Catalytic Activity of a Zr MOF Containing POCOP-Pd Pincer Complexes

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O O

Catalytic Activity of a Zr MOF Containing P C P‑Pd Pincer

Complexes

Abebu A. Kassie and Casey R. Wade*

Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00164

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sı Supporting Information

ABSTRACT: A metal−organic framework assembled from

P C P-Pd pincer complex metallolinkers (1-PdBF ,

Zr O (OH) (L-PdMeCN) (BF ) , L = (2,6-(OPAr ) C H , Ar =

4 3

2 2

−

p-C H CO ) has been generated via postsynthetic oxidative I /

BF₄ligand exchange with NOBF . 1-PdBF catalyzes a range of

−

organic transformations, including transfer hydrogenation of

unsaturated organic substrates, terminal alkyne hydration, and

intramolecular hydroarylation of alkynes. The homogeneous

analogue, Bu P C P-PdBF , shows poor catalytic activity for

transfer hydrogenation and alkyne hydration and decomposes

under the catalytic reaction conditions. Solubility limitations and

catalyst deactivation pathways observed for the homogeneous pincer complex propound the advantages of using porous solid

supports to immobilize organometallic species.

INTRODUCTION

Most industrial chemical processes rely on heterogeneous

catalysts owing to their stability under harsh conditions,

ing material was found to catalyze gas-phase hydrogenation of

■

ethylene to ethane. We have recently reported the synthesis

and characterization of Zr MOFs assembled from linkers based

18−22

1,2

on Co, Ru, Pd, and Pt diphosphine pincer complexes.

recyclability, and ease of product separation.

However,

Several of these pincer MOFs have shown improved catalytic

activity or selectivity compared to homogeneous counterparts,

and the diﬀerences in reactivity could be attributed to site

isolation and secondary environment eﬀects provided by the

rigid MOF scaﬀold.

homogeneous catalysts are more structurally well-deﬁned and

often provide greater selectivity and activity. Consequently,

more eﬃcient and sustainable catalytic processes could be

realized with a hybrid approach to catalyst design that

consolidates the beneﬁcial features of heterogeneous and

homogeneous systems. Metal−organic frameworks (MOFs)

with well-deﬁned crystalline structures, large internal surface

areas, and tunable functionality have emerged as an important

Novel methods have also been developed to activate the

immobilized pincer complexes for catalysis (Figure 1). For

−

example, 1-PdTFA has been accessed by postsynthetic I /

−9

−

class of materials for hybrid catalyst design. The organic and

inorganic building blocks of these solid materials provide ideal

platforms to incorporate uniform catalytic sites that mimic the

structure and function of homogeneous catalysts. In addition,

the rigid frameworks can provide site isolation for reactive

intermediates that might otherwise suﬀer from deactivation

TFA (TFA = CF CO ) oxidative ligand exchange at the

3 2

P C P-PdX linkers using PhI(TFA) . This material showed a

dramatic increase in catalytic activity for transfer hydro-

genation of aldehydes compared to both the parent MOF, 1-

PdX, and the homogeneous analogue, Bu P C P-PdTFA.

−

NOBF₄was found to facilitate I /BF oxidative ligand

,8,10−13

processes in solution.

Diphosphine pincer ligands are widely used in homogeneous

exchange at P N P-PdI linkers in 2-PdI to cleanly generate 2-

PdBF₄. The latter showed superior catalytic activity for the

intramolecular cyclization of o-alkynyl anilines compared to the

TFA-exchanged analogue 2-PdTFA. The improved catalytic

14−17

catalysis owing to their stability and versatility.

Recently,

we and others have been interested in immobilizing

diphosphine pincer complexes in MOFs and other porous

materials to investigate the eﬀects of site isolation and

heterogenization on catalytic activity, stability, and recycla-

activity of 2-PdBF can be linked to the presence of the more

8−24

Received: March 6, 2020

bility.

