Bulky guanidinato nickel(I) complexes: synthesis, characterization, isomerization, and reactivity studies

Reactions of lithium complexes of the bulky guanidinates [ (Dip)N 2CNR2]a?? (Dip=C6H3iPr2-2,6; R=C6H11 (Gisoa??) or iPr (Prisoa??), with NiBr2 have afforded the nickel(II) complexes [ Ni(L)(I?-Br) 2] (L=Gisoa?? or Prisoa??), the latter of which was crystallographically characterized. Reduction of [ Ni(Priso)(I?-Br) 2] with elemental potassium in benzene or toluene afforded the diamagnetic species [ Ni(Priso) 2(I?-C6H5R)] (R=H or Me), which were shown, by X-ray crystallographic studies, to possess nonplanar bridging arene ligands that are partially reduced. A similar reduction of [ Ni(Priso)(I?-Br) 2] in cyclohexane yielded a mixture of the isomeric complexes [ Ni(I?-I?1-N-,I?2-Dip-Priso) 2] and [ Ni(I?-I?2-N,Na??-Priso) 2], both of which were structurally characterized. These complexes were also formed through arene elimination processes if [ Ni(Priso) 2(I?-C6H5R)] (R=H or Me) were dissolved in hexane. In that solvent, diamagnetic [ Ni(I?-I?1-N-,I?2-Dip-Priso) 2] was found to slowly convert to paramagnetic [ Ni(I?-I?2-N,Na??-Priso) 2], suggesting that the latter is the thermodynamic isomer. Computational analysis of a model of [ Ni(I?-I?2-N,Na??-Priso) 2] showed it to have a NiNi bond that has a multiconfigurational electronic structure. An analogous copper(I) complex [ Cu(I?-I?2-N,Na??-Giso) 2] was prepared, structurally authenticated, and found, by a theoretical study, to have a negligible Cua??a??a??Cu bonding interaction. The reactivity of [ Ni(Priso) 2(I?-C6H5Me)] and [ Ni(I?-I?2-N,Na??-Priso) 2] towards a range of small molecules was examined and this gave rise to diamagnetic complexes [ Ni(Priso)(I?-CO) 2] and [ Ni(Priso)(I?-N3) 2]. Taken as a whole, this study highlights similarities between bulky guanidinate ligands and the I?-diketiminate ligand class, but shows the former to have greater coordinative flexibility.