A Mixed-Ring Sandwich Complex from Unexpected Ring Contraction

Article pubs.acs.org/IC

A Mixed-Ring Sandwich Complex from Unexpected Ring Contraction in [Re(η6‑C6H5Br)(η6‑C6R6)](PF6) Giuseppe Meola, Henrik Braband, Daniel Hernández-Valdés, Carla Gotzmann, Thomas Fox, Bernhard Spingler, and Roger Alberto* Department of Chemistry, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland S Supporting Information *

ABSTRACT: The contraction of coordinated aromatic hydrocarbons is a rare reactivity pattern in organometallic chemistry. We describe the conversion of a bromobenzene coordinated to a ReI center into a cyclopentadienyl aldehyde. Under mildly alkaline conditions, the expected phenol complex is formed with Re and 99Tc but under strong basic conditions; ring contraction occurs in close to quantitative yields for Re only. A mechanism for this unprecedented reaction is proposed based on 1H and 2H NMR spectra and density functional theory calculations.

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INTRODUCTION Sandwich complexes of d- and f-elements are core elements in many fields of contemporary science and are key players, namely, in homogeneous catalysis, material sciences, and in biological or medicinal-oriented research topics.1,2 These organometallics complemented coordination compounds over the recent years in life sciences, in particular. Many examples corroborate their superiority to coordination compounds, mainly due to additional options such as metal- or ligandcentered reactivities and optical imaging through fluorescence.3,4 The selection of organometallic building blocks comprising aromatic ligands is, however, relatively limited. Ferrocene (Fc) is an exceptional multimodal key player as a diagnostic and a therapeutic label at the same time due to its induction of reactive oxygen species (ROS) formation.5,6 Furthermore, the {RuII(arene)}2+ fragment coordinates directly to phenyl groups in biomolecules, a convenient way of labeling for proteomic purposes, for instance.7,8 In the bio-organometallic field, sandwich complexes with mixed aromatic ligands of various hapticities are essentially absent. Although known since the advent of organometallic chemistry, little is known about such compounds, and few have found interest beyond structures and properties. Mixed-aromatic sandwich complexes often obey the 18-electron rule, which limits the combinatorial chemistry of two different aromatic ligands. For group 4 and 5 elements, combinations between [η7-C7H7]+ and [η5-C5H5]− were reported;9 for group 6 and 8, combinations between [η6C6H6] and [η5-C5H5]− and for group 9 [η5-C5H5]− and [η4C4H4] π-complexes are prototypical.10,11 Probably best investigated are mixed-ring sandwich complexes of iron [Fe(η5-C5H5)(η6-C6H6)]+ for which all kinds of organic reactions at the arene ring are described but leaving the cyclopentadienyl untouched.12 For group 7, there is a gap in the systematics of mixed-aromatic sandwiches, and only [M(η5C5H5)(η6-C6H6)] (M = Mn, Re) are experimentally set.13−16 To obtain these mixed-aromatic sandwich complexes, the © 2017 American Chemical Society

ligands are introduced directly as neutral arenes, tropylium, or cyclopentadienyl salts by various substitution reactions. Along their studies on substitution reactions on coordinated aromatic ligands, Pauson et al. reported in the 1960s a ring contraction of tropylium, coordinated to the {Cr(CO)3} moiety, to benzene upon attack of strong nucleophiles.17,18 This uncommon reaction found only punctual interest. Easy nucleophilic attack to the positively charged tropylium ligand and concomitant formation of highly stabilized benzene was interpreted as the underlying driving force.18

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RESULTS AND DISCUSSION In our search for new organometallic building blocks for life sciences, we recently introduced sandwich complexes of the type [M(arene)2]+ (M = 99(m)Tc, Re) along a convenient route and directly from [MO4]−.19 The arene rings can be functionalized for conjugation to targeting molecules, for example, with bromide for nucleophilic substitution.20 Since phenols are frequent modalities in biomolecules, we attempted to replace the bromide group in [Re(η6-C6H5Br)(η6-C6H6)]+ (1+) by hydroxide to obtain [Re(η6-C6H5OH)(η6-C6H6)]+ (2+) (Scheme 1). This reaction proceeded as expected, and the structure of compound [2](OTf) was confirmed by X-ray structure analysis (see Supporting Information). In agreement with the electrophilic reactivity of coordinated hydrocarbons,21 complex 2+ was formed in almost quantitative yield in water under mildly alkaline conditions and elevated temperatures of 60−80 °C. However, at temperatures of 80 °C or higher (microwave), and with hydroxide being in large excess over 1+ (>100:1), complex 2+ (in solution as its conjugate base 2) formed only as a minor side product. The main product precipitated from solution. Upon isolation, we identified this compound as the mixed-aromatic sandwich Received: February 14, 2017 Published: May 11, 2017 6297

