Heteroleptic ruthenium(II) 2,2′-bipyridine complexes incorporating substituted pyrazol-1-yl-pyridazine ancillaries

1 Introduction

In recent years, ruthenium(II) complexes of the [Ru(bpy)₃]²⁺ (bpy=2,2′-bipyridine) family have attracted great attention due to their possible applications in bioinorganic chemistry and photo-induced electron transfer processes [1], [2], [3], [4], [5]. In comparison to the polypyridyl systems, research based on ruthenium-bpy complexes with the other N-containing heterocycles, e.g. imidazole, pyrazole and pyridazine, has been limited [6], [7], [8], [9]. As a matter of fact, coordination of such nitrogen-containing multi-dentate heterocyclic ligands and their derivatives to the ruthenium center alters the electron transfer properties of the obtained ruthenium complexes due to the different electron donor/acceptor properties of the coordinating nitrogen atoms [10], [11]. For example, ruthenium(II) complexes incorporating dianionic bipyrazolate ancillaries are suited for high-performance dye-sensitized solar cells [12]. On the other hand, (1-pyrazolyl)pyridazines and their derivatives are able to act as bridges between transition metal centers, such as nickel, cobalt, copper, silver and ruthenium [13]. According to searches in the Cambridge Structural Database (CSD) [14], tetradentate (1-pyrazolyl)pyridazine ligands may adopt five coordination modes when coordinated to transition metals, as shown in Chart 1 (see modes a–e), among which the μ₂,η⁴- (a) and η²- (b) fashions are more common [15], [16], [17], [18], [19]. A few ruthenium complexes bearing η²- and μ₂,η⁴-(1-pyrazolyl)pyridazine ligands have been reported, such as [(η⁵-Cp*)Ru(η²-L)(PPh₃)](PF₆) (L=3,6-bis(pyrazolyl)pyridazine) [20], [(η⁶-p-cymene)Ru(η²-L1)Cl](ClO₄), [(η⁶-C₆H₆)Ru(η²-L′)Cl](PF₆) (L′=3,6-bis(3,5-dimethylpyrazolyl)pyridazine) [21], [(η⁶-p-cymene)Ru(η²-L′)Cl](BPh₄) [22], and an oxo-centered triruthenium-acetate complex [Ru₃O(OAc)₅(py)-(μ₂,η⁴-L)](PF₆) (py=pyridine) [23]. Recently, we have reported the syntheses and phosphorescent properties of several heteroleptic ruthenium(II)/(III) polypyridine complexes with a series of substituted 2,2′-bipyridines l, Schiff base N,O-ligands and N,C-phenylphthalazines [24], [25], [26]. In this paper, we report the synthesis, structure and spectroscopic properties of three heteroleptic ruthenium(II)-bpy complexes with (1-pyrazolyl)pyridazine ligands (Scheme 2).

Chart 1:

Coordination modes of the (1-pyrazolyl)pyridazine ligands.

(a–e) Represents the five coordination modes.

2 Experimental section

3 Results and discussion

Previously, Thompson has reported an effective procedure for the synthesis of 3,6-bis(pyrazolyl)pyridazine by the treatment of pyrazole with potassium metal to produce the potassium pyrazolide, which was then reacted with 3,6-dichloropyridazine to afford the target compounds [15]. As shown in Scheme 1, condensation of 3,6-dichloropyridazine or 3,6-dichloro-4,5-dimethylpyridazine with 3-methyl-1H-pyrazole or 4-methyl-1H-pyrazole with the assistance of sodium metal afforded three substituted pyrazol-1-yl-pyridazine ligands L1‒L3, which were easily purified with good yields. As displayed in Scheme 2, interactions of cis-[Ru(bpy)₂Cl₂]·2H₂O with an equivalent of L1‒L3 in the presence of NH₄PF₆ led to the isolation of the corresponding heteroleptic ruthenium(II) complexes [Ru(L1)(bpy)₂](PF₆) (1), [Ru(L2)(bpy)₂](PF₆) (2) and [Ru(L3)(bpy)₂](PF₆) (3), respectively. The separate singlets for the two methyl groups in 1 and 2 (four methyl signals for 3) in their ¹H NMR spectra show that the structures are rigid in solution (no fluxionality of the bis(pyrazolyl)piperazine ligand). Moreover, the ³¹P and ¹⁹F NMR spectra of complexes 1–3 exhibit the fluorine signal as a doublet at about δ=–70 ppm and the phosphorus signal as a septet at about δ=–144 ppm, which are typical for (PF₆)^‒ anions.

Scheme 1:

Syntheses of ligands L1, L2 and L3.

Scheme 2:

Syntheses of heteroleptic ruthenium(II) complexes 1, 2 and 3.

