Unusual ligand transformation mediated by chromium(III): Hydrolytic disintegration of a [PNP] hybrid ligand with CH3CN insertion

Article abstract of DOI:10.1021/om800353a

Concomitant reaction among coordinated CH₃CN and a bidentate [P(N)P] aminodiphosphine ligand in [Ph₂PN(R)PPh₂]CrCl ₃(CH₃CN) (R = Cy, Ph) with H₂O results in a tridentate [ONO] amino-α-diphosphoryl ligand in [Ph₂P(O) C(CH₃)(NH₂)P(O)Ph₂]CrCl₃. The latter releases the free ligand upon treatment with excess water or gives a cationic complex, {[Ph₂P(O)C(CH₃)(NH₂)P(O)Ph ₂]CrCl₂[P(O)(H)Ph₂]}⁺SbF ₆^-, when treated with AgSbF₆. All the new compounds have been crystallographically established. The catalytic activities of these complexes toward ethylene oligomerization and polymerization are described and compared. 2008 American Chemical Society.

Full text of DOI:10.1021/om800353a

4188
Organometallics 2008, 27, 4188–4192
Unusual Ligand Transformation Mediated by Chromium(III):
Hydrolytic Disintegration of a [PNP] Hybrid Ligand with CH3CN
Insertion
Shihui Teo, Zhiqiang Weng,* and T. S. Andy Hor*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Kent Ridge, Singapore 117543
ReceiVed April 20, 2008
Concomitant reaction among coordinated CH3CN and a bidentate [P(N)P] aminodiphosphine ligand
in [Ph2PN(R)PPh2]CrCl3(CH3CN) (R ) Cy, Ph) with H2O results in a tridentate [ONO] amino-R-
diphosphoryl ligand in [Ph2P(O)C(CH3)(NH2)P(O)Ph2]CrCl3. The latter releases the free ligand upon treatment
with excess water or gives a cationic complex, {[Ph2P(O)C(CH3)(NH2)P(O)Ph2]CrCl2[P(O)(H)Ph2]}+SbF6-,
when treated with AgSbF6. All the new compounds have been crystallographically established. The catalytic
activities of these complexes toward ethylene oligomerization and polymerization are described and
compared.
additional stabilization to the unsaturated intermediate.5 Our
Introduction
recent isolation of CrCl3(CH3CN)(PNP),6 which catalyzes
Bi- and multidentate hybrid ligands are ligands of choice in
Cr(III)-catalyzed ethylene oligomerization.1 The aminodiphos-
phines RN[(CH2)nPR′2]2 (n g 0) (PNP) are typical examples
of such ligands.2 They offer excellent support to the metal center
by balancing the key chemical stability, catalytic activity, and
site selectivity.3 Their uses also allow modification of the donor
characteristics, ligand substituents, and skeletal backbone.
Modification of such hybrid ligands provides a systematic means
to tune the electronic and steric features of the precatalyst. This
approach to catalyst design is complemented by ongoing
vigorous mechanistic research in olefin oligomerization. Current
knowledge points to a metallacyclic pathway involving coor-
dinatively unsaturated intermediates and perhaps a reversible
Cr(I)/Cr(III) process.4 If the hybrid character of the multidentate
ligand is expressed in terms of a dynamic formation and
cleavage of the M-L bond, the resultant hemilability could offer
ethylene polymerization, is an illustration of this model. It is
formed directly from ligand addition to CrCl3(THF)3 possibly
via a saturated dinuclear species Cr2Cl4(µ-Cl)2(PNP)2.3c This
mononuclear CrCl3(CH3CN)(PNP) is a 15-electron complex and
would be 13-electron, five-coordinate, and heavily unsaturated
without the solvate (CH3CN) or ligation from the internal donor
(i.e., N) of the hybrid ligand. Coordination of CH3CN therefore
provides a simple model for the access of a range of potentially
unsaturated and catalytically active Cr(III) complexes supported
by the hybrid ligand. This type of catalyst in olefin oligomer-
ization and polymerization has many advantages. We herein
report an unexpected complication that could lead to catalyst
degradation. It arises from the unusual metal-mediated ligand
interaction between the PNP ligand and solvate, viz., CH3CN.
This results in an hitherto unknown ligand transformation and
formation of a new R-aminophosphine oxide type of ligand,7
thus turning a PNP into an ONO-type of ligand in a single step.
To our knowledge, such interligand interaction and transforma-
tions are unprecedented.
* Corresponding author. E-mail: chmwz@nus.edu.sg and andyhor@
nus.edu.sg.
