Transition Metal (II) Coordination Chemistry Ligated by a New Coplanar Tridentate Ligand, 2,6-Bis(5-isopropyl-1H-pyrazol-3-yl)pyridine

Abstract

Transition metal (II) complexes stabilized by 2,6-di(pyrazol-3-yl)pyridine as a novel coplanar tridentate nitrogen-donor ligand have been reported for their unusual structures and photoluminescent properties. In this work, the ligand 2,6-bis(5-isopropyl-1H-pyrazole-3-yl)pyridine (denoted as L) and its transition metal (II) halogenido complexes viz [ZnCl₂(L)] (1), [ZnBr₂(L)] (2), [CuCl₂(L)] (3), and [CuCl(L)(thf)](PF₆) (4) were synthesized and characterized by single crystal X-ray crystal analysis. Its structures contained N–H groups in its pyrazole rings and hydrogen bonds between these N–H donors and the coordinated halogenide ions and lattice solvent molecules. Tautomers between 3-pyridyl and 5-pyridyl substitutes were also observed. In L, the N–H group at the pyrazole nitrogen was located adjacent to the pyridine ring to form hydrogen bonds with adjacent pyrazoles. However, on complexation, the H atoms at the pyrazole nitrogens are shifted remotely to the pyridine. The zinc (II) complexes [ZnCl₂(L)] (1) and [ZnBr₂(L)] (2) possessed distorted trigonal pyramidal structures in the solid state. By comparison, the copper (II) complexes [CuCl₂(L)] (3) and [CuCl(L)(thf)](PF₆) (4) adopted square pyramidal geometry with a Jahn–Teller distortion resulting from their d⁹ electron configurations.

1. Introduction

In recent years, N-heterocyclic ligands derived from pyridine, pyrazole, and imidazole have been investigated for their coordination behavior [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. N-heterocycles are π-electron deficient, behaving as π-acceptors and soft donor sites in transition metal complexes. By comparison, the π-excessive pyrazole is a poorer π-acceptor and hard donor [3].

N-heterocyclic ligand 2,2’:6’,2’’-terpyridine (terpy) has been extensively investigated over 30 years, paralleling the maturity of supramolecular chemistry [1,2,4,5,6,10,12,13,14] (Figure 1a). The terpy motif is a typical metal-binding domain containing three nitrogen-donor atoms. Another coplanar nitrogen-donor ligand is 2,6-bis(pyrazolyl)pyridine [3,7,11,17,18], which substitutes two pyrazole rings for the two terminal pyridine rings of the terpy structure, leading to differences in basicity and π-orbital energy, and therefore differences in kinetic stability, coordination structures, and physicochemical properties of its coordination metal complexes [7].

2,6-Bis(pyrazolyl)pyridines can be classified into two types: 2,6-bis(N-pyrazolyl)pyridine derivatives with covalent N_pyrazole–C_pyridine bonds (bispzpy) (Figure 1b), and 2,6-bis(1H-pyrazol-3-yl)pyridine derivatives with covalent C_pyrazole–C_pyridine bonds (bispzHpy) (Figure 1c). The former bispzpy ligand is aprotic and provides only one of the two pyrazole nitrogen atoms for coordination, while the latter bispzHpy is neutral on coordination with the N–H available for hydrogen bonding, or deprotonated, in which case both pyrazole nitrogens are available for coordination to metal ions [11].

Although these two types of ligands have been widely studied [7,11,17,18], in this work, we have focused on 2,6-bis(1H-pyrazol-3-yl)pyridine derivatives (bispzHpy) (Figure 1c). This ligand has a Lewis basic donor together with a Lewis acid N–H group and so can form many hydrogen bonds in the solid state. Complexes of 2,6-bis(1H-pyrazol-3-yl)pyridine (bispzHpy) derivatives have been employed as catalysts [18,19,20,21] and functional materials [11,22,23,24]. Most common derivatives possess methyl, trifluoromethyl, phenyl, or tertiary butyl substituents on the pyrazolyl rings [18,19,20,21,22,23,24], but the isopropyl substituted ligand has not hitherto been reported. We previously studied different substituents on the pyrazolyl rings of the tripodal nitrogen-containing hydrotris(pyrazolyl)borate ligands to explore the influence of their bulkiness and electronic characteristics on the structure and reactivity of metal complexes [25,26,27,28,29]. Copper (II) complexes of bispzpy ligands bearing methyl and isopropyl-substituted pyrazoles have been reported [17].

We herein report the synthesis and characterization of a new 2,6-bis(1H-pyrazol-3-yl)pyridine (bispzHpy) ligand (Figure 1c), viz 2,6-bis(5-isopropyl-1H-pyrazole-3-yl)pyridine (denoted as L), and of four transition metal (II) halogenido complexes, [ZnCl₂(L)] (1), [ZnBr₂(L)] (2), [CuCl₂(L)] (3), and [CuCl(L)(thf)](PF₆) (4), in order to evaluate the influence of this bulky substituent on the structural, physicochemical, and spectroscopic properties of the resulting metal (II) complexes.

