Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron(III) and iron(IV) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO₂ reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design.

α-Alkylation of arylacetonitriles with primary alcohols catalyzed by backbone modified N-heterocyclic carbene iridium(i) complexes

α-Alkylation of arylacetonitriles with primary alcohols was achieved by using backbone-modified NHC–Ir^I complexes as catalysts with turnover numbers of up to 960.

Effect of Ancillary Ligand in Cyclometalated Ru(II)-NHC-Catalyzed Transfer Hydrogenation of Unsaturated Compounds

In an effort to develop efficient Ru(II)-NHC-based catalyst considering their stereoelectronic effect for hydride-transfer reaction, we found that the ancillary NHC ligand can play a significant role in its catalytic performance. This effect is demonstrated by comparing the activity of two different types of orthometalated precatalysts of general formula [( p-cymene)(NHC)Ru^II(X)] (NHC = an imidazolylidene-based ImNHC, compound 2a-c, or a mesoionic triazolylidene-based tzNHC, compound 4) in transfer hydrogenation of carbonyl substrates. The electron-rich precatalyst, 2c, containing p-OMe-substituted NHC ligand performed significantly better than both unsubstituted complex 2a and p-CF₃ substituted electron-poor complex 2b in ketone reduction. Whereas bulky mesoionic triazolylidene ligand containing complex 4 was found to be superior catalyst for aldehyde reduction and the precatalyst 2a is more suitable for the selective transfer hydrogenation of a wide range of aromatic aldimines to amines. To the best of our knowledge, this is the first systematic study on the effect of stereoelectronic tuning of ancillary orthometalated NHC ligand in Ru(II)-catalyzed transfer hydrogenations of various types of unsaturated compounds with broad substrate scope.

Fine-tuning the efficiency of para-hydrogen-induced hyperpolarization by rational N-heterocyclic carbene design

Iridium N-heterocyclic carbene (NHC) complexes catalyse the para-hydrogen-induced hyperpolarization process, Signal Amplification by Reversible Exchange (SABRE). This process transfers the latent magnetism of para-hydrogen into a substrate, without changing its chemical identity, to dramatically improve its nuclear magnetic resonance (NMR) detectability. By synthesizing and examining over 30 NHC containing complexes, here we rationalize the key characteristics of efficient SABRE catalysis prior to using appropriate catalyst-substrate combinations to quantify the substrate's NMR detectability. These optimizations deliver polarizations of 63% for 1H nuclei in methyl 4,6-d2-nicotinate, 25% for 13C nuclei in a 13C2-diphenylpyridazine and 43% for the 15N nucleus of pyridine-15N. These high detectability levels compare favourably with the 0.0005% 1H value harnessed by a routine 1.5 T clinical MRI system. As signal strength scales with the square of the number of observations, these low cost innovations offer remarkable improvements in detectability threshold that offer routes to significantly reduce measurement time.

Small "Yaw" Angles, Large "Bite" Angles and an Electron-Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride-Transfer Activity

Cyclometalated complexes are an important class of (pre)catalysts in many reactions including hydride transfer. The ring size of such complexes could therefore be a relevant aspect to consider while modulating their catalytic activity. However, any correlation between the cyclometalating ring size and the catalytic activity should be drawn by careful assessment of the pertinent geometrical parameters, and overall electronic effects thereof. In this study, we investigated the vital role of key stereoelectronic functions of two classes of iridacyclic complexes-five-membered and six-membered cycles-in manupulating the catalytic efficiency in a model hydride-transfer reaction. Our investigation revealed that there exists an interesting multidimensional synergy among all the relevant stereoelectronic factors-yaw angle, bite angle, and the electronic properties of both the ligand and the metal center-that governs the hydride donor ability (hydricity) of the complexes during catalysis. Thus the six-membered chelate complexes with small yaw and large bite angles, strong donor ligand, and electron-rich metal were found to be better catalysts than their five-membered analogues. A frontier molecular orbital analysis supported the significant role of the above stereoelectronic synergistic effect associated with the chelate ring to control the hydride donor ability of the complexes.

Porous aromatic frameworks (PAFs) as efficient supports for N-heterocyclic carbene catalysts

Polymeric aromatic frameworks (PAFs), easily obtained from organic platforms, as excellent supports to yield robust well-defined organometallic heterogenized catalysts.

Air stable NHCs: a study of stereoelectronics and metallorganic catalytic activity

IPrBr: an air stable NHC. A stereoelectronic and catalytic study of IPrBr is reported.

Radioiodinated and astatinated NHC rhodium complexes: synthesis

INTRODUCTION

The clinical development of radioimmunotherapy with astatine-211 is limited by the lack of a stable radiolabeling method for antibody fragments. An astatinated N-heterocyclic carbene (NHC) Rhodium complex was assessed for the improvement of radiolabeling methodologies with astatine.

METHODS

Wet harvested astatine-211 in diisopropyl ether was used. Astatine was first reduced with cysteine then was reacted with a chlorinated Rh-NHC precursor to allow the formation of the astatinated analogue. Reaction conditions have been optimized. Astatine and iodine reactivity were also compared. Serum stability of the astatinated complex has been evaluated.

RESULTS

Quantitative formation of astatide was observed when cysteine amounts higher than 46.2 nmol/μl of astatine solution were added. Nucleophilic substitution kinetics showed that high radiolabeling yields were obtained within 15 min at 60°C (88%) or within 5 min at 100°C (95%). Chromatographic characteristics of this new astatinated compound have been correlated with the cold iodinated analog ones. The radioiodinated complex was also synthesized from the same precursor (5 min. at 100°C, up to 85%) using [(125)I]NaI as a radiotracer. In vitro stability of the astatinated complex was controlled after 15 h incubation in human serum at 4°C and 37°C. No degradation was observed, indicating the good chemical and enzymatic stability.

CONCLUSION

The astatinated complex was obtained in good yield and exhibited good chemical and enzymatic stability. These preliminary results demonstrate the interest of this new radiolabeling methodology, and further functionalizations should open new possibilities in astatine chemistry.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Although there are many steps and pitfalls before clinical use for a new prosthetic group from the family of NHC complexes, this work may open a new path for astatine-211 targeting.

Electronic tuning of a carbene center via remote chemical induction, and relevant effects in catalysis

The present report develops the idea that an N-heterocyclic carbene incorporating a remote anionic functionality--here, a malonate group--as a backbone component of its heterocyclic framework, can be "post-functionalized" directly from its transition-metal complexes, upon simple addition of a variety of electrophiles interacting directly with the malonate group in the outer coordination sphere. From a palette of selected electrophilic reagents, it was thus possible to modulate the electronic donor properties of the carbene center over a rather broad range. Both the zwitterionic complex [Rh{malo-NHC}(cod)] and the cationic derivatives [Rh{malo-NHC(E)}(cod)](+) (where "malo-NHC(E)" represents the ligand modified by a selected electrophile "E") were used as pre-catalysts in two types of catalytic reactions, namely, the polymerization of phenylacetylene and the hydroboration of styrene. The results indicate that, in both cases, the zwitterionic species is by far the best catalyst, whereas a decrease in the ligand donicity induced by the added electrophile results in a concomitant reduction of catalytic activity. Apparent deviations to such a trend in the case of the hydroboration of styrene were rationalized in terms of an interaction between the reactive catecholborane substrate and the remote functionality of the N-heterocyclic carbene leading to an in situ modification of the nature of the active species. These observations serve as a useful basis to define the scope and limitations of the present conceptual approach in catalysis.