Exploring the limits of redox non-innocence: pseudo square planar [{κ4-Me2C(CH2N=CHpy)2}Ni]n (n=2+, 1+, 0,-1,-2) favor Ni(II)

Treatment of Ni(COD)(2) with Me2C(CH2N=CHpy)(2) (dmp(PI)(2)) afforded pseudo-square planar, diamagnetic {dmp(PI)(2)(2-)}Ni-II (Ni[0]), which contains radical anion pyridine-imine (PI) units bound to Ni(II). Ag+ oxidation of Ni[0] provided [{dmp(PI)(2)}Ni](+) (Ni[+]) whose assignment as Ni(I) or Ni(II) was made on the basis of ab initio calculations, and EPR spectroscopy. Upon crystallization, Ni[+] formed an asymmetric dimer, [{dmp(PI)(2)}(2)(NiNi0)-Ni-II](2+) (Ni[2+]Ni[0]), consisting of pseudo-octahedral Ni(II) and pseudo-tetrahedral Ni(0). Ferrocenium oxidation (2 equiv.) of Ni[0] provided [{dmp(PI)(2)}(MeCN)(2)Ni-II](2+) (Ni[2+]), while KC8 reduction provided [{dmp(PI)(2)(3-)}Ni-II](-)(K(THF)(2))(+) (Ni[-]) in polymeric form (K+ counterion) or as ion-pairs (with K+(crypt-2.2.2)). Excess K-0 or 2 equiv. Cs-0 and Ni[0] provided highly sensitive [{dmp(PI)(2)(4-)}Ni-II](2-) (Ni[2-]), which disproportionates with C=C bond formation and dehydrogenation to afford [{2-py,3-PI,4-Me-2-azacyclopent-2-ene(2-)}(2)Ni](2-) [K+(crypt-2.2.2)](2) (Ni[AcPP]) upon addition of crypt-2.2.2. All redox events may be construed as shuttling electrons in and out of the ligand scaffold. While radical character is implicated in the calculations of Ni[+], Ni[0], and Ni[-], the pseudo-square planar species are remarkably resilient to further reactivity, thereby highlighting the stability provided by this geometry.