Manganese Tricarbonyl Complexes Research Articles

Unveiling the Unusual Mn(CO)3 Migration in a Manganese Cyclohexenyl Complex by DFT Computations.

Homogeneous catalysis involving a transition metal agostic interaction (TM…H…C) is an attractive strategy for C-H bond activation, in which the transition metal agostic intermediates serve as the critical component. To investigate the roles of manganese agostic intermediates in the unusual migration of the Mn(CO)3 fragment in the (exo-phenyl)(η3-cyclohexenyl)manganese tricarbonyl [(Ph)(η3-C6H8)Mn(CO)3] (complex 1) under the protonation of tetrafluoroboric acid-diethyl ether (HBF4.Et2O), a comprehensive density functional theory (DFT) theoretical study was performed. The computational results showed that formation of the [(cyclohex-3-enyl)-η6-benzene]manganese tricarbonyl complex [(C6H9)(η6-Ph)Mn(CO)3+][BF4] (complex 2) was achieved via a series of mono-agostic and di-agostic intermediates. The overall rate-limiting step for this unusual migration of the Mn(CO)3 fragment is the formation of the di-agostic (η2-phenyl)manganese complex 8 (4 → 5 → 8) with a Gibbs barrier of 15.4 kcal mol-1. The agostic intermediates with TM…H…C agostic interactions were well-characterized by geometry parameters, Atoms-In-Molecules (AIM) analyses, and the Natural Adaptive Orbitals (NAdOs). The located pathways in the current study successfully explained the experimental observations, and the findings on the TM…H…C agostic interaction provided a new aspect of the catalytic reaction with the manganese complex.

Discovery of antibacterial manganese(i) tricarbonyl complexes through combinatorial chemistry

To accelerate the discovery of novel metalloantibiotics we applied combinatorial synthesis to the preparation of 420 novel manganese tricarbonyl complexes.

Photoinduced behaviour of N,N-bidentate manganese(I) and rhenium(I) tricarbonyl complexes for singlet oxygen generation and CO release.

The combined action of singlet oxygen (1O2) and photoinduced carbon monoxide (CO) released by tricarbonyl metal complexes is a promising synergic treatment against multi-resistant bacterial infections. In this work, we explore the use of a polydentate ligand (bpm = 2,2-bipyrimidine) that offers the opportunity to accommodate two metal centers exhibiting both singlet oxygen generation and carbon monoxide releasing properties in a single molecule. A series of monometallic ([(bpm)M(CO)3Br]; M = Mn, Re) and homo or hetero bimetallic ([Br(CO)3M(bpm)M'(CO)3Br]; M = Mn, Re) compounds were synthesized in moderate to good yields by modulating the metal precursor or the stoichiometry, also the syn:anti isomers ratio for the bimetallic complexes was dependent on the experimental conditions used. DFT modelling shows the anti-isomer is more stable than the syn-isomer by less than 8 kJ mol-1, which is consistent with those experimentally observed in terms of majority product and the effect of experimental conditions over the anti-syn ratio. The HOMO-LUMO gap is lower for the mono and bimetallic rhenium(I) compounds compared to the values for the manganese(I) analogues, while the heterometallic complex shows intermediate values for the anti-isomer. The photophysical characterization shows typical absorption and emission bands with MLCT character. In addition, CO-release and 1O2 generation quantum yields were evaluated for the monometallic Mnbpm and Rebpm homologues and compared with values obtained for the homo- and hetero-bimetallic complexes. Interestingly the replacement of a Mn(CO)3Br moiety in MnbpmMn by a Re(CO)3Br one makes the heterometallic MnbpmRe molecule a molecular oxygen sensitizer and partially retaining its carbon monoxide releasing ability.

Antimicrobial Activity of Manganese(I) Tricarbonyl Complexes Bearing 1,2,3-Triazole Ligands.

