Metal-centered Oxidation Research Articles

Chromium(0) and Molydenum(0) Complexes with a Pyridyl-Mesoionic Carbene Ligand: Structural, (Spectro)electrochemical, Photochemical, and Theoretical Investigations.

This work reports on the synthesis and in-depth electrochemical and photochemical characterization of two chromium(0) and molydenum(0) metal complexes with bidentate pyridyl-mesoionic carbene (MIC) ligands of the 1,2,3-triazol-5-ylidene type and carbonyl coligands. Metal complexes with MIC ligands have turned out to have very promising electrocatalytic and photochemical properties, but examples of MIC-containing complexes with early-transition-metal centers remain extremely rare. The electrochemistry of these new MIC complexes was studied by cyclic voltammetry and especially spectroelectrochemistry in the IR region consistent with a mainly metal-centered oxidation, which is fully reversible in the case of the chromium(0) complex. At the same time, the two reduction steps are predominantly ligand-centered according to the observed near-IR absorbance, with the first reduction step being reversible for both systems. The results of the electron paramagnetic resonance studies on the oxidized and reduced species confirm the IR spectroelectrochemistry experiments. The photochemical reactivity of the complexes with a series of organic ligands was investigated by time-resolved (step-scan) Fourier transform infrared (FTIR) spectroscopy. Interestingly, the photoreactions in pyridine and acetonitrile are fully reversible with a slow dark reverse reaction back to the educt species over minutes and even hours, depending on the metal center and reagent. This reversible behavior is in contrast to the expected loss of one or several CO ligands known from related homoleptic as well as heteroleptic M(CO)4L2 α-diimine transition-metal complexes.

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes.

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges. Herein, we explore a copper complex (12-) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (22- and 32-). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using 12- that is absent in 22- and 32-. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 12- compared to 22- and 32- and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a kobs of 140 s-1 at an overpotential of only 200 mV. In contrast, 22- degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 32- prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.

Photovoltaic performance of bipyridine and dipyridophenazine ligands anchored ruthenium complex sensitizers for efficient dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSCs) can be influenced by the sensitizer. Therefore, three heteroleptic ruthenium complexes, cis-[Ru(dcbpy)(bpy)(NCS)2] (Ru-bpy), cis-[Ru(dcbpy)(dppz)(NCS)2] (Ru-dppz) and cis-[Ru(dcbpy)(dppx)(NCS)2] (Ru-dppx) (dcbpy = 4,4′-dicarboxylic-2,2′-bipyridine, NCS = isothiocyanate) sensitizers, were synthesized using one pot synthesis. The structures and functionalization of the ligands in each complex sensitizer were confirmed using FT-IR and NMR analyses. They were then systematically studied to determine their light absorption spectrum, the metal-centered oxidation peak, and electron recombination properties using a UV–vis spectrophotometer and electroanalytical techniques (i.e., CV, EIS, and LSV). The criteria for an efficient DSSC ruthenium sensitizer were drawn based on the structure-property relationship. It was found that the power conversion efficiency (ɳ) of the sensitizers were in the sequence of Ru-dppz (ɳ = 0.40%) < Ru-bpy (ɳ = 0.58%) < Ru-dppx (ɳ = 0.61%) in comparison to a commercialized sensitizer N719 (ɳ = 0.93%), The introduction of the dimethyl group altered the LUMO to enhance the electron injection to the TiO2 conduction band and thus increased the power conversion efficiency of the DSSC.

Tuning of Persulfate Activation from a Free Radical to a Nonradical Pathway through the Incorporation of Non-Redox Magnesium Oxide.

Nonradical-based advanced oxidation processes for pollutant removal have attracted much attention due to their inherent advantages. Herein we report that magnesium oxides (MgO) in CuOMgO/Fe3O4 not only enhanced the catalytic properties but also switched the free radical peroxymonosulfate (PMS)-activated process into the 1O2 based nonradical process. CuOMgO/Fe3O4 catalyst exhibited consistent performance in a wide pH range from 5.0 to 10.0, and the degradation kinetics were not inhibited by the common free radical scavengers, anions, or natural organic matter. Quantitative structure-activity relationships (QSARs) revealed the relationship between the degradation rate constant of 14 substituted phenols and their conventional descriptor variables (i.e., Hammett constants σ, σ-, σ+), half-wave oxidation potential (E1/2), and pKa values. QSARs together with the kinetic isotopic effect (KIE) recognized the electron transfer as the dominant oxidation process. Characterizations and DFT calculation indicated that the incorporated MgO alters the copper sites to highly oxidized metal centers, offering a more suitable platform for PMS to generate metastable copper intermediates. These highly oxidized metals centers of copper played the key role in producing O2•- after accepting an electron from another PMS molecule, and finally 1O2 as sole reactive species was generated from the direct oxidation of O2•- through thermodynamically feasible reactions.

