Crystal structure of (2-(2-pyridine)-benzimidazole-κ² N,N′)-bis(1-phenylpyrazole-κ² C,N)iridium(III) hexafluorophosphate, C₃₀H₂₂F₆IrN₇P

1 Source of materials

All the reagents were A. R. grade commercially available and used as received without further purification. The cyclometalated chloro-bridged iridium(III) dimer, [(ppz)₂Ir(μ–Cl)]₂ (ppz = 1-phenylpyrazole), was synthesized following the reported literature procedures [6] by heating IrCl₃·3H₂O (1 equiv) and 1-phenylpyrazole (2.3 equiv) in a mixed solution of 2-ethoxyethanol and water (v/v = 3/1) at 135 ^∘C. 540.70 mg o-phenylenediamine was dissolved in 30 mL of ethanol with the help of sonication. After that, 0.48 mL of pyridine-2-formaldehyde was added to the above solution and reacted at 80 °C for 2 h to obtain 839.41 mg 2-(2-pyridine)-benzimidazole (bzp). The synthesized iridium(III) dimer [(ppz)₂Ir(μ–Cl)]₂ (103.00 mg, 0.1 mmol), auxiliary ligand 2-(2-pyridine)-benzimidazole (39.04 mg, 0.2 mmol), and potassium hexafluorophosphate (36.81 mg, 0.2 mmol) were added together to a mixed solvent containing dichloromethane (50 mL) and methanol (10 mL). Subsequently, the mixture was heated to 85 °C and refluxed in a dark N₂ atmosphere for 24 h to obtain the target compound. After filtration, the obtained solid product was dissolved in chloroform (10 mL), and then 10 mL of buffer solution (V_chloroform/V_n-hexane = 1/1) and 20 mL of n-hexane were added successively. Under the diffusion effect of solution, bright yellow block crystals were obtained after a week at room temperature in the dark with a yield of 65.40 mg (49 % based Ir). Anal. Calcd. for C₃₀H₂₂F₆IrN₇P: C, 44.06 %; H, 2.71 %; N, 11.99 %. Found C, 44.35 %; H, 2.92 %; N, 11.46 %. IR (KBr, /cm⁻¹): 3049(m), 2924(s), 1661(m), 1595(m), 1468(s), 1433(s), 1259(m), 1053(m), 844(vs), 747(s), 552(s).

2 Experimental details

The structure was solved by Direct Methods and refined using the SHELX software [3]. All the hydrogen atoms were added by theoretical method and isotropic displacement parameters were given (U_iso = 1.2 U_eq, U_eq is the equivalent isotropic displacement parameter of the parent atom) [5].

3 Comment

The design and construction of cyclometalated iridium(III) complexes, which were assembled from iridium ions and organic conjugate ligands, have attracted much attention because of their favorable properties arising from strong spin-orbit coupling induced by the Ir(III) ion [7], [8], [9], [10]. The cyclometalated Ir(III) complex is usually formed by the coordination of C–N chelate ligand and N–N auxiliary ligand with Ir(III) atom, and has a hexahedral spatial configuration [11, 12]. The p* orbital of the N–N auxiliary ligand in the cyclometalated Ir(III) complex is the main contributor to the lowest unoccupied molecular orbital (LUMO), while the p orbital of the C–N chelate ligand and the d orbital of the Ir(III) center mainly affect the highest occupied molecular orbital (HOMO) [13, 14]. The conjugation adjustment of N–N auxiliary ligands or the introduction of electron withdrawing substituents can reduce the LUMO energy level of the system, while the introduction of electron donating substituents can increase the LUMO energy level [15]. Among the N–N auxiliary ligands, the Schiff base ligands which contain –RC=N– characteristic groups with lone pair electrons on the N atoms have strong ability of coordination with the metal ions and p-conjugation structure. In this context, we selected the 2-(2-pyridine)-benzimidazole (bzp) as the auxiliary ligand in the preparation of cyclometalated Ir(III) complex.

The crystal structure consists of one Ir³⁺ cation, two ppz chelate ligands, one bzp auxiliary ligand, and one [PF₆]⁻ counter anion (see the Figure). The Ir(III) center is 6-coordinated by four N atoms and two C atoms, in which two N atoms and two C atoms come from the ppz chelate ligands while the rest two N atoms from the bzp ancillary ligand, forming an octahedral plane. The metal-carbon bond lengths (Ir–C) are 1.991(10) Å and 2.017(10) Å, respectively, while the distances of Ir–N bonds range from 2.019(8) Å to 2.192(9) Å. The bond angles about the Ir(III) ion range from 76.3(3)^∘ to 174.4(4)^∘, and these values match with the previously reported cyclometalated Ir(III) complexes [16]. There are C–H … F hydrogen bonding interactions.