N‐Heterocyclic Carbene Self‐assembled Monolayers on Copper and Gold: Dramatic Effect of Wingtip Groups on Binding, Orientation and Assembly

Abstract Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of E des=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing.

Self-assembled monolayers of N-heterocyclic carbenes (NHCs) on coppera re reported. The monolayer structure is highly dependento nt he N,N-substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surfacewhilemethyl-or ethylsubstituted NHCs lie flat. Te mperature-programmed desorption studies show that the NHC binds to Cu(111)w ith ad esorption energy of E des = 152 AE 10 kJ mol À1 .N HCs that bind upright desorb cleanly,w hile flat-lying NHCs decompose leaving adsorbed organic residues.S canning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC) 2 Cu species by extraction of ac opper atom from the surface after annealing.
Controlo ver the orientation of molecules within self-assembled monolayers (SAMs)iscritical. In thiolate SAMs on gold, solution deposition methods give upright binding modes throughatwo-step process, with flat-lying speciesp resent at low coverage transformingi nto dense upright SAMsa th igher concentrations and extended times. [1] The ability to prepare well-defined SAMs with predictable properties (e.g. hydrophobicity/hydrophilicity) is ah allmark of thiol-based SAMs and is critically dependent on molecular orientation [2,3] Recently, N-heterocyclic carbenes (NHCs) have emergeda s promisinga lternatives to thiols for the formation of robust, ordered SAMs on Au. [4][5][6][7][8] Seminalw ork by Siemeling [4] and Johnson [5] showedt hat NHCs bind to planar Au surfaces. Crudden and Horton demonstrated that NHCs form clean, well-ordered monolayers with exceptionally high stability, surviving treat-ment with boilingo rganic solvent, acid, base, oxidant, [6] and high temperatures. [9] These conditions would destroy typical thiol-based SAMs. [10][11] Other studies of NHC films on Au have shownr emarkable effects on work function, [12] and highly ordered structures can be imaged by low temperature scanning tunneling microscopy (STM). [7] In addition, the formation of strongC ÀSi bonds has recently been reported following NHC adsorption on Si(111). [13] Despite the impact of these studies, the effect of NHCs tructure on SAM formation has barely been examined. Additionally, the ability of NHCs to form monolayers on other, more reactive metalshas received no attention outside the realm of nanoparticles. [14] Thiol-based SAMs on more reactive metals such as Cu, Ag, Pt, Ni, etc. are prone to decomposition and generationo f metal sulfides, creatingapressing need to find alternatives to S-based ligands for these metals. [15] Herein we presentt he preparation of highlyo rdered,t hermally stable NHC films on Cu(111). Through STM, high resolution electron energy loss spectroscopy (HREELS), and temperature programmed desorption (TPD) studies, we demonstrate that small differences in the size of the NHC wingtip groups lead to two distinct binding modes. In addition, we find that NHCs bind to Cu(111)w ith as imilarly high adsorption energy to that observed on Au(111).
Bench-stable benzimidazolium bicarbonates bearing methyl, ethyl, or isopropyl wing tipg roups (1 to 3)w ere vapor deposited onto cleanC u(111)a nd Au(111)s urfaces ( Figure 1a). The resultingS AMs weree xamined by HREELS in the specular geometry where spectra are dominated by dipole scattering such that the surfaced ipole selection rule provides experimental information on adsorbate orientation. [1617] Spectra obtained following adsorption of NHCs 1 and 2 on both Cu(111)a nd Au(111)w ere dominated by av ery strong peak at 730 cm À1 ,a ssigned to the out-of-plane aromatic CÀH bending mode, whose dipole moment is normalt ot he molecular plane. Features at 2940 and 3075 cm À1 (assigned to the alkyl and aromatic ring CÀHs tretches, respectively)a nd at 1250-1600 cm À1 (C-N and C=Cs tretches, and CÀHb ending modes)a ll appear very weak. The relative intensities of these energy losses provide strong evidencet hat the molecular planes of 1 and 2 are aligned approximately parallel to both the Au(111)and Cu(111)s urface planes. (Figure 1b).
