Parent Complex Research Articles

Deciphering the Mechanism of Base-Triggered Conversion of Ammonia to Molecular Nitrogen and Methylamine to Cyanide.

We report an in-depth investigation into the ammonia oxidation mechanism by the catalyst [RuIII(tpy)(dmabpy)NH3]3+ ([Ru(NH3)]3+). Stoichiometric reactions of [Ru(NH3)]3+ were carried out with exogenous noncoordinating bases to trigger a proposed redox disproportionation reaction, which was followed using variable-temperature NMR spectroscopy. An intermediate species was identified as a dinitrogen-bridged complex using 15N NMR and Raman spectroscopy on isotopically labeled complexes. This intermediate is proposed to derive from coupling of nitridyl species formed upon sequential redox disproportion reactions. Acetonitrile displaces the dinitrogen bridge to yield free N2. DFT calculations support this lower-energy pathway versus that previously reported for ammonia oxidation by the parent [RuIII(tpy)(bpy)NH3]3+ complex. These experimental and computational results are consistent with the interpretation of redox disproportionation involving sequential hydrogen atom transfer reactions by an amide/aminyl intermediate, [Ru(NH2)-]+ ⇔ [Ru(NH2)•]+, formed upon deprotonation of the parent complex. Control experiments employing a large excess of ammonia as a base indicate this new proposed lower-energy pathway contributes to the oxidation of ammonia to dinitrogen in conditions relevant to electrocatalysis. In addition, analogous methylamine complexes, [Ru(NH2CH3)]2+/3+, were prepared to further test the proposed mechanism. Treating [Ru(NH2CH3)]3+ with a base cleanly yields two products [Ru(NH2CH3)]2+ and [Ru(CN)]+ in an ∼3:1 ratio, fully consistent with the proposed cascade of hydrogen atom transfer reactions by an intermediate.

Effect of cyclic substituents on the anti-cancer activity and DNA interaction of ruthenium(II) bis-phenanthroline dipyridoquinoline.

Introduction: Ruthenium(II) complexes have emerged recently as candidates for anti-cancer therapy, where activity is related to lipohilicity, cellular localization, and specific interactions with biomolecules. Methods: In this work, two novel complexes were synthesized and are reported based on the [Ru(phen)2(dipyrido[3,2-f:2',3'-h]quinoxaline]2+ framework. Results: Compared to the parent complex, annealing of cyclopenteno and cyclohexeno rings to the extended ligand substantially increased cytotoxicity towards a number of cancer cell lines, and induced apoptosis. The complexes localize in the nuclei of cancer cells and co-locate with DAPI on DNA. DNA binding studies show that both complexes bind strongly to DNA and one complex intercalates DNA like the parent, whilst the other appears to have multiple modes of interaction. Discussion: It is likely that the increased lipophilicity of the novel complexes is a key factor for increasing their cytotoxicity, rather than their DNA binding mode.

Locking the Conformation of a Paddlewheel Rhodium Complex: Design, Synthesis, and Applications in Catalytic Nitrene Transfers

AbstractThe exponential proliferation of conformers makes it impossible to examine the entire population in most systems. Controlling conformational ensembles is thus pivotal in many areas of chemistry. Rh2(esp)2, a dicarboxylate‐derived paddlewheel rhodium complex, is one of the most effective catalysts for nitrene chemistry. Its enormous success has led to preparing many analogous complexes. However, there has been little consideration for the conformational dynamics of the parent catalyst. Herein, we report a new ligand modification principle that prevents conformer interconversion. The resulting complex comprises two isolable conformers, whose structures have been determined by X‐ray diffraction. Combined experimental and computational data has revealed similarities and dissimilarities between the conformationally confined and parent complexes. Three model cases have demonstrated the utility of conformational fixation in the development of stereoselective catalysts for nitrene transfer reactions. The design principle described in this study can be combined with other established modification strategies, serving as a springboard for further advancement of the chemistry of paddlewheel metal complexes.

