Cobalt(II) tris(2-pyridylmethyl)amine complexes [Co(TPA)X]+ bearing coordinating anion (X = Cl−, Br−, I− and NCS−): synthesis and application for carbon dioxide reduction

Synthesis, Characterization, and Reactivity of a Synthetic End-On Cobalt(II) Alkyl Persulfide Complex as a Model Platform for Thiolate Persulfidation

Persulfides (RSS-) are ubiquitous source of sulfides (S2-) in biology, and interactions between RSS- and bioinorganic metal centers play critical roles in biological hydrogen sulfide (H2S) biogenesis, signaling, and catabolism. Here, we report the use of contact-ion stabilized [Na(15-crown-5)][tBuSS] (1) as a simple synthon to access rare metal alkyl persulfide complexes and to investigate the reactivity of RSS- with transition metal centers to provide insights into metal thiolate persulfidation, including the fundamental difference between alkyl persulfides and alkyl thiolates. Reaction of 1 with [CoII(TPA)(OTf)]+ afforded the η1-alkyl persulfide complex [CoII(TPA)(SStBu)]+ (2), which was characterized by X-ray crystallography, UV-vis spectroscopy, and Raman spectroscopy. RSS- coordination to the Lewis acidic Co2+ center provided additional stability to the S-S bond, as evidenced by a significant increase in the Raman stretching frequency for 2 (vS-S = 522 cm-1, ΔvS-S = 66 cm-1). The effect of persulfidation on metal center redox potentials was further elucidated using cyclic voltammetry, in which the Co2+ → Co3+ oxidation potential of 2 (Ep,a = +89 mV vs SCE) is lowered by nearly 700 mV when compared to the corresponding thiolate complex [CoII(TPA)(StBu)]+ (3) (Ep,a = +818 mV vs SCE), despite persulfidation being generally seen as an oxidative post-translational modification. The reactivity of 2 toward reducing agents including PPh3, BH4-, and biologically relevant thiol reductant DTT led to different S2- output pathways, including formation of a dinuclear 2Co-2SH complex [CoII2(TPA)2(μ2-SH)2]2+(4).

A Co(III)-peroxo-arylboronate complex formed by nucleophilic reaction of a Co(III)-peroxo species

In this work, we report the generation and characterization of two new Co(III)-peroxo complexes 2 and 3. 2 is best described as a mononuclear CoIII-(O2) complex that exhibits an 18O-isotope sensitive OO bond stretching vibration at 845(-49) cm-1, indicating a relatively weak peroxo moiety compared to those of other CoIII-(O2) complexes reported previously. Complex 3 is a CoIII-peroxo-arylboronate species having a rare {CoIIIOOBO} five-membered metallocycle, which is structurally characterized using X-ray crystallography. Investigations of the reaction mechanism using density functional theory calculations show that 2 likely undergoes a nucleophilic attack to an arylboronic acid, which is generated by hydrolysis of the BPh4- anion in wet acetonitrile solution, to first form a CoIII-peroxo-arylboronic acid adduct, followed by the loss of one benzene molecule to generate the five-membered metallocycle. The entire reaction is thermodynamically favorable. Taken together, the conversion of 2 to 3 represents the discovery of a novel nucleophilic reactivity that can be carried out by mononuclear Co(III)-peroxo complexes.

Electrocatalytic CO2 Reduction: Monitoring of Catalytically Active, Downgraded, and Upgraded Cobalt Complexes