2. Results and Discussion

2.1. Synthesis of Ligand

2,6-Bis(5-isopropyl-1H-pyrazol-3-yl)pyridine (L) was synthesized by a modification of a procedure reported by Ikariya et al. [30] (Figure 2). The bis-β-diketone intermediate, 2,6-bis(1,3-dioxo-4,4-dimethylpentyl)pyridine, was synthesized by condensation of 3-methyl-2-butanone and diethyl 2,6-pyridinedicaraboxylate in an approximate 1:20 ratio of β-keto/enol tautomers in chloroform, judging from the integration of the broad signals at 15.8 ppm, which is assignable as the enol OH proton (Figure S1). This assignment was confirmed by the broad peak at ca. 16 ppm of 2,6-bis(1,3-dioxo-3-phenyl-1,3-propyl)pyridine [22]. Further treatment of this bis-β-diketone intermediate with hydrazine monohydrate in ethanol yielded the expected 2,6-bis(5-isopropyl-1H-pyrazol-3-yl)pyridine (L). This ligand was insoluble in dichloromethane, chloroform, and n-heptane but soluble in polar solvents such as acetone and methanol because of the N–H groups on the pyrazolyl rings. The broad ¹H NMR signal in (CD₃)₂CO solution at 12.37 ppm is assignable to these N–H groups (Figure S2). This value is commensurate with the corresponding peaks observed at 12.57 ppm for 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine and at 13.61 ppm for 2,6-bis(5-phenyl-1H-pyrazol-3-yl)pyridine [22]. The ¹³C-NMR spectrum of this ligand is shown in Figure S3. However, in the CD₃OD solution, more complicated ¹H NMR signals were observed because the 2,6-di(pyrazol-3-yl)pyridine framework exists as a mixture of 3-pyridyl and 5-pyridyl tautomers (Figure 3) [7,11,31,32,33,34,35,36,37].

2.2. Synthesis of Complexes

The reaction of ligand L with one equivalent of zinc (II) chloride, zinc (II), bromide, and copper (II) chloride dihydrate in methanol afforded the dichlorido complexes [ZnCl₂(L)] (1), [ZnBr₂(L)] (2) and [CuCl₂(L)] (3), respectively, in high yields (Figure 4). In addition, the reaction of 3 with an equimolar amount of the AgPF₆ in methanol afforded the monochlorido copper (II) complex [CuCl(L)(thf)](PF₆) (4) after recrystallization from thf/heptane. These complexes were insoluble in nonpolar solvents and soluble in polar solvents, mirroring the solubility of ligand L. The ¹H NMR spectra of the zinc (II) complexes 1 and 2 in (CD₃)₂CO solution showed the appropriate signals, which were shifted relative to L (vide infra). EPR spectra were recorded for the paramagnetic copper (II) complexes 3 and 4 and indicated mononuclear structures (vide infra).

2.3. Structure of Ligand

A single crystal X-ray diffraction study showed that L crystallized in a tetragonal system with I4₁/a (#88) space group (Table S1, Figure 5). Both pyrazole rings adopted the pyrazol-5-yl tautomer in the solid state (Figure 3), with the N–H group adjacent to the pyridine ring. This trend has been previously reported for 2,6-bis(3-phenyl-1H-pyrazol-5-yl)pyridine [34] with a cis-cis configuration of the terminal nitrogen atoms relative to the central pyridine ring [1,33,34,35]. This contrasts with the coplanar trans-trans configuration of terpy [35] and 2,6-bis(N-pyrazolyl)pyridine (bispzpy) [1,17]. Hydrogen bonding in the solid state results in a cis-cis configuration for bispzHpy (Figure 5). In bispzHpy, this configuration is likely driven by the interlocked hydrogen bonding network that results (vide infra). Complexes are arranged in tetranuclear stacks, with each offset from its neighbors giving rise to effective offset face-to-face interactions (Figures S4 and S5).

The bonds linking the rings in both L and 2,6-bis(3,5-diisopropyl-N-pyrazolyl)pyridine (denoted as L1py) [17] were in the single bond range for sp² carbons and carbons C6–C13, 1.4636(18) and C12–C17, 1.4623(19) Å. The N–N bond lengths were N1–N2, 1.3574(16); N3–N4, 1.3503(17) for L. The C–N bond length in the pyridine ring was N5–C13, 1.3399(17), N5–C17, 1.3434(17), also matching the literature values [1,33,34,35]. The angles between these bonds linking the rings were different. The pyrazolyl rings were almost coplanar with the pyridine ring, with dihedral angles of 2.2 and 4.6°. By comparison, the dihedral angles between the two pyrazole rings were 55.9° for 2,6-bis(3,5-diisopropyl-N-pyrazolyl)pyridine [17] and 7.1° for the equivalent angle in terpy [1]. Therefore, this bispzHpy type ligand would lead to planar coordination with metal (II) ions.

2.4. Structures of Halogenido Metal (II) Complexes

The solid-state structure of [ZnCl₂(L)] (1) is similar to that of [ZnBr₂(L)] (2). Both 1 and 2 crystallized in the monoclinic space group I2/a (#15) with 2 equiv. of MeOH as a solvate (Table S1, Figure 6 and Figures S6–S9) and the crystal structure of both complexes is bisected by a mirror plane through Zn, N5, and C9 with symmetry operators: –X + 1/2, Y, –Z. Both complexes were five-coordinated structures involving the three nitrogen atoms of N2, N2’, and N5 and two halogenide ions, Cl1 and Cl1’, for 1 and Br1 and Br1’ for 2. The coordination geometry was a distorted trigonal bipyramidal geometry, which was characterized by the τ₅ structural parameter, 0.43 for 1 and 0.42 for 2, where the structural parameter τ₅ was calculated using the equation (β − α)/60, where α and β are the largest angles (β > α) around a five-coordinate metal center [36]. The three nitrogen atoms were in the equatorial plane, and the two halogenide anion atoms occupied the axial positions.

