Recently, iridium catalysts in the aerobic oxidation of primary and secondary alcohols,  catalytic H/D-exchange reactions  by iridium complexes contain NHC (N-heterocyclic carbene) ligand have been studied by our group. Iridium complexes bearing Cp* and NHC ligands showed good reactivities in the aerobic oxidation reactions of primary and secondary alcohols, so a variety of iridium complexes have been utilized in the aerobic oxidation of HMF. Biomass-derived carbohydrate feeds are usually treated at mild temperatures, and most of these processes are carried out in the liquid phase.  Using water as solvent alleviates the environmental problems associated with organic solvent and reduces the cost. HMF is highly soluble in water, so water was used as solvent in our catalytic system. The aerobic oxidations of HMF in water under atmospheric pressure of pure oxygen formed 2, 5-diformylfuran (DFF) in good yields.(Table 4.1) These reactions were catalyzed by three different iridium carbene complexes at 150 0 C for 12 hours. Triethyl amine was used as a base in this system. The products were identified by 1 H NMR and 13 C NMR spectroscopy in CDCl 3 . 1 H NMR
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bonds on the cyclometalating ligands. It has been posited that the highly electron deficient C^N ligands make them susceptible to chemical degradation by nucleophilic aromatic substitution of the fluorine substituent. A study by Bolink and co-workers[16a] on the stability of fluorine-containing green-emitting iridium complexes will be outlined in more detail in Sect. 4, but it is worth noting here that they demonstrated that of the four complexes studied, the complex bearing four fluorine substituents was far less stable than others bearing just two fluorine atoms, providing indirect evidence that indeed (multiply) fluorinated aromatic rings can be implicated in the electrochemical degradation of the emitter in the device. Similar degradation processes are believed to be operative in OLEDs [59, 60], but the harsher environment in the emissive layer of a LEEC means that this effect is more pronounced in this class of electroluminescent device. Thus, there is interest in designing new emitters that emit blue light without the need for fluorine substituents that might negatively impact the stability. In addition, there is interest in adopting hydrophobic substituents within the ligand framework to impede nucleophiles from coordinating to the iridium center and quenching the emission. These two strategies are exemplified by complexes 8 and 9, with 9 in particular representing an all-in-one effort to achieve blue emission without impacting the device stability.
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Relaxation of higher excited states plays a crucial role in energy and charge transfer processes, 10 and understanding the nature and rates of relaxation from such 1 MLCT states may enable more efficient extraction of energy from systems employing transition metal complexes. Intersystem crossing in iridium complexes has not been much explored. Formation of the lowest 3 MLCT state has been found to be very fast 11, 12 when looking at transient absorption data. Ultrafast luminescence experiments – which would give a much clearer assignment of ISC – have been limited in iridium complexes due to the generally higher emission energy, hence forcing observation of singlet emission to wavelengths that were experimentally difficult. The development and interest in iridium complexes for use in organic light emitting diodes (OLEDs) has driven the synthesis of materials that can cover the entire visible spectrum, 13 therefore red emitting iridium materials are available for study. 14 This was the pursued direction to enable observation of fluorescence, and hence the deduction of ISC rates in iridium materials; as if the material has red-shifted emission and all excited state electronic features are also red-shifted then the singlet emission region should become observable with upconversion spectroscopy.
