A strategy for modulating the coordination mode and redox properties of nickel complexes is introduced, using a 2,2'-bipyridine derivative featuring a flexidentate and cation-responsive aza-crown ether. The bipyridine-aza-crown ether ligand can reversibly change denticity from κ4 to κ6 by the addition/removal of chloride ion or acetonitrile. A key impact of the crown ether pendant is that the electrochemical response of a nickel complex is tunable by addition of redox-inactive ions. Redox-switchable cation binding occurs after the first reduction, with strong Na+ binding induced upon reduction from nickel(II) to nickel(I). Even though Na+ has a low charge and is a weak Lewis acid, large anodic shifts (ranging from 72 to 236 mV) are observed for the electrochemical features. Even larger anodic shifts (160 to 428 mV) are observed in the presence of Ca2+ ions. The potential shifts of some features can be reversed by cation removal via the addition of free crown ether. A mechanism for changes in Ni primary coordination sphere under different electrochemical conditions is proposed, providing insight into how intersecting design features of flexidenticity and cation-crown interactions can introduce reversibly tunable properties of nickel complexes relevant to a range of catalytic reactions.

Tuning metal-ligand covalency offers promising design strategies to tailor the photoluminescence (PL) emission energies of metal-centered, spin-flip excited states. Herein, we report a series of tris(quinolinolate) chromium(III) complexes that exhibit record-low near-infrared II emission energies in a fluid solution at room temperature, afforded by their strong metal-ligand covalencies. Appending π-conjugated arylethynyl substituents to quinolinolate ligands resulted in minimal changes to metal-ligand bond lengths, facilitating nearly indiscriminate metal-ligand covalencies for each tris(quinolinolate) chromium(III) complex. As a result, near-infrared II emission was observed across the series, while simultaneously affording careful tuning of the 4LMCT/4(1ILCT) electronic transition energies across the visible region. Detailed analyses of ground-state electronic structures and excited-state electronic transitions revealed that PL arises from a ligand-field, 2E excited state, admixed with charge-transfer character. This assignment was supported by solvent-dependent near-infrared emission energies and long excited-state lifetimes, consistent with spin-forbidden relaxation (ranging from 115 ± 5 to 123 ± 1 ns across the series). Utilizing spectroscopic data, ligand-field parameters were quantified, further rationalizing the role of strong metal-ligand covalency in enabling near-infrared II emission. Supported by these results, this study presents a new class of ligands that enable room-temperature near-infrared II emission in chromium(III)-based spin-flip excited states.

The D1 dopamine receptor (D1R) ligand BMS Compound A is a positive allosteric modulator (PAM) of D1R signaling. BMS Compound A binds specifically to one of three known allosteric sites on the D1R and is a useful chemical tool for the investigation of D1R pharmacology. Despite the utility of BMS Compound A, even small quantities of the molecule are not commercially available. Here we report two variations on a convergent synthetic approach to construct this tool molecule. The first route is a four-step sequence to afford the four stereoisomers of BMS Compound A. The second route leverages a diastereoselective cyclopropanation on a chiral ketal substrate. Carrying this single enantiomer intermediate through our previous convergent reaction sequence affords two separable diastereomers of BMS Compound A-each a single enantiomer. We characterized the four stereoisomers of BMS Compound A, assigned their absolute configuration, specific rotation, 13C nuclear magnetic resonance (NMR) spectra. We further evaluated their ability to activate D2R signaling or potentiate dopamine-mediated D1R signaling. We found the greatest difference in D1R PAM activity for syn vs. anti configurations and less drastic differences between enantiomers. We also determined that the four stereoisomers exhibited agonist activity at the D2R.

[This corrects the article DOI: 10.1039/D4SC05396F.].

