Cyclometalated Iridium(III) Complexes Containing Hydroxide/Chloride Ligands: Isolation of Heterobridged Dinuclear Iridium(III) Compounds Containing μ‑OH and μ‑Pyrazole Ligands

The reaction of the cyclometalated chloro-bridged iridium­(III)
dimers [(ppy)2 Ir­(μ-Cl)]2 (ppyH = 2-phenyl
pyridine) and [(tpy)2Ir­(μ-Cl)]2 (tpyH
= 2-p-tolylpyridine) with 3,5-diphenylpyrazole (Ph2PzH) in the presence of sodium methoxide resulted in the formation
of heterobridged dimers [(ppy)2Ir­(μ-OH)­(μ-Ph2Pz)­Ir­(ppy)2] (1) and [(tpy)2Ir­(μ-OH)­(μ-Ph2Pz)­Ir­(tpy)2] (2). Interestingly, the reaction of [(ppy)2Ir­(μ-Cl)]2 with 3(5)-methyl-5(3)-phenylpyrazole (PhMePzH) afforded both
a heterobridged dimer, [(ppy)2Ir­(μ-OH)­(μ-PhMePz)­Ir­(ppy)2] (3), and the monomer [(ppy)2Ir­(PhMePz)­Cl]
(4). The compound [(ppy)2Ir­(PhMePz)­OH] (5) containing a terminal OH was obtained in a hydrolysis reaction
involving 4, sodium methoxide, and PhMePzH. Complexes 1–5 were characterized by X-ray crystallography
and electrospray ionization high-resolution mass spectrometry. All
of the complexes are luminescent at room temperature in their dichloromethane
solutions. The luminescence of the dinuclear complexes is characterized
by a single structureless band centered at λmax =
550 nm (1 and 3) and 546 nm (2). The emission spectra of the mononuclear complexes 4 and 5 display vibronic structures with their λmax values at 497 nm (4) and 513 nm (5). In each case, the main emission bands are accompanied by shoulder
bands at 526 (4) and 534 nm (5). The quantum
yields, calculated with reference to [Ir­(ppy)2(bpy)]­PF6 (ΦCH3CN = 0.0622), range from
0.11 to 0.17 for the dinuclear complexes and 0.045 to 0.048 for the
mononuclear complexes. The lifetimes of the emission are in the microsecond
region, suggesting the phosphorescent nature of the emission. Density
functional theory (DFT) and time-dependent DFT calculations were performed
on complexes 1 and 4 in the ground state
to gain insight into the structural, electronic, and photophysical
properties. Electrochemical studies on complexes 1–3 showed the presence of two consecutive one-electron-oxidation
processes, assigned as the stepwise oxidation of the two IrIII centers, i.e., IrIII–IrIII/IrIII–IrIV and IrIII–IrIV/IrIV–IrIV couples, respectively. The
monomers displayed single-oxidation peaks. No reduction process was
observed within the solvent cathodic potential limit.