A tetranuclear nickel cluster isolated in multiple high-valent states.

We report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(ii) ([Ni4]) neutral cluster. Electrochemical analysis of [Ni4] reveals three reversible sequential oxidations at 0.248 V (1e-), 0.678 V (1e-), and 0.991 V (2e-) vs. Fc/Fc+ corresponding to mono-, di-, and tetra-oxidized species, [Ni4]+, [Ni4]2+, [Ni4]4+, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(iii) cluster, [Ni4]4+.

Photo-accelerated fast charging of lithium-ion batteries

Due to their exceptional high energy density, lithium-ion batteries are of central importance in many modern electrical devices. A serious limitation, however, is the slow charging rate used to obtain the full capacity. Thus far, there have been no ways to increase the charging rate without losses in energy density and electrochemical performance. Here we show that the charging rate of a cathode can be dramatically increased via interaction with white light. We find that a direct exposure of light to an operating LiMn2O4 cathode during charging leads to a remarkable lowering of the battery charging time by a factor of two or more. This enhancement is enabled by the induction of a microsecond long-lived charge separated state, consisting of Mn4+ (hole) plus electron. This results in more oxidized metal centers and ejected lithium ions are created under light and with voltage bias. We anticipate that this discovery could pave the way to the development of new fast recharging battery technologies.

Bioinspired iron(II)-β-diketonate and iron(II)-α-hydroxy ketone complexes of a carbanionic N3C ligand: Oxidation of metal center vs C[sbnd]C bond cleavage of co-ligand with dioxygen

Two bioinspired low-spin iron(II)-β-diketonate complexes, [(L1)FeII(BA)] (2) and [(L1)FeII(DBM)] (3), and an iron(II)-α-hydroxy ketone complex [(L1)FeII(HAP)] (4) (L1 = tris(2-pyridylthio)methanido anion, BA = monoanionic benzoylacetone, DBM = monoanionic dibenzoylmethane and HAP = monoanionic 2-hydroxyacetophenone) of a tripodal carbanionic ligand were prepared from an iron(II) precursor complex [(L1)FeII(CH3CN)2](ClO4) (1). The dioxygen reactivity of the complexes (2, 3 and 4) was investigated to evaluate the effect of ligand geometry and of the spin state of iron on the CC bond cleavage of the co-ligands. Complexes 2 and 3 react with dioxygen to yield benzoic acid as a minor product. The major pathway involves the formation of the corresponding iron(III)-β-diketonate complexes, [(L1)FeIII(BA)](ClO4) (2ox-ClO4) and [(L1)FeIII(DBM)](ClO4) (3ox-ClO4). Complex 4, however, undergoes the CC bond cleavage of the iron-coodinated HAP to form benzoic acid as the major product. In the reaction, phenylglyoxal is formed as the minor product with concomitant generation of an iron-oxygen oxidant. The oxidant is able to transfer an oxygen atom to thioanisole and can exchange its oxygen atom with water. The reactivity patterns of the low-spin iron(II) complexes reported here are distinctly different from that of the corresponding high-spin complexes supported by a monoanionic facial N3 ligand. The results emphasize the role of spin-state of iron, the denticity of the supporting ligand, and the nature of co-ligand in affecting the CC bond cleavage reaction.

Synthesis, structures, photophysical properties, and theoretical study of four cationic iridium(III) complexes with electron-withdrawing groups on the neutral ligands

Four cationic iridium(III) complexes Ir1–Ir4 with the general form of [Ir(C^N)2(N^N)]PF6 (where C^N = 2-(thiophen-2-yl)pyridine or 2-(benzo[b]thiophen-2-yl)pyridine, N^N = 4,4′-dibromo-2,2′-bipyridine or 5,5′-dibromo-2,2′-bipyridine) have been synthesized and characterized. X-ray diffraction analysis of complexes Ir1–Ir4 indicate the coordination of the iridium atoms are distorted octahedral geometry. All complexes exhibit strong red phosphorescence (λem = 599–633 nm) with quantum efficiencies of 0.16–0.37 in oxygen-free dichloromethane solution at room temperature. The introduction of electron-withdrawing groups on neutral ligands was found to be useful in red-shifting the emission maxima of the complexes compared to that of the complexes with electron-donating substituents on neutral ligands. Electrochemical study shows a quasireversible, metal-centered oxidation with potentials at 0.76–0.90 V (vs. Ag+/Ag). Density functional theory (DFT) calculation shows that the LUMOs are mainly localized on the neutral ligands and the electron-withdrawing groups on neutral ligands can stabilize both the LUMO and HOMO though the former is affected to a much more appreciable degree than the latter.

