A chromium-diphosphine system for catalytic ethylene trimerization: Synthetic and structural studies of chromium complexes with a nitrogen-bridged diphosphine ligand with ortho-methoxyaryl substituents

Article abstract of DOI:10.1021/om050605+

To gain molecular-level insight into the important features of a chromium-diphosphine catalytic system for ethylene trimerization, the coordination chemistry of chromium with PNP ligands (PNP ^OMe = (o-MeOC₆H₄)₂PN(CH ₃)P(o-MeOC₆H₄)₂, P ^t-BuN^i-amylP^OMe = (2-MeO-4-t-BuC ₆H₃)₂PN(i-amyl)P(2-MeO-4-t-BuC ₆H₃)₂) has been explored. Chromium(0) carbonyl complexes have been synthesized by CO displacement with diphosphine. Oxidation of Cr(CO)₄{κ²-(P,P)-(PNP^OMe)} with I ₂, Br₂, and PhICl₂ generates the corresponding chromium(III) halide complexes. Chromium(III) complexes CrCl ₃{(κ³-(P,P,O)-(PNP^OMe)}, CrCl ₃{κ³-(P,P,O)-(P^t-BuN ^i-amylP^OMe)}, and CrCl₂(CH₃) {κ³-(P,P,O)-(PNP^OMe)} can be synthesized by metalation with CrCl₃(THF)₃ or CrCl₂(CH ₃)(THF)₃. Reaction of CrCl₃{κ ³-(P,P,O)-(PNP^OMe)} with o,o′-biphenyldiyl diGrignard affords CrBr(o,o′-biphenyldiyl){κ³-(P,P,O)- (PNP^OMe)}. Single-crystal X-ray diffraction studies show that the Cr-O and Cr-P distances can vary significantly as a function of metal oxidation state and the other ligands bound to chromium. Variable-temperature ²H NMR spectroscopy studies of chromium(III) complexes supported by PNP ligands indicate fluxional behavior with the ether groups interchanging at higher temperatures. Low-temperature ²H NMR spectra are consistent with solution structures similar to the ones determined in the solid state.

Full text of DOI:10.1021/om050605+

Organometallics 2006, 25, 2733-2742
2733
Articles
A Chromium-Diphosphine System for Catalytic Ethylene
Trimerization: Synthetic and Structural Studies of Chromium
Complexes with a Nitrogen-Bridged Diphosphine Ligand with
ortho-Methoxyaryl Substituents
Theodor Agapie, Michael W. Day, Lawrence M. Henling, Jay A. Labinger,* and
John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
ReceiVed July 19, 2005
To gain molecular-level insight into the important features of a chromium-diphosphine catalytic system
for ethylene trimerization, the coordination chemistry of chromium with “PNP” ligands (PNPOMe ) (o-
MeOC6H4)2PN(CH3)P(o-MeOC6H4)2, Pt-BuNi-amylPOMe ) (2-MeO-4-t-BuC6H3)2PN(i-amyl)P(2-MeO-4-
t-BuC6H3)2) has been explored. Chromium(0) carbonyl complexes have been synthesized by CO
displacement with diphosphine. Oxidation of Cr(CO)4{κ2-(P,P)-(PNPOMe)} with I2, Br2, and PhICl2
generates the corresponding chromium(III) halide complexes. Chromium(III) complexes CrCl3{(κ3-(P,P,O)-
(PNPOMe)}, CrCl3{κ3-(P,P,O)-(Pt-BuNi-amylPOMe)}, and CrCl2(CH3){κ3-(P,P,O)-(PNPOMe)} can be synthe-
sized by metalation with CrCl3(THF)3 or CrCl2(CH3)(THF)3. Reaction of CrCl3{κ3-(P,P,O)-(PNPOMe)}
with o,o′-biphenyldiyl diGrignard affords CrBr(o,o′-biphenyldiyl){κ3-(P,P,O)-(PNPOMe)}. Single-crystal
X-ray diffraction studies show that the Cr-O and Cr-P distances can vary significantly as a function of
metal oxidation state and the other ligands bound to chromium. Variable-temperature 2H NMR spectroscopy
studies of chromium(III) complexes supported by PNP ligands indicate fluxional behavior with the ether
groups interchanging at higher temperatures. Low-temperature 2H NMR spectra are consistent with solution
structures similar to the ones determined in the solid state.
Scheme 1
Introduction
The oligomerization of ethylene typically leads to a broad
range of R-olefins, requiring fractional distillation to isolate
individual R-olefins.1 There is increasing interest in developing
catalytic systems that give better selectivity toward desirable
alkenes. One of the most commercially useful olefins is
1-hexene, a comonomer for the synthesis of linear low-density
polyethylene (LLDPE). Several recent reports describe catalytic
systems that trimerize ethylene to 1-hexene with high selec-
tivity.2-16 Some ethylene trimerization catalysts are based on
titanium and tantalum,5,9,16 but the most numerous and successful
systems are based on chromium.2-4,6-8,10-14
(1) Skupinska, J. Chem. ReV. 1991, 91, 613.
(2) Briggs, J. R. Chem. Commun. 1989, 674.
(3) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm, S.; Wasserscheid,
P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 4337.
(4) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass,
D. F. Chem. Commun. 2002, 858.
(5) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A. J. Am.
Chem. Soc. 2001, 123, 7423.
(6) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert,
U.; Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334.
(7) Wu, F.-J. (Amoco Corporation) US 5,811,618, 1998.
(8) Reagen, W. K.; Pettijohn, T. M.; Freeman, J. W. (Philips Petroleum
Company) US 5,523,507, 1996.
A catalyst generated from CrCl3(THF)3, a diphosphine ligand
(1), and methaluminoxane (MAO) (eq 1) trimerizes ethylene
to 1-hexene with unprecedented selectivity (99.9% 1-hexene in
(11) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon,
J. T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc.
2003, 125, 5272.
(9) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002,
21, 5122.
(10) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.;
Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.;
Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem.
Soc. 2004, 14712.
(12) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F.;
Killian, E.; Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S. Chem.
Commun. 2005, 622.
(13) Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F. M.; Killian, E.;
Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M. J. Chem.
Commun. 2005, 620.
10.1021/om050605+ CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/28/2006

2734 Organometallics, Vol. 25, No. 11, 2006
Agapie et al.
Scheme 2
the hexenes fraction) and productivity (∼2 × 106 turnovers/
h).4 The effect of changing the diphosphine on trimerization
activity was investigated (Scheme 1); key features appear to be
the ortho-methoxy-substituted aryl groups and the PNP back-
bone. Changing the amide in the PNP backbone to a methylene
group (6) or replacing the ortho-methoxy groups with ethyl
groups (2) greatly diminishes the catalytic activity. Recent
studies involving a variety of related PNP ligands have shown
that trimerization and tetramerization of ethylene can be
achieved in the absence of ortho-donors, but these catalysts
operate at higher pressures of ethylene and with lower selectivity
for the terminal olefin in the corresponding Cn (n ) 6 or 8)
fraction.10,12,13
Scheme 3
mechanistic possibilities such as a Cossee-type chain growth
mechanism.23 Further mechanistic investigations and synthetic
work with related chromium complexes are expected to provide
additional understanding of the important features of the Cr/
PNP catalyst system.