The coordination bond distances for Zn–Npz were in the range of 2.2670(19) for 1 and 2.252(2) Å for 2, and the bond distances for Zn–Npy were 2.095(2) for 1 and 2.096(2) Å for 2. These Zn–Npz bond lengths were only slightly longer than those of Zn–Npy. This trend has previously been observed in other complexes with ligands of similar frameworks [21,22,30,35]. The Npz–Zn–Npz bond angles were 148.76(6)° for 1 and 148.85(7)° for 2, which was not influenced by the different halogenide ions. The Zn–Cl distance was 2.2655(4) Å with a Cl–Zn–Cl’ angle of 114.54(2)° compared to the longer Zn–Br distance of 2.4030(4) Å, and the more acute Br–Zn–Br’ angle of 113.25(2)°. These differences are caused by the relative size of these halogenide anions. This Zn–Cl distance (2.2655(4) Å) and Cl1–Zn1–Cl1’ angle (114.54(2)°) in 1 is almost the same with the corresponding distances (2.25 and 2.27 Å) and angle (112°) in [ZnCl₂(terpy)] [37].

In the molecular packing of these compounds, there were two types of hydrogen bonds, as illustrated in Figures S8 and S9: (i) O–H···X (O1–H1···Cl1’: 3.131(2) Å, 164.97° for 1 with symmetry operator X, –Y + 1, +1, and O1–H1D···Br1’: 3.255(3) Å, 141.75° for 2 with symmetry operator –X + 1, –Y + 1, –Z + 1) between the oxygen from the crystalline methanol molecule and the halogenide anions and (ii) N–H···O (N1–H1D···O1: 2.763(2) Å, 172.84° for 1 and N1–H1A···O1: 2.765(3) Å, 172.74° for 2 between the nitrogen from pyrazolyl ring and the oxygen atom from the crystalline methanol molecule. These hydrogen bonds stabilize the solid-state structures and create an infinite zig-zag chain structure.

Complex 3 crystallized in an orthorhombic space group with P2₁2₁2₁ (#19), including 3 equiv. of MeOH as crystalline solvates (Table S2 and Figure 7a and Figures S10–S11). The coordination environments of the copper atom are depicted in Figure 6a. The copper (II) is coordinated by three nitrogen atoms (N2, N4, N5) from L and two Cl atoms to form a square pyramidal geometry, which is best described based by the τ₅ parameter = 0.08 [36]. In the molecular structure, the planar three nitrogen atoms from L and one Cl anion (Cl2) occupy the equatorial (basal) plane, and the other Cl atom (Cl1) is in the axial position. The distance between the copper (II) ion and its basal plane is 0.30 Å. The Cu–Npz bond lengths (2.022(2) and 2.013(2) Å) are slightly longer than those of Cu–Npy (1.986(2) Å), as observed in the corresponding zinc (II) complexes 1 and 2. However, these bond lengths are considerably shorter than those of the Zn (II) complexes due to the ionic radius difference. The Npz–Cu–Npz bond angle is 155.14(10)° and the Cl1–Cu1–Cl2 angle is 108.18(3)°. The Cu1–Cl1 distance (2.5963(8) Å) is much longer than that of Cu1–Cl2 (2.2096(8) Å) due to the Jahn–Teller distortion in this d⁹ complex [38]. Moreover, the Cu–Cl distances (2.5963(8) and 2.2655(4) Å) and the Cl1–Cu1–Cl1’ angle (108.18(3)°) in 1 are almost the same as the corresponding distances (2.469 and 2.252 Å) and angle (104.5°) of [CuCl₂(terpy)] [38] and the corresponding structural parameters (2.4167(7) and 2.2587(7) Å, 115.96(3)°) of [CuCl₂(L1py)] [17]. The dihedral angles between the two pyrazole rings were 9.88° with planar coordination to the copper (II) ion.

The molecular packing of 3 (Figure S11) contains three hydrogen bonds, (i) O–H···X (O2–H2···Cl1: 3.134(3) Å, 171.22° and O1–H1D···Cl1’: 3.083(5) Å, 112.39° with symmetry operator: X + 1/2, –Y + 1/2 + 2, –Z) between the oxygen from the crystalline methanol molecule and the halogenide anions, (ii) N–H···O (N1–H1···O3: 2.693(4) Å, 174.29° and N3–H3···O2’’: 2.767(4) Å, 163.33° with symmetry operator –X + 1/2 + 1, –Y + 2, Z + 1/2–1) between the nitrogen from the pyrazolyl ring and the oxygen atom of the crystalline methanol molecule, and (iii) O–H···O (O1–H1D···O3: 2.592(6) Å, 84.31° and O3–H3A···O1: 2.592(6) Å, 161.04°) between oxygens from two crystalline methanol molecules.

