An outer-sphere two-electron platinum reagent

Mechanism guided two-electron energy storage for redox-flow batteries using nickel bis(diphosphine) complexes

The storage of multiple electrons per molecule can greatly enhance the energy density of redox-flow batteries (RFBs). Here, we show that nickel bis(diphosphine) complexes efficiently store multiple electrons through either sequential 1e- redox waves or a concerted 2e- redox wave, depending on their coordination environment. Mechanistic studies comparing ligand sterics (-Me vs. -Ph) and coordination of monodentate ligands (MeCN vs. Cl-) allow for selective control of the electron transfer pathway, steering electron storage toward the more favorable 2e- wave. Continuous charge-discharge cycling experiments show more negative charge-discharge potentials and improved capacity retention in the presence of Cl-, thus improving the energy storage of nickel bis(diphosphine) complexes as anolytes in RFBs. This work shows how mechanistic understanding of 2e- redox cycles for transition metal complexes can create new opportunities for multi-electron storage in RFBs.

Role of Solvent Coordination in the Multi-electron Redox Cycle of Nickel Diethyldithiocarbamate

Nickel(II) diethyldithiocarbamate, NiII(dtc)2, is known to undergo a 2e- ligand-coupled electron transfer (LCET) oxidation to form [NiIV(dtc)3]+. However, the thermodynamics and kinetics of this 2e- process can be greatly affected by solvent coordination. For low coordinating solvents like acetonitrile and acetone, 2e- oxidation is observed via cyclic voltammetry (CV) at a single potential while stronger coordinating solvents like methanol, N,N-dimethylformamide, dimethyl sulfoxide, and pyridine exhibit a 1e- oxidation wave by formation of [NiIII(dtc)2(sol)x]+ intermediates. The decay of these complexes to eventually yield [NiIV(dtc)3]+ was monitored as a function of CV scan rate and temperature to extract rate constants and activation parameters. A thorough analysis of activation parameters revealed that ΔHapp⧧ generally increased with solvent coordination ability, suggesting solvent dissociation was a key factor in the rate limiting step. However, ΔSapp⧧ was found to be negative for all solvents, suggesting an associative mechanism in line with dimer formation with NiII(dtc)2 to facilitate ligand exchange. Density function theory calculations supported the competitive nature of dissociative and associative steps. Using these calculations, we propose two paths for decay of [NiIII(dtc)2(sol)x]+ species based on the coordination strength of the solvent. These studies point to the ability of solvents to either aid or hinder multielectron LCET reactions.

Efficiently luminescent heteroleptic neutral platinum(II) complexes based on N^O and N^P benzimidazole ligands

A series of new luminescent cycloplatinated(II) complexes (5a-8a and 5b-8b) with formulas Pt(bt)(N^O) and Pt(bt)(N^P) have been synthesized [bt = phenylbenzothiazole, N^O = (2-(1H-benzimidazole)-phenyl)diphenylphosphine oxide derivatives for 1a-4a and N^P = (2-(1H-benzimidazole)-phenyl)diphenylphosphine derivatives for 1b-4b]. The crystal structures of the complexes show distorted square planar geometries around the platinum centers. There are no obvious π-π and Pt-Pt intermolecular interactions in the crystal lattice due to the presence of sterically bulky ancillary ligands. Consequently, these complexes exhibit structured monomeric emissions in the range of 527-540 nm in CH₂Cl₂ solution. The photoluminescent quantum yields of Pt(bt)(N^O) (5a-8a) in CH₂Cl₂ solution at room temperature are higher than those of Pt(bt)(N^P) (5b-8b). The above result is well consistent with the crystal structural characteristics of the complexes. The structured emission with microsecond radiative lifetimes and the result of TD-DFT calculations indicate that the emissions of these complexes are mainly attributed to a mixed ³LC-MLCT state.

Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate

The unique redox cycle of Ni^II(dtc)₂, where dtc^- is N,N-diethyldithiocarbamate, in acetonitrile displays 2e^- redox chemistry upon oxidation from Ni^II(dtc)₂ → [Ni^IV(dtc)₃]⁺ but 1e^- redox chemistry upon reduction from [Ni^IV(dtc)₃]⁺ → Ni^III(dtc)₃ → Ni^II(dtc)₂. The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate Ni^II(dtc)₂ and six-coordinate [Ni^IV(dtc)₃]⁺. Cyclic voltammetry (CV) experiments show that these 1e^- and 2e^- pathways can be controlled by the addition of pyridine-based ligands (L) to the electrolyte solution. Specifically, the addition of these ligands resulted in a 1e^- ligand-coupled electron transfer (LCET) redox wave, which produced a mixture of pyridine-bound Ni(III) complexes, [Ni^III(dtc)₂(L)]⁺, and [Ni^III(dtc)₂(L)₂]⁺. Although the complexes could not be isolated, electron paramagnetic resonance (EPR) measurements using a chemical oxidant in the presence of 4-methoxypyridine confirmed the formation of trans-[Ni^III(dtc)₂(L)₂]⁺. Density functional theory calculations were also used to support the formation of pyridine coordinated Ni(III) complexes through structural optimization and calculation of EPR parameters. The reversibility of the LCET process was found to be dependent on both the basicity of the pyridine ligand and the scan rate of the CV experiment. For strongly basic pyridines (e.g., 4-methoxypyridine) and/or fast scan rates, high reversibility was achieved, allowing [Ni^III(dtc)₂(L)_x]⁺ to be reduced directly back to Ni^II(dtc)₂ + xL. For weakly basic pyridines (e.g., 3-bromopyridine) and/or slow scan rates, [Ni^III(dtc)₂(L)_x]⁺ decayed irreversibly to form [Ni^IV(dtc)₃]⁺. Detailed kinetics studies using CV reveal that [Ni^III(dtc)₂(L)]⁺ and [Ni^III(dtc)₂(L)₂]⁺ decay by parallel pathways due to a small equilibrium between the two species. The rate constants for ligand dissociation ([Ni^III(dtc)₂(L)₂]⁺ → [Ni^III(dtc)₂(L)]⁺ + L) along with decomposition of [Ni^III(dtc)₂(L)]⁺ and [Ni^III(dtc)₂(L)₂]⁺ species were found to increase with the electron-withdrawing character of the pyridine ligand, indicating pyridine dissociation is likely the rate-limiting step for decomposition of these complexes. These studies establish a general trend for kinetically trapping 1e^- intermediates along a 2e^- oxidation path.

pH-Mediated Colorimetric and Luminescent Sensing of Aqueous Nitrate Anions by a Platinum(II) Luminophore@Mesoporous Silica Composite

Increased levels of nitrate (NO₃^-) in the environment can be detrimental to human health. Herein, we report a robust, cost-effective, and scalable, hybrid material-based colorimetric/luminescent sensor technology for rapid, selective, sensitive, and interference-free in situ NO₃^- detection. These hybrid materials are based on a square-planar platinum(II) salt [Pt(tpy)Cl]PF₆ (tpy = 2,2';6',2″-terpyridine) supported on mesoporous silica. The platinum salt undergoes a vivid change in color and luminescence upon exposure to aqueous NO₃^- anions at pH ≤ 0 caused by substitution of the PF₆^- anions by aqueous NO₃^-. This change in photophysics of the platinum salt is induced by a rearrangement of its crystal lattice that leads to an extended Pt···Pt···Pt interaction, along with a concomitant change in its electronic structure. Furthermore, incorporating the material into mesoporous silica enhances the surface area and increases the detection sensitivity. A NO₃^- detection limit of 0.05 mM (3.1 ppm) is achieved, which is sufficiently lower than the ambient water quality limit of 0.16 mM (10 ppm) set by the United States Environmental Protection Agency. The colorimetric/luminescence of the hybrid material is highly selective to aqueous NO₃^- anions in the presence of other interfering anions, suggesting that this material is a promising candidate for the rapid NO₃^- detection and quantification in practical samples without separation, concentration, or other pretreatment steps.

