C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

Abstract

Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105–111 kcal/mol) C-H bonds of pyridine substrates are unknown. In this work, we report the rapid room-temperature cleavage of C-H bonds in pyridine, 4-tert-butylpyridine, and 2-phenylpyridine by an iron(I) species, to give well-characterized iron(II) products. In addition, 4-dimethylaminopyridine (DMAP) undergoes room-temperature C-N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl-bridged tetrairon(II) square. These facile bond-cleaving reactions are proposed to occur through intermediates having a two-electron reduced pyridine that bridges two iron centers. Thus, the redox non-innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions.

Reduction of 2,2′-Bipyridine by Quasi-Linear 3d-Metal(I) Silylamides—A Structural and Spectroscopic Study

“…For comparison of the structural changes of the bipyridine ligand, the analogous zinc complexes were also synthesized. Overall our results indicate that anionic metal(I) silylamides are capable of reducing and ligate substrates, even when the electrochemical reduction potential of the latter is by up to 1 V higher.Inorganics 2019, 7, 117 2 of 22 metal(I) fragments [1,[16][17][18][19][20][21]. Given this observed reluctance to obtain further electron density upon Lewis base coordination, we were interested in the formal transfer of an electron from the metal(I) ion onto substrates to form metal(II) complexes bearing an anionic radicaloid ligand.…”

“…In addition, we were interested in elaborating the synthetic usability of the elusive manganese(I) silylamide [Mn I (hmds) 2 ] − (Scheme 1), which cannot be isolated due to its extreme sensitivity. Inorganics 2019, 7, x FOR PEER REVIEW 2 of 22 Inorganics 2019, 7, x; doi: FOR PEER REVIEW www.mdpi.com/journal/inorganicsphosphites, NHCs or THF (exemplarily shown for iron) [6], which is in contrast to the behavior of other two-coordinate metal(I) fragments [1,[16][17][18][19][20][21]. Given this observed reluctance to obtain further electron density upon Lewis base coordination, we were interested in the formal transfer of an electron from the metal(I) ion onto substrates to form metal(II) complexes bearing an anionic radicaloid ligand.…”

“…Inorganics 2019, 7, 117 2 of 22 metal(I) fragments [1,[16][17][18][19][20][21]. Given this observed reluctance to obtain further electron density upon Lewis base coordination, we were interested in the formal transfer of an electron from the metal(I) ion onto substrates to form metal(II) complexes bearing an anionic radicaloid ligand.…”

Bond Activations Assisted by Redox Active Ligand Scaffolds

“…A yet another remarkable example of regioselective bond activation was accounted in 2017. Holland and co‐workers documented the reduction of the redox active pyridine moiety which in turn allowed for the activation of C–H and C–N [on reaction with 4‐dimethylaminopyridine (DMAP)] bonds of pyridine by the iron in the coordinated metal compound (Figure b) . A previous report by Holland and co‐workers efficiently demonstrated facile two electron reduction of pyridine leading to a pyridine bridged (η 1 :η 3 ) dimeric Fe II Fe II complex (6) .…”

“…Holland and co-workers documented the reduction of the redox active pyridine moiety which in turn allowed for the activation of C-H and C-N [on reaction with 4-dimethylaminopyridine (DMAP)] bonds of pyridine by the iron in the coordinated metal compound (Figure 2b). [11] A previous report [12] by Holland and co-workers efficiently demonstrated facile two electron reduction of pyridine leading to a pyridine bridged (η 1 :η 3 ) dimeric Fe II Fe II complex (6). The reduced pyridine directs the η 3 -coordinated Fe II group either to its ortho-position which facilitates -hydrogen abstraction by iron(II) giving a C-H activated dimeric complex, 7 or concomitantly, C-N activated 8 is favoured if the η 3 -coordinated Fe II group is directed towards the para-position of the pyridine framework which thereby gateways the abstraction of the -NMe 2 group by the precursor Fe I complex.…”