Cobalt(II) Complexes with N, N, N-Scorpionates and Bidentate Ligands: Comparison of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate To^M to Control the Structural Properties and Reactivities of Cobalt Centers

Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and To^M (tris(4,4-dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp* and To^M coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N₃O₂ donor set in the Tp* complex is a square-pyramid, whereas that in the To^M complex is close to a trigonal-bipyramid. Both Tp*- and To^M-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (L^Ph) complexes, Tp* retains the tridentate (к³) mode, whereas To^M functions as the bidentate (к²) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on To^M lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)₂ and Co(acac)₂. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both To^M and Tp* complexes function as catalysts for hydroxylation. The higher electron-donating To^M complexes show faster initial reaction rates compared to the corresponding Tp* complexes.