Complexes of Platinum Group Metals with a Conformationally Locked Scorpionate in a Metal-Organic Framework: An Unusually Close Apical Interaction of Palladium(II)

Article abstract of DOI:10.1021/acs.inorgchem.1c00941

We report synthetic strategies for installing platinum group metals (PGMs: Pd, Rh, Ir, and Pt) on a scorpionate-derived linker (TpmC*) within a metal-organic framework (MOF), both by room-temperature postsynthetic metalation and by direct solvothermal synthesis, with a wide range of metal loadings relevant for fundamental studies and catalysis. In-depth studies for the palladium adduct Pd(II)@Zr-TpmC? by density-functional-theory-assisted extended X-ray absorption fine structure spectroscopy reveals that the rigid MOF lattice enforces a close Pd(II)-Napical interaction between the bidentate palladium complex and the third uncoordinated pyrazole arm of the TpmC? ligand (Pd-Napical = 2.501 ± 0.067 ?), an interaction that is wholly avoided in molecular palladium scorpionates.

Full text of DOI:10.1021/acs.inorgchem.1c00941

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Complexes of Platinum Group Metals with a Conformationally
Locked Scorpionate in a Metal−Organic Framework: An Unusually
Close Apical Interaction of Palladium(II)
̌
Michael T. Payne, Constanze N. Neumann, Eli Stavitski, and Mircea Dinca*
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ABSTRACT: We report synthetic strategies for installing platinum group metals
(PGMs: Pd, Rh, Ir, and Pt) on a scorpionate-derived linker (TpmC*) within a
metal−organic framework (MOF), both by room-temperature postsynthetic
metalation and by direct solvothermal synthesis, with a wide range of metal loadings
relevant for fundamental studies and catalysis. In-depth studies for the palladium
adduct Pd(II)@Zr-TpmC* by density-functional-theory-assisted extended X-ray
absorption fine structure spectroscopy reveals that the rigid MOF lattice enforces a
close Pd(II)−Napical interaction between the bidentate palladium complex and the
third uncoordinated pyrazole arm of the TpmC* ligand (Pd−Napical = 2.501 0.067
Å), an interaction that is wholly avoided in molecular palladium scorpionates.
of the scorpionate. Metal−organic frameworks (MOFs) are
privileged among solid-state materials because of their modular
structure and molecular tunability, offering a capacity for
design and optimization ideal for supporting well-defined metal
complexes.5 In the case of first-row transition metals, the
heterogenization of scorpionate-based catalysts in the MOFs
MFU-4l and CFA-1 led to long-lived catalysts that replicated
the characteristic reactivity of their molecular analogues while
eschewing bimolecular deactivation pathways.6−12 The requi-
site catalytically active transition-metal centers were installed in
the secondary building unit (SBU) by the cation exchange of
structural metals, but the widespread success of this strategy
for first-row transition metals has not translated to platinum
group metals (PGMs: Pd, Rh, Ir, and Pt). To our knowledge,
there is only one reported introduction of a second- or third-
row transition metal containing a partially filled d shell into an
SBU.13
INTRODUCTION
■
The scorpionate ligands tris(pyrazolylborate) (Tp) and
tris(pyrazolylmethane) (Tpm), introduced by Trofimenko in
the late 1960s,1 can tightly chelate a metal by facial κ3
coordination while retaining open cis-oriented coordination
sites. The resulting stable metal complexes are capable of
concerted oxidative addition and reductive elimination. This
has enabled fundamental studies of the properties of second-
and third-row transition metals, exemplified by Canty’s use of
scorpionates to stabilize high-oxidation-state organometallic
complexes of palladium and platinum,2 as well as novel
reactivity, such as the C−H activation chemistry of rhodium,
iridium, and ruthenium scorpionates.3
The coordination chemistry of scorpionates, however, can
be challenging to control in molecular systems: coordination
sites required for catalysis are often blocked via intermolecular
pathways such as homoleptic ML2 complex formation and
(ML)2 dimerization (Figure 1a). Undesired intermolecular
interactions are exacerbated by the rotational lability of the
pyrazole rings, which allows the scorpionate to bridge multiple
metals while also opening additional ligand degradation
pathways via intramolecular C−H activation. These destructive
pathways can be blocked by the installation of bulky
substituents on the pyrazole rings,4 but such ligands are
often difficult to synthesize, and steric encumbrance of the
active site limits both metal ligation and overall catalyst
activity.
Alternatively, single-site installation of a reactive metal onto
the linker of a MOF produces a well-defined heterogeneous
catalyst that can be studied and tuned with molecular
precision.14−16 Beyond the simple translation of homogeneous
systems to solid supports, site isolation in a MOF can confer
Special Issue: Postsynthetic Modification of Metal-
Organic Frameworks
Received: March 26, 2021
In principle, bimolecular deactivation or decomposition
pathways can be eliminated altogether by immobilizing the
ligand in a solid matrix, locking the conformation and position
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Figure 1. (a) Deactivation modes of molecular metal scorpionates by homoleptic ML2 formation and (ML)2 dimerization. (b) Synthetic routes for
accessing site-isolated PGM scorpionates using the Zr-TpmC* MOF. (c) Generic scheme for remetalation of Zr-TpmC*.
new catalytic properties by modulating the catalyst micro-
environment, restricting the ligand flexibility, and stabilizing
against biomolecular decomposition;17 thus, methodologies for
producing new MOF-supported metal species offer the means
to access new reactivity with existing catalysts. Recently, our
group reported a MOF composed of a Tpm-based linker,
{(TpmC*)8[Zr6O4(OH)4(X)4(H2O)4]3} (Zr-TpmC*; X =
HO− or HCOO−).18 Attempts at the direct solvothermal
synthesis of Zr-TpmC* led to the exclusive formation of
amorphous materials, which we attributed to the conforma-
tional flexibility of the free ligand. However, TpmC* could be
templated in situ with rigid C3v symmetry, required for the Zr-
TpmC* framework, via metalation with CuI during MOF
synthesis to produce crystalline CuI@Zr-TpmC*. Subsequent
treatment of CuI@Zr-TpmC* with 1 M HCl removed
templating equivalents of CuI to yield crystalline free-base
Zr-TpmC* (Scheme S2). We hypothesized that free-base Zr-
TpmC* would be capable of supporting PGM scorpionates
without the need for SBU cation exchange. Indeed, we found
that the free-base MOF could be readily metalated with
suitable precursors to yield the corresponding palladium,
rhodium, iridium, and platinum scorpionate complexes within
a heterogeneous host (Figure 1b,c). PGM@Zr-TpmC*
represents the first report of MOF-supported second- and
third-row transition-metal scorpionates. Both high PGM
loadings, ideal for mechanistic and structural studies, and low
metal loadings that minimize mass-transfer limitations in
catalytic applications could be accessed.