Complex 4 crystallized in a monoclinic space group with C2/c (#15) with 2 equiv. of THF as coordinated solvent and crystalline solvate (Table S2 and Figure 7b and Figures S12–S13). The coordination environments of the copper atom are depicted in Figure 6b. The copper (II) is coordinated by three nitrogen atoms (N2, N4, N5) from L, one Cl atom in the equatorial (basal) plane, and an oxygen atom of thf (O1) in the axial position to form a square pyramidal geometry, which is best described by τ₅ parameter = 0.17 [36]. The distance between the copper (II) ion and its basal plane is 0.20 Å. The Cu–Npz bond lengths (2.0086(17) and 2.0096(17) Å) are slightly longer than those of Cu–Npy (1.9770(16) Å), as observed in zinc (II) and copper (II) complexes 1–3. However, these bond lengths are considerably shorter than those of the Zn (II) complexes due to the difference in ionic radius. The Npz–Cu–Npz bond angle is 156.43(7)°. In 4, the Cu1–O1 distance (2.2905(19) Å) is much longer than that of all other bond distances due to the Jahn–Teller distortion in this d⁹ complex. The dihedral angle between the two pyrazole rings was 6.04° with planar coordination to the copper (II) ion.

In the dimeric structure of 4 (Figure S13), there are two hydrogen bonds, (i) N–H···Cl (N3–H3A···Cl1’: 3.1947(17) Å, 159.14° with symmetry operator –X, Y, –Z + 1/2) between the nitrogen from the pyrazolyl ring and the halogenide anions, and (ii) N–H···O (N1–H1···O2: 2.694(3) Å, 157.67°) between the nitrogen from the pyrazolyl ring and the oxygen atom from the crystalline thf molecule. These hydrogen bonding interactions stabilize the solid-state dimeric structure.

2.5. Infrared and Far-Infrared Spectroscopy

IR spectral data for the free ligand L and the zinc (II) complexes [ZnCl₂(L)] (1) and [ZnBr₂(L)] (2) and copper (II) complexes [CuCl₂(L)] (3) and [CuCl(L)(thf)](PF₆) (4) together with their assignments, are listed in the experimental section, and these spectra are shown in Figure S14. Raman spectra are shown in Figure S15. Spectral assignments were made with reference to the corresponding spectra of [ZnCl₂(2,6-benzimidazol-2-yl)] [39]. The N–H stretching of the ligand L at 3069 cm⁻¹ undergoes a slight shift on complexation at 3181 cm⁻¹ for 1, 3197 cm⁻¹ for 2, 3152 cm⁻¹ for 3, and 3151 cm⁻¹ for 4, suggesting that the pyrazole protons remain attached at the N1 and N3 positions. The broad stretching vibration is likely due to intramolecular hydrogen bonding. In addition, broad O–H stretching bands were also observed around 3400 cm⁻¹. The characteristic C–H stretching modes of the ring residues are about 3100 cm^−1, with the C–H stretching vibrations of the isopropyl residue below 3000 cm⁻¹.

The lower frequency region (Figure 8) is characteristic of M–X (M = Zn and Cu, X = Cl and Br) stretching. The most intense bands appeared at 301 and 278 cm⁻¹ for [ZnCl₂(L)] (1), 232 and 200 cm⁻¹ for [ZnBr₂(L)] (2), 341 and 307 cm⁻¹ for [CuCl₂(L)] (3), and 351 cm⁻¹ for [CuCl(L)(thf)](PF₆) (4). The corresponding stretching bands are observed at 287 and 278 cm⁻¹ for [ZnCl₂(terpy)], 222 and 213 cm⁻¹ for [ZnCl₂(terpy)] [40], 331 and 296 cm⁻¹ for [ZnCl₂(py)₂] and 260 and 213 cm⁻¹ for [ZnBr₂(py)₂] [41]. For 3, the higher energy shift at 341 and 307 cm⁻¹ is due to the different metal (II) ion. Only one Cu–Cl band at 351 cm⁻¹ was observed in the spectrum of 4 due to the different coordination atom pair. The ν(P-F) absorption bands in 4 appeared at 840 and 558 cm⁻¹ [42].

2.6. ¹H-NMR Spectroscopy

¹H and ¹³C NMR data for the free ligand L and the zinc (II) complexes [ZnCl₂(L)] (1) and [ZnBr₂(L)] (2) together with their assignments are listed in the experimental section, and the corresponding spectra are shown in Figures S2, S3 and S16–S19. As mentioned above, the ligand L exhibits a singlet broad peak at 12.3 ppm assigned to the N–H group. Signals for the two equivalent isopropyl groups are located at 3.04 and 1.31 ppm, while the pyridine moiety was associated with two resonances, a triplet at 7.84 and a doublet at 7.77 ppm. After coordination of zinc (II) halogenide, the broad signals at 12.8 ppm for both 1 and 2 (Figures S16 and S17) were slightly shifted due to the change of the N–H moiety from the 5-pyridyl tautomer to the 3-pyridyl tautomer (Figure 3). The corresponding signals also shifted from 3.04 and 1.31 ppm for L to 3.23 and 1.40 ppm for 1, and 3.24, 1.40 ppm for ,; and from 7.84 and 7.77 ppm for L to 8.23 and 7.98 ppm for 1 and 8.26, 8.01 ppm for 2. However, the ¹H-NMR signals of L in protonic solvents such as CD₃OD were clearly broadened (Figure S18) due to pyrazole protons in the 3-pyridyl and 5-pyridyl tautomers (Figure 3). To see if the hydrogen bonds to the pyrazole caused this broadening, the reaction of the bispzHpy ligand with NaH was performed. These signals narrowed by the reaction of 2 equivalents of NaH with L (Figure S19).

