Arene–Ruthenium(II) and −Iridium(III) Complexes with “Click”-Based Pyridyl-triazoles, Bis-triazoles, and Chelating Abnormal Carbenes: Applications in Catalytic Transfer Hydrogenation of Nitrobenzene

The complexes [(Cym)­Ru­(<b>L</b>)­Cl]­PF₆, <b>2</b>–<b>4</b>, and [Cp*Ir­(<b>L</b>)­Cl]­PF₆, <b>6</b>–<b>8</b> (Cym = <i>p</i>-cymene, Cp* = pentamethylcyclopentadienyl), with <b>L</b> = “click”-derived pyridyl-triazol, bis-triazole, or bis-abnormal carbene, were synthesized and spectroscopically characterized. Structural elucidation of the complexes shows a half-sandwich, piano-stool type of coordination around the metal centers and a delocalized situation within the triazolylidene rings. All the complexes were tested for their catalytic efficiency in the transfer hydrogenation of nitrobenzenes, and the results were compared with their 2,2′-bipyridine (bpy) Ru counterpart <b>1</b> and Ir counterpart <b>5</b>. Remarkably, the nature of the final catalytic product is strongly dependent on the chosen metal center, with aniline being preferentially formed with the Ru complexes and azobenzenes with the Ir complexes. Judicious selection of catalyst and reaction conditions also facilitates the isolation of azoxybenzene. To the best of our knowledge, this is a rare example of a homogeneous catalytic synthesis of azobenzene from nitrobenzene. The influence of ligand substitution, metal substitution, and temperature variation on catalytic activity and selectivity has been investigated, whereby a systematic variation of the ligands from bpy, to pyridyl-triazole, to bis-triazole, to bis-abnormal carbene has been carried out. We also present a mechanistic investigation for this transformation with the aim of understanding reaction behavior

Mono- and Digold(I) Complexes with Mesoionic Carbenes: Structural Characterization and Use in Catalytic Silver-Free Oxazoline Formation

Triazolylidenes are a prominent class of mesoionic carbenes that have found use as supporting ligands in homogeneous catalysis in recent years. We present here the syntheses of three new mononuclear gold­(I) chlorido and two new dinuclear gold­(I) chlorido complexes. The ligands in the aforementioned complexes are derived from either the corresponding monotriazolium or the bitriazolium salts. All complexes have been characterized by ¹H and ¹³C­{¹H} NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. Structural characterization delivers a delocalized bonding situation within the triazolylidene ligands and a linear coordination at the gold­(I) centers. The gold­(I) centers in all cases are bound to one triazolylidene-<i>C</i> donor and a chlorido ligand. Additionally, for the digold­(I) complexes large Au–Au distances were observed, ruling out the existence of aurophilic interactions in these digold complexes in the solid state. All of the gold­(I) complexes were tested as (pre)­catalysts for the cyclization reaction of propargylic amides to form oxazolines. We show here that the steric bulk of the substituents on the triazolylidene ligands plays a decisive role in the catalytic efficiency of the gold­(I) complexes. Copper­(II) triflate is shown as a viable alternative to silver­(I) salts as an additive for the oxazoline formation. Mechanistic studies show the detection of a gold­(I) triazolylidene vinyl complex as an intermediate in the catalytic synthesis of oxazoline with these complexes. These results thus establish copper­(II) triflate as an alternative to silver­(I) salts as an additive in gold­(I) triazolylidene catalysis. Furthermore, it also shows that steric tuning of triazolylidene ligands can indeed be utilized for increasing the catalytic efficiency of the corresponding complexes

Di- and Trinuclear Iridium(III) Complexes with Poly-Mesoionic Carbenes Synthesized through Selective Base-Dependent Metalation

Mutidentate carbene ligands based on a rigid aromatic platform are valuable synthons for generating carbene complexes with higher nuclearity. We present here the selective, base-dependent synthesis of a dinuclear or a trinuclear Ir^III complex from the 1,3,5-substituted benzene derived tris-triazolium salt. The dinuclear Ir^III complex features an unreacted triazolium unit which enables us to compare the metric parameters between the bonded 1,2,3-triazol-5-ylidene to their parent triazolium salt present in the same molecule. Single crystal X-ray diffraction studies confirm the di- and trinuclear nature of the complexes and establish their configuration and conformation. Both the di- and trinuclear Ir^III complexes have been used for catalytic transfer hydrogenation, and these complexes are potent precatalysts delivering good to excellent yields for the reduction of benzaldehyde, acetophenone, benzophenone, and cyclohexanone. Furthermore, they show a preference for reducing nitrobenzene to either azoxybenzene or azobenzene. Mercury poisoning tests conclusively prove the homogeneous nature of the reported catalysis. The lack of orthometalation in these complexes and the possible effect thereof on catalysis are discussed

Gauging Donor/Acceptor Properties and Redox Stability of Chelating Click-Derived Triazoles and Triazolylidenes: A Case Study with Rhenium(I) Complexes

Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other N-heterocyclic carbene counterparts. We present here seven rhenium­(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium­(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through ¹H and ¹³C NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV–vis–near-IR/IR/electron paramagnetic resonance spectroelectrochemistry and density functional theory calculations are used to address questions related to the electronic structures of the complexes in various redox states, their redox stability, and the understanding of chemical reactivity following electron transfer in these systems. The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components. Additionally, these results point to the fact that MIC-containing ligands confer remarkable redox stability to their <i>fac</i>-Re­(CO)₃-containing metal complexes. These findings will probably be useful for fields such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry, where <i>fac</i>-Re­(CO)₃ complexes of triazoles/triazolylidenes are likely to find use

Non-Innocence of 1,4-Dicyanamidobenzene Bridging Ligands in Dinuclear Ruthenium Complexes.

Four dinuclear complexes, [{Ru^II(ttpy)­(bpy)}₂(μ-L)]­[PF₆]₂, where bpy is 2,2′-bipyridine, ttpy is 4-(<i>tert</i>-butylphenyl)-2,2′:6′,2″-terpyridine, and L is 2,5-dimethyl-, 2,5-dichloro-, 2,3,4,5-tetrachloro- and unsubstituted 1,4-dicyanamidebenzene dianion have been synthesized and characterized. Electron paramagnetic resonance (EPR) spectroscopy of electrogenerated [{Ru­(ttpy)­(bpy)}₂(μ-L)]³⁺ ions shows largely ligand centered spin and thus the complexes’ oxidation states are best formulated as [Ru­(II), L^•–, Ru­(II)]³⁺. Visible-NIR and IR spectra of [{Ru­(ttpy)­(bpy)}₂(μ-L)]^3+,4+ ions were also obtained by spectroelectrochemical methods. For the [{Ru­(ttpy)­(bpy)}₂(μ-L)]³⁺ ions, the significant variations in the spectra were rationalized in terms of an increased ruthenium contribution to the singly occupied molecular orbital with increasing number of chloro substituents on the bridging ligand L

Substituent-Induced Reactivity in Quinonoid-Bridged Dinuclear Complexes: Comparison between the Ruthenium and Osmium Systems

The ligand 2,5-bis­[2-(methylthio)­anilino]-1,4-benzoquinone (<b>L</b>) was used in its doubly deprotonated form to synthesize the complex [{Cl­(η⁶-Cym)­Os}₂(μ<i>-</i>η²:η²-<b>L</b>_<b>‑2H</b>)] (<b>1</b>; Cym = <i>p</i>-cymene = 1-isopropyl-4-methylbenzene). Spectroscopic characterization and elemental analysis confirms the presence of the chloride ligands in <b>1</b>, which indirectly shows that the bridging ligand <b>L</b>_<b>‑2H</b> acts in a bis-bidentate fashion in <b>1</b>, with the thioether substituents on the bridge remaining uncoordinated. Abstraction of the chloride ligands in <b>1</b> by AgBF₄ in CH₃CN leads not only to the release of those chloride ligands but also to a simultaneous substituent-induced release of Cym with the bridging ligand changing its coordination mode to bis-tridentate. In the resulting complex [{(CH₃CN)₃Os}₂(μ-η³:η³-<b>L</b>_<b>‑2H</b>)]²⁺ (<b>2</b>^<b>2+</b>), the thioether groups of <b>L</b>_<b>‑2H</b> are now coordinated to the osmium centers with the bridging ligand coordinating to the metal center in a bis-meridional form. The coordination mode of <b>L</b>_<b>‑2H</b> in <b>2</b>^<b>2+</b> was confirmed by single-crystal X-ray diffraction data. A structural analysis of <b>2</b>^<b>2+</b> reveals localization of double bonds within the “upper” and “lower” parts of the bridging ligand in comparison to bond distances in the free ligand. Additionally, the binding of the bridge to the osmium centers is seen to occur through O^– and neutral imine-type N donors. The complexes <b>1</b> and <b>2</b>^<b>2+</b> were investigated by cyclic voltammetry and UV–vis–near-IR and EPR spectroelectrochemistry. This combined approach was used to unravel the redox-active nature of the ligand <b>L</b>_<b>‑2H</b>, to determine the sites of electron transfer (ligand radical versus mixed valency), and to compare the present systems with their ruthenium analogues <b>3</b> and <b>4</b>^<b>2+</b> (Schweinfurth, D. Inorg. Chem. 2011, 50, 1150). The effect of replacing ruthenium by its higher homologue osmium on the reactivity and the electrochemical and spectroscopic properties were explored, and the differences were deciphered by taking into account the intrinsic dissimilarities between the two homologues. The usefulness of incorporating additional donor substituents on potentially bridging quinonoid ligands was probed in this work

