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

摘要

Reactions of lithium complexes of the bulky guanidinates [{(Dip)N} _2CNR_2]- (Dip=C_6H_3iPr _2-2,_6; R=C_6H_(11) (Giso-) or iPr (Priso-), with NiBr_2 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-C_6H_5R)] (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-κ~1-N-,ν~2-Dip-Priso)}_2] and [{Ni(Iμ-κ~2-N,N′-Priso)}_2], both of which were structurally characterized. These complexes were also formed through arene elimination processes if [{Ni(Priso)}_2(Iμ-C_6H _5R)] (R=H or Me) were dissolved in hexane. In that solvent, diamagnetic [{Ni(Iμ-κ~1-N-,ν~2-Dip-Priso)} _2] was found to slowly convert to paramagnetic [{Ni(Iμ- κ~2-N,N′-Priso)}_2], suggesting that the latter is the thermodynamic isomer. Computational analysis of a model of [{Ni(Iμ-κ~2-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-κ~2-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 _6H_5Me)] and [{Ni(Iμ-κ~2-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-N_3)}_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.