MetalâLigand Cooperative Reactivity in the (Pseudo)-Dearomatized

Article pubs.acs.org/JACS

Metal−Ligand Cooperative Reactivity in the (Pseudo)-Dearomatized PNx(P) Systems: The Influence of the Zwitterionic Form in Dearomatized Pincer Complexes Théo P. Gonçalves and Kuo-Wei Huang* KAUST Catalysis Center and Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia S Supporting Information *

ABSTRACT: The concept of aromaticity in pincer ligands and complexes was discussed in order to provide insights into their metal−ligand cooperative activities. The aromatic PNx(P) and dearomatized PNx(P)* pincer ligands and the corresponding transition metal complexes were studied with the nucleus-independent chemical shift (NICSzz), anisotropy of the current (induced) density (ACID), isochemical shielding surfaces (ICSSzz), harmonic oscillator model of aromaticity (HOMA), MCBO, Shannon aromaticity, and natural bond order (NBO) analyses. The study on the model systems showed that for the dearomatized species the decrease of the NICS(1)zz value comes with the larger contribution of the aromatic zwitterionic mesomeric form. In all examples, the incorporation of the metal center into the pincer ligand decreases the NICS(1)zz values. The DFT calculations support the dearomatized pyridine ring in PNP* or PNN* ligand indeed being nonaromatic, in contrast to the PN3(P)* ligand which has partial aromatic character due to the larger contribution of the zwitterionic resonance structure. The difference in aromaticity between the rings contributes to the thermodynamic balance of the metal ligand cooperative reactions, changing the energetics of the process when different dearomatized pincer ligands are used. This was further exemplified by aromaticity analysis of the heterolytic hydrogen cleavage reaction of ruthenium PNN complexes of Milstein and the PN3 of Huang, with similar geometries but distinctive thermodynamic preference.

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INTRODUCTION Thirty-five years after the initial report on the synthesis and coordination chemistry of 2,6-lutidine-derived PNP pincer complexes in 1971 by Nelson, 1 the discovery of the dearomatization/rearomatization process was made by Milstein2 in 2005 as a key step in the catalytic dehydrogenative coupling of alcohols into esters. Such an activation mode via metal−ligand cooperation has led to extraordinary applications with an explosive growth in various dehydrogenative/hydrogenative and bond activation reactions.3 Analogous studies on the coordination chemistry of PN3P pincer complexes containing the NH spacers instead of CH2 groups were first reported by Haupt4 in 1987 and extended by Kirchner in 2006.5 While potential applications on catalysis have been explored, the dearomatization/rearomatization of the central pyridine ring via deprotonation/reprotonation of the N−H arm for catalytic applications was unknown until our discovery in 2011.6,7 Strikingly, this seemingly small change from CH to N in the dearomatized structures has led to different kinetic and thermodynamic properties (Figure 1).6d−f For example, the ruthenium complex reported by Milstein efficiently catalyzes the dehydrogenative acylation of amines with alcohols,8 while PN3P*−Ru promotes the dehydrogenative coupling of amines to imines.6b Both systems catalyze the dehydrogenative © 2017 American Chemical Society

homocoupling of alchohols to esters, but compared to the Milstein’s, the Huang complexes in general require a higher reaction temperature.2b,6b Moreover, it was found that those two systems follow different pathways for the heterolytic hydrogen cleavage reaction. The proton shuttle mechanism is not required for Milstein’s PNN*−Ru complex (4a)6d,9 in contrast to PN3*−Ru complex (4b) which requires two water or protic molecules to connect the reactivities of the imine arm and the Ru center. In fact, 4b was found to effectively catalyze the ester hydrogenation even in the presence of water.6e While numerous applications in catalysis have been developed,3,6,7,10 little effort has been made to understand the origin of the driving force and to rationalize the different thermodynamic preference particularly between the Milstein and Huang systems (Figure 1). In the protonation of analogous dearomatized pincer complexes,11 the influence on the metal center was observed, effectively changing enthalpy of protonation via the aromatization process. Conceivably, such metal−ligand cooperation processes with a carefully manipulated central pyridine ring can be valuable for the design of catalytic reactions as the additional driving force for the Received: June 17, 2017 Published: September 1, 2017 13442

DOI: 10.1021/jacs.7b06305 J. Am. Chem. Soc. 2017, 139, 13442−13449

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Journal of the American Chemical Society

