The structures and catalytic behaviors of two new complexes based on bifunctional tetrazole-carboxylate connector

Abstract

Two transition metal coordination complexes [Co(Tppebc)₂(Py)(H₂O)]_n (1) and [Ni(Tppebc)₂(Py)(H₂O)]_n (2) (Py = pyridine, Tppebc = 3-(4-(2-(1H-tetrazol-5-yl)phenyl-4-yl) phenyl)-2-ethoxy-3H-benzimidazole-4-carboxylic acid) have been prepared by a hydrothermal method. The structural unit of complexes 1 and 2 adopts similar distorted octahedra geometry. Furthermore, both of those complexes feature a 1D chain and can be further extended to a higher-dimensional architecture through π···π stacking and hydrogen bonding. The catalytic property of complexes 1 and 2 were researched in the green catalytic process of the oxidative 2,6-di-tert-butylphenol (DBP). The high conversion and good selectivity results strongly indicate that both of the title complexes are catalytically active under the optimized reaction conditions.

Keywords:
tetrazole-carboxylate connector; crystal structures; 1D chains; catalytical active

1 Introduction

Enormous attentions have been paid to coordination complexes, because of diverse fascinating functionalities and excellent applications, such as catalysis, luminescence, molecular magnetism and conductivity (Liu et al., 2017; Chang et al., 2011; Feng et al., 2010; Pan et al., 2011). Among thousands of coordination complexes investigated, a significant number of them are appropriative to the rational design of intriguing structural motifs with rich physical and chemical performances using multidentate organic ligands such as N-heterocyclic ligands and carboxylate ligands (Burrows et al., 2011; Cheng et al., 2012; Yang et al., 2009). Recently, considerable efforts have been made on the architectures of complexes according to the tetrazole and imidazole derivates (Li et al., 2009; Ni et al., 2011). Among them, 5-substituted tetrazoles turn out to be an excellent class of building blocks in the synthesis of structurally versatile complexes, which can strongly tune properties and performances of complexes (Pachfule et al., 2010; Qiu et al., 2010). Tetrazolyl-carboxylate ligands as excellent building blocks bearing various coordination modes and variable configurations, can give rise to intriguing MOFs with beautiful aesthetics and useful functionalization (Naik et al., 2010; Zhang et al., 2013). Accordingly, the tetrazole-carboxylate derivative 3-(4-(2-(1H-tetrazol-5-yl) phenyl-4-yl) phenyl)-2-ethoxy-3H-benzimidazole-4-carboxylic acid (Tppebc) is a good candidate for greater tunability of structural frameworks. It is well-known that coordination complexes based on mixed-ligands exhibit unusual architectures, as combining different ligands could satisfy the coordination geometries requirement of metal centers during the assembly procedure (Guo et al., 2019; Geng et al., 2022; Kanoo et al., 2009). It is worth noting that the proficient introduction of the neutral pyridine analogs as auxiliary ligands into the reaction systems involving multifarious carboxylic acid ligands was concerned in recent years, which may form attractive coordination architectures (Kumar et al., 2006; Zhang et al., 2013).

Taking the above-mentioned factors into considerations, the two novel complexes [Co(Tppebc)₂(Py)(H₂O)]_n (1) and [Ni(Tppebc)₂(Py)(H₂O)]_n (2) have been prepared through the reaction of corresponding Co^II/ Ni^II salts with the primary ligand Tppebc and the auxiliary ligand pyridine under similar hydrothermal conditions. The results manifest that the coordination modes of tetrazole moiety and pyridine can significantly affect the structure of complexes. In addition, the catalysis-structure relationships indicate that the coordinated environments of the Co^II/Ni^II centers play a vital role in their catalytic activities.

2 Experimental

2.1 Materials and measurements

All chemical reagents were purchased from commercial sources and directly used. The elemental composition of complexes 1 and 2 were performed on a Flash EA 1112 elemental analyzer. Infrared radiation (IR) data spectra were measured on a BRUKER TENSOR 27 spectrophotometer. Thermogravimetric analysis was performed on a METTLER TOLEDO TGA/SDTA instrument under a nitrogen atmosphere upon 10 °C·min^-1 heating rate. The X-ray diffraction data was recorded on a SuperNova with graphite mono-chromated Cu-Kα radiation (λ = 1.54184 Å).