Locking the Conformation of a Paddlewheel Rhodium Complex: Design, Synthesis, and Applications in Catalytic Nitrene Transfers.

The exponential proliferation of conformers makes it impossible to examine the entire population in most systems. Controlling conformational ensembles is thus pivotal in many areas of chemistry. Rh2(esp)2, a dicarboxylate-derived paddlewheel rhodium complex, is one of the most effective catalysts for nitrene chemistry. Its enormous success has led to preparing many analogous complexes. However, there has been little consideration for the conformational dynamics of the parent catalyst. Herein, we report a new ligand modification principle that prevents conformer interconversion. The resulting complex comprises two isolable conformers, whose structures have been determined by X-ray diffraction. Combined experimental and computational data has revealed similarities and dissimilarities between the conformationally confined and parent complexes. Three model cases have demonstrated the utility of conformational fixation in the development of stereoselective catalysts for nitrene transfer reactions. The design principle described in this study can be combined with other established modification strategies, serving as a springboard for further advancement of the chemistry of paddlewheel metal complexes.

Synthesis, crystal structure, thermal properties of a homoleptic Schiff base cobalt complex bis(N-tert-pentyl salicylaldimine)cobalt(II) as precursor for coating Co3O4 thin film by a plasma enhanced MOCVD

A Schiff base complex of cobalt namely, bis(N-tert-pentyl-salicylaldimine)cobalt(II) (2) was synthesized by treating the parent complex, bis(salicylaldehydato)cobalt(II) (1) with n-tert-pentylamine in a methanolic medium. Complex 2 was analysed by dynamic TG/DTA using high purity Ar as purge gas at ambient pressure and was found to be volatile without undergoing decomposition leaving a negligible amount of (1.8 %) residue at 322 °C after melting sharply at 155 °C. The parent complex (1) wherein four oxygen atoms are coordinated to cobalt yielded a residue of 58 % at 400 °C. The dissimilarity in thermal property of 2 (CoN2O2) from that of 1 (CoO2O2) arises from the bonding of nitrogen atoms to cobalt by introducing n-tert-pentylamine which caused replacement of aldehyde O (–CH = O) with azomethine nitrogen (–CH = N). Single crystal XRD analysis of complex 2 revealed it to crystallise in orthorhombic crystal system with a space group of P212121. Further 2 was characterised by FTIR, elemental analysis and mass spectrometry. Transpiration TG technique was employed to evaluate the temperature dependence of saturated vapour pressure (pe) and enthalpy of vaporization (ΔvapH° = 102.3 ± 2.5 kJ mol−1) for 2 in the temperature range of 443–545 K. Complex 2 was transformed in to Co3O4 thin film at 350 °C in an Ar plasma by using a lab made plasma assisted chemical vapour deposition (CVD) process at 1.2 mbar deposition pressure. XRD revealed a preferential growth of cubic Co3O4 in the (311) plane. The SEM images confirmed a dense but scattered growth of cauliflower shaped structures. The particle size was found to be 19.3 nm. Kubelka-Munk function yielded a band gap of 2.01 eV.

Electrocatalytic Atom Transfer Radical Addition with Turbocharged Organocopper(II) Complexes.

The utility and scope of Cu-catalyzed halogen atom transfer chemistry have been exploited in the fields of atom transfer radical polymerization and atom transfer radical addition, where the metal plays a key role in radical formation and minimizing unwanted side reactions. We have shown that electrochemistry can be employed to modulate the reactivity of the Cu catalyst between its active (CuI) and dormant (CuII) states in a variety of ligand systems. In this work, a macrocyclic pyridinophane ligand (L1) was utilized, which can break the C-Br bond of BrCH2CN to release •CH2CN radicals when in complex with CuI. Moreover, the [CuI(L1)]+ complex can capture the •CH2CN radical to form a new species [CuII(L1)(CH2CN)]+ in situ that, on reduction, exhibits halogen atom transfer reactivity 3 orders of magnitude greater than its parent complex [CuI(L1)]+. This unprecedented rate acceleration has been identified by electrochemistry, successfully reproduced by simulation, and exploited in a Cu-catalyzed bulk electrosynthesis where [CuII(L1)(CH2CN)]+ participates as a radical donor in the atom transfer radical addition of BrCH2CN to a selection of styrenes. The formation of these turbocharged catalysts in situ during electrosynthesis offers a new approach to the Cu-catalyzed organic reaction methodology.