Chemical Oxidation of a Coordinated PNP-Pincer Ligand Forms Unexpected Re-Nitroxide Complexes with Reversal of Nitride Reactivity.

Because of the thermodynamic demands of N2 cleavage, N2-derived nitride complexes are often unreactive. The development of multistep N2 functionalization reactions hinges on methods for modulating nitride reactivity with supporting ligands. Here, we describe the reactions of N2-derived Re-nitride complexes, including the first Re nitrides supported by a nitroxide-containing pincer ligand, and unusual examples of Re6+-nitride complexes. The previously reported N2-derived complex (PNP)Re(N)(Cl) (PNP = N(CH2CH2PtBu2)2) can be oxidized by O atom transfer to the backbone amide to form a novel nitroxide-pincer complex or by 1e- to form a rare S = 1/2 Re6+-nitride complex. The Re-nitrido interaction in a series of Re- and ligand-oxidized complexes is characterized using 15N NMR spectroscopy, IR spectroscopy, and DFT calculations, and shows changes in the Re-N bond order from both ligand- and metal-centered oxidations. Chemical oxidation of the supporting ligand to form a nitroxide-pincer ligand results in subtle electronic changes at Re and a more electron-deficient nitride ligand. Combined ligand- and metal-centered oxidation to form a Re6+-nitroxide complex results in a reversal of reactivity at the nitride ligand from nucleophilic to electrophilic. These systematic electronic structure and reactivity studies demonstrate methods for inducing reactivity in N2-derived nitride complexes.

Long-Lived Trimetallic Complexes of Fe(II), Ru(II), and Os(II) Based on a Heteroditopic Bipyridine-Terpyridine Bridge: Synthesis, Photophysics, and Electronic Energy Transfer.

Synthesis, characterization, and investigation of photophysical and redox behaviors of a new class of homo- and heterotrimetallic complexes of composition [(bpy/phen)2Ru(dipy-Hbzim-tpy)M(tpy-Hbzim-dipy)Ru(bpy/phen)2]6+ (M = FeII, RuII, and OsII) derived from a conjugated heteroditopic bipyridine-terpyridine bridge were carried out in this work. Trimetallic RuZnRu complexes of composition [(bpy/phen)2Ru(dipy-Hbzim-tpy)Zn(tpy-Hbzim-dipy)Ru(bpy/phen)2]6+ were also synthesized in situ as their photophysical properties are of particular interest in demonstrating the absorption and emission spectra of the complexes in the presence of a metal (Zn2+) that has neither metal-to-ligand charge transfer (MLCT) nor metal-centered (3MC) states. Complexes display intense absorption bands spanning almost the entire UV and visible region. The complexes also exhibit rich electrochemical behaviors with a number of metal-centered reversible oxidation and ligand-centered reduction waves. All complexes are luminescent at room temperature, and time-resolved emission spectral studies indicate that peripheral RuII-centered emissive 3MLCT states are quantitatively quenched, by intramolecular energy transfer to the low lying 3MLCT (for central Ru and Os) or 3MC states of the FeII center (nonluminescent). Interestingly, Fe(II) does not adversely deteriorate the photophysics of the RuFeRu assembly. Thus, multicomponent complexes in the present work can serve as well-organized light-harvesting antennas as the light absorbed by multiple chromophoric subunits is efficiently channeled to the distinct component having the lowest-energy excited state.

