Earth-abundant Metal Complexes Research Articles

Mononuclear Iron Pyridinethiolate Complex Promoted CO2 Photoreduction via Rapid Intramolecular Electron Transfer.

Designing earth-abundant metal complexes as efficient molecular photocatalysts for visible light-driven CO2 reduction is a key challenge in artificial photosynthesis. Here, we demonstrated the first example of a mononuclear iron pyridine-thiolate complex that functions both as a photosensitizer and catalyst for CO2 reduction. This single-component bifunctional molecular photocatalyst efficiently reduced CO2 to formate and CO with a total turnover number (TON) of 46 and turnover frequency (TOF) of 11.5 h-1 in 4 h under visible light irradiation. Notably, the quantum yield was determined to be 8.4 % for the generation of formate and CO at 400 nm. Quenching experiments indicate that high photocatalytic activity is mainly attributed to the rapid intramolecular quenching protocol. The mechanism investigation by DFT calculation and electrochemical studies revealed that the protonation of Febpy(pyS)2 is indispensable step for photocatalytic CO2 reduction.

Sustainable Preparation of an Earth-abundant Metal Acetylacetonate Complex

This experiment highlights the synthesis of the Earth-abundant metal complex, Fe(acac)3 (acac = acetylacetonate), using an approach that is greener than traditional published syntheses. Students will also have an opportunity to determine the magnetic susceptibility of this complex and to learn about NMR spectroscopy.

CO2 Activation with Manganese Tricarbonyl Complexes by an H-Atom Responsive Benzimidazole Ligand.

Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI (HL)(CO)3 Br (1 a-Br) and MnI (MeL)(CO)3 Br (1 b-Br) (where HL=2-(2'-pyridyl)benzimidazole; MeL=1-methyl-2-(2'-pyridy)benzimidazole) and assayed their electrocatalytic properties for CO2 reduction. A redox-active pyridine benzimidazole ancillary ligand in complex 1 a-Br displayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO2 conversion at a markedly lower reduction potential than that observed for 1 b-Br. Notably, a one-electron reduction of 1 a-Br yields a structurally characterized H-bonded binuclear Mn(I) adduct (2 a') rather than the typically observed Mn(0)-Mn(0) dimer, suggesting a novel method for CO2 activation. Combining advanced electrochemical, spectroscopic, and single crystal X-ray diffraction techniques, we demonstrate the use of an H-atom responsive ligand may reveal an alternative, low-energy pathway for CO2 activation by an earth-abundant metal complex catalyst.

Interfacial Self-Assembly of Organized Ultrathin Films of Tripodal Metal-Terpyridyl Coordination Polymers as Luminophores and Heterogeneous Catalysts for Photocatalytic CO2 Reduction.

Metal-directed interfacial self-assembly of well-defined coordination polymer (CP) ultrathin films can control the metal complex arrangement and distribution at the molecular level, providing a convenient route for the design and fabrication of novel opto-electrical devices and heterogeneous catalysts. Here, we report the assembly of two series of CP multilayers with the transition-metal ions of Fe2+, Co2+, Zn2+ and Tb3+ as connectors and tripodal terpyridyl ligands of 4,4',4″-(1,3,5-triazine-2,4,6-triyl)tris(1-(4-([2,2':6',2″-terpyridin]-4'-yl)benzyl)pyridin-1-ium) (TerPyTa) and 4,4',4″-(benzene-1,3,5-triyl)tris(1-(4-([2,2':6',2″-terpyridin]-4'-yl)benzyl)pyridin-1-ium) (TerPyBen) as linkers at the air-water interface. The as-prepared Langmuir-Blodgett (LB) films display strong luminescence, with the emission wavelength and relative intensity dependent on both the metal ions and linkers; among them, the Zn-TerPyTa and Zn-TerPyBen CPs give off the strongest luminescent emission centered at about 370 nm with an emission lifetime of approximately 0.2-0.3 ns. The Tb-TerPyTa CPs can give off emission at approximately 490, 546, 586, and 622 nm, attributed to the 5D4 to 7F3-6 electron transitions of typical Tb3+ ions. Finally, these CP LB films can act as efficient heterogeneous photocatalysts for the CO2 reduction to selectively produce CO. The catalytic efficiency can be optimized by adjusting the experimental conditions (light sensitizer, electron donor, and water content) and CP composition (metal ion and ligand) with an excellent yield of up to 248.1 mmol g-1. In particular, it is revealed that, under the same conditions, the catalytic efficiency of the Fe-TerPyTa CP LB film is nearly 2 to 3 orders of magnitude higher than that of the other metalated complexes investigated in the homogeneous system. UV-vis spectroscopy and cyclic voltammetry studies demonstrated that the dual active sites of Fe-terpyridine and TerPyTa units contribute to the enhanced catalytic activity. This work provides an effective method to introduce the earth-abundant metal complexes into CP films to construct efficient noble-metal-free photocatalysts for the CO2 reduction.

