6,6′-Di-(8″-quinoline)-2,2′-bipyridine Cobalt(II) Complex

Abstract

This short note describes the synthesis of a novel compound, 6,6′-di-(8″-quinoline)-2,2′-bypyridine (2), that bears a bipyridyl backbone and quinoline donors. Compound 2 coordinates with a cobalt(II) ion in a tetradentate manner and affords the complex [Co^II(2)(TfO)₂], whose molecular structure has been identified by single crystal X-ray diffraction crystallographic analysis. The coordination features of 2 were compared with those of 2,2′:6′,2″:6″,2‴-quaterpyridine (qtpy, 1), a well-studied tetradentate polypyridine ligand. Compound 2 presents a new example of tetradentate chelators for 3d metal ions.

1. Introduction

The 2,2′:6′,2″:6″,2‴-quaterpyridine (qtpy, 1) is a well-known N-heterocyclic scaffold and has been widely utilized as a tetradentate ligand for transition metal cations with respect to the coordination chemistry [1]. The interest in the quaterpyridine type of transition metal complexes has been rising in the past decade because of their emerging application in the development of sustainable energy [2,3,4,5]. The photovoltaic and catalytic performances of quaterpyridine metal complexes and derivatives are highly related to their coordination structures.

While the 4d metal ions such as Ru(II) show a tendency to coordinate with all four N donors of qtpy in a tetradentate manner [6,7]. The 3d metal ions can bind qtpy in either a tridentate or a tetradentate fashion [8,9,10]. This diverse coordination manner of qtpy partly originates from its chelate cavity that fuses three five-member chelate rings with the metal core (Scheme 1) and accommodates larger 4d metal ions better than smaller 3d metal ions [11]. Another reason is ascribed to the structural flexibility of qtpy, of which certain distortions may thermodynamically contribute to the tetradentate coordinating geometry between the metal center and qtpy, on the one hand. On the other hand, rotation of the terminal pyridines allows a tridentate binding mode of qtpy without a significant steric barrier. We reasoned that introducing quinoline groups as an alternative to pyridine subunits could reduce the cavity size of this ligand and facilitate its tetradentate coordination with 3d metal ions. This directs us to the design of the tetradentate 6,6′-di-(8″-quinoline)-2,2′-bipyridine ligand (2) that could form two six-member and one five-member chelate rings with the metal core (Scheme 1). Ligand 2 possesses the same extent of flexibility as qtpy, considering the freedom of quinoline group twisting.

This short note will describe the synthesis of the novel tetradentate ligand 6,6′-di-(8″-quinoline)-2,2′-bipyridine (2) that is able to chelate with a Co(II) center as an example of 3d metal ions in a tetradentate fashion.

2. Results and Discussion

The title compound (2) was synthesized in a one-step procedure (Scheme 2) from the commercially available starting materials 6,6′-dibromo-2,2′-bipyridine and 8-quinolineboronic acid (Scheme 2). Typical Suzuki–Miyaura coupling conditions were applied using [Pd(PPh₃)₄] as a catalyst and potassium carbonate as a base. The reaction proceeded smoothly in anaerobic toluene and afforded compound 2 as a white powder with a decent yield. This compound was characterized by both ¹H- and ¹³C-NMR spectroscopy (Figures S1 and S2).

We chose the cobalt(II) ion as a representative of 3d metal ions to study the coordination behavior of 2. Stirring equimolar amounts of 2 and [Co^II(CH₃CN)₂(TfO)₂] in acetonitrile at room temperature afforded a light orange crystalline solid as the major product (Scheme 2). Diffusion of diethyl ether into the acetonitrile solution of this product yielded prism crystals suitable for X-ray diffraction crystallographic analysis (Tables S1 and S2). The molecular structure of [Co^II(2)(TfO)₂] in its solid state clearly illustrates a tetradentate binding mode of 2 to the Co(II) center (Figure 1). The elemental analysis further verified the purity of the complex [Co^II(2)(TfO)₂]. The coordination geometry of this complex is distorted octahedral, with the 6,6′-di-(8″-quinoline)-2,2′-bipyridine (2) in the equatorial position and the two axial positions being occupied by two triflate anions. The four Co–N bond distances range from 2.078(4) to 2.108(4) Å. The two co-triflate bonds are 2.194(3) Å (Co–O2) and 2.226(3) Å (Co–O1) long. Significant twisting between the quinoline and bipyridine moieties could be observed, with dihedral angles of 39.0° (N2–C52–C59–C58) and 32.7° (C36–C37–C50–N3), respectively. The dihedral angle of the bipyridine backbone (N2–C56–C46–N3) is 18.2°.

