Iron deposition

Focusing on these issues and following the indications from experimentalists, a first set of simulations targeted the approach of a single Fe atom to the upper Cp–ring of one of the two ferrocene molecules adsorbed on the Cu(111) surface.

These simulations have shown that the approach of iron, initially placed at a distance of about 3.5 ˚A above the upper C5H5 ring, proceeds in a barrierless way. The Fe atom gradually approaches the hydrocarbon ring and eventually it reaches an equilibrium distance of 2.0 ˚A from the Cp–ring. However, contrary to the case of Cu deposition, a destabilization of the ferrocene molecule occurs.

The upper C5H5 ring coordinates with the deposited Fe atom rather strongly and forms a stable complex. This upper Cp–ring then leaves the ferrocene to which it belongs at the beginning of the simulation and this newly formed Fe–C5H5 system becomes an independent moiety that departs from the substrate, leaving exposed the Fe atom belonging to the originally physisorbed ferrocene. This process is sketched in panels (a) and (b) of Figure 3.5. In these conditions, a physisorbed Fe–C5H5 complex remains on the Cu(111) surface as a new and different building block for which the ferrocene molecule represents just a precursor, in line with the suggestions for transformation and reassembling of deposited ferrocene reported by Paul and coworkers [67].

The destabilization process can also involve a hydrogen transfer from the depart-ing Cp–rdepart-ing to the exposed Fe atom, as shown in panel (c) of Figure 3.5. Agostic interaction is a term coined by Malcolm Green in the year 1983 which specifically defines “the situations in which a hydrogen atom is covalently bonded simultane-ously to both a carbon atom and to a transition metal atom” [205]. The bond

length of metal to hydrogen is typically in the range of 1.8-2.3 ˚A with metal-hydrogen-carbon bond angles of 90-140 degrees [206].

Figure 3.5: Main snapshots of the deposition process of an atom of iron on one of the two ferrocene molecules physisorbed on a Cu(111) substrate.

(a) Approach of the Fe atom. (b) Destabilization of the upper Cp ring by the approaching Fe atom. (c) Departure of the upper Cp–ring / Fe complex accompanied by H transfer. The color code is cyan for H, gray for C, orange

for Fe and brown for Cu.

This results after a strong agostic interaction taking place between the Fe metal center and the distorted Cp ring. Such an interaction is rather common in metal-hydrocarbon systems and often represents the trigger for catalytic reactions pro-moted by transition metals [206, 207]. Indeed, despite the fact that for Fe de-position the realization of a junction is jeopardized, the unshielded Fe atom can become an active catalytic site well suited to bind subsequent deposited molecules.

This destabilization process is due to the chemical nature of the atom used for the deposition with respect to the metal already present in the selected double–decker molecule. In fact, in this case, the same atom, namely Fe, is present in the fer-rocene complex. Thus, a second Fe atom that is not sandwiched between the two Cp–rings is therefore prone to bind to another available moiety and starts com-peting with the Fe of the Fe(C5H5)2 present on the Cu(111) surface. As a ”fair”

result of this competition, the bottom Cp–ring, interacting with the 2D electronic state at the metal surface, remains in its place carrying the coordinated Fe. At the same time, the upper Cp–ring forms an analogous complex with the incoming Fe atom and two Fe–C5H5 are formed. The upper one being not physisorbed, leaves the vicinity to search for a suitable location on Cu(111) where no adsorbents and no steric repulsions would hinder its own adsorption.

As mentioned, this destabilization, although having an evident negative conse-quence for the practical realization of a nanojunction at first glance, should not be regarded as a pure failure. In fact, cyclopentadienyl–iron complexes represent useful precursors for iron carbene complexes and an important catalytic moiety [208, 209]. Hence, the deposition of Fe, in contrast with Cu, represents a viable way not to form nanojunctions, but a route to realize nanocatalysts where the control of the active, or to be activated, sites is granted by the ordered array of pristine ferrocene molecules physisorbed onto a Cu(111) substrate. By a proper selection of deposited atoms, such as Fe in this case, the metal atom sandwiched between two cyclic hydrocarbon molecules can be exposed, thus becoming an ac-tive catalytic center.

Figure 3.6: Highest occupied (left panel) and lowest unoccupied (right panel) Kohn-Sham states for the final configuration of the system after the deposi-tion of iron. Positive and negative amplitudes are represented in red and blue, respectively, at isosurfaces values of ± 9×10⁻³ (e/˚A³)^1/2. The color code is

identical to that of Figure 3.5.

The analysis of the highest occupied Kohn-Sham (HOKS) orbital and the lowest unoccupied one (LUKS) is summarized in Figure 3.6for the resulting destabilized structure. Namely, the HOKS orbital is a mixture of the surface state, the d_x²−y²

state of the (now) unshielded Fe of the destabilized ferrocene, plus ad_z² contribu-tion from the deposited Fe. Minor contribucontribu-tions from the removed Cp ring are also

visible. The corresponding eigenvalues for these two states are (HOKS -4.178 and LUKS -4.177 eV) characterized by a tiny energetic separation of about 0.001 eV, reminding clearly that the system has a strong metallic character, as expected also on the basis of the former works [69]. Yet, this makes the unoccupied states easily accessible by electron donors, thus making the system a potentially highly reactive nanocatalyst. The fact that surface states mix with the ferrocene (dissociated or not) molecular orbitals is not a surprise, since this feature was already observed and assessed also experimentally [70]. Instead, contrary to the case of pure fer-rocene [69, 70], the LUKS of this destabilized system, does no longer resembles a purely ferrocene molecular orbital, but again a mixing of the Cu(111) surface state and a d_x²_−y² orbital belonging to the deposited Fe atom. Being this Fe, in the present configuration, the most unshielded (by Cp rings) one, this indicates that this Fe site is able to accept electrons and, as such, can become and active catalytic center. It can be inferred that an identical fate will be true for the Fe atom of the ferrocene which has lost its upper Cp ring.