LOW-VALENT NICKEL THIAPORPHYRINS - NUCLEAR-MAGNETIC-RESONANCE AND ELECTRON-PARAMAGNETIC-RESONANCE STUDIES

The H-2 NMR spectra of one-electron reduction product of nickel(II) tetraphenyl-21-thiaporphyrin have been recorded. The following selectively deuterated thiaporphyrins have been used: 5,20-diphenyl-10,15-bis(phenyl-d5)-21-thiaporphyrin (STPPH-d10),5,10,15,20-tetraphenyl-21-thiaporphyrin (STPPH-d6) deuterated at pyrrole beta-positions, 5,20-diphenyl-10,15-bis(p-tolyl)-21-thiaporphyrin (SDPDTPH-d2) deuterated at the thiophene beta-position. Two characteristic patterns of chemical shifts for one-electron-reduced species have been established as exemplified by Ni(STPP-d6) (pyrrole, -52.3, 25.6, 21.0 ppm; 363 K), Ni(SDPDTP-d2) (thiophene; 41 ppm; 293 K) and Ni(STPP-d6)(1-MeIm)(pyrrole, 61.3, 42.3, 20.8 ppm; 298 K). The coordination of pyridine, methyldiphenylphosphine, and 1-methylimidazole resulted in similar spectral patterns. The H-2 NMR spectra of Cu(II)(STPP-d6) established a standard pattern for the well-defined d9 electronic structure (pyrrole, 49.9, 38.5, 29.6 ppm; thiophene, -9.0 ppm). The isotropic shift of nickel thiaporphyrins have been discussed in terms of the contribution of two canonical forms: nickel(I) thiaporphyrin-Ni(II) and thiaporphyrin anion radical. The sigma- and pi-delocalization mechanisms of the spin density have been considered to account for the isotropic shift pattern. Ni-61 hyperfine coupling constants have been measured by means of EPR spectroscopy for the series of nickel thiaporphyrins enriched in the Ni-61 isotope. The typical hyperfine coupling constants (A/10(-4) cm-1) are as follows: Ni-61(STPP), A1 = 6.5, A2 = 32.2, A3 = 3.2; Ni-61(STPP)(SO2), A1 = 49.3, A2 = 15.0, A3 = 18.5; Ni(STPP)(1-MeIm), A1 = 6.9, A2 = 4.6, A3 = 16; Ni-61(STPP)(py)2, A1 = 18.3, A2 = 11.1, A3 = 15.5. The coordination of P(OEt)3 by Ni(STPP) has been confirmed by the characteristic superhyperfine splitting (A1P =124, A2P = 133, A3P = 127). A parallel analysis of Ni-61 hyperfine coupling constants and H-2 NMR isotropic shifts provided direct insight into the electron and spin density distribution in one-electron-reduced nickel tetraphenylthiaporphyrin complexes.