Redox Activity of IrIII Complexes with Multidentate Ligands Based on Dipyrido-Annulated N-Heterocyclic Carbenes: Access to High Valent and High Spin State with Carbon Donors.

Synthetic strategies to access high-valent iridium complexes usually require use of π donating ligands bearing electronegative atoms (e. g. amide or oxide) or σ donating electropositive atoms (e. g. boryl or hydride). Besides the η5 -(methyl)cyclopentadienyl derivatives, high-valent η1 carbon-ligated iridium complexes are challenging to synthesize. To meet this challenge, this work reports the oxidation behavior of an all-carbon-ligated anionic bis(CCC-pincer) IrIII complex. Being both σ and π donating, the diaryl dipyrido-annulated N-heterocyclic carbene (dpa-NHC) IrIII complex allowed a stepwise 4e- oxidation sequence. The first 2e- oxidation led to an oxidative coupling of two adjacent aryl groups, resulting in formation of a cationic chiral IrIII complex bearing a CCCC-tetradentate ligand. A further 2e- oxidation allowed isolation of a high-valent tricationic complex with a triplet ground state. These results close a synthetic gap for carbon-ligated iridium complexes and demonstrate the electronic tuning potential of organic π ligands for unusual electronic properties.

Regulation of the rate of dinucleation of a monocopper(I) complex containing bipyrimidine rotary units by restricted double pyrimidine rotation.

New copper(I) complexes with coordinated 2-(4'-methyl)pyrimidinyl moieties were fabricated, and the isomerism of their pyrimidine ring linkage was investigated. The ligands bis[2-(diphenylphosphino)phenyl] ether (DPEPhos) and 4,4'-dimethyl-2,2'-bipyrimidine (dmbpm) were used to synthesize a heteroleptic copper(I) complex, [Cu(I)(DPEPhos)(dmbpm)]·BF4 (1·BF4), and a dinuclear copper(I) complex, [(Cu(I))2(DPEPhos)2(µ-dmbmp)](BF4)2 [2·(BF4)2]. The X-ray crystallographic structures, UV-vis absorption spectra, and luminescence properties of the complexes were analyzed. The thermodynamic and kinetic aspects of the isomerism of 1·BF4 were examined by variable-temperature NMR. Double pyrimidine ring rotation was found to be restricted sterically by the bulky DPEPhos ligands. This limited the number of the possible isomers: 1·BF4 showed only isomers with either one (io isomer) or both (oo isomer) of the two methyl groups positioned away from the copper center, while dinuclear 2·(BF4)2 was only found as a symmetric (io-io) isomer, with each of the two methyl groups positioned toward different copper centers. The addition of [Cu(MeCN)2(DPEPhos)] (3·BF4) allowed both isomers of 1·BF4 to form 2·(BF4)2, although at different rates and via different pathways, which were analyzed using time-dependent UV-vis spectroscopy. The io isomer dinucleated more quickly than the oo isomer owing to it being able to form 2·(BF4)2 (i) without bond dissociation and (ii) without a sterically congested ii configuration around the copper center. In contrast, oo-1·BF4 required (i) recombination of the bipyrimidine coordination bonds or (ii) formation of a product with higher thermodynamic energy, unsymmetric (ii-oo) 2·(BF4)2. These findings are interpreted as demonstrating a novel kinetic property: a conversion rate determined by pyrimidine ring inversion.

Spectrometric monitoring of CO2 electrolysis on a molecularly modified copper surface.

Since copper has been extensively studied due to its unique ability to reduce carbon dioxide to hydrocarbons and alcohols, it tends to yield a mixture of products. Among various efforts to improve the selectivity and efficiency of this catalysis, the introduction of organic molecules and polymers on the copper/electrolyte interface has proven to be an effective and promising way to improve surface activity, considering the variation and precise designability of organic structures. The role of surface molecular modifiers, however, is not as simple as that in homogeneous catalysts, and an understanding of a wide scale of interactions from the atomic scale to the whole electrode structure is required. This feature article classifies those different scale interactions caused by organic modifiers on copper catalysts, together with the experimental support by in situ vibrational spectroscopy which directly observes surface species and events. Based on these recent understandings, novel fabrication methods of organic structures on copper catalysts are also discussed.

