Synthesis, characterization, crystal structures and magnetic exchange in dinuclear copper complexes with 3-amino-1-propanol as terminal and bridging ligand

Abstract

The synthesis, X-ray structures and spectroscopic and magnetic properties are described for two groups of dinuclear Cu(II) compounds with the ligand 3-amino-1-propanol (Hap). The formulae of the compounds are for group A: [Cu(ap)(anion)] 2, in which ap is the dehydronated Hap and the anions are formate, nitrate, chloride and bromide and for group B: [Cu(ap)(Hap)] 2(anion) 2, with anion = iodide, bromide, chloride, nitrate and tetrafluoroborate. The structure of group A compounds consists of dinuclear units with the co-planar centrosymmetric chromophore ANCuOO′CuNA, in which the A ligands (anions) bridge to neighbouring units as axial ligands, thereby forming infinite chains. Dimer Cu…Cu distances are about 295 pm. The X-ray structure of [Cu(ap)(NO 3)] 2 ( I) is described in detail. Dark blue crystals of I were studied with single crystal X-ray measurements and refined to a final R value of 0.025 for 1440 reflections. The compound crystallizes in the monoclinic space group P2 1/ c with two dimeric molecules in a cell of dimensions a = 886.56(5), b = 815.92(6), c = 928.71(3) pm and β = 104.671(4)°. The structure consists of centrosymmetric di-alkoxy bridged dimers in which the Cu…Cu distance is 293.91(3) pm. The dimers are polymerized along the c axis into chains via NHO hydrogen bonds. These chains are joined together along the b axis by CuONOCu bridges and weak NHO hydrogen bonds. This results in layers, which are held together by van der Waals forces. The copper(II) ions have a distorted square pyramidal geometry, which is close to a tetragonally distorted octahedral coordination sphere. The structure of group B compounds consists of dinuclear units with the centrosymmetric chromophore [LNCuOO′CuNL] in which L is an N-coordinating Hap ligand. The alcoholic function of Hap coordinates as an axial ligand to neighbouring dimeric units, forming again an infinite chain structure. Dimeric Cu…Cu distances are 303 pm. The anions in structure B are not coordinated to copper, but are hydrogen bonded to the ligand NH and OH groups. Ligand-field spectra agree with square-pyramidal (4 + 1) or distorted octahedral (4 + 1 + 1) coordination geometries for Cu(II) in all cases. Small differences in ligand-field spectra are ascribed to differences in anions and axial CuO distances. The compounds are all very strongly magnetically coupled, as seen from the fact that they are diamagnetic at low temperatures. Both groups of compounds have been studied with magnetic susceptibility measurements in the 2–400 K region and the data could be fitted with the Bleaney-Bowers equation for dimers. Compounds of type A have much smaller values of -2 J than those of type B. This nicely agrees with the larger value of the CuOCu angles in the case of group B (99.5–7° in A, against 103.5°). EPR spectra of the compounds show mainly small paramagnetic impurities (below 1%) in agreement with the dimeric structure.