Synthesis and characterization of bis(pyrazolyl)borate Ni(ii) complexes: ligand rearrangement and transformation

Five examples of bis(pyrazolyl)borate Ni(ii) complexes 2-5, exhibiting C-HNi interactions, were readily prepared from the reactions of K[BBN(3-R¹-4-R²-pz)₂] with Ni(ii) precursors (Ni(acac)₂ or NiCl₂(PPh₃)₂) in dichloromethane or toluene. When R¹ = R² = H, complex 2a with square-planar geometry around the Ni centre and showing an unusual C-HNi anagostic interaction was obtained. In contrast, when R¹ = Me, R² = H or R¹ = Me, R² = Br, tetrahedral complexes 3 or 4 were formed preferentially with strong C-HNi agostic interactions, respectively. Additionally, some differences in the formation and transformation of 3 and 4 were also found including a 1,2-borotropic shift during the formation of 3 and a further geometrical transformation from tetrahedral 3 to square-planar 2b by the second 1,2-borotropic shift under continuous heating; in contrast, no ligand change and further conversion were found in 4. When the more hindered 3-ⁱPr-substituted ligand 1d was introduced in the reaction, the hydrolysis and cleavage of one B-N bond in the ligand occurred, leading to the singly hydroxo-bridged complex 5. The experimental and theoretical results indicate that the preference to form a thermodynamically stable complex and then balancing with orbital energy should be the intrinsic reason for the reaction selectivity.