2.2 Synthesis of [Co(Tppebc)(Py)(H₂O)]_n (1)

Firstly, 0.0146 g Co(NO₃)₂.6H₂O (0.05 mmol), 0.0220 g Tppebc (0.05 mmol) and 1 mL pyridine was treated by water (4 mL) and methanol (8 mL) mixture solution. The reaction was performed in a Teflon-lined stainless-steel container (25 mL) for 3 days at 150 °C. Then, the reaction medium was gradually cooled at a rate of 5 °C·h^-1. The product was obtained as a pink rhombic block crystal. Yield: 55% (based on Co). Elemental analysis (%) calcd for C₂₇H₂₁CoN₇O₄: C, 57.25; H, 3.74; N, 17.31. Found: C, 57.12; H, 3.93; N, 17.26. IR(KBr/pellet; ν, cm^-1): 3375(m), 1698(s), 1604(w), 1570(s), 1489(w), 1452(m), 1396(s), 1301(w), 1141(m), 1070(m), 957(w), 814(w).

2.3 Synthesis of [Ni(Tppebc)(Py)(H₂O)]_n (2)

The synthetic procedure of 2 is similar with that of complex 1. Briefly, 0.0145 g Ni(NO₃)₂.6H₂O (0.05 mmol) was replaced with equimolar of Co(NO₃)₂.6H₂O. The complex 2 was synthesized as a green block crystal. Yield: 63% (based on Ni). Elemental analysis (%) calcd for C₂₇H₂₁NiN₇O₄: C, 57.28; H, 3.74; N, 17.32. Found: C, 57.33; H, 3.69; N, 17.16. IR(KBr/pellet; ν, cm^-1): 3207(m), 1700(s), 1619(m), 1571(s), 1490(w), 1450(m), 1302(w), 1188(w), 1219(w), 1023(m), 997(w), 760(m).

2.4 Crystal structure determination

The suitable crystal chunks of complexes 1 and 2 were equipped mounted on glass fibres and then they were measured on a Super Nova with graphite mono-chromated Cu-Kα radiation (λ = 1.54184 Å) at 293(2) K. The structures of complexes 1 and 2 were analyzed via direct methods and expanded with Fourier techniques (Martins et al., 2019). The structural refinements were performed on an OLEX2 crystallographic program (Dolomanov et al., 2010). Crystal data containing space group, lattice parameters and other relevant information for the title complexes are listed in Table 1. Relevant bond lengths and bond angles are summerized in Table 2.

Table 1
Crystal data and structure refinement for complexes 1 and 2.

Complex	1	2
Empirical formula	C27H21CoN7O4	C27H21NiN7O4
Formula weight	566.44	566.22
Temperature (K)	293(2)	293(2)
Wavelength (Mo-Kα) (Å)	1.54184	1.54184
Crystal system	triclinic	triclinic
Space group	P-1	P-1
a	8.1543(3)	8.0870(3)
b	11.5919(5)	11.6192(5)
c	13.3086(6)	13.2622(6)
α	97.015(4)	97.320(4)
β	94.562(3)	94.120(3)
γ	99.713(3)	100.042(3)
Volume (Å3), Z	1224.20(9), 2	1211.43(9), 2
F (000)	582	584
θ range for data collection(o)	3.36 to 73.52	3.38 to 73.49
Goodness-of-fit on F2	1.071	1.026
Final R1a, wR2b	0.0342, 0.0886	0.0344, 0.0884

• ^a
  R₁ = [|F₀|-|F_c|]/|F₀|;
• ^b
  wR₂ = [w(F₀²-F_c²)²]/[w(F₀²)²]^1/2. w = 1/[σ²(F_o)²+ 0.0297 P²+27.5680P], where P = ( F²_o+ 2 F²_c)/3.

Table 2
Selected bond lengths and angles for complexes 1 and 2.

