A PCP Pincer Ligand for Coordination Polymers with Versatile Chemical Reactivity: Selective Activation of CO2Gas over CO Gas in the Solid State

Article abstract of DOI:10.1002/anie.201604730

A tetra(carboxylated) PCP pincer ligand has been synthesized as a building block for porous coordination polymers (PCPs). The air- and moisture-stable PCP metalloligands are rigid tetratopic linkers that are geometrically akin to ligands used in the synth

Full text of DOI:10.1002/anie.201604730

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International Edition: DOI: 10.1002/anie.201604730
German Edition: DOI: 10.1002/ange.201604730
Coordination Polymers
A PCP Pincer Ligand for Coordination Polymers with Versatile
Chemical Reactivity: Selective Activation of CO2 Gas over CO Gas in
the Solid State
Junpeng He, Nolan W. Waggoner, Samuel G. Dunning, Alexander Steiner, Vincent M. Lynch,
and Simon M. Humphrey*
Abstract: A tetra(carboxylated) PCP pincer ligand has been
synthesized as a building block for porous coordination
polymers (PCPs). The air- and moisture-stable PCP metal-
loligands are rigid tetratopic linkers that are geometrically akin
to ligands used in the synthesis of robust metal–organic
frameworks (MOFs). Here, the design principle is demon-
strated by cyclometalation with PdIICl and subsequent use of
the metalloligand to prepare a crystalline 3D MOF by direct
reaction with CoII ions and structural resolution by single
induce stronger guest binding, for example in the adsorption
of CO2 by amine-decorated materials.[6]
The observation of stronger physisorption at vacant metal
sites in PCPs and MOFs is perhaps not surprising, since this is
a prerequisite of chemical catalysis. In fact, there are an
increasing number of examples of organic reactions catalyzed
by MOFs that are based on analogues of homogeneous
catalysts.[7] For example, in 2010 Champness and co-workers
were able to utilize a Mn(CO)3Cl-chelated building block to
prepare a photoreactive MOF;[7a] more recently via a similar
route, Lin and co-workers used an Ir-functionalized MOF to
À
crystal X-ray diffraction. The Pd Cl groups inside the pores
are accessible to post-synthetic modifications that facilitate
chemical reactions previously unobserved in MOFs: a Pd
CH3 activated material undergoes rapid insertion of CO2 gas to
activate C H bonds;[7b] and Fujita and co-workers structurally
À
À
characterized a Pd-containing MOF that could brominate
aromatic molecules.[7c] However, chemisorption and activa-
tion of common light gas molecules (for which PCPs and
MOFs display very high storage capacities) remains largely
elusive.[8] Arguably, the ability to sequester, activate, and
convert gas molecules such as CO2 inside high-surface-area
MOFs would be both economically and environmentally
more appealing than targeting organic conversions, for which
large-scale homogeneously-catalyzed processes already exist.
The activation of small molecules by organometallic
À
give Pd OC(O)CH3 at 1 atm and 298 K. However, since the
material is highly selective for the adsorption of CO2 over CO,
À
a Pd N3 modified version resists CO insertion under the same
conditions.
E
fforts to prepare PCPs and MOFs with advanced solid-
state chemical reactivity are driven by the desire to access
materials that can adsorb guest molecules more strongly,[1]
and that could potentially catalytically activate adsorbates
inside the pores.[2] The vast majority of known PCPs and
MOFs adsorb small molecules (for example, N2, O2, H2, CO2)
via simple physisorption (dipolar) interactions, typically with
complexes is an important ongoing area of research.[9]
A
number of systems have been shown to be proficient in
difficult chemical transformations, including the activation of
N2.[10] The reactivity of such molecular complexes is extremely
sensitive to the identity of the metal and its ligand donor set.
This chemistry relies primarily on later 4d and 5d metals that
engage in increased metal–ligand covalency; the donor sets
are correspondingly soft atoms, such as phosphines and
carbenes.[11] This presents a synthetic challenge for the
synthesis of PCPs and MOFs containing sites of potential
catalytic reactivity, since their assembly is usually conducted
in solution via the formation of coordination bonds between
hard 3d transition metal ions and carboxylic acid donors.
