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

Abstract

We have investigated the spin distribution and determined the magnetic exchange coupling J(ab) (defined according to the following Hamiltonian: H(spin) = -2J(ab)S(a).S(b)) for three arylethynyl-bridged organoiron(III) diradicals containing [(eta(2)-dppe)(eta(5)-C(5)Me(5))Fe(III)](+) fragments. Considering the distance separating the Fe(III) centers (>or=11 A), remarkably large intramolecular magnetic interactions between unpaired spins were found for two of them. Thus, an antiferromagnetic coupling (J(ab)) of ca. -190 cm(-1) was experimentally determined for the binuclear Fe(III) species featuring a 1,4-diethynylbenzene bridge 1[PF(6)](2), while a ferromagnetic interaction of over +150 cm(-1) was evidenced for its 1,3-substitued analogue 2[PF(6)](2). We also show that a much weaker interaction (0 > J(ab) >or= -1 cm(-1)) takes place in the 4,4'-biphenyl analogue of 1[PF(6)](2) (3[PF(6)](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 Fe(III) relatives 4[PF(6)] and 5[PF(6)]. 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[PF(6)](2), a dominant diradicaloid character dominates the valence-bond description of the singlet state unpaired electrons.

From Antiferromagnetic to Ferromagnetic Interaction in Cyanido-Bridged Fe(III)–Ru(II)–Fe(III) Complexes by Change of the Central Diamagnetic Cyanido-Metal Geometry

“…When paramagnetic impurities α are considered, the experimental data can be closely fitted to eq 1 from modified Bleaney−Bowers equation (N, g, k, and β are the Avogadro number, the Zeeman factor, the Boltzmann constant, and the Bohr magneton, respectively). 13 The fit gives J = −13.6 cm −1 , g = 2.00, α = 0.01, and an agreement factor…”

Triphenylamine Derivatives with Para-Disposed Pendant Electron-Rich Organoiron Alkynyl Substituents: Defining the Magnetic Interactions in a Trinuclear Iron(III) Trication

“…Actually, the averaged spin density present on the organic spacer of 1[PF 6 ] 3 significantly exceeds one-third of that of 2[PF 6 ], the value expected for a pure doublet state where the unpaired spin is delocalized over three metallic end groups (Scheme 2) but is clearly below that expected for a pure quartet species. 7 This observation is probably attributable to the presence of a thermal equilibrum between polyradicals in the doublet and quartet states, dominated by the doublet state. 27 Thus, the NMR data obtained for these Fe(III) compounds strongly support the idea that, in the organometallic triradical 1[PF 6 ] 3 , the nitrogen bridging atom acts as an antiferromagnetic coupler between two of the branches.…”

Electron‐Rich Iron/Ruthenium Arylalkynyl Complexes for Third‐Order Nonlinear Optics: Redox‐Switching between Three States

“…It is mainly localized on the iron atom (0.52e) and, to a lesser extent, on the conjugated carbon chain and the ruthenium center (0.13e) and is in line with earlier calculations on related complexes. [19,29] The optimized (gas-phase) structure shows slight modifications of the torsion angles along the conjugated chain when compared with those of the experimental (solid-state) arrangement. To evaluate the role of these changes of orientation of the Fe-A C H T U N G T R E N N U N G (dppe)Cp* and RuA C H T U N G T R E N N U N G (dppe) 2 fragments in localizing the unpaired electron, a partial conformational study was performed.…”

“…[36] However, DFT calculations on 3-H 2 + dications reveal that these two magnetic states will have very similar electronic structures, [29] so it is likely that the NLO response of the system would not be modified significantly upon magnetically switching between these states. Of more interest is the possibility of effecting photoinduced switching between MO-1 and MO-2 compounds featuring mixed-valent Fe III / Ru II units, which should possess significantly different electronic properties.…”