Redox Non-innocent Ligands Research Articles

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes.

The redox non-innocent behavior of the diaryl-azo-oxime ligand LNOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(LNO-)Cl(CO)(PPh3)2]+2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(LNO-)Cl(CO)(PPh3)2]+3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(LNO-)Cl2(PPh3)2] 4 and [Ir(LNO-)2Cl(PPh3)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(LNO-)Cl(CO2H)(PPh3)2] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO2H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Metal-ligand cooperative approaches in homogeneous catalysis using transition metal complex catalysts of redox noninnocent ligands.

Catalysis offers a straightforward route to prepare various value-added molecules starting from readily available raw materials. The catalytic reactions mostly involve multi-electron transformations. Hence, compared to the inexpensive and readily available 3d-metals, the 4d and 5d-transition metals get an extra advantage for performing multi-electron catalytic reactions as the heavier transition metals prefer two-electron redox events. However, for sustainable development, these expensive and scarce heavy metal-based catalysts need to be replaced by inexpensive, environmentally benign, and economically affordable 3d-metal catalysts. In this regard, a metal-ligand cooperative approach involving transition metal complexes of redox noninnocent ligands offers an attractive alternative. The synergistic participation of redox-active ligands during electron transfer events allows multi-electron transformations using 3d-metal catalysts and allows interesting chemical transformations using 4d and 5d-metals as well. Herein we summarize an up-to-date literature report on the metal-ligand cooperative approaches using transition metal complexes of redox noninnocent ligands as catalysts for a few selected types of catalytic reactions.

MINIREVIEW: Crystalline organic metal chalcogenides for thermoelectric conversion

Organic metal chalcogenides (OMCs) refer to coordination polymers (CPs) with continuous one, two or three dimensional metal-chalcogen (M-X) networks formed by metal nodes and chalcogen atoms from the organic ligands. Among the conductive CPs ever reported, OMCs display superior electrical transport behaviors with conductivities ranging from 10-6 to over 2000 Scm-1, especially those derived from benzenehexathiol (BHT) containing 2D or 3D M − S networks. The highly tunable chemical structure as well as electronic structures of OMCs have inspired interests on their thermoelectric performance, and some newly emerged cases with the ZT exceeding 0.1 show the great potential of OMCs as new candidate for the development of TE materials, especially those materials constructed with redox non-innocent ligands. Here, the recent progresses in this direction are summarized.

The Impact of Ligand Carboxylates on Electrocatalyzed Water Oxidation.

ConspectusFossil fuel shortage and severe climate changes due to global warming have prompted extensive research on carbon-neutral and renewable energy resources. Hydrogen gas (H2), a clean and high energy density fuel, has emerged as a potential solution for both fulfilling energy demands and diminishing the emission of greenhouse gases. Currently, water oxidation (WO) constitutes the bottleneck in the overall process of producing H2 from water. As a result, the design of efficient catalysts for WO has become an intensively pursued area of research in recent years. Among all the molecular catalysts reported to date, ruthenium-based catalysts have attracted particular attention due to their robust nature and higher activity compared to catalysts based on other transition metals.Over the past two decades, we and others have studied a wide range of ruthenium complexes displaying impressive catalytic performance for WO in terms of turnover number (TON) and turnover frequency (TOF). However, to produce practically applicable electrochemical, photochemical, or photo-electrochemical WO reactors, further improvement of the catalysts’ structure to decrease the overpotential and increase the WO rate is of utmost importance. WO reaction, that is, the production of molecular oxygen and protons from water, requires the formation of an O–O bond through the orchestration of multiple proton and electron transfers. Promotion of these processes using redox noninnocent ligand frameworks that can accept and transfer electrons has therefore attracted substantial attention. The strategic modifications of the ligand structure in ruthenium complexes to enable proton-coupled electron transfer (PCET) and atom proton transfer (APT; in the context of WO, it is the oxygen atom (metal oxo) transfer to the oxygen atom of a water molecule in concert with proton transfer to another water molecule) to facilitate the O–O bond formation have played a central role in these efforts.In particular, promising results have been obtained with ligand frameworks containing carboxylic acid groups that either are directly bonded to the metal center or reside in the close vicinity. The improvement of redox and chemical properties of the catalysts by introduction of carboxylate groups in the ligands has proven to be quite general as demonstrated for a range of mono- and dinuclear ruthenium complexes featuring ligand scaffolds based on pyridine, imidazole, and pyridazine cores. In the first coordination sphere, the carboxylate groups are firmly coordinated to the metal center as negatively charged ligands, improving the stability of the complexes and preventing metal leaching during catalysis. Another important phenomenon is the reduction of the potentials required for the formation of higher valent intermediates, especially metal-oxo species, which take active part in the key O–O bond formation step. Furthermore, the free carboxylic acid/carboxylate units in the proximity to the active center have shown exciting proton donor/acceptor properties (through PCET or APT, chemically noninnocent) that can dramatically improve the rate as well as the overpotential of the WO reaction.