We report herein the synthesis, characterization, and reactivity
of chromium carbonyl, halide, alkyl, and aryl complexes
supported by PNP ligands with ortho-methoxyaryl substituents.
Part of this work was communicated previously.23 The effects
of varying the nature of the backbone linker atom and the ortho-
substituents on the coordination chemistry of the diphosphine
to chromium, their effects on the catalytic system, and mecha-
nistic investigations of the ethylene trimerization and reactions
with other olefins will be reported separately. A second study
that describes closely analogous (PNP)chromium(III) phenyl
derivatives appears in the following paper.24
CrCl3(THF)3 + 1
300 equiv MAO, toluene8 1-hexene
ethylene
(1-20 bar)
(1)
Although details concerning the mechanisms of the trimer-
ization (or tetramerization) reaction are not yet clear, the most
popular proposed mechanism to account for the high selectivity
involves metallacyclic intermediates (Scheme 2).2,4,5,17 Initial
coordination of 2 equiv of ethylene to a ligated Crn species
followed by oxidative coupling forms a chromacyclopentane
of oxidation state Crn+2. The transition state for â-hydrogen
elimination from the chromacyclopentane leading to 1-butene
is expected to be rather strained; hence, ring expansion by
ethylene insertion dominates. The resulting chromacycloheptane
is flexible enough to undergo rapid â-hydrogen elimination,
giving a chromium-alkenyl-hydride species that reductively
eliminates 1-hexene to regenerate Crn and closes the catalytic
cycle. In agreement, Jolly and co-workers have reported well-
characterized chromacyclopentane and chromacycloheptane
complexes; the latter decomposes more readily and yields
1-hexene.18 More recently, it has been suggested that the release
of 1-hexene from the metallacycloheptane intermediate proceeds
via a concerted 3,7-hydrogen shift with formal two-electron
reduction of the metal.17,19-22 We recently reported experimental
support for the chromacyclic pathway and ruled out alternative
Results and Discussion
Synthesis of PNP Ligands. 2H NMR spectroscopy provides
a valuable handle for the study of paramagnetic compounds.25
Deuterium labeling can be achieved readily at the ortho-
methoxyaryl position by methylating the appropriate phenol with
CD3I (Scheme 3). Two routes have been used for the synthesis
of the diphosphine ligands (Schemes 3 and 4). For both routes
31P NMR spectroscopy is a useful tool for monitoring the
progress of the reaction. Scheme 3 outlines a linear protocol,
(14) McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J. T.
Organometallics 2005, 24, 552.
(15) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet.
Chem. 2004, 689, 3641.
(16) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem., Int.
Ed. 2001, 40, 2516.
(19) Blok, A. N. J.; Budzelaar, P. H. M.; Gal, A. W. Organometallics
2003, 22, 2564.
(20) de Bruin, T. J. M.; Magna, L.; Raybaud, P.; Toulhoat, H.
Organometallics 2003, 22, 3404.
(17) Yu, Z.-X.; Houk, K. N. Angew. Chem., Int. Ed. 2003, 42, 808.
(18) Emrich, R.; Heinemann, O.; Jolly, P. W.; Kruger, C.; Verhovnik,
G. P. J. Organometallics 1997, 16, 1511.
(21) Janse van Rensburg, W.; Grove, C.; Steynberg, J. P.; Stark, K. B.;
Huyser, J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207.
(22) Tobisch, S.; Ziegler, T. Organometallics 2003, 22, 5392.

Chromium-Diphosphine System for Ethylene Trimerization
Organometallics, Vol. 25, No. 11, 2006 2735
Scheme 4
smaller P-Cr-P angle (67.54(2)° vs 70.78(3)°) and a larger
P-N-P angle (101.24(7)° vs 95.74°) for P-C-P.32
Oxidation of Cr(CO)4{(P,P)-K2-(PNPOMe)}: Accessing
(PNP)chromium(III) Complexes. Considering that chromium-
(I)/chromium(III) is one of the possible redox couples perform-
ing the trimerization reaction (Scheme 2), chromium(III)- or
chromium(I)-PNP complexes could furnish useful information
regarding the relevant coordination chemistry of the catalyst
and could provide good synthetic entries for mechanistic studies.
The oxidation of 16 with C6H5ICl2, Br2, or I2 affords CrX3-
(PNPOMe) (eq 3). The products with X ) Cl (18) and I (20)
were structurally characterized (Figures 2 and 3), confirming
the structures shown with the PNP ligand bound in a κ3-(P,P,O)
fashion. While all the above products are paramagnetic (2H
NMR), diamagnetic byproducts are present as well in the crude
reaction mixture. Recrystallization from CH2Cl2/petroleum ether
mixture affords the desired chromium(III) products cleanly.
involving reaction of the Grignard reagent from deuterated
2-bromomethylphenyl ether (8) with PBr3 to give bromodi-
arylphosphine.26 Condensation with (CH3)NH3Cl in the presence
of excess tertiary amine provides the desired product. Even
though this route could provide clean product in 60-70% yields,
it was found that the last step was difficult to reproduce.
A convergent route was found to work well for the synthesis
of a variety of PNP ligands (Scheme 4). This synthetic protocol
takes advantage of the facile preparation of the PNP backbone
unit (12 or 13),27,28 which can be subsequently treated with a
Grignard of choice (prepared from bromides such as 8, 10, or
11) to give the desired diphosphines. Compound 14 was targeted
as a more hydrocarbon-soluble version of 9, as well as a system
for comparison. During the last step in the preparation of 14,
intermediates such as the partially arylated species (Ar2PN(R2)-
PArCl and ArClPN(R2)PArCl) were observed by 31P NMR
spectroscopy. It may be possible to use partially arylated species
to obtain asymmetric diphosphines with both ligating (such as
o-OMe) and nonligating (such as o-Et) aryl groups. An
investigation of such a mixed ligand may provide information
about the relation between the number of donors on the aryl
groups and the catalytic activity.
Synthesis of CrCl3{K3-(P,P,O)-(PNPOMe)} (18), CrCl3{κ3-
(P,P,O)-(Pt-BuNi-amylPOMe)} (21), and CrCl2(CH3){κ3-(P,P,O)-
(PNPOMe)} (22) from Chromium(III) Starting Materials.