Metal formyl complexes are critical intermediates in the reduction of CO to valuable products such as methanol and higher alcohols/hydrocarbons, yet examples of formyl generation via the catalytic hydrogenation of transition metal carbonyl complexes under mild conditions are lacking. The catalytic hydrogenation of a ruthenium carbonyl complex with H2 to produce a formyl complex is reported here. Two classes of hydrogenation catalysts were compared: bis(diphosphine)-ligated complexes that proceed via termolecular H2 splitting with an external base and pincer-ligated complexes that proceed via an H2 splitting mechanism involving metal-ligand cooperativity. The hydride transfer and H2 splitting steps were evaluated for both classes of catalysts, revealing advantages for catalysts that utilize metal-ligand cooperativity and elucidating conditions to promote formyl generation. Only the pincer-ligated Ir and Ru complexes capable of reacting via pathways involving metal-ligand cooperativity were suitable for catalysis. Using 1-10 mol % of the catalysts (PNP)Ir(H)2 and (HPNP)Ru(H)2(CO) (PNP = ( i Pr2PC2H4)2N-), which use metal-ligand cooperation to activate H2, up to 10 turnovers or up to 71% yield were achieved for the conversion of [Ru(bpy)2(CO)2]2+ (bpy = 2,2 '-bipyridine) to the formyl complex [Ru(bpy)2(CO)(CHO)]+. The Lewis acid B(C6F5)3 was required as an additive to achieve high yields of the formyl complex using (HPNP)Ru(H)2(CO) as a catalyst. The catalytic route avoids the use of expensive stoichiometric reagents, such as borohydride, instead generating metal formyls that are key intermediates in CO reduction schemes with H2 gas.

Metal nitride complexes are key intermediates in ammonia synthesis. Although water would be an ideal source of protons, the synthesis of ammonia from well-defined nitride complexes in water is underexplored. The aqueous reactivity of rhenium nitride complexes supported by tridentate and tetradentate phosphinite-based ligands is examined here. The tetradentate ligand 2,9-bis(diisopropylphosphinitomethyl)-1,10-phenanthroline (6POphenOP) was found to support a nitride complex that is water-soluble and resists ligand hydrolysis from pH 2-13. Structural analysis enabled by hanging-drop crystallization was combined with NMR spectroscopic analysis to elucidate how the rhenium coordination sphere changes when dissolved in aqueous buffer and to identify networks of water molecules in the solid state, including an example of water hydrogen bonding with the nitride. Addition of SmI2 to buffered water solutions of (6POphenOP)Re nitride complexes produced aqueous ammonium. Buffer concentration influences ammonium yields, an effect that is proposed to arise from coordination of buffer to Re and/or Sm, providing insight into the selection of appropriate conditions for aqueous N2 fixation.

Cation-switchable catalysis is a promising strategy for modulating reaction rates or selectivity using external stimuli, with potential applications including polymer synthesis and tandem catalysis. This study establishes a deeper understanding of the mechanistic origins of cation rate promotion in systems where a hemilabile crown ether affixed to the catalyst serves as a cation receptor site, which in turn enables reactivity extensions of this platform to introduce cation-switchable alkene hydrogenation. The mechanistic study hinges on the synthesis of two previously unreported iridium(iii) complexes, bearing either bis(2-methoxyethyl)amine or diethylamine groups, which could be compared in catalysis with and without cation promoters. Only the catalyst containing a crown ether exhibits switchable catalysis, providing support for a "substrate gating" mechanism in which cations tune the hemilability of the crown ether donors to enable alkene binding. The generality of the substrate gating mechanism is established through the extension of catalytic reactivity to cation-switchable alkene hydrogenation. The crown-ether-containing catalyst was found to be an effective hydrogenation catalyst capable of reducing internal and trisubstituted olefins. The isomerization and hydrogenation reactions occur at similar rates in some cases, and kinetic and labeling studies provide insight into the implications of competitive reactivity.

We present the first approach to controlled metal chelation of peptide backbones, where the anchoring site is an aza-amino acid nitrogen and the directionality of chelation events is dictated by the acidity of neighboring NHs. Selective backbone chelation precludes the need for metal-binding side chains and/or free N- or C-termini in peptides. We show that the presence and location of an aza-amino acid impact complex formation and report the first X-ray crystal structures of azapeptides bound to palladium and nickel. Evidence of atropisomerism in metallo-azapeptides is also presented.

