Rhenium Tricarbonyl Complexes Research Articles

Polypyridyl coordinated rhenium(I) tricarbonyl complexes as model devices for cancer diagnosis and treatment

This study reports on the solid-state structures, solution-state photoluminescence and in vitro biological applications of fac-[Re(CO)3(N,N′-Bid)(X)]n where N,N′-Bid = 4,4′-dimethyl-2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine and 4,4′-dimethoxy-2,2′-bipyridine; X = triphenylphosphine, dicyclohexylphenylphosphine, cyclohexyldiphenylphosphine, 1,3,5 triaza-7-phosphaadamantane, OH2 and Br, and n = 0, +1. All the synthesised complexes (1–18) were spectroscopically characterized using infrared, nuclear magnetic resonance (1H, 13C and 31P) and ultraviolet/visible techniques, and single crystal X-ray diffraction. An increasing trend was noted in the IR CO stretching frequencies, fac-[Re(CO)3(N,N′-Bid)(Br)] > fac-[Re(CO)3(N,N′-Bid)(H2O)]+ >fac-[Re(CO)3(N,N′-Bid)(P)]+. The study reports three new crystal structures: fac-[Re(CO)3(5,5′-DiMBpy)(NO3)] (2a), fac-[Re(CO)3(4,4′-DiMoxBpy)(NO3)][(CH3)2CO] (3a) and fac-[Re(CO)3(4,4′-DiMBpy)(Br)] (4). The ligand bite angles (N-Re-N) are 74.8(2)° for 2a, 74.57(13)° for 3a and 74.80(2)° for 4. All eighteen complexes displayed excellent luminescent properties, with an emission range between 505 and 609 nm, and the most significant Stokes shift of 270 nm is noted for 3. In vitro biological screening against breast cancers revealed two viable complexes, fac-[Re(CO)3(4,4′-DiMoxBpy)(Br)] (6) with IC50 = 10.92 ± 2.3 µM against MDA-MB-231 cells (SI = 1.49) and IC50 = 16.25 ± 1.9 µM against MRC-5 cells; fac-[Re(CO)3(5,5′-DiMBpy)(CyPh2P]+ (13) with IC50 = 5.74 ± 2.5 µM against MCF-7 cells (SI = 2.62) and IC50 = 15.04 ± 2.6 µM against MRC-5 cells.

Luminescent fac-[ReX(CO)3(phenyl-pyta)] (X = Cl, Br, I) complexes: influence of the halide ligand on the electronic properties in solution and in the solid state.

Tricarbonylrhenium(I) complexes that incorporate a chloride ligand are promising photoluminescent materials, but those incorporating a bromide or iodide ligand have received very little attention regarding their solid-state properties. In this work, three rhenium(I) complexes differing only by the nature of their halide ligand (X = Cl, Br, and I) were compared. They are based on a fac-[ReX(CO)3(N^N)] framework where the N^N bidentate ligand is a 3-(2-pyridyl)-1,2,4-triazole unit functionalized by an appended phenyl group. DFT calculations showed that the character of the lowest energy transitions progressively changes from Re → N^N ligand (MLCT) to X → N^N ligand (XLCT) when increasing the size of the halogen atom. Regarding the electrochemical behavior, the chloride and bromide complexes 1-Cl and 1-Br were similar, while the iodide complex 1-I exhibited a strikingly different electrochemical signature in oxidation. From a spectroscopic viewpoint, all three complexes emitted weak red-orange phosphorescence in dichloromethane solution. However, in the solid state, marked differences appeared. Not only was 1-Cl a good emitter of yellow light, but it had strong solid-state luminescence enhancement (SLE) properties. In comparison, 1-Br and 1-I were less emissiveand they showed better mechanoresponsive luminescence (MRL) properties, probably related to a loose molecular arrangement in the crystal packing and to the opening of vibrational non-radiative deactivation pathways. This study highlights for the first time how the nature of the halide ligand in this type of complex allows fine tuning of the solid-state optical properties, for potential applications either in bio-imaging or in the field of MRL-active materials.