Prolonged exposure of NHC 1 to Cu(111)l ed to significant enhancement of the weakers ignals, implying an increasing amount of upright speciesath igherc overage ( Figure S1). However,t he 730 cm À1 peak was not suppressed;t herefore, the spectrum reflects the coexistence of upright and flat-lying species. Such behavior is not uncommon [18] -a similar interpretation was proposed for the coverage-dependence of benzoic acid adsorption on Cu(110). [19] Spectra recorded for the bulkier NHC 3 on Cu(111)a nd Au(111)a re significantly differentf rom those obtained for NHCs 1 and 2,w ith strong peaks observed with 1 and 2 appearingw eak for 3,a nd vice versa (Figure 1b). NHC 3 must, therefore, adopt an upright geometry.T his is consistent with the DFT optimized geometry (Figure 1c), and analogoust or esults obtained from NHC 3 on Au(111). [6,9,12] STM revealed the presence of highly-orderedm olecules of NHC 1 on Cu(111)a t3 00 K, along with trenches of ad epth equivalent to as ingle Cu atom. Elliptical features of length 3.4 AE 0.7 ,w ere resolved in the trenches (Figure 2a). Adjacent features were separated by 3.8 AE 0.6 ,c onsistentw ith individ-ual upright species derived from NHC 1 stacked via intermolecular p-p interactions. [20] These upright speciesc oexist with features assigned to flatlying molecules periodically arranged (Figure 2b)i nto as uperstructure consistentw ith ac ommensurate (4 À4 j 82 )u nit cell containing 40 Cu atoms and two dimeric features of length 0.91 AE 0.09 nm which are tentatively ascribed to enetetramine speciesr esulting from the dimerization of NHCs. [21] Enetetramines were employed by Siemeling [4] as potential precursors to NHC-functionalized surfaces, but have never previously been observed intact on asurface.
The inset in Figure 2b displays molecular features whose periodicity is consistent with ac ommensurate (1 À3 j 94 )s tructure containing one enetetramine speciesp er unit cell (31 Cu atoms). When imaging at 300 K, the islands fluctuated in shape due to the high mobility of individual speciesw ith images being acquired slightly below saturation coverage.
Annealing the sample to 365 Kresulted in the disappearance of upright speciesa nd the formation of an ew ordered molecular arrangement. Analysiso ft he Fourier transform of the image revealed unit cell vectors a = 1.55 nm; b = 2.43 nm, with an included angle q(a,b) = 77.68.T hese dimensions conform with ac ommensurate (7 4 j 110) superstructure (a = 1.555 nm, b = 2.438 nm, q(a,b) = 80.088, ( Figure 3a). Each unit cell contains two distinct molecular features and 66 Cu atoms.
The SAM is dynamic in nature,a lthough less so than its precursor prior to annealing. Reflectional and rotational domains, and aM oirØ pattern were identified. Additionally,i ns itu defect correction and growth of ap redominant domain were visible    ( Figure 3b and Figure S2). The growth of ap referential domain is likely directed by the crystallographic direction of steps at which the SAM nucleates.T he features observed are too large to be attributed to single NHC molecules or enetetramines, and are insteada ssigned to pairs of (NHC) 2 Cu complexes coadsorbed with Cu adatoms (Figure3c, d).
Extraction of Cu atoms from steps andi ncorporation into molecular assemblies is at hermally activated process. [19] Lifting of one atom from the (111)s urfacep lane has precedenti nt he Au chemistryo fi sonitriles [22] andt hiols [3] ,w ith recent studies reporting similare ffects for NHCs. [7] Rodríguez-Castillo et al. concluded from DFT calculations that upright NHCs restructure Au surface atoms, probably as an intermediate step in the formation of (NHC) 2 Au I complexes. [23] Additionally,T ang and Jiang recently reported that (NHC) 2 Au complexes containing less sterically bulky NHCs favor flat lying NHC geometries driven by vdW interactions between the NHCs and the Au surface. [24] Furthermore, (NHC) 2 Cu 0 complexesh aveb een isolated and characterized. [25] Further work (e.g. XPS) would be required to identify the formal oxidation state of Cu in these complexes. Some evidencew as found for ordereda rrangements of NHC 1 on Au(111)( FigureS4), though the speciesw ere too mobile to image clearly.N oo rdering of NHC 3 was found on either Cu(111)o rA u(111)a ta ny coverage when imaginga t3 00 K. It is likely that the upright NHCs exhibit similar "ballbot-type motion"t ot hat described by Glorius and Fuchs for NHCs on Au. [7] Essentially,u pright NHC 3s kates aroundt he surface riding on top of aC ua datom. Glorius and Fuchs reported high mobility when imaging even at 77 K. [7] TPD data further confirmed that the two typeso fa dsorbed NHCs displayd istinctly different behavior.W hile upright NHC 3 desorbed cleanly from both Cu(111)a nd Au(111), flat-lying NHCs 1 and 2 underwent more complex decomposition and desorption processes (See Figure S4 for data on Au). Figure4a shows coverage dependentT PD spectra for m/z = 39 (C 3 H 3 + from the benzene moiety) following the adsorption of NHC 3 onto Cu(111)a t3 00 K.
