Kinetics and Mechanism of Water Substitution at Half-Sandwich Iridium(III) Aqua Cations Cp*Ir(A−B)(H2O)2+/+ in Aqueous Solution (Cp* = η5-Pentamethylcyclopentadienyl Anion; A−B = Bidentate N,N or N,O Ligand)

Abstract

The perchlorate complexes of a series of half‐sandwich monoaqua cations Cp*Ir(A−B)(H2O)2+/+ with A−B = prol (D/L‐proline anion), picac (picolinic acid anion), R,R‐dach [(−)‐(1R,2R)‐1,2‐diaminocyclohexane], R,R‐dpen [(+)‐(1R,2R)‐1,2‐diphenylethylenediamine], phen (o‐phenanthroline), and bpy (2,2′‐bipyridine) (Cp* = η5‐pentamethylcyclopentadienyl anion) have been prepared and characterized. An X‐ray structure analysis of Cp*Ir(R,R‐dach)(H2O)(ClO4)2·H2O has revealed that the cation Cp*Ir(R,R‐dach)(H2O)2+ has a distorted pseudo‐octahedral coordination geometry. In the case of A−B = prol, crystallization from water led to the trinuclear complex [Cp*Ir(D‐prol)]3(ClO4)3, which has also been characterized by X‐ray structure analysis. The experimental data suggest that in aqueous solution the trinuclear proline complex dissociates to form the cation Cp*Ir(D‐prol)(H2O)+. The proton dissociation constants of the coordinated water in Cp*Ir(A−B)(H2O)2+/+ have been determined as pKa = 7.5 (A−B = bpy) and pKa = 7.1 (A−B = R,R‐dach and picac). Substitution of the water in Cp*Ir(A−B)(H2O)2+/+ by the monodentate ligands L = py (pyridine), DMS (dimethyl sulfide), TU (thiourea), and monodentate anions according to the Equation Cp*Ir(A−B)(H2O)2+/+ + L → Cp*Ir(A−B)L2+/+ + H2O has been studied by multi‐wavelength stopped‐flow spectrophotometry in aqueous solution at I = 0.2 M. This kinetic investigation, carried out at different concentrations, temperatures, and pressures, showed that the process obeys second‐order kinetics, where rate = kL[Cp*Ir(A−B)H2O2+/+][L]. The magnitude of the second‐order rate constant kL depends on the nature of both A−B and L. The data for kL have been found to range from 6.4 × 104 M−1s−1 (A−B = D‐prol; L = TU) to 10.5 M−1s−1 (A−B = bpy; L = py) at 298 K. The activation parameters for water substitution at Cp*Ir(A−B)(H2O)2+/+ (A−B = bpy, R,R‐dach, and picac) by L = TU have been evaluated. The activation volumes of ΔV≠ = +2.3, +7.4, and +7.3 cm3 mol−1, respectively, are supportive of an Id mechanism. The results regarding the kinetic lability of the coordinated water in the monoaqua cations Cp*Ir(A−B)(H2O)2+/+ are compared to those obtained for the triaqua cation Cp*Ir(H2O)32+.

Ligand Exchange and Redox Processes in Iridium Triazolylidene Complexes Relevant to Catalytic Water Oxidation

“…The pK a of compound 2 was determined by spectroscopic titration using aqueous solutions buffered at different pHs. 38,39 Representative UV-vis spectra of the acidic and basic forms are depicted in Fig . 40 The pK a values have also been used to extrapolate the donor ability of the bidentate (L,L) ligand, 35,36 with a higher pK a value corresponding to an increased donor capacity of the chelating ligand.…”

Coligand tuning of the DNA binding properties of half-sandwich organometallic intercalators: Influence of polypyridyl (pp) and monodentate ligands (L=Cl, (NH2)2CS, (NMe2)2CS) on the intercalation of (η5-pentamethylcyclopentadienyl)-iridium(III)- dipyridoquinoxaline and -dipyridophenazine complexes

“…Moreover, as demonstrated for L = Cl À , the release of the monodentate ligand can lead to a change in the complex binding mode and to significant alterations in the DNA structure. As it should be possible to tune the time required for such changes by varying the nucleophilicity of the ligand L [24], this opens opportunities for the use of such organometallic complexes for controlled two-stage reactions with DNA.…”

Half-sandwich iridium complexes—Synthesis and applications in catalysis

“…Mononuclear Ir-Cp* complexes may aggregate in solution through self-recognition. Thus, the mononuclear [Cp*Ir( 2 -C,N-d-proline)] self assembled in water to give a trinuclear complex [Cp*Ir(d-proline)] 3 155, which contains nine chiral centers (3Ir,3N,3␣-C) of R-configuration (Scheme 40) [82]. Similarly, treatment of the mononuclear complex [Cp*Ir(␣-aminoacidate)Cl] with AgBF 4 led to new chiral trimers [83].…”