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.

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.

Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands.

The present study reports that the ground- and excited-state Re6(23e)/Re6(24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N-heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re6(µ3-S)8Cl6]4- with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re6(µ3-S)8Cl5(L)]3- (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4'-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans- and cis-[Re6(µ3-S)8Cl4(lut)2]2- and mer-[Re6(µ3-S)8Cl3(lut)3]-, were synthesized by the reaction of [Re6(µ3-S)8Cl6]3- with an excess amount of lut in refluxed N,N-dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E 1/2[Re6(23e)/Re6(24e)] = 0.49-0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the pK a of the N-heteroaromatic ligand [pK a(L)], the 1H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re6(µ3-S)8X6-n (L) n ] n-4 complexes (n = 0, X = Cl, Br, I, or NCS; n = 1-3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (ΣE L). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E 1/2(L0/L-) = -1.28 to -1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state (3CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (3MLCT) for L = bpy, cpy, and pz. The complexes with the 3CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with 3MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E 1/2[Re6(23e)/Re6(24e)] and E 1/2(L0/L-). The calculated oxidation potential of the excited hexanuclear rhenium complex with the 3CC character was linearly correlated with pK a(L), σ, and ΣE L. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.

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.

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].

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.

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.

Bridging-arylene effects on spectroscopic and photophysical properties of arylborane-dipyrrinato zinc(ii) complexes.

Novel bis(dipyrrinato)zinc(ii) derivatives having 4-[bis(2,4,6-trimethylphenyl)boryl]phenyl (ZnBph) or 4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-tetramethylphenyl groups (ZnBdu) at the 5-position of the dipyrrinato ligands were designed and synthesized. In ZnBph with the smaller dipyrrinate-arylene and arylene-dimesitylboryl dihedral angles, an intramolecular charge transfer arising from the presence of the vacant p orbital on the boron atom participates in the ππ* excited state in character in contrast to the pure ππ* excited state of ZnBdu. The synergistic ππ*/ILCT excited state was weakly fluorescent, and the fluorescence was enhanced upon binding of fluoride to the boron atom.

Anion-mediated encapsulation-induced emission enhancement of an IrIII complex within a resorcin[4]arene hexameric capsule.

Understanding of encapsulation processes in confined inner spaces of self-assembled hosts is important for the rational creation of supramolecular systems showing unusual reactivities and physical properties through molecular recognition. Herein we report the formation of luminescent supramolecular host-guest complexes consisting of a hydrogen-bonded resorcin[4]arene hexameric capsule and a variety of emissive Ir complex salts. The Ir complexes, accompanied by small counter anions (Cl-, Br-, NO2-, I- and NO3-), are trapped effectively to show large encapsulation-induced emission enhancement (EIEE) behavior, while Ir complexes having large counter anions (ClO4-, PF6- and OTf-) are not stabilized within the capsule, suggesting that the Ir complex cation is trapped together with its counter anion to form an ion-pair in the capsule. Hydrogen-bonding capabilities of the counter anions also contribute to stabilize host-guest association, because the counter anions trapped within the capsule were in contact with the hydrophilic surfaces of the capsule.

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.

Synthesis and Photoluminescence of Tetracyanidonitridorhenium(V) Complexes with Five-Membered N-Heteroaromatic Ligands and Photoluminescence-Intensity Change.

Novel tetracyanidonitridorhenium(V) complexes with five-membered N-heteroaromatic ligands, (PPh4)2[ReN(CN)4L] [L = imidazole (Him) (2), 1-methylimidazole (Mim) (3), and pyrazole (pyz) (4)] and (PPh4)2[ReN(CN)4L]·L [L = Him (5) and Mim (6)], were synthesized by the reactions of (PPh4)2[ReN(CN)4] (1) with Him, Mim, and pyz, and their structures were determined by single-crystal X-ray analysis. The complexes 2, 3, 4, and 6 showed intense photoluminescence, with the emission quantum yields (Φem) being 0.65-0.75 in the solid state at 296 K. In contrast, the Φem and τem values of 5 are significantly smaller and shorter, respectively, than the relevant values of 2. The interconversion reactions among 1, 2, and 5 accompanied by large photoluminescence-intensity changes were accomplished by solvent-free reactions and exposure of water. The mechanochemical reaction of 2 with 1 mol equiv of Him in the solid state gave 5. Complex 5 was also obtained by the mechanochemical reaction of 1 with 2 mol equivalents of Him in the solid state. By placing solid of 5 in water, the solid showed intense photoluminescence to give 2. Complex 1 was produced under vacuum at 185 °C from 2 or 5.

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.

Encapsulation condition dependent photophysical properties of polypyridyl Ru(ii) complexes within a hydrogen-bonded capsule.

Controlling the encapsulation equilibrium is a key strategy to affect host-guest associations. Ruthenium(ii) polypyridyl complex salts suspended in a chloroform solution of resorcin[4]arene afforded a host-guest complex which showed structured emission spectra even in the solution state. In contrast, a host-guest complex obtained through homogeneous encapsulation conditions by using soluble ruthenium(ii) polypyridyl complex salts showed broadened emission spectra which strongly depended on the amount of the host owing to the encapsulation equilibrium. These results demonstrate that a simple modulation of the encapsulation technique is indeed promising and a facile approach to control the photophysical properties of supramolecular complexes.

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.

U- to Z-shape isomerization in a Pt2Ag2 framework containing pyridyl-NHC ligands.

Synthesis and isomerization of a heteropolynuclear Pt2Ag2 complex containing pyridyl N-heterocyclic carbene (NHC) ligands are reported. The Pt2Ag2 complex could take two geometrical isomers possessing a twisted U-shaped structure and a Z-shaped structure. The rate of the isomerization reaction depended on the concentration of the solution, implying that the reaction took place through an intermolecular process.

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.

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.

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.

Reversible formation and cleavage of Pt→Ag dative bonds in a pre-organized cavity of a luminescent heteropolynuclear platinum(ii) complex.

A U-shaped Pt2Ag2 complex [Pt2Ag2(ppy)2(Ph2pz)4] with a pre-organized cavity (ppy = 2-phenylpridinate and Ph2pz = 3,5-diphenylpyrazolate) and related complexes have been prepared. The Pt2Ag2 complexes react with Ag(i) ions to give the corresponding Pt2Ag3 complexes containing Pt→Ag dative bonds. It became obvious that the existence of the C(ipso) atom in the chelate ligand is important as the driving force for forming Pt→Ag dative bonds. However, once the Pt2Ag3 complex is formed, the trapped Ag(i) ion is mainly stabilized by the Pt→Ag dative bonds, which are stronger than the Ag-C(ipso) bond. The trapped Ag(i) ion can be abstracted from the cavity selectively by adding an equivalent amount of chloride ion into the solution of Pt2Ag3 complexes to reproduce the original Pt2Ag2 complexes.

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.