Î±-Diazo Ketones in On-Surface Chemistry

Article Cite This: J. Am. Chem. Soc. 2018, 140, 6000−6005

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α‑Diazo Ketones in On-Surface Chemistry Lacheng Liu,†,‡,# Henning Klaasen,§,# Alexander Timmer,†,‡,# Hong-Ying Gao,*,†,‡ Dennis Barton,§,∥,⊥ Harry Mönig,*,†,‡ Johannes Neugebauer,§,∥ Harald Fuchs,*,†,‡ and Armido Studer*,§ †

Center for Nanotechnology, Heisenbergstraße 11, 48149 Münster, Germany Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany § Organisch-Chemisches Institut and ∥Center for Multiscale Theory and Simulation, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany ⊥ Physics and Materials Science Research Unit, University of Luxembourg, 162 A, Avenue de la Faïencerie, L-1511 Luxembourg ‡

S Supporting Information *

ABSTRACT: Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-paraphenylenes is reported. Polymerization on Cu(111) occurs via initial N2 fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,β-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cubiscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers. introduce α-diazo ketones as precursors for an on-surface reaction sequence proceeding via metallo carbenes yielding two different 1D polymers (Scheme 1). After deposition on the

1. INTRODUCTION On-surface chemistry where reactions are conducted in a twodimensional (2D) environment under ultrahigh vacuum conditions on a substrate has matured to an independent research domain over the past few years.1 Solution phase reactions like the Ullmann2 or Glaser coupling3 have been achieved on surfaces along with many other transformations such as imine4 and imide5 bond formation, decarboxylative coupling,6 and azide alkyne cycloaddition.7 Protective group strategies8 have been implemented, new “clean” reactions9 yielding interesting structures10 have been introduced, and reaction selectivity has been addressed.11 The development of new processes will help extending the tool box of activation modes, which is currently restricted to thermal annealing, UVirradiation, or single-electron reduction.1 To date, σ- and πstabilized radicals,1,12 as well as coordinating anionic ligands13 have been proposed and visualized as intermediates on surfaces. Carbenes have been suggested to be intermediates and were also formed ex situ as well as at the surface,14 but to our knowledge, visualized biscarbene intermediates were not used for on surface reactions to date. α-Diazo carbonyl compounds are important building blocks in organic synthesis, because of their versatile reactivity15 and ease of preparation. They are generally accessed by diazo group transfer to an activated methylene compound16 or by oxidation of a hydrazone.17 α-Diazo carbonyl compounds serve as precursors for in situ generation of carbenes that are highly useful intermediates in organic synthesis.15 Herein, we will © 2018 American Chemical Society

Scheme 1. Schematic Illustration of the Reaction Pathways of On-Surface Polymerization of α-Diazo Ketones

Cu(111) surface, the diazo groups of the precursor biphenyl bis α-diazo ketone 1 first react upon N2 fragmentation to polymeric Cu-biscarbene chains at room temperature. Polymerization to poly-α,β-unsaturated 1,4-diketones then occurs after thermal annealing. In contrast, on Au(111) polymerization occurs at room temperature. Further increase of temperature Received: March 7, 2018 Published: April 8, 2018 6000

DOI: 10.1021/jacs.8b02599 J. Am. Chem. Soc. 2018, 140, 6000−6005

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Journal of the American Chemical Society leads to alternating furandiyl biphenyl copolymers by an unprecedented 2D reaction. The precursor diazo ketone, reaction intermediates, and two different 1D polymers are studied by scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and X-ray photoelectron spectroscopy (XPS). The experimental results are further supported by density functional theory (DFT) calculations.