Complex 1
Co(1)-O(1) #1	2.0572(14)	Co(1)-O(2)	2.1144(13)
Co(1)-O(3)	2.2086(15)	Co(1)-N(2)#2	2.1069(16)
Co(1)-N(3)#1	2.1486(16)	Co(1)-N(7)	2.1466(17)
3#Co(1)-N(2)	2.1069(16)	1#Co(1)-N(3)	2.1486(16)
1#O(1)-Co(1)-O(2)	97.42(5)	1#O(1)-Co(1)-O(3)	87.71(6)
1#O(1)-Co(1)-N(3)#1	88.70(6)	1#O(1)-Co(1)-N(7)	175.54(6)
O(2)-Co(1)-O(3)	85.81(5)	O(2)-Co(1)-N(3)#1	169.94(6)
O(2)-Co(1)-N(7)	87.00(6)	2#N(2)-Co(1)-O(2)	92.17(6)
2#N(2)-Co(1)-N(3)#1	96.29(6)	1# N(3)-Co(1)-O(3)	86.47(6)
Complex 2
1#Ni(1)-O(1)	2.0473(14)	Ni(1)-O(2)#2	2.1231(13)
1#Ni(1)-O(3)	2.1283(14)	1#Ni(1)-N(4)#2	2.0583(16)
1#Ni(1)-N(5)	2.0949(16)	1#Ni(1)-N(7)	2.0925(17)
1#Ni(1)-O(2)	2.1231(13)	2#Ni(1)-N(4)	2.0583(16)
O(1)-Ni(1)-O(2)#1	95.52(6)	O(1)-Ni(1)-O(3)	87.61(6)
O(1)-Ni(1)-N(4)#2	84.49(6)	O(1)-Ni(1)-N(5)	88.43(6)
2#N(4)-Ni(1)-O(2) #1	91.56(6)	1#O(2)-Ni(1)-O(3)	85.26(5)
2#N(4)-Ni(1)-O(3)	171.17(6)	2#N(4)-Ni(1)-N(5)	96.03(6)
2#N(4)-Ni(1)-N(7)	95.22(7)	N(5)-Ni(1)-O(2)#1	171.76(6)
N(7)-Ni(1)-O(2)#1	87.80(6)	N(7)-Ni(1)-N(5)	88.30(7)

• Symmetry transformations used to generate equivalent atoms: #3 1+X,+Y,+Z for 1; #1-X,1-Y,-Z; #2 1-X,1-Y,-Z for 2.

2.5 Catalytic reactions

Complex 1 was choosed as a represent catalyst under the optimized reaction conditions, in which 0.02 mmol of the catalyst and 1 mmol of the co-catalyst NaOCH₃ was added to different reaction mediums and stirred at 45 °C. Afterwards, 200 μL of H₂O₂ (of 30% aqueous solution) was gradually added into the mixture at an interval of 25 minutes (four times in all) to minimize the decomposition of H₂O₂. After 3 h, the mixture was condensed in vacuum, and the products were separated out by preparative TLC conducted on dry silica gel plates with dichloromethane-petroleum ether (1:4 v/v) as the elution solvents. The products were gathered and arid in vacuo. The main product, 3,3',5,5'-tetra-tert-butyl-4,4'-diphenoquinone (DPQ): IR(cm⁻¹): 3439(m), 3010(w), 2960(s), 2864(m), 1608(s), 1460(m), 1267(w), 1090(s), 1038(m), 847(w). The byproduct: 2,6-ditert-butylbenzoquinone (BQ), IR(cm⁻¹): 3301(w), 2959(m), 2870(m), 1653(s), 1560(m), 1321(m), 1243(w), 1081(m), 920(m), 819(w).

3 Results and discussions

3.1 Crystal structure of [Co(Tppebc)(Py)(H₂O)]_n (1)

The unit of this complex includes one Co(II) center, one Tppebc, one pyridine and one water. As illustrated in Figure 1, the Co(II) center features a six-coordinated mode with a distorted octahedra geometry, coordinating with two N atoms (N2, N3) from two Tppebc, one N atom (N7) from pyridine and two O atoms (O1, O2) from another two Tppebc and O3 atom from the coordinated water molecular. The bonding lengths of Co-O and Co-N are ranging from 2.0572 to 2.2086 Å, which are in accordance with those previously observed in other Co(II) complexes (Bai et al., 2010; Yang et al., 2009). Bridging by two carboxylate groups from two Tppebc ligands, every two adjacent Co(II) metal centers are constructed into a binuclear unit, which are extended by Tppebc to result in a 1D chain (Figure 2).

