**1. Introduction**

Functional coordination polymers (CPs) and derived materials have been of a special focus in recent years owing to important structural characteristics of these compounds [1–3], intrinsic properties [4,5], and a broad diversity of applications [6–10] including in the field of catalysis [11–17]. The development of new catalytic systems incorporating coordination polymers with target structures and functionalities continues to be a challenging area, since the assembly of CPs can be affected by a diversity of factors. These include the nature of metal centers, organic linkers and supporting ligands, stoichiometry, and various reaction conditions [18–24].

Aromatic carboxylic acids with several COOH groups are the most common building blocks for constructing functional CPs [14,15,17,19]. Within a diversity of such carboxylic

**Citation:** Li, Y.; Liang, C.; Zou, X.; Gu, J.; Kirillova, M.V.; Kirillov, A.M. Metal(II) Coordination Polymers from Tetracarboxylate Linkers: Synthesis, Structures, and Catalytic Cyanosilylation of Benzaldehydes. *Catalysts* **2021**, *11*, 204. https:// doi.org/10.3390/catal11020204

Academic Editors: Jorge Bedia and Carolina Belver

Received: 26 December 2020 Accepted: 29 January 2021 Published: 3 February 2021

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acids, semi-flexible linkers are especially captivating due to unique geometrical arrangements and conformational flexibility, which may lead to crystallization of unexpected metal-organic architectures [21–23,25].

On the other hand, cyanosilylation of carbonyl substrates is an interesting reaction for C-C bond formation [26,27], which is used for the preparation of cyanohydrins—key precursors for some pharmaceutical and fine chemistry products [28,29]. The use of transition metal complexes or coordination polymers in cyanosilylation reactions is gaining relevance as these compounds can behave as low-cost, efficient, and recyclable catalysts [25,30,31].

Considering our research focus on the design of CPs and catalytic systems on their basis [14,15,25], the main goal of this study consisted in the preparation of new metal-organic architectures, followed by their characterization and catalytic application in cyanosilylation of benzaldehydes. Thus, we selected an unexplored ether-linked tetracarboxylic acid, 3-(2,4-dicarboxylphenoxy)phthalic acid (H4dpa, Scheme 1), and tested it as a principal building block for generating copper(II), manganese(II), and zinc(II) CPs. The use of H4dpa as a linker can be justified by a number of relevant features of this tetracarboxylic acid. These features include (i) up to 9 possible sites for coordination (eight O-carboxylate sites and an O-ether site); (ii) two aromatic rings that can provide certain spatial flexibility and conformational adaptation owing to their separation by O-ether group; (iii) the fact that this carboxylic acid possesses good stability under hydrothermal conditions and remains little-explored in the synthesis of CPs.

**Scheme 1.** Structural formulae of H4dpa and bipy.

Thus, this work reports on the hydrothermal synthesis, characterization, thermal behavior, crystal structures, and catalytic application of 2D CPs derived from H4dpa as a linker and bipy as a crystallization mediator. The obtained products [Cu2(μ4-dpa)(bipy)2 (H2O)]n·6nH2O (**1**), [Mn2(μ6-dpa)(bipy)2]n (**2**), and [Zn2(μ4-dpa)(bipy)2(H2O)2]n·2nH2O (**3**) were also screened as potential catalysts for mild cyanosilylation of benzaldehydes into cyanohydrins. The influence of different reaction conditions and substrate scope was investigated, showing that the zinc(II) CP **3** is a particularly effective and recyclable heterogeneous catalyst.

#### **2. Results and Discussion**

### *2.1. Hydrothermal Synthesis*

Aqueous medium mixtures composed of Cu(II), Mn(II), or Zn(II) chlorides with H4dpa as a linker, NaOH as a base for deprotonation of carboxylic acid groups, and 2,2-bipyridine as a mediator of crystallization were subjected to hydrothermal synthesis (3 days, 160 ◦C), resulting in the formation of three 2D coordination polymers as crystalline solids. These were formulated as [Cu2(μ4-dpa)(bipy)2(H2O)]n·6nH2O (**1**), [Mn2(μ6-dpa)(bipy)2]n (**2**), and [Zn2(μ4-dpa)(bipy)2(H2O)2]n·2nH2O (**3**) on the basis of standard solid-state characterization methods, namely infrared spectroscopy (IR), elemental analysis (EA), thermogravimetric analysis (TGA), powder (PXRD) and single-crystal X-ray diffraction. All compounds represent 2D metal-organic networks that are driven by ether-bridged tetracarboxylate nodes that show two distinct coordination modes, namely <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> in **1** and **3**, or <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> in **2** (Scheme 2).

