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

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posted on 12.10.2015 by Brian M. Lindley, Peter T. Wolczanski, Thomas R. Cundari, Emil B. Lobkovsky
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.