The bond dissociation free energy (BDFE) of the element-hydrogen bonds of protic substrates have been found to decrease upon metal coordination. Herein, an early/late heterobimetallic complex is used to examine the impact on the BDFEN-H when the substrate binding site and the redox-active site are two different metals that are spatially separated. A tris(phosphinoamide) framework is used to link a d0 ZrIV center with an accessible substrate binding site to a coordinatively saturated redox-active Co center, which serves as an appended electron reservoir. A series of aniline, amido, and imido Zr/Co model compounds were synthesized starting from the ZrIV/Co-I aniline adduct PhH2N-Zr(MesNP i Pr2)3CoCN t Bu (2). 2,4,6-tris-tert-butylphenoxyl radical ((Bu3ArO center dot)-Bu-t) was used to abstract one or two H atoms and produce the amido and imido complexes PhHN-Zr(MesNP i Pr-2)(3)CoCN t Bu (3) and PhN equivalent to Zr(MesNP i Pr-2)(3)CoCN t Bu (4), respectively. Using open-circuit potential measurements, the BDFEN-H within 2 and 3 were determined to be 37 kcal/mol (2) and 55 kcal/mol (3). Cyclic voltammetry measurements were conducted to determine the CoI/0 and Co0/-I redox potentials. The pK as were then estimated using the Bordwell equation to provide further insight into the thermochemical aspects of the observed proton coupled electron transfer (PCET) reactions.

Square pyramidal cobalt-(III)-alkyl complexes ((PNCH2CH2NP)-CoIII-R (1-R) and (PNCHCHNP)-CoIII-R (2-R), R = CH3, n Bu, or Bn) were synthesized via oxidative addition of alkyl halides to [(PNCH2CH2NP)-CoI]-[Na-(THF)n] (1-Na) or [(PNCHCHNP)-CoI]-[Na-(THF)n] (2-Na). The redox-active ligand of (PNCHCHNP)-CoII (2) provides access to the one-electron oxidized species [(PNCHCHNP)-Co-(THF)]-[PF6] (2-PF 6 ), allowing synthesis of 2-R compounds through alkylation of 2-PF 6 with carbanions. Radical trapping experiments with TEMPO and analysis of the thermal decomposition products suggest cobalt-carbon bond homolysis as the primary decomposition pathway for these molecules, although the homolysis products are dependent on the equatorial ligand framework and the identity of the alkyl substituent. The stability of the cobalt-carbon bond was evaluated through kinetic and computational methods. We suggest a more distorted equatorial ligand destabilizes the cobalt-carbon bond, while the electron-rich phosphine substituents stabilize the cobalt-carbon bond compared to previously reported cobalamin model compounds. Catalytic investigations into radical Heck-type cross-coupling found (PNCH2CH2NP)-CoII (1) to be a more active catalyst than 2, although unwanted side products resulted in reduced yields for the desired cross-coupled product.

Biological methylation is a fundamental regulatory process in gene expression, biomolecule modification, and cell repair. Many enzymatic C-H methylation reactions proceed through sequential one-electron steps mediated by distinct redox cofactors, such as Fe4S4 clusters and methylcobalamin. However, structurally faithful model complexes of these cofactors have thus far been unable to replicate the characteristic C-H methylation reactivity. Herein, we report the first isolable copper(II/III)-methyl complexes that undergo C─H methylation via a radical mechanism analogous to SAM enzymes. The copper(II)-methyl complex undergoes reversible one-electron oxidation to a formal copper(III)-methyl species, which serves as a methyl radical reservoir capable of both generating and capturing carbon radicals. The CuIII-CH3 complex mediates C-H methylation through hydrogen atom transfer (HAT) and methyl radical transfer, affording methylated products from substrates similar to those targeted by radical SAM methyltransferases. By merging the characteristic HAT and radical rebound reactivity within a single organometallic center, this copper(II/III)-methyl species provides a synthetic platform that mirrors key mechanistic features of enzymatic C-H methylation.