Synthesis and characterization of trigonal bipyramidal FeIII complexes and their solution behavior

A series of air-stable trigonal bipyramidal FeIII complexes supported by a redox non-innocent NNN pincer ligand, CztBu(PyrR)2− (R = iPr, Me, or H), were synthesized, fully characterized, and utilized for the investigation of the interaction between acetone and the FeIII center. The magnetic moments determined from the paramagnetic 1H NMR spectra in conjunction with EPR and Mössbauer spectroscopy indicate the presence of a high-spin ferric center. Cyclic voltammetry studies feature two quasi-reversible events corresponding to a metal-centered FeIII/II reduction around −0.40 V (vs. Fc) and a ligand-centered CztBu(PyrR)2/CztBu(PyrR)2+ oxidation at potentials near +0.70 V (vs. Fc). UV–Visible spectroscopy in CH2Cl2 showcases ligand–metal charge transfer (LMCT) bands, as well as coordination of acetone to CztBu(PyrH)2FeCl2. In situ IR spectroscopy and solution conductivity (̺κ) measurements of CztBu(PyrR)2FeCl2 with varied equivalents of acetone reveal that acetone is weakly associated with the iron center.

A diamagnetic iron complex and its twisted sister - structural evidence on partial spin state change in a crystalline iron complex.

We report here the syntheses of a diamagnetic Fe complex [Fe(HL)2] (1), prepared by reacting a redox non-innocent ligand precursor N,N'-bis(3,5-di-tert-butyl-2-hydroxy-phenyl)-1,2-phenylenediamine (H4L) with FeCl3, and its phenoxazine derivative [Fe(L')2] (2), which was obtained via intra-ligand cyclisation of the parent complex. Magnetic measurements, accompanied by spectroscopic, structural and computational analyses show that 1 can be viewed as a rather unusual Fe(III) complex with a diamagnetic ground state in the studied temperature range due to a strong antiferromagnetic coupling between the low-spin Fe(III) ion and a radical ligand. For a paramagnetic high-spin Fe(II) complex 2 it was found that, when crystalline, it undergoes a thermally induced process where 25% of the molecules in the material change to a diamagnetic low-spin ground state below 100 K. Single crystal X-ray studies conducted at 95 K afforded detailed structural evidence for this partial change of spin state of 2 showing the existence of crystallographically distinct molecules in a 3 : 1 ratio which exist in high- and low-spin states, respectively. Also, the magnetic behaviour of 2 was found to be related with the crystallinity of the material as demonstrated by near-IR radiation to unpaired electrons conversion ability of amorphous sample of 2.

Metal-Ligand Cooperativity via Exchange Coupling Promotes Iron- Catalyzed Electrochemical CO2 Reduction at Low Overpotentials.

Biological and heterogeneous catalysts for the electrochemical CO2 reduction reaction (CO2RR) often exhibit a high degree of electronic delocalization that serves to minimize overpotential and maximize selectivity over the hydrogen evolution reaction (HER). Here, we report a molecular iron(II) system that captures this design concept in a homogeneous setting through the use of a redox non-innocent terpyridine-based pentapyridine ligand (tpyPY2Me). As a result of strong metal-ligand exchange coupling between the Fe(II) center and ligand, [Fe(tpyPY2Me)]2+ exhibits redox behavior at potentials 640 mV more positive than the isostructural [Zn(tpyPY2Me)]2+ analog containing the redox-inactive Zn(II) ion. This shift in redox potential is attributed to the requirement for both an open-shell metal ion and a redox non-innocent ligand. The metal-ligand cooperativity in [Fe(tpyPY2Me)]2+ drives the electrochemical reduction of CO2 to CO at low overpotentials with high selectivity for CO2RR (>90%) and turnover frequencies of 100 000 s-1 with no degradation over 20 h. The decrease in the thermodynamic barrier engendered by this coupling also enables homogeneous CO2 reduction catalysis in water without compromising selectivity or rates. Synthesis of the two-electron reduction product, [Fe(tpyPY2Me)]0, and characterization by X-ray crystallography, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), variable temperature NMR, and density functional theory (DFT) calculations, support assignment of an open-shell singlet electronic structure that maintains a formal Fe(II) oxidation state with a doubly reduced ligand system. This work provides a starting point for the design of systems that exploit metal-ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned through secondary metal-dependent interactions.

