The recent years have witnessed the development of metal-catalyzed reductive carboxylation of organic (pseudo)halides with CO₂ as C1 source, representing potential powerful alternatives to existing methodologies for preparing carboxylic acids, privileged motifs in a myriad of pharmaceuticals and molecules displaying significant biological properties. While originally visualized as exotic cross-coupling reactions, a close look into the literature data indicates that these processes have become a fertile ground, allowing for the utilization of a variety of coupling partners, even with particularly challenging substrate combinations. As for other related cross-electrophile scenarios, the vast majority of reductive carboxylation of organic (pseudo)halides are characterized by their simplicity, mild conditions, and a broad functional group compatibility, suggesting that these processes could be implemented in late-stage diversification. This perspective describes the evolution of metal-catalyzed reductive carboxylation of organic (pseudo)halides from its inception in the pioneering stoichiometric work of Osakada to the present. Specific emphasis is devoted to the reactivity of these coupling processes, with substrates ranging from aryl-, vinyl-, benzyl- to unactivated alkyl (pseudo)halides. Despite the impressive advances realized, a comprehensive study detailing the mechanistic intricacies of these processes is still lacking. Some recent empirical evidence reveal an intriguing dichotomy exerted by the substitution pattern on the ligands utilized; still, however, some elementary steps within the catalytic cycle of these reactions remain speculative, in many instances invoking a canonical cross-coupling process. Although tentative, we anticipate that these processes might fall into more than one distinct mechanistic category depending on the substrate utilized, suggesting that investigations aimed at unraveling the mechanistic underpinnings of these processes will likely bring new and innovative research grounds in this vibrant area of expertise.

A review of all existing N-heterocyclic silylene (NHSi) complexes involved in catalysis. Remarkably few examples exist, but already show promise as an emerging new generation of catalysts with the potential to tune catalyst activities and selectivities.

The first neutral bis(metallylene)pyridine pincer-type [ENE] ligands (E = Si^II, Ge^II) were synthesized, and their coordination chemistry and reactivity toward iron was studied. First, the unprecedented four-coordinate complexes κ²E,E′-[ENE]FeCl₂ were isolated. Unexpectedly and in contrast to other related pyridine-based pincer-type Fe(II) complexes, the N atom of pyridine is reluctant to coordinate to the Fe(II) site due to the enhanced σ-donor strength of the E atoms, which disfavors this coordination mode. Subsequent reduction of κ²Si,Si′-[SiNSi]FeCl₂ with KC₈ in the presence of PMe₃ or direct reaction of the [ENE] ligands using Fe(PMe₃)₄ produced the highly electron-rich iron(0) complexes [ENE]Fe(PMe₃)₂. The reduction of the iron center substantially changes its coordination features, as shown by the results of a single-crystal X-ray diffraction analysis of [SiNSi]Fe(PMe₃)₂. The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination environment, with a coordinative (pyridine)N→Fe bond, and a trimethylphosphine ligand occupying the apical position. This geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature ¹H and ³¹P NMR spectra of the [ENE]Fe(PMe₃)₂ complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0) complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of [ENE]Fe(PMe₃)₂ with CO resulted in the isolation of [ENE]Fe(CO)₂ and the hitherto unknown κ²E,E′-[ENE]Fe(CO)₂L (L = CO, PMe₃) complexes. All complexes were fully characterized (NMR, MS, XRD, IR, and ⁵⁷Fe Mössbauer spectroscopy), showing the highest electron density on the iron center for pincer-type complexes reported to date. DFT calculations and ⁵⁷Fe Mössbauer spectroscopy confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active precatalysts for the hydrosilylation of ketones.

Combined experimental and theoretical analysis of the carbonyl hydrosilylation catalysed by an iron(0) pincer complex reveals an unprecedented mechanism of action. The iron(0) complex is in fact a precatalyst that is converted into an iron(II) catalyst through oxidative addition of a hydrosilane. Neither the hydrogen atom nor the silicon atom bound to the iron(II) centre are subsequently transferred onto the carbonyl acceptor, instead remaining at the sterically inaccessible iron(II) atom throughout the catalytic cycle. A series of labelling, crossover and competition experiments as well as the use of a silicon-stereogenic hydrosilane as a stereochemical probe suggest that the iron(II) site is not directly involved in the hydrosilylation. Strikingly, it is the silyl ligand attached to the iron(II) atom that acts as a Lewis acid for carbonyl activation in this catalysis. The whole catalytic process occurs on the periphery of the transition metal. Computation of the new peripheral as well as plausible alternative inner and outer sphere mechanisms support the experimental findings.