Direct metallation with chromium(III) precursors provides an
alternative pathway for the synthesis of PNP-supported chro-
mium(III) complexes. Chromium(III) starting materials are
commercially available (CrCl3(THF)3) or easy to prepare (CrCl2-
(CH3)(THF)3),33 facilitating the access to the complexes and
their characterization (eq 4). Compounds 18, 21, and 22 were
prepared by displacement of THF with the diphosphine ligand
in CH2Cl2. Because coordinating solvents, such as THF, compete
with the diphosphine for coordination to chromium(III), their
Synthesis of Cr(CO)4{(P,P)-K2-(PNPOMe)} (16). Chromium-
(0) carbonyls provide a possible entry into chromium-diphos-
phine chemistry, and indeed, a chromium tetracarbonyl complex
(16) supported by diphosphine 9 can be prepared readily starting
from Cr(CO)6 (eq 2). Spectroscopic data are consistent with a
(P,P)-κ2 coordination mode, without involvement of the ether
groups. The average stretching frequency for the four CO normal
modes is 1916 cm-1 (CH2Cl2) in 16, about 10 cm-1 lower than
those reported for Cr(CO)4{(C6H5)2PCH2P(C6H5)2} or Cr(CO)4-
{(C6H5)2PCH2CH2P(C6H5)2} (∼1927 cm-1) (Table 1).29 On the
other hand, the average CO stretching frequency for the
tetracarbonyl complex (17) supported by the PNP phosphine
without ether substituents (15) is 1925 cm-1, very similar to
the values for (C6H5)2PCH2P(C6H5)2 and (C6H5)2PCH2CH2P-
(C6H5)2. This suggests that the presence of more electron-rich
aryl groups bearing ethers in 9 is the main reason for the greater
electron density at the metal center in 16.30 The amide in the
ligand backbone has a smaller effect.
2
use has been avoided or minimized. H NMR spectroscopy is
very useful in the characterization of the paramagnetic reaction
products. Broad peaks downfield from the diamagnetic region
indicate the formation of paramagnetic species.
Solution magnetic susceptibility measurements obtained by
the method of Evans34,35 for the isolated compounds are
(23) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 1304.
(24) Schofer, S. J.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. 2006, 25, 2743.
(25) La Mar, G. N.; Horrocks, W. D., Jr.; Holm, R. H. NMR of
Paramagnetic Molecules; Academic Press: New York, 1973.
(26) Dossett, S. J.; Gillon, A.; Orpen, A. G.; Fleming, J. S.; Pringle, P.
G.; Wass, D. F.; Jones, M. D. Chem. Commun. 2001, 699.
(27) King, R. B.; Gimeno, J. Inorg. Chem. 1978, 17, 2390.
(28) Nixon, J. F. J. Chem. Soc. A 1968, 2689.
(29) Gabelein, H.; Ellermann, J. J. Organomet. Chem. 1978, 156, 389.
(30) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(31) Bennett, M. J.; Cotton, F. A.; LaPrade, M. D. Acta Crystallogr.
1971, B27, 1899.
A single-crystal X-ray diffraction study confirmed the
spectroscopic assignment of 16 (Figure 1). The Cr-P bond
lengths are similar to the ones reported for analogous com-
pounds.31,32 Compared to Cr(CO)4(Me2PCH2PMe2), 16 has a

2736 Organometallics, Vol. 25, No. 11, 2006
Agapie et al.
Table 1. Carbonyl Stretching Frequencies for Selected Complexes
ν(CO) (CH2Cl2, cm-1
)
ν(CO)av (CH2Cl2, cm-1
)
Cr(CO)4{MeN(P(o-MeOC6H4)2)2}
Cr(CO)4{MeN(PPh2)2}
Cr(CO)4{CH2(PPh2)2}
2003, 1906, 1886, 1867
2008, 1917, 1895, ∼1881
2011, 1918, 1903, 1878
2011, 1917, 1903, 1878
1916
1925
1927
1927
Cr(CO)4{C2H4(PPh2)2}
consistent with a quartet ground state (µeff ) 3.8 µB for 18, 3.9
µB for 21, and 3.7 µB for 22). Compounds 18 (Figure 3), 21
(see Supporting Information), and 22 (Figure 4) were structurally
characterized by single-crystal X-ray diffraction; all display κ3-
(P,P,O) coordination of the diphosphine to yield a coordination
number of six around the metal center. In 22, the methyl group
coordinates trans to the oxygen, rather than to the phosphorus,
presumably due to a smaller trans influence for the ether donor.
Due to the presence of a few disordered solvent molecules in
the crystal, reliable structural parameters were not obtained for
21; however, connectivity similar to 18 was observed. The fact
that 18 and 21 have similar molecular structures may indicate
that making changes at the periphery of the ligand (i.e., replacing
a distal aryl-H with CMe3 or replacing the N-methyl with N-i-
amyl), while keeping the basic ligand framework constant (i.e.,
preserving the PNP backbone and o-ether-substituted aryls), does
not substantially alter the binding mode of the methoxy-
substituted diphosphine ligands. Comparisons among the struc-
tures of 18, 20, and 22 are presented in a later section.
Chromium Alkyls and Metallacyclopentanes as Entry
Points for Mechanistic Studies: Synthesis of CrBr(o,o′-
biphenyldiyl){K3-(P,P,O)-(PNPOMe)} (23). The proposed mech-
anism involves a chromacyclopentane intermediate;18,36 isolation
of such a complex could provide, upon activation, direct access
to models of the species in the proposed catalytic cycle. Attempts
to prepare analogues of metallacyclopentane have been suc-
cessful, probably because chelation makes the products stable
to side reactions. Compound 23 has been prepared from o,o′-
biphenyldiyl Grignard reagent and chromium trichloride 18 (eq
5). The solid-state structure of 23 (Figure 5) shows distorted
octahedral coordination with the biphenyldiyl group trans to
Figure 3. Structural drawing of 18 with displacement ellipsoids
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): Cr-P1 2.3855(7); Cr-P2 2.5098(7); Cr-O1 2.1562(15); Cr-
Cl1 2.2937(6); Cr-Cl2 2.2776(6); Cr-Cl3 2.3210(7); N-P1 1.6920-
(18); N-P2 1.6978(18); P2-Cr-P1 66.56(2); O1-Cr-Cl2 165.62-
(4); P2-N-P1 104.95(10).
Figure 1. Structural drawing of 16 with displacement ellipsoids
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): Cr-P1 2.3557(6); Cr-P2 2.3712(5); Cr-C2,C4 (av) 1.841;
Cr-C1,C3 (av) 1.875; C2-O2,C4-O4 (av) 1.164; C1-O1,C3-O3 (av)
1.152; N-P1 1.6946(14); N-P2 1.7046(14); P1-N-P2 101.24(7);
P1-Cr-P2 67.536(18).
Figure 2. Structural drawing of 20 with displacement ellipsoids
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): Cr-P1 2.3891(6); Cr-P2 2.5205(6); Cr-O1 2.1820(14); Cr-
I1 2.6604(3); Cr-I2 2.6765(3); Cr-I3 2.6481(3); N-P1 1.6990-
(16); N-P2 1.7020(16); P2-Cr-P1 66.825(18); O1-Cr-I3 163.77-
(4); P1-N-P2 105.42(9).
Figure 4. Structural drawing of 22 with displacement ellipsoids
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): Cr-P1 2.4010(10); Cr-P2 2.5204(10); Cr-O1 2.435(2); Cr-
C
30 2.061(4); Cr-Cl1 2.2939(10); Cr-Cl2 2.3011(9); N-P1 1.701-
(2); N-P2 1.691(3); P2-Cr-P1 66.54(3); O1-Cr-C30 166.44(15);
P1-N-P2 105.58(14).

Chromium-Diphosphine System for Ethylene Trimerization
Organometallics, Vol. 25, No. 11, 2006 2737
Table 2. Coalescence Temperatures for Compounds 18, 21,
22, and 23 in CH2Cl2
1st coalescence (°C)
2nd coalescence (°C)
18
21
22
23
-50
-5
<-80
-10
-10
18
40a
15
a In chlorobenzene.
the static solid-state structure. This suggests a dynamic process
that allows the exchange of ether groups, and a variable-
2
temperature H NMR study was therefore undertaken (Figure
6).
