Magnetic and Phonon Excitations in Iron(III) Porphyrin Complexes Probed by Inelastic Neutron Scattering: Impact of the Perdeuteration of the Porphyrin Ligand

Magnetic and phonon excitations, among the fundamental properties of metal complexes, have been of intense interest. Phonons include both intramolecular and intermolecular (or lattice) vibrations. Few techniques effectively probe excitations below 400 cm^–1, where many transition metal complexes exhibit magnetic transitions. Inelastic neutron scattering (INS) is ideal for studying both magnetic and phonon excitations. Unlike IR and Raman spectroscopies, INS has no symmetry-based selection rules, thereby revealing all vibrational modes. We have employed INS by VISION (Vibrational Spectrometer) at Oak Ridge National Laboratory to investigate Fe(TPP)X [TPP^2–= meso-tetraphenylporphyrinate; X^–= F^–(Fe–F); Cl^–(Fe–Cl); Br^–(Fe–Br); I^–(Fe–I)] and perdeuterated Fe(TPP-d₂₈)Cl (Fe–Cl-d₂₈). Density functional theory (DFT) phonon calculations provide calculated INS spectra for comparison with experimental spectra, giving phonon symmetries, energies, and movies. At 5 K, the magnetic transition in Fe–Cl-d₂₈(M_S= ±1/2 → M_S= ±3/2 at 12.7 cm^–1) is 3.2(1) times more intense than Fe–Cl, as incoherent neutron scattering of the H atoms in the latter leads to larger background and reduced signal/noise ratios in Fe–Cl. The DFT calculations also yield atomic spin densities in Fe(TPP)X. Additionally, the crystal structure of D₂(TPP-d₂₈) was determined by single-crystal X-ray diffraction.