Electrolyte Solvent Mixtures for a Symmetric, Non-Aqueous Redox Flow Battery Based on [Fe(bpy)3][Fsi

Redox flow batteries (RFBs) store energy in liquid redox-active electrolytes which are charged and discharged while flowing through an electrochemical cell. This separation of storage and reaction sites enables flexible scalability of power and energy density. [1] Research suggests that non-aqueous RFB systems offer a promising combination of cost and performance for stationary applications, such as storage of excess renewable energy from domestic- to grid-scale [2]. In this work, we investigate a RFB based on the iron(II)-tris(2,2’-bipyridine) bis(fluorosulfonyl)imide complex, which shows one metal-centred oxidation process and up to three ligand-centred reduction processes, providing 2.4 V cell potential [3]. Using the same active material on both positive and negative sides of the cell mitigates the detrimental effects of cross-over through the separator membrane. These redox processes are solvent-dependent [3] and their electrochemistry was studied in a range of aprotic single and mixed solvents, some already used in lithium-ion batteries [4]. Since energy density is determined by the cell potential and the concentration of the active material, the solubility of the reduced and oxidated states of the complex was examined by UV/vis spectroscopy. Furthermore, conductivity and viscosity were measured over a range of temperatures. In RFBs, the dynamic viscosity of the electrolyte influences not just the conductivity but also affects the power required to pump the electrolyte. Cost, safety and environmental impact were considered as well.

Synthesis, Structural, Spectral and Electrochemical Redox Properties of Monofused Porphyrins Derived from Trans-Chlorins

The core of photosynthetic pigments consist of chlorophylls having chlorin skeleton and associated pigments which are responsible for light harvesting on Earth. Chlorins are utilized as photosensitizers in PDT since they exhibit intense long wavelength absorption ≥ 600 nm [1]. The advancement in synthetic methodologies over the past few years brought classical methods to bring functionalized chlorins [2,3]. Several methods are available in literature for the synthesis of tetraphenylchlorins (TPCs) from tetraphenylporphyrins (TPPs). However, there are only a few reports on smooth conversion of functionalized chlorins into porphyrins [4,5]. The interconversion of chlorins into porphyrins is of great interest since it opens a new synthetic route to access a wide variety of nonviable β-functionalized porphyrinoids [4,5]. In this regard, several ruffled β-to-ortho-phenyl monofused metalloporphyrins (MIIMFPs) with 1,3-indanedione functionality have been synthesized by oxidative fusion of free base trans-chlorins via nickel insertion followed by the skeletal rearrangement of macrocycle. The synthesized monofused porphyrins exhibited red shifted electronic spectral features as compared to precursor, trans-chlorins or structurally related unfused porphyrins (mono/tri-β-substituted porphyrins) due to extended π-conjugation and nonplanar conformation of the macrocyclic core. Four of the synthesized porphyrins were structurally characterized by X-ray diffraction analysis. Ring fusion resulted into twisted macrocyclic conformation. NiMFP(IND)Ph2 (3b) exhibited modestly ruffled conformation of the macrocyclic core which was further confirmed by higher Δ24 (0.362 Å) and ΔC β (0.279 Å) values. Free base monofused porphyrins have shown upfield shifted inner core NHs as compared to trans-chlorins, whereas much downfield shifted as compared to H2TPP. Electron rich Ni(II) complexes exhibited metal centered oxidation (NiII/NiIII) due to electron donating β-substituents and extended π-conjugation of macrocyclic core. In this presentation, we will describe the facile synthesis, structural, spectral and intriguing electrochemical redox properties of monofused porphyrins. Figure 1

Synthesis, spectral and electrochemical studies of mononuclear samarium (II), europium (II) and ytterbium (II) complexes with a NNO donor Schiff base derived from 4-methyl-2,6-dibenzoylphenol and propane-1,3-diamine

Three mononuclear divalent lanthanide complexes, [Sm(L)2(THF)2] (1), [Eu(L)2(THF)2] (2) and [Yb(L)2(THF)2] (3), (where HLMe = 2-{N-(3-aminopropyl)benzimidoyl}-6-benzoyl-4-methylphenol) have been synthesized and characterized by elemental analyses, conductivity measurements, mass spectrometry, IR, electronic and 1H NMR spectroscopy and cyclic voltammetry. The spectral studies suggest that the LnN4O4 coordination polyhedron is a distorted dodecahedral. The electrochemical behavior of 1–3 shows irreversible metal centered oxidation wave at ca. 0.544–0.643 V and ligand centered reduction at ca. −1.085 to −1.260 V versus Ag/Ag+. Interaction of 1, 2 and 3 with calf thymus DNA suggests an intercalative binding mode.

Oxidatively Induced Reductive Elimination: Exploring the Scope and Catalyst Systems with Ir, Rh, and Ru Complexes.