The mononuclear qtpy (1) cobalt(II) complex, [Co^II(1)(H₂O)(SO₃)]⁺, has been reported in the literature with an established crystal structure [12]. Comparing the molecular structures of [Co^II(2)(TfO)₂] and [Co^II(1)(H₂O)(SO₃)]⁺ reveals some distinct differences between these two ligands binding with a Co(II) core. First of all, the four Co–N(1) bond distances (1.854~1.979 Å) in [Co^II(1)(H₂O)(SO₃)]⁺ are much shorter than those Co–N(2) bonds in [Co^II(2)(TfO)₂]. Secondly, the dihedral angles between the two external pyridines and the bipyridine backbone in [Co^II(1)(H₂O)(SO₃)]⁺ are 8.0° and 1.1°, respectively, dramatically smaller than the corresponding dihedral angles between quinolines and the bipyridine in [Co^II(2)(TfO)₂]. These findings indicate a greater distortion of the Co-2 coordination plan than the Co-1 plan. This higher extent of distortion for Co-2 is mainly attributed to the steric repulsion of the two H atoms at the two positions of the quinoline moieties in [Co^II(2)(TfO)₂].

The UV-vis absorption spectra of 2 and [Co^II(2)(TfO)₂] were recorded in acetonitrile and displayed in Figure S3. Compound 2 shows absorbance bands at λ_max = 240 and 303 nm, which derive from the π → π * electron excitation of the pyridyl and quinoline moieties. Metallization of 2 with a Co(II) center results in enhancement of the UV-vis absorption profile and a slight red-shift of the 303 nm band to 330 nm. An absorbance band at λ_max = 350 nm emerges in the spectrum of [Co^II(2)(TfO)₂]. The cyclic voltammogram (CV) of [Co^II(2)(TfO)₂] in DMF (Figure S4) shows a quasi-reversible feature at E_1/2 = −0.57 V (all potentials in this paper are referred to as the redox potential of the ferrocenium/ferrocene couple, Fc⁺/Fc, unless otherwise noted), which is assigned to a metal-based Co^III/II redox event. Further voltammetric scanning toward the cathode triggered three redox features at E = −1.16 V, E = −1.36 V, and E = −1.66 V (all peak potentials of irreversible waves). We propose that both the Co center and the ligand 2 contribute to these redox processes of [Co^II(2)(TfO)₂], considering the CV of 2 shows a reversible redox wave at E_1/2 = −2.38 V. The irreversibility of the series of redox features in the CV of [Co^II(2)(TfO)₂] is ascribed to the dissociation of triflate ligands and the association of solvent DMF molecules [13].

3. Materials and Methods

All air- and moisture-sensitive experiments were performed under a dry argon atmosphere using standard Schlenk techniques. The cobalt complex was prepared in a glovebox containing an atmosphere of purified dinitrogen. Dry solvents for moisture-sensitive experiments were purchased from commercial sources (water content ≤ 10 ppm) and used as received without further purification. [Co^II(MeCN)₂(CF₃SO₃)₂] was prepared according to the literature method [14]. Microwave syntheses were carried out using an Anton-Parr Monowave 200 microwave reactor. Water for syntheses and analysis was purified by the Milli-Q technique (18.2 MΩ). Thin-layer chromatography analyses were performed on silica gel-coated glass plates with the fluorescence indicator UV254.

¹H- and ¹³C-NMR spectra were recorded on a Bruker (Zurich, Switzerland) Avance NEO (600 MHz) spectrometer, operating at a probe temperature of room temperature. Chemical shifts, δ, are reported in ppm relative to the peak of SiMe₄, using the ¹H chemical shifts of the residual solvents as references [15]. UV-vis absorption spectra were recorded with a compact OTO Photonics (Taiwan) UV-vis spectrometer (SE2030-050-FUV). Elemental analysis (C N H) was performed on the Vario EL Cube (Langenselbold, Germany). Electrochemical voltammetry measurements were conducted on a Chinstruments CHI700E potentiostat/galvanostat with a glass carbon working electrode (ø = 3 mm), a Pt counter electrode, and an Ag pseudo reference electrode (a silver wire suspended in a 0.1 M DMF solution of TBAPF₆ and separated from the analyte solution by a frit). Tetra-n-butylammonium hexafluorophosphate (TBAPF₆) was used as a supporting electrolyte (0.1 M), and ferrocene was applied as an internal reference. All reported potentials in organic electrolytes were referred to the Fe^III/II redox potential of ferrocene (Fc).

