Metal–organic frameworks (MOFs) are an emerging class of microporous materials that have potential applications in a wide range of areas. As a subclass of MOFs, ionic MOFs, especially charge-separated MOFs, have been relatively less studied but possess unique features including strong host–guest interactions from the exposed charged centers. We report the synthesis and single-crystal structural characterization of five new charge-separated MOFs (UNM 1–5) based on two tetrapodal borate ligands: tetrakis(4-(4-pyridineethynyl)-2,3,5,6-tetrafluorophenyl)borate (T1) and tetrakis(4-(4-pyridyl)-2,3,5,6-tetrafluorophenyl)borate (T2) having rigid arms of different lengths and pyridine groups at the end of each arm. Coordination of these tetrapods with Cu(I), Cu(II), and Ag(I) ions under specific conditions led to a series of new charge-separated MOFs in single crystalline forms. UNM-1 and UNM-2/UNM-3, which crystallize respectively in tetragonal I4̅ space group and monoclinic C2/c space group, are derived from Cu(CH₃CN)₄BF₄ and Cu(NO₃)₂ upon coordination with T1. On the other hand, coordination of T2 with Cu(CH₃CN)₄BF₄ and AgBF₄ respectively yielded UNM-4 and UNM-5 in the monoclinic I2/a space group. All these MOFs possess several degrees of interpenetration that are correlated with the arm lengths of ligands. Noticeably, UNM-1 is 4-fold interpenetrated, leading to the highest stability among all five MOFs, while still displaying an impressive Brunauer–Emmett–Teller (BET) surface area (SA_BET) of ca. 621 m²/g. Our findings highlight the versatility of tetrapodal borate ligands in engineering charge-separated MOFs with diverse structures and controlled functionality.

The hydrogen isotopes deuterium (D) and tritium (T) have become essential tools in chemistry, biology and medicine¹. Beyond their widespread use in spectroscopy, mass spectrometry and mechanistic and pharmacokinetic studies, there has been considerable interest in incorporating deuterium into drug molecules¹. Deutetrabenazine, a deuterated drug that is promising for the treatment of Huntington’s disease², was recently approved by the United States’ Food and Drug Administration. The deuterium kinetic isotope effect, which compares the rate of a chemical reaction for a compound with that for its deuterated counterpart, can be substantial^1,3,4. The strategic replacement of hydrogen with deuterium can affect both the rate of metabolism and the distribution of metabolites for a compound⁵, improving the efficacy and safety of a drug. The pharmacokinetics of a deuterated compound depends on the location(s) of deuterium. Although methods are available for deuterium incorporation at both early and late stages of the synthesis of a drug^6,7, these processes are often unselective and the stereoisotopic purity can be difficult to measure^7,8. Here we describe the preparation of stereoselectively deuterated building blocks for pharmaceutical research. As a proof of concept, we demonstrate a four-step conversion of benzene to cyclohexene with varying degrees of deuterium incorporation, via binding to a tungsten complex. Using different combinations of deuterated and proteated acid and hydride reagents, the deuterated positions on the cyclohexene ring can be controlled precisely. In total, 52 unique stereoisotopomers of cyclohexene are available, in the form of ten different isotopologues. This concept can be extended to prepare discrete stereoisotopomers of functionalized cyclohexenes. Such systematic methods for the preparation of pharmacologically active compounds as discrete stereoisotopomers could improve the pharmacological and toxicological properties of drugs and provide mechanistic information related to their distribution and metabolism in the body.

Key steps in the functionalization of an unactivated arene often involve its dihaptocoordination by a transition metal followed by insertion into the C–H bond. However, rarely are the η²-arene and aryl hydride species in measurable equilibrium. In this study, the benzene/phenyl hydride equilibrium is explored for the {WTp(NO)(PBu₃)} (Bu = n-butyl; Tp = trispyrazoylborate) system as a function of temperature, solvent, ancillary ligand, and arene substituent. Both face-flip and ring-walk isomerizations are identified through spin-saturation exchange measurements, which both appear to operate through scission of a C–H bond. The effect of either an electron-donating or electron-withdrawing substituent is to increase the stability of both arene and aryl hydride isomers. Crystal structures, electrochemical measurements, and extensive NMR data further support these findings. Static density functional theory calculations of the benzene-to-phenyl hydride landscape suggest a single linear sequence for this transformation involving a sigma complex and oxidative cleavage transition state. Static DFT calculations also identified an η²-coordinated benzene complex in which the arene is held more loosely than in the ground state, primarily through dispersion forces. Although a single reaction pathway was identified by static calculations, quasiclassical direct dynamics simulations identified a network of several reaction pathways connecting the η²-benzene and phenyl hydride isomers, due to the relatively flat energy landscape.

