Stepwise Sulfite Reduction on a Dinuclear Ruthenium Complex Leading to Hydrogen Sulfide.

Sulfite reduction by dissimilatory sulfite reductases is a key process in the global sulfur cycle. Sulfite reductases catalyze the 6e- reduction of SO32- to H2S using eight protons (SO32- + 8H+ + 6e- → H2S + 3H2O). However, detailed research into the reductive conversion of sulfite on transition-metal-based complexes remains unexplored. As part of our ongoing research into reproducing the function of reductases using dinuclear ruthenium complex {(TpRu)2(µ-Cl)(µ-pz)} (Tp = HB(pyrazolyl)3), we have targeted the function of sulfite reductase. The isolation of a key SO-bridged complex, followed by a sulfite-bridged complex, eventually resulted in a stepwise sulfite reduction. The reduction of a sulfite to a sulfur monoxide using 4H+ and 4e-, which was followed by conversion of the sulfur monoxide to a disulfide with concomitant consumption of 2H+ and 2e-, proceeded on the same platform. Finally, the production of H2S from the disulfide-bridged complex was achieved.

Integrative Assembly of Heteroleptic Tetrahedra Controlled by Backbone Steric Bulk.

A bent fluorenone-based dipyridyl ligand LA reacts with PdII cations to a solvent-dependent dynamic library of [PdnL2n] assemblies, constituted by a [Pd3LA6] ring and a [Pd4LA8] tetrahedron as major components, and a [Pd6LA12] octahedron as minor component. Introduction of backbone steric hindrance in ligand LB allows exclusive formation of the [Pd6LB12] octahedron. Combining equimolar amounts of both ligands results in integrative self-sorting to give an unprecedented [Pd4LA4LB4] heteroleptic tetrahedron. Key to the non-statistical assembly outcome is exploiting the structural peculiarity of the [Pd4L8] tetrahedral topology, where the four lean ligands occupy two doubly bridged edges and the bulky ligands span the four remaining, singly bridged edges. Hence, the system finds a compromise between the entropic drive to form an assembly smaller than the octahedron and the enthalpic prohibition of pairing two bulky ligands on the same edge of the triangular ring. The emission of luminescent LA is maintained in both homoleptic [Pd3LA6] and heteroleptic [Pd4LA4LB4].

A Heteropolynuclear Pt-Ag System Having Cycloplatinated Rollover Bipyridyl Units.

The synthesis and photophysical properties of the heteropolynuclear Pt-Ag complex having cyclometalated rollover bipyridine ligands (bpy*) and bridging pyrazolato ligands are reported. The Pt2Ag2 complex was synthesized by two step reactions from a Pt(II) complex precursor having the rollover bpy* ligand, [Pt(bpy*)(dmso)Cl], with 3,5-dimethylpyrazole (Me2pzH) and a subsequent replacement of NH protons in the Me2pzH moieties with the Ag(I) ion. The Pt2Ag2 complex exists as a mixture of U- and Z-shaped isomers in solution, whose structures were clearly determined by single-crystal X-ray structural analyses. NMR studies using the single crystals revealed rapid isomerization of the Pt2Ag2 complexes in solution, although the Pt2Ag2 structures were supported effectively by the multiple metal-metal interactions. Furthermore, the Pt2Ag2 framework can capture a Ag(I) ion during the U-Z isomerization to afford a Pt2Ag3 core with the formation of Pt → Ag dative bonds. The Pt2Ag3 complex showed further aggregation to form a dimer structure in the presence of coordinating solvent via the crystallization process. The formation of Pt → Ag dative bonds significantly changes the emission energy from the Pt2Ag2 complex, while the emission spectra of U- and Z-isomers of Pt2Ag2 complex almost coincide with each other and their emissive properties are very similar. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations revealed the effects of additional Ag(I) ion on the photophysical properties of the heteropolynuclear Pt-Ag complexes bearing the rollover bpy* ligands.

Multinuclear Ag Clusters Sandwiched by Pt Complex Units: Fluxional Behavior and Chiral-at-Cluster Photoluminescence.

