After more than 40 years, it was finally confirmed!

Demand for linear α-olefins is booming.
Linear α-olefins (LAOs) refer to a type of long-carbon chain olefins with more than 6 carbon atoms and vinyl functional groups at the chain ends.
They mainly refer to 1-hexene and 1-octene, which are used in large quantities.
As a comonomer for ethylene polymerization, it can produce linear low-density polyethylene (LLDPE), polyethylene-octene copolymer (POE) and other high value-added polyethylene materials containing short-chain branches, which are used in food packaging, coatings, toys, It is widely used in pipelines and other fields
.
According to reports, the global production of LAOs in 2012 was 3.
5 million tons, increasing at an annual rate of 3.
3%.
By 2025, the global production value of LAOs will reach 11.
5 billion U.
S.
dollars
.
? At the earliest, people used the Ziegler process left by the ancestors to produce LAOs with even carbon through ethylene oligomerization.
This process was originally used to synthesize high molecular weight polyethylene, but now it is necessary to use it to prepare only a few carbons.
Atomic LAOs are really reluctant to do so
.
Although through various transformations, it is inevitable that polyethylene products with a high degree of polymerization will be generated in the oligomerization process, which seriously affects the continuous production of the process
.
? Subsequently, people developed the ethylene selective oligomerization process, which really achieved the goal of "getting what you want", and wanted to allow several ethylene molecules to polymerize and become the main source of 1-hexene and 1-octene.
Production process
.
In order to allow ethylene to "obediently" carry out selective oligomerization, the biggest secret must be the catalyst used.
The process uses transition metal chromium catalysts, which have the advantages of high activity and good selectivity
.
The research on the selective oligomerization mechanism is that the selective oligomerization mechanism of ethylene is very complicated.
Based on the polymerization kinetics and deuterium isotope labeling technology, the researchers inferred the mechanism of a metal cyclization trimer: (i) η2 of two ethylene molecules Coordination; (ii) oxidative coupling to produce metal cyclopentane; (iii) inserting another ethylene molecule to form a larger metal ring; (iv) H-shift or β-hydride elimination and reduction elimination to form LAOs, such as Shown in Figure 1
.
? Figure 1.
Hypothetical selective oligomerization mechanism of ethylene
.
? In this widely accepted mechanism study, people have discovered many intermediate products, such as chromium(I)-ethylene complexes, five- and seven-membered chromium(III) metal ring complexes, but the most critical The chromium(I)-diethylene complex has never been found in the experiment, and can only remain in the guess of the researchers.
This has also become the biggest regret in the study of the selective oligomerization mechanism of ethylene
.
? Make up for the biggest regret, the research group of Professor BEBode from the University of St Andrews in the United Kingdom uses Et6Al2 activated [Cr(CO)4(PNP)] [Al(OC(CF3)3)4] as the catalytic system, using electronic paramagnetic resonance technology ( EPR) discovered the existence of chromium(I)-diethylene complex in the experiment for the first time, and confirmed its composition as [Cr(C2H4)2(CO)2(PNP)]+, which made up for the study of the selective oligomerization mechanism of ethylene The biggest regret in China, this is of milestone significance for further promoting the development of ethylene selective oligomerization process
.
?? Clues found in EPR Figure 2.
The formation mechanism of chromium(I)-diethylene complex
.
? In order to study the key intermediate product formed in the selective oligomerization of ethylene: chromium(I)-diethylene complex, the researchers used [Cr(CO)4(PNP)] [Al(OC(CF3)3)4 ] Is a pre-catalyst.
Five times the equivalent of Et6Al2 is added in an Ar atmosphere of 273 K, and the various intermediate products produced are monitored every 1 minute using EPR at 295 K for a total of 16 hours
.
? Figure 3.
Three different intermediate products in the EPR spectrum over time
.
The researchers found that within a few minutes of the reaction, three different superimposed signals appeared in the EPR spectrum
.
Two of these signals can be attributed to the A and CrI-bisarene complexes (CrI-bisarene) that have been reported in the literature.
At 30 minutes, the researchers found a complex with a medium g factor.
Not reported in the literature
.
Although the g factor of the D complex is similar, its 31P coupling constant is small, and there is a hyperfine coupling (Aiso=8.
2 MHz).
These are coupling modes that D will not appear, so this complex is not D, the researchers labeled it as species X, and suspected to be the legendary chromium(I)-diethylene complex
.
Finally confirmed chromium (I) - Ethylene bis complex continuous wave X 4.
FIG substance (CW) EPR spectra
.
? In order to verify that the substance labeled X is the legendary chromium(I)-diethylene complex, the researchers injected ethylene gas (1 bar) under the same experimental conditions, and monitored the experimental process by EPR.
The signal of X was observed in ~3 minutes, but the intensity was higher
.
After replacing the hydrogen atoms in the two ethylene gases with deuterium, the researchers performed an EPR simulation and found that the experimental spectrum can be reproduced
.
At first, researchers speculated that X may be d8 (deuterated) metal cyclopentane, but in this structure, the alpha and beta protons are not the same, and there will be differences in the ultrafine coupling structure, and more importantly, after oxidative cyclization Chromium changes from 1 valence to 3 valence, and there will be no double ground state, so the possibility that X is d8 metal cyclopentane is ruled out
.
In the end, the researchers believe that X is a Cr/PNP complex combined with two ethylene molecules, which is the legendary chromium(I)-diethylene complex
.
Through mass spectrometry analysis, the researchers also found 1-hexene and 1-octene in the product, indicating that this intermediate has a strong ability to catalyze LAOs
.
Continuing the research after getting X as a diethylene complex, in order to further study the complete structure of this complex, the researchers used 13C labeling combined with mass spectrometry and infrared analysis and found that X initially contained 4 carbonyl ligands.
After being activated by ethylene, two of the carbonyl groups are replaced by two ethylene molecules
.
Therefore, the researchers not only confirmed that X is a chromium(I)-diethylene complex, but also determined that its structure is [Cr(C2H4)2(CO)2(PNP)]+
.
This complex has cis and trans isomers.
According to density functional theory (DFT) calculations, the researchers found that the energy difference between the cis and trans isomers is 84 kJ/mol, so the trans complex is more stable
.
? Summary In order to confirm the existence of chromium(I)-diethylene complexes in the selective oligomerization mechanism of olefins, the research group of Professor BEBode from the University of St.
Andrews in the United Kingdom discovered a new type that has not yet been reported in the literature.
The X structure, after excluding the possibility of D complex and d8 metal cyclopentane, the researchers finally determined that the X structure is the legendary chromium(I)-diethylene complex, and further determined that its complete structure is reverse The formula of [Cr(C2H4)2(CO)2(PNP)]+
.
This research not only adds the most important piece of the puzzle in the metal cyclization mechanism of olefin selective oligomerization, but more importantly, it will promote the industrial application of olefin selective oligomerization to prepare LAOs
.