We further demonstrate that Ir@Zr-TpmC* and Rh@Zr-
TpmC* are readily prepared not only by postsynthetic
modification of free-base Zr-TpmC* but also by direct
solvothermal synthesis of the MOF in the presence of IrCl3
and RhCl3, which coordinate to TpmC* and template the
ligand with C3v symmetry, thereby avoiding laborious copper
templation and demetalation steps (vide supra). The two
complementary synthetic pathways of postynthetic metalation
and direct synthesis provide access to a diverse suite of PGM@
Zr-TpmC* systems (Figure 1b) and lay the foundation for the
introduction of a wide range of elements into MOF-supported
scorpionates. Detailed further investigation by X-ray absorp-
tion spectroscopy (XAS) and complementary density func-
tional theory (DFT) calculations revealed that the MOF
support can impart a unique PGM coordination environment
that is distinct from the corresponding molecular complex, as
was the case for the geometrically frustrated interaction of the
TpmC* metalloligand with square-planar Pd(II). We observe
that the rigid MOF lattice enforces an unusually short Pd(II)−
Napical interaction (2.501 Å), which we attribute to the
metalloligand being conformationally locked in C3v symmetry
by the MOF framework. In molecular systems, short
interatomic distances between square-planar Pd(II) centers
and elements capable of acting as axial ligands are reported to
lower the oxidation potential of Pd(II) and concomitantly
stabilize the III+ and IV+ oxidation states.19,20 The short Pd−
Napical interaction enforced by the MOF support and limited
molecular motion required for the coordination of an axial
ligand suggests that Pd(II)@Zr-TpmC* may display a low
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Figure 2. (a) PXRD patterns of PGM@Zr-TpmC*. (b) TEM/EDS images of PGM@Zr-TpmC*. PSM = postsynthetic metalation.
barrier for the oxidation of Pd, a desirable feature in high-
valent Pd catalysis.
metalation of the TpmC* site (and the absence of Pd
nanoparticles) is corroborated by transmission electron
microscopy/energy-dispersive X-ray spectroscopy (TEM/
EDS) imaging of 1, which showed even incorporation of Pd
throughout the Zr-TpmC* crystallites (Figure 2b).
RESULTS AND DISCUSSION
■
Scorpionate Sites in Zr-TpmC* That Are Quantita-
tively Metalated by Pd(II) via Postsynthetic Modifica-
tion. We first pursued the synthesis of a Pd(II)@Zr-TpmC*
system using [Pd(CH3CN)4](BF4)2 as a Pd(II) source, based
on the assumption that the labile acetonitrile (CH3CN)
ligands and weakly coordinating anions would facilitate PGM
installation. Activated free-base Zr-TpmC* was soaked in an
CH3CN solution of [Pd(CH3CN)4](BF4)2 containing 1 equiv
of Pd complex per 1 equiv of an open TpmC* binding site
over a period of 5 days. During this time, the yellow-orange
color of the solution faded and the MOF sample changed in
color from white to yellow, suggesting that the Pd complex had
been absorbed into the framework. The product was
recovered, washed, and dried to yield a yellow microcrystalline
powder, which we assign as [Pd(CH3CN)2](BF4)2@Zr-
TpmC* (1). Characterization of 1 by powder X-ray diffraction
(PXRD) shows that the crystallinity of Zr-TpmC* is
maintained (Figure 2a) with no signature of Pd nanoparticle
formation (Figure S9).
Further experimental characterization of 1 provides evidence
that the metalation reagent Pd(CH3CN)4(BF4)2 is ultimately
incorporated at the scorpionate sites, losing two CH3CN
ligands to form [Pd(CH3CN)2(TpmC*)]2+ units, whose
−
charge is balanced by BF4 presumably residing in the pores.
A PXRD experiment gives direct evidence for the uniform
introduction of Pd into the scorpionate site because the 7.8°
peak, which corresponds to the (200) reflection, is observed to
systematically decrease in intensity with increasing Pd loading
(Figure 3a). We attribute this to a phase cancellation
phenomenon caused by heavy atoms in the scorpionate sites,
which lie directly between the (200) planes and thus diffract
180° out-of-phase with the (200) reflection (Figure S13). The
CH3CN ligands bound to Pd in 1 were also detected by IR
spectroscopy as weak bands at 2310 and 2340 cm−1 (Figure
3b). These CN stretch bands are red-shifted by approx-
imately 10 cm−1 in comparison with free [Pd(CH3CN)4]-
(BF4)2. We ascribe the red shift to the exchange of two weakly
π-accepting nitrile ligands for the framework pyrazole ligands,
which would effectively behave only as σ donors due to a fixed
orientation that gives poor d−π* overlap (Figure 4b); back-
bonding to the remaining nitrile ligands would thus increase,
and the C−N triple bond would concomitantly weaken.
Characterization of 1 digested in D2SO4/deuterated dimethyl
sulfoxide (DMSO-d6) by 19F NMR spectroscopy confirmed
In our initial report on Zr-TpmC*, we observed that the
remetalation of Zr-TpmC* by [Cu(CH3CN)4](BF4) plateaued
at 4 equiv of Cu per formula unit (50% of the available TpmC*
sites). In contrast, we achieved nearly complete metalation of
Zr-TpmC* using [Pd(CH3CN)4](BF4)2, with up to 7 equiv of
Pd installed per formula unit (vs eight available TpmC* sites),
as determined by inductively coupled plasma mass spectrom-
etry (ICP-MS). Furthermore, while cation exchange typically
requires large excesses of metal for significant incorporation,
metalation of Zr-TpmC* by Pd is nearly quantitative, with
>80% Pd incorporation at a range of substoichiometric
loadings (Figure S11). Such an efficient incorporation is
essential when preparing catalysts from precious metals such as
Pd, given their high costs. Reaction with an excess of
Pd(CH3CN)4(BF4)2 does not increase the metal loading to
exceed 7 equiv of Pd per formula unit, suggesting that excess
Pd does not deposit elsewhere in the framework. Successful
−
the presence of BF4 as the charge-compensating anion
(Figure S8). Although CH3CN was detected by IR, no
CH3CN was observed in the 1H NMR spectrum of digested 1.
We hypothesize that strong acid and trace water in the
digestion conditions may induce hydrolysis of CH3CN to
acetic acid, as the latter was observed in the 1H NMR spectrum
(Figure S7).21
The successful incorporation of Pd at the TpmC*
scorpionate site in 1 is further evidenced by the X-ray
photoelectron spectroscopy (XPS) data, which shows an
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(Figure S10). Qualitative comparisons of the X-ray absorption
near-edge structure (XANES) with reference Pd compounds
are also consistent with a II+ oxidation state and reflect a
decrease in the Pd binding energy.
