Synthesis and Spin-State Studies in Solution of γ-Substituted Tris(β-diketonato) iron(III) Complexes and Their Spin-Equilibrium β-Ketoimine Analogues Derived from Triethylenetetramine

Eric V. Dose, Kenneth M.M. Murphy, Lon J. Wilson

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Abstract

Six tris(β-diketonato)iron(III) complexes, viz., [Fe(acacX)3], and eight new magnetically anomalous β-ketoimineiron(III) complexes containing hexadentate ligands derived from triethylenetetramine (trien) and various β-diketones ([Fe- ((acac)2trien)](Y) from acetylacetone with Y- = PF6-, BPh4-, Br-, and I-, [Fe((bzacCl)2trien)](PF6) and [Fe((bzacCl)2trien)](PF6) from benzoylacetone, and [Fe((tfac)2trien)](PF6) from trifluoroacetylacetone) have been synthesized and studied. The tris chelates have been synthesized by new or modified methods, in most cases from the parent [Fe(acac)3] complex, and are found to exist as typical pseudooctahedral high-spin (S = 5/2) iron(III) complexes. On the basis of a radical-trap experiment, the chlorination of [Fe(acac)3] by N-chlorosuccinimide (NCS) to form [Fe(acacCl)3] is postulated to undergo an ionic (Cl+) intermediate. In solution, the β-ketoimine complexes are hexacoordinate, uni-univalent electrolytes which exhibit variable-temperature magnetic susceptibility data and electronic spectra which are consistent, in six cases, with a 2T ⇄ 6A spin-equilibrium process arising from iron(III) in a tetragonally distorted N4O2 ligand field environment like that found for the previously studied [Fe((XSal)2trien)](Y) series (Sal = salicylaldehyde). Two of the complexes, [Fe((acacCl)2trien)](PF6) and [Fe((bzacCl)2trien)](PF6), have been synthesized by direct chlorination of the β-ketoimine rings of [Fe((acac)2trien)](PF6) and [Fe((bzac)2trien)](PF6) using NCS in a manner similar to that used for the preparation of [Fe(acacCl)3], The two chlorinated β-ketoimine complexes are found to favor the low-spin electronic state over that of their parent complexes, suggesting that the spin state is dependent on the electron-withdrawing capability of the chelate ring substituent. The 2T ⇄ 6A spin equilibrium for the parent [Fe((acac)2trien)](Y) complex is strongly solvent dependent and exhibits an anion dependence which is surprisingly large in comparison with that of [Fe(Sal2trien)](Y). The solvent dependency on the magnetic moment cannot be completely attributable to a secondary amine [N-H…solvent] hydrogen-bonding interaction involving the trien backbone as previously noted for the [Fe(Sal2trien)]+ complex. Laser Raman temperature-jump kinetic measurements, conducted in collaboration with Dr. Norman Sutin, yield spin-state lifetimes for the [Fe((acac)2trien)] (PF6) salt in a methanol/10% acetone solution of τ(2T) = 6.7 × 10-8 s and τ(6A) = 1.5 × 10-7 s at 298 K which are similar in magnitude to those previously reported for other iron ΔS = 2 spin-equilibrium processes. The room-temperature Mossbauer spectrum of the 2T ⇄ 6A [Fe((acac)2trien)](BPh4) complex (μeff = 3.04 μβ) shows separate signals attributable to the 6A and 2T spin states, indicating that τ(2T) and τ(6A) in the solid state for a dynamic spin-equilibrium process are >10-7 s.

Original languageEnglish
Pages (from-to)2622-2630
Number of pages9
JournalInorganic Chemistry
Volume15
Issue number11
DOIs
StatePublished - Nov 1 1976

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