10‐Heterocorroles: Ring‐Contracted Porphyrinoids with Fine‐Tuned Aromatic and Metal‐Binding Properties

Abstract

A current trend in porphyrinoid chemistry is the development of ring-contracted macrocycles and the study of their metal complexes. [1] Corroles clearly play a major role in this field, particularly with respect to species containing a porphyrinlike N 4 metal binding site. [2] However, many other noncontracted porphyrinoids, such as the porphyrin isomers and the azaporphyrins, are in fact also characterized by a smaller N 4 cavity. [3] Reports from coordination compounds of such macrocycles clearly prove the importance of the size of the metal binding site for spectacular results in terms of electronic structure, [4] small-molecule binding, [5] and catalysis. [6] Conceptually, the class of 10-heterocorroles can be regarded as intermediate between porphyrins (dianionic ligand) and corroles (ring-contracted ligand; Scheme 1). 10-Heterocorroles with an oxygen, sulfur, or nitrogen atom in the backbone have been known for a long time, [7] and have recently gained renewed interest. [8] However, the literature procedures to synthesize these macrocycles are either long and cumbersome, or yield chelate complexes of Group 10 elements which cannot be demetalated without disintegration of the porphyrinoid ligand.We are interested in free-base 10-heterocorroles with a Group 16 element (O, S, Se) in the backbone, as such ligands may allow a fine-tuning of the electronic properties of the transition metal through the size of the N 4 cavity. For this purpose, a general and efficient entry into this class of porphyrinoids (or better: corrinoids) was needed. We report here the successful syntheses and ligand properties of freebase 10-oxa-, 10-thia-, and 10-selenacorroles.The known synthetic protocols leading to 10-heterocorroles are only of limited use for a general preparative entry into this class of porphyrinoids, as they either require a more than stoichiometric amount of supported palladium as a reagent, or a thioether precursor, the oxygen and selenium homologues of which are unavailable. To overcome these drawbacks we attempted to develop a metal-promoted twostep macrocyclization approach from a,w-dibrominated dipyrrins such as 1 (Scheme 2). These precursors were first coupled to bromine-terminated linear tetrapyrrole copper(II) chelates by using a nBuLi/Cu II protocol, and then cyclized in a second step to the desired copper(II) complexes such as 2-4, by using different nucleophilic O, S, or Se transfer reagents, respectively. During the course of our investigations it Scheme 1. 10-Heterocorroles H 2 (XCor) as hybrid structures from porphyrin and corrole frameworks (with the 18p main conjugation pathways shown in bold).Scheme 2. Syntheses of 10-heterocorroles H 2 L 5-7 via the copper(II) complexes 2-4; a) Cu(OAc) 2 ·H 2 O, wet DMF, 110 8C, 15 min; b) Cu(OAc) 2 ·H 2 O, DMF, 5 min, then Na 2 S·9 H 2 O, 110 8C, 15 min, then TFA, toluene, 110 8C, 3 h; c) Cu(OAc) 2 ·H 2 O, THF, 15 min, then KSeCN, 65 8C, 12 h; d) SnCl 2 ·2 H 2 O, HCl(aq), acetonitrile/dichloromethane, 15 min. TFA = trifluoroacetic acid.

Metal‐Assisted One‐Pot Synthesis of Isoporphyrin Complexes

“…Thes lightly rhombic X-band EPR spectrum of this copper chelate (71 K; see the Supporting Information) shows g values of g 1 = 2.217, g 2 = 2.049, and g 3 = 2.048, and hyperfine coupling parameters A 1 (Cu) = 177 G, A 2 (Cu) = 15 G, and A 3 (Cu) = 15 G. Ac omparison of these values with those of copper complexes of tetraarylporphyrins and of several ring-contracted porphyrinoids clearly demonstrates the presence of the spacious isoporphyrin N 4 cavity through the large g 1 and the small A 1 (Cu) values. [25] This new option for transmetalation reveals new facets of the coordination chemistry of the intriguing isoporphyrin ligand.…”

meso‐Pyrrole‐Substituted 22‐Oxacorroles: Building Blocks for the Synthesis of BODIPY‐Bridged 22‐Oxacorrole Dyads

“…4 Core‐modified corroles, resulting from the replacement of one of the pyrroles of corrole with heterocycles such as thiophene5 and furan,6–8 possess distinct properties that are different from aza analogues. However, because of a lack of proper synthetic protocols and instability of heterocorroles,9–11 particularly core‐modified oxa and thia corroles,5–8 the chemistry of these corroles is still at an early stage of development. Recently, we reported the synthesis of meso ‐pyrrole‐substituted 22‐oxacorroles by a “3+2” condensation of 16‐oxatrypirrane and meso ‐aryl dipyrromethane under acid‐catalyzed conditions, followed by oxidation with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) 12.…”

Heterocorrole Conformations: Little Saddling, Much Ruffling

“…[6] Later contributions further supported the prediction, mainly by additional structural insight from four-coordinate silver and gold corroles. [7] 10-Heterocorroles [12] offer the opportunity to vary the size of the binding cavity while maintaining comparable electronic and steric conditions.T his feature is established by using heteroatoms of Group 16 elements [9] as the connecting bridge in the 10-position ( Figure 2). Thes tepwise decrease in the atomic radius of the heteroatom bridge enforces the reduction of the N 4 cavity from the large 10-selenacorrole via the medium-sized 10-thiacorrole to the small 10-oxacorrole.W e have now studied the adaptation of 10-heterocorrole ligands to divalent ions of different sizes,n amely,t oNi( ionic radius 63 pm), Cu (71 pm), and Pd (78 pm), and found that the unexpected ruffling modes are present in the corrole series, and that they even dominate the out-of-plane conformation modes for most heterocorrole complexes.…”

“…Nickel-, copper-, and palladium complexes 1-9 were prepared by stirring the free-base ligand [9] with the metal acetate in am ethanol/chloroform mixture ( Figure 3). Analytically pure crystalline material was obtained for all compounds after recrystallization in 83-92 %yield.…”

“…[13] Thec rystals of nickel and palladium 10-oxacorroles 1 and 7 showed severe [8] for porphyrin, corrole, and 10-heterocorroles. [9][10][11] disorder,a nd the structures could not be refined satisfactorily.C ocrystallization with one equivalent of tetracyanoquinodimethane (TCNQ), however, gave wellordered crystals of both compounds and good crystallographic data. Selected details of the molecular structures of 1-9 are summarized in Table 1.…”