Luminescent Pt(2,6-bis(N-methylbenzimidazol-2-yl)pyridine)X+: a comparison with the spectroscopic and electrochemical properties of Pt(tpy)X+ (X = Cl, CCPh, Ph, or CH3)

A series of platinum(ii) pincer complexes of the formula Pt(mbzimpy)X+, 1(a-d), (mbzimpy = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine; X = Cl; (a), CCPh; (b), Ph; (c), or CH3; (d), CCPh = phenylacetylide, and Ph = Phenyl) have been synthesized and characterized. Electronic absorption and emission, as well as electrochemical properties of these compounds, have been investigated. Pt(tpy)X+ analogs (tpy = 2,2';6'2''-terpyridine), 2(a-d), have also been investigated and compared. Electrochemistry shows that 1 and 2 analogs undergo two chemically reversible one-electron reduction processes that are shifted cathodically along the a < b < c < d series. Notably, these reductions occur at slightly higher negative potentials in the case of 1. The absorption spectra of 1 and 2 in acetonitrile exhibit ligand-centered (1LC) transitions (ε ≈ 104 M-1 cm-1) in the UV region and metal-to-ligand-charge transfer (1MLCT) transitions (ε ≈ 103 M-1 cm-1) in the visible region. The corresponding visible bands of 1b and 2b have been assigned to 1(LLCT/MLCT) mixed state (LLCT: ligand-to-ligand-charge transfer). The preceding 1LC and 1MLCT transitions of 1 occur at lower energies than that of 2. These 1LC transitions have distinctly been blue-shifted along a < c < d in 2, but occur at nearly identical energies in 1. Conversely, 1MLCT transitions are red-shifted along a < c < d in both the analogs. The 77 K glassy solutions of 1 and 2 exhibit an intense vibronically-structured emission band at λmax(0-0) in the 470-560 nm range. This band is red-shifted along b < a ≤ c < d in 1 and along a ≤ d ≈ c ≪ b in 2. The main character of these emissions is assigned to 3LLCT emissive state in 1b and 2b, whereas to 3LC in the rest of the compounds. Relative stabilization of these spin-forbidden emissive states is discussed by invoking configuration mixing with the higher-lying 3MLCT state.

Electrochemical Reoxidation Enables Continuous Methane-to-Methanol Catalysis with Aqueous Pt Salts

The direct conversion of methane to methanol would enable better utilization of abundant natural gas resources. In the presence of stoichiometric Pt^IV oxidants, Pt^II ions are capable of catalyzing this reaction in aqueous solutions at modest temperatures. Practical implementation of this chemistry requires a viable strategy for replacing or regenerating the expensive Pt^IV oxidant. Herein, we establish an electrochemical strategy for continuous regeneration of the Pt^IV oxidant to furnish overall electrochemical methane oxidation. We show that Cl-adsorbed Pt electrodes catalyze facile oxidation of Pt^II to Pt^IV at low overpotential without concomitant methanol oxidation. Exploiting this facile electrochemistry, we maintain the Pt^II/IV ratio during Pt^II-catalyzed methane oxidation via in situ monitoring of the solution potential coupled with dynamic modulation of the electric current. This approach leads to sustained methane oxidation catalysis with 70% selectivity for methanol.