Singly reduced intermediates have recently been implicated as photoactive intermediates in a number of important reactions; however, their photophysical properties remain poorly understood. A series of dirhodium(II,II) complexes, cis-[Rh2(p-R-Form)2(bncn)2]2+ (bncn = benzo[c]cinnoline; p-R-Form = N,N'-di-p-R-phenylformamidinate), where R = -OCH3 (1), -CH3 (2), -H (3), -F (4), -Cl (5), and -CF3 (6), and their respective radical anions were prepared and their excited state properties were investigated. Complex 3 acted as a single-molecule photocatalyst for H2 production with red light. Substitution on the formamidinate ligands in 1-6 affected the energy of the Rh2(δ*)/Form(π,nb) highest occupied molecular orbital (HOMO), consistent with the metal/ligand-to-ligand charge transfer (1ML-LCT) absorption maxima and the 3ML-LCT excited state lifetime, ranging from 1.6 ns in 1 to 54 ns in 6 in CH3CN. The highest turnover number for photocatalytic H2 evolution was observed for 3, and the lowest values were for 1 and 6. The radical anion, [Rh2]-, formed during photocatalysis, was shown to absorb a photon and undergo a second reduction. The lifetimes of the doublet excited states of [3]- and [6]- were 0.49 and 0.24 ns, respectively. Calculations showed that the lowest energy excited state in [3]- was 2ML-LCT, whereas that in [6]- was a bncn- → Rh2(σ*) ligand-to-metal charge transfer (2LMCT) state. The 2LMCT state stabilized across the series from [1]- to [6]-, pointing at its role in modulating the photophysical properties. This work highlights the importance of the reductive quenching of [Rh2]- and the generation of the doubly reduced species to effectively catalyze hydrogen evolution.

An example of a molecular Ti2+/Ti4+ complex is reported, whereby the metal centers are bridged by the redox-active tetrakis(imino)pyracene ligand (TIP). Spectroscopic, electrochemical, electronic structure calculations, and magnetic analyses were used to evaluate the degree and mechanism of electronic communication and magnetic exchange between the two metal centers. It was found that the complex is best described as a borderline Class II/III mixed valence complex, whereby the population of thermally accessible, close-lying excited states gives rise to extremely strong antiferromagnetic coupling (|J|> 250 cm-1 at ∼11.3 Å).

The substituent on the central cyclic diamido phosphorus fragment of a tridentate bis-(phosphine)-pincer ligand framework (PPRP) was varied systematically (R = NEt2, N i Pr2, OEt, O i Pr, OCH2CF3, OCH-(CF3)2, (-)-menthoxide, Me, CF3) to tune the steric profiles as well as the σ-donor and π-acceptor properties of the pincer ligands. A series of (PPRP)-CoI2 compounds were synthesized and characterized using 1H NMR and EPR spectroscopy and single crystal X-ray diffraction. The differences in electron density at the cobalt centers imparted by the varying ligand substituents were evaluated using cyclic voltammetry of the (PPRP)-CoI2 compounds and calculated ν-(CO) stretching frequencies of hypothetical (PPRP)-Co-(CO)-(H) compounds. In an attempt to deconvolute variations in σ-donor and π-acceptor properties, the 1 J P-Se coupling constants of a series of phosphine selenide compounds were used to evaluate basicity as a direct measure of σ-donor strength. The catalytic activities of the (PPRP)-CoI2 compounds for the hydroboration of styrene and α-methylstyrene at room temperature were evaluated quantitatively through kinetic studies to determine the rate constant (k obs ) for each precatalyst. In general, it was found that compounds featuring a less electron-rich cobalt center were more active alkene hydroboration catalysts, with steric properties playing a more prominent role with the bulkier 1,1'-disubstituted alkene substrate.