Modeling of Re(I) tricarbonyl complexes against SARS-CoV-2 receptor via DFT, in-silico molecular docking, and QSAR

The quest for potential antiviral drug for the ongoing SARS-CoV-2 pandemic has posed a serious challenge to the scientific community. While several potential drugs have been proposed from organic molecular perspective, the uses of 3D metal complexes of bipyridine ligand have been recently proven to be potential coordinate covalent inhibitors for the SARS-CoV-2 main 3-Chymotrypsin-like protease (3CLpro). Herein, we present detailed DFT studies, in silico molecular docking, and multilinear regression analysis (MLRA) investigations of eight (8) selected biologically active Rhenium Tricarbonyl complexes designed and modeled based on the results of Karges and co-workers. The atomistic DFT modeling was conducted to investigate the reactivity, structural stability, and electronic properties based on frontier molecular orbitals (FMO), natural bond orbitals (NBO), interaction energies, density of states (DOS), charge distributions, and molecular thermochemical parameters. Molecular docking simulations were performed to study the binding interactions between the selected biologically active complexes and the target SARS-CoV-2 viral protein, 3CLPRO. The best quantitative structure-activity relationship (QSAR) was established to demonstrate the correlations between the DFT calculated descriptors and the in vitro biological activities (IC50) of structures.

Neutral Formazan Ligands Bound to the fac-(CO)3Re(I) Fragment: Structural, Spectroscopic, and Computational Studies.

Metal complexes with ligands that coordinate via the nitrogen atom of azo (N=N) or imino (C=N) groups are of interest due to their π-acceptor properties and redox-active nature, which leads to interesting (opto)electronic properties and reactivity. Here, we describe the synthesis and characterization of rhenium(I) tricarbonyl complexes with neutral N,N-bidentate formazans, which possess both N=N and C=N fragments within the ligand backbone (Ar1-NH-N=C(R3)-N=N-Ar5). The compounds were synthesized by reacting equimolar amounts of [ReBr(CO)5] and the corresponding neutral formazan. X-ray crystallographic and spectroscopic (IR, NMR) characterization confirmed the generation of formazan-type species with the structure fac-[ReBr(CO)3(κ2-N2,N4(Ar1-N1H-N2=C(R3)-N3=N4-Ar5))]. The formazan ligand coordinates the metal center in the ‘open’ form, generating a five-membered chelate ring with a pendant NH arm. The electronic absorption and emission properties of these complexes are governed by the presence of low-lying π*-orbitals on the ligand as shown by DFT calculations. The high orbital mixing between the metal and ligand results in photophysical properties that contrast to those observed in fac-[ReBr(CO)3(L,L)] species with α-diimine ligands.

Sono-ReCORMs for synergetic sonodynamic-gas therapy of hypoxic tumor

Carbon monoxide (CO) gas therapy, a novel anti-tumor technique based on the cytotoxicity from the CO released in situ, has become one of the hot topics in cancer treatment. Since the technique is oxygen-independent, it displays promising therapeutic effect for hypoxic tumor where traditional photodynamic therapy shows limited efficacy and insufficient penetration depth. To fully address these limitations of PDT, we propose a synergetic sonodynamic-CO gas releasing strategy for the therapy of hypoxic tumor. In this work, two rhenium(I) tricarbonyl complexes with different substituted ligands are investigated for US-triggered ROS generation and CO release. Our results indicated that the electron-donating NMe2-substituted complex (Re-NMe2) exhibits stronger luminescence intensity and generates more singlet oxygen (1O2) than the electron-withdrawing NO2-substituted complex (Re-NO2). In addition, Re-NMe2 displays release of CO triggered by US, thus showing high sono-cytotoxicity to tumor cells in-vitro and in-vivo. The strong ROS-generating capability combined with rapid CO-releasing feature from Re-NMe2 has made it a powerful tool for the efficient treatment of hypoxic tumor.

