A Structurally Characterized CuIII Complex Supported by a Bis(anilido) Ligand and Its Oxidative Catalytic Activity

X-ray absorption and emission spectroscopy of N2S2 Cu(II)/(III) complexes

This study investigates the influence of ligand charge on transition energies in a series of CuN2S2 complexes based on dithiocarbazate Schiff base ligands using Cu K-edge X-ray absorption spectroscopy (XAS) and Kβ valence-to-core (VtC) X-ray emission spectroscopy (XES). By comparing the formally Cu(II) complexes [CuII(HL1)] (HL12- = dimethyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and [CuII(HL2)] (HL22- = dibenzyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and the formally Cu(III) complex [CuIII(L2)], distinct changes in transition energies are observed, primarily attributed to the metal oxidation state. Density functional theory (DFT) calculations demonstrate how an increased negative charge on the deprotonated L23- ligand stabilizes the Cu(III) center through enhanced charge donation, modulating the core transition energies. Overall, significant shifts to higher energies are noted upon metal oxidation, emphasizing the importance of scrutinizing ligand structure in XAS/VtC XES analysis. The data further support the redox-innocent role of the Schiff base ligands and underscore the criticality of ligand protonation levels in future spectroscopic studies, particularly for catalytic intermediates. The combined XAS-VtC XES methodology validates the Cu(III) oxidation state assignment while offering insights into ligand protonation effects on core-level spectroscopic transitions.

High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand

High-valent metal species are often invoked as intermediates during enzymatic and synthetic catalytic cycles. Anionic donors are often required to stabilize such high-valent states by forming strong bonds with the Lewis acidic metal centers while decreasing their oxidation potentials. In this report, we discuss the synthesis of two high-valent metal complexes [ML]+ in which the NiIII and CuIII centers are ligated by a new tetradentate, tetraanionic bis(amidateanilido) ligand. [ML]+, obtained via chemical oxidation of ML, exhibits UV-vis-NIR, EPR, and XANES spectra characteristic of square planar, high-valent MIII species, suggesting the locus of oxidation for both [ML]+ is predominantly metal-based. This is supported by theoretical analyses, which also support the observed visible transitions as ligand-to-metal charge transfer transitions characteristic of square planar, high-valent MIII species. Notably, [ML]+ can also be obtained via O2 oxidation of ML due to its remarkably negative oxidation potentials (CuL/[CuL]+: -1.16 V, NiL/[NiL]+: -1.01 V vs Fc/Fc+ in MeCN). This demonstrates the exceptionally strong donating nature of the tetraanionic bis(amidateanilido) ligation and its ability to stabilize high-valent metal centers..

Distorted copper(ii) radicals with sterically hindered salens: electronic structure and aerobic oxidation of alcohols

The sterically hindered salen ligands featuring biphenyl and tetramethyl putrescine linkers were synthesized and chelated to copper. The resulting complexes CuL^bp,tBu, CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe were structurally characterized, showing a significanty tetrahedrally distorted metal center. The complexes show two reversible oxidation waves in the range 0.2 to 0.8 V vs. Fc⁺/Fc. A further reduction wave is detected in the range -1.4 to -1.7 V vs. Fc⁺/Fc. It is reversible for CuL^bp,tBu and CuL^bp,OMe and assigned to the Cu^II/Cu^I redox couple. One-electron oxidation of CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe was performed chemically and electrochemically. It is accompanied by a quenching of the EPR resonances. Phenoxyl radical formation was established by X-Ray diffraction on the cations [CuL^bp,OMe]⁺ and [CuL^pu,OMe]⁺, whereby the coordination sphere is elongated upon oxidation with quinoidal distributions of bond distances. The cations exhibit a NIR band of moderate intensity in their optical spectrum, supporting their classification as class II mixed-valent radical species according to the Robin Day classification. The proposed electronic structures are supported by DFT calculations. The cations [CuL^bp,OMe]⁺, [CuL^pu,tBu]⁺ and [CuL^pu,OMe]⁺ were active towards aerobic oxidation of the unactivated alcohol 2-phenylethanol, with TON numbers up to 58 within 3 h.

Shedding light on the use of Cu(ii)-salen complexes in the A3 coupling reaction

One Cu(ii) complex, {Cu(ii)L} (1S), has been synthesised, in two high yielding steps under ambient conditions, and characterised by single-crystal X-Ray diffraction (SXRD), IR, UV-Vis, circular dichroism (CD), elemental analysis, thermogravimetric analysis (TGA) and electron spray ionization mass spectroscopy (ESI-MS). This air-stable compound enables the generation, at room temperature and in open-air, of twenty propargylamines, nine new, from secondary amines, aliphatic aldehydes and alkynes with a broad scope with yields up to 99%. Catalyst loadings can be as low as 1 mol%, while the recovered material retains its structural integrity and can be used up to 5 times without loss of its activity. Control experiments, SXRD, cyclic voltammetry and theoretical studies shed light on the mechanism revealing that the key to success is the use of phenoxido salen based ligands. These ligands orchestrate topological control permitting alkyne binding with concomitant activation of the C-H bond and simultaneously acting as a template temporarily accommodating the abstracted acetylenic proton, and continuously generating, via in situ formed radicals and a Single Electron Transfer (SET) mechanism, a transient Cu(i) active site to facilitate this transformation. The scope and limitations of this protocol are discussed and presented.

