Photocatalytic Generation of Hydrogen from Water Using a Platinum(II) Terpyridyl Acetylide Chromophore

The cationic complex [Pt(tolylterpyridine)(phenylacetylide)]+ has been used as a photosensitizer for the reduction of aqueous protons in the presence of a sacrificial electron donor to make H2. In this system, triethanolamine (TEOA) acts as the sacrificial reducing agent, methyl viologen (MV2+) serves as an electron transfer agent, and colloidal Pt stabilized by polyacrylate functions as the catalyst for H2 generation. The Pt(II) chromophore undergoes both oxidative and reductive quenching, but H2 is only seen when both TEOA and MV2+ are present. Irradiation of the reaction solution for 10 h with λ > 410 nm leads to 85 turnovers and an overall yield of 34% based on TEOA. While H2 evolution is maximized for the system at pH 7, it is also seen at pH 5 and 9, in contrast with earlier reports using Ru(bpy)32+ as the photosensitizer. This is the first time that a Pt diimine or terpyridyl complex has been used as the photosensitizer for H2 generation from aqueous protons

Synthesis and Structural Characterization of a New Vapochromic Pt(II) Complex Based on the 1-Terpyridyl-2,3,4,5,6-pentaphenylbenzene (TPPPB) Ligand

A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)2Cl2, to produce the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be brightly luminescent in the solid state at room temperature and at 77 K, with no change in λemmax. The complex exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red (5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the presence of non-interacting Pt···Pt separations with distances of 3.9092(9) and 4.5483(11) Å and a zigzag arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt···Pt separations of 3.2981(14) and 3.3427(14) Å, indicative of metallophilic interaction. The change in the emitting state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d8−d8 metallophilic interactions in the red form and its metal−metal-to-ligand charge transfer (MMLCT) excited state to a more-localized Pt(dπ)-to-tpy(π*) metal-to-ligand charge transfer (MLCT) excited state in the green form

Synthesis and Structural Characterization of a New Vapochromic Pt(II) Complex Based on the 1-Terpyridyl-2,3,4,5,6-pentaphenylbenzene (TPPPB) Ligand

A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)2Cl2, to produce the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be brightly luminescent in the solid state at room temperature and at 77 K, with no change in λemmax. The complex exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red (5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the presence of non-interacting Pt···Pt separations with distances of 3.9092(9) and 4.5483(11) Å and a zigzag arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt···Pt separations of 3.2981(14) and 3.3427(14) Å, indicative of metallophilic interaction. The change in the emitting state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d8−d8 metallophilic interactions in the red form and its metal−metal-to-ligand charge transfer (MMLCT) excited state to a more-localized Pt(dπ)-to-tpy(π*) metal-to-ligand charge transfer (MLCT) excited state in the green form

Synthesis and Structural Characterization of a New Vapochromic Pt(II) Complex Based on the 1-Terpyridyl-2,3,4,5,6-pentaphenylbenzene (TPPPB) Ligand

A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)2Cl2, to produce the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be brightly luminescent in the solid state at room temperature and at 77 K, with no change in λemmax. The complex exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red (5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the presence of non-interacting Pt···Pt separations with distances of 3.9092(9) and 4.5483(11) Å and a zigzag arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt···Pt separations of 3.2981(14) and 3.3427(14) Å, indicative of metallophilic interaction. The change in the emitting state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d8−d8 metallophilic interactions in the red form and its metal−metal-to-ligand charge transfer (MMLCT) excited state to a more-localized Pt(dπ)-to-tpy(π*) metal-to-ligand charge transfer (MLCT) excited state in the green form

Photogeneration of Hydrogen from Water Using an Integrated System Based on TiO₂ and Platinum(II) Diimine Dithiolate Sensitizers

Two platinum(II) diimine dithiolate complexes, Pt(dcbpy)(met) (1) and Pt(dcbpy)(bdt) (2) (dcbpy = 4,4‘-dicarboxyl-2,2‘-bipyridine; met = cis-1,2-dicarbomethoxyethylene-1,2-dithiolate; bdt = 1,2-benzenedithiolate) were used as sensitizers for platinized TiO2 (TiO2/Pt), and systems composed of 1/TiO2/Pt and 2/TiO2/Pt were found to generate hydrogen from aqueous protons and a sacrificial electron donor, triethanolamine (TEOA), under visible light irradiation. Turnover numbers reached 84 after 95 h of irradiation for the first system and 72 after 73 h of irradiation for the second. Additional photolyses with light of wavelength longer than 455 nm showed that the systems are photostable with no loss of activity. Parallel experiments with Pt(diimine)(met) and Pt(diimine)(bdt) complexes that do not bind to TiO2 reveal that attachment of the sensitizer to TiO2 is essential for effective hydrogen generation

Visible Light-Driven Hydrogen Production from Aqueous Protons Catalyzed by Molecular Cobaloxime Catalysts

