Ferromagnetic interactions mediated by syn–anti carboxylate bridging in tetranuclear copper(II) compounds

The tridentate ligand N-(2-hydroxypropyl)aminopropionic acid (H2hap) and the tripod ligand nitrilodi(propionic acid) acetic acid (H3ndpa) have been synthesised and found to interact with Cu(II) to produce tetranuclear compounds. In the cyclic complex [Cu(Hhap)(NO3)]4 (1), the four Cu(II) centres form a slightly buckled square linked via syn–anti carboxylate bridges. In [Cu4(ndpa)2(H2O)8(NO3)2]·2H2O (2), and [Cu4(ndpa)2(H2O)10]SO4·6H2O (3), two Cu(II) centres are each chelated by a ndpa3− ligand which in turn are linked by coordination of the outer carboxylate oxygens on the longer propionate arms of the ligands to two further {Cu(H2O)3}2+ units resulting in a rhomboid array of Cu(II) centres. In the closely related complexes [Cu4(ndpa)2(H2O)6Cl2]·4H2O (4), and [Cu4(ndpa)2(H2O)6Br2]·4H2O (5), replacement of one aqua ligand by an outer carboxylate oxygen from the shorter acetate arm of an adjacent complex results in the formation of infinite sheets of linked tetranuclear units. Magnetic studies performed between 1.8 and 300 K on these complexes indicate that ferromagnetic or antiferromagnetic coupling between Cu(II) ions can be observed depending on the geometry of the carboxylate bridging mode. Compound 1 shows a paramagnetic behaviour with a ferromagnetic interaction (J/kB=+3.9 K, S=2 ground state) between adjacent Cu(II) ions through equatorial–equatorial syn–anti carboxylate bridges. In compound 2, within the rhombus motif ferromagnetic (J1/kB=+12.1 K) and antiferromagnetic (J2/kB=−0.4 K) interactions through syn–anti bridges but in equatorial–equatorial and equatorial–axial modes, respectively, are present. Finally, in the 2-D 4 and 5 the linkage of the rhombus motif, which is isolated in 2, by equatorial–axialanti–anti carboxylate bridges, leads to dominating ferromagnetic interactions (J′=+0.2 K) between the ferromagnetically interacting Cu(II) dimers (J1=+8.8 K).