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. 2023 May 23;62(22):8467–8471. doi: 10.1021/acs.inorgchem.3c00632

Rhenium Biscorrole Sandwich Compounds: XAS Evidence for a New Coordination Motif

Abraham B Alemayehu †, Macon Jedediah Abernathy ‡, Jeanet Conradie †,§, Ritimukta Sarangi ‡,*, Abhik Ghosh †,*
PMCID: PMC10245377  PMID: 37219484

Abstract

graphic file with name ic3c00632_0005.jpg

The interaction of three free-base meso-tris(p-X-phenyl)corroles H3[TpXPC] (X = H, CH3, OCH3) with Re2(CO)10 at 235 °C in the presence of K2CO3 in o-dichlorobenzene has led to putative rhenium biscorrole sandwich compounds with the formula ReH[TpXPC]2. Density functional theory calculations and Re L3-edge extended X-ray absorption fine structure measurements suggest a seven-coordinate metal center, with the “extra” hydrogen located on one of the corrole nitrogens. The complexes can be deprotonated by a base such as 1,8-diazabicyclo[5.4.0]undec-7-ene, resulting in a substantial sharpening of the UV–vis spectra and split Soret bands, consistent with the generation of C2-symmetric anions. Both the seven-coordinate neutral and eight-coordinate anionic forms of the complexes represent a new coordination motif in the field of rhenium–porphyrinoid interactions.

Short abstract

The high-temperature interaction of free-base meso-triarylcorroles with Re2(CO)10 has led to putative rhenium biscorrole sandwich compounds with the formula ReH[Cor]2 (Cor = corrole). Density functional theory calculations and Re L3-edge extended X-ray absorption fine structure measurements suggest a seven-coordinate metal center, with the “extra” hydrogen residing on one of the corrole nitrogens.

The interaction of rhenium with porphyrin-type ligands has resulted in a growing variety of coordination motifs in recent years.1,2 Thus, porphyrins3 and related ligands (such as sapphyrin4,5 and triphyrin6) act as tridentate ligands toward the [Re(CO)3]+ fragment to yield unusual “capped” organometallic complexes that obey the 18-electron rule. Likewise, 99Tc(CO)3-capped porphyrin derivatives are also well-known.7 Pentavalent rhenium oxo8 and rhenium nitrido9 porphyrins have also been known for decades. The next major developments came in the 1980s in the form of monomeric rhenium(II) porphyrins10 and metal–metal triple-bonded rhenium(II) porphyrin dimers.11,12 Rhenium corroles are of more recent provenance: although one was serendipitously isolated some time ago,13 a general synthetic route to ReVO corroles14 (as well as to 99TcO corroles15) emerged only a few years ago.16 The simplicity of latter route has also allowed for peripheral functionalization of ReVO corroles via electrophilic aromatic substitution reactions such as halogenation17 and formylation.18 Another exciting, recent development has been the synthesis of metal–metal quadruple-bonded rhenium corrole dimers.19 Detailed electrochemical20 and density functional theory (DFT)21 studies of these complexes have yielded a host of new insights into metal–metal quadruple bonding. Herein we provide strong spectroscopic evidence for yet another coordination motif for rhenium in the form of rhenium biscorrole sandwich compounds with the molecular formula ReH[TpXPC]2, where TpXPC denotes a generic meso-tris(p-X-phenyl)corrole. One of the corroles in these complexes is thought to be monoprotonated, which results in a seven-coordinate rhenium(V) center.

Rhenium biscorrole sandwich compounds, ReH[TpXPC]2, were serendipitously discovered as we attempted to optimize the current synthetic protocol for metal–metal quadruple-bonded rhenium corrole dimers.19 Heating Re2(CO)10 and free-base corroles H3[TpXPC] (X = H, CH3, OCH3) in the presence of a base in 1,2-dichlorobenzene at 150–190 °C resulted in the exclusive formation of ReV[TpXPC](O).14 Increasing the temperature to 190–220 °C resulted in the formation both ReVO corroles and metal–metal quadruple-bonded rhenium corrole dimers {Re[TpXPC]}2.19 Given the clear role of temperature in determining the product profile, we reasoned that the use of even higher temperatures might lead to higher yields of rhenium corrole dimers. Increasing the temperature to 235 °C (Caution!), to our surprise, led to yet a third product in addition to ReVO corrole and the rhenium corrole dimer. High-resolution electrospray ionization mass spectrometry (HR-ESI-MS) in the positive mode indicated the molecular formula ReH[TpXPC]2 for the new products, which, gratifyingly, could be isolated for each of the three corrole ligands examined (Figure 1).

