Accessing high-spin configurations of transition metal phosphides defines a dividing line that prevents common properties of solid-state materials from being replicated within multiple-bonded molecular analogs. Here, we report the synthesis of a VIII phosphaethynolate complex, [(pyrNdipp)2V(PCO)] (2) in a halide metathesis with Na(OCP). Exposure of 2 to Lewis-basic ligands induces a one-electron reductive elimination of the PCO- moiety, generating VII complexes [(pyrNdipp)2V(L)2] (L = THF, DMAP; 3THF, 3DMAP). When 2 is instead photolyzed, a cascade of reduction, decarbonylation, and multiple-bond formation steps affords a high-spin and mixed-valent vanadium phosphide, [(pyrNdipp)2V═P═V(pyrNdipp)2] (4) comprising formal [V2III, IV] nodes. Structural characterization coupled with vibrational, UV-visible, and X-ray spectroscopic studies reveals an S4 symmetrical [V═P═V] centered architecture conforming to a fully delocalized, mixed-valency description. Theoretical studies demonstrate that 4 evades spin-pairing by leveraging the weak ligand-field splitting at the vanadium nodes, leading to a high-spin, ST = 3/2 ground state of this multiple-bonded, weakly Jahn-Teller distorted system.

Using an Earth-abundant transition metal to mediate formation and splitting of C-C σ-bonds, in response to electrical stimuli, constitutes a promising strategy to construct complex organic skeletons. Here, we showcase how [ n Bu4N][N3] reacts with an isocyanide adduct of a tetrahedral and high-spin TiII complex, [(Tp tBu,Me)TiCl] (1), to enact N-atom transfer, C-N bond formation, and C-C coupling, to form a dinuclear complex, [(Tp tBu,Me)Ti{AdN(N)C-C(N)NAd}Ti(Tp tBu,Me)] (3), with two TiIII ions bridged by a disubstituted oxalimidamide ligand ( n Bu = n-butyl, Tp tBu,Me = hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate, Ad = 1-adamantyl). Magnetic and computational studies reveal two magnetically isolated d1 TiIII ions, and electrochemical studies unravel a reversible two-electron oxidation at -0.87 V vs. [FeCp2]0/+. Despite these observations, chemical oxidation of 3, ultimately, leads to rupture of the oxalimidamide moiety with C-C bond splitting to form [(Tp tBu,Me)Ti{1,3-μ2-AdNCN}2Ti(Tp tBu,Me)][B(C6F5)4]2 (4), which displays an antiferromagnetically coupled Ti2 III,III configuration, mediated by superexchange through its bridging carbodiimide ligands. A comparative reactivity study of isocyanide toward a transient vanadium nitride [(Tp tBu,Me)V[triple bond, length as m-dash]N(THF)] (5) gives further insight into the structure of putative intermediates involved in the coupling sequence.

The interstellar diatomic molecule, phosphorus mononitride (P≡N), is highly unstable under conditions typical on Earth, and its utility for constructing elusive P-N π-bonded motifs has remained uncertain. Here, we show how Na(OCP) transfers a P atom to an electrophilic osmium nitride complex to form a metal-bound P≡N ligand. Quantum chemical calculations and X-ray absorption spectroscopy unveil a cumulenic [OsIV=N=P] electronic structure comprising orthogonal Os=N and N=P π-bonding. On reaction with elemental sulfur, the highly reduced P≡N ligand, formally [PN]2-, forms a trigonal planar [NPS2]2- motif. Chlorination instead transforms the P≡N ligand to a bent [NPCl]- group coordinated to OsIII (S = ½). [3 + 2] cycloaddition of this radical with azide forms an aromatic interpnictide, [PN4]-, that is inaccessible from the parent P≡N system. These findings provide a rare glimpse of the divergent reactivity of the alien P≡N molecule, paving the way to long-sought P-N multiple-bonded archetypes.

