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Lawrence Berkeley National Laboratory Recent Work Title Stable Platinum(IV) Corroles: Synthesis, Molecular Structure, and Room-Temperature Near-IR Phosphorescence. Permalink https://escholarship.org/uc/item/9n05q3k2 Journal ACS omega, 3(8) ISSN 2470-1343 Authors Alemayehu, Abraham B McCormick, Laura J Gagnon, Kevin J et al. Publication Date 2018-08-17 DOI 10.1021/acsomega.8b01149 Peer reviewed eScholarship.org Powered by the California Digital Library University of California This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: ACS Omega 2018, 3, 9360−9368 http://pubs.acs.org/journal/acsodf Stable Platinum(IV) Corroles: Synthesis, Molecular Structure, and Room-Temperature Near-IR Phosphorescence † ‡ ‡ § Abraham B. Alemayehu, Laura J. McCormick, Kevin J. Gagnon, Sergey M. Borisov, † and Abhik Ghosh*, † Department of Chemistry, UiTThe Arctic University of Norway, N-9037 Tromsø, Norway ‡ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8229, United States § Institute of Analytical Chemistry and Food Chemistry, NAWI Graz, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria *S Supporting Information ABSTRACT: A series of stable Pt(IV) corrole complexes with the general IV 3− formula Pt [TpXPC](m/p-C6H4CN)(py), where TpXPC is the trianion of a tris(p-X-phenyl)corrole and X = CF3, H, and CH3, has been synthesized, affording key physicochemical data on a rare and elusive class of metallocorroles. Single-crystal X-ray structures of two of the complexes revealed very short equatorial Pt−N distances of 1.94−1.97 Å, an axial Pt−C distance of ∼2.03 Å, and an axial Pt−N distance of ∼2.22 Å. The complexes exhibit Soret maxima at ∼430 nm, which are essentially independent of the meso-aryl para substituents, and strong Q bands with the most intense peak at 595−599 nm. The substituent-independent Soret maxima are consistent with an innocent PtIV−corrole3− description for the complexes. The low reduction potentials (−1.45 ± 0.08 V vs saturated calomel reference electrode) also support a highly stable Pt(IV) ground state as opposed to a noninnocent corrole•2− description. The reductions, however, are irrever- sible, which suggests that they involve concomitant cleavage of the Pt−aryl bond. Unlike Pt(IV) porphyrins, two of the IV complexes, Pt [TpXPC](m-C6H4CN)(py) (X = CF3 and CH3), were found to exhibit room-temperature near-IR phosphorescence with emission maxima at 813 and 826 nm, respectively. The quantum yield of ∼0.3% is comparable to those observed for six-coordinate Ir(III) corroles. ■ INTRODUCTION X-ray structure determination.8 Here, we report that in situ The 5d metallocorroles constitute a unique class of size- exposure of the initially formed Pt(IV)-PhCN products to pyridine leads to a new class of stable, nonradical Pt(IV) mismatched complexes that incorporate a large 5d transition- corroles with the general formula PtIV[TpXPC](m/p- metal ion within a sterically constrained macrocyclic ligand.1 C H CN)(py), which have been variously characterized with Despite a steric mismatch inherent in their structures, the 6 4 single-crystal X-ray structure determination, electrochemical See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. majority of them exhibit remarkable chemical and photo- studies, and UV−vis−NIR absorption and emission spectros- chemical stabilities. A number of them also exhibit room- Downloaded via LAWRENCE BERKELEY NATL LABORATORY on January 28, 2019 at 17:42:41 (UTC). copy (Figure 1). Although the results represent modest temperature near-IR phosphorescence,2 which has led to 3−5 progress from a synthetic viewpoint, the physicochemical applications as oxygen sensors and as photosensitizers in ff fi 6,7 measurements a ord signi cant insight into the electronic photodynamic therapy and dye-sensitized solar cells. properties of a rare and elusive class of substances. Platinum(IV) corroles, of which there has been only a single report,8 are particularly intriguing because of their potential for RESULTS AND DISCUSSION axial reactivity. They are, however, only accessible via a low- ■ yielding, serendipitously discovered reaction, which involves As mentioned above, the Pt(IV) corroles PtIV[TpXPC](m/p- · the interaction of a free-base corrole and Pt4(OAc)8 2HOAc in C6H4CN)(py) (X = CF3, H, and CH3) were obtained rather benzonitrile at high temperature. The initially formed Pt(IV) simply by the addition of pyridine to the reaction mixture at IV products, Pt [TpXPC](m/p-C6H4CN)(PhCN), where the end of the Pt insertion. For all compounds, purity and TpXPC is the trianion of a meso-tris(para-X-phenyl)corrole composition were established via thin-layer chromatography, (X = CF3, H, and CH3) and the m/p-C6H4CN group derives from the