One effective approach to this problem has been demon-
strated by Cohen[12a,b] and others,[12c] and involves the post-
synthetic modification of MOFs to incorporate secondary
coordination sites within the pores, which can be doped with
catalytically-active metal species. This type of method offers
extensive design flexibility. However, it is relatively common
for post-synthetically modified materials to lose crystallinity,
which impedes absolute structural resolution of the materi-
als.[13]
binding energies in the range 5–40 kJmolÀ1 [3]
. Adsorption and
desorption processes in this regime are conveniently rever-
sible, but the adsorption capacity is limited. Recent research
by several groups has shown that the zero-coverage binding
energy (Qst) of adsorbates can be increased by the inclusion of
stronger adsorption sites in the pores. Lewis acid sites tend to
induce stronger host–guest binding interactions, as demon-
strated for materials with vacant metal coordination sites,[4] or
via the inclusion of polarizable cations.[5] It has also been
shown that pore functionalization with Lewis bases can
[*] J. He, N. W. Waggoner, S. G. Dunning, Dr. V. M. Lynch,
Prof. S. M. Humphrey
Department of Chemistry, The University of Texas at Austin
NHB 6.336, 100 E. 24th St. Stop A1590, Austin TX 78712 (USA)
E-mail: smh@cm.utexas.edu
Dr. A. Steiner
Department of Chemistry, University of Liverpool
Crown St., Liverpool L69 7ZD (UK)
An alternative synthetic approach, which is more likely to
result in the formation of highly crystalline materials, involves
the use of pre-formed catalyst complexes as chemically robust
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604730.
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metalloligands for the direct assembly of PCPs/MOFs. This
can be achieved by strategic positioning of carboxylic acid
groups in ancillary positions of a given complex. The resulting
metalloligands are closely related to water-soluble versions of
homogeneous catalysts, originally designed to facilitate oper-
ation under biphasic conditions.[14] Using this approach, we
previously synthesized a tetratopic building block by para-
carboxylation of the well-known 1,2-bis(diphenylphosphino)-
benzene ligand, and then formed complexes with PdCl2 and
PtCl2. The chelating nature of the ligand resulted in geo-
metrically rigid and thermally stable metalloligands, which
underwent direct reaction with Zn(NO3)2 in a mixed organic/
aqueous solvent to provide isostructural porous solids, PCM-
18 (PCM = phosphine coordination material).[15] Notably, the
coordinatively unsaturated square-planar PdII and PtII metal
sites showed unusual adsorption behavior towards H2 at
elevated temperatures, recently elaborated computationally
by Head-Gordon and co-workers.[16]
We then looked to apply the same strategy to the
preparation of cyclometalated metalloligands to further
improve the resistance to leaching of the active site during
framework assembly, as well as to access more interesting
reactivity toward small molecules. PCP pincer ligands were an
obvious choice to achieve both of these aims.[17] Our approach
to the design and synthesis of a suitable PCP-pincer is shown
in Scheme 1. Briefly, the p-brominated diaryl chlorophos-
phine (1) was prepared by a known method[18] and converted
by lithiation and direct work-up with solid CO2, followed by
precipitation with HCl.
The ligand 5 is suitable for metalation with a range of
transition metal ions. One caveat is that metalation reactions
need to be conducted under conditions that disfavor depro-
tonation of the CO2H groups. Fortunately, metalation using
organometal halides in aprotic organic solvents involves
À
activation of an aromatic C H bond and the resulting
elimination of HX (X = Cl, Br, I) decreases the reaction
pH. This negates ligand deprotonation that would otherwise
result in the formation of unwanted oligomers. As a simple
first example of this strategy, 5 was refluxed directly with
PdCl2(MeCN)2 in tetrahydrofuran (THF) to yield the cyclo-
metalated complex [PdCl(5)] (6), which is suitable for
purification by column chromatography using acidified
silica. Single crystals of 6 were grown by slow evaporation
of a concentrated ethanol solution, allowing for structural
determination by single-crystal X-ray diffraction (SCXRD;
Scheme 1). This study reveals important information regard-
ing the geometry of the metalloligand as a building block for
the subsequent formation of polymers. The Cl-Pd-C axis is
located on a two-fold symmetry site, resulting in a symmetric
molecule in which the ancillary carboxylates are rigidly
locked into a rectangular orientation (Scheme 1; bottom left).
