Real-time single-molecule imaging of oxidation catalysis at a liquid–solid interface

Abstract

Many chemical reactions are catalysed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. A variety of conventional spectroscopic techniques are available for the study of reaction mechanisms at the ensemble level, and, only recently, fluorescence microscopy techniques have been applied to monitor single chemical reactions carried out on crystal faces and by enzymes. With scanning tunnelling microscopy (STM) it has become possible to obtain, during chemical reactions, spatial information at the atomic level. The majority of these STM studies have been carried out under ultrahigh vacuum, far removed from conditions encountered in laboratory processes. Here we report the single-molecule imaging of oxidation catalysis by monitoring, with STM, individual manganese porphyrin catalysts, in real time, at a liquid-solid interface. It is found that the oxygen atoms from an O2 molecule are bound to adjacent porphyrin catalysts on the surface before their incorporation into an alkene substrate.

Detection of different oxidation states of individual manganese porphyrins during their reaction with oxygen at a solid/liquid interface

“…Such a distribution points at a correlated process, and might be explained by a similar preferential pair-wise generation of state Mn1-C (illustrated in Fig. 1) as was previously observed for the reaction of oxygen with another type of Mn porphyrins adsorbed on a Au(111) surface 18 . At higher surface coverages of Mn1-C of 10-16%, this correlation appears not so pronounced when compared to the corresponding simulation (Fig.…”

“…The majority of these studies use ensemble techniques in solution, such as electrochemistry and absorption, electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The scanning tunneling microscope (STM) has proven to be a promising tool to study reactions at the molecular level [8][9][10][11][12] , and initial experiments have been reported in which the reactive properties of metal porphyrins were studied on a surface at the single molecule level by STM in ultrahigh vacuum (UHV) [13][14][15][16][17] and under ambient conditions 18,19 .…”

“…As a result, it is difficult for the manganese atom to come in close contact with the surface, which will decrease the probability of the donation of an electron from the surface (see also the Supplementary Note). Previous research on a monolayer of Mn(III) porphyrins that lacked meso-phenyl substituents revealed that their adsorption to a Au(111) surface led to a quantitative reduction of the manganese centres from Mn(III) to Mn(II) 18 . While a difference in work function between the two surfaces might play a role in this process, it was reasoned that in the case of Au(111) a direct contact between the surface and the metal centre was possible, and that gold adatoms could act as axial ligands to the porphyrin metals 41,42 .…”

“…An overview of the possible reaction pathways involving manganese porphyrins and oxygen, based on the literature [1][2][3]18,[20][21][22][23][24][25][26][27][28][29] and the observations described here, is depicted in Figure 1. A reduction of the manganese centre of the porphyrin from Mn(III) to Mn(II) with loss of the counterion 20 is a requirement for the binding of an oxygen molecule to give a dioxygen adduct 21 .…”

Two-Dimensional Molecular Patterning by Surface-Enhanced Zn-Porphyrin Coordination

“…We suggest that the latter phenomenon is likely always playing a role in the final mechanism. Recently, Elemans et al have demonstrated that in the presence of oxygen Mn-porphyrins physisorbed on Au (111) have different catalytic properties than their counterparts in solution, 3 thus giving the first indication that the physicochemical properties of functional porphyrins are modified upon physisorption. Moreover, since the n-dodecyl tails of the Zn-TDPs are solvated by n-tetradecane, coordination of the zinc core might be disfavored in solution.…”