Synthesis, Crystal Structure, and Catalytic Property of a Copper Coordination Compound Based on In Situ Generated 2-Hydroxynicotinic Acid

Abstract

The copper coordination complex [CuCl(2-OHNA)H₂O]·H₂O (1) was synthesized by the reaction of CuCl₂·2H₂O with in situ generated 2-hydroxynicotinic acid and its crystal structure was determined by single X-ray diffraction methods. It was further characterized by FT-IR spectroscopy, elemental analyses, and thermogravimetric analysis. Complex 1 crystallizes in monoclinic space group P2₁/c with a = 8.9797(13), b = 14.196(2), c = 7.0738(11) Å, β = 96.897(2), V = 895.2(2) ų, M_r = 273.12, D_c = 2.027 g/cm³, and Z = 4. In the structure, complex 1 is linked into 2D sheets via intermolecular hydrogen bonding [N1–H1···O2 (−x+1, y − 1/2, − z+1/2); O5–H10···O2 (− x+1, y + 1/2, − z+1/2); O4–H9···O5 (x, y − 1, z); O4–H8···O5 (− x+2, y + 1, − z)]. The catalytic property of 1 was also investigated in the selective oxidation of benzyl-alkanes using TBHP as oxidant. Under optimized conditions, 1 exhibited high catalytic activity and selectivity toward the corresponding aryl ketones.

Graphic Abstract

The copper coordination complex [CuCl(2-OHNA)H₂O]·H₂O (1) was synthesized by the reaction of CuCl₂·2H₂O with in situ generated 2-hydroxynicotinic acid and the catalytic property of 1 was also investigated in the selective oxidation of benzyl-alkanes using TBHP as oxidant.

Introduction

In recent decades, transition-metal complexes have obtained long-lasting attention due to their variety of architectures and the potential applications as functional materials [1,2,3,4]. Among transition metals, copper is an important element in catalysis chemistry, coordination chemistry, magnetochemistry and its relevance as the active-site structure of metalloproteins [5,6,7,8]. Moreover, its flexible coordination modes make the copper ion to form mononuclear, dinuclear and multinuclear complexes. Functionalized carboxylates can easily be applied to construct functionalized metal–organic coordination compounds with supramolecular architectures. Functional groups like –OH, –NH₂ are frequently used for this purpose [9]. 2-Hydroxynicotinic acid (2-OHNAH) is one of the most successful functionalized carboxylates due to its versatile modes of coordination [10].

Herein, we present a Cu(II) coordination complex [CuCl(2-OHNA)H₂O]·H₂O (1) with in situ generated 2-hydroxynicotinic acid. Complex 1 was fully characterized by single crystal X-ray diffraction (SXRD), FT-IR spectroscopy, elemental analysis (EA), and thermogravimetric (TGA) analysis. The metal contents of 1 were determined using inductively coupled plasma (ICP) on a JY-ULTIMA2 analyzer. Complex 1 was obtained by mixing CuCl₂·2H₂O and 2-chloronicotinic acid (2-ClNAH) in 1:2 molar ratio in a 1:2 water–methanol mixture under basic conditions. In 1, the original 2-ClNAH ligand underwent to a base hydrolysis reaction followed by keto-enol tautomerization (Scheme 1). Importantly, complex 1 was further investigated as catalyst for the oxidation of benzylic sp³ C–H bonds with TBHP as an oxidant.

Scheme 1

Hydrolysis of 2-chloronicotinic acid in presence of base and successive keto-enol tautomerization

Experimental

All reagents and solvents were purchased from commercial sources and used without further purification. The C, H and N elemental analyses were conducted on a Perkin-Elmer 240C elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm⁻¹ on a Nicolet 170 SXFT/IR spectrometer. The GC analyses were performed on Shimadzu GC-2014C with a FID detector equipped with an Rtx-1701 Sil capillary column. TGA experiments were carried out on a Perkin-Elmer TGA 7 analyzer with a heating rate of 10 °C/min from room temperature to 700 °C under nitrogen atmosphere.

