Adatom mediated adsorption of N‐heterocyclic carbenes on Cu(111) and Au(111)

The adsorption of N‐heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density‐functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail. Flat‐laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so‐called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC)2Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC)2Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.


| INTRODUCTION
N-heterocyclic carbenes (NHCs) 1,2 have been recently attracted much attention as suitable ligands for stabilization and modification of metal surfaces and nanoparticles. [3][4][5][6] NHCs are organic molecules, which include in their central ring one or more N atoms and the so-called carbene C atom, that is, a neutral carbon atom with a valence of two and two unshared valence electrons (see Figure 1), giving rise to a high reactivity. They find applications in various chemistry fields 2 as well as in the electronic device technology. 7 In many instances, NHCs form highly ordered self-assembled monolayers on planar metal [7][8][9][10][11][12][13][14][15][16][17][18][19] and silicon surfaces 20 with considerable thermal and chemical stability 7,8,[20][21][22] and a remarkable influence on the surface properties. 2,15,19,20,[23][24][25] In particular, such systems appear to be promising in connection with doping of (organic) semiconductors, as similar molecules have been shown to allow immobilization of charge carriers via covalent anchoring. [26][27][28] In the last decade, NHCs have been widely studied on flat Au surfaces. [7][8][9]13,14,22,23,25,29,30 However, the interaction with more reactive surfaces has received less attention. Hitherto, there is merely a handful of investigations on Cu and Ag surfaces. 10,12,16,31,32 Recently, particular attention has been paid to the different binding modes of the NHCs on metallic surfaces and to the role of their side groups in determining these modes. Jiang et al. 10 and Wang et al. 9 reported flatlaying adsorption configurations for NHCs with relatively small Nbound organic substituents on coinage metals 10 and Au(111), 9 respectively. It has been furthermore shown that NHCs can easily bind to under coordinated metal adatoms on the respective metal surfaces. 9,12,32 NHCs with bulkier N-substituents prefer upright adsorption geometries, resulting in so-called ballbot configurations, displaying high surface mobility. 9 In line with these studies, but for another class of NHC molecules, Larrea et al. 12 reported the selfassembled monolayers of three NHCs, namely benzimidazolium bicarbonates bearing methyl, ethyl, and isopropyl wing tip groups on Au (111) and Cu(111) surfaces (denoted NHC1, NHC2, and NHC3 in Reference [12] and in the following, see Figure 1). They found again that bulky isopropyl substituents (in case of NHC3) are responsible for allowing the molecules adopt upright geometries on both surfaces, while the less bulky NHCs (NHC1 and NHC2) lie flat on planar surfaces. Very recently, the work of Deng et al. 32 was published, which reports on DFT calculations on NHC1 adsorption (labeled NHC Me in Reference [32] at different solid surfaces, including Au(111) and Cu(111), suggesting that adsorption at adatoms plays a decisive role for the formation of (NHC1) 2 dimer complexes.
In the present work, we extend this DFT study to NHC2 and NHC3 in order to investigate the influence of the steric bulk effect of the side groups, which has been frequently discussed in experimental studies. [7][8][9][10]14,16 Surface adatoms are confirmed to play a decisive role in determining the adsorption configuration independent on the size of the molecular side groups. Furthermore adatom attached NHC are characterized as bistable complexes with both upright and flat-laying configurations, providing an efficient precursor for the observed formation of bis(NHC) dimers.