Multinuclear Ag clusters sandwiched by Pt complex units were synthesized and characterized by single crystal X-ray diffraction and NMR studies. The sandwich-shaped multinuclear Ag complexes showed two different types of fluxional behavior in solution: rapid slippage of Pt complex units on the Ag3 core and a reversible demetalation-metalation reaction by the treatment with Cl anion and Ag ion, respectively. The Ag2 complex obtained by demetalation reaction from the Ag3 complex displayed U to Z isomerization. These multinuclear Ag complexes showed strong photoluminescence whose properties depended on the existence of Pt→Ag dative bonds. The Ag3 complex, identified to be "chiral-at-cluster", was optically resolved by the formation of a diastereomeric salt with a chiral anion. The enantiomers show circular dichroism (CD) and circularly polarized luminescence (CPL) properties which is unprecedented for compounds based on a chiral sandwich structure. Theoretical calculations allow to understand their structural features and photophysical properties.

Photocatalytic CO2 Reduction under Visible-Light Irradiation by Ruthenium CNC Pincer Complexes.

Photocatalytic CO2 reduction using a ruthenium photosensitizer, a sacrificial reagent 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H), and a ruthenium catalyst were carried out. The catalysts contain a pincer ligand, 2,6-bis(alkylimidazol-2-ylidene)pyridine (CNC) and a bipyridine (bpy). The photocatalytic reaction system resulted in HCOOH as a main product (selectivity 70-80 %), with a small amount of CO, and H2 . Comparative experiments (a coordinated ligand (NCMe vs. CO) and substituents (tBu vs. Me) of the CNC ligand in the catalyst) were performed. The turnover number (TONHCOOH ) of carbonyl-ligated catalysts are higher than those of acetonitrile-ligated catalysts, and the carbonyl catalyst with the smaller substituents (Me) reached TONHCOOH =5634 (24 h), which is the best performance among the experiments.

Controlling the Electronic Structures and Excited-State Characteristics of Dipyrrinatoiridium(III) Complexes by an Arylborane or an Arylamino Unit.

Novel dipyrrinatoiridium(III) complexes, in which a typical element substituent (i.e., arylborane or arylamino group) was introduced at the meso position of the 5-phenyldipyrrinato ligand, were designed and synthesized. The (dimesitylboryl)phenyl complex 1 showed fascinating photophysical properties arising from a synergistic interaction between metal-to-ligand charge transfer/ligand-to-ligand charge transfer and charge transfer from the π orbital of the aryl group to the vacant p orbital on the boron atom [π(aryl)-p(B) charge transfer]. On the other hand, the (N,N-diphenylamino)phenyl complex 2 showed the most intense and longest-lived emission from the 3ππ*-type excited state in the dipyrrinato moiety among the complexes in the present study. These characteristics of the complexes were evaluated in terms of the structures and spectroscopic and photophysical properties.

Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing Ammonia.

The fundamental biogeochemical cycle of nitrogen includes cytochrome c nitrite reductase, which catalyzes the reduction of nitrite ions to ammonium with eight protons and six electrons (NO2- + 8H+ + 6e- → NH4+ + 2H2O). This reaction has motivated researchers to explore the reduction of nitrite. Although a number of electrochemical reductions of NO2- have been reported, the synthetic nitrite reduction reaction remains limited. To the best of our knowledge, formation of ammonia has not been reported. We report a three-step nitrite reduction cycle on a dinuclear ruthenium platform {(TpRu)2(µ-pz)} (Tp = HB(pyrazol-1-yl)3), producing ammonia. The cycle comprises conversion of a nitrito ligand to a NO ligand using 2H+ and e-, subsequent reduction of the NO ligand to a nitrido and a H2O ligand by consumption of 2H+ and 5e-, and recovery of the parent nitrito ligand. Moreover, release of ammonia was detected.

Emission Tuning of Heteroleptic Arylborane-Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler-Berg-Type Relation.

