The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well-behaved redox-chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to "metallaverdazyl" radicals. The stability of these ligand-based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L2Zn0/-1/-2) were fully characterized by single-crystal X-ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well-known β-diketiminates, but the nitrogen-rich (NNCNN) backbone of formazanates opens the door to redox-chemistry that is otherwise not easily accessible. N is better than C: Bis(formazanate) zinc complexes (see picture; Zn yellow, N blue, O red, Na green) show sequential and reversible redox chemistry in which the formazanate ligands are reduced to metallaverdazyl radicals. These ligands are very similar to β- diketiminates, but the nitrogen-rich NNCNN backbone of formazanates opens the door to redox chemistry that is otherwise difficult to access

Triggering redox activity in a thiophene compound: radical stabilization and coordination chemistry

The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L* on reaction with FeBr2. In contrast, reaction of L− with CuI forms the unique, neutral Cu2I2(L*) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metal-free radical dication L*2+

Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry

The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L* on reaction with FeBr2. In contrast, reaction of L− with CuI forms the unique, neutral Cu2I2(L*) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metal-free radical dication L*2+

Inner-sphere vs. outer-sphere reduction of uranyl supported by a redox-active, donor-expanded dipyrrin

The uranyl(VI) complex UO2Cl(L) of the redox-active, acyclic diimino-dipyrrin anion, L− is reported and its reaction with inner- and outer-sphere reductants studied. Voltammetric, EPR-spectroscopic and X-ray crystallographic studies show that chemical reduction by the outer-sphere reagent CoCp2 initially reduces the ligand to a dipyrrin radical, and imply that a second equivalent of CoCp2 reduces the U(VI) centre to form U(V). Cyclic voltammetry indicates that further outer-sphere reduction to form the putative U(IV) trianion only occurs at strongly cathodic potentials. The initial reduction of the dipyrrin ligand is supported by emission spectra, X-ray crystallography, and DFT; the latter also shows that these outer-sphere reactions are exergonic and proceed through sequential, one-electron steps. Reduction by the inner-sphere reductant [TiCp2Cl]2 is also likely to result in ligand reduction in the first instance but, in contrast to the outer-sphere case, reduction of the uranium centre becomes much more favoured, allowing the formation of a crystallographically characterised, doubly-titanated U(IV) complex. In the case of inner-sphere reduction only, ligand-to-metal electron-transfer is thermodynamically driven by coordination of Lewis-acidic Ti(IV) to the uranyl oxo, and is energetically preferable over the disproportionation of U(V). Overall, the involvement of the redox-active dipyrrin ligand in the reduction chemistry of UO2Cl(L) is inherent to both inner- and outer-sphere reduction mechanisms, providing a new route to accessing a variety of U(VI), U(V), and U(IV) complexes

Synthesis of Polysubstituted 3-Methylisoquinolines through the 6π-Electron Cyclization/Elimination of 1-Azatrienes derived from 1,1-Dimethylhydrazine

A convenient one pot microwave-assisted 6π-electron cyclization/aromatization approach toward 3-methylisoquinolines is reported. The starting 1-azatriene derivatives were prepared in situ by reaction of 2-propenylbenzaldehydes with 1,1-dimethylhydrazine, which exhibited superior performance when compared with other hydrazine derivatives. Minor amounts of the related 3,4-dihydro isoquinolines were formed concomitantly with the isoquinolines, and a mechanism for their generation was proposed. The reaction conditions were optimized, and its scope and limitations were explored. In general, the transformation proceeded in moderate to good yields.Fil: Vargas Vargas, Didier Farley. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Química Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Instituto de Química Rosario; ArgentinaFil: Larghi, Enrique Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Química Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Instituto de Química Rosario; ArgentinaFil: Kaufman, Teodoro Saul. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Química Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Instituto de Química Rosario; Argentin

Localized mixed-valence and redox activity within a triazole-bridged dinucleating ligand upon coordination to palladium

The new dinucleating redox-active ligand (LH4), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of PdII resulted in the formation of paramagnetic (S=inline image ) dinuclear Pd complexes with a κ2-N,N′-bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin–Day Class II mixed-valence within the redox-active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one-electron oxidation allowed for isolation of the corresponding cationic ligand-based diradical species. SQUID (super-conducting quantum interference device) measurements of these compounds revealed weak anti-ferromagnetic spin coupling between the two ligand-centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox-active ligand framework thus allows for efficient electronic communication between the two binding pockets

