Organometallics
Article
this temperature was reached, ethylene was added through a Brooks
thermal mass flow meter to the desired reaction pressure. When the
reactor temperature and pressure had stabilized, the catalyst shot tank
was pressurized with nitrogen to 200 psi over the reactor pressure,
and the precatalyst solution was injected into the reactor, beginning
the reaction. Ethylene was fed on demand through the Brooks thermal
mass flow controller, and the temperature was controlled by adjusting
the mantle temperature and the flow through the internal cooling coil.
After the 30 min reaction time, the ethylene feed was stopped, and
the reactor was cooled to 35 °C and then vented at a rate of 1−4 psi
per second, until 10 psi was reached. At this point, the reactor was
returned to the nitrogen-filled glovebox, where it was unsealed. The
contents of the reactor were sampled for gas chromatography (GC)
analysis, and then the reactor contents were emptied. The reactor was
thoroughly cleaned to remove any polymer not suspended in the
solvent. The bulk of the solvent was allowed to evaporate, and the
remaining residue was dried in a vacuum oven. The resulting residue
was weighed to give the polymer yield for the reaction.
Synthesis of i-Pr-PNP-CrCl3(THF) (1-Cr). A 20 mL vial was
charged with solid N-(diphenylphosphino)-N-(1-methylethyl)-P,P-
diphenyl-phosphinous amide (1) (0.100 g, 0.234 mmol) and solid
CrCl3(THF)3 (0.087 g, 0.213 mmol). Toluene (8 mL) was added,
and the vial was shaken. A deep blue-black color developed within 5
min, with undissolved solids still present. The reaction mixture was
shaken well and was allowed to stand overnight to give a deep blue-
black solution with almost no precipitate. The solution was filtered via
syringe filter into a 20 mL vial and allowed to stand overnight. Several
very large crystals formed. Elemental analysis for iPr-PNP-
CrCl3(THF)·toluene: calculated: C, 60.85; H, 5.78; N, 1.87; found:
C, 60.28; H, 5.69, N, 1.62%.
It is evident that bis(phospholanes) are a special ligand class
for chromium-catalyzed ethylene oligomerization, and it is
likely that additional derivatives based on the phospholane
motif might lead to catalysts with even better activity and
selectivity. Our results demonstrate that ligands that do not
contain the widely used diphenylphosphine motif can be useful
for ethylene tri- and tetramerization and that the range of
relevant ligand motifs for successful catalysts is much wider
than previously thought, which should lead to new research
directions and discoveries.
EXPERIMENTAL SECTION
■
General Methods. All reagents were obtained from commercial
sources and used as received unless otherwise noted. All syntheses
and manipulations of air-sensitive materials were performed under an
inert atmosphere (nitrogen). Solvents (toluene, hexane, diethyl ether)
were first saturated with nitrogen and then dried by passage through
activated alumina and Q-5 catalyst (available from Engelhard
Corporation) prior to use. C6D6 was dried over Na/K, and CDCl3
was dried over activated 4 Å molecular sieves before use. NMR
spectra were recorded on Varian MR-400, VNMRS-500, and Bruker-
1
400 spectrometers. H NMR data are reported as follows: chemical
shift (multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q =
quartet, p = pentet, and m = multiplet), integration, and assignment).
1
Chemical shifts for H NMR data are reported in parts per million
downfield from internal tetramethylsilane (TMS, δ scale) using
residual protons in the deuterated solvents (C6D6, 7.15 ppm: CDCl3,
1
7.25 ppm) as references. 13C NMR data were determined with H
Synthesis of Complex 9-Cr. CrCl3(THF)3 (294.7 mg, 0.79
mmol) was dissolved in 5 mL of THF giving a purple solution. To this
solution was added a solution of 200 mg of ligand 9 (0.79 mmol)
dissolved in 5 mL of THF. The reaction mixture was allowed to stir
overnight at ambient temperature. The reaction mixture was filtered
through a disposable frit, and the solvent was removed from the
filtrate in vacuo, yielding 66.8 mg of a blue solid. Yield 17.5%. Crystals
suitable for a single-crystal X-ray diffraction study were grown by
evaporation of a THF/hexanes solution of the compound at ambient
temperature. Elemental analysis: Calculated: C, 44.60; H, 6.65.
Found: C, 44.12; H, 6.96%.