Novel heteroleptic arylborane-ruthenium(II) complexes having a series of ancillary ligands L' ([Ru(B2bpy)L'2]2+) in CH3CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru(B2bpy)L'2]2+ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru(B2bpy)L'2]2+ in CH3CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler-Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes.

Chiral-at-Metal Phosphorescent Square-Planar Pt(II)-Complexes from an Achiral Organometallic Ligand.

The synthesis and characterization of a new kind of cis- and trans-cyclometalated square-planar platinum(II) complexes is reported. Uncharged organometallic compounds carrying one or two of the C∧N-donor ligand LCN were prepared. Due to the heterobidentate coordination of the achiral chelate ligand, the formed [PtLCNCl(SEt2)], cis- and trans-[PtLCN2] complexes are chiral with the metal serving as the stereo center. The enantiomers of complex trans-[PtLCN2] could be separated and their absolute configuration was determined by anomalous X-ray diffraction, in accordance with CD spectroscopic results and TD-DFT calculations. All compounds were fully characterized by NMR spectroscopy, mass spectrometry and X-ray structure determination. The photophysical properties of trans-[PtLCN2] have been investigated showing phosphorescence in solution and in the solid state with a moderate quantum yield. For the enantiomers, strong circular dichroism (CD) and circularly polarized luminescence (CPL) effects were observed, rendering this new structural motif suitable for application in chiroptical and luminescent materials.

Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}(n-C3F7COO)6]2- (X = Cl, Br, or I).

Temperature (T)-dependent emission from [{Mo6X8}(n-C3F7COO)6]2- (X = Cl (1), Br (2), and I (3)) in optically transparent polyethylene glycol dimethacrylate matrices were studied in 3 K < T < 300 K to elucidate the spectroscopic and photophysical properties of the clusters, in special reference to zero-magnetic-field splitting (zfs) in the lowest-energy excited triplet states (T1) of the clusters. The cluster complexes 1 and 2 showed the T-dependent emission characteristics similar to those of [{Mo6Cl8}Cl6]2-, while 3 exhibited emission properties different completely from those of 1 and 2. Such T-dependent emission characteristics of 1, 2, and 3 were explained successfully by the excited triplet state spin-sublevel (Φn, n = 1-4) model. The zfs energies between the lowest-energy (Φ1) and highest-energy (Φ4) spin sublevels, ΔE14, resulted by the first-order spin-orbit coupling, were evaluated to be 650, 720, and 1000 cm-1 for 1, 2, and 3, respectively. The emission spectra of 1, 2, and 3 in CH3CN at 298 K were reproduced very well by the ΔE14 values and the population percentages of Φn at 300 K. We also report that the ΔE14 values of the clusters correlate linearly with the fourth power of the atomic number (Z) of X: ΔE14 ∝ {Z(X)}4.

Encapsulation and Enhanced Luminescence Properties of IrIII Complexes within a Hexameric Self-Assembled Capsule.

Encapsulation and luminescence studies of [Ir(ppy)2 (bpy)]Cl (ppy=2-phenylpyridinate, bpy=2,2'-bipyridine) within a hexameric resorcinarene capsule are reported. One IrIII complex cation was encapsulated within the capsule, as demonstrated by NMR and dynamic light scattering (DLS) studies. The emission color of the IrIII complex was drastically changed from orange to yellow by encapsulation, in contrast with the lack of significant changes in the absorption spectrum. The hexameric capsule effectively hampers the non-radiative pathway to increase both the luminescence quantum yield and the exited state lifetime. The luminescent properties of the encapsulated IrIII complex depend on the ratio of IrIII complex to the resorcinarene monomer as well as the concentration of resorcinarene monomer owing to the reversible process of self-assembly of the hexameric capsule. Quenching experiments revealed that the IrIII complex in the capsule was effectively separated from quenchers.

Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}(4+) Core Cluster Complexes by Terminal Carboxylate Ligands.