Rhenium(I) derivatives of aminoquinoline and imidazolopiperidine-based ligands: Synthesis, in vitro and in silico biological evaluation against Plasmodium falciparum

A small library of aminoquinoline and imidazolopiperidine (IMP)-based ligands, containing the 1,2,3-triazole moiety, and their corresponding tricarbonyl rhenium complexes were synthesised and their inhibitory activities evaluated against the chloroquine-sensitive (CQS) and multidrug-resistant (MDR) strains (NF54 and K1, respectively) of P. falciparum. The quinoline-based compounds (L1, L2, ReL1, and ReL2) were at least six-fold more potent than their IMP-based counterparts (L3, L4, ReL3, and ReL4) against both strains of P. falciparum, with the most promising compound (L1) displaying activity comparable to chloroquine diphosphate (CQDP) in the MDR strain. Additionally, all of the synthesised compounds have resistance indices less than CQDP. To gain insight into a possible mechanism of action, in silico hemozoin docking simulations were performed. These studies proposed that the tested compounds may act via hemozoin inhibition, as the new aminoquinoline-derivatives, with the exception of complex ReL2 (binding affinity: −12.62 kcal/mol), showed higher binding affinities than the reference drug chloroquine (CQ, −13.56 kcal/mol). Furthermore, the ligands exhibited superior binding affinity relative to their corresponding Re(I) complexes, which is reflected in their antiplasmodial activity.

ELECTROCHEMICAL CO2 REDUCTION OF RHENIUM TRICARBONYL COMPLEX

Carbon dioxide is considered as a primary reason for global climate change, thus CO2 needs to be urgently reduced. Catalytic conversion of CO2 into chemical fuels is one of the most crucial technologies that can address both global warming and the depletion of fossil fuels. Rhenium tricarbonyl complex [Re(bpy)(CO)3Cl] (bpy: 2,2’ bipyridine) possesses a great potential of capturing and highly selective converting CO2 to carbon monoxide. In the current study, we synthesized and characterized the structure of [Re(bpy)(CO)3Cl] by 1H NMR, ESI-MS, FITR, and PL spectroscopy. The electrochemical properties and the electrochemical CO2 reduction of [Re(bpy)(CO)3Cl] in the absence and presence of an electron donor source were carried out using cyclic voltammetric measurements. The cyclic voltammogram of [Re(bpy)(CO)3Cl] in N2-saturated DMF solution displayed one irreversible reduction wave at -1.33 V. [Re(bpy)(CO)3Cl] expressed its electrocatalytic behavior in CO2 atmosphere by the enhancement of the cathodic current density. The current increased approximately twofold in CO2-saturated DMF solution (from 0.15 to 0.32 mA/cm2) and more enhancement when adding TEOA solvent. With the presence of an electron donor, the CO2 reduction efficiency of [Re(bpy)(CO)3Cl] was improved and represented by an approximately fourfold increase in cathodic current from 0.32 to 1.12 mA/cm2. One-electron reduced species of [Re(bpy)(CO)3Cl] observed at 1.33 V in N2 and CO2-saturated electrolytes contributed to the reaction with CO2.

Effect of carbazole and pyrrolidine functionalization of phenanthroline ligand on ground- and excited-state properties of rhenium(I) complexes. Interplay between 3MLCT and 3IL/3ILCT

New rhenium(I) tricarbonyl complexes based on the phenanthroline ligand were designed and synthesized to investigate the role of carbazole (cbz) and pyrrolidine (pyrr) substituents in determining thermal, electrochemical and optoelectronic properties. Both pyrr and cbz are electron-releasing substituents, but they differ in the steric hindrance and electron delocalization ability. The impact of pyrr and cbz on ground- and excited-state properties of [ReCl(CO)3(cbz2-CH3phen)] (1) and [ReCl(CO)3(pyrr2-CH3phen)] (2) were investigated with a range of techniques including cyclic voltammetry, absorbance and emission spectroscopies, transient absorption spectroscopy, and they were discussed in comparison with the model chromophore [ReCl(CO)3(CH3phen)] (3). Experimental results were supported by theoretical calculations. The most striking difference between the pyrr and cbz groups was seen in their impact on the LUMO energy level. While the attached carbazole groups decreased the LUMO energy of 1, the pyrrolidine ones in 2 resulted in the rise of LUMO energy level compared to the model chromophore. For both 1 and 2, the HOMO got energetically destabilized in agreement with electro-donor abilities of pyrr and cbz substituents. Structural modification of phen backbone resulted also in changes of distribution of occupied MOs of 1 and 2 in relation to 3. Experimentally, it was manifested in noticeable changes of electrochemical and photophysical behaviour of the resulting Re(I) complexes in relation to the model chromophore.