Synthesis of (R,R)-MeDuPhos-CrCl3(THF) (10-Cr). To a vial
containing (R,R)-MeDuphos (10) (0.60 g, 1.96 mmol) and
CrCl3(THF)3 (0.56 g, 1.51 mmol) was added 15 mL of THF. The
resulting solution was stirred for 30 min at ambient temperature and
then heated for 1 h at 60 °C. The THF was concentrated under
formation of crystalline material. The supernatant was pipetted away
from the solids. The solids were dried under reduced pressure to give
violet-blue crystalline material, 0.4776 g, 59.0%. Crystals suitable for
X-ray diffraction analysis were grown by slow evaporation of a THF
solution at ambient temperature. Elemental analysis: Calculated: C,
49.22; H, 6.76; Found: C, 49.24; H, 6.69%.
Synthesis of (R,R)-EtDuPhos-CrCl3(THF) (11-Cr).
CrCl3(THF)3 (513 mg, 1.38 mmol) was dissolved in 5 mL of THF
giving a purple solution. To this solution was added dropwise a
solution of the (R,R)-EtDuPhos ligand (11) (500 mg, 0.60 mmol)
dissolved in 5 mL of THF. Almost immediately upon addition of the
ligand, the solution color changed to a deep cobalt blue. The reaction
mixture was stirred for 8 h (homogeneous solution), and then the
solvent was removed in vacuo. The resulting blue solid was dried
under vacuum at 60 °C overnight. Yield 506.7 mg, 62%. Crystals
suitable for X-ray diffraction analysis were grown by evaporation of a
THF/hexanes solution at ambient temperature. Elemental analysis:
Calculated: C, 52.67; H, 7.48. Found: C, 52.43; H, 7.26%.
Synthesis of (S,S)-i-PrDuPhos-CrCl3(THF) (12-Cr).
CrCl3(THF)3 (224 mg, 0.60 mmol) was dissolved in 5 mL of THF
resulting in a purple solution. To this solution was added dropwise a
solution of the (S,S)-iPrDuPhos ligand (12) (250 mg, 0.60 mmol) in
5 mL of THF. Almost immediately upon ligand addition, the solution
color changed to a deep cobalt blue. The reaction mixture was stirred
decoupling, and the chemical shifts are reported in parts per million
versus tetramethylsilane (C6D6, 128 ppm, CDCl3, 77 ppm). Elemental
analyses were performed at Midwest Microlab, LLC. Syntheses of
representative complexes are shown below, and the syntheses of the
remaining compounds are given in the Supporting Information. The
high-throughput catalyst evaluation procedure can be found in the
Supporting Information.7
Semi-Batch Reactor Experimental Information. The ethylene
tetramerization reactions were conducted in a 300 mL Parr semibatch
reactor equipped with a 10 mL catalyst shot tank, an agitator, and a
thermocouple. The reactor was heated by an electrical resistive
heating mantle, and temperature was controlled through the use of
the heating mantle as well as by an internal cooling coil. Both the
reactor and the temperature-control system were controlled and
monitored by a Camile TG automation system. The nitrogen and
ethylene feeds, as well as the methylcyclohexane solvent, were passed
through purification columns containing A2 alumina and Q-5
reagents. Chlorobenzene and nonane were passed through activated
alumina. The methylcyclohexane, chlorobenzene, and nonane were
stored over activated 4 Å molecular sieves.
All reactor manipulations and solution preparations were
performed in a nitrogen-purged glovebox. For precatalysts prepared
in situ, a solution of 1.2 equiv of the ligand in methylcyclohexane (2
mM) was added in a dropwise fashion to a solution of 1 equiv of the
Cr precursor (CrCl3(THF)3) in chlorobenzene (2 mM), and the
resulting solution was stirred for 30 min. For precatalysts comprising
preformed ligand-chromium complexes, a solution of the complex was
prepared in chlorobenzene (2 mM). Methylcyclohexane (100 mL)
was added to the reactor body in a glovebox, along with MMAO-3A
(1.77 M in heptane) and nonane (the internal standard for GC
analysis). The precatalyst solution was loaded into a 10 mL catalyst
shot tank, and the vial and syringes containing the precatalyst solution
were washed with 2.5 mL of methylcyclohexane that was added to the
shot tank. The reactor was then sealed and removed from the
glovebox.
The reactor was then transferred to a reactor stand equipped with
the heating mantle, and connections were made to the nitrogen and
ethylene feed lines, cooling lines, and a vent line. The reactor was
pressure-tested to 750 psi nitrogen. After the pressure test, the reactor
was slowly vented to ∼10 psi and then slowly heated to 70 °C. When
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Organometallics XXXX, XXX, XXX−XXX