The premise of most studies on the homogeneous electrocatalytic CO2 reduction reaction (CO2RR) is a good understanding of the reaction mechanisms. Yet, analyzing the reaction intermediates formed at the working electrode is challenging and not always attainable. Here, we present a new, general approach to studying the reaction intermediates applied for CO2RR catalyzed by a series of cobalt complexes. The cobalt complexes were based on the TPA-ligands (TPA = tris(2-pyridylmethyl)amine) modified by amino groups in the secondary coordination sphere. By combining the electrochemical experiments, electrochemistry-coupled electrospray ionization mass spectrometry, with density functional theory (DFT) calculations, we identify and spectroscopically characterize the key reaction intermediates in the CO2RR and the competing hydrogen-evolution reaction (HER). Additionally, the experiments revealed the rarely reported in situ changes in the secondary coordination sphere of the cobalt complexes by the CO2-initiated transformation of the amino substituents to carbamates. This launched an even faster alternative HER pathway. The interplay of three catalytic cycles, as derived from the experiments and supported by the DFT calculations, explains the trends that cobalt complexes exhibit during the CO2RR and HER. Additionally, this study demonstrates the need for a molecular perspective in the electrocatalytic activation of small molecules efficiently obtained by the EC-ESI-MS technique.

Cleaner and stronger: how 8-quinolinolate facilitates formation of Co(III)-thiolate from Co(II)-disulfide complexes

The formation of Co(III)-thiolate complexes from Co(II)-disulfide complexes using the anionic ligand 8-quinolinolate (quin^-) has been studied experimentally and quantum chemically. Two Co(II)-disulfide complexes [Co₂(L^xSSL^x)(Cl)₄] (x = 1 or 2; L¹SSL¹ = 2,2'-disulfanediylbis(N,N-bis(pyridin-2-ylmethyl)ethan-1-amine; L²SSL² = 2,2'-disulfanedylbis (N-((6-methylpyridin-2-yl)methyl)-N-(pyridin-2-ylmethyl) ethan-1-amine) have been successfully converted with high yield to their corresponding Co(III)-thiolate complexes upon addition of the ligand 8-quinolinolate. Using density functional theory (DFT) computations the d-orbital splitting energies of the cobalt-thiolate compounds [Co(L¹S)(quin)]⁺ and [Co(L²S)(quin)]⁺ were estimated to be 3.10 eV and 3.07 eV, indicating a slightly smaller ligand-field strength of ligand L²SSL² than of L¹SSL¹. Furthermore, the orientation of the quin^- ligand in the thiolate compounds determines the stability of the thiolate complex. DFT computations show that the thiolate structure benefits from more electrostatic attraction when the oxygen atom of the quin^- ligand is positioned trans to the sulfur atom of the [Co(L¹S)]²⁺ fragment. Quin^- is the first auxiliary ligand with which it appeared possible to induce the redox-conversion reaction in cobalt(II) compounds of the relatively weak-field ligand L²SSL².

Ni(I)-TPA Stabilization by Hydrogen Bond formation on the Second Coordination Sphere: a DFT Characterization

Ni(I) compounds are less common than those of either Ni(0) or Ni(II). Recently, a series of Ni(I) tris(2 pyridylmethyl)amine (TPA) complexes were synthetized through the reduction of Ni(II)-TPA complexes and...

Evaluating Metal-Ligand Interactions of Metal-Binding Isosteres Using Model Complexes

Bioisosteres are a useful approach to address pharmacokinetic liabilities and improve drug-like properties. Specific to developing metalloenzyme inhibitors, metal-binding pharmacophores (MBPs) have been combined with bioisosteres, to produce metal-binding isosteres (MBIs) as alternative scaffolds for use in fragment-based drug discovery (FBDD). Picolinic acid MBIs have been reported and evaluated for their metal-binding ability, pharmacokinetic properties, and enzyme inhibitory activity. However, their structural, electronic, and spectroscopic properties with metal ions other than Zn(II) have not been reported, which might reveal similarities and differences between MBIs and the parent MBPs. To this end, [M(TPA)(MBI)]⁺ (M = Ni(II) and Co(II), TPA = tris(2-pyridylmethyl)amine) is presented as a bioinorganic model system for investigating picolinic acid, four heterocyclic MBIs, and 2,2'-bipyridine. These complexes were characterized by X-ray crystallography as well as NMR, IR, and UV-vis spectroscopies, and their magnetic moments were accessed. In addition, [(Tp^Ph,Me)Co(MBI)] (Tp^Ph,Me = hydrotris(3,5-phenylmethylpyrazolyl)borate) was used as a second model compound, and the limitations and attributes of the two model systems are discussed. These results demonstrate that bioinorganic model complexes are versatile tools for metalloenzyme inhibitor design and can provide insights into the broader use of MBIs.

