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

Abstract

The synthesis and structural comparison are reported herein for a series of late first-row transition metal complexes using a macrocyclic pyridinophane ligand, <i>N</i>,<i>N</i>′-di-<i>tert</i>-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane (^tBuN4). The ^tBuN4 ligand enforces a distorted octahedral geometry in complexes [(^tBuN4)­M^II(MeCN)₂]­(OTf)₂ (M = Fe^II, Co^II, Ni^II, Cu^II), [(^tBuN4)­Zn^II(MeCN)­(OTf)]­(OTf), and [(^tBuN4)­Fe^III(OMe)₂]­(OTf), with elongated axial M–N_amine distances compared to the equatorial M–N_py distances. The geometry of [(^tBuN4)­Cu^I(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 Co^I, Ni^I, Co^III, Fe^III solvato-complexes, while the electrochemical oxidation of (^tBuN4)­NiCl₂ complex leads to formation of a Ni^III species, supporting the ability of the ^tBuN4 ligand to stabilize first row transition metal complexes in various oxidation states. Importantly, the [(^tBuN4)­M^II(MeCN)₂]²⁺ complexes exhibit two available <i>cis</i> coordination sites and thus can mediate reactions involving exogenous ligands. For example, the [(^tBuN4)­Cu^II(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)­Fe^II(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)­M^II systems should be able to promote redox transformations involving exogenous substrates