Magnetic exchange in polynuclear transition metal system:: Ab initio CASPT2 and density functional theory study on triangular copper(II) complexes

The magnetic exchange interactions for five representative triangular Copper(II) complexes: antiferromagnetic Cu-3(TiPB)(6) (1), [Cu-3(mu(3)-OH)(aaat)(3)(H2O)(3)](2+) (2), [PPN](2)[Cu-3(mu(3)-O)(mu-pz)(3)Cl-3] (3), [PPN][Cu-3(mu 3-OH)(mu-Pz)(3)Cl-3] (4) and ferromagnetic [Cu-3(2-CH3C6H4CO2)(4){(C2H5)(2)NC2H4O}(2)H2O] (5) are investigated by using density functional theory combined with broken-symmetry approach (DFT-BS) and ab initio CASPT2 method. Our calculated results show that DFT-BS has remarkable dependence on the particular chosen XC functionals and is system-dependent, while the calculations at CASPT2 level of theory are able to give the accurate magnetic coupling constants. (qualitatively, the two theoretical methods reproduce consistently the linear correlation between the magnetic coupling constants and the departure of the (mu(3)-O) oxygen atom from the {Cu3} plane in the complexes (3) and (4). Spin population analyses reveal that the DFT-BS method overestimates the spin electronic delocalization from the Cu(II) center to the bridging ligands.