A series of cobaloxime complexes([Co(dmgH)2pyCl] (1), [Co(dmgH)2(4-COOMe-py)Cl] (2), [Co(dmgH)2(4-Me2N-py)Cl] (3), [Co(dmgH)(dmgH2)Cl2] (4), [Co(dmgH)2(py)2](PF6) (5), [Co(dmgH)2(P(n-Bu)3)Cl] (6), and [Co(dmgBF2)2(OH2)2] (7), where dmgH = dimethylglyoximate monoanion, dmgH2 = dimethylglyoxime, dmgBF2 = (difluoroboryl)dimethylglyoximate anion, and py = pyridinewere synthesized and studied as molecular catalysts for the photogeneration of hydrogen from systems containing a Pt terpyridyl acetylide chromophore and triethanolamine (TEOA) as a sacrificial donor in aqueous acetonitrile. All cobaloxime complexes 1−7 are able to quench the luminescence of the Pt(II) chromophore [Pt(ttpy)(CCPh)]ClO4 (C1) (ttpy = 4′-p-tolyterpyridine). The most effective electron acceptor for hydrogen evolution is found to be complex 2, which provides the fastest luminescence quenching rate constant for C1 of 1.7 × 109 M−1 s−1. The rate of hydrogen evolution depends on many factors, including the stability of the catalysts, the driving force for proton reduction, the relative and absolute concentrations of system components (TEOA, Co molecular catalyst, and sensitizer), and the ratio of MeCN/water in the reaction medium. For example, when the concentration of TEOA increases, the rate of H2 photogeneration is faster and the induction period is shorter. Colloidal cobalt experiments and mercury tests were run to verify that the system is homogeneous and that catalysis does not occur from in situ generated colloidal particles during photolysis. The most effective system examined to date consists of the chromophore C1 (1.1 × 10−5 M), TEOA (0.27 M), and catalyst complex 1 (2.0 × 10−4 M) in a MeCN/water mixture (24:1 v/v, total 25 mL); this system has produced ∼2150 turnovers of H2 after only 10 h of photolysis with λ > 410 nm

Synthesis, Electrochemistry, Photophysics, and Solvatochromism in New Cyclometalated 6-Phenyl-4-(<i>p</i>-R-phenyl)-2,2′-bipyridyl (R = Me, COOMe, P(O)(OEt)₂) (C^∧N^∧N) Platinum(II) Thiophenolate Chromophores

Three new cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (C∧N∧N) Pt(II) thiophenolate complexes (R = Me (2a), COOMe (2b), and P(O)(OEt)2 (2c)) have been synthesized and studied. The new C∧N∧N ligands L2 (R = COOMe) and L3 (R = P(O)(OEt)2) undergo cyclometalation with a Pt(II) source to give the Pt(II) chloro complexes 1b and 1c, respectively, which are luminescent in fluid solution with λmax ∼ 575 nm, assigned to a metal-to-ligand charge-transfer (3MLCT) emissive state. Reaction of the chloro complexes 1a (R = Me), 1b, and 1c with sodium thiophenolate gives 2a−2c, respectively, in good yields. The novel thiophenolate complexes have two interesting absorption bands in their electronic spectra tentatively assigned to a charge-transfer to C∧N∧N (1CT) (λabs ∼415 nm) transition and a mixed metal/ligand-to-ligand′ charge-transfer (MMLL′CT, λabs ∼ 555 nm) transition, respectively. The MMLL′CT band is solvatochromic with absorption maxima in the range of 496 nm in MeOH to 590 nm in toluene (ε ∼ 4000 dm3 mol−1 cm−1), which correlate well with an empirical charge-transfer-based solvent scale. Excitation of 2a−2c into the MMLL′CT band gives emission maxima around 680 nm in frozen CH2Cl2 solution, and no emission in fluid solution. Ligand L2 and complexes 1a·MeCN, 1b, and 2b·CH2Cl2 have been characterized by single crystal X-ray crystallography. The electrochemical properties of ligands L1 (R = Me) and L2 and complexes 1a−1c and 2a−2c have been examined by cyclic voltammetry and are shown to exhibit reversible and quasi-reversible reductions and irreversible oxidations

Synthesis, Electrochemistry, Photophysics, and Solvatochromism in New Cyclometalated 6-Phenyl-4-(<i>p</i>-R-phenyl)-2,2′-bipyridyl (R = Me, COOMe, P(O)(OEt)₂) (C^∧N^∧N) Platinum(II) Thiophenolate Chromophores

Three new cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (C∧N∧N) Pt(II) thiophenolate complexes (R = Me (2a), COOMe (2b), and P(O)(OEt)2 (2c)) have been synthesized and studied. The new C∧N∧N ligands L2 (R = COOMe) and L3 (R = P(O)(OEt)2) undergo cyclometalation with a Pt(II) source to give the Pt(II) chloro complexes 1b and 1c, respectively, which are luminescent in fluid solution with λmax ∼ 575 nm, assigned to a metal-to-ligand charge-transfer (3MLCT) emissive state. Reaction of the chloro complexes 1a (R = Me), 1b, and 1c with sodium thiophenolate gives 2a−2c, respectively, in good yields. The novel thiophenolate complexes have two interesting absorption bands in their electronic spectra tentatively assigned to a charge-transfer to C∧N∧N (1CT) (λabs ∼415 nm) transition and a mixed metal/ligand-to-ligand′ charge-transfer (MMLL′CT, λabs ∼ 555 nm) transition, respectively. The MMLL′CT band is solvatochromic with absorption maxima in the range of 496 nm in MeOH to 590 nm in toluene (ε ∼ 4000 dm3 mol−1 cm−1), which correlate well with an empirical charge-transfer-based solvent scale. Excitation of 2a−2c into the MMLL′CT band gives emission maxima around 680 nm in frozen CH2Cl2 solution, and no emission in fluid solution. Ligand L2 and complexes 1a·MeCN, 1b, and 2b·CH2Cl2 have been characterized by single crystal X-ray crystallography. The electrochemical properties of ligands L1 (R = Me) and L2 and complexes 1a−1c and 2a−2c have been examined by cyclic voltammetry and are shown to exhibit reversible and quasi-reversible reductions and irreversible oxidations