Five Nitrogen Oxidation States from Nitro to Amine: Stabilization and Reactivity of a Metastable Arylhydroxylamine Complex.

Redox noninnocent ligands enhance the reactivity of the metal they complex, a strategy used by metalloenzymes and in catalysis. Herein, we report a series of copper complexes with the same ligand framework, but with a pendant nitrogen group that spans five different redox states between nitro and amine. Of particular interest is the synthesis of a unprecedented copper(I)-arylhydroxylamine complex. While hydroxylamines typically disproportionate or decompose in the presence of transition metal ions, the reactivity of this metastable species is arrested by the presence of an intramolecular hydrogen bond. Two-electron oxidation yields a copper(II)-(arylnitrosyl radical) complex that can dissociate to a copper(I) species with uncoordinated arylnitroso. This combination of ligand redox noninnocence and hemilability provides opportunities in catalysis for two-electron chemistry via a one-electron copper(I/II) shuttle, as exemplified with an aerobic alcohol oxidation.

Octahedral Iron Complexes of Pyrazine(diimine) Pincers: Ligand Electronic Effects and Protonation.

Redox noninnocent ligands are known to be involved in altering the overall electronic nature of organometallic complexes by serving as an electron reservoir. Pyrazine(diimine) backbones in these complexes introduce enhanced π acidity over their more well-studied pyridine(diimine) analogues and open up the opportunity for functionalization of the nitrogen at the 4-position of the ring. Herein we report the characterization of bis-chelated pyrazine(diimine) [(PzDI)2Fe]n+ (n = 0, 1, and 2) complexes for electronic and structural comparison to pyridine(diimine) complexes (PDI) with similar architectures. Cyclic voltammetry studies show three reductions, two of which are ligand-based and reversible. Reduction of [(PzDI)2Fe]2+ (1) to [(PzDI)2Fe]+ (2) and (PzDI)2Fe (3) gives rise to characteristic structural changes, such as imine C═N bond lengthening, indicating the formation of a ligand radical, a conclusion which is further supported by electron paramagnetic resonance (EPR) and electronic structure calculations. Comparisons between the PzDI and PDI systems are highlighted. Complex 1 can be protonated at the uncoordinated pyrazine nitrogen, resulting in changes to its spectroscopic and redox properties; efforts to further functionalize the ligand are discussed.

Impact of Bis(imino)pyridine Ligands on Mesoscale Properties of CdSe/ZnS Quantum Dots

We investigate the effect of surface modification of CdSe/ZnS quantum dots (QDs) with bis(imino)pyridine (BIP) ligands. BIPs are a class of redox non-innocent ligands known to facilitate charge transfer in base metals on the molecular scale, but their behavior in nano- to meso-scale systems has been largely unexplored. Using electron microscopy, crystallography and ultrafast spectroscopy, we reveal that structure specific π-π stacking of the BIP molecules alters inter-dot separation in QD films, thereby leading to changes in optical and electronic properties. The three variations used are unsubstituted (BIP-H), dimethyl (BIP-Me) and diisopropyl (BIP-Ipr) bis(imino)pyridine, and when compared with the native octadecylamine (ODA) ligand, we find that both energy and charge transfer efficiencies between QDs are increased post-ligand exchange, the highest achieved through BIP-Ipr, despite its larger unit cell volume. We further investigate charge transfer from QD films to conducting (indium tin oxide, ITO) and semiconducting (zinc oxide, ZnO) substrates using time-resolved spectroscopy, and determine that the influence of the ligands is QD band gap dependent. In QDs with large band gap (2.3 eV) the BIP ligands facilitate charge transfer to both ITO and ZnO substrates, but in dots with small band gap (1.9 eV) they pose a hindrance when ZnO is used, resulting in reduced recombination rates. These results highlight the importance of investigating multiple avenues in order to optimize surface modification of QDs based on the end goal. Finally, we verify that BIP ligands hasten the rate of QD photo brightening under continuous illumination, allowing the ensemble to achieve stable emission faster than in their native configuration. Our study sets the stage for novel charge transfer systems in the meso and nanoscale, yielding a diverse selection of new surface ligands for applications such as conductive materials and energy production/storage devices employing QDs.

