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, C 9 H 18 N 2 ) 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(C 2 H 4 ) 3 with dabn affords yellow TP -3 (C 9 H 18 N 2 )Ni(C 2 H 4 ) ( 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 1 J (CH) = 142 Hz. Reaction of 1 with butadiene yields the red mononuclear T -4 complex (C 9 H 18 N 2 )Ni(η 2 -C 4 H 6 ) 2 ( 2a ) in solution, from which the dinuclear derivative {(C 9 H 18 N 2 )Ni(η 2 -C 4 H 6 )} 2 (μ-η 2,η 2 -C 4 H 6 ) ( 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 (C 9 H 18 N 2 )Ni(C 2 H 2 ) ( 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 1 J (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 (C 9 H 18 N 2 )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.