Compounds 18-23 display one peak in the 2H NMR spectra
at the fast exchange limit (temperatures higher than 40 °C).
Upon cooling, two decoalescence processes are apparent (Table
2). At the lowest temperatures one broad, presumably para-
magnetically shifted peak corresponding to one ether and one
(or two for 23) sharper peak(s) is observed (Figure 6). At
intermediate temperatures two peaks in a one-to-one ratio of
intensities are observed, one broader and more paramagnetically
downfield shifted (Figure 6, -25 °C spectrum and Figure 7).
The proposed processes that allow the stepwise, dynamic
exchange of the ether groups are shown in Schemes 5 and 6
for compounds 18 and 22. The lower barrier corresponds to
uncoordination of the ether group, allowing two sets of two
ether groups to interchange. The higher barrier corresponds to
Berry pseudorotations that exchange the “top” and “bottom”
of the molecule. In the case of complex 18, only one Berry
pseudorotation is necessary for this exchange (Scheme 5), while
complex 22 requires three such processes (Scheme 6). Thus, at
the highest temperatures, where this barrier is overcome, the
2H NMR spectra display only one peak corresponding to fast
interchange of the ether groups (Figure 6, 17 °C and 40 °C
spectra). Similar variable-temperature behavior was observed
for compounds 21 and 23, indicating that ether exchange occurs
in a variety of related complexes displaying different anionic
ligands and peripheral ligand framework. The overlap of the
noncoordinated ether signals even at low temperature (Figure
6, 60 °C spectrum) is probably due to smaller paramagnetic
shifts and, hence, a smaller difference in their chemical shifts
along with a broadening of the peaks due to the sample
paramagnetism and increased viscosity.
Quantifying the thermodynamic barriers, for example by
fitting the chemical shifts and/or line widths to predicted values,
was not attempted, due to paramagnetic contributions in
broadening and chemical shift. However, a qualitative analysis
of some of the factors that could influence those energies
suggests that the relative size of the barriers may correlate with
the differences in the coalescence temperatures. For instance,
comparing 18 and 22, the first barrier is expected to be lower
in 22 because of the stronger trans influence of the methyl group
(ground state destabilized). The second barrier is expected to
be higher in 22 because that process involves placing a methyl
rather than a chloride trans to a phosphine (intermediate
destabilized, more than the ground state).
Figure 5. Structural drawing of 23 with displacement ellipsoids
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): Cr-P1 2.6606(14); Cr-P2 2.4287(14); Cr-O1 2.3331(33);
Cr-C26 2.0361(51); Cr-C8 2.0611(49); Cr-Br 2.4811(9); N-P1
1.7055(40); N-P2 1.7049(38); P2-Cr-P1 64.97(4); C26-Cr-C8
82.04(21); P1-N-P2 106.93(19).
the weakly bound donors: the ether and the outside phosphine.
The structure of 23 will be discussed in further detail in a later
section. Bromide abstraction from 23 may be effected with Na-
[B(3,5-(CF3)2-C6H3)4] in order to access a cationic chromium
o,o′-biphenyldiyl complex. Although reaction occurred readily,
giving one major product as assessed by 2H NMR spectroscopy,
its structure has not yet been fully characterized.
The structures of 18, 21, and 22 show κ3-coordination mode
of the ligand. Since MAO is commonly used to generate a
cationic transition metal alkyl from halide precursors, the active
species in the proposed catalytic cycle (Scheme 2) may be a
cationic chromium(III) dialkyl complex with one or more
additional ether groups coordinated. Halide abstraction reactions
from 18 and 22 occur readily, even with mild reagents such as
Na[B(3,5-(CF3)2-C6H3)4], but the resulting species have not been
structurally characterized. Preparations of alkyl complexes,
starting from 18, 21, and 22 and alkylating agents such as
Grignard reagents and lithium, aluminum, and zinc alkyls, have
been attempted, but have been unsuccessful thus far. An
alternative route to (PNP)Cr-phenyl complexes is reported in
the following article.24
Ether Exchange Processes in Complexes Supported by
PNP Ligands. 2H NMR spectroscopy proved to be instrumental
in understanding the solution behavior of the (PNP)chromium-
(III) complexes. The number of peaks observed in the 2H NMR
spectra of all reported compounds is smaller than expected for
Structural Comparisons between Cr/PNPOMe Complexes.
In all the characterized compounds, the metal center is six-
coordinate. Moving from chromium(0) to chromium(III), the
Cr-P bond lengths increase significantly (Table 3). The same
general trend was found for other reported chromium(0) and
chromium(III) complexes.31,32,37-39 This may be due to a
(32) Jones, P. G.; Jager, S. Z. Kristallogr. 1997, 212, 85.
(33) Nishimura, K.; Kuribayashi, H.; Yamamoto, A.; Ikeida, S. J.
Organomet. Chem. 1972, 37, 317.
(34) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(35) Evans, D. F. J. Chem. Soc. 1959, 2003.
(36) Theopold, K. H. Eur. J. Inorg. Chem. 1998, 15.
(37) Arif, A. M.; Jones, R. A.; Hefner, J. G. J. Crystallogr. Spectrosc.
Res. 1986, 16, 673.

2738 Organometallics, Vol. 25, No. 11, 2006
Agapie et al.
2
Figure 6. Variable-temperature H NMR (CH2Cl2) spectroscopic study of 18.
replacement of a chloride (in 18) with a methyl (in 22) leads to
Cr-O bond elongation by almost 0.3 Å, likely due to a strong
trans influence from the methyl group. The effect on the Cr-P
bond lengths is small (<0.02 Å), suggesting that the PPO
framework is quite flexible, so that each donor is able to adjust
its distance to chromium without affecting the others signifi-
cantly. In 23, the presence of two different ligands (halide and
aryl) in the P-Cr-P plane causes a larger difference between
the two Cr-P bond lengths (∼0.23 Å). The Cr-Pb bond trans
to the aryl groups elongated to 2.6606(14) Å, the longest Cr-P
bond length to our knowledge.40 The relatively small increase
of the P-N-P angle and decrease of the P-Cr-P angle from
18 to 23 correlate with the increase in the Cr-P bond lengths,
probably to minimize the overall ring deformation. Since the
species involved in the catalytic cycle are presumably chromium
alkyls, it is important to note that the methyl in 22 prefers to
ligate trans to the ether, not to the phosphine, and that the o,o′-
biphenyldiyl in 23 prefers to ligate trans to a phosphine and an
ether rather than two phosphines.
Trimerization Trials with Well-Defined (PNP)chromium-
(III) Precursors. With (PNP)chromium(III) complexes in hand,
ethylene trimerization tests were performed. Chromium com-
plexes 18, 21, and 23 were activated with excess methyl
aluminoxane (MAO, 300 equiv) in toluene under ethylene to
generate systems active for catalytic ethylene trimerization (see
the Supporting Information). The catalysts generated under these
conditions are not stable catalyst systems, however, and ethylene
trimerization activity decreases to ∼10% of the initial value
within 30 min. Considering that the large excess of MAO
hinders the utilization of 1H NMR spectroscopy for mechanistic
studies, stoichiometric activators have been investigated as well.