(1) (a) Junges, F.; Kuhn, M. C. A.; Santos, A. H. D. P. dos; Rabello,
C. R. K.; Thomas, C. M.; Carpentier, J.-F.; Casagrandre, O. L., Jr.
Organometallics 2007, 26, 4010. (b) McGuinness, D. S.; Rucklidge, A. J.;
Tooze, R. P.; Slawin, A. M. Z. Organometallics 2007, 26, 2561. (c) Jabri,
A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R. Organo-
metallics 2006, 25, 715.
(2) (a) Hierso, J.-C.; Amardeil, R.; Bentabet, E.; Broussier, R.; Gauth-
eron, P.; Meunier, P.; Kalck, P. Coord. Chem. ReV. 2003, 236, 143. (b)
Zubiri, M. R. I.; Woollins, J. D. Comments Inorg. Chem. 2003, 24, 189.
(c) Cotton, F. A.; Hong, B. Prog. Inorg. Chem. 1992, 40, 179. (d) Mayer,
H. A.; Kaska, W. C. Chem. ReV. 1994, 94, 1239. (e) Jiang, T.; Zhang, S.;
Jiang, X.; Yang, C.; Niu, B.; Ning, Y. J. Mol. Catal. A: Chem. 2008, 279,
90.
Results and Discussion
Synthesis and Structures. The PNP ligands with Cr precur-
sors and their related dinuclear Cr(III) PNP complexes {[Ph2PN
(R)PPh2]CrCl2(µ2-Cl)]}2 (1) developed by Bollmann et al.3a are
active ethylene tetramerization catalysts. In a coordinating
solvent such as CH3CN, the Cr(III) complex 1 dissolves but
also undergoes Cr-Cl-Cr bridge cleavage to form a solvate-
(3) (a) 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, 126, 14712. (b) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon,
J. T.; Haasbroek, D.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan,
D. H. J. Am. Chem. Soc. 2005, 127, 10723. (c) Elowe, P. R.; McCann, C.;
Pringle, P. G.; Spitzmesser, S. K.; Bercaw, J. E. Organometallics 2006,
25, 5255. (d) 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. (e) Overett, M.; Blann, K.; Bollmann, A.;
Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.; Morgan, D. H.;
Neveling, A.; Otto, S. Chem. Commun. 2005, 622.
(5) (a) Weng, Z.; Teo, S.; Hor, T. S. A. Acc. Chem. Res. 2007, 40, 676.
(b) Weng, Z.; Teo, S.; Hor, T. S. A. Organometallics 2006, 25, 4878. (c)
Weng, Z.; Teo, S.; Koh, L. L.; Hor, T. S. A. Angew. Chem., Int. Ed. 2005,
44, 7560.
(6) Weng, Z.; Teo, S.; Hor, T. S. A. Dalton. Trans. 2007, 3493.
(7) (a) Santos, J. M.; de los Ignacio, R.; Aparicio, D.; Palacios, F. J.
Org. Chem. 2007, 72, 5202. (b) Bhagat, S.; Chakraborti, A. K. J. Org.
Chem. 2007, 72, 1263. (c) Nemoto, T; Fukuyama, T.; Yamamoto, E.;
Tamura, S.; Fukuda, T.; Matsumoto, T.; Akimoto, Y.; Hamada, Y. Org.
Lett. 2007, 9, 927. (d) Oliana, M.; King, F.; Horton, P. N.; Hursthouse,
M. B.; Hii, K. K. J. Org. Chem. 2006, 71, 2472. (e) Rahman, M. S.; Oliana,
M.; Hii, K. K. Tetrahedron: Asymmetry, 2004, 15, 1835.
(4) Wass, D. F. Dalton Trans. 2007, 816.
10.1021/om800353a CCC: $40.75
2008 American Chemical Society
Publication on Web 07/24/2008

Unusual Ligand Transformation
Scheme 1
Organometallics, Vol. 27, No. 16, 2008 4189
Figure 2. ORTEP diagram of complex 3 with thermal ellipsoids
at the 40% probability level.
of νN-H at 3082 and 3058 cm-1 is in agreement with the
formation of a new -NH2 function.
