Steric strain in cobalt(III) compounds. Crystal and molecular structure of dibenzoylmethanato-O,O′-[NN′-o-phenylenebis(salicylideneiminato)]cobalt(III)

Abstract

A preparative study is reported of the mixed-ligand cobalt(III) compounds with β-diketonate or related ester anions and the quadridentate ligand of the title compound. Although the quadridentate ligand was expected to remain in a planar configuration in such compounds, several β-diketonate species can be made in which the diketone is O,O-bidentate and the quadridentate ligand is in the same non-planar configuration that NN′-ethylenebis(salicylideneiminate) adopts in related compounds. This is proven, and the details of the steric strain in the molecule are revealed, by an X-ray structural study of the title compound. The orange-brown crystals are monoclinic with a= 16.89(2), b= 16.47(1), c= 9.84(1)Å, β= 90.86(5)°, space group P2₁/n, and Z= 4. A total of 3 091 independent reflections were obtained on a diffractometer. The structure was solved by conventional Patterson and Fourier techniques and was refined to a final R of 0.059. The octahedral cobalt(III) molecular species contains an O,O-bidentate β-diketonate anion, and the quadridentate Schiff-base ligand is in the non-planar cis-β configuration. The steric strain is taken up not only in the expected bond-angle distortions, but also in significant bond-length changes. Whereas the two Co–N distances are not significantly different [1.906(4) and 1.902(5)Å], nor are the two Co–O distances for the β-diketonate [1.897(4) and 1.894(4)Å], yet the two Co–O distances for the quadridentate ligand differ by 8σ. The distance to the ‘out-of-plane’ oxygen is long [1.912(4)Å] and that to the ‘in-plane’ oxygen is shorter than normal [1.880(4)Å]. Bond angles at the cobalt range between 83.5(2) and 96.5(2)°. In contrast to the diketonates, both ethyl acetoacetate and diethyl malonate form C-bonded unidentate compounds, with the quadridentate ligand in the planar configuration.