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) te traphenyl-21-thiaporphyrin have been recorded. The following selective ly deuterated thiaporphyrins have been used: ,20-diphenyl-10,15-bis(ph enyl-d5)-21-thiaporphyrin STPPH-d10),5,10,15,20-tetraphenyl-21-thiapor phyrin (STPPH-d6) deuterated at pyrrole beta-positions, 5,20-diphenyl- 10,15-bis(p-tolyl)-21-thiaporphyrin (SDPDTPH-d2) deuterated at the thi ophene beta-position. Two characteristic patterns of chemical shifts f or one-electron-reduced species have been established as exemplified b y 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, methyldiphenylphosphi ne, and 1-methylimidazole resulted in similar spectral patterns. The H -2 NMR spectra of Cu(II)(STPP-d6) established a standard pattern for t he well-defined d9 electronic structure (pyrrole, 49.9, 38.5, 29.6 ppm ; thiophene, -9.0 ppm). The isotropic shift of nickel thiaporphyrins h ave 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 fo r the series of nickel thiaporphyrins enriched in the Ni-61 isotope. T he 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 = 4 9.3, 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 coordinatio n of P(OEt)3 by Ni(STPP) has been confirmed by the characteristic supe rhyperfine splitting (A1P =124, A2P = 133, A3P = 127). A parallel anal ysis of Ni-61 hyperfine coupling constants and H-2 NMR isotropic shift s provided direct insight into the electron and spin density distribut ion in one-electron-reduced nickel tetraphenylthiaporphyrin complexes.