Figure 1. Effect of the pincer arm on the heterolytic hydrogen cleavage reaction of the Milstein and Huang system via direct and proton shuttle mechanisms.

stabilization energy together with magnetic, geometric and reactivity properties, it is difficult to define precisely.25 NICS(0)26 and NBO analyses have been employed to study the aromaticity of the relevant double-deprotonated PNP* pincer ligand,27 suggesting that the central pyridine ring is not aromatic, even though the ring has some significant contribution from the aromatic mesomeric form, enforced by the phosphorus-assisted hyperconjugation, i.e., lone pair Cα‑P → σ*P‑CMe. The NICS analysis has been found to be useful in correlation with the activation energy and thermodynamic of the reactions,28 but it is noted that in comparison to NICS(0)iso NICSzz(1) based on the total contribution to the out-of-plane component of the NICS tensor is more reliable28c,29 when studying aromaticity, pseudoaromaticity, and antiaromaticity.30 The NICSzz(1) methodology has been proven to be extremely useful for identification of leading resonance structures and recognizing aromatic rings in complex polyaromatic hydrocarbons.29c,e,31 Moreover, the aromatic stabilization energy (ASE) correlates better with NICS(1)zz than with the NICSiso, given by equation EASE = −0.604*NICS(1)zz, as suggested by von Ragué Schleyer et al.29b based on the five-member ring analysis, even though the correlation between NICS and ASE for different ring sizes is not as straightforward.23,32 In order to understand the role of aromaticity in metal PNx(P) complexes, we started our investigation on simple systems with zwitterionic resonance forms and analyzed the effects of the substituents on the pyridine ring and the influence of the arms on the aromaticity of ligands/complexes. The energetic contribution from aromatization was quantified by correlating the experimental aromatic stabilization energies with NICS(1)zz values. With these tools in hand, the Milstein’s PNN and Huang’s PN3−Ru complexes were then compared in details from the analysis of the aromaticity, aromatization energy and changes of the aromaticity by means of 3D aromatic descriptors. In the first example of N-methylformamide, the molecule can be represented by the two most populated mesmeric forms, taking into account the π system (Figure 2, 5). In one form (35%), the molecule can be described by zwitterionic structure 5b, and this similar distribution of zwitterionic structures was

reaction can be modulated by the change of aromatization energies.12 Herein, to provide theoretical insights to the catalysis field, we disclose our computational investigation of the aromaticity in PNx(P) and PNx(P)* metal complexes with the aim to understand the impact of the aromaticity in order to offer new and complementary view of the reactivity rising from the aromatization/dearomatization process.

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COMPUTATIONAL DETAILS

All geometry optimizations were carried out at the M06/6-311G(d,p) level of theory together with UltraFine integration grid. For transition metals, the SDD pseudopotential was applied with corresponding basis set.13 The out-of-plane nucleus-independent chemical shift (NICSzz) scans from 0 to 5.0 Å were calculated at CAM-B3LYP/6-311+G(d,p)/ SDD level at geometrical center of the ring. In that analysis the aromatic rings have negative values and nonaromatic rings have positive NICS(1)zz value. To complement the NICS(1)zz interpretation, additional analysis was provided. The Shannon aromaticity analysis (SA) which is based on the electron density, successfully predicts the order of aromaticity in both linear and angular polyacenes.14 The boundary for the aromaticity is 0.003 < SA < 0.005, and lower values are given for more aromatic compounds. In some cases, the multicenter bond order (MCBO)15 with natural atomic orbital (NAO) analysis was also applied. In this analysis the higher values are given for more aromatic compounds. The appropriate file for such calculation was generated with the help of Natural Bond Orbital (NBO6) and DMNAO keyword at M06/6311+G(d,p) level of theory. In addition, harmonic oscillator model of aromaticity (HOMA) analysis was also performed.16 Except for the NICS calculations and anisotropy of the current (induced) density (ACID), all aromaticity analysis were carried out with MultiWFN program,17 and natural bond orbital analysis NRT was carried with the NBO6 program.18 The natural resonance theory calculations were obtained at M06/6-311+G(d,p) method level with multireference NRT analysis by NBO procedure taking into account just three resonance structures: one neutral and two zwitterionic structures with aromatic ring.19 All DFT calculations taken were carried with Gaussian 09.20 The ACID21 and isochemical shielding surfaces (ICSSzz)17,22 calculations were carried with M06/6-311+G(d,p) level of theory.23