It has been found that compound 23 can trimerize ethylene
catalytically upon activation with Na[B(3,5-(CF3)2-C6H3)4].
When 23 is exposed to ethylene without halide abstraction, no
1-hexene formation is observed (1H NMR). This may be due
2
Figure 7. Room-temperature H NMR (CH2Cl2) spectrum of 22.
mismatch between the hard chromium(III) center and the soft
phosphines, which counterbalances the contraction of the metal
in the higher oxidation state. The P-Cr-P and P-N-P angles
are each quite similar for the reported compounds; the most
notable difference is in the P-N-P angle for chromium(0)
versus chromium(III).
The two Cr-P bond lengths in all the chromium(III)
compounds are significantly different, with Cr-Pb (Pb corre-
sponds to the phosphine whose pendant ether coordinates to
the metal) being shorter due to oxygen chelation. In the
compounds containing homoleptic anionic ligand sets, the
difference between the two Cr-P bond lengths is quite constant,
between 0.12 (in 18) and 0.14 Å (in CrPh3(PNPOMe)).23 The
halide complexes show similar Cr-P as well as Cr-O bond
lengths. Compared to the halide complexes, CrPh3(PNPOMe
)
shows elongation of all the Cr-PNPOMe contacts, probably due
to an increased trans influence from the phenyl groups.
When the anionic ligand sets are heteroleptic, the differences
in structural parameters are more substantial. For instance,
(38) Gray, L. R.; Hale, A. L.; Levason, W.; McCullough, F. P.; Webster,
M. J. J. Chem. Soc., Dalton Trans. 1984, 47.
(39) Gardner, T. G.; Girolami, G. S. J. Chem. Soc., Dalton Trans. 1987,
1758.
(40) Cotton, F. A.; Duraj, S. A.; Powell, G. L.; Roth, W. J. Inorg. Chem.
Acta 1986, 113, 81.

Chromium-Diphosphine System for Ethylene Trimerization
Organometallics, Vol. 25, No. 11, 2006 2739
Scheme 5
Scheme 6
Table 3. Selected Structural Parameters for 16, 18, 20, Cr(PNPOMe)Ph3, 22, and 23
Cr-Pa (Å)a
Cr-Pb (Å)
Cr-O (Å)
P-N-P (deg)
P-Cr-P (deg)
16
18
20
2.3712(5)
2.5098(7)
2.5205(6)
2.6381
2.3557(6)
2.3855(7)
2.3891(6)
2.4971
101.24(7)
104.95(10)
105.42(9)
105.72
67.54(2)
66.56(2)
66.83(2)
64.02
2.1562(15)
2.1820(14)
2.286
CrPh3(PNPOMe
)
b
22
23
2.5204(10)
2.6606(14)
2.4010(10)
2.4287(14)
2.435(2)
2.3331(33)
105.58(14)
106.93(19)
66.54(3)
64.97(4)
a Pa corresponds to the outside phosphine (not chelated through an ether). b Average for the two molecules in the asymmetric unit.
to various factors including decreased electrophilicity or a
smaller number of coordination sites available to an incoming
olefin in the neutral complex.
Coordination of the pendant ether draws the corresponding
phosphorus donor ca. 0.12-0.23 Å closer to Cr. One of the
halide ligands is found to be labile in chromium(III)(PNPOMe
)
complexes, facilitating the generation of cationic species. This
has allowed the generation of a catalytic system for ethylene
trimerization based on a chromium(III)(PNPOMe) biphenyldiyl
complex upon activation with stoichiometric amounts of Na-
[B(3,5-(CF3)2-C6H3)4]. Activation of well-defined precursors
with excess MAO also generates systems active for catalytic
trimerization of ethylene. 2H NMR spectroscopy has been very
useful for the characterization of paramagnetic compounds.
Conclusions
A series of chromium(0) and chromium(III) complexes
supported by PNP ligands have been synthesized and character-
ized. It has been found that the PNP ligand can display κ2-
(P,P) and fac-κ3-(P,P,O) coordination modes. In all the cases
investigated, a coordination number of six is observed for the
chromium(III) center. The Cr-O bond length is very dependent
(2.16 to 2.44 Å) on the nature of the ligand trans to the ether.
2
Variable-temperature H NMR spectroscopy studies indicate

2740 Organometallics, Vol. 25, No. 11, 2006
Agapie et al.
and was collected by filtration. This procedure afforded 4.26 g (8
mmol, 85% yield) of spectroscopically pure desired product 9.
that, in solution, the ether groups are involved in a dynamic
exchange process.
(i-amyl)NPCl2 (12). Compound 12 was synthesized using the
published procedure used for 13.27,28 A mixture of i-amylamine (5
mL, 43 mmol, 1 equiv), PCl3 (15 mL, 172 mmol, 4 equiv), and
C2H2Cl4 (30 mL) was refluxed, under argon, in an oil bath
maintained at 150 °C. After 21 h the temperature was increased to
165 °C, and the mixture was refluxed for an additional 48 h. Then,
the reaction mixture was allowed to cool and the desired product
was collected by low-pressure distillation (60 °C, 0.03 Torr) as a
colorless liquid, analytically pure by NMR spectroscopy (6.12 g,
51% yield). 1H NMR (300 MHz, C6D6) δ: 0.73 (d, 6H, CH3), 1.31
(nonet, 1H, (CH3)2CHCH2), 1.64 (m, 2H, CHCH2), 3.56 (m, 2H,
NCH2). 13C NMR (75 MHz, C6D6) δ: 22.7, 27.3, 41, 47.7 (t, 2JCP
) 4.9 Hz, NCH2). 31P NMR (121 MHz, CDCl3) δ: 166.25.
Experimental Section
General Considerations. All air- and moisture-sensitive com-
pounds were manipulated using standard vacuum line, Schlenk, or
cannula techniques or in a drybox under a nitrogen atmosphere.
Solvents for air- and moisture-sensitive reactions were dried over
sodium benzophenone ketyl or calcium hydride, or by the method
33
of Grubbs.41 Compound 1327,28 and CrCl2(CH3)(THF)3 were
prepared as described previously. Dichloromethane-d2 was pur-
chased from Cambridge Isotopes and distilled from calcium hydride.
Other materials were used as received. Methylaluminumoxane was
purchased from Aldrich. UV-vis measurements were taken on a
Hewlett-Packard 8452A diode array spectrometer using a quartz
crystal cell. Elemental analyses were performed by Desert Analytics,
(2-MeO-4-t-BuC6H3)2PN(i-amyl)P(2-MeO-4-t-BuC6H3)2, Pt-Bu
-
Ni-amylPOMe (14). Procedure 2 was employed: 38% isolated yield.