Reversible ligand-centered reduction in low- coordinate iron formazanate complexes

Coordination of redox‐active ligands to metals is a compelling strategy for making reduced complexes more accessible. In this work, we explore the use of redox‐active formazanate ligands in low‐coordinate iron chemistry. Reduction of an iron(II) precursor occurs at milder potentials than analogous non‐redox‐active β‐diketiminate complexes, and the reduced three‐coordinate formazanate‐iron compound is characterized in detail. Structural, spectroscopic, and computational analysis show that the formazanate ligand undergoes reversible ligand‐centered reduction to form a formazanate radical dianion in the reduced species. The less negative reduction potential of the reduced low‐coordinate iron formazanate complex leads to distinctive reactivity with formation of a new N−I bond that is not seen with the β‐diketiminate analogue. Thus, the storage of an electron on the supporting ligand changes the redox potential and enhances certain reactivity

Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand

A series of rhodium(III) complexes of the redox-active ligand, H(L = bis(4-methyl-2-(1H-pyrazol-1-yl)phenyl)amido), was prepared, and the electronic properties were studied. Thus, heating an ethanol solution of commercial RhCl3·3H2O with H(L) results in the precipitation of insoluble [H(L)]RhCl3, 1. The reaction of a methanol suspension of [H(L)]RhCl3 with NEt4OH causes ligand deprotonation and affords nearly quantitative yields of the soluble, deep-green, title compound (NEt4)[(L)RhCl3]·H2O, 2·H2O. Complex 2·H2O reacts readily with excess pyridine, triethylphosphine, or pyrazine (pyz) to eliminate NEt4Cl and give charge-neutral complexes trans-(L)RhCl2(py), trans-3, trans-(L)RhCl2(PEt3), trans- 4, or trans-(L)RhCl2(pyz), trans-5, where the incoming Lewis base is trans- to the amido nitrogen of the meridionally coordinating ligand. Heating solutions of complexes trans-3 or trans-4 above about 100 °C causes isomerization to the appropriate cis-3 or cis-4. Isomerization of trans-5 occurs at a much lower temperature due to pyrazine dissociation. Cis-3 and cis- 5 could be reconverted to their respective trans- isomers in solution at 35 °C by visible light irradiation. Complexes [(L)Rh(py)2Cl](PF6), 6, [(L)Rh(PPh3)(py)Cl](PF6), 7, [(L)Rh(PEt3)2Cl](PF6), 8, and [(L)RhCl(bipy)](OTf = triflate), 9, were prepared from 2·H2O by using thallium(I) salts as halide abstraction agents and excess Lewis base. It was not possible to prepare dicationic complexes with three unidentate pyridyl or triethylphosphine ligands; however, the reaction between 2, thallium(I) triflate, and the tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (ttpy) afforded a high yield of [(L)Rh(ttpy)]- (OTf)2, 10. The solid state structures of nine new complexes were obtained. The electrochemistry of the various derivatives in CH2Cl2 showed a ligand-based oxidation wave whose potential depended mainly on the charge of the complex, and to a lesser extent on the nature and the geometry of the other supporting ligands. Thus, the oxidation wave for 2 with an anionic complex was found at +0.27 V versus Ag/AgCl in CH2Cl2, while those waves for the charge-neutral complexes 3−5 were found between +0.38 to +0.59 V, where the cis- isomers were about 100 mV more stable toward oxidation than the trans- isomers. The oxidation waves for 6−9 with monocationic complexes occurred in the range +0.74 to 0.81 V while that for 10 with a dicationic complex occurred at +0.91 V. Chemical oxidation of trans-3, cis-3, and 8 afforded crystals of the singly oxidized complexes, [trans- (L)RhCl2(py)](SbCl6), cis-[(L)RhCl2(py)](SbCl4)·2CH2Cl2, and [(L)Rh(PEt3)2Cl](SbCl6)2, respectively. Comparisons of structural and spectroscopic features combined with the results of density functional theory (DFT) calculations between nonoxidized and oxidized forms of the complexes are indicative of the ligand-centered radicals in the oxidized derivatives

C–H Functionalization Reactivity of a Nickel–Imide

This article discusses C-H functionalization reactivity of a Nickel-Imide