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With respect to the performance of iridium-based emitters in OLEDs, a similar problem exists. Although exceptional performance red 2b and green 8 OLEDs based on iridium have been reported, a corresponding deep blue emitter remains elusive. 2a This is due in no small part to the host–guest configuration of the emissive layers in OLEDs – the requirement thus being that the high triplet energies required for blue emitters (42.8 eV) necessitate even higher triplet energies for the host materials. At these energy regimes, realising triplet host energies that are compatible with the emitter and suitable functional device materials (charge transport layers, electrodes) has become increasingly difficult to achieve. Thus, even devices that show deep blue emission using iridium complexes show poorer efficiencies compared to their red- and green-emitting counterparts (B30% EQE); a recent review on blue emitters in OLEDs identified a champion true blue device based on iridium as having CIE coordinates of (0.14, 0.10), with an EQE of 7.6%, 2a while a recent report 9 outlined the use of a tris- cyclometalated NHC iridium complex that achieved CIE coor- dinates of (0.16, 0.09) at an EQE of 10.1% – a different story from the higher efficiencies (420%) reported for sky-blue emitters. 2a,10
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catalysed cross-coupling reactions, the coordination chemistry of IBiox ligands has remained largely unexplored. With the aim of investigating low-coordinate NHC complexes of rhodium and iridium, we have recently begun to expand the coordination chemistry of IBiox ligands, seeking to exploit their conformational rigidity to avoid intramolecular cyclometalation reactions that can occur via C−H bond activation of the downward pointing alkyl and aryl NHC appendages. 2,3 In particular, we have focused our
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Electrochemiluminescence or electrogenerated chemiluminescence, ECL, generates luminescence in solution 16 , while electroluminescent devices, generate luminescence in the solid state. ECL involves light emission that is produced by an energetic electron transfer reaction between electrochemically generated radicals in the vicinity of an electrode. 17 Two general methods for producing ECL are “annihilation” and “co-reactant” reactions. 17 In annihilation systems, radical cations and anions are generated in solution and light emission results when they combine. Co-reactant studies are useful when a system does not give stable radical cations or anions. Co-reactant intermediates are either strong reducing agents in oxidative-reduction ECL or strong oxidizing agents in reductive-oxidation ECL. 17 Benzoyl peroxide (BPO) is a common co- reactant and produces a strong oxidizing agent when reduced; this has been selected for the purpose of our studies. 17 Previously, we have studied the ECL of iridium(III) complexes containing aryltriazoles that emitted blue light. 18 We wanted to investigate if the prepared thienyltriazole ligands were electrochemiluminescent. Here, we report the electrochemical properties of 3.1-3.4 in Table 3.1, and the ECL spectra and corresponding ECL efficiencies, from annihilation and co-reactant pathways, in Table 3.3. Furthermore, using ECL to detect light is of importance for possible applications in biosensors, OLED displays, optoelectronics, microelectronics and bioanalytical chemistry. 1,3,19-26
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An important feature of these complexes, particularly for LEECs, is that their emission can be readily tuned across the visible spectrum. In many cases, the emissive excited states of these complexes are a mixture of metal-to-ligand charge transfer ( 3 MLCT) between the metal and N^N ligands and ligand-to-ligand charge transfer ( 3 LLCT) between the phenyl rings of the C^N ligands and the N^N ligands. Thus, to a first approximation, the energies of the HOMOs and LUMOs of these complexes can be independently modulated as a function of appropriate substituent modification of the C^N and N^N ligands. Such facile colour tuning is in stark contrast to the narrow orange-red emission range of ruthenium(II) polypyridyl complexes. For reference, we include several reviews from other groups exploring colour tuning of iridium complexes. 5b, 8
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Neutral arene ligands do not stabilize Ir III . In contrast, negatively-charged pentamethylcyclopentadienyl (Cp*) a is an excellent stabilizing ligand for Ir III . In the work reported here we apply the design concepts discovered for Ru II and Os II arene complexes to Ir III Cp* and functionalized Cp* complexes [(η 5 -Cp x )Ir(XY)Cl] 0/+ containing N,N-bound 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), ethylenediamine (en), and N,O-bound picolinate (pico) as chelating ligands. Iridium(III) Cp* complexes have attracted recent attention as catalysts, for example in hydrogen transfer reactions. 13 Only a few iridium complexes containing functionalized Cp* ligands have been reported previously. 14 We have studied the effect of Cp* functionalization on the rate of hydrolysis, acidity of the aqua adducts, interactions with nucleobases, hydrophobicity (octanol/water partition), cell accumulation (the net effect of uptake and efflux) and distribution, interaction with DNA, and cytotoxicity to cancer cells. 15 We show that such complexes can be thermodynamically stable and yet kinetically labile towards substitution reactions and that substituents on the cyclopentadienyl ring and chelating ligand can have a dramatic effect on chemical and biological activity. This appears to be the first time that Cp xph and Cp xbiph have been used as ligands in iridium complexes.