2. RESULTS AND DISCUSSION 2.1. STM Characterization of α-Diazo Ketone 1 and Polymerization on Au(111) and Cu(111) Surfaces. Experiments were performed with bis α-diazo ketone 1. In situ mass spectrometry and cryo XPS measurements on Au(111) revealed that 1 is stable up to 110 °C upon thermal evaporation and reaches the surface in an intact configuration (see Figure S1 and XPS discussion below). Diazo ketones 1 were first deposited on Au(111) at room temperature and subsequently cooled to 78 K for STM analysis. The STM data did not show any defined ordering of the molecules but oligomeric structures (Figure S2), indicating that most monomers directly reacted on Au(111) at room temperature. However, deposition of 1 onto a precooled Au(111) surface at −16 °C led to a self-assembled structure of intact diazo compounds (Figure 1a and b). After annealing to 135 °C, polymeric structures presumably containing α,β-unsaturated 1,4-diketone moieties as linking entities were formed (Figure 1c and d). Lateral STM tip manipulation confirmed the existence of covalently bound polymers (Figure S3). It is suggested that poly-α,β-unsaturated 1,4-diketones are formed on Au(111) by N2-fragmentation to give the corresponding carbenes that further undergo CC coupling to the polymeric alkenes. We found selective trans-alkene formation in all cases as supported by DFT calculations (see below). The experimental center-tocenter distance between the enone moieties is 1.20 ± 0.01 nm. The corresponding statistical analysis as well as the method are shown in Figure S4. Notably, the structure of these polymeric 1,4-diketones is the same as the one obtained after deposition on Au(111) at room temperature. Upon increasing the temperature to 240 °C, a follow up transformation was observed on Au(111) (Figure 1e). The equidistant connecting units were assigned to the biphenyl moieties (Figure 1f). Since some of the proposed 1,4-diketone functionalities remained, reaction after the second annealing step did not occur quantitatively. We found that the biphenyl groups at the polymer termini were moving during scanning (Figure S5). Furthermore, two neighboring biphenyl groups form an angle of 125 ± 5° as derived from corresponding measurements on 20 corner linkages (see statistical analysis provided in Figure S5). Based on these findings, it is suggested that the α,βunsaturated 1,4-diketone moiety cyclizes after initial CC double bond isomerization, by nucleophilic attack of one carbonyl oxygen atom to the carbonyl carbon in γ-position followed by surface assisted deoxygenation to a furan ring at 240 °C (see also Scheme 1). We assume that the oxygen atom will desorb after cyclization from the surface based on STM studies, where we could not find any evidence that these oxygen atoms remain at the surface. Diazo ketone 1 was also evaporated onto a Cu(111) substrate at room temperature where long oligomeric structures were formed (Figure 2a and b). The bright spots between two entities are likely Cu atoms. We assume that diazo ketone 1 first reacts upon N2 fragmentation to Cu-biscarbene organometallic polymers as intermediate structures (see Figure 2b). In analogy

Figure 1. Reaction steps of α-diazo ketone 1 on Au(111) surface. (a) Overview STM image of the self-assembled structure of α-diazo ketones as deposited on a cold (−16 °C) surface (−1 V, 10 pA). (b) Enlarged image (−0.2 V, 30 pA, periodic unit cell: a = 2.11 ± 0.01 nm, b = 1.05 ± 0.01 nm, θ = 60 ± 1°) with the structure sketch of the precursor. (c) Overview STM image after annealing up to 135 °C (−1 V, 30 pA) and (d) high resolution image (−0.02 V, 200 pA) with the structure sketch of the proposed polymer. (e) Overview STM image after further annealing up to 240 °C (−0.5 V, 20 pA) and (f) high resolution image (−0.5 V, 20 pA) with the suggested structure of the product.

to the reactions on Au(111), covalent organic polymers were formed on Cu(111) after annealing to 135 °C (Figure 2c and d). However, the contrast of the α,β-unsaturated 1,4-diketone is less pronounced as compared to the case on Au(111). It is assumed that the interaction between the oxygen atoms of the diketones with the Cu(111) substrate is much stronger as compared to the Au substrate, orienting the carbonyl groups toward the Cu-substrate. Upon further annealing to 165 °C, polymers, presumably covalently bound by furan-2,5-diyl type linkages, were formed (Figure 2e and f). 2.2. nc-AFM Characterization of Polymeric Products Derived from α-Diazo Ketone 1 on Cu(111). To further confirm the suggested structures of the polymeric Cubiscarbene intermediates as well as the downstream organic polymers, nc-AFM measurements were performed on Cu(111) with an oxygen-terminated tip in constant-height mode.18 As mentioned, the furandiyl polymers on the Au(111) surface show slight lateral movements during STM scanning. Therefore, nc-AFM measurements were exclusively performed on Cu(111), where structures show a stronger coupling to the surface. Similar to previous nc-AFM measurements,19 our suggested organometallic intermediates could be clearly identified (Figure 3a). The biphenyl groups are bend toward the surface due to the interaction between the carbonyl group 6001


