Synthesis and catalytic oligomerization activity of chromium catalysts of ligand systems with switchable connectivity

Article abstract of DOI:10.1021/om300673u

Nucleophilic attack of in situ generated bis(diphenylphosphino)methane (DPPM^-) anion at CO₂, CyNCO, t-BuNCO, 2,6-(i-Pr) ₂PhNCO, and 2,4,6-(Me₃)PhNCO resulted in the formation of the novel anionic ligands {[(Ph₂P)₂CHCO ₂]Li(THF)₂}₂ (1), {[(Ph₂P) ₂C=CNH(R)O]Li(OEt)₂}₂ (R = Cy (2), R = t-Bu (3)), [Ph₂PCH=P(Ph₂)C=N(2,6-i-Pr₂C ₆H₃)O]Li(OEt₂)₂ (4), and {[(Ph ₂P)₂C=CNH(2,4,6-Me₃C₆H ₂)O]Li]_n (5), respectively. Ligand 4, however, showed a connectivity resulting from a nonclassical type of attack where the P atom acted as a nucleophilc center, thus affording a mixed-valent P(III)/P(V) species. Instead, the closely similar 5 showed a classical type of connectivity. The reaction of the in situ generated DPPM^- anion with 1 and 0.5 equiv of CrCl₃(THF)₃ gave the chelated chromium complexes [HC(PPh₂)₂]Cr[(μ-Cl)₂Li(THF) ₂]₂ (6) and [HC(PPh₂)₂] ₂Cr(μ-Cl)₂Li(THF)₂]·1.5THF (7), respectively. The reaction of ligand 1 with CrCl₂(THF)₂ afforded the dimeric [{[(Ph₂P)₂C(H)CO₂] ₂}Cr(THF)]₂ (8), whereas the reaction of 3 with CrCl ₃(THF)₃ resulted in the octahedral complex [(Ph ₂P)₂C(H)C=N(t-Bu)O]CrCl₂(THF) ₂·0.5THF·0.5(toluene) (9). The complexation of ligand 4 with CrCl₃(THF)₃ switched the connectivity to classical form and afforded the octahedral chromium complex [(Ph₂P)C(H)C=N(2,6- i-Pr₂C₆H₃)O]CrCl₂(THF) ₂·1.5THF (10). In contrast, the reaction of the classical ligand 5 with CrCl₃(THF)₃ resulted in [(Ph ₂P)C(H)=P(Ph₂)C=N(2,4,6-Me₃C₆H ₂)O]Cr(THF)₂Cl₂ (11) with a nonclassical type of connectivity. Reaction of 11 with DEAC switched the connectivity back to a classical type, affording {(EtCl₂Al)[(Ph₂P) ₂C(H)C=N(2,4,6-Me₃C₆H₂)OAlEt ₂](μ-Cl)Cr}₂(μ-Cl)₂· (toluene) (12). The catalytic behavior of all of these complexes has been assessed under different oligomerization conditions, and it was found that the modification of the DPPM framework with cumulenes considerably enhances their catalytic performance in comparison to catalysts 6 and 7. In any event, a Schultz-Flory distribution of oligomers was obtained. However, the in situ catalytic testing of ligands 2-4 using Cr(acac)₃ as metal precursor and DMAO as cocatalyst, in methylcyclohexane, switched the catalytic behavior to selective formation of 1-hexene and 1-octene (no higher liquid oligomers) along with a significant amount of narrowly dispersed, low-molecular-weight polyethylene wax. Interestingly, the precatalyst 12 showed self-activating trimerization capability with moderate activity.

Full text of DOI:10.1021/om300673u

Article
pubs.acs.org/Organometallics
Synthesis and Catalytic Oligomerization Activity of Chromium
Catalysts of Ligand Systems with Switchable Connectivity
Shaneesh Vadake Kulangara,† Chris Mason,‡ Michael Juba,‡ Yun Yang,† Indira Thapa,‡
Sandro Gambarotta,*,‡ Ilia Korobkov,§ and Rob Duchateau*,†
†Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands
‡Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
§X-ray Core Facility, Faculty of Science, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
S
* Supporting Information
ABSTRACT: Nucleophilic attack of in situ generated bis(diphenylphosphino)methane (DPPM−) anion at CO2, CyNCO, t-
BuNCO, 2,6-(i-Pr)2PhNCO, and 2,4,6-(Me3)PhNCO resulted in the formation of the novel anionic ligands {[(Ph2P)2CHCO2]-
Li(THF)2}2 (1), {[(Ph2P)2CCNH(R)O]Li(OEt)2}2 (R = Cy (2), R = t-Bu (3)), [Ph2PCHP(Ph2)CN(2,6-i-
Pr2C6H3)O]Li(OEt2)2 (4), and {[(Ph2P)2CCNH(2,4,6-Me3C6H2)O]Li]n (5), respectively. Ligand 4, however, showed a
connectivity resulting from a nonclassical type of attack where the P atom acted as a nucleophilc center, thus affording a mixed-
valent P(III)/P(V) species. Instead, the closely similar 5 showed a classical type of connectivity. The reaction of the in situ
generated DPPM− anion with 1 and 0.5 equiv of CrCl3(THF)3 gave the chelated chromium complexes [HC(PPh2)2]Cr[(μ-
Cl)2Li(THF)2]2 (6) and [HC(PPh2)2]2Cr(μ-Cl)2Li(THF)2]·1.5THF (7), respectively. The reaction of ligand 1 with
CrCl2(THF)2 afforded the dimeric [{[(Ph2P)2C(H)CO2]2}Cr(THF)]2 (8), whereas the reaction of 3 with CrCl3(THF)3
resulted in the octahedral complex [(Ph2P)2C(H)CN(t-Bu)O]CrCl2(THF)2·0.5THF·0.5(toluene) (9). The complexation of
ligand 4 with CrCl3(THF)3 switched the connectivity to classical form and afforded the octahedral chromium complex
[(Ph2P)C(H)CN(2,6-i-Pr2C6H3)O]CrCl2(THF)2·1.5THF (10). In contrast, the reaction of the classical ligand 5 with
CrCl3(THF)3 resulted in [(Ph2P)C(H)P(Ph2)CN(2,4,6-Me3C6H2)O]Cr(THF)2Cl2 (11) with a nonclassical type of
connectivity. Reaction of 11 with DEAC switched the connectivity back to a classical type, affording {(EtCl2Al)[(Ph2P)2C-
(H)CN(2,4,6-Me3C6H2)OAlEt2](μ-Cl)Cr}2(μ-Cl)2·(toluene) (12). The catalytic behavior of all of these complexes has been
assessed under different oligomerization conditions, and it was found that the modification of the DPPM framework with
cumulenes considerably enhances their catalytic performance in comparison to catalysts 6 and 7. In any event, a Schultz−Flory
distribution of oligomers was obtained. However, the in situ catalytic testing of ligands 2−4 using Cr(acac)3 as metal precursor
and DMAO as cocatalyst, in methylcyclohexane, switched the catalytic behavior to selective formation of 1-hexene and 1-octene
(no higher liquid oligomers) along with a significant amount of narrowly dispersed, low-molecular-weight polyethylene wax.
Interestingly, the precatalyst 12 showed self-activating trimerization capability with moderate activity.
tri-, and tetramerization catalytic systems.6 The commonly
INTRODUCTION
■
accepted mechanism for selective ethylene oligomerization is a
Catalytic ethylene oligomerization is an industrially relevant
redox metallacycle mechanism,8 where a two-electron oxidative
process.1 The fact that the fractionation of the oligomerization
addition of ethylene to chromium forms an expandable chroma
mixture to extract the most requested α-olefins (1-hexene and
metallacycle. The active oxidation state of chromium
1-octene) is an energy-intensive process motivates the search
responsible for the selectivity has been the center of a debate
for selective catalytic systems. As a result of intense research
due to the ambiguities generated by the redox dynamism of this
metal.8f,9−13 Different redox couples10−12 have been proposed,
but recent work has clearly pointed out that Cr(I) is most likely
activity in this field, several selective ethylene trimerization
catalysts,2−6 as well as a few ethylene tetramerization systems,
have been discovered.7
Among the elements displaying catalytic behavior for
ethylene oligomerization/polymerization, chromium occupies
a unique position, since it provides commercially viable poly-,
Received: July 19, 2012
Published: August 27, 2012
© 2012 American Chemical Society
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Article
the species responsible for catalyst selective behavior.13
Therefore, if a selective tri- or tetramerization system is being
sought, the main challenge is to stabilize the highly reactive
monovalent oxidation state. In turn, this relates to a judicious
choice of the ancillary ligand framework and donor atom
combination. However, designing catalysts which may dis-
tinguish between selective tri- and tetramerization remains a
great challenge.