Direct conversion of C-H bonds into C-C bonds is a promising alternative to the conventional cross-coupling reactions, thus giving rise to a wide range of efficient catalytic C-H functionalization reactions. Among the elementary stages in the catalytic C-C bond formation, reductive elimination constitutes a key step of the catalytic cycle, and, therefore, extensive studies have been made to facilitate this process. In this regard, oxidation on the metal center of a post-transmetalation intermediate would be an appealing approach. Herein, we have explored the substrate scope, catalyst systems, and oxidation tools to prove that the oxidatively induced reductive elimination ( ORE) plays a critical role in the product-releasing C-C bond formation. Notably, we have demonstrated that ORE broadly operates with a series of half-sandwich d6 Ir(III)-, Rh(III)-, and Ru(II)-aryl complexes. We have described that the metal center oxidation of the isolable post-transmetalation intermediates by means of chemical- or electro-oxidation can readily deliver the desired arylated products upon reductive elimination even at ambient temperature. Computational studies delineated the thermodynamics of the reductive elimination, where the activation barriers are shown to be significantly reduced upon increasing the oxidation states of the intermediates. We were also successful in corroborating this ORE in the corresponding Rh- methyl complex. In addition, catalytic conditions were optimized to incorporate this mechanistic understanding into the Ir-, Rh-, and Ru-catalyzed C-C bond formations under mild conditions.

β-Functionalized Dibenzoporphyrins with Mixed Substituents Pattern: Facile Synthesis, Structural, Spectral, and Electrochemical Redox Properties.

A new series of mixed β-substituted dibenzoporphyrins were synthesized, and the effect of β-substitution on the spectral and electrochemical redox properties of the macrocycle was elucidated. The synthetic route to β-tetrasubstituted dibenzoporphyrins begins with the regioselective bromination of NiTPP(Benzo)2 to afford NiTPP(Benzo)2Br4, followed by Pd catalyzed coupling reaction to access NiTPP(Benzo)2(R)4 (R = phenyl (Ph) and phenylethynyl (PE)). Synthesized benzoporphyrins exhibited red-shifted absorption spectral features with tunable redox properties. These benzoporphyrins displayed pronounced electronic effects of β-substituents on the macrocyclic skeleton. NiTPP(Benzo)2(PE)4 exhibited the lowest HOMO-LUMO gap among the series due to extended π-conjugation. Intrestingly, metal-centered oxidation of Ni(II)/Ni(III) was observed for NiOPP(Benzo)2 and NiOPP(Benzo)Br2 after an initial conversion of the neutral porphyrin to its dicationic form under electrochemical conditions.

Fluorenylporphyrins functionalized by electrochromic ruthenium units as redox-triggered fluorescence switches.

Two dyads containing tris- and tetrakis-meso-fluorenyl-substituted porphyrin and ethynylruthenium units, 1 and 2, were investigated by emission spectro-electrochemical (SEC) methods for their potential use as fluorescence switches. The ruthenium group as a potential electron donor and the porphyrin as a potential electron acceptor are connected by a phenylene bridge in 1 and by a fluorenylene bridge in 2. The new fluorenyl-linked dyad 2 was probed by UV-visible, near-infrared (NIR) and infrared (IR) absorption SEC methods, and the properties interpreted with the aid of hybrid-DFT computations, for comparison with reported data for 1. The porphyrin-based fluorescence of 1 decreased in intensity upon oxidation to 1+ and decreased further on oxidising 1+ to 12+. A much weaker change in the fluorescence intensity of 2 was observed upon oxidation to 2+ but the intensity decreased upon subsequent oxidation of 2+ to 22+. These findings contrast with data reported for some other porphyrins appended with redox-active ruthenium or iron units, where fluorescence intensities increase upon oxidation of the peripheral metal centers, but they match data reported more recently for closely related arrays. A rationale for these apparently contrasting observations is proposed.

Trapping of a Pseudotetrahedral CoIIO4 Core in Mixed-Valence Mixed-Geometry [Co5] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism.

