Mechanochemical activation of latent N-heterocyclic carbene catalysts (2013)

The use of mechanical forces to activate chemical bonds and carry out chemical transformations is called "mechanochemistry". Mechanochemistry is an alternative method for the activation of chemical reactions, next to activation by heat or by means of (photo)chemical stimuli. Although being discovered centuries ago, mechanochemistry recently experiences a strong revival. It has been shown that mechanochemically activated reactions may proceed via different energetic pathways that result in different reaction products compared to their thermal analogues. In this thesis, the use of mechanical forces to activate latent catalysts is explored. For this purpose, organometallic complexes containing N-heterocyclic carbene (NHC) ligands, coordinated to a silver(I) metal center, have been synthesized. These latent catalyst complexes were functionalized with poly(tetrahydrofuran) (PTHF). These polymer chains act as a handle for transfer of macroscopic forces onto individual chemical bonds. The most convenient way of applying forces on a polymer chain is by exposing them to ultrasound in solution. Ultrasound in solution generates an acoustic pressure wave that passes through the solution. Locally, small gas bubbles (cavities) are formed after nucleation around dissolved gas molecules. The cavitation bubbles grow and when they eventually become too large, they will be unstable and collapse. Collapse occurs within microseconds and gives rise to a variety of sonochemical effects. Next to the thermal effects within the hotspot of the collapsing cavitation bubble, where extreme temperatures and pressures arise, these effects comprise of mechanical effects that occur as a direct result of large a velocity gradient in the solution close to the retracting bubble/liquid interface. When a polymer chain is present in this high strain region, hydrodynamic forces unfold and stretch the polymer chain. When forces are high enough, individual bonds along the polymer backbone are stretched and broken. Mechanochemical scission of polymer chains is a nonrandom process. Forces accumulate along the stretched polymer chain and reach a maximum value in the center. Therefore, mechanochemical scission takes place around the midpoint of the polymer chain. The selectivity of mechanochemical scission can be increased by incorporation of a weak bond ("mechanophore") near the polymer chain midpoint. Furthermore, since the chain breaks at its weakest point, the positioning of this mechanophore allows precise tailoring of the location of mechanochemical bond scission. This thesis can be divided into two parts, each of them covering one of the main aims that were set out for this work. In the first part of this thesis, the aim is to gain more detailed insights in the physical aspects, mechanisms and underlying processes of mechanochemical activation of latent catalysts ("mechanocatalysis") by ultrasound in solution. In studies prior to the work described in this thesis, conditions had already been established under which catalyst activation by polymer scission through thermal effects could be excluded. In this thesis, the possibility of radical-induced polymer scission is excluded as well. By varying the saturation gas used in sonication experiments, the radical production was lowered by one order of magnitude as the heat capacity of the gas increased from argon to nitrogen to methane and isobutane. However, the percentage of scission during a fixed sonication time remained the same within experimental error for all these gases. This result clearly demonstrates that the formation of radicals during cavitation in ultrasound does not affect the mechanochemical scission (i.e., activation) of silver(I)–NHC polymer catalysts. In addition to this, the importance of creating the right hydrodynamic conditions of cavitation is clearly highlighted in this work. For example, the solubility of isobutane in toluene is so high that the collapse of cavitation bubbles was "cushioned", which, in turn, led to a reduction in polymer scission efficiency. Having established the true mechanochemical nature of mechanocatalyst activation, the stretching of polymer chains under the influence of hydrodynamic forces and the subsequent chain scission are further investigated. In previous work, it had already been shown that the silver(I)–NHC supramolecular polymers have a significantly lowered limiting molecular weight for mechanochemical chain scission compared to non-functionalized, fully covalent polymers. The question to be answered is if such a low limiting molecular weight is compatible with a scission mechanism that requires unfolding and stretching of individual polymer chains in solution prior to scission. The longest characteristic relaxation times (~10–7 s) of these supramolecular polymers in solution were determined by viscosity measurements in order to verify that the critical conditions for coil-to-stretch transition of these