Complex chemistry of 2,2,6,6-tetramethyl-4-(2,2 ': 6 ',2 ''-terpyridin-4 '-yloxy)piperidin-1-oxyl, a spin-labelled terpyridine

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 degrees 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 (L-1) in 70% recrystallised yield. The complexes [ML21][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) A and Mn ... N(aminoxyl) distances of 9.740(4) and 9.530(4) Angstrom. The structure shows an unusual O ... O contact between aminoxyl centres on adjacent molecules. Voltammetric measurements of L-1 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/L-1 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 chi(m)T below 10 K, which cannot be fitted by these models; it is proposed that this reflects the intermolecular O ... O contacts in the solid.