Topological Dependence of the Magnetic Exchange Coupling in Arylethynyl-Bridged Organometallic Diradicals Containing [(η²-dppe)(η⁵-C₅Me₅)Fe^III]⁺ Fragments

We have investigated the spin distribution and determined the magnetic exchange coupling Jab (defined according to the following Hamiltonian: Ĥspin = −2JabŜa·Ŝb) for three arylethynyl-bridged organoiron(III) diradicals containing [(η2-dppe)(η5-C5Me5)FeIII]+ fragments. Considering the distance separating the FeIII centers (≥11 Å), remarkably large intramolecular magnetic interactions between unpaired spins were found for two of them. Thus, an antiferromagnetic coupling (Jab) of ca. −190 cm−1 was experimentally determined for the binuclear FeIII species featuring a 1,4-diethynylbenzene bridge 1[PF6]2, while a ferromagnetic interaction of over +150 cm−1 was evidenced for its 1,3-substitued analogue 2[PF6]2. We also show that a much weaker interaction (0 > Jab ≥ −1 cm−1) takes place in the 4,4′-biphenyl analogue of 1[PF6]2 (3[PF6]2), evidencing that insertion of an additional 1,4-phenylene unit in the bridge severely disrupts the magnetic communication in these diradicals. With the help of NMR and density functional theory, the magnetic properties of these compounds were rationalized and compared to those of the corresponding mononuclear FeIII relatives 4[PF6] and 5[PF6]. Finally, it is shown that, for all of these dinuclear Fe(III) complexes, the structural changes between singlet and triplet spin isomers remain very small regarding the carbon-rich bridge. Thus, even for a strongly coupled diradical such as 1[PF6]2, a dominant diradicaloid character dominates the valence-bond description of the singlet state unpaired electrons.