For example, Byers and Tsung have shown that

encapsulation of PNP-Ru complexes in UiO-66 improves their

activity toward CO hydrogenation and oﬀers protection from

thiol poisoning. P C P-Ir complexes have also been

immobilized at the Zr metal nodes of NU-1000 using a

solvent-assisted ligand-incorporation approach, and the result-

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 1. Postsynthetic Ligand-Exchange Reactions

ately becomes dark orange, indicating the generation of soluble

iodine species. After a series of successive washes with MeCN,

the X-ray ﬂuorescence (XRF) and X-ray photoelectron (XPS)

spectra of 1-PdBF showed that signals corresponding to

iodine completely disappeared (Figures S12 and S13). Powder

X-ray diﬀraction (PXRD) analysis conﬁrmed that the MOF

retains crystallinity (Figure S14 ). The acid-digested P NMR

spectrum of 1-PdBF contains two major resonances at 147

and 144 ppm along with an unidentiﬁed minor species at 91

ppm (Figure S4 ). The peak at 147 ppm appears at a chemical

shift similar to that observed for Bu P C P-PdBF (147.5

ppm in CDCl ), which has been prepared via reaction of

O O

Bu P C P-PdI and AgBF (Scheme 1b). The resonance at

44 ppm is likely due to H P C P-PdX (X = TFA or OH )

−

species arising from ligand substitution under the digestion

conditions (CF CO H/C D ). This assignment is supported

Figure 1. (a) Structures of 1-PdX and 2-PdI. (b) Postsynthetic

^o^xⁱ^d^a^tⁱ^v^e^lⁱ^g^aⁿ^d^e^x^c^h^aⁿ^g^e^u^s^e^d^t^o^g^eⁿ^e^r^a^t^e¹^-^P^d^T^F^A^aⁿ^d²^-^P^d^B^F4^.

by the P NMR spectrum of Bu P C P-PdBF which shows

4 4

similar speciation in the CF CO H/C D solvent mixture

_w_e_a_k_l_y_c_o_o_r_d_i_n_a_t_i_n_g_B_F− ^aⁿⁱ^oⁿ^a^s^w^e^l^l^a^s^t^h^e^a^bⁱ^lⁱ^t^y^o^f^N^O^B^F4

to aﬀord clean and complete exchange of iodide.

(Figure S9 ). P C P-Pd species have been proposed as

intermediates in oxidative Heck reactions with iodonium

These ﬁndings have prompted us to further explore the

scope of catalytic reactions available to pincer MOFs. Ligand

platforms used in homogeneous catalysis often require

optimization for diﬀerent types of reactions. In contrast,

heterogeneous systems potentially oﬀer more latitude owing to

site-isolation eﬀects that relieve the need to ﬁne-tune ligand

steric proﬁles and the absence of solubility limitations. In this

regard, the ability to employ a recyclable heterogeneous

catalyst for a range of organic transformations is potentially

appealing for multistep or tandem reaction sequences. Herein

salts. In the present case, NOBF is a potent oxidant and

might be expected to eﬀect Pd-based oxidation. Although a

stoichiometric excess of NOBF was used for the oxidative

ligand exchange, we do not observe any obvious spectroscopic

evidence of oxidized Pd species in 1-PdBF . The lack of an

observed spectroscopic signature does not rule out the

formation of Pd intermediates during the ligand exchange

process. The acid-digested H NMR spectrum of a dried

sample of 1-PdBF closely resembles that of Bu P C P-

PdBF in the same solvent mixture and shows the presence of

−

we apply NOBF for postsynthetic I /BF oxidative ligand

∼1 equiv of MeCN per pincer complex, suggesting that the

solvent molecule serves as a ligand toward the cationic Pd

exchange at P C P-PdI metallolinkers. The resulting MOF, 1-

PdBF , has been evaluated as a catalyst for a series of reactions

center (Figures S5 and S8). Elemental analysis of 1-PdBF is in

including transfer hydrogenation of unsaturated organic

substrates, hydration of terminal alkynes, and intramolecular

hydroarylation reactions.

agreement with nearly complete exchange of the iodide ligand

(see the Experimental Section for details).

N adsorption isotherms measured at 77 K for samples of 1-

−

PdI and 1-PdBF activated at 100 °C and 10 Torr for 20 h

gave calculated BET surface areas of 1060 and 880 m g ,

RESULTS AND DISCUSSION

2 −1

■

-PdX was prepared following the previously reported

respectively. The BET surface area of 1-PdI is consistent with

2 −1

procedure. A suspension of 1-PdX in water was treated

with an aqueous solution of NaI to aﬀord 1-PdI as a yellow

that previously reported for 1-PdX (1164 m g ) and the slight

decrease observed for 1-PdBF is likely due to the presence of

−

microcrystalline powder (Scheme 1a). The acid-digested

pore-occluding BF₄counterions.

