Carboxyl-Bonded Low-Spin Iron(III). Chemistry of a Family of Coordination Type cis-FeN₄O₂

Abstract

Three tridentate ligands H2PhL (Ar = Ph), H2ToL (Ar = p-tolyl), and H2NpL (Ar = α-naphthyl) of structural type HONC(Ar)NNC6H4CO2H have been synthesized. These (general abbreviation H2ArL) react with iron(III) chloride, affording the pink Et4N[Fe(ArL)2] (s = 1/2; μeff = 1.98−2.04 μB) in which azo oxime chelation ensures spin-pairing while carboxylate coordination stabilizes the trivalent state. The EPR spectra of the complexes in frozen dimethylformamide−toluene glass (77 K) are rhombic with g values in the range 1.997−2.173. The X-ray structure of Et4N[Fe(NpL)2]·H2O·CH2Cl2 has been determined, revealing meridional binding of the two ligands affording cis-FeN4O2 geometry. The effect of spin-pairing is expressed in the relatively short (∼1.90 Å) Fe−O and Fe−N lengths. The water of crystallization is hydrogen-bonded to carboxyl O atoms. The iron(III)−iron(II) reduction potentials lie near −0.1 V vs SCE. Crystal data for Et4N[Fe(NpL)2]·H2O·CH2Cl2: crystal system triclinic, space group P1̄, a = 11.539(9) Å, b = 13.306(13) Å, c = 14.60(2) Å, α = 89.30(9)°, β = 82.65(9)°, γ = 79.90(8)°, V = 2189(4) Å3, Z = 2, R = 6.59%, R w = 6.23%.

First Examples of Carboxyl-Bonded Low-Spin Manganese(III) Complexes

“…On the other hand the azo-oxime function strongly promotes spin-pairing as demonstrated for trischelated manganese(II)such chelation, however, fails to stabilize manganese(III). 3b,c In H 2 ArL the azo−oxime and carboxylate functions together produce the desired effectsthe former promotes spin-pairing and the latter stabilizes manganese(III) by lowering reduction potentials. We have recently shown that the H 2 ArL ligands are also effective in inducing low-spin character in iron(III) …”

“…Synthesis of Ligands and Complexes. The ligands were synthesized as previously described . The complexes were synthesized by a general method.…”

“…We have been interested in realizing uncommon spin-configurations of 3d ions with the help of suitably designed ligands. − The ion of specific concern here, manganese(III), is generally high-spin ( s = 2; 3d 4 ) but for a few exceptions. ,, In particular, all members in the large family of complexes incorporating Mn III −O(carboxylate) binding are high-spin …”

A Redox‐Induced Spin‐State Cascade in a Mixed‐Valent Fe3(μ3‐O) Triangle

“…The properties of their one‐electron reduced counterparts, formally 2Fe 3+ Fe 2+ species, are highly temperature dependent as a result of the transition from valence trapped to fully delocalized regimes. All three Fe centers, however, retain local high‐spin configurations across the entire temperature range, as a combined result of the weak field character of the carboxylate ligand and the large electron–electron repulsion in Fe 3+ : low‐spin configurations at Fe 3+ are in general uncommon …”

Regiospecific Oximato Coordination at the Oxygen Site: Ligand Design and Low-Spin MnII and FeII/III Species

“…In principle, [Mn(RL) 2 ] has rhombic symmetry, but in practice, the rhombic component of the ligand field is not resolved. The EPR spectrum of [Fe(RL) 2 ]PF 6 in frozen glass is also axial, Figure .…”