Abstract:

Density functional theory at the BP86 level and natural bond orbital theory were used to investigate the influence of bridging ligands on the Ni―Ni interactions and magnetic coupling properties of metal string complexes [Ni₃(L)₄(NCS)₂] (L = 1: dpa^- (dipyridylamine), 2: mpta^- (4-methylpyridyl-thiazolylamine), 3: mdpa^- (4-methyl-dipyridylamine), 4: mppa-(4-methylpyridyl-3H-pyrrolylamine)) with potential applications in molecular wires. The following conclusions can be drawn. (1) The ground states of the complexes are antiferromagnetic (AF) singlet states, which correspond to the quintet state (HS). The energy and structure of HS is similar to AF. There are three-center-four-electron σ bonds (σ₂σ_nb¹σ^*1) along the Ni₃⁶⁺ chains. (2) The Ni―Ni and Ni―N distances are unaffected by methyl substituents on the pyridine ring of dpa^- ligands. However, substitution of the 3H-pyrrole ring or thiazole ring by the pyridine ring in mdpa^- lengthens the N1―N2 and Ni―Ni distances but shortens the Ni2―N2 distance. These effects of the thiazole ring are weaker than those of the 3H-pyrrole ring. Therefore, the strength of the Ni―Ni interaction is 1 ≈ 3 > 2 > 4. (3) The predicted J_ab values of 3 and 4 are -103 and -88 cm^-1, respectively. The AF magnetic coupling effects of the complexes increase with increasing Ni―Ni interaction strength: the stronger the Ni―Ni interaction, the greater the direct magnetic coupling in the σ orbitals along the Ni₃⁶⁺ chains. In addition, the stronger the Ni2―N2 interaction, the larger the indirect magnetic coupling involving the bridging ligand. The direct magnetic coupling is stronger than the indirect magnetic coupling.

Key words: Metal string complex, Bridging ligand, Density functional theory, Magnetic coupling, M olecular wire