Rhenium(I) Triscarbonyl Complexes with Redox-Active Amino- and Iminopyridine Ligands: Metal–Ligand Cooperation as Trigger for the Reversible Binding of CO₂ via a Dearmomatization/Rearomatization Reaction Sequence

Abstract

We describe rhenium(I) triscarbonyl compounds (3 and 4) decorated with simple-to-use 2-iminomethyl-pyridine (1, impy) and 2-aminomethyl-pyridine (2, ampy) ligands, respectively, which can serve as cooperative ligand scaffolds enabling CO 2 binding via a formal [1,3] addition under Re−O and C−C bond formation. fac-[Re(impy)-(CO) 3 Br] (3) is readily prepared by stirring (1-(pyridin-2yl)-N-(p-tolyl)methanimine (impy, 1) and [Re(CO ) 5 Br] in refluxing THF. Alternatively, complex 3 can be readily obtained when a mixture of [Re(CO) 5 Br], p-toluidine, and picolinaldehyde is refluxed in ethanol. Complex 3 is reduced with excess potassium metal in THF (two-electron reduction) to give the anionic amido complex K[Re(amidopy*)(CO) 3 ] (5b, the asterisk indicates the dearomatized ligand). Analysis of the 1 H and 13 C{ 1 H} NMR spectra of 5b suggest the dearomatization of the pyridine unit. Complex 5b is highly reactive and gives rise to the facile [1,3] addition of CO 2 . The addition of the CO 2 and thus the formation of K[Re(amidopy-COO)(CO) 3 ] ( 6) is characterized by the concomitant formation of a Re−O and a C−C bond. The addition is triggered by the rearomatization of the pyridine unit in 6. Remarkably, isotopic labeling experiments involving 13 CO 2 suggest a reversible binding of CO 2 to the complex. The related amine complex fac-[Re(ampy)(CO) 3 Br] (4) is similarly prepared by stirring (4-methyl-N-(pyridin-2ylmethyl)aniline ( 2) and [Re(CO) 5 Br] in THF at 60 °C. Upon addition of excess base (LiHMDS), complex 3 is readily deprotonated twice to give likewise the anionic amido complex Li[Re(amidopy*)(CO) 3 ] (5a). The latter reaction gives rise to a facile access to the reactive dearomatized anionic fragment [Re(amidopy*)(CO) 3 ] − with no need for the application of strong reducing agents. The ion pair M + /[Re(amidopy*)(CO) 3 ] − is highly reactive and combines MLC (metal−ligand cooperation) via a dearomatization/rearomatization scheme and bifunctional reactivity enabled by the nucleophilic nature of the Re complex and the Lewis acidic counter alkali cation.

Computational Study of an Iron(II) Polypyridine Electrocatalyst for CO2 Reduction: Key Roles for Intramolecular Interactions in CO2 Binding and Proton Transfer

“…This was observed in several other cases e.g. pincer-type complexes [113][114][115] zirconium metallocene phosphinoaryloxide complex 116 as well as in catalytic processes. 117,118 There are two conceivable ligand-based binding sites for CO 2 in this ligand framework; it can be inserted into the amine group yielding a carbamate or can bind via a reversible C-C bond to the ligand framework (see fig.…”

Cooperative Binding of SO2 under M–O and C–S Bond Formation in a Rhenium(I) Complex with Activated Amino- or Iminopyridine Ligand

“…Both strategies give rise to the activated complex 1 , which is characterized by an increased nucleophilicity of the benzylic carbon atom (dearomatization of the pyridine unit). As a result, 1 reacts reversibly with CO 2 via a MLC pathway under Re–O and C–C bond formation and concomitant rearrangement of the ligand π-system (rearomatization) . Kubiak and co-workers observed a similar reaction scheme in a molybdenum compound .…”

“…Complex 1 is prepared by two-electron reduction of the iminopyridine ligand in fac-[Re(impy)(CO) 3 Br] (i) using potassium metal or, alternatively, via a milder route, by double deprotonation of the related aminopyridine complex fac-[Re(ampy)(CO) 3 Br] (ii). 27 Both strategies give rise to the activated complex 1, which is characterized by an increased nucleophilicity of the benzylic carbon atom (dearomatization of the pyridine unit). As a result, 1 reacts reversibly with CO 2 via a MLC pathway under Re−O and C−C bond formation and concomitant rearrangement of the ligand π-system (rearomatization).…”

“…As a result, 1 reacts reversibly with CO 2 via a MLC pathway under Re−O and C−C bond formation and concomitant rearrangement of the ligand π-system (rearomatization). 27 Kubiak and co-workers observed a similar reaction scheme in a molybdenum compound. 28 The activation of CO 2 involving Cnucleophilic actor ligands has attracted increased attention and was recently reviewed by Song and co-workers.…”

“…MLC in transition metal complexes involving pyridine-based ligands was developed into an important tool in bond-activation chemistry. − Along these lines, we recently described an anionic Re­(I) triscarbonyl complex, which includes a redox active bidentate amidopyridine ligand (K­[Re­( amidopy )­(CO) 3 ] (Scheme , 1 ). Complex 1 is prepared by two-electron reduction of the iminopyridine ligand in fac -[Re­( impy )­(CO) 3 Br] (i) using potassium metal or, alternatively, via a milder route, by double deprotonation of the related aminopyridine complex fac- [Re­( ampy )­(CO) 3 Br] (ii) . Both strategies give rise to the activated complex 1 , which is characterized by an increased nucleophilicity of the benzylic carbon atom (dearomatization of the pyridine unit).…”