Synthesis of a Redox-Active NNP-Type Pincer Ligand and Its Application to Electrocatalytic CO2 Reduction With First-Row Transition Metal Complexes

Introducing Novel Redox-Active Bis(phenolate) N-Heterocyclic Carbene Proligands: Investigation of Their Coordination to Fe(II)/Fe(III) and Their Catalytic Activity in Transfer Hydrogenation of Carbonyl Compounds

A simple and efficient procedure for synthesizing novel pincer-type tridentate N-heterocyclic carbene bisphenolate ligands is reported. The synthesis of pincer proligands with N,N'-disubstituted imidazoline core, 5 and 6, was carried out via triethylorthoformate-promoted cyclization of either N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)cyclohexanediamine, 3, or N,N'-bis(2-hydroxyphenyl)cyclohexanediamine, 4, in the presence of concentrated hydrochloric acid. Cyclic voltammograms of the ligands revealed ligand-centered redox activity, indicating the noninnocent nature of the ligands. The voltammograms of the ligands exhibit two successive one-electron oxidations and two consecutive one-electron reductions. In contrast to previous reports, the redox-active ligands in this study exhibit one-electron oxidation and reduction processes. All products were thoroughly characterized by using 1H and 13C NMR spectroscopy. The base-promoted deprotonation of the proligands and subsequent reaction with iron(II) and iron(III) chlorides yielded compounds 7 and 8. These compounds are binuclear and tetranuclear iron(III) complexes that do not contain carbene functional groups. Complexes 7 and 8 were characterized by using elemental analysis and single-crystal X-ray crystallography. At low catalyst loadings, both 7 and 8 exhibited high catalytic activity in the transfer hydrogenation of selected aldehydes and ketones.

Palladium-anchored donor-flexible pyridylidene amide (PYA) electrocatalysts for CO2 reduction

The conversion of CO2 into CO as a substitute for processing fossil fuels to produce hydrocarbons is a sustainable, carbon neutral energy technology. However, the electrochemical reduction of CO2 into a synthesis gas (CO and H2) at a commercial scale requires an efficient electrocatalyst. In this perspective, a series of six new palladium complexes with the general formula [Pd(L)(Y)]Y, where L is a donor-flexible PYA, N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, N2,N6-bis(1-butylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, or N2,N6-bis(1-benzylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, and Y = OAc or Cl-, were utilized as active electrocatalysts for the conversion of CO2 into a synthesis gas. These palladium(ii) pincer complexes were synthesized from their respective H-PYA proligands using 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) or sodium acetate as a base. All the compounds were successfully characterized by various physical methods of analysis, such as proton and carbon NMR, FTIR, CHN, and single-crystal XRD. The redox chemistry of palladium complexes toward carbon dioxide activation suggested an evident CO2 interaction with each Pd(ii) catalyst. [Pd(N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide)(Cl)]Cl showed the best electrocatalytic activity for CO2 reduction into a synthesis gas under the acidic condition of trifluoracetic acid (TFA) with a minimum overpotential of 0.40 V, a maximum turnover frequency (TOF) of 101 s-1, and 58% FE of CO. This pincer scaffold could be stereochemically tuned with the exploration of earth abundant first row transition metals for further improvements in the CO2 reduction chemistry.

First-Row Transition Metal Complexes of a Phosphine-Silylene-Based Hybrid Ligand

We have prepared two new silylene-phosphine-based hybrid ligands Si{N(R)C₆H₄(PPh₂)}{PhC(N^tBu)₂} [R = TMS {trimethylsilyl} (1) and TBDMS {tert-butyldimethylsilyl} (2)], which possess two donor sites. Furthermore, the treatment of the bidentate ligand 1 with base metal halides {FeBr₂, CoBr₂, NiCl₂·dme [nickel chloride(II) ethylene glycol dimethyl ether]} and 2 with NiBr₂·dme [nickel bromide(II) ethylene glycol dimethyl ether] afforded four-coordinate six-membered metal complexes 3-6, respectively, which feature coordination from both Si(II) and P(III) sites. Subsequently, complexes 3 [(FeBr₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], 4 [(CoBr₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], 5 [(NiCl₂)Si{N(SiMe₃)C₆H₄(PPh₂)}{PhC(N^tBu)₂}], and 6 [(NiBr₂)Si{N(Si^tBuMe₂)C₆H₄(PPh₂)}{PhC(N^tBu)₂}] are studied for their redox and magnetic properties with the help of UV-vis spectroscopy, cyclic voltammetry, SQUID magnetometry, and theoretical calculations. Complexes 3-6 were found to display a paramagnetic behavior. All the compounds are well established by single-crystal X-ray diffraction studies.

