Exploring the limits of redox non-innocence: pseudo square planar [{κ4-Me2C(CH2NCHpy)2}Ni]n (n = 2+, 1+, 0, −1, −2) favor Ni(ii)

Abstract

Treatment of Ni(COD)₂ with Me₂C(CH₂NCHpy)₂ (dmp(PI)₂) afforded pseudo-square planar, diamagnetic {dmp(PI)₂²⁻}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}₂Ni^IINi⁰]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)₂Ni^II]²⁺ (Ni[2+]), while KC₈ reduction provided [{dmp(PI)₂³⁻}Ni^II]⁻(K(THF)₂)⁺ (Ni[−]) in polymeric form (K⁺ counterion) or as ion-pairs (with K⁺(crypt-2.2.2)). Excess K⁰ or 2 equiv. Cs⁰ and Ni[0] provided highly sensitive [{dmp(PI)₂⁴⁻}Ni^II]²⁻ (Ni[2−]), which disproportionates with CC bond formation and dehydrogenation to afford [{2-py,3-PI,4-Me₂-azacyclopent-2-ene²⁻}₂Ni]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.