CrCl2(THF)2 and CrCl3(THF)3 were prepared according to published
procedures.19 Bis(diphenylphosphino) methane, CyNCO, t-BuNCO,
2,4,6-(Me3)PhNCO, 2,6-(i-Pr)2PhNCO, and lithium and aluminum
reagents were purchased from Sigma Aldrich and used as received.
1
Unless stated otherwise, the H, 13C{1H}, and 31P{1H} NMR spectra
were recorded on a Bruker 300 spectrometer at 300.13, 75.47, and
121.49 MHz, respectively, at 298 K. DMSO-d6, dried with 4 Å
molecular sieves and stored under nitrogen, was used for all the NMR
measurements. All chemical shifts are given in ppm and referenced to
SiMe4. Elemental analyses were carried out with a Perkin-Elmer 2400
CHN analyzer. Molecular weights and molecular weight distributions
of the polyethylenes were determined by means of high-temperature
SEC on a PL-GPC210, equipped with refractive index and viscosity
detectors and a 3 × PLgel 10 μm MIXED-B column set, at 160 °C
with 1,2,4-trichlorobenzene as solvent. BHT and Irganox have been
used as antioxidants. The molecular weights of the polyethylenes
produced were referenced to linear polyethylene standards. Oligome-
The BP Chemicals and Sasol PNP chromium complexes,
stabilized by neutral RN(PAr2)2 ligands, have marked a
milestone in this field. These catalysts oligomerize ethylene
with high selectivity toward either 1-hexene or 1-octene,
depending on the ligand substituents (Ar = 2-OMe-C6H4,
C6H5, respectively).4,7a Further replacements of the heteroatom
combinations or modifications of the ligand frameworks14 also
produced highly selective ethylene trimerization catalysts. We
also have reported a series of Cr(III) and Cr(II) complexes of
NPN15 and NP16 ligands, which showed switchable catalytic
behavior. Last but not least, SK-Energy reported a selective
ethylene tetramerization system based on substituted bis-
(diphenylphosphino)ethane ligands, producing 1-octene with
77% selectivity.7b Recently, we found that the chromium(III)
complexes of the [PPh2NR(CH2)3NRPPh2] ligand are also
capable of producing 1-octene with record selectivity.7c
From this collection of results, it is clear that the presence of
phosphorus donor atoms in the ligand scaffold of these
heteroditopic ligands is an important prerequisite. However,
the simpler bis(diphenylphosphino)methane (DPPM) ligand,
in combination with CrCl3(THF)3 and activated with MMAO,
only produces a Schultz−Flory distribution of oligomers with
very low activity.17 This is in spite of its similarity to the PNP
system in terms of bite angle, steric encumbrance, and donor
atoms and also to the relationship with the SK-Energy
diphosphine system. Conversely, Wass and co-workers18
reported that activation of bis(diphenylphosphino)methane-
stabilized tetracarbonylchromium via one-electron oxidation
provides a selective ethylene oligomerization catalyst producing
mainly 1-hexene and 1-octene with a higher selectivity toward
1-octene in comparison to 1-hexene.
We are currently engaged in a systematic screening of
families of diversified ligand systems containing various
combinations of donor functions and ligand scaffolds
containing the ability for retaining alkyl aluminate residues.
This last point is of interest in the ultimate view of obtaining
self-activating catalytic systems.9,13,15 For this work, we have
examined deprotonated bis(diphenylphosphino)methane as the
starting point and reacted it with CO2 and isocyanate to probe
geometrical arrangements not previously examined. It was
argued that the phosphorus donor atoms might be central to
the stabilization of the highly desirable lower oxidation states of
chromium. At the same time, hard donors such as oxygen and
nitrogen could aid in the retention of aluminum resi-
dues.13,14c,15,16 The anionic character of these ligands, in
combination with the ability of the system to form zwitterionic
species, was also regarded as beneficial for imparting robustness
to the complexes and providing sufficient electrophilicity to the
metal center. Herein we describe our observations.
1
rization data were analyzed by H NMR spectroscopy for activity and
GC-MS for reaction mixture composition. Gas chromatography of
oligomerization products was conducted on a Varian 450-GC
equipped with an autosampler.
General Oligomerization Procedure. All oligomerizations were
performed in a 250 mL Buchi reactor. The reactor was dried in an
̈
oven at 120 °C for 2 h prior to each run and then evacuated for 1/2 h
and rinsed with argon three times. After that, the reactor was loaded
with toluene and the desired amount of cocatalyst. After the solution
was stirred for 10 min, it was saturated with ethylene. The reactor was
temporarily depressurized to allow injection of the catalyst solution
into the reactor under an argon flow, after which the reactor was
immediately repressurized to the desired set point. The temperature of
the reactor was kept as constant as possible by a thermostat bath. After
30 min reaction time and cooling to 0 °C, the reaction mixture was
depressurized and a mixture of ethanol and dilute hydrochloric acid
was subsequently injected to quench the reaction. The polymer was
separated by filtration and dried at 60 °C for 18 h under reduced
pressure prior to molecular weight determination.
Computational Method. Geometry optimizations were per-
formed using the Gaussian 09 program package without symmetry
constraints, using the unrestricted B3LYP hybrid functional and
double-ζ basis set 6-31G(d,p). Frequency analyses were carried out on
the resulting geometries to verify the nature of stationary points (no
imaginary frequencies).
X-ray Crystallography. Suitable crystals were selected, mounted
on a thin, glass fiber with paraffin oil, and cooled to the data collection
temperature. Data were collected on a Bruker AXS SMART 1 k CCD
diffractometer. Data collection was performed with three batch runs at
ψ = 0.00° (600 frames), at ψ = 120.00° (600 frames), and at ψ =
240.00° (600 frames). Initial unit-cell parameters were determined
from 60 data frames collected at different sections of the Ewald sphere.
Semiempirical absorption corrections based on equivalent reflections
were applied. The systematic absences and unit-cell parameters were
consistent for the reported space groups. The structures were solved
by direct methods, completed with difference Fourier syntheses, and
refined with full-matrix least-squares procedures based on F2. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were treated as idealized contribu-
tions. All scattering factors and anomalous dispersion factors are
contained in the SHELXTL 6.12 program library.
Synthesis of {[(Ph2P)2C(H)CO2]Li(THF)2}2 (1). A solution of
bis(diphenylphosphino)methane (13.4 mmol, 5.1 g) in THF (130
mL) was treated with n-BuLi (13.4 mmol, 2.5 M, 5.3 mL). The
resulting yellow solution was stirred for 3 h. The solution was then
saturated with CO2 for a period of 3 h and then kept at −35 °C for 2
days, thus affording colorless crystals of 1. Yield: 3.2 g, 5.5 mmol, 41%.
1H NMR: δ 7.08−7.55 (m, 20H; Ar H), 4.00 (br, 1H; P2CH).
13C{1H} NMR: δ 48.05 (PCP), 128.3, 133.8, 139.7 (Ar C, Ph), 170.2
(t-C, CO2). 31P{1H} NMR: δ −16.13.
EXPERIMENTAL SECTION
■
General Procedures. All air- and/or water-sensitive reactions
were performed under a nitrogen atmosphere, in oven-dried flasks
using standard Schlenk type techniques. Anhydrous solvents were
obtained by means of a multiple column purification system.