Polypyridyl Co complex-based water reduction catalysts: why replace a pyridine group with isoquinoline rather than quinoline?

The electronic effect of the substituent has been fully leveraged to improve the activity of molecular water reduction catalysts (WRCs). However, the steric effect of the substituents has received less attention. In this work, a steric hindrance effect was observed in a quinoline-involved polypyridyl Co complex-based water reduction catalyst (WRC), which impedes the formation of Co(iii)-H from Co(i), two pivotal intermediates for H₂ evolution, leading to significantly impaired electrocatalytic and photocatalytic activity with respect to its parent complex, [Co(TPA)Cl]Cl (TPA = tris(2-pyridinylmethyl)-amine). In sharp contrast, two isoquinoline-involved polypyridyl Co complexes exhibited significantly improved H₂ evolution efficiencies compared to [Co(TPA)Cl]Cl, benefitting mainly from the more basic and conjugated features of isoquinoline over pyridine. The dramatically different influences caused by the replacement of a pyridine group in the TPA ligand by quinoline and isoquinoline fully demonstrates the important roles of both the electronic and steric effects of a substituent. Our results may provide novel insights for designing more efficient WRCs.

Electronic effects on polypyridyl Co complex-based water reduction catalysts

Three new isomeric cobalt complexes of TPA (tris(2-pyridylmethyl)amine) based on methoxy substitution at the ortho, meta and para positions, respectively, were constructed and their photocatalytic proton reduction efficiencies were compared. It was found that there are good linear correlations with the Hammett constants of the substituents for the computed Co–N bond lengths, redox potentials of Co^II/I and Co^I/0 events, and the photocatalytic activities of the complexes. The ortho-substituted Co complex distinguished itself from the others remarkably in all these comparisons, demonstrating the presence of a steric effect besides the electronic effect. For other examined complexes, a stronger electron-donating substituent may lead to a higher hydrogen evolution efficiency, suggesting that the formation of a Co(iii) hydride intermediate is the rate-limiting step.

Three isomeric Co complexes showed a significant substituent electronic effect in photocatalytic hydrogen production.

p-Substituted Tris(2-pyridylmethyl)amines as Ligands for Highly Active ATRP Catalysts: Facile Synthesis and Characterization

A facile and efficient two-step synthesis of p-substituted tris(2-pyridylmethyl)amine (TPMA) ligands to form Cu complexes with the highest activity to date in atom transfer radical polymerization (ATRP) is presented. In the divergent synthesis, p-Cl substituents in tris(4-chloro-2-pyridylmethyl)amine (TPMA^3Cl ) were replaced in one step and high yield by electron-donating cyclic amines (pyrrolidine (TPMA^PYR ), piperidine (TPMA^PIP ), and morpholine (TPMA^MOR )) by nucleophilic aromatic substitution. The [Cu^II (TPMA^NR2 )Br]⁺ complexes exhibited larger energy gaps between frontier molecular orbitals and >0.2 V more negative reduction potentials than [Cu^II (TPMA)Br]⁺ , indicating >3 orders of magnitude higher ATRP activity. [Cu^I (TPMA^PYR )]⁺ exhibited the highest reported activity for Br-capped acrylate chain ends in DMF, and moderate activity toward C-F bonds at room temperature. ATRP of n-butyl acrylate using only 10-25 part per million loadings of [Cu^II (TPMA^NR2 )Br]⁺ exhibited excellent control.