Structural confirmation of complex 3 comes from single-
crystal X-ray diffraction studies (Figure. 2 and Table 1). It has
a distorted octahedral geometry with a fac-[CrCl3] core com-
pleted by [ONO] tridentate coordination. The Cr-O lengths
(2.003(6) and 2.039(6) Å) are significantly shorter than that of
the related PNP complex CrCl3{[η3-(P,P,O)-(PNPOMe)]} with
an “agostic” Cr-O interaction (2.1562(15) Å).8 The Cr-N bond
(2.141(7) Å) is comparable to those in the trimerization catalysts,
viz., CrCl3[HN(CH2CH2PPh2)2] (2.137(3) Å)9 and CrCl3[HN(CH2
CH2SEt)2] (2.1059(18) Å).10 This hybrid tridentate [ONO]
ligand coordinates to a fac-Cr(III) core with two fused five-
membered rings such that two phenyl groups across the two
rings are oriented face-to-face with some degree of π · · · π
stacking (interplanar distance ) 3.52 Å).
In a single reaction, a bidentate [P(N)P] aminodiphosphine
remarkably transforms to a tridentate [ONO] amino-R-diphos-
phoryl ligand. This is driven by the favorable conversion of
the reactive [CrCl3L2(solvate)] to the stable [CrCl3(L2L′)] core
with extrusion of free amine RNH2. To our knowledge, there
is no precedent of such ligand transformation. A possible
pathway is the hydro-heterolytic cleavage of the reactive PIII-N
bonds in 2, followed by CH3CN insertion into the void between
the two phosphoryl functions (A), resulting in proton and
electron transfer (B) to create the macrocyclic ligand (3),
Scheme 2. Effectively, the first step is ligand dissection, whereas
the second is a ligand reconstruction step.
This ligand transformation has several favorable driving
forces. A strained four-membered ring is replaced by the more
stable fused five-membered rings with additional macrocyclic
effect. The reactive (P(1)-Cr(1)-P(2) 65.95(4)°) P-N bonds
are replaced by the more robust C-P and C-N with additional
P-O bonds. Removal of the labile CH3CN and ejection of stable
amine RNH2 also contribute to the thermodynamic drive.
The potential use of this ligand transformation as a synthetic
method for new [ONO] ligand Ph2P(O)C(CH3)(NH2)P(O)Ph2
(4) is demonstrated in the hydrolytic reaction of 3. In the
presence of excess H2O, complex 3 breaks down to release the
[NON] ligand 4 (65% yield), which has been spectroscopically
Figure 1. ORTEP diagram of complex 2 (R ) Cy) with thermal
ellipsoids at the 40% probability level. Hydrogen atoms have been
omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of Complexes
2 and 3
2
3
Cr(1)-P(1)
2.4887(13)
2.4850(13)
2.051(4)
Cr(1)-P(2)
Cr(1)-N(1)
2.141(7)
2.307(3)
2.278(3)
2.313(3)
2.039(6)
2.003(6)
1.511(11)
1.523(10)
1.521(6)
1.505(6)
Cr(1)-Cl(1)
Cr(1)-Cl(2)
Cr(1)-Cl(3)
Cr(1)-O(1)
2.2780(12)
2.2995(14)
2.2953(13)
Cr(1)-O(2)
C(1)-C(2)
N(1)-C(1)
1.501(5)
P(1)-O(1)
P(2)-O(2)
P(1)-N(1)-P(2)
P(1)-Cr(1)-P(2)
P(1)-C(1)-P(2)
O(1)-Cr(1)-O(2)
104.74(18)
65.95(4)
109.3(4)
87.7(2)
stabilized mononuclear complex, [Ph2PN(R)PPh2]CrCl3(CH3
CN) (2), Scheme 1.
The solid state three-dimensional structure of 2 (R )
cyclohexyl) was established using X-ray crystallography (Figure
1 and Table 1). The related analogue with R ) CH3-S-C3H6-
has been reported earlier.6 Complex 2 is a six-coordinated
complex with a P,P′-bidentate [PNP] ligand with a spectating
amine but coordinated CH3CN. It is stable in pure and dry
CH3CN, but in the presence of trace H2O, the [PNP] ligand
breaks down and transforms to a new class of tridentate [ONO]
hybrid ligand in [Ph2P(O)C(CH3)(NH2)P(O)Ph2]CrCl3 (3). IR
analysis indicates the loss of the nitrile function. The presence
(8) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2006, 25, 2733.
(9) McGuiness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert,
U.; Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334.
(10) Dixon, J. T.; Wasserscheid, P.; Keim, W.; Morgan, D.; Bollmann,
A.; Maumela, F.; Hess, F.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272.

4190 Organometallics, Vol. 27, No. 16, 2008
Teo et al.