DOI: 10.1021/acs.inorgchem.7b00394 Inorg. Chem. 2017, 56, 6297−6301

Article

Inorganic Chemistry Scheme 1. Reaction Pathwaysa of [Re(η6-C6H5Br)(η6C6R6)]+ with (i) Small and (ii) Large Excess of NaOH

Cp-aldehyde complex [Re(η5-C5H4CHO)(η6-C6Me6)] (5) albeit in lower yields due to decreased water solubility of [4]Br. Common chemical analyses and an X-ray structure determination confirmed the structure of 5 (see Supporting Information). No mixed sandwich complexes are known so far for 99Tc. To elucidate if this ring contraction is of more general nature, and would also apply for 99Tc, we prepared [99Tc(η6-C6H5Br)(η6C6Me6)]+ (6+) and subjected it to the same conditions as 4+. However, in contrast to 4+, no ring contraction was observed, and only the phenol complex [99Tc(η6-C6H5OH)(η6-C6Me6)] (7+) was formed. Thus, the observed ring contraction tolerates different opposite arene ligands but (so far) not another element of the group 7 triad. The fact that this ring contraction has not been observed previously in other d-element bis-arene complexes may be related to their respective instabilities under strongly alkaline conditions. Still, it is worthwhile to extend the study also to the neighboring elements and to iron or ruthenium, in particular.12 The reported reactivity of the 99Tc complex entails the question about the availability of the 99mTc homologues. At very high dilution with 99mTc, kinetics often governs product formation, which might be different from thermodynamically driven 99Tc chemistry. We emphasize that the successful reaction with 99Tc does not necessarily stand for an identical behavior with 99mTc. We prepared [99mTc(η6-C6H5Br)(η6C6Me6)]+ as described before19 and transferred it into a strongly alkaline solution of pH ≈ 12. Heating to 100 °C led to a slow but close to quantitative formation of the 99mTc analogue of 7+ (deprotonated in this solution) indeed. Comparison of high-performance liquid chromatography (HPLC) retention time confirmed its nature (see Supporting Information). The mixed-ring sandwich complexes 3 and 5 are neutral 18electron species, comparable to, for example, ferrocene (Fc) or [Cr(η6-C6H6)2]. Since Fc, in particular, is frequently applied in bio-organometallic chemistry and other fields, a comparison of the respective electrochemical data is of interest. A cyclic voltammogram of 3 is shown in Figure 2 as an overlay with the ones of Fc and [Re(η6-C6H6)2]+. Figure 2 shows that the two compounds Fc and 3 display under identical conditions about the same E°1/2, namely, 0.45 V for Fc/Fc+ and 0.44 V for 3°/3+ versus Ag/AgCl in acetonitrile. We note that this potential is substantially shifted as compared to the isoelectronic complex [Re(η6-C6H6)2]+, for which E°1/2

a Under the shown conditions, the −OH group is deprotonated in the reaction.

complex [Re(η5-C5H4CHO)(η6-C6H6)] (3). Along the reaction pathway, the coordinated bromobenzene ligand obviously contracted into a cyclopentadienyl system, and the “excess” carbon from the former benzene ring was transformed into the pendent aldehyde group. Besides common analytical characterization (see Supporting Information), we confirmed the authenticity of 3 by X-ray structure analysis (Figure 1).

Figure 1. ORTEP of [Re(η5-C5H4CHO)(η6-C6H6)] (3, left) and [99Tc(η6-C6H5O)(η6-C6Me6)] (7, deprotonated form, right). Relevant bond lengths (Å) are for 3: Re1−Cp 1.888(8), Re1−benzene(centroid) 1.705(7) and for 7: Tc1−C6Me6 1.7137(6), Tc1−C6H5O 1.7321(7).

To our knowledge, the formation of 3 by ring contraction of an arene to a cyclopentadienyl is unprecedented. Although aromatic systems coordinated to metals cannot directly be compared to free aromatic molecules, it is unexpected that the prototypical and highly resonance-stabilized aromatic bromobenzene is converted to cyclopentadienyl. We assume, however, that its interaction with the formally positive charge of the rhenium center in conjunction with resonance stabilization by the formyl group still favors this ring contraction. Since the ReI center partially polarizes the coordinated bromobenzene, it becomes more prone to nucleophilic attack, or, in other words, the coordinated hydrocarbon becomes more electrophilic. Hypothesizing that the phenol complex 2+ or its conjugated base would be a preceding intermediate in the ring contraction reaction, we first determined its pKa value in aqueous solution. Potentiometric titration gave an acidity constant of pKa = 5.34(1), which is almost 5 orders of magnitude lower than phenol itself. This distinct acidification is consistent with strong π-bonding from the arene into the rhenium center as well as with the 1H NMR data, evidencing substantial deshielding of the aromatic protons (see Supporting Information). Ring contraction is not limited to compound 1+ but tolerates alterations in the second arene ring. We chose [Re(η6C6H5Br)(η6-C6Me6)]+ (4+) as an another “extreme”. Applying similar conditions as for 1+ led directly to the corresponding

