near-IR spectra of the model system. We note that BMK is
one of the best-performing DFT methods for the computation of
electronic transitions, as found by Goerigk and Grimme20 in a
comprehensive benchmark study. Nonetheless, BMK has a slight
tendency to overestimate excitation energies (in general, by ∼900
cm−1). We also note that the use of the 6-31G(d) basis set is likely to
lead to an overestimation of excitation energies in comparison with
results obtained with larger triple-ζ basis sets. In the closely related
system of McCarthy et al., this difference is ∼600 cm−1.2 In our
preliminary benchmark study on related aromatic diimides, we found a
notably larger deviation (blue shift) for higher-energy transitions
(>20000 cm−1) with TD-BMK/6-31G(d) (Supporting Information,
Figure S12). Thus, a total correction of −1500 cm−1 was applied to the
TD-BMK/6-31G(d) spectra for low-energy transitions (<20000 cm−1)
and −3000 cm−1 for bands >20000 cm−1. The simulated spectra were
modeled by Gaussian functions with a half-width of 500 cm−1, which is
comparable to the resolution of the experimental spectra. Assignment
of the molecular orbitals involved in the transitions was based on the
dominant contribution to each excited state and the visualization of
the associated orbitals (Supporting Information, Table S7).
RESULTS AND DISCUSSION
■
The crystal structure of (Zn(DMF)NO3)2(NDC)(DPMNI)
was determined using X-ray diffraction data collected at 150 K
from single crystals obtained at 80 °C. The structure features
one-dimensional coordination polymers comprised of dis-
torted-trigonal-bipyramidal Zn2+ centers coordinated to one
DMF molecule and one NO3− anion (via their oxygen atoms),
a single DPMNI ligand via the pyridyl nitrogen, and an NDC
ligand via chelation of the carboxylate group, as shown in
Figure 1a (crystallographic information is provided in the
Supporting Information). Adjacent chains are closely packed by
virtue of π−π* stacking of alternating NDC and DPMNI
aromatic cores which are separated by a distance of 3.580(2) Å
(Figure 1b), forming layers on the (001) planes. The dense
packing of the 1-D chains due to π−π* stacking between the
DPMNI and NDC ligand cores of neighboring chains results in
no solvent-accessible pore volume, as confirmed via PLATON.
This is in agreement with the low BET surface area of 22.0
0.2 m2 g−1 determined from the N2 adsorption isotherm
collected at 77 K, which is indicative of surface adsorption, and
the minimal CO2 uptake at 298 K (Supporting Information).
Le Bail refinement of the PXRD pattern obtained from a bulk
sample of the as-synthesized framework at 298 K was in good
agreement with the single-crystal data, whereby the fitted unit
cell parameters of a = 7.728(2) Å, b = 8.024(2) Å, and c =
17.524(4) Å at 298 K are very similar to those determined from
the single-crystal data at 150 K (Supporting Information).
Thermogravimetric analysis revealed an approximately 11%
weight loss in the region 140−200 °C due to DMF (Supporting
Information), which was in reasonable agreement with the
DMF content of the crystal structure (14% by mass). Attempts
to exchange the coordinated DMF molecules for lower boiling
point solvents such as methanol, ethanol, THF, and CHCl3, led
to a loss of crystallinity which may result from weakening of the
intermolecular interactions between adjacent chains and the
destruction of the crystal structure. Indeed, the persistence of
the characteristic DMF ν(CO) stretching frequency
(Supporting Information) suggests that the DMF molecules
remain coordinated to the Zn2+ metal centers following the
attempted solvent exchange procedures.
Solid-state cyclic voltammetry was performed on (Zn-
(DMF)NO3)2(NDC)(DPMNI) as a microcrystalline powder
which was mechanically immobilized onto a glassy-carbon-disk
working electrode. The voltammogram revealed two reduction
processes at −0.85 and −1.30 V vs Fc0/Fc+ at 500 mV/s, which
are characteristic of the formation of the monoradical anion and
dianion forms of the DPMNI ligand (Figure 2).8 These data are
consistent with previous results for ligands containing the
redox-active NDI core.5,6 Due to poor diffusivity of counterions
through the material as a result of dense packing, the redox
processes observed via solid-state electrochemistry are likely to
be localized at the surface of the crystallites. The reversibility of
these processes is dependent on the transfer of counterions of
opposite charge through the material to compensate for the
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dx.doi.org/10.1021/ic402173z | Inorg. Chem. 2013, 52, 14246−14252
Leong, Chanel F.