This study investigates the impact of structural isomerism on the excited state lifetime and redox energetics of heteroleptic [Ir(ppy)2(bpy)]+ and homoleptic Ir(ppy)3 photoredox catalysts using ground-state and time-dependent density functional theory methods. While the ground- and excited-state reduction potentials differ only slightly among the isomers of these complexes, our findings reveal significant variations in the radiative and non-radiative decay rates of the reactivity-controlling triplet 3MLCT states of these closely related species. The observed differences in radiative decay rates could be traced back to variations in the transition dipole moment, vertical energy gaps, and spin-orbit coupling of the isomers. In [Ir(ppy)2(bpy)]+, transition dipole moment differences play a significant role in controlling the relative lifetime of the triplet states, which we rationalized by a vectorial analysis of permanent dipole moments of the ground and excited states. Regarding the two isomers of Ir(ppy)3, changes in radiative decay rates were primarily attributed to variations in vertical energy gaps and intensity borrowing from other singlet-singlet transitions driven by spin-orbit coupling. Non-radiative decay variations were assessed in terms of differences in reorganization energies, adiabatic energy gap, and spin-orbit coupling. For both complexes, reorganization energies associated with low-energy molecular vibrations and metal-ligand bond length changes following the de-excitation process were major contributors. These insights provide a deeper understanding of how molecular design can be leveraged to optimize the performance of iridium-based photoredox catalysts, potentially guiding the development of more efficient catalytic systems for future applications.

The nitrido-ate complex [(PN) 2 Ti(N){μ 2 -K(OEt 2 )}] 2 ( 1 ) (PN − =(N-(2-P i Pr 2 -4-methylphenyl)-2,4,6-Me 3 C 6 H 2 ) reductively couples CO and isocyanides in the presence of DME or cryptand (Kryptofix222), to form rare, five-coordinate Ti II complexes having a linear cumulene motif, [K(L)][(PN) 2 Ti(NCE)] (E=O, L=Kryptofix222, ( 2 ); E=NAd, L=3 DME, ( 3 ); E=N t Bu, L=3 DME, ( 4 ); E=NAd, L=Kryptofix222, ( 5 )). Oxidation of 2 – 5 with [Fc][OTf] afforded an isostructural Ti III center containing a neutral cumulene, [(PN) 2 Ti(NCE)] (E=O, ( 6 ); E=NAd ( 7 ), N t Bu ( 8 )) and characterization by CW X-band EPR spectroscopy, revealed unpaired electron to be metal centric. Moreover, 1e − reduction of 6 and 7 in the presence of Kryptofix222cleanly reformed corresponding discrete Ti II complexes 2 and 5 , which were further characterized by solution magnetization measurements and high-frequency and -field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with [Fc*][B(C 6 F 5 ) 4 ] resulted in a ligand disproportionated Ti IV complex having transoid carbodiimides, [(PN) 2 Ti(NCNAd) 2 ] ( 9 ). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their 15 N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as Ti II →Ti III →Ti IV and increasing order of π-donation as Ti II →Ti III →Ti IV , thus displaying more covalency in Ti III species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N-atom to CO and CNAd.

To reveal, quantify, and rationalize the effect of backbone pi-extension on ligand redox activity, we studied the ground- and excited-state reduction potentials of eight ruthenium photoredox catalysts with the formula Ru(ppy)(2)L (L is the redox-active ligand of the bipyridine family) using density functional theory. Our research underlines the profound importance of the fusion position of backbone aromatic C-6 rings on the redox activity of ligands in transition metal photoredox catalysts. Namely, certain fusion positions lead to the dearomatization of C-6 rings in ligand-centered electron transfer events, resulting in a thermodynamic penalty equivalent to a half-volt negative shift in the reduction potential. Contrarily, the extent of backbone delocalization shows a minimal impact on redox energetics, which can be explained by the charge concentration at the nitrogen contact atoms in ligand-centered reductions. Grounded in Caulton's conceptual framework, we reaffirm the predictive potency of Lewis structures in ligand-centered redox energetics with qualitative and quantitative data. Our hypothesis regarding the effect of backbone ring dearomatization on redox energetics is further corroborated using magnetic and structure-based aromaticity indicators. Highlighting fusion-dependent dearomatization as a determining factor of ligand-centered electron transfer energetics, our findings hold implications for molecular-level design in advanced electroactive materials and catalysts.

Vertical electron transfer controls the thermodynamics of Cu( i ) photoredox catalysis. Structural relaxation indirectly enhances metal-centered reductions by preventing flattening distortions destabilizing oxidized Cu( ii ) species.