Synthesis of Complex [CuCl(2-OHNA)H₂O]·H₂O (1)

To a 40 mL methanolic solution of 2-ClNAH (0.95 g, 6 mmol) was slowly added a 20 mL aqueous solution of CuCl₂·2H₂O (0.52 g, 3 mmol) with continuous stirring. The mixture was stirred for 1 h with constant heating at 60 °C. Subsequently, the solution was allowed to cool and the pH was adjusted to about 7.5 with an aqueous solution of NaOH. The mixture was then heated over a water bath for 2 h and subsequently filtered. The solution was left to over a week at room temperature to evaporate the solvent and until blue colored translucent crystals formed (ca. 52% yield based on 2-ClNAH). Anal. calcd. (%) for C₆H₈ClCuNO₅: C, 26.38; H, 2.95; N, 5.13. Found: C, 26.24; H, 3.11; N, 5.06. IR (KBr) v: 3442, 3338, 1664, 1616, 1552, 1473, 1247, 1148, 1092, 672, 577, 542, 441 cm⁻¹.

X-ray Crystallography

Single-crystal X-ray diffraction data for complex 1 were collected on a Bruker Smart APEX II CCD diffractometer using graphite-monochromated Mo Ka radiation (λ = 0.71073 Å) at 298 K. The data were corrected for absorption using the multi-scan technique [11]. The structure was solved by the direct method and refined through full-matrix least-squares techniques method on F² using the SHELXTL 97 crystallographic software package [12, 13]. The hydrogen atoms connected to C and N atoms were placed in calculated positions and refined using a riding model. Hydrogen atoms on O atoms were located in the difference Fourier maps and then refined using a riding model and a dependent displacement parameter U_iso(H) = 1.5U_eq(O). The distances of O–H bonds and bond angles of H–O–H were further restrained using the DFIX command. Crystallographic data for complex 1 are summarized in Table 1 and selected bond lengths and bond angles are listed in Table 2.

Table 1 Crystal data and structure refinement of 1

Table 2 Selected bond length and bond angles (Å, °) of 1

Catalytic Oxidation of Benzylic sp³ C–H Bonds

In a typical experiment, the required amount of catalyst (1.5 mol%) and benzyl hydrocarbon (0.125 mmol), 70% TBHP (0.35 mmol) and benzonitrile (1 mL) were successively added to a round-bottom flask. The reaction mixture was subsequently heated at 80 °C in a Wattecs Parallel Reactor for 24 h with stirring. The conversion and selectivity of these reactions were determined by GC analyses with nitrobenzene as an internal standard. All of the products were confirmed by GC analysis in combination with GC–MS.

Results and Discussion

Thermogravimetric Analysis of [CuCl(2-OHNA)H₂O]·H₂O (1)

Thermogravimetric analysis (TGA) was performed to study the stability of complex 1 (Fig. 1). Three separate weight loss steps were observed: The first weight loss began at 90 °C and was completed at 160 °C. The observed weight loss of 19.90% corresponds to the loss of one lattice water molecule and –Cl⁻ group (calcd 19.95%). The second weight loss of 24.72% occurred in the range of 310–540 °C, corresponding to the loss of one coordinated water molecule and one –COO⁻ group (calcd 24.17%). The third and continuous weight loss step began at 543 °C and can be attributed to the further decomposition of the 2-OHNA ligand.

Fig. 1

TGA curve for complex 1

Crystal Structure of [CuCl(2-OHNA)H₂O]·H₂O (1)

Compound 1 including the hydrogen-bonded solvent water is shown in Fig. 2. SXRD analysis shows that complex 1 crystallizes in the monoclinic space group P2₁/c. The Cu²⁺ ion is square planar coordination as expected. Chlorine atoms in the neighboring sheet are at a 2.80 Å distance from Cu. The ligand 2-OHNA exists in keto form and bidentately coordinates to the metal centre in O, O′-salicylate manner. The typical square planar coordination around the Cu(II) ion includes three O atoms (two from the 2-OHNA ligand and one from the coordinated water molecule) and one Cl atom. The chelate bite angle in the five-membered ring formed by O1 (carboxyl group) and O3 (carbonyl group) is 92.9°. The distances of Cu1–O1 and Cu1–O3 are 1.920(7) Å and 1.914(9) Å respectively, which are all considerably shorter than the Cu1–O4 (1.937(1) Å) distance indicating that the coordination bond between copper and water molecule (O4) is relatively weaker than the chelate bonds between copper and the 2-OHNA ligand (O1 and O3). The bond lengths and angles between the nicotinic acid and Cu are comparable with structurally characterized complexes previously reported [14,15,16].