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives.

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

2+1 Rhenium Tricarbonyl Complexes with N,N′-Bidentate Ligands and Ethyl Isocyanoacetate: Synthesis, Structure, and Properties

2+1 Rhenium Tricarbonyl Complexes with N,N′-Bidentate Ligands and Ethyl Isocyanoacetate: Synthesis, Structure, and Properties

Synthesis and Characterization of Novel [2 + 1] Tricarbonyl Rhenium Complexes with the Hydrophilic Phosphine Ligands PTA and CAP.

In the pursuit of hydrophilic model fac-[Re(CO)3]+ complexes for (radio) pharmaceutical applications, six novel [2 + 1] mixed-ligand complexes of the general type fac-[Re(CO)3(bid)P] were synthesized and characterized, where bid is a bidentate ligand bearing either (N, O) or (S, S′) donor atom sets and P is the hydrophilic phosphine 1,3,5-triaza-7-phosphoadamantane (PTA) or its macrocyclic homologue 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP). The (N, O) ligands used in this study were picolinic and quinaldic acid, while the (S, S′) ligand was diethyldithiocarbamate. The complexes were synthesized in generally high yields and purity and the characterization was performed by spectroscopic methods, IR, NMR, and elemental analysis. Detailed X-ray crystallographic study of molecular packing by using Hirshfeld analysis tools revealed a plethora of intermolecular interactions such as hydrogen bond, π⋯π, C-H⋯π, and carbonyl-carbonyl interactions. To our knowledge, the CAP complexes reported herein are the first example of [2 + 1] mixed-ligand fac-[Re(CO)3]+ complexes with CAP. The new complexes might have the potential to serve as platforms for the design of target-specific complexes with favorable pharmacokinetics.

Photocatalytic CO2 reduction sensitized by a special-pair mimic porphyrin connected with a rhenium(i) tricarbonyl complex.

Zn porphyrins with an imidazolyl group at the meso position generate a highly stable porphyrin dimer by complementary coordination from the imidazolyl to the Zn ion in noncoordinating solvents such as chloroform, which mimics the natural special pair in photosynthesis. In this work, we have synthesized an imidazolyl-substituted Zn porphyrin connected with a Re 2,2-bipyridine tricarbonyl complex as a CO2 reduction catalyst via a p-phenylene linker, affording a homodimer with two Re complexes on both sides (ReDRe). The dimeric structure is easily dissociated into the corresponding monomers in coordinating solvents. Therefore, we prepared a mixture containing a heterodimer with the Re carbonyl complex on one side (ReD) by simple mixing with an imidazolyl Zn porphyrin and evaporating the solvent. Using the Grubbs catalyst, the subsequent olefin metathesis reaction of the mixture gave covalently linked porphyrin dimers through the allyloxy side chains, enabling the isolation of the stable hetero- (ReD′) and homo-dimers (ReD′Re) with gel permeation chromatography. The Zn porphyrin dimers have intense absorption bands in the visible light region and acted as good photosensitizers in photocatalytic CO2 reduction in a mixture of N,N-dimethylacetamide and triethanolamine (5 : 1 v/v) containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor, giving CO with high selectivity and durability. Under irradiation with strong light intensity, the reaction rate in ReD′ exceeded that of the previous porphyrin Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Re complex dyad, ZnP-phen=Re. For instance, after irradiation at 560 nm for 18 h, the turnover number (TONCO) of ReD′ reached 2800, whereas the TONCO of ZnP-phen=Re was 170. The high activity in the system using the porphyrin dimer originates from no accumulation of the one-electron reduced species of the porphyrin that inhibit light absorption due to the inner-filter effect.

Target-specific mononuclear and binuclear rhenium(i) tricarbonyl complexes as upcoming anticancer drugs.