Tuning two-electron transfer in terpyridine-based platinum(ii) pincer complexes

An important factor in obtaining reversible multi-electron transfer is overcoming large changes in coordination geometry. One strategy is to use ligands that can support the geometries favored before and after the electron transfer. Pip₂NCN⁻ pincer and terpyridine ligands are used to support square planar Pt(ii) and octahedral Pt(iv). For the Pt(ii) complexes, [Pt(Z-pip₂NCN)(R-tpy)]⁺ (Z = NO₂, MeO, H; R = H, tertyl butyl, tolyl), ¹H NMR spectroscopy shows that the Z-pip₂NCN⁻ ligand is monodentate whereas the R-terpyridyl ligand is tridentate. The availability of flanking piperidyl groups of the monodentate pincer ligand is essential for the stabilization of the metal center upon oxidation. Pt(Z-pip₂NCN)(R-tpy)⁺ complexes undergo two-electron platinum centered oxidation near 0.4 V and two Pt(tpy) centered reductions near −1.0 V and −1.5 V. An estimate of n_ox/n_red = 1.8 is consistent with an oxidation that involves two-electron transfer per Pt center. Variation in the pincer-(Z) and terpyridine-(R) substituents allows for tuning of the oxidation process over a 260 mV range and the two reduction processes over ranges of 230 mV (first reduction) and 290 mV (second reduction step).

Square planar Pt(ii) terpyridine complexes with pincer ligands undergo two-electron oxidation and variation in the ligand substituents allows for tuning of the two-electron oxidation process over a 260 mV range.

Excited-State Processes of Cyclometalated Platinum(II) Charge-Transfer Dimers Bridged by Hydroxypyridines

A series of four anti-disposed dinuclear platinum(II) complexes featuring metal-metal-to-ligand charge-transfer (MMLCT) excited states, bridged by either 2-hydroxy-6-methylpyridine or 2-hydroxy-6-phenylpyridine and cyclometalated with 7,8-benzoquinoline or 2-phenylpyridine, are presented. The 2-hydroxypyridine bridging ligands control intramolecular d⁸-d⁸ metal-metal σ interactions, affecting the frontier orbitals' electronic structure, resulting in marked changes to the ground- and excited-state properties of these complexes. Three of these molecules possess reversible one-electron oxidations in cyclic voltammetry experiments as a result of strong intramolecular metallophilic interactions. In this series of molecules, X-ray crystallography revealed Pt-Pt distances ranging between 2.815 and 2.878 Å; the former represents the shortest reported metal-metal distance for platinum(II) dimers possessing low-energy MMLCT transitions. All four molecules reported here display visible absorption bands beyond 500 nm and feature MMLCT-based red photoluminescence (PL) above 700 nm at room temperature with high PL quantum yields (up to 4%) and long excited-state lifetimes (up to 341 ns). The latter were recorded using both transient PL and transient absorption experiments that self-consistently yielded quantitatively identical excited-state lifetimes. The energy-gap law was successfully applied to this series of chromophores, documenting this behavior for the first time in molecules possessing MMLCT excited states. The combined data illustrate that entirely new classes of MMLCT chromophores can be envisioned using bridging pyridyl hydroxides in cooperation with various C^N cyclometalates to achieve photophysical properties suitable for excited-state electron- and energy-transfer chemistry.

Catalytic Methane Monofunctionalization by an Electrogenerated High-Valent Pd Intermediate

Electrophilic high-valent metal ions are potent intermediates for the catalytic functionalization of methane, but in many cases, their high redox potentials make these intermediates difficult or impossible to access using mild stoichiometric oxidants derived from O2. Herein, we establish electrochemical oxidation as a versatile new strategy for accessing high-valent methane monofunctionalization catalysts. We provide evidence for the electrochemical oxidation of simple PdSO4 in concentrated sulfuric acid electrolytes to generate a putative Pd2III,III species in an all-oxidic ligand field. This electrogenerated high-valent Pd complex rapidly activates methane with a low barrier of 25.9 (±2.6) kcal/mol, generating methanol precursors methyl bisulfate (CH3OSO3H) and methanesulfonic acid (CH3SO3H) via concurrent faradaic and nonfaradaic reaction pathways. This work enables new electrochemical approaches for promoting rapid methane monofunctionalization.

Gold(iii) assisted C–H activation of 1,4,7-trithiacyclononane: synthesis and spontaneous resolution of a bicyclic chiral sulfonium salt

A trithiamacrocyclic ligand complex of Au(iii) undergoes a redox-mediated thermal reaction to form a chiral bicyclic sulfonium salt.