Correction for ‘Molecular bowls for inclusion complexation of toxic anticancer drug methotrexate’ by Pratik Karmakar et al. , Chem. Sci. , 2024, 15 , 10155–10163, https://doi.org/10.1039/D3SC05627A.

Maintaining stable drug concentrations in the bloodstream is a challenge for injectable hydrophobic progestin contraceptives. This work investigates porous silicon dioxide (pSiO2) microparticles as a delivery vehicle for progestins via melt-infiltration of drugs into the mesopores. The pSiO2 is prepared through electrochemical anodization of single-crystalline silicon followed by thermal oxidation, yielding vertically oriented pores (≈50 nm diameter) with porosity varied (between 35-75%) to optimize drug loading and release. Among the progestins tested, etonogestrel and levonorgestrel (LNG) decompose near their melting points, preventing melt infiltration. However, addition of 20% cholesterol by mass suppresses the melting point of LNG sufficiently to enable loading without degradation. Mass loadings exceeding 50% (drug: drug + carrier) are achieved for segesterone acetate (SEG) and LNG, retaining drug crystallinity as confirmed by X-ray diffraction. In vitro, both SEG and LNG-loaded pSiO2 display sustained drug release for up to 3 months, with reduced burst release, more constant steady-state concentrations, and a substantially reduced tail compared to pure LNG or SEG, or SEG loaded into pSiO2 from a chloroform solution. In a pilot in vivo study, SEG-loaded pSiO2 microparticles are well tolerated in 20-week-old female rats over a 25-week period, with no signs of toxicity.

Oxidative addition is a reactivity pathway that is not generally invoked for phosphorus(III) compounds; however, recent reports highlight the transition metal-like behavior of some tertiary organophosphines. In this study, kinetic, mechanistic, and quantum chemical studies were integrated to demonstrate that a formal oxidative addition, whereby phosphorus(III) centers activate C-F bonds or weakened C-H bonds, is general reactivity class for these compounds. It was found that tertiary organophosphines react with polyfluorinated compounds to yield λ5-phosphanes, ylides, hydrodefluorination products, and/or homocoupled products. Moreover, it was found that the activation parameters from these reactions may serve as an alternative for gauging the cone angle and the Tolman electronic parameter of phosphine ligands. From these reactions, a new mechanism for phosphine catalyzed hydrodefluorination was developed and validated.

The Co(III) complexes, cis-[Co(ppy)2(L)]PF6, where ppy = 2-phenylpyridine and L = bpy (2,2'-bipyridine; 1), phen (1,10-phenanthroline; 2), and DAP (1,12-diazaperylene; 3), are reported and their photophysical properties were investigated to evaluate their potential as sensitizers for applications that include solar energy conversion schemes and photoredox catalysis. Calculations show that cyclometallation in the cis-[Co(ppy)2(L)]PF6 series affords strong Co(dπ)/ppy(π) orbital interactions that result in a Co/ppy(π*) highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) localized on the diimine ligand, L(π*). Complexes 1-3 exhibit relatively invariant oxidation potentials, whereas the reduction event is dependent on the identity of the diimine ligand, L, consistent with the theoretical predictions. For 3 a broad Co/ppy(π*) → L(π*) metal/ligand-to-ligand charge transfer (ML-LCT) absorption band is observed in CH3CN with a maxima at 507 nm, extending beyond 600 nm. Upon excitation of the 1ML-LCT transition, transient absorption features consistent with the population of a 3ML-LCT excited state with lifetimes, τ, of 3.0 ps, 4.6 and 42 ps for 1, 2 and 3 in CH3CN respectively are observed. Upon irradiation with 505 nm, 3 is able to reduce methyl viologen (MV2+), an electron acceptor commonly in photocatalytic schemes. To our knowledge, 3 represents the first heteroleptic molecular Co(III) complex that combines cyclometallation with a diimine ligand with lowest-lying metal-to-ligand charge transfer excited states able to undergo photoinduced charge transfer with low-energy green light. As such, the structural design of 3 represents an important step toward d6 photosensitizers based on earth abundant metals.