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RESULTS AND DISCUSSION Analysis of Model Compounds. Although aromaticity is an important concept in chemistry24 referring to additional 13443


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contributions do not reflect the changes in the aromaticity of 8 and 9. Interestingly, the delocalization of the phosphorus lone pairs, LPp‑rich → Pσ*P−Me in 9b (2.73 kcal/mol) is lower than that of 8b (3.57 kcal/mol). Furthermore, when the NH arm is introduced, the LPp‑rich → Pσ*P−Me delocalization in 10b has an even lower value (2.50 kcal/mol), and the molecule exhibits the highest contribution of the zwitterionic form in this series. Once again, this trend is reflected by the NICS, SA, HOMA, and MCBO values, and those aromaticity indexes provide the same aromatic picture. From other model studies, the aromaticity of the monosubstituted pyridines (11) decreases in the following order: H > Me > OH > NH2, reflecting the π donating ability of the substituents (Figure 3). This is in alignment with

Figure 3. NICS(1)zz values for the substituted pyridine rings.

observation by Alonso et al. that in contrast to the benzene derivatives, all substituents in pyridine decrease the aromaticity of the ring.32b Furthermore, the position of the substitution also plays an important role, and the aromaticity decreases in the following order: 2-position >4-position >3-position; this is trend similar to that in the literature.33 Interestingly, when two substituents are added (12), the effects of the substitutions on the aromaticity are additive, changing the NICS(1)zz linearly as the number of substituents increases. This analysis indicates that the aromaticity of the dearomatized pyridine ring NICS(1)zz values decreases in the presence of the iminic arm. However, the aromaticity of the aromatic pyridine ring is decreased in the presence of the π-donating moieties. These observations open the discussions on two important questions: (a) Is the dearomatized pyridine ring in the seemingly “dearomatized” pincer complexes truly dearomatized? (b) What is the relationship between aromaticity and reactivity of these “dearomatized” pincer complexes? With the results from simple pyridine systems, we then analyzed the aromaticity of the pyridine-based pincer ligands in order to evaluate the influence of the arms (Figure 4). The two di-tbutylphosphinomethyl substituents give a higher aromaticity in 13 than that in 14 with one amino arm. The decrease in aromaticity by NICS(1)zz is even larger when two amino arms are present, giving the lowest values for the aromaticity in 15 among this pyridine group. The addition of a methoxy group in the 4-position also decreases the aromaticity from NICS(1)zz value of −24.5 in 13 to −21.4 in 16. For the dearomatized pyridine rings, the NICS(1)zz values are positive if the CH arm is present (17, 20, and 21), which is in alignment with chemical intuition. Surprisingly, if an iminic arm, N, is in place, the values of NICS(1)zz are negative (18 and 19). This may imply that the rings manifest aromaticity through a larger contribution of the zwitterionic resonance form. The value is slightly higher in 19 than in 18 which reflects the stronger influence of the amino groups as supported by our model study.

Figure 2. Contribution of the mesomeric forms in prototypic PNx(P)* ligands.

observed by Weinhold.19a The replacement of the oxygen by nitrogen in 6 decreases the zwitterionic contribution to 27%. This trend reflects the change of the electronegativity of the atoms and their ability to accommodate the negative charge. However, if the imidamide moiety is incorporated in to the pyridine ring 7, then the contribution of the zwitterionic structure 7b is more pronounced, likely due to additional stabilization gained from aromaticity. Indeed, by NBO analysis, the zwitterionic form has a higher contribution (54%) and represents the molecule better than the neutral resonance form 7a. This is supported by the negative NICS value, SA, HOMA, and MCBO, suggesting the presence of a very weak aromatic resonance. The negative value of NICS was also observed in the pyridin-2(1H)-one system,33 and it was applied to design highly electron-donating ligand.34 When the CH−Me arm is attached (8), the contribution of zwitterionic form (8b) becomes slightly lower, favoring the nonaromatic form 8a. This is supported by higher NICS, SA, and lower HOMA MCBO value. Once again, this arises from the fact that stabilization of the negative charge is better on the nitrogen than on the carbon center. In the case of dimethylphosphine moiety attached to the CH arm (9), the contribution of the zwitterionic form (9b) is higher compared to that of 8. This trend is also reflected by the NICS, SA, HOMA and MCBO values. It should be noted, however, that likely rising from limitations of NBO the resonance 13444


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Figure 5. NICS(1)zz values for ligands and complexes. Figure 4. NICS(1)zz values for the PNx(P) ligands.

