Applying the Macrocyclic Effect to Smaller Ring Structures. N,N‘-Dimethyl-3,7-diazabicyclo[3.3.1]nonane Nickel(0) Complexes

The two N donor atoms in the tertiary diamine N,N‘-dimethyl-3,7-diazabicyclo[3.3.1]nonane (dabn, C₉H₁₈N₉) are ideally positioned in the bicyclic structure for chelation to a metal center. This feature was utilized to synthesize unusual diamine nickel(0)−ethene and −ethyne complexes, which represent limiting cases of the Pearson hard-soft acid-base concept. Thus, the reaction of Ni(C₂H₄)₃ with dabn affords yellow TP-3 (C₉H₁₈N₂)Ni(C₂H₄) (1) (dec. 0 °C) in which the ethene ligand displays extreme high-field NMR shifts at δ(H) 0.27 and δ(C) 20.4 and an exceptionally small coupling constant ¹J(CH) = 142 Hz. Reaction of 1 with butadiene yields the red mononuclear T-4 complex (C₉H₁₈N₂)Ni(η²-C₄H₆)₂ (2a) in solution, from which the dinuclear derivative {(C₉H₁₈N₂)Ni(η²-C₄H₆)}₂(μ-η²,η²-C₄H₆) (2) (dec. 20 °C) is isolated. Complexes 2 and 2a are more soluble than 1 and thus better suited for further reactions. When ethyne is added to a solution of 2 or 2a at −78 °C, the yellow TP-3 complex (C₉H₁₈N₂)Ni(C₂H₂) (3) (dec. −30 °C) is obtained. The ethyne ligand of 3 exhibits the lowest IR C⋮C stretching frequency (1560 cm^-1) and by far the smallest NMR coupling constant ¹J(CH) = 178 Hz yet reported for a mononuclear nickel(0)−ethyne complex. In addition, Ni(CO)₄ reacts with dabn to yield orange T-4 (C₉H₁₈N₂)Ni(CO)₂ (4). The results demonstrate that tertiary diamines, which are hard Lewis bases and which a priori are expected to coordinate poorly to the soft Lewis acid Ni(0), may be supported in such a coordination by the stabilizing principle of preorganization and consequently act as very powerful donor ligands