Alkene hydroboration provides a convenient route to generate organoborane synthons and recent efforts to develop catalysts for this and many other organic transformations have involved a shift to Earth-abundant first row transition metals. Herein, we report the synthesis of a new bench-stable Coii precatalyst, (PPCF3P)CoI2 (1), which was found to function as a highly active alkene hydroboration catalyst in the presence of an activator. The substrate scope was probed through exploring a collection of electronically and sterically distinct alkenes with a wide range of substitution patterns and functional groups. A single species is spectroscopically observed during catalysis, and activation of the Coii precatalyst with KBEt3H in the presence of styrene and in the absence of HBpin affords this species, (PPCF3P)Co(η2-styrene)H (2), which has been isolated, characterized, and demonstrated to function as an active catalyst for alkene hydroboration in the absence of additional activators. A plausible mechanism involving a CoI-hydride active species is proposed based on catalytic and stoichiometric experiments.

Metal-ammine (NH3) and metal-amide (NH2) species are common intermediates in ammonia oxidation, typically designed with weak N-H bonds to facilitate the sequential breakage of N-H bonds. Since the σ-bonding interaction between a metal center and an NH3 ligand weakens the N-H bonds, M-NH3 complexes with high N-H bond dissociation free energy (BDFE) are rare. However, we are particularly interested in M-NH3 complexes with high BDFEN-H for their potential in the conversion of C(sp3)-H bonds to C(sp3)-NH2 bonds. Herein, we report the synthesis and characterization of copper(II) and formal copper(III) NH3 and NH2 complexes derived from ammonia (NH3). The N-H bond dissociation free energy (BDFE) of the copper(II) ammine complex (LCuIINH3, L = [N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamido]2-) was measured to be 92.8(1.5) kcal/mol, the highest N-H bond strength reported for an M-NH3 species. The copper(III) amide congener (LCuIIINH2) is reactive toward C(sp3)-H functionalization, efficiently mediating the C(sp3)-H activation and C(sp3)-N bond formation, producing a range of functionalized hydrocarbon products, including nitriles, ketones, and primary amines.

[Ru(tpy)(L)(py)]2+ (tpy = 2,2;6'2″-terpyridine; py = pyridine), where L represents 1,10-phenanthroline (1; phen), 2-phenyl-1,10-phenanthroline (2; phenyl-phen), and 2-(1'-pyrenyl)-1,10-phenanthroline (3; pyrenyl-phen), were synthesized and characterized by 1D and 2D 1H NMR and through X-ray crystallography. These methods show that 2 and 3 exhibit intramolecular, interligand π-stacking between the phenyl and pyrenyl groups of the substituted phen ligands in 2 and 3, respectively, and the tpy ligand in each complex. DFT calculations show that the deviation from octahedral geometry around the metal in 2 and 3 leads to a lower energy eg-type orbital set with Ru-py(σ*) character, making the dissociative metal-centered (MC) excited state(s) more accessible from the metal-to-ligand charge transfer (MLCT) states in these complexes relative to 1. The lifetimes of the lowest energy Ru→tpy 3MLCT excited states in 2 and 3 are shorter than that of 1, consistent with the more facile 3MLCT deactivation through the 3MC state(s) in 2 and 3 as compared to 1. Complexes 2 and 3 undergo pyridine ligand exchange with a Cl- ion in CH2Cl2 with quantum yields, Φ450, of 0.024(2) and Φ450 = 0.019(3), respectively (λirr = 450 nm), representing a > 100-fold increase as compared to that of 1, Φ450 < 10-4. The increase in photoinduced ligand exchange in 2 and 3 is attributed to the greater population of the dissociative 3MC state(s) in these complexes. This represents the first demonstration that intramolecular π-stacking in Ru(II) complexes can be used to enhance ligand release upon irradiation, highlighting a new method to achieve more efficient delivery of pyridine-containing drugs for photochemotherapy.

