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. 2022 Oct 25;7(44):40275–40278. doi: 10.1021/acsomega.2c05199

Porphyryne

Abhik Ghosh †,*, Jeanet Conradie †,‡,*
PMCID: PMC9647813  PMID: 36385855

Abstract

graphic file with name ao2c05199_0005.jpg

Density functional theory calculations with the B3LYP*-D3 method with large STO-QZ4P basis sets unambiguously predict a singlet ground state for Zn-porphyryne. However, the calculations also predict a low singlet–triplet gap of about 0.4 eV and a high adiabatic electron affinity of 2.4 eV. Accordingly, the reactivity of porphyryne species may be dominated by electron transfer, hydrogen abstraction, and proton-coupled electron transfer processes.

Introduction

Although the existence of benzyne (1,2-dehydrobenzene) was surmised in the early part of the 20th century,1 strong evidence for the species emerged decades later, most notably through the work of Wittig2−5 and Roberts.6−11 Subsequently, benzyne has been intensively studied via gas-phase spectroscopic studies, especially photoelectron spectroscopy,12−14 and quantum chemical calculations.15−18 More recently, benzyne has been trapped in a container molecule19 and even imaged with STM.20 As powerful Diels–Alder dienophiles, benzynes and other arynes are widely employed as highly reactive synthetic reagents and intermediates.21−24 Benzynes are key intermediates of the hexadehydro-Diels–Alder reaction, a synthetic reaction recently developed by Hoye and coworkers.25,26 Interestingly, in spite of major advances in the functionalization of porphyrin-type compounds,27−29 an aryne based on a porphyrin, i.e., a porphyryne, remains unknown. Herein, following a long-standing tradition among chemical theoreticians in studying reasonable-looking but nonexistent molecules,30−35 we have considered the viability of such an intermediate based on a comparative density functional theory (DFT) study of zinc porphyryne and benzyne.

Results and Discussion

While a variety of popular exchange-correlation functionals were examined (which generally yielded very similar results), the results quoted (Table 1 and Figures 1–3) are those for the well-tested36−40 hybrid functional B3LYP* (with 15% Hartree–Fock exchange)41,42 augmented with Grimme’s D3 dispersion corrections43 as implemented in the ADF 2019 program system.44,45 The optimized geometry of Zn-porphyryne corresponds to C2v symmetry and reveals unremarkable skeletal bond distances, except for that of the C–C triple bond (1.233 Å), which is similar to that in benzyne (1.243 Å; Figure 1). Kohn–Sham MO energy level diagrams facilitate further comparative discussion of the two molecules (Figure 2). For benzyne, although the HOMO corresponds to the in-plane triple-bond π-MO, it is accidentally degenerate with two other benzene π-HOMOs. Thus, our calculations predict three near-degenerate IPs, an interesting facet of benzyne that appears to have been overlooked in the literature until now (Table 1).12−18 In the case of Zn-porphyryne, the two HOMOs correspond to the classic Gouterman a1u and a2u HOMOs,46−49 while the in-plane triple-bond π-MO corresponds to HOMO-2. Accordingly, the vertical IP corresponding to ionization from the latter MO (7.9 eV) is about an eV higher than the two lowest IPs (6.9 eV, which is essentially the same as that calculated for Zn-porphyrin and experimentally observed for unsubstituted free-base porphine50).51−55 For both benzyne and Zn-porphyryne, the LUMO clearly corresponds to the in-plane triple-bond π-antibonding MO. Another point of similarity is that the triplet state of both molecules involves an excitation from the in-plane triple-bond π-MO to the triple-bond π*-MO and is thus characterized by similar spin density profiles.