1H NMR (300 MHz, C6D6) δ: 0.59 (d, 6H, CH(CH3)2), 1.75 (br
m, 2H, CHCH2), 1.23 (s, 36H, C(CH3)3), 3.75 (br m, 2H, NCH2),
6.58 and 7.23 (app d, 8H, aryl-H), 7.76 (app s, aryl-H). 13C NMR
(75 MHz, C6D6) δ: 23.4, 27.8, 32.1, 34.7, 41.2 (t, 3JCP ) 3.9 Hz,
1
Tucson, AZ, and by Midwest Microlab, Indianapolis, IN. H and
13C NMR spectra were recorded on Varian Mercury 300 or Varian
INOVA-500 spectrometers at room temperature, unless indicated
otherwise. Chemical shifts are reported with respect to internal
solvent: 5.32 (t) ppm and 54.00 (t) ppm (CD2Cl2); 7.27 ppm (s)
2
NCH2CH2), 53.9 (t, JCP ) 11.1 Hz, NCH2), 110.7, 126.8, 129.5
1
2
and 77.23 ppm (t) (CDCl3) for H and 13C data. H NMR spectra
were recorded on a Varian INOVA-500 spectrometer; the chemical
shifts are reported with respect to an external D2O reference (4.8
ppm). 31P chemical shifts are reported with respect to an external
H3PO4 85% reference (0 ppm).
(t), 132.1 (t), 142.9, 159.9 (t). 31P NMR (121 MHz, C6D6) δ: 44.7.
If the last step in the preparation of 14 is performed at room
temperature, intermediates are observed by 31P NMR spectroscopy
(121 MHz, CDCl3) δ: 138 (s, ArClPN(i-amyl)PArCl); 54 (d, 2JPP
) 330 Hz, Ar2PN(i-amyl)PArCl), 141 (d, Ar2PN(i-amyl)PArCl).
Diphosphine Synthesis: Procedure 1.42 Compound 8 (8.36 g,
44 mmol, 4 equiv) in tetrahydrofuran (THF, 100 mL) was added
to magnesium turnings (1.32 g, 55 mmol, 5 equiv) using a pressure-
equalizing funnel. The reaction mixture was stirred at 40 °C for
∼12 h. After cooling to room temperature, excess Mg was removed
by filtration and the filtrate was added to a cold (-78 °C) solution
of PBr3 (2.09 mL, 22 mmol, 2 equiv) in THF (total volume ∼300
mL). After stirring for 1 h at low temperature, the mixture was
allowed to reach room temperature and stir for 6 h (31P NMR (121
MHz) of PAr2Br δ: 63.2). NEt3 (17 mL) was vacuum transferred
to the reaction mixture followed by solid MeNH3Cl (0.743 g, 11
mmol, 1 equiv). The resulting mixture was stirred at room
temperature overnight. Volatiles were removed under vacuum, and
MeOH (70 mL) was added. The resulting slurry was cannula
transferred to a sintered glass funnel, and the desired product 9
Cr(CO)4(PNPOMe) (16). A toluene solution (30 mL) of Cr(CO)6
(0.468 g, 2.12 mmol, 1 equiv) and PNPOMe (1.13 g, 2.12 mmol, 1
equiv) was stirred at 110 °C in a sealed Schlenk tube for 36 h. The
color of the solution gradually changed from colorless to bright
yellow. Volatile materials were removed in vacuo. The yellow
residue was dissolved in CH2Cl2 and filtered to remove a brown
impurity. Upon cooling to -35 °C crystallization occurred. The
mother liquor was decanted, and the residue was dried in vacuo to
provide 1.166 g (1.68 mmol, 80% yield) of crystalline yellow
product in two crops. 1H NMR (300 MHz, CDCl3) δ: 2.86 (t, 3JHP
) 8.7 Hz, 3H, NCH3), 3.47 (s, 12H, OCH3), 6.85 and 7.65 (app
dd, 8H, 3- and 6-aryl-H), 7.02 and 7.40 (app td, 8H, 4- and 5-aryl-
H). 13C NMR (75 MHz, CDCl3) δ: 37.1 (t, 2JCP ) 6.8 Hz, NCH3),
55.1 (s, OCH3), 110, (s, aryl), 120.5 (app t, aryl), 125.8 (app t,
aryl), 131.8 (s, aryl), 133.2 (app t, aryl), 160.0 (s, aryl), 222.4 (t,
2JCP ) 13.4 Hz, CO), 229.8 (app t, 2JCP ) 9.1 Hz, CO). 31P NMR
(121 MHz, CDCl3) δ: 102.6. νCO (cm-1, KBr plates, CH2Cl2
solution): 1867, 1886, 1906, 2002. Anal. Calcd for C33H31NO8P2-
Cr‚CH2Cl2 (%): C, 53.14; H, 4.29; N, 1.82. Found: C, 53.64; H,
4.65; N, 1.74.
1
was collected by filtration (3.7 g, 7 mmol, 63% yield). H NMR
(300 MHz, CDCl3) δ: 2.43 (t, 3JHP ) 3.3 Hz, 3H, NCH3), 3.61 (s,
12H, OCH3, unlabeled version), 6.86 (app t, 8H, aryl-H), 7.07-
7.11 (m, 4H, aryl-H), 7.31 (td, 4H, aryl-H). 13C NMR (75 MHz,
CDCl3) δ: 33.9 (t, 2JCP ) 5.7 Hz, NCH3), 55.2 (OCH3), 110, 120.1,
127.0 (t), 130, 133 (t), 160.7 (t). 31P NMR (121 MHz, CDCl3) δ:
52.2.
Cr(CO)4{NMe(PPh2)2} (17). A procedure similar to the one
used for the preparation of 16 was employed. Yield: 54%. 1H NMR
(300 MHz, C6D6) δ: 2.32 (t, 3JHP ) 8.4 Hz, 3H, NCH3), 7.02 and
7.47 (m, 20H, aryl-H). 13C NMR (75 MHz, C6D6) δ: 33.8 (t, 2JCP
) 6.1 Hz, NCH3), 129.2 (t, aryl), 131.1 (s, aryl), 132.4 (t, aryl),
Diphosphine Synthesis: Procedure 2. Compound 8 (7.14 g,
37.6 mmol, 4 equiv) was added via syringe to a mixture of
magnesium turnings (1.21 g, 50 mmol, 5.2 equiv) and tetrahydro-
furan (40 mL). The reaction mixture was stirred at 50 °C for ∼12
h. After cooling to room temperature, excess Mg was removed by
filtration. The filtrate was added to a THF solution of 13 (2.19 g,
9.4 mmol, 1 equiv). The transfer was completed with the aid of
some THF (total solution volume ∼250 mL). The reaction mixture
was stirred at 60 °C for ∼18 h. The reaction mixture was quenched
with water and extracted with CH2Cl2. The combined organic
fractions were dried over MgSO4 and then filtered. Volatile
materials were removed via rotary evaporation. The residue was
dissolved in CH2Cl2, and half a volume of methanol was added.
Upon concentration under vacuum, a white solid precipitated out
137.3 (app t, aryl), 223.1 (t, 2JCP ) 12.9 Hz, CO), 229.1 (t, 2JCP
)
9.5 Hz, CO). 31P NMR (121 MHz, C6D6) δ: 114. νCO (cm-1, KBr
plates, CH2Cl2 solution): ∼1881 (shoulder), 1895, 1917, 2008.
Anal. Calcd for C29H25NO4P2Cr (%): C, 61.82; H, 4.11; N, 2.49.