A systematic one-step one-pot multicomponent reaction of Co(ClO4)2·6H2O, H3L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO2Na; R = CH3, C2H5) resulted in the two structurally novel coordination aggregates [CoIICoIII4L2(μ1,3-O2CCH3)2(μ-OH)2](ClO4)4·4H2O (1) and [CoIICoIII4L2(μ1,3-O2CC2H5)2(μ-OH)(μ-OMe)](ClO4)4·5H2O (2). At room temperature, reactions of H3L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {CoII2} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(μ1,3-O2CCH3)(μ-OH/OMe)Co2}+ fragments were utilized to trap a central CoII ion during the formation of [Co5] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo- T d CoII ion present. The structural distortion leads to an easy-axis magnetic anisotropy ( D = -31.31 cm-1 for 1 and -21.88 cm-1 for 2) and a small but non-negligible transverse component ( E/ D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O-Co-O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co2(μ-L)} hosts controls the distortion of the central {CoO4} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.

Nickel-Induced Skeletal Rearrangement of Free Base trans-Chlorins into Monofused NiII-Porphyrins: Synthesis, Structural, Spectral, and Electrochemical Redox Properties.

Several ruffled β-to- ortho-phenyl monofused metalloporphyrins (MIIMFPs) with 1,3-indanedione functionality have been synthesized by oxidative fusion of free base trans-chlorins via nickel insertion followed by the skeletal rearrangement of macrocycle. The synthesized monofused porphyrins exhibited red-shifted electronic spectral features as compared to precursor, trans-chlorins or structurally related unfused porphyrins (mono/tri-β-substituted porphyrins) due to extended π-conjugation and nonplanar conformation of the macrocyclic core. Four of the synthesized porphyrins were structurally characterized by X-ray diffraction analysis. Ring fusion resulted in twisted macrocyclic conformation, and the twist angles (the angle between 24-atom core and fused part) were found to be in the range of 20.97° to 27.97°. NiMFP(IND)Ph2 (3b) exhibited modestly ruffled conformation of the macrocyclic core which was further confirmed by higher Δ24 (0.362 Å) and ΔCβ (0.279 Å) values. Free base monofused porphyrins have shown upfield shifted (Δδ = 0.27-0.29 ppm) inner core NHs as compared to precursors ( trans-chlorins), whereas much downfield shifted (Δδ = 0.9 ppm) as compared to H2TPP. Electron-rich Ni(II) complexes, that is, NiMFP(IND)R2 (where R = H and Ph) (1b and 3b), exhibited metal centered oxidation (NiII/NiIII) due to extended π-conjugation of macrocyclic core and electron-donating β-substituents. Facile synthesis with good yields (60-90%), unexpected nickel-induced oxidative fusion, and selective conversion of trans-chlorins into monofused NiII-porphyrins are the significant features of the present work.

Chalcogen Impact on Covalency within Molecular [Cu3(μ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study.

Reaction of the tricopper(I)-dinitrogen tris(β-diketiminate) cyclophane, Cu3(N2)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu3(μ3-O)L (2) or the corresponding Cu3(μ3-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L2,3-edge X-ray absorption data quantify 49.5% Cu 3d character in the lowest unoccupied molecular orbital of 2, with Cu 3d participation decreasing to 33.0% in 4 and 40.8% in the related sulfide cluster Cu3(μ3-S)L (3). Multiedge XAS and UV/visible/near-IR spectra are employed to benchmark density functional theory calculations, which describe the copper-chalcogen interactions as highly covalent across the series of [Cu3(μ-E)]3+ clusters. This result highlights that the metal-ligand covalency is not reserved for more formally oxidized metal centers (i.e., CuIII + O2- vs CuII + O-) but rather is a significant contributor even at more typical ligand-field cases (i.e., Cu3II/II/I + E2-). This bonding is reminiscent of that observed in p-block elements rather than in early-transition-metal complexes.

Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study.

The three- and four-membered redox series [Cr(MePDP)2]z (z = 1-, 2-, 3-) and [Mo(MePDP)2]z (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand MePDP2- (H2MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by X-ray crystallography, UV/vis/NIR spectroscopy, and magnetic susceptibility measurements. Experimental and density functional theory (DFT) studies are consistent with closed-shell MePDP2- ligands and +III oxidation states (d3, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(MePDP)2]1- (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo(MePDP)2 which was characterized by X-ray crystallography, UV/vis/NIR, and magnetic susceptibility measurements. The experimental and computational data indicate metal-centered oxidation to a MoIV complex (d2, S = 1) with two MePDP2- ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of MePDP•3- radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established experimentally by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/vis/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calculations. X-ray crystallographic analyses of the one-electron-reduced species [M(MePDP)2]2- (S = 1, M = Cr, Mo) established structural indicators for pincer reduction and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/vis/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as containing +III metal ions (d3) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, respectively.