[(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)(C6H4)][PF6](2) (2(2+)) and [1,3,5-{Cp*(dppe)Fe(C=C-)}(3)(C6H3)] [PF6] (3(3+)) 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)(2)Fe][PF6], 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, H-1 NMR, Mossbauer, 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 H-1 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 pi-bound Cp* ligand. ESR spectra of the bi- and triradicals showed broad and unresolved signals at g = 2.10 (2(2+), Delta H-pp = 550 G) and 2.13 (3(3+), Delta H-pp = 170 G) in addition to signals at g = 4.55 and 4.46, respectively due to Delta m(s) = 2 transition. The Delta 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+) (2J = 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 toba energetically favored with the spin density mainly localized on the iron centers supporting the experimental results.