Efficient Hydrogen Production at pH 7 in Water with a Heterogeneous Electrocatalyst Based on a Neutral Dimeric Cobalt-Dithiolene Complex

The development of efficient hydrogen production technologies is fundamental for replacing fossil-fuel-based energies. As such, electrocatalysts derived from Earth-abundant metal complexes are appealing, and interesting performances have typically been disclosed under acidic conditions in organic solvents. However, their applicability under relevant pH-neutral conditions has been underexplored. Herein, we demonstrate that nonionic, dimeric cobalt-dithiolene complexes supported on a multiwalled carbon nanotube (MWCNT)/carbon paper (CP) electrode are powerful electrocatalysts for hydrogen production in aqueous media at pH 7. The high turnover numbers encountered (TON up to 50980) after long reaction times (up to 16 h) are explained by the increased electroactive cobalt concentration on the modified electrode, which is ca. 4 times higher than that of a state-of-the-art cobalt porphyrin electrocatalyst. These findings point out that immobilizing well-defined, multinuclear, low-cost metal complexes on carbon material is a promising strategy to design highly electroactive electrodes enabling production of green energies.

Are Zn(II) pincer complexes efficient apoptosis inducers? a deep insight into their activity against A549 lung cancer cells.

To expand the array of chemotherapeutic drugs, earth-abundant metal complexes are found to be the future direction. In this regard, new zinc(II) complexes 1-3 of 8-aminoquinoline-based pincer ligands were synthesized, characterized and tested for their anticancer activity. The IC50 values of these complexes were estimated by an MTT assay to be 16.35-17.95 μM and 33.35-40 μM against A549 lung and MCF-7 breast cancer cells respectively. Among them, 3 was slightly better than the other complexes and, thus, subjected to detailed studies. Moreover, the ligand corresponding to 3 was less active against both the cell lines than the complex. Further, 3 showed no toxicity against normal fibroblast cell line L929, which instantly elevated the drug characteristic of our complex. An AO-EB staining assay revealed that 3 can induce apoptosis in A549, and it was quantified by flow cytometry as 22.77%. Moreover, the depolarization of the mitochondrial membrane potential determined by JC-1 staining indicated excess ROS production sites in the mitochondria, which was confirmed by carboxy-H2DCFDA staining. Interestingly, the present complexes show better activity than that of the standard drug cisplatin against A549 cells. Overall, the studies provided promising results that can be extended for clinical applications.

Photochemical charge accumulation in a heteroleptic copper(i)-anthraquinone molecular dyad via proton-coupled electron transfer.

