Redox addressable ligands in copper(i) coordination chemistry: thione and oligosulfide-bridged 6-methyl-2,2′-bipyridines

Abstract

The ligands 6-(6-methylpyridin-2-yl)pyridine-2(1H)-thione (1) and 1,2-bis(6′-methyl-2,2′-bipyridin-6-yl)disulfane (2) have been prepared and structurally characterized. Compound 2 is synthesized by the oxidative coupling of 1 using I₂ or H₂O₂, but it also occurs spontaneously in solution in the absence of dithiothreitol. Under some conditions, oligosulfide bridges with three or four sulfur atoms (ligands 3 and 4, respectively) are produced in addition to 2. In the presence of dithiothreitol, 1 binds copper(I), functioning as a monodentate S-donor; the four ligands in [Cu(1)₄]⁺ encapsulate the copper ion so that the sulfur atoms exhibit S⋯S contacts. The presence of the NH⋯N hydrogen bonded unit in the bpy domain in 1 prevents it acting as an N,N′-chelate. In the absence of dithiothreitol, copper(I) reacts with 1 to give [(1)₂Cu(µ-2)Cu(1)₂]²⁺ in addition to [Cu(1)₄]⁺. While the thione ligand functions as an S-donor, the disulfide ligand 2, freed of the NH⋯N hydrogen bond, acts as a bis(bpy) chelate. Ligand 2 itself reacts with [Cu(NCMe)₄]⁺ to give an orange solid, 5, tentatively proposed to be {[Cu(2)][PF₆]}_n. When 5 is treated with concentrated aqueous HCl, the insoluble [H₂2][CuCl₄]·H₂O is formed. By using a mixture of ligands 2, 3 and 4 with [Cu(NCMe)₄]⁺, it is possible to isolate an example of a dinuclear copper(I) complex containing bridging 1,2-bis(6′-methyl-2,2′-bipyridin-6-yl)trisulfane (3) ligands. The complex [Cu₂(3)₂][PF₆]₂ features double helical P- and M-[Cu₂(3)₂]²⁺ cations. The role of face-to-face π-interactions, and the effect of the length and helical twist in the backbone of the oligosulfide-bridged bpy ligands in each of the structures have been evaluated.