Electrocatalytic CO2 Reduction by Molecular Ruthenium Complexes with Polypyridyl Ligands

Two series of ruthenium complexes with various polypyridyl ligands have been prepared. One series of complexes (5 examples) are featured with tetradentate polypyridyl ligands and two acetonitrile molecules at the axial positions of the coordination sphere; the other series (3 examples) include combinations of a tridentate polypyridyl ligand, one 2,2'-bipyridine (bpy) or two picolines, and one acetonitrile ligand. All these complexes were fully characterized by their NMR spectra as well as X-ray single crystal structures. Their electronic absorption and redox data were measured and reported. Of the 8 complexes, three candidates effectively catalyze electrochemical CO 2 reduction reaction (CO 2 RR) in wet acetonitrile medium, generating CO as the major product. All these three catalytically active complexes contain a 2,2':6',2″:6″,2‴-quaterpyridine (qpy) ligand scaffold. A maximum turnover frequency (TOF max ) of > 1000 s -1 was achieved for the electrocatalytic CO 2 reduction at a modest overpotential. On the basis of electrochemical and spectroelectrochemical evidences, the CO 2 substrate was proposed to bind with the ruthenium center at the two-electron reduced state of the complex and then enter the catalytic cycle.

Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO₂). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO₂, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO₂ in their reduced states. The Ni(0) d¹⁰ species associates with CO₂ to form a putative Aresta-type Ni-η²-CO₂ complex, where electron transfer to CO₂ through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d⁹ intermediate binding CO₂ can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d⁷-to-d⁹ configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO₂ activation for complexes based on redox-innocent ligands.

A series of 3d transition-metal complexes (M = Mn, Fe, Co, Ni, Cu, and Zn) coordinated by a new redox-innocent PNP pincer ligand system were synthesized and structurally as well as electrochemically analyzed to illuminate the role of the metal center in molecular electrochemical carbon dioxide (CO₂) activation.

Unraveling the Effect of Aromatic Groups in Mn(I)NNN Pincer Complexes on Carbon Dioxide Activation Using Density Functional Study

Carbon dioxide utilization is necessary to reduce carbon footprint and also to synthesize value-added chemicals. The transition metal pincer complexes are attractive catalysts for the hydrogenation of carbon dioxide to formic acid. There is a need to understand the factors affecting the catalytic performance of these pincer complexes through a structure-activity relationship study using computational methods. It is a well-established fact that aromatic functionalities offer stability and selectivity to transition metal catalysts. However, their impact on the performance of the catalysts is lesser known in the case of metal pincer complexes. Hence, it is necessary to investigate the catalytic performance of Mn(I)NNN pincer complexes with variably activated aromatic functionalities. In this context, 15 catalysts are designed by placing different types of aromatic rings at the pincer carbons and two terminal nitrogen of Mn(I)NNN pincer complexes. A benzene moiety, placed at C2-C3 carbons of Mn(I)NNN pincer complex with identical aromatic groups at the terminal nitrogen, is found to be most efficient toward CO₂ hydrogenation than the rest of the catalysts. On the other hand, when N,N-dimethyl aniline is placed at C2-C3 carbons of Mn(I)NNN pincer complexes, then the catalytic performance is significantly decreased. Thus, the present study unravels the impact of aromatic groups in Mn(I)NNN pincer complexes toward the catalytic hydrogenation of carbon dioxide.

Transition Metal Complexes as Catalysts for the Electroconversion of CO2 : An Organometallic Perspective

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Electro- and Photochemical Reduction of CO2 by Molecular Manganese Catalysts: Exploring the Positional Effect of Second-Sphere Hydrogen-Bond Donors

A series of molecular Mn catalysts featuring aniline groups in the second-coordination sphere has been developed for electrochemical and photochemical CO₂ reduction. The arylamine moieties were installed at the 6 position of 2,2'-bipyridine (bpy) to generate a family of isomers in which the primary amine is located at the ortho- (1-Mn), meta- (2-Mn), or para-site (3-Mn) of the aniline ring. The proximity of the second-sphere functionality to the active site is a critical factor in determining catalytic performance. Catalyst 1-Mn, possessing the shortest distance between the amine and the active site, significantly outperformed the rest of the series and exhibited a 9-fold improvement in turnover frequency relative to parent catalyst Mn(bpy)(CO)₃ Br (901 vs. 102 s^-1 , respectively) at 150 mV lower overpotential. The electrocatalysts operated with high faradaic efficiencies (≥70 %) for CO evolution using trifluoroethanol as a proton source. Notably, under photocatalytic conditions, a concentration-dependent shift in product selectivity from CO (at high [catalyst]) to HCO₂ H (at low [catalyst]) was observed with turnover numbers up to 4760 for formic acid and high selectivities for reduced carbon products.

