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

The bi- and trinuclear iron(III) complexes [1,3-{Cp*(dppe)Fe(C⋮C−)} 2 (C 6 H 4 )][PF 6 ] 2 ( 2 2+ ) and [1,3,5-{Cp*(dppe)Fe(C⋮C−)} 3 (C 6 H 3 )] [PF 6 ] ( 3 3+ ) were prepared by oxidation of [1,3-{Cp*(dppe)Fe(C⋮C−)} 2 (C 6 H 4 )] or [1,3,5-{Cp*(dppe)Fe(C⋮C−)} 3 (C 6 H 3 )] with 2 or 3 equiv of [(C 5 H 5 ) 2 Fe][PF 6 ], respectively. Complexes 2 2+ and 3 3+ 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, 1 H NMR, Mössbauer, and ESR spectroscopies. The three organoiron groups of 3 3+ are not located on the same side of the molecule, and its two faces are therefore magnetically nonequivalent. The 1 H NMR isotropic shifts are expected to be essentially contact shifts and the zfs (zero field splitting) parameter D is expected to be small for 2 2+ and 3 3+ 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 ( 2 2+, Δ H pp = 550 G) and 2.13 ( 3 3+, Δ H pp = 170 G) in addition to signals at g = 4.55 and 4.46, respectively due to Δ m s = 2 transition. The Δ m s = 3 transition was observed at g = 7.97 in the spectrum of 3 3+ . 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 2 2+ (2 J = 130.6 ± 0.2 cm - 1 ) and a quartet ground state for triradical 3 3+ . 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 2 2+ and 3 3+ 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.