Investigations on the reactivity of (pseudo-)macrocyclic copper complexes

In this thesis, two topics are being discussed: the investigation of the reactivity of \(Cu-H_2dmg\) complexes and the synthesis and characterization of photoswitchable dinuclear copper complexes with azobenzene-based ligands. In the first chapter, the formation and reactivity of various mono- and dinuclear copper dimethylglyoxime complexes is studied. It was determined that the pH value strongly influences the nuclearity of the complexes formed: under basic conditions the mononuclear complex \([Cu(Hdmg)_2]\) is favored, while neutral and slightly acidic conditions yield the dinuclear complex \([Cu_2(H_2dmg)(Hdmg)(dmg)]X (X = ClO_4^-, BF_4^-, OTf^-)\). However, for the conversion between these two species an adjustment of the pH value as well as addition of extra ligand is needed. Additionally, changing the copper salts used affect the complex formation: the slightly basic \(Cu(OAc)_2\) leads to \([Cu(Hdmg)_2]\), \(CuCl_2·2H_2O\) yields \([Cu(H_2dmg)Cl_2]\), while \(Cu(ClO_4)_2·6H_2O\), \(Cu(BF_4)_2·6H_2O\), and \(Cu(OTf)_2\) form dinuclear complexes of the type \([Cu_2(H_2dmg)(Hdmg)(dmg)]X (X = ClO_4^-, BF_4^-, OTf^-)\). Interestingly, starting from \(Cu(BF_4)_2·6H_2O\) additionally forms O-O-bidentate \(BF_2\)-bridged complexes, such as \([Cu_2(dmg ⋅ BF_2)(Hdmg)(H_2dmg)]BF_4\), \([Cu(dmg ⋅ BF_2)_2(MeCN)]\), and \([Cu(dmg ⋅ BF_2)(Hdmg)]\) via the decomposition of \(BF_4^-\). This provides a facile synthetic route for these types of complexes without the usage of the highly toxic \(BF_3·Et_2O\). Attempts to use the resulting fluoride ions for the fluorination of organic substrates were not successful. The solvent-dependent reactivity of \([Cu_2(H_2dmg)(Hdmg)(dmg)]ClO_4\) was also examined. Unlike its mononuclear analogue, it is sensitive towards \(H_2O\), leading to the hydrolysis of \(H_2dmg\) forming 2,3-butanedione monoxime and hydroxylamine \((NH_2OH)\). This reaction is enabled by cooperative effects of the two copper centers, which increase the electrophilicity of the \(µ_2-O-N=C\) group. Depending on the solvent, \(NH_2OH\) is either reduced to \(NH_4^+\) (in ethanol) or oxidized to \(N_2O\) (in acetonitrile). The latter reaction yields \([Cu(MeCN)_4]X (X = ClO_4^-, BF_4^-, OTf^-)\), providing a simple method to prepare Cu(I) salts. Overall, the reactivity of \([Cu_2(H_2dmg)(Hdmg)(dmg)]X (X = ClO_4^-, BF_4^-, OTf^-)\) exemplifies functional, enthalpic, and entropic cooperativity. In the second, chapter the synthesis and characterization of the new azobenzene and azoxybenzene based ligands \(3,3'-Azobenz(metPA)_2\) and \(3,3'-Azoxybenz(metPA)_2\) as well as their (macro-)cyclic dinuclear Cu(I) complexes are described. \(3,3'-Azobenz(metPA)_2\) was synthesized in a three-step synthesis starting from 3-nitrobenzyl alcohol and exhibits reversible photoisomerization, as confirmed by spectroscopic studies and quantum-chemical calculations. The complex \([Cu_2(3,3'-Azobenz(metPA)_2)_2](BF_4)_2\) also remains reversibly photoswitchable – representing the first known reversibly photoswitchable (3d-)metallocycle. This photoswitchable copper complex offers promising potential for light-controlled host guest systems and electron transfer applications, as isomerization alters the cavity size and the coordination geometry of the Cu(I)-center. Geometry optimizations suggest a shift toward a more tetrahedrally distorted Cu(I) coordination \((τ_4 = 0.59–0.68)\), potentially facilitating reversible Cu(I)/Cu(II) interconversion - an entatic state-like behavior. The photoreactivity of the azoxybenzene analogue of the ligand, \(3,3'-Azoxybenz(metPA)_2\), depends on the wavelength used: 365 nm induces a Wallach rearrangement to a 2-hydroxyazobenzene derivative instead of E-Z isomerization, thus, hindering the subsequent Z–E isomerization, while irradiation at 310 nm favors E–Z isomerization. However, re-isomerization using 448 nm causes small parts of the compound to undergo a Wallach rearrangement, which limits photoisomerization as with high numbers of cycles the corresponding 2-hydroxyazobenzole derivative is enriched. Additionally, the corresponding Cu(I) complex \([Cu_2(3,3'-Azoxybenz(metPA)_2](BF_4)_2\) could be synthesized. However, due to the observed Wallach rearrangement, it was not investigated towards its photophysical properties.