Scheme 2. Proposed Conversion Pathway to 2 to 3
alternative one-step methodology to amino-diphosphine oxides
and diphosphonates such as R2P(O)-C(NH2)-R′-P(O)R2
under ambient conditions. Attempts to develop the use of Cr
catalysts to promote the hydrolysis of PNP compounds to form
new amino-diphosphonates are currently in progress.
Chloride abstraction reaction of 3 with AgSbF6 in CH3CN
was successful but did not give the expected product
[{Ph2P(O)C(CH3)(NH2)P(O)Ph2}CrCl2(CH3CN)]+; presumably
the [CrIIICl2(solvate)] core is too reactive to be isolated. Its
formation triggers a ligand cleavage to release Ph2POH, which
subsequently attacks the intermediate to yield {[Ph2P(O)C
-
(CH3)(NH2)P(O)Ph2]CrCl2[P(O)(H)Ph2]}+SbF6 (5) (Scheme
1). Other decomposition byproducts are not isolated or identified.
The structure of 5 has been confirmed by X-ray single-crystal
crystallographic analysis, but the quality of data is too poor to
justify any meaningful structural discussions.
Ethylene Oligomerization and Polymerization. Typical
ONO ligands, represented by amine bis(phenolate), N-alkoxy-
ꢀ-ketoiminate, and Schiff base, have shown good catalytic
activity in the polymerization of alkenes and enantioselective
catalysis.13 However, we are not aware of any report on metal
complexes with the [ONO] donor set on a P-C-P backbone.
The ethylene oligomerization activities of 2-5 were investigated
in toluene with MAO as coactivator (Table 2). Preliminary data
collected at rt under 430 psi pressure of ethylene with a [Al]/
[Cr]ratioof420indicatedlowtomoderateactivities(1300-13 700
g prod./g Cr) of 2-5 toward oligomerization and polymerization
(Table 2).
The catalytic activities are significantly influenced by the
ligand environment. The combination of Cr(III) with the hybrid
ligand is essential to achieve ethylene oligomerization and
polymerization and influence their distribution as well as the
R-alkene selectivity. Without these ligands, Cr precursors such
as Cr(acac)3 and CrCl3(THF)3 give little or no oligomers. They
give polymers but with mediocre activities (Table 2, entries 3
and 5). While Cr-PNP complexes are reported to be excellent
in selective ethylene trimerization and tetramerization,3 the
introduction of MeCN into the Cr-PNP coordination sphere
(giving precatalyst 2) will drastically alter the selectivity to
exclusive production of polyethylene (PE) with an activity of
13 700 g prod./g Cr when dry toluene is used as solvent (Table
2, entry 1). The role of CH3CN is unclear, but it could stabilize
the metallacyclic intermediates8,14 by suppressing the ꢀ-hydro-
gen elimination and therefore encourage growth of the metal-
lacyclic ring. The catalytic activity and production of PE
decrease significantly when the bidentate PNP-Cr precatalyst 2
is replaced by the tridentate ONO-Cr complex 3 and a crude
form of 5 (1300 and 4500 g prod./g Cr, respectively) (Table 2,
entries 2 and 7). A tentative comparison between a neutral (3)
and ionic complex 5 of similar makeup of hybrid ligands shows
similar activities toward oligomerizations, but 5 shows an at
least 7-fold increase compared to 3 in terms of PE activity. These
data should be treated with caution since 5 is invariably
contaminated by other unknown impurities. In general both
Figure 3. ORTEP diagram of complex 4 with thermal ellipsoids
at the 40% probability level. Selected bond lengths [Å] and bond
angles [deg]: P(1)-O(1) 1.490(3), P(1)-C(1) 1.879(3), C(1)-N(1)
1.476(11), O(1)-P(1)-C(1) 112.47(12), N(1)-C(1)-C(2) 107.4(5),
N(1)-C(1)-P(1) 112.6(4), C(2)-C(1)-P(1) 107.6(4).
and crystallographically established (Scheme 1). The dihedral
angle between the O(1)-P(1)-C(1) and P(1)-C(1)-P(1A)
planes expands beyond perpendicular, viz., 113.7(3)°. The anti-
orientation of the two PdO bonds moves the phenyl rings away
from each other and removes the π stacking seen in the
complexed state (3).
In the absence of Cr, the free [PNP] ligand Ph2PN(Ph)PPh2
in CH3CN can react with excess water (>1000 fold) to give 4
but only very slowly (∼30 days), with much lower yield (ca.
20%) and also giving another unidentified byproduct. There is
no evidence of the formation of 4 when the PNP ligand is added
to a stoichiometric mix of CH3CN and water under ambient
conditions, even after prolonged stirring of >30 days.
