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A panel of iridium(iii) porphyrin complexes containing axial N-heterocyclic carbene (NHC) ligand(s) were synthesized and characterized. X-ray crystal structures of the bis-NHC complexes [Ir III (ttp)(IMe) 2 ] + ( 2a ), [Ir III (oep)(BIMe) 2 ] + ( 2d ), [Ir III (oep)(I i Pr) 2 ] + ( 2e ) and [Ir III (F 20 tpp)(IMe) 2 ] + ( 2f ) display ruffled porphyrin rings with mesocarbon displacements of 0.483-0.594 Å and long Ir-C NHC bonds of 2.100-2.152 Å. Variable-temperature 1 H NMR analysis of 2a reveals that the macrocycle porphyrin ring inversion takes place in solution with an activation barrier of 40 ± 1 kJ mol -1 . The UV-vis absorption spectra of Ir III (por)-NHC complexes display split Soret bands. TD-DFT calculations and resonance Raman experiments show that the higher-energy Soret band is derived from the 1 MLCT dπ(Ir) → π*(por) transition. The near-infrared phosphorescence of Ir III (por)-NHC complexes from the porphyrin-based 3 (π, π*) state features broad emission bands at 701-754 nm with low emission quantum yields and short lifetimes ( Φ em < 0.01; τ < 4 μs). [Ir III (por)(IMe) 2 ] + complexes (por = ttp and oep) are efficient photosensitizers for 1 O 2 generation ( Φ so = 0.64 and 0.88) and are catalytically active in the light-induced aerobic oxidation of secondary amines and arylboronic acid. The bis-NHC complexes exhibit potent dark cytotoxicity towards a panel of cancer cells with IC 50 values at submicromolar levels. The cytotoxicity of these complexes could be further enhanced upon light irradiation with IC 50 values as low as nanomolar levels in association with the light-induced generation of reactive oxygen species (ROS). Bioimaging of [Ir III (oep)(IMe) 2 ] + ( 2c ) treated cells indicates that this Ir complex mainly targets the endoplasmic reticulum. [Ir III (oep)(IMe) 2 ] + catalyzes the photoinduced generation of singlet oxygen and triggers protein oxidation, cell cycle arrest, apoptosis and the inhibition of angiogenesis. It also causes pronounced photoinduced inhibition of tumor growth in a mouse model of human cancer.

A new class of cyclometalated Ir(iii) complexes supported by various bidentate C-deprotonated (C^N) and cis -chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized. These complexes display strong emission in deaerated solutions at room temperature with photoluminescence quantum yields up to 89% and emission lifetimes up to 96 μs. A photo-stable complex containing C-deprotonated fluorenyl-substituted C^N shows no significant decomposition even upon irradiation for over 120 h by blue LEDs (12 W). These, together with the strong absorption in the visible region and rich photo-redox properties, allow the bis-NHC Ir(iii) complexes to act as good photo-catalysts for reductive C-C bond formation from C(sp 3 /sp 2 )-Br bonds cleavage using visible-light irradiation ( λ > 440 nm). A water-soluble complex with a glucose-functionalized bis-NHC ligand catalysed a visible-light-driven radical cyclization for the synthesis of pyrrolidine in aqueous media. Also, the bis-NHC Ir(iii) complex in combination with a cobalt catalyst can catalyse the visible-light-driven CO 2 reduction with excellent turnover numbers (>2400) and selectivity (CO over H 2 in gas phase: >95%). Additionally, this series of bis-NHC Ir(iii) complexes are found to localize in and stain endoplasmic reticulum (ER) of various cell lines with high selectivity, and exhibit high cytotoxicity towards cancer cells, revealing their potential uses as bioimaging and/or anti-cancer agents.

The macrocyclic [Fe III (L1)Cl 2 ] + ( 1 , L1 = N , N '-dimethyl-2,11-diaza[3,3](2,6)pyridinophane) complex is an active catalyst for the oxidation of water to oxygen using [NH 4 ] 2 [Ce IV (NO 3 ) 6 ] (CAN), NaIO 4 , or Oxone as the oxidant. The mechanism of 1 -catalysed water oxidation was examined by spectroscopic methods and by 18 O-labelling experiments, revealing that Fe IV [double bond, length as m-dash]O and/or Fe V [double bond, length as m-dash]O species are likely to be involved in the reaction. The redox behaviour of 1 and these high-valent Fe[double bond, length as m-dash]O species of L1 has been examined by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the cyclic voltammograms of 1 at different pH show a pH-dependent reversible couple ( E 1/2 = +0.46 V vs. SCE at pH 1) and an irreversible anodic wave ( E pa = +1.18 V vs. SCE at pH 1) assigned to the Fe III /Fe II couple and the Fe III to Fe IV oxidation, respectively. DFT calculations showed that the E value of the half reaction involving [Fe V (L1)(O)(OH)] 2+ /[Fe IV (L1)(O)(OH 2 )] 2+ is +1.42 V vs. SCE at pH 1. Using CAN as the oxidant at pH 1, the formation of an Fe IV [double bond, length as m-dash]O reaction intermediate was suggested by ESI-MS and UV-vis absorption spectroscopic measurements, and the rate of oxygen evolution was linearly dependent on the concentrations of both 1 and CAN. Using NaIO 4 or Oxone as the oxidant at pH 1, the rate of oxygen evolution was linearly dependent on the concentration of 1 , and a reactive Fe V [double bond, length as m-dash]O species with formula [Fe V (L1)(O) 2 ] + generated by oxidation with NaIO 4 or Oxone was suggested by ESI-MS measurements. DFT calculations revealed that [Fe V (L1)(O) 2 ] + is capable of oxidizing water to oxygen with a reaction barrier of 15.7 kcal mol -1 ., (This journal is © The Royal Society of Chemistry 2015.)