Synthesis of {[(Ph2P)2CCNH(Cy)O]Li(OEt)2}2 (2). A solution
of bis(diphenylphosphino)methane (10 mmol, 3.8 g) in diethyl ether
(100 mL) was treated with n-BuLi (10.1 mmol, 2.5 M, 4.1 mL) at
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Article
room temperature. After the addition was completed, the solution was
stirred for 3 h. Cyclohexyl isocyanate (10 mmol, 1.3 mL) was added to
the yellow solution, and the mixture was stirred for 3 h and
subsequently kept at −35 °C for 2 days, affording colorless crystals of
2. Yield: 5.4 g, 4.6 mmol, 46%. 1H NMR: δ 0.5−1.3 (m, 10H; Cy H),
3.3 (m, 1H; ipso H Cy), 4.8(br, 1H; NH), 7.1−7.3 (m, 20H; Ar
H).13C{1H} NMR: δ 25.9 (Cy CH2), 47.5 (ipso C Cy), 132, 126, 125
(Ar C), 144.8 (PCP), 172.8 (t-C-NCO). 31P{1H} NMR: δ −12.04.
Synthesis of {[(Ph2P)2CCNH(t-Bu)O]Li(OEt)2}2 (3). A solution
of bis(diphenylphosphino)methane (12.8 mmol, 4.9 g) in THF (100
mL) was treated with n-BuLi (13.5 mmol, 2.5 M, 5.6 mL). The
resulting yellow solution was stirred for 3 h. tert-Butyl isocyanate (12.8
mmol, 1.3 g) was then added, resulting in the immediate precipitation
of a white powder. The precipitate was washed three times with cold
hexanes and analyzed after drying under vacuum for 12 h. Yield: 4.8 g,
the supernatant liquid was separated and concentrated and kept at −35
°C, thus affording pale blue crystals of 8. Yield: 837 mg, 0.35 mmol,
35%. Anal. Calcd (found) for C136H148Cr2O16P8: C, 68.28 (68.21); H,
6.19 (6.21). μeff = 3.91 μB.
S y n t h e s i s o f [ ( P h 2 P ) 2 C ( H ) C  N ( t - B u ) O ] -
CrCl2(THF)2·0.5THF·0.5(toluene) (9). A white suspension of 3
(489 mg, 1.0 mmol) in THF was mixed with a suspension of
CrCl3(THF)3 (375 mg, 1 mmol) in THF, and the resulting pale green
solution was stirred for 1 h. The solvent was then removed under
vacuum, and the residue was suspended in toluene. After
centrifugation, the supernatant liquid was separated, concentrated,
and kept at room temperature for 7 days, thus affording pale green
crystals of 9. Yield: 400 mg, 0.48 mmol, 48%. Anal. Calcd (found) for
C43.50H53Cl2CrNO3.50P2: C, 62.83 (62.86); H, 6.38 (6.29); N, 1.68
(1.70). μeff = 3.89 μB.
Synthesis of [(Ph2 P)C(H)CN(2,6-i-Pr2 C6 H3 )O]-
CrCl2(THF)2·1.5THF (10). A colorless solution of 4 (741 mg, 1
mmol) in THF was treated with CrCl3(THF)3 (375 mg, 1 mmol), and
the resulting green mixture was stirred for 4 h. The solvent was
removed under vacuum, and the residue was suspended in toluene.
After centrifugation, the supernatant liquid was evaporated and the
green solid redissolved in THF. Green crystals of 10 were obtained by
keeping the solution at −30 °C for 1 week. Yield: 560 mg, 0.58 mmol,
58%. Anal. Calcd (found) for C52H66Cl2CrNO4.50P2: C, 64.89 (64.72);
H, 6.90 (6.88); N, 1.45 (1.49). μeff = 3.82 μB.
Synthesis of [(Ph2P)C(H)P(Ph2)CN(2,4,6-Me3C6H2)O]Cr-
(THF)2Cl2 (11). A suspension of 5 (2 mmol, 1.29 g) in THF (60 mL)
was mixed with CrCl3(THF)3 (2 mmol, 748 mg). Upon completion of
the addition, the solution was stirred for 15 min, affording a dark
brown solution. The solution was centrifuged, and the supernatant
liquid was concentrated in vacuo and allowed to crystallize at −37 °C,
which afforded brown crystals of 11. Yield: 0.99 g, 1.2 mmol, 61%.
Anal. Calcd (found) for C51H64Cl2CrNO5P2: C, 64.04 (64.08); H, 6.69
(6.72); N, 1.46 (1.48). μeff = 3.82 μB.
Synthesis of {(EtCl2Al)[(Ph2P)2C(H)CN(2,4,6-Me3C6H2)-
OAlEt2](μ-Cl)Cr}2(μ-Cl)2·(toluene) (12). Method A. A suspension
of 5 (643 mg, 1 mmol) in THF (30 mL) was mixed with
CrCl3(THF)3 (1 mmol, 374 mg). After completion of the addition,
the solution was stirred for 15 min, producing a dark brown solution.
The solvent was then evaporated under vacuum, leaving a brown
powder. The powder was dissolved in toluene (30 mL) and mixed
with (CH3CH2)AlCl (631 mg, 5 mmol). After the addition, the
solution was centrifuged and the supernatant liquid was layered with
hexanes and allowed to crystallize blue crystals of 12 (0.12 g, 0.13
mmol, 13%). Anal. Calcd (found) for C88H108Al4Cl8Cr2N2O2P4: C,
57.28 (57.32); H, 5.86 (5.78); N, 1.53 (1.49). μeff = 3.32 μB.
Method B. A suspension of 11 (1.0 g, 1.2 mmol) in toluene (30
mL) was treated with (CH3CH2)AlCl (650 mg, 5.3 mmol). After the
addition, the solution was centrifuged and the supernatant liquid was
layered with hexanes and allowed to crystallize blue crystals of 12
(0.25 g, 0.27 mmol, 45%).
1
9.8 mmol, 76%. H NMR: δ 0.79 (s, 9H; (CH3)3C), 3.79 (br, 1H;
NH), 7.1−7.6 (m, 20H; Ar H). 13C{1H} NMR: δ 29.16 (CH3), 49.4
(CCH3), 126.85, 127.34, 132.58 (Ar C, Ph), 145.23 (PCP), 174.0
(NCO). 31P{1H} NMR: δ −13.9.
Synthesis of [Ph2PCHP(Ph2)CN(2,6-i-Pr2C6H3)O]Li(OEt2)2
(4). A solution of bis(diphenylphosphino)methane (10 mmol, 3.8 g)
in diethyl ether (100 mL) was treated with n-BuLi (10 mmol, 2.5 M,
4.1 mL). After the addition, the suspension was stirred for 1 h,
resulting in a clear yellow solution. Neat 2,6-diisopropylphenyl
isocyanate (10 mmol, 2.1 g) was added, and the mixture was stirred
for another 5 h. The resulting yellow solution was then kept at −35 °C
for 2 days, affording colorless crystals of 4. Yield: 3.4 g, 4.6 mmol, 46%.
1H NMR: δ 0.61 (d, 12H; CH3), 2.63 (m, 2H; ipso-CH), 4.26 (t, 1H;
PCHP), 6.5−7.7 (m, 23H; Ar H). 13C{1H} NMR: δ = 24.2 (i-Pr CH3),
26.89 (i-Pr CHCH3), 46.01 (PCP), 164.9 (t-C, ArNCO), 151.2, 140.6,
134.4, 127.8 (ArCNCO), 127.8, 127.4, 133.8 (PPh). 31P{1H} NMR: δ
−8, −24.