| METHODOLOGY
Total-energy density functional calculations are performed using the Quantum-ESPRESSO package. 33,34 The electron-electron exchangecorrelation energy is modeled using the Perdew-Burke-Ernzerhof  Tables SI and SII).
Atomic structure optimization is performed with convergence criteria of 3 meV/Å for the atomic forces and 10 À7 eV for total energy difference. The adsorption energies (E ads ) are given by where E sys , E sur , and E mol are the energies of the adsystem, the surface (with and without adatoms) and the gas-phase molecule, respectively.    Figure S1 and Tables SIII and SIV for details).
In agreement with previous works, 9,16,30 the most stable structures of the three NHCs on ideal surfaces are characterized by a formation of a single covalent bond between the NHC's carbene carbon atom (see Figure 1) and one atom from the metal surfaces (see Figure 2, Figure S3, and the charge density difference plots in Figure S6). Such geometries are denoted as surface-bound species in the following. Table 1 compares the characteristic structural parameters, the adsorption energies and the contributions of the dispersion interactions to the adsorption energies for the three surface-bound NHCs in their most stable states. Our calculations clearly show, however, that the adsorption energies are increasing with the size of the side groups. Apparently, this is related to the increased contribution of the dispersion interactions of 53% (45%), 58% (52%) and 63% (58%) for NHC1, NHC2, and NHC3 on Cu(111) (Au(111)), respectively. The vertical shift of the metal atom on which the molecule anchors (d 2 ) shows the same tendency as well, further highlighting the crucial role of the dispersion interactions to determine adequate molecular geometries and reliable adsorption energies, also in case of covalently bound species.
It is important to note that our calculations found no indications for any flat-laying geometries. Irrespective of the start structure, the geometries of all studied molecules have been finally converged to upright configurations (with θ < 10 ; see Table 1). Vertical geometries (θ = 0 ) can be achieved within a few meV. In contrast, forcing the molecules to adopt flat-laying geometries would require energies in the order of 1 eV (see Figure S2 for details). This is true not only on Au(111), 14,30 but also on Cu(111). In both cases, flat-laying adsorption geometries can be safely ruled out on pristine metallic substrates and thus cannot account for the different binding modes reported by Larrea et al. 12 for these NHCs. Adatoms are also reported to play a key role for the formation of NHC self-assembled monolayers. 40 The presence of adatoms explains the high mobility of NHCs on Au(111) surface despite strong NHC Au bond formation. As suggested by Wang et al. 9 for 1,3-dimethylimidazol-2-ylidene (IMe) molecule and by Larrea et al. 12 for NHC1, NHC2, and NHC3, the formation of NHC-adatom complexes enables the so-called 'ballbot'-type motion of these complexes. It offers low barriers to diffuse, much lower than surface-bound molecules and even lower than the adatom alone, thereby promoting the molecular self-assembly. As a F I G U R E 2 Side-views of the most stable adsorption configuration of (exemplarily shown) NHC1 on the ideal Au(111  Figure S4).  Figure 3). Table 2 presents the structural geometries and the adsorption energies of the most stable structures of the three NHCs on Cu (111) in the upright and flat-laying binding modes (see also Figure S5). Compared to the surface-bound species, NHC ballbot species in the T A B L E 1 A comparison of most stable structures of NHC1, NHC2, and NHC3 on Cu(111) and Au(111). The contributions of the van der Waals (vdW) interactions (E vdW ) to the adsorption energy (E ads ) are also given. The adsorption geometry parameters (d 1 , d 2 , and θ) have been defined in Figure 2 Configuration F I G U R E 3 (A) Side-view of, for example, an Au adatom residing on the fcc hollow site of planar surface. Its height assuming no molecular adsorption is considered as a reference (d 0 ). (B,C) side-views of the most stable structures of a N-heterocyclic carbenes (NHC) (e.g., NHC2) adsorbed on an adatom (ballbot species) in an upright/a flat-laying geometry. The vertical upwards shift of the adatom after the NHC adsorption is denoted as d 3 . (D) Is like (C) but in a top-view. The azimuthal orientation of the molecular symmetry axis (SA) with respect to the <110> direction of the surfaces is denoted as (ϕ). The adsorption parameters of NHC1, NHC2, and NHC3 are given in Table 2 Figure 5A). This can be related to the fact that the metal sand d-orbitals are more accessible in the ballbot species, enabling a stronger donor-acceptor bond. 14 Thereby, the length of the formed C Cu bond (about 1.93 Å)

| Adsorption at Cu/Au adatoms
is apparently shorter than that for the surface-bound species (2.05 Å).
This tendency is also reflected in the far smaller upwards shift of the adatom (d 3 ) compared to that (d 2 ) shown in Table 1 for the same NHCs in surface-bound species.
In parallel, the flat-laying ballbot geometries become possible and even most stable for NHC1 and NHC2 (see also Figure 5A). In fact, The results drawn for Cu(111) can be more or less transferred to the Au(111) surface, see Table 3. Compared to the surface-bound species, NHC ballbot species in the upright geometries are more stable by about 1 eV, in agreement with previous studies. 14,16,29,30 The length of the formed C Au bond (2.03 Å) is again reduced (2.15 Å in the surface-bound species). The calculated length of d 1 is in a very good agreement with values presented in References [13,14,16,23,29].  Figure 5B).
Notably, a weak dependence of the adsorption energies for NHC3 on the tilting angle was previously reported. 14  the same surface atom (NHC-adatom-NHC, see Figure 4). For different small-sized NHCs (IMe and NHC1), such species have been previously reported experimentally on Au(111) as well as on Cu (111) surfaces, see for example, References [9,10,12,13]. There are some indications that the formation of such complexes is promoted by thermal activation or by intermolecular interactions. 12,14 Tables 2 and 4). They are, thus, again much larger than those obtained for surface-bound NHC (see also  per molecule for the (NHC) 2 Au dimers are even slightly higher (by 10-80 meV) than the adsorption energies of the single NHC flat-lying ballbot species. In other words, the (NHC) 2 Au dimers are highly probable and expected to be formed even prior to a separate ballbot configuration at already present, but still 'unoccupied' adatoms.
On Cu (111), the situation is more complex. The adsorption energies per molecule for (NHC) 2 Cu are slightly smaller than for the respective most stable ballbot species (cf. Figure 5A) Figure 4B, where the central Au atom lay in plane with the two NHC molecules, in agreement with NEXAFS measurements reported in Reference [14].
On Cu(111) our calculations find that a similar planar configuration is less stable, by about 0.2 eV higher in energy than the most stable structure, where the Cu atom within the (NHC) 2 Cu dimer is still covalently bound to two surface atoms (bridging position, see also  [Corrections added on 05 Jan 2022, after the first online publication: ProjektDEAL funding statement has been added.]