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Figure 2. Reaction steps of α-diazo ketones on Cu(111) surface. (a) Overview STM image of the intermediate structure of α-diazo ketones as deposited on Cu(111) surface (−0.5 V, 30 pA). (b) Enlarged STM image (−0.5 V, 30 pA) with the suggested structure of the Cubiscarbene chain. (c) STM image after annealing up to 135 °C, (−0.1 V, 50 pA) and (d) high-resolution image (−0.2 V, 30 pA) with the proposed structure of the unsaturated diketone polymer. (e) STM image after further annealing up to 240 °C (−0.5 V, 30 pA) and (f) a high-resolution image (−0.1 V, 50 pA) with the structure of the proposed furandiyl polymer.

Figure 3. High-resolution nc-AFM images and corresponding STM data of different reaction products of the α-diazo ketones on Cu(111), recorded with an oxygen-terminated tip. (a) nc-AFM image of an intermediate Cu-biscarbene chain. (z0: −1 V, 10 pA, Δz = −0.18 nm). (b) Corresponding STM image of the same intermediate Cubiscarbene chain as shown in (a) (−1 V, 10 pA). (c) nc-AFM image of an α,β-unsaturated 1,4-diketone polymer (z0: −1 V, 10 pA, Δz = −0.75 nm). (d) Corresponding STM image of the same α,βunsaturated 1,4-diketone polymer as shown in (c) (−1 V, 10 pA). (e) nc-AFM image of a furandiyl polymer (z0: −1 V, 10 pA, Δz = −0.56 nm). (f) The corresponding STM image of the same furandiyl polymer as shown in (e) (−1 V, 10 pA). All the nc-AFM images were processed by Laplacian of Gaussian filtering.

and the Cu(111) substrate. Thus, the distance between the tip and connecting Cu atom is larger at this position than it is on top of a molecule. As a result, the tip−sample interaction is attractive at this location and the Cu atoms in the organometallic polymer appear as distinct black spots indicating that they are significantly pulled out of the Cu(111) plane. In Figure 3a, the lower black spot appears slightly larger than the upper one as a result of the little z drift that was present during the measurement. The center-to-center distance between two biphenyl groups of the organometallic polymer was measured to be 1.54 ± 0.01 nm in the nc-AFM image (Figure 3a) and 1.55 ± 0.01 nm in the STM image (Figure 3b), respectively. Unfortunately, it was not possible to resolve any feature of the carbonyl groups due to their bending toward the substrates. Nevertheless, the images confirm the formation of metallo polymeric Cu-biscarbene structures. Figure 3c shows the nc-AFM image of a part of an organic polymer, in which the feature of the alkene group of the α,β-unsaturated 1,4-diketone moiety is visible. The center-to-center distance between two biphenyl groups of the alkene polymer was measured to be 1.33 ± 0.01 nm in the nc-AFM image (Figure 3c) and 1.35 ± 0.01 nm in the STM image (Figure 3d). The feature of a furandiylbiphenyl copolymer is also shown in Figure 3e, in which the chemical structure of the biphenyl entities and the furan rings at

