Low-energy NIR Research Articles

Photocatalytic CO2 Reduction by Near‐Infrared‐Light (1200 nm) Irradiation and a Ruthenium‐Intercalated NiAl‐Layered Double Hydroxide

AbstractNear‐infrared light‐driven photocatalytic CO2 reduction (NIR‐CO2PR) holds tremendous promise for the production of valuable commodity chemicals and fuels. However, designing photocatalysts capable of reducing CO2 with low energy NIR photons remains challenging. Herein, a novel NIR‐driven photocatalyst comprising an anionic Ru complex intercalated between NiAl‐layered double hydroxide nanosheets (NiAl‐Ru‐LDH) is shown to deliver efficient CO2 photoreduction (0.887 μmol h−1) with CO selectivity of 84.81 % under 1200 nm illumination and excellent stability over 50 testing cycles. This remarkable performance results from the intercalated Ru complex lowering the LDH band gap (0.98 eV) via a compression‐related charge redistribution phenomenon. Furthermore, transient absorption spectroscopy data verified light‐induced electron transfer from the Ru complex towards the LDH sheets, increasing the availability of electrons to drive CO2PR. The presence of hydroxyl defects in the LDH sheets promotes the adsorption of CO2 molecules and lowers the energy barriers for NIR‐CO2PR to CO. To our knowledge, this is one of the first reports of NIR‐CO2PR at wavelengths up to 1200 nm in LDH‐based photocatalyst systems.

Photocatalytic CO2 Reduction by Near-Infrared-Light (1200 nm) Irradiation and a Ruthenium-Intercalated NiAl-Layered Double Hydroxide.

Near-infrared light-driven photocatalytic CO2 reduction (NIR-CO2PR) holds tremendous promise for the production of valuable commodity chemicals and fuels. However, designing photocatalysts capable of reducing CO2 with low energy NIR photons remains challenging. Herein, a novel NIR-driven photocatalyst comprising an anionic Ru complex intercalated between NiAl-layered double hydroxide nanosheets (NiAl-Ru-LDH) is shown to deliver efficient CO2 photoreduction (0.887 μmol h-1) with CO selectivity of 84.81 % under 1200 nm illumination and excellent stability over 50 testing cycles. This remarkable performance results from the intercalated Ru complex lowering the LDH band gap (0.98 eV) via a compression-related charge redistribution phenomenon. Furthermore, transient absorption spectroscopy data verified light-induced electron transfer from the Ru complex towards the LDH sheets, increasing the availability of electrons to drive CO2PR. The presence of hydroxyl defects in the LDH sheets promotes the adsorption of CO2 molecules and lowers the energy barriers for NIR-CO2PR to CO. To our knowledge, this is one of the first reports of NIR-CO2PR at wavelengths up to 1200 nm in LDH-based photocatalyst systems.

Panchromatic NIR-Absorbing Sensitizers with a Thienopyrazine Auxiliary Acceptor for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are a cost-effective and highly tunable technology for harvesting solar energy. Current research is focused on the design of near-infrared (NIR, >700 nm) absorbing organic sensitizers to efficiently harvest low-energy photons to increase the efficiency of DSCs. Herein, two NIR sensitizers containing a proaromatic thienopyrazine (TPZ) auxiliary acceptor are synthesized. The optical, electrochemical, and device properties of the sensitizers were studied. The two dyes probe the placement of the TPZ auxiliary acceptor (A′) either between the π-bridge and the acceptor (WM1, D−π–A′−A) or between the donor and the π-bridge (WM2, D–A′−π–A). WM1 and WM2 exhibit absorption maxima at 631 and 604 nm in dichloromethane (DCM), respectively. Both dyes also have higher energy absorption features, which contributes to panchromatic absorption into the NIR region. Optimized devices were sequentially sensitized with Y123 to control the surface conformation of the TPZ dyes, yielding high photocurrent values of 20.3 and 14.4 mA/cm2 for WM1 and WM2, respectively. WM1 exhibits a low-energy NIR incident photon-to-current efficiency (IPCE) onset at ∼900 nm.