The reactions between the tetra-n-butylammonium salt of [{Mo6I8}I6](2-) and silver carboxylates RCOOAg (R = CH3 (1), C(CH3)3 (2), α-C4H3O (3), C6H5 (4), α-C10H7 (5), or C2F5 (6)) in CH2Cl2 afforded new carboxylate complexes [{Mo6I8}(RCOO)6](2-). The complexes were characterized by X-ray single-crystal diffraction and elemental analysis, cyclic/differential pulse voltammetry, and IR, NMR, and UV-visible spectroscopies. The emission properties of the complexes 1-6, and those of the earlier reported complexes with R = CF3 (7) and n-C3F7 (8), were studied both in acetonitrile solution and in the solid state. In deaerated CH3CN at 298 K, all of the complexes 1-8 exhibit intense and long-lived emission with the quantum yield and lifetime being 0.48-0.73 and 283-359 µs, respectively. The oxidation (Eox)/reduction (Ered) potentials of the complexes correlate linearly with the pKa value of the terminal carboxylate ligands L = RCOO (pKa(L)). Reflecting the pKa(L) dependences of Eox/Ered, the emission energy (νem) of the complexes was also shown to correlate with pKa(L). The present study successfully demonstrates synthetic tuning of the redox, spectroscopic, and photophysical characteristics of a {Mo6I8}(4+)-based cluster complex with pKa(L).

Remarkably Intense Emission from Ruthenium(II) Complexes with Multiple Borane Centers.

The electrochemical, spectroscopic, and phophysical properties of a series of Ru(II) complexes having a triarylborane-appended 2,2'-bipyridine (bpy) ligand(s) (RuBbpys: [Ru(Bbpy)n(bpy)(3-n)](2+) (B1n) and [Ru(B2bpy)n(bpy)(3-n)](2+) (B2n), B = (dimesityl)boryldurylethynyl group(s) at the 4- or 4,4'-position(s) in bpy, n = 1-3) are described. In the excited states of the complexes, the intramolecular charge transfer transitions between the π-orbital of the aryl group and the vacant p-orbital on the boron atom (π(aryl)-p(B) CT) synergistically interact with the metal-to-ligand charge transfer (MLCT) transitions. The molar absorption coefficient of the MLCT band (ε(MLCT)) of the complex increased with increasing n, and B23 showed extremely intense absorption with ε(MLCT) = 5.6 × 10(4) M(-1) cm(-1) at 488 nm. Furthermore, B23 showed the highest emission quantum yield (0.43) among those of the polypyridine Ru(II) complexes hitherto reported. As one of the interesting results, we report that the radiative rate constant of B2n shows the correlation with ε(MLCT). The effects of the synergistic MLCT/π(aryl)-p(B) CT interactions on the spectroscopic and photophysical characteristics of RuBbpys are discussed in detail.

Dual emissions from ruthenium(II) complexes having 4-arylethynyl-1,10-phenanthroline at low temperature.

A [Ru(phen)3](2+) complex (phen = 1,10-phenanthroline) having an arylethynyl group at the 4-position of one of the three phen ligands (aryl: (dimesityl)borylduryl group = [RuBE](2+) or duryl group = [RuDE](2+)) showed dual emissions at low temperature in propylene carbonate (PC). The shorter emission lifetime components (τ(em)(s) ≈ 13 µs) that originated from the lowest-energy excited triplet states (T1) of the complexes were almost independent of temperature (T = 77-320 K), while the longer emission lifetime components (τ(em)(l)), as the T2 emissions appeared below the fluid-to-glass transition temperature (Tg ≈ 220 K) in PC, were almost constant at 27 µs in the range of T = 77-220 K. The τ(em)(s) components of the complexes were assigned to the emissions from the metal-to-ligand charge transfer (MLCT) excited states possessing relatively large ligand-centered (LC) excited-state characters (T1(MLCT/LC)), while the τ(em)(l) components were shown to be originating from the T2(MLCT) excited states. The T2(MLCT) states of the complexes became nonemissive above Tg in PC due to fast nonradiative decay through solvent reorganization around the T2(MLCT) excited states. The photophysical properties of the complexes were also shown to be characterized by the presence of the arylethynyl units at the 4-positions of the phen ligands.

