Coordination of the Strong Field Terimine System 6-Triazol-3-yl-2,2′-bipyridine and Substituted Derivatives. Electronic and Structural Properties of Bis(ligand)iron(II) Complexes

Abstract

A terminal pyridine ring in terpyridine has been replaced by a triazole or substituted triazole moiety to give the tridentates 6-(1,2,4-triazol-3-yl)-2,2´-bipyridine, 6-(1-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine, 6-(5-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine and 6-(1,5-dimethyl-1,2,4-triazol-3-yl)-2,2´-bipyridine. The effects of coordination of these on the electronic and structural properties of the [FeN₆]²⁺ species are considered. The primary effect is a reduction in the ligand field but salts of the [FeN₆]²⁺ derivatives are essentially low spin (singlet ground state) at room temperature. A small and temperature-dependent population of quintet state species is indicated by Mössbauer spectra but in no instance is a complete singlet → quintet transition observed up to 383 K. The electronic spectra of the [FeN₆]²⁺ species are similar to that of [Fe(trpy)₂]²⁺ but the energy of the charge-transfer transition is higher, indicating less effective metal ion → donor π-interaction than in the terpyridine complex. Consistent with this, the Mössbauer isomer-shift values are greater than for [Fe(trpy)₂]²⁺ and the E_½ values for the [FeN₆]³⁺ /[FeN₆]²⁺ couples are less positive than for [Fe(trpy)₂]²⁺. Structural data reveal average Fe–N distances normal for low-spin iron(II) but with greater distortion of the coordination environment than in [Fe(trpy)₂]²⁺. When present, the >NH group of the triazole moiety is hydrogen-bonded to solvate water and the anion in the hydrated tetrafluoroborate salts. Bis(6-(1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) tetrafluoroborate tetrahydrate: monoclinic, space group P2₁/c, a 15·917(7), b 18·432(5), c 10·845(5) Å, β 94·36(2)°, Z 4; bis(6-(1-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) tetrafluoroborate: triclinic, space group P−1, a 11·68(1), b 11·96(2), c 12·46(2) Å, α 82·93(8), β 72·30(9), γ 68·49(10)°, Z 2; bis(6-(5-methyl-1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) tetrafluoroborate monohydrate: monoclinic, space group P2₁/c, a 10·715(4), b 25·818(5), c 11·039(4) Å, β 90·08(2)°, Z 4; bis(6-(1,5-dimethyl-1,2,4-triazol-3-yl)-2,2´-bipyridine)iron(II) triflate tetrahydrate: orthorhombic, space group P bcn; a 20·654(6), b 11·618(3), c 16·080(6) Å, Z 4.