{[(Ph2P)2CCNH(2,4,6-Me3C6H2)O]Li]n (5). A solution of bis-
(diphenylphosphino)methane (13.4 mmol, 5.13 g) in diethyl ether
(130 mL) was treated with n-BuLi (13.4 mmol, 2.5 M, 5.3 mL). After
the addition, the solution was stirred for 1 h, affording a clear yellow
solution. 2,4,6-Trimethylphenyl isocyanate (13.4 mmol, 2.15 g) was
added to the stirred solution. After 1 h, 5 separated out as a white
precipitate, which was isolated by filtration and recrystallized from
THF (3.69 g, 5.7 mmol, 43%). 1H NMR (500 MHz, 20 °C): δ 6.9−7.9
(20H, P-(C5H6)2), 6.4 (2H, C6H2(CH3)3), 4.5 (1H, NH), 1.6 (9H,
C6H2(CH3)3). 31P{1H} NMR: δ −12.5.
Synthesis of [HC(PPh2)2]Cr[(μ-Cl)2Li(THF)2]2 (6). A solution of
bis(diphenylphosphino)methane (1.0 mmol, 382 mg) in THF (6 mL)
was treated with n-BuLi (1.1 mmol, 2.5 M, 0.4 mL), and the resulting
yellow solution was stirred for 3 h at room temperature. A suspension
of CrCl3(THF)3 (1 mmol, 375 mg) in THF was then added, resulting
in a green mixture which was stirred for 2 h. After centrifugation, the
resulting green solution was then kept at room temperature for 4 days,
forming pale green crystals of 6. Yield: 580 mg, 0.66 mmol, 66%. Anal.
Calcd (found) for C41H53Cl4CrLi2O4P2: C, 55.9 (56.01); H, 6.02
(6.10). μeff = 3.92 μB.
RESULTS AND DISCUSSION
Synthesis of [HC(PPh2)2]2Cr(μ-Cl)2Li(THF)2]·1.5THF (7). A
solution of bis(diphenylphosphino)methane (1.0 mmol, 382 mg) in
THF (6 mL) was treated with n-BuLi (1.1 mmol, 2.5 M, 0.4 mL), and
the resulting yellow solution was stirred for 3 h at room temperature.
A THF suspension of CrCl3(THF)3 (0.5 mmol, 190 mg) was then
added to this solution, affording a brown mixture which was stirred for
8 h. The solvent was removed under vacuum, and the residue was
redissolved in toluene. After centrifugation, the supernatant liquid was
evaporated. The dark brown residue was redissolved in THF and kept
at −35 °C for 7 days, affording brown crystals of 7. Yield: 280 mg, 0.24
mmol, 24%. Anal. Calcd (found) for C64H70Cl2CrLiO3.50P4: C, 66.82
(66.90); H, 6.09 (6.10). μeff = 3.82 μB.
Synthesis of [{[(Ph2P)2C(H)CO2]2}Cr(THF)]2 (8). A colorless
solution of the ligand 1 (1.157 g, 1.0 mmol) in THF was mixed
with a suspension of CrCl2(THF)2 (266 mg, 1 mmol) in THF, and the
resulting pale blue solution was stirred for 4 h. The solvent was then
removed under vacuum, the residue was redissolved in fresh THF (4
mL), and the resulting mixture was centrifuged. After centrifugation,
■
The bis(diphenylphoshino)methane anion (DPPM−) was
prepared in situ by treating commercially available DPPM
with 1 equiv of n-BuLi in THF or Et2O. Reacting the anion
with CO2, CyNCO, and t-BuNCO afforded the corresponding
lithium salts {[(Ph2P)2CHCO2]Li(THF)2}2 (1), {[(Ph2P)2C
CNH(Cy)O]Li(OEt)2}2 (2), and {[(Ph2P)2CCNH (t-Bu)-
O]Li(OEt)2}2 (3), respectively (Scheme 1). The 31P{1H}
NMR spectra of the ligands 1−3 showed a single resonance at
−16.13, −12.04, and −13.9 ppm, respectively, in agreement
with the presence of two identical P atoms. In these reactions,
the carbon atom located between the two phosphorus atoms of
the DPPM− anion acted as a nucleophile, attacking the
electrophilic carbon atom of the cumulenes and in the process
forming a carbon−carbon bond. Interestingly, this species may
exist in a tautomeric form, where the hydrogen atom may have
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Scheme 1
Figure 2. ORTEP drawings of 2. Selected bond distances (Å) and
angles (deg): P(1)−C(1), = 1.784(2), P(2)−C(1) = 1.789(2), C(1)−
C(2) = 1.415(3), Li(1)−O(1) = 1.885(4), Li(1)−O(2) = 1.917(5),
N(1)−C(2) = 1.359(2), Li(1)−P(1) = 2.569(4); O(2)−Li(1)−O(1a)
= 113.1(2), O(1)−Li(1)−O(2) = 115.4(2), O(1)−Li(1)−O(1a) =
93.62(18), O(2)−Li(1)−P(1a) = 112.80(18).
been transferred from the C to the N atom (Scheme 1) and the
double bond from the CN to the CC position. In the
cases of 1 and 2 it was possible to obtain single crystals of
suitable quality for X-ray diffraction.
The X-ray structure of 1 consists of a dimer (Figure 1) with
two ligand moieties bridged by two tetrahedrally coordinated
oxygen atom of diethyl ether (Li(1)−O(2) = 1.917(5) Å). The
trigonal-planar geometry of the carbon bridging the two
phosphorus atoms (P(1)−C(1)−P(2) = 119.14(12)°, C(2)−
C(1)−P(1) = 115.11(15)°, C(2)−C(1)−P(2) = 125.48(15)°)
and the rather short C−C distance (C(1)−C(2) = 1.415(3) Å)
also implies some multiple-bond character with the isocyanate
residue carbon atom.
The crystallographic parameters clearly indicate that the
hydrogen atom, originally present at the methyne carbon atom
of the DPPM− anion, has been transferred to the nitrogen atom
of the isocyanate, forming an N−H and a CC bond. The
presence of this as a predominant tautomeric form is further
confirmed by the absence of methyne correlation peaks in the
1H{13C}-HMQC experiments (see the Supporting Informa-
1
tion) and the presence in the H NMR spectrum of a broad
peak at 4.8 ppm without carbon correlation characteristic of the
N−H function.
When tert-butyl isocyanate was used, the lithiated ligand
{[(Ph2P)2CCNH(t-Bu)O]Li(OEt)2}2 (3), possessing a
structure similar to that of 2, was isolated and fully
characterized by NMR. However, when 2,6-diisopropylphenyl
isocyanate was employed, the reaction took a different pathway.
The structure of the lithium salt [Ph2PCHP(Ph2)CN(2,6-
i-Pr2C6H3)O]Li(OEt2)2 (4) showed that a nonclassical type of
nucleophilic attack of the DPPM− phosphorus instead of the
carbon atom had occurred (Scheme 2).
Figure 1. ORTEP drawing of 1. Selected bond distances (Å) and
angles (deg): P(1)−C(1) = 1.866(2), P(2)−C(1) = 1.865(2), C(1)−
C(26) = 1.542(3), O(1)−C(26) = 1.245(2), O(2)−C(26) = 1.239(3),
Li(1)−O(3) = 1.947(4), Li(1)−O(4) = 1.992(5); O(3)−Li(1)−O(4)
= 100.6(2), O(1)−Li(1)−O(2) = 134.6(2), O(1)−Li(1)−O(3) =
103.30(19), O(2)−Li(1)−O(3) = 102.7(2).
This resulted in the formation of a phosphorus−carbon bond
instead of the expected carbon−carbon bond. In addition, the
formal oxidation state of one of the two phosphorus atoms
changed from +3 to +5 as a result of the consequent formation
of the CP double bond within the DPPM− anion residue.
Accordingly, the 31P NMR spectra of the ligand display two
lithium atoms. Two of the coordination sites around each
lithium are occupied by the oxygen atoms of two THF
molecules (Li(1)−O(3) = 1.947(4) Å, Li(1)−O(4) = 1.992(5)
Å), while the other two coordination sites are filled by two
oxygen atoms, each from one carboxylate moiety (Li(1)−O(1)
= 1.891(4) Å, Li(1)−O(2) = 1.991(4) Å).
