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Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity.

ABSTRACT: We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR'-pyOR (OR = OMe, OtBu, OH and R' = Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1-4); two silver complexes of these NHCR'-pyOR ligands (5-7); six ruthenium complexes of the type [?6-(p-cymene)Ru(NHCR'-pyOR)Cl]X with R' = Me, Et and R = Me, tBu, H and X = OTf-, PF6- and PO2F2- (8-13); and two iridium complexes, [Cp*Ir(NHCMe-pyOtBu)Cl]PF6 (14) and [Cp*Ir(NHCMe-pyOH)Cl]PO2F2 (15). The complexes are air stable and were isolated in moderate yield. However, for the PF6- salts, hydrolysis of the PF6- counter anion to PO2F2- during t-butyl ether deprotection was observed. Most of the complexes were characterized by 1H and 13C-NMR, MS, IR, and X-ray diffraction. The ruthenium complexes [?6-(p-cymene)Ru(NHCMe-pyOR)Cl]OTf (R = Me (8) and tBu (9)) were tested for their ability to accelerate CO2 hydrogenation and formic acid dehydrogenation. However, our studies show that the complexes transform during the reaction and these complexes are best thought of as pre-catalysts.

SUBMITTER: Gerlach DL

PROVIDER: S-EPMC5714516 | biostudies-literature | 2017 Sep

REPOSITORIES: biostudies-literature

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Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity.

Gerlach Deidra L DL Siek Sopheavy S Burks Dalton B DB Tesh Jamie M JM Thompson Courtney R CR Vasquez Robert M RM White Nicholas J NJ Zeller Matthias M Grotjahn Douglas B DB Papish Elizabeth T ET

Inorganica chimica acta 20170701

We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR'-pyOR (OR = OMe, OtBu, OH and R' = Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1-4); two silver complexes of these NHCR'-pyOR ligands (5-7); six ruthenium complexes of the type [η6-(p-cymene)Ru(NHCR'-pyOR)Cl]X with R' = Me, Et and ...[more]

PMID: 29217867

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Project description:Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6'-dihydroxybipyridine (6,6'-dhbp)) for both their proton-responsive features and for metal-ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = t Bu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6'-dxbp)Cl]OTf, where x = hydroxy (4Ir ) or methoxy (5Ir ); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(?6-cymene)Ru(6,6'-dxbp)Cl]OTf, where x = hydroxy (4Ru ) or methoxy (5Ru ); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir , and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = t Bu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1-5. In general, NHC-pyOR complexes 1-3 were modest precatalysts for both reactions. NHC complexes 1-3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP , which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir ) is 5-8 times more active than x = methoxy (5Ir ). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir . For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ? 4Ru and 4Ir ? 5Ir . Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6'-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation.

Project description:The synthesis, photophysics, and photobiological activities of a series of novel neutral heteroleptic cyclometalated iridium(iii) complexes incorporating boron dipyrromethene (BODIPY) substituted N-heterocyclic carbene (NHC) ligands (Ir1-Ir5) are reported. The effect of the substitution position of BODIPY on the NHC ligands, either on C4 of the phenyl ring (Ir1-Ir3) or C5 of the benzimidazole unit (Ir4 and Ir5), and its linker type (single or triple bond) on the photophysical properties was studied. Ir1-Ir5 exhibited BODIPY-localized intense 1IL (intraligand transition)/1MLCT (metal-to-ligand charge transfer) absorption at 530-543 nm and 1,3IL/1,3CT (charge transfer) emission at 582-610 nm. The nanosecond transient absorption results revealed that the lowest triplet excited states of these complexes were the BODIPY-localized 3?,?* states. Complexes Ir4 and Ir5 exhibited blue-shifted 1IL absorption and 1,3IL/1,3CT emission bands compared to the corresponding absorption and emission bands in complexes Ir1 and Ir3. However, replacing the methyl substituents on N3 of benzimidazole in complexes Ir1 and Ir4 with oligoether substituents in Ir3 and Ir5, respectively, did not impact the energies of the low-energy absorption and emission bands in the corresponding complexes. Water-soluble complexes Ir3 and Ir5 have been explored as photosensitizers for in vitro photodynamic therapy (PDT) effects toward human SKMEL28 melanoma cells. Ir3 showed no dark cytotoxicity (EC50 > 300 ?M) but good photocytotoxic activity (9.66 ± 0.28 ?M), whereas Ir5 exhibited a higher dark cytotoxicity (20.2 ± 1.26 ?M) and excellent photocytotoxicity (0.15 ± 0.01 ?M). The phototherapeutic indices with visible light (400-700 nm) activation were >31 for Ir3 and 135 for Ir5. Ir3 and Ir5 displayed 1O2 quantum yields of 38% and 22% in CH3CN, respectively, upon 450 nm excitation. Ir5 was more effective at generating reactive oxygen species (ROS) in vitro. Ir5 was also active against Staphylococcus aureus upon visible light activation, with a phototherapeutic index of >15 and EC50 value of 6.67 ?M. These photobiological activities demonstrated that these neutral Ir(iii) complexes are promising in vitro PDT reagents, and substitution at C5 on the benzimidazole group of the NHC ligand was superior to C4 substitution on the phenyl ring.

Project description: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 [IrIII(ttp)(IMe)2]+ (2a), [IrIII(oep)(BIMe)2]+ (2d), [IrIII(oep)(I i Pr)2]+ (2e) and [IrIII(F20tpp)(IMe)2]+ (2f) display ruffled porphyrin rings with mesocarbon displacements of 0.483-0.594 Å and long Ir-CNHC bonds of 2.100-2.152 Å. Variable-temperature 1H 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 IrIII(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 1MLCT d?(Ir) ? ?*(por) transition. The near-infrared phosphorescence of IrIII(por)-NHC complexes from the porphyrin-based 3(?, ?*) state features broad emission bands at 701-754 nm with low emission quantum yields and short lifetimes (? em < 0.01; ? < 4 ?s). [IrIII(por)(IMe)2]+ complexes (por = ttp and oep) are efficient photosensitizers for 1O2 generation (? 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 IC50 values at submicromolar levels. The cytotoxicity of these complexes could be further enhanced upon light irradiation with IC50 values as low as nanomolar levels in association with the light-induced generation of reactive oxygen species (ROS). Bioimaging of [IrIII(oep)(IMe)2]+ (2c) treated cells indicates that this Ir complex mainly targets the endoplasmic reticulum. [IrIII(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.

Dataset Information

Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity.

Publications

Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity.

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