Solvated-ion-pairing-sensitive molecular bistability based on copper(I)-coordinated pyrimidine ring rotation.

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both 2·BF(4) {2(+) = [Cu(Mepypm)(dppp)](+), where Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino)propane} and previously reported 1·BF(4), which possesses a bulky diphosphine ligand (1(+) = [Cu(Mepypm)(DPEphos)](+), where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two rotational isomers of 2(+) were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of 2·BF(4) in CDCl(3) (ΔH = 6 kJ mol(-1); ΔS = 25 J K(-1) mol(-1)) were more positive than that tested under other conditions, such as in more polar solvents CD(2)Cl(2), acetone-d(6), and CD(3)CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2(+) with a bulky counterion, such as B(C(6)F(5))(4)(-). The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.

Structural modification on copper(I)-pyridylpyrimidine complexes for modulation of rotational dynamics, redox properties, and phototriggered isomerization.

The redox properties of copper pyridylpyrimidine complexes, which undergo linkage isomerism based on pyrimidine ring rotation, were compared under different coordination environments. A newly synthesized compound, [Cu(Mepypm)(L(Mes))]BF4 (1·BF4, Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine, L(Mes) = 2,9-dimesityl-1,10-phenanthroline) was compared with previously reported complexes of [Cu(MepmMepy)(L(Mes))]BF4 (2·BF4, MepmMepy = 4-methyl-2-(6'-methyl-2'-pyridyl)pyrimidine), Cu(Mepypm)(DPEphos)]BF4 (3·BF4, DPEphos = bis[2-(diphenylphosphino)phenyl]ether), [Cu(Mepypm)(L(Anth))]BF4 (4·BF4, L(Anth) = 2,9-bis(9-anthryl)-1,10-phenanthroline), and [Cu(Mepypm)(L(Macro))]BF4 (5·BF4). Isomer ratios, isomerization dynamics, redox properties, and photoelectron conversion functions varied with the coordination structure. Methyl substituents on the 6-position of the pyridine moiety increased steric repulsion and contributed to quicker rotation, enhanced photoluminescence, and increased photodriven rotational isomerization.

Stimuli-responsive pyrimidine ring rotation in copper complexes for switching their physical properties.

This paper summarizes the results of our recent studies on the development of an artificial molecular rotor system that exhibits a change in redox potential and photoluminescence in response to external stimuli such as heat and photons. The molecular rotor is made of copper complexes bearing two bidentate ligands; the rotor is described here as [Cu(Rpmpy)(L(x))](+), where Rpmpy and L(x) are a 4-methyl-2-(2'-pyridyl)pyrimidine derivative and a bidendate ligand with bulky moieties, x, respectively, and the pyrimidine ring can rotate beside the copper centre while maintaining the pyridine-copper connection. The simplicity of the system enabled us to design the rotating motion more accurately. We expected that placing a wall in the rotational trajectory in the L(x) moiety would decrease the rate of the rotational dynamics. This slow rate of rotation was a key factor in achieving an external-stimuli-induced switching from thr equilibrium to metastable states. This switching was based on four stable isomers derived from the rotation and oxidation states, the behaviours of which were characterized for isolated copper(I) complexes using spectroscopic and electrochemical measurements at several temperatures. The steric shifts arising from the ring rotation were exploited not only to exhibit well-established oxidation-triggered motion but also to modulate the rest potential of the electrode, to manipulate the intramolecular electron transfer, to develop a redox potential switch based on photo-driven rotation, and to demonstrate the dual-luminescence behaviour.

