Late First-Row Transition Metal Complexes of a Tetradentate Pyridinophane Ligand: Electronic Properties and Reactivity Implications

The
synthesis and structural comparison are reported herein for a series
of late first-row transition metal complexes using a macrocyclic pyridinophane
ligand, N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane (tBuN4). The tBuN4 ligand enforces a distorted octahedral geometry in complexes
[(tBuN4)­MII(MeCN)2]­(OTf)2 (M = FeII, CoII, NiII, CuII), [(tBuN4)­ZnII(MeCN)­(OTf)]­(OTf), and [(tBuN4)­FeIII(OMe)2]­(OTf), with elongated
axial M–Namine distances compared to the equatorial
M–Npy distances. The geometry of [(tBuN4)­CuI(MeCN)]­(OTf) is pentacoordinate with weak axial
interactions with the amine N-donors of tBuN4. Complexes
[(tBuN4)­M­(MeCN)2]­(OTf)2 (M = Fe,
Co) exhibit magnetic properties that are intermediate between those
expected for high spin and low spin complexes. Electrochemical studies
of (tBuN4)­M complexes suggest that tBuN4 is
suitable to stabilize CoI, NiI, CoIII, FeIII solvato-complexes, while the electrochemical oxidation
of (tBuN4)­NiCl2 complex leads to formation of
a NiIII species, supporting the ability of the tBuN4 ligand to stabilize first row transition metal complexes in various
oxidation states. Importantly, the [(tBuN4)­MII(MeCN)2]2+ complexes exhibit two available cis coordination sites and thus can mediate reactions involving
exogenous ligands. For example, the [(tBuN4)­CuII(MeCN)2]2+ species acts as an efficient Lewis
acid and promotes an uncommon hydrolytic coupling of nitriles. In
addition, initial UV–vis and electron paramagnetic resonance
(EPR) studies show that the [(tBuN4)­FeII(MeCN)2]2+ complex reacts with oxidants such as H2O2 and peracetic acid to form high-valent Fe transient
species. Overall, these results suggest that the (tBuN4)­MII systems should be able to promote redox transformations
involving exogenous substrates.