Zinc(II) and Copper(II) Metal–Organic Frameworks Constructed from a Terphenyl-4,4′′-dicarboxylic Acid Derivative: Synthesis, Structure, and Catalytic Application in the Cyanosilylation of Aldehydes

A Journal of

Accepted Article

Title: Zn(II) and Cu(II) Metal Organic Frameworks Constructed from Terphenyl-4,4´´-

dicarboxylic Acid Derivative: Synthesis, Structure and Catalytic Application in

Aldehyde Cyanosilylation

´

Authors: ANIRBAN KARMAKAR; Anup Paul; Guilherme M. D. M. Ru´bio; M. Fatima

C. Guedes Da Silva; Armando J.L. Pombeiro

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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201600902

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European Journal of Inorganic Chemistry

10.1002/ejic.201600902

Zn(II) and Cu(II) Metal Organic Frameworks Constructed from

Terphenyl-4,4′′-dicarboxylic Acid Derivative: Synthesis, Structure

and Catalytic Application in Aldehyde Cyanosilylation

Anirban Karmakar*, Anup Paul, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva* and

Armando J. L. Pombeiro*

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,

1049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com ; fatima.guedes@tecnico.ulisboa.pt ;

pombeiro@tecnico.ulisboa.pt.

Abstract

Solvothermal reaction of zinc(II) and copper(II) salts with 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-

dicarboxylic acid (H₂L) gave rise to the 1D and 2D MOFs [Zn(L)(DMF)₂]_n(1), [Zn₂(L)(NO₃)₂(4, 4’-

Bipyridine)₂]_n.n(DMF) (2), [Cu(L)(H₂O)₂]_n.2n(H₂O) (3) and [Cu₃(OH)₂(L)₂]_n.n(H₂O) (4) which were

characterized by elemental analysis, FT-IR spectroscopy and X-ray single-crystal diffraction. 1 and 3

have a zig-zag and a linear type 1D structure, respectively, whereas 2 and 4 possess a rectangular

grid and a layer type 2D structure. In 2 and in 4, a dinuclear Zn(II) cluster (C₂O₄Zn₂) or a trinuclear µ³-

hydroxo bridged Cu(II) cluster [Cu₃(OH)₂]_n, respectively, acts as a building block unit. These

frameworks act as heterogeneous catalysts (the Zn-MOFs being more active than the Cu-MOFs, and

1 acting as the most efficient one) for the cyanosilylation reaction of aldehydes with trimethylsilyl

cyanide (TMSCN) at room temperature. These MOF-based heterogeneous catalysts are of easy

recovery and can be reused, at least for a few consecutive cycles, without losing activity.

Introduction

The design and synthesis of microporous metal–organic frameworks (MOFs) have become a subject

of extensive investigation due to their intriguing architectures and functional properties, namely in

gas storage and separation, catalysis, sensing, luminescence and drug delivery etc¹. The applications

of such materials largely depend on the metal ions, linkers size, shape and other properties of the

frameworks²and, e.g., a small change in ligand conformation can produce significant differences in

the final structure³.

In this context, rigid polycarboxylate ligands and their derivatives have been extensively utilized to

synthesize various MOFs due to their strong coordination ability in diverse modes, being able to

satisfy the geometric requirement of the metal centres leading to higher dimensional frameworks

and topologies⁴. Rigid aromatic carboxylic acid ligands, such as terphenyl-4,4′′-dicarboxylic acid⁵and

a few substituted (methyl and methoxy) derivatives⁶have received considerable attention for the

construction of interesting MOFs. The introduction of a flexible side functional arm, such as the

propoxy group, in terphenyl-4,4′′-dicarboxylic acid can influence the planarity of the ligand with a

considerable effect on the MOF structure and interpenetration.

In this context, we have selected 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid (H₂L) as

an organic linker due to the following features: (a) the multiple coordination sites may generate

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multidimensional structures; (b) this species contains side functional arms (propoxy groups) which

may affect the ligand conformation during the self-assembling process and also influence the

network topology. Thus, in the present work we aim to get an insight on how the conformational

preference, flexibility of side functional group and rigidity of the dicarboxylate framework could be

utilized in MOF synthesis.

Cyanosilylation is a fundamental carbon–carbon bond forming reaction in organic chemistry⁷, which

is used to synthesize, e.g., cyanohydrins which are key derivatives in the synthesis of fine chemicals

and pharmaceuticals⁸. Cyanohydrins are versatile organic compounds for both synthetic chemistry

and biological processes and can be transformed into a wide variety of important species such as α-

hydroxy acids, α-hydroxy aldehydes and β-aminoalcohols. For the preparation of cyanohydrins, the

use of a catalyst with a Lewis acid or base character, capable of activating both the substrate and the

cyanide precursor, is necessary, affording highly versatile cyanohydrin trimethylsilyl ethers. The most

commonly used cyanide source is trimethylsilyl cyanide (TMSCN). Cyanosilylation reactions have

been extensively studied in homogeneous media using discrete complexes and coordination

polymers containing various d-block,⁹f-block¹⁰and a few p-block¹¹metal ions, but only a few studies

have employed heterogeneous catalysts¹². Recently, a few MOFs have been used for cyanosilylation

of aldehydes¹³, but the development of efficient MOF based catalysts (in particular heterogeneous

ones) is still an understudied subject that deserves to be explored.

Therefore, the general objectives of this study are the following: (i) design, preparation and

characterization of multi-dimensional MOFs using 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-

dicarboxylic acid (H₂L) as a linker; (ii) investigation of the catalytic activity of the synthesized

complexes in the cyanosilylation reaction of aldehydes. In accord, by using H₂L as a pro-ligand, we

report the preparation and characterization of four different 1D and 2D polymers, viz. [Zn(L)(DMF)₂]_n

(1), [Zn₂(L)(NO₃)₂(4, 4’-Bipyridine)₂]_n.n(DMF) (2), [Cu(L)(H₂O)₂]_n.2n(H₂O) (3) and [Cu₃(OH)₂(L)₂]_n.n(H₂O)

(4), and their heterogeneous catalytic activities towards the cyanosilylation reaction of aldehydes

with TMSCN (trimethylsilyl cyanide).