Cobalt polypyridyl complexes stand out as efficient catalysts for electrochemical proton reduction, but investigations into their operating mechanisms, with broad-reaching implications in catalyst design, have been limited. Herein, we investigate the catalytic activity of a cobalt(II) polypyridyl complex bearing a pendant pyridyl base with a series of organic acids spanning 20 pKa units in acetonitrile. Structural analysis, as well as electrochemical studies, reveals that the Co(III) hydride intermediate is formed through reduction of the Co(II) catalyst followed by direct metal protonation in the initial EC step despite the presence of the pendant base, which is commonly thought of as a more kinetically accessible protonation site. Protonation of the pendant base occurs after the Co(III) hydride intermediate is further reduced in the overall ECEC pathway. Additionally, when the acid used is sufficiently strong, the Co(II) catalyst can be protonated, and the Co(III) hydride can react directly with acid to release H2. With thorough mechanistic understanding, the appropriate electroanalytical methods were identified to extract rate constants for the elementary steps over a range of conditions. Thermodynamic square schemes relating catalytic intermediates proposed in the three electrocatalytic HER mechanisms were constructed. These findings reveal a full description of the HER electrocatalysis mediated by this molecular system and provide insights into strategies to improve synthetic fuel-forming catalysts operative through metal hydride intermediates.

Using a diverse array of thermally robust phosphine enediyne ligands (dxpeb, X = Ph, Ph-pOCH3, Ph-pCF3, Ph-m2CH3, Ph-m2CF3, iPr, Cy, and tBu) a novel suite of cisplatin-like Pt(ii) metalloenediynes (3, Pt(dxpeb)Cl2) has been synthesized and represents unique electronic perturbations on thermal Bergman cyclization kinetics. Complexes 3e (Ph-m2CF3) and 3f (iPr) are the first of this structure type to be crystallographically characterized with inter alkyne termini distances (3e: 3.13 & Aring;; 3f: 3.10 & Aring;) at the lower end of the widely accepted critical distance range within which enediynes should demonstrate spontaneous ambient temperature cyclization. Despite different electronic profiles, these metalloenediynes adopt a rigid, uniform structure suggesting complexes of the form Pt(dxpeb)Cl2 have orthogonalized geometric and electronic contributions to thermal Bergman cyclization. Kinetic activation parameters determined using 31P NMR spectroscopy highlight the dramatic reactivity and thermal tunability of these complexes. At room temperature, the half-life (t1/2) of cyclization spans a range of similar to 35 hours and for the aryl phosphine derivatives, cycloaromatization rates are 10-30 times faster for complexes with electron donating substituents (3b: Ph-pOCH3; 3d: Ph-m2CH3) compared to those with electron withdrawing substituents (3c: Ph-pCF3; 3e: Ph-m2CF3). Computational interrogation of the aryl phosphine metalloenediynes 3a-3e reveals that the origin of this precise electronic control derives from electronic withdrawing group-mediated alkyne carbon polarization that amplifies coulombic repulsion increasing the cyclization barrier height. Additionally, mixing between the in-plane pi-orbitals and the phosphine aryl ring system is pronounced for complexes with electron donating substituents which stabilizes the developing C-C bond and lowers the activation barrier. This pi-orbital mixing is negligible however, for complexes with electron withdrawing substituents due to an energetic mismatch of the orbital systems. Overall, this work demonstrates that for geometrically rigid frameworks, even remote enediyne functionalization can have pronounced effects on activation barrier.

The importance of data curation has been recognized in multiple areas of research; however, the discussion of this important issue is only beginning to emerge in materials science. In this Perspective, we highlight the benefits of using the standardized data curation protocols in materials science and discuss current gaps in accurate and reproducible data reporting using case studies drawn from high-impact materials science papers and well-known databases such as the Crystallography Open Database (COD) and the Cambridge Structural Database (CSD). We argue that both experimental and computational materials scientists need to embrace a culture of rigorous data curation as part of modern research data management. We propose a sample data curation pipeline for materials chemistry and illustrate its use by creating two new materials chemistry databases. We hope that this perspective will serve to catalyze further discussion and promote the continuous development of rigorous data curation practices within the materials science research community. We posit that adherence to best practices of data curation will promote and enhance the reliability, reproducibility, and integrity of materials research and enable the development of reliable AI and machine learning models that critically depend on the use of quality data.