Synthetic control of spectroscopic and photophysical properties of triarylborane derivatives having peripheral electron-donating groups.

The spectroscopic and photophysical properties of triarylborane derivatives were controlled by the nature of the triarylborane core (trixylyl- or trianthrylborane) and peripheral electron-donating groups (N,N-diphenylamino or 9H-carbazolyl groups). The triarylborane derivatives with and without the electron-donating groups showed intramolecular charge-transfer absorption/fluorescence transitions between the π orbital of the aryl group (π(aryl)) and the vacant p orbital on the boron atom (p(B), π(aryl)-p(B) CT), and the fluorescence color was tunable from blue to red by the combination of peripheral electron-donating groups and a triarylborane core. Detailed electrochemical, spectroscopic, and photophysical studies of the derivatives, including solvent dependences of the spectroscopic and photophysical properties, demonstrated that the HOMO and LUMO of each derivative were determined primarily by the nature of the peripheral electron-donating group and the triarylborane core, respectively. The effects of solvent polarity on the fluorescence quantum yield and lifetime of the derivatives were also tunable by the choice of the triarylborane core.

Excited-state characteristics of tetracyanidonitridorhenium(V) and -technetium(V) complexes with N-heteroaromatic ligands.

Six-coordinate tetracyanidonitridorhenium(V) and -technetium(V) with axial N-heteroaromatic ligands, (PPh4)2[MN(CN)4L] [M = Re, L = 4-(dimethylamino)pyridine (dmap), 3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine (py), 3-benzoylpyridine (3bzpy), 4,4'-bipyridine (bpy), pyrazine (pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc, L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized. The crystal structures of 11 complexes were determined by single-crystal X-ray analysis. All of the complexes showed photoluminescence in the crystalline phase at room temperature. The emission maximum wavelengths (λ(em)) of the rhenium complexes with dmap, lut, pic, ppy, or py were similar to one another with a quite high emission quantum yield (Φ(em)): λ(em) = 539-545 nm, Φ(em) = 0.39-0.93, and emission lifetime (τ(em)) = 10-45 µs at 296 K. The emission spectra at 77 K exhibited vibronic progressions, and the emissive excited state is characterized as (3)[(d(xy))(1)(dπ*)(1)] (dπ* = d(xz), d(yz)). On the other hand, the emission maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy, or 4bzpy was significantly dependent on the nature of the axial ligand in the crystalline phase: λ(em) = 564-669 nm, Φ(em) ≤ 0.01-0.36, and τ(em) = 0.03-13.3 µs at 296 K. The emission spectra at 77 K in the crystalline phase did not show vibronic progressions. The emissive excited state of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to originate from the metal-to-N-heteroaromatic ligand charge-transfer (MLCT)-type emission with a spin-triplet type. The change in the excited-state characteristics of rhenium complexes by the N-heteroaromatic ligand is a result of stabilization of the π* orbital of the N- heteroaromatic ligand to a lower energy level than the dπ* orbitals. The emission spectral shapes of technetium complexes were almost independent of the nature of the N-heteroaromatic ligand with λ(em) = 574-581 nm at room temperature. The different emission characteristics between the pz and cpy coordinate rhenium complexes and the technetium analogues would be due to stabilization of technetium-centered orbitals compared with the rhenium ones in energy.

Symmetry-breaking host-guest assembly in a hydrogen-bonded supramolecular system.

Bio-inspired self-assembly is invaluable to create well-defined giant structures from small molecular units. Owing to a large entropy loss in the self-assembly process, highly symmetric structures are typically obtained as thermodynamic products while formation of low symmetric assemblies is still challenging. In this study, we report the symmetry-breaking self-assembly of a defined C1-symmetric supramolecular structure from an Oh-symmetric hydrogen-bonded resorcin[4]arene capsule and C2-symmetric cationic bis-cyclometalated Ir complexes, carrying sterically demanding tertiary butyl (tBu) groups, on the basis of synergistic effects of weak binding forces. The flexible capsule framework shows a large structural change upon guest binding to form a distorted resorcin[4]arene hexameric capsule, providing an asymmetric cavity. Location of the chiral guest inside the anisotropic environment leads to modulation of its Electric Dipole (ED) and Magnetic Dipole (MD) transition moments in the excited state, causing an increased emission quantum yield, longer emission lifetime, and enhancement of the dissymmetry factor (glum) in the circularly polarized luminescence.

