Axial Pyridine Research Articles

Synthesis, general physicochemical behaviour and magnetic studies of the macrocyclic bis-pyridine derivatives [TTDPzM(py)2]·2H2O (M = MnII, CoII, NiII)

In our extensive work on phthalocyanine-like macrocycles carrying peripherally attached strongly electron-withdrawing 1,2,5-thia/selenodiazol rings we recently reported on the FeII species of formula [TTDPzFe(py)2].2H2O (TTDPz = tetrakis(thiadiazole)porphyrazinato dianion). The present work deals with the synthesis and extended physicochemical studies of the series of related macrocycles [TTDPzM(py)2].2H2O (M = MnII, CoII, NiII). The diffractograms of three solid samples of the NiII complex [TTDPzNi(py)2]·2H2O, obtained from different synthetic procedures, indicate that the species is reproducibly obtained. All three samples are isomorphous with each other as shown by their X-ray powder diffractograms. Based on the comparison of the IR spectra of [TTDPzNi(py)2].2H2O and the related desolvated [TTDPzNi], the IR absorption peaks belonging to the pyridine molecules could be identified. The systematic presence of the two water molecules in all three species suggests that forms of contact do exist in the form of hydrogen bonds between them and the S and N atoms present peripherally in the thiadiazole groups of the macrocyclic units. As regard to the elimination of water, the thermograms (in inert atmosphere in the range 25–600 °C) indicate in all cases that loss of water occurs below 100 °C but loss of pyridine occurs over a broad range 100–300 °C for the MnII and CoII species whereas for the NiII complex it takes place in the narrow range 200–220 °C. In relation to the parallel series of metallophthalocyanines, i.e. [PcM] (M = MnII, FeII, CoII), it is noted that the NiII analog has not been reported in the literature and in a direct experiment it has been proved that [PcNi] heated in pyridine is obtained unchanged, the different behavior for the NiII TTDPz derivative due to the electron withdrawing effect of the external thiadiazole substituents, which makes favorable axial pyridine coordination in the complex. Completely insoluble in water, the triad of TTDPz derivatives are extremely low-soluble in non-donor or low donor solvents. This precluded any attempt to isolate single crystals for single-crystal X-ray work. Their UV–visible spectra show essential similarities in whatever organic solvent is used with clearly positioned Soret (300–400 nm) and Q-band absorptions (630–650 nm). Variable temperature magnetic susceptibility and magnetization studies reveal that the [TTDPzM(py)2].2H2O complexes all show paramagnetism with ground state spins of 3/2 (MnII), ½ (CoII) and 1 (NiII), supported by fitting the data to spin Hamiltonian formalism, vide infra. Comparisons are made with the magnetism of [PcM] analogues with key differences noted for M = NiII.

Expedited Proton Relay in Enzyme-Inspired Cobaloximes Facilitate Organic Transformations.

Developing a water-soluble, oxygen-tolerant, and acid-stable synthetic H2 production catalyst is vital for renewable energy infrastructure. To access such an effective catalyst, we strategically incorporated enzyme-inspired, multicomponent outer coordination sphere elements around the cobaloxime (Cl-Co-X) core with suitable axial coordination (X). Our cobaloximes with axial imidazole or L-histidine coordination in photocatalytic HAT including the construction of anilines via a non-canonical cross-coupling approach is found superior compared to commonly used cobaloxime catalysts. The reversible Co(II)/Co(I) process is influenced by the axial N ligand's nature. Imidazole/L-histidine with a higher pKa promptly produces H2 upon irradiation, leading to the improved reactivity compared to previously employed axial (di)chloride or pyridine analogue.

Electrochemical ammonia oxidation catalyzed by ruthenium complexes: investigation of substituent effect of axial pyridine ligands

Abstract We have examined catalytic ammonia oxidation using ruthenium complexes bearing 2,2′-bipyridine-6,6′-dicarboxylate ligand and axial 3- and 4-substituted pyridine ligands under electrochemical conditions to study the substituent effect of the axial pyridine ligands. Ruthenium complex bearing 3,4-dibromopyridine ligands shows the highest catalytic activity to produce up to 145 equivalents of dinitrogen based on the catalyst.