Stable M(II)-Radicals and Nickel(III) Complexes of a Bis(phenol) N-Heterocyclic Carbene Chelated to Group 10 Metal Ions

The tetradentate ligand based on (1-imidazolium-3,5-di tert-butylphenol) units was prepared and chelated to group 10 metal ions (Ni(II), Pd(II), and Pt(II)), affording complexes 1, 2, and 3, respectively. The X-ray crystal structures of 1-3 show a square planar metal ion coordinated to two N-heterocyclic carbenes and two phenolate moieties. The cyclic voltammetry curves of complexes 1-3 show two reversible oxidation waves in the range 0.11-0.21 V ( E_1/2¹) and 0.55-0.65 V ( E_1/2²) vs Fc⁺/Fc, which are assigned to the successive oxidations of the phenolate moieties. One-electron oxidation affords mononuclear ( S = 1/2) systems. Complex 1⁺·SbF₆^- was remarkably stable, and its structure was characterized. The coordination sphere is slightly dissymmetric, while the typical patterns of phenoxyl radicals were observed within the ligand framework. Complex 1⁺ exhibits a rhombic signal at g = 2.087, 2.016, and 1.992, confirming its predominant phenoxyl radical character. The g-values are slightly smaller for 2⁺ (2.021, 2.008, and 1.983) and larger for 3⁺ (2.140, 1.999, and 1.885) yet consistent with phenoxyl radical species. The electronic spectra of 1⁺-3⁺ display an intervalence charge-transfer (IVCT) transition at 2396, 2600, and 2294 nm, respectively. Its intensity supports the description of cations 1⁺ and 3⁺ as mixed-valent (Class II/III) compounds according to the Robin Day classification. Complex 2⁺ behaves as a mixed-valent class II radical compound. In the presence of pyridine, radical species 1⁺ is successively converted into stable mono and bis(adducts), which are both Ni(III) complexes. Dications 1⁺²-3⁺² were prepared electrochemically. They are electron paramagnetic resonance (EPR)-silent and do not show IVCT transition in their NIR spectra, consistent with a bis(radical) formulation. The proposed electronic structures are fully supported by density functional theory calculations.

Tuning of Thiyl/Thiolate Complex Near-Infrared Chromophores of Platinum through Geometrical Constraints

The chemistry of radical-ligand complexes of the transition metals has developed into a vibrant field of research that spans from fundamental studies on the relationship between the chemical and electronic structures to applications in catalysis and functional materials chemistry. In general, fine-tuning of the relevant properties relies on an increasingly diversifying pool of radical-proligand structures. Surprisingly, the variability of the conformational freedom and the number of distinct bonding modes supported by many radical proligands is limited. This work reports on the angular constraints and relative geometric alignment of metal and ligand orbitals as key parameters that render a series of chemically similar thiyl/thiolate complexes of platinum(II) electronically and spectroscopically distinct. The use of conformational flexible thiophenols as primary ligand scaffolds is essential to establishing a defined radical-ligand [(^areneS)₂Pt^II]^•+ core whose electronic structure is modulated by a series of auxiliary coligands at platinum.

Synthesis, Characterization, Crystal Structure, and DFT Study of a New Square Planar Cu(II) Complex Containing Bulky Adamantane Ligand

A copper complex with square planar geometry, [(L)CuBr₂] (1), (L = N′-(furan-2-ylmethylene)adamantne-1-carbohydrazide) has been synthesized and characterized by Fourier transfer infrared (FTIR) spectroscopy, elemental analysis, mass spectrometry, and single crystal X-ray diffraction. The crystal of 1 is solved as monoclinic, space group P2₁/m with unit cell parameters: a = 10.8030(8), b = 6.6115(8), c = 12.1264(12) Å, β = 101.124(8)°, V = 849.84(15) ų, Z = 2, and R₁ = 0.0751 with wR₂ = 0.1581 (I > 2 σ). The structure of 1 shows intramolecular hydrogen bonding between the N–H and the furan oxygen which stabilizes the configuration of the complex. Furthermore, inside the lattice there are other weak interactions between bromo ligands and the ligand L. DFT calculations where performed to study the stability of this geometry.

Electronic Structures of Mono-Oxidized Copper and Nickel Phosphasalen Complexes

Non-innocent ligands render the determination of the electronic structure in metal complexes difficult. As such, a combination of experimental techniques and quantum chemistry are required to correctly elucidate them. This paper deals with the one-electron oxidation of copper(II) and nickel(II) complexes featuring a phosphasalen ligand (Psalen), which differs from salen analogues by the presence of iminophosphorane groups (P=N) instead of imines. Various experimental techniques (X-ray diffraction, cyclic voltammetry, NMR, EPR, and UV/Vis spectroscopies, and magnetic measurements) as well as quantum chemical calculations were used to define the electronic structure of the oxidized complexes. These can be modified by a small change in the ligand structure, that is, the replacement of a tert-butyl group by a methoxy on the phenoxide ring. The different techniques have allowed quantifying the amount of spin density located on the metal center and on the Psalen ligands. All complexes were found to possess a multi-configurational ground state, in which the ratio of the +II versus +III oxidation state of the metal center, and therefore the phenolate versus phenoxyl radical ligand character, varies upon the substituents. The tert-butyl group favors a strong localization on the metal center whereas with the methoxy group the metallic configurations decrease and the ligand configurations increase. The importance of the geometrical considerations compared with the electronic substituent effect is highlighted by the differences observed between the solid-state (EPR, magnetic measurements) and solution characterizations (EPR and NMR data).