the corners can be identified. The signature of the fivemembered rings is even more clearly visible in the overview nc-AFM image (Figure S6). These results confirm the formation of furan moieties during polymerization. The angle between two neighboring biphenyl groups of the furandiyl polymer was measured to be 120 ± 1° in the nc-AFM image (as shown in Figure 3e) and 121 ± 3° in the STM image (as shown in Figure 3f, see also Figure S7). The nc-AFM data further suggests that the furan rings are bent with the oxygen pointing toward the surface. 2.3. Theoretical Studies of the α,β-Unsaturated 1,4Diketone Polymer and the Furandiyl-Biphenyl Polymer on Au(111) and Cu(111). To further support the structural assignments, DFT calculations were performed on the isomeric model compounds 2 and 3 resembling repeating moieties of the α,β-unsaturated 1,4-diketone polymers (Figure 4). It is emphasized, that the relative orientation of the CO bond with respect to the CC bond leads to either s-cis (CO and CC are cis oriented, see model compound enone 2) or strans (CO and CC are trans oriented, see model enone 3) 6002


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We also calculated a model structure representing a repeating moiety of the furandiyl-biphenyl polymer on Cu(111) (Figure S7). The experimental distance (1.25 ± 0.01 nm) on Cu(111) is in good agreement with the theoretical distance (1.24 nm). However, the experimental angle between two biphenyl groups on Cu(111) (120 ± 1° by nc-AFM and 121 ± 1° by STM) is slightly smaller than its theoretical value (128°) due to the bending. 2.4. XPS Characterization of α-Diazo Ketone on Cu(111) and Au(111) Surfaces. As the self-assembly structure of intact diazo ketones on Au(111) can only be obtained on a cold substrate (Figure 1a and b), cryo-XPS measurements were conducted, in order to fully analyze the chemical states of the molecular assembly and reaction products (Figure 5). The sample was held at 100 K during

Figure 4. DFT studies of the two possible isomers. (a) Sketch of s-cis enone 2. (b) Sketch of s-trans enone 3. (c−f) Optimized structures (in alphabetical order) of enone 2 and enone 3 on Au(111) and of enone 2 and enone 3 on Cu(111). Top view (top) and side view (below). The distances are given in Å.

configurations (Figure 4a and b). Figure 4c−f shows the corresponding DFT-optimized structures of models 2 and 3 on Au(111) and Cu(111), respectively, revealing different distances between the biphenyl moieties in all four depicted cases. The comparison of the experimental distance of 1.20 ± 0.01 nm of the poly α,β-unsaturated 1,4-diketone on Au(111) (Figure 1) with the calculated distances for s-cis enone 2 (1.33 nm, Figure 4c) and s-trans enone 3 (1.21 nm, Figure 4d) clearly indicates that the enone moieties are s-trans configured on Au(111), with good agreement between the theoretically and experimentally determined values. On Cu(111), the experimental distance is significantly larger (1.33 ± 0.01 nm by nc-AFM and 1.35 ± 0.01 nm by STM, Figure 3c,d). Therefore, the polymer must have been formed with a different relative orientation of the substituents at the enone moieties, probably as s-cis configured isomers. Indeed, the calculated distance of 1.34 nm for the s-cis configured model compound 2 (Figure 4e) fits well with the experimental values. Notably, the DFT calculated structure of model enone 2 on Cu(111) further shows that the enone moiety is not adsorbing flat on the surface. Rather both oxygen atoms are pointing downward, as can be seen from the side view (Figure 4e). The bending of the carbonyl group toward the surface, which was found theoretically more pronounced on Cu(111), explains why the enone moieties could not be imaged by STM.

Figure 5. X-ray photoelectron spectra and curve-fitting of the N 1s (top), C 1s (middle), and O 1s (bottom) core levels of α-diazo ketone 1 on Au(111). The surface was held at 100 K during sublimation and measurement. The N 1s spectra exhibits two distinct peaks indicating that the molecules reach the surface intact and only react upon thermal activation at the surface. All sample states were checked by STM before the XPS measurement.

