Effects of Electric Field on the Structures of Metal String Complexes M<sub>3</sub>(dpa)<sub>4</sub>Cl<sub>2</sub> (M=Co, Rh, Ir; dpa=dipyridylamide)

Abstract

As potential molecular wire species, the geometrical and electronic structures of metal string complexes M 3 (dpa) 4 Cl 2 (1: M=Co, 2: M=Rh, 3: M=Ir; dpa=dipyridylamide) were investigated theoretically using density functional theory with the PBE0 functional by considering the interaction of an external electric field along the M 3 6+ linear metal chain. The results show that the ground states of the complexes are all doublets. There is a 3-center-3-electron σ bond delocalized over the M 3 6+ chain for 1 and 2, while there isa 3-center-4-electron σ bond and a weak δ bond among the Ir 3 6+ chain in 3. Moving down the column of Co, Rh, and Ir elements in the periodic table, the complexes with the corresponding metals showed some regular trends, such as stronger M - M bonds, smaller LUMO-HOMO gaps, weaker anti-ferromagnetic spin coupling among the M 3 6 + chains, and stronger spin delocalization from M 3 6 + to ligands. In the external electric field along the Cl4→Cl5 direction, the M3 ― Cl5 bonds at the low potential side tend to be shortened, while the M2―Cl4 distances at the high potential side increase. With the increase of electric field, the average M―M distances slightly decrease, which is beneficial for electron transport. When the electric field increases, the molecular energy decreases and the dipole moment linearly increases. Moreover, the negative charge moves from Cl5 at the low potential end towards Cl4 at the high potential end, and the spin electron moves from M3 at the low potential end to M1 and M2 at the high potential end, while the positive charges transfer in the opposite direction along the M 3 6 + chain of 3. However, there is no charge transfer between dpa - ligands and M 3 6+ chain or Cl - ligands. The LUMO-HOMO gaps decrease with increasing electric field, which is beneficial for electron transfer. The sensitivity of the frontier orbitals to the electric field is different, which leads to the orbital level crossing for LUMO or HOMO. Moving down the column of metal elements in the periodic table, the complexes with the corresponding metals showed weaker orbital level crossing for LUMO or HOMO and smaller deviation of average M―M distances due to the effect of the electric field.