Metal complexes have gradually been attracting interest from researchers worldwide as potential cancer therapeutics. Driven by the many side effects of the popular platinum-based anticancer drug cisplatin, the tireless endeavours of researchers have afforded strategies for the design of appropriate metal complexes with minimal side effects compared to cisplatin and its congeners to limit the unrestricted propagation of cancer. In this regard, transition metal complexes, especially rhenium-based complexes are being identified and highlighted as promising cancer theranostics, which are endowed with the ability to detect and annihilate cancer cells in the body. This is attributed the amazing photophysical properties of rhenium complexes together with their ability to selectively attack different organelles in cancer cells. Therefore, this review presents the properties of different rhenium-based complexes to highlight their recent advances as anticancer agents based on their cytotoxicity results.

Electrochromic behavior of fac-tricarbonyl rhenium complexes

Tricarbonyl rhenium complex shows good electrochromic performance with a colored stage of green, rapid response and good switching stability.

Theoretical probe of absorption and fluorescence emission characteristics of highly luminescent ReL(CO)3X (L = 12H-indazolo[5,6-f][1,10]phenanthroline and X = F, Cl, Br, I): a DFT/TD-DFT study

Four novel rhenium (Re) tricarbonyl complexes (ReL(CO)3X; X = F, Cl, Br, I and L = 12H-indazolo[5,6-f][1,10]phenanthroline) with promising luminescent properties were investigated using a DFT/TD-DFT approach at the ωB97X-D3/6-31 + G(d,p)//LANL2DZ level of theory. The effects of halogen co-ligands (F, Cl, Br and I) and solvents (acetonitrile (ACN), dichloromethane (DCM) and dimethylsulfoxide (DMSO)) on luminescent properties of the complexes were examined. The studied Re(I) tricarbonyl complexes assumed distorted octahedral geometry with an N–Re–N bite angle of ca. 75°, and showed possible p–π interactions between the halogen (X) atom and the N-donor ligand (L) ring. Prominent electronic excitations (with high oscillator strengths) are S0→S3, attributed to the hole–electron orbital transition based on the natural transition orbitals analyses, and characterised as MLCT/LMCT. The complexes exhibited prominent Stokes fluorescence with Stokes shifts ranging from 9204 to 19490 cm−1. ReL(CO)3Cl in ACN showed the largest Stokes shift, whereas ReL(CO)3I in ACN displayed the smallest Stokes shift. Fluorescence emission wavelengths varied with X and solvent, though the four complexes emit in the cyan/blue range in DCM. ReL(CO)3F (in ACN) exhibited the highest fluorescence lifetime and quantum yield. The study demonstrates the tunability of photophysical properties and promotes the theoretical probe of optical characteristics of materials as corroborative efforts to experimental findings.

Azo–hydrazone tautomerism in organometallic complexes triggered by a -Re(CO)3(L) core: A spectroscopic and theoretical study

The spectroscopic properties and tautomeric behavior of five novel Rhenium (I) tricarbonyl complexes bearing an azo ligand are presented. The organic ligand is stable in solution as the hydrazone tautomer. It remains as hydrazone in different medium conditions (solvent, concentration, pH, etc.) without the formation of detectable amounts of the azo tautomer. However, the complexation of this ligand to the strong electron-withdrawing fragment -Re(CO)3X (X = Cl−, Br−) causes tautomerism to appear in the organic moiety. Two well-defined regions in the electronic spectra for both tautomers were observed, allowing the azo/enol-keto/hydrazone equilibrium to be followed. TD-DFT calculations indicate that for the keto/hydrazone form, the main absorption band is attributed to an IL transition. In contrast, the azo/enol species shows a major contribution of the 1MLCT (dπ(Re) → dπ*(NN)) transition. The tautomeric equilibrium is easily shifted using solvents with different dielectric constant and hydrogen bond donor/acceptor (HBD/HBA) abilities. Also, the tautomerization process is deeply influenced by the electronic properties of the axial ligand. Therefore, this behavior represents a different strategy for the design of novel materials with optical properties. Furthermore, tautomerism is affected by the concentration of complexes and the presence of water in solvents. The acid-base behavior of these compounds in a 50% v/v ethanol-buffer system showed that the azo/enol tautomer is stabilized in acidic media. At the same time, an increase of pH promotes tautomerization toward the keto/hydrazone, followed by forming the anionic form at pH > 8.

Prolonging the Triplet State Lifetimes of Rhenium Complexes with Imidazole-Pyridine Framework for Efficient CO2 Photoreduction.

