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

Reaction of a hydroxopalladium complex bearing the Tp iPr 2 ligand, (Tp iPr 2 )(py)Pd−OH ( 1 iPr 2 ), with active methylene compounds, X−CH 2 −Y 2 [dicyanomethane ( 2a ), methyl cyanoacetate ( 2b ), benzoylacetonitrile ( 2c )], resulted in dehydrative condensation to afford the N/O-bound enolates, (Tp iPr 2 )(py)Pd−X−CH−Y 3 i Pr 2 a − c . When the hydroxo complex 1 Me 2 with the less bulky Tp Me 2 ligand was allowed to react with 2 at 0 °C, similar N/O-bound enolato complexes, (Tp Me 2 )(py)Pd−X−CH−Y ( 3 Me 2 a − c ), were obtained as kinetic products, which were gradually converted to the more stable C-bound enolato complexes (Tp Me 2 )(py)Pd−CHXY ( 4 Me 2 a − 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 (Tp R )(py)Pd−NH−C(CH 2 Y) CCN(Y) ( 6 ), whereas the C-bound enolates 4 Me 2 a,b showed no indication of the dimerization. Thus, the present study reveals that the reactivity of transition-metal enolates is dependent on their structures.