2.7. EPR Spectroscopy

The EPR spectra of [CuCl₂(L)] (3) and [CuCl(L)(thf)](PF₆) (4) were acquired as MeOH glass samples at 130 K (Figure 9) and in the solid state (Figure S20). The g values of the MeOH samples at 130 K were g_‖ > g_⊥ > 2.00, having a large A_‖ value (15.8 mT for 3 and 16.7 mT for 4). All EPR signals were consistent with typical square-based copper (II) complexes, having a dx²−y² ground state, matching the crystal structures of 3 and 4 [17,36,38]. This is consistent with the square pyramidal geometry and τ₅ parameter = 0.08 for 3 and 0.17 for 4. In the solid-state, EPR signals of both [CuCl₂(L)] (3) and [CuCl(L)(thf)](PF₆) (4) were broadened with g values g_‖ 2.26; g_⊥ 2.07 for 3 and g_‖ 2.21; and g_⊥ 2.08 for 4 (Figure S20).

2.8. UV-Vis Absorption Spectroscopy

Solution UV-Vis spectra of ligand L, [ZnCl₂(L)] (1), [ZnBr₂(L)] (2), [CuCl₂(L)] (3), and [CuCl(L)(thf)](PF₆) (4) are shown in Figure 10 and Figure S21. The π-π* and n-π* transition bands of the pyridine and pyrazole at 242 and 307 nm were red-shifted on complexation [17,19,20], viz 254 and 319 nm for 1, 254 and 319 nm for 2, 256 and 326 nm for 3, and 255, 324 nm for 4. The d–d transition bands appeared at 700 nm for 3 and 713 nm for 4. Solid UV-Vis and diffuse reflectance (DR) spectra for all complexes were plotted (Figures S22–S25), and the d-d transition peaks of [CuCl₂(L)] (3) and [CuCl(L)(thf)](PF₆) (4) were slightly shifted to 724 nm for 3 and 670 nm for 4 from the equivalent d-d transition absorption bands in MeOH solution (~710 nm). This small difference is likely due to distortion in the solid-state structures caused by hydrogen bonding, resulting in a small geometrical change. In solution, the influence of this hydrogen bonding would be weak. This difference was also observed in DR spectra (Figure S25) at 724 nm for 3 and 690 nm for 4.

3. Materials and Methods

3.1. Material and General Techniques

Ligand L and metal (II) complexes 1–4 were prepared and handled using Schlenk tubes under an argon atmosphere. Tetrahydrofuran (THF) was dried by refluxing over sodium benzophenone ketyl and distilled before use. Super dehydrated methanol (MeOH), ethanol (EtOH), and ethyl acetate (EtOAc) were degassed by argon bubbling before use. Other reagents were commercially available and used without further purification.

3.2. Instrumentation

Elemental analyses (C, H, and N) were performed by the Open Facility Centre for Research of Ibaraki University. IR spectra were acquired as KBr pellets in the 4000–400 cm⁻¹ region using a JASCO FT/IR-6300 spectrophotometer (JASCO, Tokyo, Japan). Far-IR spectra were recorded as CSI pellets in the 680–150 cm⁻¹ region using a JASCO FT/IR-6700 spectrophotometer (JASCO, Tokyo, Japan). FT-Raman spectra were recorded as powders on slide glass in the 4000–100 cm⁻¹ region using a JASCO FT/IR-6300 spectrophotometer (JASCO, Tokyo, Japan) with a 600 mW YAG laser. ¹H (500.13 MHz) NMR spectra were acquired on a Bruker AVANCE III-500 NMR spectrometer (Bruker Japan, Yokohama, Japan) at room temperature (298 K) in CDCl₃-d₁ or (CD₃)₂CO-d₆. ¹H NMR shifts are relative to residual, CD₃OD (δ 3.31) and (CD₃)₂CO (δ 2.05) signals, respectively. ¹³C (125.77 MHz) NMR spectra were obtained on the same instrument, and shifts are relative to the signal of the residual CD₃OD (δ 49.00). Room temperature UV-Vis spectra (MeOH solution in 200–1040 nm) were measured with a JASCO V-570 spectrophotometer using a quartz cell (0.10 cm path length) (JASCO, Tokyo, Japan). Solid samples for UV-Vis spectroscopy were prepared as Nujol mulls (Nujol, poly(dimethylsiloxane), viscosity 10,000) (Aldrich, Milwaukee, WI, USA)) and run on quartz plates equipped with an integrating sphere apparatus (JASCO ISN-470 Tokyo, Japan). Diffuse reflectance (DR) spectra were obtained by finely grinding microcrystalline material into powders at 200–1000 nm on a JASCO V-570 spectrophotometer equipped with an integrating sphere apparatus (JASCO ISN-470) (JASCO, Tokyo, Japan). EPR spectra were acquired on a JEOL JES-X320 EPR spectrometer (JEOL, Tokyo, Japan) in frozen MeOH glass at 130 K in quartz tubes (diameter 5 mm) at a frequency of ~9.2 GHz, a microwave power of 1 mW, a field modulation 0.1 mT, and time constant 0.03 sec. Solid samples were run as fine powders in 5 mm quartz tubes.

