Jahn-Teller distortion, ferromagnetic coupling, and electron delocalization in a high-spin Fe–Fe bonded dimer

Torsion Effects Beyond the δ Bond and the Role of π Metal-Ligand Interactions

Previous studies on bimetallic paddlewheel compounds have established a direct correlation between metal-metal distance and ligand torsion angles, leading to the rule that higher torsion results in longer metal-metal bond distances. Here, the new discovery based on diarylformamidinate Ru₂⁵⁺ paddlewheel compounds [Ru2Cl(DArF)4] that show an opposite behavior is reported: higher torsions lead to shorter metal-metal distances. This discovery challenges the assumption that internal rotation solely impacts the δ bond. By combining experimental and theoretical techniques, it is demostrated that this trend is associated with previously overlooked π metal-ligand interactions. These π metal-ligand interactions are a direct consequence of the paddlewheel structure and the conjugated nature of the bidentate ligands. This findings offer far-reaching insights into the influence of equatorial ligands and their π-conjugation characteristics on the electronic properties of paddlewheel complexes. That this effect is not exclusive of diruthenium compounds but also occurs in other bimetallic cores such as ditungsten or dirhodium is demonstrated, and with other ligands showing allyl type conjugation. These results provide a novel approach for fine-tuning the properties of these compounds with significant implications for materials design.

Computational Investigation of the Metal and Ligand Substitution Effects on the Structure and Electronic States of the Phosphoranimide Tetramer Complexes of Cu(I), Ni(I), Co(I), and Fe(I)

The structurally unique saddle-shaped paramagnetic tetrametallic clusters of Co(I) and Ni(I) with phosphoranimide ligands have been synthesized and proposed as catalyst precursors. The analogous Cu(I) nanocluster is planar and diamagnetic. These notable variations in geometry and ground electronic states indicate that the effect of metal and ligand substituents on the structure and electronic properties of these complexes requires investigation. We present a computational study of a series of these novel homoleptic complexes containing Co(I), Ni(I), and Cu(I) as well as Fe(I) coordinated to phosphoranimides with electron-donating and withdrawing substituents, conducted at the relativistic density functional theory level using ZORA-PBE/TZP. The optimized structures of the saddle-shaped Co(I) and Ni(I) and planar Cu(I) tetramers with linear N-M-N coordination are validated with respect to X-ray diffraction determinations. The ground-state analysis indicates that Cu(I) complexes are diamagnetic, whereas Ni(I) and Co(I) complexes are in high-spin states, in agreement with magnetic susceptibility measurements. The computational results show that Fe(I) complexes are saddle shaped and high spin. The Co(I) complex is stabilized by a metal macrocycle distortion from square to diamond, as elucidated from its Walsh diagram. The effects of metals and ligand substituents on the ground electronic state, metal center coordination environment, and energy of the complexes are investigated. The bulky tertiary butyl substituent causes the largest saddle-shape distortion of the tetramer marcocycle, which partially offsets its electron-donating effect. Macrocycle distortions with N-M-N site angles ranging from obtuse to alternating obtuse reflex are correlated with the increasing number of unpaired electrons. The phenyl-substituted complexes are expected to have the highest reactivity toward electrophiles. Understanding the interplay between structural and electronic parameters is intended to guide the development of synthetic cooperative systems for multielectron redox reactions, models of biological systems, and molecular magnets.

Extraordinarily Large Ferromagnetic Coupling (J≥150 cm-1 ) by Electron Delocalization in a Heterometallic Mo≣Mo-Ni Chain Complex

The new heterometallic chain compounds Mo₂ Ni(dpa)₄ Cl₂ (1) and [Mo₂ Ni(dpa)₄ Cl₂ ]OTf (2) (dpa=2,2'-dipyridylamine) have been prepared and studied by crystallography and magnetic susceptibility, among other methods. Oxidation of 1 to 2 removes an electron from the multiply bonded Mo₂ unit, consistent with the formulation of 2 containing a (Mo₂ )⁵⁺ ⋅⋅⋅(Ni)²⁺ core. While 1 contains an S=1, pseudo-octahedral Ni^II ion, 2 has an S=3/2 ground state, in which the two Ni^II unpaired electrons, one in a localized δ-orbital and one in a heavily delocalized σ_nb -orbital are joined by an unpaired electron in a Mo-Mo δ-orbital. The S=3/2 ground state is persistent to 300 K, evidencing strong ferromagnetic coupling of the Mo₂ and Ni spins with J≥150 cm^-1 . This ferromagnetic interaction occurs via delocalization of a σ_nb -electron across all three metal atoms, forcing ferromagnetic alignment of electrons in orthogonal Ni and Mo₂ δ-symmetry orbitals. We anticipate that this new means of coupling spins can be used as a design principle for the preparation of new compounds with high spin ground states.

Not all density functionals are created equal: the case of the missing electron in the oxidized [W-W≡O](7+) core

The location of the unpaired electron in the new mixed-valent (W2)(IV,V) trication [W2O(dpa)4](3+) presents a challenge for DFT methods. EPR spectroscopy confirms the unpaired electron to be in the W(V)-oxo unit, in agreement with the predictions of hybrid functionals B3LYP and TPSSh, but contrary to the predictions of non-hybrid functionals.