DFT-Assisted Extended X-ray Absorption Fine Struc-
ture Spectroscopy (EXAFS) Characterization of Pd(II)@
Zr-TpmC* That Reveals an Unusually Close Pd−N
Apical Interaction. In addition to a confirmation of the
metal oxidation state, an XAS study of Pd@Zr-TmpC* was
pursued to shed light on the coordination environment of
Pd(II), a d8 metal with a strong electronic preference for
square-planar geometry, placed in the scorpionate binding site
of 2, which is conformationally locked in C3v symmetry by the
MOF lattice (Figure 4a). In molecular palladium(II)
scorpionates, the tridentate ligand is coordinated via two of
2
the three pyrazoles in an edge-on κ fashion, with the third
pyrazole donor rotated 90° to avoid an unfavorable filled−
filled interaction with the Pd dz orbital.2,22 Upon two-electron
2
oxidation of Pd(II) to Pd(IV) d6, the electronic preference
shifts to favor an octahedral geometry, allowing the formation
of stable κ3-scorpionate complexes. Because rotation of the
pyrazoles in Zr-TpmC* is inhibited by carboxylate linkages to
the SBU, we hypothesized that this rigidity would enforce an
unusually close apical interaction between the palladium(II)
κ2-scorpionate and the third pyrazole arm of the ligand. Such
an interaction could promote oxidation to a Pd(IV) complex
by preconfiguring the Pd for an octahedral geometry.
Figure 3. (a) PXRD overlay of Zr-TpmC* samples with various Pd
loadings (percentage is versus the total accessible TpmC* ligand). (b)
IR spectrum overlay of Zr-TpmC* samples with various Pd loadings.
The Pd loading was determined as the Pd content versus the available
TpmC* sites by ICP-MS.
To directly probe the local coordination environment of Pd
in 1, we employed DFT-assisted EXAFS analysis, an
established technique for understanding the local structure of
metal atoms in MOFs as well as zeolites, organometallic
complexes, and inorganic salts.23 The initial cluster model used
for EXAFS fitting was based on the crystallographic data of
CuI@Zr-TpmC*,1 8 Pd(Tpm)2 (BF4 )2 , and Pd-
(CH3CN)4(BF4)2,24 and the model was structurally optimized
increase of 0.2−0.3 eV in the N 1s binding energy associated
with the scorpionate pyrazoles upon metalation and a
concomitant decrease of more than 1 eV in the Pd 3d5/2
binding energy compared with [Pd(CH3CN)4](BF4)2 (Table
S2). The observed Pd 3d5/2 binding energy is consistent with
the assignment of the Pd oxidation state in 1 as II+, an
assignment that is also supported by solid-state XAS
measurements of 1 collected at the Pd K-edge (24350 eV)
Figure 4. (a) Coordination of TpmC* to Zr-oxo nodes in 1 (protons omitted for visual clarity). (b) Optimized DFT model of the
[Pd(CH3CN)2TpmC*]2+ fragment used for EXAFS fitting. (c) Close Pd−Napical interaction in 2, determined by EXAFS fitting. (d) k3-weighted
EXAFS signal and corresponding modulus (e) and imaginary (f) parts of the phase-uncorrected FT of the experimental and best fits for
[Pd(CH3CN)2TpmC*]2+.
D
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Figure 5. (a) Example structure for [Pd(CH3CN)2(TpmC*)]2+ highlighting the Pd−Napical bond. (b) Relative energy (kcal/mol) versus Pd−Napical
distance (Å). A shallow PES surface is observed, which is within the error of DFT calculations.
using DFT (Figure 4a,b). During geometry optimization, the
position of the carboxylic acid was kept fixed to account for the
rigidity of the solid structure; analogous protocols have been
employed for structure optimization of SBUs in several
MOFs.25−27 Optimized structures that did not include this
restraint gave one imaginary frequency involving rotation of
the apical pyrazole away from Pd, consistent with the
previously mentioned behavior of molecular palladium(II)
scorpionate systems (Figure S39).
revealed a shallow PES between the Pd−Napical distances from
EXAFS fitting and DFT optimization, indicating that the
experimental data are compatible with calculations.
We thus draw a comparison to molecular complexes of
Pd(II) with tetradentate diazapyridinophane ligands reported
by Mirica et al., where the shortest Pd(II)−Napical distance
from X-ray crystal structures (2.525 Å) is slightly longer than
the Pd(II)−Napical distance that we observed in 1.19,20 The
close Pd−Napical interaction in the Mirica systems is reported to
lower the oxidation potential of Pd and stabilize Pd in the III+
and IV+ oxidation states, and given the structural similarities,
an analogous effect may be present in 1. High-valent Pd
systems find application in catalysis, enabling unprecedented
organic transformations not accessible with conventional
Pd(0)/Pd(II) systems.31,32 Yet, to our knowledge, no well-
defined Pd(III) or Pd(IV) species have been reported on a
solid support, although transient Pd(III) or Pd(IV) has been
proposed in the solid-state fluorination catalysis of Pd-
functionalized metal−organic layers.33 Our study of 1 suggests
that the immobilization of ligands in a MOF in a geometry that
preferentially stabilizes Pd(IV) over Pd(II) may be a viable
strategy for the development of heterogeneous high-valent Pd
catalysts.
Postsynthetic Installation of Rh, Pt, and Ir in Zr-
TpmC*. Inspired by the successful synthesis of the MOF-
based palladium scorpionate 1, we pursued the metalation of
Zr-TpmC* by other PGMs. We prioritized metalation
precursors that have slim steric profiles to allow for facile
mass transport into the MOF pores and/or labile ligands that
are easily displaced by the framework-bound scorpionate. On
the basis of these criteria, we chose the metalation precursor
[Rh(CO)2Cl]2 for Rh, used to generate scorpionate catalysts
for C−H bond activation,3,34,35 and precursors [Ir-
(CH3CN)2(COD)](BF4) and [Pt(CH3CN)4](BF4)2 for Ir
and Pt, respectively. When Zr-TpmC* was soaked in an
CH3CN solution of [RhCl(CO)2]2 over 4 days, the MOF
sample turned bright yellow to form a product assigned as
[RhCl(CO)2]@Zr-TpmC* (2). Samples of 2 were prepared
with varying amounts of Rh (0.1−2.0 equiv/TpmC* site). In
contrast to metalation by [Pd(CH3CN)4](BF4)2, the color of
the Rh solution did not completely fade, even when
substoichiometric quantities of Rh were employed. Corre-
spondingly, metalation of Zr-TpmC* by Rh was not
quantitative, with a maximum of 0.4 Rh/TpmC* site installed
in 2 (Figure S12). The surface area of Zr-TpmC* also
decreases substantially upon metalation by [Rh(CO)2Cl]2,
The signal/noise ratio of the collected EXAFS data allowed
us to fit single scattering (SS) paths from both the first and
second coordination shells in the R-space interval ΔR = 1.0−
3.1 Å. The first coordination shell consists of two acetonitrile
N atoms and two pyrazole N atoms in square-planar geometry,
which were modeled as an averaged four-atom shell of SS paths
(N1) with a single ΔR and Debye−Waller (σ2) parameter. The
second coordination shell consists of the apical imine N atom
of the third pyrazole arm (Napical) and the amine N atoms of
the two coordinated pyrazoles (N2). The obtained fit
reproduces our experimental data well (Figure 4c−f), giving
Pd−N1 and Pd−N2 bond distances that are in good agreement
with both the DFT calculations and crystallographic data of
analogous Pd compounds (Table S7). The EXAFS fitting
revealed an unusually short Pd−Napical distance of 2.501
0.067 Å (Figure 4c) compared with other square-planar Pd(II)
complexes. In the analogous molecular complex, a 90° rotation
of the apical pyrazolate to both avoid the short Pd−Napical
distance and minimize steric interactions between adjacent
pyrazolates would be expected, and such a rotation is observed
in the reported crystal structures of palladium(II) scorpio-
nates.2,22,28−30 In contrast, the close Pd−Napical distance is
preserved in 1 by the rigid Tmp* binding pocket enforced by
the MOF.
