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

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr) 2 PO-Ar}Ni(μ-Br)] 2 , 1a-d (Ar: C 6 H 4 , a; 3-Cl-C 6 H 3 , b; 3-OMe-C 6 H 3 , 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 Et 2 NOH 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) 2 PO-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) 2 PO-C 6 H 4 }Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr) 2 PO-C 6 H 4 }Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr) 2 PO-C 6 H 4 }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)CF 3 results in the isolation of [{κP,κC-(i-Pr) 2 PO-C 6 H 4 }Ni{OC(O)CF 3 }(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.