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Nearly all of the photoactive iridium(III) complexes that are used as emitters in electroluminescent devices, 1−6 as dyes in solar cells, 7−9 in nonlinear optics (NLO), 10−12 as photoredox catalysts, 13,14 as solar fuels, 15 and in bioimaging 16,17 contain conjugated ﬁ ve-membered chelated ligands, such as the commonly used 2-phenylpyridine (ppyH), 2,2 ′ -bipyridine (bpy), acetylacetonate (acac), and picolinate (pic). Photoactive iridium complexes containing a six-membered cyclometalating chelate are very rare, and the few reported examples can be categorized into two families of complexes: those containing conjugated 18−21 or nonconjugated 22−24 bidentate cyclometalat- ing ligands. For instance, in 2008, Song et al. 22 obtained a phosphorescent Ir(III) complex [Ir(dfb-pz) 2 (fptz)] (where
Isomerisation reactions are, by their very nature, atom-efficient transformations, and allow for the interconversion of useful functional groups. Allylic alcohol rearrangements to form ketones comprise one such type of isomerisation reaction (Scheme 1). Allylic alcohol isomerisation can be catalysed by a variety of metal systems, 1 including iridium, 2, 3 although we have recently reported a highly-active ruthenium system for this transformation. 4 With the appropriate ligand sphere, asymmetric variants are also possible, albeit with high loadings of expensive catalyst complexes (ca. 5 mol%). 5 Many iridium-catalysed methodologies require the use of dihydrogen in order to generate the active iridium(III) dihydride species in situ; 6-8 the reaction solution must then be degassed to remove excess dihydrogen and avoid competing hydrogenation reactions. While this is achievable without specialist equipment, it requires a cylinder of hydrogen on hand and is less operationally straightforward than the simple mixing of a catalyst and substrate. Recently, Ahlsten et al. reported the elegant tandem allylic alcohol isomerisation/α-chlorination or α-fluorination of a range of substrates by employing typically 0.25 – 1.0 mol% of iridium(III) precursor [IrCl 2 (Cp*)] 2 (Cp* = 1,2,3,4,5-
Table S3. Hydrogen Bond Lengths (Å) and Angles ( o ) for Complex 4 S7 Table S4. Antimicrobial Activity of Complexes 1-14 and HPLC retention times S8 Table S5. Antibacterial Activity of Complexes 4-10 against S. aureus and S. pyogenes under Anaerobic and Aerobic Conditions S9 Table S6. Selectivity Factors (SF) for Complexes 4-14 S9 Table S7. Dependence of Antibacterial Activity of Complexes 4-10 on Time of Storage of Solutions of Complexes in CAMHB Medium at Various Temperatures for 21 Days S10 Table S8. S. aureus Biofilm Disruption by Complexes 4-9 S10 Table S9. Results of ANOVA S11 Table S10. MBC/MIC Ratios of for Complexes 1-14 S11 Figure S1. Elution condition for hydrophobicity measurements by RP-HPLC S12 Figure S2. Plot of retention time versus MICs for complexes 1-4 against MRSA S12 Figure S3. Reaction of complex 4 with L-cysteine monitored by 1 H NMR S13
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The antioxidant activity of complexes Ir-TEMPO1 and Ir- TEMPO2 was determined in A2780 ovarian cancer cells using 2 0 ,7 0 -dichlorouorescein diacetate (DCFH-DA) and ROS induc- tion by hydrogen peroxide as well as the organic hydroperoxide TBHP (tert-butyl hydroperoxide), as shown in Fig. 4. The uo- rescence based-experiment conrmed that, as expected, Ir- TEMPO1 and Ir-TEMPO2 do not induce intracellular ROS per se, as there is no increased DCFH-DA uorescence upon drug exposure. Furthermore, it also shows a reduction in uores- cence when cells have been pre-treated with Ir-TEMPO1 and Ir- TEMPO2 for 24 h prior to induction of ROS by hydrogen peroxide or TBHP. This reduction of uorescence is concentration-dependent, as the values for cells treated with equipotent IC 50 concentrations of the TEMPO-appended
2.3. Synthesis of Rh(III) and Ir(III) Complexes A hot ethanolic solution (20 mL) the ligand (1 mmol) was added slowly to the hot ethanolic solution (20 mL) of the corresponding metal salts (1 mmol) with continu- ous stirring. The resultant colored solution was refluxed for 10 - 12 h at 80˚C. On cooling the colored precipitate of complex was obtained, which was filtered, washed thoroughly with cold ethanol and dried under vacuum. Purity of the complexes were checked by TLC.
