N-Heterocyclic Carbenes Reduce and Functionalize Copper Oxide Surfaces in One Pot

Article abstract of DOI:10.1002/chem.202002308

Benzimidazolium hydrogen carbonate salts have been shown to act as N-heterocyclic carbene precursors, which can remove oxide from copper oxide surfaces and functionalize the resulting metallic surfaces in a single pot. Both the surfaces and the etching products were fully characterized by spectroscopic methods. Analysis of surfaces before and after NHC treatment by X-ray photoelectron spectroscopy demonstrates the complete removal of copper(II) oxide. By using ¹³C-labelling, we determined that the products of this transformation include a cyclic urea, a ring-opened formamide and a bis-carbene copper(I) complex. These results illustrate the potential of NHCs to functionalize a much broader class of metals, including those prone to oxidation, greatly facilitating the preparation of NHC-based films on metals other than gold.

Full text of DOI:10.1002/chem.202002308

Accepted Article
Accepted Article
Title: N-Heterocyclic Carbenes Reduce and Functionalize Copper
Oxide Surfaces in One Pot
Authors: Alex Veinot, Abrar Al-rashed, J. Daniel Padmos, Ishwar
Singh, Dianne S. Lee, Mina R. Narouz, Paul A. Lummis,
Christopher J. Baddeley, Cathleen Marie Crudden, and J.
Hugh Horton
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To be cited as: Chem. Eur. J. 10.1002/chem.202002308
Link to VoR: https://doi.org/10.1002/chem.202002308
01/2020

10.1002/chem.202002308
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N-Heterocyclic Carbenes Reduce and Functionalize Copper Oxide
Surfaces in One Pot
Alex J. Veinot, [a,†] Abrar Al-Rashed, [a,†] J. Daniel Padmos,[a] Ishwar Singh,[a] Dianne S. Lee,[a] Mina R.
Narouz,[a] Paul A. Lummis,[a] Christopher J. Baddeley,[b] Cathleen M. Crudden,*[a,c] J. Hugh Horton*[a]
Abstract: Benzimidazolium hydrogen carbonate salts have been
shown to act as N-heterocyclic carbene precursors which can remove
oxide from copper oxide surfaces and functionalize the resulting
metallic surfaces in a single step. Both the surfaces and the etching
products are fully characterized by spectroscopic methods. Analysis
of surfaces before and after NHC treatment by X-ray photoelectron
spectroscopy demonstrates the complete removal of copper(II) oxide.
Using 13C-labelling, we determine that the products of this
transformation include a cyclic urea, a ring-opened formamide and a
bis-carbene copper(I) complex. These results illustrate the potential
of NHCs to functionalize a much broader class of metals, including
those prone to oxide treatment, greatly facilitating the preparation of
NHC-based films on metals other than gold.
handling copper surfaces in a typical laboratory environment is
consistently challenged by the presence of surface oxide.
Herein, we describe the treatment of oxidized copper
surfaces with common NHC precursors,[2b,8] resulting in removal
of copper oxide and deposition of NHCs on the resulting metallic
surface. Through several methods, we characterize the oxidation
product of the NHC (Scheme 1). These results demonstrate that
NHCs can be used not just to form monolayers on metal surfaces,
but to remove oxide from reactive metals, and simultaneously
functionalize the resulting metallic surfaces. This simple,
straightforward method should significantly increase the ease with
which more reactive, difficult to handle metals can be
functionalized.
N-Heterocyclic carbenes (NHCs) are beginning to be recognized
as powerful ligands for the stabilization of nanomaterials.[1] NHC–
based monolayers have been reported to have considerably more
thermal and chemical stability than thiol analogs,[2] and recent
studies have shown that NHCs can also functionalize more
reactive metal surfaces, including copper,[3] magnesium,[4]
platinum[5] and silver.[3b]
Gold is the most widely employed metal in organic-on-metal
type devices because of its high resistance to oxidation and ease
of handling. However, more oxidation-sensitive metals often have
superior properties and are typically much cheaper. For example,
silver has a more sensitive surface plasmon response[6] and
copper is widely applied in the semiconductor industry because of
its conductivity and cost.[7]
Previous work from our groups[3a] and Papageorgiou and
co-workers[3b] have shown that NHCs are fully capable of
functionalizing metallic copper surfaces under ultra-high vacuum
Scheme 1. Benzimidazolium hydrogen carbonate salts remove copper oxides
and functionalize metallic copper surfaces. (NHC
=
1,3-diisopropyl-
benzimidazolylidene, 1).
