Antiferromagnetic coupling of transition metal spins across pyrimidine and pyrazine bridges in dinuclear manganese(II), cobalt(II), nickel(II) and copper(II) 1,1,1,5,5,5-hexauoropentane-2,4-dionate complexes

Dinuclear manganese(), cobalt(), nickel(), and copper() complexes bridged by pyrimidine and pyrazine derivatives, L[M(hfac)(2)](2) [L = 4,6-di(2-pyridyl) pyrimidine (DPPM), 2,3-di(2-pyridyl) pyrazine (DPPZ); M = Mn, Co, Ni, Cu; hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate], were synthesized and their magnetic properties were studied. Antiferromagnetic couplings across the pyrimidine ring were observed for the DPPM complexes with the exchange parameters, 2J/k(B), of -0.40, -3.1, -9.1 and -46 K for M = Mn, Co, Ni and Cu, respectively. The pyrimidine nitrogen atoms are coordinated at the axial position of each metal ion for M = Mn, Co and Ni, and coordinated equatorially for M = Cu. The DPPZ complexes also exhibited antiferromagnetic interactions, which are weaker than those of the DPPM complexes. Crystal structure analysis indicated that the molecular structures of the four DPPZ complexes are essentially the same in spite of various space groups. Ab initio unrestricted Hartree-Fock calculations on DPPM[Cu(hfac)(2)](2) predicting a positive effective exchange integral (J) are inconsistent with the experiments, because of overestimation of the role of pi-type spin-polarization in DPPM. The J value from the density functional UB3LYP calculations is close to the experimentally determined value, which arises from a sigma-type exchange pathway across the pyrimidine ring.