N/O- and C-Bound (Enolato)palladium Complexes with Hydrotris(pyrazolyl)borato Ligands (Tp^R: R = iPr₂, Me₂) Obtained via Dehydrative Condensation between the Hydroxo Complexes Tp^RPd(Py)OH and Active Methylene Compounds: Factors Determining the Isomer Distribution and Dimerization of Cyano Compounds

Reaction of a hydroxopalladium complex bearing the TpiPr2 ligand, (TpiPr2)(py)Pd−OH (1iPr2), with active methylene compounds, X−CH2−Y 2 [dicyanomethane (2a), methyl cyanoacetate (2b), benzoylacetonitrile (2c)], resulted in dehydrative condensation to afford the N/O-bound enolates, (TpiPr2)(py)Pd−X−CH−Y 3iPr2a−c. When the hydroxo complex 1Me2 with the less bulky TpMe2 ligand was allowed to react with 2 at 0 °C, similar N/O-bound enolato complexes, (TpMe2)(py)Pd−X−CH−Y (3Me2a−c), were obtained as kinetic products, which were gradually converted to the more stable C-bound enolato complexes (TpMe2)(py)Pd−CHXY (4Me2a−c) upon warming to 50 °C. X-ray crystallography of the N/O- and C-bound enolates reveals that (1) the Pd center adopts the square-planar or square-pyramidal geometry, (2) the structure of the C-bound isomer is consistent with the canonical structure with the localized bonding scheme, and (3) in the N/O-bound isomers the negative charge is delocalized over the X−CH−Y linkage to form the zwitterionic structure. The N-bound enolato complexes 3a,b obtained from cyano compounds further reacted with the cyano compounds to give the 1:2 condensates: i.e., the 2-cyanoethenylamido complexes (TpR)(py)Pd−NH−C(CH2Y)CCN(Y) (6), whereas the C-bound enolates 4Me2a,b showed no indication of the dimerization. Thus, the present study reveals that the reactivity of transition-metal enolates is dependent on their structures.