Distinctive features of the structure of hemihexaphyrazine complexes with Y, La, and Lu according to quantum chemical data

The molecular structure of Y, La, and Lu complexes with hemihexaphyrazine (Нр), a six-link heteroazaporphyrinoid of АВАВАВ type, has been studied by DFT. It has been found that these MHp (M = Y, La, Lu) complexes have a non-planar structure of Cs symmetry with the metal atom dislocated from the center of the cavity to one of the thiadiazole rings. The average lengths of coordination bonds formed by the metal atom with six nitrogen atoms of the thiadiazole moieties are 2.389, 2.566, and 2.358 A in YHp, LaHp, and LuHp, respectively. Different degrees of deviation from planar structure have been determined in La complex and Y and Lu complexes, characterized by the values of the N N...N N torsion angle between nonequivalent N N bonds in two thiadiazole rings (38.2, 56.0, and 58.6°, respectively). At the same time the internal circumferences of the macrocycle are the same in all three complexes. It has been shown that the planar structures of the complexes of the D3h symmetry are saddle points on the PES. The transition barriers between the structures of Cs and D3h symmetries have been computed (40.5, 4.6, and 52.4 kJ/mol for YHp, LaHp, and LuHp, respectively). An NBO analysis of the electron density distribution in MHp complexes has been carried out and the nature of coordination bonds in the complexes has been examined. We suppose the coordination bonds in the complexes as three-center ones involving atom M and two nitrogen atoms of Nt-Nt bond in the thiadiazole moieties, and the Hp ligand as a quasi tridentate one, despite the fact that the metal atom is bonded to six nitrogen atoms of the ligand. The value of the lanthanide contraction ΔrLn (0.208 A) in the examined LnHp complexes with one ligand has been determined, which turned out to be close to ΔrLn values in the series of lanthanide halogenides with three monovalent ligands and in the series of lanthanide compounds with three bidentate ligands.