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 ((tBu)N4). The (tBu)N4 ligand enforces a distorted octahedral geometry in complexes [((tBu)N4)M(II)(MeCN)2](OTf)2 (M = Fe(II), Co(II), Ni(II), Cu(II)), [((tBu)N4)Zn(II)(MeCN)(OTf)](OTf), and [((tBu)N4)Fe(III)(OMe)2](OTf), with elongated axial M-N(amine) distances compared to the equatorial M-N(py) distances. The geometry of [((tBu)N4)Cu(I)(MeCN)](OTf) is pentacoordinate with weak axial interactions with the amine N-donors of (tBu)N4. Complexes [((tBu)N4)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 ((tBu)N4)M complexes suggest that (tBu)N4 is suitable to stabilize Co(I), Ni(I), Co(III), Fe(III) solvato-complexes, while the electrochemical oxidation of ((tBu)N4)NiCl2 complex leads to formation of a Ni(III) species, supporting the ability of the (tBu)N4 ligand to stabilize first row transition metal complexes in various oxidation states. Importantly, the [((tBu)N4)M(II)(MeCN)2](2+) complexes exhibit two available cis coordination sites and thus can mediate reactions involving exogenous ligands. For example, the [((tBu)N4)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 [((tBu)N4)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 ((tBu)N4)M(II) systems should be able to promote redox transformations involving exogenous substrates.