Compounds with the P-C(N)-P functions have been shown
to be biologically active.11 For example, R-aminophosphates
can mimic the action of natural amino acids and act as enzyme
inhibitors.11a However, only a few methods of their preparation
are known.12 This metal-mediated pathway could provide a new
(13) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Weitman, N; Goldschmidt,
Z. Chem. Commun. 2000, 379. (b) Chmura, A. J.; Davidson, M. G.; Jones,
M. D.; Lunn, M. D.; Mahon, M. F. Dalton Trans. 2006, 887. (c) Douthwaite,
R. E. Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem. 2004, 100, 385. (d)
Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 2005, 127, 11037.
(14) (a) Briggs, J. R. Chem. Commun. 1989, 674. (b) Carter, A.; Cohen,
S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass, D. F. Chem. Commun.
2002, 858. (c) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A.
J. Am. Chem. Soc. 2001, 123, 7423. (d) Yu, Z.-X.; Houk, K. N. Angew.
Chem., Int. Ed. 2003, 42, 808. (e) Tomov, A. K.; Chirinos, J. J.; Jones,
D. J.; Long, R. J. J. Am. Chem. Soc. 2005, 127, 10166.
(11) (a) Aminophosphonic and Aminophosphosphinic Acids. Chemistry
and Biological ActiVity; Kukhar, V. P., Hudson, H. R., Eds.; Wiley, and
Sons: Chichester, NY, 2000; pp 1-634. (b) Green, J. R. J. Organomet.
Chem. 2005, 690, 2439.
(12) (a) Hirai, T.; Han, L-B. J. Am. Chem. Soc. 2006, 128, 7422. (b)
Hamilton, R.; Mckervey, M. A.; Rafferty, M. D.; Walker, B. J. J. Chem.
Soc., Chem. Commun. 1994, 37. (c) Vanderhoydonck, B.; Stevens, C. V.
Synthesis 2004, 722.

Unusual Ligand Transformation
Organometallics, Vol. 27, No. 16, 2008 4191
Table 2. Activities and Selectivities of Different Catalytic Systems toward Polymerization and Oligomerization of Ethylenea
oligomer distribution (wt %)
entry
catalyst
activityb total product (mg) PE (%)c oligomers (%)
C4(R-C4)
76 (83)
C6(R-C6)
C8(R-C8)
C10(R-C10) C12+
1
2
2 (R ) Cy)
13700
1300
600
2500
1500
7400
4500
12 300
286
28
13
52
32
100
86
100
100
94
0
14
0
0
6
3
0
2 (43)
7 (42)
19
3
Cr(THF)3Cl3
4/Cr(THF)3Cl3
Cr(acac)3
4/Cr(acac)3
5
4d
5
3 (60)
8 (43)
70 (14)
0
8 (40)
15 (73)
2 (45)
13 (72)
18 (26)
0
11 (42)
21 (60)
76
43
6d
7
154
94
255
10
84
96
90
16
4
0
0
0
0
12
100
8e
5
a Conditions: 0.4 µmol of Cr loading, toluene, 430 psi of ethylene, 420 equiv of MAO, room temperature, 1 h. Average at least 3 runs. b Activity )
g prod./g Cr. c % ) weight %. d 0.4 µmol of 4 is used. e Chlorobenzene is used as solvent instead of toluene.
NMR spectrometers (1H at 300.14 MHz, 13C at 75.43 MHz, and
complexes are not very active, probably because of their low
31P at 121.49 MHz). Mass spectra were obtained on a Finnigan
solubility in the reaction solvent, toluene. When it is replaced
Mat 95XL-T spectrometer. Elemental analyses were performed by
the microanalytical laboratory in-house. All chemicals were obtained
from Sigma-Aldrich or Strem Chemicals unless stated otherwise.
{[Ph2PN(Cy)PPh2]CrCl3}2, Ph2PN(Cy)PPh2, and Ph2PN(Ph)PPh2
were prepared with modifications of literature procedures.3a In all
the examples, the molar mass of methylaluminoxane (MAO) (10
wt % in toluene solution) was taken to be 58.016 g/mol, corre-
sponding to the (CH3-Al-O) unit, in order to calculate the molar
quantities of MAO used in the preparation of the catalysts described
below.