Hydrogen bonding networks are ubiquitous in biological systems and play a key role in controlling the conformational dynamics and allosteric interactions of enzymes. Yet in small organometallic catalysts, hydrogen bonding rarely controls ligand binding to the metal center. In this work, a hydrogen bonding network within a well-defined organometallic catalyst works in concert with cation-dipole interactions to gate substrate access to the active site. An ammine ligand acts as one cofactor, templating a hydrogen bonding network within a pendent crown ether in the secondary coordination sphere, an interaction which prevents the binding of nitriles to the nickel center. Sodium ions are a second cofactor, disrupting hydrogen bonding to enable ligand substitution reactions and substrate binding. Thermodynamic analyses provide insight into the energetic requirements of the different supramolecular interactions enabling substrate gating. Switchable ligand substitution and switchable hydroamination catalysis illustrate the dual cofactor approach.

We report on the latest advancements in Microcrystal Electron Diffraction (3D ED/MicroED), as discussed during a symposium at the National Center for CryoEM Access and Training housed at the New York Structural Biology Center. This snapshot describes cutting-edge developments in various facets of the field and identifies potential avenues for continued progress. Key sections discuss instrumentation access, research applications for small molecules and biomacromolecules, data collection hardware and software, data reduction software, and finally reporting and validation. 3D ED/MicroED is still early in its wide adoption by the structural science community with ample opportunities for expansion, growth, and innovation.

Bipyridine ligands have been extensively employed in nickel catalysis, with ligand modifications focused on steric or electronic tuning. In this work, we explore modifications designed to modulate the coordination mode using a 2,2'-bipyridine derivative with an appended aza-crown ether macrocycle capable of flexidentate binding to nickel. A series of complexes varying in charge from neutral to dicationic demonstrates the flexibility of the macrocycle, with bipyridine-aza-crown ether denticity changing from к4 to к6 upon sequential abstraction of chloride ligands. The changes in binding mode can be reversed by addition of chloride ion. Comparisons between the macrocycle-containing ligand and an analogous ligand with a non-macrocyclic diethylamine donor provide insight into the role of the crown ether, including in electrochemical reductions probed via cyclic voltammetry.

Reduction of the cobalt(II) chloride complex, Ph2B((t)BuIm)(2)Co(THF)Cl (1) in the presence of (BuN)-Bu-t equivalent to C affords the diamagnetic, square planar cobalt(I) complex Ph2B((t)BuIm)(2)Co(C equivalent to(NBu)-Bu-t)(2) (2). This is a rare example of a 16-electron cobalt(I) complex that is structurally related to square planar noble metal complexes. Accordingly, the electronic structure of 2, as calculated by DFT, reveals that the HOMO is largely d(z)(2) in character. Complex 2 is readily oxidized to its cobalt(II) congener [Ph2B((t)BuIm)(2)Co(C=(NBu)-Bu-t)(2)]BPh4 (3-BPh4), whose EPR spectral parameters are characteristic of low-spin d(7) with an unpaired electron in an orbital of d(z)(2) parentage. This is also consistent with the results of DFT calculations. Despite its 16-electron configuration and the d(z)(2) parentage of the HOMO, the only tractable reactions of 2 involve one electron oxidation to afford 3.