the deposition and XPS measurements. The N 1s data for 1 deposited on cold Au(111) exhibits two clear peaks at binding energies Eb[N(+)](N 1s) = 402.9 eV and Eb[N(−)](N 1s) = 400.8 eV, which originate from the intact diazo group (Figure 5 top). In the C 1s region a broad peak at Eb[C−C](C 1s) = 284.0 eV, assigned to all backbone carbons, and two peaks at Eb[C−N](C 1s) = 285.1 eV and Eb[CO](C 1s) = 286.8 eV corresponding to diazo and oxygen connected carbon atoms, are visible. The O 1s spectrum consists of a single peak centered at Eb[CO](O 1s) = 530.4 eV. These results in combination with the mass spectrometry data (Figure S1) provide strong evidence that 1 is sublimated intact and alkene formation occurs as a result of elevated substrate temperature. The complete XPS data set for 1 on Au(111) including all annealing temperatures is depicted in Figure S8. 6003


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3. CONCLUSIONS In summary, we have introduced diazo ketone 1 as a precursor for on surface polymerization on Cu(111) and Au(111) surfaces. Our combined STM, nc-AFM, DFT, and XPS analysis conclusively shows that the observed reactions at room temperature proceed via initial N2-fragmentation to give polymeric metallo carbene intermediates. These carbenes then undergo thermal CC coupling to form poly-α,βunsaturated 1,4-diketones. Whereas on Au(111) these initial two steps proceed at room temperature, carbene coupling to the α,β-unsaturated 1,4-diketone moiety occurs at elevated temperature on Cu(111). Further annealing leads to deoxygenative cyclization of the 1,4-diketone entities to give polymeric structures containing furan-2,5-diyl connections. This work introduces α-diazo ketones as valuable starting materials having the potential to open new avenues in the field of on-surface synthesis.

The evolution of the XPS core-level spectra of diazo ketones on Cu(111) as a function of annealing temperature are shown in Figure 6. XPS measurements were performed on three

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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.8b02599. Chemical synthesis; experimental and DFT methods; detailed descriptions of experiments and analysis; mass spectra of α-diazo ketone 1; additional STM images and distance measurements, complete XPS spectra; 1H NMR spectra (PDF)

Figure 6. X-ray photoelectron spectra and curve-fitting of the O 1s (a) and C 1s (b) core levels of α-diazo ketone 1 on Cu(111). From top to bottom, the data in each tile corresponds to the as deposited state at room temperature, first annealing step to 135 °C and the final annealing step to 165 °C, respectively. All sample states were checked by STM before the XPS measurement.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] *[email protected]

samples of 1 on Cu(111): as deposited at room temperature (Cu-biscarbene chains as product), after annealing at 135 °C (α,β-unsaturated 1,4-diketone polymers as products), and after annealing at 165 °C (furandiyl polymers as products). For the data corresponding to the room-temperature deposition, no obvious N 1s peak was identified (Figure S9) revealing that the diazo group already reacted at room temperature and N2 desorbed. The corresponding O 1s data show a clear single peak at Eb[CO](O 1s) = 530.5 eV, which can be assigned to the oxygen atom of the ketone within the proposed Cubiscarbene. The C 1s data can be fitted with four peaks at binding energies 284.4 eV (aromatic), 285.3 eV (CC), 286.1 eV (CO), and 283.2 eV (C−Cu). The C 1s component at 283.2 eV is commonly ascribed to carbon atoms connected to a metal atom of the surface, proving the presence of an organometallic intermediate.20 After the first annealing to 135 °C, the Cu bound carbon component is significantly reduced, whereas the O 1s peak remains as a single peak appearing slightly shifted with respect to the previous state (Eb[CO](O 1s) = 530.3 eV). After the second annealing to 165 °C, the O 1s spectrum finally shows a second peak at Eb[furan](O 1s) = 533.6 eV which can be assigned to the oxygen atom within the furan ring of the final product.21 The remaining peak at Eb [carbonyl](O 1s) = 530.3 eV indicates that not all 1,4-diketone moieties cyclized to form the furan rings, in agreement with the nc-AFM observations discussed above.

ORCID

Hong-Ying Gao: 0000-0001-6831-9112 Harry Mönig: 0000-0003-2639-9198 Armido Studer: 0000-0002-1706-513X Author Contributions #

L.L., H.K., and A.T. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (SFB 858, TRR 61, FU 299/19, GA 2430/1-1, and MO 2345/4-1) for financial support.

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