Selective Synthesis and Photophysical Properties of Phosphorescent Heteroleptic Iridium(III) Complexes with Two Different Bidentate Groups and Two Different Monodentate Ligands

Synthesis, Structure, and Properties of Nontrivial Iridium(III) Complexes Based on Anthracene-Decorated Benzimidazole Ligand

Reactions of iridium trichloride hydrate with bulky 2-(9-anthracenyl)-1-phenyl-benzimidazole (anbi) in the presence of N-donor ligands afforded a number of unique noncyclometalated complexes, while attempts to prepare a common μ-chloro-bridged bis-cyclometalated dimer systematically gave a monocyclometalated complex cis-[Ir(C,N-anbi)(N-anbi)Cl2] instead. The obtained complexes were characterized by 1H NMR, high-resolution mass spectrometry, single-crystal and powder X-ray diffraction, UV-vis spectroscopy, and cyclic voltammetry. The noncyclometalated complexes fac-[Ir(N-anbi)(N^N)Cl3)], where N^N are 4,4'-disubstituted 2,2'-bipyridines, are octahedral and contain the anthracene and 2,2'-bipyridine units in a close cofacial arrangement. These complexes were found to be exceptionally inert to the chloride ligand exchange even in the presence of silver triflate, forming a rare trinuclear Ir-μ-Cl3-Ag-μ-Cl3-Ir structure instead. In the monocyclometalated complex, the Ir(III) ion is pentacoordinated in a rare square-pyramidal geometry, where the bulky anthracene fragment is involved in the steric shielding of the metal center. This is in line with the results of gas-phase density functional theory calculations, demonstrating that the experimentally observed structure is energetically most preferable. The monocyclometalated complex is deeply colored due to intense charge-transfer absorption bands in the range 450-650 nm with ε = 2000-5000 M-1 cm-1, superior to the noncyclometalated complexes. The synthesis, structures, and properties of the new complexes are discussed in the context of the related mono-, bis-, and noncyclometalated iridium(III) compounds.

C-H, N-H, and O-H Bond Activations to Prepare Phosphorescent Hydride-Iridium(III)-Phosphine Emitters with Photocatalytic Achievement in C-C Coupling Reactions

Complex IrH5(PiPr3)2 (1) activates two different σ-bonds of 3-phenoxy-1-phenylisoquinoline, 2-(1H-benzimidazol-2-yl)-6-phenylpyridine, 2-(1H-indol-2-yl)-6-phenylpyridine, 2-(2-hydroxyphenyl)-6-phenylpyridine, N-(2-hydroxyphenyl)-N'-phenylimidazolylidene, and 1,3-di(2-pyridyl)-4,6-dimethylbenzene to give IrH{κ3-C,N,C-[C6H4-isoqui-O-C6H4]}(PiPr3)2 (2), IrH{κ3-N,N,C-[NBzim-py-C6H4]}(PiPr3)2 (3), IrH{κ3-N,N,C-[Ind-py-C6H4]}(PiPr3)2 (4), IrH{κ3-C,N,O-[C6H4-py-C6H4O]}(PiPr3)2 (5), IrH{κ3-C,C,O-[C6H4-Im-C6H4O]}(PiPr3)2 (6), and IrH{κ3-N,C,C-[py-C6HMe2-C5H3N]}(PiPr3)2 (7), respectively. The activations are sequential, with the second generally being the slowest. Accordingly, dihydride intermediates IrH2{κ2-C,N-[C6H4-isoqui-O-C6H5]}(PiPr3)2 (2d), IrH2{κ2-N,N-[NBzim-py-C6H5]}(PiPr3)2 (3d), IrH2{κ2-N,N-[Ind-py-C6H5]}(PiPr3)2 (4d), and IrH2{κ2-N,C-[py-C6HMe2-py]}(PiPr3)2 (7d) were characterized spectroscopically. Complexes 3 and 5 are green phosphorescent emitters upon photoexcitation, exhibiting good absorption over a wide range of wavelengths, emission quantum yields about 0.70 in solution, long enough lifetimes (10-17 μs), and reversible electrochemical behavior. In agreement with these features, complex 3 promotes the photocatalytic α-amino C(sp3)-H arylation of N,N-dimethylaniline and N-phenylpiperidine with 1,4-dicyanobenzene and 4-cyanopyridine under blue LED light irradiation. The C-C coupling products are isolated in high yields with only 2 mol % of photocatalyst after 24 h.

