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) , b= 15.529 (6) , c = 15.153 (4) , = 104.42 (3), V = 3962 3, and Z = 8. Complex 4b [Ni(ether)I]BPh4, crystallizes in the orthorhombic P212121 (No. 18) space group with a = 10.016 (2) , b = 11.834 (3) , c = 30.127 (8) , V = 3570 (1) 3, and Z = 4. In pseudo-square-planar 3a (a 2.947 (2) 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) and in 3a of 2.188 (3) and 2.201 (3) . 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: E(I-)>E(Br-) > E(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.