Results and Discussion

Syntheses and characterization

The pro-ligand 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid (H₂L) was synthesised via

Pd-mediated cross-coupling reactions between methyl 4-iodo-2-propoxybenzoate and 1,4-bis(5,5-

dimethyl-1,3,2-dioxaborinan-2-yl)benzene by following the reported procedure^14a,band subsequent

hydrolysis with excess of KOH (Scheme 1). The solvothermal reaction of Zn(NO₃)₂.6H₂O or

Cu(NO₃)₂.3H₂O with H₂L in DMF:MeOH or DMF:MeOH:H₂O led to the formation of the MOFs

[Zn(L)(DMF)₂]_n(1), [Zn₂(L)(NO₃)₂(4,4’-Bipyridine)₂]_n.n(DMF) (2), [Cu(L)(H₂O)₂]_n.2n(H₂O) (3) and

[Cu₃(OH)₂(L)₂]_n.n(H₂O) (4), respectively (Scheme 1).

H₂L and the synthesised complexes 1-4 were characterised by elemental analyses, FT-IR and

multinuclear (¹H and ¹³C only in the case of H₂L in view of the non-solubility of the MOFs) NMR

techniques, and single crystal X-ray diffraction analysis (in the case of MOFs). The ¹H NMR spectrum

of H₂L exhibits characteristic resonances owing to –COOH at δ 12.55 and to the alkoxy chain -O-CH₃-

CH₂-CH₃attached to the aromatic ring which exhibits the resonances due to –CH₃, -CH₃-CH₂- and –O-

CH₂- protons at δ 1.04, 1.79 and 4.15, respectively (Fig. S7). Resonances due to phenyl ring protons

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European Journal of Inorganic Chemistry

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are observed in the range of δ 7.39-7.86. In its ¹³C NMR spectrum, the signals due to –COOH, -O–

CH₂-, CH₃-CH₂- and -CH₃appear at δ 167.7, 70.28, 22.36 and 10.68, respectively, along with other

resonances at their usual positions (Fig. S8).

Scheme 1. Syntheses of ligand H₂L and MOFs 1-4.

In the IR spectra of 1-4, the characteristic strong bands of the coordinated carboxylate

groups appear at 1609–1600 cm^-1for the asymmetric stretching and 1384-1391 cm^-1for the

symmetric one^14c. The C–O stretching of coordinated carboxylate group is observed between 1215

and 1224 cm^-1. A medium broad band between 2963 and 2967 cm^-1is due to ν_CHof the propoxy

groups. For H₂L, the strong band at 1685 cm^-1is due to ν_COof uncoordinated carboxylic acid groups,

whereas ν_CHof the aliphatic groups appears at 2966 cm^-1.

Crystal structure analyses

According to the crystal structure analyses, 1-4 crystallize in different space groups (1 in C2/c, 2 and

3 in P-1 and 4 in P21/n). Compounds 1 and 3 have zig-zag and linear type one dimensional structure,

respectively, while compounds 2 and 4 possess a rectangular grid and layer type 2D structure.

Selected bond distances and angles of 1 are listed in Table S3 (supporting information).

Compound 1 comprises one Zn²⁺ions, one L^2-ligand and two coordinated dimethylformamide

molecules (Figures 1A and 1B) and features a zig-zag type one-dimensional coordination polymeric

chain (Figure 1C) which expands to 2D by means of H-bond interactions (Figure 1D). The Zn1 center

presents a distorted octahedral environment and binds to four carboxylate oxygen atoms of two

neighboring L^2-units in a chelating bidentate fashion [Zn1–O1, 2.1861(13) Å and Zn1–O2, 2.1200(11)

Å]. The remaining two coordination sites of zinc are engaged to two oxygen atoms from two DMF

molecules [Zn1–O4, 2.0771(13) Å] which are displayed in mutual cis position. The two carboxylate

groups are coordinated to the Zn(II) center almost in perpendicular fashion (dihedral angle between

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European Journal of Inorganic Chemistry

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the two binding COO^-groups of 74.92^o). The L^2-ligand is not planar; the carboxylate containing

phenyl groups (C2-C7 and C8-C10) make angles of 32.04^owith the central phenyl ring. In addition,

the angle between the least-square planes of the carboxylate moiety and the attached phenyl group

is of 33.93 ^o. The two propoxy arms (-OCH₂CH₂CH₃) are oriented in opposite direction with C_phenylOCC

torsion angles of 172.75^o. In a chain of 1 the metal cations are 19.23 Å apart and the shortest

intermolecular ZnZn distance equals the unit cell b dimension (7.5333 Å).

A

B

C

D

Figure 1: (A) The molecular structure of framework 1 with partial atom labelling scheme. Symmetry

codes to generate equivalent atoms: ‘) 1-x, y, 1/2-z. (B) Schematic representation of 1. (C) One

dimensional structure of 1. (D) A packing view of framework 1.