Competition between N,C,N-Pincer and N,N-Chelate Ligands in Platinum(II)

Replacement of the chloride ligand of PtCl{κ3-N,C,N-[py-C6HR2-py]} (R = H (1), Me (2)) and PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (3) by hydroxido gives Pt(OH){κ3-N,C,N-[py-C6HR2-py]} (R = H (4), Me (5)) and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (6). These compounds promote deprotonation of 3-(2-pyridyl)pyrazole, 3-(2-pyridyl)-5-methylpyrazole, 3-(2-pyridyl)-5-trifluoromethylpyrazole, and 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole. The coordination of the anions generates square-planar derivatives, which in solution exist as a unique species or equilibria between isomers. Reactions of 4 and 5 with 3-(2-pyridyl)pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[R'pz-py]} (R = H; R' = H (7), Me (8). R = Me; R' = H (9), Me (10)), displaying κ1-N1-pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N1-to-N2 slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[CF3pz-py]} (R = H (11a), Me (12a)) and Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N2-[CF3pz-py]} (R = H (11b), Me (12b)). 1,3-Bis(2-pyridyloxy)phenyl allows the chelating coordination of the incoming anions. Deprotonations of 3-(2-pyridyl)pyrazole and its substituted 5-methyl counterpart promoted by 6 lead to equilibria between Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[R'pz-py]} (R' = H (13a), Me (14a)) with a κ-N1-pyridylpyrazolate anion, keeping the pincer coordination of the di(pyridyloxy)aryl ligand, and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[R'pz-py]} (R' = H (13c), Me (14c)) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[CF3pz-py]} (15a), Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N2-[CF3pz-py]} (15b), and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[CF3pz-py]} (15c). The N1-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of 4-6 with 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole yield Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[(CF3)2C4(py)HN]} (R = H (16), Me (17)) or Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[(CF3)2C4(py)HN]} (18), displaying κ1-N1-pyrrolate coordination. Complexes 7-10 are efficient green phosphorescent emitters (488-576 nm). In poly(methyl methacrylate) (PMMA) films and in dichloromethane, they experience self-quenching, due to molecular stacking. Aggregation occurs through aromatic π-π interactions, reinforced by weak platinum-platinum interactions.

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates

The hexahydride complex OsH₆(PⁱPr₃)₂ promotes the C-H bond activation of the 1,3-disubstituted phenyl group of the [BF₄]^- and [BPh₄]^- salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ²-C,N- or κ²-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ³-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ²-C,N- and κ²-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF₄]^- inhibits the azolium reactions, [BPh₄]^- favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C-H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Insertion of Unsaturated C-C Bonds into the O-H Bond of an Iridium(III)-Hydroxo Complex: Formation of Phosphorescent Emitters with an Asymmetrical β-Diketonate Ligand