In multimetallic compounds, N2 typically bridges late transition metals in an end-on (η1:η1) fashion while early transition metals often bind N2 side-on (η2), with the latter resulting in more significant N-N bond elongation. Herein, N2 fixation is accomplished by using a well-defined scaffold featuring a heterobimetallic combination of Zr and Co. We report a heterotetrametallic Zr2Co2 cluster in which N2 is bound side-on to the two Co centers and end-on to Zr in a μ3-η1:η2:η2 binding mode that defies established paradigms for N2 binding. The heterometallic approach is shown to facilitate catalytic reductive silylation of N2 with turnover numbers far exceeding those of reported cobalt catalysts.

Chiral organic-inorganic metal halide (OIMH) materials are gaining increasing attention as candidates for asymmetric materials due to their unique photoelectric, chiral optic, and spintronic properties. The introduction of chirality into OIMHs is usually achieved by the use of chiral organic cations, while previous studies often focus on primary ammonium cations derived from commercially available chiral amines, limiting the tunability of the OIMH materials. Herein, we report the use of Zincke reactions to synthesize chiral N-substituted pyridinium salts, namely, (R)/(S)-methylbenzylpyridinium (R/S-MBnP) chloride and the corresponding 1D chiral OIMHs, (R/S-MBnP)PbX3 (X = Cl, Br, and I). The chirality of the pyridinium salts and the corresponding OIMHs is confirmed by circular dichroism (CD) spectroscopy, and the crystal structure is revealed by single-crystal X-ray diffraction (XRD). The photoluminescence (PL) and PL decay lifetimes were measured. The stability against water is monitored by powder XRD. This study demonstrates that Zincke reactions offer high tunability of organic cations for chiral OIMH materials.

The scorpionate sandwich complex [(TpMe,Me)2Fe], where TpMe,Me is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, has long been of interest because it exhibits spin crossover in the solid state. Previous X-ray crystallographic studies revealed that the complex crystallizes on a triclinic lattice with a nearly ideal D 3d geometry. The structure is consistent with high-spin (S = 2) Fe(II) at 298 K, but becomes low-spin (S = 0) at low temperature. We report an alternative trigonal polymorph, in which the complex adopts a distorted conformation of S 6 symmetry with a large pyrazolyl ring torsion. This distortion inhibits spin crossover. An X-ray crystal structure performed at 298 K confirms a high-spin state, and this is retained in a second structure at 100 K. Meta-analysis of known [(TpMe,Me)2M] complexes indicates that every example crystallized in either a distorted conformation on an isomorphous trigonal lattice, or an undistorted conformation on a triclinic lattice, depending on the size of the central metal ion. High-spin Fe(II) falls exactly at the dividing line; structures with longer M-N bonds are trigonal, and structures with shorter M-N bonds are triclinic, including low-spin Fe(II). The ligand conformation is coupled to lattice packing, and to spin crossover. Different intermolecular contacts between the lattices are elucidated.

The realization and discovery of quantum spin liquid (QSL) candidate materials are crucial for exploring exotic quantum phenomena and applications associated with QSLs. Most existing metal-organic two-dimensional (2D) quantum spin liquid candidates have structures with spins arranged on the triangular or kagome lattices, whereas honeycomb-structured metal-organic compounds with QSL characteristics are rare. Here, we report the use of 2,5-dihydroxy-1,4-benzoquinone (X2dhbq, X = Cl, Br, H) as the linkers to construct cobalt(II) honeycomb lattices (NEt4)2[Co2(X2dhbq)3] as promising Kitaev-type QSL candidate materials. The high-spin d7 Co2+ has pseudospin-1/2 ground-state doublets, and benzoquinone-based linkers not only provide two separate superexchange pathways that create bond-dependent frustrated interactions but also allow for chemical tunability to mediate magnetic coupling. Our magnetization data show antiferromagnetic interactions between neighboring metal centers with Weiss constants from -5.1 to -8.5 K depending on the X functional group in X2dhbq linkers (X = Cl, Br, H). No magnetic transition or spin freezing could be observed down to 2 K. Low-temperature susceptibility (down to 0.3 K) and specific heat (down to 0.055 K) of (NEt4)2[Co2(H2dhbq)3] were further analyzed. Heat capacity measurements confirmed no long-range order down to 0.055 K, evidenced by the broad peak instead of the λ-like anomaly. Our results indicate that these 2D cobalt benzoquinone frameworks are promising Kitaev QSL candidates with chemical tunability through ligands that can vary the magnetic coupling and frustration.

