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 CO2 binding via a formal [1,3] addition under Re–O and C–C bond formation. fac-[Re(impy)(CO)3Br] (3) is readily prepared by stirring (1-(pyridin-2-yl)-N-(p-tolyl)methanimine (impy, 1) and [Re(CO)5Br] in refluxing THF. Alternatively, complex 3 can be readily obtained when a mixture of [Re(CO)5Br], 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 1H and 13C{1H} 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 CO2. The addition of the CO2 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 13CO2 suggest a reversible binding of CO2 to the complex. The related amine complex fac-[Re(ampy)(CO)3Br] (4) is similarly prepared by stirring (4-methyl-N-(pyridin-2-ylmethyl)aniline (2) and [Re(CO)5Br] 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.