Dicopper(II) complexes of a macrocyclic ligand containing single hydroxo-, methoxo-, or 1,1-azido-bridges: synthesis, magnetic properties, electron spin resonance spectra, and the crystal and molecular structure of a µ-hydroxo-derivative

Abstract

Reaction of 2,6-diacetylpyridine with 1,3-diaminopropane in methanol in the presence of M[ClO₄]₂(M = Sr^II or Ba^II) in 2:1:1 molar proportions yields the complexes [ML²(ClO₄)₂] of the new 20-membered macrocyclic tetraimine ligand L² derived from the [2 + 2] condensation of 2 mol of diketone with 2 mol of diamine. The single Ba²⁺ in [BaL²(ClO₄)₂] may be replaced by two Cu²⁺ to afford, depending on the conditions, the complexes Cu₂L²(OH)(ClO₄)₃·2H₂O and Cu₂L²(OMe)(ClO₄)₃˙2H₂O formulated as containing two Cu²⁺ linked by, respectively, single hydroxo- and methoxo-bridges. Crystals of the µ-hydroxo-complex are tetragonal with a= 15.20(1), c= 14.50(1)Å, Z= 4, and space group P4₂/n. 1 191 Reflections above background were measured by diffractometer and refined by Fourier methods to R 0.078. Each copper atom in the binuclear [Cu₂L²(OH)(OH₂)]³⁺ cation is bonded to the three nitrogen atoms of a trimethine group of the folded macrocycle [Cu–N 1.916(13)–2.066(12)Å]. The approximate co-ordination square plane about each metal atom is completed by a shared oxygen atom, O(1)(presumed to be from OH^–), at 1.916(9)Å. The two Cu atoms are also linked in a shared axial position by a second oxygen atom, O(2)(presumed to be from H₂O), at 2.519(12)Å. The Cu–O(1)–Cu and Cu–O(2)–Cu angles are 110.3(7) and 77.3(4)°, respectively, and the Cu ⋯ Cu separation is 3.145(4)Å. Magnetic susceptibility measurements in the temperature range 90–300 K establish that the two Cu²⁺ in each binuclear complex are antiferromagnetically coupled with J=–32 and –53 cm^–1 for the µ-hydroxy- and µ-methoxo-derivatives, respectively, where 2J is the singlet–triplet separation. The appearance of a ‘ triplet ’ e.s.r. spectrum at g≈ 2 and a seven-line ΔM= 2 half-band spectrum at g≈ 4 for frozen methanol–dimethyl sulphoxide solutions of the µ-methoxo-complex indicate retention of the bridge in solution. Reaction of Cu₂L²(OMe)(ClO₄)₃·2H₂O with Na[N₃] affords Cu₂L²(N₃)(ClO₄)₃ for which i.r. spectra suggest that the Cu²⁺ are intramolecularly linked by one nitrogen atom of the azide ion (µ-1,1-N₃ bridging mode). E.s.r. spectra of frozen solutions of the monoazide show that the Cu²⁺ are weakly antiferromagnetically coupled. The structures and physical properties of the complexes are discussed in relation to the cavity size of the macrocycle L².