The crystal structures of complexes 1·EtOH, 2·EtOH and 3 have been established by X-ray crystallography. The molecular structures of the heteroleptic ruthenium(II)-bpy complex cations are shown in Figs. 1–3. The central ruthenium atoms of the three complexes are in an octahedral coordination environment, with two bpy units and one L ligand. The pyrazol-1-yl-pyridazine (L) ligands in the three complexes all adopt the η²-coordination mode (Chart 1b), forming a stable five-membered metallocycle of RuCN₃, similar to those in other related half-sandwich ruthenium(II) complexes [20], [21], [22]. The Ru–N(bpy) bond lengths in 1·EtOH, 2·EtOH and 3 range from 2.021(12) to 2.089(11) Å. The average Ru–N(L) bond length of 2.051(8) Å in complex 3 is similar to those in complexes 1·EtOH (2.065(4) Å) and 2·EtOH (2.055(6) Å). The chelate bond angles of N(L)–Ru–N(L) in 1·EtOH, 2·EtOH and 3 are 78.23(17)°, 78.1(2)° and 77.5(4)°, respectively, which are slightly larger than those in the reported half-sandwich ruthenium(II) complexes (75.43°‒75.90°) [20], [21], [22]. A tendency for increased ruthenium–nitrogen bond lengths in the trans position with respect to the Ru–N(pz) bond is observed in 1·EtOH (2.064(4) Å vs 2.047(5) Å) and 2·EtOH (2.053(6) Å vs 2.047(6) Å), which is identical with that in related [Ru(bpy)₂(C^N)]⁺ complexes [31]. The three complexes have small N(bpy)–Ru–N(bpy) bond angles (78.47(19)°‒79.5(2)°), due to the rigidity of the bpy units. Views of the packing of the complexes 1·EtOH, 2·EtOH and 3 in the crystals are shown in Figs. 1b, 2b, and 3b, respectively. The organization of the components of 1·EtOH, 2·EtOH and 3 seems to be influenced by very weak interionic and intermolecular C–H⋯F and C–H⋯N hydrogen-bonding interactions (see Table 3). The separations H⋯N in the three compounds are in the range 2.45–2.66 Å, and the H⋯F distances in 1·EtOH, 2·EtOH and 3 are between 1.97 and 2.38 Å. The hydrogen bond angles C–H⋯N are 116.4° and 153.5° for complex 1·EtOH, 118.2° for complex 2·EtOH and 121.8° for complex 3. The C–H⋯F bond angles are in the range 153.5°–164.4° for complexes 1·EtOH, 2·EtOH and 3. Hydrogen bonding with the ethanol molecules certainly contributes to the stabilization of the crystal structures of complexes 1·EtOH and 2·EtOH.

Fig. 1:

(a) Molecular structure of the cation [Ru(L1)(bpy)₂]²⁺ of the complex 1·EtOH. The ethanol solvent molecule and the counter anions of PF₆^‒ are omitted for clarity. (b) Packing diagram of 1·EtOH in a unit cell, viewed along the crystallographic bc plane. C‒H···F and C‒H···N hydrogen bonds are shown as dashed lines.

Fig. 2:

(a) Molecular structure of the cation [Ru(L2)(bpy)₂]²⁺ of the complex 2·EtOH. The ethanol solvent molecule and the counter anions of PF₆^‒ are omitted for clarity. (b) Packing diagram of 2·EtOH in a unit cell, viewed along the crystallographic ab plane. C‒H···F and C‒H···N hydrogen bonds are shown as dashed lines.

Fig. 3:

(a) Molecular structure of the cation [Ru(L3)(bpy)₂]²⁺ of complex 3. The ethanol solvent molecule and the counter anions of PF₆^‒ are omitted for clarity. (b) Packing diagram of 3 in a unit cell, viewed along the crystallographic ab plane. C‒H···F and C‒H···N hydrogen bonds are shown as dashed lines.

The UV/Vis absorption spectra of the new heteroleptic ruthenium(II) complexes 1–3, together with that of the parent complex [Ru(bpy)₃](PF₆)₂, in CH₂Cl₂ at room temperature are shown in Fig. 4. At a first glance, the spectra are grossly similar and reveal three intense transitions located at 230–260, 270–330 and 410–470 nm. The strong absorption bands around 250 and 300 nm are assigned to typical spin-allowed π‒π* transitions of the ligands, and the comparatively less intense broad bands around 450 nm are ascribed to the metal-to-ligand charge transfer. These spectra are similar to that of [Ru(bpy)₃](PF₆)₂ and related ruthenium(II)-bpy complexes with additional N,N-donors [12], [24]. It is interesting to note that there is a red-shift of about 60 nm observed for the low-lying transition depending on the presence of coordination of the L ligands. This considerable shift suggests that the low-energy transition is the main signature of the charge transfer arising between the ruthenium atom and bpy ligand. The room temperature PL spectra of the heteroleptic ruthenium(II) complexes 1–3 and [Ru(bpy)₃](PF₆)₂ in the CH₂Cl₂ solution are illustrated in Fig. 5. The emission maximum for the complexes 1–3 appears at about 595 nm but with weaker intensity compared to the parent complex [Ru(bpy)₃](PF₆)₂, possibly due to a reduced intracationic π-electron conjugation system.

Fig. 4:

The UV/Vis absorption spectra of complexes 1–3 and [Ru(bpy)₃](PF₆)₂ in the CH₂Cl₂ solution.

Fig. 5:

Luminescence emission spectra of complexes 1–3 and [Ru(bpy)₃](PF₆)₂ in the CH₂Cl₂ solution (λ_ex=460 nm).

In summary, three new heteroleptic ruthenium(II) complexes incorporating substituted pyrazol-1-yl-pyridazine ancillaries were synthesized by the reaction of cis-[RuCl₂(bpy)₂]·2H₂O with 3,6-bis(pyrazolyl)pyridazine-type ligands in moderate yields. According to the CSD search results, only five ruthenium(II) complexes with 3,6-bis(pyrazolyl)pyridazine-type ligands have previously been reported [20], [21], [22], [23]. The structure determination of the three polypyridyl ruthenium(II) complexes confirmed that the 3,6-bis(pyrazolyl)pyridazine ligand coordinates to the ruthenium(II)-bpy moiety in the usual η²-coodination mode (Chart 1b) as an enrichment of the structural diversity of the limited series of this type of ruthenium complexes highlighting the specific features of the Ru–N(pz) and trans-Ru–N(py) bonds. The three crystal structures appear to be stabilized by C–H⋯N and C–H⋯F hydrogen-bonding interactions. The optical properties of the three dicationic complexes in solution show close similarities, but a significant difference from the parent [(bipy)₃Ru]²⁺ dication both in absorption and emission. No significant effect of the methyl substituents has been observed.