On the magneto-structural role of the coordinating anion in oxamato-bridged copper(II) derivatives

We herein present the synthesis, spectroscopic analysis, description of the crystal structures and magnetic properties of four new complexes of the formula [{Cu(opba)(H2O)1.2}{Cu(dmphen)(SCN)}2]·dmf (1), [{Cu(opba)}2{Cu(dmphen)Cl}4]·1.5dmf·2.5dmso (2), [{Cu(opba)}2{Cu(dmphen)Br}4]·dmf·2.3dmso (3) and [{Cu(opba)}{Cu(dmphen)(dca)}2]n (4) [H4opba = N,N′-1,2-phenylenebis(oxamic acid), dmphen = 2,9-dimethyl-1,10-phenanthroline and dca = dicyanamide anion]. 1 is a neutral tricopper(II) complex where an inner [Cu(opba)]2− fragment adopts a bis-bidentate coordination mode towards two outer [Cu(dmphen)(NCS)]+ units. 2 and 3 are bis-trinuclear species where two oxamato-bridged [Cu(opba){Cu(dmphen)X}2] [X = Cl− (2) and Br− (3)] tricopper(II) entities are connected by two single X ions involving the central and one of the peripheral copper(II) ions. 4 is a neutral chain made up of oxamato-bridged [Cu(opba){Cu(dmphen)(dca)}2] fragments linked through a single end-to-end dicyanamide ligand that connects the central copper(II) ion with one of the peripheral copper(II) ions. Magnetic susceptibility measurements on polycrystalline samples of 1–4 in the temperature range 1.9–300 K show the occurrence of strong antiferromagnetic interactions between the copper(II) ions through the oxamate bridge [J = −328(2) (1), −288(2) (2), −431(2) (3) and −370(1) cm−1 (4), the Hamiltonian being defined as H = −J(S1·S2 + S1·S3)] and a weak ferromagnetic coupling across the equatorial-axial exchange pathway provided by the single halide bridge [j = +3.08(3) (2) and +2.34(1) cm−1 (3)]. These values are analyzed by simple orbital symmetry considerations and compared with those reported in the literature for polynuclear copper(II) complexes with these bridging ligands.