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(i mpy )(CO) 3 Br] ( 3 ) is readily prepared by stirring (1-(pyridin-2-yl)- 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-2-ylmethyl)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.