In vitro cytotoxicity of Mn(I) and Ru(II) carbonyls with a diphenyl pyridyl phosphine coligand towards leukaemia

Human acute monocytic leukaemia cells were tested under both dark and light conditions for their susceptibility to Mn(I) and Ru(II) carbonyl complexes with a diphenyl pyridyl phosphine coligand. The Ru(II) complex (IC₅₀ = 7.13 ± 0.8 μM) displayed higher outstanding potency against leukaemia than the Mn(I) analogue (54.58 ± 4.1 μM) in the dark and both complexes were completely harmless to healthy mouse bone marrow cells.

Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes

Six rhenium(I) κ³N-dicarbonyl complexes with 4'-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe₂-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σ_p substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ²N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Redox-Active Manganese Pincers for Electrocatalytic CO2 Reduction

The decrease of total amount of atmospheric CO2 is an important societal challenge in which CO2 reduction has an important role to play. Electrocatalytic CO2 reduction with homogeneous catalysts is based on highly tunable catalyst design and exploits an abundant C1 source to make valuable products such as fuels and fuel precursors. These methods can also take advantage of renewable electricity as a green reductant. Mn-based catalysts offer these benefits while incorporating a relatively cheap and abundant first-row transition metal. Historically, interest in this field started with Mn(bpy-R)(CO)3X, whose performance matched that of its Re counterparts while achieving substantially lower overpotentials. This review examines an emerging class of homogeneous Mn-based electrocatalysts for CO2 reduction, Mn complexes with meridional tridentate coordination also known as Mn pincers, most of which contain redox-active ligands that enable multi-electron catalysis. Although there are relatively few examples in the literature thus far, these catalysts bring forth new catalytic mechanisms not observed for the well-established Mn(bpy-R)(CO)3X catalysts, and show promising reactivity for future studies.

Luminescent PhotoCORMs: Enabling/Disabling CO Delivery upon Blue Light Irradiation

The new luminescent carbonyl compounds [Mn(Oxa-H)(CO)₃Br] (1) and [Mn(Oxa-NMe₂)(CO)₃Br] (2) were synthesized and fully characterized. Complexes 1 and 2 showed CO release under blue light (λ₄₅₃). Spectroscopic techniques and TD-DFT and SOC-TD-DFT calculations indicated that 1 and 2 release the Oxa-H and Oxa-NMe₂ coligands in addition to the carbonyl ligands, increasing the luminescence during photoinduction.

Selective synthesis and crystal structures of manganese(I) complexes with a bi- or tridentate terpyridine ligand

The crystal structures of two manganese(I) complexes with a different coordination mode of the supporting ligand are reported: fac-bromido-tricarbon-yl(4'-phenyl-2,2':6',2''-terpyridine-κ2 N,N')manganese(I), [MnBr(C21H15N3)(CO)3], I, and cis-bromido-dicarbon-yl(4'-phenyl-2,2':6',2''-terpyridine-κ3 N,N',N'')manganese(I), [MnBr(C21H15N3)(CO)2], II. In both complexes, the manganese(I) atom is coordinated by terminal carbonyl ligands, a bromide ion, and a 4'-phenyl-2,2':6',2''-terpyridine ligand within a distorted octa-hedral environment. In I, the metal ion is facially coordinated by three carbonyl ligands and the terpyridine ligand binds in a bidentate fashion. The non-coordinating nitro-gen atom in the terpyridine ligand is positioned on the side opposite to the bromido ligand. In II, the metal ion is coordinated by two carbonyl ligands in a cis configuration and the terpyridine ligand binds in a tridentate fashion; notably, one carbonyl and the trans bromido ligand are mutually disordered over two positions. In I, the complex mol-ecules are linked by C-H⋯Br hydrogen bonds. In II, aromatic π-π contacts are present, as well as pairs of C-H⋯Br and C-H⋯O hydrogen bonds.

2,2':6',2''-Terpyridine switches from tridentate to monodentate coordination in a gold(iii) terpy complex upon reaction with sodium azide

Reaction of [AuCl(terpy-κ³-N^1,1',1'')]Cl₂ with an excess of sodium azide did not result in the expected exchange of the chlorido by an azido ligand to give [Au(N₃)(terpy-κ³-N^1,1',1'')]²⁺. Instead, X-ray structure analysis showed that the isolated product is [Au(N₃)₃(terpy-κ¹-N¹)], in which the terpyridine ligand is in a very rare monodentate coordination mode. This is also the dominant species in solution, together with a minor amount of [Au(N₃)₂(terpy-κ²-N^1,1')]⁺. The stability of the tris(azido)gold(iii) moiety relative to other possible species was also confirmed by DFT calculations.

Immobilization of Mn(i) and Ru(ii) polypyridyl complexes on TiO2 nanoparticles for selective photoreduction of CO2 to formic acid

Under visible irradiation in DMF/TEOA, the hybrid nanomaterial Ru(ii)/TiO₂/Mn(i), lead to the selective reduction of CO₂ into HCOO⁻.