Electronically Asynchronous Transition States for C-N Bond Formation by Electrophilic [CoIII(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle.

The oxidation state of the redox noninnocent tetra-amido macrocyclic ligand (TAML) scaffold was recently shown to affect the formation of nitrene radical species on cobalt(III) upon reaction with PhI=NNs [van LeestN. P.; J. Am. Chem. Soc.2020, 142, 552−56331846578]. For the neutral [CoIII(TAMLsq)] complex, this leads to the doublet (S = 1/2) mono-nitrene radical species [CoIII(TAMLq)(N•Ns)(Y)] (bearing an unidentified sixth ligand Y in at least the frozen state), while a triplet (S = 1) bis-nitrene radical species [CoIII(TAMLq)(N•Ns)2]– is generated from the anionic [CoIII(TAMLred)]– complex. The one-electron-reduced Fischer-type nitrene radicals (N•Ns–) are formed through single (mono-nitrene) or double (bis-nitrene) ligand-to-substrate single-electron transfer (SET). In this work, we describe the reactivity and mechanisms of these nitrene radical complexes in catalytic aziridination. We report that [CoIII(TAMLsq)] and [CoIII(TAMLred)]– are both effective catalysts for chemoselective (C=C versus C–H bonds) and diastereoselective aziridination of styrene derivatives, cyclohexane, and 1-hexene under mild and even aerobic (for [CoIII(TAMLred)]–) conditions. Experimental (Hammett plots; [CoIII(TAML)]-nitrene radical formation and quantification under catalytic conditions; single-turnover experiments; and tests regarding catalyst decomposition, radical inhibition, and radical trapping) in combination with computational (density functional theory (DFT), N-electron valence state perturbation theory corrected complete active space self-consistent field (NEVPT2-CASSCF)) studies reveal that [CoIII(TAMLq)(N•Ns)(Y)], [CoIII(TAMLq)(N•Ns)2]–, and [CoIII(TAMLsq)(N•Ns)]– are key electrophilic intermediates in aziridination reactions. Surprisingly, the electrophilic one-electron-reduced Fischer-type nitrene radicals do not react as would be expected for nitrene radicals (i.e., via radical addition and radical rebound). Instead, nitrene transfer proceeds through unusual electronically asynchronous transition states, in which the (partial) styrene substrate to TAML ligand (single-) electron transfer precedes C–N coupling. The actual C–N bond formation processes are best described as involving a nucleophilic attack of the nitrene (radical) lone pair at the thus (partially) formed styrene radical cation. These processes are coupled to TAML-to-cobalt and cobalt-to-nitrene single-electron transfer, effectively leading to the formation of an amido-γ-benzyl radical (NsN––CH2–•CH–Ph) bound to an intermediate spin (S = 1) cobalt(III) center. Hence, the TAML moiety can be regarded to act as a transient electron acceptor, the cobalt center behaves as a spin shuttle, and the nitrene radical acts as a nucleophile. Such a mechanism was hitherto unknown for cobalt-catalyzed hypovalent group transfer and the more general transition-metal-catalyzed nitrene transfer to alkenes but is now shown to complement the known concerted and stepwise mechanisms for N-group transfer.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands.

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble NiII complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiOx . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO4 2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [NiIII (TAML2- )-O. ] is formed via several proton-coupled electron transfer processes and reacts with H2 O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.

Designing a Cr-catalyst bearing redox non-innocent phenalenyl-based ligand towards hydrosilylative CO2 functionalization.

