Organometallic Nickelamacrocycles of the Type [(R₂R‘P)Ni(C₂H₄COO)]_n:  Synthesis and Self-Assembly to Form Different Molecular Architectures Tuned by the Phosphine

The reaction of succinic anhydride with a 1:2 mixture of (cod)₂Ni and a monodentate phosphine generates reactive monomeric nickelalactones, which undergo rapid aggregation to form cyclic oligomers of the composition [(R₂R‘P)Ni(C₂H₄COO)]_n (R₂R‘P = EtPh₂P (2), Me₃P (4), (i-Pr)₃P (5), Cy₃P (6), Et₃P (7)). The complexes were fully characterized by elemental analyses (except 4), NMR and IR spectroscopy, and X-ray crystallography of single crystals. Depending on the bulkiness of the phosphines, three types of nickelamacrocycles with different ring size and different connectivity pattern of the monomeric units are formed. The small Me₃P stabilizes a cyclic tetramer (n = 4) in which the units are linked by Ni−O−Ni bonds. This bridge is stable in thf solution. The bulkiest phosphines stabilize another type of cyclotetrameric architecture in 2, 5, and 6, in which the monomeric units are connected by Ni−O−CO bridges. In contrast, 7, stabilized by the Et₃P ligand, forms a hexacyclic compound (n = 6) with Ni−O−CO bridges. The IR spectrum of 7 in thf shows two CO valence frequencies, indicating that at least two species are present in which the carboxylate group is differently coordinated (Ni−O and Ni−O−CO−Ni coordination).