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. 2026 Feb 20;6(3):1488–1494. doi: 10.1021/jacsau.5c01770

Nitric Oxide Insertion into a Metalloporphyrin–Carbon Bond

Jennifer Londoño-Salazar †, Guan Ye †, Chance W Lander †, Tsitsi A Kapfunde †, Nobuyuki Yamamoto †, Novruz G Akhmedov †, Yong Zhang ‡, Yihan Shao †,∥,*, George B Richter-Addo †,*
PMCID: PMC13014249  PMID: 41889735

Abstract

Nitric oxide (NO) insertion into a metalloporphyrin rhodium–carbon bond is observed when crystals of (TPP)­Rh­(C6H4Cl)­(CH2Cl2) x (TPP = tetraphenylporphyrinato dianion) are exposed to NO gas over a two week period. The NO-inserted product was identified by X-ray crystallography as (TPP)­Rh­(ONC6H4Cl-o)­(CH2Cl2)0.5·(TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2)0.5, in which NO inserted selectively into the ortho-Cl aryl isomer but not into the para-/meta-Cl aryl isomers. Density functional theory calculations predict a higher reactivity for the ortho-Cl isomer and thus provide a rationale for the preferred NO insertion into the potentially more reactive ortho-Cl isomer under our reaction conditions. Whereas NO insertion reactions are well-known for coordination–organometallic compounds, this result represents the first such NO insertion into a metalloporphyrin metal–carbon bond for any metal.

Keywords: nitric oxide, metalloporphyrin, organometallic, insertion, X-ray structure

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Insertions of small molecules (E) into metal–carbon bonds ([M]–R; [M] = metal complex, R = alkyl/aryl) serve as pathways for the generation of new E–R bonds in the resulting [M]–E–R derivatives. Such insertions of various small molecules have been well-documented for coordination and nonporphyrin organometallic complexes in both stoichiometric , and catalytic reactions.

The porphyrin macrocycle is a common equatorial N4-ligand in bioinorganic chemistry, and several metalloporphyrins have been utilized in catalysis. However, reactions involving small molecule insertions into the metal–carbon bonds of organometallic porphyrins have remained much less common. Although a handful of examples of insertions of CO, , CO2, , and SO2 , into metalloporphyrin-carbon bonds are known, the related insertions of any NOx species into metalloporphyrin-carbon bonds to generate new C–N bonds have yet to be reported, although such insertions are well-known in nonporphyrin systems (reviewed in ref ).

Reactions of NO with organometallic porphyrins can, in principle, result in one or more of the products shown in Figure . Simple adduct formation (left of Figure ) has been reported for organoiron porphyrins. , In addition, two NO molecules may couple at a metal site to generate hyponitrite ([N2O2]2–) derivatives. −

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Likely products resulting from the reaction of NO with an organometallic compound [M]–R (M = metal complex; R = alkyl or aryl).

Reactions of NO with organometallic porphyrins have the potential of inserting into the metal–carbon bond (middle of Figure ). As mentioned above, although NO insertions into nonporphyrin organometallic complexes are well-documented, this reaction type has yet to be reported for organometallic porphyrins. A double NO insertion to generate a diazeniumdiolate (NONOate) ligand (right of Figure ) is also a possible pathway, as this reaction type has been demonstrated for a variety of nonporphyrin complexes. − We note that although such a double NO insertion has yet to be reported for metalloporphyrins, a small handful of such metalloporphyrin–NONOate complexes have been synthesized from preformed NONOate reagents, − with solid-state structures determined for metalloporphyrin derivatives of Fe (κ2-NONOate) , and Ru (κ1-NONOate).

In this Letter, we report the interaction of NO with a synthetic organorhodium porphyrin to result in the first observed NO insertion reaction for any metalloporphyrin (i.e., middle of Figure ).