We describe a novel method to synthesize 2,5-dialkyl-4,6,7-tricyanoindole derivatives from a base-catalyzed reaction of 1,3-diketones with fumaronitrile. The reaction proceeds by the condensation of two molecules of fumaronitrile and one molecule of 1,3-diketone in a remarkable process that involves the cleavage of one C(sp³)–C(sp²) bond in 1,3-diketones and the formation of one carbon–nitrogen bond and four carbon–carbon bonds to construct both the aryl and pyrrole rings of the indole in one step.

Previously, we reported an iron(III) complex with 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(2,4-ditertbutyl-phenol) as a ligand (Fe(^tbudhbpy)Cl, 1) as catalytically competent for the electrochemical reduction of CO₂ to formate (Faradaic efficiency FE_HCO2^– = 68 ± 4%). In mechanistic experiments, an essential component was found to be a pre-equilibrium reaction involving the association of the proton donor with the catalyst, which preceded proton transfer to the Fe-bound O atoms upon reduction of the Fe center. Here, we report the synthesis, structural characterization, and reactivity of two iron(III) compounds with 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(2-methoxy-4-methylphenol) (^mecrebpy[H]₂, Fe(^mecrebpy)Cl, 2) and 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(4-(tert-butyl)benzene-1,2-diol) (^tbucatbpy[H]₄, Fe(^tbucatbpy), 3) as ligands, where pendent −OMe and −OH groups are poised to modify the protonation reaction involving the Fe-bound O atoms. Differences in selectivity and activity for the electrocatalytic reduction of carbon dioxide (CO₂) to formate (HCO₂^–) between complexes 1–3 were assessed via cyclic voltammetry and controlled potential electrolysis (CPE) experiments in N,N-dimethylformamide. Mechanistic studies suggest that the O atoms in the secondary coordination sphere are important for relaying the exogenous proton donor to the active site through a preconcentration effect, which leads to the J_HCO2⁻ (partial catalytic current density for formate) increasing by 3.3-fold for 2 and 1.2-fold for 3 in comparison to the J_HCO2⁻ of ₁. These results also suggest that there is a difference in the strength of the interaction between the pendent functional groups and the sacrificial proton donor between 2 and 3, resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased FE_HCO2^– = 85 ± 2% for 2, whereas 3 had a lower FE_HCO2^– = 71 ± 3%, with CO and H₂ generated as co-products in each case to reach mass balance. These results indicate that using secondary sphere moieties to modulate metal–ligand interactions and multisite electron and proton transfer reactivity in the primary coordination sphere through reactant preconcentration can be a powerful strategy for enhancing electrocatalytic activity and selectivity.

The complex MoTp(NO)(DMAP)(η²-naphthalene) (1; DMAP = 4-(dimethylamino)pyridine; Tp = tris(pyrazolyl)borate) is demonstrated to undergo Michael–Michael ring-closure (MIMIRC) reactions promoted by trimethylsilyltriflate. The resulting hexahydrophenanthrenes are formed stereoselectively, with isolation of a single dominant isomer. Combining the MIMIRC sequence with an oxidative decomplexation step, the final tricyclics can be synthesized from the naphthalene complex with overall yields between 19 and 50% (for four steps). This reaction sequence is shown to be capable of producing a steroidal core directly from naphthalene, providing access to a biologically relevant carbon framework.