Iron-Catalyzed Parahydrogen Induced Polarization.

Parahydrogen induced polarization (PHIP) can address the low sensitivity problem intrinsic to nuclear magnetic resonance spectroscopy. Using a catalyst capable of reacting with parahydrogen and substrate in either a hydrogenative or nonhydrogenative manner can result in signal enhancement of the substrate. This work describes the development of a rare example of an iron catalyst capable of reacting with parahydrogen to hyperpolarize olefins. Complexes of the form (MesCCC)Fe(H)(L)(N2) (L = Py (Py = pyridine), PMe3, PPh3) were synthesized from the reaction of the parent complexes (MesCCC)FeMes(L) (Mes = mesityl) with H2. The isolated low-spin iron(II) hydride compounds were characterized via multinuclear NMR spectroscopy, infrared spectroscopy, and single crystal X-ray diffraction. (MesCCC)Fe(H)(Py)(N2) is competent in the hydrogenation of olefins and demonstrated high activity toward the hydrogenation of monosubstituted terminal olefins. Reactions with p-H2 resulted in the first PHIP effect mediated by iron which requires diamagnetism throughout the reaction sequence. This work represents the development of a new PHIP catalyst featuring iron, unlocking potential to develop more PHIP catalysts based on first-row transition metals.

Nucleosome density shapes kilobase-scale regulation by a mammalian chromatin remodeler.

Nearly all essential nuclear processes act on DNA packaged into arrays of nucleosomes. However, our understanding of how these processes (for example, DNA replication, RNA transcription, chromatin extrusion and nucleosome remodeling) occur on individual chromatin arrays remains unresolved. Here, to address this deficit, we present SAMOSA-ChAAT: a massively multiplex single-molecule footprinting approach to map the primary structure of individual, reconstituted chromatin templates subject to virtually any chromatin-associated reaction. We apply this method to distinguish between competing models for chromatin remodeling by the essential imitation switch (ISWI) ATPase SNF2h: nucleosome-density-dependent spacing versus fixed-linker-length nucleosome clamping. First, we perform in vivo single-molecule nucleosome footprinting in murine embryonic stem cells, to discover that ISWI-catalyzed nucleosome spacing correlates with the underlying nucleosome density of specific epigenomic domains. To establish causality, we apply SAMOSA-ChAAT to quantify the activities of ISWI ATPase SNF2h and its parent complex ACF on reconstituted nucleosomal arrays of varying nucleosome density, at single-molecule resolution. We demonstrate that ISWI remodelers operate as density-dependent, length-sensing nucleosome sliders, whose ability to program DNA accessibility is dictated by single-molecule nucleosome density. We propose that the long-observed, context-specific regulatory effects of ISWI complexes can be explained in part by the sensing of nucleosome density within epigenomic domains. More generally, our approach promises molecule-precise views of the essential processes that shape nuclear physiology.

Phototoxicity of Tridentate Ru(II) Polypyridyl Complex with Expanded Bite Angles toward Mammalian Cells and Multicellular Tumor Spheroids.