Further DFT calculations support the experimentally
determined Pd−Napical interatomic separation. Although the
original DFT-optimized model predicted a Pd−Napical distance
that was approximately 0.2 Å longer than that determined by
EXAFS fitting, a subsequent DFT structure optimization in
which the Pd−Napical distance was constrained to 2.501 Å
raised the structure energy by less than 1 kcal/mol, a marginal
increase that falls within the error of DFT calculations.
Additionally, we conducted a relaxed potential energy surface
(PES) scan, in which geometry optimization occurs at each
step prior to energy calculation, across the Pd−Napical bond
distances RPd−N = 2.301−2.801 Å in 0.04 Å steps at the M06/
SDD-6-311G(d,p) level of theory (Figure 5). The calculation
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from 1768 40 m2/g (Zr-TpmC*) to 701 1 m2/g (2, 0.4
Rh/TpmC* site). When characterized by PXRD, the intensity
of the (200) reflection at 7.8° did not vary with Rh loading, but
increasing Rh loading correlated with a pronounced increase in
the intensity of the (100) reflection at 3.8° (Figure 6a). Given
vibrations at 2087, 2064, 2023, and 1992 cm−1. We
hypothesize that these four vibrations correspond to the four
vibrations of the dimeric [Rh(CO)2Cl]2 metalation precursor
and thus that [Rh(CO)2Cl]2 maintains its dimeric structure
upon incorporation into the MOF. If the TpmC* sites were
metalated by [Rh(CO)2Cl]2, the dimer would be expected to
break, as is the case for molecular TpmC* complexes. In total,
the PXRD, surface area, and FTIR analyses suggest that
attempts to generate supported rhodium scorpionates in Zr-
TpmC* with [Rh(CO)2Cl]2 instead deposit the Rh dimer
intact within the pores.
Metalation of Zr-TpmC* with the third-row PGMs Ir and Pt
was accomplished under conditions analogous to those of Pd
and Rh to yield [Ir(COD)BF4]@Zr-TpmC* (3, x ≈ 0−1.5)
and [Pt(CH3CN)2(BF4)2]x@Zr-TpmC* (4, x ≈ 0−0.5). As
with Pd and Rh, the crystallinity of the MOF is retained and
incorporation occurs evenly throughout the material, as
determined by PXRD and TEM/EDS, respectively (Figure
1b). However, incorporation of the third-row metals Ir and Pt
was substantially lower than that of the second-row metals,
with a maximum of 0.19 Ir/TpmC* site in 3 and a maximum
of 0.063 Pt/TpmC* site in 4. Characterization of 3 and 4 by
IR showed no discernible features of the molecular complex, a
possible consequence of the low metal loadings.
Direct Synthesis That Provides an Alternative Route
to Access PGM@Zr-TpmC*. Motivated to find synthetic
routes that would result in higher metal incorporation for Rh,
Ir, and Pt, we hypothesized that TpmC* could be templated in
situ by metals other than Cu for direct MOF synthesis.
Anticipating that a d6 metal would provide the proper
geometry for MOF synthesis, we attempted direct synthesis
using IrCl3. Indeed, the reaction of TpmC* with IrCl3, ZrOCl2,
and benzoic acid in N,N-dimethylformamide (DMF) at 120 °C
over 5 days yields off-white microcrystalline solids whose
PXRD patterns match Zr-TpmC* and display no signatures of
nanoparticle formation. We assigned these as IrCl3@Zr-
TpmC* (5) based on ICP-MS data, which gives a metal
loading of 7.1 equiv of Ir per formula unit, equivalent to the full
metalation achieved postsynthetically with Pd-
(CH3CN)4(BF4)2.
The Rh analogue, RhCl3@Zr-TpmC* (6), can be similarly
prepared by the direct synthesis of Zr-TpmC* in the presence
of RhCl3 (Figure S15). In some preparations, reaction
temperatures above 60 °C resulted in blackened material,
but no signature of Rh nanoparticles was visible in the powder
diffractograms for these samples. The visible blackening of the
solids, however, suggests that there may still be nanoparticles
present in the sample that are too small to detect by PXRD.
Characterization of 6 by TEM/EDS reveals an irregular
crystallite morphology in contrast to the cubic crystallites of
Zr-TpmC* and the PGM-metalated MOF samples (Figure
1b). Rh loading in these samples is also subquantitative, with
3.5 equiv of Rh per formula unit installed compared to the
theoretical maximum of 8.
Figure 6. (a) PXRD overlay of Zr-TpmC* samples with varied Rh
loading (percentage is vs the total accessible TpmC* ligand). (b)
FTIR spectrum overlay of Zr-TpmC* samples with varied Rh loading
versus metalation precursor [Rh(CO)2Cl]2. The Rh loading was
determined as the Rh content versus the available TpmC* sites by
ICP-MS.
that the intensities of low-angle reflections in the PXRD of
MOFs are highly sensistive to disordered species such as the
solvent in the pores,36 the increasing intensity of the (100)
reflection for increasing Rh loading in 2 suggests that
[Rh(CO)2Cl]2 is encapsulated in the pores rather than
incorporated into the TpmC* site. The small but systematic
shift of the reflections to lower angles with increasing Rh
loading could also be consistent with the pore filling, which can
subtly distort the MOF structure and increase the size of the
unit cell; however, we note that the PXRD patterns in Figure
6a were not referenced to an external standard and the samples
were dried at room temperature (and thus not fully
desolvated), so there is a minor experimental variance in the
peak position from sample to sample.