The structure of 2 (Figure 2) is also dimeric, with two
monomeric units linked together by two tetrahedrally
coordinated lithium atoms (O(1)−Li(1)−O(2) = 115.4(2)°,
O(2)−Li(1)−O(1a) = 113.1(2)°, O(1)−Li(1)−O(1a) =
93.62(18)°, O(2)−Li(1)−P(1a) = 112.80(18)°). Each lithium
atom is bonded to two bridging oxygen atom of the isocyanate
moieties (Li(1)−O(1) = 1.885(4) Å, Li(1)−O(1a) = 1.904(4)
Å), a phosphorus atom (Li(1)−P(1a) = 2.569(4) Å), and one
Scheme 2
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equally intense peaks at −8.0 and −24.0 ppm, as expected for
the presence of two nonequivalent P atoms in different
oxidation states.
The crystal structure of 4 (Figure 3) shows the distorted-
tetrahedral lithium atom surrounded (O(2)−Li(1)−O(3) =
Figure 4. Different approaches of aliphatic and aromatic isocyanates to
the DPPM anion (B3LYP-optimized structures).
understand this apparent discrepancy, one must keep in mind
the dynamic behavior observed for the two aromatic derivatives
(see below). In other words, the mesityl derivative clearly
switches the connectivity from classical to nonclassical and
back, as a function of the presence of lithium or chromium
metal and the aluminate residues (see below). Therefore, it
seems reasonable to propose that 5 might possess in the gas
phase or the solid state the same structure as the
diisopropylphenyl derivative 4. However, the connectivity
might well switch in solution as a result of the solvation and
different state of aggregation and solvation of the alkali-metal
cation.
Figure 3. ORTEP drawing of 4. Ellipsoids are drawn at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): P(1)−C(1) = 1.7510(18), P(2)−C(1)
= 1.6932(18), P(2)−C(26) = 1.8557(17), C(26)−O(1) = 1.261(2),
N(1)−C(26) = 1.285(2); O(2)−Li(1)−O(3) = 106.75(19), O(1)−
Li(1)−O(3) = 109.20(2), O(1)−Li(1)−O(2) = 113.20(2), O(1)−
Li(1)−P(1) = 91.03(14).
To understand the behavior of this family of ligands, we have
conducted a preliminary exploration of the behavior of the basic
DPPM− anion with CrCl3(THF)3 with variable stoichiometric
ratios. The reaction with a 1:1 molar ratio resulted in the
formation of the chromium complex [HC(PPh2)2]Cr[(μ-
Cl)2Li(THF)2]2 (6), in which the octahedral coordination
sphere of chromium contains one bidentate monoanionic
chelating DPPM− frame and four chlorine atoms bridging
THF-solvated lithium atoms (Figure 5a). However, the
reaction of the DPPM− anion with 0.5 equiv of CrCl3(THF)3
afforded the octahedral chromium complex [HC(PPh2)2]2Cr-
(μ-Cl)2Li(THF)2]·1.5THF (7), whose six coordination sites
are occupied by two bidentate-monoanionic chelating DPPM
frames and two chlorine atoms bridging THF-solvated lithium
atoms (Figure 5b).
The X-ray structures of the two complexes showed similarly
distorted octahedral geometries around chromium, with four
chlorine atoms bridging between two solvated lithium centers
in the case of complex 6 (Cr(1)−Cl(1) = 2.3500(6) Å, Cr(1)−
Cl(2) = 2.3959(6) Å). In the case of 7, only two chlorine atoms
bridge between a THF-solvated lithium atom (Cr(1)−Cl(1) =
2.3634(12) Å, Cr(1)−Cl(2) = 2.3741(12) Å). The Cr(1)−
Li(1) distance in 6 (Cr(1)−Li(1) = 3.206(4) Å) was found to
be slightly shorter than the corresponding bond distance in 7
(Cr(1)−Li(1) = 3.2989(1) Å). In addition, the chromium to
phosphorus distances in the case of 6 (Cr(1)−P(1) =
2.4506(6) Å, Cr(1)−P(1a) = 2.4506(6) Å) were shorter than
the corresponding bond distances in 7 (Cr(1)−P(1) =
2.4626(12) Å, Cr(1)−P(3) = 2.4700(12) Å, Cr(1)−P(2) =
2.4978(12) Å, Cr(1)−P(4) = 2.5149(12) Å).
106.75(19)°, O(1)−Li(1)−O(3) = 109.20(2)°, O(1)−Li(1)−
O(2) = 113.20(2)°) by one of the two phosphorus atoms
(Li(1)−P(1) = 2.703(4) Å), the oxygen atom (Li(1)−O(1) =
1.813(4) Å) of the isocyanate moiety, and two coordinated
diethyl ether molecules (Li(1)−O(2) = 1.948(4) Å, Li(1)−
O(3) =1.977(4) Å). The two fairly short20 carbon−phosphorus
distances are in agreement with the presence of a significant
extent of double-bond character and substantial electronic
delocalization within the P−C−P frame (C(1)−P(1) =
1.751(18) Å, C(1)−P(2) = 1.6932(18) Å).
To rationalize the discrepancy of behavior between the
aliphatic and aromatic isocyanates, DFT calculations were
performed to understand the origin of the nonclassical bonding
mode. It was found that the two different bonding modes may
be attributed to the different electronic configurations of the
two isocyanates (Figure 4) and steric interactions within the
final products. The straight OCN− functionality of the
aromatic isocyanate tends to reside in plane with the rigid
aromatic ring, forming a conjugated system. On the other hand,
in the case of aliphatic isocyanate, the alkyl group is bent away,
allowing the carbon atom to approach the central carbon of the
DPPM− anion. Instead, the carbon atom of the aromatic
isocyanate can only reach the DPPM− anion through the
phosphorus atom to minimize the steric repulsions.
The behavior of the mesityl derivative [{[(Ph2P)2C
CNH(Mes)O]Li]n (5) is in striking contrast with the above
arguments. Although single crystals suitable for X-ray analysis
could not be grown in this case, the NMR spectra are
completely consistent with the classical type of reaction. To
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Figure 5. ORTEP drawings of 6 and 7. Ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity. Selected bond distances
(Å) and angles (deg) are as follows. For 6: Cr(1)−Cl(1) = 2.3500(6), Cr(1)−Cl(2) = 2.3959(6), Cr(1)−P(1) = 2.4560(6), Cr(1)−P(1a) =
2.4560(6), P(1)−C(1) = 1.7236; Cl(1)−Cr(1)−Cl(1a) = 177.97(4), Cl(1)−Cr(1)−Cl(2a) = 90.76(2), Cl(1)−Cr(1)−Cl(2) = 90.58(2), Cl(1)−
Cr(1)−P(1) = 89.14(2), Cl(1)−Cr(1)−P(1a) = 89.14(2), P(1a)-Cr(1)−P(1) = 67.13(3). For 7: Cr(1)−Cl(1) = 2.3634(12), Cr(1)−Cl(2) =
2.3741(12), Cr(1)−P(1) = 2.4626(12), Cr(1)−P(2) = 2.4978(12), Cr(1)−P(3) = 2.4700(12), Cr(1)−P(4) = 2.5149(12); Cl(1)−Cr(1)−Cl(2) =
90.31(4), Cl(1)−Cr(1)−P(1) = 95.64(4), Cl(1)−Cr(1)−P(3) = 93.10(4), Cl(1)−Cr(1)−P(4) = 92.52(4).