NMR spectrum of 1-PdI shows a single sharp resonance at 148

With 1-PdBF in hand, we set out to evaluate its activity for

ppm, which is consistent with that observed for H P C P-PdI

transfer hydrogenation of aldehydes in comparison to the

previously reported 1-PdTFA. Benzaldehyde was used as a

benchmark substrate with 3 mol % catalyst loading (based on

Pd) and formic acid as the hydrogen source (Table 1). The

(

148 ppm in DMSO-d ) (Figure S1). The H NMR spectrum

of the acid-digested sample displays all resonances expected for

a single H P C P-PdI pincer complex (Figure S2). Reaction

of 1-PdI with NOBF (2 equiv per Pd) in acetonitrile results in

reaction yields were determined by H NMR using 1,3,5-

the formation of 1-PdBF as a white microcrystalline powder.

trimethoxybenzene as an internal standard. Under these

Upon addition of NOBF , the reaction supernatant immedi-

conditions, 1-PdBF catalyzes the transfer hydrogenation of

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Article

Table 1. Catalytic Transfer Hydrogenation of Aldehyde,

Alkene, Alkyne, and Nitroarene Substrates with 1-PdBF₄

hydrogenation of unsaturated organic substrates provides a

mild alternative to direct hydrogenation, which often requires

high-pressure H₂and complicated reaction setups.

multitude of homogeneous catalysts have been developed for

alkene transfer hydrogenation with reducing agents such as

29−31

isopropanol, formic acid, or ethanol.

However, relatively

few heterogeneous systems have been developed despite

interest in catalysts that are reusable and operate under mild

32−34

conditions.

In addition, catalytic reduction of nitroarenes

is used to prepare amines which are intermediates en route to

pharmaceutical and agricultural products. Both homoge-

neous and heterogeneous catalysts have been developed for the

hydrogenation of nitro groups, but selective reduction to

35−37

amines still presents challenges.

Under reaction conditions similar to those used for transfer

hydrogenation of aldehydes, styrene was fully converted to

ethylbenzene in 6 h with 1-PdBF as a catalyst (Table 1, entry

). 1-PdBF could be recycled once with no loss of activity,

but a slight decline was observed in the third run (72% yield).

This loss of activity is likely due to partial framework

degradation resulting in catalyst deactivation. Two notable

observations support this hypothesis: (1) The MOF changed

color from oﬀ-white to dark brown after the third run. (2)

PXRD analysis after the third run showed a signiﬁcant loss of

framework crystallinity and the appearance of new reﬂections

attributable to zirconium oxide (Figure S28). Nonetheless, 1-

PdBF₄outperforms 1-PdX and 1-PdI as well as the

O O

homogeneous complex Bu P C P-PdBF for transfer hydro-

genation of styrene (Figures S29 −S31). Further eﬀorts to

expand the substrate scope to terminal aliphatic oleﬁns using 1-

dodecene as a substrate gave a mixture of starting material,

internal alkenes, and only a small amount of dodecane product

(

Figure S34 ). Alkene isomerization is not uncommon for Pd

catalysts, and the low yield of hydrogenated product suggests

that 1-PdBF favors chain walking over hydrogenation and is

38−40

less active toward internal alkenes.

The decreased activity

toward 1-dodecene and its internal isomers may also be a

function of the size of the substrate since 1-PdBF catalyzes

the transfer hydrogenation of 1,5-cyclooctadiene to a mixture

of cyclooctene and cyclooctane with 60% overall conversion of

the alkene groups (Figures S32 and S33). Encouraged by the

Reaction conditions: substrate (0.2 mmol), catalyst (0.006 mmol

Pd), NaO CH (0.2 mmol), HCO H (0.6 mmol), H O/MeOH (1:1,

activity of 1-PdBF for alkene transfer hydrogenation, we

v/v), 6 h, 60 °C. Yields are for hydrogenated products and were

determined by H NMR with respect to an internal standard (1,3,5-

trimethoxybenzene). A longer reaction time (12 h) was used. The

reaction was carried out in MeOH without added H O. Increased

investigated the α,β-unsaturated carbonyl substrate benzylide-

neacetone. There has been considerable interest in chemo-

41−44

selective hydrogenation of this class of compounds.

amounts of HCO H (1.2 mmol) and catalyst (0.01 mmol) were used.