To better interrogate the Rh speciation in 2, Fourier
transform infrared (FTIR) spectra were collected for both
metalated Rh@MOF samples and the [Rh(CO)2Cl]2 metal-
ation precursor (Figure 6b). The IR spectrum of [Rh-
(CO)2Cl]2 predominantly consists of four strong vibrations
related to the carbonyl groups, namely, A1 (2100 cm−1), B2
(2086 cm−1), B1 (2037 cm−1), and A2 (2025 cm−1).37 While
the carbonyl vibrations are too weak to resolve for samples of 2
at lower loading, the sample of 2 at 40% loading (equivalents
of Rh vs equivalents of the TpmC* sites) displays four clear
Attempts at the direct synthesis of Pd@Zr-TpmC* with the
Pd(II) precursor Na2PdCl4 yielded only Pd nanoparticles, with
analogous results for the Pt(II) precursor K2PtCl4. We
attribute this outcome to two factors. First, the high
temperature, acidic conditions, and trace water in the MOF
synthesis can decompose DMF to produce amines that readily
reduce Pd(II) and Pt(II), leading to the rapid formation and
aggregation of Pd0 and Pt0 nanoparticles. Second, because of
the strong electronic preference of d8 metals for square-planar
F
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mA, respectively. Samples for PXRD were prepared by placing a thin
layer of the appropriate material on a zero-background Si crystal plate.
N2 Adsorption Measurements. N2 adsorption isotherms were
measured by a volumetric method using a Micromeritics ASAP 2020
Plus gas sorption analyzer. Samples were activated at 110 °C (for
demetalated Zr-TpmC* samples) or 95 °C (for metalated Zr-TpmC*
samples) under high dynamic vacuum (<10−4 mbar) for 18 h before
analysis. All N analyses were performed using a liquid-nitrogen bath at
77 K. Oil-free vacuum pumps were used to prevent contamination of
the sample or feed gases. Fits to the Brunauer−Emmett−Teller39
equation satisfied the published consistency criteria.40
geometry, coordination of Pd(II) and Pt(II) with TpmC*
renders the C3v geometry of the ligand higher in energy during
synthesis, inhibiting formation of the Zr-TpmC* lattice.
Preliminary attempts to access Pt@Zr-TpmC* by direct
synthesis using the Pt(IV) precursor K2PtCl6 yielded gray
solids whose PXRD pattern consists of broad peaks that
correspond to the Zr-TpmC* structure (Figure S16), but
similarly to the Rh direct synthesis, the reaction temperature
and time were limited by Pt reduction and aggregation.
Fortunately, postsynthetic modification avoids the reducing
conditions of the MOF synthesis and thus allows for Pd/Pt
incorporation into the scorpionate framework without the loss
of metal to nanoparticle formation.
1
NMR. H, 13C, and 19F NMR spectra were measured on a JEOL
JNM-ECZ500R/S1 spectrometer (500 MHz).
XPS. XPS was conducted at Dow Chemical on a PHI VersaProbe II
spectrometer equipped with an Al source using a 100 μm X-ray spot
size. Samples were mounted using double-sided Cu tape. The charge
shift was calibrated by setting either the hydrocarbon C 1s to 285.0
eV or the Si 2p of SiO2 to 103.7 eV. Each point of the analysis was
energy-corrected independently of the others.
Scanning Tunneling Electron Microscopy (STEM)/EDS. STEM/
EDS analysis was conducted at Dow Chemical. Samples for TEM
were prepared by mixing a small amount of sample powder with
methanol. The fines were collected onto a standard Cu mesh TEM
grid with a lacy C support. Data collection was performed using a FEI
Themis field-emission-gun aberration-corrected (probe) transmission
electron microscope. The microscope was operated at an accelerating
voltage 200 keV. STEM images were collected at 2048 × 2048 image
size. The Themis microscope has Bruker AXS XFlash EDS detectors
with an energy resolution of 137 eV/channel for elemental
identification and quantitative analysis. Instrument conditions:
STEM 55−100 pA 70 μm C2 Spot 9 for imaging; ∼300−500 pA
for EDS map collection; 700−1500 s collection time per map; 1024 ×
1024 map size.
XAS. XAS measurements in the solid state were carried out at the
Inner Shell Spectroscopy (ISS) beamline of the NSLS-II at BNL
(Upton, NY). Data were collected at the Pd K-edge (24350 eV) using
fluorescence detection with a passivated implanted planar silicon
detector. Pellets of 5 mm diameter were pressed from a Pd-
(CH3CN)2(BF4)2@Zr-TpmC* MOF and poly(ethylene glycol)
(50:50 ratio by mass).
XAS data reduction and EXAFS extraction procedures were
performed using the Athena code, while EXAFS analysis was done
with the Artemis software from the Demeter package.41 Once
extracted, the k3-weighted Δ(k) functions were Fourier-transformed
in the k-space interval Δk = 2.0−12.0 Å−1 and fitted in the R space in
the interval ΔR = 1.0−3.1 Å, potentially resulting in 13 independent
parameters (2ΔkΔR/π > 13; Table S6). The phases and amplitudes
were calculated by the FEFF6 code,42 implemented in the Artemis
software, using the DFT-optimized structures.
CONCLUSION
■
In summary, this work establishes synthetic methodologies for
the installation of Pd, Rh, Ir, and Pt into the scorpionate sites
of a MOF, while maintaining crystallinity and porosity. We
further report the geometrically frustrated interaction between
Pd(II) and a scorpionate ligand conformationally locked in C3v
symmetry by the MOF superstructure, as explored by DFT-
assisted EXAFS analysis. An unusually close 2.5 Å Pd(II)−
Napical distance was observed that preconfigures the Pd for
octahedral geometry and may lower its oxidation potential; a
further study of the Pd@Zr-TpmC* systems may enable access
to supported palladium scorpionates in higher oxidation states.
Thus, this work not only extends the existing and well-defined
chemistry of PGM scorpionates to MOFs but also signals
emergent properties in such systems that can be probed and
tuned with high fidelity thanks to the exceptional structural
uniformity of MOFs.
EXPERIMENTAL METHODS
■
Reagents. ZrOCl2·8H2O and [Pd(CH3CN)4](BF4)2 were pur-
chased from Alfa Aesar. CuI, AgBF4, Na2PdCl4, [Rh(CO)2Cl]2, RhCl3
hydrate, IrCl3 hydrate, [Ir(1,5-COD)Cl]2, PtCl2, K2PtCl4, and
K2PtCl6 were purchased from Sigma-Aldrich. Ethyl diacetoacetate
was purchased from TCI. N,N-Dimethylformamide (DMF), benzoni-
trile, acetonitrile (CH3CN), acetone, ethanol, and chloroform were
used as received without further purification. Ethyl 3,5-dimethylpyr-
azole-4-carboxylate was synthesized according to a reported
procedure.38 The MOF linker TpmC* was prepared following a
previously reported procedure.18
Dry, deaerated CH3CN and dichloromethane (DCM) were
obtained by passing the solvent through two silica columns in a
Glass Contour Solvent System and degassing with a flow of Ar gas for
30 min, followed by three freeze−pump−thaw cycles. All other
commercial reagents were used as received without further
purification.