solvent (i.e., PhCN), proved unstable, but could be Received: May 26, 2018 • − derivatized to stable, paramagnetic products PtIV[TpXPC 2 ]- Accepted: August 1, 2018 (m/p-C6H4CN)(Ar), which proved amenable to single-crystal Published: August 17, 2018 © 2018 American Chemical Society 9360 DOI: 10.1021/acsomega.8b01149 ACS Omega 2018, 3, 9360−9368 ACS Omega Article Figure 1. Current status of Pt−corrole chemistry; the complexes prepared in the course of this study are schematically depicted in blue. 1 Figure 2. Representative H NMR spectrum: Pt[TpCF3PC](m-C6H4CN)(py). high-resolution electrospray ionization mass spectrometry, and the two axial ligands, m-C6H4CN and pyridine, were found to 1H NMR spectroscopy (Figures 2 and 3). Elemental analyses, occupy symmetry-equivalent sites in the crystal, each with 50% however, could not be obtained because of the very small occupancy, and were modeled such that the atoms of the two quantities available. Single-crystal X-ray structures could be six-membered rings were superimposed (Figure 4). Accord- obtained for two of the complexes, providing unambiguous ingly, the axial Pt−C/N distances for this structure only proof of structure (Table 1). Both structures revealed a Pt represent an average of the “true” Pt−C and Pt−N distances. IV atom located exactly or nearly exactly in the mean plane of a Fortunately, the second structure, Pt [TPC](m-C6H4CN)- IV planar corrole ligand. For Pt [TpCF3PC](m-C6H4CN)(py), (py) (TPC = triphenylcorrolato), was found to be fully 9361 DOI: 10.1021/acsomega.8b01149 ACS Omega 2018, 3, 9360−9368 ACS Omega Article 1 Figure 3. Representative H NMR spectrum: Pt[TpCF3PC](p-C6H4CN)(py). − − ordered (Figure 5). The structures exhibit some of the shortest potentials, which range from 1.53 V (for X = CH3)to 1.37 − IV − Pt N distances known, which for the equatorial nitrogens are V (for X = CF3), underscore the high stability of the Pt Ar 1.955 ± 0.015 Å, reflecting the sterically constrained character corrole unit toward reduction. That said, although the of 5d metallocorroles. The axial Pt−C and Pt−N distances in electrochemical HOMO-LUMO gap of 2.1 eV is typically the TPC complex are longer, 2.033(7) and 2.216(6) Å, indicative of a redox-inactive metal center and of ligand- − respectively. centered oxidation and reduction,10 13,16,36 the fact that the All six complexes exhibit slightly split Soret bands (Table 2 reduction is irreversible suggests concomitant cleavage of the and Figures 6 and 7), which are essentially unaffected by the Pt−Ar bond. para substituents on the meso-aryl groups as well as strong Q Photophysical measurements were carried out on two of the IV bands. Over a long series of studies, we have shown that such complexes, Pt [TpXPC](m-C6H4CN)(py) for X = CF3 and substituent-insensitive Soret maxima are indicative of an CH3 (Table 3 and Figures 9 and 10). Both are clearly innocent, nonradical corrole macrocycle, which is typical of phosphorescent, which was confirmed by almost complete the great majority of stable 4d and 5d metallocorroles, quenching of the emission in the presence of oxygen (Figure − including MoO,9 RuN,10 OsN,11 TcO,12 ReO,13 and Au14 17 9b,d), measurement of the decay time (Figure 10) and by corroles as well as Mo18 and W biscorroles.19 In contrast, the acquisition of luminescence excitation spectra (Figure 9a,c). IV •2− Soret maxima of the Pt [TpXPC ](m/p-C6H4CN)(Ar) The latter are essentially identical to the absorption spectra; were found to redshift dramatically in response to increasing the small deviations are due to nonlinearities ascribable to electron-donating character of the para substituent X,7 a strong absorption in the Soret region (the concentration used phenomenon that is also observed for other noninnocent was necessary for obtaining high-quality emission spectra with metallocorroles, such as MnCl,20,21 FeCl,22,23 FeNO,24,25 excitation in the Q-band). The NIR phosphorescence is rather μ 26 27−35 Fe2( -O), and Cu corroles. weak, but the quantum yields are in the same order of Cyclic voltammetry measurements were carried out for the magnitude as those observed for Ir(III) corroles.2 This meta-cyanophenyl series PtIV[TpXPC](m-C H CN)(py), observation is interesting, considering that Pt(IV) porphyr- 6 4 − which could be obtained in somewhat higher yields than the ins,37 in contrast to Pt(II) porphyrins,38 41 have been reported para series (Figure 8 and Table 2). Given the instability of the to be nonemissive.42 Weak red fluorescence (not quenchable Pt(V) state, the oxidation potentials, which range from 0.56 V by oxygen) was also clearly detected for the two compounds fl (for X = CH3) to 0.74 V (for X = CF3), may be safely assigned studied.

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