Table 1. Scalar-Relativistic All-Electron B3LYP*-D3/ZORA-STO-QZ4P Results on Benzyne, Zn-Porphyryne, and Zn-Porphyrin: IPs, EAs, and Singlet–Triplet Gaps (eV).

  vertical
 adiabatic
compound IP1 IP2 IP3 EA ES–T IP1 IP2 IP3
benzyne 9.610 9.637 9.616 0.645 1.457 9.480 9.481 9.483
Zn-porphyryne 6.928 6.963 7.936 2.397 0.441 6.886 6.939 7.790
Zn-porphyrina 6.759 6.764 7.915 1.201 1.830 6.691    
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a

The adiabatic IP and EA of Zn-porphyrin take into account their different point group symmetries relative to the neutral state, as a result of Jahn–Teller-type symmetry-breaking phenomena in the ionized states.

Figure 1.

Figure 1

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Symmetry-distinct bond distances (Å) in all-electron B3LYP*-D3/ZORA-STO-QZ4P-optimized geometries of (a) benzyne and (b) Zn-porphyryne. Note that a C2v minimum was obtained for each molecule. Note also very similar C–C triple-bond distances in the two molecules.

Figure 3.

Figure 3

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B3LYP*-D3/ZORA-STO-QZ4P spin density profiles (0.002 e·Å–3) of selected adiabatically ionized/excited states of (a) benzyne and (b) Zn-porphyryne.

Figure 2.

Figure 2

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B3LYP*-D3/ZORA-STO-QZ4P Kohn–Sham MO energy level diagrams for (a) benzyne and (b) Zn-porphyryne.

Our calculations also underscore major differences between benzyne and Zn-porphyryne as well as between Zn-porphyryne and Zn-porphyrin. Thus, Zn-porphyryne is expected to exhibit a high electron affinity (2.4 eV) and a low singlet–triplet gap (0.44 eV). For reference, a simple closed-shell porphyrin such as Zn-porphyrin (Table 1) exhibits a much lower electron affinity56−60 of around 1.2 eV and a much higher singlet–triplet gap of ∼2.0 eV.61 Both features of Zn-porphyryne appear to be a consequence of the low LUMO energy level (well below the Gouterman π-LUMOs46,47) and the lower HOMO–LUMO gap relative to benzyne, which in turn presumably reflect the greater destabilization of a triple bond in a five-membered ring relative to a six-membered ring (in spite of similar triple-bond distances, as shown in Figure 1).

Conclusions

High-quality DFT calculations clearly indicate Zn-porphyryne as a ground-state singlet species. Its high electron affinity, however, may thwart classic reaction pathways such as the Diels–Alder reaction. Instead, major reaction pathways may involve electron transfer, C–H abstraction, and proton-coupled electron transfer. Nevertheless, the overall calculated energetics suggest the existence of porphyryne species as reactive intermediates, and we accordingly encourage experimentalists to attempt their generation and trapping under carefully controlled conditions.

Acknowledgments

This work was supported by grant no. 324139 of the Research Council of Norway (A.G.) and grant nos. 129270 and 132504 of the South African National Research Foundation (J.C.).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c05199.

  • Optimized Cartesian coordinates (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c05199_si_001.pdf (158.1KB, pdf)

