Mechanistic Studies of the Reaction of Ir(III) Porphyrin Hydride with 2,2,6,6-Tetramethylpiperidine-1-oxyl to an Unsupported Ir−Ir Porphyrin Dimer

Luminescent iridium(III) porphyrin complexes as near-infrared-emissive biological probes

We report herein the design, synthesis and characterisation of a series of luminescent iridium(III) porphyrin complexes [Ir(ttp)(CH2CH2OH)] (H2ttp = 5,10,15,20-tetra-4-tolylporphyrin) (1), [Ir(tpp-Ph-NO2)(CO)Cl] (H2tpp-Ph-NO2 = 5-(4-((4-nitrophenoxy)carbonyloxymethyl)phenyl)-10,15,20-triphenylporphyrin) (2), [Ir(tpp-COOMe)(Py)2](Cl) (H2tpp-COOMe = 5-(4-methoxycarbonylphenyl)-10,15,20-triphenylporphyrin; Py = pyridine) (3) and [Ir(tpp-COOH)(Py)2](Cl) (H2tpp-COOH = 5-(4-carboxylphenyl)-10,15,20-triphenylporphyrin) (4). All the complexes displayed long-lived near-infrared (NIR) emission attributed to an excited state of mixed triplet intraligand (3IL) (π → π*) (porphyrin) and triplet metal-to-ligand charge transfer (3MLCT) (dπ(Ir) → π*(porphyrin)) character. The cytotoxicity of the complexes toward HeLa cells was examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cationic complexes 3 and 4 exhibited higher cytotoxic activity toward HeLa cells than their neutral counterparts 1 and 2. Cellular uptake studies by inductively coupled plasma-mass spectrometry (ICP-MS) and laser-scanning confocal microscopy (LSCM) indicated that complexes 3 and 4 showed higher cellular uptake efficiencies than complexes 1 and 2 due to their cationic charge, and they were enriched in the perinuclear region of the cells with negligible nuclear uptake. Additionally, the carboxyl complex 4 was used to label a model protein bovine serum albumin (BSA) via an amidation reaction. The resultant luminescent protein conjugate 4-BSA displayed similar photophysical properties and intracellular localisation behaviour to its parent complex. The results of this work will contribute to the development of luminescent iridium(III) porphyrin complexes and related bioconjugates as NIR-emissive probes for bioimaging applications.

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H2 Reductive Elimination

Using the doubly protic bis-pyrazole-pyridine ligand (N(NN^H)₂), we have synthesized an octahedral Ir^III-H [HIr(κ³-N(NN^H)(NN^-))(CO)(^tBuPy)]⁺ ([1-MH]⁺) from an Ir^I starting material. This hydride was generated by adding sufficient electron density to the metal center such that it became the thermodynamically preferred site of protonation. It was observed via UV-vis spectroscopy that [1-MH]⁺ establishes a [^tBuPy] dependent equilibrium with a ligand protonated square-planar Ir^I [Ir(N(NN^H)₂)(CO)]⁺ ([2-LH]⁺). This example of metal/ligand proton tautomerism is unusual in that the position of the equilibrium can be controlled by the concentration of exogeneous ligand (i.e., ^tBuPy). This equilibrium was shown to be key to the reactivity of the Ir^III-H; 2 equiv of [1-MH]⁺ release H₂, converting to the Ir^II dimer [[Ir(N(NN^-)(NN^H))(CO)(^tBuPy)]₂]²⁺ ([7]²⁺) under mild conditions (observable at room temperature). Mechanistic evidence is presented to support that this dinuclear reductive elimination occurs by tautomerization of the metal hydride [1-MH]⁺ to a ligand protonated species [1-LH]⁺, from which ligand dissociation is facile, generating [2-LH]⁺. Subsequent reaction of [2-LH]⁺ with [1-MH]⁺ allows for production of H₂ and the Ir^II dimer [7]²⁺. The tautomerization between the metal-hydride and the ligand protonated species provides a low energy pathway for ligand dissociation, opening the needed coordination site. The ability to control the interconversion between a metal-hydride and a ligand-protonated congener using an exogeneous ligand introduces a new strategy for catalyst design with proton responsive ligands.

Iridium porphyrins in CD3OD: reduction of Ir(III), CD3-OD bond cleavage, Ir-D acid dissociation and alkene reactions

Methanol solutions of iridium(III) tetra(p-sulfonatophenyl)porphyrin [(TSPP)Ir(III)] form an equilibrium distribution of methanol and methoxide complexes ([(TSPP)Ir(III)(CD3OD)(2-n)(OCD3)n]((3+n)-)). Reaction of [(TSPP)Ir(III) with dihydrogen (D2) in methanol produces an iridium hydride [(TSPP)Ir(III)-D(CD3OD)](4-) in equilibrium with an iridium(I) complex ([(TSPP)Ir(I)(CD3OD)](5-)). The acid dissociation constant of the iridium hydride (Ir-D) in methanol at 298 K is 3.5 × 10(-12). The iridium(I) complex ([(TSPP)Ir(I)(CD3OD)](5-)) catalyzes reaction of [(TSPP)Ir(III)-D(CD3OD)](4-) with CD3-OD to produce an iridium methyl complex [(TSPP)Ir(III)-CD3(CD3OD)](4-) and D2O. Reactions of the iridium hydride with ethene and propene produce iridium alkyl complexes, but the Ir-D complex fails to give observable addition with acetaldehyde and carbon monoxide in methanol. Reaction of the iridium hydride with propene forms both the isopropyl and propyl complexes with free energy changes (ΔG° 298 K) of -1.3 and -0.4 kcal mol(-1) respectively. Equilibrium thermodynamics and reactivity studies are used in discussing relative Ir-D, Ir-OCD3 and Ir-CD2- bond energetics in methanol.