Studies of nuclear magnetic relaxation processes in paramagnetic metalloporphyrin complexes

Temperature dependence of Nuclear Magnetic Resonance (NMR) chemical shifts and longitudinal and transverse relaxation times (T₁ and T₂) was studied for the pyrrole protons in a number of six-coordinate S = 1/2 iron(III) tetraphenylporphyrin (TPP) and tetramesitylporphyrin (TMP) complexes in the temperature range 190--310 K. In all complexes, temperature behavior of the chemical shifts and relaxation times is consistent with the presence of a low spin - high spin exchange caused by the dissociation of one axial ligand. In symmetric sterically hindered complexes, cyclic exchange induced by the synchronous rotation of axial ligands is also present. In all complexes, T₂s are considerably shorter than T₁s. Relaxation times in the TMP complexes are generally longer than corresponding values for the TPP complexes. Estimate of the electronic T₁ is given and mechanisms of nuclear relaxation are discussed. The rate of NOE buildup for a pair of pyrrole protons in [TMPFe(2-MeImH)₂]⁺ was measured; it is consistent with the Stokes rotational correlation time. A method is proposed to predict the detectability and optimum detection conditions of NOE between a pair of structurally rigid protons in similar complexes. Contrary to previous studies, no NOE is detected between pyrrole protons of two unsymmetrically substituted bis-N-methylimidazole Fe(III) TPP complexes. Two NMR approaches were utilized to measure the rate constant of axial ligand rotation in the TMP complex. Saturation transfer measurements yield overestimated rate constant. The measurement based on the temperature dependence of the T₂s(ΔH‡ = 48 ± 1 kJ/mol, ΔS‡ = -10 ± 6 J/K · mol) is consistent with previous studies. Modified MM2 potentials were also used to study the rotation of axial ligands in [TMPFe(1,2-Me₂Im)₂]⁺ and [TPPFe(1-MeIm)₂]⁺. Adiabatic potential energy surfaces (PES) for rotation of axial ligands were constructed for both complexes. Synchronous rotation of the axial ligands (ΔH‡ = 48 kJ/mol) is highly preferable in the TMP complex. For the TPP complex, the enthalpy barriers to synchronous and asynchronous rotation are 3.3 and 5.4 kJ/mol, respectively. The relationship between the orientation of axial ligands, distortion of metalloporphyrin core from planarity, and the bulkiness of axial ligands and porphyrin substituents is discussed