The complex Co­(dmpe)2H catalyzes the hydrogenation of CO2 at 1 atm and 21 °C with significant improvement in turnover frequency relative to previously reported second- and third-row transition-metal complexes. New studies are presented to elucidate the catalytic mechanism as well as pathways for catalyst deactivation. The catalytic rate was optimized through the choice of the base to match the pK a of the [Co­(dmpe)2(H)2]+ intermediate. With a strong enough base, the catalytic rate has a zeroth-order dependence on the base concentration and the pressure of hydrogen and a first-order dependence on the pressure of CO2. However, for CO2:H2 ratios greater than 1, the catalytically inactive species [(μ-dmpe)­(Co­(dmpe)2)2]2+ and [Co­(dmpe)2CO]+ were observed.

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, reversible heterolytic cleavage of H2 in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H(+)/H(-) exchange at an estimated rate of >10(7) s(-1) at 25 °C.

Catalysts that are able to reduce carbon dioxide under mild conditions are highly sought after for storage of renewable energy in the form of a chemical fuel. This study describes a systematic kinetic and thermodynamic study of a series of cobalt and rhodium bis­(diphosphine) complexes that are capable of hydrogenating carbon dioxide to formate under ambient temperature and pressure. Catalytic CO2 hydrogenation was studied under 1.8 and 20 atm of pressure (1:1 CO2/H2) at room temperature in tetrahydrofuran with turnover frequencies (TOF) ranging from 20 to 74 000 h–1. The catalysis was followed by 1H and 31P NMR spectroscopy in real time under all conditions to yield information about the rate-determining step. The cobalt catalysts displayed a rate-determining step of hydride transfer to CO2, while the hydrogen addition and/or deprotonation steps were rate limiting for the rhodium catalysts. Thermodynamic analysis of the complexes provided information on the driving force for each step of catalysis in terms of the catalyst hydricity (ΔG°H– ), acidity (pK a), and free energy for H2 addition (ΔG°H2 ). Linear free-energy relationships were identified that link the kinetic activity for catalytic hydrogenation of CO2 to formate with the thermodynamic driving force for the rate-limiting steps of catalysis. The catalyst exhibiting the highest activity, Co­(dmpe)2H, was found to have hydride transfer and hydrogen addition steps that were each downhill by approximately 6 to 7 kcal mol–1, and the deprotonation step was thermoneutral. This indicates the fastest catalysts are the ones that most efficiently balance the free energy relationships of every step in the catalytic cycle.

4-Phosphoryloxy-N,N-dimethyltryptamine (psilocybin) is a naturally occurring tertiary amine found in many mushroom species. Psilocybin is a prodrug for 4-hydroxy-N,N-dimethyltryptamine (psilocin), which induces psychedelic effects via agonist activity at the serotonin (5-HT) 2A receptor (5-HT2A). Several other 4-position ring-substituted tryptamines are present in psilocybin-containing mushrooms, including the secondary amine 4-phosphoryloxy-N-methyltryptamine (baeocystin) and the quaternary ammonium 4-phosphoryloxy-N,N,N-trimethyltryptamine (aeruginascin), but these compounds are not well studied. Here, we investigated the structure–activity relationships for psilocybin, baeocystin, and aeruginascin, as compared to their 4-acetoxy and 4-hydroxy analogues, using in vitro and in vivo methods. Broad receptor screening using radioligand binding assays in transfected cells revealed that secondary and tertiary tryptamines with either 4-acetoxy or 4-hydroxy substitutions display nanomolar affinity for most human 5-HT receptor subtypes tested, including the 5-HT2A and the serotonin 1A receptor (5-HT1A). The same compounds displayed affinity for 5-HT2A and 5-HT1A in mouse brain tissue in vitro and exhibited agonist efficacy in assays examining 5-HT2A-mediated calcium mobilization and β-arrestin 2 recruitment. In mouse experiments, only the tertiary amines psilocin, psilocybin, and 4-acetoxy-N,N-dimethyltryptamine (psilacetin) induced head twitch responses (ED50 0.11–0.29 mg/kg) indicative of psychedelic-like activity. Head twitches were blocked by 5-HT2A antagonist pretreatment, supporting 5-HT2A involvement. Both secondary and tertiary amines decreased body temperature and locomotor activity at higher doses, the effects of which were blocked by 5-HT1A antagonist pretreatment. Across all assays, the pharmacological effects of 4-acetoxy and 4-hydroxy compounds were similar, and these compounds were more potent than their 4-phosphoryloxy counterparts. Importantly, psilacetin appears to be a prodrug for psilocin that displays substantial serotonin receptor activities of its own.

A series of iron hydride complexes featuring PRNR′ PR (PRNR′ PR = R2PCH2N(R′)CH2PR2 where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, t Bu) and cyclopentadienide (CpX = C5H4X where X = H, C5F4N) ligands has been synthesized; characterized by NMR spectroscopy, X-ray diffraction, and cyclic voltammetry; and examined by quantum chemistry calculations. Each compound was tested for the electrocatalytic oxidation of H2, and the most active complex, (CpC5F4N)Fe(PEtNMePEt)(H), exhibited a turnover frequency of 8.6 s–1 at 1 atm of H2 with an overpotential of 0.41 V, as measured at the potential at half of the catalytic current and using N-methylpyrrolidine as the exogenous base to remove protons. Control complexes that do not contain pendant amine groups were also prepared and characterized, but no catalysis was observed. The rate-limiting steps during catalysis are identified through combined experimental and computational studies as the intramolecular deprotonation of the FeIII hydride by the pendant amine and the subsequent deprotonation by an exogenous base.

Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation.

H2 oxidation by a molecular electrocatalyst is dramatically improved by controlling proton movement from iron to the outer coordination sphere.

A balance of metal electrophilicity and ligand steric influences is required for facile, reversible H–H heterolytic cleavage in Mn complexes with pendant amines.