The metalloligand 6 is topologically comparable to the 1,2,4,5-
tetrasubstituted aromatic carboxylates used widely in MOF
synthesis by Hupp and Farha and others.[19]
Initial attempts to prepare porous polymers based on 6
were conducted using 3d transition-metal ions. Along with the
identification of several new materials with 1D (chain) and
2D (layered) structures, large purple crystals of an infinitely
porous 3D coordination material were obtained by reaction
of 6 with Co(BF4)2 in a mixture of N,N-dimethylformamide
(DMF), ethanol, and H2O (2:3:1) at 508C. SCXRD revealed
the material (henceforth named PCM-36) to have the formula
composition
[H2N(CH3)2]3[Co8(OH)3(6)4(OH2)17]·(solv).
PCM-36 contains two symmetry-unique PCP–PdCl building
blocks that are multiply coordinated to CoII ions through all
available carboxylate groups (Figure 1A). There are two
distinct inorganic nodes in this material: a 5-connected
[Co3(m3-OH)(OH2)5]5+ node, and a 3-connected [Co2(m2-OH)-
(OH2)6]3+ node (Supporting Information). Rotational disor-
der of two of the P-aryl groups results in some of the CoII sites
having partial occupancies. The formula unit obtained from
the SCXRD study is in excellent agreement with results
obtained by elemental microanalysis of a bulk crystalline
sample that was subjected to treatment under vacuum to
remove all residual solvent: observed (calculated): C 44.22
(44.15), H 3.04 (3.68); N 0.99 (1.03); Cl, 3.24 (3.48)%.
Crystalline PCM-36 is air- and moisture-stable over months
and can be re-submerged in alcohols or aqueous environ-
ments post-evacuation, resulting in retention of bulk crystal-
linity (Supporting Information).
Scheme 1. Phosphine 1 was obtained via a reported route.[18] Condi-
tions: i) 1 equiv. NaOEt in EtOH with 1 in Et2O, À208C, 30 min; ii) 2
and 0.42 equiv. 1,3-(CH2Br)C6H4, p-xylene reflux, 18 h; iii) 15 equiv.
HSiCl3, toluene reflux, 18 h, NaHCO3 quench; iv) 5 equiv. n-BuLi, THF,
À788C, 1 h, then added excess CO2, warmed to r.t., dissolution in
degassed H2O, acidification with 2.0m HCl to pH 1; v) 1 equiv. PdCl2-
(MeCN)2, THF reflux, 3 d. Bottom left: single-crystal structure of
metalloligand 6;[28] inset: view along Cl-Pd-C, showing the C2 symmetry
and rectangular orientation of CO2H groups.
into the diaryl ethyphosphinite (2) by slow addition of sodium
ethoxide at low temperature. Phosphine 2 was then reacted
with 0.5 equivalents of 1,3-bis(bromomethyl) benzene via the
Arbuzov reaction to yield the bis(phosphine oxide) (3), which
was subsequently cleanly reduced to the bis(phosphine) (4) by
excess HSiCl3. The final ligand 5 was obtained as the free acid
The extended structure of PCM-36 reveals micropores in
all three crystallographic directions (Figure 1B,C); the largest
oval-shaped pore openings (seen in the ac-plane; Figure 1B)
À
have two accessible Pd Cl sites, separated by 15.4 ꢀ (Pd···Pd
distance).
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quently immersed in dry organic solvents and reagents were
slowly added dropwise and allowed to stand for 2–24 h under
N2 and without stirring. The treated crystals were then
subjected to cycles of solvent exchange using the same
solvent, before drying under vacuum prior to characterization
(see PXRD; Supporting Information).
