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Formally Ferric Heme Carbon Monoxide Adduct Atanu Rana, Sk Amanullah, Pradip K. Das, Ashley B. McQuarters, Nicolai Lehnert, and Abhishek Dey J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09067 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Journal of the American Chemical Society

Formally Ferric Heme Carbon Monoxide Adduct Atanu Rana1, Sk Amanullah1, Pradip K Das1, Ashley B. McQuarters2, Nicolai Lehnert2,*, Abhishek Dey1,*. 1 Department

of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India – 700032 2Department of Chemistry, The University of Michigan, 930 N. University, Ann Arbor, MI 48109, USA

Email: [email protected] Supporting Information Placeholder ABSTRACT: Formally ferric carbonyl adducts are reported in a series of thiolate-bound iron porphyrins. Resonance Raman data indicate the presence of both Fe-S and Fe-CO bonds, and EPR data of this S=1/2 species indicate a ligand-based electron hole, giving this complex an Fe(II)-thiyl radical electronic ground state. The FTIR data show that the C-O vibrations are substantially higher than in the corresponding ferrous-thiolate CO adducts. DFT calculations reproduce the spectroscopic features and indicate that backbonding to the low lying * orbitals of the bound CO stabilizes the Fe 3d orbitals resulting in a stabilization of the ferrous-thiyl radical ground state compared to the five-coordinate ferric-thiolate precursor complexes. Access to stable thiyl radicals will help understand these elusive species that are mostly encountered as short-lived reactive reaction intermediates.

Carbon monoxide (CO) is an important small molecule in nature which is generated in the human body during the degradation of heme by heme oxygenases.14 While CO is toxic to humans due to its ability to inhibit respiration, small amounts of CO are needed as a primary signaling molecule that works as a neurotransmitter.5-6 Apart from its biological significance, the small molecule CO has been a matter of great interest to the inorganic chemistry community as a −acidic ligand that is capable of stabilizing low valent complexes of first row transition metals.7 CO is also an important ligand in many important organometallic catalysts, e.g. Fischer carbenes, Collman reagent, etc.89 Finally, metal carbonyls are found in the active sites of hydrogenases, and metal carbonyl species are known intermediates in the artificial CO2 reduction process.10-11 Binding of CO to a metal entails backbonding to its low lying * orbitals from the occupied metal 3d orbitals. Such interactions are favored in low-valent metal centers due to their high energy filled valence orbitals. Hence, CO generally binds to transition metals having

formal oxidation states of two and lower.12 Correspondingly, there are innumerable reports on CO binding to metals in their formal +2 to -1 oxidation states.13-15 However, to date there are no reports of CO binding to metal centers having a formal +3 oxidation state.

Scheme 1: Schematic representation of the tautomeric ferric thiolate and ferrous thiyl species.

Figure 1: Crystal structure of CO-bound CytP450CAM.

Recently, a unique case of valence tautomerism was reported for thiolate-bound five-coordinate ferric porphyrin model complexes.16 At room temperature, a formally ferric thiolate complex exists in an equilibrium of two valence tautomers: the ferric thiolate and ferrous thiyl states (scheme 1). The ferric thiolate state has a favorable ΔH, allowing it’s stabilization at low temperature, while the ferrous thiyl state has a higher ΔS (derived from the population of low lying vi-

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brational states). Hence, the equilibrium is shifted towards the ferrous thiyl state at higher temperatures. The ferrous thiyl state is capable of activating O2 which results in the eventual oxidation of the thiolate sulfur to sulfoxide. Thus, the valence tautomerism present in five-coordinate ferric thiolate model complexes makes them uncharacteristically O2 sensitive.16 It was also discovered that hydrogen bonding to the thiolate, present in the Cyt P450 (Fig.1) active site, stabilizes the ferric thiolate state which is not O2 sensitive.17-19 The presence of the ferrous thiyl valence tautomer in a formally ferric thiolate species may allow CO binding to the same. In this manuscript, we report two CO-bound ferric thiolate complexes (Scheme 2) which are characterized by EPR, resonance Raman and FTIR spectroscopy. These complexes exhibit S=1/2 ground states and are characterized by unique Fe-S, Fe-CO and C-O vibrations.

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FeTPPSPh-CO and PPSR-CO show CO at 1970 cm-1 and 1965 cm-1, respectively (Fig.S5A). The CO stretching frequencies are substantially higher than the corresponding CO modes for the reduced PPSR-CO complex, at 1950 cm-1 (Fig.S5B) and for CO bound ferrous Cyt P450 (at 1948 cm-1).20-22 The higher energy CO in the ferric thiolate complexes relative to the reduced ferrous thiolate species is consistent with lower back bonding from the formally ferric center relative to the thiolate bound ferrous center. Nevertheless, the question remains how the formally ferric heme is able to mediate CO binding after all.

Scheme 2: Schematic representation of the ferric thiolate model complexes FeTPP-SPh and PPSR.