polymers is indeed fulfilled under typical hydrodynamic conditions in ultrasound experiments, where strain rates typically exceed 107 s–1 . In the next step, molecular dynamics (MD) simulations, combined with COGEF calculations were performed to estimate the forces required for mechanochemical scission of the silver(I)–NHC coordination bond. The typical forces were estimated to be between 400 and 500 pN, which is indeed much lower than the forces that are typically required for scission of covalent bonds (up to several nN). The modeling rationalizes the strong reduction of limiting molecular weight for mechanochemical scission of these supramolecular polymers compared to covalent polymers. The effect of radicals and their secondary products (referred to as sonochemical impurities) on the mechanocatalytic activity of the active catalyst species is investigated as well. Although it was established that radicals, and/or other sonochemical impurities, do not influence mechanocatalyst activation, it is anticipated that these reactive species may still be able to deactivate the highly nucleophilic NHC active catalyst species. The transesterification of vinyl acetate with benzyl alcohol to form benzyl acetate was used as a benchmark reaction to determine the catalytic activity. It was shown that the mechanocatalytic activity was significantly increased when sonication experiments are performed under conditions that suppress the formation of sonochemical impurities. Catalytic conversions of the transesterification reaction increased dramatically, from less than 1% to ca. 11% after 30 minutes of sonication when the sonication was performed under radical-suppressing conditions using methane instead of argon as saturation gas. It is shown that the deactivating species in case of the silver(I)–NHC mechanocatalyst is a persistent species, not the radicals themselves. More specifically, the sonochemical impurity was identified as trace amounts of a weak Brønsted acid, possibly acetic acid that is formed during thermal degradation of the reactant vinyl acetate in the hotspot of the collapsing cavitation bubble. Furthermore, a potential metal-free NHC-based mechanocatalyst complex is designed and synthesized by replacing the silver(I)–NHC coordination complex by the adduct of an NHC and aromatic isothiocyanate. In literature, it had been shown that these NHC–isothiocyanate (NHC–NCS) adducts have a reversible nature, albeit with an extremely high equilibrium constant of ~1014 M–1. At elevated temperatures, between ca. 70 and 100 °C depending on the nature of the isothiocyanate, free NHCs were generated, leading to (thermal) catalytic activity. It is anticipated that incorporation of the NHC–NCS adduct within the center of a PTHF chain results in a latent catalyst that can be activated by mechanical forces. Indeed, near-midpoint scission was observed when the polymer adduct in solution was subjected to ultrasound irradiation. However, mechanochemical scission did not result in the desired mechanocatalytic activity in the benchmark transesterification reaction of vinyl acetate and benzyl alcohol. More careful analysis revealed that the product of mechanochemical scission of the polymer NHC–NCS adduct is not the free NHC. Instead, non-selective, near-midpoint scission of the PTHF chain takes place with a similar scission rate as for non-functionalized PTHF of the same molecular weight. Apparently, the difference in bond strength between the NHC–NCS adduct bond and covalent bonds within the polymer backbone is not large enough to yield a selective mechanochemical scission process on the microsecond timescale of ultrasound experiments. In the second part of this thesis, the use of alternative techniques for mechanocatalyst activation was studied. Attempts to use well-defined hydrodynamic flows in microfluidic devices have not been successful. Based on the results of relaxation time measurements of silver(I)–NHC supramolecular polymers as described earlier, it is reasoned that the strain rates obtained in such devices (max. 105 s–1) are not sufficient to fulfill the coil-to-stretch criterion, which is a prerequisite for chain scission. More successes are obtained when using atomic force microscopy (AFM) as an alternative mechanochemical activation technique. Although, in ultrasound experiments, the NHC–NCS adduct does not display mechanocatalytic activity, it is used in these single molecule force experiments owing to its synthetic accessibility. A molecular link between the AFM cantilever and NHC–NCS surface construct was established through thiourea bond formation. Upon retraction of the cantilever, molecular rupture events were observed with a maximum in the rupture force distribution at Frup = 690 pN. Clearly, this force is too low to correspond to rupture of a covalent bond (several nN are required for this); in addition, control experiments showed that these rupture events are not present in the absence of the NHC–NCS adduct in the AFM surface construct. So, the