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 compounds, CH2XY

Abstract Reaction of hydroxo-cobalt and -nickel complexes, (Tp iPr 2 M) 2 (μ-OH) 2 ( 1 ) [M=Co ( 1 Co ), Ni ( 1 Ni ); Tp iPr 2 : hydrotris(3,5-diisopropylpyrazolyl)borato], with active methylene compounds, CH 2 XY ( 2 ) [X/Y=CN/COPh ( 2a ); CN/CN ( 2b ); COCH 2 CMe 2 CH 2 CO ( 2d : dimedone); COMe/COOMe ( 2e ); COOMe/COOMe ( 2f )], in the presence of a drying agent (e.g. Na 2 SO 4 ) results in the dehydrative condensation to give N/O-bound enolato complexes with monomeric chelated structure Tp iPr2 MXCHY ( 3e and 3f ), or dimeric cyclic structure (Tp iPr 2 M) 2 (μ-XCHY) 2 ( 3a , 3b , and 3d ). In contrast, treatment of 1 with methyl cyanoacetate ( 2c ) gives the κ 2 -carboxylato complex, Tp iPr 2 M(κ 2 -OOCCH 2 CN) ( 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 3b Co -2py, 3e Co -MeCN, 3f Ni and 4c Ni 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, Tp R M-CHXY, is observed, even when a less sterically hindered ligand, Tp Me 2 , is employed. Reaction of 3a Co and 3a Ni with benzaldehyde carried out as a preliminary study of the reactivity of the obtained enolato complexes 3 affords the metallacyclic products Tp iPr2 C oO-C(Ph)-C(CN-CoTpiPr2)-CHPh-C(CN)-C(Ph)-O ( 5 ) (characterized by X-ray crystallography as a 2MeCN adduct) and Tp iPr2 N iO-C(Ph)-C(CN-H)-CHPh-C(CN)-C(Ph)-O ( 6 ), respectively, by way of double condensation of PhCHO with two enolato functional groups.