Purging CO through solutions of the model complexes FeTPP-SPh and FePPSR results in an immediate change in color from brown to bright red. The absorption spectra show comparatively sharp Soret bands at 425 nm (Fig.S1). Upon further reduction by one electron with Na2S, the Soret band shifts to 451 nm, which is characteristic of thiolate bound ferrous CO adducts, implying that the species preceding the ferrous CO complex is of formally ferric CO character. The EPR spectra of the resting ferric thiolate precursor complexes show either a S=5/2 ground state (GS) with a signal at g=6.0 in FeTPP-SPh (Fig.S3) or a S=1/2 GS in PPSR (due to MeOH coordination) with a rhombic signal with gx=2.26, gy=2.17 and gz=1.93. Upon addition of CO gas into the THF solution of the above mentioned complexes, the Fe(III) EPR signals are decreased and replaced by new rhombic signals with g1=2.07, g2=2.02 (A2=12.2x10-4 cm-1) and g3=1.96 (A3=16.3x10-4 cm-1) for FeTPP-SPh-CO. For PPSR-CO these are g1=2.06, g2 =1.98 (A2=16.3x10-4 cm-1) and g3=1.96 (A3=18.5x10-4 cm-1), respectively (Fig.2). The these complexes are S=1/2 even at room temperature (RT) as indicated by their EPR spectra at RT (Fig.S2) CO binding to the FeTPP-SPh and PPSR complexes was further monitored using FTIR spectroscopy.

Figure 2: Overlay of the EPR spectra of A) FeTPP-SPh, B) PPSR CO-bound species in comparison to their corresponding ferric precursor complexes. The blue lines indicate the ferric precursors and the red lines correspond to their CO-adducts.

Resonance Raman (rR) data of ferric FeTPP-SPh at 77K show oxidation state marker () and spin state marker bands at () 1361 cm-1 and 1552 cm-1, respectively, indicating a ferric high spin species (Fig.S4) consistent with the EPR data. Upon addition of CO into the solution, a distinct change is observed both in the spin state as well as the oxidation state marker band, with  at 1367 cm-1 and  at 1565 cm-1 (along with some residual Fe(III) high-spin species, Fig.S4). The  and  marker bands for the CO-bound FeTPP-SPh complex are substantially higher than those of CO-

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bound ferrous porphyrins, which are observed for  at 1363 cm-1 and  at 1561 cm-1. Overall, there is a dramatic increase in the rR intensity (when normalized with respect to solvent vibrations). In addition, several new vibrations are observed between 200-700 cm-1 in the rR spectra of the FeTPP-SPh-CO adduct. These appear at 210 cm-1, 343 cm-1, 524 cm-1, 595 cm1 and 776 cm-1 (Fig.3). To identify these vibrations, an isotopically labelled sulphur atom (34S) was used in the FeTPP-SPh complex.

Figure 3: rR spectra overlay of FeTPP-SPh and of CO-bound FeTPP-SPh in the low-energy region.

Unique vibrations for FeTPP-SPh-CO, located at 210 cm-1, 343 cm-1, and 369 cm-1, shift to 201 cm-1, 336 cm1 and 366 cm-1, respectively, upon 34S substitution (Fig.4). This indicates a significant contribution of FeS vibrations to these normal modes. A weak vibrational band observed at 664 cm-1, which is shifted to 657 cm-1 on 34S isotopic substitution (Fig.4), likely represents a C-S vibration of the ferric CO adduct.

Figure 4: Overlay of the rRaman spectra of the CO-bound 32S and 34S FeTPP-SPh complexes (inset: C-S region). The blue lines represent the spectra of FeTPP-32SPh-CO and the red lines correspond to FeTPP-34SPh-CO.

The 525 cm-1 mode appears in the rRaman data of both FeTPP-SPh and PPSR only after CO purging, unambiguously identifying it as a Fe-CO mode. The rR spectrum of PPSR-CO assists in confirming the assignment of the vibrational mode at 525 cm-1, which shifts to 523 cm-1 upon 57Fe substitution. This feature corresponds to the Fe-C-O bending vibration (Fig.5) which is substantially higher than that reported for CO bound CytP450 which appears at 481 cm-1.20-21

Figure 5: Overlay of the rR spectra of CO-bound 56Fe and 57Fe versions of complex PPSR. The blue line represents the spectrum for PPSR-56Fe-CO, and the red line represents the data for PPSR-57Fe-CO.

DFT calculations provide significant insight into the electronic structures of the CO-bound ferric porphyrin species with axial thiolate ligation. The structures of the ferric precursors and their CO adducts were optimized using G09-D (BP86, 6-311G*) (Fig.6).23-25 As shown in Fig.6, structure (A) is ligated by a water molecule as an axial ligand in the sixth position and (B), (C) both have the neutral -acidic ligand CO as the sixth ligand. Further analysis of the electronic properties of these complexes considers their vibrational frequencies, orbital wave functions, and spin density distributions. The calculated vibrational frequencies (using BP86 with dispersion correction) of the oxidation state (4) and spin state (2) marker bands of Fe-porphyrins are in good agreement with experimentally observed 4 and 2 frequencies (within ±7 cm−1) for the ferric thiolate complexes and their CO adducts (Table S1). The (C-O) for FeIIIPPSR-CO and FeIIITPP-SPh-CO are computed to be at 2013 cm-1 and 1995 cm-1, respectively, relative to the experimental values of 1965 cm-1 and 1970 cm-1 (Fig. S3A), respectively. The (C-O) frequencies for the corresponding FeIIPPSR-CO and FeIITPPSPh-CO complexes are computed at 1957 cm-1 and 1948 cm-1, respectively. The DFT-predicted lowering