3.3. Preparation of Ligand and Complexes

• 2,6-Bis(3-isopropyl-1H-pyrazol-5-yl)pyridine (bispzHpy) (L)

3-Methyl-2-butanone (3.50 mL, 32.7 mmol) was added to a suspension of sodium hydride (0.768 g, 32.0 mmol) in thf (40 mL) and the mixture was stirred for 40 min at room temperature and then heated to reflux for 20 min. Diethyl 2,6-pyridinedicaraboxylate (2.384 g, 10.7 mmol) in thf (10 mL) was added to the boiling mixture over 15 min, and the mixture was maintained at reflux for 30 min. After cooling, a saturated sodium chloride solution (25 mL) was added, and 1 M hydrochloric acid was added until the pH reached 7 before extraction with ether (total 110 mL). The combined organic layer was washed with saturated sodium chloride solution (30 mL × 3) and dried over MgSO₄. Evaporation of the solvent in vacuo afforded 2,6-bis(1,3-dioxo-4-methylpenthyl)pyridine as a yellow oil (2.216 g, 7.31 mmol, 68%). Hydrazine monohydrate (2.0 mL, 41.23 mmol) in EtOH (10 mL) was added to a boiling solution of the bis-β-diketone intermediate in EtOH (30 min) over the course of 10 min, and the mixture was maintained at reflux for an additional 2 h. The solvent was evaporated to dryness, and the residue was triturated with dichloromethane until a colorless precipitate appeared. The resulting precipitate was collected and afforded L as a white powder (1.360 g, 4.60 mmol, 43%). Single crystals suitable for X-ray diffraction were obtained by slow recrystallization from EtOH.

Anal. Calcd for C₁₇H₂₁N₅: C 69.12, H 7.17, N 23.71. Found: C 68.94, H 7.20, N 23.50. IR (KBr) ν/cm⁻¹: 3214 (br, O–H, N–H, C–H), 2960 (C–H), 2929 (C–H), 2870 (C–H), 1602, 1579, 1562, 1479, 1465, 1306, 1280, 1173, 1150, 1107, 996, 981, 813, 798, 655. Far–IR (CsBr) ν/cm⁻¹: 658, 491, 427, 329. Raman (neat) ν/cm⁻¹: 3131 (O–H), 3069 (N–H), 2964 (C–H), 2929 (C–H), 2870 (C–H), 1602, 1581, 1482, 1463, 1457, 1410, 1373, 1282, 994, 959. ¹H NMR ((CD₃)₂CO) δ/ppm (assignment): 12.37 (br, 2H, N–H), 7.84 (t, 1H, 8 Hz, 4–pyH), 7.77 (d, 2H, 8 Hz, 3,5–pyH), 6.83 (s, 2H, 4–pzH), 3.04 (m, 2H, 7 Hz, CH(CH₃)₂), 1.31 (d, 12H, 7 Hz, CH(CH₃)₂). ¹³C-NMR ((CD₃)₂CO) δ/ppm (assignment): 151.1 (2–py), 138.4 (4–py), 118.7 (3–py), 100.9 (4–pz), 27.9 (CH(CH₃)₂), 23.1 (CH(CH₃)₂), 3,5-pz were not detected. UV-Vis (solution, MeOH) λ_max/nm (ε/M⁻¹cm⁻¹): 242 (24,000), 307 (12,900). UV-Vis (solid, Nujol) λ_max/nm: 280, 332. DR (neat) λ_max/nm: 288, 332.

• Reaction of L with NaH

L (164.5 mg, 0.557 mmol) in THF (10 mL) was added to a suspension of sodium hydride (26.6 mg, 1.108 mmol) in THF (6 mL) over 30 min. The mixture was stirred for 3 h at room temperature, and evaporation of the solvent in vacuo afforded a white powder.

¹H NMR (CD₃OD) δ/ppm (assignment): 7.84 (t, 1H, 8 Hz, 4–pyH), 7.70 (d, 2H, 8 Hz, 3,5–pyH), 6.77 (s, 2H, 4–pzH), 3.08 (m, 2H, 7 Hz, CH(CH₃)₂), 1.35 (d, 12H, 7 Hz, CH(CH₃)₂).

• [ZnCl₂(L)] (1)

BispzHpy (50.00 mg, 0.167 mmol) in MeOH (10 mL) was added to ZnCl₂ (22.5 mg, 0.171 mmol) in MeOH (5 mL), and the solution was stirred for 1 day at room temperature. Evaporation of the solvent under reduced pressure afforded a white residue. Recrystallization from MeOH (6 mL)/ EtOAc (8 mL) afforded [ZnCl₂(bispzHpy)] as colorless crystals (60.2 mg, 0.139 mmol, 84%). Single crystals suitable for X-ray diffraction were obtained by slow recrystallization from the same solvent.