Synthesis of [Ph2PN(Cy)PPh2]CrCl3(CH3CN) (2). To the solid
of {[Ph2PN(Cy)PPh2]CrCl3}2 (81 mg, 0.13 mmol) in a test tube
was added CH3CN (1 mL) to give rapid precipitation of blue crystals
of 2. The solution was kept at -30 °C for 1 day to maximize the
crystal deposit. After removing the mother liquid, the crystals were
collected and washed by hexane and dried under vacuum. Yield:
41 mg, 48%. MS (FAB+): m/z 589 [M - Cl - CH3CN]+, 554 [M
- 2Cl - CH3CN]+, 468 [Ph2PN(Cy)PPh2]+, 384 [Ph2PNPPh2]+,
185 [PPh2]+. Anal. (%) Calcd for C32H34N2P2CrCl3: C 57.63, H
5.14, N 4.20. Found: C 56.26, H 4.94, N 4.53.
by a more polar solvent such as chlorobenzene, in which 5 is
significantly more soluble, the PE activity improves by nearly
2-fold to 12 300 g/g (Table 2, entry 8). Attempts to characterize
the polymer product by GPC analysis were not successful, as it
is insoluble under standard analytical procedure (PE dissolution
in 1,3,5-trichlorobenzene at 145 °C). This polymer product is
likely to be high-density polyethylene (HDPE).15
The nature of the metal complex also influences the oligo-
meric selectivity. Although the ONO ligand 4 with CrCl3(THF)3
catalyzes ethylene polymerization (entry 4), it works with
Cr(acac)3 to promote good activity (7400 g prod./g Cr) toward
ethylene oligomerization in which the distribution of the liquid
products (C4-C20) (entry 6) follows a typical Schulz-Flory
distribution with a K value of 0.82.16 Activation of the
4/Cr(acac)3 system with other Lewis acid cocatalysts such as
EtAlCl2, AlMe3, and BEt3 or variation of the ethylene pressure
will affect the overall catalytic activity but not the oligomer
distribution. At higher temperatures (up to 80 °C), the activity
increases by 2-fold, but the selectivity favors PE over oligomers,
with the latter production greatly reduced to 15%.
One-Pot Synthesis of [Ph2P(O)C(CH3)(NH2)P(O)Ph2]CrCl3
(3). Method A: To a solution of Ph2PN(Cy)PPh2 (1.13 g, 2.42
mmol) in CH2Cl2 (5 mL) was added CrCl3(THF)3 (0.906 g, 2.42
mmol) and the mixture stirred at rt for 30 min to give a green
solution. The solvent was removed in Vacuo. The dark green solid
obtained was dissolved in CH3CN to give a dark blue solution of
2. Water (0.1 mL) was then added to the solution and the mixture
kept at rt for 10 days to give crystalline purple deposits of 3. The
mother liquor was decanted and the solid product collected and
dried in Vacuo [yield: 0.767 g, 53% based on CrCl3(THF)3].
Conclusion
We have demonstrated an unusual ligand destruction followed
by reconstruction process during which the ligand is reconfigured
from a bidentate [PNP] to a tridentate [ONO] donor set. As this
transformation is facilitated by coordinated CH3CN and water, it
implies that in principle all P-X-P (X ) N, O, Si, S, etc.) hybrid
ligands with reactive X-P bonds currently used in ethylene
oligomerization are potentially vulnerable to such hydro-heterolytic
cleavage, and hence such catalytic reactions could not be used in
CH3CN solvent, especially when strict exclusion of H2O is difficult.
This serendipitous discovery has revealed a direct pathway to
prepare a new type of catalytically active hybrid ligand. It also
prompted us to design a new generation of ligands that can support
Cr(III) in olefin oligo- and polymerization in more polar solvents
such as nitriles and alcohols.
Method B:
A procedure similar to method A, using
Ph2PN(Ph)PPh2 instead of Ph2PN(Cy)PPh2, yielded 3 [yield: 82.9
mg, 57% based on CrCl3(THF)3].
Method C: 2 (0.237 g, 0.36 mmol) was dissolved in MeCN and
H2O (15 µL) was injected. The reaction mixture was stirred for 7
days under nitrogen and left to stand overnight before the mother
liquor was decanted. The purple solid obtained was dried in Vacuo
[yield: 0.208 g, 96%].
Data for 3: IR [Nujol, cm-1]: ν(N-H) 3082 m, 3058 m. MS
(FAB+): m/z 567 [M - Cl]+, 532 [M - 2Cl]+, 185 [PPh2]+. Anal.
(%) Calcd for C26H25NP2O2CrCl3 · H2O: C 50.22, H 4.38, N 2.25.
Found: C 50.28, H 4.46, N 2.30.
Synthesis of Ph2P(O)C(CH3)(NH2)P(O)Ph2 (4). Purple, solid
3 (180 mg, 0.298 mmol) was suspended in water (10 mL) and kept
for 2 days until a crystalline white product of 4 was precipitated.