The bulk sample of the diamagnetic (TPP)­Rh­(C6H4Cl)­(CH2Cl2) x precursor used in this work consists of all three axial aryl-Cl isomers, namely, the C6H4Cl-p/m/o isomers, in a 74:10:16 ratio in CDCl3 as determined by 1H NMR spectroscopy (Figures S1–S2 in the SI). ,

Exposure of a solution of (TPP)­Rh­(C6H4Cl)­(CH2Cl2) x in CH2Cl2 to NO gas for ∼5 min resulted in a product mixture whose IR spectra revealed new bands at 1321 cm–1 and 1118 cm –1 (KBr pellet; Figure S3a). Employing 15NO in the reaction shifted these bands to 1298 cm–1 (Δ = −23 cm–1) and 1114 cm–1 (Δ = −4 cm–1) (Figure S3b). We note that the former band at 1321 cm–1 is in the range for coordinated N-bound nitrosoarenes, , and is similar to the υNOs reported for the ruthenium compounds (TPP)­Ru­(PhNO)­(1-MeIm) (υNO = 1321 cm–1) and (TPP)­Ru­(PhNO)­(py) (υNO = 1328 cm–1). It is unclear from the IR spectra which of the axial p-/m-/o-Cl isomers may have reacted with NO to result in the proposed liganded ONC6H4Cl derivative, or if any of the axial p-/m-/o-Cl isomers were unreactive with the added NO. The DFT-calculated υNO bands for the isomeric (TPP)­Rh­(ONC6H4Cl-p) and (TPP)­Rh­(ONC6H4Cl-o) nitrosoarene derivatives are similar, at 1340 cm–1 and 1336 cm–1, respectively (Figure S4; ωB97X-D/LANL2DZ­(Rh)/6-31+G*­(non-Rh); a band at 890 cm–1 is associated primarily with the aryl group of the nitrosoarene in the complex). We thus assign the experimentally observed 1321 cm–1 band to the υNO of a Rh-coordinated nitrosoarene ONC6H4Cl ligand. Unfortunately, numerous attempts to crystallize and structurally characterize the Rh-containing product(s) from the solution reaction have, to date, only resulted in the generation of crystals that were shown by crystallography to be unreacted (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2) x . We also find that a very small amount (∼1% yield) of the nitroarene p-O2NC6H4Cl, identified by its characteristic UV–vis and 1H NMR experimental and calculated spectra (Figure S5), − was formed when the workup of the product mixture was performed aerobically, indicative of likely nitrosoarene formation for the para-Cl isomer and subsequent air-oxidation to the identified nitroarene product.

We have previously reported successes with a “solid crystal + NO” synthesis methodology to generate kinetically unstable nitrosyl metalloporphyrins that could not be readily crystallized intact from solution. − Exposure of randomly selected plate-shaped crystals of (TPP)­Rh­(C6H4Cl)­(CH2Cl2) x , placed in a vial under nitrogen, to NO gas for ∼2 weeks resulted in the generation of the NO-inserted product (TPP)­Rh­(ONC6H4Cl-o)­(CH2Cl2)0.5 together with unreacted (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2)0.5 in a 1:1 ratio in the same crystal as determined by X-ray crystallography (eq and Figures and S6).

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X-ray crystal structure of the asymmetric unit showing the relative positioning of the NO-inserted product (TPP)­Rh­(ONC6H4Cl-o)­(CH2Cl2)0.5 (bottom) and unreacted (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2)0.5 (top) in a 1:1 ratio. The coordinated CH2Cl2 solvates refined to 50% occupancy, and the p-Cl/m-Cl ratio in the latter unreacted complex is 91:9. Thermal ellipsoids are drawn at 50% [CCDC 2480161].

Interestingly, the observed NO-insertion product for this formally Rh­(II) compound contained the nitrosoarene possessing only the ortho-Cl subsequent on the aryl ring. To the best of our knowledge, no other (por)­Rh­(RNO/ArNO)- containing complexes have been reported or structurally characterized.

The “unreacted” molecule in the asymmetric unit contained the para-Cl and meta-Cl substituents (i.e., no ortho-Cl) on the axial aryl ligand in a 91:9 ratio. The axial Rh–C bond length of 1.967(14) Å in this formally Rh­(III) compound is similar to those determined for the independently crystallized complexes (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2) x (1.988(4) Å; Figure S7; p-/m-Cl ratio of 91:9) and (TPP)­Rh­(C6H4Cl-p/m) (1.970(7) Å for p-Cl, 2.007(16) Å for m-Cl; Figure S8; p-/m-Cl ratio of 81:19).