New expanded porphyrinoids: Synthesis, structure and properties of hemihexaphyrazines and their reduced metal containing derivatives

Hemihexaphyrazines can be considered as hexamembered porphyrinoids of the ABABAB type, where thiadiazole rings (A) and pyrrole-containing subunits (B) joined via aza bridges to form a macrocyclic system with an expanded coordination cavity capable of holding at once three atoms of transition metals that are formed a triangular cluster. It was established for the first time that unsubstituted hemihexaphyrazine and its metal complexes can be reduced to form crystalline salts. These salts were characterized by UV-Vis-NIR and IR spectroscopy, MS, and X-Ray diffraction data. The magnetic properties of these compounds were studied for the first time using EPR and SQUID techniques within the [Formula: see text] K temperature range. It was shown that the three-electron reduction of starting [Formula: see text] yields [Formula: see text] and [Formula: see text] crystalline salts. An unusual doublet-quartet transition was found for the [Formula: see text] dianions. Salt [Formula: see text] also shows an unusually strong low energy NIR absorption, which was observed at 1000–2200 nm with maxima located at 1283 and 1980 nm. A phenomenon consisting of equalization of bond lengths among exocyclic nitrogen atoms ([Formula: see text] and neighboring atoms C(pyrrole) and C(thiadiazole) was observed under reduction conditions.

Regulating the trap distribution of ZnGa2O4:Cr3+ by Li+/Ga3+ doping for upconversion-like trap energy transfer NIR persistent luminescence

Doping Li+/Ga3+ in ZnGa2O4:Cr3+ splits the energy level of electron traps into shallow traps and deep traps, which makes the developed phosphors be successfully excited by a low-energy NIR light and suitable for long-term biological imaging.

Rhenium Tricarbonyl Complexes of Azodicarboxylate Ligands

The excellent π-accepting azodicarboxylic esters adcOR (R = Et, iPr, tBu, Bn (CH2-C6H5) and Ph) and the piperidinyl amide derivative adcpip were used as bridging chelate ligands in dinuclear Re(CO)3 complexes [{Re(CO)3Cl}2(µ-adcOR)] and [{Re(CO)3Cl}2(µ-adcpip)]. From the adcpip ligand the mononuclear derivatives [Re(CO)3Cl(adcpip)] and [Re(CO)3(PPh3)(µ-adcpip)]Cl were also obtained. Optimised geometries from density functional theory (DFT) calculations show syn and anti isomers for the dinuclear fac-Re(CO)3 complexes at slightly different energies but they were not distinguishable from experimental IR or UV-Vis absorption spectroscopy. The electrochemistry of the adc complexes showed reduction potentials slightly below 0.0 V vs. the ferrocene/ferrocenium couple. Attempts to generate the radicals [{Re(CO)3Cl}2(µ-adcOR)]•- failed as they are inherently unstable, losing very probably first the Cl- coligand and then rapidly cleaving one [Re(CO)3] fragment. Consequently, we found signals in EPR very probably due to mononuclear radical complexes [Re(CO)3(solv)(adc)]•. The underlying Cl-→solvent exchange was modelled for the mononuclear [Re(CO)3Cl(adcpip)] using DFT calculations and showed a markedly enhanced Re-Cl labilisation for the reduced compared with the neutral complex. Both the easy reduction with potentials ranging roughly from -0.2 to -0.1 V for the adc ligands and the low-energy NIR absorptions in the 700 to 850 nm range place the adc ligands with their lowest-lying π* orbital being localised on the azo function, amongst comparable bridging chelate N^N coordinating ligands with low-lying π* orbitals of central azo, tetrazine or pyrazine functions. Comparative (TD)DFT-calculations on the Re(CO)3Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF) and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries showed a markedly better agreement of the latter with the experimental XRD, IR and UV-Vis absorption data.

Through Bridge Spin Coupling in Homo- and Heterobimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation.

We have described copper(II)-iron(III) and copper(II)-manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV-visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV-visible spectroscopic studies show that upon 2e-oxidation of the heterobimetallic complexes only ring-centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low-energy NIR band obtained for the 2e-oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π-cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π-cation radical. Moreover, the coupling between two π-cation radicals are much stronger in the 2e-oxidized complexes of dimanganese(III) and copper(II)-manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)-iron(III) analogs. Furthermore, coupling between the unpaired spins of a π-cation radical and copper(II) is much stronger in the 2e-oxidized complex of copper(II)-iron(III) porphyrin dimer as compared to its copper(II)-manganese(III) analog. The Mulliken spin density distributions in 2e-oxidized homo- and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e-oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.

