Probing the magnetic superexchange couplings between terminal Cu^II ions in heterotrinuclear bis(oxamidato) type complexes

The reaction of one equivalent of [n-Bu₄N]₂[Ni(opboR₂)] with two equivalents of [Cu(pmdta)(X)₂] afforded the heterotrinuclear Cu^IINi^IICu^II containing bis(oxamidato) type complexes [Cu₂Ni(opboR₂)(pmdta)₂]X₂ (R = Me, X = NO₃^- (1); R = Et, X = ClO₄^- (2); R = n-Pr, X = NO₃^- (3); opboR₂ = o-phenylenebis(NR-substituted oxamidato); pmdta = N,N,N',N",N"-pentamethyldiethylenetriamine). The identities of the heterotrinuclear complexes 1-3 were established by IR spectroscopy, elemental analysis and single-crystal X-ray diffraction studies, which revealed the cationic complex fragments [Cu₂Ni(opboR₂)(pmdta)₂]²⁺ as not involved in any further intermolecular interactions. As a consequence thereof, the complexes 1-3 possess terminal paramagnetic [Cu(pmdta)]²⁺ fragments separated by [Ni^II(opboR₂)]^2- bridging units representing diamagnetic S_Ni = 0 states. The magnetic field dependence of the magnetization M(H) of 1-3 at T = 1.8 K has been determined and is shown to be highly reproducible with the Brillouin function for an ideal paramagnetic spin = ¹/₂ system, verifying experimentally that no magnetic superexchange couplings exists between the terminal paramagnetic [Cu(pmdta)]²⁺ fragments. Susceptibility measurements versus temperature of 1-3 between 1.8-300 K were performed to reinforce the statement of the absence of magnetic superexchange couplings in these three heterotrinuclear complexes.