**Scheme 2.** Coordination modes of μ4- or <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> linkers in **1**–**3**.

#### *2.2. Structure of [Cu2(μ4-dpa)(bipy)2(H2O)]n*·*6H2O (1)*

The structure of a 2D CP **1** (Figure 1) comprises two Cu(II) centers (Cu1, Cu2), one <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> linker, two bipy moieties, and one terminal H2O ligand per asymmetric unit. The Cu1 center is 4-coordinated and exhibits a distorted {CuN2O2} seesaw geometry. It is formed by two carboxylate O atoms from a pair of <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> ligands and two bipy N atoms (Figure 1a). The Cu2 atom is five-coordinated and features a distorted {CuN2O3} square-pyramidal geometry. It is constructed from two oxygen atoms from two <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> blocks, two bipy N atoms, and a terminal H2O ligand.

**Figure 1.** Structural fragments of **1**. (**a**) Coordination environment around Cu(II) atoms; H atoms are omitted for clarity. Symmetry codes: i = *x* + 1, *y*, *z* + 1; ii = *x* + 1, *y*, *z*. (**b**) 2D metal-organic layer; view along the *b* axis. (**c**) Topological view of 2D layer with a **sql** topology; view along the *b* axis; Cu centers (green balls), centroids of <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> nodes (gray).

The Cu–O [1.942(2)–2.345(3) Å] and Cu–N [1.992(3)–2.058(3) Å] bonds are typical for such a type of compounds [14,18,30]. The dpa<sup>4</sup>− ligand behaves as a μ4-linker with carboxylate moieties being monodentate (Scheme 2, mode I). In <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup>, a dihedral angle between aromatic cycles and a Car–Oether–Car angle are 86.34 and 115.93◦, correspondingly. The <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> linkers connect four Cu atoms to assemble a 2D metal-organic layer (Figure 2b) which, after simplification, is described as a mononodal 4-linked net. It possesses a **sql** (Shubnikov tetragonal plane net) topology with a (44.62) point symbol (Figure 1c) [32,33].

**Figure 2.** Structural fragments of **2**. (**a**) Coordination environment around Mn(II) atoms; H atoms are omitted for clarity. Symmetry codes: i = *x* + 1, *y*, *z*; ii = *x* + 1, −*y* + 3/2, *z* + 1/2. (**b**) Mn2 subunit. (**c**) 2D metal-organic layer; view along the *b* axis. (**d**) Topological representation of 2D layer with a **3,6L66** topology; view along the *b* axis; Mn(II) nodes (turquoise balls), centroids of <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> nodes (gray).