Tridentate ligand-coordinated ruthenium (II) polypyridyl complexes with large N-Ru-N bite angles have been shown to promote ligand field splitting and reduce singlet-triplet state mixing leading to dramatically extended emission quantum yields and lifetimes under ambient conditions. These effects are anticipated to enhance their photoinduced singlet oxygen production, promoting prospects for such complexes as type II phototherapeutics. In this contribution, we examined this putative effect for [Ru(bqp)(bqpCOOEt)]2+, Ru-bqp-ester, a heteroleptic complex containing bqp = [2,6-bi(quinolin-8-yl)pyridine], a well-established large bite angle tridentate ligand, as well as its peptide conjugates [Ru(bqp)(bqpCONH-ahx-FrFKFrFK(Ac)-CONH2)]5+ (Ru-bqp-MPP) and [Ru(bqp) (bqp)(CONH-ahx-RRRRRRRR-CONH2)]10+ (Ru-bqp-R8) that were prepared in an effort to promote live cell/tissue permeability and targeting of the parent. Membrane permeability of both parent and peptide conjugates were compared across 2D cell monolayers; A549, Chinese hamster ovary, human pancreatic cancer (HPAC), and 3D HPAC multicellular tumor spheroids (MCTS) using confocal microscopy. Both the parent complex and peptide conjugates showed exceptional permeability with rapid uptake in both 2D and 3D cell models but with little distinction in permeability or distribution in cells between the parent or peptide conjugates. Unexpectedly, the uptake was temperature independent and so attributed to passive permeation. Both dark and photo-toxicity of the Ru(II) complexes were assessed across cell types, and the parent showed notably low dark toxicity. In contrast, the parent and conjugates were found to be highly phototoxic, with impressive phototoxic indices (PIs) toward HPAC cell monolayers in particular, with PI values ranging from ∼580 to 760. Overall, our data indicate that the Ru(II) parent complex and its peptide conjugates show promise at both cell monolayers and 3D MCTS as photosensitizers for photodynamic therapy.

Synthesis, characterization, and biological evaluation of some metal complexes containing N and S donor atoms

Eight new bipyridine adducts of Zn(II) and Ni(II) dithiocarbamate complexes have been prepared from the reaction of Zn(II) and Ni(II) complexes of N-ethyl-N-phenyldithiocarbamate and 4,4′ dipyridine, 4,4′-dimethyl-2,2′-bipyridine, 4,4′-dimethoxy-2,2′-bipyridine, 4,4′-ditertbutyl-2,2′-bipyridine. The resulting adducts were represented as [M(L1)2(DP)], [M(L1)2(DMeB)], [M(L1)2(DMxB)] and [M(L1)2(DTB)] respectively (where M = Zn or Ni). The structural configuration of these adducts was established using various spectroscopic techniques. The obtained data from the spectroscopic studies revealed a change in the tetrahedral configuration of the parent complexes to octahedral coordination in the adducts due to an additional metal-nitrogen bond. The formation of this bond was established by a considerable shift in the peaks around the metal-nitrogen (M−N) coordination upon the formation of the adducts. The study of biological properties including cytotoxicity, antioxidant, and anti-inflammatory properties were carried out. No noticeable trends were reported in all the studies despite the concentration-dependent profile shown in the assays and the structural similarities of the compounds. These adducts, nevertheless, showed good antioxidant activity in the DPPH assay, with IC50 ranging between 3.78 and 4.87 µg/mL which is better than the standard ascorbic acid (4.96 µg/mL). Also, in the cytotoxicity study, the adducts [Zn(L)2DMeBp] (1) and [Ni(L)2 DTBp] (6) showed the best activity against the mutating human cervical cancer (HeLa) and embryo kidney (HEK 293) cell lines and better than the standard 5-Flurouracil. The molecular docking studies further revealed that the adducts had a good binding affinity to the drug targets used for the study.

Investigating Metal-Metal Bond Polarization in a Heteroleptic Tris-Ylide Diiron System.