Dianionic States of Trithiadodecaazahexaphyrin Complexes with Homotrinuclear MII3O Clusters (M = Ni and Cu): Crystal Structures, Metal- Or Macrocycle-Centered Reduction, and Doublet-Quartet Transitions in the Dianions.

Metal complexes of trithiadodecaazahexaphyrin (Hhp) that contain MII3O clusters inside a π-extended trianionic (Hhp3-) macrocycle have been prepared. Studies of the magnetic properties of NiII3O(Hhp) and CuII3O(Hhp) reveal a diamagnetic and EPR-silent trianionic (Hhp3-) macrocycle and diamagnetic NiII3(O2-) or paramagnetic CuII3(O2-) tetracations. The positive charge of MII3O(Hhp) is compensated by one acetate anion {MII3O(Hhp)}+(CH3CO2-). The three-electron reduction of {MII3O(Hhp)}+ yields {cryptand(Cs+)}2{NiII2NiIO(Hhp5-)}2-·2C7H8 (1) and {cryptand(Cs+)}2{CuII3O(Hhp•6-)}2-·C7H8 (2) crystalline salts. The magnetic properties of 1 reveal the formation of Hhp5- and the reduction of nickel(II) to the paramagnetic NiI ion (S = 1/2), which is accompanied by the formation of the {NiII2NiIO(Hhp5-)}2- dianion. As a result, the magnetic moment of 1 is 1.68 μB in the 20-220 K range, and a broad EPR signal of NiI was observed. The Hhp5- macrocycle has a singlet ground state, but the increase in the magnitude of the magnetic moment of 1 above 220 K is attributed to the population of the triplet excited state in Hhp5-. The {NiII2NiIO(Hhp5-)}2- dianion is transferred from the doublet excited state to the quartet excited state with an energy gap of 1420 ± 50 K. Salt 1 also shows an unusually strong low-energy NIR absorption, which was observed at 1000-2200 nm. In 2, a highly reduced Hhp•6- radical hexaanion (S = 1/2) coexists with a CuII3(O2-) cluster (S = 1/2) in the {CuII3O(Hhp•6-)}2- dianions. The dianions have a triplet ground state with antiferromagnetic exchange between two S = 1/2 spins with J = -6.4 cm-1. The reduction of Hhp in both salts equalizes the initially alternated C-N bonds, supporting the increase in the Hhp macrocycle electron delocalization.

Synthesis of branched WO3@W18O49 homojunction with enhanced interfacial charge separation and full-spectrum photocatalytic performance

Interfacial charge separation is a fundamental and crucial process in photoelectric conversion for composite photocatalyst. In this work, the interfacial charge separation performance was investigated on a nonmetallic branched homojunction, which is fabricated through solvothermal growth of W18O49 nanofiber (as branches) onto WO3 microrods (as backbones). The ultrafast transfer of photogenerated electrons from the WO3 backbones to the W18O49 branches across the contact interface was demonstrated by a series of experiments and characterizations. The contrast experiment showed that the WO3@W18O49 homojunction exhibited superior interfacial electron transfer capacity to the BiVO4@W18O49 heterojunction, the calculated interfacial charge separation efficiency of WO3@W18O49 was 51.3%, which was more than twice as that of BiVO4@W18O49 (24.2%). Upon localized surface plasmon resonance excitation by low-energy NIR photons, the full-spectrum light driven photo-degradation for 2,4-DCP was realized. The branched structure favors the enhancement of light scattering and absorbing. Meanwhile, the homojunction structure leads to a low impedance interface and increased electric conductivity. Thus, the WO3@W18O49 exhibited an enhanced photocatalytic performance under both full-spectrum and NIR light irradiation. This work provides a promising approach to design and fabricate novel photocatalysts with full-spectrum response ability and enhanced charge separation.

Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry.