References

  1. Stoermer R.; Kahlert B. Über das 1- und 2-Brom-cumaron. Ber. Dtsch. Chem. Ges. 1902, 35, 1633–1640. 10.1002/cber.19020350286. [DOI] [Google Scholar]
  2. Wittig G.; Pieper G.; Fuhrmann G. Über die Bildung von Diphenyl aus Fluorbenzol und Phenyl-lithium (IV. Mitteil. über Austauschreaktionen mit Phenyl-lithium). Ber. Dtsch. Chem. Ges. 1940, 73, 1193–1197. 10.1002/cber.19400731113. [DOI] [Google Scholar]
  3. Wittig G. Phenyl-lithium, der Schlüssel zu einer neuen Chemie metallorganischer Verbindungen. Naturwissenschaften 1942, 30, 696–703. 10.1007/BF01489519. [DOI] [Google Scholar]
  4. Wittig G. Fortschritte auf dem Gebiet der organischen Aniono-Chemie. Am. Ethnol. 1954, 66, 10–17. 10.1002/ange.19540660103. [DOI] [Google Scholar]
  5. Wittig G.; Pohmer L. Intermediäre Bildung von Dehydrobenzol (Cyclohexadienin). Am. Ethnol. 1955, 67, 348–348. 10.1002/ange.19550671306. [DOI] [Google Scholar]
  6. Roberts J. D.; Simmons H. E. Jr.; Carlsmith L. A.; Vaughan C. W. Rearrangement in The Reaction of Chlorobenzene-1-C14 With Potassium Amide. J. Am. Chem. Soc. 1953, 75, 3290–3291. 10.1021/ja01109a523. [DOI] [Google Scholar]
  7. Roberts J. D.; Semenow D. A.; Simmons H. E. Jr.; Carlsmith L. A. The Mechanism of Aminations of Halobenzenes. J. Am. Chem. Soc. 1956, 78, 601–611. 10.1021/ja01584a024. [DOI] [Google Scholar]
  8. Roberts J. D.; Vaughan C. W.; Carlsmith L. A.; Semenow D. A. The Chemistry of Benzyne. J. Am. Chem. Soc. 1956, 78, 611–614. 10.1021/ja01584a025. [DOI] [Google Scholar]
  9. Bunnett J. F. J. Chem. Educ. 1961, 38, 278. 10.1021/ed038p278. [DOI] [Google Scholar]
  10. Heaney J. H. The Benzyne and Related Intermediates. Chem. Rev. 1962, 62, 81–97. 10.1021/cr60216a001. [DOI] [Google Scholar]
  11. Wentrup C. The Benzyne Story. Aust. J. Chem. 2010, 63, 979–986. 10.1071/CH10179. [DOI] [Google Scholar]
  12. Dewar M. J. S.; Tien T.-P. Photoelectron spectrum of benzyne. J. Chem. Soc., Chem. Commun. 1985, 1243–1244. 10.1039/c39850001243. [DOI] [Google Scholar]
  13. Leopold D. G.; Miller A. E. S.; Lineberger W. C. J. Am. Chem. Soc. 1986, 108, 1379–1384. 10.1021/ja00267a003. [DOI] [Google Scholar]
  14. Wenthold P. G.; Squires R. R.; Lineberger W. C. Ultraviolet Photoelectron Spectroscopy of the o-, m- and p-Benzyne Negative Ions. Electron Affinities and Singlet–Triplet Splittings for o-, m- and p-benzyne. J. Am. Chem. Soc. 1998, 120, 5279–5290. 10.1021/ja9803355. [DOI] [Google Scholar]
  15. Lindh R.; Bernhardsson A.; Schütz M. Benzyne Thermochemistry: A Benchmark ab Initio Study. J. Phys. Chem. A 1999, 103, 9913–9920. 10.1021/jp991919b. [DOI] [Google Scholar]
  16. Johnson W. T. G.; Cramer C. J. Influence of Hydroxyl Substitution on Benzyne Properties. Quantum Chemical Characterization of the Didehydrophenols. J. Am. Chem. Soc. 2001, 123, 923–928. 10.1021/ja002250l. [DOI] [PubMed] [Google Scholar]
  17. Vanovschi V.; Krylov A. I.; Wenthold P. G. Structure, vibrational frequencies, ionization energies, and photoelectron spectrum of the para-benzyne radical anion. Theor. Chem. Acc. 2008, 120, 45–58. 10.1007/s00214-007-0305-7. [DOI] [Google Scholar]
  18. Smart S. E.; Boyn J.-N.; Mazziotti D. A. Resolving correlated states of benzyne with an error-mitigated contracted quantum eigensolver. Phys. Rev. A 2022, 105, 022405 10.1103/PhysRevA.105.022405. [DOI] [Google Scholar]