This article describes the synthesis, characterization, and S-atom transfer reactivity of a series of C3v-symmetric diiron complexes. The iron centers in each complex are coordinated in distinct ligand environments, with one (FeN) bound in a pseudo-trigonal bipyramidal geometry by three phosphinimine nitrogens in the equatorial plane, a tertiary amine, and the second metal center (FeC). FeC is coordinated, in turn, by FeN, three ylidic carbons in a trigonal plane, and, in certain cases, by an axial oxygen donor. The three alkyl donors at FeC form through the reduction of the appended N═PMe3 arms of the monometallic parent complex. The complexes were studied crystallographically, spectroscopically (NMR, UV-vis, and Mössbauer), and computationally (DFT, CASSCF) and found to be high-spin throughout, with short Fe-Fe distances that belie weak orbital overlap between the two metals. Further, the redox nature of this series allowed for the determination that oxidation is localized to the FeC. S-atom transfer chemistry resulted in the formal insertion of a S atom into the Fe-Fe bond of the reduced diiron complex to form a mixture of Fe4S and Fe4S2 products.

Iron(III)-Complexes with N-Phenylpyrazole-Based Ligands

The use of iron as a replacement for noble metals in photochemical and photophysical applications is challenging due to the typically fast deactivation of short-lived catalytically active states. Recent success of a cyclometalated iron(III) complex utilizing a bis-tridentate ligand motif inspired the use of phenyl-1H-pyrazole as a bidentate ligand. Five complexes using the tris(1-phenylpyrazolato-N,C2)iron(III) complex scaffold are presented. In addition to the parent complex, four derivatives with functionalization in the meta-position of the phenyl ring are thoroughly investigated by single crystal diffractometry, UV-Vis-spectroscopy, and cyclic voltammetry. Advanced X-ray spectroscopy in the form of X-ray absorption and emission spectroscopy allows unique insights into the electronic structure as well as DFT calculations. The ligand design leads to overlapping MLCT and LMCT absorption bands, and emissive behavior is suppressed by low-lying MC states.

Cover Feature: Electrochemical CO2 Reduction with a Heterogenized Iridium−Pincer Catalyst in Water (ChemCatChem 12/2023)

The Cover Feature illustrates the integration of a well-defined molecular electrocatalyst onto carbon-based electrodes leading to a new hybrid material active for the CO2 reduction reaction in fully aqueous medium. In their Research Article, M. Koepf, M. Gennari and co-workers discuss the heterogenization of an iridium-pincer complex via π-π stacking interactions between the pyrene tagged-complex and multiwalled carbon nanotubes. Following the in-depth investigation of the parent complex under homogeneous conditions, they are able to show that the selectivity towards formate production is retained after immobilization. More information can be found in the Research Article by M. Koepf, M. Gennari and co-workers.

Ligand Tuning in Cu(pyalk)2 Water Oxidation Electrocatalysis

Molecular copper water oxidation electrocatalysts have been extensively studied in recent years for their potential use in artificial photosynthetic systems for solar energy conversion. Although ligand modification and its ability to influence catalytic properties is a key advantage of molecular systems, there are, as yet, few examples of systematic studies of these effects. Our oxidatively resistant pyalk (2-pyridyl-2-propanoate) ligand forms a complex with copper(II) that catalyzes water oxidation and provides an attractive scaffold for systematic ligand tuning. Here, we report a series of analogous copper complexes with electron-donating (methoxy-) and -withdrawing (methoxycarbonyl-) groups at the para-position of the pyalk ligand. Trends in the pKa and redox potential align with first-principles predictions for the electron-withdrawing and electron-donating groups. While the modified complexes show good activity for water oxidation, lowered faradaic efficiency in comparison to the parent complex highlights the importance of stability considerations for catalyst tuning.