Fig. 2

ORTEP diagram (50% probability) of complex 1 with atom numbering

Further, hydrogen bondings are observed as weak supramolecular interactions to stabilize the lattice structure (Fig. 3). First, the adjacent mononuclear structures are connected by the hydrogen bonding interaction formed by N1 of the pyridine ring and O2 derived from the carboxyl group of the adjacent 2-OHNA anion with the N1–H1···O2 (− x + 1, y − 1/2, − z + 1/2) distance of 2.825(2) Å to form a 1D supramolecular chain. Second, the solvent water molecules along with the coordinated water molecules connect the neighboring 1D chains to generate 2D layers by the hydrogen bonding interactions (O5–H10···O2 (− x + 1, y + 1/2, − z +1 /2) = 2.798(2) Å, O4 − H9···O5 (x, y − 1, z) = 2.732(2) Å, O4–H8···O5 (− x + 2, y + 1, − z) = 2.758(2) Å). At last, the 2D layers are stacked by solvent water hydrogen-bonding to Cl to form a 3D supramolecular structure with the O5–H11···Cl1 (− x + 2, y + 1/2, − z + 1/2) distance of 3.193(6) Å. All hydrogen bonds are shown in Table 3.

Fig. 3

a 2D layer along (1 0 2) formed by hydrogen bonds. b Stacked layers with solvent water hydrogen-bonding to Cl. c Packing and unit cell, view along a (left) and c (right)

Table 3 Hydrogen bonds lengths and bond angles (Å, °) of 1

Catalytic Activity of [CuCl(2-OHNA)H₂O]·H₂O (1)

Selective oxidation of benzyl-alkanes is a powerful tool to generate high value organic compounds from less expensive raw materials and represents one of the formidable challenges in organic chemistry [17, 18]. However, a few catalysts have shown catalytic activity in the oxidation of benzylic C–H bonds with unsatisfactory catalytic activity [19,20,21]. With the aim to understand the catalytic property of the title coordination polymer for developing a new oxidation system in organic chemistry, the oxidation of diphenylmethane to benzophenone was selected as a model reaction (Scheme 2). While molecular oxygen and hydrogen peroxide were inefficient to promote diphenylmethane oxidation, TBHP was a good oxidizing reagent in terms of the percentage yield of benzophenone. Thus, TBHP was chosen as the best oxidant in the reaction. After the preliminary optimization, benzophenone is obtained as the almost sole product in 97.5% conversion using TBHP as oxidant in the presence of complex 1 at 80 °C for 24 h. And indeed, the catalytical activity of complex 1 outperforms those of some transition-metal-based complexes reported to date, i.e. [Cu₄L₄]₄X (L = 1,3,5-tris(1-benzylbenzimidazol-2-yl)benzene, X = ClO₄, OTs, OTf) [22], NHPI/Cu@PILC [23].

Scheme 2

The oxidation of diphenylmethane to benzophenone with TBHP

The excellent performance in the transformation of diphenylmethane to benzophenone prompted us to extend the scope of complex 1 as catalyst for other benzylic sp³ C–H bonds. As illustrated in Table 4, complex 1 exhibits excellent catalytic activity for oxidation of benzyl sp³ C–H bonds, fluorene and its derivates, 9H-xanthenes are all oxidized to the corresponding aryl ketones with great conversion rate (98.5–99.6%) and exceptional selectivity (97.5–99.5%). That is to say, complex 1 could facilitate the oxidation of benzylic sp³ C–H bonds and may serve as highly efficient and selective catalyst.

Table 4 Results of selective oxidation of benzylic sp³ C–H bonds compounds catalyzed by complex 1

Conclusion

In summary, we have synthesized a copper coordination complex [CuCl(2-OHNA)H₂O]·H₂O (1) by the reaction of CuCl₂·2H₂O with in situ generated 2-hydroxynicotinic acid. The introduction of Cu²⁺ cation make complex 1 more stable and can be used as catalysts in the selective oxidation of alkylbenzenes with high catalytic activity and high selectivity toward the corresponding aryl ketones. Intramolecular hydrogen-bonds help to form a 2D layers. Complex 1 shows excellent generality in converting benzylic sp³ C–H bonds compounds to corresponding aromatic ketones efficiently. Investigations on the use of this complex for other potential catalytic reactions are in progress.