Copper(II) and Nickel(II) Complexes of Binucleating Macrocyclic Bis(disulfide)tetramine Ligands

Novel macrocyclic bis(disulfide)tetramine ligands and several Cu(II) and Ni(II) complexes of them with additional
ligands have been synthesized by the oxidative coupling of linear tetradentate N2S2 tetramines with iodine. Facile
demetalation of the Ni(II) oxidation products affords the free 20-membered macrocycles meso-9 and rac-9 and
the 22-membered macrocycle 16, all of which are potentially octadentate N4S4 ligands. X-ray structure analyses
reveal distinctly different conformations for the two isomers of 9; meso-9 shows a stepped conformation in profile
with the disulfide groups corresponding to the rise of the step, whereas rac-9 exhibits a V conformation with the
disulfide groups near the vertex of the V. No metal complexes of rac-9 have been isolated. Crystallographic
studies of three Cu(II) complexes reveal that depending upon the size of the macrocyclic ligand and the nature
of the additional ligands (I-, NCO-, and CH3CN), the Cu(II) coordination geometry shows considerable variation
(plasticity), with substantial changes in the Cu(II)−disulfide bonding. Thus, a diiodide salt contains six-coordinate
Cu(II) to which all four bridging disulfide sulfur atoms form strong equatorial bonds. In contrast, isocyanato
complexes of the 20- and 22-membered macrocycles exhibit trigonal-bipyramidal Cu(II) and distorted cis-octahedral
Cu(II) geometries, respectively, having only one and no short equatorially bound sulfur atoms. The coordination
geometry of the latter complex can also be described as four-coordinate seesaw with two semicoordinated
S(disulfide) ligands. Disulfide → Cu(II) ligand-to-metal charge transfer absorptions of both isocyanato-containing
Cu(II) species appear too weak to observe, probably because of poor overlap of the sulfur orbitals with the Cu(II)
d-vacancy. The dual disulfide-bridged Ni(II) units of the crystallographically characterized octahedral Ni(II) complex
of meso-9 with axial iodide and acetonitrile ligands promote substantial antiferromagnetic coupling (J = −13.0(2) cm-1).