Desorption occurred in as ingle peak (T max = 570 K) independento fc overage;t ypical of first-order kinetics. [26] Coincident desorptions of H 2 ,H CN, andC 3 H 5 + (assigned to isopropyl groups)w ere also detected ( Figure S3), suggesting that NHC 3 desorbs intact from Cu(111). AR edhead analysis yields ad esorptione nergy of 152 AE 10 kJmol À1 ;i ndistinguishable within error from the reported value of 158 AE 10 kJmol À1 on Au(111). [927] DFTanalysiso fasingle NHC 3 specieso naC u(111) slab predicts ab inding energy highert han that observedo n Au (182.6 kJmol À1 (see SI)). However,e rrors in such calculations are typically % 20 kJmol À1 ,s ot heory and experiment are mutually consistent within error.R epulsive lateral interactions (not accounted for by the DFT calculations) may explain the lower value measured by TPD, though the fact that T max is independent of coverage points to such interactions being relatively weak. These resultsc onfirmt hat NHCs form highly stable SAMs on Cu surfaces.
For films derived from NHC 1,H 2 evolution (m/z = 2) occurs in two overlapping peaks at 532 Ka nd 590 Kw hich can be assignedt os equential thermally activated dehydrogenation steps. [28] Desorption of HCN (m/z = 27) occurs concurrently with H 2 evolution, signifying that thesed esorptione ventsa re related. Unlike the TPD spectra for NHC 3,t he relative intensity of the fragments did not correlate with the exposure, suggesting am ore complexs urface chemistry.T he yield for m/z = 39 is marginal, implying that the ring moiety remains adsorbed (Figure 4b). It can be concluded that NHC 1 is stable on Cu(111) up to the onset temperature for the first dehydrogenation step ( % 475 K), contrasting starkly with the behavior of NHC 3.
STM imagingo fC u(111)t erraces after heating NHC 3 films resultedi nc lean surfaces, consistent with NHC desorption via asimple C-Cu bond cleavage (Figure 4c). In contrast, annealing NHC 1 and 2 films at high temperatures showede videncef or decomposition products, indicating ac omplex decomposition of flat-lying NHCs (Figure 4d). TPD and STM imaging of films derived from 2 ( Figure S3) resembled closely the behavior observed for 1,a nd an analogousi nterpretationi sp roposed. Thermal stabilityo ft he SAMsw as also assessed by HREELS ( Figure S1). Overall attenuation of all spectral features occurs above 560 Kfor all NHCs, consistentwith the TPD findings.
In conclusion, the substituents at the N,N-positions of the NHC are criticali nd etermining the adsorption geometry and fate of the NHC-basedS AMs on Au and Cu. Small NHCs with dimethyl substituents form films with mixtures of flat and upright orientations, while the diisopropyl NHC stands upright only.T he adsorption energy of NHC 3 on Cu(111)w as found to be the same as on Au(111)w ithin error. Upright NHCs desorb cleanly,w hile flat-lying NHC films dissociatel eaving surface contamination. For NHC 1,o rdered arrangements of (NHC) 2 Cu complexes were imaged.T he fact that appropriately designed NHCs bind to Cu with high bond energies is as ignificant discovery and paves the way for future work on the practical ap-

Experimental Section
See the Supporting Information for experimental details.