Nickel(II) complexes of quinquedentate macrocyclic ligands and the crystal and molecular structure of the ring-opened hydrolysis product [11-(6-acetyl-2-pyridyl)-3,7,10-triazadodec-10-enylamine-NN′N″N‴N″″]-aquanickel(II) diperchlorate

Abstract

In contrast to several other first-row transition-metal (and non-transition-metal) ions, Ni^II is ineffective as a template for the cyclic condensation of 2,6-diacetylpyridine with 4,7-diazadecane-1,10-diamine and related tetraamines to yield quinquedentate macrocyclic ligands. However, nickel(II) complexes of the 17-membered macrocycle L³ may be prepared by replacement of Ag^I from the complex [AgL³][ClO₄]. On the basis of electronic spectra and other physical measurements, these complexes [NiL³(X)][ClO₄](X = ClO₄, NCS, or N₃) are shown to have six-co-ordinate structures in which L³ adopts a new conformation, i.e. one in which it occupies five sites of a distorted octahedron. Replacement of Ag^I from the cavity of the less flexible 16-membered macrocycle L² in dry ROH (R = Me or Et) yields distorted octahedral complexes of the new, more flexible, macrocycles L⁹ or L¹⁰ formed by addition of ROH across one azomethine bond of L². The driving force for these reactions, which do not occur with (pentagonal) complexes of L² with other metal ions, is believed to arise from a strong preference for the d⁸ ion for an orthogonal disposition of donor atoms. If the metal exchange is carried out in the presence of water, or if the L⁹ and L¹⁰ complexes are treated with water, a ready hydrolysis occurs to yield octahedral complexes of the ring-opened ligand L¹¹. Crystals of the title complex [NiL¹¹(OH₂)][ClO₄]₂ are monoclinic, with a= 15.292(12), b= 19.807(15), c= 8.432(8)Å, β= 106.54(8)°, Z= 4, space group P2₁/a. 1 514 Reflections above background have been measured by diffractometer and refined by full-matrix least squares to R 0.076. The ligand occupies five sites of a distorted octahedron around the metal ion [Ni–N 2.099(12), 1.999(14), 2.312(13), 2.084(11), and 2.085(12)Å]. A water molecule at 2.126(8)Å completes the co-ordination sphere.