General Procedures. ICP-MS. ICP-MS analyses were conducted
at the MIT Center for Environmental Health Sciences (CEHS) using
an Agilent 7900 ICP-MS spectrometer. Calibration standards were
prepared for ICP-MS analysis using analytical standard solutions
purchased from Ricca Chemicals and a 2% HNO3 solution (prepared
from EMD Millipore Omnitrace HNO3 and ultrafiltered water).
Digestion of PGM-containing samples was performed in aqua regia
(HCl + HNO3, OmniTrace Ultra, EMD Millipore) to ensure
digestion of any metallic PGM.
The first coordination shell of Pd consists of two CH3CN N atoms
and two pyrazole N atoms in square-planar geometry and was
modeled as an averaged four-atom shell of SS paths with a single ΔR
and Debye−Waller (σ2) parameter because of the close similarity in
the bond lengths. The SS paths involving N atoms in the second
coordination shell were modeled using two sets of ΔR and σ2
parameters: one for the two interior N atoms of the coordinated
pyrazole rings and one for the apical N atom of the uncoordinated
pyrazole ring.
Synthesis. Triethyl 1,1′,1″-Methanetriyltris(3,5-dimethyl-1H-
pyrazole-4-carboxylate) (TpmC*-Et). To a 2 L round-bottomed
flask containing ethyl 3,5-dimethylpyrazole-4-carboxylate (50.0 g, 297
+
mmol, 1 equiv) and NBu4 Br− (9.60 g, 29.8 mmol, 0.1 equiv) was
added deionized water (320 mL). Sodium carbonate (157 g, 1.48 mol,
5 equiv) was added to the flask in portions. After all of the sodium
carbonate was added, chloroform (360 mL) was added to the flask,
and the reaction mixture was heated to reflux for 3 days. After the
reaction mixture cooled to room temperature, toluene (400 mL) was
added to the flask, and the mixture was filtered. The organic phase
was collected via a separatory funnel, and the aqueous phase was then
extracted with toluene (200 mL) three times. The combined organic
IR Spectroscopy. Benchtop IR spectra were recorded on a Bruker
Tensor 37 instrument with either a Ge or a diamond attenuated-total-
reflectance sample holder.
PXRD. Laboratory PXRD patterns were collected on a Bruker D8
diffractometer fitted with a Goebel mirror and a Lynxeye Si-strip
position-sensitive detector, operating in reflection mode with Ni-
filtered Cu Kα1,2 radiation (Kα1 = 1.5406 Å, Kα2 = 1.5444 Å, and
Kα2/Kα1 = 0.5). The tube voltage and current were 40 kV and 40
G
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phases were dried over magnesium sulfate and concentrated to a
minimum volume in vacuo, after which methanol (ca. 10 mL) was
added to induce precipitation of the product. The flask was stored at 4
°C overnight and filtered the following day to recover brown solids.
The crude product was washed by sonication in cold methanol (50
mL) and then filtered and dried in air to afford TpmC*-Et as a white
powder. Yield: 18.8 g (36.8%). 1H NMR (500 MHz, C6D6): δ 8.17 (s,
1H), 4.02 (q, J = 7.1 Hz, 6H), 2.42 (s, 9H), 2.36 (s, 9H), 0.93 (t, J =
7.1 Hz, 9H). 13C NMR (126 MHz, C6D6): δ 163.40, 151.16, 145.98,
112.49, 80.86, 59.42, 14.46, 13.88, 10.57.
through a glass frit to remove AgCl. An additional ca. 5 mL of DCM
was used to rinse the vial and frit. To the yellow filtrate was added
∼50 mL of diethyl ether, which caused the instantaneous precipitation
of fine yellow crystals of [Ir(1,5-COD)(CH3CN)2](BF4). The crystals
were collected under vacuum on a 15 mL glass frit, washed with two
15 mL aliquots of ether, and left for 5 min to dry. The crystals were
transferred to a 20 mL scintillation vial and dried under vacuum
1
overnight (286 mg, 0.609 mmol, 81.8% yield). H NMR (400 MHz,
CDCl3): δ 1.72 (m, CH2), 2.26 (m, CH2), 2.57 (s, CH3), 4.22 (s, br,
CH). 13C NMR (101 MHz, CDCl3): δ 3.50, 31.27, 70.68, 123.19.
Pt(CH3CN)2Cl2. PtCl2(CH3CN)2 was prepared from PtCl2 following
a slightly modified literature procedure.44 Under an N2 atmosphere,
PtCl2 (199 mg, 0.748 mmol) was suspended in 25 mL of CH3CN in a
100 mL Schlenk flask. A magnetic stir bar was added, and the reaction
mixture was stirred and heated to reflux overnight, during which the
PtCl2 fully dissolved to give a pale-yellow solution. The solution was
allowed to cool to room temperature and then concentrated in vacuo
to ca. 10 mL in volume, precipitating yellow solids. Under air, the
solids were recovered on a 15 mL glass frit, washed with two 15 mL
aliquots of hexanes, and dried overnight under vacuum (203 mg,
0.583 mmol, 78% yield).
[Pt(CH3CN)4](BF4)2. The Pt(II) metalation complex [Pt-
(CH3CN)4](BF4)2 was prepared with a slight modification of the
reported procedure.45 In a N2 glovebox, a 100 mL Schlenk flask was
charged with AgBF4 (230. mg, 1.18 mmol), 20 mL of dry CH3CN,
and a magnetic stir bar. The flask was transferred to a Schlenk line,
and PtCl2(CH3CN)2 (203 mg, 0.583 mmol) was added under
positive pressure of N2. The hood light was shut off, the flask was
wrapped in Al foil to mitigate photodegradation of AgBF4, and the
reaction mixture was heated to reflux for 4 h under N2. The reaction
mixture was then allowed to cool to room temperature and the foil
removed, revealing a yellow solution with a white-gray precipitate of
AgCl. Under air, the solution was filtered over a Celite pad to remove
AgCl, and the yellow filtrate was concentrated in vacuo to ca. 5 mL. A
50 mL portion of ether was poured into the flask, which caused the
instantaneous precipitation of fine white crystals of [Pt(CH3CN)4]-
(BF4)2. The product was collected on a 15 mL glass frit, washed with
two 10 mL portions of ether, and dried overnight under vacuum. The
final product was then transferred to a 20 mL scintillation vial and
stored under a N2 atmosphere (169 mg, 0.317 mmol, 54.4% yield).