  19. Warmuth R. o-Benzyne: Strained Alkyne or Cumulene?—NMR Characterization in a Molecular Container. Angew. Chem., Int. Ed. 1997, 36, 1347–1350. 10.1002/anie.199713471. [DOI] [Google Scholar]
  20. Pavliček N.; Schuler B.; Collazos S.; Moll N.; Pérez D.; Guitián E.; Meyer G.; Peña D.; Gross L. Nat. Chem. 2015, 7, 623–628. 10.1038/nchem.2300. [DOI] [PubMed] [Google Scholar]
  21. Wenk H. H.; Winkler M.; Sander W. One Century of Aryne Chemistry. Angew. Chem., Int. Ed. 2003, 42, 502–528. 10.1002/anie.200390151. [DOI] [PubMed] [Google Scholar]
  22. Tadross P. M.; Stoltz B. M. A Comprehensive History of Arynes in Natural Product Total Synthesis. Chem. Rev. 2012, 112, 3550–3577. 10.1021/cr200478h. [DOI] [PubMed] [Google Scholar]
  23. Modern Aryne Chemistry; Biju A. T., ed.; Wiley-VCH: Weinheim, 2021; pp. 528. [Google Scholar]
  24. Comprehensive Aryne Synthetic Chemistry; Yoshida H., ed.; Elsevier, Amsterdam, 2022; pp. 392. [Google Scholar]
  25. Hoye T. R.; Baire B.; Niu D.; Willoughby P. H.; Woods B. P. The hexadehydro-Diels–Alder reaction. Nature 2012, 490, 208–212. 10.1038/nature11518. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Fluegel L. L.; Hoye T. R. Hexadehydro-Diels–Alder Reaction: Benzyne Generation via Cycloisomerization of Tethered Triynes. Chem. Rev. 2021, 121, 2413–2444. 10.1021/acs.chemrev.0c00825. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Vicente M.; Smith K. Syntheses and Functionalizations of Porphyrin Macrocycles. Curr. Org. Synth. 2014, 11, 3–28. 10.2174/15701794113106660083. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Hiroto S.; Miyake Y.; Shinokubo H. Synthesis and Functionalization of Porphyrins through Organometallic Methodologies. Chem. Rev. 2017, 117, 2910–3043. 10.1021/acs.chemrev.6b00427. [DOI] [PubMed] [Google Scholar]
  29. Silva A. M. G.; Cavaleiro J. A. S. Porphyrins in Diels-Alder and 1,3-dipolar cycloaddition reactions. Prog. Heterocycl. Chem. 2008, 19, 44–69. 10.1016/S0959-6380(08)80004-9. [DOI] [Google Scholar]
  30. Dasent W. E.Nonexistent Compounds: Compounds of Low Stability; Marcel Dekker: New York, 1965, pp. 182. [Google Scholar]
  31. Hoffmann R. Why Think Up New Molecules?. Am. Sci. 2008, 96, 372–374. 10.1511/2008.74.372. [DOI] [Google Scholar]
  32. Ghosh A.; Taylor P. R. Iron(IV) Porphyrin Difluoride Does Not Exist: Implications for DFT Calculations on Heme Protein Reaction Pathways. J. Chem. Theory Comput. 2005, 1, 597–600. 10.1021/ct050086s. [DOI] [PubMed] [Google Scholar]
  33. Ghosh A.; Conradie J. Pentafluorosulfanyltrimethylsilane: A Nonexistent Molecule?. Eur. J. Inorg. Chem. 2015, 207–209. 10.1002/ejic.201402928. [DOI] [Google Scholar]
  34. Demissie T. B.; Conradie J.; Vazquez-Lima H.; Ruud K.; Ghosh A. Rare and Nonexistent Nitrosyls: Periodic Trends and Relativistic Effects in Ruthenium and Osmium Porphyrin-Based {MNO}7 Complexes. ACS Omega 2018, 3, 10513–10516. 10.1021/acsomega.8b01434. [DOI] [PMC free article] [PubMed] [Google Scholar]
