Deconvoluting capping ligand influence on photophysical properties in tetrathiafulvalene-based diradicaloids.

Tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt) complexes are synthetically tunable and emit brightly in the near-infrared II region (NIR II, 1000-1700 nm). Their emission/absorption energies respond to the identity of the capping ligands on the metal center, but a detailed understanding of how ligand bonding interactions dictate photophysical properties is key to predictive design optimization. Here we assess the relative influence of ligand pi (π) backbonding sigma (σ) donation in these complexes across a new series of olefin- and phosphite-capped complexes.

Synthesis of a potassium capped terminal cobalt-oxido complex.

An unusual example of a potassium capped terminal cobalt-oxido complex has been isolated and crystallographically characterized. The synthesis of [DHP]CoOK proceeds from a previously reported parent compound, [DHP]CoOH, deprotonation with KOBu. Structural and electronic characterization suggest a weakly coupled dimer in a distinct seesaw geometry with a Co(III) oxidation state and a non-innocent radical ligand.

Isolation and Crystallographic Characterization of an Octavalent CoO Diamond Core.

High-valent cobalt oxides play a pivotal role in alternative energy technology as catalysts for water splitting and as cathodes in lithium-ion batteries. Despite this importance, the properties governing the stability of high-valent cobalt oxides and specifically possible oxygen evolution pathways are not clear. One root of this limited understanding is the scarcity of high-valent Co(IV)-containing model complexes; there are no reports of stable, well-defined complexes with multiple Co(IV) centers.

Variable Peripheral Ligand Donation Tunes Electronic Structure and NIR II Emission in Tetrathiafulvalene Tetrathiolate Diradicaloids.

Near-infrared (NIR) lumiphores are promising candidates for numerous imaging, communication, and sensing applications, but they typically require large, conjugated scaffolds to achieve emission in this low-energy region. Due to the extended conjugation and synthetic complexity required, it is extremely difficult to tune the photophysical properties of these systems for desired applications. Here, we report facile tuning of deep NIR-emitting diradicaloid complexes through simple modification of peripheral ligands.

Metal-ligand cooperativity in chemical electrosynthesis.

Electrochemistry has been an increasingly useful tool for organic synthesis, as it can selectively generate reactive intermediates under mild conditions using an applied potential. Concurrently, synergistic activity of a metal and a ligand has been used in thermal catalysis and electrocatalytic renewable fuel generation for substrate selectivity and improved catalyst activity. Combining these synthetic strategies is an attractive approach for mild, selective, and sustainable electrosynthesis.

Selective Cobalt-Mediated Formation of Hydrogen Peroxide from Water under Mild Conditions via Ligand Redox Non-Innocence.

Despite the broad importance of hydrogen peroxide (HO) in oxidative transformations, there are comparatively few viable routes for its production. The majority of commercial HO is currently produced by the stepwise reduction of dioxygen (O) via the anthraquinone process, but direct electrochemical formation from water (HO) would have several advantages─namely, avoiding flammable gases or stepwise separations. However, the selective oxidation of HO to form HO over the thermodynamically favored product of O is a difficult synthetic challenge.

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons.

Alkyne semihydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semihydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H equivalent (2H + 2e) on the ligand backbone. This method is chemoselective for the semihydrogenation of terminal alkynes over internal alkynes or alkenes.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand.

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating.

Origin of Weak Magnetic Coupling in a Dimanganese(II) Complex Bridged by the Tetrathiafulvalene-Tetrathiolate Radical.

Magnetic exchange coupling () between different spin centers plays a crucial role in molecule-based magnetic materials. Direct exchange coupling between an organic radical and a metal is frequently stronger than superexchange through diamagnetic ligands, and the strategy of using organic radicals to engender desirable magnetic properties has been an area of active investigation. Despite significant advances and exciting bulk properties, the magnitude of for radical linkers bridging paramagnetic centers is still difficult to rationally predict.

Electrochemical Reduction of NO with a Molecular Copper Catalyst.

Deoxygenation of nitrous oxide (NO) has significant environmental implications, as it is not only a potent greenhouse gas but is also the main substance responsible for the depletion of ozone in the stratosphere. This has spurred significant interest in molecular complexes that mediate NO deoxygenation. Natural NO reduction occurs via a Cu cofactor, but there is a notable dearth of synthetic molecular Cu catalysts for this process.

Metal-Ligand Cooperative Transfer of Protons and Electrons.

Realizing cooperativity between ligands and metal centers in the transfer of proton and electron equivalents has the potential to facilitate faster, selective, and novel transformations. Recent advances in the synthesis and application of ligands with these design features illustrate the value of this biomimetic strategy in synthetic chemistry.

Bright, Modular, and Switchable Near-Infrared II Emission from Compact Tetrathiafulvalene-Based Diradicaloid Complexes.

Near-infrared (NIR)-emitting molecules are promising candidates for biological sensing and imaging applications; however, many NIR dyes are large conjugated systems which frequently have issues with stability, solubility, and tunability. Here, we report a novel class of compact and tunable fluorescent diradicaloid complexes which are air-, water-, light-, and temperature-stable. These properties arise from a compressed π manifold which promotes an intense ligand-centered π-π transition in the NIR II (1000-1700 nm) region and which subsequently emits at ∼1200 nm.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen.

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations.

Generation and Aerobic Oxidative Catalysis of a Cu(II) Superoxo Complex Supported by a Redox-Active Ligand.

Cu systems feature prominently in aerobic oxidative catalysis in both biology and synthetic chemistry. Metal ligand cooperativity is a common theme in both areas as exemplified by galactose oxidase and by aminoxyl radicals in alcohol oxidations. This has motivated investigations into the aerobic chemistry of Cu and specifically the isolation and study of Cu-superoxo species that are invoked as key catalytic intermediates.

A cobalt adduct of an N-hydroxy-piperidinium cation.

Cooperativity between organic ligands and transition metals in H-atom (proton/electron) transfer catalysis has been an important recent area of investigation. Tetramethylpiperidine-N-oxyl (TEMPO) radicals feature prominently in this area, prompting us to examine cooperativity between its hydrogenated congener, TEMPOH, and Co centers ligated by dihydrazonopyrrole ligands which have previously been shown to also store H-atom equivalents. Addition of TEMPOH to ( DHP)CoOTf results in formation of an unusual Co-adduct of 1-hydroxy-2,2,6,6-tetramethylpiperidin-1-ium (TEMPOH ) which has been characterized with IR spectroscopy and single crystal X-ray diffraction.

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate Via a Preorganized Secondary Coordination Sphere.

Enzymes exert control over the reactivity of metal centers with precise tuning of the secondary coordination sphere of active sites. One particularly elegant illustration of this principle is in the controlled delivery of proton and electron equivalents in order to activate abundant but kinetically inert oxidants such as O for oxidative chemistry. Chemists have drawn inspiration from biology in designing molecular systems where the secondary coordination sphere can shuttle protons or electrons to substrates.