À
First, we attempted to generate Pd H groups via reaction
with NaBH4 in methanol by the method of Goldberg and co-
workers,[20] but PCM-36 underwent decomposition and ele-
mental Co was observed. In contrast, slow addition of a dilute
solution of methyllithium (MeLi) in THF (ca. 10 equiv per Pd;
Milstein and co-workers[21]) did not cause decomposition and
À
appeared to result in the generation of Pd Me groups via
elimination of LiCl. Quantitative analysis of this transforma-
tion is not straightforward to perform because, unlike in the
molecular regime, NMR spectroscopy is not a generally
applicable method in the solid state, especially when there are
multiple paramagnetic centers present (for example, high-
spin CoII, S = 3/2). However, solid-state FTIR provides some
important insights: a comparison of the far-IR spectra for the
parent PCM-36 and MeLi-treated samples shows the appear-
ance of a new band at about 279 cmÀ1 that is attributed to the
Figure 1. A) The asymmetric unit of the PCM-36 polymer framework
showing CoII-carboxylate connectivity (pore constituents are omitted
for clarity).[28] B) Space-filling and superimposed ball-and-stick repre-
sentation in the crystallographic ac-plane showing the largest oval
shaped pores. C) Alternative view in the ab-plane.
Thermogravimetric analysis (TGA) of as-synthesized
PCM-36 indicated that free solvent was removed below
1208C after which no further mass loss occurred until the
onset of framework decomposition at approximately 3708C
(Supporting Information). The bulk surface area of evacuated
PCM-36 was relatively low (112 m2 gÀ1; BET method using
CO2 as the probe gas), but the type-I form of the adsorption–
desorption isotherm is indicative of bulk microporosity
(Figure 2A). The crystallinity of samples of PCM-36 was
also assessed by powder X-ray diffraction (PXRD; Support-
ing Information), which confirmed phase purity with the
SCXRD result (Supporting Information).
À
methyl Pd C stretch, based on reported values for a series of
square-planar PdCl2X2 complexes (Supporting Informa-
2
tion).[22] Furthermore, the Pd C(sp ) stretching band (ca.
À
410 cmÀ1) becomes broadened toward lower wavenumbers in
the Me-modified material, which is expected due to the
stronger trans-influence of CH3 versus ClÀ. The elemental
À
microanalysis of the product showed a reduction in the total
amount of Cl (to < 1%, see above). Collectively, these data
À
suggest that a fraction of the Pd Cl sites in PCM-36 were
successfully modified.
Wendt and co-workers have shown that CO2 can undergo
À
Next, reactions were conducted to explore the potential
activation of PdII sites inside the pores, by substitution of ClÀ
ligands with more weakly coordinating anions. There are
many such precedents for analogous molecular Pd-based
pincer complexes, whereby replacement of Cl anions has been
shown to facilitate chemical reactivity towards various small
molecule adsorbates.[20,21,23,24b] In all of the studies, fresh
crystalline samples of PCM-36 were desolvated by heating at
808C overnight under vacuum. The crystals were subse-
insertion into Pd C bonds of pincer complexes in benzene
solution; insertion is commonly rapid at room temperature
for allyl substitutents and occurs at 808C for CH3.[23] When
evacuated PCM-36-Me was exposed to dry CO2 for 6 h at
298 K, an intense new band was observed in the FTIR
spectrum at about 1645 cmÀ1 that was not observed when the
parent PCM-36 was exposed to CO2 gas (Figure 3A). We
suspected this might be due to the carbonyl asymmetric
stretch (n’CO) of a Pd-OC(O)Me moiety obtained by CO2
Figure 2. A) Adsorption–desorption isotherms for CO2 and CO gas in PCM-36. B) Post-synthetic modifications applied to PCM-36 and reaction
outcomes upon exposure of dry crystalline samples of modified PCM-36 to CO2 and CO gas.
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when incorporated into the MOF. However, this explanation
À
is directly contradicted by the apparent reactivity of the Pd
Me material towards CO2, which appears to proceed at lower
temperature than in the molecular regime. An alternative
explanation is that CO gas is not permitted access to the pores
À
and therefore only a minority of Pd N3 groups at (or near)
the surface of the PCM-36 crystallites may undergo reaction.