The hydrothermal reaction of 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid (H₂L) with

zinc (II) nitrate hexahydrate in presence of 4, 4’-bipyridine as auxiliary ligand led to the formation of

a two dimensional network [Zn₂(L)(NO₃)₂(4, 4’-Bipyridine)₂]_n.n(DMF) (2) (Figure 2B). Single-crystal X-

ray diffraction analysis disclose that the asymmetric unit of 2 contains one Zn²⁺ions, half a L^2-ligand,

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one chelating nitrate anion, one coordinated 4, 4’-bipyridine molecule and one non-coordinated

DMF molecule (Figures 2A and 2D). The Zn1 ion has a distorted octahedral geometry with its

equatorial plane occupied by one of the O-carboxylate atoms from two neighboring L^2-ligand [Zn1-

O1, 2.010(4) Å and Zn1-O2, 2.004(5) Å] and two O-atom of the chelating nitrate anion [Zn1-O4, 2.294(6) Å

and Zn1-O5, 2.238(6) Å], whereas the axial sites are occupied by the N-atoms from two 4, 4’-bipyridine

molecules [Zn1-N1, 2.142(5) Å and Zn1-N2, 2.165(5) Å]. In this framework the carboxylate groups are

bound in a syn-syn bridigning bidentate fashion thus giving rise to a dinuclear C₂O₄Zn₂cluster which

acts as a secondary building block (Figure 3B) where each of these unit is associated with two L^2-

ligands, four 4, 4’-bipyridine molecules and two nitrate anions. The minimum Zn···Zn distance

reaches 3.770(2) Å. The L^2-ligand is also not planar, the carboxylate containing rings being twisted

22.35^ofrom the central phenyl group, and the dihedral angle between the carboxylate group and the

attached phenyl is of 58.42^o. The two propoxy arms are oriented in opposite direction but the

C_phenylOCC torsion angles are now much shorter (64.91^o).

A

B

C

D

Figure 2: The molecular structure of 2 with partial atom labelling scheme (hydrogen atoms are

omitted for clarity) (A), and its schematic representation (B). Two dimensional arrangement of

framework 2 showing the open rectangular channels emptied (C), and with encapsulated DMF

molecules indicate as space fill model (D).

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The 2D polymeric architecture of 2 also has open rectangular channels with approximate dimension

of 11.4 x 19.4 Å², which are occupied by the DMF molecules (Figure 2D).

A

B

C

Figure 3. Schematic representation of the asymmetric unit of 3 (A), its one dimensional structure (B)

and two dimensional hydrogen bonded packing view (C). Symmetry codes to generate equivalent

atoms: B: ‘) -x, 2-y, 1-z; C: ‘) -x, 1-y, 1-z.

The single crystal analysis of 3 (Figure 3) reveals a linear 1D framework constructed from Cu(II) ions,

one L^2-ligands and two coordinated water molecules; the compound crystallized with two non-

coordinated water molecules. The Cu(II) centre displays a square planner geometry (τ₄= 0)^15a, with

two of the trans positions occupied by one of the O-atoms from L^2-ligands [Cu1-O2 1.9659(19) Å]

and the remaining ones occupied by two water molecules [Cu1-O4 1.993(3) Å]. The Cu-O_waterbond

distance is slightly larger than the Cu-O_carboxylatebond distance. The planarity of the L^2-ligand is

greater than that found for 1 and 2, as indicated by the angle of 15.34^obetween the least-square

planes of the carboxylate-containing and the central- phenyl rings. In contrast, the twisting of the

COO^-moiety relative to the attached phenyl, assumes the highest value (50.60^o). Additionally, the

propoxy substituents are nearly co-planar (C_phenylOCC torsion angle of 175.82^o).

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The intermolecular organization in 3 is characterized by extensive hydrogen bond interactions. The

carboxylate O1- and the propoxy-O3 atoms act as acceptors from the coordinated water molecules

leading to the expansion of the 1D chains to a 2D hydrogen bonded network (Figure 3B). Such

interactions are further reinforced by contacts linking the non-coordinated O5-water molecule with

the carboxylate O1-atoms of vicinal chains, thus forming a close square with graph set (8)^15b[O5-

H5A·····O1, d_D–A2.850(4) Å, <D–H····A 158.5^o; O5-H5B·····O1, d_D–A2.872(5) Å, <D–H····A 151.7^o]. Other

strengthening contacts comprise the O6-water molecule which simultaneously binds the O5-water

molecule and the O2-carboxylate atom [O6-H6B·····O5, d_D–A2.902(5) Å, <D–H····A 155.3^o; O6-

H5A·····O2, d_D–A2.989(4) Å, <D–H····A 161.7^o].

The use of Cu(II) salts for coordination polymerization with H₂L in the presence of DMF, 1,4-dioxane

and water has led to a unique structure (4) in which the Cu²⁺ions aggregate via O_hydroxo, O_aquaand

O_carboxylateatoms into one-dimensional chains, which are doubly crosslinked with deprotonated ligand

(L^2-) giving rise to a two-dimensional framework (Figure 4C). In 4, the asymmetric unit contains three

Cu²⁺ion, two deprotonated ligands (L^2-), two hydroxo anions, two coordinated water molecules and

one non-coordinated water molecule (Figure 4A). The three crystallographically independent Cu(II)

ions have distorted square pyramidal geometry (τ₅= 0.21, 0.06 and 0.07 for Cu1, Cu2 and Cu3,

respectively)^15b. They are interconnected with each other via µ³-hydroxo and µ²-carboxylate bridged

(Figure 4B). The coordination sphere of each metal centres formed by two carboxylate oxygen atom

of a L^2-unit and two O-atoms from hydroxo anion, which occupies four of the equatorial sites. The

axial position is occupied by other carboxylate oxygen. As expected and due to Jahn–Teller

distortion¹⁶the axial Cu-O bond distance is slightly larger [Cu1–O11, 2.615(4) Å; Cu2–O2, 2.564(4) Å;

Cu3–O7, 2.511(4) Å] than the equatorial ones.

A

B

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European Journal of Inorganic Chemistry

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C

Figure 4: Schematic representation of 4 (A); One dimensional chain of Cu(II) ions bridged by hydroxo

and carboxylate anions in the framework 4 (B); Two dimensional polymeric structure of framework 4

(C). Symmetry codes to generate equivalent atoms: B: ‘) -x, 1-y, 1-z.

The shortest interatomic distances between the copper ions along the polymeric copper-oxygen

cluster are of 2.9651 or 3.0297 Å, involving symmetry related Cu3 or Cu2 cations, followed by 3.0796

Å and concerning a Cu1⋯Cu3 length. While the hydroxo anion O14 bridges two Cu2 leading to a 4-

membered Cu₂O₂ring (Cu-O-Cu angle of 99.6^o), and additionally to a Cu1 centre, the O15-hydroxo

make similar connections but between Cu3 (forming the 4-membered Cu₂O₂core with Cu-O-Cu

angle of 97.9^o) and then to the cited Cu1. The angle between the least-square planes of those four

membered rings is of 18.30^o, and they are 1.710 and 1.627 Å apart from the common Cu1 cation,

respectively.