A One‐Pot Three‐In‐One Synthetic Strategy to Immobilize Cobalt Corroles on Carbon Nanotubes for Oxygen Electrocatalysis

AbstractMolecular electrocatalysis requires the immobilization of molecular catalysts on electrode materials for practical uses. Direct adsorption of catalyst molecules on electrode materials is simple, but grafting molecules through chemical bonds can significantly improve electrocatalytic efficiency and durability. However, designing and synthesizing catalyst molecules to simultaneously improve catalytic activities and realize easy grafting on electrodes is challenging. The study herein reports on a one‐pot strategy to graft Co corroles on pyridine‐modified carbon nanotubes (CNTs) for oxygen electrocatalysis. By modifying CNTs with pyridines, the resulting pyridine‐modified CNTs can function as platforms to grab in situ generated Co corroles through the axial pyridine ligation on Co. Such an immobilization strategy combines three effects for the regulation of electrocatalysis, including the axial ligand effect, the electron transfer effect, and the chemical bond effect. The resulting hybrid materials show high efficiency for oxygen electrocatalysis, and the Zn–air battery assembled using these materials displays comparable performance as the Pt/Ir‐based battery. Moreover, the electrocatalytic efficiency of these hybrid materials can be systematically tuned by using different pyridines, highlighting the controllable feature of this strategy. Therefore, this work is significant to present a one‐pot three‐in‐one strategy to immobilize catalyst molecules on CNTs for electrocatalysis.

Unraveling the mechanism of pyrrole and N-defect regulating CoN4 single atom catalysts as a pH-universal bifunctional electrocatalyst for OER and ORR

The development of bifunctional electrocatalysts for fuel cells and rechargeable metal-air cells was very urgent, but it still faced enormous challenges. Single atom catalysts had achieved satisfactory performance in these research fields, however, the coordination environment and catalytic mechanism of the active center were still unclear. Herein, density functional theory (DFT) calculations were used to systematically investigated the effects of heterolateral pyridine or pyrrole ligand and/or N-defect on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) of CoN4 catalysts both in acidic and alkaline media. Compared with CoN4, the N-defect and axial pyrrole or pyridine ligand jointly modified CoN4 catalysts had improve activities for OER, the N-defect and heterolateral pyrrole ligand modified CoN4 catalysts had improve activities for ORR. Additionally, CoN3-pyrrole also exhibited moderate intrinsic barriers for the kinetics rate-limiting step (KRLS) in both acidic and alkaline media for OER and ORR. In conclusion, CoN3-pyrrole was considered as the optimal candidate for bifunctional catalysts due to its relatively superior bifunctional theoretical overpotential and reaction barrier of KRLS in acidic and alkaline media. The theoretical overpotential was not the only evaluation criteria, the intrinsic barrier of KRLS must be considered as another evaluation criteria of electrocatalytic performance.

Soluble Ruthenium Phthalocyanines as Semiconductors for Organic Thin‐Film Transistors

AbstractRuthenium phthalocyanine (RuPcs) are multipurpose compounds characterized by their remarkable reactivity and photoelectronic properties, which yield a broad synthetic scope and easy derivatization at the axial position. However, RuPcs have been underexplored for use in organic thin‐film transistors (OTFTs), and therefore new studies are necessary to provide basic insight and a first approach in this new application. Herein, two novel RuPc derivatives, containing axial pyridine substituents with aliphatic chains (RuPc(CO)(PyrSiC6) (1) and RuPc(PyrSiC6)2 (2), were synthesized, characterized, and tested as the organic semiconductor in OTFTs. RuPc thin‐films were characterized by X‐ray diffraction (XRD), and atomic force microscopy (AFM) to assess film morphology and microstructure. 1 displayed comparable p‐type device performance to other phthalocyanine‐based OTFTs of similar design, with an average field effect mobility of 2.08×10−3 cm2 V−1 s−1 in air and 1.36×10−3 cm2 V−1 s−1 in nitrogen, and threshold voltages from −11 V to −20 V. 2 was found to be non‐functional as the semiconductor in the device architecture used, likely as a result of significant differences in thin‐film formation. The results of this work illustrate a promising starting point for future development of RuPc electronic devices, particularly in this new family of OTFTs.

Single-Atom Cobalt Catalysts Coupled with Peroxidase Biocatalysis for C-H Bond Oxidation.