The observation of formal NO insertion into the likely aryl ortho-Cl isomer and not the para-/meta-Cl isomers of the (TPP)­Rh­(C6H4Cl)­(CH2Cl2) x precursor in this “crystal+gas” reaction led to us pursue examination of any differences in the geometrical parameters of these isomers. Unfortunately, all of our numerous efforts to date at reproducing the plate-shaped crystals (with the P21 space group) with the presumed 1:1 ratio of the precursor (TPP)­Rh­(C6H4Cl-o)­(CH2Cl2) x and (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2) x isomers that resulted in the NO insertion reaction of eq and Figure have not been successful. Almost all of our crystallization attempts of the starting (TPP)­Rh­(C6H4Cl-p/m/o)­(CH2Cl2) x reagent (with the bulk p-/m-/o-Cl isomeric ratio of 74:10:16) have resulted in block or needle crystals containing only the (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2) x isomers without the ortho-Cl isomer present (e.g., Figures S7 and S8, with the para-Cl:meta-Cl isomers in a ∼8–9:2–1 ratio). ,, We find, however, that these crystals with the p-/m-Cl isomers are unreactive with NO under the same crystal+NO gas reaction conditions used for the process in eq .

This past summer, and after a multiyear effort since early 2014 when we first discovered the NO insertion reaction of Figure , we were fortunate to obtain plate-shaped crystals whose subsequent X-ray structural determination revealed the desired 1:1 ratio of the ortho-Cl:para-/meta-Cl isomers (Figures and S9) but in the P-1 space group with a different molecular packing than that observed in the P21 space group structure of Figure . However, exposure of this batch of crystals to NO gas did not result in an NO-inserted product. Examination of the molecular packing ligand in this P-1 crystal revealed that the axial ortho-Cl ligand was sterically constrained so as not to easily bend to form the ONC6H4Cl-o derivative. Curiously, the X-ray structural determination of other crystals from the same crystallization batch revealed a similar P-1 molecular packing as in Figure , but now with a 1:1 ratio of (TPP)­Rh­(C6H4Cl-p)­(CH2Cl2) x and (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2/MeOH) x (i.e., with no ortho-Cl isomer present; Figure S11). We have, to date, not been able to find a reproducible crystallization condition to generate the plate-shaped P21 crystal form with the desired “parallel porphyrin” packing with the 1:1 ratio of o-Cl to p-/m-Cl isomers for the crystal+NO reaction to generate the NO-inserted product of Figure . This remains a crystallization challenge.

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X-ray crystal structure of the asymmetric unit showing the relative positioning of (TPP)­Rh­(C6H4Cl-o)­(CH2Cl2) (left) and (TPP)­Rh­(C6H4Cl-p/m)­(CH2Cl2/MeOH) (right). The axially coordinated CH2Cl2 (left) and CH2Cl2/MeOH (right) ligands are not shown; other coordinated solvent molecules could contribute to site occupancies, but these were not modeled due to diffuse electron densities (see SI). Thermal ellipsoids are drawn at 50% [CCDC 2480163].

Importantly, and to the best of our knowledge, the formation of (TPP)­Rh­(ONC6H4Cl-o)­(CH2Cl2)0.5 (eq and Figure ) represents the first experimental demonstration of NO insertion into a metalloporphyrin–carbon bond for any metal. In particular, we were intrigued that the NO insertion appeared to have occurred for the ortho-Cl isomer but not the para-/meta-Cl isomers in the selected crystal. Our success at generating and determining the X-ray structure of a crystal form with the desired 1:1 ratio (although in the alternate P-1 space group) has allowed us to experimentally probe the geometrical differences of these isomers at the molecular level to complement our DFT studies.