1H NMR (400 MHz, CD3CN): δ 2.61 (s, CH3). 13C NMR (101
1,1′,1″-Methanetriyltris(3,5-dimethyl-1H-pyrazole-4-carboxylic
acid) (TpmC*). In a 1 L round-bottomed flask, Na metal (19.0 g) was
dissolved in ethanol (470 mL) stepwise by fully dissolving small
pieces of Na (ca. 0.5 g) one at a time. Caution!It is important to
perform this step slowly and with caution to avoid overheating. Na is
highly reactive and poses fire/explosion hazards, and the dissolution of Na
in alcohol is exothermic and produces flammable hydrogen gas. After
complete dissolution of all Na, TpmC*-Et (9.18 g, 17.8 mmol) and
deionized water (9.40 mL) were added to the solution. The reaction
mixture was then heated to reflux for 30 min, during which the
reaction mixture first cleared up and then became turbid again. After
the reaction mixture cooled to room temperature, the solid was
separated by filtration and redissolved in a minimal amount of
deionized water. The mixture was acidified with glacial acetic acid (7.1
mL) and stored at 4 °C overnight. The product TpmC* was isolated
by filtration and dried in air overnight to give a white powder. Yield:
1
6.52 g (84.9%). H NMR (500 MHz, methanol-d4): δ 8.49 (s, 1H),
2.38 (m, 18H). 13C NMR (126 MHz, methanol-d4): δ 167.10, 152.98,
147.87, 113.26, 80.85, 14.46, 10.89.
[CuI(TpmC*)8][Zr6O4(OH)4(C6H5COO)4]3 (CuI@Zr-TpmC*). In a 1
L glass bottle, TpmC* (1.49 g) and benzoic acid (74.0 g, 175 equiv)
were sonicated in DMF (500 mL) until dissolved to form a colorless
solution. Separately, copper(I) iodide (2.7 g, 4.0 equiv) and ZrOCl2·
8H2O (2.79 g, 2.50 equiv) were dissolved in DMF (100 mL) in a 250
mL glass beaker. The two solutions were then combined in the 1 L
glass bottle, which was capped and placed in an oven preheated to
120 °C for 4 days. After cooling to room temperature, the reaction
mixture was filtered through a fine glass frit, collecting fine white
solids. The solids were washed on the frit three times with DMF (100
mL) and dried on the frit to give a white powder, which was taken
directly to the next step. Yield: 3.57 g of crude product. In the original
published procedure,18 the reaction mixture was heated to 120 °C for
only 2 days, but we found that extending this reaction time tended to
improve the crystallinity of the product. This procedure, however, still
yielded only a microcrystalline product with reaction times as long as
11 days.
[Zr6O4(OH)4(C6H5COO)4]3 (Zr-TpmC*). In a 1 L glass bottle, 3.57 g
of as-synthesized Zr-TpmC* was suspended in a mixture of DMF
(350 mL) and 4 M HCl (15 mL). With the bottle capped, the mixture
was heated in an oven at 100 °C for 18 h. After the bottle cooled to
room temperature, the solution was decanted. The acid treatment was
repeated three more times, and then the demetalated MOF was
isolated by filtration. The solids were washed by soaking in DMF (50
mL) for 4 h, with the solvent decanted and replaced each hour, and
then the solids were transferred to a 20 mL vial and soaked in DMF
overnight. The following day, the solvent was decanted and the solids
were soaked in acetone (20 mL) for 4 h, with the solvent decanted
and replaced every hour. The MOF was then activated under a
dynamic vacuum at 110 °C for 18 h to give a white powder. Yield:
1.204 g (47% vs TpmC*); Cu:Zr = 0.001:1.000 as determined by
ICP-MS.
MHz, CD3CN): δ 4.41.
General Method for Postsynthetic Metalation of Zr-TpmC*. In
an N2-filled glovebox, 100 mg of activated Zr-TpmC* was charged
into a 20 mL vial. A PGM metalation complex (1.0 equiv/TpmC*
site) was dissolved in CH3CN (10 mL) and added into the vial
containing Zr-TpmC*. The vial was capped and left undisturbed for 1
week. The metalation solution was decanted and the MOF soaked in
20 mL of fresh CH3CN overnight to remove unbound metal complex.
The supernatant was decanted and replaced with fresh CH3CN every
24 h, and the MOF was soaked in fresh CH3CN for 3 days. The MOF
was then soaked in fresh DCM following the same soaking procedure
for 3 days. The MOF was then activated at room temperature under
dynamic vacuum overnight to give a microcrystalline powder.
Metalation of Zr-TpmC* by [Pd(CH3CN)4](BF4)2 to Produce
Pd(CH3CN)2(BF4)2@Zr-TpmC* (1). The preparation followed the
generic procedure, using 100 mg of MOF (0.0171 mmol) and 64.6
mg of [Pd(CH3CN)4](BF4)2 (0.145 mmol). Elemental analysis:
Pd:Zr = 0.378:1.000, as determined by ICP-MS.
Metalation of Zr-TpmC* by [RhCl(CO)2]2 to Produce RhCl(CO)2@
Zr-TpmC* (2). The preparation followed the generic procedure, using
50 mg of MOF (8.5 μmol) and 13.3 mg of [RhCl(CO)2]2 (0.0342
mmol). Elemental analysis: Rh:Zr = 0.184:1, as determined by ICP-
MS.
Metalation of Zr-TpmC* by [Ir(1,5-COD)(CH3CN)2]BF4 to
Produce Ir(1,5-COD)BF4@Zr-TpmC*(3). The preparation followed
the generic procedure, using 100 mg of MOF (0.0171 mmol) and 69
mg of [Ir(1,5-COD)(CH3CN)2](BF4) (0.15 mmol). Elemental
analysis: Ir:Zr = 0.020:1.000, as determined by ICP-MS.
[Ir(1,5-COD)(CH3CN)2]BF4. The Ir(I) metalation complex was
prepared by following a literature procedure.43 In a N2 glovebox,
[Ir(1,5-COD)Cl]2 (250 mg, 0.372 mmol) was dissolved in 6 mL of
DCM in a 20 mL scintillation vial, forming an orange solution. After 5
min of stirring, 1.5 mL of CH3CN was added to the solution, which
quickly turned yellow. The solution was stirred for 5 min to ensure all
components were fully dissolved, and then AgBF4 (150 mg, 0.771
mmol) was added, immediately forming a white-gray precipitate of
AgCl. The reaction mixture was stirred for 10 min and then filtered
H
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Metalation of Zr-TpmC* by [Pt(CH3CN)4](BF4)2 to Produce
Pt(CH3CN)2(BF4)2@Zr-TpmC* (4). The preparation followed the
generic procedure, using 100 mg of Zr-TpmC* (0.0171 mmol) and
78 mg of [Pt(CH3CN)4](BF4)2 for the metalation solution. Elemental
analysis: Pt:Zr = 0.023:1.000, as determined by ICP-MS.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00941.