A family of six cobalt(II) compounds of the general formula (nMeO-C₆H₄NH₂)₂CoX₂, (X = Cl, Br; nMeO-C₆H₄NH₂ = 2-MeO-, 3-MeO-, or 4-MeO-aniline) has been prepared and the compounds characterized by combustion analysis, IR, single-crystal X-ray diffraction and variable temperature magnetization measurements [(n-MeOC₆H₄NH₂)₂CoX₂, (1, n = 2, X  = Cl; 2, n = 2, X  = Br; 3, n = 3, X  = Cl; 4, n = 3, X  = Br; 5, n = 4, X  = Cl; 6, n = 4, X  = Br]. Although the six compounds crystallize in a variety of space groups, they are all isocoordinate. The compounds are slightly distorted tetrahedra. In all compounds, hydrogen bonds and short intermolecular X…Co contacts link the molecules into chains. For 3–6, short halide…halide contacts, in some cases short enough to be considered halogen bonds, further link the chains into layer structures. The complexes have also been studied via variable temperature magnetic susceptibility measurements. All six compounds exhibit antiferromagnetic exchange and show maxima in χ in the range 2–5 K. They have been fit to a variety of models incorporating single-ion anisotropy (SIA), and/or Heisenberg or Ising 1D- and 2D-systems. The results indicate that both antiferromagnetic superexchange and SIA are likely present in all compounds to some degree, but their relative contributions vary greatly. The potential superexchange pathways are discussed in terms of the observed magnetic properties and structures.

Because of their rigidity, polycyclic aromatic hydrocarbons (PAHs) have become a significant building block in molecular materials chemistry. Fusion or doping of boron into PAHs is known to improve the optoelectronic properties by reducing the LUMO energy level. Herein, we report a comprehensive study on the syntheses, structures, and photophysical properties of a new class of fused N‐heterocyclic boranes (NHBs), pyrene‐ and benzene‐linked in a “Janus‐type” fashion (2–4, 6–9, and 11). Remarkably, these examples of fused NHBs display fluorescent properties, and collectively their emission spans the visible spectrum. The pyrene‐fused NHBs all display blue fluorescence, as the excitations are dominated by the pyrene core. In notable contrast, the emission properties of the benzene‐fused analogues are highly tunable and are dependent on the electronics of the NHB fragments (i.e., the functional group directly bound to the boron atoms). Pyrene‐fused 2–4 and 11 represent the only molecules in which the K‐region of pyrene is functionalized with NHB units, and while they exhibit distorted (twisted or stair‐stepped) pyrene cores, benzene‐fused 6–9 are planar. The electronic structure and optical properties of these materials were probed by computational studies, including an evaluation of aromaticity, electronic transitions, and molecular orbitals.

Earth-abundant transition-metal catalysts capable of reducing CO₂ to useful products have been gaining attention to meet increasing energy demands and address concerns of rising CO₂ emissions. Group 6 molecular compounds remain underexplored in this context relative to other transition metals. Here, we present a molecular chromium complex with a 2,2′-bipyridine-based ligand capable of selectively transforming CO₂ into CO with phenol as a sacrificial proton donor at turnover frequencies of 5.7 ± 0.1 s^–1 with a high Faradaic efficiency (96 ± 8%) and a low overpotential (110 mV). To achieve the reported catalytic activity, the parent Cr(III) species is reduced by two electron equivalents, suggesting an approximate d⁵ active species configuration. Although previous results have suggested that low-valent species from the Cr/Mo/W triad are nonprivileged for CO₂ reduction in synthetic molecular systems, the results presented here suggest that reactivity analogous to late transition metals is possible with early transition metals.

Oxidative addition and reductive elimination reactions are central steps in many catalytic processes, and controlling the energetics of reaction intermediates is key to enabling efficient catalysis. A series of oxidative addition and reductive elimination reactions using (^RPNP)RhX complexes (R = tert-butyl, isopropyl, mesityl, phenyl; X = Cl, I) was studied to deduce the effect of the size of the phosphine substituents. Using (^RPNP)RhCl as the starting material, oxidative addition of MeI was observed to produce (^RPNP)Rh(Me)(I)Cl, which was followed by reductive elimination of MeCl to form (^RPNP)RhI. The thermodynamics and kinetics vary with the identity of the substituent R on phosphorus of the PNP ligand. The presence of large steric bulk (e.g., R = tert-butyl, mesityl) on the phosphine favors Rh(I) in comparison to the presence of two smaller substituents (e.g., R = isopropyl, phenyl). An Eyring plot for the oxidative addition of MeI to (^tBuPNP)RhCl in THF-d₈ is consistent with a polar two-step reaction pathway, and the formation of [(^tBuPNP)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show that the steric bulk affects the reaction energies of addition reactions which generate six-coordinate complexes by tens of kcal mol^–1. The ligand steric bulk is calculated to have a reduced effect (a few kcal mol^–1) on S_N2 addition barriers, which only require access to one side of the square plane.