A series of heteroleptic Rh-2(II,II) complexes, cis-[Rh-2(mu-DPhF)(2)(mu-bncn)(2)](2+) (1; bncn = benzo[c]cinnoline), cis-[Rh-2(mu-DPhF)(mu-OAc)(mu-bncn)(2)](2+) (2), and cis-[Rh-2(mu-OAc)(2)(mu-bncn)(2)](2+) (3), is presented, and the excited state and redox properties of each complex was characterized for the photo- and electrocatalytic production of H-2. The oxidation potentials shift anodically from 1 to 3, consistent with a highest occupied molecular orbital (HOMO) with significant metal-ligand mixing, Rh-2(delta*)/DPhF(pi/nb). In contrast, modest differences in the first two bncn-localized reversible reduction potentials were observed in 1 - 3. The lowest energy metal/ligand-to-ligand charge transfer ((ML)-M-1-LCT) transition, Rh-2(delta*)/DPhF(pi/nb) -> bncn(pi*), shifts from 633 nm in 1 to 553 nm in 2, and the metal-to-ligand charge transfer ((MLCT)-M-1) Rh-2(pi*) -> bncn(pi*) absorption in 3 appears at 462 nm in CH3CN. Although the (ML)-M-3-LCT excited state of 2 is shorter lived than that of 1, 2.7 ns as compared to 19 ns, respectively, photocatalytic hydrogen generation is observed for the former upon 595 nm irradiation in the presence of 0.1 M TsOH (p-tolylsulfonic acid) and 0.1 M BNAH (1-benzyl-1,4-dihydronicotinamide). The temperature dependence of the (ML)-M-3-LCT lifetimes of 1 and 2 shows the presence of a thermally accessible deactivating state. In addition, the singly reduced intermediate, [2](-), is photoactive and able to generate hydrogen in the presence of TsOH. Importantly, the electrocatalytic currents generated by equimolar concentrations of 1 - 3 in CH3CN are nearly identical, consistent with a mechanism of catalysis that is localized on the bncn ligand and does not require a Rh-H hydride intermediate. This finding can be used to develop earth-abundant first-row transition metal complexes for photo- and electrocatalytic H-2 production.

We establish the synthesis, physical properties, and highly-frustrated magnetism of Mn2In2Se5 and Mn2Ga2S5 van der Waals crystals.

Conjugated organic cations are intriguing for organic-inorganic halide perovskites due to their direct participation in the optoelectronic properties of hybrid materials. In conjugated cations, the dihedral angle, or torsion angle, between adjacent aromatic rings is a critical secondary structural element. This angle influences not only the shape of the cations but also the overlap between the π-orbitals on adjacent rings, thereby affecting their electronic properties. Understanding how variations in the dihedral angle impact the structure and properties of hybrid organic-inorganic metal halides (HOIMHs) is fundamentally important. In this study, we utilized 2,2'-dimethyl bipyridinium as the organic cation, reacting it with PbI₂ to form hybrid lead iodides. Remarkably, variations in the dihedral angle between the two pyridinium rings resulted in the formation of two distinct crystal structures with different band gaps. Our findings demonstrate that manipulating the dihedral angle offers a novel approach to controlling the structures and properties of hybrid metal halides with conjugated cations.