The complexation of CrCl2(THF)2 with 1 resulted in the
formation of the dimeric chromium complex [{[(Ph2P)2C(H)-
CO2]2}Cr(THF)]2 (8). The molecular structure of 8 (Figure
6) showed a dinuclear chromium complex with a rather short
Cr−Cr bond distance (Cr−Cr = 2.337(8) Å) in the
characteristic arrangement of the axially coordinated, paddle-
wheel complexes of divalent chromium.21 Four of the
coordination sites around each chromium atom are occupied
by four oxygen atoms of the carboxylate moiety (Cr(1)−O(3)
= 1.9985(18) Å, Cr(1)−O(1) = 2.016(2) Å), while the fifth
coordination site is occupied by oxygen atom of the THF
molecule (Cr(1)−O(5) = 2.247(2) Å). Complex 8 shows the
reduced paramagnetism typical of the paddlewheel divalent
chromium dimers (μeff = 3.91 μB per dimer).21
DPPM− residue. Unfortunately, the isolation of single crystals
of the corresponding Cr(II) complex was not successful, due to
the poor solubility of this blue microcrystalline material,
including in highly polar solvents such as dichloromethane
and acetonitrile.
Complexation of ligand 4, which resulted from a nonclassical
isocyanate/diphosphine anion reaction, with CrCl3(THF)3
afforded the complex [(Ph2P)C(H)CN(2,6-i-Pr2C6H3)O]-
CrCl2(THF)2·1.5THF (10), where the ligand has apparently
switched the connectivity toward the classical type of
isocyanate−diphosphino anion aggregation (Scheme 3). The
Scheme 3
Attempts to crystallize complexes of CrCl3(THF)3 with 2
invariably led to powdery products. Conversely, reaction of 3
with CrCl3(THF)3 afforded [(Ph2P)2C(H)CN(t-Bu)O]-
CrCl2(THF)2·0.5THF·0.5(toluene) (9) in crystalline form.
The molecular structure of 9 (Figure 7) showed the octahedral
coordination geometry of the chromium atom as defined by
one phosphorus (Cr(1)−P(1) = 2.4280(12) Å), one oxygen
(Cr(1)−O(1) = 1.937(3) Å), two chlorine atoms (Cr(1)−
Cl(1) = 2.2991(12) Å, Cr(1)−Cl(2) = 2.2998(12) Å), and two
molecules of THF (Cr(1)−O(2) = 2.069(3) Å, Cr(1)−O(3) =
2.079(3) Å). The long bond distance of the C−C bond
connecting the two DPPM− and isocyanate residues and the
pyramidality of the PCP’s C atom indicate that the ligand is
present in the tautomeric form containing the CN double
bond and the hydrogen atom on the central carbon of the
resulting octahedral chromium complex is very similar to 9.
Accordingly, the phosphorus oxidation state has been reduced
from +5 to +3 through an internal redox transformation.
The crystal structure of 10 (Figure 8) shows an octahedrally
coordinated chromium atom bonded to one oxygen atom
(Cr(1)−O(1) = 1.939(2) Å, O(1)−Cr(1)−Cl(1) = 90.41(7)°)
and one phosphorus atom (Cr(1)−P(1) = 2.4334(9) Å,
Cl(1)−Cr(1)−P(1) = 91.44(3)°) of the ligand, two chlorine
atoms (Cr(1)−Cl(2) = 2.2980(10) Å, Cr(1)−Cl(1) =
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Figure 8. ORTEP drawing of 10. Ellipsoids are drawn at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): P(1)−C(1) = 1.844(3), P(2)−C(1) =
1.870(3), C(1)−C(26) = 1.539(4), Cr(1)−Cl(2) = 2.2980(10),
Cr(1)−Cl(1) = 2.3016(10), Cr(1)−O(1) = 1.939(2), Cr(1)−P(1) =
2.4334(9); O(1)−Cr(1)−Cl(1) = 90.41(7), Cl(1)−Cr(1)−P(1) =
91.44(3), O(3)−Cr(1)−Cl(1) = 90.52(9), Cl(2)−Cr(1)−Cl(1) =
92.09(4), O(2)−Cr(1)−Cl(1) = 174.60(8).
Figure 6. ORTEP drawing of 8. Ellipsoids are drawn at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): Cr(1)−Cr(1a) = 2.3337(8), Cr(1)−
O(3) = 1.9985(18), Cr(1)−O(1) = 2.016(2), Cr(1)−O(5) =
2.247(2); O(3)−Cr(1)−O(5) = 89.40(8), O(1)−Cr(1)−O(5) =
91.78(8), O(3)−Cr(1)−O(1) = 93.59(8), O(3)−Cr(1)−O(2) =
86.99(8).
change of the phosphorus oxidation state from +5 to +3 is also
reflected in the lengthening of P−C bond distances (P(1)−
C(1) = 1.844(3) Å, P(2)−C(1) = 1.870(3) Å) in comparison
to the corresponding bond distances in the ligand 4 (P(1)−
C(1) = 1.751(18) Å, P(2)−C(1) = 1.693(2) Å). Also the C−C
bond distance of the link between the isocyanate and
diphosphine residues of complex 10 (C(1)−C(26) =
1.539(4) Å) compares well to that of 9 (C(1)−C(26) =
1.542(5) Å). The pyramidality of the PCP’s central C atom also
indicates the presence of the H atom in that position and
consequent CN double-bond character.
As mentioned above, the behavior of the mesityl analogue is
diametrically opposite to that of the diisopropyl derivative.
First, the lithium derivative 5 possesses the classical
connectivity, as is clearly indicated by the symmetry of its
NMR spectra. Second, upon reaction with CrCl3(THF)3 and
similar to the case of 10, the ligand connectivity also switched
but toward the non-classical form, affording [(Ph2P)C(H)
P(Ph2)CN(2,4,6-Me3C6H2)O]Cr(THF)2Cl2 (11) (Scheme
4).
Complex 11 is monomeric (Figure 9) with the chromium
metal center in a rather regular octahedral coordination
(Cl(2)−Cr(1)−O(3) = 89.60(19)°, O(2)−Cr(1)−O(1) =
88.7(3)°, O(1)−Cr(1)−P(1) = 90.77(19)°). Two of the
coordination sites are occupied by two chlorine atoms located
in a cis orientation with respect to one another (Cl1−Cr1 =
2.274(3) Å, Cl(2)−Cr(1) = 2.313(3) Å, Cl(1)−Cr(1)−Cl(2) =
94.64(11)°). Two oxygen atoms from two molecules of THF
occupy two other coordination sites also in cis positions relative
to one another (O(2)−Cr(1) = 2.093(7) Å, O(3)−Cr(1) =
2.065(6) Å, O(3)−Cr(1)−O(2) = 85.0(3)°). The final two
coordination sites are occupied by an oxygen atom (O(1)−
Cr(1) = 1.930(6) Å) and one of the two phosphorus atoms
(P(1)−Cr(1) = 2.445(3) Å) of the ligand.
Figure 7. ORTEP drawing of 9. Ellipsoids are drawn at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): Cr(1)−Cl(1) = 2.2991(12), Cr(1)−
Cl(2) = 2.2998(12), Cr(1)−P(1) = 2.4280(12), Cr(1)−O(1) =
1.937(3), Cr(1)−O(2) = 2.069(3), Cr(1)−O(3) = 2.079(3), C(1)−
C(26) = 1.542(5); O(1)−Cr(1)−Cl(2) = 91.47(8), O(3)−Cr(1)−
Cl(2) = 89.71(9), O(1)−Cr(1)−Cl(2) = 91.47(8), Cl(2)−Cr(1)−
P(1) = 92.87(4), Cl(2)−Cr(1)−Cl(1) = 91.67(5), O(2)−Cr(1)−
Cl(2) = 174.17(9).
2.3016(10) Å, Cl(2)−Cr(1)−Cl(1) = 92.09(4)°), and two
THF molecules (O(2)−Cr(1)−Cl(1) = 174.60(8)°). The
In an attempt to understand the catalytic behavior as a
function of the particular nature of the activator (see below),
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Scheme 4
Figure 9. ORTEP drawing of 11. Ellipsoids are drawn at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): O(1)−Cr(1)−P(1) = 90.77(19);
P(2)−C(2) = 1.859(9); O(1)−C(2)−N(1) = 128.3(9), O(1)−C(2)−
P(2) = 117.9(7), N(1)−C(2)−P(2) = 113.8(7); N(1)−C(2) =
1.283(11), O(1)−C(2) = 1.299(10); P(1)−C(1)−P(2) = 125.0(6);
C(1)−P(1) = 1.734(9), C(1)−P(2) = 1.700(10).