A synthetic methodology to prepare iridium(III) emitters of the class [3b+3b+3b'] with two ortho-metalated 1-phenylisoquinolines and an asymmetrical β-diketonate has been discovered. The abstraction of the chloride ligands of the dimer [Ir(μ-Cl){κ²-C,N-(C₆H₄-isoqui)}₂]₂ (1, C₆H₅-isoqui = 1-phenylisoquinoline) with AgBF₄ in acetone and the subsequent addition of water to the resulting solution affords the water solvate mononuclear complex [Ir{κ²-C,N-(C₆H₄-isoqui)}₂(H₂O)₂]BF₄ (2), which reacts with KOH to give the dihydroxo-bridged dimer [Ir(μ-OH){κ²-C,N-(C₆H₄-isoqui)}₂]₂ (3). Treatment of the latter with dimethyl acetylenedicarboxylate leads to Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CO₂CH₃)CHC(OCH₃)O]} (4), as a result of the anti-addition of the O-H bond of a mononuclear [Ir(OH){κ²-C,N-(C₆H₄-isoqui)}₂] fragment to the C-C triple bond of the alkyne and the coordination of one of the carboxylate substituents to the metal center. Complex 3 also reacts with α,β-unsaturated ketones. The reaction with 3-(4-methylphenyl)-1-phenylprop-2-en-1-one affords Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(C₆H₅)CHC(p-C₆H₄Me)O]} (5), whereas methyl vinyl ketone gives a mixture of Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CH₃)CHCHO]} (6) and Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CH₃)CHC(CH═CH₂)O]} (7). Complexes 5 and 6 are the result of the addition of the O-H bond of the mononuclear [Ir(OH){κ²-C,N-(C₆H₄-isoqui)}₂] fragment to the C-C double bond of the α,β-unsaturated ketones and the coordination of the carbonyl group to the iridium center, to generate O,O-chelates which lose molecular hydrogen to aromatize into the asymmetrical β-diketonate ligands. Complexes 4-7 are phosphorescent emitters in the red spectral region (599-672 nm) in doped poly(methyl methacrylate) (PMMA) film at 5 wt % at room temperature and 2-methyltetrahydrofuran at room temperature and 77 K. They display short lifetimes (0.8-2.5 μs) and quantum yields in both doped PMMA films and in 2-methyltetrahydrofuran at room temperature depending on the substituents of the β-diketonate: about 0.6-0.5 for 4 and 6 and ca. 0.35 for 5 and 7.

Theoretical insight into the photophysical properties of six heteroleptic Ir(iii) phosphorescent complexes bearing ppy-type ligands

By using density functional theory and time-dependent density functional theory, the geometrical, electronic and photophysical properties of six complexes with two ppy-type ligands and one acetylacetone anion around the Ir center have been explored. The lowest energy absorption wavelengths are located at 414 nm for 1, 434 nm for 2, 434 nm for 3, 421 nm for 4, 436 nm for 5, and 425 nm for 6, respectively. The lowest energy emissions of these complexes are localized at 617, 492, 633, 634, 491 and 491 nm, respectively, for complexes 1-6, simulated in CH2Cl2 medium at the M062X level. The calculated lowest lying absorption wavelength and the lowest energy emission wavelength for complex 3 are very close to the available experimental values. The position and number of the incorporated electron-withdrawing fluorine substituents have some effect on the electronic and photophysical properties of these studied complexes.

Emissive Iridium(III) Complexes with Phosphorous-Containing Ancillary

Ir(III) metal complexes and related emitters bearing all kind of cyclometalated chromophoric chelates and non-chromophoric ancillary are extensively studied during the past three decades. Many of them have been found to display bright room temperature phosphorescence from triplet excited states in both solution and solid states, offering a possible application in contemporary optoelectronic technologies, including organic light emitting diodes, electrochemiluminescence, biological imaging and chemical sensing. Among reported materials, there are Ir(III) complexes with at least one phosphorus (P)-containing ligand and/or ancillary chelate, together with cyclometalates or equivalents that are in control of the actual emission energy. Particularly, possession of P-based donor can lead to divergent structural and photophysical properties compared to the traditional designs. This review aims to provide a literature overview as well as the authors' personal account to the development of relevant Ir(III) based phosphors bearing these P-donors. To the readers' convenience, the contents are subdivided into six sessions, according to whether or not they are charge natural, or with mono- or dianionic electronic character, and in accordance to their divergent bonding modes, i. e. monodentate, bidentate and tripodal coordination. In many cases, the P-based ancillaries offer an easy accessible route to the formation of efficient sky-blue and true-blue emitters due to their π-accepting property, together with enlarged emission energy gap and destabilized upper lying quenching state.

Acetylene in Organic Synthesis: Recent Progress and New Uses

Recent progress in the leading synthetic applications of acetylene is discussed from the prospect of rapid development and novel opportunities. A diversity of reactions involving the acetylene molecule to carry out vinylation processes, cross-coupling reactions, synthesis of substituted alkynes, preparation of heterocycles and the construction of a number of functionalized molecules with different levels of molecular complexity were recently studied. Of particular importance is the utilization of acetylene in the synthesis of pharmaceutical substances and drugs. The increasing interest in acetylene and its involvement in organic transformations highlights a fascinating renaissance of this simplest alkyne molecule.