Figure 1
Schematic illustration of the crystal structure of 1.

Figure 2
Schematic illustration of the 1D chain structure of 1.

In addition, the structure of complex 1 is further stabilized by face-to-face π···π interactions between the pyridine ring and imidazole ring with centroid separation of 3.6257(16) Å and the dihedral angle of 7.70(14)°. C-H···π interaction between the C5 atom and the centroid of benzene is existed with the separation of 3.628(2) Å. Moreover, adjacent 1D chains are stabilized through strong hydrogen bonds (O4···N6, 2.877(2) Å; O3···O2, 2.834(2) Å; O3···N4, 2.752(2) Å; C9-···O2, 2.989(2) Å; C9-O4, 2.858(2) Å). Thus, complex 1 is finally formed a 3D supra-molecular structure by π···π interactions and hydrogen bonds (Figure 3).

Figure 3
Schematic illustration of 3D supra-molecular structure of 1.

3.2 Crystal Structure of [Ni(Tppebc)(Py)(H₂O)]_n (2)

When the Co(II) center was replaced by Ni(II) center, a similar 1D coordination complex 2 was obtained. The asymmetric unit consists of a Ni(II) center, one Tppebc, one pyridine and one water. As shown in Figure 4, the Ni(II) center displays a slightly distorted octahedral geometry, including two tetrazole nitrogen atoms, one pyridine nitrogen atom, two carboxylate oxygen atoms, and one water oxygen atom. Both Ni-N and Ni-O distances are located in the similar scope with those in other Ni(II) complexes (Ghannam et al., 2018; Zianna et al., 2016). Each Tppebc ligand applies two oxygen atoms from two carboxylate ligands to bridge two Ni(II) centers, constructing a large Ni-Tppebc-Ni-Tppebc ellipse hole with Ni1···Ni1 distance of 4.5130(5) Å. In addition, the Ni(II) centers are connected by Tppebc to result in a 1D chain (Fig. S1).

Figure 4
Schematic illustration of the crystal structure of 2.

For imidazole ring and pyridine ring, the centroid-to-centroid separation distance is 3.6096(15) Å with a dihedral angle of 8.18(14)°. The separation of the C-H···π force between the centroid of benzene and the C5 is 3.622(3) Å. Besides, the 1D chains are benefited from the hydrogen bonds (O4···N1, 2.880(2) Å; O3···O2, 2.760(2) Å; C9···O2, 2.997(2) Å; C15·· N2, 2.927(3) Å; C9···O4, 2.857(3) Å). From hydrogen-bonding and π···π interactions, complex 2 is finally connected into a 3D supra-molecular framework (Fig. S2).

3.3 Thermal analyses

Both compounds 1 and 2 were maintained and are able to maintain stability in ambient condition. Complex 1 remains stable until 201 °C with two exothermic peaks at 314, 490 °C through the DSC curve, indicating the decomposition of this complex (Fig. S3). As for 2, it was rather at 277 °C. The DSC demonstrated that there was an exothermic peak at around 503 °C, illustrate sing the framework decomposition (Fig. S4).

3.4 Catalytic properties of complexes

In recent years, the application of MOFs as catalysts has gained a great number of research attentions, since MOFs are featured with following advantages: 1) the controllable oxidation state and electron density of metal center can be introducing different ligands, associating with selectivity coordinated geometry and catalytic behaviors (Lan et al., 2019; Wang et al., 2015). Therefore, we studied the catalytic behaviors of 1 and 2 with DBP clean oxidant H₂O₂ as oxidant together with NaOCH₃ as co-catalyst in different solvents at a certain temperature (Hu et al., 2012; Kennemur et al., 2019). The standard reaction system of DBP selective catalytic substrate H₂O₂ oxidation was selected in this work (Scheme S1, Supplementary Material). The relevant catalytic results are summarized in Table 3 and Table 4. Regarding to those experimental data, the catalytic substrate reaction is strongly dependent on the applied solvent, among which acetonitrile turned out to be the suitable reaction medium, in accordance with other papers reported before (Santos et al., 2020). DBP was oxidized by complexes 1 and 2 with H₂O₂ as oxidant, respectively, resulting in the coupled main product DPQ and the by-product BQ. Under the optimized reaction medium, the yield of rates of both complexes 1 and 2 were 81% and 80%, respectively. Also, the conversion rate of DBP is approximately 90% for the two catalytic substrate reaction. Importantly, the experiment to oxidize DBP only in the presence of H₂O₂ resulting in much less catalytic product (35% or 64%, respectively), in line with previous reports (Turk et al., 2005; Mu et al., 2011). These results illustrate that complexes 1 and 2 are featured with high catalytic activity to synthesize di-benzoquinone product. In addition, complexes 1 and 2 manifest similar catalytic activities under the same reaction conditions, owning to their same geometric configurations.