Found: C, 62.79; H, 4.30; N, 2.39.
Oxidation of 16: General Procedure. Dichloromethane solu-
tions of oxidant (I2, Br2, or C6H5ICl2, 1-1.6 equiv) were added
dropwise to dichloromethane solutions of yellow 16 (1 equiv) and
stirred from 15 min to 6 h. The color of the reaction mixture
changed upon addition to bright green (for Br2) or dark blue
(PhICl2). When the oxidation was performed with I2 (red), the color
of the mixture changed slightly, to red-brown. In some cases gas
(41) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(42) Cooley, N. A.; Green, S. M.; Wass, D. F.; Heslop, K.; Orpen, A.
G.; Pringle, P. G. Organometallics 2001, 20, 4769.
2
evolution was observed. Generally, the H NMR spectra of the
reaction mixtures display a broad peak downfield from the
diamagnetic region and some diamagnetic peaks. Recrystallization

Chromium-Diphosphine System for Ethylene Trimerization
Organometallics, Vol. 25, No. 11, 2006 2741
Table 4. Crystal and Refinement Data for Complexes 16, 18, 20, 22, and 23
16
18
20
22
23
empirical formula
fw
C
33H31NO8P2Cr‚C4H8O C29H31Cl3NO4P2Cr‚CH2Cl2
C
29H31NO4P2I3Cr‚CH2Cl2
C
30H34Cl2NO4P2Cr‚CH2Cl2
C
41H39NO4P2BrCr‚CH2Cl2
755.63
762.76
1037.11
742.35
888.51
T (K)
98(2)
98(2)
98(2)
100(2)
98(2)
a, Å
b, Å
c, Å
11.7105(5)
16.3059(8)
19.3321(9)
10.6977(7)
15.0041(10)
21.4755(14)
13.5277(3)
13.5151(3)
20.3801(5)
9.7683(6)
16.6273(10)
20.9934(13)
9.9167(6)
11.9935(8)
17.9725(11)
77.371(1)
76.328(1)
70.670(1)
1936.8(2)
2
R, deg
â, deg
102.333(1)
96.172(1)
107.892(1)
94.990(1)°
γ, deg
volume, Å3
Z
3606.3(3)
4
3427.0(4)
4
3545.85(14)
4
3396.8(4)
4
cryst syst
space group
d
monoclinic
P21/n (# 14)
1.392
monoclinic
P21/n (# 14)
1.478
monoclinic
P21/n (# 14)
1.943
monoclinic
P21/c (# 14)
1.452
triclinic
P1h (# 2)
1.524
calc, g/cm3
θ range, deg
µ, mm-1
abs corr
GOF
1.65 to 28.43
0.462
none
1.66 to 28.17
0.853
none
1.579
0.0370, 0.0621
1.61 to 35.00
3.211
none
1.405
0.0335, 0.0499
1.56 to 28.33
0.782
none
2.760
0.0618, 0.0988
1.82 to 28.29
1.592
none
1.731
0.0416, 0.0744
1.755
R1,a wR2b [I > 2σ(I)] 0.0394, 0.0620
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]1/2
.
from CH2Cl2 at -35° C followed by collection on a sintered glass
funnel provided pure products according to 2H NMR spectroscopy.
Compound 19: 50% yield. 2H NMR (76 MHz, CH2Cl2) δ: 9.6
(br s, OCD3). µeff ) 3.6 µB. λmax (CH2Cl2, nm): ∼550 (br shoulder,
ꢀ ) 361 M-1 cm-1), 665 (ꢀ ) 681 M-1 cm-1). Anal. Calcd for
C29H19D12Br3NO4P2Cr‚CH2Cl2 (%): C, 39.67; H, 3.63; N, 1.54.
Found: C, 40.11; H, 3.82; N, 2.21.
Compound 20: 37% yield. 2H NMR (76 MHz, CH2Cl2) δ: 9.0
(br s, OCD3). µeff ) 3.6 µB. λmax (CH2Cl2, nm): 381 (shoulder, ꢀ )
8100 M-1 cm-1), 679 (ꢀ ) 953 M-1 cm-1). Anal. Calcd for
C29H19D12I3NO4P2Cr‚CH2Cl2 (%): C, 34.34; H, 3.14; N, 1.33.
Found: C, 34.57; H, 3.42; N, 1.33.
petroleum ether. Storing at -35 °C caused product crystallization
as dark brown needles. The mother liquor was decanted, and the
crystals were washed with a cold CH2Cl2/petroleum ether mixture.
This procedure afforded 109 mg (82% yield) of desired product.
2H NMR (76 MHz, CH2Cl2) δ: 6.3 (br s, 6D, OCD3), 13.6 (br s,
6D, OCD3); at -90 °C: 6.9 (br s), ∼25 (v br s); at 71° C (PhCl):
7.3 (br s). Coalescence temperatures (°C): r80, 40. µeff ) 3.7 µB.
λ
max (CH2Cl2, nm): 462 (ꢀ ) 495 M-1 cm-1), 495 ((ꢀ ) 379 M-1
cm-1) shoulder), 700 (ꢀ ) 261 M-1 cm-1). Anal. Calcd for
C30H22D12Cl2NO4P2Cr‚CH2Cl2 (%): C, 49.37; H, 4.77; N, 1.85.
Found: C, 48.98; H, 5.13; N, 1.76.
CrBr(o,o′-biphenyldiyl)(PNPOMe) (23). Magnesium turnings
(12.3 mg, 1.4 mmol, 8 equiv) were added to a diethyl ether solution
(10 mL) of o,o′-dibromobiphenyl (53.5 mg, 0.17 mmol, 1 equiv)
and stirred overnight at room temperature or with heating. The
solution was decanted and concentrated to approximately 5 mL to
cause precipitation of a white solid, which was dissolved by addition
of dichloromethane. The resulting solution was cooled to almost
freezing, then added to a thawing dichloromethane solution (20
mL) of 18 (0.1183 mg, 0.17 mmol, 1 equiv). The color of the
reaction mixture gradually changed from dark blue to forest green
upon warming. The reaction mixture was stirred for 2 h, then
volatile materials were removed in vacuo. Dichloromethane was
added to the green residue and the mixture filtered through Celite
to remove a brown solid. The green filtrate was concentrated,
layered with petroleum ether, and cooled to -35 °C to cause
crystallization of 23 as a green material. The green solid was
collected on a sintered glass frit, washed with cold dichloromethane,
and dried under vacuum to provide 104.2 mg of desired product
(0.12 mmol, 75% yield). In cases when a white powder (probably
magnesium salts) precipitated out along with the desired product,
an additional recrystallization provided clean product. 2H NMR (76
MHz, CH2Cl2) δ: 5.8 (br s, OCD3), 9 (v br s, OCD3); at 45 °C:
7.3 (br s); at -87 °C: 2.7 (br s), 5.7 (br s), 29.2 (v br s).
Coalescence temperatures (°C): -10, 15. µeff ) 3.8 µB. λmax (CH2-
Cl2, nm): 382 (ꢀ ) 1937 M-1 cm-1), 449 (ꢀ ) 809 M-1 cm-1),
598 (ꢀ ) 364 M-1 cm-1). Anal. Calcd for C41H39NO4P2BrCr (%):
C, 61.28; H, 4.89; N, 1.74. Found: C, 61.38; H, 4.93; N, 2.10.