In accordance with this hypothesis, it was found that CO gas
was not adsorbed inside an evacuated sample of PCM-36 that
was subsequently able to adsorb CO2 without impediment
(Figure 2A).
Figure 3. A) Comparison of solid-state FTIR spectra in the carbonyl
stretching region as a function of PCM-36-Me exposure to CO2. B) The
N3 symmetric stretching band region. C) Appearance of NCO stretch-
ing band upon high pressure treatment of PCM-36-N3 to CO.
Selective adsorption of small molecule adsorbates by
MOFs and PCPs has been widely documented and studied.[27]
Most commonly, CO2 is preferentially adsorbed over other
gases such as N2, O2, and CO.[27] This unique property of
microporous materials could be exploited in selective catal-
ysis using crude mixtures of unrefined reagents, in which the
MOF-catalyst acts as its own filter, to prevent unwanted side-
reactions and/or catalyst poisoning. The intriguing prelimi-
nary results discussed here for this new class of PCP-pincer-
based MOFs lends some support to this premise. We are
presently investigating the preparation of other PCMs based
on Ru-, Rh-, and Ir-PCP pincer building blocks for funda-
mental studies of other important small molecule activations,
insertion into Pd-Me bonds (Figure 2B). By comparison, m2-
bridging acetates in Pd(OAc)2 have n’CO = 1593 cmÀ1 while
free methylacetate has n’CO = 1740 cmÀ1. The intermediate
n’CO value observed here is indicative of a pseudo-mono-
À
dentate acetate binding mode, in which one Pd O bond is
much shorter than the other; this is expected based on known
À À
crystal structures of PCP M OAc complexes (M = Ni,
Pd).[23b,24] It should also be noted that the carbonyl stretch
for lithium acetate occurs in the solid state at 1588 cmÀ1.
Therefore, it is highly unlikely that reaction of any residual
MeLi with CO2 in the pores of PCM-36 is responsible for the
new feature at 1645 cmÀ1 (Figure 3A). To support this
premise, a sample of PCM-36-Me was then exposed to
99.5% 13CO2 for 4 h and analyzed by direct excitation 13C-
MAS-NMR: a single broad peak was observed at 167 ppm
accompanied by intense side bands, which is directly in the
including C H bond activation.
À
Acknowledgements
The authors acknowledge the NSF for funding under grant
number DMR-1506694, and the Welch Foundation (F-1738).
We thank Dr. Steve Swinnea (Univeristy of Texas) for
assistance with PXRD analyses and Dr. Vladimir Bakhoutov
(Texas A&M University) for MAS-NMR analysis.
À
À
range of other known PCP Pd OAc complexes (152–
175 ppm; Supporting Information).[25] Furthermore, the cor-
responding carbonyl-carbon peak for LiOAc occurs signifi-
cantly more up-field, at 181 ppm.
Keywords: CO2 activation · coordination polymers · metal–
organic frameworks · PCP pincer ligands · selective adsorption
In an alternative experiment, treatment of PCM-36 with
trimethylsilyl azide (TMS-N3; 15 equiv) in THF gave a parti-
À
ally Pd N3-substituted material via elimination of TMSCl.
The FTIR spectrum of this material shows an intense new
band due to the characteristic N3 symmetric stretching mode
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Received: May 14, 2016
Revised: July 13, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Coordination Polymers
J. He, N. W. Waggoner, S. G. Dunning,
A. Steiner, V. M. Lynch,
S. M. Humphrey*
&&&& — &&&&
A PCP Pincer Ligand for Coordination
Polymers with Versatile Chemical
Reactivity: Selective Activation of CO2
Gas over CO Gas in the Solid State
Poly-PCP-pincers: A new synthetic strat-
egy for the formation of crystalline,
porous versions of PCP-pincer complexes
is presented. These complexes can
selectively activate gas molecules, such
as CO2 over CO, in the solid state.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!