Briefly, 5000 cells were seeded per well in 96-well plates. The cells were pre-incubated in the corresponding drug-free media at 37°C for 48 h before adding different concentrations of the compounds to be tested. Stock solutions of the Ir(III) complexes themselves were firstly prepared in 5% v/v DMSO and 95% v/v either PBS or cell culture medium, for the ‘conjugated complexes’ or polymer/conjugate controls the use of DMSO was omitted. The concentration of Ir solutions was determined by ICP-OES before drug administration. In all cases, stock solutions were further diluted in cell culture medium until working concentrations were achieved, maintaining the total amount of DMSO below 1%. The drug exposure period was 24 h. After this, supernatants were removed by suction and each well was washed with PBS. A further 72 h was allowed for the cells to recover in drug-free medium at 37°C. The SRB assay was used to determine cell viability. Absorbance measurements of the solubilised dye (on a BioRad iMark microplate reader using a 470 nm filter) allowed the determination of viable treated cells compared to untreated controls. IC 50 values
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The ruthenium picolinamide complexes, 9-12, show a different trend to their iridium-Cp* analogues whereby the cytotoxicities are in the order 12>10>11>9 (where the phenyl ring substituents are 2,5-diCl, 3-Cl, 2,4-diCl and 2-Cl respectively) for all cell lines. The quinaldamide complex 13 has similar activity to the picolinamide complex 10. Compound 12 is the most cytotoxic compound of the series, by an order of magnitude, for both cell lines after a five day exposure, particularly for HT-29 cells with higher activity than cisplatin (IC 50 value of 6 M compared to 10 M). As expected, all compounds display
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Crystal structures of all new complexes were determined; these are shown in Figure 2 and in Table 1. Crystals for X-ray diffraction work were grown from either acetone, dichloromethane or 1,2- dichloroethane at room temperature. Compounds 1a, 1c, 2a–2c have two or three independent molecules in the asymmetric units all of which have very similar geometries. Values presented in the discussion of these molecules are the mean values. Data for 2a is of somewhat lower quality, but is sufficient to demonstrate the connectivity of the molecule; the bond lengths and angles are in line with those observed in the other molecules. All of the complexes (1a–c and 2a–c) are isostructural and attain piano stool geometry (around the metal centre) with the η 5 –Cp* ring slightly tilted, as shown by the slight variation in the M–C bond lengths (see Table 1 and Figure 2). The M-E and M-C (M = Ir, Rh; E = As, Sb) bonds are all as expected.  The pnictogen ligands adopt tetrahedral geometry, again, with normal As–C and Sb–C bond lengths and angles.