Reversible copper(II)/(I) electrochemical potential switching driven by visible light-induced coordinated ring rotation.

We here describe the first metal complex system in which electronic signals can be repeatedly extracted by converting bistable states related to an intramolecular ligand rotational motion, which is fueled by visible light. The molecular structure for relating an electron transfer and a motion consists of a copper center and a coordinated unsymmetrically substituted pyrimidine derivative, whose rotational isomerization causes an electrochemical potential shift. To harness light energy effectively through metal-to-ligand charge transfer (MLCT) excitation, we prepared a simple copper(I) complex coordinated by a 4-methyl-2-(6'-methyl-2'-pyridyl)pyrimidine and a bulky diimine. The thermodynamic and kinetic parameters of redox and rotational reactions were analyzed by cyclic voltammograms at variable temperatures, by considering four stable isomers related to copper(II)/(I) states and rotational isomeric states. The key feature of this compound is that the rotation is frozen in the copper(I) state (rate constant for the rotation, k(Ii→o) = 10(-4) s(-1)) but is active in the copper(II) state (k(IIi→o) = 10(-1) s(-1)) at 203 K. The compound makes a bypass route to the isomeric metastable copper(I) state, via a tentative copper(II) state formed by photoelectron transfer (PET) in the presence of a redox mediator, decamethylferrocenium ion (DMFc(+)), or upon a partial oxidation of the complex. Light- and heat-driven rotation in the copper(I) state with a potential shift (ΔE°' = 0.14 V) was analyzed by electrochemical measurements of the complex in the solution state. The rotor could be reset to the initial state by heating, thereby completing the cycle and enabling repeated operation fueled by light energy. A significant redox potential shift associated with the copper(II)/(I) transition accompanied the rotation, thereby providing a new type of molecular signaling system.

Ferrocene-dithiolene hybrids: control of strong donor-acceptor electronic communication to reverse the charge transfer direction.

We prepared a novel class of ferrocene-dithiolene hybrid molecules, FcS4dt(Me)2 and FcS4dt[Pt((t)Bu2bpy)] (where FcS4dt indicates 2-(1,3-dithia[3]ferrocenophane-2-ylidene)-1,3-dithiole-4,5-dithiolate and (t)Bu2bpy indicates 4,4'-di-tert-butyl-2,2'-bipyridine), in which the ferrocene moiety was bound to the planar conjugated dithiolene skeleton via two sulfur atoms such that the cyclopentadienyl rings were perpendicular to the dithiolene backbone. The physical properties and electronic structures of the complexes and their oxidized species [FcS4dt(Me)2](•+) and [FcS4dt[Pt((t)Bu2bpy)]](•+) were investigated by means of single-crystal X-ray diffraction (XRD) analysis, cyclic voltammetry, electron paramagnetic resonance (EPR), and UV-vis near infrared (UV-vis-NIR) spectroscopy. The electron density distributions of the highest occupied molecular orbitals (HOMOs) of FcS4dt(Me)2 and FcS4dt[Pt((t)Bu2bpy)] differed remarkably in that the HOMO of the former was ferrocene-based whereas that of the latter was dithiolene-based. The differences in the HOMO distributions originated from the energy level of the dithiolene-based π-orbital in each of the complexes, which was controlled by changing R in FcS4dt(R)2 (R = Me for FcS4dt(Me)2; 2R = Pt((t)Bu2bpy) for FcS4dt[Pt((t)Bu2bpy)]). We succeeded in analyzing the crystal structure of [FcS4dt[Pt((t)Bu2bpy)]](F4TCNQ)·C6H14·CH2Cl2 (where F4TCNQ indicates 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), which provided a rare example of the crystal structure of a [Pt(diimine)(dithiolate)](•+) ion-based complex. A comparison of the bond lengths in FcS4dt[Pt((t)Bu2bpy)] and [FcS4dt[Pt((t)Bu2bpy)]](•+) suggested that the latter complex displayed a conjugated dithiolene-based π-radical character. These considerations agreed well with the electronic structures calculated using density functional theory (DFT) and time-dependent(TD)-DFT methods. Significant electronic communication between the ferrocene and dithiolene moieties was detected for both [FcS4dt(Me)2](•+) and [FcS4dt[Pt((t)Bu2bpy)]](•+) in the appearance of an intramolecular charge transfer band, which was hardly observed for previously reported ferrocene-dithiolene hybrid molecules. The charge transfer direction was reversed between the two cations. The electron coupling parameter HAB and the potential energy curves of the oxidized complexes were estimated based on the classical two-state Marcus-Hush theory. These results suggest that FcS4dt-based metalladithiolenes can exhibit controllable electronic structures expressed as double-minimum potential energy curves.