The solid was collected, washed with cold water, then dissolved in
toluene, filtered through a layer of Celite, and dried over anhydrous
MgSO4. The mixture was then filtered and the solvent evaporated
carefully to give a solid, which was collected and dried in Vacuo.
Yield: 87 mg (65%). IR [Nujol, cm-1]: ν(N-H) 3065 w. 1H NMR
(CDCl3): δ 8.10 (s, 4 H, Ph), 7.98 (s, 4 H, Ph), 7.40-7.31 (m, 12
Experimental Section
General Comments. All reactions were carried out using
conventional Schlenk techniques under an inert atmosphere of
nitrogen or argon with an M. Braun Labmaster 130 inert gas system.
NMR spectra were measured on Bruker ACF300 300 MHz FT
(15) (a) Li, K.; Ilia, J. D.; Guzei, A.; Mapolie, S. F. J. Organomet. Chem.
2002, 660, 108. (b) Grinshpun, V.; Rudin, A. J. Appl. Polym. Sci. 1985,
30, 2413.
(16) (a) Schulz, G. V. Z. Phys. Chem., Abt. B 30 1935, 379. (b) Schulz,
G. V. Z. Phys. Chem., Abt. B 43 1939, 25. (c) Flory, P. J. J. Am. Chem.
Soc. 1940, 62, 1561.

4192 Organometallics, Vol. 27, No. 16, 2008
Teo et al.
Table 3. Crystal Data and Structure Refinement Parameters for Complexes 2, 3, and 4
2
3
4
empirical formula
C
32H34Cl3CrN2P2
C
54H53Cl6Cr2N3O6 P4
C26H25NO2P2
Mr
666.90
1280.57
445.41
temp, K
223(2)
blue
223(2)
purple
223 (2)
colorless
0.26 × 0.20 × 0.14
monoclinic
C2/c
cryst color
cryst size, mm3
0.20 × 0.08 × 0.06
0.10 × 0.08 × 0.06
cryst syst
space group
orthorhombic
monoclinic
Pbca
P2(1)/c
a, Å
b, Å
c, Å
10.8132(9)
17.3988(13)
34.178(3)
90
90
90
6430.1(9)
8
20.8878(15)
18.7658(14)
15.4625(10)
90
110.622(2)
90
5672.6(7)
4
20.424(15)
8.974(7)
12.342(9)
90
99.544(15)
90
2231(3)
4
1.326
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
density, Mg/m3
abs coeff, mm-1
F(000)
1.378
0.729
2760
1.499
0.830
2624
0.219
936
θ range for data collection, deg
2.23 to 25.00
1.04 to 25.00
2.02 to 27.50
index ranges
-12 e h e 12, -20 e k e 19,
-40 e l e 35
35 585
-24 e h e 23, -22 e k e 21,
-18 e l e 17
32 273
-26 e h e 26, -5 e k e 11,
-16 e l e 15
no. of reflns collected
7201
no. of indep reflns
5666 (Rint ) 0.1468)
0.9575 and 0.8678
5666/0/362
R1 ) 0.0626, wR2 ) 0.1120
R1 ) 0.1026, wR2 ) 0.1243
1.032
9999 (Rint ) 0.1160)
0.9519 and 0.9216
9999/4/689
R1 ) 0.1034, wR2 ) 0.2122
R1 ) 0.1626, wR2 ) 0.2383
1.091
2551 (Rint ) 0.0751)
0.9700 and 0.9453
2551/21/151
R1 ) 0.0827, wR2 ) 0.2262
R1 ) 0.1079, wR2 ) 0.2453
1.068
max. and min. transmn
no. of data/restraints/params
final R indices [I > 2σ(I)]ab
R indices (all data)
goodness-of-fit on F2c
large diff peak and hole, e Å-3
0.357 and -0.326
1.054 and -0.877
0.871 and -0.459
2 1/2
a R ) (∑|Fo| - |Fc|)∑|Fo|. b wR2 ) [(∑w|Fo| - |Fc|)2/∑w|Fo| ]
.
c GoF ) [(∑w|Fo| - |Fc|)2/(Nobs - Nparam)]1/2
.