Figure 2. Cyclic voltammograms of ferrocene (red), [Re(η6-C6H6)2]+ (blue), and the mixed-ligand sandwich complex 3 (black). Acetonitrile, 0.1 M [TBA][PF6] (electrolyte), glassy carbon working electrode (i.d. = 3 mm), Pt auxiliary electrode, and Ag/AgCl reference electrode, analyte concentrations 1 mM, voltage step: 6 mV, sweep rate: 0.1 V/s. 6298

DOI: 10.1021/acs.inorgchem.7b00394 Inorg. Chem. 2017, 56, 6297−6301

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Inorganic Chemistry is +1.34 V versus Ag/AgCl.19 Identifying trends for comparable isoelectronic mixed-ring sandwich complexes along triads or within the same period but with neighboring elements is difficult, since most of these electrochemical data are missing or were acquired under different conditions. On the basis of electrochemical data from Figure 2, the chemical reactivity of 3 parallels the one of Fc, which might have implications for application in life sciences. Whereas 3 is stable in solution over an extended period in the absence of oxygen, it starts to decompose slowly when exposed to air, comparable to Fc and in agreement with electrochemical data. Whether reactive oxygen species are formed as in case of Fc remains to be clarified. The mechanism of this ring contraction is a key question. We assumed that the phenol complex 2+ or its conjugate base 2 might be an intermediate along the reaction pathway (vide infra and Scheme 1). To (dis)confirm this hypothesis, we exposed the separately synthesized and isolated complex 2+ to the same reaction conditions as in the direct synthesis of 1+ to 3. Even at T > 100 °C in water and at pH = 14, 2+ (present in its deprotonated form) did not react to 3 but remained completely stable. Although density functional theory (DFT) calculations show that 3 is by 151 kJ/mol more stable than 2+, the formation of intermediate D seems to be a key step to cross the high energetic barrier to obtain product 3 (see Supporting Information). The intermediate D is only accessible from 1+, as the formation of 3 from 2+ is not possible even in the presence of excess NaBr and NaOH and at 80 °C. Pauson proposed a mechanism for the tropylium-to-benzene rearrangement in [Cr(η7-C7H7) (CO)3]+ with strong nucleophiles.17 In a first step, they hypothesized a nucleophilic attack (of, e.g., cyclopentadienyl) to the tropylium under formation of a cycloheptadienyl complex as a first intermediate. In a second step, “electronic shifts” lead to ring contraction.17 In a similar approach as the one of Pauson et al., we showed previously that the nucleophilic attack of n-butyl lithium (nBuLi) to [Re(η6C6H6)2]+ gave the cyclohexadienyl complex [Re(η5-C6H6-nBu)(η6-C6H6)] but did not lead to any ring contractions.20 Reaction of nBuLi with 1+ resulted expectedly in a nucleophilic aromatic substitution of the bromide. The same reactivity pattern was observed with low concentrations of [OH]− and at temperatures up to 80 °C; only the phenol complex 2+ was obtained as mentioned before. To gain a closer insight into a potential mechanism for the ring contraction, we performed the reaction 1+→3 in D2O/[OD]− to assess by deuterium 2H NMR spectroscopy which of the positions in the final product would carry a “D” instead of an “H”. The D(2H) together with a 1H NMR spectra of the product 3 at the end of this reaction are shown in Figure 3. Since both spectra were recorded from the very same sample tube, the integrals give the correct respective ratios of H/D distributions (see Supporting Information for NMR details). The 1H NMR spectrum of the product displays no 1H on the aldehyde (9.53 ppm) (Figure 3). Therefore, the 2H integral of −CDO was set to 1. The 2H strength of the α-CD is 1.2 relative to −CDO, and the α-CH signal accounts for 0.8. The ratio between −CDO and α-CD is thus ∼1:1 taking some self-exchange into consideration. The β-CD amount is small (0.2) and sums up with the corresponding β-CH (1.8) to the required integral of 2. Since the product precipitates from solution, further self-exchange H→D occurs to a minor amount only, if at all. In a separate experiment, we noted that heating of nondeuterated, essentially insoluble 3 in D2O led to a small

Figure 3. D(2H) (blue) and 1H NMR spectra (red) of 3 after reaction in D2O/NaOD (recorded on a 500 MHz (2H, 76.79 MHz) device in acetonitrile as solvent).