The photocatalytic reduction of CO2 into fuels offers the prospect for creating a new CO2 economy. Harnessing visible light-driven CO2 -to-CO reduction mediated by the long-lived triplet excited state of rhenium(I) tricarbonyl complexes is a challenging approach. We here develop a series of new mononuclear rhenium(I) tricarbonyl complexes (Re-1-Re-4) based on the imidazole-pyridine skeleton for photo-driven CO2 reduction. These catalysts are featured by combining pyridyl-imidazole with the aromatic ring and different pendant organic groups onto the N1 position of 1,3-imidazole unit, which display phosphorescence under Ar-saturated solution even at ambient conditions. By contrast, {Re[9-(pyren-1-yl)-10-(pyridin-2-yl)-9H-pyreno[4,5-d]imidazole)](CO)3 Cl} (Re-4) by introducing pyrene ring at the N1 position of pyrene-fused imidazole unit exhibits superior catalytic performance with a higher turnover number for CO (TONCO =124) and >99.9 % selectivity, primarily ascribed to the strong visible light-harvesting ability, long-lived triplet lifetimes (164.2 μs) and large reductive quenching constant. Moreover, the rhenium(I) tricarbonyl complexes derived from π-extended pyrene chromophore exhibit a long lifetime corresponding to its ligand-localized triplet state (3 IL) evidenced from spectroscopic investigations and DFT calculations.

Efficient Direct Nitrosylation of α-Diimine Rhenium Tricarbonyl Complexes to Structurally Nearly Identical Higher Charge Congeners Activable towards Photo-CO Release.

The reaction of rhenium α-diimine (N-N) tricarbonyl complexes with nitrosonium tetrafluoroborate yields the corresponding dicarbonyl-nitrosyl [Re(CO)2(NO)(N-N)X]+ species (where X = halide). The complexes, accessible in a single step in good yield, are structurally nearly identical higher charge congeners of the tricarbonyl molecules. Substitution chemistry aimed at the realization of equivalent dicationic species (intended for applications as potential antimicrobial agents), revealed that the reactivity of metal ion in [Re(CO)2(NO)(N-N)X]+ is that of a hard Re acid, probably due to the stronger π-acceptor properties of NO+ as compared to those of CO. The metal ion thus shows great affinity for π-basic ligands, which are consequently difficult to replace by, e.g., σ-donor or weak π-acids like pyridine. Attempts of direct nitrosylation of α-diimine fac-[Re(CO)3]+ complexes bearing π-basic OR-type ligands gave the [Re(CO)2(NO)(N-N)(BF4)][BF4] salt as the only product in good yield, featuring a stable Re-FBF3 bond. The solid state crystal structure of nearly all molecules presented could be elucidated. A fundamental consequence of the chemistry of [Re(CO)2(NO)(N-N)X]+ complexes, it that the same can be photo-activated towards CO release and represent an entirely new class of photoCORMs.

Luminescent Rhenium-containing Ionic Liquid Exhibiting Photoinduced Vapochromism

Abstract A luminescent ionic liquid containing a cationic rhenium tricarbonyl complex was synthesized. UV photoirradiation of this liquid in the presence of an organic solvent vapor caused a color change and luminescence quenching via photochemical ligand exchange, thereby enabling the photocontrol of vapochromism.

Dinuclear Re(I) Complexes as New Electrocatalytic Systems for CO2 Reduction

AbstractA family of dinuclear tricarbonyl rhenium (I) complexes containing bridging 1,2‐diazine ligand and halide anions as ancillary ligands and able to catalyze CO2 reduction is presented. Electrochemical studies show that the highest catalytic efficiency is obtained for the complex containing the 4,5‐bipenthyl‐pyridazine and iodide as ancillary halogen ligands. This complex gives rise to TOF=15 s−1 that clearly outperforms the values reported for the benchmark mononuclear Re(CO)3Cl(bpy) (11.1 s−1). The role of the substituents on the pyridazine ligand and the nature of the bridging halide ligands on the catalytic activity have been deeply investigated through a systematic study on the structure‐properties relationship to understand the improved catalytic efficiencies of this class of complexes.