Accessibility and Selective Stabilization of the Principal Spin States of Iron by Pyridyl versus Phenolic Ketimines: Modulation of the ⁶A₁ ↔ ²T₂ Ground-State Transformation of the [FeN₄O₂]⁺ Chromophore

Abstract

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2'-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH(2)CH(2)NH(2), Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe(II)(apRen)(2)]X(2) (R = H, Me; X(-) = ClO(4)(-), BPh(4)(-), PF(6)(-)) and [Fe(III)(hapRen)(2)]X (R = Me, Et; X(-) = ClO(4)(-), BPh(4)(-)). Single-crystal X-ray analyses of [Fe(II)(apRen)(2)](ClO(4))(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe(II)-N bond distances (1.896-2.041 Å) pointing to the (1)A(1) (d(π)(6)) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe(III)(hapMen)(2)]ClO(4), determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the (2)T(2) (d(π)(5)) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe(III)N(4)O(2)](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) ↔ (2)T(2) transformations in [Fe(III)(hapMen)(2)]ClO(4) and [Fe(III)(hapEen)(2)]ClO(4), respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(π) → p(π)* and p(π) → d(π) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H(2)hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe(III)(hapen)}(2)(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm(-1)).

Unusual Magneto‐Structural Features of the Halo‐Substituted Materials [FeIII(5‐X‐salMeen)2]Y: a Cooperative [HS‐HS]↔[HS‐LS] Spin Transition

“…The room temperature values of χ M T for 1 – 4 , which occur in the range 4.06–4.28 cm 3 K mol −1 , are consistent with the spin‐only value for HS iron(III) (χ M T ≈4.38 cm 3 K mol −1 ). They also concur with literature values in the range 3.80–4.50 cm 3 K mol −1 for closely related ferric spin‐crossover materials; although some of these reported values are somewhat low . The spin conversion in 1 – 4 , evidenced by the decrease in the values of χ M T with temperature, culminates in a plateau with a width of 115 K (140‐25 K) for 2 – 4 and 85 K (110‐25 K) for 1 .…”

“…The azomethine proton (‐C H =N‐) is associated with the singlet at δ 8.56 ppm. The most de‐shielded proton is that of the phenolic ‐OH group with a chemical shift of δ 13.47 ppm; crystallographic elucidation of closely related salicylaldimine‐based Schiff bases invariably shows H‐bonding interaction between the phenolic proton and the imine nitrogen . The resonances at δ 7.78 (d; J= 2.31 Hz), δ 7.59 (dd; J= 8.68, 2.31 Hz) and δ 6.72 ppm (d; J= 8.61 Hz) are ascribable to the aromatic protons H 6 , H 4 and H 3 (Figure ), respectively, consistent with the para ‐substitution of the phenolic ring.…”

“…The charge‐transfer spectra for [Fe III (5‐Cl‐salMeen) 2 ]ClO 4 ( 2 ) and [Fe III (5‐Br‐salMeen) 2 ]ClO 4 ( 3 ), depicted in Figure a, demonstrate a remarkable correlation between the electronic spectra and the Hammett parameters (σ p , Table ) . The UV LMCT bands ( λ ≈363 nm) are attributable to phenolate (p π )→iron(III) (d σ *) electronic transitions whereas the broad lopsided visible LMCT bands are phenolate (p π )→iron(III) (d π ) in nature …”

One-pot synthesis, structural characterization, UV–Vis and electrochemical analyses of new Schiff base complexes of Fe(III), Ni(II) and Cu(II)

“…The first process at −0.205 V can be assigned to the oxidation process corresponding to the redox couple Ni II /Ni III [17], while the following reversible process at −1.295 V could be assigned to the reduction process corresponding to the redox couple Ni II /Ni I [30]. In contrast, an irreversible process was observed at E pc equal to −2.140 V and can be assigned to the reduction of phenoxyl radical species [26].…”

“…In the spectrum of complex 2 two bands are observed: a characteristic charge-transfer MLCT transition Fe(III) → O phenolate at higher energy (380 nm) and a broad band at 632 nm, a charge-transfer LMCT transition (pπ orbital to an iron(III) dπ orbital) [25,26]. Thus, the high intensity MLCT transition [27], band observed at 380 nm can be responsible for the oxidation of the metal center Fe (II) → Fe(III) during the course of the reaction.…”