Figure 1.

Figure 1

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Summary of the interactions between rhenium and corroles as currently elucidated. Inset: this work.

As for molybdenum and tungsten biscorrole complexes,22−24 the exceedingly crowded 1H NMR spectra of the new compounds, aside from confirming their diamagnetic nature, proved indecipherable. No hydridic protons were observed, and the NH proton signals in general were also found to be broad and not readily discernible. For one compound, ReH[TpOCH3PC]2, natural abundance 15N–1H heteronuclear single quantum coherence did lead to plausible identification of the NH proton to a broad singlet at −0.64 ppm. All three complexes also failed to yield single-crystal X-ray structures. Fortunately, mass spectrometry, optical and X-ray absorption (XAS) spectroscopies, and DFT modeling studies25−27 paint a fair picture of the structure and nature of the compounds. As for molybdenum and tungsten biscorroles,22,23 the Soret maxima of ReH[TpXPC]2 in dichloromethane and toluene proved to be significantly blue-shifted relative to those of free-base corroles, indirectly lending support to the sandwich formulation (Figure 2). Unusually for electronically innocent metallocorroles,16 the Soret maxima were found to exhibit significant redshifts with increasing electron-donating character of the substituent X, a behavior typically observed for noninnocent metallocorroles.27,28 We tentatively ascribe this observation to the unusual geometry of the complex and to charge-transfer character involving the empty Re 5dz2 orbital. Upon stirring with 1,8-diazabicyclo[5.4.0]undec-7-ene in anhydrous toluene, the UV–vis spectra sharpened dramatically, consistent with the formation of a C2-symmetric {Re[TpXPC]2}− anion. The UV–vis spectra of the putative anion are characterized by a deeply split Soret band, with the main feature at ∼438 nm and a prominent, left shoulder at ∼368 nm for X = H and CH3 and at ∼379 nm for X = OCH3, again reflecting a significant substituent effect. Importantly, HR-ESI-MS analysis of the putative anions (in methanolic solution) in the negative mode revealed a molecular ion with a mass 1 Da lower than that observed for the neutral compounds (see the Supporting Information).

Figure 2.

Figure 2

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UV–vis spectra of (a) ReH[TpXPC]2 and (b) putative {Re[TpXPC]2}− (X = H, CH3, and OCH3) anions in anhydrous toluene.

To determine the most probable location of the “extra” hydrogen in the neutral sandwich compounds, all-electron scalar-relativistic DFT (OLYP29,30-D331/ZORA-STO-TZ2P, as implemented in the ADF program system32) geometry optimizations were carried out on a variety of potential tautomeric forms of ReH[TPC]2. Assuming an approximate square antiprism of the corrole nitrogens (with the corrole rings rotated approximately 135° relative to each other22,23), the global minimum appears to be an nitrogen-protonated tautomer, where the protonated nitrogen belongs to one of the pyrrole rings distal with respect to the direct pyrrole–pyrrole linkage (Figure 3). A rhenium-protonated form could not be located as a local minimum because it spontaneously evolved to the global minimum over the course of the geometry optimization. meso-Carbon-protonated forms were also found to be >1.5 eV above the global minimum and so were not considered realistic contenders for the actual structure.

Figure 3.

Figure 3

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All-electron OLYP-D3/ZORA-STO-TZ2P-optimized geometry of ReH[TPC]2: (a) side and top views; (b) selected distances (Å).