Antimicrobial resistance is one of the most pressing health issues of our time. The increase in the number of antibiotic-resistant bacteria allied to the lack of new antibiotics has contributed to the current crisis. It has been predicted that if this situation is not dealt with, we will be facing 10 million deaths due to multidrug resistant infections per year by 2050, surpassing cancer-related deaths. This alarming scenario has refocused attention into researching alternative drugs to treat multidrug-resistant infections. In this study, the antimicrobial activities of four manganese complexes containing 1,2,3,-triazole and clotrimazole ligands have been evaluated. It is known that azole antibiotics coordinated to manganese tricarbonyl complexes display interesting antimicrobial activities against several microbes. In this work, the effect of the introduction of 1,2,3,-triazole-derived ligands in the [Mn(CO)3(clotrimazole)] fragment has been investigated against one Gram-positive bacterium and five Gram-negative bacteria. The initial antimicrobial activity of the above-mentioned complexes was assessed by determining the minimum inhibitory and bactericidal concentrations using the broth microdilution method. Growth curves in the presence and absence of the complexes were performed to determine the effects of these complexes on the growth of the selected bacteria. A possible impact on cellular viability was determined by conducting the MTS assay on human monocytes. Three of the Mn complexes investigated (4-6) had good antimicrobial activities against all the bacteria tested, with values ranging from 1.79 to 61.95 µM with minimal toxicity. Due to the increased problem of antibiotic resistance and a lack of new antibacterial drugs with no toxicity, these results are exciting and show that these types of complexes can be an avenue to pursue in the future.

CO2 Activation with Manganese Tricarbonyl Complexes by an H-Atom Responsive Benzimidazole Ligand.

Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI (HL)(CO)3 Br (1 a-Br) and MnI (MeL)(CO)3 Br (1 b-Br) (where HL=2-(2'-pyridyl)benzimidazole; MeL=1-methyl-2-(2'-pyridy)benzimidazole) and assayed their electrocatalytic properties for CO2 reduction. A redox-active pyridine benzimidazole ancillary ligand in complex 1 a-Br displayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO2 conversion at a markedly lower reduction potential than that observed for 1 b-Br. Notably, a one-electron reduction of 1 a-Br yields a structurally characterized H-bonded binuclear Mn(I) adduct (2 a') rather than the typically observed Mn(0)-Mn(0) dimer, suggesting a novel method for CO2 activation. Combining advanced electrochemical, spectroscopic, and single crystal X-ray diffraction techniques, we demonstrate the use of an H-atom responsive ligand may reveal an alternative, low-energy pathway for CO2 activation by an earth-abundant metal complex catalyst.

Rational design of co-ordination compounds in combination of bipyridine type of ligands and group 7 metal (M = Mn, Re) for photoCORM: a DFT study.

A large number of manganese and rhenium tricarbonyl complexes are known in literature along with various applications in different fields. CO-releasing molecules (CORMs) got recent research attention because CO can act as a prodrug for different diseases. CORMs offer the promising prospect of a safe and controllable amount of CO release. In this research work, we have explored the electronic properties of compounds such as bipyridine-related [Mn(CO)3] and [Re(CO)3] and we have compared the electronic properties of both manganese and rhenium tricarbonyl complexes in the light of carbon monoxide releasing tendency. The chosen Mn and Re metals have enough possibility to vary or play with ligands and design a new and novel CORM molecule. In this context, we have taken a range of 4,4'-disubstituted 2,2' bipyridyl ligands (Rbpy, where R = NH2, tBu, OCH3, H, CF3, CN, NO2) to investigate CO's liberation ability to identify and study such molecules. The calculated absorbance of designed complexes (1-14) shows visible/near-IR region (350-850 nm). The HOMO-LUMO energy gap of 7 (ΔE=2.40 eV) complex and for complex 14 (ΔE=2.28 eV) which is lesser in all complexes but the MLCT percentage is greater in Mn tricarbonyl complexes in comparison to Re tricarbonyl complexes. The calculated results of the FMO approach revealed that complex 7 and 14 have the lowest energy gap which is also in good agreement with DOSs and TDM results. The theoretically calculated results revealed that the both Mn and Re tricarbonyl complexes have a tendency for labialization of CO, but Mn tricarbonyl complexes are more prone to CO release because they have higher MLCT percentage. In this research work, we have performed density functional theory (DFT) calculations to explore the physical properties of compounds such as bipyridine-related [Mn(CO)3] and [Re(CO)3] and we have compared the physical properties of both manganese and rhenium tricarbonyl complexes in the light of carbon monoxide releasing tendency. DFT-based calculations were performed by using B3LYP/LANL2DZ basis set followed by acetonitrile solvent using the conductor-like polarizable continuum model (CPCM) for different calculations. Various geometrical calculations were performed using the Gaussian16 suite of programs and the output results obtained from Gaussian16 were visualized using GaussView 5.0.16. The same level of theory was used for various calculations, including frontier molecular orbital (FMO) analysis, metal to ligand charge transfer (MLCT), density of state (DOS) calculations, and transition density of matrix (TDM) calculations. For specific calculations, GaussSum 2.2 software package was used to calculate the density of states, and the Multiwfn 3.8 program was used to analyze the transition density matrix, which is presented using heat maps for both electrons and holes.