(UHV) conditions. Temperature programed
desorption
experiments demonstrated that the strength of the NHC–Cu bond
is identical to that of the NHC–Au bond within error.[3a] However,
To examine the reaction of NHCs with copper oxide,
polycrystalline copper surfaces were cleaned using glacial acetic
acid and then oxidized using 30% H2O2.[9] The oxidized copper
surfaces were then treated with 10 mM solutions of the NHC
precursor, 1,3-diisopropylbenzimidazolium hydrogen carbonate
(1HC), in 1,2-dichloroethane at room temperature for 48 hours.
Analysis of the resulting surfaces by X-ray photoelectron
spectroscopy (XPS) confirmed the complete removal of CuO as
seen by the loss of the Cu(II) shake-up peaks between 938.5 and
946.5 eV (compare Cuox and Cuox-1HC, Figure 1a). The resulting
Cu 2p3/2 XPS signal appears at 932.6 eV, in the region expected
for a Cu(0) surface.[10] This signal appears at the same position as
independently prepared Cu(0) surfaces, and is identical in intensity
to NHC-modified Cu(0) surfaces prepared by either treatment with
free NHC 1 (Cu(0)-1) or hydrogen carbonate salt 1HC (Cuox-1HC).
This provides a preliminary indication that functionalization of the
surface takes place after oxide etching (Figure 1a). It should be
noted that thiols have also been shown to recued copper oxide
[a]
A. J. Veinot, Dr. A. Al-Rashed, Dr. J. D. Padmos, I. Singh, D. Lee, Dr.
M. R. Narouz, Dr. P. A. Lummis, Prof. Dr. C. M. Crudden, Prof. Dr. J. H.
Horton
Department of Chemistry
Queen’s University
Chernoff Hall, Kingston, Ontario, Canada, K7L 3N6
E-mail: jhh@queensu.ca, cruddenc@chem.queensu.ca
Prof. Dr. C. J. Baddeley
[b]
EaStCHEM School of Chemistry
University of St Andrews
St Andrews, Fife, United Kingdom, KY16 9ST
E-mail: cjb14@st-andrews.ac.uk
[c]
[†]
Prof. Dr. C. M. Crudden
Institute of Transformative Bio-Molecules (WPI-ITbM)
Nagoya University
Furo, Chikusa, Nagoya 464-8602, Japan
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
1
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10.1002/chem.202002308
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surfaces, although this method would not be applicable to the
formation of NHC-terminated surfaces.[11]
of NHC 1HC with powdered Cu2O, we are able to clearly
demonstrate the ability of 1HC to reduce this metal oxide.
The presence of NHC ligand on the surface after oxide
removal was confirmed by analysis of the C 1s and N 1s XPS data.
Although C 1s signals were not as helpful due to the presence of
adventitious carbon, the change in the nitrogen region was highly
diagnostic. As shown in Fig. 2a, Cu(0) modified with NHC 1 or
precursor 1HC were characterized by a strong signal at 400.0 eV,
similar to what has been observed for NHC on Au.[2] The Cuox
samples treated under the same conditions (Fig. 2b) also have a
strong signal at 400.0 eV, identical to the functionalization of
metallic surfaces. These observed changes strongly suggest that
the removal of oxide from the surface is followed by protection of
the newly revealed metallic surface with NHC.
Figure 1. Stacked Cu 2p3/2 XPS (a) and Cu LMM Auger (b) plots illustrating the
removal of metal oxides by treating cleaned (Cu(0)) and oxidized (Cuox) copper
surfaces with solutions of the free NHC 1 or its hydrogen carbonate salt 1HC
.
Figure 2. (a) Stacked N 1s XPS illustrating the functionalization of Cu(0)
surfaces with NHC 1 and NHC precursor 1HC, and (b) N 1s XPS plots illustrating
the sample after treatment of Cuox surfaces with NHC precursor 1HC. In both
cases, treatment was accomplished with 10 mM DCE solutions of the hydrogen
carbonate salt 1HC or free NHC 1.[16]
While the data in Figure 1a show conclusively that Cu2+
species have been removed by treatment with 1HC, the ability of
the NHC to reduce Cu2O cannot be assessed since the signals
for Cu1+ and Cu(0) overlap in XPS.[12a] However, Auger
spectroscopy in the Cu LMM region can give qualitative
information about the presence of Cu1+ and Cu(0) species (Figure
1b).[12]
When analyzed by Auger spectroscopy, the oxidized
sample (Cuox) exhibits a broad peak at 917.9 eV, characteristic of
Cu2+.[9b] By contrast, the Cu(0) sample exhibits a major peak at
918.8 eV, characteristic of metallic copper.[10, 13] Small amounts of
Cu1+ species are also observed, which will be discussed below.
