Stability, electronic spectra, and structure of transition-metal ion complexes of a novel mixed-donor (nitrogen–sulphur) macrocycle, 1-thia-4,7-diazacyclononane

Abstract

The synthesis of the novel ligand 1-thia-4,7-diazacyclononane (L¹) is described. A potentiometric study yields pK₁ as 9.67, and pK₂ as 3.98 in 0.1 mol dm^–3 NaNO₃ at 25 °C; complex-formation constants logK₁ and logK₂ are found to be 10.45 and 9.6 for Ni^II and 12.42 and 9.87 for Cu^II in 0.1 mol dm^–3 NaNO₃ at 25 °C. Comparison of these constants shows a stabilisation relative to the complexes of the open-chain analogue 3-thiapentane-1,5-diamine (L²) which is similar to that found for comparable cyclononane macrocycles such as those with N₃ and N₂O donors. The electronic spectra of [NiL¹₂]²⁺ and [NiL²₂]²⁺ are discussed, and it is shown that, if correction is made for spin–orbit coupling effects, 10Dq is larger in the former complex. The crystal structure of [NiL¹₂][NO₃]₂ has been determined and refined to a conventional R factor of 0.0398. Average bond lengths of Ni–N 2.115 ± 0.007 Å and Ni–S 2.418 ± 0.001 Å are found, which are very similar to those in the open-chain analogue. The structure has the sulphur donor atoms placed trans to each other, in contrast to that of the L² complex where they are cis. The structures of the complexes are discussed in relation to possible origins of the high ligand-field strengths of the macrocycles.