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(1) (a) Pincer Compounds: Chemistry and Applications; Morales- Morales, D., Ed.; Elsevier: 2018; Vol. 1. (b) Kumar, A.; Bhatti, T. M.; Goldman, A. S. Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes. Chem. Rev. 2017, 117, 12357− 12384. (c) Andrew, R. E.; González-Sebastián, L.; Chaplin, A. B. NHC-Based Pincer Ligands: Carbenes with a Bite. Dalton Trans. 2016, 45, 1299−1305. (d) Organometallic Pincer Chemistry; van Koten, G., Milstein, D., Eds.; Topics in Organometallic Chemistry; Springer: 2013; Vol. 40. (e) van der Boom, M. E.; Milstein, D. Cyclometalated Phosphine-Based Pincer Complexes: Mechanistic Insight in Catalysis, Coordination, and Bond Activation. Chem. Rev. 2003, 103, 1759−1792. (f) Albrecht, M.; van Koten, G. Platinum Group Organometallics Based on “Pincer” Complexes: Sensors, Switches, and Catalysts. Angew. Chem., Int. Ed. 2001, 40, 3750−3781. (2) (a) Feller, M.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Milstein, D. Direct Observation of Reductive Elimination of MeX (X = Cl, Br, I) From Rh(III) Complexes: Mechanistic Insight and the Importance of Sterics. J. Am. Chem. Soc. 2013, 135, 11040−11047. (b) Feller, M.; Iron, M. A.; Shimon, L. J. W.; Diskin-Posner, Y.; Leitus, G.; Milstein, D. Competitive C-I Versus C-CN Reductive Elimination From a Rh(III) Complex. Selectivity Is Controlled by the
inertness arises from their strong C-C and C-H bonds. 10 Another problem is the over oxidation of alkanes which give rise to undesired low value oxygenates such as carbon monoxides and carbon dioxides during the alkane oxidation process. Therefore addressing these two major problems, namely C-H bond activation and controlled or selective incorporation of oxygen into these activated C-H bonds are of paramount importance in utilizing selective oxidation as a viable alternative on the commercial scale. Solutions to these problems have become major targets for numerous fundamental researches these days. The work in this thesis is an attempt to address these two major problems through the careful selection of suitable metal atoms and the design and synthesis of a new class of compounds: Iridium-Bismuth carbonyl cluster complexes and modifying them by incorporating other catalytically important transition metals like gold and ruthenium and main group elements like tin and germanium which are also known to be valuable modifiers for heterogeneous catalysts, and then transforming these new bi- and multi-metallic cluster compounds into bi- and multi-metallic nanoparticles which could eventually serve as new catalysts for selective oxidation reactions.
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between δ(-) 14.60 – (-) 15.04 ppm for pyridines complexes and in the range of δ (-) 15.46 – (-) 19.98 ppm for imidazole’s complexes (Table 2). The imines C-H signals for the starting free imines appear at δ 9.01-9.44ppm and after complexation these signals are absent, providing evidence for insertion of Ir metal into the C-H bond of the imines. Strong confirmation evidence comes from appearance of the resonance of the hydride signal in each complex at high field (Foot and Heaton, 1973 and Wentzel, 2011), ca. (average) δ -17.29 ppm. The hydride signals in the complexes are split by coupling to two un-equivalents 31 P nuclei. As both of these spin-spin couplings are ca. 13.20-16.14Hz, frequently, 2J ( 31 P- 1 H), (Table 2). The hydride doublet of doublet often appears as a triplet due to the coupling between 31 P and 1 H, 2 J ( 31 P- 1 H) ca. 13.20-16.14Hz, (Figure 2 and 3), complexes (2 and 4). The phosphine (PPh 3 )
42 Morris and co-workers have developed a series of iron complexes with tetradentate PNNP ligands (Scheme 1.51). [129-131] Complex 76 represents the first well defined iron catalyst capable of ATH of aromatic ketones. Using only 0.5 mol% catalyst most of the aromatic ketones were fully reduced within half hour, although the enantioselectivities obtained were relatively low.  Complex 77, prepared with a chiral diphenylethylenediamine backbone significantly improved the activity and selectivity, affording a TOF up to 4900 h -1 and enantioselectivities up to 99% for the TH of ketones.  The analogous 78 is highly enantioselective for the ATH of N- (diphenylphosphinoyl) and N-(p-tolylsulphonyl) ketimines (Scheme 1.51).  These iron complexes represent viable alternative to precious metal catalytic systems for TH; however they are still in their infancy. The catalytic activity and substrate scope have to be further improved in order to compete with the catalysts based on precious metals. In addition, their sensitivity to air and moisture makes them difficult for industrial applications.
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