Earth-abundant Metal Complexes Research Articles

Tuning the Triplet Excited State of Bis(dipyrrin) Zinc(II) Complexes: Symmetry Breaking Charge Transfer Architecture with Exceptionally Long Lived Triplet State for Upconversion.

Zinc(II) bis(dipyrrin) complexes, which feature intense visible absorption and efficient symmetry breaking charge transfer (SBCT) are outstanding candidates for photovoltaics but their short lived triplet states limit applications in several areas. Herein we demonstrate that triplet excited state dynamics of bis(dipyrrin) complexes can be efficiently tuned by attaching electron donating aryl moieties at the 5,5'-position of the complexes. For the first time, a long lived triplet excited state (τT =296 μs) along with efficient ISC ability (ΦΔ =71 %) was observed for zinc(II) bis(dipyrrin) complexes, formed via SBCT. The results revealed that molecular geometry and energy gap between the charge transfer (CT) state and triplet energy levels strongly control the triplet excited state properties of the complexes. An efficient triplet-triplet annihilation upconversion system was devised for the first time using a SBCT architecture as triplet photosensitizer, reaching a high upconversion quantum yield of 6.2 %. Our findings provide a blueprint for the development of triplet photosensitizers based on earth abundant metal complexes with long lived triplet state for revolutionary photochemical applications.

Electrocatalytic hydrogen production and carbon dioxide conversion by earth abundant transition metal complexes of the Schiff base ligand: (E)-1-((2-dimethylamino)-propylimino)methyl)naphthalene-2-ol

The present work describes an electrocatalytic study for the conversion of carbon dioxide to environmentally benign and industrially important products, as well as the production of hydrogen (green energy) from acids using earth abundant transition metal complexes as catalysts. One cobalt and one nickel complex of the Schiff base ligand (E)-1-((2-(dimethylamino)ethylimino)methyl)naphthalene-2-ol have been used. The cobalt complex (1) has been tested as an electrocatalyst for the reduction of protons in non-aqueous solvent using trifluoro acetic acid (TFA) as a substrate. Complex 1 catalyses the acid reduction at a relatively high potential (−0.7 V) with a turn over frequency of 7.05 s−1 at a 10:1 ratio of the acid to the catalyst. Production of hydrogen gas was detected by gas chromatography. The nickel complex (2) was examined for the electrocatalytic reduction of carbon dioxide. At an applied potential of −2.1 V, the main reduction product is oxalic acid. The acid produced was quantitatively determined by HPLC. The rate of oxalic acid formation is 1.3495 × 10−7 M/min. with a kcat value of 58.03 s−1.

Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption

Exploiting earth-abundant iron-based metal complexes as high-performance photosensitizers demands long-lived electronically excited metal-to-ligand charge-transfer (MLCT) states, but these species suffer typically from femtosecond timescale charge-transfer (CT)-state quenching by low-lying nonreactive metal-centered (MC) states. Here, we engineer supermolecular Fe(II) chromophores based on the bis(tridentate-ligand)metal(II)-ethyne-(porphinato)zinc(II) conjugated framework, previously shown to give rise to highly delocalized low-lying 3MLCT states for other Group VIII metal (Ru, Os) complexes. Electronic spectral, potentiometric, and ultrafast pump-probe transient dynamical data demonstrate that a combination of a strong σ-donating tridentate ligand and a (porphinato)zinc(II) moiety with low-lying π*-energy levels, sufficiently destabilize MC states and stabilize supermolecular MLCT states to realize Fe(II) complexes that express 3MLCT state photophysics reminiscent of their heavy-metal analogs. The resulting Fe(II) chromophore archetype, FeNHCPZn, features a highly polarized CT state having a profoundly extended 3MLCT lifetime (160 ps), 3MLCT phosphorescence, and ambient environment stability. Density functional and domain-based local pair natural orbital coupled cluster [DLPNO-CCSD(T)] theory reveal triplet-state wavefunction spatial distributions consistent with electronic spectroscopic and excited-state dynamical data, further underscoring the dramatic Fe metal-to-extended ligand CT character of electronically excited FeNHCPZn. This design further prompts intense panchromatic absorptivity via redistributing high-energy absorptive oscillator strength throughout the visible spectral domain, while maintaining a substantial excited-state oxidation potential for wide-ranging photochemistry--highlighted by the ability of FeNHCPZn to photoinject charges into a SnO2/FTO electrode in a dye-sensitized solar cell (DSSC) architecture. Concepts enumerated herein afford opportunities for replacing traditional rare-metal-based emitters for solar-energy conversion and photoluminescence applications.

