“Post-80s” Nobel Prize winner’s Groundbreaking Work published in Nature

2022-04-26 0 By

It is well known that the electron is a kind of elementary particle.We all learned in high school that in a chemical reaction, the gain or loss of electrons in an atom or molecule usually means that the chemical reaction takes place.Therefore, we generally consider electrons to be the most important agents directly involved in chemical reactions.In addition, electrons have more meaning and value for chemical reactions.Back in 2014, scientists showed that electrons can also act as catalysts in chemical reactions.This idea not only expands the scope of catalyst research, but also raises the importance of electrons in chemical reactions to a new level.Similar to the formation and fracture of covalent and coordination bonds, some complexation and discomplexation between host and guest can be regarded as “chemical reaction” with low energy barrier.In response to certain stimuli host-guest complex, people can through additional stimulus signal, through adjusting the subject or object of electron cloud density or charge to control the formation and destruction of the weak key, so we can see that electrons in the weak interaction leading non covalent bond formation and dissociation of aspect also occupies the important position.J. F. Stoddart, Nobel Laureate, born in 1942, is 80 years old.Inspired by the Electron catalytic model, j. F. Stoddart and his team at Northwestern University recently realized the host-guest complex based on Electron catalysis. The work was published in Nature.The lead authors of this work are DRS. Yang Jiao and Yunyan Qiu.The authors have designed to regulate the complexation between host and object which could not otherwise be complexed by using a small number of additional “electrons”.As shown in the figure below, the subject still chooses the pyridine macroring developed by the research group for a long time, except that in the initial state, the macroring is in the double cation double radical state (R2(·+)).The object is designed as a large steric hindrance group of 3, 5-diisopropyl phenyl at one end, and a terminal group of 3, 5-dimethylpyridine cation (PY+) at the other end, and a pyridine salt (D+(·+) in the form of single cation and single radical on its axis.It is possible to convert R2(·+) or D+(·+) to R·+ or D+ by supplying electrons with an external electron source reagent.This transformation can reduce the Coulomb repulsion between subject and object.Through quantum chemical calculation, it can be known that the energy barrier of R2(·+) passing through the PY+ end group of D+ and R·+ passing through the PY+ end group of D+(·+) is 8.8 or 9.3 kcal mol-1 respectively after adding electrons to make the subject or guest reduce.Compared with the R2(·+) -d +(·+) state before adding electrons, the binding energy barrier is 15.0 kcal mol-1.The lower complexation energy barrier promotes the smooth complexation of host-guest complex to form a key intermediate.The intermediate can then release electrons to transform into a triradical complex, and the released electrons can go on to reduce the next host-guest pair.In this way, a small number of electrons can catalyze a large number of complexations between host and guest, even though the complexation between host and guest is forbidden dynamically under normal conditions.In order to verify the feasibility of this mechanism, the complexation between R2(·+) and D+(·+) under conventional conditions was tested.The two were dissolved in acetonitrile. After about 10 hours, only a very small amount of complexation occurred between the host and guest.This indicates that the energy barrier on the object hinders the complexation between the host and object.By adding 4 mol% cobalt dicene CoCp2 as the electron donor, we observed the appearance of an absorption peak at 1080 nm, which is the characteristic peak of the tri-radical complex, and the simultaneous appearance of an absorption peak at 600 nm for BIPY·+.This further confirms the feasibility of the above principle.Within 70 minutes, the reaction reached stability.Through kinetic calculation, it can be found that the complexation rate between host and guest is increased by 640 times after adding cobalt cene.Increasing the amount of cobalt dicene to 4 mol% to 8 mol% can significantly accelerate the complexation rate, and the reaction can be completed in 10 minutes.When the amount of cobalt dicene is reduced to 4 mol%, 2 mol% and then 1 mol%, the complexation rate slows down.Figure 2. Formation of host-guest complex detected by INFRARED spectroscopy In order to further explore the mechanism of molecular recognition, the whole electron catalytic process was divided into separate steps.The first step is to add 1 equivalent cobalt cene to the mixture of R2(·+) and D+(·+) in equal quantities.In the near infrared spectrum, the absorption peak at 1700 nm can be instantly observed, which is a double radical complex, and the principle of some key intermediates characteristic absorption peak.The second step is to further add excessive host R2(·+) and guest D+(·+). By the displacement of absorption peak, the transformation of diradical complex to triradical complex in the solution can be judged.In order to further verify the existence of the key intermediate of diradical complex, we designed and synthesized a carboxyl Cat6+.The six positively charged hydrocarbon Cat6+ can be converted to the tri radical form Cat3(·+) by gradual dropping of cobalt dicene.With further drops of cobalt dicene, the color of the solution changed from blue-purple to brown, and the infrared absorption peak gradually changed from 1080 nm to the characteristic absorption of diradical at 1640 nm.In addition, the single crystal structure of diradical hydrocarbon was obtained, which further verified the existence of intermediates.In this process, the electrons are the catalyst, and cobalt cene is just a “vehicle” for the electrons.Therefore, as long as the REDOX potential of each species is properly matched, more species can act as reducing agents in this process in principle.FIG. 3. Exploration of complexation mechanism and verification of key intermediates Due to the catalytic effect of electrons, the oxidation of epoxy can be carried out without the addition of chemicals during the regulation process, and the process can also be controlled by the addition of electrodes.The authors hypothesize that if BIPY(0) and BIPY2+ live longer than the time required for molecular recognition, then electron injection and removal at the electrode can promote the complexation between R2(•+) and D+(•+).Since both R2(·+) and D+(·+) contain BIPY·+ radical cations that can accept/lose electrons from the electrode, there are several possible molecular recognition pathways during electrochemical processes.One possibility is that both cathodic reduction and anodic oxidation are applied to R2(·+), producing equal amounts of R·+ and R2+(·+), respectively.After electrochemistry is excited, part of R·+ can rapidly combine with D+(·+) to form [D⊂R]+2(·+) diradical complex.This intermediate can then undergo a single electron transfer (SET) with R2(·+) to generate [D⊂R]+3(·+) tri-radical complex as the end product and to generate a new R·+ to restart the cycle.As [D⊂R]+2(·+) diradical complex and R2+(·+) SET to recover R2(·+) as the starting material or to return electrons (BET) to the anode, the catalytic cycle is terminated and the [D⊂R]+3(·+) triradical complex is generated.By increasing electrolysis conditions such as current and stirring speed, the molecular recognition speed can be significantly accelerated.For example, increasing the electrolytic current from 0.5 mA to 1.0 and 2.0 mA can increase the recognition speed of the host-guest complex.Increasing the stirring speed will slow down the recognition rate.FIG. 4. Summary of electrode-catalyzed complexation. This work proposes a new host-guest identification method, which breaks the traditional research and application strategies of host-guest complexes.Will make a pioneering contribution to the field of supramolecular chemistry.- Cellulose —- help test technology – source: Frontiers of Polymer Science