We report the syntheses, structures, and luminescence properties of a series of copper-iodo cuboids supported by L-type antimony ligands. The cuboids are of general formula [(SbR3)4Cu4(I)4] (1-4, 8), where SbR3 is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, iPr, tBu, Ph); triphenyl, iPr2Ph, and Me2Ph stibines resulted in the formation of dimers of type [(SbR3)4(Cu)2(I)2] (5-7). While similar luminescent copper-halide cubes have been studied, the corresponding "heavy-atom" congeners have not been studied, and ligation of such heavy-atom moieties is often associated with long-lived triplet states and low-energy absorption and emission profiles. Overall, two obligate parameters are found to imbue NIR emission: (i) short Cu-Cu bonds and (ii) high crystallographic symmetry; both of these properties are found only in [(SbiPr3)4Cu4(I)4] (1, in I23; λem = 711 nm). The correlation between NIR emission and high crystallographic symmetry (which intrinsically includes high molecular symmetry)-versus only molecular symmetry-is confirmed by the counterexample of the molecularly symmetric tBu-substituted cuboid [(SbtBu3)4Cu4(I)4] (3, λem = 588 nm, in R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of the SbtBu3 ligand should red-shift emission beyond that of the SbiPr3-supported cuboid, the emission of 3 is limited to the visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for 1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S0) and excited triplet state (T1) demonstrate two key factors necessary for low-energy NIR emission: (i) a significant contraction of the interconnected Cu4 intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A1 symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu4 core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T1, thereby inhibiting low-energy NIR emission.

Radical formation in polymeric nickel complexes with N2O2 Schiff base ligands: An in situ ESR and UV–vis–NIR spectroelectrochemical study

A series of electrochemically generated polymeric nickel (II) Schiff base complexes was investigated by in situ electron spin resonance (ESR)/ultraviolet–visible–near infrared (UV–vis–NIR) spectroelectrochemistry and the combination of cyclic voltammetry and quartz crystal microbalance (QCM) techniques. The comparative analysis of the in situ ESR and UV–vis–NIR (including low-energy NIR range) spectra of poly-[Ni(Schiff)] films recorded during their oxidation at ambient temperature was performed, which allowed us to monitor the formation of different charge carriers in the polymer at different doping levels and elucidate their structure as well as the extent of their delocalization. The polymer doping level achieved by its oxidation within selected range of potentials was determined by combining the results of cyclic voltammetry and QCM measurements. The influence of the substituents in the diamine backbone and phenolate moieties of the ligand on the spectroscopic properties of the oxidized polymers and number of electrons per monomer unit exchanged in the redox processes was studied in detail. Integration of data from all techniques allowed us to propose the existence of both inter- and intrachain delocalization of radical cations generated in the doped polymers and show that the highest doping levels (up to two positive charges per a monomer unit) are attained in the poly-[Ni(Schiff)] films with highly localized biphenoxyl radical cations.

N-TiO2/g-C3N4/Up-conversion phosphor composites for the full-spectrum light-responsive deNO x photocatalysis

The ternary composites consisted of nitrogen-doped titanium dioxide, carbon nitride and up-conversion phosphors (UP) were successfully prepared by a solvothermal method. The heterojunction could be formed when N-TiO2 and g-C3N4 were combined together. The composite of N-TiO2/g-C3N4@UP had excellent ultraviolet, visible and infrared light absorption, indicating the possibility for the utilization of full spectrum of solar light. When N-TiO2 was coupled with g-C3N4 and up-conversion phosphors to form a composite, the visible light and NIR light absorption of the samples increased. The ternary composite N-TiO2/g-C3N4@G-UP presented reasonable deNO x performance of about 8.0% under the irradiation of IR light of 980 nm. The intensification of the photocatalysis might be realized by utilizing up-conversion phosphors, which could convert low-energy NIR light into high-energy photons (visible light) and increase the efficient irradiation on the surface of photocatalyst.