Geometry optimizations of the three isomeric (TPP)­Rh­(C6H4Cl-p/m/o) compounds using DFT reveal, perhaps not surprisingly, that the Rh–C bond length of the ortho-Cl isomer is slightly elongated relative to those of the other two para-/meta-Cl isomers (Figure ), as the ortho-Cl atom will be expected to bump up against the porphyrin macrocycle; such steric hindrance has been noted for other metalloporphyrins with “ortho”-substituted ligands such as (TPP)­Fe­(2-MeHIm). Consistent with this steric hindrance is the observation that the calculated apical displacement of the Rh atom (ΔRh) from the 24-atom porphyrin plane is the largest for the ortho-Cl isomer.

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DFT-calculated structures of the parent (TPP)­Rh­(C6H4Cl-p/m/o) isomers showing the axial Rh–C bond lengths and relative single-point energies of the complexes; the equatorial porphyrin aryl groups have been deleted (in the figure) for clarity while retaining the bound aryl C atoms [M06/LANL2DZ/6–31+G*]. The calculations with the unsubstituted porphine ligands gave the same trends in Rh–C bond lengths and relative energies.

Variations in the tilting of the axial–C6H4Cl ligand with respect to the porphyrin plane are evident when considering that the calculated axial ∠Rh–(C–C)­aryl bond angles for the para-Cl and meta-Cl isomers are ∼120° (left of Figure ), whereas the related ∠Rh–C–C­(Cl) and ∠Rh–C–C­(H) bond angles for the ortho-Cl isomer (right of Figure ) are ∼127° and ∼117°, respectively. This calculated axial tilt for the ortho-Cl isomer is consistent with what we observed experimentally (left of Figure ), where the ∠Rh–C–C­(Cl) and ∠Rh–C–C­(H) bond angles are 131(2)° and 109.9(19)°, respectively. The associated energies show the order to be para-Cl ≈ meta-Cl < ortho-Cl, with the ortho-Cl isomer being the highest in energy with respect to the other isomers (i.e., by ∼7 kcal/mol and ∼8 kcal/mol when the unsubstituted porphine macrocycle is used in the calculations).

The calculated structural and energy data appear to support our initial hypothesis that the ortho-Cl isomer is likely the more reactive isomer in its reaction with NO gas, consistent with our experimental observation that the NO insertion occurred in the solid crystal for the ortho-Cl isomer but not for the para-/meta-Cl isomers (Figure ).

We computationally probed the NO insertion pathway for all three (porphine)­Rh­(C6H4Cl-p/m/o) (porphine = unsubstituted porphyrin) isomers using DFT calculations. Scheme (top) shows the molecular structures of the identified intermediate and transition state along the reaction pathway, using the experimentally observed ortho-Cl isomer as the framework for the discussion. We utilized the freezing string method as implemented in Q-Chem to locate the transition states for the three isomers starting with an intermediate I that places both the Rh–C6H4Cl moiety and the NO molecule on the same side of the porphine, with the NO ligand ∼6 Å from the Rh-aryl fragment (placing the NO on the same side of the Rh-aryl fragment avoids the generation of the simple Rh–NO adduct). Calculated relative energies and selected geometrical parameters for the reagents, intermediates, transition states, and products for all three isomers are shown in Table S11.

1. Calculated Reaction Pathway Illustrated for the (Porphine)­Rh­(C6H4Cl-o Isomer), Associated Energies, And Geometrical Parameters ,

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a Calculated for the unsubstituted porphine model. The NPA charges (in units of e) for selected atoms were calculated using M06/LANL2DZ/6–31+G*.

b The energy diagrams for the three -p/-m/-o isomers employing two other functionals are shown in the Supporting Information (Figures S12 (ωB97X-D/LANL2DZ/6–31+G*) and S13 (B3LYP/LANL2DZ/6–31+G*)).

c Energies are relative to those of the reactants.

d Energy difference with respect to the intermediate I o.