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes.

Chiral molecules are essential for the development of advanced technological applications in spintronic and photonic. The best systems should produce large circularly polarized luminescence (CPL) as estimated by their dissymmetry factor (glum), which can reach the maximum values of −2 ≤ glum ≤ 2 when either pure right- or left-handed polarized light is emitted after standard excitation. For matching this requirement, theoretical considerations indicate that optical transitions with large magnetic and weak electric transition dipole moments represent the holy grail of CPL. Because of their detrimental strong and allowed electric dipole transitions, popular chiral emissive organic molecules display generally moderate dissymmetry factors (10−5 ≤ glum ≤ 10−3). However, recent efforts in this field show that glum can be significantly enhanced when the chiral organic activators are part of chiral supramolecular assemblies or of liquid crystalline materials. At the other extreme, chiral EuIII- and SmIII-based complexes, which possess intra-shell parity-forbidden electric but allowed magnetic dipole transitions, have yielded the largest dissymmetry factor reported so far with glum ~ 1.38. Consequently, 4f-based metal complexes with strong CPL are currently the best candidates for potential technological applications. They however suffer from the need for highly pure samples and from considerable production costs. In this context, chiral earth-abundant and cheap d-block metal complexes benefit from a renewed interest according that their CPL signal can be optimized despite the larger covalency displayed by d-block cations compared with 4f-block analogs. This essay thus aims at providing a minimum overview of the theoretical aspects rationalizing circularly polarized luminescence and their exploitation for the design of chiral emissive metal complexes with strong CPL. Beyond the corroboration that f–f transitions are ideal candidates for generating large dissymmetry factors, a special attention is focused on the recent attempts to use chiral CrIII-based complexes that reach values of glum up to 0.2. This could pave the way for replacing high-cost rare earths with cheap transition metals for CPL applications.

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

Room-Temperature Chemoselective Reductive Alkylation of Amines Catalyzed by a Well-Defined Iron(II) Complex Using Hydrogen.

A transition-metal frustrated Lewis pair approach has been envisaged to enhance the catalytic activity of tricarbonyl phosphine-free iron complexes in reduction of amines. A new cyclopentadienyl iron(II) tricarbonyl complex has been isolated, fully characterized, and applied in hydrogenation. This phosphine-free iron complex is the first Earth-abundant metal complex that is able to catalyze chemoselective reductive alkylation of various functionalized amines with functionalized aldehydes. Such selectivity and functionality tolerance (alkenes, esters, ketones, acetals, unprotected hydroxyl groups, and phosphines) have been demonstrated also for the first time at room temperature with an Earth-abundant metal complex. This alkylation reaction was also performed without any preliminary condensation and generated only water as a byproduct. The resulting amines provided rapid access to potential building blocks, metal ligands, or drugs. Density functional theory calculations highlighted first that the formation of the 16 electron species, via the activation of the tricarbonyl complex Fe3, was facilitated and, second, that the hydrogen cleavage did not follow the same pathway as bond breaking, usually described with the known cyclopentadienone iron tricarbonyl complexes (Fe1 and Fe4). These calculations highlighted that the new complex Fe3 does not behave as a bifunctional catalyst, in contrast to its former congeners.

Is Iron the New Ruthenium?

Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, for example, in lighting, sensing, solar cells, and photoredox catalysis. There is a long-standing interest in replacing ruthenium with iron because ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap. However, it is very difficult to obtain iron complexes with an electronic structure similar to that of ruthenium(II) polypyridines. The latter typically have a long-lived excited state with pronounced charge-transfer character between the ruthenium metal and ligands. These metal-to-ligand charge-transfer (MLCT) excited states can be luminescent, with typical lifetimes in the range of 100 to 1000 ns, and the electrochemical properties are drastically altered during this time. These properties make ruthenium(II) polypyridine complexes so well suited for the abovementioned applications. In iron(II) complexes, the MLCT states can be deactivated extremely rapidly (ca. 50 fs) by energetically lower lying metal-centered excited states. Luminescence is then no longer emitted, and the MLCT lifetimes become much too short for most applications. Recently, there has been substantial progress on extending the lifetimes of MLCT states in iron(II) complexes, and the first examples of luminescent iron complexes have been reported. Interestingly, these are iron(III) complexes with a completely different electronic structure than that of commonly targeted iron(II) compounds, and this could mark the beginning of a paradigm change in research into photoactive earth-abundant metal complexes. After outlining some of the fundamental challenges, key strategies used so far to enhance the photophysical and photochemical properties of iron complexes are discussed and recent conceptual breakthroughs are highlighted in this invited Concept article.

Photoredox catalysts based on earth-abundant metal complexes

Visible light photoredox catalysis has exploded into the consciousness of the synthetic chemist. We critically review Earth-abundant metal complexes photocatalysts including Cu(i), Zn(ii), Ni(0), V(v), Zr(iv), W(0), W(vi), Mo(0), Cr(iii), Co(iii) and Fe(ii).

Synthetic Applications Based on Dehydrogenative Alcohol Activation Catalyzed by Mn-, Fe and Co-Complexes

Background and Objectives: An environmentally benign method for the activation of an alcohol is based on its temporary conversion to a more reactive carbonyl compound by transition metal-catalyzed dehydrogenation. The use of homogeneous complexes in which noble metals were recently replaced by cheap, less toxic, earth-abundant metals (Mn, Fe, Co) enabled sustainable dehydrogenative coupling reactions of alcohols and X-nucleophiles (X: C, N, O). Greener synthesis of valuable chemicals (imines, amides, esters) occurred under acceptorless conditions, while direct alkylation of amines, alcohols and carbonyl compounds was achieved through borrowing hydrogen processes. Successful applications in the preparation of pharmaceutically interesting molecules (i.e. Piribedil) are discussed. The synthetic utility was further explored in the heterocycle synthesis. Aromatic (quinolines, pyrimidines, pyrroles) and non-aromatic heterocycles (lactones, lactams, cyclic amines and imides) were built up through single or multiple C-X bond formation. Keywords: Alcohol activation, oxidation, earth-abundant metal-complexes, acceptorless dehydrogenative coupling, borrowing hydrogen methodology, nucleophiles.

Monitoring Ultrafast Spin Crossover Intermediates in an Iron(II) Complex by Broad Band Stimulated X-ray Raman Spectroscopy.

The photoinduced spin crossover dynamics of transition metal complexes is of fundamental scientific importance and is used for sensor device applications and solar energy harvesting. Current X-ray and optical spectroscopy experiments for [FeII(bpy)3], an archetypal earth-abundant metal complex, show conflicting spin dynamics. We have simulated the broad band transient X-ray absorption and hybrid (broad + narrow band) X-ray stimulated Raman signals at the N and Fe K-edges of the key excited state intermediates involved in the spin crossover process of this complex. We find that these signals are much more sensitive to electron and spin populations than transition absorption and may be useful in the design of photovoltaic and artificial photosynthetic systems.