Photophysical properties of cyclometalated Pt(II) complexes: counterintuitive blue shift in emission with an expanded ligand π system

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C(^)N)Pt(O(^)O) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (C(^)N = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); O(^)O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C(^)N ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (E(0-0) = 479 nm, 1; E(0-0) = 495 nm, 2; E(0-0) = 470 nm, 3) with disparate radiative rate constants (kr = 1.4 × 10(5) s(-1), 1; kr = 0.10 × 10(5) s(-1), 2; kr = 2.6 × 10(5) s(-1), 3). Zero-field splittings of the triplet states (ΔE(III-I) = 11.5 cm(-1), 1'; ΔE(III-I) < 2 cm(-1), 2; ΔE(III-I) = 46.5 cm(-1), 3) were determined using high resolution spectra recorded in Shpol'skii matrices. The fact that the E0-0 energies do not correspond to the extent of π-conjugation in the aromatic C(^)N ligand is rationalized on the basis of structural distortions that occur upon cyclometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the C(^)N ligands in the triplet state alter the spin-orbit coupling in the complexes.

X-ray and synchrotron diffraction studies of 2-(pyridin-2-yl)-1,10-phenanthroline in the role of ligand for two copper polymorphs or hydrogen bonded with 2,2,6,6-tetramethyl-4-oxopiperidinium hexafluorophosphate

Different extended packing motifs of dichlorido[2-(pyridin-2-yl)-1,10-phenanthroline]copper(II), [CuCl2(C17H11N3)], are obtained, depending on the crystallization conditions. A triclinic form, (I), is obtained from dimethylformamide-diethyl ether or methanol, whereas crystallization from dimethylformamide-water yields a monoclinic form, (II). In each case, the Cu(II) centre is in a five-coordinate distorted square-pyramidal geometry. The extended packing for both forms can be described as a highly offset π-stacking arrangement, with interlayer distances of 3.674 (3) and 3.679 (3) Å for forms (I) and (II), respectively. The reaction of diprotonated Pt(tmpip2NCN)Cl [tmpip2NCN = 2,6-bis(2,2,6,6-tetramethylpiperidylmethyl)benzyl] with AgPF6 under acidic conditions, followed by the addition of 2-(pyridin-2-yl)-1,10-phenanthroline, results in a hydrogen-bonded cocrystal, 2,2,6,6-tetramethyl-4-oxopiperidinium hexafluorophosphate-2-(pyridin-2-yl)-1,10-phenanthroline (1/1), C9H18NO(+)·PF6(-)·C17H11N3, (III). The extended packing maximizes π-π interactions in a parallel face-to-face arrangement, with an interlayer stacking distance of 3.4960 (14) Å.

Photooxidation of a platinum-anthracene pincer complex: formation and structures of Pt(II)-anthrone and -ketal complexes

Irradiating the cyclometalated pincer complex Pt(II)(DPA)Cl (1, DPA = 1,8-bis(diphenylphosphino)anthracene) in the presence of O(2) led to three sequential oxidations of the anthracenyl ring. The first photoproduct, a Pt(II)-9,10-endoperoxide complex, was converted photochemically to a Pt(II)-9-hydroxyanthrone complex A which was further oxygenated to a Pt(II)-hemiketal (B). The oxidation of A, which could be accelerated by light irradiation, probably involved a Pt(II)-anthraquinone intermediate. B underwent acid-catalyzed ketalization to form a binuclear Pt(II)(2)-diketal (B1). The photolysis was followed by UV-vis absorption and NMR spectroscopy, and the structures of A and B1 were characterized by single crystal X-ray diffraction.