Synthesis and characterization of new neutral Mn(I) tricarbonyl complexes with 8‐hydroxyquinoline and imidazole ligands as CO releasing molecules

A series of manganese(I) tricarbonyl complexes formulated as [Mn(8‐HQ)(CO)3Imd] (HQ = hydroxyquinoline and Imd = imidazole derivatives) was synthesized and fully characterized. These structures could be easily accessed from a common dimeric edifice formulated as [Mn2(8‐HQ)2(CO)6]. The structure of this complex has been confirmed by X‐ray diffraction studies. Its reaction with a range of imidazole derivatives (Imd) yielded the monomeric complexes [Mn(8‐hydroxyquinoline)(CO)3Imd] with various functional groups as an anchoring point for a vector. Photoactivation of all these complexes by blue‐light irradiation led to the liberation of 3 mol of CO per mol of complex as determined experimentally. Spectroscopic and computational methods were employed to explore the mechanism of CO release. Finally, a manganese complex including the more elaborate imidazole ligand N‐ and C‐di‐protected histidine was successfully synthesized, opening the way to its direct incorporation in peptides.

Synthesis, structure, CO releasing, and biological activities of new 1-D chain Mn(I)/Mn(II) visible light activated CO-releasing molecules (CORMs)

The manganese tricarbonyl complex {[Mn2(CO)6Mn(H2O)4(BPDC)2]·2H2O}n (1) (BPDC = 2,2′-bipyridyl-4,4′-dicarboxylic acid) was synthesized from the reaction of MnBr(CO)5 with 2,2′-bipyridyl-4,4′-dicarboxylic acid. Various fac-Mn(CO)3 cores based on mononuclear complexes were achieved and only a handful of the Mn(I)-based dinuclear metallastirrups or supramolecular rectangles were reported. Interestingly, our compound is the first example of a CORM with 1-D ladder-like chain and Mn(I)/Mn(II) mixed valence state. Studies show that 1 can release CO under three different wavelengths of light (Green: λ = 520–530 nm; Blue: λ = 465–470 nm; UV: λ = 365–370 nm) and CO releasing rate can be adjusted by changing the wavelength of the irradiation light. Density functional theory (DFT) calculations were conducted to analyze the effect of metal-to-ligand charge transfer (MLCT) on CO releasing behaviors. Furthermore, cellular viability assay and anti-inflammatory activities showed that 1 possessed biological compatibility but no systemic cytotoxicity, and could prevent LPS-induced secretion of TNF-α and NO in RAW264.7 cells. The results suggest that 1 may provide a therapeutic approach for inflammation.

Manganese(I) Tricarbonyl Complexes with Bidentate Pyridine-Based Actor Ligands: Reversible Binding of CO2 and Benzaldehyde via Cooperative C–C and Mn–O Bond Formation at Ambient Temperature

We report manganese(I) tricarbonyl complexes decorated with imino- and amino-pyridine ligands [Mn(impy)(CO)3Br] and [Mn(ampy)(CO)3Br], respectively. Both compounds can be transformed either via two-electron reduction for the former or double deprotonation for the latter into anionic species with a disturbed (“dearomatized”) π-electron system of the pyridine ring M[Mn(amidopy*)(CO)3] (M = alkali metal). The newly formed five-coordinated complex is anionic and encompasses a nucleophilic carbon center within its metalla cycle. This leads to noteworthy reactivity: [Mn(amidopy*)(CO)3]− readily reacts with C═O double bonds. Specifically, CO2 and benzaldehyde can bind to the complex via a metal–ligand cooperative [1,3]-addition under C–C and Mn–O bond formation and concomitant rearomatization of the pyridine ring. Remarkably, we found that this addition is reversible. Exchange reactions using isotopically labeled 13CO2 indicate reversible C–C and Mn–O bond formation at ambient temperature. Likewise, bonded benzaldehyde is exchanged from the complex under a CO2 atmosphere. Density functional theory calculations suggest a significant role for the cationic counter ion in the bond activation reactions that can make this bond activation feasible.

