A novel type of tetranuclear cluster with the trinucleating ligand 1,2-Bis(benzimidazol-2-yl)-1-hydroxyethane. Crystal and molecular structure of Ni(HRSL) 2(CF 3SO 3) 2 and [Cu 4(SL) 4(NO 3) 2(EtOH)] (NO 3) 2(EtOH) 3(H 2O) 2; spectroscopy and magnetism of these and related transition metal compounds

Abstract

A number of nickel, copper and zinc compounds with the tridentate chelating ligand 1,2-bis(benzimidazot-2-yl)-l-hydroxyethane (abbreviated HL, and HSL or HRSL for the S isomer and the RS mixture, respectively) is described. The ligand forms mononuclear complexes of general formula M(HRSL) 2(anion) 2 for all metal ions and a variety of anions, both for the racemic mixture of ligands ( R,S) and - in a few cases - for the chiral ( S) ligand. A dinuclear species of formula [CuCl(RSL)](H 2O) 2Cl 2: appears to be formed with a bridging ligand and a bridging chloride anion, again both for the chiral and the ( S) forms of the ligands. Tetranuclear species are formed only for copper nitrate and copper perchlorate, with formula [Cu 4(RSL) 4](anion) 4(EtOH) x. Crystal structures have been solved for a monomeric and for a tetrameric species. The compound [Cu 4(SL) 4(NO 3) 2(EtOH)](NO 3) 2(EtOH) 3 -(H 2O) 2 (I) crystallizes in the orthorhombic space group P2 12 12 with cell dimensions a=28.633(9), b=18.599(4), c=15.509(8) Å and Z=4. The crystal structure has been determined from X-ray diffraction data by direct and Fourier methods and refined by full-matrix least-squares techniques to a conventional R value of 0.077 for 3046 independent reflections. The compound consists of a tetrameric distorted cubane-type Cu 4O 4 core formed by four copper(II) and four triply bridging oxygen atoms of SL - ligands with CuCu distances of 3.069(4)–3.734(4) Å. Each copper(II) atom has a distorted octahedral environment, two oxygen and two nitrogen atoms of three SL ligands forming the equatorial coordination with CuO and CuN bonds of 1.95(1)–2.03(2) Å. The axial sites are occupied by SL oxygen, ethanol or nitrate oxygen atoms with CuO distances varying from 2.5 6(2) to 2.93(2) Å. Ni(HRSL) 2(CF 3SO 3) 2 ( II) crystallizes in the monoclinic space group C2/ c, with a=21.985(3), b=11.786(2), c=14.816(2) Å,β=94.67(1) o, Z=4 (i.e. four HRL units and four HSL units). The nickel(II) ion is octahedrally coordinated by 2 oxygen atoms and 4 nitrogen atoms in a cis geometry. NiN distances are 2.04–2.06 Å, whereas NiO 2.15 Å. The octahedral coordination cis angles vary from 75 to 101 o. The spectroscopic properties of the monomeric species agree with a strong tridentate chelating behaviour, in agreement with the somewhat distorted geometry found for the Ni(II) triflate. In fact the ligand field spectra show two groups of Ni(HRSL) 2 compounds, with slightly but significantly different ligand field parameters. No Ni(HSL) 2 2+ salts could be isolated. The magnetic properties of the (supposed) dimer and the tetramers are in agreement with expectation for a dimer of dimers, since the Cu(II) ions have pairwise strong interactions.