Complexes of mesoionic carbene (MIC) ligands are gaining immense popularity in organometallic chemistry and homogeneous catalysis. We present here a series of palladium(II) complexes that are comprised of MIC donor ligands, and we demonstrate their applications in α-arylation, α-methylation, and Suzuki–Miyaura coupling reactions. All the complexes have been structurally characterized by X-ray crystallographic analysis. These palladium(II) complexes are potent precatalysts and they deliver good to excellent yields for both α-arylation and Suzuki–Miyaura coupling reactions. A palladium(II) complex bearing two MIC units in a trans fashion is used for chemoselective Suzuki–Miyaura coupling reaction. This complex delivers lower yields in α-arylation reactions compared with PEPPSI-type complexes, however, it gives an α-methylated product when the reaction is conducted in N,N-dimethylformamide. Mercury poisoning experiments suggest that the Suzuki–Miyaura coupling reaction likely proceeds via Pd nanoparticles. However, the α-arylation reaction proceeds homogeneously, as shown by the negative mercury poison test. The present results thus open up new avenues for MIC ligands in catalytic α-arylation and α-methylation reactions.

We describe a novel approach to highly functionalized ferrocenyl-substituted pyridine derivatives. Key reaction is the cyclocondensation of an easily available β-ketoenamide bearing an N-ferrocenylcarbonyl group. The resulting 4-hydroxypyridine derivatives were converted into the corresponding pyrid-4-yl nonaflates, which were employed in a range of palladium-catalyzed coupling reactions. The reaction sequence delivers a library of new 2-ferrocenyl-substituted pyridine derivatives, including compounds with two ferrocene moieties, a butadiyne derivative, a 2,4-bisferrocenylpyridine, and a compound with a thiophene core and two 2-ferrocenylpyridine groups. The compounds were investigated by UV/Vis spectroscopy and by (spectro)electrochemistry. The (spectro)electrochemical results conclusively prove that the reaction takes place through a ferrocene-centered oxidation and a reduction based on the heterocyclic part of the molecule. Intriguing effects on the electrochemical responses of these systems upon changing the electrolyte are also observed.

The reactions of the precursor complex [Fe₂(CO)₆(μ-bdt)] F with PPh₃, PPh₂Me, PPh₂H have been investigated. Treatment of F with the phosphine ligands yielded both mono- and disubstituted complexes [Fe₂(CO)₅(μ-bdt)(PPh₃)] (1), [Fe₂(CO)₄(μ-bdt)(PPh₃)₂] (2), [Fe₂(CO)₅(μ-bdt)(PPh₂Me)] (3), [Fe₂(CO)₄(μ-bdt)(PPh₂Me)₂] (4), [Fe₂(CO)₅(μ-bdt)(PPh₂H)] (5) and [Fe₂(CO)₄(μ-bdt)(PPh₂H)₂] (6). Crystal structures have been reported for complexes 1–3. Complexes 1, 3 and 5 participate in electrocatalytic proton reduction in the presence of two distinct acids of varying strengths: HClO₄ and CF₃CO₂H.

Die üblichen Oxidationsstufen des Kupfers in stabilen Komplexen sind +I und +II. Cu^III-Komplexe werden oft als Zwischenstufen in der Bio- und Homogenkatalyse in Betracht gezogen. In letzter Zeit wurden Cu^IV-Spezies als mögliche Intermediate in der Oxidationskatalyse postuliert. Trotz der Bedeutung der höheren Oxidationsstufen des Kupfers gibt es nur wenig spektroskopische Untersuchungen solcher Verbindungen, wobei für Cu^IV-Komplexe keine derartigen Informationen vorliegen. Wir berichten hier über die Synthese und Charakterisierung dreier Kupfercorrole. Die Kombination von Elektrochemie, UV-vis-NIR/EPR Spektroelektrochemie, XANES Messungen und DFT-Rechnungen weist auf drei definierte Redoxzustände dieser Moleküle hin, mit Kupfer in den Oxidationsstufen +II, +III und +IV. Die vorliegenden Ergebnisse sind die erste spektroskopische und theoretische Untersuchung einer Cu^IV-Verbindung und beschreiben eine Redoxserie, bei der Cu^II, Cu^III und Cu^IV in derselben molekularen Umgebung diskutiert werden.