#### *2.3. Structure of [Mn2(μ6-dpa)(bipy)2]n (2)*

The structure of **2** also shows a 2D coordination polymer network (Figure 2). Per asymmetric unit, there are two manganese(II) atoms, one <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> block, and two bipy ligands. The Mn1 center is five-coordinated and shows a distorted {MnN2O3} trigonal-bipyramidal environment. It is built from three oxygen atoms from three <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> linkers and a pair of bipy N donors (Figure 2a). The six-coordinate Mn2 center is bound by four oxygen atoms from three <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> ligands and a pair of bipy N atoms, thus creating a distorted {MnN2O4} octahedral geometry. The Mn–O [2.087(3)–2.217(4) Å] and Mn–N [2.226(4)–2.277(4) Å] bonds agree with those in related compounds [19,21,22]. The tetracarboxylate dpa<sup>4</sup>− moiety behaves as a μ6-linker (Scheme 2, mode II) with the carboxylate functionalities being monodentate or μ-bridging bidentate. In <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup>, the relevant angles are 79.83◦ (dihedral angle among aromatic moieties) and 116.37◦ (Car–Oether–Car). The manganese(II) centers are held together by three carboxylate groups from three <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> blocks, thus generating a dimanganese(II) subunit [Mn1·Mn2 3.4679(6) Å] (Figure 2b). Such Mn2 subunits are additionally connected by carboxylate groups of <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> to form a 2D metal-organic layer (Figure 2c). Regarding topology, this 2D layer is built from 3-linked Mn1/Mn2 nodes as well as the 6-linked <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> nodes (Figure 2d). The resulting net is thus classified as a binodal 3,6-linked layers of a **3,6L66** topological type [34]. It is described by a (43.612)(43)2 point symbol, wherein the (43.612) and (43) indices correspond to the Mn(II) and <sup>μ</sup>6-dpa<sup>4</sup><sup>−</sup> nodes, respectively.

#### *2.4. Structure of [Zn2(μ4-dpa)(bipy)2(H2O)2]n*·*2H2O (3)*

In the structure of a 2D coordination polymer **3** (Figure 3), there are two zinc(II) atoms (Zn1, Zn2), one <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> block, two bipy moieties, two terminal water ligands and a couple of lattice H2O molecules. Both Zn atoms are five-coordinated, showing

distorted {ZnN2O3} square pyramidal geometries. These are constructed from two O donors from a pair of <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> linkers, a water ligand, and a pair of bipy nitrogen atoms. The Zn–O [1.973(3)–2.093(4) Å] and Zn–N [2.119(3)–2.181(3) Å] bond lengths are typical for compounds having an O–Zn–N environment [2,30,35,36]. The dpa<sup>4</sup>− block adopts a μ4-coordination fashion (Scheme 2, mode I). In <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup>, the relevant angles are 72.59◦ (dihedral angle among two aromatic moieties) and 117.72◦ (Car–Oether–Car). The <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> blocks interconnect four Zn(II) centers to furnish a 2D layer (Figure 3b). Its topological classification discloses an uninodal 3-linked layer with an **hcb** (Shubnikov hexagonal plane net/(6, 3)) topology and a (63) point symbol (Figure 3c) [37,38].

**Figure 3.** Structural fragments of **3**. (**a**) Coordination environment around Zn(II) atoms; H atoms are omitted for clarity. Symmetry codes: i = <sup>−</sup>*x*, −*y* + 1, −*z*; ii = −*x* + 1/2, *y* − 1/2, −*z* + 1/2. (**b**) 2D metal-organic layer; view along the *a* axis. (**c**) Topological representation of 2D layer with an **hcb** topology; view along the *a* axis; Zn(II) centers (cyan balls), centroids of <sup>μ</sup>4-dpa<sup>4</sup><sup>−</sup> nodes (gray).

#### *2.5. TGA & PXRD*

Thermal behavior of compounds **1**–**3** was investigated by TGA on gradual heating from 30 to 800 ◦C under nitrogen flow (Figure S2). CP **1** reveals a loss of six lattice and one coordinated H2O molecules in the 33–104 ◦C interval (calcd. 13.8%; exptl. 13.5%). After dehydration, the network of **1** maintains its stability until 220 ◦C. CP **2** does not encompass water as a lattice solvent or ligand and shows thermal stability up to 338 ◦C. For CP **3**, a mass loss in the 86–171 ◦C window refers to an elimination of two crystallization and two coordinated H2O moieties (calcd. 8.4%; exptl. 8.6%); the obtained sample maintains stability until 218 ◦C.

To confirm a phase purity, the microcrystalline powders of **1**–**3** were analyzed by PXRD (Figure S2). The obtained experimental plots well agree with the patterns calculated from single-crystal X-ray diffraction data, what confirms a purity of the bulk solids **1**–**3** prepared via hydrothermal synthetic procedure.