One of the doubly deprotonated ligand (L^2-) is interconnected with four different copper ions and

one of carboxylate arms are coordinated with two Cu(II) centers (Cu1 and Cu3) via a µ²–η¹–η¹-

bridging mode, while the another part of the ligand are connected with other two copper (Cu1’ and

Cu3’) ions in a µ²–η¹–η⁰-bridging fashion. However other ligand coordinated with three Cu(II) centres

and for this one carboxylate coordinated with two Cu(II) centers (Cu1 and Cu2) via a µ²–η¹–η¹-

bridging mode and other carboxylate group coordinated to the Cu2 centre in monodentate fashion.

In this framework a trinuclear µ³-hydroxo bridged Cu(II) cluster [Cu₃(OH)₂] act as a building blocks

unit which are forming a one dimensional chain and these 1D chains are further connected via

doubly deprotonated ligands to forming a two-dimensional architecture (Figure 4C).

To improve the description of the crystal structures of 1-4 we performed their topological analysis by

reducing their multidimensional structures to simple node-and-linker nets where the metallic nodes

and the organic linkers represent secondary building units (SBUs). We have carried out these

analyses by using TOPOS 4.0.¹⁷Topological analysis of frameworks 1 and 3 reveals that they have a

2-connected uninodal net with a topological type 2C1 net (Figure 5A and 5C). In the particular case

of 2, to represent the simplified net structure of this framework we consider dinuclear Zn(II) unit as a

single node which subsequently coordinated via 2 different carboxylate linkers and four 4,4’-

bipyridine. Each ligand as well as 4, 4’-bipyridine are connected with two different metal and acts as

a 2-connected nodes. The polymeric chain structure thus generated is a 2, 2, 6-connected trinodal

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net (Figure 5B). However, the 2D framework 4 has more complex 3,3,4,4,4,5,5-connected 7-nodal

network topology (Figure 5D).

A

B

C

D

Figure 5 Node-and-linker-type descriptions of: (A) 1D zig-zag MOF of 1; (B) the 2D coordination

frameworks in 2; (C) the 1D linear type coordination framework in 3; (D) the 2D coordination

frameworks in 4. The zinc nodes are represented in green whereas the copper nodes are in blue

color, the carboxylate linker in red, the 4,4’-bipyridine are in pink and the hydroxyl groups are in

yellow colour.

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Catalysis

We have tested the activity of the synthesized frameworks (1-4) as solid heterogeneous catalysts in

the cyanosilylation reaction of various aldehydes, namely 4-nitrobenzaldehyde as a model substrate,

and performed the cyanosilylation reaction by using a mixture of aldehyde, trimethylsilyl cyanide

and a Zn or Cu-catalyst in CH₂Cl₂(DCM) at room temperature (Scheme 2, Table 1).

Scheme 2

When framework 1 is used as catalyst (2 mol%), a high conversion of 94% of 4-nitrobenzaldehyde

into 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile is reached after 10

h reaction in

dichloromethane, at room temperature (entry 6, Table 1). However, with frameworks 2, 3 and 4,

lower conversions of 85%, 79% and 65% were obtained, respectively (entries 13-15, Table 1). The

plot of yield vs. time (until 10 h) for the cyanosilylation reaction of 4-nitrobenzaldehyde with TMSCN

and 1 in DCM is presented in Fig. 6A. Upon further extending the reaction time to 24 h, only a slight

increase of the product yield to 96% occurred. Moreover, no other products were detected and the

yield of this product is considered to be the conversion of 4-nitrobenzaldehyde.

The four MOFs activities follow the order of 1 > 2 > 3 > 4, showing that the Zn(II) MOFs are more

active than the Cu(II) ones. Although a relationship between structure and catalytic activity in the

present study cannot be clearly established, the highest conversation shown by the compound 1

may eventually be associated to its 1D zig-zag structure or favourable packing arrangement for easily

accessible metal centres, together with the higher Lewis acidity of the zinc sites.

Table 1 Optimization of the parameters of the cyanosilylation reaction between 4-nitrobenzaldehyde and

trimethylsilyl cyanide (TMSCN) with Zn- and Cu-MOF as catalysts^a

Entry

Catalyst

Time (h)

Amount of

Solvent

Yield^b(%)

TON^c

Catalyst (mol%)

1

2

4

6

8

2.0

DCM

32

61

72

81

87

94

16

30

36

40

43

47

2

3

4

5

6

10

7

8

1

10

1.0

5.0

DCM

75

95

75

19

9

1

10

2.0

CH₃CN

THF

CHCl₃

MeOH

82

73

90

87

41

36

45

43

10

11

12

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13

14

15

2

3

4

10

2.0

DCM

85

79

65

42

39

32

16

17

18

19

Blank

Zn(NO₃)₂.6H₂O

Cu(NO₃)₂.3H₂O

10

-

DCM

12

18

16

13

-

9

8

-

2.0

H₂L

^aReaction conditions: 4-nitrobenzaldehyde (0.50 mmol), trimethylsilyl cyanide (1.0 mmol), 2.0 mol%

b

1

of catalyst 1 and solvent (DCM) (2.0 mL), at room temperature.; Calculated by H-NMR as the

number of moles of product 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile per mole of

c

aldehyde X 100; Number of moles of product 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile

per mole of catalyst.