Plugging the 3MC Sink in RuII‐Based Photocatalysts

AbstractQuantum chemical methods have been utilised to explore the kinetics and thermodynamics of a prominent charge recombination pathway in a series of RuII based molecular photocatalysts. Selective tuning of the RuII coordination sphere, replacing the tbbpy ligands of the hydrogen evolving parent photocatalyst with electron rich, biimidazole based ligands, promotes unidirectional charge transfer towards the bridging ligand during initial photoexcitation. These electronic effects are also significant in the triplet manifold, where the predicted rate of the undesired deactivation process from the 3MLCT state on the bridging ligand to a 3MC state on the ruthenium centre, is decreased relative to the parent complex, by 1–2 orders of magnitude, alongside a decrease in electronic coupling. This design methodology could be utilised to promote targeted (light‐driven) electron transfer pathways, as well as to potentially reduce 3MC deactivation pathways in commonly used polypyridyl‐based RuII photocentres, thus enhancing the quantum efficiency of light driven catalysis.

Luminescent iridium(III) porphyrin complexes as near-infrared-emissive biological probes.

We report herein the design, synthesis and characterisation of a series of luminescent iridium(III) porphyrin complexes [Ir(ttp)(CH2CH2OH)] (H2ttp = 5,10,15,20-tetra-4-tolylporphyrin) (1), [Ir(tpp-Ph-NO2)(CO)Cl] (H2tpp-Ph-NO2 = 5-(4-((4-nitrophenoxy)carbonyloxymethyl)phenyl)-10,15,20-triphenylporphyrin) (2), [Ir(tpp-COOMe)(Py)2](Cl) (H2tpp-COOMe = 5-(4-methoxycarbonylphenyl)-10,15,20-triphenylporphyrin; Py = pyridine) (3) and [Ir(tpp-COOH)(Py)2](Cl) (H2tpp-COOH = 5-(4-carboxylphenyl)-10,15,20-triphenylporphyrin) (4). All the complexes displayed long-lived near-infrared (NIR) emission attributed to an excited state of mixed triplet intraligand (3IL) (π → π*) (porphyrin) and triplet metal-to-ligand charge transfer (3MLCT) (dπ(Ir) → π*(porphyrin)) character. The cytotoxicity of the complexes toward HeLa cells was examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cationic complexes 3 and 4 exhibited higher cytotoxic activity toward HeLa cells than their neutral counterparts 1 and 2. Cellular uptake studies by inductively coupled plasma-mass spectrometry (ICP-MS) and laser-scanning confocal microscopy (LSCM) indicated that complexes 3 and 4 showed higher cellular uptake efficiencies than complexes 1 and 2 due to their cationic charge, and they were enriched in the perinuclear region of the cells with negligible nuclear uptake. Additionally, the carboxyl complex 4 was used to label a model protein bovine serum albumin (BSA) via an amidation reaction. The resultant luminescent protein conjugate 4-BSA displayed similar photophysical properties and intracellular localisation behaviour to its parent complex. The results of this work will contribute to the development of luminescent iridium(III) porphyrin complexes and related bioconjugates as NIR-emissive probes for bioimaging applications.

Spin crossover in mixed-anion Fe(NH2trz)3(BF4)(SiF6)0.5 crystalline rod-shaped particles: the strength of the solid-liquid post synthetic modification.

A pure mixed-anion Fe(NH2trz)3(BF4)(SiF6)0.5 spin crossover complex is obtained implementing a solid-liquid post synthetic modification approach from the Fe(NH2trz)3(BF4)2 parent complex. This method allows obtaining highly crystalline powder samples incorporating homogeneous micrometric (1 μm long) rod-shaped particles. This compound presents an abrupt spin crossover behaviour with a narrow (10 K) hysteresis loop centred just above room temperature (320 K) which makes it very interesting for future integration into devices for various applications.

A convenient route to mixed cationic group 13/14/15 compounds.

