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(iPr2) ligand, (Tp(iPr2))(py)Pd-OH (1(iPr2)), 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, (Tp(iPr2))(py)Pd-X-CH-Y 3(iPr2)a-c. When the hydroxo complex 1(Me2) with the less bulky Tp(Me2) ligand was allowed to react with 2 at 0 degreesC, similar N/O-bound enolato complexes, (Tp(Me2))(py)Pd-X-CH-Y (3(Me2)a-c), were obtained as kinetic products, which were gradually converted to the more stable C-bound enolato complexes (Tp(Me2))(py)Pd-CHXY (4(Me2)a-c) upon warming to 50 degreesC. 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(CH2Y)=CCN(Y) (6), whereas the C-bound enolates 4(Me2)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.