Ultrafast Intersystem Crossing vs Internal Conversion in α-Diimine Transition Metal Complexes: Quantum Evidence

Whereas third row transition metal carbonyl α-diimine complexes display luminescent properties and possess low-lying triplet metal-to-ligand charge transfer (MLCT) states efficiently accessible by a spin-vibronic mechanism, first row analogues hold low-lying metal-centered (MC) excited states that could quench these properties. Upon visible irradiation, different functions are potentially stimulated, namely, luminescence, electron transfer, or photoinduced CO release, the branching ratio of which is governed by the energetics, the character, and the early time dynamics of the photoactive excited states. Simulations of ultrafast nonadiabatic quantum dynamics, including spin-vibronic effects, of [M(imidazole)(CO)₃(phenanthroline)]⁺ (M = Mn, Re) highlight the role of the metal atom. An ultrafast intersystem crossing process, driven by spin-orbit coupling, populates the low-lying triplet states of [Re(imidazole)(CO)₃(phen)]⁺ within the first tens of fs. In contrast, efficient internal conversion between the two lowest ¹MLCT states of [Mn(imidazole)(CO)₃(phen)]⁺ is mediated within 50 fs by vibronic coupling with upper MC and MLCT states.

Electrocatalytic reduction of carbon dioxide with Mn(terpyridine) carbonyl complexes

The behavior of a series of Manganese (Mn) carbonyl compounds with 2,2':6',2''-terpyridine (tpy) in κ²-N,N' and κ³-N,N',N'' coordination modes under electrochemically reducing conditions is reported. In the presence of carbon dioxide (CO₂) and Brønsted acid (phenol), two-electron reduction to carbon monoxide (CO) and water (H₂O) is observed. Based on data obtained from cyclic voltammetry and infrared spectroelectrochemistry, the active state of the catalyst in the proposed mechanism for all cases is [Mn(κ³-N,N',N''-tpy)(CO)₂]^-. Under these conditions, competing decomposition reactions limit the overall Faradaic efficiency. These results suggest the possibility of developing new tridentate ligand frameworks suitable for catalytic systems with Mn carbonyl cores.

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-R)(CO)3 Br] Complexes

The electrochemical behavior of fac-[Mn(pdbpy)(CO)₃ Br] (pdbpy=4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine) (1) in acetonitrile under Ar, and its catalytic performances for CO₂ reduction with added water, 2,2,2-trifluoroethanol (TFE), and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V versus the standard calomel electrode (SCE) in CO₂ -saturated acetonitrile, reveal that the process selectivity is extremely sensitive to the acid strength, producing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV/Vis) study under Ar and CO₂ atmospheres shows that 1 undergoes fast solvolysis; however, dimer formation in acetonitrile is suppressed, resulting in an atypical reduction mechanism in comparison with other reported Mn^I catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO₂ reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)₃ Br] (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst (2), bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO₂ .

pH-Sensitive metal-free carbon monoxide prodrugs with tunable and predictable release rates

Carbon monoxide prodrugs with triggered release profiles are highly desirable for targeted CO delivery to minimize their untoward side-effects.

The CO release properties of κ2N1,N2Mn(i) tricarbonyl photoCORMs with tridentate benzimidazole coligands

The κ²N¹,N²bidentate mode of tridentate benzimidazole ligand is changed into meridional, tridentate one upon illumination at 468 nm.

Improving the efficiency of electrochemical CO2 reduction using immobilized manganese complexes

Immobilization of [Mn(bpy)(CO)3Br], (1) and [Mn(bpy((t)Bu)2)(CO)3Br] (2, where (bpy((t)Bu)2) = 4,4'-di-tert-butyl-2,2'-bipyridine) in Nafion/multi-walled carbon nanotubes (MWCNT) on glassy carbon yielded highly active electrodes for the reduction of CO2 to CO in aqueous solutions at pH 7. Films incorporating have significantly improved selectivity towards CO2, with CO : H2 ∼ 1 at -1.4 V vs. SCE, exceeding that for the previously reported /MWCNT/Nafion electrode. Furthermore, we report the synthesis and subsequent electrochemical characterization of two new substituted Mn(i) bipyridine complexes, [Mn(bpy(COOH)2)(CO)3Br] (3) and [Mn(bpy(OH)2)(CO)3Br] (4) (where (bpy(COOH)2) = 4,4'-di-carboxy-2,2'-bipyridine and (bpy(OH)2) = 4,4'-di-hydroxy-2,2'-bipyridine). Both 3 and 4 were found to have some activity towards CO2 in acetonitrile solutions; however once immobilized in Nafion membranes CO2 reduction was found to not occur at significant levels.

Photo-activated CO-releasing molecules (PhotoCORMs) of robust sawhorse scaffolds [μ(2)-OOCR(1), η(1)-NH2CHR(2)(C = O] OCH3, Ru(I)2CO4]

A class of sawhorse-type ruthenium(i) complexes featuring a stable CORM sphere with diverse carboxylic and amino acid derivatives were synthesized and validated as lead structures for photo-activated CO-releasing molecules (PhotoCORMs). The CO release of these CORMs was triggered by 365 nm UV irradiation. Cell viability studies indicated that 3a and 3f were non-toxic both in the dark and in UV light, making them excellent lead structures for therapeutic CORMs.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

Application of the energetic span model to the electrochemical catalysis of proton reduction by a diiron azadithiolate complex

A method for the computation of TOF of catalysis of electrochemical reaction as a function of the potential was developed.

A kinetic analysis of CO release from a diiron hexacarbonyl complex promoted by amino acids

CO release from [Fe₂{μ-SCH₂CH(OH)CH₂(OH)}(CO)₆] initiated by amino acids depends highly on the nature of the acids. Among the examined amino acids, glutamate, a ligand with a chelating effect and an additional functional group, exhibits the best efficiency in promoting the CO release.

Tuning coordination modes of pyridine/thioether Schiff base (NNS) ligands to mononuclear manganese carbonyls

Manganese carbonyls are ligated by pyridine/thioether Schiff base (NNS) ligands. Coordination of the thioether-S donor to the Mn(i) center is determined by subtle steric changes at the ligand periphery.