The formation of novel cationic mixed main group compounds is reported revealing a chain composed of different elements of group 13, 14, and 15. Reactions of different pnictogenylboranes R2EBH2·NMe3 (E = P, R = Ph, H; E = As, R = Ph, H) with the NHC-stabilized compound IDipp·GeH2BH2OTf (1) (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene) were carried out, yielding the novel cationic, mixed group 13/14/15 compounds [IDipp·GeH2BH2ER2BH2·NMe3]+ (2a E = P; R = Ph; 2b E = As; R = Ph; 3a E = P; R = H; 3b E = As; R = H) by the nucleophilic substitution of the triflate (OTf) group. The products were analysed by NMR spectroscopy and mass spectrometry and for 2a and 2b also by X-ray structure analysis. Further reactions of 1 with H2EBH2·IDipp (E = P, As) resulted in the unprecedented parent complexes [IDipp·GeH2BH2EH2BH2·IDipp][OTf] (5a E = P; 5b E = As), which were studied by X-ray structure analysis, NMR spectroscopy and mass spectrometry. Accompanying DFT computations give insight into the stability of the formed products with respect to their decomposition.

Synthesis and Characterization of Late Transition Metal Complexes of Mono-Acetate Pendant Armed Ethylene Cross-Bridged Tetraazamacrocycles with Promise as Oxidation Catalysts for Dye Bleaching

Ethylene cross-bridged tetraazamacrocycles are known to produce kinetically stable transition metal complexes that can act as robust oxidation catalysts under harsh aqueous conditions. We have synthesized ligand analogs with single acetate pendant arms that act as pentadentate ligands to Mn, Fe, Co, Ni, Cu, and Zn. These complexes have been synthesized and characterized, including the structural characterization of four Co and Cu complexes. Cyclic voltammetry demonstrates that multiple oxidation states are stabilized by these rigid, bicyclic ligands. Yet, redox potentials of the metal complexes are modified compared to the "parent" ligands due to the pendant acetate arm. Similarly, gains in kinetic stability under harsh acidic conditions, compared to parent complexes without the pendant acetate arm, were demonstrated by a half-life seven times longer for the cyclam copper complex. Due to the reversible, high oxidation states available for the Mn and Fe complexes, the Mn and Fe complexes were examined as catalysts for the bleaching of three commonly used pollutant model dyes (methylene blue, methyl orange, and Rhodamine B) in water with hydrogen peroxide as oxidant. The efficient bleaching of these dyes was observed.

Visible-Light-Driven Photocatalytic CO2 Reduction by Re(I) Photocatalysts with N-Heterocyclic Substituents

The properties and reactivity of metal complex photocatalysts for solar-to-fuel conversion can be conveniently controlled by the molecular design of their ligands. In this work, rhenium(I) tricarbonyl polypyridyl compounds, fac-[Re(NN)(CO)3Cl], where NN is a 1,10-phenanthroline ligand having pyrrole (Re-pyr), indole (Re-ind), or carbazole (Re-cbz) groups attached to its 4 and 7 positions, were investigated as photocatalysts for reducing CO2 to CO. These complexes can harvest light in the visible region and displayed an exceptionally high photocatalytic performance, with an up to 10-fold increase in the TONCO values compared to the unsubstituted Re-phen parental complex. Re-pyr was the best performing photocatalyst in this series, with a TONCO of 125 after 24 h and ΦCO = 0.22. The TONCO of Re-pyr, Re-ind, and Re-cbz surpassed even the most efficient Lehn-type photocatalysts reported to date to promote the photoreduction of CO2 using triethanolamine as the sacrificial electron donor, without a second molecule working as a light absorber. The excellent photocatalytic performance of this series was intimately related to the presence of N-heterocyclic substituents, which enhanced the visible-light absorption properties of the compounds, favored the kinetics and thermodynamics of each individual step of the photocatalysis, inhibited deactivation of the photocatalysts by dimerization, stabilized reduced intermediates, and unlocked an alternative CO2 photoreduction pathway that proceeds predominantly through a two-electron reduced species [Re(NN)(CO)3]−.