activity of 3 after treatment with ammonium
borate (Fig. 1C) revealed an increase of the
a-value or a decrease of the 1-butene selectiv-
ity with increasing temperature. Ethylene oligo-
merization catalysts usually show a decrease
of thea-value with increasing temperature
( 19 – 21 ).Catalysts with an inverse temperature
dependence of thea-value are reported as
well ( 22 , 23 ).1-Butene selectivity greater than
90 mol % has been observed below 20°C. Turn-
over frequencies (TOFs) of greater than 5 mil-
lion per hour have been observed for all six
runs (Fig. 1C), with a maximum of 4800 s−^1
bar−^1 at 30°C. This is an extremely high TOF.
The TOF is in the range between 10−^2 and 10^2
s–^1 for most relevant industrial applications.TOFs between 10^3 and 10^7 s–^1 were recorded
for highly active enzymes as catalysts ( 24 ).
Very high TOFs for ethylene dimerization have
been achieved with vanadium-based cata-
lysts ( 25 ), for instance, 370 s−^1 bar−^1 ( 26 ).The
catalyst system based on 3 is also long-term
stable. We carried out a 3-hour run to demon-
strate it. The ethylene consumption over time1022 4 MARCH 2022•VOL 375 ISSUE 6584 science.orgSCIENCE
Fig. 1. Catalyst synthesis and selective 1-butene formation.(A) Synthesis
of 4 starting from the titanium precursor 1. After a toluene elimination
yielding 2 , a salt metathesis reaction is carried out [Ap-H,N-mesityl-6-(2,4,6-
triisopropylphenyl)pyridin-2-amine; LiImi, lithium salt of 1,3-bis(2,6-dimethyl-
phenyl)imidazolidin-2-imine]. Compound 3 reacts withN,N-dimethylanilinium
tetr akis(pentafluorophenyl)borate to form the cationic complex 4. Ar^1 , 2,4,6-
triisopropylphenyl; Ar^2 , 2,4,6-trimethylphenyl. (B) Molecular structures of
compounds 1 to 3 , as determined by x-ray crystallography. Color coding is as
follows: C, orange; Cl, gray; N, blue; and Ti, red. (C) Plot of the temperature
dependence of the chain-growth probability [a-value:a=nC+2/nC, with C being the
carbon number of thea-olefins obtained (and determined asnoctene/nhexene)] and
the TOF obtained with the precatalyst 3 after activation with an ammonium borate
cocatalyst. The TOF was calculated based on ethylene consumption. Thea-value
increases with increasing temperature. Selective 1-butene formation >90% is
possible below 20°C. The activity is extremely high and has an optimum at around
30°C. Conditions are as follows: ethylene pressure (Peth)=2.0bar;time(t)=15min;
precatalyst isn 3 = 0.05mmol; ammonium borate activator is [R 2 N(CH 3 )H]+[B
(C 6 F 5 ) 4 ]−(R = C 16 H 33 to C 18 H 37 ); Ti/B = 1/1.1; toluene volume (Vtoluene) = 150 ml;
and scavenger is 300mmol triisobutylaluminum (TIBA). (D) Plot of the
ethylene consumption (black, data points) of a 3-hour run to demonstrate the
combination of high activity and long-term stability of the catalyst system
based on 3. At the end of the reaction, the 300-ml reactor that was used is
completely filled (fig. S20 and safety note there); the catalyst loading corresponds
to about 0.03 parts per million (ppm). The ethylene consumption is constant for
more than 2 hours, as shown by a linear fit (blue) that indicates a negligible catalyst
decomposition over multiple hours (R^2 , coefficient of determination). Conditions
are as follows:Peth= 4.0 bar; temperature (T) = 15°C; precatalyst isn 3 = 0.2mmol;
ammonium borate activator is [R 2 N(CH 3 )H]+[B(C 6 F 5 ) 4 ]−(R = C 16 H 33 to C 18 H 37 );
Ti/B = 1/1.1;Vtoluene= 30 ml; and scavenger is 300mmol TIBA. (E) Observed (black)
and simulated (blue) Schulz-Flory distribution to confirm a Cossee-ArlmanÐtype
mechanism. Conditions are as listed for (C) except forT= 40°C,Peth= 7 bar, and
t= 1 min.RESEARCH | REPORTS