Kinetics of the Same Reaction Monitored over Nine Orders of Magnitude in Concentration: When Are Unique Subensemble and Single‐Turnover Reactivity Displayed?

Abstract

Essentially no information is known about the behavior of individual molecular catalysts under reaction conditions. This is a result of the averaging inherent to traditional analytical techniques. Herein, a combined fluorescence microscopy and H NMR spectroscopy study reveals that unique (that is, non-ensemble averaged) distributions and time-variable kinetics from molecular ruthenium catalysts within growing polynorbornene occur and are detectable between 10 m and 10 m of substrate, surprisingly just 1000-fold less concentrated than a typical laboratory bench-scale reaction. The kinetic states governing single-turnover events are determinable by overlay of the signal arising from individual monomer insertion reactions with that from polymer growth from neighboring catalysts.

Does Selectivity of Molecular Catalysts Change with Time? Polymerization Imaged by Single‐Molecule Spectroscopy

“…We became interested whether single‐molecule/single‐turnover imaging techniques could address these analytical challenges [15–24] . We started our studies by designing a pair of fluorescent molecules that could differentiate between two different reaction pathways [25] in the living metathesis polymerization reaction at aggregate 1 (Figure 1 a).…”

“…In this competition experiment, each single flash denotes the instantaneous chemoselectivity of a single catalyst. The low concentration of imaging agents and the high concentration of living strands within 1 means that each flash likely occurs at a different individual catalyst [16, 17, 26, 27] . Further, the high initial concentration of untagged norbornene (prior to precipitation of 1 and subsequent rinsing) and the low concentration of imaging agents 2 and 3 mean that most polymer is composed of untagged (dark) norbornene.…”

“…We became interested whether single-molecule/singleturnover imaging techniques could address these analytical challenges. [15][16][17][18][19][20][21][22][23][24] We started our studies by designing a pair of fluorescent molecules that could differentiate between two different reaction pathways [25] in the living metathesis polymerization reaction at aggregate 1 (Figure 1 a). This aggregate formed upon association of numerous individual living polymer strands, from ruthenium catalyst and untagged (dark) norbornene (5 mM), each strand of which contained potentially one catalyst.…”

Kinetics of initiation of the third generation Grubbs metathesis catalyst: convergent associative and dissociative pathways

“…9 Exquisite detail regarding catalyst turnover has been elucidated using techniques such as heteronuclear NMR magnetisation transfer, 10 dynamic nuclear polarisation labelling, 11 and single-molecule uorescence. 12 However, much of the work in the primary phases of the development of new ruthenium precatalysts, and the investigation of their relative efficiency and activity, focussed on the initiation event. 1,4,5 A now classic example of such initiation studies is the series of experiments reported by Grubbs, Sanford and Love et al in which a large excess of ethyl vinyl ether (EVE) was used to generate styrene and a stable Fischertype alkylidene product [Ru] ¼ CHOEt, in an apparently irreversible manner.…”

Kinetics of Palladium(0)‐Allyl Interactions in the Tsuji‐Trost Reaction, derived from Single‐Molecule Fluorescence Microscopy

“…Over the last decade, single‐molecule fluorescence spectroscopy (SMFS) emerged as a tool to bring insights into the microscopic behavior of molecular, chemical systems . When ensemble measurements conceal uncommon events, single‐molecule observation has the potential to provide a deeper understanding of chemical processes . In these first studies, it was shown that even text book reactions can show furcations of reaction pathways, still leading to an identical product .…”