Variable modes of nickel(II) coordination with macrocyclic ligands based on 1,5-bis(2-mercaptoethyl)-1,5-diazacyclooctane (BME-DACO)
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The synthesis, structural characterization, and spectroscopic and magnetic properties of the NiII complexes containing macrocyclic ligands derived from 1,5-bis(2-mercaptoethyl)-1,5-diazacyclooctane are described. The cis orientation of thiolate sulfurs rigidly held in (BME-DACO)NiII (1) permits high-yield template syntheses of macrocycle ligands. Thus reaction of 1 with 1,3-dibromopropane gives a four-coordinate N2S2Ni complex, (4,8-dithia-1,11-diazabicyclo-[9.3.3]heptadecane)nickel(II) bromide, [Ni(bicycle)]Br2 (3a). Alkylation of 1 with bis(2-iodoethyl) ether results in a six-coordinate complex, containing a pentadentate, N2S2O donor macrocyclic ligand, (7-oxa-4,10-dithia-l,13-diazabicyclo [11.3.3] nonadecane)iodonickel(II) iodide, [(Ni(ether)I] I (4a). Complex 3a crystallizes in the monoclinic P21/c (No. 14) space group with a = 17.384 (7) Å, b= 15.529 (6) Å, c = 15.153 (4) Å, β = 104.42 (3)°, V = 3962 Å3, and Z = 8. Complex 4b [Ni(ether)I]BPh4, crystallizes in the orthorhombic P212121 (No. 18) space group with a = 10.016 (2) Å, b = 11.834 (3) Å, c = 30.127 (8) Å, V = 3570 (1) Å3, and Z = 4. In pseudo-square-planar 3a (a 2.947 (2) Å bromide interaction with Ni11 creates a virtual square pyramid), the metalladiazacyclohexane rings are in the chair/boat configurations. However, in pseudooctahedral 4b, the folding back of the eight-membered NiS2C4O ring to accommodate Ni-O bonding and the presence of an iodide ligand trans to the oxygen enforce the uncommon chair/chair configuration of the fused metalla rings. The larger macrocycle of 4 creates a larger N2S2 cavity, with Ni-S distances in 4b of 2.405 (2) and 2.362 (2) Å and in 3a of 2.188 (3) and 2.201 (3) Å. The relative accessibility of the Ni1 oxidation state determined by cyclic voltammetry reflects this cavity size, with the larger yielding more positive values of the Ni11/1 couple. Electrochemical studies of the ion-exchanged derivatives of 4 found the following dependence of the Ni11/1 redox couple on the nature of the nickel-bound halide ligand: E½(I-)>E½(Br-) > E½(Cl-1). Factors which influence the interconversion between four- and six-coordination of 3 and 4 are explored. © 1992, American Chemical Society. All rights reserved.
author list (cited authors)
Darensbourg, M. Y., Font, I., Mills, D. K., Pala, M., & Reibenspies, J. H.