Reversal of H₂O and OH^- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes

Abstract

Contrary to expectations based on the spectrochemical series, H 2 O is found to be a significantly weaker field ligand than OH -in the magnetochemical series ranking of ligand field strengths based on the spin states of iron(III) tetraphenylporphyrin complexes. The preparation and characterization of the [Fe(H 2 O)-(TPP)] + ion and the spectroscopic identification of Fe(OH)(TPP) have made this assessment possible. These two species were previously thought to be unattainable because of the facile formation of the well-known µ-oxo dimer, (TPP)Fe-O-Fe(TPP). However, the special characteristics of single equivalents of water under high acidity, relevant to metalloenzyme active sites and superacidity, make them accessible in benzene solution. Their 1 H NMR -pyrrole chemical shifts at -43 and +82 ppm indicate admixed-intermediate S ) 3 / 2 , 5 / 2 and high S ) 5 / 2 spin states for the aqua and hydroxo species, respectively. The X-ray crystal structure of the aqua complex has been determined for [Fe(H 2 O)(TPP)][CB 11 H 6 Cl 6 ] and is consistent with the high degree of S ) 3 / 2 character indicated by the NMR measurement, Mössbauer spectroscopy (∆E q ) 3.24 mm‚s -1 ), and magnetic susceptibility (µ eff ) 4.1 µ B ). The anhydrous precursor to these species is the "nearly bare" iron(III) porphyrin complex Fe(CB 11 H 6 Br 6 )(TPP). Judged by its magnetic parameters (δ pyrrole ) -62 ppm, ∆E q ) 3.68 mm‚s -1 , µ eff ) 4.0 µ B ) it attains the long sought essentially "pure" S ) 3 / 2 spin state. The magnetochemical ranking of ligand field strengths in five-coordinate high-spin and admixed-intermediate-spin iron(III) porphyrins is useful because it more closely reflects the intuitive field strengths of crystal field theory than does the usual spectrochemical ranking, which is controlled largely by π effects in octahedral low-spin d π 6 complexes.

A Four-Coordinate Fe(III) Porphyrin Cation

“…4,6,7 On the basis of the β-pyrrole proton NMR isomer shift, Reed has established a magnetochemical series for different anions. 8 In summary, the first four-coordinate Fe(III) porphyrin complex has been isolated and fully characterized in both solution and solid state and shows a pure S = 3 / 2 intermediate spin state.…”

Synthesis, characterization, and luminescence properties of mononuclear copper(I) halide complexes of 2-(2-benzimidazolyl)-6-methylpyridine

“…It is interesting to note that only the Cu\N pyridyl distance depends on the halide ancillary ligand, whereas the Cu\N imidazolyl bond length is largely identical in 1-3, implying that the halide probably has certain effect on only the pyridyl unit, and not the entire Hbmp ligand. Additionally, it is noted that the Cu\N pyridyl (2.168 Å) and Cu\P (2.203 Å) distances in 1 are longer than the corresponding values (2.141 and 2.1839 Å) of 2, slightly shorter than those (2.176 and 2.193 Å) of 3, which most likely originates from both the increasing ligand field strength (I − b Br − b Cl − ) and the decreasing electrondonating nature (I − >Br − >Cl − ) of the halide [26]. The Cu\X distances of 1-3 elongate with an increase in the van der Waals of X.…”

Spin–spin interactions in iron(III) porphyrin radical cations with ruffled and saddled structure

“…If field strength of axial ligand is extremely weak, they adopt even intermediate-spin (S = 3/2) state [3]. One electron oxidation of iron(III) porphyrins produces either iron(IV) poprphyrins or iron(III) porphyrin radical cations depending mainly on the nature of axial ligands [4].…”