New tricks for an old dog: Visible light-driven hydrogen production from water catalyzed by fac- and mer- geometrical isomers of tris(thiosemicarbazide) cobalt(III)

Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of platinum for the electrocatalytically and photocatalytically driven hydrogen evolution reaction. In this work, we report on molecular H2-evolving catalysts based on two octahedral complexes of cobalt thiosemicarbazide, fac-[Co(Htsc)3]Cl3·3H2O (C1, Htsc = thiosemicarbazide) and mer-[Co(Htsc)3]Cl3·4H2O (C2), which have facial (fac) and meridional (mer) geometry, respectively. Electrochemical studies confirmed that both C1 and C2 are active electrocatalysts in MeOH solution using acetic acid as the proton source, with the same overpotential of ∼ 640 mV and TOF of ∼ 210 s−1. The complex C1 also exhibits electrocatalytic activity for hydrogen evolution reaction in aqueous media free of organic solvent with a moderate overpotential (560 mV). Visible light-driven hydrogen evolution experiments were carried out in combination with fluorescein as photosensitizer and triethylamine as sacrificial reductant in homogeneous environments. Our studies showed that both C1 and C2 can be used as efficient proton-reduction catalysts in purely aqueous solution and have the same photocatalytic activities. A TOF of 125 h−1 with a TON of 900 for photocatalytic H2 generation using C1 and C2 in water were achieved for the noble-metal-free homogeneous system. It should be noted that this is the first reported study investigating the effect on the catalytic hydrogen production performance of using fac- and mer-isomers of molecular catalysts.

Efficient electrocatalytic and photocatalytic hydrogen evolution using a linear trimeric thiolato complex of nickel

Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of noble-metal platinum and biological catalysts hydrogenases for catalyzing the hydrogen evolution reaction. In this study, we report on a molecular H2-evolving catalyst based on a linear trimeric thiolato complex of nickel Ni3(LN2S2)2 (LN2S2 = N,N′-dimethyl-N-N′-bis(2-mecaptoethyl)-ethylenediaminato). Electrochemical studies showed that the trinuclear nickel complex Ni3(LN2S2)2 can electrocatalyze hydrogen evolution from weakly acidic solutions with remarkable turnover frequencies (3495 s−1 at −1.98 V and 715 s−1 at −1.58 V vs SCE). An efficient noble-metal-free homogeneous photocatalytic system for hydrogen generation from water working under visible light irradiation was further constructed by using the target nickel complex as photocatalyst, fluorescein (Fl) as photosensitizer (PS), and triethylamine (TEA) as sacrificial electron donor. Our studies showed that Ni3(LN2S2)2 can be used in purely aqueous solution and gave a turnover number (TON, vs catalyst) for H2 evolution of 790, corresponding to a TOF 60 h−1. The results show that multinuclear nickel(II) complexes are a promising new direction for molecular catalysts for the electro- and photoreduction of protons.

