Copper complexes with bibracchial lariat ethers: from mono- to binuclear structures

Copper(II) complexes with a series of bibracchial lariat ethers are describ ed. Independently of the nature of the counterion present (nitrate or perch lorate), the lariat ether N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L- 1) always forms mononuclear complexes, whereas the lariat ethers N,N'-bis(2 -aminobenzyl)-4, 13-diaza-18-crown-6 (L-2) and N,N'-bis(2-salicylaldiminobe nzyl)4,13-diaza-18-crown-6 (L-3) only give binuclear compounds. The X-ray c rystal structure of [CuL1](ClO4)(2) shows a seven-coordinated copper (II) i on in a distorted (axially compressed) pentagonal-bipyramidal geometry. The X-ray crystal structure of [Cu-2(L-3-2H)](ClO4)(2) confirms the binuclear nature of the compound with both metal ions having identical coordination e nvironments and each one placed out of the crown hole but efficiently encap sulated by the corresponding pendant arm; each copper(II) ion is five-coord inated with an intermediate geometry between trigonal-bipyramidal and squar e-pyramidal (tau = 0.40). The EPR spectra in frozen solution samples are in accordance with a stable coordinate pattern for the metal centre of ligand L-1, yielding a rhombic distorted complex with axial compression in soluti on, in agreement with the X-ray crystal structure of [CuL1](ClO4)(2). For t he binuclear complexes of L-2 and L-3, the Cu(II) centres in solution can b e distorted from their tetragonally elongated structures via interaction wi th ethanol and/or the nitrate counterion, leading to more than one species. (C) 2001 Elsevier Science B.V. All rights reserved.