This paper reports a robust strategy to catalyze in situ C-H oxidation by combining cobalt (Co) single-atom catalysts (SACs) and horseradish peroxidase (HRP). Co SACs were synthesized using the complex of Co phthalocyanine with 3-propanol pyridine at the two axial positions as the Co source to tune the coordination environment of Co by the stepwise removal of axial pyridine moieties under thermal annealing. These structural features of Co sites, as confirmed by infrared and X-ray absorption spectroscopy, were strongly correlated to their reactivity. All Co catalysts synthesized below 300 °C were inactive due to the full coordination of Co sites in octahedral geometry. Increasing the calcination temperature led to an improvement in catalytic activity for reducing O2, although molecular Co species with square planar coordination obtained below 600 °C were less selective to reduce O2 to H2O2 through the two-electron pathway. Co SACs obtained at 800 °C showed superior activity in producing H2O2 with a selectivity of 82-85% in a broad potential range. In situ production of H2O2 was further coupled with HRP to drive the selective C-H bond oxidation in 2-naphthol. Our strategy provides new insights into the design of highly effective, stable SACs for selective C-H bond activation when coupled with natural enzymes.

New Mn(III) complexes with tetradentate cis-N2O2, acacen-type ligands. Synthesis and characterization, single-crystal X-ray diffraction and DFT studies

One of the challenges of manganese(III) magnetochemistry is the design and synthesis of new building block systems to study and develop molecular magnets. In this context, two new Mn(III) complexes were synthesized using new tetradentate cis-N2O2 acacen ligands from the condensation of 1-anisyl-1,3-butanedione and 1,2-diaminoethane (2:1 ratio). All compounds were fully characterized by conventional spectroscopic techniques, such as FT-IR, HRMS, and UV–Vis, and rationalized with DFT and TD-DFT studies. The resulting cationic complexes were composed of N4O2 coordination spheres using a tetradentate ligand (N2O22−) in the equatorial plane and two axial pyridines with general formulae as [MnIII(L)(Py)2](X) (X = BF4−; ClO4−). Single-crystal X-ray diffraction revealed that both complexes are isostructural with typical axial anisotropy due to the Jahn-Teller effect by tetragonal distortion. A complete analysis of the crystalline structures revealed a nonclassical intermolecular interaction of CH···F and CH···O. Supported by Hirshfeld surfaces and 2D- fingerprint plots, it was possible to isolate and calculate the contribution of intermolecular interactions in the supramolecular arrangement. For both complexes, the magnetic susceptibility confirmed a high spin state (HS) with 4.8 µB and 4.9 µB related to the spin-only contribution to the magnetic moment.

Redox properties of bis-cobalt(III) complex of 3,3′-linked N-confused porphyrin dimer with axial pyridine ligands

Novel cobalt(III) complexes of N-confused porphyrin (5,10,15,20-tetrakis(penta-fluorophenyl)-2-aza-21-carbaporphyrin) and its 3,3[Formula: see text]-linked dimer, Co1 and Co2, were synthesized and their structures were elucidated by X-ray crystallographic analysis and NMR spectroscopy. The redox properties of the complexes were examined by electrochemical approaches, and the corresponding oxidized/reduced species were analyzed by spectroelectrochemistry and DFT calculations. Notably, due to the facile redox feature of the N-confused porphyrins, ligand-centered oxidative/reductive processes were assumed for Co1 and Co2 presumably as inferred from the spectral analyses. These results indicate the unique multi-electron reservoir capability of the metal complexes of the N-confused porphyrin dimer.

Multitargeted Platinum(IV) Anticancer Complexes Bearing Pyridinyl Ligands as Axial Leaving Groups.

Although multitargeted PtIV anticancer prodrugs have shown significant activities in reducing drug resistance, the types of bioactive ligands and drugs that can be conjugated to the Pt center remain limited to O-donors. Herein, we report the synthesis of PtIV complexes bearing axial pyridines via ligand exchange reactions. Unexpectedly, the axial pyridines are quickly released after reduction, indicating their potential to be utilized as axial leaving groups. We further expand our synthetic approach to obtaining two multitargeted PtIV prodrugs containing bioactive pyridinyl ligands: a PARP inhibitor and an EGFR tyrosine kinase inhibitor; these conjugates exhibit great potential for overcoming drug resistance, and the latter conjugate inhibits the growth of Pt-resistant tumor in vivo. This research adds to the array of synthetic methods for accessing PtIV prodrugs and significantly increases the types of bioactive axial ligands that can be conjugated to a PtIV center.

