Thermally Induced Two-Step, Two-Site Incomplete ~6A_1 reversible ~2T_2 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 Asy

摘要

The six-coordinate mononuclear iron(III) complexes [Fe(salpm)2]ClO4 centre dot 0.5EtOH, [Fe(salpm)2]Cl, [Fe{(3,5-~tBu2)-salpm}_2]X (X = ClO_4~- or Cl~-), and [Fe{(3,5-~tBu2)-salpm}2]NO3 centre dot 2H20 [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 [(8chi_MT)~(1/2)~5.85-5.90 mu_B] of these complexes are consistent with the high-spin (S = 5/2) ground state. These complexes are intensely colored on account of the strong p_(pi) -> d_(pi)~* LMCT visible absorptions. Definitive evidence for the structures of [Fe(salpm)2]ClO4 centre dot 0.5EtOH and [Fe{(3,5-~tBu2)-salpm}_2]NO3centre dot 2H20 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 centre dot 0.5EtOH undergoes a thermally induced ~6A_1reversible ~2T_2 crossover, [Fe{(3,5-~tBu2)-salpm}_2]NO3centre dot 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 centre dot 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]-CIO4 centre dot 0.5EtOH remain in the high-spin state down to 5 K.