Sterically Hindered Nickel(II) and Iron(II) Lacunar Cyclidene Complexes Containing gem-Dimethyl Groups on Their Saturated Chelate Rings

The structure of the cyclidene ligand in the corresponding iron(II)-centered dioxygen carrier has been altered by the addition of gem-dimethyl groups to increase the bulk on the saturated rings of the parent macrocycle. Dramatic responses are found in the dioxygen affinities of the resulting complexes. Placing the bulky group to the rear of the cavity, within which the O₂ binds, enhances the O₂ affinity, while placing the group at the entry greatly decreases the affinity. The presence of bulky groups at both positions essentially stops the binding of O₂. Interpretations are based on the crystal structure data and molecular modeling. X-ray structure data for [DMNiDM(Me₂Me₂Mxyl[16]cyclidene)](PF₆)₂: NiC₃₂H₄₈N₆O₂F₁₂, monoclinic, P2₁/c, a = 13.541 (4) Å, b = 15.408 (5) Å, c = 20.471 (7) Å, β = 106.10°, V = 4103 (2) ų, Z = 4, R = 0.083, and R_w = 0.091 for 3302 reflections with I > 2.0σ.