Multitargeted Platinum(IV) Anticancer Complexes Bearing Pyridinyl Ligands as Axial Leaving Groups

AbstractAlthough multitargeted PtIV anticancer prodrugs have shown significant activities in reducing drug resistance, the types of bioactive ligands and drugs that can be conjugated to the Pt center remain limited to O‐donors. Herein, we report the synthesis of PtIV complexes bearing axial pyridines via ligand exchange reactions. Unexpectedly, the axial pyridines are quickly released after reduction, indicating their potential to be utilized as axial leaving groups. We further expand our synthetic approach to obtaining two multitargeted PtIV prodrugs containing bioactive pyridinyl ligands: a PARP inhibitor and an EGFR tyrosine kinase inhibitor; these conjugates exhibit great potential for overcoming drug resistance, and the latter conjugate inhibits the growth of Pt‐resistant tumor in vivo. This research adds to the array of synthetic methods for accessing PtIV prodrugs and significantly increases the types of bioactive axial ligands that can be conjugated to a PtIV center.

Structural and Magnetic Investigation of Cobalt Valence Tautomeric Complexes with Sulfur-Containing Ligands

We report a series of heteroleptic cobalt complexes of the general formula [Co(tBu2sq)2(L)2], where tBu2sq = 3,5-di(tert-butyl)-o-semiquinonato ligand and L represents pyridines functionalized with sulfur-containing substituents. The octahedral coordination of the Co metal center in these mononuclear complexes is formed by two chelating tBu2sq ligands in the equatorial plane and two axial pyridines in the axial positions. All complexes exhibit layered crystal packings, with S-containing substituents providing cohesion within the layers and the tBu substituents of dioxolenes protruding into the interlayer space, where disordered solvent molecules are located. The presence of thienyl substituents in the para position of the pyridine ring leads to the formation of molecular chains within the layers, due to efficient π–π interactions between the L ligands. Even more efficient packing with two-dimensional intermolecular π–π interactions is achieved when two thienyl substituents are present in the meta positions of the pyridine ring. Introduction of a terminal cyano or 1,3-dithiole-2-one substituent on the opposite end of the pyridyl-bound thienyl group leads to the disruption of the π–π interactions, thus decreasing the crystal packing efficiency. Such disruption, however, is not observed when the thienyl group is terminated with methyl-carboxylate. Magnetic measurements reveal an onset of valence-tautomeric spin-crossover (VT-SCO) from the [LS-CoIII(tBu2sq•)(tBu2cat)(L)2] state (low-spin, S = 1/2) to the [HS-CoII(tBu2sq•)2(L)2] state (high-spin, S = 5/2) above 300 K for the complexes with para-thienyl-substituted pyridines, while the complexes with a bithienyl substituent in the para position or two thienyl substituents in the meta positions exhibit only LS state up to 400 K. Adding a terminal group to the para-thienyl substituent lowers the VT-SCO temperature, leading to two-step spin-state conversion.

Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a β,β-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H3tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [FeIIItpfc(py)]2COT and 6-coordinate [FeIIItpfc(py)2]2COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [FeIIItpfc(py)]2COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

Ruthenium Complexes of Rigid, Dianionic, Tetradentate N‐Donor Ligands and their Potential as Catalysts for Water Oxidation

AbstractTwo mononuclear ruthenium(II) complexes based on dianionic {N4} ligands and with axial pyridines have been prepared and characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The {N4} ligands provide a constrained equatorial coordination with one large N−Ru−N angle, and additional non‐coordinating N atoms in case of 2. Their redox properties have been investigated (spectro)electrochemically, and their potential to serve as water oxidation catalysts has been probed using cerium ammonium nitrate (CAN) at pH 1.0. Complex 1 undergoes rapid degradation, likely via ligand oxidation, whereas 2 is more rugged and exhibits 80 % efficiency in the CeIV‐driven water oxidation, with a high initial turnover frequency (TOFi) of 3.07×10−2 s−1 (at 100 equiv. CAN). The initial rate of O2 evolution exhibits 1st order dependence on catalyst concentration, suggesting a water nucleophilic attack mechanism. Repeated addition of CAN and control experiments show that high ionic strength conditions (both NO3− and CeIII) significantly decrease the TOF.