A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr)6]PF6 (CNAr = 2,6-dimethylphenylisocyanide, λmax = 300 nm). Upon oxidation to [Re(CNAr)6](PF6)2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λmax = 650 nm). TD-DFT was used to identify the participating ligand-based orbitals in the LMCT transition, revealing that the majority of the donor orbital is based on the aryl ring (85%) as opposed to the CN bond (14%). For both [Re(CNAr)6]+ and [Re(CNAr)6]2+, structural characterization by X-ray diffraction reveals deviations from Oh geometry at the central Re ion, with larger reduction in symmetry observed for Re(II). For [Re(CNAr)6]+, these structural changes lead to a broadening of the strong ν(C≡N) stretch (2065 cm-1), as the degeneracy of the T1u IR-active mode is broken. Furthermore, a shoulder is observed for this ν(C≡N) stretch, resulting from deviation of the C-N-Ar bond from linearity. By contrast, [Re(CNAr)6]2+ has two weak bands in the ν(C≡N) region (2065 and 2121 cm-1). DFT calculations indicate that reduction of symmetry at the central rhenium ion manifests in the decrease in intensity and the observed split of the ν(C≡N) band. Stability of both complexes are limited by light-induced decomposition where Re(I) dissociates a isocyanide ligand upon irradiation and Re(II) absorbance decays under ambient light. These data provide new insights to the electronic structure of [Re(CNAr)6]2+, enhancing our understanding of LMCT excited states and the versatility of isocyanide ligands.

Seven-coordinate rhenium oxo complexes supported by a tetradentate bipyridine carboxamide/carboxamidate ligand are reported. The neutral dicarboxamide H2Phbpy-da ligand initially coordinates in an L4 (ONNO) fashion to an octahedral rhenium oxo precursor, yielding a seven-coordinate rhenium oxo complex. Subsequent deprotonation generates a new oxo complex featuring the dianionic (L2X2) carboxamidate (NNNN) form of the ligand. Computational studies provide insight into the relative stability of possible linkage isomers upon deprotonation. Structural studies and molecular orbital theory are employed to rationalize the relative isomer stability and provide insight into the rhenium-oxo bond order.

Oxidative addition is an essential elementary reaction in organometallic chemistry and catalysis. While a diverse array of oxidative addition reactions has been reported to date, examples of P-O bond activation are surprisingly rare. Herein, we report the ligand-templated oxidative addition of a phosphinite P-O bond in the diphosphinito aniline compound HN(2-OPiPr2-3,5-tBu-C6H2)2 [H(P2ONO)] at Ni0 to form (PONO)Ni(HPiPr2) after proton rearrangement. Notably, the P-O cleavage occurs selectively over an amine N-H bond activation. Additionally, the ligand cannibalization is reversible, as addition of XPR2 (X = Cl, Br; R = iPr, Cy) to (PONO)Ni(HPiPr2) readily produces either symmetric or unsymmetric (P2ONO)NiX species and free HPiPr2. Finally, the mechanisms of both the initial P-O bond cleavage and its subsequent reconstruction are investigated to provide further insight into how to target P-O bond activation.

When irradiated with blue light in the presence of a Lewis base (L), [CpW(CO)3]2 undergoes metal-metal bond cleavage followed by a disproportionation reaction to form [CpW(CO)3L]+ and [CpW(CO)3]-. Here, we show that in the presence of pyridinium tetrafluoroborate, [CpW(CO)3]- reacts further to form a metal hydride complex CpW(CO)3H. The rection was monitored through in situ photo 1H NMR spectroscopy experiments and the mechanism of light-driven hydride formation was investigated by determining quantum yields of formation. Quantum yields of formation of CpW(CO)3H correlate with I-1/2 (I = photon flux on our sample tube), indicating that the net disproportionation of [CpW(CO)3]2 to form the hydride precursor [CpW(CO)3]- occurs primarily through a radical chain mechanism.

Functionalized molecular design has been employed to selectively bind and signal the presence of perchlorate ions in aqueous-type solutions. Pyrene-benzimidazole-based bipodal molecular scaffolds were studied to understand their response to perchlorate detection in acidic buffered media. The molecules showed a significant, selective "turn-on" response towards perchlorate ion, even in the presence of heavy metal ions. The structure and characteristics of the molecule were determined through 1H nuclear magnetic resonance (NMR), 13C NMR, NMR titration studies, single-crystal X-ray crystallography, computational modeling and predictions, absorption spectroscopy, and fluorescence spectroscopy. Furthermore, the molecule was found to be sensitive to changes in pH, making it useful for detecting changes in a local environment.