Highly Reactive CoIII,IV2(μ-O)2 Diamond Core Complex That Cleaves C-H Bonds

The selective activation of strong sp³ C-H bonds at mild conditions is a key step in many biological and synthetic transformations and an unsolved challenge for synthetic chemists. In nature, soluble methane monooxygenase (sMMO) is one representative example of nonheme dinuclear iron-dependent enzymes that activate strong sp³ C-H bonds by a high-valent diiron(IV) intermediate Q. To date, synthetic model complexes of sMMO-Q have shown limited abilities to oxidize strong C-H bonds. In this work, we generated a high-valent Co^III,IV₂(μ-O)₂ complex 3 supported by a tetradentate tris(2-pyridylmethyl)amine (TPA) ligand via one-electron oxidation of its Co^III₂(μ-O)₂ precursor 2. Characterization of 2 and 3 using X-ray absorption spectroscopy and DFT calculations showed that both species possess a diamond core structure with a short Co···Co distance of 2.78 Å. Furthermore, 3 is an EPR active species showing an S = 1/2 signal with clearly observable hyperfine splittings originated from the coupling of the ⁵⁹Co nuclear spin with the electronic spin. Importantly, 3 is a highly reactive oxidant for sp³ C-H bonds, and an oxygenation reagent. 3 has the highest rate constant (1.5 M^-1 s^-1 at -60 °C) for oxidizing 9,10-dihydroanthracene (DHA) compared to diamond core complexes of other first-row transition metals including Mn, Fe and Cu reported previously. Specifically, 3 is about 4-5 orders of magnitude more reactive than the diiron analogs Fe^III,IV₂(μ-O)₂ and Fe^IV₂(μ-O)₂ supported by TPA and related ligands. These findings shed light on future development of more reactive approaches for C-H bond activation by bioinspired dicobalt complexes.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Photocatalytic CO2 Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO2, CO, or Proton

The cobalt complexes Co^IIL1(PF₆)₂ (1; L1 = 2,6-bis[2-(2,2'-bipyridin-6'-yl)ethyl]pyridine) and Co^IIL2(PF₆)₂ (2; L2 = 2,6-bis[2-(4-methoxy-2,2'-bipyridin-6'-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO₂ reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO₂ reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E_1/2 = -1.77 and -1.85 V versus Ag/AgNO₃ (or -1.86 and -1.94 V vs Fc^+/0), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)₃]²⁺ as a photosensitizer, tri- p-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H₂ were produced under visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy)₃]⁺. However, bulk electrolysis in a wet CH₃CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co-H] ⁿ⁺ ( n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to produce [Co⁰L2(CO)] with ν_CO = 1894 cm^-1 together with [Co^IIL2]²⁺ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV-vis spectroscopy indicate a slow CO binding rate with the [Co^IL2]⁺ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO₂ is much slower than the photochemical formation of [Ru(bpy)₃]⁺ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy)₃]⁺.

Electrocatalytic and Photocatalytic Reduction of CO2 to CO by Cobalt(II) Tripodal Complexes: Low Overpotentials, High Efficiency and Selectivity

The reduction of carbon dioxide (CO₂ ) has been considered as an approach to mitigate global warming and to provide renewable carbon-based fuels. Rational design of efficient, selective, and inexpensive catalysts with low overpotentials is urgently desired. In this study, four cobalt(II) tripodal complexes are tested as catalysts for CO₂ reduction to CO in a MeCN/H₂ O (4:1 v/v) solution. The replacement of pyridyl groups in the ligands with less basic quinolinyl groups greatly reduces the required overpotential for CO₂ -to-CO conversion down to 200-380 mV. Benefitting from the low overpotentials, a photocatalyst system for CO₂ -to-CO conversion is successfully constructed, with an maximum turnover number (TON) of 10 650±750, a turnover frequency (TOF) of 1150±80 h^-1 , and almost 100 % selectivity to CO. These outstanding catalytic performances are further elucidated by DFT calculations.

Artificial Photosynthetic Systems for CO2 Reduction: Progress on Higher Efficiency with Cobalt Complexes as Catalysts

The conversion of CO₂ into fuels or value-added chemicals is currently a field of great research interest. Molecular cobalt catalysts have long been used as mediators of reductive transformations of CO₂ . In this Minireview, the cobalt complex-based photocatalytic and photoelectrocatalytic systems for CO₂ reduction are discussed and summarized, alongside progress on the design of new molecular cobalt catalysts and their performance in photocatalysis.