High-field (high-frequency) EPR spectroscopy and structural characterization of a novel manganese(iii) corrole

Abstract

First published as an Advance Article on the webThe X-ray structure, magnetic susceptibility, and high-field (high-frequency) EPR spectrum of manganese 5,10,15-tris-(pentafluorophenyl) corrole unambiguously establish that the complex contains an isolated, slightly rhombic, manganese(III) center.Manganese(III) porphyrins are very efficient catalysts for functionalization of hydrocarbons in processes that involve high valent intermediates. 1,2 Spectroscopic identification of odd-spin manganese(IV) and spin-coupled manganese(V) porphyrins by magnetic resonance measurements is relatively straightforward, via utilization of EPR and NMR, respectively. 3-6 However, even-spin manganese(III) porphyrins are EPR-silent (X-band) and their NMR spectra are not easily interpretable. For example, an early assignment of the origin of the paramagnetic shifts in manganese(III) porphyrins was recently revised, 7 and there is some evidence indicating that the complexes might better be described as manganese(II) porphyrin radical cations. 8 A recent development relevant to this question is the utilization of highfield (high-frequency) EPR (HF-EPR) spectroscopy for elucidation of the electronic structures of high-spin manganese(III) complexes. 9 The close relationship of porphyrins and corroles suggests that metallocorroles could also be very interesting catalysts. This proposal was only recently explored, taking advantage of the novel electron-poor 5,10,15-tris(pentafluorophenyl)corrole, H 3 tpfc, 10 whose substitution pattern matches that of the most active porphyrin-based catalysts. First, it was demonstrated that the iron and rhodium complexes of H 3 tpfc are potent catalysts for oxygen and carbene transfer to olefins and alkanes, 11 followed by full characterization of H 3 tpfc 12 and its Cr V O, Fe IV Cl and Rh III (PPh 3 ) complexes. 13,14 The manganese corrole [(tpfc)Mn] was also found to be an epoxidation catalyst, as well as an excellent precursor to a relatively stable oxomanganese(V) corrole. 1b,15 The apparent importance of this result together with the rather limited information on the electronic structures of manganese corroles 16,17 were the driving forces for the current investigations. A combination of HF-EPR spectroscopy, X-ray crystallography, and magnetic susceptibility measurements, led to the conclusion that [(tpfc)Mn(OPPh 3 )] is an authentic manganese(III) complex that does not experience significant intermolecular interactions.Obtaining X-ray quality crystals of [(tpfc)Mn] was a difficult task; NMR examination of material from different crystallization batches revealed significant variations in chemical shifts. Since we suspected that these differences were due to solvent coordination, we added external ligands to the recrystallization mixtures. These attempts finally met with success: with triphenylphosphine oxide as additive, X-ray quality crystals of [(tpfc)Mn(OPPh 3 )] were isolated. 18 The structure of [(tpfc)Mn(OPPh 3 )] (Fig. 1) is quite different from that of the previously reported square planar ma...

Constructing Single‐Chain Magnets by Supramolecular π–π Stacking and Spin Canting: A Case Study on Manganese(III) Corroles

“…The computed D value of the Mn III ion is À2.51 cm À1 , which is very close to the experimental result of À2.69 cm À1 for the similar compound [Mn(tpfc)(OPPh 3 )] with the same N 4 O coordination sphere. [36] The spin-orbit couplings of ground and excited spin state with multiplicities 2S + 1 of 5 and 3 both contributes to the overall zero-field splitting at the ratio of about 1:3. The corresponding…”

High-frequency EPR: An occasion for revisiting ligand field theory

“…Manganese(III) complexes have been increasingly studied since the availability of high frequency and high fields in recent years [5,6,9,13,15], and further work is currently in progress to gain better insights in key factors determining magnetic anisotropy. In much of the cases reported so far the parameters obtained by simulations of HFEPR spectra have been explained on the basis of simple considerations based on the foundations of ligand field theory outlined in the previous paragraph.…”

“…Some papers have now appeared where the HFEPR spectra of some so-called EPR-silent ions have been reported [5][6][7][8][9][10][11][12][13][14][15]. For their interpretation ligand field techniques have been mainly used.…”

Recent Advances in the Synthesis of Corroles and Core-Modified Corroles

“…Simple and efficient synthetic methods have already opened a new avenue for in-depth studies of the coordination chemistry of corroles. A number of complexes of meso-as well as β-substituted corroles with Fe III , [11,12] Fe IV , [12Ϫ15] Co III , [13,16Ϫ18] Mn III , [19,20] Mn IV , [21,14] Mn V , [19,21] Cr V , [22Ϫ24] Ru III , [25] Rh III , [13,12] As III and As V , [26] Sb III and Sb IV , [26] Bi III and Bi IV , [26] Ge IV , [12] Sn IV , [12] and P V [12,27] were described within a short period of time. [24,28] Stabilization of exceptionally high metal oxidation states is relevant to catalysis by heme enzymes and synthetic (porphyrin)iron in which Fe IV complexes are key intermediates.…”