Thermally Induced Two-Step, Two-Site Incomplete ⁶A₁ ↔ ²T₂ 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

Abstract

The six-coordinate mononuclear iron(III) complexes [Fe(salpm)2]ClO(4).0.5EtOH, [Fe(salpm)2]Cl, [Fe{(3,5-tBu2)-salpm}2]X (X=ClO4- or Cl-), and [Fe{(3,5-tBu2)-salpm}2]NO(3).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 [(8chiMT)1/2 approximately 5.85-5.90 microB] of these complexes are consistent with the high-spin (S=5/2) ground state. These complexes are intensely colored on account of the strong ppi-->dpi* LMCT visible absorptions. Definitive evidence for the structures of [Fe(salpm)2]ClO(4).0.5EtOH and [Fe{(3,5-tBu2)-salpm}2]NO(3).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]ClO(4).0.5EtOH undergoes a thermally induced 6A1<-->2T2 crossover, [Fe{(3,5-tBu2)-salpm}2]NO(3).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]ClO(4).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 approximately 200-100 K, whereas the other converts very nearly completely between 100 and 65 K; approximately 10% of the complex cations in [Fe(salpm)2]ClO(4).0.5EtOH remain in the high-spin state down to 5 K.

Symmetry Breaking in Iron(II) Spin-Crossover Molecular Crystals

“…There are two distinct situations that account for stepwise transitions. One is observed when different SCO sites, characterized by distinct transition temperatures (T 1/2 or T c ), are present in the solid [10][11][12][13][14][15][16][17]. The other arises from subtle balances between different elastic interactions in the crystal lattice, which spontaneously generate two or more The crystal structures of the anhydrous and the hydrate species (x = 0, 1/3) were investigated at 200 K, 130 K and 90 K. At 200 K, the crystals are monoclinic (space group C2/c, (Z = 8)) and contain one crystallographically independent Fe II site, which is HS.…”

Theoretical study on second‐order nonlinear optical properties of spin crossover Fe(III) phenolate‐pyridyl Schiff base complexes

“…The molecular backbones of Schiff base ligand 1a and Fe(III) complex 1b were taken from the structures based on the crystallographic data [24], under the basic structure, we designed a series of Schiff base ligands and complexes through donor/acceptor exchange and Ni metal substitution (see Fig. 1).…”

“…Ϫ [24] with high-spin state (S ϭ 6) and low-spin state (S ϭ 2), and compared with the properties of corresponding ligands and closed-shell Ni(II) metal complexes. Meanwhile, the variation law of second-order NLO responses in the different spin states and the open, closed shell complexes were revealed.…”

Synthesis, characterization and catalytic study of a novel iron(III)-tridentate Schiff base complex in sulfide oxidation by UHP

“…Also an absorption band at about 317 nm is attributed to a phenolate (p p ) ? iron(III) (d r * ) chargetransfer transition [40][41][42][43]. The intense higher-energy bands at around 270 nm can be attributed to intraligand p ?…”

“…By the addition of UHP (0.4 mmol) to the reaction solution of the iron complex catalyzed oxidation of methylphenylsulfide, the absorption band in the region 317 nm that can be assigned to a ligand to metal chargetransfer (phenolate (p p ) ? iron(III) (d r * )) [40][41][42][43] of the complex [Fe(L)(acac)(EtOH)] disappeared gradually and the solution turned from brown to pale yellow. The mark decrease of the absorption intensity at 317 nm after 15 min (Fig.…”