A Computational Study on the Mechanism of Catalytic Cyclopropanation Reaction with Cobalt N-Confused Porphyrin: The Effects of Inner Carbon and Intramolecular Axial Ligand.

The factors that affect acceleration and high trans/cis selectivity in the catalytic cyclopropanation reaction of styrene with ethyl diazoacetate by cobalt N-confused porphyrin (NCP) complexes were investigated using density functional theory calculations. The reaction rate was primarily related to the energy gap between the cobalt-carbene adduct intermediates, A and B, which was affected by the NCP skeletons and axial pyridine ligands more than the corresponding porphyrin complex. In addition, high trans/cis stereoselectivity was determined at the TS1 and, in part, in the isomerization process at the carbon-centered radical intermediates, Ctrans and Ccis.

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N 5 Site for Enhanced Electrocatalytic Reaction

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N ₅ Site for Enhanced Electrocatalytic Reaction

A combined QTAIM/IRI topological analysis of the effect of axial/equatorial positions of NH2 and CN substituents in the [(PY5Me2)MoO]+ complex

By means of the Interaction Region Indicator (IRI) and Quantum Theory of Atoms in Molecules (QTAIM), the influence exerted by NH2 (amino) and CN (cyano) as electron donor and electron acceptor substituent groups, respectively, located at para–positions of axial and equatorial pyridine rings of derivatized complexes coming from the [(PY5Me2)MoO]+ complex during the hydrogen molecular release in the gas phase was analyzed. In any case, a H–H covalent bond is forming at the transition state, with a strengthening of the electron density of 5.5% when the substituent group involved is NH2 at the para–position of the axial pyridine ring. However, there was no difference between NH2 and CN when these substituent groups are located at the para–positions of the equatorial pyridine rings. The topological properties of electron densities from the QTAIM are not perturbed by the electron donor and electron acceptor nature of the substituents, even when these substituent groups are located at the axial or equatorial pyridine rings of the Mo–based complex.

Pyridine-grafted nitrogen-doped carbon nanotubes achieving efficient electroreduction of CO2to CO within a wide electrochemical window

Grafted axial pyridine molecules can promote CO2ER and suppress the occurrence of competitive HER. Prepared composite metal-free catalyst exhibits excellent CO selectivity and considerable current density within a wide potential window.

Enhancing the electrocatalytic activity of Fe phthalocyanines for the oxygen reduction reaction by the presence of axial ligands: Pyridine-functionalized single-walled carbon nanotubes

We have examined the electrocatalytic activity of iron phthalocyanine (FePc) and perchlorinated iron phthalocyanine 16(Cl)FePc for the oxygen reduction reaction (ORR) in alkaline medium with the two molecules either adsorbed on the external surface of single-wall carbon nanotubes (SWCNT) or covalently anchored via an axial pyridine ligand on pyridine-functionalized single-wall carbon nanotubes (py-SWCNT). Regardless of the particular phthalocyanine type, the ORR activity is higher when the substrate is py-SWCNT rather than SWCNT. The Tafel slopes for ORR are very similar for the two Fe macrocyclic complexes attached to SWCNTs in the two different configurations, suggesting a common rate-determining step for the ORR for all four catalysts. It is also observed that, for both the SWCNT and py-SWCNT supports, the ORR activity is higher for 16(Cl)FePc than for FePc. This is attributed to the electron-withdrawing effect of the peripheral and non-peripheral chlorine atoms in the macrocyclic ligand. While FeN4 macrocycles are known to be located on the strong binding side of a volcano correlation including several MN4 species, the chlorine substituents decrease the binding energy of O2 on the central Fe cation, thereby moving up the macrocycle catalyst towards the apex of the volcano correlation. Both the carbon surface and macrocyclic ligand effects were optimized with 16(Cl)FePc attached to py-SWCNT, which is more active than both FePc on SWCNT and FePc on py-SWCNT. Here we show that both the axial ligand and the electron withdrawing groups (-Cl) have a combined collaborative effect in increasing the catalytic activity for ORR of Fe-phthalocyanines confined on the external walls of single-wall carbon nanotubes.

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts