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

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%.