High-valent corrole metal complexes

Abstract

This commentary concentrates on corrole complexes with the three metal ions that are most relevant to oxidation catalysis: chromium, manganese, and iron. Particular emphasis is devoted to the only recently introduced meso-triarylcorroles and a comparison with the traditionally investigated b-pyrrole-substituted corroles. Based on a combination of spectroscopic methods, electrochemistry, and X-ray crystallography, it is concluded that in most high-valent metallocorroles the corrole is not oxidized. Both experimental (for (oxo)chromium(V) corrole) and computational (for (oxo)manganese(V) corrole) evidence indicate that the stabilization of high-valent metal ions by corroles originates from a combination of short metal-nitrogen bonds and large metal out-of-plane displacements in the corrole, which lead to quite unexpected interactions of the oxo-metal p* orbitals with the in-plane orbitals of the corrole.Keywords Corrole á Iron á Manganese á Chromium á Cation radicals Corroles may be considered as the non-natural analogs of the cobalt-coordinating corrin ring in Vitamin B 12 with which they share an identical skeleton, or as one meso-carbon short porphyrins with whom they share aromaticity (Scheme 1) [1]. Corrole chemistry, which started in 1964 [2], also led to the ®rst structural characterization of a free-base corrole by Hodgkin and coworkers in 1971 in course of their Vitamin B 12 project [3]. The main research eorts during the years to come were devoted to the synthesis of corroles [4] and to investigation of the coordination chemistry of their metal complexes [5,6]. Still, compared to porphyrins, the research activity in corrole chemistry remained very low. For example, a computer search of``article titles, keywords, or abstract'' in the ISI database for 1998 produced 1143 hits for porphyrin*, but only 10 hits for corrol*. There was also no reported application of corroles or their metal complexes in any scienti®c journal or in patent databases up to 1999, and it took 28 years to obtain the second ever X-ray crystal structure of a metalfree corrole [7]. This situation is however to experience a major change, as much more ecient and simple methodologies for the preparation of corroles are constantly being introduced (for the most recent developments that are not covered by the review [4], see [7,8,9]).One main dierence between the corrin, porphyrin, and corrole macrocycles (Scheme 1) is the number of ionizable protons in their N 4 coordination core, which is 1, 2, and 3, respectively. Accordingly, in their coordination complexes they act as mono-, di-, and trianionic ligands, respectively. This rather naive distinction is actually of importance, as may be appreciated by the following facts. The catalytic action of Vitamin B 12 relies on stabilization of cobalt(I) by the monoanionic corrinate ligand, the most common oxidation states of porphyrin-supported metals are +2 and +3, and the trianionic corrolate ligand supports unusually high metal oxidation states. Within this commentary we will examine the coordinati...

The Electronic Structure of Iron Corroles: A Combined Experimental and Quantum Chemical Study

“…This can be traced back to the exceptionally strong metal-ligand s interactions in corrole systems. [14] They render the Fe d x 2 Ày 2 orbital too high in energy to be occupied and thus yield an intermediate-spin state for the iron center (S Fe = 3/2) in the electronic ground state. By contrast, this orbital is singly occupied in the corresponding porphyrin system in which the iron center locally exists in a high-spin state (S Fe = 5/2).…”

“…[13] One apparently unresolved issue, however, is the assignment of the electronic states in corrole-metal complexes, the stable oxidation states of which are almost invariably higher by one unit than those of analogous metalloporphyrins. [14,15] A particularly intense debate has centered around complexes with the general formula of [FeClA C H T U N G T R E N N U N G (Cor)], (5 a-8 a; Scheme 2).…”

Experimental and Theoretical Investigations of the Existence of CuII, CuIII, and CuIV in Copper Corrolato Complexes

“…[11] In many cases,u nusually high oxidation states that are unlikely to be stabilized by other ligand environments are found to be stable inside the corrolato cavity.T he trianionic nature of the corrolato ligand is certainly helpful for this purpose.Some of the higher oxidation states of metals that have been stabilized in corrolato environments include Cu ,and Cr VI . [12] We have recently presented unambiguous experimental and theoretical evidence for the existence of Ag II and Ag III within the same corrolato framework. [13] In the following,we present the synthesis of three new copper complexes [5,10,15- An approach combining synthetic, electrochemical, UV/ Vis/NIR/EPR spectroelectrochemical, crystallographic,a nd DFT studies has been adapted to establish the origin of Cu II , Cu III ,a nd Cu IV within the three-step redox series of these complexes.…”

Recent Advances in the Synthesis of Corroles and Core-Modified 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. The Gross group has examined iron, manganese, and rhodium complexes as catalysts for epoxidation, [19,21,29] cyclopropanation, [12,29] hydroxylation, [29] and aziridination [15] with very encouraging results.…”

“…The area prior to 1999 has been widely reviewed [1] and a commentary about metal complexes of triarylcorroles has also recently been published. [24] Historical and synthetic aspects to that date are well covered, so this review will focus on synthetic corrole chemistry over the last three years. The striking synthetic advances lead to the view that examination of many novel aspects of the coordination chemistry of (corrole)metal complexes will quickly become more accessible, especially with respect to high metal oxidation states.…”