exchange of the α-CH to α-CD, corroborating the previous statements. The observed isotope distribution pattern (Scheme 2) after the reaction implies that one α-CD and the aldehyde Scheme 2. Observed Isotope Exchange Pattern in the Ring Contraction Reaction

proton (2H) originate from a D2O protonation step in one of the intermediates or to an intramolecular D-shift during the reaction. To account for this isotope pattern, we propose the reaction mechanism depicted in Scheme 3. Reversible, nucleophilic Scheme 3. Proposed Ring Contraction Mechanism to 3 Based on the Observed Isotope Patterna

{Re} represents the {Re(η6-C6R6)}+ (R = −H, −CH3) fragment, the middle scheme the underlying isotope exchange reactions. a

attack of a hydroxide to coordinated bromobenzene leads to the relatively stable cyclohexadienyl complex A (reaction (i). Its mesomeric form B can reversibly be protonated (D+) to C. If fast enough, these steps can happen twice to finally reach D. All forms A−D are in a dynamic acid−base equilibrium, including a further but less stable intermediate with a D in the para position (not shown in Scheme 3) and giving rise to the small 2H signal of β-CD in the 2H NMR spectrum of 3 (vide supra). Since the reaction was run in D2O, all intermediates must ultimately be 6299

DOI: 10.1021/acs.inorgchem.7b00394 Inorg. Chem. 2017, 56, 6297−6301

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Inorganic Chemistry present in the doubly deuterated mesomere D or its conjugate acid C. From C or D, slow reactions may branch into (ii), which leads to bromide dissociation and the side product 2+ or into the ring-contraction mechanism (iii). The main direction the reaction takes depends on the excess hydroxide, since C and B are in a protolysis equilibrium; large excess favors (iii), and small excess favors (ii). To assess that the relative rates for establishing the equilibria in processes leading from A to D are much faster than dissociation of bromide (ii) or hydroxide, we run the reaction in D2O/NaOD under conditions leading to exclusive formation of soluble 2+. After 12 h at 70 °C, ultraperformance liquid chromatography-mass spectrometry (UPLC-MS) analysis revealed m/z = 361.1, which accounted for the molecular mass of 2++2 of the final main product, evidencing the replacement of two hydrogens by two deuterons. In addition, we detected one single peak in the UPLC, assigned to 1+ but with two m/z values of 423.0 and 424.1, respectively. These masses are assigned to 1++1 and 1++2, respectively, with one or two protons exchanged for deuterons, confirming the reversibility of reaction (i) and the rapid equilibria A−D (see Supporting Information). In an attempt to support reaction (iii), DFT calculations were performed. The intermediate structures D and E as well as the product 3 and the transition state TS were optimized. For the step D→E, the resulting Gibbs free energy of activation was found as ΔG‡ = 150 kJ/mol. (Supporting Information shows the structure of the transition state.) The high value of the activation barrier explains the importance of heating over 80 °C to obtain product 3. Natural population analysis shows a negative partial charge on the ortho-carbons in complex D (see Supporting Information). This and the transition state found support the hypothesis of a ring contraction with a concomitant, intramolecular proton (deuteron) transfer as a key step in the formation of the Cp−aldehyde complex. Thermodynamic seems to be the driving force for the last step (E→3), since the final product is by 274 kJ/mol more stable than the intermediate E.

Experimental details for the syntheses and analytical data for all new complexes, 1H and 2H NMR spectra, UPLC MS data and instrument descriptions together with crystallographic data and computational details (PDF) Accession Codes

CCDC 1520793−1520796 and 1529872−1529873 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

*E-mail: [email protected]. ORCID

Roger Alberto: 0000-0001-5978-3394 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the State Secretariat for Education Research and Innovation SERI under the European Cooperation in Science and Technology COST Program CM1105 with the Project C13.0080 and the Univ. of Zürich. We thank Dr. Olivier Blacque for supporting the theoretical calculations.

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REFERENCES

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CONCLUSION Reactivities of metal-coordinated molecules or ions are key in organometallic chemistry. Many basic reactivity patterns have been described since its advent, including ring contractions from coordinated tropylium to benzene. These reactions are of fundamental relevance and the base for their utility in directed organic syntheses and chemical transformations. We elucidated in this report an unprecedented ring contraction of a coordinated bromobenzene to a cyclopentadienyl ligand. This conversion applies for rhenium but not for technetium but still implies a more general scheme, likely to be observed for other d-element arene complexes as well. This reaction belongs therefore to a basic series, supporting metal-mediated activation of arenes toward the syntheses of organic compounds. On the more applied side, the physicochemical similarity of key compound 3 to ferrocene might induce bio-organometallic studies with a building block appropriate for imaging (99mTc) and therapy (Re) in the field of theranostics in future.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00394. 6300

DOI: 10.1021/acs.inorgchem.7b00394 Inorg. Chem. 2017, 56, 6297−6301

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on May 11, 2017. The Supporting Information was updated. The revised paper was reposted on May 23, 2017.

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DOI: 10.1021/acs.inorgchem.7b00394 Inorg. Chem. 2017, 56, 6297−6301