  35. Conradie J.; Alemayehu A. B.; Ghosh A. Iridium(VII)–Corrole Terminal Carbides Should Exist as Stable Compounds. ACS Org. Inorg. Au 2022, 2, 159–163. 10.1021/acsorginorgau.1c00029. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Jacobsen H.; Cavallo L. Re-evaluation of the Mn(salen) mediated epoxidation of alkenes by means of the B3LYP* density functional. Phys. Chem. Chem. Phys. 2004, 6, 3747–3753. 10.1039/b402188f. [DOI] [Google Scholar]
  37. Conradie M. M.; Conradie J.; Ghosh A. Capturing the spin state diversity of iron(III)-aryl porphyrins OLYP is better than TPSSh. J. Inorg. Biochem. 2011, 105, 84–91. 10.1016/j.jinorgbio.2010.09.010. [DOI] [PubMed] [Google Scholar]
  38. Hirao H. T Functional Performs Well in the Calculation of Methylcobalamin? Comparison of the B3LYP and BP86 Functionals and Evaluation of the Impact of Empirical Dispersion Correction. J. Phys. Chem. A 2011, 115, 9308–9313. 10.1021/jp2052807. [DOI] [PubMed] [Google Scholar]
  39. Conradie J.; Ghosh A. DFT Calculations on the Spin-Crossover Complex Fe(salen)(NO): A Quest for the Best Functional. J. Phys. Chem. B 2007, 111, 12621–12624. 10.1021/jp074480t. [DOI] [PubMed] [Google Scholar]
  40. Siegbahn P. E. M.; Blomberg M. R. A. A Systematic DFT Approach for Studying Mechanisms of Redox Active Enzymes. Front. Chem. 2018, 6, 644. 10.3389/fchem.2018.00644. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Reiher M.; Salomon O.; Artur Hess B. Reparameterization of hybrid functionals based on energy differences of states of different multiplicity. Theor. Chem. Acc. 2001, 107, 48–55. 10.1007/s00214-001-0300-3. [DOI] [Google Scholar]
  42. Salomon O.; Reiher M.; Hess B. A. Assertion and validation of the performance of the B3LYP* functional for the first transition metal row and the G2 test set. J. Chem. Phys. 2002, 117, 4729–4737. 10.1063/1.1493179. [DOI] [Google Scholar]
  43. Grimme S.; Antony J.; Ehrlich S.; Krieg H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. 10.1063/1.3382344. [DOI] [PubMed] [Google Scholar]
  44. Te Velde G.; Bickelhaupt F. M.; Baerends E. J.; Fonseca Guerra C.; van Gisbergen S. J. A.; Snijders J. G.; Ziegler T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931–967. 10.1002/jcc.1056. [DOI] [Google Scholar]
  45. Relativistic effects were taken into account with the zeroth-order regular approximation (ZORA) applied as a scalar correction and specially optimized all-electron ZORA STO-QZ4P basis sets:; van Lenthe E.; Baerends E. J.; Snijders J. G. Relativistic Total Energy Using Regular Approximations. J. Chem. Phys. 1994, 101, 9783–9792. 10.1063/1.467943. [DOI] [Google Scholar]
  46. The π-MOs of Zn-porphyryne are visually indistinguishable from those of Zn-porphyrin:; Gouterman M.; Wagnière G. H.; Snyder L. C. Spectra of Porphyrins. Part II. Four-Orbital Model. J. Mol. Spectrosc. 1963, 11, 108–127. 10.1016/0022-2852(63)90011-0. [DOI] [Google Scholar]
  47. Gouterman M.Optical Spectra and Electronic Structure of Porphyrins and Related Rings. In The Porphyrins; Dolphin D., Ed.; Academic Press: New York, 1978; Vol. III, Part A, pp. 1–165. [Google Scholar]
  48. Ghosh A. Martin Gouterman: a gay rights pioneer. ChemistryWorld 2020, 36–37. [Google Scholar]
  49. Ghosh A. An Exemplary Gay Scientist and Mentor: Martin Gouterman (1931–2020). Angew. Chem., Int. Ed. 2021, 60, 9760–9770. 10.1002/anie.202012840. [DOI] [PubMed] [Google Scholar]