Anal. Calcd for C₁₇H₂₁Cl₂N₅Zn·(H₂O): C 45.40, H 5.16, N 15.57. Found: C 45.18, H 4.94, N 15.37. IR (KBr) ν/cm⁻¹: 3466 (br, O–H), 3182 (N–H), 3128 (C–H), 3100 (C–H), 3070 (C–H), 2962 (C–H), 2934 (C–H), 2873(C–H), 1613, 1577, 1511, 1455, 1290, 1014, 818, 787. Far–IR (CsI) ν/cm⁻¹: 671, 640, 505, 302 (Zn–Cl), 276 (Zn–Cl). Raman (neat) ν/cm⁻¹: 3136 (O–H), 3058 (N–H), 2976 (C–H), 2933 (C–H), 2895 (C–H), 1613, 1576, 1421, 1020, 975, 270. ¹H NMR ((CD₃)₂CO) δ/ppm (assignment): 12.83 (br, 2H, N–H), 8.23 (t, 1H, 8 Hz, 4–pyH), 7.98 (d, 2H, 8 Hz, 3,5–pyH), 6.90 (s, 2H, 4–pzH), 3.23 (m, 2H, 7 Hz, CH(CH₃)₂), 1.40 (d, 12H, 7 Hz, CH(CH₃)₂). UV-Vis (solution, MeOH) λ_max/nm (ε/M⁻¹cm⁻¹): 237 (19,800), 254 (18,500), 319 (11,900). UV-Vis (solid, Nujol) λ_max/nm: 256, 332. DR (neat) λ_max/nm: 266, 338.

• [ZnBr₂(L)] (2)

BispzHpy (97.9 mg, 0.331 mmol) in MeOH (10 mL) was added to ZnBr₂ (75.9 mg, 0.337 mmol) in MeOH (5 mL) and the solution was stirred for 1 day at room temperature. Evaporation of the solvent in vacuo afforded a white residue. Recrystallization from THF afforded [ZnBr₂(bispzHpy)] as a colorless solid (96.6 mg, 0.186 mmol, 56%). Crystals, suitable for X-ray analysis, were obtained by recrystallization from MeOH/EtOAc.

Anal. Calcd for C₁₇H₂₁Br₂N₅Zn: C 39.22, H 4.07, N 13.45. Found: C 39.08, H 3.91, N 13.39. IR (KBr) ν/cm⁻¹: 3498 (br, O–H), 3303 (br, O–H), 3197 (N–H), 3125 (C–H), 3096 (C–H), 3066 (C–H), 2968 (C–H), 2932 (C–H), 2874 (C–H), 1613, 1575, 1508, 1455, 1280, 1010, 813, 796. Far–IR (CsI) ν/cm⁻¹: 671, 631, 588, 503, 375, 232 (Zn–Br), 201 (Zn–Br). Raman (neat) ν/cm⁻¹: 3314 (O–H), 3197 (O–H), 3133 (O–H), 3070 (N–H), 3056 (C–H), 2975 (C–H), 2932 (C–H), 2896 (C–H), 1612, 1575, 1452, 1421, 1020, 975, 883. ¹H NMR ((CD₃)₂CO) δ/ppm (assignment): 12.83 (br, 2H, N–H), 8.26 (t, 1H, 8 Hz, 4–pyH), 8.01 (d, 2H, 8 Hz, 3,5–pyH), 6.92 (s, 2H, 4–pzH), 3.24 (m, 2H, 7 Hz, CH(CH₃)₂), 1.40 (d, 12H, 7 Hz, CH(CH₃)₂). UV-Vis (solution, MeOH) λ_max/nm (ε/M⁻¹cm⁻¹): 237 (20,100), 254 (18,800), 319 (12,100). UV-Vis (solid, Nujol) λ_max/nm: 264, 330. DR (neat) λ_max/nm: 274, 338.

• [CuCl₂(L)] (3)

BispzHpy (75.4 mg, 0.255 mmol) in MeOH (5 mL) was added to CuCl₂‧2H₂O (43.8 mg, 0.255 mmol) in MeOH (10 mL) and the solution was stirred for 3 h at room temperature. Evaporation of the solvent in vacuo afforded a green powder. Recrystallization from MeOH/EtOAc afforded [CuCl₂(bispzHpy)] as green crystals (95.1 mg,0.204 mmol, 80%).

Anal. Calcd for C₁₇H₂₁Cl₂N₅Cu·2(H₂O): C 43.83, H 5.41, N 15.03. Found: C 44.01, H 5.33, N 14.78. IR (KBr) ν/cm⁻¹: 3520 (br, O–H), 3412 (br, O–H), 3152 (N–H), 3128 (C–H), 3092 (C–H), 3056 (C–H), 2968 (C–H), 2929 (C–H), 2877 (C–H), 1614, 1574, 1518, 1464, 1303, 1025, 821, 797. Far–IR (CsI) ν/cm⁻¹: 647, 584, 514, 475, 434, 377, 341 (Cu–Cl), 308 (Cu–Cl), 213, 201. UV-Vis (solution, MeOH) λ_max/nm (ε/M⁻¹cm⁻¹): 254 (17,300), 323 (6940), 700 (80). UV-Vis (solution, THF) λ_max/nm (ε/M⁻¹cm⁻¹): 256 (18,650), 326 (6890), 706 (70). UV-Vis (solid, Nujol) λ_max/nm: 342, 382, 432, 724. DR (neat) λ_max/nm: 226, 260, 342, 724. EPR (MeOH, 130 K): g_‖ 2.25 (A_‖ 15.8 mT); g_⊥ 2.07. EPR (solid, 130 K): g_‖ 2.26; g_⊥ 2.07.