#### *2.6. Cyanosilylation of Benzaldehydes*

Coordination polymers **1**−**3** were tested as catalysts in the mild, heterogeneous cyanosilylation of benzaldehyde substrates with TMSCN (trimethylsilyl cyanide) [33,35]. 4-Nitrobenzaldehyde was used as model substrate and converted into the corresponding cyanohydrin product, 2-(4-nitrophenyl)-2-[(trimethylsilyl)oxy]acetonitrile (Table 1, Scheme 3). An influence of catalyst loading, solvent type, reaction time, catalyst recycling and substrate scope was investigated.

The cyanosilylation reaction almost does not undergo when catalyst is absent or when using a carboxylic acid ligand or a metal salt precursor as potential catalysts (3−8% yields; Table 1, entries 1−3). The product yields are higher when using **1** (37%, entry 4) and **2** (27%, entry 5) as catalysts, and significantly higher in the presence of **3** (92%, entry 6). Although it is not possible to establish a clear relationship between structural features and catalytic activity of the obtained 2D compounds, we can speculate that a superior

activity exhibited by the coordination polymer **3** may eventually be related to the presence of accessible and unsaturated zinc(II) centers on a surface of catalyst particles, together with a higher Lewis acidity of the zinc sites [30,39]. Given a superior activity of zinc(II) derivative **3**, the reaction catalyzed by this CP was optimized further.


**Table 1.** Cyanosilylation of 4-nitrobenzaldehyde with TMSCN catalyzed by coordination polymers. a

a Conditions: 4-nitrobenzaldehyde (0.5 mmol), TMSCN (1.0 mmol), solvent (2.5 mL), room temperature (~25 ◦C). b Yields based on 1H NMR (nuclear magnetic resonance) analysis: (moles of product per mol of benzaldehyde substrate) × 100%.

**Scheme 3.** Cyanosilylation of model substrate(4-nitrobenzaldehyde) into the corresponding cyanohydrin.

We found that there is an yield growth from 40 to 92% on increasing the time of reaction in the 1–12 h interval (Table 1, entries 6−12; Figure S7). The catalyst **3** loading of 2, 3, or 4 mol% has also a notable effect as attested by an yield increase from 73 to 92 and 93%, respectively (entries 6, 13, 14). Although dichloromethane (CH2Cl2) was used as a standard solvent to achieve the highest yield (92%), the cyanosilylation reaction also undergoes quite effectively in alternative solvents such as chloroform (CHCl3, 82%), tetrahydrofuran (THF, 68%), acetonitrile (CH3CN, 76%), and methanol (CH3OH, 80%) (Table 1, entries 15−18).

Furthermore, in the reactions catalyzed by **3**, substrate scope was studied using different functionalized benzaldehydes (Table 2). The corresponding cyanohydrin products are obtained in yields ranging from 51 to 87%. The substrates with an electron-withdrawing group (R = NO2, Cl) generally show a higher reactivity (Table 2, entries 2−5), which can be explained by an increased substrate electrophilicity. As expected, benzaldehyde (R = H) and substrates with an electron-donating group (R = OH, CH3) reveal lower product yields (entries 6, 7).

**Table 2.** Substrate scope in cyanosilylation of various benzaldehydes with TMSCN catalyzed by **3**. a


 a Conditions: functionalized benzaldehyde (0.5 mmol), TMSCN (1.0 mmol), catalyst **3** (3.0 mol.%), CH2Cl2 (2.5 mL), room temperature (~25 ◦C). b Yields based on 1H NMR analysis: (moles of product per mol of functionalized benzaldehyde substrate) × 100%.

Given a remarkable activity of CP **3** in these cyanosilylation reactions, the stability of the catalyst and its possible recycling were studied. Thus, in the end of the cyanosilylation of 4-nitrobenzaldehyde (conditions of entry 6, Table 1), the catalyst was isolated via centrifugation, washed by CH2Cl2, air-dried, and reused in the next reaction runs. These tests indicate that the coordination polymer **3** behaves as a stable catalyst which maintains its activity for a minimum of 4 cyanosilylation cycles, as attested by resembling product yields in the 92 −87% range (Figure S8). In addition, PXRD data of the used catalyst (Figure S9) show that its metal-organic structure remains stable.