We have also tested the effect of catalyst amount and solvents in the cyanosilylation reaction

catalysed by 1. The increase of the catalyst amount from 1.0 to 2.0 mol% enhances the product yield

from 75 to 94%, respectively (entries 7 and 6, Table 1), but a further increase of that amount (up to

5%) results only in an insignificant yield improvement of 1% (entry 8, Table 1), with a marked

decrease of the TON value. We have also checked the catalytic reaction in various solvents, such as

CH₃CN, THF, CHCl₃, MeOH and CH₂Cl₂, and the results indicate that CH₂Cl₂(94% yield) is the best one

for such a transformation, whereas the worst one is THF (73% yield) (entries 9-12, Table 1).

A

B

Figure 6 (A) Plot of 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile yield vs. time for the

cyanosilylation reaction, catalyzed by 1, of 4-nitrobenzaldehyde with TMSCN at room temperature

(dotted line concerns the catalyst removal after 2 h: see text). (B) Effect of the catalyst recycling on

the yield of 2-(4-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile.

A blank test was carried out with 4-nitrobenzaldehyde in the absence of any catalyst at room

temperature and only 12% of conversion detected after a reaction time of 10 h. We have also

checked the reactivity of Zn(NO₃)₂.6H₂O, Cu(NO₃)₂.3H₂O and free ligand (H₂L) in CH₂Cl₂and the

obtained reaction yields are only 18%, 16% and 13%, respectively (entries 16-18, Table 1).

We have also compared the activities of catalyst 1 in the reactions of other substituted aromatic and

aliphatic aldehydes with trimethylsilyl cyanide, producing the corresponding cyanohydrin derivatives

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with yields ranging from 65 to 91% (Table 2). Aryl aldehydes bearing strong electron-withdrawing

substituents (nitro and chloro) exhibit the highest reactivities (entries 2 and 3, Table 2), that may be

related with an increase of the electrophilicity of the substrate. The aldehydes containing electron-

donating groups (methyl or methoxy) show lower reaction yields (entries 5 and 6, Table 2), as

expected. The ortho-substituted aldehyde shows a lower reactivity possibly due to steric hindrance.

The aliphatic aldehydes give good reaction yields in the 84-88% range (entries 9 and 10, Table 2).

Table 2: Cyanosilylation reaction between various aldehydes and trimethylsilyl cyanide with catalyst 1^a

Entry

Reactant

Product

Yield^b(%)

TON^c

Benzaldehyde

3-nitrobenzaldehyde

4-chlorobenzaldehyde

4-hydroxybenzaldehyde

4-methylbenzaldehyde

4-methoxybenzaldehyde

2-nitrobenzaldehyde

Salicylaldehyde

2-phenyl-2-((trimethylsilyl)oxy)acetonitrile

2-(3-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile

2-(4-chlorophenyl)-2-((trimethylsilyl)oxy)acetonitrile

2-(4-hydroxyphenyl)-2-((trimethylsilyl)oxy)acetonitrile

2-(p-tolyl)-2-((trimethylsilyl)oxy)acetonitrile

2-(4-methoxyphenyl)-2-((trimethylsilyl)oxy)acetonitrile

2-(2-nitrophenyl)-2-((trimethylsilyl)oxy)acetonitrile

2-(2-hydroxyphenyl)-2-((trimethylsilyl)oxy)acetonitrile

2-((trimethylsilyl)oxy)propanenitrile

1

2

3

4

5

6

7

8

9

74

91

86

81

71

65

83

63

88

84

37

45

43

40

35

32

41

31

44

42

Acetaldehyde

Propionaldehyde

2-((trimethylsilyl)oxy)butanenitrile

10

a

Reaction conditions: 2.0 mol% of catalyst 1, solvent (CH₂Cl₂) (2.0 mL), trimethylsilyl cyanide (1.0

mmol) and aldehyde (0.50 mmol) for 10 h at room temperature; ^bCalculated by ¹H NMR; ^cNumber of

moles of product per mole of catalyst.

We have performed a competition cyanosilylation reaction in the presence of 1 involving equal

molar amounts of 3-nitrobenzaldehyde and acetaldehyde, substrates with a marked difference in

size, and a higher product yield was observed in the former case, as when using them separately.

Moreover, within aliphatic aldehydes, no substantial yield decrease was observed with the increase

of the substrate size. Hence, the catalysis does not appear to be mainly occurring within the MOF

pores.

In order to examine the stability of 1 in the cyanosilylation reaction, we have tested the recyclability

of this heterogeneous catalyst. For this purpose, upon completion of a reaction cycle, we have

separated the catalyst by centrifugation, washed with dichloromethane and dried at room

temperature before further use. We have repeatedly recycled the catalyst 1 and the catalytic system

maintained the full activity for at least five consecutive cycles as shown in Fig. 6B.

To further clarify if the catalytic process is heterogeneous or homogeneous, the following test¹⁸for

the cyanosilylation reaction was carried out. We performed a controlled experiment with catalyst 1

until an intermediate yield (ca. 62%) was observed (2 h), then removed the catalyst by centrifugation

and kept the catalyst-free reaction solution under the same conditions stirred for additional 10 h. As

shown in Fig. 6A (dotted line), after removal of the solid catalyst, the yield of 2-(4-nitrophenyl)-2-

((trimethylsilyl)oxy)acetonitrile did not increase significantly. These results demonstrate that the

catalysis is heterogeneous in nature. Additionally, the filtrated solution, after the separation of the

catalyst, was evaporated to dryness and the amount of zinc determined was only 0.025% of the

amount used in the reaction, thus ruling out any significant leaching of the catalyst. In order to check

the structural integrity of the catalysts 1-4, we have performed FT-IR and powder X-ray diffraction

12

European Journal of Inorganic Chemistry

10.1002/ejic.201600902

analyses of the catalyst before and after the catalysis reaction. They indicate that the structure of

the solid was retained (Figures S1-S4).

Our catalysts appear to be usually much more efficient for the aldehyde cyanosilylation than other

reported heterogeneous ones. In fact, for example, the zinc(II) MOFs of 1,2-bis(4-pyridyl)ethane lead

to overall yields of only 14-18% after 24 h reaction time in the cyanosilylation reaction of

benzaldehyde and TMSCN^19a. Moreover, the use of 1D, 2D and 3D MOFs of Zn(II) based on 4,4’-

bipyridine and acetate, results into an yield of only 21-22%^19b

.

