Ion-Pair Charge-Transfer Complexes Based on (o-Phenylenebis(oxamato))cuprate(II) and Cyclic Diquaternary Cations of 1,10-Phenanthroline and 2,2'-Bipyridine: Synthesis, Crystal Structure, and Physical Properties.

The ion pair charge-transfer (IPCT) complexes (C(14)H(12)N(2))[Cu(opba)].3H(2)O (1) and Na(2)(C(12)H(12)N(2))Cu(opba).4H(2)O (2), opba = o-phenylenebis(oxamate), have been isolated. Both compounds crystallize in the triclinic space group P&onemacr;. Crystal data for the compounds 1, a = 7.3216(13) Å, b = 10.1756(9) Å, c = 16.4890(16) Å, alpha = 107.786(8) degrees, beta = 94.79(1) degrees, gamma = 104.16(1) degrees, V = 1117.4(3) Å(3), Z = 2, R = 0.033 for 5332 observed reflections with I > 3sigma(I); 2, a = 6.732(3) Å, b = 11.169(3) Å, c = 12.126(2) Å, alpha = 111.01(2) degrees, beta = 102.71(3) degrees, gamma = 93.57(3) degrees, V = 820.1(5) Å(3), Z = 2, R = 0.065 for 3354 observed reflections with I > 3sigma(I). The crystal structures consist of alternated anion complex and diquaternary cation layers pi-pi interacting through the corresponding aromatic rings. The metallic layers contain the Cu(opba)(4)(-) dimeric group where the copper(II) ions are in square-pyramidal environment. For both compounds an ion-pair charge-transfer (IPCT) band is observed at 517 nm. Their thermal behavior has also been explained on the basis of the different crystal arrangements. The dimeric nature of compound 1 is clearly confirmed by the observation of temperature-dependent ESR transitions between its singlet and triplet states. The |J| value, determined from the positions of these transitions, is 0.085 cm(-)(1) at room temperature and increases notably with decreasing temperature being 0.14 cm(-)(1) at 4.2 K. Comparison of these exchange constants with those observed for other carboxylate-bridged copper compounds allow us to deduce that the temperature dependence of J is due to lattice shrinkage leading to appreciable change of the Cu-O(ap) distance. The ESR spectrum of compound 2 is characteristic of an axial g-tensor with g( parallel) = 2.178 and g( perpendicular) = 2.049, showing a weak half-field DeltaM(s) = 2 transition in agreement with the dimeric nature of the complex. The thermal evolution of the magnetic susceptibility corresponds to a compound with weak antiferromagnetic interactions. The best fit of the magnetic data to a dimer S = (1)/(2) model gives J = -0.8 cm(-)(1), in good agreement with the topology of the bridging unit with the magnetic orbitals in parallel planes and a Cu-O-Cu angle of 95.4 degrees.