Organic Molecular Catalyst for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) is fundamentally important and critical in water electrolysis and electrochemical hydrogen pumping.1-3 Recently, molecular HER catalysts based on earth-abundant metal complexes have received much research interest for their role as a low-cost alternative to Pt-based catalysts.4-7 However, such molecular catalysts often show poor stability in harsh acidic aqueous environment, which makes them not viable for the most common polymer electrolyte membrane (PEM) electrolyzer. The HER catalysts based on organic molecules and polymers, including N-containing molecules and polymers, have also attracted attention but they suffer from slow kinetics restricting their practicality.8-11 In this presentation, we will demonstrate a new HER catalyst based on a small organic molecule. This catalyst, which is different from materials based on metal complexes, has been evaluated in both aqueous acidic electrolyte and Nafion membrane electrode assembly (MEA), at room temperature and at 80 °C. The interaction of this molecule with acidic electrolytes, including sulfuric acid and Nafion, has also been studied using Fourier-transform Infrared (FTIR) spectroscopy. The results indicate that a unique molecular structure of the catalyst is responsible for significant acceleration in the HER rate, resulting in the onset potential of less than 0.05 V vs. RHE. The durability of this molecule has also been evaluated in H2 pump experiment in MEA revealing respectable stability during a 60 h test at a bias voltage of -0.2 V, with faradaic efficiency close to 100%. Based on the experimental data and density functional theory (DFT) calculations, the reaction mechanism will be proposed. It will involve an estimate of the Gibbs free energy change along the HER pathway and determination of the rate-limiting reaction steps and corresponding activation energies by transition state searching along the reaction coordinate. Finally, the activity-structure relationship for the studied catalyst will be analyzed and compared with those for similar molecular catalysts. Acknowledgements Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. References (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, 146. (2) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Nat Mater 2017, 16, 57. (3) Fishel, K.; Qian, G.; Eisman, G.; Benicewicz, B. C. In High Temperature Polymer Electrolyte Membrane Fuel Cells: Approaches, Status, and Perspectives; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing: Cham, 2016, p 527. (4) Coutard, N.; Kaeffer, N.; Artero, V. Chem. Commun. 2016, 52, 13728. (5) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Chem. Sci. 2014, 5, 865. (6) Wang, M.; Chen, L.; Sun, L. Energy Environ. Sci. 2012, 5, 6763. (7) Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. Science 2011, 333, 863. (8) Uchida, T.; Mogami, H.; Yamakata, A.; Sasaki, Y.; Osawa, M. J. Am. Chem. Soc. 2008, 130, 10862. (9) Văduva, C. C.; Vaszilcsin, N.; Kellenberger, A. Int. J. Hydrogen Energy 2012, 37, 12089. (10) Liu, L.; Zha, D.-W.; Wang, Y.; He, J.-B. Int. J. Hydrogen Energy 2014, 39, 14712. (11) Kurys, Y. I.; Mazur, D. O.; Koshechko, V. G.; Pokhodenko, V. D. Theor. Exp. Chem. 2016, 52, 163.

Iron-based metal–organic frameworks (MOFs) for visible-light-induced photocatalysis

Photocatalysis is one of the best solutions to solve the problems of global energy shortage and environmental deterioration since it can utilize the abundant and ubiquitous solar light for chemical transformations. In addition to traditional semiconductor-based photocatalysts, metal–organic frameworks (MOFs), a class of micro–mesoporous hybrid materials, are recently emerging as a new type of photocatalytic materials due to their unique structural characteristics. Among all the MOFs, Fe-containing MOFs are extremely appealing in photocatalysis since most of the Fe-based MOFs are visible light responsive due to the existence of extensive iron–oxo (Fe–O) clusters. In addition, Fe is an earth-abundant metal and Fe-containing complexes are commonly used in catalysis and photocatalysis. In this short review, we summarized the recent progress in using Fe-based MOFs for several photocatalytic applications, including pollutants degradation, water oxidation/reduction, CO2 reduction and organic transformations, with a focus on their different light-harvesting properties as compared with that of Ti- and Zr-containing MOFs as well as the structural influence on photocatalysis due to the existence of the diversified Fe-based MOFs. We hope that such a review can stimulate intensive research for rational design of Fe-based MOFs, one of the most promising MOF-based systems, for photocatalysis.

Electrons, Photons, Protons and Earth-Abundant Metal Complexes for Molecular Catalysis of CO2 Reduction

Electrochemical and photochemical reduction of CO2, or a smart combination of both, are appealing approaches for the storage of renewable, intermittent energies and may lead to the production of fuels and of value-added chemicals. By using only earth-abundant metal (Cu, Ni, Co, Mn, Fe) complexes, cheap electrodes and/or cheap sacrificial electron donors and visible light sensitizers, systems functioning with molecular catalysts have been recently designed, showing promising results, in particular, for the two-electron reduction of the carbon dioxide. By combining experimental and mechanistic studies, key parameters controlling the catalysis efficiency have been deciphered, opening the way to the design of future, more efficient and durable catalysts, as well as to the development of electrochemical or photoelectrochemical cells, all being key steps for the emergence of applied devices. The most recent advances related to these issues are discussed in this review.

