An intermediate-spin iron(III) porphyrin complex with a vinylidene group inserted into a iron-nitrogen bond: paramagnetic susceptibility, EPR, and Moessbauer properties

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Mössbauer study of the three-coordinate planar Fe(II) thiolate complex [Fe(SR)(3)](-) (R = C(6)H(2)-2,4,6-tBu(3)): model for the trigonal iron sites of the MoFe(7)S(9):homocitrate cofactor of nitrogenase

The cofactor (M-center) of the MoFe protein of nitrogenase, a MoFe(7)S(9):homocitrate cluster, contains six Fe sites with a (distorted) trigonal sulfido coordination. These sites exhibit unusually small quadrupole splittings, Delta E(Q) approximately 0.7 mm/s, and isomer shifts, delta approximately 0.41 mm/s. Mössbauer and ENDOR studies have provided the magnetic hyperfine tensors of all iron sites in the S = 3/2 state M(N). To assess the intrinsic zero-field splittings and hyperfine parameters of the cofactor sites, we have studied with Mössbauer spectroscopy two salts of the three-coordinated Fe(II) thiolate complex [Fe(SR)(3)](-) (R = C(6)H(2)-2,4,6-tBu(3)). One of the salts, [Ph(4)P][Fe(SR)(3)] x 2MeCN x C(7)H(8), 1, has a planar geometry with idealized C(3h) symmetry. This S = 2 complex has an axial zero-field splitting with D = +10.2 cm(-1). The magnetic hyperfine tensor components A(x) = A(y) = -7.5 MHz and A(z) = -29.5 MHz reflect an orbital ground state with d(z(2)) symmetry. A(iso) = (A(x) +A(y) +A(z))/3 = -14.9 MHz, which includes the contact interaction (kappa P = -21.9 MHz) and an orbital contribution (+7 MHz), which is substantially smaller than A(iso) approximately -22 MHz of the tetrahedral Fe(II)(S-R)(4) sites of both rubredoxin and [PPh(4)](2)[Fe(II)(SPh)(4)]. The largest component of the electric field gradient (EFG) tensor is negative, as expected for a d(z(2)) orbital. However, Delta E(Q) = -0.83 mm/s, which is smaller than expected for a high-spin ferrous site. This reduction can be attributed to a ligand contribution, which in planar complexes provides a large positive EFG component perpendicular to the ligand plane. The isomer shift of 1, delta = 0.56 mm/s, approaches the delta-values reported for the six trigonal cofactor sites. The parameters of 1 and their importance for the cofactor cluster of nitrogenase are discussed.

Five-coordinate iron(III) porphycenes: (1)H NMR, magnetic, and structural studies

Five-coordinate iron(III) 2,7,12,17-tetrapropylporphycene (TPrPc)Fe(III)X (X = C(6)H(5)O(-), Cl(-), Br(-), I(-), ClO(4)(-)) complexes have been investigated. The (1)H NMR spectra demonstrate downfield shifts for pyrrole resonances [(TPrPc)Fe(III)(C(6)H(5)O), 65.3 ppm; (TPrPc)Fe(III)Cl, 28.5 ppm] but large upfield ones for (TPrPc)Fe(III)Br (-7.8 ppm), (TPrPc)Fe(III)I (-49.4 ppm), and (TPrPc)Fe(III)ClO(4) (-77.1 ppm) (294 K, CD(2)Cl(2)). The pyrrole chemical shifts span the remarkable +70 to -80 ppm range. The variable-temperature (1)H NMR spectra of (TPrPc)Fe(III)X demonstrate anti-Curie behavior with a sign reversal for (TPrPc)Fe(III)Cl. These behaviors are consistent with the admixed S = 3/2, 5/2 ground electronic state with a dominating contribution of the S = 3/2 one. In terms of the chemical shift, (TPrPc)Fe(III)(ClO(4)) can be considered as an example of the purest S = 3/2 state in the investigated series. The extent of the S = 5/2 contribution in the admixed S = 3/2, 5/2 ground electronic state, as gradated solely the basis of the pyrrole proton paramagnetic shifts, is controlled by the strength of the axial ligand, following the magnetochemical series (Evans, D. R.; Reed, C. A. J. Am. Chem. Soc. 2000, 122, 4660). Significantly iron(III) 2,7,12,17-tetrapropylporphycene, soluble in typical organic solvents, can be considered as a universal framework to classify the ligand strength in a magnetochemical series, consistently using the beta-H pyrrole paramagnetic shifts as a fundamental criterion. The structure of (TPrPc)Fe(III)Cl has been determined by X-ray crystallography. The iron is five-coordinate with bonds of nearly equal length to the four pyrrole nitrogen atoms (Fe-N in the range 1.983(5)-2.006(6) A). The iron lies 0.583(1) A out of the mean plane of the macrocycle and 0.502(5) A out of the mean N(4) plane. In the solid, pairs of molecules are positioned about the center of symmetry so there is face-to-face pi-pi contact. The mean plane separation is 3.38 A, and the lateral shift of the porphycene center along the Fe-N bond is 4.490 A. The distance from one porphycene center to the other is 5.62 A, and the iron-iron separation is 6.304(2) A.

An integrated approach to the mid-spin state (S = 3/2) in six-coordinate iron(III) chiroporphyrins

An intermediate-spin state very close to the mid-spin state (S = 3/2) can be stabilized in a ferric porphyrin by an integrated approach which combines the favorable effects of a weak axial field strength and of a small macrocycle hole. Axial ligand exchange by reaction of chloroiron(III)tetramethylchiroporphyrin [(TMCP)FeCl] with silver perchlorate in ethanol-chloroform leads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline products containing a bisethanol complex [[(TMCP)FeIII(EtOH)2]ClO4] and three variants of a mixed ethanol-water complex [[(TMCP)FeIII(EtOH)(H2O)]ClO4] have been structurally characterized in the solid state. The small hole of the ruffled chiroporphyrin and the weak axial oxygen ligation result in strongly tetragonally distorted complexes. The six-coordinate species exhibit long axial Fe-O bond distances (2.173(5)-2.272(4) A) and the shortest equatorial Fe-N(av) distances (1.950(5)-1.978(7) A) found as yet in a ferric porphyrin, reflecting a singly occupied dz2 orbital and a largely depopulated dx2-y2 orbital. An intriguing case of bond-stretch isomerism is seen for the axial Fe-O bonds in two crystallographically independent mixed ethanol-water species, and it is accounted for by their distinct intra- and intermolecular hydrogen-bond arrays. The Mössbauer spectrum (delta = 0.35(1) mm s-1 and delta EQ = 3.79(1) mm s-1 at 77 K) indicates a strong tetragonal distortion around the ferric ion, in agreement with the structural data. The value of the magnetic moment (mu eff = 3.8 mu B in the range 50-300 K) strongly supports a mid-spin state (S = 3/2). The EPR spectrum at 80 K (g perpendicular approximately 4.0, g parallel approximately 2.00) is consistent with a nearly pure mid-spin state (4A2) with little rhombic distortion. The 1H NMR spectra in CDCl3-EtOH exhibit upfield-shifted resonances for the pyrrole protons (delta approximately -30 ppm) which are consistent with the depopulated iron dx2-y2 orbital. Solution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussed on the basis of the NMR data. The bisethanol and ethanol-water species are potential models of unknown hemoprotein ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obtained by site-directed mutagenesis.

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

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.