Table 3
The catalytic activity analyses of complex 1 with DBP^A.

Solvent	Acetone	Methanol	Ethanol	Acetonitrile	Water
Conversion of DBP (%)	93	89	85	95	86
Yield (%)B DPQ	81	79	73	87	76
Yield (%)B BQ	9	7	9	6	8
SelectivityC DPQ (%)	90	92	89	94	90

• ^A
  Reaction conditions: 1 mmol of DBP and NaOCH₃, 0.02 mmol of catalyst and 20 μL of H₂O₂ in 2 mL reaction medium at 45 °C;
• ^B
  Conversions and yield rates for DPQ;
• ^C
  Selectivity = ([DPQ]×100)/([DPQ] + [BQ]).

Table 4
The catalytic activity analyses of complex 2 with DBP^A.

Solvent	Acetone	Methanol	Ethanol	Acetonitrile	Water
Conversion of DBP (%)	92	91	83	96	88
Yield (%)B DPQ	80	81	70	86	72
Yield (%)B BQ	8	6	10	7	11
SelectivityC DPQ (%)	91	93	86	95	87

• ^a
  Reaction conditions: 1 mmol of DBP and NaOCH₃, 0.02 mmol of catalyst and 20 μL of H₂O₂ in 2 mL reaction medium at 45 °C;
• ^b
  Conversions and yield rates for DPQ;
• ^c
  Selectivity = ([DPQ]×100)/([DPQ] + [BQ]).

4 Conclusions

To sum up, two novel metal coordination complexes have been designed and synthesis through the synergetic effect of the mixed connectors. The catalytic activities of the complexes indicate that both of complexes 1 and 2 are good candidates as the catalyst for the oxidative coupling of DBP.

Supplementary Material

Detailed structural parameters for 1 and 2 were submitted to the Cambridge Crystallographic Data Center with CCDC reference numbers 1914149 and 1914150. The crystallographic data can be acquired this free network Data Centre.

Fig. S1 The 1D chain of 2 (all hydrogen atoms are omitted for clarity). Fig. S2 View of the 3D crystal packing of the structure of 2 (all hydrogen atoms have been omitted for clarity) Fig. S3 Thermogravimetric (TG) and differential scanning calorimetric (DSC) curves for complex 1 Fig. S4 Thermogravimetric (TG) and differential scanning calorimetric (DSC) curves for complex 2 Scheme S1 Oxidation reaction of 2,6-di-tert-butylphenol.

This material is available as part of the online article from https://doi.org/10.1590/fst.88122

Acknowledgements

We gratefully acknowledge the financial support by the key program of He’nan colleges and universities (Nos. 21B150009 and 22B530002), the He’nan Key Science and Technology Research (No. 222102320421), the He’nan Teaching Reform Program (No. 2021SJGLX881) and He’nan Educational Science Planning Program (No. 2022YB0565) and the Youth Backbone Program of Henan Vocational College of Applied Technology (No. 2022-GGJS-H005).

• Practical Application: Two novel metal complexes based on Co and Ni have been synthesized and demonstrated that they feature good catalytic property in the green catalytic process. These complexes have great potential to be applied as a class of novel catalysis in the synthetic and industrial chemistry.

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Publication Dates

• Publication in this collection
  14 Nov 2022
• Date of issue
  2022

History

• Received
  01 Aug 2022
• Accepted
  19 Sept 2022