A Different Perspective on Tuning the Photophysical and Photochemical Properties: The Influence of Constitutional Isomers in Group 6 Carbonyl Complexes with Pyridyl-Mesoionic Carbenes.

Exploring novel and existing design principles to tune the photochemical and photophysical properties of transition-metal complexes is an important goal in contemporary research. Here, we highlight the influence of constitutional isomers of pyridyl-1,2,3-triazolylidene mesoionic carbene (MIC) ligands on the photophysical and photochemical properties of the corresponding tetracarbonyl group 6 metal complexes (M = Cr, Mo, W). All new complexes [M(C-C)] presented herein incorporate a C-C linked pyridyl-MIC ligand and were fully characterized by X-ray diffraction analysis, elemental analysis, and 1H NMR and IR spectroscopy. Detailed photophysical investigations reveal a single emission in the VIS region, which extends into the NIR with lifetimes of up to 3.5 µs in the solid state at lower temperatures. The quantum yields were determined for all three complexes, and, in particular, the W0 complex shows an unusually high quantum yield of 29% compared to the values of 0.02% obtained for the [M(C-N)] isomers investigated in earlier works. Beyond this, the investigated W0 complex also exhibits an emission at 717 nm in a fluid solution. The combination of luminescence and FTIR-step scan spectroscopy with theoretical calculations reveals an emissive 3MLCT state. Irradiation of the presented complexes leads to a clean cleavage of one axial CO ligand. A metastable 16 VE species with a vacant axial coordination site was detected in the solid state at low temperatures. In solution, the respective solvato complexes are formed. A dark reverse reaction is observed, as previously described for the [M(C-N)] analogues. The increased electron density induced by the C-C linked pyridyl-MIC ligand leads to an increased kinetic rate constant for the reformation of the starting species and is also reflected in the lower photodissociation quantum yields.

Mechanistic and Kinetic Investigations of ON/OFF (Photo)Switchable Binding of Carbon Monoxide by Chromium(0), Molybdenum(0) and Tungsten(0) Carbonyl Complexes with a Pyridyl-Mesoionic Carbene Ligand.

This work tackles the photochemistry of a series of mononuclear Cr0 , Mo0 and W0 carbonyl complexes containing a bidentate mesoionic carbene ligand of the 1,2,3-triazol-5-ylidene type. FTIR spectroscopy, combined with density functional theory calculations, revealed a clean photo-induced reaction in organic solvents (acetonitrile, pyridine, valeronitrile) to give mainly one photoproduct with monosubstitution of a carbonyl ligand for a solvent molecule. The highest photodissociation quantum yields were reached for the Cr0 complex under UV irradiation (266 nm). Based on previous investigations, the kinetics of the dark reverse reactions have now been determined, with reaction times of up to several hours in pyridine. Photochemical studies in the solid state (KBr matrix, frozen solution) also showed light-induced reactivity with stabilization of the metastable intermediate with a free coordination site at very low temperature. The identified reactive species emphasizes a mechanism without ligand-sphere reorganization.

Mechanistic and Kinetic Investigations of ON/OFF (Photo)Switchable Binding of Carbon Monoxide by Chromium(0), Molybdenum(0) and Tungsten(0) Carbonyl Complexes with a Pyridyl-Mesoionic Carbene Ligand.

Invited for the cover of this issue are Gereon Niedner-Schatteburg, Biprajit Sarkar and co-worker at TU Kaiserslautern and the University of Stuttgart. The image depicts the selective dissociation of an axial CO from a metal complex. Read the full text of the article at 10.1002/chem.202201038.

The Antibacterial Drug Candidate SBC3 is a Potent Inhibitor of Bacterial Thioredoxin Reductase.

Antibiotic resistance is a growing problem for public health and associated with increasing economic costs and mortality rates. Silver and silver-related compounds have been used for centuries due to their antimicrobial properties. In this work, we show that 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene silver(I) acetate/NHC*-Ag-OAc (SBC3) is a reversible, high affinity inhibitor of E. coli thioredoxin reductase (TrxR; Ki =10.8±1.2 nM). Minimal inhibition concentration (MIC) tests with different E. coli and P. aeruginosa strains demonstrated that SBC3 can efficiently inhibit bacterial cell growth, especially in combination with established antibiotics like gentamicin. Our results show that SBC3 is a promising antibiotic drug candidate targeting bacterial TrxR.

Unexpected reactions of NHC*-CuI and -AgI bromides with potassium thio- or seleno-cyanate.

The reactions of N-heterocyclic carbene CuI and AgI halides with potassium thio- or seleno-cyanate gave unexpected products. The attempted substitution reaction of bromido-(1,3-dibenzyl-4,5-di-phenyl-imidazol-2-yl-idene)silver (NHC*-Ag-Br) with KSCN yielded bis-[bis-(1,3-dibenzyl-4,5-di-phenyl-imidazol-2-yl-idene)silver(I)] tris-(thio-cyanato)-argentate(I) diethyl ether disolvate, [Ag(C29H24N2)2][Ag(NCS)3]·2C4H10O or [NHC*2Ag]2[Ag(SCN)3]·2Et2O, (1), while reaction with KSeCN led to bis-(µ-1,3-dibenzyl-4,5-diphenyl-2-seleno-imidazole-κ2 Se:Se)bis-[bromido-(1,3-dibenzyl-4,5-diphenyl-2-seleno-imid-azole-κSe)silver(I)] di-chloro-methane hexa-solvate, [Ag2Br2(C29H24N2Se)4]·6CH2Cl2 or (NHC*Se)4Ag2Br2·6CH2Cl2, (2), via oxidation of the NHC* fragment to 2-seleno-imidazole. This oxidation was observed again in the reaction of NHC*-Cu-Br with KSeCN, yielding catena-poly[[[(1,3-dibenzyl-4,5-diphenyl-2-seleno-imidazole-κSe)copper(I)]-µ-cyanido-κ2 C:N] aceto-nitrile monosolvate], {[Cu(CN)(C29H24N2Se)]·C2H3N} n or NHC*Se-CuCN·CH3CN, (3). Compound (1) represents an organic/inorganic salt with AgI in a linear coordination in each of the two cations and in a trigonal coordination in the anion, accompanied by diethyl ether solvent mol-ecules. The tri-blade boomerang-shaped complex anion [Ag(SCN)3]2- present in (1) is characterized by X-ray diffraction for the first time. Compound (2) comprises an isolated centrosymmetric mol-ecule with AgI in a distorted tetra-hedral BrSe3 coordination, together with di-chloro-methane solvent mol-ecules. Compound (3) exhibits a linear polymeric 1 ∞[Cu-C≡N-Cu-] chain structure with a seleno-imidazole moiety additionally coordinating to each CuI atom, and completed by aceto-nitrile solvent mol-ecules. Electron densities associated with an additional ether solvent mol-ecule in (1) and two additional di-chloro-methane solvent mol-ecules in (2) were removed with the SQUEEZE procedure [Spek (2015 ▸). Acta Cryst. C71, 9-18] in PLATON.