• [CuCl(L)(thf)](PF₆) (4)

BispzHpy (51.3 mg, 0.174 mmol) in MeOH (10 mL) was added to CuCl₂‧2H₂O (30.7 mg, 0.180 mmol) in MeOH (5 mL) and the solution was stirred for 24 h at room temperature. To this mixture was added AgPF₆ (49.7 mg, 0.197 mmol) in MeOH (10 mL) and the solution was stirred for 5 h at room temperature. Evaporation of the solvent in vacuo afforded a green powder. Recrystallization from thf/heptane afforded [CuCl(bispzHpy)(thf)](PF₆) as blue crystals (98.0 mg, 0.160 mmol, 92%). Single crystals suitable for X-ray diffraction were obtained by slow recrystallization under the same mixed solvent.

Anal. Calcd for C₂₁H₂₈ClN₅CuF₆OP·2(H₂O): C 38.95, H 5.14, N 10.82. Found: C 39.23, H 4.63, N 10.55. IR (KBr) ν/cm⁻¹: 3527 (br, O–H), 3409 (br, O–H), 3151 (N–H), 3058 (C–H), 2971 (C–H), 2934 (C–H), 2878 (C–H), 1615, 1576, 1558, 1518, 1465, 1304, 1026, 840 (P–F), 558 (P–F). Far–IR (CsI) ν/cm⁻¹: 654, 558 (P–F), 469, 431, 379, 351 (Cu–Cl), 308, 202. UV-Vis (solution, MeOH) λ_max/nm (ε/M⁻¹cm⁻¹): 253 (15,400), 323 (7600), 710 (100). UV-Vis (solution, THF) λ_max/nm (ε/M⁻¹cm⁻¹): 255 (21,200), 324 (7800), 713 (90). UV-Vis (solid, Nujol) λ_max/nm: 256, 334, 670. DR (neat) λ_max/nm: 252, 260, 340, 690. EPR (MeOH, 130 K): g_‖ 2.25 (A_‖ 16.7 mT); g_⊥ 2.07. EPR (solid, 130 K): g_‖ 2.21; g_⊥ 2.08.

3.4. X-Ray Crystal Structure Determinations

Diffraction data of ligand L and its metal (II) complexes 1–4 were obtained on a Rigaku XtaLAB P200 diffractometer (Rigaku Oxford Diffraction, Oxfordshire, UK) using multilayer mirror monochromated MoKα radiation (λ = 0.71073 Å) at −95 ± 2 °C. A crystal of suitable size and quality was coated with Paratone-N oil (Hampton Research, Aliso Viejo, CA, USA) and mounted on a Dual-Thickness MicroLoop LD (200 µM) (MiTeGen, New York, NY, USA). The unit cell parameters were determined using CrystalClear from 18 images [43]. Data were collected at 0.5° intervals in ϕ and ω to a maximum 2θ value of 55.0°. The crystal-to-detector distance was ca. 45 mm. Data reduction, including empirical absorption correction, was accomplished with CrysAlisPro (Rigaku Oxford Diffraction, Oxfordshire, UK) [44]. The structures were solved by direct methods (SIR2008 [45]) and refined (anisotropic displacement parameters and C-bound H atoms in the riding model approximation) on F² [46]. All calculations were performed with the CrystalStructure [47] crystallographic software package except for refinement, which was performed using SHELXL 2013 [46]. Hydrogen atoms were placed in calculated positions. A weighting scheme of the form w = 1/[σ²(F_o²) + (aP)² + bP], where P = (F_o² + 2F_c²)/3, was applied toward the latter stages of each refinement. The absolute configuration of [CuCl₂(L)] (3) was confirmed from the values of the Flack parameters (0.002(2)) refined using 2408 Parsons’ quotients pairs [48]. The solvent molecules in [ZnCl₂(L)] (1) and [ZnBr₂(L)] (2) were too disordered to be modeled properly; thus, the program SQUEEZE, a part of the PLATON package of crystallographic software [49], was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. Crystallographic data and structure refinement parameters, including the final discrepancies (R and Rw), are listed in Tables S1 and S2. All crystallographic data have been deposited in the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, and copies can be obtained on request, free of charge, by quoting the publication citation and the deposition numbers (see Tables S1 and S2).

4. Conclusions

We have reported the synthesis of tridentate planar ligand 2,6-bis(5-isopropyl-1H-pyrazol-3-yl)pyridine and observed the influence of the ligand substituents on the resulting zinc (II) and copper (II) complexes. This ligand has an N–H group on its pyrazole ring, leading to 3-pyridyl and 5-pyridyl tautomers. This N–H group also reduced solubility in nonpolar solvents and increased solubility in polar solvents. Various hydrogen bonds between N–H groups and the coordinated halogenide ions/some lattice solvent molecules were observed in the solid-state structures. On complexation, the positions of the H atoms on the pyrazole nitrogens were shifted remotely to the pyridine (i.e., N1 and N3) from adjacent to the pyridine ring (N2 and N4). In the zinc (II) complexes, the coordination geometry of 1 and 2 are both distorted trigonal bipyramidal with the three nitrogen atoms (N2, N2’, and N5) from L in the equatorial plane and two halogenide anion atoms in the axial positions. By contrast, in the copper (II) complexes, the coordination geometry of 3 and 4 is square pyramidal, with the basal plane occupied by three nitrogen atoms (N2, N4, and N5) from L, and the chloride ions are in the equatorial plane.

The structure and properties of these complexes provide useful data for further understanding of this tridentate planar ligand system and pave the way for the synthesis of new coordination complexes based on N3 tridentate planar ligands for improved catalytic performance and as models of the active sites of transition metal containing proteins with hydrogen bonding.