Structural and Electronic Properties of Iron(II) and Nickel(II) Complexes of 2-Triazolyl-1,10-phenanthroline Derivatives

Abstract

Salts of the [FeN₆]²⁺ derivatives of the tridentate systems 2-(triazol-3-yl)-1,10-phenanthroline, 2-(1-methyltriazol-3-yl)-1,10-phenanthroline, 2-(5-methyltriazol-3-yl)-1,10-phenanthroline and 2-(1,5-dimethyltriazol-3-yl)-1,10-phenanthroline undergo temperature-induced singlet (¹A₁) ↔ quintet (⁵T₂) transitions in both the solid state and in solution. Both the nature of the associated anion and the extent of solvation of the salts influence the extent of the transition in solid samples. The solution behaviour has been interpreted in terms of a simple thermal equilibrium involving the two spin states. Electronic spectra of corresponding nickel(II) complexes establish that the field strengths for all ligands fall within a narrow range, but suggest that the order is influenced by electronic effects of the methyl substituents . Structural data indicate that a 5-methyl substituent exerts a small steric barrier to coordination of the triazolyl moiety. Data were obtained at 298 K for [FeL₂] [ClO₄]2.H₂O.C₂H₅OH (L = 2-(1-methyltriazol-3-yl)-1,10-phenanthroline) and [FeL2] [ClO4]2.3H2O (L = 2-(5-methyltriazol-3-yl)-1,10-phenanthroline). The average Fe-N distance in the former complex is 1.96 Ǻ and in the latter 1.97 Ǻ, indicating that in both instances the metal is essentially in the singlet state in these crystals. The ligands coordinate in meridional planes of an octahedron and give a distorted environment about the metal atom, the Fe- N_central distances being significantly shorter than the Fe-N_distal distances in both instances. This distortion results in unusually large quadrupole splitting in the Mossbauer spectra for the complexes in the singlet state. Bis [2-(1-methyltriazol-3-yl)-1,10-phenanthroline]iron(II) perchlorate monohydrate monoethanolate : triclinic, space group Pī , a 9.423(4), b 10.536(4), c 19.402(8) Ǻ, α 75.59(3), β 85.80(3), γ 68.76(3)°, Z 2. Bis [2-(5-methyltriazol-3-yl)-1,10-phenanthroline ]iron(II) perchlorate trihydrate : tetragonal, space group P41212, a 14.676(3), c 16.169(3) Ǻ, Z 4.