Characterization of double modified internal gate pixel by 3D simulation study

We have developed a novel detector concept based on Modified Internal Gate Field Effect Transistor (MIGFET) wherein a buried Modified Internal Gate (MIG) is implanted underneath a channel of a FET. In between the MIG and the channel of the FET there is a depleted semiconductor material forming a potential barrier between charges in the channel and similar type signal charges located in the MIG. The signal charges in the MIG have a measurable effect on the conductance of the channel. In this paper a double MIGFET pixel is investigated comprising two MIGFETs. By transferring the signal charges between the two MIGs Non-Destructive Correlated Double Sampling Readout (NDCDSR) is enabled.The proposed MIG radiation detector suits particularly well for low-light-level imaging, X-ray spectroscopy, as well as synchrotron and X-ray Free Electron Laser (XFEL) facilities. The reason for the excellent X-ray detection performance stems from the fact that interface related issues can be considerably mitigated since interface generated dark noise can be completely avoided and interface generated 1/f and Random Telegraph Signal (RTS) noise can be considerably reduced due to a deep buried channel readout configuration.Electrical parameters of the double MIGFET pixel have been evaluated by 3D TCAD simulation study. Simulation results show the absence of interface generated dark noise, significantly reduced interface generated 1/f and RTS noise, well performing NDCDSR operation, and blooming protection due to an inherent vertical anti-blooming structure. In addition, the backside illuminated thick fully depleted pixel design provides a homogeneous radiation entry window, low crosstalk due to lack of diffusion, and good quantum efficiency for low energy X-rays and NIR light.These facts result in excellent Signal-to-Noise Ratio (SNR) and very low crosstalk enabling thus excellent X-ray energy and spatial resolution. The simulation demonstrates the charge to current conversion gain for source current readout to be 1.4 nA/e.

Tuning the near-IR band energy and redox potentials of magnesium tetra(ferrocenyl)tetraazaporphyrins

Magnesium 2(3),7(8),12(13),17(18)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin (3), and magnesium 2(3),7(8),12(13),17(18)-tetracyano-3(2),8(7),13(12),18(17)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin (4) complexes were prepared using template condensation reaction between magnesium butoxide and (Z)-dicyanovinylferrocene (1) and tricyanovinylferrocene (2), respectively. Redox and unusual optical properties of macrocyclic compounds 3 and 4 were investigated in details using UV-vis, MCD, electro-, and spectroelectrochemical methods as well as DFT and TDDFT calculations. It was shown that the HOMO in both compounds resembles Goutermans' a1u orbital (D4h symmetry), which is different from meso-ferrocenyl containing porphyrins. In both macrocycles 3 and 4 overlapping oxidation waves for ferrocene substituents were observed in electrochemical and spectroelectrochemical experiments. Electrochemically observed oxidation waves are rather broad because of the presence of positional isomers and span only ~300 mV range in DCM/0.05 M TBAF system. Based on the excellent agreement between experimental UV-vis spectra and TDDFT predicted excitation energies, it was suggested that the low-energy NIR and visible regions are predominantly composed by the MLCT transitions with a significant π–π* mixing.

An efficient light converter YAB:Cr3+,Yb3+/Nd3+ with broadband excitation and strong NIR emission for harvesting c-Si-based solar cells

The Cr3+–Yb3+ co-doped YAB could convert almost high-energy UV-Vis photons into low-energy NIR photons efficiently, which benefits the enhancement of silicon solar photovoltaic conversion efficiency.

A new approach to highly sensitive determination of retinoic acid isomers by preconcentration with CdSe quantum dots

Unusual amounts of retinoic acid (RA) isomers play an important role in abnormal morphological development of mammals; such as rat embryos. Each isomer of RA has a unique function in first steps of embryonic life. In the current study, a new method for preconcentration and simultaneous determination of all-trans retinoic acid, 13-cis retinoic acid, 9-cis retinoic acid and 9,13-di-cis retinoic acid in rat whole rudimentary embryo culture (RWEC) has been developed. RA isomers were extracted from samples by conjugation to appropriate amount of surface modified CdSe quantum dots (QDs) prior to HPLC/UV determination. In order to quickly release of the analytes with unchanged form, separated RA-QD conjugation were irradiated by intensive near infrared wavelength (NIR). Low energy NIR irradiation results in maintaining the primary forms of RA isomers during the release. The conjugation and release mechanisms were described and experimental parameters were investigated in detail. Under optimized conditions, the method was linear in the range of 0.040-34.600 pmol g(-1) for all-trans RA (R(2)=0.9996), 0.070-34.200 pmol g(-1) for 13-cis RA (R(2)=0.9992), 0.050-35.300 pmol g(-1) for 9,13-di-cis RA (R(2)=0.9998) and 0.050-32.900 pmol g(-1) for 9-cis RA (R(2)=0.9990). The present method can be useful for retinoic acid monitoring in clinical studies.

