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Bulky Guanidinato Nickel(I) Complexes: Synthesis, Characterization, Isomerization and Reactivity Studies

Jones, C.; Schulten, C.; Fohlmeister, L.; Stasch, A.; Murray, K. S.; Moubaraki, B.; Kohl, S.; Ertem, M. Z.; Gagliardi, L.; Cramer, C. J.
Chem. Eur. J. 2011, 17, 1294. (doi:10.1002/chem.201002388)

Reactions of lithium complexes of the bulky guanidinates, [{(Dip)N}2CNR2]- (R = C6H11 (Giso-) or iPr (Priso-), Dip = C6H3iPr2-2,6), with NiBr2 have afforded the nickel(II) complexes, [{(L)Ni(μ-Br)}2] (L = Giso- or Priso-), the latter of which was crystallographically characterized. Reduction of [{(Priso)Ni(μ-Br)}2] with elemental potassium in benzene or toluene afforded the diamagnetic species, [{(Priso)Ni}2(μ-C6H5R)] (R = H or Me), which X-ray crystallographic studies showed to possess non-planar bridging arene ligands that are apparently partially reduced. A similar reduction of [{(Priso)Ni(μ-Br)}2] in cyclohexane yielded a mixture of the isomeric complexes, [{Ni(μ-κ1-N-,η2-Dip-Priso)}2] and [{Ni(μ-κ2-N,N'-Priso)}2], both of which were structurally characterized. These complexes were also formed via arene elimination processes when [{(Priso)Ni}2(μ-C6H5R)] (R = H or Me) were dissolved in hexane. In that solvent, diamagnetic [{Ni(μ-κ1-N-,η2-Dip-Priso)}2] was found to slowly convert to paramagnetic, spin coupled [{Ni(μ-κ2-N,N'-Priso)}2], suggesting the latter is the thermodynamic isomer. A computational analysis of a model of [{Ni(μ-κ2-N,N'-Priso)}2] showed it to have a Ni-Ni bond which has a multiconfigurational electronic structure. An analogous copper(I) complex, [{Cu(μ-κ2-N,N'-Giso)}2], was prepared, structurally authenticated, and found to have a negligible Cu---Cu bonding interaction by a theoretical study. The reactivity of [{Ni(μ-κ2-N,N'-Priso)}2] and [{Ni(κ1-N-,η2-Dip-Priso)}2] towards a range of small molecules was examined and this gave rise to the diamagnetic complexes, [{(Priso)Ni(μ-CO)}2] and [{(Priso)Ni(μ-N3)}2]. Taken as a whole, this study highlights close similarities between bulky guanidinate ligands and the β-diketiminate ligand class, but shows the former to have a greater coordinative flexibility.