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

Reactions of lithium complexes of the bulky guanidinates [{(Dip)N} ₂CNR₂]^- (Dip=C₆H₃iPr ₂-2,6; R=C₆H₁₁ (Giso^-) or iPr (Priso^-), with NiBr₂ have afforded the nickel(II) complexes [{Ni(L)(Iμ-Br)}₂] (L=Giso^- or Priso ^-), the latter of which was crystallographically characterized. Reduction of [{Ni(Priso)(Iμ-Br)}₂] with elemental potassium in benzene or toluene afforded the diamagnetic species [{Ni(Priso)} ₂(Iμ-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)(Iμ-Br)} ₂] in cyclohexane yielded a mixture of the isomeric complexes [{Ni(Iμ-κ¹-N-,ν²-Dip-Priso)}₂] and [{Ni(Iμ-κ²-N,N′-Priso)}₂], both of which were structurally characterized. These complexes were also formed through arene elimination processes if [{Ni(Priso)}₂(Iμ-C₆H ₅R)] (R=H or Me) were dissolved in hexane. In that solvent, diamagnetic [{Ni(Iμ-κ¹-N-,ν²-Dip-Priso)} ₂] was found to slowly convert to paramagnetic [{Ni(Iμ- κ²-N,N′-Priso)}₂], suggesting that the latter is the thermodynamic isomer. Computational analysis of a model of [{Ni(Iμ-κ²-N,N′-Priso)}₂] showed it to have a NiNi bond that has a multiconfigurational electronic structure. An analogous copper(I) complex [{Cu(Iμ-κ²-N, N′-Giso)}₂] was prepared, structurally authenticated, and found, by a theoretical study, to have a negligible Cua⋯Cu bonding interaction. The reactivity of [{Ni(Priso)}₂(Iμ-C ₆H₅Me)] and [{Ni(Iμ-κ²-N,N′- Priso)}₂] towards a range of small molecules was examined and this gave rise to diamagnetic complexes [{Ni(Priso)(Iμ-CO)}₂] and [{Ni(Priso)(Iμ-N₃)}₂]. Taken as a whole, this study highlights similarities between bulky guanidinate ligands and the β-diketiminate ligand class, but shows the former to have greater coordinative flexibility.