Single crystal X-ray intensity data of [Co^II(2)(TfO)₂] were collected on an XtaLAB Synergy HyPix diffractometer equipped with Cu-Kα radiation (λ = 1.54184 Å) at 94.99(10) K. Absorption corrections were applied using the multi-scan program SADABS (version 2.03). The structure of [Co^II(2)(TfO)₂] was solved using the SHELXT-97 structure solution program (intrinsic phasing) and refined by least-squares procedures on F² using SHELXL [16]. Anisotropic thermal parameters were applied to all non-hydrogen atoms.

• Synthesis of 6,6′-di-(8″-quinoline)-2,2′-bipyridine (2).

8-Quinolinylboronic acid (332 mg, 1.92 mmol) was added to a solution of 6,6′-dibromo-2,2′-bipyridine (200 mg, 0.64 mmol) in a mixture of toluene (10 mL) and EtOH (10 mL). After degassing the solution with Ar, K₂CO₃ (600 mg, 4.34 mmol) and Pd(PPh₃)₄ (70 mg, 0.096 mmol) were added to this solution, and the resulting mixture was heated by a microwave reactor to 140 °C for 15 min under stirring. The solution was allowed to cool to room temperature, and the volatile components were removed under a vacuum. The residue was extracted with methylene chloride (50 mL × 3) three times. The combined organic layers were dried with anhydrous sodium sulfate. The solid salt was removed by filtration. The volume of the filtrate was reduced under vacuum. The slow addition of petroleum ether into the concentrated solution precipitated pure product 2 as a white powder (169.3 mg, 64.8%). ¹H-NMR (600 MHz, chloroform-d) δ 9.01 (dd, J = 4.1, 1.8 Hz, 2H); 8.63 (dd, J = 7.8, 1.1 Hz, 2H); 8.40 (dd, J = 7.2, 1.5 Hz, 2H); 8.28–8.21 (m, 4H); 7.96–7.91 (m, 4H); 7.74 (dd, J = 8.1, 7.2 Hz, 2H); 7.46 (dd, J = 8.3, 4.1 Hz, 2H). ¹³C-NMR (101 MHz, chloroform-d) δ 156.47, 156.06, 150.38, 139.20, 136.62, 136.34, 131.81, 128.89, 127.18, 126.73, 121.12, 119.84.

• Synthesis of [Co^II(2)(TfO)₂].

A solution of [Co^II(CH₃CN)₂(TfO)₂] (135.8 mg, 0.24 mmol) in about 3 mL CH₃CN was slowly added into a suspension of 2 (100 mg, 0.24 mmol) in about 7 mL CH₃CN under stirring. The color of the solution turned orange instantly. The reaction mixture was kept stirring overnight at room temperature. The insoluble solid was removed by filtration through a plug of celite. The solvent was removed under a vacuum. The crude product was dissolved in a small amount of CH₃CN again, and slow vapor diffusion of Et₂O into the CH₃CN solution afforded complex [Co^II(2)(TfO)₂] as light orange prism crystals (97 mg, 54.1%) suitable for single crystal X-ray diffraction analysis. Anal. Calcd. for [Co^II(2)(TfO)₂], (C₃₀H₁₈N₄CoF₆O₆S₂): C, 46.95; H, 2.36; N, 7.30. Found: C, 46.74; H, 2.17; N, 7.37.

4. Conclusions

The compound 6,6′-di-(8″-quinoline)-2,2′-bipyridine (2) and the corresponding complex [Co^II(2)(TfO)₂] have been synthesized and characterized. The ligand (2) coordinates with the Co(II) center in a tetradentate manner, forming two six-member and one five-member chelate ring as expected. Compared with the well-studied quaterpyridine ligand, ligand 2 distorts to a greater extent when chelating with the cobalt core. These findings here indicate compound 2 is a promising tetradentate ligand for 3d transition metals.