Figure 10. ORTEP drawing of 12. Ellipsoids are drawn at the 50%
probability level, and H atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): Cr(1)−P(2) = 2.480(2); P(2)−
C(1)−C(2) = 109.6(4); N(1)−C(2)−C(1) = 122.0(6); Al(1)−O(1)
= 1.810(5); Al(2)−N(1) = 1.944(5), Al(2)−Cl(3) = 2.149(3), Al(2)−
Cl(4) = 2.142(3), Al(2)−C(6) = 1.922(7).
we have attempted the isolation of complexes as arising from
the reaction of 6−11 with a variety of aluminum alkyls. The
reactions almost invariably afforded very air-sensitive ill-defined
materials, often oily in nature. Just in the case of the reaction of
11 with DEAC (Scheme 5), it was possible to isolate a complex
in crystalline form formulated as {(EtCl2Al)[(Ph2P)2C(H)C
N(2,4,6-Me3C6H2)OAlEt2](μ-Cl)Cr}2(μ-Cl)2 (12) on the
basis of its crystal structure (Figure 10).
The same complex could also be conveniently obtained via
one-pot synthesis by reacting 5 with CrCl3(THF)3 followed by
the addition of DEAC. The salient feature of this species is that
the ligand, for the second time, has switched the ligand
connectivity from the nonclassical back to the classical. This is a
possible result of the retention of the aluminate residue,
requiring from the ligand an ampler chelating bite that in turn
can be accommodated by a switching of connectivity.
pyramidal chromium atoms of each of the two identical units
(Cl(1)−Cr(1)−Cl(1A) = 92.19(6)°, Cl(1)−Cr(1)−P(2) =
95.86(7)°, P(2)−Cr(1)−P(1) = 67.93(6)°, P(1)−Cr(1)−
Cl(2) = 111.78(7)°) are bridged by two chlorine atoms
(Cr(1)−Cl(1) = 2.3763(19) Å, Cr(1)−Cl(1A) = 2.3879(19)
Å) located on two of the coordination sites of the equatorial
plane. A terminally bonded chlorine atom (Cr(1)−Cl(2) =
2.584(2) Å) occupying the axial position is in turn bonded to
one of the two aluminum atoms (Cl(2)−Al(1) = 2.267(3) Å).
Two phosphorus atoms from the DPPM moiety of the ligand
chelate the last two equatorial coordination sites of chromium
(Cr(1)−P(1) = 2.467(2) Å). The central carbon on the DPPM
moiety is sp3 hybridized (P(1)−C(1)−P(2) = 94.4(3)°, P(1)−
C(1)−C(2) = 110.5(4)°), since it is also bonded to one
hydrogen atom and to the central carbon atom of the
isocyanate moiety (C(1)−C(2) = 1.505(8) Å). The carbon
on the isocyanate moiety is instead trigonal planar (O(1)−
Complex 12 is a symmetry-generated dimer containing two
chromium and four aluminum metals. The two square-
Scheme 5
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a
Table 1. Ethylene Oligomerization Results for Complexes 6−12
cat. ID (loading,
amt of oligomer,
mL
activity, g/((mmol of Cr)
h)
C6,
C8,
≥C10,
μmol)
cocat. (amt, equiv)
MAO (500)
amt of PE, g
mol %
mol %
mol %
α
6 (30)
7 (30)
9 (1)
9 (1)
9 (1)
9 (1)
21.7
14.3
21.4
17
3.0
1180
663
48.2
49.8
47.4
47.6
46.8
23.6
26.5
28.8
29.1
28.7
28.2
23.7
23.8
23.1
24.5
0.58
MAO (500)
MAO (300)
MAO (500)
MAO (1000)
0
0.55
0.60
0.63
0.69
trace
trace
4.2
29820
24000
18340
12000
7.2
MAO (1000) + TMA
(250)
6.0
9 (1)
DMAO (500)
DMAO (500)
1.2
2.2
2.8
2400
440
b
9 (10)
b
9 (10)
DMAO (500) + TEA
(250)
6.2
1410
50.7
29.4
19.8
0.65
10 (10)
10 (1)
10 (1)
10 (10)
MMAO (500)
MAO (500)
MAO (1000)
MMAO (500)
DMAO
0.7
0.5
0.8
140
9520
21480
0
6.4
42.9
43.7
31.4
30.8
25.2
25.3
0.68
0.65
14.6
b
10 (10)
18.6
18.5
1.5
0.2
2596
2599
2233
53.5
55.0
99.9
30
15.63
9.9
0.63
11 (10)
12 (10)
12 (10)
DMAO
0.3
35.1
0.1
DMAO (500) + TEA
(250)
traces
a
b
Oligomerization conditions: solvent, 100 mL of toluene; ethylene pressure, 30 bar; reaction time, 30 min; reaction temperature, 60 °C. The
solvent is methylcyclohexane.
a
Table 2. Ethylene Oligomerization Results of in Situ Catalyst Testing: 1,2/CrCl3(THF)3
cat. ID (loading,
μmol)
amt of cocat., equiv amt of oligomer, mL atm of PE, g activity, g/((mmol of Cr) h) C6, mol % C8, mol % ≥C10, mol %
α
1 (5)
1 (5)
1 (5)
MAO (500)
MAO (1000)
MMAO (500)
MMAO (500)
MAO (500)
MAO (1000)
MMAO (500)
MMAO
12.1
18.3
7.0
11
9
7808
8680
47.6
48.2
44.5
29.1
28.7
24.9
23.2
23.1
30.2
0.59
0.61
0.59
23
11160
b
1 (10)
2 (5)
2 (5)
2 (5)
2 (10)
12.1
30.4
6.2
0.4
0.7
3560
8800
1704
49.1
53.8
51.9
31.7
30.2
31.9
18.9
15.2
16.1
0.62
0.55
0.53
b
a
CrCl3(THF)3 and 1 or 2 were mixed in toluene and the in situ prepared complexes were injected in the reactor. Reaction conditions: solvent, 100
b
mL of toluene; ethylene pressure, 30 bar; reaction time, 30 min; reaction temperature, 60 °C. The solvent is methylcyclohexane.
a
Table 3. Effect of Catalyst Precursor and Cocatalyst on Selectivity
ligand (loading,
μmol)
amt of cocat., equiv
amt of oligomer, mL amt of PE, g activity, g/((mmol of Cr) h) C6, mol % C8, mol % ≥C10, mol %
1 (1)
1 (1)
2 (5)
2 (5)
3 (5)
3 (5)
4 (5)
4 (5)
DMAO (500)
7.8
18800
15600
0
0
0
0
DMAO (500)/TEA (100)
DMAO (500)
9.4
2.3
2.6
4.5
4.9
2.3
2.6
0
0
6.2
9.4
1.2
2.8
1.9
2.4
3120
4400
1760
2400
1400
1600
52.8
51.9
61.8
61.7
50.5
48.8
47.2
48.1
38.9
38.3
49.5
51.2
0
DMAO (500)/TEA (100)
DMAO (500)
0
0
0
0
DMAO (500)/TEA (100)
DMAO (500)
DMAO (500)/TEA (100)
a
Oligomerization conditions: solvent, 100 mL of methylcyclohexane; ethylene pressure, 30 bar; reaction time, 30 min; reaction temperature, 60 °C;
Cr(acac)3/ligand = 1/1.
C(2)−N(1) = 123.4(6)°, O(1)−C(2)−C(1) = 114.6(6)°) with
delocalization of the double bond over both the carbon−
nitrogen (N(1)−C(2) = 1.306(7) Å) and carbon−oxygen
bonds (O(1)−C(2) = 1.295(7) Å). The oxygen of the
isocyanate moiety is also bound to the aluminum atom of an
Et2Al unit, which is in turn bonded to the chlorine located at
the apical position. The nitrogen atom of the isocyanate moiety
is also bonded to the aluminum atom of an EtAlCl2 residue.
