A supported CO-FePc system was prepared by dosing CO molecules onto FePc adsorbed on a Cu (111) surface at 4.8 K. The scanning tunneling microscope (STM) image in Figure 1a shows two distinct features for the FePc molecule and the complex (CO-FePc), which are similar to previously reported STM images on other surfaces25. AFM images were obtained using a CO-terminated tip, confirming their respective structures (Figures 1b and c). The AFM image in Figure 1b of the CO-FePc complex featured a protruding center due to the CO attached to Fe. This was confirmed by comparing with AFM images of FePc molecules on the surface (1c), and further verified by our simulated images (Figures 1d and e).
The dative CO-FePc bond is known to be formed via σ-donation from the CO 5σ orbital and π-back donation from Fe dπ26-29. We studied the rupture of this dative bond by applying mechanical forces using the scanning tip of the AFM. The same CO-terminated tip employed for imaging was used first because it is known to be chemically inert1. By decreasing the tip height, the repulsive interactions increased, as indicated by the increased contrast in the images (Figure 2a (ii-iv)). At a lower tip height (+40 pm), the peripheral aromatic rings of FePc became visible, while the center of the image became distorted due to strong repulsions with the tip. Upon further reducing the tip height (+30 pm), a sudden change of the image occurred during scanning, as indicated by a line created with a different contrast. Subsequent scans showed the repulsion had disappeared, indicating that the CO attached to FePc was dislodged due to the strong repulsion with the tip. The chemical structure of FePc revealed from subsequent scanning of the lower part of the molecule confirmed that a free FePc was left after CO removal and that the tip remained intact during the dissociation. Comparison of the contrast in the lower part to the upper part of the same AFM image (iv) obtained at the same tip height reveals a downward shift of FePc by ~ 30 pm upon CO removal. This shift is likely due to the trans effect by the Cu substrate on the FePc complex26-30, whereby the binding or removal of one ligand respectively reduces or enhances the strength of the bond to the ligand on the opposite side31. This observation confirmed the rupture of the dative bond between CO and FePc induced by the increased interactions during tip scanning.
To elucidate the CO-FePc bond rupture, we performed detailed measurements of the interaction forces during the entire bond rupture process. Figure 2b shows a 3D force map representing the original frequency shift (Δf) obtained at different tip heights (z) by scanning across the center of the CO-FePc complex (shown in the insert). The dislodging of the CO was indicated by a break point (x = 0) with decreasing the tip height during scanning and by the discontinuity in the frequency shift (Δf) curve (red curve in Fig. 2c). The deconvoluted force curve along the tip height (z) at the breaking point (x = 0), in Fig. 2d, shows that the dative bond ruptured with a force of 220±30 pN, after passing a maximal force at ~300 pN.
Dislodging experiments were also performed with a bare metal tip terminated by a Cu atom under similar experimental conditions as using the CO-terminated tip. The Cu atom tip is known to be a chemically active tip1. When the Cu-tip was used, only attractive interactions between the tip and CO-FePc were detected (Figure 2e), until the rupture of the dative bond took place. At this point, the attractive force reached 150±30 pN by reducing the tip height (Figures 2f, g).
The observation that both a repulsive force of +220 pN and an attractive force of -150 pN are capable of breaking the same dative bond indicates the important role of the probe tip. It is difficult to rationalize how a compressive force could break a chemical bond as the single bond strength is often correlated with the bond length,32 unless it is through a displacement. We performed real-space DFT calculations to address these questions to and shed light on the details of the bond-breaking process. Both model tips were initially positioned at relatively large heights (~500 pm) to compute the relative frequency shifts using a finite difference method26. The calculated frequency shift curves (scaled) for both tips agreed very well with the measurements (dashed curves in Figures 2c and 2f).
Due to the multidimensional interactions between the two CO molecules for the CO-tip (vide infra), the Cu-tip is examined first (Figure 3a). The calculated maximum force (-156 pN) at the Cu tip apex agrees closely with the measured force of -150±30 pN (Figure 3b). Furthermore, as the force on the tip apex approaches a maximum at a Fe-C bond distance of ~1.9-2.1 Å (shaded area), a low spin to high spin transition of the system occurs (Figure 3b). The change in spin states suggests the breaking of the dative bond, as demonstrated by a previous DFT study of the CO-FePc complex on a Au(111) surface31. For a CO-tip, simply applying a compressive vertical force from above the CO-FePc was not sufficient to break the bond. Therefore, we simulated the experimental process where the tip approaches the CO-FePc complex horizontally (Figure 3c), similar to the AFM scanning along the x direction (see Supplementary Materials for the equilibrium geometry). Using this method, while a small increase of the vertical force acting on the tip apex was observed (Figure 3d), a lateral force emerged at the same time acting on the C attached to FePc and rapidly increased to a few nN (Figure 3e). High resolution calculations (dashed curves inside the red boxes) indicate the bond was likely broken when the shear force approaches ~400 pN on the bottom C atom of the CO-FePc complex. Bond rupture by a shear force of 400 pN is reasonable given the fact that the Fe-C dative bond is weaker than typical single bonds21. Therefore, our calculations confirm that the lateral force is most likely responsible for the breaking of the dative bond when the compressive force (220 pN) was applied by the CO-terminated tip.
The calculated spin-polarized local density of states (LDOS) projected onto the center Fe atom of the system shows that the removal of CO results in an increase (from 0.00 to 1.20 µB) in the net magnetic moment (Figures 4a and b). This change in spin state is consistent with Figure 3b and a previous DFT study of the CO-FePc complex on the Au(111) surface31. The total electron density and the highest occupied molecular orbital (HOMO) of the CO-FePc system demonstrate that the Fe center interacts with the two bridge Cu atoms underneath via the dz2 orbital (Figures 4a and d). Figures 4a and c show the presence of the Cu(111) substrate weakens the Fe-C dative bond by shifting the dz2 orbital toward the HOMO. These results confirm the rupture of the dative bond.