Crystal structure, electrochemical and spectroscopic investigation of mer-tris[2-(1H-imidazol-2-yl-κN 3)pyrimidine-κN 1]ruthenium(II) bis(hexafluoridophosphate) trihydrate

The first example of a homoleptic RuII complex with heteroaryl-imidazoles is reported in the meridional stereochemistry, exclusively. The supramolecular hydrogen-bonded network reveals mutual N—H⋯N bonds between adjacent complexes.


Chemical context
Since the first preparation of the tris(2,2-bipyridine) ruthenium(II) complex by Burstall (1936), its interesting electrochemical and photochemical properties have stimulated the preparation and characterization of numerous analogous ruthenium(II) complexes (Le-Quang et al., 2018;Dong et al., 2018;Linares et al., 2013). When asymmetric bidentate ligands are used to obtain homoleptic complexes, facial and meridional isomers can be obtained, depending on steric and electronic properties with important implications on chemical reactivity and spectroscopy (Metherell et al., 2014). An interesting class of asymmetric ligands are heteroaryl-imidazoles, since a combination of electron-rich and electron-poor rings can be used to tune the electronic properties of the final complexes (Ratier de Arruda et al., 2017;Nakahata et al., 2017).
In this context, we have devised a synthetic procedure to obtain exclusively the meridional isomer of the first reported homoleptic Ru II complex with the bidentate 2-(1H-imidazol-2-yl)pyrimidine (impm) ligand containing imidazole (im) and pyrimidine (pm) rings in the same unit. ISSN 2056-9890

Structural commentary
The title complex crystallizes with two hexafluoridophosphates counter-anions and three lattice water molecules. The total +2 charge for the complex is in very good agreement with molar conductivity and mass spectrometry measurements. We can conclude that all three ligands in the complex are neutral, not showing the typical ionization of the imidazole hydrogen atom. The molecular structure of the cationic complex is shown in Fig. 1. It reveals a distorted octahedral configuration with meridional stereochemistry, with two imidazole units trans to each other as well as two pyrimidine units trans to each other. There is no correlation between the trans-cis orientation and bond lengths. For example, all Ru-N im bond lengths are essentially the same within their standard uncertainties, and the same observation is valid for Ru-N pm bond lengths. However, Ru-N im bond lengths are systematically shorter than Ru-N pm bonds by 0.03 Å , as expected from the stronger Lewis basicity of the imidazole unit. Averaged bond lengths are 2.054 (10) Å for Ru-N im and 2.083 (8) Å for Ru-N pm . As a result of the bidentate nature of the ligands, coordination angles differ from the ideal 90 value with N im -Ru-N pm angles ranging from 78.5 (2) to 78.7 (2) , the latter being the main cause for the distorted octahedral configuration.

Supramolecular features
Although hydrogen atoms were not modelled for the three water molecules present in the crystal structure, it is clear that a three-dimensional hydrogen-bonded network is formed by all species. Water molecules cluster in triads and are close to two hexafluoridophosphate anions in the lattice. The supramolecular arrangement of water molecules and PF 6 À anions may result in different hydrogen-bonded patterns, and the disorder in hydrogen-atom positions may explain the absence of electron densities close to oxygen atoms in difference maps. Possible donor-acceptor pairs involving the water oxygen atoms are included in Table 1. One of the water molecules (O3) is hydrogen bonded to two N-H imidazole units, N6 and N10, Fig. 2  The molecular structure of the homoleptic cationic complex [Ru(L) 3 ] 2+ (L = C 7 H 6 N 4 ) with the atom-numbering scheme. Displacement ellipsoids are plotted at the 50% probability level.

Electrochemistry and electronic spectroscopy
The Ru III /Ru II potential for the [Ru(impm) 3 ] 2+ complex (0.87 V versus Ag/AgCl) was found to be intermediate between those reported for [Ru(im) 6 ] 2+ (0.295 V; Clarke et al., 1996), and [Ru(bpm) 3 ] 2+ (1.72 V; Ernst & Kaim, 1989), in which bpm stands for 2,2 0 -bipyrimidine. Since the reduction potential can be directly related to the t 2g orbitals of the complex, i.e. the HOMO (Possato et al., 2017;Eberlin et al., 2006;Nunes et al., 2006), the changes in potential can be accounted for by the high imidazole electron -donor ability, which tends to increase the energy of the HOMO, leading to a decrease of the reduction potential. Conversely, pyrimidine is a better -receptor, decreasing the HOMO energy, therefore increasing the reduction potential (Lever, 1990). The electrochemical results reveal that the impm ligand was successfully used to tune these effects by combining them, as we had intended. The electronic spectrum of [Ru(impm) 3 ] 2+ revealed an asymmetric band centered at 421 nm (log " = 4.14), indicating that two superimposed metal-to-ligand charge-transfer (MLCT) bands may be present. This could be explained if two transitions from the Ru II t 2g to two * orbitals are observed. Moreover, the MLCT in [Ru(bpm) 3 ] 2+ is observed at 454 nm (Ernst & Kaim, 1989); this is an indication that the * orbitals involved in the [Ru(impm) 3 ] 2+ transitions lie higher in energy.

Synthesis and crystallization
The ligand was synthesized following the same procedure as reported in the literature (Nakahata et al., 2017). The Ru II complex was prepared by a mixture of one equivalent of RuCl 3 Á3H 2 O (50 mg), 3.3 equivalents of the ligand (92 mg) and 10 ml of DMF. The mixture was stirred and heated to 423 K for 5 min, until the colour turned to green. After the addition of 45 ml of triethylamine, the reaction mixture was kept under reflux for three h, resulting in a reddish purple mixture. This reaction mixture was filtered while still hot using a sintered glass funnel (G4). The filtrate was processed further with constant addition of ethanol and evaporation using a rotary evaporator until the volume reduced to almost 1.5 ml. The resulting reduced mixture was added dropwise to an aqueous solution of NH 4 PF 6 (200 mg in 5 ml of milliQ water) and left in the refrigerator overnight to induce precipitation. Subsequently, the precipitate was filtered, washed with icecold water to remove excess NH 4 PF 6 and dried in a desiccator. Yield: 83.42%. Analysis calculated for [Ru(C 7 H 6 N 4 ) 3 ](PF 6 ) 2 : C,30.41;H,2.19;N,20.26. Found: C,30.51;H,2.55;N,19.78. Ã M (S cm 2 mol À1 ): 162.44, within the typical range for a 1:2 electrolyte in water, 150-310 S cm 2 mol À1 (Geary, 1971 796,844,1102,1409,1629,1590,1551,1471. Crystals of the title compound were obtained by slow evaporation of a methanol:water solution of the complex.
artefacts and the multi-scan method gave the best results. However, a residual positive density was still found close to ruthenium (less than 1 Å ) as a consequence of this insufficient absorption correction (Spek, 2018

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (8)