Molecular and electronic structure of MM quadruply bonded complexes containing O2CC6H4N(Ph)2 supporting ligands

The complexes trans-M2(TiPB)2(O2CC6H4-4-NPh2)2 where M=Mo (I), and M=W (II) were prepared from the reaction between M2(TiPB)4 and the carboxylic acid Ph2N-4-C6H4CO2H (∼2equiv.) in toluene (TiPB=2,4,6-triisopropylbenzoate). The compounds were isolated as microcrystalline powders that are yellow, I or purple II, and are air-sensitive. Compound I was characterized by single-crystal X-ray crystallography and shown to have a centrosymmetric structure and a planar central C6H4-CO2M2O2C-C6H4 unit. Calculations indicate that the HOMO is principally M2δ with mixing of the filled π-orbitals of the N,N-diphenyl-4-aminobenzoate. Oxidation of I and II yield cationic species that have been characterized by EPR and UV–Vis–NIR spectroscopy. The radical cations show EPR spectra characteristic of M25+ centers having the MM configuration σ2π4δ1 and compound I+ shows a low energy NIR transition assignable to a ligand to metal charge transfer transition. The homoleptic compound Mo2(O2CC6H4-4-N-Ph2)4, III, was prepared from the reaction between Mo(CO)6 and Ph2N-4-C6H4CO2H in refluxing ortho-dichlorobenzene and is shown to exhibit similar spectroscopic features to those of I.

Marked Stabilization of Redox States and Enhanced Catalytic Activity in Galactose Oxidase Models Based on Transition MetalS-Methylisothiosemicarbazonates with −SR Group in Ortho Position to the Phenolic Oxygen

Reactions of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde S-methylisothiosemicarbazone and 5-tert-butyl-2-hydroxy-3-phenylsulfanylbenzaldehyde S-methylisothiosemicarbazone with pentane-2,4-dione (Hacac) and triethyl orthoformate in the presence of M(acac)2 as template source at 107 °C afforded metal complexes of the type M(II)L(1) and M(II)L(2), where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H2L(1)) and/or thiophenyl (H2L(2)) group in the ortho position of the phenolic moiety. Demetalation of NiL(1) in CHCl3 with HCl(g) afforded H2L(1). The latter reacts with Zn(OAc)2·2H2O with formation of ZnL(1). The effect of -SR groups and metal ion identity on stabilization of phenoxyl radicals generated electrochemically was studied in detail. A marked stabilization of phenoxyl radical was observed in one-electron-oxidized complexes [ML(2)](+) (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV-vis-NIR measurements. In solution, the oxidized CuL(2) and NiL(2) display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL(2) complex shows reversible reduction, reduction of NiL(2), CuL(1), and NiL(1) is irreversible. EPR measurements in conjunction with density functional theory calculations provided insights into the extent of electron delocalization as well as spin density in different redox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the (3)[CuL(2)](+) and (2)[NiL(2)](+) oxidation ground states. The catalytic activity of synthesized complexes in the selective oxidations of alcohols has been studied as well. The remarkable efficiency is evident from the high yields of carbonyl products when employing both the CuL(2)/air/TEMPO and the CuL(2)/TBHP/MW(microwave-assisted) oxidation systems.

Detailed evaluation of the geometric and electronic structures of one-electron oxidized group 10 (Ni, Pd, and Pt) metal(II)-(disalicylidene)diamine complexes.

The geometric and electronic structures of a series of one-electron oxidized group 10 metal salens (Ni, Pd, Pt) have been investigated in solution and in the solid state. Ni (1) and Pd (2) complexes of the tetradentate salen ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (H(2)Salcn) have been examined along with the Pt (3) complex of the salen ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediamine (H(2)Salen). All three oxidized compounds exist as ligand radical species in solution and in the solid state. The solid state structures of [1](+) and [3](+) exhibit a symmetric coordination sphere contraction relative to the neutral forms. By contrast, the coordination sphere of the Pd derivative [2](+) exhibits a pronounced asymmetry in the solid state. In solution, the oxidized derivatives display intense low-energy NIR transitions consistent with their classification as ligand radical compounds. Interestingly, the degree of communication between the phenolate moieties depends strongly on the central metal ion, within the Ni, Pd, and Pt series. Electrochemical measurements and UV-vis-NIR spectroscopy, in conjunction with density functional theory calculations provide insights into the degree of delocalization of the one-electron hole in these systems. The Pd complex [2](+) is the least delocalized and is best described as a borderline Class II/III intervalence complex based on the Robin-Day classification system. The Ni [1](+) and Pt [3](+) analogues are Class III (fully delocalized) intervalence compounds. Delocalization is dependent on the electronic coupling between the redox-active phenolate ligands, mediated by overlap between the formally filled metal d(xz) orbital and the appropriate ligand molecular orbital. The degree of coupling increases in the order Pd < Ni < Pt for the one-electron oxidized group 10 metal salens.