Gratifyingly, 1-PdBF shows selectivity for transfer hydro-

genation of the alkene over the ketone functional group after 6

h reaction time (Table 1, entry 5). Increasing the reaction time

to 12 h gave 27% of the fully hydrogenated product, 4-phenyl-

2-butanol (Figures S35 −S38 ).

benzaldehyde to benzyl alcohol in 98% yield, while 1-PdTFA

gives only 73% yield (Table 1, entry 1). This comparison

−

reveals that the more weakly coordinating BF₄counteranion

oﬀers a distinct improvement in catalytic activity. In line with

Initial screening of phenylacetylene as a substrate with 1-

our previous ﬁndings, the homogeneous complex, Bu P°C P-

PdBF under the optimized conditions provided a mixture of

PdBF , displays poor catalytic activity (9% yield) and shows

ethylbenzene (37%) and acetophenone (60%) as hydro-

genated and hydrated products, respectively (Figures S39

and S40 ). When water was excluded from the reaction,

ethylbenzene and acetophenone were produced as major

(80%) and minor (17%) products, respectively (Figures S41

and S42 ). The hydration side reaction could not be completely

inhibited owing to the presence of adventitious water in formic

acid and methanol. Moreover, the semihydrogenation product,

styrene, was not observed. However, an internal alkyne,

diphenylacetylene, aﬀorded a ∼1:3 mixture of stilbene and 1,2-

diphenylethane after 5 h, and extending the reaction time to 12

signs of decomposition under the reaction conditions (Figure

S22). However, 1-PdBF₄remains crystalline after the

catalytic reaction, corroborating the beneﬁcial eﬀect of catalyst

immobilization. Moreover, 1-PdBF catalyzed transfer hydro-

genation of both aryl and aliphatic aldehyde substrates in good

yields (Table S1 , entries 1−4).

Catalytic Transfer Hydrogenation of Alkene, Alkyne,

and Nitroarene Substrates. The scope and generality of

catalytic transfer hydrogenation with 1-PdBF has been tested

with alkene, alkyne, and nitroarene substrates. Transfer

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

h provided nearly full conversion to 1,2-diphenylethane

oxidized diarylphosphine fragments associated with degrada-

tion of the pincer ligand (Figure S57). Although pincer

complexes are generally quite robust, pincer ligands containing

P−O and P−N linkages are known to be susceptible to

Figures S43 −S45). Similar to 1-dodecene, 4-octyne provided

mixtures of octene isomers under the optimized transfer

hydrogenation conditions (Figure S46). 1-PdBF catalyzes the

52,53

transfer hydrogenation of nitrobenzene and 2-nitrotoluene in

good yields after 6 h with 5 mol % catalyst loading. However,

full conversion of the nitroarene substrates required increased

amounts of formic acid (6 equiv), which also results in

formation of N-aryl formamides as side products (Table 1,

entries 8 and 9).

hydrolysis.

In the present case, MOF immobilization of

P C P-PdBF appears to ameliorate this sensitivity, leading to

an eﬃcient and recyclable catalyst.

Synthesis of Coumarins via Intramolecular Hydro-

arylation. The functionalization of arene C−H bonds to form

new C−C bonds remains a synthetically challenging trans-

formation. In this regard, catalytic hydroarylation oﬀers an

Catalytic Hydration of Alkynes. Catalytic hydration of

alkynes has historically been facilitated by Hg(II) and strong

atom-economic approach to couple unsaturated organic

54−59

Brønsted acids. Alternative catalyst systems have been sought

owing to the toxicity of Hg(II), and some very eﬃcient Au, Pt,

Fe, and Co catalysts have emerged that require only mild

substrates (alkenes and alkynes) and arenes.

The

synthesis of biologically relevant coumarins via intramolecular

hydroarylation of alkynes is one practical application of this

synthetic transformation. The Pechmann condensation has

been one of the most widely used methods for preparation of

coumarins, but requires stoichiometric amounts of Brønsted or

Lewis acid, making it less attractive from an atom economy

46−51

Brønsted acid or acid-free conditions.

The stability and

recyclability of 1-PdBF₄makes it a potentially appealing

catalyst for alkyne hydration. Indeed, 1 mol % 1-PdBF₄

catalyzes the hydration of phenylacetylene to acetophenone

in 95% yield after 5 h at 60 °C using a 1/1 (v/v) mixture of

MeOH and water as solvent (Table 2). The catalyst could be

perspective. As a result, there has been interest in developing

transition metal catalysts for intramolecular hydroarylation

reactions, and Pd-based catalysts have received considerable

54,57

Table 2. Catalytic Hydration of Alkynes

attention.

Thus, the ability of the Lewis acidic Pd sites in

-PdBF to activate alkynes toward hydration prompted us to

investigate intramolecular hydroarylation as more challenging

class of reactions. Gratifyingly, the intramolecular hydro-

arylation of 3 with 5 mol % 1-PdBF as catalyst proceeded

smoothly to generate coumarin 6 in 81% yield after 12 h in

toluene at 90 °C (Table 3). 1-PdBF could be recycled twice

Table 3. Synthesis of Simple Coumarins via Intramolecular

Hydroarylation

Entry

Catalyst

1-PdX

% Yield

MeO

¹^-^P^d^B^F4

Reaction conditions: substrate (0.4 mmol), catalyst (0.004 mmol),

^tBu

H O/MeOH, 5 h, 60 °C. Yields were determined by H NMR with

respect to an internal standard (1,3,5-trimethoxybenzene). THF/

P C P-PdBF

4 4

1-PdBF (run 2)

H O (5:1, v/v) solvent mixture was used.

1-PdBF (run 3)

1-PdBF (run 4)

1-PdBF₄

recycled up to 4 times with no loss of activity (Figure S53),

and PXRD analysis con ﬁ rmed that the MOF remains

crystalline after recycling (Figure S54). Under similar reaction

conditions, 1-PdI and UiO-67 gave <5% yield of product

NO₂

Reaction conditions: substrate (0.1 mmol), catalyst (0.005 mmol),

toluene-d , 12 h, 90 °C. 1,4-Dioxane was used instead of toluene-d .

Yields were determined by H NMR with respect to an internal

(Table 2, entries 2 and 4), indicating that the catalytic activity

standard (hexamethylbenzene or hexaethylbenzene).

observed for 1-PdBF is the result of activated Pd pincer sites

rather than coordinatively unsaturated Zr species arising from

framework defects. 1-PdBF is also selective for hydration of

terminal alkynes, and no substrate conversion was observed

with diphenylacetylene and 4-octyne (Table 2, entries 6 and

with no appreciable loss of activity. A modest decrease in

product yield was observed in the fourth run, but this is

attributed to catalyst loss during the recycling process since

PXRD analysis showed that the MOF retained its crystallinity.

The heterogeneous nature of the catalyst was further

supported by a hot ﬁltration test which showed an abrupt

halt in substrate conversion after removing the MOF from the

reaction mixture (Figures S64 and S65). As expected, 1-

PdXwas inactive (<5% yield of 6) under identical reaction

conditions.

). Catalytic alkyne hydration studies with the homogeneous

complex, Bu P C P-PdBF , were complicated by its poor

solubility in MeOH/H O mixtures, but a catalytic reaction

carried out in a THF/H O (5:1, v/v) mixture that fully

solubilizes the complex revealed no substrate conversion

(

Table 2, entry 3). Unexpectedly, ³¹P NMR analysis of the

reaction mixture showed decomposition of the pincer complex

with resonances around 20−35 ppm signaling the formation of

Organometallics XXXX, XXX, XXX−XXX

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Article

Pd-catalyzed hydroarylation reactions have been proposed to

proceed via activation of the alkyne group by the Lewis acidic

metal site followed by nucleophilic attack of the arene. This

mechanism is akin to electrophilic aromatic substitution and as

a result becomes unfavorable for electron deﬁcient arenes.

Technologies. Other solvents and reagents were purchased from

commercial suppliers and used as received. Routine powder X-ray

diﬀraction (PXRD) patterns for phase identiﬁcation were collected

using a Rigaku Miniﬂex 600 diﬀractometer with nickel-ﬁltered Cu Kα

radiation (λ = 1.5418 Å). ATR-IR spectra were measured using a

Bruker Alpha II spectrometer with a diamond ATR accessory. N₂

adsorption isotherms (77 K, liquid nitrogen bath) were measured

using a Micromeritics 3Flex Surface Characterization Analyzer. Prior

to analysis, samples (100−200 mg) were heated under reduced

pressure until the outgas rate was less than 2 mTorr/min. GC-MS

analysis was performed using an Agilent 7890B GC system equipped

with the HP-5 Ultra Inert column (30 m, 0.25 mm, 0.25 μm) and a

FID detector. For MS detection an electron ionization system was

used with an ionization energy of 70 eV. Elemental analyses (C, H, N,

and I) were performed by Robertson Microlit Laboratories (Ledge-

Indeed, while 1-PdBF showed only a small decrease in activity

with 4 as a substrate, no hydroarylation product was observed

with the electron-deﬁcient substrate 5 (Table 3, entries 7 and

). Bu P C P-PdBF again proved to be poorly soluble under

4 4

the catalytic reaction conditions optimized for 1-PdBF . After

solvent screening, 1,4-dioxane was found to be a suitable

solvent, and Bu P C P-PdBF gave 61% yield of 6 after 12 h

at 90 °C (Table 3, entry 3). Consequently, immobilization of

the pincer complex in 1-PdBF₄not only provides slight

improvement in catalytic activity but also obviates solubility

limitations encountered for the homogeneous system.

−

wood, NJ). XRF data for I /BF exchange reactions were obtained

using an Innov-X Systems X-500 spectrometer with a Co Kα radiation

source. High-resolution XPS analysis was performed at The Ohio

State Surface Analysis lab using a Kratos Axis Ultra X-ray

photoelectron spectrometer with a monochromatic Al Kα X-ray

source. The binding energy of all photoelectron spectra were

referenced to adventitious carbon at 284.5 eV.

CONCLUSION

■

−

In summary, NOBF has been applied as a reagent for I /BF

oxidative ligand exchange of P C P-PdI complexes immobi-

Solution-state NMR spectra were measured using Bruker 400 MHz

lized as linkers in 1-PdI. The resulting material, 1-PdBF , is

spectrometer. For H NMR spectra, the solvent resonance was

more eﬀective than 1-PdTFA for transfer hydrogenation of a

benchmark substrate, benzaldehyde, emphasizing the impor-

tance of optimizing precatalyst activation procedures. Similar

to our previous observations, the homogeneous analogue,

referenced as an internal standard. For P NMR spectra, 85% H PO

was used as an external standard (0 ppm). Solvent-suppressed H

NMR spectra were collected using 180° water selective excitation

sculpting with default parameters and pulse shapes.

Synthesis of 1-PdI. A solution of NaI (0.070 g, 0.47 mmol) in

Bu P C P-PdBF , decomposes under the catalytic conditions

H O (2 mL) was added to a suspension of 1-PdX (0.126 g, 0.116

without appreciable substrate conversion. 1-PdBF₄also

catalyzes transfer hydrogenation of alkene, alkyne, and

nitroarene substrates, albeit with modest activity. More

mmol) in H O (5 mL) and stirred gently at room temperature for 16

h. The solid was collected by centrifugation, washed with H O (5 ×

0 mL) and MeCN (5 × 10 mL), and dried in vacuo to give a faint

31 1

notably, 1-PdBF proved to be an eﬃcient and recyclable

yellow microcrystalline powder (0.121 g). P{ H} NMR (162 MHz,

catalyst for alkyne hydration and intramolecular hydroarylation

CF CO H/C D ) δ 146.8 (s, 2P).

of alkynes. Bu P C P-PdBF exhibits poor solubility in the

Synthesis of 1-PdBF . A solution of NOBF (0.060 g, 0.50

aqueous solvent mixture used for alkyne hydration with the

mmol) in MeCN (1 mL) was added to a suspension of 1-PdI (0.275

g, 0.25 mmol) in MeCN (5 mL) and stirred gently at room

temperature for 16 h. The reaction supernatant turned orange upon

MOF (MeOH/H O) and was found to decompose when

catalytic reactions were attempted in THF/H O solvent

addition of NOBF . The solid was collected by centrifugation, washed

mixture. This behavior indicates that the complex is sensitive

to hydrolysis in aqueous solvent mixtures and incompatible

with MeCN (3 × 5 mL), and dried in vacuo to aﬀord 1-PdBF as a

white microcrystalline powder (0.25 g). Anal. calcd for 1-PdBF₄:

with the NaO CH/HCO H reagent mixture used in the

Zr O (OH) (OH)₁_.₂(POCOP-Pd-MeCN)₂_.₇(BF₄)₂_.₇(H O) : C,

catalytic transfer hydrogenation reactions. However, immobi-

lization in the MOF signiﬁcantly reduces this sensitivity to the

7.02; H, 2.19; N; 1.2. Found: C, 35.49; H, 2.45; N, 0.48; I, 0.23.

The empirical formula given above is consistent with the presence of

extent that 1-PdBF can be recycled 4 times without loss of

missing linker defects which have been found in related

62−66

alkyne hydration activity. While 1-PdBF and Bu P C P-

materials.

