A Bis-(carbone) Pincer Ligand and Its Coordinative Behavior toward Multi-Metallic Configurations

Carbones are divalent carbon(0) species that contain two lone pairs of electrons. Herein, we have prepared the first known stable and isolable free bis-(carbone) pincer framework with a well-defined solid-state structure. This bis-(carbone) ligand is an effective scaffold for forming monometallic (Ni and Pd) and trinuclear heterometallic complexes with Au-Pd-Au, Au-Ni-Au, and Cu-Ni-Cu configurations. Sophisticated quantum-theoretical analyses found that the metal-metal interactions are too weak to play a significant role in upholding these multi-metallic configurations; rather, the four lone pairs of electrons within the bis-(carbone) framework are the main contributors to the stability of the complexes.

Redox-Active Supramolecular Heteroleptic M4 L2 L'2 Assemblies with Tunable Interior Binding Site

Three Pt4 L2 L'2 heteroleptic rectangles (1-3), containing ditopic redox-active bis-pyridine functionalized perylene bisimide (PBI) ligands PBI-pyr2 (L) are reported. Co-ligand L' is a dicarboxylate spacer of varying length, leading to modified overall size of the assemblies. 1 H NMR spectroscopy reveals a trend in the splitting and upfield chemical shift of the PBI-hydrogens in the rectangles with respect to free PBI, most pronounced with the largest strut length (3) and least with the smallest strut length (1). This is attributed to increased rotational freedom of the PBI-pyr2 ligand over its longitudinal axis (Npy -Npy ), due to increased distance between the PBI-surfaces, which is corroborated by VT-NMR measurements and DFT calculations. The intramolecular motion entails desymmetrization of the two PBI-ligands, in line with cyclic voltammetry (CV) data. The first (overall two-electron) reduction event and re-oxidation for 1 display a subtle peak-to-peak splitting of 60 mV, whilst increased splitting of this event is observed for 2 and 3. The binding of pyrene in 1 is probed to establish proof of concept of host-guest chemistry enabled by the two PBI-motifs. Fitting the binding curve obtained by 1 H NMR titration with a 1:1 complex formation model led to a binding constant of 964±55 m-1 . Pyrene binding is shown to directly influence the redox-chemistry of 1, resulting in a cathodic and anodic shift of approximately 46 mV on the first and second reduction event, respectively.

Enhanced Metallophilicity in Metal-Carbene Systems: Stronger Character of Aurophilic Interactions in Solution

Metallophilicity is an essential concept that builds upon the attraction between closed shell metal ions. We report on the [M₂ (bisNHC)₂ ]²⁺ (M=Au^I , Ag^I ; NHC=N-heterocyclic carbene) systems, which display almost identical features in the solid state. However, in solution the Au₂ cation exhibits a significantly higher degree of rigidity owed to the stronger character of the aurophilic interactions. Both Au₂ and Ag₂ cationic constructs are able to accommodate Ag⁺ ions via M-M interactions, despite their inherent Coulombic repulsion. When electrostatic repulsion between host and guest is partially diminished, M-M distances are substantially shortened. Quantum chemical calculations estimate intermetallic bond orders up to 0.2. Although at the limit of (or beyond) the van der Waals radii, metallophilic interactions are responsible for their behavior in solution.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

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.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

Circumventing Intrinsic Metal Reactivity: Radical Generation with Redox-Active Ligands

Nickel complexes have gained sustained attention as efficient catalysts in cross-coupling reactions and co-catalysts in dual systems due to their ability to react with radical species. Central to this reactivity is nickel's propensity to shuttle through several accessible redox states from Ni⁰ to Ni^IV . Here, we report the catalytic generation of trifluoromethyl radicals from a nickel complex bearing redox-active iminosemiquinone ligands. This unprecedented reactivity is enabled through ligand-based oxidation performing electron transfer to an electrophilic CF₃⁺ source while the nickel oxidation state is preserved. Additionally, extension of this reactivity to a copper complex bearing a single redox equivalent is reported, thus providing a unified reactivity scheme. These results open new pathways in radical chemistry with redox-active ligands.

Controlled Interconversion of a Dinuclear Au Species Supported by a Redox-Active Bridging PNP Ligand Facilitates Ligand-to-Gold Electron Transfer

Redox non-innocent ligands have recently emerged as interesting tools to obtain new reactivity with a wide variety of metals. However, gold has almost been neglected in this respect. Here, we report mechanistic investigations related to a rare example of ligand-based redox chemistry in the coordination sphere of gold. The dinuclear metal-centered mixed-valent AuI -AuIII complex 1, supported by monoanionic diarylamido-diphosphine ligand PNPPr and with three chlorido ligands overall, undergoes a complex series of reactions upon halide abstraction by silver salt or Lewis acids such as gallium trichloride. Formation of the ultimate AuI -AuI complex 2 requires the intermediacy of AuI -AuI dimers 5 and 7 as well as the unique AuIII -AuIII complex 6, both of which are interconverted in a feedback loop. Finally, unprecedented ortho-selective C-H activation of the redox-active PNP ligand results in the carbazolyldiphosphine derivative PN*PPr via ligand-to-metal two-electron transfer. This work demonstrates that the redox-chemistry of gold may be significantly expanded and modified when using a reactive ligand scaffold.

Localized Mixed-Valence and Redox Activity within a Triazole-Bridged Dinucleating Ligand upon Coordination to Palladium

The new dinucleating redox-active ligand (L^H4 ), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of Pd^II resulted in the formation of paramagnetic (S=1/2 ) 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.

Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox-active pyridine aminophenol ligand to Ru(II) followed by aerobic oxidation generates two diamagnetic Ru(III) species [1 a (cis) and 1 b (trans)] with ligand-centered radicals. The reaction of 1 a/1 b with excess NaN3 under inert atmosphere resulted in the formation of a rare bis(nitrido)-bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru-N-Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a/1 b supports the oxidation of a nitride (N(3-) ) to half an equivalent of N2 . The trinuclear omplex is reactive toward TEMPO-H, tin hydrides, thiols, and dihydrogen.