Synthesis and crystal structures of dinuclear zinc complexes with the 1,3-bis(2-pyridylmethyl)acetamidinato ligand

The reaction of an equimolar mixture of N,N′-bis(2-pyridylmethyl)acetamidine (1) and di(tert-butyl)phosphane with dimethylzinc yields dinuclear bis(methylzinc) bis(2-pyridylmethyl)acetamidinate di(tert-butyl)phosphanide (2). A similar protocol allows the preparation of bis(alkylzinc) bis(2-pyridylmethyl)acetamidinate tert-butylamide [zinc-bound methyl (3) or trimethylsilylmethyl group (4)]. The reactions of 3 and 4 with diphenylsilane lead to the formation of insoluble dimeric bis(alkylzinc) N,N′-bis(2-pyridylmethyl)acetamidinate hydrides [zinc-bound methyl (5) or trimethylsilylmethyl group (6)]. These zinc hydrides decompose once dissolved under formation of elemental zinc thus hampering catalytic applications. Molecular structures of [(1)ZnCl2] as well as of the zinc complexes 2 to 6 are discussed.

N,N′-Bis(pyridylmethyl)acetamidinates are tetradentate ligands which can adopt Z,Z- as well as E,Z-configuration. The nuclearity of the complexes depend on the configuration. These complexes are catalytically active in silylation of ketones. However, catalytic cycles containing hydride anions are disadvantageous due to the insolubility of these zinc hydride species (Zn green, N blue, C gray).Figure optionsDownload as PowerPoint slideHighlights
► Acetamidinate ligands can adopt Z,Z- as well as E,Z-configurations leading to dinuclear or tetranuclear complexes.
► The catalytic activity of zinc amidinates is enhanced compared to complexes with a bis(2-pyridylmethyl)amido backbone (also leading to side reactions discussed in the article).
► Zinc hydrides are less effectively stabilized with acetamidinates than with dipicolylamides due to a larger Zn⋯Zn separation.