of (C-O) upon reduction is consistent with the experimental data, where the (C-O) mode is lowered from 1965 cm-1 in FeIIIPPSR-CO to 1950 cm-1 in FeIIPPSR-CO (Fig. S5B). A few key vibrational modes are identified for FeIIIPPSR-CO in the calculations as well: the Fe-S bent at 189 cm-1, the Fe-S stretch at 347 cm-1, the FeCO stretch at 425 cm-1, the Fe-C-O bent at 529 cm-1, the C-S stretch at 683 cm-1, and the C-O stretch at 2013 cm1 (Table S2). Similarly, for FeIIITPP-SPh-CO, the Fe-S bent is found at 195 cm-1, the Fe-S stretch at 335 cm-1, the Fe-CO stretch at 446 cm-1, the Fe-CO bent at 541 cm-1, the C-S stretch at 692 cm-1, and the C-O stretch at 1995 cm-1 (Table S2) in the DFT calculations. Therefore, the computed values are in good agreement with the experimentally observed vibrational frequencies and their 34S and 57Fe isotope shifts, which supports our vibrational assignments (Table 1 and Table S2).

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The SOMO of the 6C FeIIIPPSR-OH2 complex mostly corresponds to the t2 dyz orbital (Fig.S11A), while the SOMO of both FeIIIPPSR-CO (Fig.S11B) and FeIIITPPSPh-CO (Fig.S11C) is largely localized on the ligands (S3p, porphyrin* and CO) (Table S3). This represents an inversion of the bonding scheme, where conventionally the antibonding orbitals have a greater contribution from the metal 3d orbitals (Scheme 3, left)26-28. However, the orbital energies of dxz and dyz are lowered (normalized with respect to the nonbonding dxy orbital, Scheme 3, right) in the CO-bound ferric thiolate complexes relative to the H2O bound ferric complex due to backbonding of the t2 orbitals with the low lying * orbitals of the bound CO (Fig.S12). Logically, the spin density distribution reflects the same (Fig.7). In other words, binding of CO stabilizes the ferrousthiyl state of the complexes and locks the complexes into this electronic structure (Table S4). This is consistent with the observed change in the EPR spectra upon forming the CO adducts. The FeIII-PPSR-OH2 complex has a t25 GS which results in a typical Fe(III) low-spin (LS) rhombic EPR signal (gx=2.26, gy=2.17 and gz=1.93). However, due to the stabilization of the t2 dxz and dyz orbitals by d-backbonding with CO, much of the spin resides on the ligands, resulting in an EPR signal with g1=2.07, g2=2.02 and g3=1.96. Table 1: Experimental and theoretical vibrational frequencies (cm-1) after dispersion correction CO

Theory

complex

Figure 6: Optimized structures of the ferric complexes A) PPSROH2, B) PPSR-CO and C) FeTPP-SPh-CO.

PPSR

Fe-C

Fe-S

C-S

C-O

Fe-C-S

529

347

683

2013

189

PPSR

Experimental 525

1965

FeTPP

Theory 541

335

FeTPP

692

1995

195

Experimental

32S

524

343

664

1970

210

34S

524

336

657

1970

201

Figure 7: Spin density plots of species A) FeIIIPPSR-OH2, B) FeIIIPPSR-CO, C) FeIIITPP-SPh-CO, all with S=1/2 ground states. Mulliken spin densities are listed.


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Scheme 3: Normal bonding scheme for ferric thiolate porphyrins (left) and inverted bonding scheme in CO bound ferric thiolates (right). The dashed lines represent the dominant fragment orbital contribution in the final molecular orbital. The interaction of a hypothetical square planar ferric porphyrin with CO (blue dash), the interaction of a thiolate with the resulting hypothetical ferric CO.

In summary, CO is found to bind thiolate-bound ferric porphyrins to result in a ferrous-thiyl CO adduct. The resonance Raman data show clear evidence of the thiyl radical and CO coordination to the ferrous porphyrin. In this sense, the CO coordination locks the complex into the ferrous-thiyl valence tautomer. Hence, the coordination of CO results in an inverted bonding scheme where the singly unoccupied hole resides primarily on the ligand instead of the metal. Such ligand radicals are invoked in compound I intermediates. Thus, access to stable ligand radical intermediates will allow us to interrogate the nature of these species and factors that stabilize them in detail. ASSOCIATED CONTENT Supporting Information

The experimental details, additional Raman and EPR data, optimized coordinates are available free of charge at pubs.acs.org

AUTHOR INFORMATION Corresponding Author

Abhishek Dey ([email protected]) and Nicolai Lehnert ([email protected])

ACKNOWLEDGMENT This research is funded by NSF (1464696) and DST SERB (EMR/2016/008063). We acknowledge Debabrata Halder for the help in DFT-D3 calculation.

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