Synthesis and Characterization of Multiply Substituted Lacunar Macrobicyclic Complexes of Nickel(II), a Representative Crystal Structure, and Preparation of Corresponding Free Ligands

A general synthetic procedure for the production of a wide variety of substituted versions of previously reported lacunar macrobicyclic nickel(II) complexes is reported. These substitutions lead to complexes with a protected cavity in the vicinity of one axial coordination site and with controlled steric and electronic properties at the metal center and its axial ligands. The X-ray crystal structure of one such multiply substituted complex, (3,ll-dibenzyl-14,20-dimethyl-2,12-diphenyl-3,ll,15,19,22,26-hexaazatricyclo[11.7.7.1^5,9]octacosa-l,5,7,9(28),12,14,19,21,16-nonaene-K⁴N)nickel(II) hexafluorophosphate, has been completed. The complex crystallizes in the monoclinic space group P2₁/c, with a = 10.126 (2) Å, b = 17.629 (4) Å, c = 30.666 (7) Å, and β = 100.87 (2)°, and the structure was solved by the heavy-atom method to R = 8.3%, R_w= 10.7% for 3363 reflections. The structure illustrates the demanding steric requirements placed upon the lacuna size and shape by the peripheral substituents of the macrocycle. The ligands reported may be removed from nickel(II) for transferral, intact, to iron(II) or cobalt(II) for investigation of the effect of steric and electronic factors on the strength and durability of their dioxygen adducts. The spectroscopic and electronic properties of a selection of nickel complexes with a range of substitution patterns are presented for comparative purposes.