Synthesis, spectroscopic, CO‐releasing ability, and anticancer activity studies of [Mn(CO)3(L–L)Br] complexes: Experimental and density functional theory studies

AbstractTwo different series of manganese(I) tricarbonyl complexes containing phosphine‐based (1–5) and 4′‐substituted 2,2′:6′,2″‐terpyridine‐based ligands (6–10) have been synthesized in order to study their CO‐releasing ability and to investigate their anticancer activity. All the synthesized complexes (1–10) have been fully characterized using standard spectroscopic and analytical techniques. Further 5, 7, and 9 have also been characterized by single‐crystal X‐ray diffraction studies. Although both the sets of ligands are π‐acceptors, they tend to change the Mn—CO bond strength upon complexation, thus affecting the CO release. Photoactivation of 1–5 and 6–10 has been achieved using 365 nm UV irradiation and low intensity visible light, respectively. The MnCO bond strength has been examined using the density functional theory (DFT) and time‐dependent DFT (TD‐DFT) calculations. In order to find the therapeutic viability of the visible light activated complexes, their cytotoxicity has been investigated both in the dark and under irradiation. The MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide) assay reveals the potential application of some of the synthesized complexes especially towards the lung cancer cells in the dark condition.

Manganese(I) tricarbonyl complexes as potential anticancer agents.

The antiproliferative activity of [Mn(CO)3(N^N)Br] (N^N = phendione 1, bipy 3) and of the two newly synthesized Mn complexes [Mn(CO)3(acridine)(phendione)]OTf (2) and [Mn(CO)3(di-triazole)Br] (4) has been evaluated by MTS against three tumor cell lines A2780 (ovarian carcinoma), HCT116 (colorectal carcinoma), HCT116doxR (colorectal carcinoma resistant to doxorubicin), and in human dermal fibroblasts. The antiproliferative assay showed a dose-dependent effect higher in complex 1 and 2 with a selectivity toward ovarian carcinoma cell line 21 times higher than in human fibroblasts. Exposure of A2780 cells to IC50 concentrations of complex 1 and 2 led to an increase of reactive oxygen species that led to the activation of cell death mechanisms, namely via intrinsic apoptosis for 2 and autophagy and extrinsic apoptosis for 1. Both complexes do not target DNA or interfere with cell cycle progression but are able to potentiate cell migration and neovascularization (for 2) an indicative that their application might be directed for initial tumor stages to avoid tumor invasion and metastization and opening a new avenue for complex 2 application in regenerative medicine. Interestingly, both complexes do not show toxicity in both in vivo models (CAM and zebrafish).

Phototriggered cytotoxic properties of tricarbonyl manganese(I) complexes bearing α-diimine ligands towards HepG2.

Reaction between bromo tricarbonyl manganese(I) and N,N'-bis(phenyl)-1,4-diaza-1,3-butadiene ligands, bearing different electron-donating and electron-withdrawing groups R=OCH3, Cl, and NO2 in the ortho- and para-positions on the phenyl substituent, afforded [MnBr(CO)3(N-N)] complexes. The influence of the character and position of the substituent on the dark stability and carbon monoxide releasing kinetics was systematically investigated and correlated with the data of the time-dependent density functional theory calculations. The combined UV/Vis and IR data clearly revealed that the aerated solutions of [MnBr(CO)3(N-N)] in either coordinating or noncoordinating solvents are dark stable and the fluctuations observed during the incubation period especially in the case of the nitro derivatives may be attributed to the exchange of the axial bromo ligand with the coordinating solvent molecules. The free ligands and nitro complexes were non-cytotoxic to HepG2 cells under both the dark and illumination conditions. In the dark, Mn(I) compounds, incorporating o-OCH3 and o-Cl, exhibited excellent cytotoxicity with IC50 values of 18.1 and 11.8μM, while their para-substituted analogues were inactive in the dark and active upon the irradiation at 365nm with IC50 values of 5.7 and 6.7μM, respectively.

