First-Row Transition Metal and Lithium Pyridine-ene-amide Complexes Exhibiting N- and C‑Isomers and Ligand-Based Activation of Benzylic C–H Bonds

Ene-amines Z-3-(2-pyridyl)-1-aza­(2,6-iPr2-Ph)­propene,
(pynac)­H, and 2-(2-pyridyl)-1-aza­(2,6-R,R′-Ph)­propene, (pyEA-ArRR′)­H,
were synthesized by condensation procedures; corresponding lithium
or potassium ene-amides were prepared via standard deprotonation protocols.
Addition of 2 equiv of (pynac)H to {(Me3Si)2N}2Fe­(THF) or 2 Li­(pynac) to FeBr2(THF)2 afforded (pynac)2Fe (1), while treatment
of CrCl2(THF)2, MnCl2, FeBr2(THF)2, and CoCl2py4 with 2 equiv
of (pyEA-AriPr2)K afforded pseudotetrahedral
(pyEA-AriPr2)2M (2-M,
M = Cr, Mn, Fe) and (pyEA-AriPr2)2Co-py (2-Co-py). Diamagnetic (κ-C,N-pyEA-AriPr2)3Co (3) was prepared in low yield (∼7%) from CoCl2, and its Co–C­(sp3) linkages are unusually
low in field strength. Reactivity studies yielded little clean reactivity,
but thermolysis of 2-Co-py afforded the bis-indolamide
derivative {κ-N,N-N­(C6H3(2-iPr)­CMe2C­(Me)­(2-py)}2Co (5-Co), and related thermolyses of 2-M (M = Cr, Mn, Fe), conducted on NMR tube scales, generated related 5-M (M = Cr, Mn, Fe) at roughly the same rates. This observation
prompted thermolyses of (pyEA-ArRR′)­Li, which rearrange to
their corresponding indolamides in >90% yields. Rate studies, accompanied
by KIE and EIE observations, revealed that an initial hydrogen transfer
is reversible and is likely to correspond to an anionic rearrangement,
whereas C–C bond formation is rate-determining, as suggested
by accompanying calculations. X-ray structure determinations of 1, 2-Fe, 2-Co-py, 3, and 5-Co were conducted.