Developing efficient photocatalysts that perform multi electron redox reactions is critical to achieving solar energy conversion. One can reach this goal by developing systems which mimic natural photosynthesis and exploit strategies such as proton-coupled electron transfer (PCET) to achieve photochemical charge accumulation. We report herein a heteroleptic Cu(i)bis(phenanthroline) complex, Cu-AnQ, featuring a fused phenazine-anthraquinone moiety that photochemically accumulates two electrons in the anthraquinone unit via PCET. Full spectroscopic and electrochemical analyses allowed us to identify the reduced species and revealed that up to three electrons can be accumulated in the phenazine-anthraquinone ring system under electrochemical conditions. Continuous photolysis of Cu-AnQ in the presence of sacrificial electron donor produced doubly reduced monoprotonated photoproduct confirmed unambiguously by X-ray crystallography. Formation of this photoproduct indicates that a PCET process occurred during illumination and two electrons were accumulated in the system. The role of the heteroleptic Cu(i)bis(phenanthroline) moiety participating in the photochemical charge accumulation as a light absorber was evidenced by comparing the photolysis of Cu-AnQ and the free AnQ ligand with less reductive triethylamine as a sacrificial electron donor, in which photogenerated doubly reduced species was observed with Cu-AnQ, but not with the free ligand. The thermodynamic properties of Cu-AnQ were examined by DFT which mapped the probable reaction pathway for photochemical charge accumulation and the capacity for solar energy stored in the process. This study presents a unique system built on earth-abundant transition metal complex to store electrons, and tune the storage of solar energy by the degree of protonation of the electron acceptor.

Rational development of molecular earth-abundant metal complexes for electrocatalytic hydrogen production

AbstractFor public health and environmental protection reasons, there is an urgent need to replace traditional fossil fuels with clean and renewable sources. Electrochemical splitting of water has the potential to produce clean hydrogen as a possible solution to global energy problems. This review article introduces the rational design of molecular metal complexes based on earth-abundant metals for electrocatalytic hydrogen production in water or water-organic media. Emphasis is placed on providing insight into structure-function relationships in catalytic properties for future ligand and catalyst design.

Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)–NHC Complexes with Long-Lived 2LMCT Excited States

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)2]PF6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]-, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]-, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]-. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]- by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (2LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state (2GS), completely in line with the parent complex [Fe(phtmeimb)2]PF6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.

Manganese-Catalyzed Reformation of Vicinal Glycols to α-Hydroxy Carboxylic Acids with the Liberation of Hydrogen Gas

Conversion of readily available feedstocks to valuable platform chemicals via an eco-friendly catalytic pathway has always been one of the key focuses of synthetic chemists. In this context, herein, we report selective transformation of readily available feedstock, vicinal glycols, to value-added α-hydroxycarboxylic acid molecules that are prevalent in bioactive molecules and biodegradable polymers. A bench stable Earth-abundant metal complex, {[HN(C2H4PPh2)2]Mn(CO)2Br}, Mn-I catalyzed the reformation reaction at low temperature in high selectivity with a turnover number reaching 2400, surpassing previously used homogeneous catalysts for such a reaction. Hydrogen gas is evolved as a byproduct without needing an acceptor. The developed protocol is applicable for both aromatic and aliphatic vicinal glycols, delivering the α-substituted hydroxycarboxylic acids in high yields and selectivities. Detailed mechanistic studies elucidated the involvements of different manganese(I)-species during this acceptorless dehydrogenation catalysis.

Coordination Polymers with 2,2′:6′,2″-Terpyridine Earth-Abundant Metal Complex Units for Selective CO2 Photoreduction

Combining molecular metal complexes into coordination polymers (CPs) is an effective strategy for developing photocatalysts for CO2 reduction; however, most such reported catalysts are noble metal-containing CPs. Herein, two novel Zr-containing bimetallic CPs, Co-Zr and Ni-Zr, were designed and successfully synthesized by connecting 2,2':6',2″-terpyridine-based molecular earth-abundant metal (Co or Ni) complexes with ZrO8 nodes. Both CPs were applied as catalysts for CO2 photoreduction to selectively produce CO. The catalytic performance of Co-Zr is better than that of Ni-Zr with a yield of 3654 μmol (g of catalyst)-1 for CO in 6 h (TON = 18.2). The difference between these two catalysts was analyzed with respect to band structure and charge migration ability. This work provides an effective way to introduce molecular earth-abundant metal complexes into coordination polymers for the construction of efficient noble metal-free CO2 photocatalysts.

