Synthesis and crystal structure of a homoleptic diruthenium complex containing tetra-2-pyridyl-1,4-pyrazine (tppz)

1 Introduction

Ruthenium(II) complexes containing polypyridyl ligand systems possess excellent redox and photophysical properties which have been studied in a broad range of optic and electronic application fields [1], [2], [3]. Tetra-2-pyridyl-1,4-pyrazine (tppz) is one of the most prominent examples for this class of ligands, and its synthesis and some transition metal complexes were first reported by Goodwin and Lions [4]. In this context, they also reported a compound of the composition [Ru(tppz)₂](ClO₄)₂·6H₂O, but the characterization was based on elemental analyses only. Later, another paper described the synthesis and spectroscopic characterization of a mononuclear and a dinuclear Ru(II) complex bearing tppz ligands [5]. The latter report is based mainly on spectroscopic investigations lacking a crystal structure determination. Furthermore, in 2004, Bernhard and co-workers described a combined experimental and computational density functional theory study of polynuclear [Ru_n(tppz)_n+1]²ⁿ⁺ complexes (n=1, 2, 3 and >5) again without crystal structure determination of any candidate of this series of complexes [6]. The latter species of interest showed properties as excellent molecular wire materials relevant to nanoelectronic applications [7]. Finally, an interesting donor–acceptor–structured diruthenium complex was described containing tppz as an electron-deficient bridging ligand along with electron-rich distal arylamines modified with long aliphatic chains [8]. This species exhibited tunable self-assembly and morphology-dependent photoconductivity effects. Since to date no crystal structure reports on homoleptic dinuclear ruthenium compounds containing the tppz ligand exist, herein, we report a modified synthetic pathway to the known complex [(tppz)Ru(μ-tppz)Ru(tppz)]⁴⁺ (1) and describe its molecular structure in the crystal.

2 Experimental section

3 Results and discussion

The synthesis of homoleptic complexes of the formula [Ru_n(tppz)_n+1]²ⁿ⁺ (n=1, 2, 3 and >5) was described in the literature reacting hydrated ruthenium(III) chloride with an excess tppz (molar ratio of Ru to tppz=1:4). First attempts were carried out in refluxing ethanol–water mixtures for 48 h [5]. Later a synthesis was reported by the reaction of the components for a short time by refluxing the educts in 1.2-ethanediol in a microwave oven [6]. The yields reported were 17%. We developed a similar preparation using refluxing ethoxyethanol as the solvent operating with a molar ratio Ru:tppz of 1:1.5 as found in the desired product complex [Ru₂(tppz)₃]⁴⁺. Adding an excess of sodium hexafluoridophosphate and purification by column chromatography with acetone as the eluent afforded the homoleptic ruthenium(II) compound 1(PF₆)₄ as a deep purple crystalline solid in 16% isolated yield. In the ¹H NMR spectrum of the title compound ([D₆]acetone), all characteristic signals were found as described elsewhere [6]. Suitable crystals for an X-ray diffraction study were grown as described above by slow diffusion overnight. Unfortunately, the compound could only be obtained as crystals containing toluene and acetonitrile as solvate molecules. The molecular structure of the cation is shown in Fig. 1, together with some selected bond lengths and bond angles in the caption.

Fig. 1:

Molecular structure of [Ru₂(tppz)₃]⁴⁺ (1) in the crystal. Displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ru1–N1 2.079(3), Ru1–N2 1.980(3), Ru1–N3 2.070(3), Ru1–N7 2.073(3), Ru1–N8 1.986(3), Ru1–N9 2.051(3), Ru2–N10 2.062(3), Ru2–N11 1.981(3), Ru2–N12 2.061(3), Ru2–N13 2.060(3), Ru2–N14 1.981(3), Ru2–N15 2.070(3), N1–Ru1–N2 78.53(11), N1–Ru1–N3 157.88(11), N1–Ru1–N7 100.15(11), N1–Ru1–N8 99.21(11), N1–Ru1–N9 88.23(11), N2–Ru1–N3 79.37(11), N2–Ru1–N7 102.02(11), N2–Ru1–N8 177.43(11), N2–Ru1–N9 99.73(11), N3–Ru1–N7 83.63(11), N3–Ru1–N8 102.90(11), N3–Ru1–N9 96.34(11), N7–Ru1–N8 79.51(11), N7–Ru1–N9 157.84(11), N8–Ru1–N9 78.90(11).

The coordination sphere around the two ruthenium atoms in the cationic complex 1 can be considered as nearly equivalent. The two ruthenium(II) centers exhibit a distorted octahedral geometry caused by the restricted bite angle of their two meridionally coordinated tridentate tppz ligands which are arranged nearly orthogonal to each other. Parts of the molecular structure are closely related to those of the known compound [(tpy)Ru(μ-tppz)Ru(tpy)](PF₆)₄ (tpy=terpyridine) which was recently described as an acetonitrile tetrasolvate [11]. Thus some important bonding parameters of both compounds should be compared (for 1 see the caption of Fig. 1). As found for the cationic complex 1 and also in [(tpy)Ru(μ-tppz)Ru(tpy)]⁴⁺, the equatorial N donor atoms of the bridging pyrazine ring exhibit slightly shorter distances to both central ruthenium atoms: Ru1–N5, 1.960(3) and Ru2–N8, 1.971(3) Å. The same is observed for the Ru–N distances in trans position to these bonds: Ru1–N2, 1.979(3) and Ru2–N11, 1.989(3) Å. All other Ru–N distances of the pyridine rings are generally slightly longer: Ru1–N1, 2.074(3); Ru1–N3, 2.071(3); Ru1–N4, 2.058(3) and Ru1–N6, 2.060(3) Å. The same is found for the second part of the dinuclear complex: Ru2–N7, 2.061(3); Ru2–N9, 2.051(3); Ru2–N10, 2.067(3) and Ru2–N6, 2.074(3) Å. Interestingly, calculated optimized geometrical parameters (Gaussian03) for [(tppz)Ru(μ-tppz)Ru(tppz)]⁴⁺ (see [6]) showed no remarkable differences concerning the corresponding bond lengths. Nearly equidistant Ru–N bond lengths have been reported (in the average 2.051 Å). Furthermore, solvent-free crystal structure determination of [(tpy)Ru(μ-tppz)Ru(tpy)](PF₆)₄ was reported [12]. In this work, the structure of the complex was further investigated by density functional theory calculations, and the results were in very good agreement with the experimental data.

In summary, we developed a modified synthetic pathway to the known homoleptic ruthenium(II) complex [(tppz)Ru(μ-tppz)Ru(tppz)]⁴⁺ (1), which has been characterized in the literature till now only by spectroscopic data. Fortunately, we were able to obtain crystals of 1(PF₆)₄ suitable for an X-ray diffraction study and confirmed its molecular structure.