Reactions of cyclonickelated complexes with hydroxylamines and TEMPO˙: isolation of new TEMPOH adducts of Ni(ii) and their reactivities with nucleophiles and oxidants

Abstract

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr)₂PO-Ar}Ni(μ-Br)]₂, 1a–d (Ar: C₆H₄, a; 3-Cl-C₆H₃, b; 3-OMe-C₆H₃, c; 4-OMe-napthalenyl, d), with hydroxylamines in the presence of TEMPO˙ (TEMPO˙ = (2,2,6,6-tetramethylpiperidinyl-1-yl)oxyl). The results of this study showed that treating 1a–d with a mixture of Et₂NOH and TEMPO˙ did not afford the desired oxidation-induced functionalization of the Ni-aryl moiety in 1a–d, giving instead the corresponding κO-TEMPOH adducts [{κP,κC-(i-Pr)₂PO-Ar}Ni(Br)(κO-TEMPOH)], 3a–d (TEMPOH = N-hydroxy-2,2,6,6-tetramethylpiperidine). The TEMPOH moiety in these zwitterionic compounds 3 can be displaced by a large excess of acetonitrile (MeCN), 10 equiv. of morpholine, or 1–2 equivalents of imidazole. Although these reactions have given the authenticated products [{κP,κC-(i-Pr)₂PO-C₆H₄}Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr)₂PO-C₆H₄}Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr)₂PO-C₆H₄}Ni(imidazole)₂]Br, 6a, a few other products were also detected by NMR, indicating that the observed reactivities are far more complex than simple substitution of the TEMPOH moiety. Similarly, treating 3a with AgOC(O)CF₃ results in the isolation of [{κP,κC-(i-Pr)₂PO-C₆H₄}Ni{OC(O)CF₃}(κO-TEMPOH)], 7a, arising from the substitution of the bromo ligand, whereas spectroscopic evidence suggests further reactivity, possibly including displacement of TEMPOH and oxidative decomposition.