Figure shows the energy diagram for the NO insertion pathway for all three (porphine)­Rh­(C6H4Cl-p/m/o) isomers (M06/LANL2DZ/6–31+G*); energies are indicated relative to the lowest energy reactant [(porphine)­Rh­(C6H4Cl-m)+NO] combination (i.e., with the meta-Cl isomer) set to 0 kcal/mol. As can be seen from the figure, all three (porphine)­Rh­(C6H4Cl-p/m/o) isomers follow similar energy trends for the conversion of reagents (R) → intermediates (I) → transition states (TS) → products (P) in these computed NO insertion reactions. The energy diagrams using different functionals (ωB97X-D/LANL2DZ/6–31+G* and B3LYP/LANL2DZ/6–31+G*) for the three isomers are shown in Figures S12–S13 in the Supporting Information. The results from all three functionals show the pathway of the ortho-Cl isomer as having the lowest ΔG ‡ energy, thus we will focus the discussion of Scheme and Figure using this isomer, consistent with our experimental observation.

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Energy diagram for the NO insertion pathway with the [(porphine)­Rh­(C6H4Cl-o/-m/-p)+NO] combination set to 0 kcal/mol with respect to the meta-Cl isomer reactants (R m ) using M06/LANL2DZ/6–31+G*. The values in the boxed area (ΔG ‡) are calculated relative to the respective intermediate ( I ).

The intermediate I o for the ortho-Cl pathway is <1 kcal/mol lower in energy than the isolated reagent molecules (R o) (Figure ). In this configuration, the nearby NO molecule in I o is essentially charge neutral as determined by the calculated NPA charges (top of Scheme ). The two most significant MOs that contribute to the Rh–Caryl interaction (Mayer bond order of 0.92; Table S12 in the SI) in the intermediate I are shown in Figure A; these involve σ interactions between the Rh d z2 orbital and the aryl group, accounting for ∼33% (HOMO–15) and ∼12% (HOMO–28) of the Rh–Caryl interaction; several other low-lying MOs contribute to this as well (Tables S13–S18). Not surprisingly, we found no significant molecular orbital interactions between the nearby NO ligand and the five-coordinate (porphine)­Rh­(C6H4Cl-o) in the intermediate complex I o.

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Molecular orbitals (MOs) that display significant Rh–axial bonding interactions for (A) the intermediate [(porphine)­Rh­(C6H4Cl-o)+NO] combination ( I ), (B) the transition state [(porphine)­Rh­(C6H4Cl-o)···NO] (TS), and (C) the NO-inserted product [(porphine)­Rh­(ONC6H4Cl-o)] (P). Percent values displayed = 40%. The MO numbering is for the α spin. Orientations were selected to better visualize the respective MOs.

The transition state (TS o ; ortho-Cl isomer) structure that was located along the NO insertion pathway was calculated to be +13.7 kcal/mol higher in energy than the intermediate I o (and +12.8 kcal/mol with respect to the starting reagents R o ; Scheme ). Similar results in terms of relative energies and ligand orientations were obtained for the analogous reaction pathways involving the meta-Cl and para-Cl isomers; the energies of the analogous TS m (for meta-Cl isomer) and TS p (for para-Cl isomer) were calculated to be 21.1 and 18.0 kcal/mol higher in energy than their intermediates I m and I p, respectively (Figure ).

There are several interesting features of the TS o in Scheme to note: (i) the axial Rh–C bond lengthens from 2.031 Å (in I o) to 2.175 Å in TS o , consistent with priming this bond for the insertion reaction, (ii) the axial C6H4Cl-o plane tilts away from the normal to the 24-atom porphine plane (from 90° in I o to 47° in TS o ; Figure S14) further exposing the Rh-bound C atom for interaction with NO, (iii) the orientation of the incoming NO ligand in TS o places its N atom closer to the Rh-bound C atom ((Rh)­C···N­(O) = 2.060 Å), and (iv) the relatively long Rh···N­(O) distance of 2.946 Å precludes direct NO coordination to the Rh center. Importantly, the NPA charge on the Rh-bound C atom in TS o is reversed (from that in I o) to become slightly negative, decreasing from +0.024e in I o to −0.046e in TS o , and the incoming NO ligand becomes more overall slightly positively charged in the TS o at +0.104e. This is consistent with the subsequent favorable C–N bond formation to yield the product P o.

