Air-stable tetraphenylporphyrinate-Cp* sandwiches of Ln(<sc>iii</sc>): how decisive is geometry in controlling magnetic anisotropy?

The first air-stable Ln(iii) organometallic mixed-ligand sandwich complexes TPPLnCp* (Ln = Y (1), Tb (2), Dy (3), Ho (4), and Er (5)) bearing both 5,10,15,20-tetraphenylporphyrinate (TPP) and pentamethylcyclopentadienide (Cp*) ligands were synthesized by the reactions of half-sandwich bis(o-N,N-dimethylaminobenzyl) precursors Cp*Ln(CH ₂C ₆H ₄NMe ₂-o) ₂ with equimolar amounts of 5,10,15,20-tetraphenylporphyrin (TPPH ₂) in toluene at ambient temperature and isolated in 62%–74% yields. X-ray analysis revealed the linear Cnt(Cp*)–Ln–Cnt(TPP) geometry with a bond angle close to 180°, a Cnt(Cp*)–Ln bond distance ranging from 2.325 to 2.365 Å and a Cnt(TPP)–Ln distance ranging from 1.012 to 1.063 Å. These complexes demonstrated remarkable thermal and oxidative stability, being sublimable under vacuum above 290 °C without decomposition (∼5 × 10 ⁻² Torr). Magnetic studies revealed field-induced single-molecule magnet (SMM) behavior for the Tb ³⁺, Dy ³⁺, and Er ³⁺ derivatives. Theoretical investigations revealed the presence of only moderately axial g tensors, a small splitting within the ground multiplet, and significant mixing between different projections of the total angular momentum, all of which contribute to the disappointing SMM behavior. A quasi-spherical Ln ³⁺ coordination environment created by the tetradentate TPP ligand and appreciable orbital interactions of its π-conjugated system with the dysprosium 4f orbitals could introduce transverse components to the crystal field.