Mixed-metal cluster chemistry. 19. Crystallographic, spectroscopic, electrochemical, spectroelectrochemical, and theoretical studies of systematically varied tetrahedral group 6-iridium clusters.

A systematically varied series of tetrahedral clusters involving ligand and core metal variation has been examined using crystallography, Raman spectroscopy, cyclic voltammetry, UV-vis-NIR and IR spectroelectrochemistry, and approximate density functional theory, to assess cluster rearrangement to accommodate steric crowding, the utility of metal-metal stretching vibrations in mixed-metal cluster characterization, and the possibility of tuning cluster electronic structure by systematic modification of composition, and to identify cluster species resultant upon electrochemical oxidation or reduction. The 60-electron tetrahedral clusters MIr(3)(CO)(11-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (5), C(5)HMe(4) (6), C(5)Me(5) (7); M = W, Cp = C(5)H(4)Me, x = 1 (13), x = 2 (14)] and M(2)Ir(2)(CO)(10-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (8), C(5)HMe(4) (9), C(5)Me(5) (10); M = W, Cp = C(5)H(4)Me, x = 1 (15), x = 2 (16)] have been prepared. Structural studies of 7, 10, and 13 have been undertaken; these clusters are among the most sterically encumbered, compensating by core bond lengthening and unsymmetrical carbonyl dispositions (semi-bridging, semi-face-capping). Raman spectra for 5, 8, WIr(3)(CO)(11)(eta(5)-C(5)H(4)Me) (11), and W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(4)Me)(2) (12), together with the spectrum of Ir(4)(CO)(12), have been obtained, the first Raman spectra for mixed-metal clusters. Minimal mode-mixing permits correlation between A(1) frequencies and cluster core bond strength, frequencies for the A(1) breathing mode decreasing on progressive group 6 metal incorporation, and consistent with the trend in metal-metal distances [Ir-Ir < M-Ir < M-M]. Cyclic voltammetric scans for 5-15, MoIr(3)(CO)(11)(eta(5)-C(5)H(5)) (1), and Mo(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (3) have been collected. The [MIr(3)] clusters show irreversible one-electron reduction at potentials which become negative on cyclopentadienyl alkyl introduction, replacement of molybdenum by tungsten, and replacement of carbonyl by phosphine. These clusters show two irreversible one-electron oxidation processes, the easier of which tracks with the above structural modifications; a third irreversible oxidation process is accessible for the bis-phosphine cluster 14. The [M(2)Ir(2)] clusters show irreversible two-electron reduction processes; the tungsten-containing clusters and phosphine-containing clusters are again more difficult to reduce than their molybdenum-containing or carbonyl-containing analogues. These clusters show two one-electron oxidation processes, the easier of which is reversible/quasi-reversible, and the more difficult of which is irreversible; the former occur at potentials which increase on cyclopentadienyl alkyl removal, replacement of tungsten by molybdenum, and replacement of phosphine by carbonyl. The reversible one-electron oxidation of 12 has been probed by UV-vis-NIR and IR spectroelectrochemistry. The former reveals that 12(+) has a low-energy band at 8000 cm(-1), a spectrally transparent region for 12, and the latter reveals that 12(+) exists in solution with an all-terminal carbonyl geometry, in contrast to 12 for which an isomer with bridging carbonyls is apparent in solution. Approximate density functional calculations (including ZORA scalar relativistic corrections) have been undertaken on the various charge states of W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (4). The calculations suggest that two-electron reduction is accompanied by W-W cleavage, whereas one-electron oxidation proceeds with retention of the tetrahedral core geometry. The calculations also suggest that the low-energy NIR band of 12(+) arises from a sigma(W-W) --> sigma*(W-W) transition.