Synthesis, structure and electrochemistry of isomeric nickel(ii) complexes of a [9]ane fused cyclam macrotricycle: evidence for a stable trans-IV and a redox induced rearrangement in a trans-I conformation

Abstract

The macrotricyclic ligand, 1,11-dithia-4,8,14,18-tetraaza-[5,2,2,5]-eicosane, L3 has been synthesized from cyclam. Both the cis- and the trans- isomers of the Ni(II) complexes have been prepared and characterized by single crystal X-ray crystallography. In the cis-[Ni(trans-I, syn-L3)](Cl₂)·4H₂O, the syn-isomer of L3 is present in which the cyclam ring adopts a trans-I stereochemistry. Crystal structure data (Pna2₁, tetragonal, a = 17.450(3), b = 12.188(2), c = 11.897(2) Å, V = 2530.2(8) ų, R₁ = 0.032 and wR₂ = 0.078) indicate that two of the Ni–N distances, Ni–N(4) = 2.164(3) Å and Ni–N(2) = 2.179(3) Å are longer than Ni–N(1) = 2.100(3) Å and Ni–N(3) = 2.101(3) Å. The Ni–S distances, 2.4265(11) Å and 2.4512(11) Å, are similar to lengths observed in many thioether complexes. The cyclic voltammogram for the Ni^2+/3+ redox couple of the cis-complex is scan rate dependent. Although quasi-reversible at a scan rate of 1000 mV s⁻¹, when the scan rate is lowered below 100 mV s⁻¹ the redox couple shows chemical irreversibility. There is evidence for rearrangement of the inner coordination sphere and adsorption of the oxidized species onto the Pt electrode. However, there is no evidence to suggest that isomerization of the cis- to the trans-complex occurs. Consistent with this electrochemical behavior, a time-dependent change in the ESR spectrum for the Ni(III) species was also observed. A mechanism is proposed where through dissociation of a thioether donor and subsequent substitution of a solvent CH₃CN, the trans-I conformation changes from a cis-type coordination to a trans-mode. In the trans-[Ni(trans-IV, anti-L3)](ClO₄)₂ complex, the anti-form of L3 is present where the cyclam moiety adopts the rare trans-IV stereochemistry. The trans-complex (C₂/_c, monoclinic, a = 22.382(9), b = 10.118(2), c = 14.300(3) Å, V = 2493.3(13) ų, R₁ = 0.051 and wR₂ = 0.142) is a minor product with all Ni–N distances at 2.124(4) Å and Ni–S distances at 2.5321(13) Å. The trans-complex shows a reversible cyclic voltammogram for the Ni^2+/3+ couple and an axial ESR spectrum for the Ni(III) species that is expected for such a symmetrical system. That there are no changes in the ESR spectroscopic features following redox cycles is consistent with the structural rigidity expected in an encapsulated system.