The Power of Ferrocene, Mesoionic Carbenes, and Gold: Redox-Switchable Catalysis

Catalysis with gold­(I) complexes is a useful route for synthesizing a variety of important heterocycles. Often, silver­(I) additives are necessary to increase the Lewis acidity at the gold­(I) center and to make them catalytically active. We present here a concept in redox-switchable gold­(I) catalysis that is based on the use of redox-active mesoionic carbenes, and of electron transfer steps for increasing the Lewis acidity at the gold­(I) center. A gold­(I) complex with a mesoionic carbene containing a ferrocenyl backbone is presented. Investigations on the corresponding iridium­(I)–CO complex show that the donor properties of such carbenes can be tuned via electron transfer steps to make these seemingly electron rich mesoionic carbenes relatively electron poor. A combined crystallographic, electrochemical, UV–vis–near-IR/IR spectroelectrochemical investigation together with DFT calculations is used to decipher the geometric and the electronic structures of these complexes in their various redox states. The gold­(I) mesoionic carbene complexes can be used as redox-switchable catalysts, and we have used this concept for the synthesis of important heterocycles: oxazoline, furan and phenol. Our approach shows that a simple electron transfer step, without the need of any silver additives, can be used as a trigger in gold catalysis. This report is thus the first instance where redox-switchable (as opposed to only redox-induced) catalysis has been observed with gold­(I) complexes

Redox Activity and Bond Activation in Iridium–Diamidobenzene Complexes: A Combined Structural, (Spectro)electrochemical, and DFT Investigation

Noninnocent ligands are special because of their ability to act as electron reservoirs and tune reactivity at a metal center “on-demand”. In the following we present two iridium­(III) complexes with a diamidobenzene ligand: one that is coordinatively unsaturated and a second one that is a coordinatively saturated, regular 18 valence electron complex. We show the electrochemical interconversion between the two complexes and propose a mechanism for the same. Both the complexes have been isolated in pure forms and characterized by spectroscopic, (spectro)­electrochemical, and crystallographic techniques. Additionally, results from DFT calculations are presented to decipher the bonding situation within the two complexes and to investigate the bond activation pathway leading to the interconversion of one form into another. In this work we make use of the increasingly popular concept of using redox steps at noninnocent ligands to tune bond activation and chemical reactivity at the metal center

Redox Activity and Bond Activation in Iridium–Diamidobenzene Complexes: A Combined Structural, (Spectro)electrochemical, and DFT Investigation

Noninnocent ligands are special because of their ability to act as electron reservoirs and tune reactivity at a metal center “on-demand”. In the following we present two iridium­(III) complexes with a diamidobenzene ligand: one that is coordinatively unsaturated and a second one that is a coordinatively saturated, regular 18 valence electron complex. We show the electrochemical interconversion between the two complexes and propose a mechanism for the same. Both the complexes have been isolated in pure forms and characterized by spectroscopic, (spectro)­electrochemical, and crystallographic techniques. Additionally, results from DFT calculations are presented to decipher the bonding situation within the two complexes and to investigate the bond activation pathway leading to the interconversion of one form into another. In this work we make use of the increasingly popular concept of using redox steps at noninnocent ligands to tune bond activation and chemical reactivity at the metal center

Heterobimetallic Cu–dppf (dppf = 1,1′-Bis(diphenylphosphino)ferrocene) Complexes with “Click” Derived Ligands: A Combined Structural, Electrochemical, Spectroelectrochemical, and Theoretical Study

Heterodinuclear complexes of the form [(dppf)­Cu­(L)]­(BF₄) (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene), where L are the chelating, substituted 4,4′-bis­(1,2,3-triazole) or 4-pyridyl­(1,2,3-triazole) ligands, were synthesized by reacting [Cu­(dppf)­(CH₃CN)₂]­(BF₄) with the corresponding “click” derived ligands. Structural characterization of representative complexes revealed a distorted-tetrahedral coordination geometry around the Cu­(I) centers, with the donor atoms being the P donors of dppf and the N donors of the substituted triazole ligands. The “local-pseudo” symmetry around the iron center in all the investigated complexes of dppf is between that of the idealized <i>D</i>_5<i>h</i> and <i>D</i>_5<i>d</i>. Furthermore, for the complex with the mixed pyridine and triazole donors, the Cu–N bond distances were found to be shorter for the triazole N donors in comparison to those for the pyridine N donors. Electrochemical studies on the complexes revealed the presence of one oxidation and one reduction step for each. These studies were combined with UV–vis–near-IR and EPR spectroelectrochemical studies to deduce the locus of the oxidation process (Cu vs Fe) and to see the influence of changing the chelating “click” derived ligand on both the oxidation and reduction processes and their spectroscopic signatures. Structure-based DFT studies were performed to get insights into the experimental spectroscopic results. The results obtained here are compared with those of the complex [(dppf)­Cu­(bpy)]­(BF₄) (bpy = 2,2′-bipyridine). A comparison is made among bpy, pyridyl-triazole, and bis-triazole ligands, and the effect of systematically replacing these ligands on the electrochemical and spectroscopic properties of the corresponding heterodinuclear complexes is investigated