  50. Dupuis P.; Roberge R.; Sandorfy C. The very low ionization potentials of porphyrins and the possible role of rydberg states in photosynthesis. Chem. Phys. Lett. 1980, 75, 434–437. 10.1016/0009-2614(80)80549-5. [DOI] [Google Scholar]
  51. Ghosh A.; Almlöf J. The ultraviolet photoelectron spectrum of free-base porphyrin revisited. The performance of local density functional theory. Chem. Phys. Lett. 1993, 213, 519–521. 10.1016/0009-2614(93)89152-8. [DOI] [Google Scholar]
  52. Ghosh A. Substituent effects on valence ionization potentials of free base porphyrins: A local density functional study. J. Am. Chem. Soc. 1995, 117, 4691–4699. 10.1021/ja00121a025. [DOI] [Google Scholar]
  53. Ghosh A. Theoretical Comparative Study of Free Base Porphyrin, Chlorin, Bacteriochlorin, and Isobacteriochlorin: Evaluation of the Potential Roles of ab Initio Hartree–Fock and Density Functional Theories in Hydroporphyrin Chemistry. J. Phys. Chem. B 1997, 101, 3290–3297. 10.1021/jp964069y. [DOI] [Google Scholar]
  54. Ghosh A.; Vangberg T. Valence ionization potentials and cation radicals of prototype porphyrins. The remarkable performance of nonlocal density functional theory. Theor. Chem. Acc. 1997, 97, 143–149. 10.1007/s002140050247. [DOI] [Google Scholar]
  55. Ghosh A. First-Principles Quantum Chemical Studies of Porphyrins. Acc. Chem. Res. 1998, 31, 189–198. 10.1021/ar950033x. [DOI] [Google Scholar]
  56. Electron affinities of metalloporphyrin-type molecules have been less intensively studied, compared with ionization potentials. DFT calculations, however, are thought to provide an excellent description of metal- versus ligand-centered reduction for such systems.
  57. Ryeng H.; Gonzalez E.; Ghosh A. DFT at Its Best: Metal- versus Ligand-Centered Reduction in Nickel Hydroporphyrins. J. Phys. Chem. B 2008, 112, 15158–15173. 10.1021/jp805486b. [DOI] [PubMed] [Google Scholar]
  58. Thomas K. E.; Vazquez-Lima H.; Fang Y.; Song Y.; Gagnon K. J.; Beavers C. M.; Kadish K. M.; Ghosh A. Ligand Noninnocence in Coinage Metal Corroles: A Silver Knife-Edge. Chem. – Eur. J. 2015, 21, 16839–16847. 10.1002/chem.201502150. [DOI] [PubMed] [Google Scholar]
  59. Alemayehu A. B.; McCormick-McPherson L. J.; Conradie J.; Ghosh A. Rhenium Corrole Dimers: Electrochemical Insights into the Nature of the Metal-Metal Quadruple Bond. Inorg. Chem. 2021, 60, 8315–8321. 10.1021/acs.inorgchem.1c00986. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Conradie J.; Vazquez-Lima H.; Alemayehu A. B.; Ghosh A. Comparing Isoelectronic, Quadruple-Bonded Metalloporphyrin and Metallocorrole Dimers: Scalar-Relativistic DFT Calculations Predict a >1 eV Range for Ionization Potential and Electron Affinity. ACS Phys. Chem. Au 2022, 2, 70–78. 10.1021/acsphyschemau.1c00030. [DOI] [PMC free article] [PubMed] [Google Scholar]
  61. Chen H. L.; Ellis P. E. Jr.; Wijesekera T.; Hagan T. E.; Groh S. E.; Lyons J. E.; Ridge D. P. Correlation between gas-phase electron affinities, electrode potentials, and catalytic activities of halogenated metalloporphyrins. J. Am. Chem. Soc. 1994, 116, 1086–1089. 10.1021/ja00082a034. [DOI] [Google Scholar]

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ao2c05199_si_001.pdf (158.1KB, pdf)

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