[(Cp*)(dppe)Fe(III)−]⁺Units Bridged through 1,3-Diethynylbenzene and 1,3,5-Triethynylbenzene Spacers: Ferromagnetic Metal−Metal Exchange Interaction

Abstract

The bi- and trinuclear iron(III) complexes [1,3-{Cp*(dppe)Fe(C⋮C−)}2(C6H4)][PF6]2 (22+) and [1,3,5-{Cp*(dppe)Fe(C⋮C−)}3(C6H3)] [PF6] (33+) were prepared by oxidation of [1,3-{Cp*(dppe)Fe(C⋮C−)}2(C6H4)] or [1,3,5-{Cp*(dppe)Fe(C⋮C−)}3(C6H3)] with 2 or 3 equiv of [(C5H5)2Fe][PF6], respectively. Complexes 22+ and 33+ were isolated as thermally and air stable blue microcrystalline solids in 95 and 80% yield, respectively. These paramagnetic compounds were characterized by cyclic voltammetry, IR, UV−vis, 1H NMR, Mössbauer, and ESR spectroscopies. The three organoiron groups of 33+ are not located on the same side of the molecule, and its two faces are therefore magnetically nonequivalent. The 1H NMR isotropic shifts are expected to be essentially contact shifts and the zfs (zero field splitting) parameter D is expected to be small for 22+ and 33+ since the Curie law is accurately obeyed for the proton resonances of the π-bound Cp* ligand. ESR spectra of the bi- and triradicals showed broad and unresolved signals at g = 2.10 (22+, ΔHpp = 550 G) and 2.13 (33+, ΔHpp = 170 G) in addition to signals at g = 4.55 and 4.46, respectively due to Δms = 2 transition. The Δms = 3 transition was observed at g = 7.97 in the spectrum of 33+. The temperature dependence of molar susceptibility obtained by SQUID measurements on microcrystalline samples suggested that the ferromagnetic interaction produces a triplet ground state in biradical 22+ (2J = 130.6 ± 0.2 cm-1) and a quartet ground state for triradical 33+. The two doublet states lie above the quartet by 18.7 ± 0.2 and 28.8 ± 0.2 cm-1. These results constitute the first examples of magnetic exchange interactions in a three-spin organometallic system with a triangular topology and the ferromagnetic coupling occurs at nanoscale distances between the metal spin carriers. The geometries of 22+ and 33+ were optimized using DFT calculations. High spin species were computed to be energetically favored with the spin density mainly localized on the iron centers supporting the experimental results.