Combined tight-binding/DFT investigation of the electronic structure of triimine-platinum(II)/TCNQ extended stacks,

. The [Pt(tbtrpy)(X)][TCNQ] (X = OH or SH) complexes form sandwich stacks with nitrile acceptors leading to extended-chain supramolecular assemblies, tbtrpy = 4,4′,4″-tert-Bu3-2,2′:6′,2″-terpyridine. Calculations with the extended Hückel tight-binding (EHTB) method are performed upon crystalline {[Pt (tbtrpy)(X)][TCNQ]}∞ species to analyze their electronic structure and consequent properties, TCNQ = 7,7,8,8-tetracyanoquinodimethane. The donor/acceptor extended chains in the solid state are predicted to exhibit metallic behavior with a large contribution from π and π* bands of TCNQ to the valence and conduction bands, respectively. Moreover, the valence band moves upward (i.e., to a less negative energy) for X = SH as compared to X = OH. Density functional theory (DFT) calculations suggest that this is due to large thiolate character in the HOMO of the square-planar donor complex, which also supports the experimental assignment of the electronic absorption bands and redox potentials. Calculations of infrared (νCN bands of TCNQ) and structural (CC bond lengths within TCNQ) data explain the metallic behavior of the stacks in terms of charge delocalization, leading to fractionally-charged species of the form [Pt(tbtrpy)X](1+δ)+[TCNQ](1+δ)- with δ > 0 and a greater δ value for X = SH vs OH.

Rhodium Dimers with 2,2-Dimethyl-1,3-diisocyano and Bis(diphenylphosphino)methane Bridging Ligands

Four rhodium dimers have been synthesized with a bridging diisocyanide ligand, dmb (2,2-dimethyl-1,3-diisocyanopropane): [Rh2(dmb)4](BPh4)2, [Rh2(dmb)4Cl2]Cl2, [Rh2(dmb)4I2](PF6)2, and [Rh2(dmb)2(dppm)2](BPh4)2 (dppm = bis(diphenylphosphino)methane). The complexes have been characterized by elemental analysis and mass spectrometry, as well as UV-visible, IR, and 1H NMR spectroscopies. X-ray crystal structures of the rhodium(I) complexes, [Rh2(dmb)4](BPh4)2 . 1.5CH3CN (3.2330(4), 3.2265(4) A) and [Rh2(dmb)2(dppm)2](BPh4)2.0.5CH3OH . 0.2H2O (3.0371(5) A), confirm the existence of short Rh...Rh interactions. The metal-metal separation for the rhodium(II) adduct, [Rh(2)(dmb)4Cl2]Cl2.6CHCl3 (2.8465(6) A), is consistent with a formal Rh-Rh bond. For the two luminescent rhodium(I) dimers and six previously investigated diisocyano-bridged dimers with and without dppm ligands, the intense spin-allowed dsigma-->psigma absorption band maximum shifts to longer wavelengths with decreasing Rh...Rh separation, and there is an approximate correlation between band energy and the inverse of the metal-metal separation cubed. Both [Rh2(dmb)4]2+ and [Rh2(dmb)4(dppm)2]2+ undergo oxidative addition in the presence of iodine. In the conversion of [Rh2(dmb)4]2+ to [Rh2(dmb)4I2]2+, the observed intermediate is tentatively assigned to a tetramer composed of two rhodium dimers. In the case of [Rh2(dmb)2(dppm)2]2+, no intermediate was detected.

A Novel Heteroditopic Terpyridine-Pincer Ligand as Building Block for Mono- and Heterometallic Pd(II) and Ru(II) Complexes

A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)2-3,5}-2,2':6',2' '-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)2, and [Ru(TPBr)2](PF6)2, synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)2 (7) and [Ru(TPPdCl)2](PF6)2 (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)2](PF6)2 (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono- and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (1)MLCT (Ru --> tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)2]2+ indicates stabilization of the luminescent 3MLCT state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)2, are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)2} chromophoric unit and the (non)metalated NCN-pincer moiety.