Photophysical and photoredox characteristics of a novel tricarbonyl rhenium(I) complex having an arylborane-appended aromatic diimine ligand.

We report the synthesis and photophysical/photoredox characteristics of a novel tricarbonyl rhenium(I) complex having a (dimesityl)boryldurylethynyl (DBDE) group at the 4-position of a 1,10-phenanthroline (phen) ligand, [Re(CO)(3)(4-DBDE-phen)Br] (ReB). ReB in tetrahydrofuran at 298 K showed the metal-to-ligand charge transfer (MLCT) emission at around 681 nm with the lifetime (τ(em)) of 900 ns. The relatively long emission lifetime of ReB compared with that of [Re(CO)(3)(phen)Br] (RePhen, τ(em) = 390 ns) was discussed on the basis of the temperature dependent τ(em) and Franck-Condon analysis of the emission spectra of the two complexes. Emission quenching studies of both ReB and RePhen by a series of electron donors revealed that the photoinduced electron transfer (PET) quenching rate constant of ReB was faster than that of RePhen at a given Gibbs free energy change of the PET reaction (ΔG(ET)(0) > -0.5 eV). All of the results on ReB were discussed in terms of the contribution of the CT interaction between the π-orbital(s) of the aryl group(s) and the vacant p-orbital on the boron atom in DBDE to the MLCT state of the complex.

Photoluminescence switching with changes in the coordination number and coordinating volatile organic compounds in tetracyanidonitridorhenium(V) and -technetium(V) complexes.

Six-coordinate distorted octahedral tetracyanidonitridorhenium(V) and -technetium(V) complexes with a volatile organic compound (VOC) coordinating at the trans position of a nitrido ligand, (PPh4)2[MN(CN)4L] (M = Re, L = MeOH, EtOH, acetone, or MeCN; M = Tc, L = MeOH), and five-coordinate square-pyramidal tetracyanidonitrido complexes without an axial ligand, (PPh4)2[MN(CN)4] (M = Re or Tc), were synthesized and characterized. Single-crystal X-ray structural analysis was carried out for (PPh4)2[MN(CN)4L] (M = Re, L = MeOH, EtOH, or acetone; M = Tc, L = MeOH) and (PPh4)2[ReN(CN)4]. All complexes studied showed photoluminescence in the solid state at room temperature. Reversible luminescence switching between six- and five-coordinate rhenium(V) complexes and between the relevant six-coordinate rhenium(V) complexes except that between the MeCN and acetone complexes was achieved by exposing them to VOC vapor in the solid state at room temperature. Luminescence changes were observed from the five-coordinate technetium(V) complexes in a MeOH vapor atmosphere in the solid state. In contrast, no vapochromic luminescence was observed from the five- and six-coordinate complexes in an acetone vapor atmosphere.

An asymmetric Pt diimine acetylide complex providing unique luminescent multinuclear sandwich complexes with Cu salts.

The formation and photophysical properties of two types of sandwich complexes supported by asymmetric Pt complex units having two different acetylide moieties are reported. The asymmetric Pt complex unit was obtained via acetylide metathesis reaction between two types of symmetric Pt complexes. The reversible acetylide exchange reaction was effectively suppressed by the incorporation of Cu ions to give unique chiral Pt4Cu3 and achiral Pt2Cu4Br4 sandwich complexes. The sandwich complexes exhibited moderate photoluminescence in the solid state, and their photophysical properties depended on the sandwich structures. These results suggest that asymmetric Pt complex units can give remarkable assembled structures by the concerted effect of labile coordination bonds and weak noncovalent interactions.