Tuning of ligand properties is at the heart of influencing chemical reactivity and generating tailor-made catalysts. Herein, three series of complexes [Ru(L)(Cl)(X)]PF₆ (X=DMSO, PPh₃, or CD₃CN) with tripodal ligands (L1–L5) containing pyridine and triazole arms are presented. Triazole-for-pyridine substitution and the substituent at the triazole systematically influence the redox behavior and photoreactivity of the complexes. The mechanism of the light-driven ligand exchange of the DMSO complexes in CD₃CN could be elucidated, and two seven-coordinate intermediates were identified. Finally, tuning of the ligand framework was applied to the catalytic oxygenation of alkanes, for which the DMSO complexes were the best catalysts and the yield improved with increasing number of triazole arms. These results thus show how click-derived ligands can be tuned on demand for catalytic processes.

The most common oxidation states of copper in stable complexes are +I and +II. Cu^III complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu^IV species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu^IV complexes being non-existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu^IV species, and describe a redox series where Cu^II, Cu^III, and Cu^IV are discussed within the same molecular platform.

Hemilabile ligands are known to impart remarkable properties to their metal complexes. Herein, we present arene half-sandwich complexes of Ru^II, Os^II, and Ir^III with “click”-derived 1,2,3-triazole (L¹) and 1,2,3-triazol-5-ylidene (L²) ligands containing a potentially hemilabile thioether donor. Structural elucidation of the complexes revealed localization of double bonds within the triazole in L¹ and a delocalized situation within the triazolylidene ring of L². For complexes with L¹, unusual coordination occurs through the less basic nitrogen “N2” of the 1,2,3-triazole. All complexes were applied for the catalytic oxidation of benzyl alcohol to benzaldehyde using N-methylmorpholine N-oxide as sacrificial oxidant. Furthermore, oxidation of diphenylmethanol to benzophenone was also achieved by using low catalyst loadings in very good yields. These are rare examples of Os^II–triazole, as well as of Os^II–triazolylidene complexes with “click”-derived ligands.

Mononuclear and halide-bridged dinuclear palladium(II) complexes with symmetrically (1 and 3) and nonsymmetrically (2, 4–6) substituted benzimidazolin-2-ylidene N-heterocyclic carbene ligands were synthesized and characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and X-ray crystallography. The mononuclear complexes exist as a mixture of cis and trans isomers, and the identity of these was ascertained by NMR spectroscopy and structural characterization. The dinuclear complexes 4–6 with nonsymmetrically substituted benzimidazolin-2-ylidene ligands form rotamers (cis/anti, cis/syn, trans/anti, and trans/syn) at room temperature, which can be transformed to a single rotamer at higher temperatures as evidenced by ¹H NMR spectroscopy. Crystal structure analyses of representative examples show that the Pd^II centers are bound in a distorted square-planar environment by halides and carbene C ligands, and the carbene ligands are perpendicular to the Pd–halide plane. All of the complexes were tested for their efficiency as (pre)catalysts in the Suzuki–Miyaura cross-coupling reaction, as well as in hydrodehalogenation reactions, and they show good activity. The Suzuki–Miyaura coupling reactions can be performed under air and in water as an environmentally benign solvent. The most active catalyst for the Suzuki–Miyaura coupling was also used in a sequence together with a “click reaction” to synthesize valuable sterically demanding tripodal triazole ligands.

A rare example of a mononuclear complex [(bpy)₂Ru(L¹_−H)](ClO₄), 1(ClO₄) and dinuclear complexes [(bpy)₂Ru(μ-L¹_−2H)Ru(bpy)₂](ClO₄)₂, 2(ClO₄)₂, [(bpy)₂Ru(μ-L²_−2H)Ru(bpy)₂](ClO₄)₂, 3(ClO₄)₂, and [(bpy)₂Ru(μ-L³_−2H)Ru(bpy)₂](ClO₄)₂, 4(ClO₄)₂ (bpy=2,2′-bipyridine, L¹=2,5-di-(isopropyl-amino)-1,4-benzoquinone, L²=2,5-di-(benzyl-amino)-1,4-benzoquinone, and L³=2,5-di-[2,4,6-(trimethyl)-anilino]-1,4-benzoquinone) with the symmetrically substituted p-quinone ligands, L, are reported. Bond-length analysis within the potentially bridging ligands in both the mono- and dinuclear complexes shows a localization of bonds, and binding to the metal centers through a phenolate-type “O⁻” and an immine/imminium-type neutral “N” donor. For the mononuclear complex 1(ClO₄), this facilitates strong intermolecular hydrogen bonding and leads to the imminium-type character of the noncoordinated nitrogen atom. The dinuclear complexes display two oxidation and several reduction steps in acetonitrile solutions. In contrast, the mononuclear complex 1⁺ exhibits just one oxidation and several reduction steps. The redox processes of 1¹⁺ are strongly dependent on the solvent. The one-electron oxidized forms 2³⁺, 3³⁺, and 4³⁺ of the dinuclear complexes exhibit strong absorptions in the NIR region. Weak NIR absorption bands are observed for the one-electron reduced forms of all complexes. A combination of structural data, electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, and DFT calculations is used to elucidate the electronic structures of the complexes. Our DFT results indicate that the electronic natures of the various redox states of the complexes in vacuum differ greatly from those in a solvent continuum. We show here the tuning possibilities that arise upon substituting [O] for the isoelectronic [NR] groups in such quinone ligands.