■
sı
Direct Synthesis of IrCl3@Zr-TpmC* (5). In a 100 mL glass bottle,
344. mg of iridium(III) chloride hydrate (0.966 mmol, 1.28 equiv)
and 324. mg of TpmC* (0.753 mmol, 1.00 equiv) were dissolved in
10 mL of DMF and stirred for 1 h, forming a deep-red/purple
solution. In a separate 100 mL Erlenmeyer flask, 555 mg of ZrOCl2·
8H2O (1.72 mmol, 2.28 equiv) and 14.7 g of benzoic acid (120 mmol,
159 equiv) were sonicated in 50 mL of DMF until dissolved to form a
colorless solution. The contents of the Erlenmeyer flask was then
transferred to the glass bottle containing Ir and TpmC*, which was
then tightly capped and heated to 120 °C for 4.5 days. After cooling
to room temperature, the yellow supernatant was decanted and the
remaining white solids were transferred to a 20 mL scintillation vial.
The solids were then washed with three 20 mL portions of DMF,
soaking in each wash for at least 1 h, followed by two 20 mL portions
of acetone, again soaking for at least 1 h. The white solids were then
activated overnight at 110 °C under high vacuum to yield 5. Yield:
258 mg of activated MOF. Elemental analysis: Ir:Zr = 0.395:1.000, as
determined by ICP-MS.
Direct Synthesis of RhCl3@Zr-TpmC* (6). In a 20 mL scintillation
vial, 37 mg of rhodium(III) chloride hydrate (0.14 mmol, 1.0 equiv)
and 62 mg of TpmC* (0.14 mmol, 1.0 equiv) were dissolved in 3 mL
of 1:1 DMF/H2O to form a deep-red solution. In a separate 20 mL
scintillation vial, 82 mg of ZrOCl2·8H2O (0.25 mmol, 1.8 equiv) and
2.5 g of benzoic acid (20 mmol, 140 equiv) were sonicated in 3 mL of
DMF until dissolved to form a colorless solution. The two solutions
were then combined in one vial, which was capped and heated to 60
°C for 24 h. After cooling to room temperature, the gray-green solids
were collected by centrifugation and washed with three 20 mL
portions of DMF, soaking in each wash for at least 1 h, followed by
two 20 mL portions of acetone, again soaking for at least 1 h. Yield:
148 mg of unactivated MOF. Elemental analysis: Rh:Zr =
0.166:1.000, as determined by ICP-MS.
NMR data, further PXRD characterization data, XANES
results, experimental details and results on Rh and Pd
loading screens, surface area measurements, XPS results,
DFT-assisted EXAFS fitting parameters and data, and
synthetic schemes (PDF)
AUTHOR INFORMATION
Corresponding Author
■
̌
Mircea Dinca − Department of Chemistry, Massachusetts
Institute of Technology (MIT), Cambridge, Massachusetts
02139, United States; orcid.org/0000-0002-1262-1264;
Phone: (617) 253-4154; Email: mdinca@mit.edu
Authors
Michael T. Payne − Department of Chemistry, Massachusetts
Institute of Technology (MIT), Cambridge, Massachusetts
02139, United States
Constanze N. Neumann − Department of Chemistry,
Massachusetts Institute of Technology (MIT), Cambridge,
Massachusetts 02139, United States; Present
Address: Department of Heterogeneous Catalysis, Max-
Planck Institut fur Kohlenforschung, Kaiser-Wilhelmplatz
̈
1, 45470 Muelheim, Germany; orcid.org/0000-0003-
1004-1140
Eli Stavitski − National Synchrotron Light Source II (NSLS-
II), Brookhaven National Laboratory (BNL), Upton, New
York 11973, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00941
Direct Synthesis of K2PtCl6@Zr-TpmC*. In a 20 mL scintillation
vial, 24.2 mg of potassium hexachloroplatinate (0.0498 mmol, 1.00
equiv) and 22.0 mg of TpmC* (0.0511 mmol, 1.03 equiv) were
dissolved in 3 mL of 1:1 DMF/H2O to form a yellow solution. In a
separate 20 mL scintillation vial, 41.3 mg of ZrOCl2·8H2O (0.128
mmol, 2.50 equiv) and 1.09 g of benzoic acid (8.93 mmol, 175 equiv)
were sonicated in 2 mL of DMF until dissolved to form a colorless
solution. The two solutions were then combined in one vial, which
was capped and heated to 60 °C for 24 h. After cooling to room
temperature, the gray solids were collected by centrifugation and
washed with three 20 mL portions of DMF, soaking in each wash for
at least 1 h, followed by two 20 mL portions of acetone, again soaking
for at least 1 h. Yield: 44.1 mg of unactivated MOF. Elemental
analysis: Pt:Zr = 0.063:1.000, as determined by ICP-MS.
Computational Methods. Computational work was performed
using the facilities provided by the Massachusetts Green High
Performance Computing Center. All calculations were performed with
the Gaussian 0346 program package. Geometry optimization was
carried out with the M06 density functional,47 using the 6-311G(d,p)
basis set48 for all nonmetal atoms and the SDD basis set49 for the Pd
atom. The M06 functional was chosen for its enhanced description of
the dispersion effects. The model structure for optimization by DFT is
a truncated cluster model of the formula [Pd(CH3CN)2(TpmC*)]2+
(Figure S38). The starting geometry of TpmC* was based on the
crystallographic data for Zr-TpmC*,18 and the geometry of Pd2+ was
assumed to be square-planar based on the crystallographic data for
[Pd(Tpm)2](BF4)2 and [Pd(CH3CN)4](BF4)2.24,28 Structures were
allowed to fully optimize such that no imaginary frequencies were
observed. The structural rigidity, imparted on TpmC* by the
extended structure of the MOF, was modeled by constraining the
position of the C and O atoms in the three terminal carboxyls of
TpmC* during geometry optimization.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
This work was supported by funding from the Dow Chemical
Company.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research used beamline 8-ID (ISS) of the NSLS-II, a U.S.
Department of Energy (DOE), Office of Science User Facility,
operated for the DOE Office of Science by BNL under
Contract DE-SC0012704. We thank Dr. Joo Kang at the Dow
Chemical Company for assistance with XPS measurements,
and Dr. Steve Rozeveld at the Dow Chemical Company for
assistance with TEM/EDS measurements. We thank Dr. James
Bour for assistance with DFT calculations, and Dr. Amanda
Stubbs for assistance with XAS measurements. We also thank
Dr. Chenyue Sun for providing ligand and MOF samples and
for scientific discussions.
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