Every chemical reaction involves the making and breaking of atomic bonds. Examining the physical nature of these bonds and the forces that alter them is crucial to understanding chemical and catalytic processes. In our article “Breaking a dative bond with mechanical forces,” published in Nature Communications, we report a detailed bonding rupture study using a qPlus type non-contact atomic force microscope (nc-AFM). Using both a metallic tip and a carbon monoxide (CO) functionalized tip, the forces involved in rupturing a dative covalent bond were measured with piconewton (pN) precision. In combination with quantum-based simulations, our results show the bond-breaking process in unprecedented detail. For example, we find that the dative bond between CO and the iron atom in a ferrous phthalocyanine molecule can be ruptured by an attractive force of ~150 pN or by a repulsive force of ~220 pN, with a significant contribution of shear forces, accompanied by changes of the spin state of the system.
A precise measurement of the bond rupture process at the single-bond scale is challenging in several ways. First, you need to be able to see a single chemical bond before you can measure it. This was not possible until 2009, when the chemical structure resolution of molecules down to the single-bond scale was demonstrated by Leo Gross et al. using qPlus type nc-AFM with a CO functionalized tip. So far, nc-AFM offers the only way to experimentally resolve and manipulate a single bond without damaging the molecular system.
Second, it is challenging to build a system that is suitable for bond strength studies since at atomic scale everything that interacts with a chemical bond could alter its nature in some way. The measurements need to be performed in ultra-high vacuum and liquid helium temperature conditions to exclude effects such as thermal perturbations, as well as unwanted interactions and bond deformations.
To tackle these challenges, we constructed a special dative bonding system by placing a single CO molecule on top of an iron phthalocyanine (FePc) molecule. Dative bonds are commonly found in transition metal complexes and play vital roles in catalysis, organometallic chemistry, and biochemistry. They are extremely sensitive to subtle environmental changes. Even the probe tip used to probe the bond can induce a charge transfer that causes a change in bonding energy. In our system, the phthalocyanine base acts as a protective frame that isolates the central iron atom, creating a much cleaner system. The CO molecule has just two atoms, is rigid, and only interacts with the central iron atom of FePc.
Figure 1a shows the chemical structure of an FePc molecule. With regular scanning tunneling microscopy (STM), we were able to resolve the cross-like structure of the molecule on the Cu(111) surface, but no detailed atomic structure could be observed (Figure 1b). An nc-AFM image taken with a CO functionalized tip was able to identify the backbone of the molecule, and the central atom could be clearly resolved (Figure 1c).
After attaching a CO molecule to FePc, STM images showed a bright spot in the center of each molecule. By gradually decreasing the tip height, the bonded system could be imaged. Further decreasing the tip height induced rupture of the bond.
Figure 2 demonstrates the process of breaking a dative bond between CO and FePc, where a) is a constant-current STM topographic image of FePc with CO sitting on top of the central Fe atom, and b-j) are constant-height AFM frequency shift images at different tip heights. The tip height was set with respect to a reference height given by the STM set point and the bare Cu(111) substrate near the molecule. The actual breaking takes place in Figure 2i, where the arrow points to a change in the image indicating before (top) and after (bottom) the CO molecule was removed from FePc. After CO removal, the FePc molecule could be imaged again as shown in Figure 2j.
With this well-controlled system, we were able to measure the bond rupture process using nc-AFM with both an inert CO functionalized tip and an active single-atom metal tip. In addition to precise measurements of the forces required to break the CO-Fe dative bond, we also examined the bond-breaking process with an unprecedented level of detail. In combination with density-functional theory calculations, we confirmed that the dative bond is weakened by the presence of the underlying Cu substrate, and the absorption of CO on FePc could also affect separation of FePc from the Cu surface.
When a CO tip is used, the bond is ruptured after the force surpasses a maximum and then breaks at a lower force value, which originates from the sequential cleavage of the σ-donation and π-back donation of the dative bond. The σ-bond has a higher symmetry than the π-bond, and hence a higher force is required to tilt and weaken the σ-bond before the dative bond is completely ruptured by the lateral force.
Our detailed information on the bond rupture process will allow us to better understand the nature of dative bonds. Moreover, our study provides a way to probe how the activities of metal-Pc molecules can be manipulated with molecular engineering. Metal-Pc molecules are widely used as model catalysts for the electrochemical reduction of CO2. Our results are therefore of great importance for engineering metal-Pc related catalyst systems, which hold potential for future carbon capture technologies.