Aryl-Containing Chelates and Amine Debenzylation to Afford 1,3-Di-2-pyridyl-2-azaallyl (smif): Structures of {κ-C,N,N^py₂-(2-pyridylmethyl)₂N(CH₂(4-^tBu-phenyl-2-yl))}FeBr and (smif)CrN(TMS)₂

Aryl-bromide ligand precursors have been prepared with the potential to afford tetradentate chelates (2-pyridylmethyl)3−xN(CH2-2-Aryl)x (x = 1, 2) containing metal-aryl linkages that promise to impart stronger fields about first row transition metals. Oxidative addition to Ni(COD)2 afforded two diamagnetic Ni(II) complexes, {κ-C,N,Npy-(2-pyridylmethyl)N(CH2(4-tBu-phenyl-2-yl))(CH2(4-tBu-phenyl-2-Br))}NiBr (1-Ni) and {(κ-C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}NiBr (2-Ni) in 96% and 67% yield, respectively. Extending these synthetic efforts to iron provided {κ-C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}FeBr (2-Fe, X-ray) in 91% yield via reduction of an adduct, {κ-N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-Br))}FeBr2 (3-Fe). 5-Coordinate 2-Fe possessed a pseudo-tbp structure, and SQUID magnetometry showed it to be S = 2 with significant zero field splitting (ZFS). 2-Fe was initially prepared via oxidative addition to Fe{N(TMS)2}2(THF) upon disproportionation to “Fe(0)” and 2 Fe{N(TMS)2}3, but when this approach was attempted with Cr{N(TMS)2}2(THF)2, the azaallyl complex {κ-N,Npy2-1,3-dipyridyl-2-azaallyl}CrN(TMS)2 ((smif)CrN(TMS)2, 4-Cr, X-ray), formed instead (>50%) via amine debenzylation. An alternative route consisting of addition of 1,3-di-2-pyridyl-2-azapropene to Cr{N(TMS)2}2(THF)2 afforded 4-Cr in 74% yield. Pseudo-square planar 4-Cr was also S = 2 (SQUID) with marked ZFS. The dipyridylazaallyl ligand “smif” imparts a remarkable optical density to 4-Cr via intraligand bands at 675 nm (ε ∼ 15 000 M−1cm−1) and 396 nm (ε ∼ 27 000 M−1cm−1). The effective fields of the chelate complexes are discussed, and a comparison of smif to isoelectronic NHC ligands is given.