Effects of Chemically-Modified Polypyridyl Ligands on the Structural and Redox Properties of Tricarbonylmanganese(I) Complexes.

Carbonyl complexes with manganese(I) as the central metal are very attractive catalysts. The introduction of redox-active ligands, such as quinones and methyl viologen analogs into these catalysts, would be expected to lead to superior catalyst performances, since they can function as excellent electron carriers. In this study, we synthesized four tricarbonylmanganese(I) complexes containing typical bidentate polypyridyl ligands, including 1,10-phenanthroline (phen) and 2,2′-bipyridine (bpy) frameworks bound to redox-active ortho-quinone/catechol or methyl viologen-like units. The molecular structures of the resulting complexes were determined by X-ray crystallography to clarify their steric features. As expected from the infrared (IR) data, three CO ligands for each complex were coordinated in the facial configuration around the central manganese(I) atom. Additionally, the structural parameters were found to differ significantly between the quinone/catechol units. Electrochemical analysis revealed some differences between them and their reference complexes, namely [MnBr(CO)3(phen)] and [MnBr(CO)3(bpy)]. Notably, interconversions induced by two-electron/two-proton transfers between the quinone and catechol units were observed in the phenanthroline-based complexes. This work indicated that the structural and redox properties in tricarbonylmanganese(I) complexes were significantly affected by chemically modified polypyridyl ligands. A better understanding of structures and redox behaviors of the present compounds would facilitate the design of new manganese complexes with enhanced properties.

Diversification of quinoline-triazole scaffolds with CORMs: Synthesis, in vitro and in silico biological evaluation against Plasmodium falciparum

A discrete series of tricarbonyl manganese and rhenium complexes conjugated to a quinoline-triazole hybrid scaffold were synthesised and their inhibitory activities evaluated against Plasmodium falciparum. In general, the complexes show moderate activity with improved inhibitory activities for the photoactivatable manganese(I) tricarbonyl complexes in the malaria parasite. All complexes are active in the dark against the NF54 CQS (chloroquine-sensitive) and K1 MDR (multidrug-resistant) strains of Plasmodium falciparum, with IC50 values in the low micromolar range. Of significance, the complexes retain their activity in the MDR strain with resistance indices ranging between 1.1 and 2.1. The Mn(I) analogues display photodissociation of all three CO ligands upon irradiation at 365 nm. More importantly, the complexes show increased antimalarial activity in vitro upon photoactivation, something not observed by the clinically used reference drug, chloroquine. As a purported mechanism of action, the compounds were evaluated as β-haematin inhibitors. To further understand the interactions of the complexes, in silico hemozoin docking simulations were performed, attesting to the fact that CO-release could be vital for blocking the hemozoin formation pathway. These results show that this strategy may be a valuable, novel route to design antimalarial agents with higher efficacy.

New fluorinated manganese carbonyl complexes for light controlled carbon monoxide (CO) release and the use of benchtop 19F-NMR spectroscopy

Carbon monoxide (CO) is used as an essential therapeutic agent. Control delivery and tracking of CO is the important concern for using in therapeutics. We report fluorinated manganese(I) tricarbonyl complexes PF-DPA.Mn and CF-DPA.Mn as a new photoCORMs for the photo-controlled CO release. For the first time, the benchtop 19F-NMR “on-off” signal has been used to monitor light-controlled CO release along with other traditional methods. 19F-NMR signal was initially “on” which attenuates on irradiating with blue light due to conversion of Mn(I) to Mn(II). CO release behaviour of new complexes also supported by Myoglobin assay, time-dependent IR study and UV–Vis experiments. Fluorinated manganese carbonyl complexes with 19F-NMR open up vast opportunities for researchers to develop a fast and reliable technique for tracking CO release.