Cu2+ species are not present.[14]
As shown in Table 1, treatment of bulk (powdered) Cu2O
and CuO with 1HC afforded urea 2, formamide 3 and copper
complex 4 as confirmed by 1H NMR spectroscopic analysis of the
supernatant (Table 1).
Table 1. 1H NMR (Cu2O, CuO) and 13C{1H} NMR (Cuox, Cu(0))
spectroscopic yields[a,b] for copper etching with 1HC
.
When Cuox is treated with 1HC, the signal for Cu2+ at 917.8
eV is lost, consistent with the XPS results, and a new signals
appears at 918.8 eV, corresponding to metallic copper (Figure 1b,
Cuox-1HC).[15] Surprisingly, the signal at 916.9 eV attributed to Cu1+
species was enhanced. Interestingly, the same results were
obtained when a metallic Cu surface was treated with 1 or 1HC
(Figure 1b). This led to the possibility that the observed Cu1+
signal results from sub-surface oxide revealed upon surface
etching.
To address this, we performed XPS studies at glancing
angles to differentiate between surface and bulk species. As
shown in Fig S4, signals due to Cu2O are only observed in bulk
analysis, not at the more surface-sensitive glancing angles. (see
supporting information). This indicates that these signals are
likely due to sub-surface lattice Cu2O, revealed by etching of the
metal oxide/metal surface. Importantly, by studying direct reaction
NH
CHO
N
N
N
N
N
N
N
Cu2O or CuO
1HC
Cu
C
O
DCE, 85 °C
18 h
2
4
3
Entry
Substrate
% Conv.[c]
82 ± 2
84 ± 8
63
2
3
4
10 ± 3
4 ± 4
7
% Rec.
96 ± 1
94 ± 2
99
1
2
3
4
Cu2O
CuO
Cuox
Cu(0)
33 ± 12
9 ± 2
9
35 ± 9
64 ± 4
46
91
15
51
24
99
[a] Measured in CD2Cl2 and are relative to C6Me6 internal standard (0.5 mmol).
Entries 1 and 2 are reported as an average of 3 replicates. [b] Entries 3 and 4
are given as relative yields for 13C quantification with the assumption that all 13
C
2
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10.1002/chem.202002308
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labelled species originate from the starting material. [c] Remainder was
has not been employed to enable the reduction and
functionalization of oxidized copper surfaces.
determined to be unreacted 1HC
.
To confirm that the same species were observed in surface
reactions, oxidized copper surfaces were treated with a labelled
NHC precursor – namely 1HC with a 13C-label at the C(2) position.
The supernatant was analyzed quantitatively using inverse-gated
(IG) 13C{1H} NMR spectroscopy. As shown in Figure 4 and Table
1, urea 2 was again observed, along with remaining 1HC and
compound 3, which results from hydrolysis of 1HC, consistent with
results obtained with bulk oxides.[18] In all cases, the
spectroscopic assignment is based on the de novo synthesis of
the compounds shown.[19] Mass spectrometry data also support
these assignments (see the supporting information).
Scheme 3. Selected literature examples of imidazolium salts reacting with bulk
Cu2O and Cu.[22, 23]
In summary, we have shown that the interaction of NHCs
with oxidized Cu surfaces reduces these surfaces to Cu(0), which
then reacts with remaining NHC to yield an NHC-functionalized
Cu(0) surface. Cyclic urea 2 is identified as the likely oxidation
product arising from the reduction of the surface. Notably NHC
complex 4 was also observed, illustrating the ability of the NHC to
etch newly generated metallic copper. Consistent with this
hypothesis, larger amounts of 4 are observed when starting with
a primarily metallic copper surface. The results of this study
provide the foundation for extending the use of NHC-based
monolayers to more reactive metal substrates that are prone to
oxidation. Finally, this work illustrates the ability of NHCs to act as
chemical etchants, which may be used as an attractive alternative
to other etchants, which are often unsafe or difficult to handle.
Figure 4. Quantitative IG-13C{1H} NMR spectra of the supernatants obtained
from Cuox etching experiments using 13C-labelled 1HC. Cu(0) etching is given in
the supporting information. Yields are given in Table 1, entries 3-4.
