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

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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 NiII and 12.42 and 9.87 for CuII 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 [NiL1 2]2+ and [NiL2 2]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 [NiL1 2][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.

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