Four new multidentate N-donor ligands L 1 -L 4 have been prepared which contain a combination of pyridyl and thiazolyl donor units. The syntheses of these ligands are facile and high-yielding, being based on reaction of an α-bromoacetyl unit with a thioamide to form the thiazolyl ring. The extended linear sequence of ortho-linked N-donor heterocycles (four for L 1 , six for L 2 ; five for L 3 ; and six for L 4 ) is reminiscent of the well-known linear oligopyridines, although these new ligands are much easier to make and have significantly different geometric coordination properties because the presence of the five-membered thiazolyl rings results in natural breaks of the ligand backbone into distinct bidentate or terdentate domains. Thus, the tetradentate ligand L 1 partitions into two bidentate domains to give dinuclear triple helicates [M 2 (L 1 ) 3 ] 4+ with six-coordinate first-row transition metal dications (M = Co, Cu, Zn). The hexadentate ligand L 2 partitions into two terdentate domains to give dinuclear double helicates [M 2 (L 2 ) 2 ] 4+ with six-coordinate metal ions (M = Cu, Zn). In the double helicate [Cu 2 (L 3 ) 2 ] 4+ the pentadentate ligand L 3 only uses its two terminal bidentate binding sites, resulting in four-coordinate Cu() centres and a non-coordinated pyridyl residue in the centre of each of the two ligand strands. These pendant pyridyl residues are directed towards each other to give a potentially two-coordinate cavity between the metal ions in the centre of the helicate. Similarly, in the double helicate [Cu 2 (L 4 ) 2 ] 4+ the metal ions are only four-coordinate, with each ligand having its central bipyridyl unit un-coordinated. This results in a potentially four-coordinate cavity between the two metal ions in the centre of the helicate. These easy-to-prepare ligands offer a great deal of scope for the development of multinuclear helicates.

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A series of first row transition metal complexes of the tripodal ligand 2,2',2"-nitrilotribenzoic acid H3L has been prepared and characterised by X-ray crystallography: Mononuclear [M(L)]- species [Cu(H2O)4]3[Cu(L)(H2O)]6.25H2O (2), [Co(H2O)6][Co(L)(H2O)].8H2O (4), [Zn(H2O)6][Zn(L)(H2O)].8H2O (5) and a neutral [M(L)] complex [Fe(III)2(L)(H2O)3].5H2O (8) are formed as well as dimeric [M(L)]2 2- species (HNEt3)2[Cu(L)]2.2CH3CN (1), (HNEt3)3[Ni(L)]2(ClO4).H2O (3), (HNEt3)2[Fe(II)(L)]2.2CH3CN (6) and (HNEt3)2[Fe(III)2(L)2(mu-O)](7). The complexes display a unique variation in the M-N distance (2.09 A for Cu(II) to 3.29 A for Fe(III)) to the bridgehead triphenylamine donor and are classified into compounds with "On","Off" and "Intermediate" N-coordination. The trigonal-bipyramidal coordination polyhedron changes towards tetrahedral in the intermediate and octahedral in the Off-state. The M-N distance of individual complexes is reversibly tuned by external chemical input such as changes of metal ion oxidation state (Fe(II)/Fe(III)) or variation of the axial coligand as a consequence of solvent or pH variation. Possible reasons for the exceptional tolerance of the M-N bond to distance variations are discussed under consideration of gas phase DFT calculations of [Zn(L)]-.

A new ferromagnetic nickel(II) hexameric structure based on the versatile N,O3-ligand 2,2',2"-nitrilotribenzoic acid has been prepared and characterised by X-ray crystallography and magnetic measurements; the compound represents a rare example of a nickel cluster with a dicubane-like core having only oxygen bridges.

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