In case of

a

Cu₃(benzenetricarboxylate)₂MOF, an yield of 57% after 72 h was obtained^19c. The 2D frameworks of

copper(II) constructed by Cu(II), 4,4’-bipyridine and formato or acetate ligands lead to an overall

yield of only 2% in the reaction of benzaldehyde and TMSCN for 5h^19d. Besides that, a Ln-MOF, Tb-

tricarboxytriphenylamine MOF, in the reaction of 4-nitrobenzaldehyde and TMSCN, results into an

yield of 47% after 4 h reaction time^19e. Recently, Huh et al. reported a 3D cadmium(II) MOF bearing

rigid 1,4-naphthalenedicarboxylate which catalyses the cyanosilylation of 4-nitrobenzaldehyde with

an overall yield of 49% after 72 h at 50^oC^19f. Moreover, incubation of benzaldehyde and TMSCN with

pristine CuBTC as the catalyst led to a conversion of 56.4% at 40^oC for 48 h^19g. Arriortua et al.

reported the heterometallic MOF [NaCu(2,4-HPdc)(2,4-Pdc)] (2,4-H₂Pdc = pyridine-2,4-dicarboxylic

acid) which catalyses the cyanosilylation of benzaldehyde with an yield of 85% after 24 h under

solvent free conditions^19h. Nevertheless, our catalyst 1 exhibits an yield of 94% for 4-

nitrobenzaldehyde cyanosilylation and 74% for benzaldehyde cyanosilylation, at room temperature.

Besides MOFs, other classes of catalysts, such as metal complexes, oxides, polyoxometalates and

nano materials, have been reported, which can give comparable yields for such a reaction. For

example,

a

mononuclear

copper(II)

complex

of

2-(2-(2,4-dioxopentan-3-

ylidene)hydrazinyl)benzenesulfonate and ethylenediamine leads to an overall yield of 85% after 16 h

reaction by using 5 mol% of catalyst in the cyanosilylation reaction of benzaldehyde and TMSCN¹⁹ⁱ. A

mononuclear Zn(II) complex with an amide-based ligand containing thiazole leads to an overall yield

of 90% in the reaction of benzaldehyde and TMSCN for 6 h by using 5 mol% of catalyst^19j. A Ce(III)

complex of D-proline, in the reaction of benzaldehyde and TMSCN, results into an yield of 90% after

40 h^19k. Moreover, cyanosilylation of benzaldehyde with TMSCN in the presence of nanocrystalline

magnesium oxide provides a reaction yield of 86% in 3 h^19l. The Preyssler P₅W₃₀polyoxometalate

based complex {[Cu₁₂(pbtz)₂(Hpbtz)₂(OH)₄(H₂O)₁₆][Na(H₂O)P₅W₃₀O₁₁₀]}·16H₂O {H₂pbtz = 5’-(pyridin-2-

yl)-1H,2’H-3,3’-bi(1,2,4-triazole)} catalyzes the cyanosilylation of benzaldehyde with TMSCN leading

to an overall yield of 62% after 24 h^19m. A bifunctional catalyst derived from Ti(OⁱPr)₄and a N-oxide

ligand was applied to the asymmetric cyanosilylation of benzaldehyde and 78% yield was obtained

by using 5 mol% catalyst at -78°C¹⁹ⁿ. Moreover, activated ceria nanorods were also used to catalyse

the cyanosilylation of benzaldehyde and ca. 92% yield was obtained after 1.5 h^19o. However, in those

systems, a longer reaction time or higher or lower (even negative) temperatures are required, what

can be considered as disadvantages in comparison with our system.

Conclusions

We synthesized and characterized four different metal organic frameworks, namely [Zn(L)(DMF)₂]_n

(1), [Zn₂(L)(NO₃)₂(4, 4’-Bipyridine)₂]_n.n(DMF) (2), [Cu(L)(H₂O)₂]_n.2n(H₂O) (3) and [Cu₃(OH)₂(L)₂]_n.n(H₂O)

(4), of Zn(II) and Cu(II) constructed from 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid

13

European Journal of Inorganic Chemistry

10.1002/ejic.201600902

(H₂L) under different solvothermal conditions. X-ray diffraction structural analyses revealed that the

compounds 1 and 3 have zig-zag and linear type 1D structures, respectively, whereas compounds 2

and 4 possess a rectangular grid and layer type 2D structure.

We have also shown that our synthesized MOFs act as heterogeneous catalysts towards the

cyanosilylation reaction of various aldehydes with TMSCN at room temperature. The Zn(II)-MOFs

show higher activity than the Cu(II) ones which may reflect the higher Lewis acidity of the former.

Among the four MOFs of this study, compound 1, with a 1D zig-zag structure, is the most active one,

conceivably on account of structural factors. We have also proved the heterogeneous character, the

stability and recyclability of the catalyst 1 and showed that it can be recycled at least five cycles

without losing activity. The above observations provide further evidence that simple MOFs can be

utilized as effective heterogeneous catalysts in important types of reactions, such as that of this

work.

