Hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands: Imidazole and oxazole ring Lewis basicity, Ni(II)/Pd(II) complex structures and spectra

Efficient [Fe-Imidazole@SiO2] Nanohybrids for Catalytic H2 Production from Formic Acid

Three imidazole-based hybrid materials, coded as IGOPS, IPS and impyridine@SiO₂ nanohybrids, were prepared via the covalent immobilization of N-ligands onto a mesoporous nano-SiO₂ matrix for H₂ generation from formic acid (FA). BET and HRTEM demonstrated that the immobilization of the imidazole derivative onto SiO₂ has a significant effect on the SSA, average pore volume, and particle size distribution. In the context of FA dehydrogenation, their catalytic activity (TONs, TOFs), stability, and reusability were assessed. Additionally, the homologous homogeneous counterparts were evaluated for comparison purposes. Mapping the redox potential of solution E_h vs. SHE revealed that poly-phosphine PP₃ plays an essential role in FA dehydrogenation. On the basis of performance and stability, [Fe²⁺/IGOPS/PP₃] demonstrated superior activity compared to other heterogeneous catalysts, producing 9.82 L of gases (VH₂ + CO₂) with TONs = 31,778, albeit with low recyclability. In contrast, [Fe²⁺/IPS/PP₃] showed the highest stability, retaining considerable performance after three consecutive uses. With VH₂ + CO₂ = 7.8 L, [Fe²⁺/impyridine@SiO₂/PP₃] activity decreased, and it was no longer recyclable. However, the homogeneous equivalent of [Fe²⁺/impyridine/PP₃] was completely inactive. Raman, FT/IR, and UV/Vis spectroscopy demonstrated that the reduced recyclability of [Fe²⁺/IGOPS/PP₃] and [Fe²⁺/impyridine@SiO₂/PP₃] nanohybrids is due to the reductive cleavage of their C-O-C bonds during catalysis. An alternative grafting procedure is proposed, applying here to the grafting of IPS, resulting in its higher stability. The accumulation of water derived from substrate's feeding causes the inhibition of catalysis. In the case of [Fe²⁺-imidazole@SiO₂] nanohybrids, simple washing and drying result in their re-activation, overcoming the water inhibition. Thus, the low-cost imidazole-based nanohybrids IGOPS and IPS are capable of forming [Fe²⁺/IGOPS/PP₃] and [Fe²⁺/IPS/PP₃] heterogeneous catalytic systems with high stability and performance for FA dehydrogenation.

Terahertz time-domain absorption spectra of Cu(i) complexes bearing tetraphosphine ligands: the bridge between the C–H⋯π and π⋯π interactions and photoluminescence properties

The terahertz spectrum bridges the luminescence and C–H⋯π and π⋯π interactions of Cu(i) complexes.

ESIPT-capable 2,6-di(1H-imidazol-2-yl)phenols with very strong fluorescent sensing signals towards Cr(iii), Zn(ii), and Cd(ii): molecular variation effects on turn-on efficiency

The derivatization influence was studied for twelve 2,6-di(1H-imidazol-2-yl)phenols, whereby a strong fluorescent sensitivity for Cr(iii) was reported, while Zn(ii)/Cd(ii) sensing potentials also appeared.

Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

A series of 2-aryloxazolo[4,5-f][1,10]phenanthroline ligands (N^N ligands) and their cationic iridium(III) complexes (1-11, aryl = 4-NO₂-phenyl (1), 4-Br-phenyl (2), Ph (3), 4-NPh₂-phenyl (4), 4-NH₂-phenyl (5), pyridin-4-yl (6), naphthalen-1-yl (7), naphthalen-2-yl (8), phenanthren-9-yl (9), anthracen-9-yl (10), and pyren-1-yl (11)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (1-5), or different aromatic substituents with varied degrees of π-conjugation (6-11) on oxazolo[4,5-f][1,10]phenanthroline ligand, we aim to understand the effects of terminal substituents at the N^N ligands on the photophysics of cationic Ir(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo[4,5-f][1,10]phenanthroline motif, while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f][1,10]phenanthroline motif. All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized ¹π,π*/¹ILCT (intraligand charge transfer) transitions, mixed with ¹LLCT (ligand-to-ligand charge transfer)/¹MLCT (metal-to-ligand charge transfer) transitions. At the range of 350-570 nm, all complexes exhibited moderately strong ¹ILCT/¹LLCT/¹MLCT transitions at 350-450 nm, and broad but very weak ³LLCT/³MLCT absorption at 450-570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes 1 and 4 that contain NO₂ or NPh₂ substituent at the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral region of 440-750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes.

One-pot syntheses of pseudopteroxazoles from pseudopterosins: a rapid route to non-natural congeners with improved antimicrobial activity

Rapid one-pot methodologies to prepare pseudopteroxazole (1) and novel congeners from abundant natural pseudopterosins have been devised. This is highlighted here with the first synthesis of the marine natural product homopseudopteroxazole (2) utilizing a novel, silver(I)-mediated catechol to benzoxazole transformation. Pseudopteroxazoles and isopseudopteroxazoles exhibit potent activity against a range of important Gram-positive pathogens including Mycobacterium spp. and vancomycin-resistant Enterococcus faecium. Several non-natural pseudopteroxazoles exhibited strong activity against methicillin-resistant Staphylococcus aureus, thereby displaying a broader spectrum of antibiotic activity compared to pseudopteroxazole.