Acknowledgements
Of the various compounds identified in the supernatants,
urea 2 is the product of oxidation of carbene 1. Importantly, this
species is not observed upon inadvertent exposure of 1HC to
oxygen. Compound 3 results from hydrolysis of starting material
1HC, a reaction that has been reported in rare cases,[20] but
appears to be enhanced by the presence of surface metal
The Natural Sciences and Engineering Research Council of
Canada (NSERC) and the Canada Foundation for Innovation
(CFI) are thanked for financial support of this work in terms of
operating and equipment grants to CMC and JHH. AJV and MRN
thank the Ontario government for OGS fellowships. AJV thanks
NSERC for a Vanier Scholarship and also the Walter C. Sumner
foundation for additional scholarship support.
hydroxides. Finally, along with unreacted starting material 1HC
,
complex 4 is the only other organic species detected.
Complex 4 is an interesting by-product that results from
abstraction of a copper atom from the surface. Previous work from
our group and others have documented the ability of NHCs to lift
metal atoms from metallic surfaces of Au, Cu and Ag resulting in
surface-bound M(NHC)2 species.[3,21] However, these compounds
have not previously been observed in solution. Consistent with
the suggestion that compound 4 arises from reaction with metallic
copper,[23] a larger amount is observed from reaction with Cu(0) in
comparison with oxidized copper (see supporting information).
These results can be compared with previous reports from
the organometallic literature, which demonstrate that reaction
between imidazolium salts and Cu2O leads to NHC–Cu
complexes.[22] The electrolysis of imidazolium salts in the
presence of Cu(0) has also been shown to produce [(NHC)2Cu]+
species similar to compound 4 observed from reaction at the
metallic surface.[23] To the best of our knowledge, this type of
reactivity has not been applied to copper surfaces until now, and
Keywords: Carbenes • Copper • Monolayers • NMR
spectroscopy • Photoelectron spectroscopy
[1]
[2]
a) C. A. Smith, M. R. Narouz, P. A. Lummis, I. Singh, A. Nazemi, C.-H.
Li, C. M. Crudden, Chem. Rev. 2019, 119, 4986-5056; b) A. V.
Zhukhovitskiy, M. J. MacLeod, J. A. Johnson, Chem. Rev. 2015, 115,
11503-11532; c) S. Engel, E.-C. Fritz, B. J. Ravoo, Chem. Soc. Rev.
2017, 46, 2057-2075.
a) C. M. Crudden, J. H. Horton, I. I. Ebralidze, O. V. Zenkina, A. B.
McLean, B. Drevniok, Z. She, H.-B. Kraatz, N. J. Mosey, T. Seki, E. C.
Keske, J. D. Leake, A. Rousina-Webb, G. Wu, Nat. Chem. 2014, 6, 409;
b) C. M. Crudden, J. H. Horton, M. R. Narouz, Z. Li, C. A. Smith, K. Munro,
C. J. Baddeley, C. R. Larrea, B. Drevniok, B. Thanabalasingam, Nat.
Commun. 2016, 7, 12654.
[3]
a) C. R. Larrea, C. J. Baddeley, M. R. Narouz, N. J. Mosey, J. H. Horton,
C. M. Crudden, ChemPhysChem 2017, 18, 3536-3539; b) L. Jiang, B.
3
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Zhang, G. Médard, A. P. Seitsonen, F. Haag, F. Allegretti, J. Reichert, B.
Kuster, J. V. Barth, A. C. Papageorgiou, Chem. Sci. 2017, 8, 8301-8308.
[4]
[5]
L. Stephens, J. D. Padmos, M. R. Narouz, A. Al-Rashed, C.-H. Li, N.
Payne, M. Zamora, C. M. Crudden, J. Mauzeroll, J. H. Horton, J.
Electrochem. Soc. 2018, 165, G139-G145.
Y. Zeng, T. Zhang, M. R. Narouz, C. M. Crudden, P. H. McBreen, Chem.
Commun. 2018, 54, 12527-12530.
[6]
[7]
[8]
H. Ehrenreich, H. Philipp, Phys. Rev. 1962, 128, 1622.
J. Rickerby, J. H. G. Steinke, Chem. Rev. 2002, 102, 1525-1550.
M. Fèvre, J. Pinaud, A. Leteneur, Y. Gnanou, J. Vignolle, D. Taton, K.
Miqueu, J.-M. Sotiropoulos, J. Am. Chem. Soc. 2012, 134, 6776-6784.
a) K. Chavez, D. Hess, J. Electrochem. Soc. 2001, 148, G640-G643; b)
M. M. Sung, K. Sung, C. G. Kim, S. S. Lee, Y. Kim, J. Phys. Chem. B
2000, 104, 2273-2277.