Experimental

The synthetic work was performed at open air, while the metal organic frameworks were prepared

under hydrothermal conditions. All the chemicals were obtained from commercial sources and used

as received. The infrared spectra (4000–400 cm^-1) were recorded on a Bruker Vertex 70 instrument

in KBr pellets; abbreviations: s = strong, m = medium, w = weak, bs = broad and strong, mb =

medium and broad. The ¹H and ¹³C NMR spectra were recorded at ambient temperature on a Bruker

Avance II + 300 (UltraShield^TMMagnet) spectrometer operating at 300.130 and 75.468 MHz for

proton and carbon-13, respectively. The chemical shifts are reported in ppm using tetramethylsilane

as the internal reference; abbreviations: s = singlet, d = doublet, t = triplet, q = quartet. Carbon,

hydrogen and nitrogen elemental analyses were carried out by the Microanalytical Service of the

Instituto Superior Técnico. Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument

(Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source. For

electrospray ionization, the drying gas and flow rate were optimized according to the particular

sample with 35 p.s.i. nebulizer pressure. Scanning was performed from m/z 100 to 1200 in methanol

solution. The compounds were observed in the positive mode (capillary voltage = 80–105 V). Powder

X-ray diffraction (PXRD) was conducted in a D8 Advance Bruker AXS (Bragg Brentano geometry)

theta-2theta diffractometer, with copper radiation (Cu Kα, λ = 1.5406 Å) and a secondary

monochromator, operated at 40 kV and 40 mA. Flat plate configuration was used and the typical

data collection range was between 5˚ and 40˚.

Synthesis of 3,3''-dipropoxy-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid (H₂L)

The pro-ligand H₂L was synthesised with a slight modification in the procedure reported earlier^14a,b

.

The intermediates a and b used for the syntheses of H₂L are described in the experimental section of

the supporting information (ESI).

Methyl 4-iodo-2-methoxybenzoate (a) was synthesised and added to the reaction mixture of b

(0.604 g, 2.0 mmol), [Pd(PPh₃)₄] (0.200 g, 0.20 mmol) and K₂CO₃(2.0 g, 14.5 mmol) dissolved in DMF

(30 mL), and the system was heated to 90 ⁰C under N₂for 24 h. The reaction mixture was then

poured into ice cold water (600 mL). The brown precipitate thus formed was extracted with CH₂Cl₂,

the solution dried with Na₂SO₄and evaporated to dryness using a rotary evaporator. The crude

14

European Journal of Inorganic Chemistry

10.1002/ejic.201600902

brown product was further purified by using column choromatagraphy (ethyl acetate:hexane) to

obtain dimethyl 3,3''-dipropoxy-(1,1':4',1''-terphenyl)-4,4''-dicarboxylate as a white product in 55%

yield.

This product was then hydrolysed by dissolving it (300 mg, 0.7 mmol) in tetrahydrofuran (THF, 20

mL), followed by addition of 4% KOH (20 mL) and stirring at room temperature overnight. The

system was then reduced to ca. 10 mL, acidified to pH~1 with conc. HCl, and the white solid product

thus obtained was washed several times with water until neutral and air dried. Yield: 80%. FT-IR

(KBr, cm^-1): 3445 (bm), 2966 (bs), 2877 (m), 1685 (s), 1606 (s), 1563 (w), 1443 (m), 1390 (bm), 1309

(bs), 1262 (m), 1221 (m), 1153 (w), 1098 (w), 1043 (w), 1009 (w), 974 (w), 824 (s), 780 (s), 691 (m). ¹H

NMR (DMSO-d6, 300 MHz, δ ppm): 7.25 (d, 4H, Ar-H), 7.74 (s, 2H, Ar-H), 7.34-7.39 (m, 4H, Ar-H),

4.15 (t, 2H, O-CH2-CH2-CH3), 1.79 (m, 2H, O-CH2-CH2-CH3), 1.04 (t, 3H, O-CH2-CH2-CH3). 13C NMR

(DMSO-d6, 75.45 MHz, δ ppm): 167.71, 158.82, 144.61, 139.31, 132.11, 127.88, 119.28, 112.34,

70.28, 22.36, 10.68. ESI-MS: Calcd. for [(M + H)]+ : m/z 435.5, found 435.0. Anal. Calcd. for C₂₆H₂₆O₆

(M = 434.48): C, 71.87; H, 6.03;; Found: C, 72.04; H, 6.15.

Synthesis of 1

A mixture of Zn(NO₃)₂.6H₂O (29 mg, 0.10 mmol) and H₂L (43 mg, 0.10 mmol) was dissolved in 4 mL of

DMF and methanol (1 : 2). To this reaction mixture we added 0.2 mL of 28% aqueous ammonia

solution and a white precipitate was obtained. The precipitate was dissolved upon further addition

of 0.3 mL of 28% aqueous ammonia solution. Then, the resulting mixture was sealed in an 8 mL glass

vessel and heated at 75 ˚C for 48 h. Subsequent gradual cooling to room temperature afforded

colorless crystals of 1. Yield: 64% (based on Zn). Anal. Calcd. for C₃₂H₃₈N₂O₈Zn (M = 644.03): C, 59.68;

H, 5.95; N, 4.35; Found: C, 59.02; H, 5.65; N, 4.05. FT-IR (KBr, cm^-1): 3361 (bs), 2963 (m), 2875 (w),

1608 (s), 1386 (bs), 1306 (w), 1215 (s), 1162 (w), 1109 (m), 1044 (w), 1012 (w), 979 (w), 896 (m), 826

(s), 792 (s), 699 (w).

Synthesis of 2

A mixture of Zn(NO₃)₂.6H₂O (29.0 mg, 0.10 mmol), H₂L (43 mg, 0.10 mmol) and 4,4’-bipyridine (31

mg, 0.20 mmol) was dissolved in 5 mL of DMF, methanol and water (1 : 4: 0.1) mixture. Then, the

resulting mixture was transferred to an 8 mL glass vessel and the sealed vessel was heated at 70 ˚C

for 48 h. It was subsequently cooled to room temperature, affording block-like colorless crystals of 2.

Yield: 42% (based on Zn). Anal. Calcd. for C₂₆H₂₇N₄O₇Zn (M = 572.88): C, 54.51; H, 4.75; N, 9.78; Found:

C, 54.11; H, 4.31; N, 9.45. FT-IR (KBr, cm^-1): 3365 (bs), 2966 (m), 2874 (w), 1609 (s), 1532 (s), 1391 (s),

1327 (m), 1304 (w), 1217 (s), 1165 (w), 1047 (m), 1018 (w), 973 (m), 900 (w), 827 (s), 798 (s), 700

(m), 681 (w).

