Complex chemistry of 2,2,6,6-tetramethyl-4-(2,2′∶6′,2″-terpyridin-4′-yloxy)piperidin-1-oxyl, a spin-labelled terpyridine ‡

Abstract

Reaction of 4′-chloro-2,2′∶6′,2″-terpyridine with 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl in the presence of 4 equivalents of powdered KOH in dmso at 50 °C for 16 h, followed by an aqueous quench, afforded 2,2,6,6-tetramethyl-4-(2,2′∶6′,2″-terpyridin-4′-yloxy)piperidin-1-oxyl (L¹) in 70% recrystallised yield. The complexes [ML¹₂][BF₄]₂ (M = Mn 1, Co 2, Ni 3, Cu 4 or Zn 5) have been prepared and characterised. Crystals of 1 grown from MeCN–Et₂O contain octahedral manganese(II) centres, with Mn–N 2.188(3)–2.263(3) Å and Mn  · · ·  N(aminoxyl) distances of 9.740(4) and 9.530(4) Å. The structure shows an unusual O  · · ·  O contact between aminoxyl centres on adjacent molecules. Voltammetric measurements of L¹ and 1–5 in MeCN–0.1 M NBu₄PF₆ showed a reversible aminoxyl/oxoammonium oxidation, which is perturbed minimally by complexation; 1–4 also showed complex metal-centred redox behaviour, which for 1–3 differs from that reported for other [M(terpy)₂]²⁺ derivatives of these metal ions. The X- and Q-band EPR spectra of 4 in MeCN–toluene (10∶1) show a broad resonance characteristic of strong Cu/L¹ exchange. Variable temperature susceptibility measurements on solid 1 and 4 revealed weakly antiferromagnetic behaviour. Data for 4 can be reproduced by the Curie–Weiss law, and by an equation describing intramolecular superexchange. However, those for 1 show a sharp drop in χ_mT below 10 K, which cannot be fitted by these models; it is proposed that this reflects the intermolecular O  · · ·  O contacts in the solid.

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