A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis

Titanium and zirconium complexes carrying two 2,6-bis(pyrrolyl)pyridine ligands have been synthesized and characterized. The neutral complexes Ti(MePDP)2 and Zr(MePDP)2 (MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr(MePDP)2 exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of -2.16 V vs Fc+/0. This reactivity was utilized to facilitate dehalogenation reactions, the reduction of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr(MePDP)2 acts as a substitute for the precious metal photosensitizer [Ru(bpy)3]2+.

ChemInform Abstract: Unprecedented Iron‐Catalyzed Selective Hydrogenation of Activated Amides to Amines and Alcohols.

The first example of hydrogenation of amides homogeneously catalyzed by an earth-abundant metal complex is reported. The reaction is catalyzed by iron PNP pincer complexes. A wide range of secondary and tertiary N-substituted 2,2,2-trifluoroacetamides were hydrogenated to form amines and trifluoroethanol.

Highly efficient photocatalytic hydrogen evolution from nickel quinolinethiolate complexes under visible light irradiation

Earth-abundant metal complexes have emerged as promising surrogates of platinum for catalyzing the hydrogen evolution reaction (HER). In this study, we report the design and synthesis of two novel nickel quinolinethiolate complexes, namely [Ni(Hqt)2(4, 4′-Z-2, 2′-bpy)] (Hqt = 8-quinolinethiol, Z = H [1] or CH3 [2], bpy = bipyridine). An efficient three-component photocatalytic homogeneous system for hydrogen generation working under visible light irradiation was constructed by using the target complexes as catalysts, triethylamine (TEA) as sacrificial electron donor and xanthene dyes as photosensitizer. We obtain turnover numbers (TON, vs. catalyst) for H2 evolution of 5923/7634 under the optimal conditions with 5.0 × 10−6 M complex 1/2 respectively, 1.0 × 10−3 M fluorescein and 5% (v/v) TEA at pH 12.3 in EtOH/H2O (1:1, v/v) mixture after 8 h irradiation (λ > 420 nm). We discuss the mechanism of H2 evolution in the homogeneous photocatalytic system based on fluorescence spectrum and cyclic voltammetry data.

Unprecedented iron-catalyzed selective hydrogenation of activated amides to amines and alcohols.

The first example of hydrogenation of amides homogeneously catalyzed by an earth-abundant metal complex is reported. The reaction is catalyzed by iron PNP pincer complexes. A wide range of secondary and tertiary N-substituted 2,2,2-trifluoroacetamides were hydrogenated to form amines and trifluoroethanol.

Molecular Catalysis of the Electrochemical and Photochemical Reduction of CO2 with Earth-Abundant Metal Complexes. Selective Production of CO vs HCOOH by Switching of the Metal Center

Molecular catalysis of carbon dioxide reduction using earth-abundant metal complexes as catalysts is a key challenge related to the production of useful products--the "solar fuels"--in which solar energy would be stored. A direct approach using sunlight energy as well as an indirect approach where sunlight is first converted into electricity could be used. A Co(II) complex and a Fe(III) complex, both bearing the same pentadentate N5 ligand (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), were synthesized, and their catalytic activity toward CO2 reduction was investigated. Carbon monoxide was formed with the cobalt complex, while formic acid was obtained with the iron-based catalyst, thus showing that the catalysis product can be switched by changing the metal center. Selective CO2 reduction occurs under electrochemical conditions as well as photochemical conditions when using a photosensitizer under visible light excitation (λ > 460 nm, solvent acetonitrile) with the Co catalyst. In the case of the Fe catalyst, selective HCOOH production occurs at low overpotential. Sustained catalytic activity over long periods of time and high turnover numbers were observed in both cases. A catalytic mechanism is suggested on the basis of experimental results and preliminary quantum chemistry calculations.