To obtain experimental support for the DFT-derived structure, Re L3-edge XAS and extended X-ray absorption fine structure (EXAFS) measurements were performed on ReH[TPC]2 (several XAS studies of metalloporphyrins and metallocorroles have been reported in recent years33−42). The Re L3-edge absorption of ReH[TPC]2 was found to be blue-shifted by 2.5 eV relative to the rhenium foil, as assessed by the first derivatives of the absorption features, consistent with the significantly oxidized state of the metal in the complex (Figure S4). The nonphase-shift-corrected EXAFS data (inset), the corresponding Fourier transform, and the best fit are presented in Figure 4. Two qualitative observations may be made from the Fourier transform data: (a) the first shell is split into multiple “subshells” and (b) single- and multiple-scattering contributions from the corrole rings dominate in the ∼2.8–3.2 Å region. FEFF fits to the data reveal a seven-coordinate first shell with 1 Re–N ∼ 1.86 Å, 2 Re–N ∼ 2.37 Å, and 4 Re–N ∼ 2.55 Å (Table 1). The peaks at and below 1 Å in the Fourier transform are artifacts of the background subtraction process in which the normalized XAS data are splined to minimize oscillatory components with periods corresponding to unphysically short metal–ligand bond distances and do not indicate rhenium–ligand backscattering interactions. The longer distance Re–N paths are correlated with one another and with the Re–C single-scattering contributions from the corrole ligand (Table 1). The Fourier transform intensity in the range R′ ∼ 3.2–5.0 Å has multiple contributions from the two corroles. In the fit presented here (Figure 4), this long-range intensity was simulated using two multiple-scattering paths from the corrole (Re–C–N) ring. That said, these paths account for less than 10% of the overall intensity and do not impact the first-shell distances reported in Table 1. Attempts to model the first shell with a total of 8 Re–N distances (split over two or three-shells) resulted in statistically worse fits. The results suggest that ReH[TPC]2 is best described as having a heterogeneous first shell with a total of 7 Re–N interactions.

Figure 4.

Figure 4

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Nonphase-shift-corrected Fourier transforms of the Re L3-edge EXAFS data for ReH[TPC]2: data (black); fit (red). The inset shows the EXAFS comparison.

Table 1. EXAFS Least-Squares Fitting Results for ReH[TPC]2.

path/shell R (Å)a σ2 (Å2)b ΔE0 (eV) Fc
1 Re–N 1.86 247 –3.40 0.09
2 Re–N 2.37 425    
4 Re–N 2.55 328    
5 Re–C 2.79 637    
10 Re–C–N 3.17 462    
24 Re–C–N 4.93 1475    
24 Re–C–N 5.16 1121    
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a

The estimated standard deviations for the distances are on the order of ±0.02 Å.

b

The σ2 values are multiplied by 105.

c

The error is given by ∑[(χobsd – χcalcd)2k6]/∑[(χobsd)2k6]. The S02 factor was set to 1.

In summary, the high-temperature interaction of Re2(CO)10 with three free-base meso-triarylcorroles has led to the isolation of neutral rhenium corrole sandwich compounds with the molecular formula ReH[TpXPC]2, a heretofore unprecedented coordination motif in the field of rhenium–porphyrinoid interactions.1,2 DFT calculations and Re L3-edge EXAFS studies support a seven-coordinate rhenium center and one loosely interacting NH group. The extra hydrogen can be removed by a base, resulting in dramatically sharper UV–vis spectra consistent with C2-symmetric {Re[TpXPC]2}− anions. Structural analyses of both the neutral and anionic forms remain a key goal for the future.

Acknowledgments

This work was supported in part by Grant 324139 of the Research Council of Norway (to A.G.) and Grants 129270 and 132504 of South African National Research Foundation (to J.C.). Use of the SSRL, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Contract DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE, Office of Biological and Environmental Research, and by the National Institutes of Health (NIH) and National Institute of General Medical Sciences (NIGMS; P30GM133894). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS or NIH.

Data Availability Statement

All data generated or analyzed in this study are included in this published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c00632.

  • Synthetic protocols, HR-ESI-MS and XAS spectra, and DFT-optimized Cartesian coordinates (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic3c00632_si_001.pdf (716.1KB, pdf)

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Associated Data

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Supplementary Materials

ic3c00632_si_001.pdf (716.1KB, pdf)

Data Availability Statement

All data generated or analyzed in this study are included in this published article and its Supporting Information.

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