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

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

Multiple Bistability in Quinonoid-Bridged Diiron(II) Complexes: Influence of Bridge Symmetry on Bistable Properties

Quinonoid bridges are well-suited for generating dinuclear assemblies that might display various bistable properties. In this contribution we present two diiron­(II) complexes where the iron­(II) centers are either bridged by the doubly deprotonated form of a symmetrically substituted quinonoid bridge, 2,5-bis­[4-(isopropyl)­anilino]-1,4-benzoquinone (<b>H</b>_<b>2</b><b>L2′</b>) with a [O,N,O,N] donor set, or with the doubly deprotonated form of an unsymmetrically substituted quinonoid bridge, 2-[4-(isopropyl)­anilino]-5-hydroxy-1,4-benzoquinone (<b>H</b>_<b>2</b><b>L5′</b>) with a [O,O,O,N] donor set. Both complexes display temperature-induced spin crossover (SCO). The nature of the SCO is strongly dependent on the bridging ligand, with only the complex with the [O,O,O,N] donor set displaying a prominent hysteresis loop of about 55 K. Importantly, only the latter complex also shows a pronounced light-induced spin state change. Furthermore, both complexes can be oxidized to the mixed-valent iron­(II)–iron­(III) form, and the nature of the bridge determines the Robin and Day classification of these forms. Both complexes have been probed by a battery of electrochemical, spectroscopic, and magnetic methods, and this combined approach is used to shed light on the electronic structures of the complexes and on bistability. The results presented here thus show the potential of using the relatively new class of unsymmetrically substituted bridging quinonoid ligands for generating intriguing bistable properties and for performing site-specific magnetic switching

Control of Complex Formation through Peripheral Substituents in Click-Tripodal Ligands: Structural Diversity in Homo- and Heterodinuclear Cobalt-Azido Complexes

The azide anion is widely used as a ligand in coordination chemistry. Despite its ubiquitous presence, controlled synthesis of azido complexes remains a challenging task. Making use of click-derived tripodal ligands, we present here various coordination motifs of the azido ligands, the formation of which appears to be controlled by the peripheral substituents on the tripodal ligands with otherwise identical structure of the coordination moieties. Thus, the flexible benzyl substituents on the tripodal ligand TBTA led to the formation of the first example of an unsupported and solely μ_1,1-azido-bridged dicobalt­(II) complex. The more rigid phenyl substituents on the TPTA ligand deliver an unsupported and solely μ_1,3-azido-bridged dicobalt­(II) complex. Bulky diiso­propyl­phenyl substituents on the TDTA ligand deliver a doubly μ_1,1-azido-bridged dicobalt­(II) complex. Intriguingly, the mononuclear copper­(II) complex [Cu­(TBTA)­N₃]⁺ is an excellent synthon for generating mixed dinuclear complexes of the form [(TBTA)­Co­(μ_1,1-N₃)­Cu­(TBTA)]³⁺ or [(TBTA)­Cu­(μ_1,1-N₃)­Cu­(TPTA)]³⁺, both of which contain a single unsupported μ_1,1-N₃ as a bridge. To the best of our knowledge, these are also the first examples of mixed dinuclear complexes with a μ_1,1-N₃ monoazido bridge. All complexes were crystallographically characterized, and selected examples were probed via magnetometry and high-field EPR spectroscopy to elucidate the electronic structures of these complexes and the nature of magnetic coupling in the various azido-bridged complexes. These results thus prove the power of click-tripodal ligands in generating hitherto unknown chemical structures and properties