Herein, we report the synthesis of a Cr(iii)-complex bearing a redox non-innocent phenalenyl-based ligand and its use as a catalyst for SET mediated hydrosilylative reduction of carbon dioxide towards formylation of primary amides under mild conditions. A preliminary mechanistic picture for this transformation has been proposed by isolation and characterization of several reactive intermediates.

Cobalt complexes of redox noninnocent azo-aromatic pincers. Isolation, characterization, and application as catalysts for the synthesis of quinazolin-4(3H)-ones.

Herein we report the synthesis, characterization and catalytic application of three new cobalt(ii)-complexes of redox noninnocent arylazo ligands, 2-(phenylazo)-1,10-phenanthroline (L1a), 2-(4-chlorophenylazo)-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) respectively. The reaction of L1a with CoIICl2·6H2O produced a μ-dichloro bridged binuclear cobalt(ii)-complex [Co(L1a)2Cl2] (1a) while the same reaction when carried out with 2-(4-chlorophenyl)azo-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) ligands produced two new mononuclear five-coordinate cobalt(ii)-complexes 1b and 2 respectively. In complex 1a and 1b, the ligands L1a and L1b are coordinated to the cobalt(ii)-center in a tridentate mode utilizing all of its nitrogen donor sites while in complex 2 one of the azo-donor sites of the ligand L2 remain pendant. All these complexes were characterized using available spectroscopic techniques and DFT studies. We further explored the potential of these complexes as catalysts for the synthesis of pharmaceutically important organic compounds via the functionalization of alcohols. A variety of substituted quinazolin-4(3H)-ones were synthesized under aerobic conditions via the coupling of alcohols and 2-aminobenzamide using 1b as the catalyst. Mechanistic investigations revealed that both cobalt and the arylazo scaffold act synergistically during catalysis.

Catalytic promiscuity of two novel cobalt(III) complexes derived from redox non-innocent Schiff base ligands: Unraveling the role of methyl groups in the ligand backbone on catalytic efficiency

Two Schiff base ligands namely (E)-4-bromo-2-(((1-hydroxy-2-methylpropan-2-yl)imino)methyl)-6-methoxyphenol(H2L1) and (E)-4-bromo-2-(((2-hydroxyethyl)imino)methyl)-6-methoxyphenol (H2L2) were designed and synthesized by varying methyl groups to prepare cobalt(III) complexes [Co(HL1)(L1)](1) and [Co(HL2)(L2)](2) using cobalt(II) acetate as metal precursor. Complexes 1 and 2 were characterized by usual physicochemical techniques and solid state structure of complex 2 was determined by X-Ray crystal structure analysis. The electrospray ionization-mass spectrometry (ESI-MS) analysis proved that complexes 1 and 2 were present as mononuclear entity in solution phase. The newly synthesized complexes were used to exploit their catalytic promiscuity. Catecholase like activity of both complexes 1 and 2 was carried out in N, N-dimethylformamide (DMF) medium using 3,5-di-tert-butyl catechol (3,5-DTBC) as model substrate (substrate : complex = 100:1) and the rate of the activity followed the trend, 1 > 2. Phosphatase like activity was studied by using disodium salt of 4-nitrophenylphosphate (4-NPP) as model substrate in DMF-H2O (substrate: complex = 20:1) mixture, indicating that the complexes were enormously accelerated the rate of phosphate monoester hydrolysis and the rate of the activity followed the reverse trend of catecholase like activity i.e.2 > 1. The role of methyl groups in the ligand backbone of H2L1 is supposed to be the determining factor to differentiate the catalytic efficiency of complexes 1 and 2 and the experimental observation has been supported by DFT calculations.

Cobalt-Catalyzed Hydrogenations via Olefin Cobaltate and Hydride Intermediates

Redox noninnocent ligands are a promising tool to moderate electron transfer processes within base-metal catalysts. This report introduces bis(imino)acenaphthene (BIAN) cobaltate complexes as hydrogenation catalysts. Sterically hindered trisubstituted alkenes, imines, and quinolines underwent clean hydrogenation under mild conditions (2–10 bar, 20–80 °C) by use of the stable catalyst precursor [(DippBIAN)CoBr2] and the cocatalyst LiEt3BH. Mechanistic studies support a homogeneous catalysis pathway involving alkene and hydrido cobaltates as active catalyst species. Furthermore, considerable reaction acceleration by alkali cations and Lewis acids was observed. The dinuclear hydridocobaltate anion with bridging hydride ligands was isolated and fully characterized.

