Abstract
Metallocorroles involving 5d transition metals are currently of interest as near-IR phosphors and as photosensitizers for oxygen sensing and photodynamic therapy. Their syntheses, however, are often bedeviled by capricious and low-yielding protocols. Against this backdrop, we describe rhenium-imido corroles, a new class of 5d metallocorroles, synthesized simply and in respectable (∼30%) yields via the interaction of a free-base corrole, Re2(CO)10, K2CO3, and aniline in 1,2,4-trichlorobenzene at ∼190 °C in a sealed vial under strict anaerobic conditions. The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H3[TPFPC], and five different meso-tris(p-X-phenyl)corroles, H3[TpXPC], where X = CF3, F, H, CH3, OCH3. Single-crystal X-ray structures obtained for two of the complexes, Re[TpFPC](NPh) and Re[TpCF3PC](NPh), revealed relatively unstrained equatorial Re–N distances of ∼2.00 Å, a ∼ 0.7-Å displacement of the Re from the mean plane of the corrole nitrogens, and an Re–Nimido distance of ∼1.72 Å. Details of the corrole skeletal bond distances, diamagnetic 1H NMR spectra, relatively substituent-independent Soret maxima, and electrochemical HOMO–LUMO gaps of ∼2.2 V all indicated an innocent corrole macrocycle. Surprisingly, unlike several other classes of 5d metallocorroles, the Re-imido complexes proved nonemissive in solution at room temperature and also failed to sensitize singlet oxygen formation, indicating rapid radiationless deactivation of the triplet state, presumably via the rapidly rotating axial phenyl group. By analogy with other metal-oxo and -imido corroles, we remain hopeful that the Re-imido group will prove amenable to further elaboration and thereby contribute to the development of a somewhat challenging area of coordination chemistry.
Highlights
The 5d metallocorroles are an unusual class of complexes that encapsulate a large 5d transition metal ion within a sterically constrained macrocyclic ligand.[1,2] Unsurprisingly in view of the structural mismatch inherent in their structures, their syntheses require highly specific conditions of reagent, solvent, and temperature and typically afford products in poor yields
The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H3[TPFPC], and five different meso-tris(p-X-phenyl)
The formation of the desired phenylimido complexes seemed immediately plausible upon mass spectrometric analysis of the crude products, an inference that was soon bolstered by 1H and 19F NMR spectroscopy
Summary
The 5d metallocorroles are an unusual class of complexes that encapsulate a large 5d transition metal ion within a sterically constrained macrocyclic ligand.[1,2] Unsurprisingly in view of the structural mismatch inherent in their structures, their syntheses require highly specific conditions of reagent, solvent, and temperature and typically afford products in poor yields. Even the best conditions for gold insertion are capricious, unusually sensitive to impurities, and afford yields of only about 20%,3−6 while the yields for platinum insertion are considerably worse,[7,8] well under 5%. These shortcomings notwithstanding, the 5d metallocorroles are of considerable interest. A number of them exhibit room-temperature near-IR phosphorescence,[9−14] which has led to applications as oxygen sensors[10−12] and as photosensitizers in photodynamic therapy[13,15] and dye-sensitized solar cells.[13] promising in this regard are the rhenium(V)-oxo corroles, which are accessible both and in fairly high yields.[16] Presented are a new class of 5d metallocorroles rhenium-imido corroles which have been synthesized in respectable yields. Like MoVO,[17] ReVO,[16] and Ta(V)-imido[18] corroles, the present complexes may serve as starting materials for new axial ligation chemistry, an aspect of 5d metallocorroles that is still in its infancy.[19,20]
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