Synthesis, Structure, Characterization, and Photophysical Studies of a New Platinum Terpyridyl-Based Triad with Covalently Linked Donor and Acceptor Groups

A new terpyridyl-containing Pt triad [Pt(pytpy)(p-CC-C6H4-NH-CO-C6H2(OMe)3)](PF6)2 (4), where pytpy = 4'-(4-pyridin-1-ylmethylphenyl)-[2,2';6',2' ']terpyridine and p-CC-C6H4-NH-CO-C6H2(OMe)3 = N-(4-ethynylphenyl)-3,4,5-trimethoxybenzamide, has been synthesized and structurally characterized. The related donor-chromophore dyad [Pt(ttpy)(p-CC-C6H4-NH-CO-C6H2(OMe)3)]PF6 2, where ttpy = 4'-p-tolyl-[2,2';6',2' ']terpyridine, and the chromophore-acceptor dyad [Pt(pytpy)(CCC6H5)](PF6)2 (3), where CCC6H5 = ethynylbenzene, have also been studied. The multistep syntheses culminate with a CuI-catalyzed coupling reaction of the respective acetylene with either [Pt(ttpy)Cl]PF6 or [Pt(pytpy)Cl](PF6)2. X-ray and spectroscopic studies support assignment of a distorted square planar environment around the Pt(II) ion with three of its coordination sites occupied by the terpyridyl N-donors and the fourth coordination site occupied by the acetylenic carbon. Although the parent compound [Pt(ttpy)(CCC6H5)]PF6 (1) is brightly luminescent in fluid solution at 298 K, dyad 2 as well as triad 4 exhibit complete quenching of the emission. The chromophore-acceptor (C-A) dyad 3 displays weak solution luminescence at room temperature with a phi(rel)(em) of 0.011 (using Ru(bpy)3(2+) as a standard with phi(rel)(em) = 0.062). Electrochemically, the donor-chromophore (D-C) dyad and the donor-chromophore-acceptor (D-C-A) triad exhibit both metal-based and donor ligand-based oxidations, whereas the triad and the C-A dyad show the expected pyridinium- and terpyridine-based reductions. Transient absorption studies of the dyad and triad systems indicate that although the trimethoxybenzene group acts as a reductive donor, in the present system, the pyridinium group fails to act as an acceptor.

Ultrafast Energy Migration in Platinum(II) Diimine Complexes Bearing Pyrenylacetylide Chromophores

The ultrafast excited-state dynamics of three structurally related platinum(II) complexes has been investigated using femtosecond transient absorption spectrometry in 2-methyltetrahydrofuran (MTHF). Previous work has shown that Pt(dbbpy)(C[triple bond]C-Ph)2 (dbbpy is 4,4'-di(tert-butyl)-2,2'-bipyridine and C[triple bond]C-Ph is ethynylbenzene) has a lowest metal-to-ligand charge transfer (3MLCT) excited state, while the multichromophoric Pt(dbbpy)(C[triple bond]C-pyrene)2 (CC-pyrene is 1-ethynylpyrene) contains the MLCT state, but possesses a lowest intraligand (3IL) excited state localized on one of the CC-pyrenyl units (Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2003, 42, 1394-96). trans-Pt(PBu3)2(C[triple bond]C-pyrene)2 serves as a model system that provides a good representation of the CC-pyrene-localized 3IL state in a Pt(II) complex lacking the MLCT excited state. Following 400 nm excitation, the formation of the 3MLCT excited state in Pt(dbbpy)(C[triple bond]C-Ph)2 is complete within 200 +/- 40 fs, and intersystem crossing to the 3IL excited state in trans-Pt(PBu3)2(C[triple bond]C-pyrene)2 occurs with a time constant of 5.4 +/- 0.2 ps. Selective excitation into the low-energy MLCT bands in Pt(dbbpy)(C[triple bond]C-pyrene)2 (lambda(ex) = 480 nm) leads to the formation of the 3IL excited state in 240 +/- 40 fs, suggesting ultrafast wire-like energy migration in this molecule. The kinetic data suggest that the presence of the MLCT states in Pt(dbbpy)(C[triple bond]C-pyrene)2 markedly accelerates the formation of the triplet state of the pendant pyrenylacetylide ligand. In essence, the triplet sensitization process is kinetically faster than pure intersystem crossing in trans-Pt(PBu3)2(CC-pyrene)2 as well as vibrational relaxation in the MLCT excited state of Pt(dbbpy)(C[triple bond]C-Ph)2. These results are potentially important for the design of chromophores intended to reach their lowest excited state on subpicosecond time scales and advocate the likelihood of wire-like behavior in triplet-triplet energy transfer.