CrCl3(PNPOMe) (18). A dichloromethane solution (20 mL) of
CrCl3(THF)3 (1.513 g, 2.9 mmol, 1 equiv) was added to a solution
of 1 (1.098 g, 2.9 mmol, 1 equiv) in dichloromethane (30 mL).
The color of the reaction mixture turned from purple to blue within
5 min of stirring. The reaction mixture was stirred for 1 h. Volatile
materials were removed in vacuo, and the blue residue was triturated
three times with dichloromethane. The resulting solid was suspended
in dichloromethane and stored at -35° C overnight. The desired
product was collected as a bright blue powder by filtration through
a sintered glass frit, washed with dichloromethane, and dried under
vacuum. The filtrate contained one or two unidentified paramagnetic
2
species, displaying peaks at 4.85 and 9.5 ppm in the H NMR
spectrum. Compound 18 obtained in this manner amounted to 1.188
2
g (53% yield). H NMR (76 MHz, CH2Cl2) δ: 8.8 (br s, OCD3);
at 40 °C: 8.4 (br s); at -60 °C: 3.8 (br s), 32 (v br s). Coalescence
temperatures (°C): -50, -10. µeff ) 3.8 µB. λmax (CH2Cl2, nm):
536 (ꢀ ) 269 M-1 cm-1), 661 (ꢀ ) 484 M-1 cm-1). Anal. Calcd
for C29H31Cl3NO4P2Cr‚CH2Cl2 (%): C, 47.23; H, 4.32; N, 1.84.
Found: C, 46.24; H, 4.29; N, 1.77.
CrCl3(Pt-BuNi-amylPOMe) (21). A procedure similar to the one
used for the preparation of 18 was employed. Recrystallization from
a CH2Cl2 solution layered with petroleum ether afforded the desired
product as a purple powder in 75% yield. 2H NMR (76 MHz, CH2-
Cl2) δ: 7 (v br, OCD3); at 40 °C: 7.1 (br s); at -90 °C: 4.4 (br
s), 28 (v br s). Coalescence temperatures (°C): -5, 18. µeff ) 3.9
µB. λmax (CH2Cl2, nm): 517 (ꢀ ) 250 M-1 cm-1), 663 (ꢀ ) 498
M-1 cm-1). Anal. Calcd for C49H59D12Cl3NO4P2Cr‚CH2Cl2 (%):
C, 59.35; H, 7.21; N, 1.38. Found: C, 59.76; H, 7.63; N, 1.34.
CrCl2(CH3)(PNPOMe) (22). A dichloromethane solution of
PNPOMe (105 mg, 0.20 mmol, 1 equiv) was added to a dichlo-
romethane solution of CrCl2(CH3)(THF)3 (70 mg, 0.20 mmol, 1
equiv) to generate a brown-green solution. The reaction mixture
was stirred for 10 min, and then the volatiles were removed under
vacuum. Upon concentration, the color of the solution turned olive
green. The residue was dissolved in CH2Cl2 and layered with
Trimerization Trials with MAO Activation. A toluene solution
(30 mL, 4 × 10-4 M) of a well-defined chromium(III)(PNP)
complex was prepared. The mixture was cooled to -78 °C in a
dry ice/acetone bath and degassed, then allowed to warm to room
temperature. The solution was magnetically stirred under ethylene,
and MAO (10% in toluene, 300 equiv) was added via syringe.
Ethylene consumption was monitored by noting the decrease in
pressure in the system over time (the pressure was kept between
771 and 655 Torr), and 1-hexene formation was documented by

2742 Organometallics, Vol. 25, No. 11, 2006
Agapie et al.
GC and GC-MS. For 1 h runs, the reaction produces 1-hexene with
a range of 1900-2800 turnovers in greater than 80% overall
selectivity for 18; 300-600 turnovers in greater than 90% overall
selectivity for 23; and 1300-2000 turnovers in greater than 70%
overall selectivity for 21. The C10 fraction is the major impurity
detectable.
additional 1 h of mixing the mixture turned brown-red, but no
1-hexene was formed according to H NMR spectroscopy.
1
X-ray Crystal Data: General Procedure. Crystals grown from
THF (16) or CH2Cl2/petroleum ether (18, 20, 22, and 23) at -35
°C were removed quickly from a scintillation vial to a microscope
slide coated with Paratone N oil. Samples were selected and
mounted on a glass fiber with Paratone N oil. Data collection was
carried out on a Bruker Smart 1000 CCD diffractometer. The
structures were solved by direct (16, 18, 22, and 23) or Patterson
methods (20) (SHELXTL-97, Sheldrick, 1990) in conjunction with
standard difference Fourier techniques. All non-hydrogen atoms
were refined anisotropically. Some details regarding refined data
and cell parameters are available in Table 4. Selected bond distances
and angles are supplied in Table 3 and in the captions of Figures
1, 2, 3, 4, and 5.
Trimerization of Ethylene with 23 upon Halide Abstraction.
Dichloromethane-d2 was vacuum transferred to a J-Young tube or
Schlenk flask charged with 23 (8-34 mg, 10-42 µmol, 1 equiv)
and NaB[C6H3(CF3)2]4 (10.5-45 mg, 12-51 µmol, 1.2 equiv). The
mixture was warmed to room temperature using a water bath
followed by mixing (via mechanical rotation for NMR tubes or
magnetic stirring for flasks) for 10 min. The mixture turned brown
as the starting materials dissolved. Ethylene (128.2 mL at 30-125
Torr, 200-860 µmol, 17.5-23 equiv) was condensed in (∼2.3-
3.8 atm in the vessel at room temperature). The reaction mixture
was mixed for 1-1.5 h at room temperature, during which the
mixture turned brown-green. o-Vinyl-biphenyl and 1-hexene were
detected by 1H NMR spectroscopy. Relative to the amount of
o-vinyl-biphenyl observed, about 3.5 equiv of 1-hexene is formed
(ca. 60%).
Reaction of 23 with Ethylene. Dichloromethane-d2 was vacuum
transferred to a J-Young tube charged with 23 (8.1 mg, 10.1 mmol,
1 equiv). Ethylene (43.48 mL at 87 Torr, 200 mmol, 20 equiv)
was condensed in (∼2 atm in the tube at room temperature). The
mixture was warmed to room temperature using a water bath, then
mixed by mechanical rotation for 1 h. During this time the mixture
achieved a brown-green color. o-Vinyl-biphenyl was detected by
1H NMR spectroscopy, but no 1-hexene was observed. After an
Acknowledgment. We thank Dr. Steven A. Cohen for
helpful discussions. We are grateful to BP Chemicals (now
Innovene) for financial support.
Supporting Information Available: Tables of bond lengths,
angles, and anisotropic displacement parameters for 16, 20, and
22 (corresponding tables for 18 and 23 were reported previously).23
Drawing of the solid-state structure of 21. Plots of ethylene
consumption over time with 18, 21, and 23 activated with MAO
(PDF). X-ray crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
OM050605+