Solid-state ligand-driven light-induced spin change at ambient temperatures in bis(dipyrazolylstyrylpyridine)iron(II) complexes.

We previously reported that an Fe(II) complex ligated by two (Z)-2,6-di(1H-pyrazol-1-yl)-4-styrylpyridine ligands (Z-H) presented a solid state ligand-driven light-induced spin change (LD-LISC) upon one-way Z-to-E photoisomerization, although modulation of the magnetism was trivial at ambient temperatures (Chem. Commun.2011, 47, 6846). Here, we report the synthesis of new derivatives of Z-H, Z-CN and Z-NO(2), in which electron-withdrawing cyano and nitro substituents are introduced at the 4-position of the styryl group to attain a more profound photomagnetism at ambient temperatures. Z-CN and Z-NO(2) undergo quantitative one-way Z-to-E photochromism upon excitation of the charge transfer band both in acetonitrile and in the solid state, similar to the behavior observed for Z-H. In solution, these substituents stabilized the low-spin (LS) states of Z-CN and Z-NO(2), and the behavior was quantitatively analyzed according to the Evans equation. The photomagnetic properties in the solid state, on the other hand, cannot be explained in terms of the substituent effect alone. Z-CN displayed photomagnetic properties almost identical to those of Z-H. Z-CN preferred the high-spin (HS) state at all temperatures tested, whereas photoirradiated Z-CN yielded a lower χ(M)T at ambient temperatures. The behavior of Z-NO(2) was counterintuitive, and the material displayed surprising photomagnetic properties in the solid state. Z-NO(2) occupied the LS state at low temperatures and underwent thermal spin crossover (SCO) with a T(1/2) of about 270 K. The photoirradiated Z-NO(2) displayed a higher value of χ(M)T and the modulation of χ(M)T exceeded that of Z-H or Z-CN. Z-NO(2)·acetone, in which acetone molecules were incorporated into the crystal lattice, further stabilized the LS state (T(1/2) > 300 K), thereby promoting large modulations of the χ(M)T values (87% at 273 K and 64% at 300 K) upon Z-to-E photoisomerization. Single crystal X-ray structure analysis revealed that structural factors played a vital role in the photomagnetic properties in the solid state. Z-H and Z-CN favored intermolecular π-π stacking among the ligand molecules. The coordination sphere around the Fe(II) nucleus was distorted, which stabilized the HS state. In contrast, Z-NO(2)·acetone was liberated from such intermolecular π-π stacking and coordination distortion, resulting in the stabilization of the LS state.

Uniform wrapping of copper(I) oxide nanocubes by self-controlled copper-catalyzed azide-alkyne cycloaddition toward selective carbon dioxide electrocatalysis.

Copper(I) oxide nanocubes were wrapped with an extremely uniform organic layer grown by self-controlled, Cu-mediated catalysis. This layer aided in retaining the initial cubic structure of the copper nanocubes during their use as a CO2 reduction electrocatalyst, resulting in high CO2 reduction selectivity by strong suppression of hydrogen evolution because of exclusion of water from the surface.