3,5-Bis(ethynyl)pyridine and 2,6-Bis(ethynyl)pyridine Spanning Two Fe(Cp*)(dppe) Units: Role of the Nitrogen Atom on the Electronic and Magnetic Couplings

The role of the nitrogen atom on the electronic and magnetic
couplings
of the mono-oxidized and bi-oxidized pyridine-containing complex models
[2,6-{Cp­(dpe)­Fe–CC−}2(NC5H3)]n+ and [3,5-{Cp­(dpe)­Fe–CC−}2(NC5H3)]n+ is theoretically tackled with the aid of density-functional theory
(DFT) and multireference configuration interaction (MR-CI) calculations.
Results are analyzed and compared to those obtained for the reference
complex [1,3-{Cp*(dppe)­Fe–CC−)}2(C6H4)]n+. The mono-oxidized
species show an interesting behavior at the borderline between spin
localization and delocalization and one through-bond communication
path among the two involving the central ring, is favored. Investigation
of the spin state of the dicationic complexes indicates ferromagnetic
coupling, which can differ in magnitude from one complex to the other.
Very importantly, electronic and magnetic properties of these species
strongly depend not only upon the location of the nitrogen atom in
the ring versus that of the organometallic end-groups but also upon
the architectural arrangement of one terminus, with respect to the
other and/or vis-à-vis the central ring. To help validate the
theoretical results, the related families of compounds [1,3-{Cp*(dppe)­Fe–CC−)}2(C6H4)]n+, [2,6-{Cp*(dppe)­Fe–CC−}2(NC5H3)]n+, [3,5-{Cp*(dppe)­Fe–CC−}2(NC5H3)]n+ (n = 0–2) were experimentally synthesized
and characterized. Electrochemical, spectroscopic (infrared (IR),
Mössbauer), electronic (near-infrared (NIR)), and magnetic
properties (electron paramagnetic resonance (EPR), superconducting
quantum interference device (SQUID)) are discussed and interpreted
in the light of the theoretical data. The set of data obtained allows
for many strong conclusions to be drawn. A N atom in the long branch
increases the ferromagnetic interaction between the two FeIII spin carriers (J > 500 cm–1), whereas, when placed in the short branch, it dramatically reduces
the magnetic exchange in the di-oxidized species (J = 2.14(5) cm–1). In the mixed-valence compounds,
when the N atom is positioned on the long branch, the intermediate
excited state is higher in energy than the different ground-state
conformers and the relaxation process provides exclusively the FeII/FeIII localized system (Hab ≠ 0). Positioning the N atom on the short branch
modifies the energy profile and the diabatic mediating state lies
just above the reactant and product diabatic states. Consequently,
the LMCT transition becomes less energetic than the MMCT transition.
Here, the direct coupling does not occur (Hab = 0) and only the coupling through the bridge (c) and the reactant
(a) and product (b) diabatic states is operating (Hac = Hbc ≠ 0).