double bonds and exhibit a “broken” aromatic system which indicates the delocalized bond system similar to a hexatriene. Inspired by previous work on the aromatic stabilization energy and NICS index,12b,38 we correlate the ASE with NICS(1)zz.38 In addition, we benefit from the experimental work of Katritzky et al. on 2-pyridone and related systems, in which related pyridine ASEs were studied by the tautomeric equilibrium.39 The aromatic stabilization energies (ΔpyASE) for seven dearomatized model structures and the pyridine ring were fitted against NICS(1)zz values, giving a correlation coefficient of 0.96 (Figure 6). The slope of 0.68 is similar to that for 5-membered rings.29b The resulting ΔpyASE [kcal/mol] = 0.68·NICS(1)zz equation allows us to provide an estimate of the aromatic stabilization energy which can be used in analyzing dearomatization/aromatization reaction energies.

Interestingly, the comparison between 18 and 20 reveals that the desaturation on the N side leads to a higher aromaticity with a more negative NICS(1)zz value. The influence of the methoxy group on the dearomatized pyridine ring (17 and 21) is negligible, in contrast to that to the aromatic tautomeric analogs (13 and 16). A similar trend can also be observed in the bipyridyl-containing ligands. The dearomatized ring in 22 has a positive value of NICS(1)zz, and that in 24 has a negative value for the dearomatized ring, indicative of the stabilization effect via π delocalization. Their aromatic tautomers, 23 and 25, reveal that the presence of an NH arm also increases the NICS(1)zz value. We continued to analyze the NICS(1)zz values of some pincer ligands, and complexes to evaluate the impact of the metal, we compared the metal complexes with corresponding protonated ligand (Figure 5). The complexation of the POCOP ligand (26) with nickel has a negligible impact on the aromaticity in 27. This complex was found to undergo the CO2 insertion.35 When the dearomatized ligand 28 with positive NICS(1)zz value is used, the complexation with ruthenium (29) results in a negative value of NICS(1)zz = −3.1. If the dearomatized PNP* ligand (17) is complexed with rhenium (30),36 then the NICS(1)zz value of +0.6 stays positive, suggesting a nonaromatic character for the central ring. Interestingly, if the same ligand binds palladium (31, 32) and platinum (33, 34),11 then NICS(1)zz values are negative, significantly lower than those of Ru and Re analogs. It was also observed that the methyl complexes (32, 34) give more negative values than do the chlorine counterparts, presumably due to the more σ-donating character of the methyl group. This may serve as the reason why the protonation enthalpy varies from metal to metal on the same ligand framework.11 In the last example, cupper(I) complex 35 synthesized by van der Vlugt et al.37 shows the most negative value of NICS(1)zz among this series. It was discussed by Milstein et al. that11 on the basis of the geometry those complexes have alternating single and

Figure 6. Correlation between ΔpyASE and NICS(1)zz for pyridine and derivatives. 13445


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Figure 7. Aromaticity analysis for hydrogentaion of Milstein and Huang system.

Figure 8. ICSS and ACID surfaces for Huang and Milstein systems.

Comparison between the Milstein and Huang Systems. On the basis of information learned from the

study on model compounds, we further investigated the heterolytic hydrogen cleavage reaction by Milstein’s PNN and 13446