The complexes [{(tmpa)Co^II}₂(μ-L¹)²⁻]²⁺ (1²⁺) and [{(tmpa)Co^II}₂(μ-L²)²⁻]²⁺ (2²⁺), with tmpa=tris(2-pyridylmethyl)amine, H₂L¹=2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, and H₂L²=2,5-di-[2-(trifluoromethyl)-anilino]-1,4-benzoquinone, were synthesized and characterized. Structural analysis of 2²⁺ revealed a distorted octahedral coordination around the cobalt centers, and cobalt–ligand bond lengths that match with high-spin Co^II centers. Superconducting quantum interference device (SQUID) magnetometric studies on 1²⁺ and 2²⁺ are consistent with the presence of two weakly exchange-coupled high-spin cobalt(II) ions, for which the nature of the coupling appears to depend on the substituents on the bridging ligand, being antiferromagnetic for 1²⁺ and ferromagnetic for 2²⁺. Both complexes exhibit several one-electron redox steps, and these were investigated with cyclic voltammetry and UV/Vis/near-IR spectroelectrochemistry. For 1²⁺, it was possible to chemically isolate the pure forms of both the one-electron oxidized mixed-valent 1³⁺ and the two-electron oxidized isovalent 1⁴⁺ forms, and characterize them structurally as well as magnetically. This series thus provided an opportunity to investigate the effect of reversible electron transfers on the total spin-state of the molecule. In contrast to 2²⁺, for 1⁴⁺ the metal–ligand distances and the distances within the quinonoid ligand point to the existence of two low-spin Co^III centers, thus showing the innocence of the quintessential non-innocent ligands L. Magnetic data corroborate these observations by showing the decrease of the magnetic moment by roughly half (neglecting spin exchange effects) on oxidizing the molecules with one electron, and the disappearance of a paramagnetic response upon two-electron oxidation, which confirms the change in spin state associated with the electron-transfer steps.

Electrochemical and photochemical bond-activation steps are important for a variety of chemical transformations. We present here four new complexes, [Ru(Lⁿ)(dmso)(Cl)]PF₆ (1–4), where Lⁿ is a tripodal amine ligand with 4−n pyridylmethyl arms and n−1 triazolylmethyl arms. Structural comparisons show that the triazoles bind closer to the Ru center than the pyridines. For L², two isomers (with respect to the position of the triazole arm, equatorial or axial), trans-2_sym and trans-2_un, could be separated and compared. The increase in the number of the triazole arms in the ligand has almost no effect on the Ru^II/Ru^III oxidation potentials, but it increases the stability of the RuS_dmso bond. Hence, the oxidation waves become more reversible from trans-1 to trans-4, and whereas the dmso ligand readily dissociates from trans-1 upon heating or irradiation with UV light, the RuS bond of trans-4 remains perfectly stable under the same conditions. The strength of the RuS bond is not only influenced by the number of triazole arms but also by their position, as evidenced by the difference in redox behavior and reactivity of the two isomers, trans-2_sym and trans-2_un. A mechanistic picture for the electrochemical, thermal, and photochemical bond activation is discussed with data from NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry.

Azocarboxamide (azcH) has been combined for the first time with [Ru–Cym] to generate metal complexes with N,N- and N,O-coordination mode, [(Cym)Ru(azc)Cl] and [(Cym)Ru(azcH)Cl]⁺[PF₆]⁻. Geometric and electronic structures of the complexes are reported along with their in vitro activities against different tumour cell lines and preliminary results on solution chemistry. Compound [(Cym)Ru(azc)Cl] exhibited remarkable cytotoxic properties. It was cell-type specific and had comparable IC₅₀ values towards both cancer cells and their drug-resistant subline. A tenfold increase in the sensitivity towards [(Cym)Ru(azc)Cl] was noted for the tumour cells with depleted intracellular glutathione (GSH) level, suggesting the essential role of GSH in cell response to this compound.

