Ligand Orientation Control in Low-Spin Six-Coordinate (Porphinato)iron(II) Species

• Published in: Inorganic chemistry 2005-06, Vol.44 (12), p.4346-4358
• Main Authors: Hu, Chuanjiang, Noll, Bruce C, Schulz, Charles E, Scheidt, W. Robert
• Format: Article
• Language: English
• Subjects: Crystallography, X-Ray; Electrochemistry; Iron - chemistry; Ligands; Metalloporphyrins - chemical synthesis; Molecular Conformation; Spectroscopy, Mossbauer
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic050320p

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)2] which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened equatorial bonds of 1.963(6) Å and slightly elongated axial Fe−N bond lengths of 2.034(9) Å that are about 0.04 Å shorter and 0.03 Å longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with parallel oriented axial ligands. The Mössbauer spectrum shows a pair of quadrupole doublets that can be assigned to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has ΔE Q = 2.25 mm/s and δ = 0.90 mm/s at 15 K. The quadrupole splitting, ΔE Q, for [Fe(TMP)(2-MeHIm)2] is 1.71 mm/s, and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mössbauer spectra thus provide a probe for ligand orientation when structural data are otherwise not available.