Sensitized Photocatalytic CO2 Reduction With Earth Abundant 3d Metal Complexes Possessing Dipicolyl-Triazacyclononane Derivatives.

Complexes based on nitrogen and sulfur containing ligands involving 3d metal centers are known for the electrocatalytic reduction of CO2. However, photocatalytical activation has rarely been investigated. We herein present results on the light-driven CO2 reduction using either Ir(dFppy)3 [Ir, dFppy = 2-(4,6-difluorophenyl)pyridine] or [Cu(xant)(bcp)]+, (Cu, xant = xantphos, bcp = bathocuproine) as photosensitizer in combination with TEA (triethylamine) as sacrificial electron donor. The 3d metal catalysts have either dptacn (dipicolyl-triazacyclononane, L N3 ) or dpdatcn (dipicolyl-diazathiocyclononane, L N2S ) as ligand framework and Fe3+, Co3+ or Ni2+ as central metal ion. It turned out that the choice of ligand, metal center and solvent composition influences the selectivity for product formation, which means that the gaseous reduction products can be solely CO or H2 or a mixture of both. The ratio between these two products can be controlled by the right choice of reaction conditions. With using Cu as photosensitizer, we could introduce an intermolecular system that is based solely on 3d metal compounds being able to reduce CO2.

Diverse emission properties of transition metal complexes beyond exclusive single phosphorescence and their wide applications

• Intersystem crossing (ISC) rate of a metal complex is determined by not only SOC constant of the metal ion. • Various photophysical mechanisms are possible for transition metal complexes. • Emission properties other than single phosphorescence can be achieved by molecular design. • Uncommon emission properties open up novel applications in many areas. • Triplet excited state of earth-abundant metal complexes can be exploited for use by the novel mechanisms. Luminescence properties and excited state reactivities of photoactive transition metal complexes have been harnessed for various applications in the fields of electroluminescent devices, luminescent sensing and bio-imaging, and solar energy conversion as well as photo-redox catalysis in organic transformation. While, beyond the conventional phosphorescence from triplet excited sates that are populated through heavy metal atom induced intersystem crossing (ISC), the past decade has witnessed blooming reports on intriguing luminescence behaviors such as sole fluorescence, dual fluorescence/phosphorescence, thermally activated delayed fluorescence (TADF), and dual phosphorescence. All these findings reveal that the possible photophysical mechanisms of transition metal complexes remain relatively underexplored. On one hand, the intersystem crossing (ISC) rate that is crucial to the depopulation of the singlet excited state and thus the population efficiency of the triplet excited state has proven to be not simply dependent on the spin-orbit coupling constant of the metal atom. On the other hand, the decay paths for triplet excited state(s) through reverse ISC (T 1 → S 1 ) and direct radiation from higher triplet excited state have been opened by appropriate ligand design. Moreover, these novel emission properties of transition metal complexes have expanded their applications into the fields of ratiometric sensing/imaging, noble-metal-free high efficiency emitters and photocatalysts and beyond. This review is aimed to discuss the unconventional emission properties and applications of transition metal complexes that were reported mainly in the past decade by focusing on the structural factors that are at play in realizing the novel excited state properties. It is conceived to provide a better understanding of the excited state properties tuning and to direct the design of new emissive metal complexes with tailored functions.

An NAD+-type earth-abundant metal complex enabling photo-driven alcohol oxidation.

For the first time, an NAD+-type earth-abundant metal complex [Zn(pbn)2(H2O)](ClO4)2 (1) was found to exhibit photo-induced oxidizing ability to convert various primary and secondary alcohols to the corresponding aldehyde and ketone compounds. In addition, a two-electron-reduced Zn(II) complex [Zn(pbnH-pbnH)(ClO4)2] (1red) comprising the novel C-C coupling ligand, obtained by the photo-induced oxidation of alcohols by 1, was successfully isolated and completely characterized. We clarified that the photochemical oxidation of alcohols by 1 to produce 1red proceeds via an electron transfer followed by proton transfer mechanism as elucidated by kinetic analysis on the basis of absorption spectroscopic measurements.

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.