In the following chapter, we discuss redox-active ligands from a theoretical and computational perspective. We center our review, especially on the redox-active behavior of bipyridine and quinone-based ligand platforms in various complexes based on different transition metals including ruthenium, iridium, iron, copper, and others. First, we present a summary of pioneering combined experimental and theoretical studies aimed at the characterization of the phenomenon of redox non-innocence. Among the chemical topics discussed where redox-active ligands play a key role: photoredox catalysis, metal-based reactivity mediated by ligands, and redox flow batteries. We provide the state-of-the-art in computing reduction potentials with density functional theory ( DFT ) in combination with implicit solvation models in the ground and excited states of complexes. Then we discuss the result of a pilot study aiming at the rational design of the redox properties of ligand-centered electron transfer processes in photoredox catalysts. The use of some advanced computational characterizing techniques, e.g., electron density difference maps, theoretical square schemes, spin population, and quasi-restricted orbitals is showcased and their unique insights into ligand redox activity are rationalized.

Fluorescent probes are a powerful tool for imaging amyloid β (Aβ) plaques, the hallmark of Alzheimer’s disease (AD). Herein, we report the synthesis and comprehensive characterization of 21 novel probes as well as their optical properties and binding affinities to Aβ fibrils. One of these dyes, 1Ae, exhibited several improvements over FDDNP, an established biomarker for Aβ- and Tau-aggregates. First, 1Ae had large Stokes shifts (138–213 nm) in various solvents, thereby reducing self-absorption. With a high quantum yield ratio (φ(dichloromethane/methanol) = 104), 1Ae also ensures minimal background emission in aqueous environments and high sensitivity. In addition, compound 1Ae exhibited low micromolar binding affinity to Aβ fibrils in vitro (Kd = 1.603 µM), while increasing fluorescence emission (106-fold) compared to emission in buffer alone. Importantly, the selective binding of 1Ae to Aβ1–42 fibrils was confirmed by an in cellulo assay, supported by ex vivo fluorescence microscopy of 1Ae on postmortem AD brain sections, allowing unequivocal identification of Aβ plaques. The intermolecular interactions of fluorophores with Aβ were elucidated by docking studies and molecular dynamics simulations. Density functional theory calculations revealed the unique photophysics of these rod-shaped fluorophores, with a twisted intramolecular charge transfer (TICT) excited state. These results provide valuable insights into the future application of such probes as potential diagnostic tools for AD in vitro and ex vivo such as determination of Aβ1–42 in cerebrospinal fluid or blood.

The interstellar diatomic molecule, phosphorus mononitride (P≡N), is highly unstable under conditions typical on Earth, and its utility for constructing elusive P–N multiply-bonded archetypes is essentially uncharted. Herein, we show how Na(OCP) transfers a P atom to an electrophilic osmium nitride complex to form a terminally bound P≡N functionality. Quantum chemical calculations and X-ray absorption spectroscopy unveil a cumulenic [Os(IV)=N=P] electronic structure comprising orthogonal Os=N and N=P π-bonding. The highly reduced P≡N ligand, formally [PN]2–, undergoes two-fold oxidation with elemental sulfur to form a trigonal planar [NPS2]2– group. On reaction with Ph3CCl, the P≡N ligand forms a bent [NPCl]– motif coordinated to Os(III) (S = ½). [3+2] cycloaddition of this radical species with Me3SiN3 forms an aromatic heterocyclic interpnictide, [PN4]–, that is inaccessible from the parent P≡N system.

Coordination of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands to palladium, gold, and boron hydride has been investigated experimentally and computationally.

In this computational study, density functional theory (DFT) and time-dependent DFT methods (TD-DFT) were employed to study the optical properties of six families of molecules with donor (D), bridge (B), and acceptor (A) fragments that have potential for use as fluorescent molecular probes for the early detection of Alzheimer's disease. After validating our computational method against experimental data, using X-ray and absorption data, the equilibrium geometries and wave functions of the ground and first singlet excited states were systematically studied. Our simulations demonstrate that the S1 states of these rod-like D-B-A fluorescent probes are twisted intramolecular charge transfer states with a predominant highest occupied molecular orbital-least unoccupied molecular orbital (HOMO-LUMO) character, the former localized primarily at the donor, whereas the latter at the acceptor site. Moreover, the influence of the bridge, donor, and acceptor fragments on molecules' absorption energies is explored, highlighting the influence of double and triple bonds and some specific modifications on the acceptor side, including the addition of electronegative atoms, pyranone derivatives, and their functionalization. By having the absorption energies of 324 probes in hand, machine learning models were trained to predict the absorption energies of molecules. The models were found to be predictive, which suggests a potential that predictive models for other crucial properties, such as emission and quantum yield, can also be trained if suitable training data sets are made available.

