Electronic Effects in Transition Metal Porphyrins. 10. Effect of Ortho Substituents on the Temperature Dependence of the NMR Spectra of a Series of Spin-Admixed Perchloratoiron(III) Tetrakis(2,6- or 2,4,6-phenyl substituted)porphyrinates

Abstract

The perchloratoiron(III) complexes of a series of 2,6-disubstituted tetraphenylporphyrin ligands, where the 2,6-phenyl substituents were -H, -F, -Cl, -Br, or -OMe, as well as two 2,4,6-phenyl-substituted complexes, where the substituents were -Me and -OMe, have been investigated as a function of temperature by 1H NMR spectroscopy. Curvature in the 1/T dependence was evident in most cases. Forced linear extrapolation of the temperature dependence observed over the range of the study yielded Curie plots that include negative slopes with very large positive 1/T intercepts (Cl approximately Br > Me > H) to negative slope with near zero intercept (tri-OMe) to positive slope with very large negative intercept (F, di-OMe). The NMR results were combined with EPR spectroscopic data and curve-fitting procedures based on an expanded Curie law to arrive at a consistent overview of the variety of temperature-dependence behaviors observed. This overview relies upon the premise that, in addition to the ground state observed by EPR spectroscopy, one (or more) thermally accessible excited state(s) are populated to varying degrees over the temperature range of the NMR measurements. If only one excited state is considered, the analysis is consistent with the ground state being a largely intermediate-spin state (S = 3/2) for the majority of the complexes but a largely high-spin state (S = 5/2) for ((2,6-F2)4TPP)FeOClO3 and ((2,6-(OMe)2)4TPP)FeOClO3.

Reductive Coupling of Nitrogen Monoxide (•NO) Facilitated by Heme/Copper Complexes

“…These shift downfield when the temperature decreases but still lie in the intermediatespin state range. [(F 8 )Fe III ]ClO 4 exhibits δ pyrrole = ~15 ppm at RT) 74. In THF- d 8 , signals for both high-spin (δ pyrrole = 54.7 at RT) and intermediate spin (δ pyrrole = 14.8 at RT) are observed at all temperatures (Figure 10).…”

“…Walker and co-workers74 have in fact previously studied [(F8)Fe III ]ClO 4 ; in CH 2 Cl 2 , this was shown to be an S = 3/2 and S = 5/2 spin admixture; the ground state was largely high-spin with the S = 3/2 state as the first excited state, based on EPR (4.2K) and NMR (CD 2 Cl 2 ) spectroscopic properties. Here, we compare these properties with our own [(F 8 )Fe III ]SbF 6 in CH2Cl 2 .…”

Synthesis and Characterization of Germanium, Tin, Phosphorus, Iron, and Rhodium Complexes of Tris(pentafluorophenyl)corrole, and the Utilization of the Iron and Rhodium Corroles as Cyclopropanation Catalysts

“…This is consistent with intermediate-spin iron(iii), since all resonances are expected at low field for pure high-spin iron(iii), while the reverse holds for low-spin complexes. [21] The EPR spectrum of 5 a with g 3.8 (Figure 1c, observable at RT in the solid state and in frozen solutions) also supports this conclusion, as may be deduced from the extensive work with iron(iii) porphyrins (high-spin: g c % 6, g k % 2, intermediate-spin: g c % 4, low-spin: g z b 2, g y % 2, g x`2 ). [22] Finally, the 19 F NMR spectrum shown in Figure 1b displays three sets of resonances, all in a 2:1 ratio due to the different magnetic environment of the central C 6 F 5 ring relative to the two other.…”

Cytochromesc‘: Biological Models for theS=3/2,5/2Spin-State Admixture?

“…According to Mitra et al ., the Maltempo model explains only the gross magnetic behavior of the complexes and mixing with low-lying excited spin states ( 4 E, 2 E, 2 B 2 ) should also be considered. As shown diagrammatically by Figure , the relative contributions of S = 3 / 2 and S = 5 / 2 states to the ground state of spin-admixed systems varies depending upon the energy of the d( x 2 − y 2 ) orbital. As the energy of this orbital increases, the ground state will change from predominantly S = 5 / 2 to predominantly S = 3 / 2 …”