Exploiting redox activity: Spin crossover, photoactivity and single molecule magnetism in mono-, di- and trinuclear complexes of diazapyridinophane and 2,2'-azobispyridine

This work describes the synthesis and characterization of mono-, di- and trinuclear complexes of first-, second- and third-row transition metal ions with the redoxactive ligand 2,2'-azobispyridine (abpy) and the macrocyclic diazapyridinophane ligands N,N'-Dimethyl-2,11-diaza[3.3](2.6)pyridinophane \((L-N_4Me_2)\) and N,N'-Di-tert-butyl-2,11-diaza[3.3](2.6)pyridinophane \((L-N_4 ^tBu_2)\). Depending on the redox state of the ligand and the metal ions, different spin states are realized, in some case leading to thermal spin crossover. Furthermore, interesting properties could be observed, including photoactivity and single molecule magnetism. In combination with iridium(III) or ruthenium(II) ions, heterodinuclear complexes \( [Ir(ppy)_2(μ-abpy1–/2–)M(L-N_4Me_2)]2+/1+\) () and \([Ru(bpy)_2(μ-abpy1–/2–)Co(L-N_4Me_2)]3+/2+\) (M = Fe(II), Co(II); ppy = 2-phenylpyridinyl; bpy = 2,2’-bipyridine) were synthesized. The heterodinuclear low-spin complexes of the radical \(abpy1–\) monoanion all display photoactive properties and were shown to undergo reductive quenching upon irradiation. This leads to complexes of the dianion \(abpy2–\) and to the activation of spin crossover properties. The ruthenium(II) complex \([Ru(L-N_4Me_2)(abpy)]²⁺\) was synthesized and compared to \([Fe(L-N_4Me_2)(abpy)]²⁺\). It could be shown that with the sterically demanding tert-butyl-substituted ligand \(L-N_4^tBu_2\) it is possible to generate three different redox states of the azopyridine ligand in the mononuclear complexes \([M(L-N_4^tBu_2)(abpy0/1–)]2+/1+\) (M = Fe(II), Co(II), Ni(II)) and \([Ni(L-N_4^tBu_2)(abpyH)]¹⁺\). In the complexes of radical \(abpy1–\), strong coupling with the metal ions and spin state changes are observed. The second reduction leads to the very basic hydrazido ligand \(abpy2–\) that in the case of \([Ni(L-N_4^tBu_2)(abpyH)]¹⁺\) is stabilized by protonation and hydrogen bonding between two complex cations. Based on this observation, trinuclear complexes \([{M(L-N_4^tBu_2)(μ-abpy²⁻)}_2Co]²⁺\) (M = Fe(II), Co(II), Ni(II), Zn(II)) around a central, pseudotetrahedral cobalt(II) ion were synthesized and investigated. It was found that, depending on the peripheral metal ions, single molecule magnetism can be observed. The extent of this effect is a function of the unpaired electrons on the peripheral metal ions. Oxidation of \([{M(L-N_4^tBu_2)(μ-abpy²⁻)}_2Co]²⁺\) quenches the single molecule magnetism and leads to what can be described as a rare case of pseudotetrahedral low-spin cobalt(III) complexes. Furthermore, unusual reactivities of azobispyridine complexes were observed and investigated, like the cobalt(II) half-sandwich complex \([Ni(L-N_4^tBu_2)(μ-abpy¹⁻)CoCp]²⁺\) (Cp = cyclopentadienyl) and aromatic hydroxylation of abpy in solid \([{Ni(L-N_4^tBu_2)(μ-abpy²⁻)}_2Co](BF_4)_2 ^. MeCN\), leading to \([{Ni(L-N_4^tBu_2)(μ-abpy-O2–)}_2Co]²⁺\) (\(abpy-O2–\) = 2-(pyridine-3-olato)-2'-azopyridine).