The oxidation of NADPH catalyzed by submitochondrial particles from beef heart in the absence and presence of NAD+ has been investigated. The data confirm earlier findings in this laboratory concerning the occurrence of an NADPH dehydrogenase with 2,6-dichlorophenolindophenol as the electron acceptor. This reaction is highly sensitive to palmityl-CoA, a feature further substantiating its possible relationship to nicotinamide nucleotide transhydrogenase. The particles also catalyzed a very low NADPH oxidase activity which probably proceeds via NADH dehydrogenase and is unrelated to transhydrogenase.

The electronic structures of the ground and excited electronic states involved in the oxidative and reductive quenching cycles of 12 relevant ruthenium, iridium, and copper photoredox catalysts (S0, T1, Dox, and Dred) are characterized using the recently developed effective oxidation state (EOS) analysis, allowing the monitoring of metal and ligand oxidation states (OSs) along the catalytic cycles. The formal oxidation state assignments derived from the EOS analysis are in agreement with those commonly assumed for these complexes in both ground and excited states. Rather clean and separate ligand- and metal-centered redox events along the different quenching cycles are observed in most of the studied molecular systems. The reliability index obtained for the OS assignations can be readily interpreted in terms of the ionic/covalent character of metal-ligand interactions and ligand non-innocent character. In addition, EOS analysis reveals the high-degree localization of the ligand-centered redox event to one or two redox-active ligand(s) in heteroleptic complexes. Ligand- and metal-condensed spin populations were also computed and analyzed for all the open-shell species involved in this study, showing promises for rapid oxidation state assignments in certain systems, especially Ru complexes, however, suffering from severe defects in other cases.

The mechanism of the gas-phase halogen-exchange reaction between boron- and aluminum-halides (i.e., BX3 + BX3 and AlX3 + AlX3, X = F, Cl, or Br) was discovered using density functional theory. The reaction takes place via a two-step mechanism with the intermediacy of a diamond-core structure analogous to diborane. Good agreement was found between the simulated reaction features and experimental observations, which demonstrate slow kinetics and an equilibrium process for boron species and dimer formation in the case of aluminum-halides. This computational and theoretical study also reveals and quantifies the effect of resonance on the thermodynamic stability of the central intermediate and conceptualizes the extreme stability difference (∼50 kcal mol-1) between boron and aluminum diamond-core bridge structures. Through an interaction energy decomposition analysis in combination with electronic structure analyses, we revealed that, beyond the resonance stabilization in free boron-halides, superior electrostatics in aluminum-halides results in the different reactivities, i.e., dimer formation for the latter species whereas substituent exchange for the former ones.

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely, [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]+ (POP = bis [2-(diphenylphosphino)phenyl]ether), are scrutinized and characterized using quasi-restricted orbitals (QROs), electron density differences, and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S0, T1, Dox, and Dred) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet metal-to-ligand charge-transfer (3MLCT) (d9L-1), Dox (d9L0), and Dred (d10L-1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu-N/Cu-P bonds, are rationalized in terms of the underlying electronic structure transformations. Among others, we reveal the molecular-scale delocalization of the ligand-centered radical in the 3MLCT (d9L-1) and Dred (d9L-1) states of homoleptic [Cu(dmp)2]+ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]+.

Decarbonylation along with E atom transfer from Na(OCE) (E=P, As) to an isocyanide coordinated to the tetrahedral TiII complex [(TptBu,Me )TiCl], yielded the [(TptBu,Me )Ti(η3 -ECNAd)] species (Ad=1-adamantyl, TptBu,Me- =hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate). In the case of E=P, the cyanophosphide ligand displays nucleophilic reactivity toward Al(CH3 )3 ; moreover, its bent geometry hints to a reduced Ad-NCP3- resonance contributor. The analogous and rarer mono-substituted cyanoarsenide ligand, Ad-NCAs3- , shows the same unprecedented coordination mode but with shortening of the N=C bond. As opposed to TiII , VII fails to promote P atom transfer to AdNC, yielding instead [(TptBu,Me )V(OCP)(CNAd)]. Theoretical studies revealed the rare ECNAd moieties to be stabilized by π-backbonding interactions with the former TiII ion, and their assembly to most likely involve a concerted E atom transfer between Ti-bound OCE- to AdNC ligands when studying the reaction coordinate for E=P.

