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Thermally Induced Two-Step, Two-Site Incomplete 6A12T2 Crossover in a Mononuclear Iron(III) Phenolate−Pyridyl Schiff-Base Complex:  A Rare Crystallographic Observation of the Coexistence of Pure S = 5/2 and 1/2 Metal Centers in the Asymmetric Unit

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journal contribution
posted on 12.11.2007, 00:00 by Musa S. Shongwe, Badria A. Al-Rashdi, Harry Adams, Michael J. Morris, Masahiro Mikuriya, Giovanni R. Hearne
The six-coordinate mononuclear iron(III) complexes [Fe(salpm)2]ClO4·0.5EtOH, [Fe(salpm)2]Cl, [Fe{(3,5-tBu2)-salpm}2]X (X = ClO4- or Cl-), and [Fe{(3,5-tBu2)-salpm}2]NO3·2H2O [Hsalpm = N-(pyridin-2-ylmethyl)salicylideneamine; H(3,5-tBu2)-salpm = N-(pyridin-2-ylmethyl)-3,5-di-tert-butylsalicylideneamine] have been synthesized and isolated in crystalline form; their chemical identities have been ascertained by elemental analyses, FAB mass spectrometry, and infrared spectroscopy. The room-temperature effective magnetic moments [(8χMT)1/2 ∼ 5.85−5.90 μB] of these complexes are consistent with the high-spin (S = 5/2) ground state. These complexes are intensely colored on account of the strong pπdπ* LMCT visible absorptions. Definitive evidence for the structures of [Fe(salpm)2]ClO4·0.5EtOH and [Fe{(3,5-tBu2)-salpm}2]NO3·2H2O has been provided by single-crystal X-ray crystallography. The monomeric complex cations in both compounds comprise two uninegative phenolate−pyridyl tridentate Schiff-base ligands coordinated meridionally to the iron(III) to afford a distorted octahedral geometry with a trans,cis,cis-[FeO2N4] core. Whereas [Fe(salpm)2]ClO4·0.5EtOH undergoes a thermally induced 6A12T2 crossover, [Fe{(3,5-tBu2)-salpm}2]NO3·2H2O retains its spin state in the solid state down to 5 K. However, EPR spectroscopy reveals that the latter complex does exhibit a spin transformation in solution, albeit to a much lesser extent than does the former. The spin crossover in [Fe(salpm)2]ClO4·0.5EtOH has resulted in an unprecedented crystallographic observation of the coexistence of high-spin and low-spin iron(III) complex cations in equal proportions around 100 K. At room temperature, the two crystallographically distinct ferric centers are both high spin; however, one [Fe(salpm)2]+ complex cation undergoes a complete spin transition over the temperature range ∼200−100 K, whereas the other converts very nearly completely between 100 and 65 K; ∼10% of the complex cations in [Fe(salpm)2]ClO4·0.5EtOH remain in the high-spin state down to 5 K.