Multinuclear, Multifield, and Multiphase Nuclear Magnetic Resonance Study of Cadmium meso-Tetraphenylporphyrin and its Pyridine Adduct

Article abstract of DOI:10.1021/ja00390a010

Multifield 113Cd, 15N, and 13C NMR spectra at 2.3, 4.7, and 9.4 T have been obtained for the pyridine adduct of 113Cd and 15N miltilabeled cadmium meso-tetraphenylporphyrin (Py-Cd-TPP).The chemical shifts, signs, and magnitudes of spin-spin coupling constants and spin-lattice relaxation parameters/mechanisms for these nuclei have been determined in solution.The sign of the 113C-15N coupling constants is found to be positive, ¹J(113Cd-15N) = 150.1 Hz.Further, solid-state 113Cd NMR experiments (with and without magic angle spinning) have been performed for Py-Cd-TPP and unliganded Cd-TPP.The latter data have been used for an evaluation of the origin of the 113Cd relaxation data for Py-Cd-TPP in solution.From these data a 33-ppm 113Cd chemical shift to lower shielding is observed upon adduct formation, i.e., on going from Cd-TPP to Py-Cd-TPP.This chemical shift is attributable to the donor properties of the fifth (pyridine) ligand plus effect of pulling the cadmium out of the porphyrin ring.The chemical shift anisotropies for axially symmetric 113Cd shielding tensors, Δ? = ?_<*> - ?_<*>, are -341+/-3 and -105+/-2 ppm for Cd-TPP and Py-Cd-TPP, respectively.Appropriate equations required for the analysis of relaxation data in terms of the anisotropic rotational diffusion for Py-Cd-TPP and librational motion of the phenyl-group substituents have been derived.From a combination of liquid- and solid-state data, the anisotropy in the molecular motion for Py-Cd-TPP in solution has been determined to be D_<*>/D_<*> = 3.2+/-0.8.Finally, the diffusion constant for the libration rotational motion of the phenyl-group substituents relative to the porphyrin-ring skeleton is determined to be D* = (2.2+/-0.8)*10⁹ s.