In addition, combustion elemental analysis has

generally been observed to show lower than expected C content for

PdBF₄both catalyze the intramolecular hydroarylation of

alkynes to form simple coumarins, the MOF catalyst proves to

be recyclable and shows greater solvent compatibility. Overall,

this work demonstrates the type of broad scope of catalytic

activity that can become available when homogeneous,

organometallic complexes are immobilized in a porous support

and optimized precatalyst activation procedures are applied. In

addition to ease of product separation and recyclability,

heterogeneous materials such as 1-PdBF₄can exhibit

drastically improved stability compared to homogeneous

analogues and obviate solubility limitations. Our ongoing

work is focused on expanding catalytic applications of more

diverse pincer MOFs.

Zr-based MOFs. Combustion ion chromatography analysis of 1-

PdBF4 shows 0.23 wt % I present in the framework, which

corresponds to ∼0.02 I per Pd site.

O O

Synthesis of Bu P C P-PdBF . A solution of AgBF (0.037 g,

.19 mmol) in MeCN (1 mL) was added to a solution of Bu P C P-

PdI (0.20 g, 0.18 mmol) in CH Cl (5 mL). Upon addition of AgBF ,

formation of white AgI salt was observed. The silver salt was ﬁltered

using a 0.45 um PTFE ﬁlter disk, and volatiles were removed under

reduced pressure. The solid was dissolved in CH Cl , ﬁltered again to

2 2

ensure complete removal of AgI, and dried in vaco to aﬀord a white

powder (0.19 g, 95%). H NMR (400 MHz, CDCl ): δ 1.57 (s, 36H,

Bu), 6.83 (d, 2H, J

= 10.42 Hz, backbone Ar−H), 7.21 (t, 1H,

H−H

J_H₋_H= 11.47 Hz, backbone Ar−H), 7.80 (m, 8H, benzoate Ar−H),

.17 (d, 8H, J

= 9.73 Hz, benzoate Ar−H). P{ H} NMR (162

H−H

MHz, CDCl ) δ 147.48 (s, 2P).

EXPERIMENTAL SECTION

■

Transfer Hydrogenation Reactions. A 1 dram screw-top vial

was charged with catalyst (0.006 mmol of Pd), sodium formate (0.2

General Considerations. 1-PdX, Bu P C P-PdI, and alkyne

substrates for intramolecular hydroarylation reactions were prepared

following literature procedures. All hydroarylation reactions were

carried out in a nitrogen-ﬁlled glovebox using solvents that were dried

mmol), MeOH (0.5 mL), H O (0.5 mL), formic acid (0.6 mmol),

substrate (0.2 mmol), and 1,3,5-trimethoxybenzene as internal

standard. The vial was sealed with Teﬂon-lined cap and heated to

60 °C for 6 h for aldehyde and alkene substrates or 5 h for alkyne

substrates. The reaction mixture was allowed to cool to room

over CaH and distilled or dried via passage through columns of

drying agents using a solvent puriﬁcation system from Pure Process

Organometallics XXXX, XXX, XXX−XXX

Organometallics

(5) Wasson, M. C.; Buru, C. T.; Chen, Z.; Islamoglu, T.; Farha, O.

Article

temperature, extracted with CDCl , and analyzed by H NMR and

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GC-MS/FID. Reaction yields were determined by integration of the

H NMR spectra of the reaction mixtures with respect to the internal

standard.

Alkyne Hydration Reactions. A 1 dram screw top vial was

charged with a catalyst (0.004 mmol), MeOH (1 mL), H O (1 mL),

substrate (0.4 mmol), and 1,3,5-trimethoxybenzene as internal

standard. The vial was sealed with Teﬂon-lined cap and heated to

(7) Rogge, S. M. J.; Bavykina, A.; Hajek, J.; Garcia, H.; Olivos-

Suarez, A. I.; Sepulveda-Escribano, A.; Vimont, A.; Clet, G.; Bazin, P.;

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0 °C for 5 h. The reaction mixture was allowed to cool to room

temperature, extracted with CDCl , and analyzed by H NMR and

GC-MS/FID. Reaction yields were determined by integration of the

H NMR spectra of the reaction mixtures with respect to the internal

standard.

