Magnetic couplings in vanadium aromatic sandwich complexes and their crystals by using DFT methods.

Using density functional theory, we have theoretically studied the origin of ferromagnetic coupling of spins in the long multiple deckers of vanadium sandwich complexes with benzene (Bz). This is done by calculating the band structure of their infinite one-dimensional (1D) crystals along the periodic (=Z) direction. We find that the magnetic coupling can be ascribed to spin polarization in the band (= those derived from d(z2)) whose k states are not involved in the formation of partial covalent bonds between vanadium atoms and benzene rings. We have also studied magnetic and electronic properties of multiple deckers of a naphthalene (Np)-vanadium complex. We find that there is a stronger covalent interaction as well as a stronger electrostatic interaction between V and Np rings than between V and Bz rings. This suggests that there is a possibility of longer multiple deckers of a Np-V complex. In addition, ferrimagnetic coupling of spins is expected, resulting in a magnetic moment 45% larger than that of the Bz-V complex at the same length. For their 1D crystals, band structure analysis also shows that the origin of magnetic coupling in long multiple deckers should be similar to that of Bz-V multiple deckers.