Reversible electron transfer in organolanthanide chemistry.

This article relates the synthesis and characterization of novel heterobimetallic complexes containing a low-valent lanthanide, a tetradentate redox non-innocent ligand, viz. the 4,5,9,10-tetraazaphenanthrene, taphen ligand and transition metal fragments of PdMe2 and PtMe2. The experimental results are supported by a theoretical study. Investigation of their reduction properties allowed the formation of isostructural original heterotrimetallic complexes containing two Cp*2Yb fragments and the (taphen)MMe2 (M = Pd and Pt) motifs. These complexes are stable in non-coordinating solvent such as toluene but decompose in coordinating solvents such as thf. Investigation of the internal electron transfer shows that the taphen ligand behaves as a two-electrons reservoir but is capable of transferring back only one electron in thf. This reversible electron(s) transfer is rare in organolanthanide chemistry and show the potential interest of these complexes in reductive chemistry. Additionally, the trinuclear complexes feature odd X-ray crystal structures in which a deviation of symmetry is observed. The latter observation was studied in depth using quantum chemistry calculations highlighting the role of non-covalent weak interactions.

Joshua S. Figueroa

Joshua S. Figueroa

Di-manganese(0) carbonyl mediated azo bond cleavage and recombination reaction in azo-aromatic ligand: An effect of extended phenolate donor

Three manganese complexes of the anionic azo-phenolate ligand [L1]−, obtained from the reaction between HL1 and Mn(OAc)2.4H2O (OAc = acetate) or Mn2(CO)10, are reported. The chosen manganese precursors are of different characters: the hydrated Mn(OAc)2 is a common source of Mn(II), while the carbonyl complex is known for its strong reducing character. The reaction between the Mn(II)-salt and HL1 (HL1 = 2,4-di-tert-butyl-6-(pyridin-2-ylazo)-phenol) produced a low-spin Mn(III) complex, ls-[MnIII(L1)−(L1)2−]; 1 via substitution and internal electron transfer (Mn(II) → [L1]−). The chemical oxidation of this molecular complex with I2 provided access to the cationic complex, ls-[MnIII(L1)−2]I3; [1]+[I3]− in quantitative yield. On the other hand, the reaction between Mn2(CO)10 and 4 equivalent of HL1 is complex. The reaction produced a heteroleptic high-spin Mn(III)-complex, hs-[MnIII(L1)−(L2)2−]; 2 in moderate yield. The ligand H2L2 (H2L2 = 2,2′-dihydroxo-3,5,3′,5′-tetra-tert-butylazobenzene) produced in situ via simultaneous azo-cleavage of [L1]− and recombination of two amido-phenolate fragments. The electronic structures of all three complexes coordinated by the two redox noninnocent ligands [L1/2]−/2− have been elucidated by using a host of physical methods: X-ray crystallography, magnetic susceptibility measurements, cyclic voltammetry, absorption spectroscopy and density functional theory (DFT). Their experimental structures are well reproduced by DFT calculations and support the overall electronic structures of the above compounds. The complexes 1, [1]+[I3]− and 2 have one, two and four unpaired electrons respectively, as evidenced by room temperature magnetic susceptibility measurements. The redox process between [1]+/1 is ligand centred which does not affect the trivalent oxidation state of the metal ion. The chemical conversion [L1]− → [L2]2− occurs only in aerobic condition and a rationale for the reaction is discussed.

Achieving Nickel Catalyzed C-S Cross-Coupling under Mild Conditions Using Metal-Ligand Cooperativity.

A simple and efficient approach of C-S cross-coupling of a wide variety of (hetero)aryl thiols and (hetero)aryl halides under mild conditions, mostly at room temperature, catalyzed by well-defined singlet diradical Ni(II) catalysts bearing redox noninnocent ligands is reported. Taking advantage of ligand centered redox events, the high-energetic Ni(0)/Ni(II) or Ni(I)/Ni(III) redox steps were avoided in the catalytic cycle. The cooperative participation of both nickel and the coordinated ligands during oxidative addition/reductive elimination steps allowed us to perform the catalytic reactions under mild conditions.