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)(H ₂ O) _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) [H ₄ opba = 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(S ₁ ·S ₂ + S ₁ ·S ₃ )] 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.