Visible-light photoredox catalysis has become a practical tool in the last years for driving energydemanding chemical reactions. Owing to their exceptional photoelectrochemical properties, classical octahedral ruthenium and iridium complexes still dominate the field of photoredox catalysis despite their drawbacks, such as sustainability and costs. Luminescent Cu (I) complexes are considered one of the most plausible candidates to replace such traditional Ru- and Ir photoredox catalysts in the near future. This review covers the development of Cu(I) complexes for photoredox catalysis with a special focus on collecting and categorizing the available photophysical and electrochemical information. The principal characteristics, advantages, drawbacks and examples of applications of the distinct classes (homoleptic, heteroleptic, in-situ generated tetra/tri/di coordinated, etc.) of Cu(I) complexes used as photoredox catalysts are presented and discussed in a systematic manner. The strategies that have been implemented to extend the lifetimes and to control the photo/electrochemical properties of Cu(I) complexes are detailed with particular emphasis on the role and potential of using dative carbon contact atoms in different ligand scaffolds. Especially, the incorporation of carbon-donor ligands such as isonitriles and carbenes has been proven to be a promising approach for controlling the photoelectrochemical properties and stability of photoredox catalysts. Visible-light photoredox catalysis has become a practical tool in the last years for driving energy demanding chemical reactions. Owing to their exceptional photoelectrochemical properties, classical octahedral ruthenium and iridium complexes still dominate the field of photoredox catalysis despite their drawbacks, such as sustainability and costs. Luminescent Cu (I) complexes are considered one of the most plausible candidates to replace such traditional Ru-and Ir photoredox catalysts in the near future. This review covers the development of Cu(I) complexes for photoredox catalysis with a special focus on collecting and categorizing the available photophysical and electrochemical information. The principal characteristics, advantages, drawbacks and examples of applications of the distinct classes (homoleptic, heteroleptic, in-situ generated tetra/tri/di coordinated, etc.) of Cu(I) complexes used as photoredox catalysts are presented and discussed in a systematic manner. The strategies that have been implemented to extend the lifetimes and to control the photo/electrochemical properties of Cu(I) complexes are detailed with particular emphasis on the role and potential of using dative carbon contact atoms in different ligand scaffolds. Especially, the incorporation of carbon-donor ligands such as isonitriles and carbenes has been proven to be a promising approach for controlling the photoelectrochemical properties and stability of photoredox catalysts. (C) 2021 Elsevier Ltd. All rights reserved.

The nitride salt [(PN)2Ti≡N{μ2-K(OEt2)}]2 (1) (PN- = (N-(2-PiPr2-4-methylphenyl)-2,4,6-Me3C6H2) can be oxidized with two equiv of I2 or four equiv of ClCPh3 to produce the phosphinimide-halide complexes (NPN')(PN)Ti(X) (X- = I (2), Cl (3); NPN' = N-(2-NPiPr2-4-methylphenyl)-2,4,6-Me3C6H22-), respectively. In the case of 2, H2 was found to be one of the other products; whereas, HCPh3 and Gomberg's dimer were observed upon the formation of 3. Independent studies suggest that the oxidation of 1 could imply the formation of the transient nitridyl species [(PN)2Ti(≡N•)] (A), which can either oxidize the proximal phosphine atom to produce the Ti(III) intermediate [(NPN')(PN)Ti] (B) or, alternatively, engage in H atom abstraction to form the parent imido (PN)2Ti≡NH (4). The latter was independently prepared and was found to photochemically convert to the titanium-hydride, (NPN')(PN)Ti(H) (5). Isotopic labeling studies using (PN)2Ti≡ND (4-d1) as well as reactivity studies of 5 with a hydride abstractor demonstrate the presence of the hydride ligand in 5. An alternative route to putative A was observed via a photochemically promoted incomplete reduction of the azide ligand in (PN)2Ti(N3) (6) to 4. This process was accompanied by some formation of 5. Frozen matrix X-band EPR studies of 6, performed under photolytic conditions, were consistent with species B being formed under these reaction conditions, originating from a low barrier N-insertion into the phosphine group in the putative nitridyl species A. Computational studies were also undertaken to discover the mechanism and plausibility of the divergent pathways (via intermediates A and B) in the formation of 2 and 3, and to characterize the bonding and electronic structure of the elusive nitrogen-centered radical in A.