Revealing the Structure of Transition Metal Complexes of Formaldoxime

Transition metal(IV) complexes of nitro-substituted 2,4,9-triazaadamantane-based tris-hydroxylamines: evidence for nitrogen α-effect on stabilization of higher oxidation states

7-Nitro-2,4,9-triazaadamantane-2,4,9-triols (NO2-TRIADs), new chelating tris-hydroxylamine ligands, were synthesized by a novel one-pot, four-component condensation involving nitromethane and nitrosoalkene precursors. Upon deprotonation, NO2-TRIADs form stable diamantane-like complexes with manganese, iron and nickel in atypical +4 oxidation state. These complexes were characterized by XRD analysis, NMR, Mössbauer spectroscopy, UV-Vis, FT-IR, HRMS, and elemental analysis. Theoretical analysis based on DFT calculations revealed a remarkable electron donating "α-effect" of the nitrogen atom in the N-O-M fragment that is responsible for the stabilization of the higher oxidation state of the metal in these complexes. Metal(IV) complexes of NO2-TRIADs exhibit high catalytic activity in oxidation reactions, in particular, in the aerobic conversion of thiols into disulfides (up to 98% yield) and oxidation of activated C-H bonds in diphenylmethanol and dihydroanthracene with mCPBA (up to 74% yield). In the oxidation of dihydroanthracene, a notable selectivity toward the formation of anthraquinone was achieved (anthraquinone/anthrone = 7).

Crown-hydroxylamines are pH-dependent chelating N,O-ligands with a potential for aerobic oxidation catalysis

Despite the rich coordination chemistry, hydroxylamines are rarely used as ligands for transition metal coordination compounds. This is partially because of the instability of these complexes that undergo decomposition, disproportionation and oxidation processes involving the hydroxylamine motif. Here, we design macrocyclic poly-N-hydroxylamines (crown-hydroxylamines) that form complexes containing a d-metal ion (Cu(II), Ni(II), Mn(II), and Zn(II)) coordinated by multiple (up to six) hydroxylamine fragments. The stability of these complexes is likely to be due to a macrocycle effect and strong intramolecular H-bonding interactions between the N-OH groups. Crown-hydroxylamine complexes exhibit interesting pH-dependent behavior where the efficiency of metal binding increases upon deprotonation of the hydroxylamine groups. Copper complexes exhibit catalytic activity in aerobic oxidation reactions under ambient conditions, whereas the corresponding complexes with macrocyclic polyamines show poor or no activity. Our results show that crown-hydroxylamines display anomalous structural features and chemical behavior with respect to both organic hydroxylamines and polyaza-crowns.

Iron(IV) Complexes with Tetraazaadamantane-based Ligands: Synthesis, Structure, Application in Dioxygen Activation and Labeling of Biomolecules

4,6,10-Trihydroxy-1,4,6,10-tetraazaadamantane (TAAD) has been shown to form a stable Fe(IV) complex having a diamantane cage structure, in which the metal center is coordinated by three oxygen atoms of the deprotonated...