[9]
[10] S. Chawla, N. Sankarraman, J. Payer, J. Electron Spectrosc. Relat.
Phenom. 1992, 61, 1-18.
[11] a) Y. Wang, J. Im, J. W. Soares, D. M. Steeves J. E. Whitten Langmuir
2016, 32, 3848−3857; b) L. Carbonell, C.M. Whelan, M. Kinsella, K.A.
Maex, Superlattices Microstruct. 2004, 36, 149−160; c) H. Keller, P.
Simak, W. Schrepp, J. Dembowski, Thin Solid Films 1994, 244,
799−805; d) M. M. Sung, K. Sung, C. G. Kim, S. S. Lee, Y. Kim, J. Phys.
Chem. B 2000, 104, 2273−2277.
[12] a) M. C. Biesinger, Surf. Interface Anal. 2017, 49, 1325-1334; b) G.
Schön, J. Electron Spectrosc. 1972, 1, 377-387.
[13] J. Fuggle, E. Källne, L. Watson, D. Fabian, Phys. Rev. B 1977, 16, 750.
[14] The smaller signal with a kinetic energy of 921 eV arises from an
unrelated Auger transition.[11]
[15] Note that the exact amounts cannot be quantified due to the qualitative
nature of Auger spectroscopy, therefore we have not deconvoluted the
signals.
[16] Reactivity between the free NHC 1 and DCE solvent was checked before
deposition. No reaction was observed on the etching time scale at room
temperature, however at 85 °C there was significant benzimidazolium
salt formation, likely from HCl abstraction.[17] We also note that the
nitrogen signal found in the bottom panel of 2b is from subsurface
impurities as shown by angle resolved XPS. See supporting information
for details.
[17] D. P. Allen, C. M. Crudden, L. A. Calhoun, R. Wang, J. Organomet. Chem.
2004, 689, 3203-3209.
[18] The formamide 3 is observed as two signals because of restricted
rotation about the amide bond.
[19] Although urea 2, resulting from oxidation of the carbene, is the expected
product, the formamide hydrolysis product 3 is actually the dominant
species. We have observed this species to result from the reaction of 1HC
with water in previous studies, but the amount observed here suggests
that perhaps some surface copper hydroxide species promote this
hydrolysis.
[20] a) J. F. DeJesus, L. M. Sherman, D. J. Yohannan, J. C. Becca, S. L.
Strausser, L. F. P. Karger, L. Jensen, D. M. Jenkins, J. P. Camden,
Angew. Chem. Int. Ed. 2020, Advance Article, 10.1002/anie.202001440
b) We note that the ability of the NHC to reduce metal surfaces should
be dependent on the reduction potential of the metal oxide in question,
relative to the redox properties of the NHC. Plese see the SI for details.
[21] a) A. Bakker, A. Timmer, E. Kolodzeiski, M. Freitag, H. Y. Gao, H. Mönig,
S. Amirjalayer, F. Glorius, H. Fuchs, J. Am. Chem. Soc. 2018, 140,
11889-11892; b) H. K. Kim, A. S. Hyla, P. Winget, H. Li, C. M. Wyss, A.
J. Jordan, F. A. Larrain, J. P. Sadighi, C. Fuentes-Hernandez, B.
Kippelen, Chem. Mater. 2017, 29, 3403-3411; c) G. Lovat, E. A. Doud,
D. Lu, G. Kladnik, M. S. Inkpen, M. L. Steigerwald, D. Cvetko, M. S.
Hybertsen, A. Morgante, X. Roy, Chem. Sci. 2019, 10, 930-935.
[22] a) C. A. Citadelle, E. L. Nouy, F. Bisaro, A. M. Z. Slawin, C. S. J. Cazin,
Dalton Trans. 2010, 39, 4489-4491; b) M. R. L. Furst, C. S. J. Cazin,
Chem. Commun. 2010, 46, 6924-6925.
[23] a) B. Liu, Y. Zhang, D. Xu, W. Chen, Chem. Commun. 2011, 47, 2883-
2885; b) B. Liu, X. Ma, F. Wu, W. Chen, Dalton Trans. 2015, 44, 1836-
1844.
4
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Entry for the Table of Contents
For the first time, a simultaneous etch-functionalize protocol has been developed for oxidized copper surfaces using N-
heterocyclic carbenes. These results demonstrate the dual functionality of N-heterocyclic carbenes when applied to more reactive metal
surfaces that are prone to oxidation and offers a new strategy for chemical etching that avoids dangerous reagents. This work also
provides an opportunity to replace precious metals such as gold with more abundant metals like copper in organic-on-metal devices.
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