Manganese and Rhenium Tricarbonyl Complexes Equipped with Proton Relays in the Electrochemical CO2 Reduction Reaction

The minireview focuses on the electrochemical CO2 reduction catalysed by manganese and rhenium tricarbonyl complexes with α‐diimine ligands having pendant proton relays attached to the α‐diimine ligand. Since the pioneering work on [Re(CO)3(Lbpy)Cl], 1Cl, as molecular electrocatalyst in the CO2 reduction reaction in the 80's, some effort has been made to understand the mechanism of the catalytic cycle and to improve the efficiency of the conversion (Lbpy = 2,2'‐bipyrdine). Later it was shown that the corresponding Mn complex, [Mn(CO)3(Lbpy)Br], 2Br, is also active in the CO2‐to‐CO conversion. As the 2 electrons reduction of CO2 is coupled to the transfer of 2 protons, pendent proton relays have been introduced in the ligand framework with the aim to improve the proton management during catalysis, to stabilize intermediates, and to alter the redox and the pH‐dependent properties of the catalyst. We summarise in this review the outcome of these studies on the performance of the catalyst, in particular, with regard to the position of the proton relay and thus, its influence on the electronic structure, the reaction steps, and the kinetics.

Cytotoxicity of Mn-based photoCORMs of ethynyl-α-diimine ligands against different cancer cell lines: The key role of CO-depleted metal fragments

A series of tricarbonyl manganese complexes bearing 4-ethynyl-2,2′-bipyridine and 5-ethynyl-1,10-phenanthroline α-diimine ligands were synthetized, characterized and conjugated to vitamin B12, previously used as a vector for drug delivery, to take advantage of its water solubility and specificity toward cancer cells. The compounds act as photoactivatable carbon monoxide-releasing molecules rapidly liberating on average ca. 2.3 equivalents of CO upon photo-irradiation. Complexes and conjugates were tested for their anticancer effects, both in the dark and following photo-activation, against breast cancer MCF-7, lung carcinoma A549 and colon adenocarcinoma HT29 cell lines as well as immortalized human bronchial epithelial cells 16HBE14o- as the non-carcinogenic control. Our results indicate that the light-induced cytotoxicity these molecules can be attributed to both their released CO and to their CO-depleted metal fragments including liberated ligands.

Bench‐Stable Manganese NHC Complexes for the Selective Reduction of Esters to Alcohols with Silanes

AbstractSelective reduction of esters to alcohols was accomplished through Mn(I)‐mediated hydrosilylation reaction. The manganese tricarbonyl complex [Mn(bis‐NHC)(CO)3Br] resulted an active pre‐catalyst for the reduction of a variety of esters using phenylsilane and the cheap and readily available polymethylhydrosiloxane. An in situ examination of the catalytic reaction using 55Mn NMR spectroscopy allowed us to detect the formation of Mn(I) intermediate active species.magnified image

Ultrafast Spectroscopy of [Mn(CO)3] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding.

Manganese tricarbonyl complexes are promising catalysts for CO2 reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)3(Rbpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (Rbpy, where R = tBu, H, CF3, NO2). Transient absorption spectroscopy measurements for Mn(CO)3(Rbpy)Br coordination compounds with R = tBu, H, and CF3 in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)3(NO2bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

Unusual Reactivity of a Thiazole-Based Mn Tricarbonyl Complex for CO2 Activation

Manganese tricarbonyl complexes have attracted considerable attention as competent electrocatalysts for CO2 reduction owing to their unique chemistry and the natural abundance of Mn. While the analogous Re complexes are well-known to activate CO2 under anhydrous conditions, most reported Mn tricarbonyl catalysts require exogenous protons. In an effort to understand the reason for the different CO2 reduction mechanisms, we investigate the reactivity of Mn(pbt)(CO)3Br, (1, where pbt = 2-(2-pyridyl)-benzothiazole) with CO2. Our combined electrochemical and spectroscopic data suggest that 1 can undergo a CO2 conversion to CO similar to Re complexes with and without the presence of a proton source but only at potentials significantly more negative than the second reduction. Furthermore, the one- and two-electron reduced species are chemically generated and characterized by NMR, UV–vis, and IR spectroscopy. By exploring sequential chemical reduction of 1, we uncover unusual reactivity of the reduced dimeric Mn–Mn species as well as the isolation of a species capable of C–H activation.