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Journal of the American Chemical Society Huang’s PN3 complexes as the examples to interrogate the role of the aromaticity on the reaction energetics. As the ring and the unsaturated arm in these Ru complexes are on the same plane, no significant alteration in geometry is expected when the rearomatization takes place. Consequently, the process relies mainly on the electronic effects through the conjugated π system with metal. Accordingly, the energetics of the aromatization/dearomatization reaction can be linked to the aromaticity indices. Milstein’s PNN complex 4a favors the heterolytic hydrogen cleavage reaction to give specie 1a.9 However, in the PN3 system, the thermodynamic equilibrium is shifted to the endergonic side, and non-hydrogenated 4b is preferred.6e Since the N−H bond is typically stronger than the C−H bond by 5.0 kcal/mol,40 there must be other factors that govern the driving force which gives Milstein system energetic advantage of 9.0 kcal/mol by electronic energy at M06/6311+G(d,p) level. First, it should be noted again that the metal complexation decreases the NICS(1) zz values on the dearomatized ring. For the Milstein system, the change for 22 → 4a is from 4.3 to 0.6, whereas for the Huang (24 → 4b) system, it is −4.1 to −7.6. At the same time, metal complexation in both 23 → 1a and 25 → 1b examples yields in decreasing aromaticity of the pyridine rings (Figures 7 and SI2). During the heterolytic hydrogen cleavage reaction, ΔNICS(1)zz of the dearomatized pyridine ring in 4a changes from −0.6 to −23.9 in 1a, resulting in a difference in ΔNICS(1)zz of −24.5 and suggesting the formation of an aromatic ring. In contrast, the change of ΔNICS(1)zz in the spectator pyridine ring is only −0.6. A significant difference was observed in the Huang system. The hydrogenation of 4b to 1b results in a much smaller change of the aromaticity of the central pyridine ring, ΔNICS(1)zz‑rxn = −12.3, with a similar value for the spectator pyridine ring (−0.7). In comparison to Milstein system, the aromatization of the Huang system is less, manifested by the value of ΔΔNICS(1)zz‑rxn = −12.2, which follows the energetic preference of ΔΔErxn = −9.0 and corresponds to −8.3 kcal/mol of difference in aromatic stabilization energy (ΔΔASErxn) between those two systems. Thus, in this simple view, the the bigger aromatization energy by ΔNICS(1)zz‑rxn, the larger the driving force present; consequently, the reaction is pushed to the exergonic side. To provide further insights to the aromaticity change, ICSSzz and ACID surfaces were examined (Figure 8). For the Milstein system, the change of the ICSSzz surface during the rearomatization process from 4a to 1 is remarkable. The nonaromatic pyridine ring of the PNN−Ru complex has paratropic (positive NICSzz) light blue surface in the center of the ring, being visible at 5 and 15 contour values (4a−a and 4a−e). The manifestation of the antiaromaticity disappears after rearomatization via hydrogenation, forming the exclusively diatropic surface with negative NICSzz value (4a−b and 4a−f). The ACID analysis confirms the antiaromatic (paratropic) ring current, i.e., anticlockwise induced current vectors, located at the dearomatized pyriding Py ring in 4a−i. After hydrogenation, rearomatization of the ring is reflected by the clockwise induced current vectors (4a−j), confirming the aromatic (diatropic) ring current. In contrast to 4a−a/−e, the ICSSzz surface for the Huang system shows no presence of the paratropic (antiaromatic) surface in the center of the “dearomatized” ring but rather shows only diatropic (aromatic) surface (4b−c and 4b−g). Similarly, the rearomatization is visualized by the presence of the large aromatic (diatropic)

surface on the ring (1b−d and 1b−h). The ACID analysis shows the mild aromatic (diatropic) ring current, i.e., clockwise induced current vectors, located at the “dearomatized” pyridine ring in (4b−k), which becomes aromatic in 1b−i. On the basis of those qualitative interpretations of the surfaces, it is clear that change of the aromaticity is much smaller for the Huang system than for the Milstein system as a consequence of the presence of the iminic arm and the contribution of the zwitterionic aromatic form.

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CONCLUSION The aromatic and dearomatized pincer ligands and complexes were studied by aromaticity indices. In all studied cases, the replacement of a proton with a metal on the model pyridine compounds results in a more aromatic character. These observations suggest that the coordination to the metal center leads to a larger contribution of the zwitterionic resonance form. This boost of ligand aromaticity through complexation is more pronounced in the PN3(P) than in the PNP or PNN based complexes. The reaction aromatic stabilization energy force ΔrxnASE can be estimated by the correlation with ΔNICS(1)zz‑rxn. In contrast to the Huang system, our estimates show that the heterolytic hydrogen cleavage reaction of Milstein PNN−Ru system has a larger change of the aromaticity and in consequence larger energy gain through rearomatization. It is clear that the aromaticity gain is not the only factor influencing the reaction energetics, but its contribution needs to be taken into account. Our results provide an explanation of the thermodynamic difference between the seemingly similar Milstein and Huang systems. Other unique reactivities are expected to be observed in these two kinetically and thermodynamically different systems and their other analogous transition metal complexes. Moreover, our work calls for more detailed theoretic studies on other catalytic systems that may involve ligand dearomatization/ rearomatization processes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06305.

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All the geometries and NICS(1)zz scans of the studied systems (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Kuo-Wei Huang: 0000-0003-1900-2658 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the service of NOOR 2, Shaheen 2 High Performance Computing Facilities, and financial support from King Abdullah University of Science and Technology (KAUST). 13447


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