Bi-copper(I) and bi-copper(II) complexes of a 30-membered macrocyclic ligand: the inclusion of substrate molecules and the crystal and molecular structures of a µ-hydroxo- and a µ-imidazolato-complex

Abstract

The preparation of the copper(II) complex [CuL][ClO₄]₂·H₂O of a 30-membered ‘N₆O₄’ macrocyclic Schiff-base ligand, L, in which the metal ion is octahedrally co-ordinated to the six nitrogen atoms, is described. In contrast to analogous complexes of the other first-row transition-metal ions, the macrocycle in [CuL]²⁺ unfolds on treatment with excess of free metal ion to yield the binuclear complex [Cu₂L][CIO₄]₄·2H₂O. This complex is the starting point for the preparation of several derivatives containing small substrate ions [OH^–, imidazolate anion (im), N₃^–, NCS^–] intramolecularly bound between the metal centres. The structures of the µ-imidazolato- and µ-hydroxo-complexes have been determined. Crystals of [Cu₂L(im)][ClO₄]₃, (I), are triclinic, with a= 12.89(1), b= 13.64(1), c= 16.51(1)Å, α= 89.9(1), β= 114.1(1), γ= 121.8(1)°, Z= 2, and space group P. Crystals of [Cu₂L(OH)][ClO₄]₃·H₂O, (II), are triclinic with a= 12.634(9), b= 14.103(8), c= 14.689(8)Å, α= 88.3(1), β= 118.4(1), γ= 115.6(1)°, Z= 2, and space group P. The two structures were solved by Patterson and Fourier methods from 1 567 and 2 786 above background reflections measured by a diffractometer and refined by full-matrix least squares to R 0.081 and 0.084 respectively. In both (I) and (II) the two Cu^II ions are each bonded to the three nitrogen atoms of the two planar trimethine groups (Cu–N 1.90–2.08 Å), the fourth position of the equatorial square plane being occupied by one nitrogen of a bridging imidazolate anion [Cu–N 1.92(2), 1.98(2)Å] or by the bridging hydroxo-oxygen atom [Cu–O 1.866(13), 1.912(10)Å]. Cu ⋯ Cu separations are 5.99 and 3.57 Å in (I) and (II) respectively. In (I), the plane of the imidazolat ring is inclined at 68.8, 79.1° to the two ‘N₄’ co-ordination planes. In (II), the Cu–O(H)–Cu angle is 141.7(7)°. In both structures the metal ions are weakly bonded to perchlorate or water oxygen atoms in axial positions. Magnetic susceptibility measurements as a function of temperature establish antiferromagnetic super-exchange in (I) and (II) with J=–21.0 and –120 cm^–1 respectively, where 2J is the singlet–triplet separation. Two heterobinuclear Cu^II/Ni^II complexes have been prepared by reaction of mononuclear [CuL]²⁺ with Ni^II salts. A bi-copper(I) complex [Cu₂L][ClO₄]₂·H₂O has also been prepared by reduction of the bi-copper(II) complex with BPh₄^– as well as by a metal-exchange method.