Synthesis of 3

Similarly, a mixture of Cu(NO₃)₂.3H₂O (23.0 mg, 0.10 mmol) and H₂L (43 mg, 0.10 mmol) was

dissolved in 4 mL of DMF, ethanol and water (1 : 4: 0.1) mixture. The resulting mixture was sealed in

an 8 mL glass vessel and heated at 75 ˚C for 48 h. It was subsequently cooled to room temperature,

affording plate-like green crystals of 3. Yield: 68% (based on Cu). Anal. Calcd. for C₂₆H₃₆CuO₁₂(M =

604.09): C, 51.69; H, 6.01. Found: C, 51.53; H, 6.32. FT-IR (KBr, cm^-1): 3435 (bs), 2965 (m), 2877 (w),

1609 (s), 1473 (m), 1384 (bs), 1312 (w), 1224 (s), 1162 (w), 1113 (m), 1075 (w), 1042 (w), 1018 (m),

896 (m), 820 (s), 795 (m), 700 (w), 643 (m).

15

European Journal of Inorganic Chemistry

10.1002/ejic.201600902

Synthesis of 4

A mixture of Cu(NO₃)₂.3H₂O (23.0 mg, 0.10 mmol) and H₂L (43 mg, 0.10 mmol) was dissolved in 5 mL

of DMF, 1,4-dioxane and water (1 : 4: 0.1) mixture. To this reaction mixture we added 0.01 mL of

concentrated HCl solution. The resulting mixture was sealed in an 8 mL glass vessel and heated at 85

˚C for 48 h. It was subsequently cooled to room temperature, affording green crystals of 4. Yield:

42% (based on Cu). Anal. Calcd. for C₅₂H₄₈Cu₃O₁₇(M = 1135.52): C, 55.00; H, 4.26. Found: C, 55.38; H,

4.01. FT-IR (KBr, cm^-1): 3546 (s), 3436 (bs), 2967 (m), 2879 (w), 1600 (s), 1420 (s), 1385 (s), 1341 (s),

1223 (w), 1113 (w), 1047 (m), 867 (m), 786 (s), 687 (s).

Procedure for cyanosilylation reaction of aldehydes

A suspension of an aromatic aldehyde (0.50 mmol), trimethylsilyl cyanide (1.0 mmol) and 2 mol%

catalyst (6.5 mg for 1, 5.7 mg of 2, 6.0 mg of 3 and 11.3 mg of 4) in dichloromethane (2.0 mL) was

stirred at room temperature. After the desired time, the catalyst was removed by centrifugation.

After evaporation of solvent from the filtrate in a rotary evaporator, a crude solid was obtained. This

was dissolved in CDCl₃and analyzed by ¹H NMR (supporting information, Fig. S6).

In order to perform the recycling experiment, the catalyst isolated by centrifugation was washed

with dichloromethane and dried at room temperature. The subsequent steps were performed as

described above.

Crystal structure determinations

X-ray quality single crystals of the compounds were immersed in cryo-oil, mounted in a nylon loop

and measured at room temperature (1-4). Intensity data were collected using a Bruker APEX-II

PHOTON 100 diffractometer with graphite monochromated Mo-Kα (λ 0.71069) radiation. Data were

collected using phi and omega scans of 0.5° per frame and a full sphere of data was obtained. Cell

parameters were retrieved using Bruker SMART²⁰software and refined using Bruker SAINT^20aon all

the observed reflections. Absorption corrections were applied using SADABS^20a. Structures were

solved by direct methods by using the SHELXS-97 package^20band refined with SHELXL-97^20b

.

Calculations were performed using the WinGX System–Version 1.80.03^20c. The hydrogen atoms

attached to carbon atoms and to the nitrogen atoms were inserted at geometrically calculated

positions and included in the refinement using the riding-model approximation; Uiso(H) were

defined as 1.2Ueq of the parent nitrogen atoms or the carbon atoms for phenyl and methylene

residues, and 1.5 Ueq of the parent carbon atoms for the methyl groups. The hydrogen atoms of

coordinated water molecules were located from the final difference Fourier map and the isotropic

thermal parameters were set at 1.5 times the average thermal parameters of the belonging oxygen

atoms. Least square refinements with anisotropic thermal motion parameters for all the non-

hydrogen atoms and isotropic ones for the remaining atoms were employed. Crystallographic data

are summarized in Table S1 (Supplementary Information file) and selected bond distances and

angles are presented in Table S2. CCDC 1481724-1481727 contain the supplementary

crystallographic data for this paper. These data can be obtained free of charge from The Cambridge

Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif .

16

European Journal of Inorganic Chemistry

10.1002/ejic.201600902

Acknowledgements

This work has been supported by the Foundation for Science and Technology (FCT) (project

UID/QUI/00100/2013), Portugal. Authors A. Karmakar and A. Paul express their gratitude to the FCT

for post-doctoral fellowships (Ref. No. SFRH/BPD/76192/2011 and SFRH/BPD/88450/2012). The

authors acknowledge the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility.

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Graphical Abstract

Zn(II) and Cu(II) Metal Organic Frameworks Constructed from

Terphenyl-4,4′′-dicarboxylic Acid Derivative: Synthesis, Structure

and Catalytic Application in Aldehyde Cyanosilylation

Anirban Karmakar*, Anup Paul, Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva* and

Armando J. L. Pombeiro*

^aCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,

1049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com ; fatima.guedes@tecnico.ulisboa.pt ;

pombeiro@tecnico.ulisboa.pt.

Metal organic frameworks of Zn(II) and Cu(II) act as recyclable heterogeneous catalysts for

cyanosilylation reaction of aldehydes at room temperature.

21

Article Doi

Zinc(II) and Copper(II) Metal–Organic Frameworks Constructed from a Terphenyl-4,4′′-dicarboxylic Acid Derivative: Synthesis, Structure, and Catalytic Application in the Cyanosilylation of Aldehydes

DOI: 10.1002/ejic.201600902

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