Synthesis, characterization, and electrocatalytic activity of bis(pyridylimino)isoindoline Cu(ii) and Ni(ii) complexes

Similar structure observed between Cu(ii) and Ni(ii) based bis(pyridylimino)isoindole complexes, yet greatly different levels of catalytic activity.

Yellow-to-brown and yellow-to-green electrochromic devices based on complexes of transition metal ions with a triphenylamine-based ligand

Transmissive-to-colored electrochromism has been achieved by combination of MLCT of transition metal complexes with the electrochromic properties of ligand molecules. The color transitions were from yellow to dark brown for the Fe(ii) complex, yellow to orange to bluish-green for the Co(ii) complex and yellow to green for the Zn(ii) complex. By using a metal ion-ligand coordination approach, the self-assembly of hydrazone-based ligands containing a triphenylamine group with appropriate metal salts (FeCl2, Co(ClO4)2 and Zn(BF4)2) produced novel complexes of the general formula [ML2]X2. The isolated complexes were characterized by spectroscopic methods, and the Co(ii) complex also by X-ray diffraction analysis. Thin films of the complexes have been obtained by a spray-coating method and they were used in the construction of electrochromic devices, which showed good electrochromic stability, a high color contrast of 47.5% for Fe(ii), 37.2% for Co(ii) and 33.7% for Zn(ii) complexes and fast coloring and bleaching times.

Enhanced Electrochemical CO2 Reduction by a Series of Molecular Rhenium Catalysts Decorated with Second-Sphere Hydrogen-Bond Donors

A series of rhenium(I) fac-tricarbonyl complexes containing pendent arylamine functionality in the second coordination sphere have been developed and studied as electrocatalysts for carbon dioxide (CO₂) reduction. Aniline moieties were appended at the 6 position of a 2,2'-bipyridine (bpy) donor in which the primary amine was positioned at the ortho- (1-Re), meta- (2-Re), and para- (3-Re) sites of the aniline substituent to generate a family of isomers. The relationship between the catalyst structure and activity was explored across the series, and the catalytic performance was compared to that of the benchmark catalyst Re(bpy)(CO)₃Cl (ReBpy). Catalysts 1-Re, 2-Re, and 3-Re outperform the benchmark catalyst both in anhydrous acetonitrile and with added trifluoroethanol (TFE) as an external proton source. In the presence of TFE, the aniline-substituted catalysts convert CO₂ to carbon monoxide (CO) with high Faradaic efficiencies (≥89%) and have superior turnover frequencies (TOFs) relative to ReBpy (72.9 s^-1), with 2-Re having the highest TOF of the series at 239 s^-1, a value that is twice that of the next most active catalyst. TOFs of 123 and 109 s^-1 were observed for the ortho- and para-substituted aniline complexes (1-Re and 3-Re), respectively. Indeed, catalytic activities vary widely across the series, showing a high sensitivity to the position of the amine functionality relative to the rhenium active site. IR and UV-vis spectroelectrochemical experiments were conducted on the aniline-substituted systems, revealing important differences between the catalysts and mechanistic insight.

Introducing Seven Transition Metal Ions into Terpyridine-Based Supramolecules: Self-Assembly and Dynamic Ligand Exchange Study

In coordination-driven self-assembly, 2,2':6',2″-terpyridine (tpy) has gained extensive attention in constructing supramolecular architectures on the basis of ⟨tpy-M-tpy⟩ connectivity. In direct self-assembly of large discrete structures, however, the metal ions were mainly limited to Cd(II), Zn(II), and Fe(II) ions. Herein, we significantly broaden the spectrum of metal ions with seven divalent transition metal ions M(II) (M = Mn, Fe, Co, Ni, Cu, Zn, Cd) to assemble a series of supramolecular fractals. In particular, Mn(II), Co(II), Ni(II), and Cu(II) were reported for the first time to form such large and discrete structures with ⟨tpy-M-tpy⟩ connectivity. In addition, the structural stabilities of those supramolecules in the gas phase and the kinetics of the ligand exchange process in solution were investigated using mass spectrometry. Such a fundamental study gave the relative order of structural stability in the gas phase and revealed the inertness of coordination in solution depending on the metal ions. Those results would guide the future study in tpy-based supramolecular chemistry in terms of self-assembly, characterization, property, and application.