Synthesis and characterization of enolato-cobalt and -nickel complexes bearing TpiPr2 ligand, [TpiPr2M(XCHY)]n [TpiPr2=hydrotris(3,5-diisopropylpyrazolyl)borato; n=1, 2], obtained from hydroxometal complexes, (μ-OH)2(MTpiPr2)2, and active methylene compou

Reaction of hydroxo-cobalt and -nickel complexes, (TpiPr2M)2(μ-OH)2 (1) [M=Co (1Co), Ni (1Ni); TpiPr2: hydrotris(3,5-diisopropylpyrazolyl)borato], with active methylene compounds, CH2XY (2) [X/Y=CN/COPh (2a); CN/CN (2b); COCH2CMe2CH2CO (2d: dimedone); COMe/COOMe (2e); COOMe/COOMe (2f)], in the presence of a drying agent (e.g. Na2SO4) results in the dehydrative condensation to give N/O-bound enolato complexes with monomeric chelated structure TpiPr2 (3e and 3f), or dimeric cyclic structure (TpiPr2M)2(μ-XCHY)2 (3a, 3b, and 3d). In contrast, treatment of 1 with methyl cyanoacetate (2c) gives the κ2-carboxylato complex, TpiPr2M(κ2-OOCCH2CN) (4c), via hydrolysis of the ester moiety by the action of the OH functional group in 1. The reaction pathway (enolato formation vs. ester hydrolysis) is a combined result of various factors including the acidity of the CH moiety in 2, rigidity of the enolato skeleton, and relative thermodynamic stability of possible structures. Molecular structures of 3bCo-2py, 3eCo-MeCN, 3fNi and 4cNi determined by X-ray crystallography reveal the formation the N/O-bound structure, where the negative charge is widely delocalized over the enolato functional group. No isomerization to C-bound enolate, TpRM-CHXY, is observed, even when a less sterically hindered ligand, TpMe2, is employed. Reaction of 3aCo and 3aNi with benzaldehyde carried out as a preliminary study of the reactivity of the obtained enolato complexes 3 affords the metallacyclic products TpiPr2C (5) (characterized by X-ray crystallography as a 2MeCN adduct) and TpiPr2N (6), respectively, by way of double condensation of PhCHO with two enolato functional groups.