Platinum(II) Terpyridyl Acetylide Complexes on Platinized TiO₂: Toward the Photogeneration of H₂ in Aqueous Media

New platinum(II) terpyridyl acetylide complexes having the ability to bind to TiO2 have been synthesized and assayed in their ability to sensitize platinized titanium dioxide for the photogeneration of H2 using visible light (λ > 410 nm). Specifically, the complexes [Pt(tpy-phen-COOH)(CC−C6H5)]Cl (1), where tpy-phen-COOH = 4′-(4-carboxyphenyl)-[2,2′;6′,2′′]terpyridine and CC−C6H5 = phenylacetylide, and [Pt(tpy-COOH)(CC−C6H5)]Cl (2), where tpy-COOH = 4′-carboxy-2,2′;6′,2′′-terpyridine, were prepared to investigate the effectiveness of attachment and proximity to the TiO2 surface on hydrogen yield. Both complexes 1 and 2 sensitize the photogeneration of hydrogen, but produce fewer turnovers than the unbound chromophore, [Pt(ttpy)(CC−C6H5)]PF6 (5). On the basis of these observations and electrochemical data, a major limitation to the effectiveness of these chromophores is their instability upon oxidation. To attempt to remedy this problem, two donor-chromophore (D-C) dyads, [Pt(tpy-phen-COOH)(CC−C6H4CH2−PTZ)]PF6 (3), where CC−C6H4CH2−PTZ = N-(4-ethynylbenzyl)-phenothiazine and [Pt(tpy-COOH)(CC−C6H4CH2−PTZ)]Cl (4) were prepared to function as TiO2-attached sensitizers. Transient absorption measurements have shown that the PTZ moiety reductively quenches the Pt center in several picoseconds. While the resultant PTZ+ radical cation is capable of oxidizing rapidly the triethanolamine sacrificial electron donor, dyads 3 and 4 attached to platinized TiO2 do not function to generate hydrogen upon irradiation, in contrast with results seen for 1 and 2

Strong Intra- and Intermolecular Aurophilic Interactions in a New Series of Brilliantly Luminescent Dinuclear Cationic and Neutral Au(I) Benzimidazolethiolate Complexes

The structural and photophysical properties of a new series of cationic and neutral Au(I) dinuclear compounds (1 and 2, respectively) bridged by bis(diphenylphosphino)methane (dppm) and substituted benzimidazolethiolate (X−BIT) ligands, where X = H (a), Me (b), OMe (c), and Cl (d), have been studied. Monocationic complexes, [Au2(μ-X−BIT)(μ-dppm)](CF3CO2), were prepared by the reaction of [Au2(μ-dppm)](CF3CO2)2 with 1 equiv of X−BIT in excellent yields. The cations 1a−1d possess similar molecular structures, each with a linear coordination geometry around the Au(I) nuclei, as well as relatively short intramolecular Au(I)···Au(I) separations ranging between 2.88907(6) Å for 1d and 2.90607(16) Å for 1a indicative of strong aurophilic interactions. The cations are violet luminescent in CH2Cl2 solution with a λemmax of ca. 365 nm, assigned as ligand-based or metal-centered (MC) transitions. Three of the cationic complexes, 1a, 1b, and 1d, exhibit unusual luminescence tribochromism in the solid-state, in which the photoemission is shifted significantly to higher energy upon gentle grinding of microcrystalline samples with ΔE = 1130 cm-1 for 1a, 670 cm-1 (1b), and 870 cm-1 (1d). The neutral dinuclear complexes, [Au2(μ-X−BIT)(μ-dppm)] (2a−2d) were formed in good yields by the treatment of a CH2Cl2 solution of cationic compounds (1) with NEt3. 2a−2d aggregate to form dimers having substantial intra- and intermolecular aurophilic interactions with unsupported Au(I)···Au(I) intermolecular distances in the range of 2.8793(4)−2.9822(8) Å, compared with intramolecular bridge-supported separations of 2.8597(3)−2.9162(3) Å. 2a-2d exhibit brilliant luminescence in the solid-state and in DMSO solution with red-shifted λemmax energies in the range of 485−545 nm that are dependent on X−BIT and assigned as ligand-to-metal−metal charge transfer (LMMCT) states based in part on the extended Au···Au···Au···Au interactions