Intramolecular Hydroarylation Reactions. In a nitrogen-ﬁlled

glovebox, a 1 dram screw-top vial was charged with a catalyst (0.005

mmol), toluene-d (1 mL), substrate (0.1 mmol), and hexaethylben-

zene as internal standard. The vial was sealed with Teﬂon-lined cap

and heated to 90 °C for 12 h. The products of the reaction were

characterized by H NMR and GC-MS/FID. Yields of catalysis were

determined by H NMR. For recycling experiments, the catalyst was

isolated from the reaction mixture via centrifugation, washed with

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Today 2019, 27, 178−197.

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Catalyst in a Robust Metal-Organic Framework. Inorg. Chem. 2015,

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4, 6821−6828.

toluene (3 × 2 mL), and resubjected to the catalytic conditions.

Achiral Linkers with Chiral Linkers in Zr-Based UiO-68 Metal-

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00164.

Organic Framework. J. Am. Chem. Soc. 2018, 140, 16229−16236.

■

(12) VanVelthoven, N.; Waitschat, S.; Chavan, S. M.; Liu, P.;

Smolders, S.; Vercammen, J.; Bueken, B.; Bals, S.; Lillerud, K. P.;

Stock, N.; De Vos, D. E. Single-Site Metal-Organic Framework

Catalysts for the Oxidative Coupling of Arenes Via C-H/C-H

Activation. Chem. Sci. 2019, 10, 3616−3622.

Spectroscopic (NMR, IR), GC-MS, PXRD, TGA, and

N gas adsorption data (PDF)

(13) Sawano, T.; Lin, Z.; Boures, D.; An, B.; Wang, C.; Lin, W.

Metal-Organic Frameworks Stabilize Mono(Phosphine)-Metal Com-

plexes for Broad-Scope Catalytic Reactions. J. Am. Chem. Soc. 2016,

138, 9783−9786.

AUTHOR INFORMATION

Corresponding Author

Casey R. Wade − Department of Chemistry and Biochemistry,

The Ohio State University, Columbus, Ohio 43210, United

States; orcid.org/0000-0002-7044-9749;

Email: wade.521@osu.edu

■

15) Kumar, A.; Bhatti, T. M.; Goldman, A. S. Dehydrogenation of

14) Selander, N.; Szabo,

Complexes. Chem. Rev. 2011, 111, 2048−2076.

K. J. Catalysis by Palladium Pincer

Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes.

Chem. Rev. 2017, 117, 12357−12384.

(16) Peris, E.; Crabtree, R. H. Key Factors in Pincer Ligand Design.

Chem. Soc. Rev. 2018, 47, 1959−1968.

Author

(

17) Valdes, H.; García-Eleno, M. A.; Canseco-Gonzalez, D.;

Abebu A. Kassie − Department of Chemistry and Biochemistry,

The Ohio State University, Columbus, Ohio 43210, United

States

Morales-Morales, D. Recent Advances in Catalysis with Transition-

Metal Pincer Compounds. ChemCatChem 2018, 10, 3136−3172.

(18) Burgess, S. A.; Kassie, A.; Baranowski, S. A.; Fritzsching, K. J.;

Schmidt-Rohr, K.; Brown, C. M.; Wade, C. R. Improved Catalytic

Activity and Stability of a Palladium Pincer Complex by Incorporation

into a Metal-Organic Framework. J. Am. Chem. Soc. 2016, 138, 1780−

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00164

(

783.

Notes

19) Reiner, B. R.; Mucha, N. T.; Rothstein, A.; Temme, J. S.; Duan,

The authors declare no competing ﬁnancial interest.

P.; Schmidt-Rohr, K.; Foxman, B. M.; Wade, C. R. Zirconium Metal −

Organic Frameworks Assembled from Pd and Pt PNNNP Pincer

Complexes: Synthesis, Postsynthetic Modification, and Lewis Acid

Catalysis. Inorg. Chem. 2018, 57, 2663−2672.

ACKNOWLEDGMENTS

■

We acknowledge the Surface Analysis Laboratory (NSF DMR-

114098) of The Ohio State University Department of

(20) Reiner, B. R.; Kassie, A. A.; Wade, C. R. Unveiling Reactive

Metal Sites in a Pd Pincer MOF: Insights into Lewis Acid and Pore

Chemistry and Biochemistry.

Selective Catalysis. Dalton Trans. 2019, 48, 9588−9595.

(21) Kassie, A. A.; Duan, P.; McClure, E. T.; Schmidt-Rohr, K.;

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