All the chromium complexes were tested for ethylene
oligomerization activity under different reaction conditions.
The DPPM−chromium complexes 6 and 7, upon activation
with MAO in toluene, showed moderate catalytic activity,
producing a Schultz−Flory (S-F) distribution of oligomers
(Table 1). Complex 7, which contains two monoanionic
DPPM ligands, displays a lower catalytic activity (663 g/
((mmol of Cr) h)) compared to 6 (1180 g/((mmol of Cr) h))
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Organometallics
Article
ligating one DPPM fragment. However, whereas 6 produced a
considerable amount of waxy low-molecular-weight PE, 7 was
found to be a polymer-free oligomerization catalyst under
identical reaction conditions. The activity of catalysts 6 and 7
was found to be higher than the activity of the in situ generated
DPPM−Cr(III) complex reported by Overett et al.,17 the poor
catalytic performance of which was attributed to the
deprotonation of the highly acidic bridging methylene proton
by metal alkyl species.22 Both catalysts 6 and 7 were found to
be inactive upon activating with different activators such as
TEA, DMAO, and MMAO either in toluene or in
methylcyclohexane.
PE. It should be noted that 1-butene was not formed by the
catalytic cycle. The venting of the reactors was always carried
out slowly at low temperature to minimize losses. To our
surprise, though, not even traces could be detected. On the
other hand, this is in agreement with the work of McGuiness
and Gibson indicating that a S-F distribution may also be
obtained via a ring expansion mechanism.25 The SEC analysis
of the PE showed that it is a low-molecular-weight wax with a
narrow molecular weight distribution (PDI = 1.7, Mw = 2050 g/
mol). When the CrCl3(THF)3/1 system was tested using
MMAO as a cocatalyst in toluene, the amount of wax
considerably increased (PDI = 1.7, Mw = 2430 g/mol) along
with small amounts of oligomers. However, the catalyst was
completely inactive when the reaction was carried out in
methylcyclohexane using MMAO as cocatalyst. The in situ
testing of ligand 2 with CrCl3(THF)3 also produced a S-F
distribution of oligomers at high Al:Cr ratio, with an activity of
8800 g/((mmol of Cr) h), but with less PE formation
compared to that for CrCl3(THF)3/1.
The in situ catalytic testing of ligand 1−4 using Cr(acac)3 as
metal precursor and DMAO as cocatalyst in methylcyclohexane
showed some interesting differences (Table 3). The ligand 1 at
an Al:Cr ratio of 500 produced only low-molecular-weight wax
with a slightly broad polydispersity index (PDI = 5.1, Mw = 12
000 g/mol). The IR analysis of the resulting PE showed that it
is vinyl terminated. The addition of 100 equiv of TEA along
with DMAO only increased the amount of PE. Interestingly,
whereas 9 and 10 produced a S-F distribution of oligomers, on
activation with DMAO (+TEA) the combination of Cr(acac)3
and ligands 2−4 produced 1-hexene/1-octene selective
catalysts (no higher liquid oligomers), albeit with a considerable
amount of low-molecular-weight waxes. The formation of a
higher amount of wax in comparison to the amount of hexene
and octene might be due to the predominant stabilization of
divalent chromium compared to the monovalent oxidation
state. Ligand 3 showed slighly higher oligomerization activity.
The addition of TEA as an external alkylating agent/reducing
agent did not result in any considerable improvement in terms
of oligomerization activity and selectivity; it only helped to
increase the amount of wax in all cases.
The modification of the anionic DPPM framework with
isocyanates or CO2 considerably enhanced the catalytic
performance of the complexes (Tables 1−3). Activation of 9
with MAO in toluene resulted in a highly active ethylene
oligomerization catalyst, again producing a S-F distribution of
oligomers with a maximum activity of 29 820 g/((mmol of Cr)
h). Interestingly, not even traces of polymer were detected. The
activity and the selectivity of this system was comparable to
those of the previously reported NPN system [(t-Bu)NP-
(Ph)2N(t-Bu)]Cr(μ2-Cl)2Li(THF)2, which also showed poly-
mer-free oligomerization behavior.23 Varying the Al:Cr ratio
affected the catalytic activity with a maximum activity at a low
Al:Cr ratio. Increasing the Al:Cr ratio had a detrimental effect
on the catalytic performance and resulted in the formation of a
considerable amount of low-molecular-weight PE. The lower
oligomerization activity at higher Al:Cr ratio might be due to
the poisoning effect of free TMA present in MAO. Indeed, the
addition of 250 equiv of TMA along with MAO switches the
catalytic behavior to polymerization, producing polyethylene
with a broad molecular weight distribution. Use of DMAO as
cocatalyst in methylcyclohexane or in toluene resulted in the
formation of different types of PE. In methylcyclohexane,
UHMWPE with narrow molecular weight distribution (PDI =
3.2, Mw = 4.1 × 106 g/mol) was obtained, while the polymer
obtained in toluene showed the characteristic broad molecular
weight waxy PE. However, the addition of 250 equiv of TEA
along with DMAO in methylcyclohexane resulted in the
formation of a S-F distribution of oligomers along with waxy
low-molecular-weight PE, implying that yet another different
active species is generated upon the addition of TEA. Upon
activation with MAO (Al:Cr = 1000), 10 also showed high
oligomerization activity (21 480 g/((mmol of Cr) h)). The
catalyst was inactive upon activation with MMAO in toluene.
However, with DMAO as cocatalyst in methylcyclohexane, 10
showed moderate oligomerization activity. Complexes 10 and
11 display very similar catalytic behavior. As anticipated,
complex 12 is a self-activating catalyst producing very small
amounts of highly pure 1-hexene. Attempts to boost catalytic
activity by adding activators only resulted in catalyst
decomposition. This could be understood in terms of
competition between the Lewis acidic Cr and Al to coordinate
Cl. As a result, the activator could replace Cl and alter the frame
of the catalytically active species.24
CONCLUSIONS
■
Attempts to functionalize the DPPM− anions by reaction with
isocyanates afforded new ligands displaying a nonanticipated
dynamism. Aside from the expected nucleophilic attack of the
negatively charged C atom at the C of the cumulenes, in some
cases it was the P atom that acted as a nucleophile, producing
ligands with a different connectivity (nonclassical). However,
the connectivity may switch back and forward between the two
classical and nonclassical forms depending on the steric
requirements of the final complexes. While we found no
evidence that this may be a fluxional behavior (at least in the
case of the diamagnetic lithium salt), this strange ligand
dynamism in principle holds some promises for catalysis. To
this end, the catalytic testing on all the chromium species
presented in this work showed mainly nonselective behavior.
Use of Cr(acac)3 as metal precursor and DMAO as cocatalyst
in methylcyclohexane resulted in the formation of 1-hexene and
1-octene (no higher liquid oligomers) along with low-
molecular-weight polyethylene wax. Only in the case of
complex 12 was an interesting selective and self-activating
behavior obtained. The fact that the complex possesses Cr in its
Since the isolation of well-defined Cr(III) complexes of
ligand 1 and 2 were not successful, these ligands were tested in
situ by mixing with CrCl3(THF)3 and the results are
summarized in Table 2.
The in situ generated chromium complex of ligand 1 showed,
upon activation with MAO (Al/Cr = 1000) in toluene, an
activity of 8680 g/((mmol of Cr) h), producing a S-F
distribution of oligomers along with a considerable amount of
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Article
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divalent state implies further reduction during the self-
activation process.
ASSOCIATED CONTENT
* Supporting Information
■
S
A figure, tables, and CIF files giving the 1H-13C HMQC
spectrum of ligand 2, crystallographic data for all crystal
structures given in the paper, and Cartesian coordinates for the
calculated structures. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sgambaro@uottawa.ca (S.G.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science and
Engineering Council of Canada (NSERC) and the Dutch
Polymer Institute (DPI, #638), Eindhoven, The Netherlands.
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(b) Albahily, K.; Fomitcheva., V.; Murugesu, M.; Gambarotta, S.;
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