NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE MOLECULAR AND ELECTRONIC-STRUCTURE OF NICKEL(II) TETRAPHENYL-21-THIAPORPHYRINS

The proton NMR spectra of high-spin (S = 1) nickel(II) monohalide complexes of tetraphenyl-21-thiaporphyrin have been recorded and assigned by means of specific deuteration, methyl substitution, and line width consideration. The characteristic pattern of pyrrole (downfield) and thiophene resonances (upfield) has been established. The specific assignment of pyrrole resonances has been achieved by NOE measurements. Replacement of chloride by perchlorate results in formation of low-spin diamagnetic complex Ni(II)(STPP)ClO4 in chloroform. Imidazole replaces the axial halide ligand to produce rive-coordinate derivatives that give resonances for the coordinated axial ligand. The formation of the six-coordinate form has been also established. It is apparent from the H-1 NMR studies of Ni(II)(STPP)Cl and related systems that the 21-thiaporphyrin macrocycle is flexible in solution. The structure of the equatorial macrocycle adjusts to the spin and electronic state of the Ni(II)(STPP) moiety, as determined by the nature and number of axial ligands. The pyrrole shift pattern has been related to the Ni(II) coordination number. The isotropic shifts of pyrroles and thiophene resonances are dominated by the sigma-contact mechanism. The direct sigma-pi-spin density transfer has been proposed to explain the upfield shift of the thiophene and a large difference in contact shifts of pyrrole resonances even for protons located in the same pyrrole ring. To account for the sulfur contribution and the geometry of 21-thiaporphyrin, the semiquantitative Fenske-Hall LCAO MO method has been used to determine molecular orbitals involved in the distribution of spin density via pi-orbitals.