Multinuclear magnetic resonance spectroscopy of spin-admixed S = 5/2, 3/2 iron(III) porphyrins

Abstract

1find Q, = -7.6 X hartree, Qy = -2.0 X lo4 hartree, and Q, = 2.7 X hartree. Then eq, = -0.44 au, eq, = -1.1 au, and eq, = 1.6 au. For 63Cu, Q, = -7.1 X hartree, Qy = -2.2 X lo4 hartree, Q, = 2.9 X lo4 hartree, eq, = -0.38 au, eq, = -1.2 au, and eq, = 1.6 au. This compound has nuclear quadrupole coupling constants consistent with the few other 63Cu(II) oxygen-coordinated compounds of this high symmetry, mostly octahedral, that have been examined (8)(9)(10) and 14 and Table 111). This system is a slightly elongated octahedron. Its QD value is at the low end of the range of values previously found for octahedral enviroiments but above values found for more elongated octahedra.14 The inclusion of the Q E parameter here could account for the observed reduction in the QD value. The large QD (or eq,) value indicates an elongated distribution of charge, mainly attributable to the d, , orbital vacancy in the Cu(I1) valence shell, with some reduction-perhaps 25%-owing to covalent charge ~p r e a d i n g .~J~J~.~~ Quantitative discussions of the relationship between QD, QE, and electronic structure are given elsewhere (e.g., ref 5 , 9, 10, and 23, and work to be published). The Q E value indicates some distortion from a completely axial charge distribution. The moderate rhombic component of that distribution (eq, -eq, = 0.8 au) must be related to a slight difference between two pairs of equatorial metal-oxygen bond distances revealed by the structural report on the host ~rysta1.l~ Finally, the utility of EPR powder spectra to determine nuclear quadrupole coupling constants has been confirmed in this study. Our predictions as well as the values initially derived from computer simulations of powder spectra were borne out by the subsequent detailed single-crystal analyses. The close agreement between the sets of EPR parameters obtained by the two methods demonstrates the adequacy of powder-spectrum simulation for the determination of nuclear quadrupole coupling constants. Acknowledgment.A variety of synthetic iron(II1) porphyrin complexes (FeII'PorX, X = S03CF3-, C104-, and C(CN),-) were examined with multinuclear NMR spectroscopy ('H, 13C, I9F, and j5Cl). Deviations from NMR Curie law behavior, diminished magnetic moments, and characteristic ESR g = 4 values support previous evidence for the quantum-mechanical admixture of S = 5/2 and S = 3/2 states. NMR studies of titrations with the corresponding tetrabutylammonium salts and dipolar shift calculations show the ligands are coordinated rather than ion paired in solution. Solvent studies indicate more S = 5/2 character is present in aromatic solvents than in chlorinated solvents. Although the tricyanomethanide complex is thought to exhibit a "pure" S = 3/2 state in crystalline form, solution measurements are consistent with spin admixture.

Synthesis, properties, and reactivity of a series of non-heme {FeNO}7/8 complexes: Implications for Fe-nitroxyl coordination

“…When [Fe(TPP)OTf] was prepared [51] and subjected to the same conditions used for [Fe(TPP)Cl] and 6, a similar 426 nm band is observed in the first spectral trace. Though the OTf − anion is considered to be a weaker field ligand than Cl − , previous reports describe [Fe(TPP)OTf] as a 5C species instead of an ion pair [51,52]. Since the resulting UV-vis spectral changes were nearly identical to that of compound 6 with [Fe(TPP) Cl], we propose that the net NO − transfer is occurring through a similar mechanism with both 5C ferric porphyrins.…”

“…[51] and compared well to known spectroscopic data including UV-vis (see Fig. S16 in the Supporting information) and FTIR [51,52]. Suitable crystals were mounted on a glass fiber.…”

Bonding Characteristics Mediated by Saddle-Shaped Porphyrin Deformation: A Theoretical Approach to the Control of Spin State of Iron(III) Porphyrins

“…This implies that no relatively stable quartet state should be found without changing the basic iron ± porphyrin coordination (that is, increasing the s-donor or p-acceptor ability of porphyrin macrocycle). However, most of the intermediate-spin iron(iii) porphyrin systems reported up to now were controlled by the axial ligand field strength (for example, ClO 4 À , CF 3 SO 3 À , and C(CN) 3 À and so on).…”

Electrochemistry and spectroelectrochemistry of iron porphyrins in the presence of nitrite

“…In the absence of nitrite, a high-spin ferric porphyrin spectrum was obtained with a resonance at 49.2 ppm, which is due to the methylene protons. Generally, two resonances are observed for the methylene protons for a five-coordinate complex, but a single methylene resonance has been observed for weak ligands such as perchlorate in the presence of a coordinating solvent (Fe(OEP)(ClO4): δ = 35.5 [17,18]). When the concentrations of Fe(OEP)(NO3) and NaNO2 were both 1 mM, the highspin spectrum disappeared and was replaced by a low-spin ferric porphyrin spectrum.…”