We note that upon forming the transition state with a slightly reduced Rh–Caryl Mayer bond order of 0.68, interactions between the Rh–Caryl, Rh–NNO and Caryl–NNO become evident as reflected by the four most significant MOs that contribute to these combined interactions (Figure B). The HOMO–15 and HOMO–27 MOs are characterized by σ interactions between the Rh d z2 orbital and the aryl group, accounting for ∼32% and ∼13%, respectively. The HOMO–53 orbital, concentrated on the Caryl–NNO fragment, reveals an interaction between a p orbital of the bound carbon atom and a p orbital of the N­(NO) ligand, the latter of which is involved in a π interaction with the O­(NO) atom; this HOMO–53 orbital accounts for ∼13% of the total Caryl–NNO bond order of 0.45. Interestingly, the HOMO displays an antibonding interaction between the Rh d z2 and the NNO atom, but with a bonding interaction between p orbitals of the NNO and Caryl atoms.

These observations above for the transition state TS o place both the aryl group in (porphine)­Rh­(C6H4Cl-o) and the NO molecule in an appropriate geometry and electronic state for formation of the resulting C–N bond in the product P o (right of Scheme ), which was calculated to be 21.7 kcal/mol lower in energy than the starting combination R o . Figure C displays the two significant MOs that contribute to the axial Rh–N bond (BO = 0.83) in the NO-inserted product, namely σ interactions between the Rh d z2 orbital and N­(O) (HOMO–46; ∼7% of axial Rh–N) and N­(C) (HOMO–45; ∼5% of axial Rh–N).

Importantly, the calculated energies in Figures and S12–S13 for all three (porphine)­Rh­(C6H4Cl-o/m/p) isomers rationalize our experimental observation that the ortho-Cl isomer was the preferred isomer that reacted with incoming NO in the crystal at room temperature under our reaction conditions.

In summary, we report, using a “solid crystal + NO” approach, the first experimental demonstration of NO insertion into a metalloporphyrin–carbon bond for any metal. Our computational results have provided insight into our observation that it was the more reactive ortho-Cl isomer (based on calculated geometries and energies) of the organorhodium porphyrin that inserted NO into its Rh–C bond in the crystal. Presumably, the slow diffusion of NO gas into the crystal allowed only the more reactive ortho-Cl isomer (but not the para-Cl or meta-Cl isomers) to insert NO within the time frame of the reaction. The calculated energies for the latter isomers do not preclude their similar reaction with NO under alternate reaction conditions. Indeed, the results of our solution studies mentioned above suggested that the para-Cl isomer was also reactive to some extent in solution to this NO insertion reaction. Interestingly, we note that although NO insertion reactions in coordination-organometallic chemistry usually proceed via migratory insertion processes (i.e., involving prior coordination of NO) the NO insertion reaction reported in this work does not suggest such a requirement for precoordination of NO to the Rh center prior to its formal into the Rh–C bond. − We are currently pursuing the syntheses and NO reactivity of other organometallic porphyrins to probe the generality of this NO insertion reaction.

Supplementary Material

au5c01770_si_001.pdf (7.9MB, pdf)

Acknowledgments

We thank Dr. Douglas R. Powell for some of the X-ray diffraction data collection and Dr. Erwin Abucayon for technical assistance. G.B.R.-A. is grateful to Dr. Michel Dupuis (University of Buffalo) for helpful discussions.

Glossary

ABBREVIATIONS

TPP

tetraphenylporphyrinato dianion

OEP

octaethylporphyrinato dianion

DFT

density functional theory

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.5c01770.

  • Experimental details, crystallography, computational methods, and data, with associated figures and tables (PDF)

#.

J.L. and G.Y. contributed equally to the work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

This material is based upon work supported by (in part while G.B.R.-A. was serving at) the U.S. National Science Foundation (NSF; CHE-2154603 and CHE-1900181). Y.S. is supported by NSF Grant No. CHE-2102071, and Y.Z. is supported by the National Institutes of Health (NIH; GM085774). Support for the acquisitions of the Bruker APEX II CCD diffractometer from the National Science Foundation (CHE-0130835) and the Bruker D8 Quest diffractometer from the NSF MRI program (CHE-1726630) is gratefully acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF or the NIH. We are grateful to the University of Oklahoma Library's Open Access Fund for financial support to cover the open access publication costs.

The authors declare no competing financial interest.

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