Macrocycles such as porphyrins and corroles have important functions in chemistry and biology, including light absorption for photosynthesis. Generation of near-IR (NIR)-absorbing dyes based on metal complexes of these macrocycles for mimicking natural photosynthesis still remains a challenging task. Herein, the syntheses of four new Ag^III corrolato complexes with differently substituted corrolato ligands are presented. A combination of structural, electrochemical, UV/Vis/NIR-EPR spectroelectrochemical, and DFT studies was used to decipher the geometric and electronic properties of these complexes in their various redox states. This combined approach established the neutral compounds as stable Ag^III complexes, and the one-electron reduced species of all the compounds as unusual, stable Ag^II complexes. The one-electron oxidized forms of two of the complexes display absorptions in the NIR region, and thus they are rare examples of mononuclear complexes of corroles that absorb in the NIR region. The appearance of this NIR band, which has mixed intraligand charge transfer/intraligand character, is strongly dependent on the substituents of the corrole rings. Hence, the present work revolves round the design principles for the generation of corrole-based NIR-absorbing dyes and shows the potential of corroles for stabilizing unusual metal oxidation states. These findings thus further contribute to the generation of functional metal complexes based on such macrocyclic ligands.

Reversible proton- and electron-transfer steps are crucial for various chemical transformations. The electron-reservoir behavior of redox non-innocent ligands and the proton-reservoir behavior of chemically non-innocent ligands can be cooperatively utilized for substrate bond activation. Although site-decoupled proton- and electron-transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod_−H)M(μ-L²⁻) M(cod_−H)] (M=Pt^II, [1] or Pd^II, [2], cod=1,5-cyclooctadiene, H₂L=2,5-di-[2,6-(diisopropyl)anilino]-1,4-benzoquinone), in which cod acts as a proton reservoir, and L²⁻ as an electron reservoir. Protonation of [2] leads to an unusual tetranuclear complex. However, [1] can be stepwise reversibly protonated with up to two protons on the cod_−H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L²⁻ ligand. The doubly protonated form of [1] is also shown to react with OMe⁻ leading to an activation of the cod ligands. The site-decoupled proton and electron reservoir sources work in tandem in a three-way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton- and electron-transfer reactions by using metal complexes of non-innocent ligands.

Orthometalation at Ir^III centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as CH activation sites, we present here mono- and dinuclear Ir^III complexes with “click”-derived 1,2,3-triazole and 1,2,3-triazol-5-ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir^III centers. Structural characterization of the complexes reveal a piano stool-type of coordination around the metal centers with the “click”-derived ligands bound either with C^N or C^C donor sets to the Ir^III centers. Furthermore, whereas bond localization is observed within the 1,2,3-triazole ligands, a more delocalized situation is found in their 1,2,3-triazol-5-ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer-pure, dinuclear cyclometalated Ir^III complexes with poly-mesoionic-carbene ligands.

Triazolium-derived N-heterocyclic carbene (aNHC) ligands, which are readily accessible by deprotonation of the corresponding triazolium salts, proved to be versatile ligands in diverse allylic substitution reactions. The corresponding triazolium salts are formed from azides and alkynes through the application of 1,3-dipolar cycloaddition and N-alkylation reactions. The unique property of these ligands is to be zwitterionic in their liberated form and to act as strong redox-active σ-donor ligands. By virtue of these qualities, these ligands enable the development of an unprecedented Fe-catalyzed regioselective aryloxylation of allylic carbonates.

Neutral, iodido-containing copper(I) complexes [Cu(aNHC)₂I] {aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 6) and 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 7)} and cationic, halide-free copper(I) complexes [Cu(aNHC)₂](BF₄) {aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 8), 3-methyl-1,4-diphenyl-1,2,3-triazol-5-ylidene (for 9), 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 10), and 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 11)}, both containing two monodentate abnormal-carbene ligands (aNHC), were synthesized from [Cu(CH₃CN)₄](BF₄) and the corresponding triazolium salts. It was possible to selectively synthesize both kinds of complexes by simply variing the counter-anion of the triazolium salts and keeping the metal precursor the same. All complexes were characterized by elemental analysis and spectroscopic methods. 6 and 11 were studied with single-crystal X-ray diffraction analyses. In 6, the copper(I) center is tricoordinated, and its geometry is in between trigonal planar and T-shaped. In halide-free 11, the copper(I) center is linearly coordinated by two abnormal-carbene ligands. All complexes were tested as catalysts in the Huisgen [3+2] cycloaddition reaction between azides and alkynes, and they showed excellent efficiencies under neat conditions. A comparison between the efficiencies of the halide-containing complexes 6 and 7 and the halide-free cases 8–11, shows that the halide-free Cu–aNHC complexes are significantly more efficient than their halide-containing counterparts. The best catalyst was used for a substrate screening by utilizing a variety of azides and a couple of alkynes. The efficiency of the catalyst was maintained with loadings as low as 0.005 mol-%. Mechanistic studies were carried out as well.