Intramolecular Metal···Sulfur Interactions of Platinum(II) 1,4,7-Trithiacyclononane Complexes with Bipyridyl Ligands: The Relationship between Molecular and Electronic Structures

Five platinum(II) 1,4,7-trithiacyclononane (ttcn) complexes with bidentate-substituted 2,2'-bipyridine ligands have been prepared and structurally characterized: [Pt(bpy)(ttcn)](PF6)2 (bpy = 2,2'-bipyridine), triclinic, P1, a = 10.2529(3) A, b = 10.7791(3) A, c = 10.7867(3) A, alpha = 83.886(1) degrees, beta = 87.565(1) degrees, gamma = 84.901(1), V = 1179.99(6) A3, Z = 2; [Pt(4,4'-dmbpy)(ttcn)](PF6)2 x CH3CN x H2O (4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridine), triclinic, P1, a = 10.1895(3) A, b = 11.8566(4) A, c = 13.1004(4) A, alpha = 77.345(1) degrees, beta = 79.967(1) degrees, gamma = 72.341(1) degrees, V = 1461.56(8) A3, Z = 2; [Pt(5,5'-dmbpy)(ttcn)](PF6)2 (5,5'-dmbpy = 5,5'-dimethyl-2,2'-bipyridine), triclinic, P1, a = 10.6397(4) A, b = 10.8449(4) A, c = 11.2621(4) A, alpha = 90.035(1) degrees, beta = 98.061(1) degrees, gamma = 91.283(1) degrees, V = 1286.32(8) A3, Z = 2; [Pt(dbbpy)(ttcn)](PF6)2 x CH3NO2 (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), triclinic, P1, a = 11.5422(7) A, b = 11.6100(7) A, c = 13.6052(9) A, alpha = 85.902(1) degrees, beta = 89.675(1) degrees, gamma = 74.942(1) degrees, V = 1755.90(19) A3, Z = 2; and [Pt(dtfmbpy)(ttcn)](PF6)2 x CH3CN (dtfmbpy = 5,5'-di-trifluoromethyl-2,2'-bipyridine): monoclinic, P2(1)/c, a = 13.1187(9) A, b = 20.9031(15) A, c = 11.3815(8) A, beta = 105.789(2) degrees, V = 3003.3(4) A3, Z = 4. For each salt, the platinum(II) center of the cation is bonded to two nitrogen atoms of the chelating diimine and two sulfur atoms of the thioether macrocycle. The third sulfur atom of ttcn forms a long apical interaction with the metal center (2.84-2.97 A), resulting in a flattened square pyramid structure. An examination of these and 17 other structures of platinum(II) ttcn complexes reveals a correlation between the apical Pt...S distance and the donor properties of the ancillary ligands, suggesting a means for using variations in ligand electronic properties to tune molecular structure. The room-temperature absorption spectra in acetonitrile solution show a broad and comparatively low-energy MLCT band maximizing near approximately 390 nm for the bpy and dialkyl-substituted bipyridyl derivatives. The maximum is dramatically red-shifted to 460 nm in the spectrum of the dtfmbpy complex as a result of the electron-withdrawing properties of the -CF(3) groups. The 3:1 EtOH/MeOH 77 K glassy solution emission spectra exhibit low-energy emission bands (lambdamax, 570-645 nm), tentatively assigned as originating from a lowest, predominantly spin-forbidden MLCT excited state that is stabilized by apical Pt...S interactions.