H, Ph), 2.13 (t, J ) 10.7 Hz, 2 H, NH2), 1.39 (t, J ) 15.5 Hz, 3 H,
CH3). 13C NMR (CDCl3): δ 132.6 (t, J ) 4.4 Hz, Ph), 132.2 (t, J
) 4.4 Hz, Ph), 131.7 (s, Ph), 131.3 (s), 128.0 (q, J ) 5.5 Hz, Ph),
58.1 (t, J ) 66.4 Hz, C(CH3)(NH2)), 21.8 (s, CH3). 31P NMR
(CDCl3): δ 32.66. MS (FAB+): m/z 446 [M + H]+, 244
[(Ph2P(O)C(CH3)(NH2)]+, 185 [PPh2]+. Anal. (%) Calcd for
C26H25NP2O2: C 70.11, H 5.61, N 3.14. Found: C 69.77, H 5.80,
N 2.77.
grams17 and for absorption effects with SADABS.18 Structure
solution and refinement were carried out with the SHELXTL suite
of programs.19 The structures were solved by direct methods to
locate the heavy atoms, followed by difference maps for the light
non-hydrogen atoms. The asymmetric unit in 3 contains two
molecules of the complex [Ph2P(O)C(CH3)(NH2)P(O)Ph2]CrCl3 and
one solvent acetonitrile, as well as two water molecules. The
hydrogen atoms of the two water molecules were not located due
to poor data quality. The final R values are R1 ) 0.103 and wR2 )
0.2365 These relatively high values could be caused by the poor
quality of the crystal (Rint ) 0.116). Other features including the
thermal parameters of the atoms are normal. In 4, the asymmetric
unit contains half a molecule. The molecule was generated by the
2-fold symmetry. The expected molecule (Ph2PO)2C(CH3)(NH2),
as evident by chemical and NMR analysis, actually does not have
2-fold symmetry. The only reasonable possibility is disorder of CH3
and NH2 groups with 50:50 occupancies. Use of space group Cc,
which does not have a 2-fold axis, could not solve the problem.
Hence, C2/c with disordered CH3 and NH2 appeared to be the best
choice. Further details of the crystallographic information are
provided in the Supporting Information. Crystal data and structure
refinement parameters for 2, 3, and 4 are given in Table 3.
Synthesis of {[Ph2P(O)C(CH3)(NH2)P(O)Ph2]CrCl2[P(O)(H)
-
Ph2]}+SbF6 (5). To a suspension of 3 (25 mg, 0.042 mmol) in
CH3CN (4 mL) was added AgSbF6 (14 mg, 0.042 mmol), and the
mixture was stirred at rt for 24 h. The resultant cloudy solution
was filtered through a layer of Celite to give a green solution. The
volatile materials were removed in Vacuo. The green residue was
redissolved in CH3CN and filtered. Et2O was carefully added and
the mixture left upon standing at -30 °C for a few weeks to give
dark green crystals of 5. Yield: ca. 12 mg (29%). IR [Nujol, cm-1]:
ν(N-H)3065w.MS(FAB+):m/z769{[Ph2P(O)C(CH3)(NH2)P(O)Ph2]
CrCl2[P(O)(H)Ph2]}+, 567 [(Ph2P(O)C(CH3)(NH2)P(O)Ph2)CrCl2]+,
532 [(Ph2P(O)C(CH3)(NH2)P(O)Ph2)CrCl]+. MS (FAB-): m/z 236
(SbF6)-. This compound was inevitably contaminated by other Cr
impurities, and hence satisfactory elemental analysis could not be
obtained.
Acknowledgment. We thank the National University of
Singapore (R-143-000-259-112 and C-143-000-003-001) and
Agency for Science, Technology & Research (R-143-000-
277-305) for support, as well as Dr. L. L. Koh and Ms. G. K.
Tan for assistance in the crystallographic analysis.
Oligomerization of Ethylene. The catalytic activities were
screened by an Endeavor parallel pressure reactor, following
recommended procedures.6 At the end of each experiment, the
vessels were cooled to ambient temperature and depressurized. The
reaction mixture was cooled to 0 °C and terminated by addition of
10% aqueous HCl. A small sample of the upper-layer solution was
filtered through a layer of Celite and analyzed by GC. The individual
products of oligomerization were identified by GC-MS. The
remainder of the upper-layer solution was filtered to isolate the
solid polymeric products. The solid products were suspended in
10% aqueous HCl and stirred for 24 h, dried under reduced pressure,
and weighed. The results are summarized in Table 2.
Supporting Information Available: X-ray structural data for
complexes 2, 3, and 4. These materials are available free of charge
via the Internet at http://pubs.acs.org.
OM800353A
(17) SMART version 5.1; Bruker Analytical X-ray Systems: Madison,
WI, 2000.
(18) Sheldrick, G. M. SADABS, a program for Empirical Absorption
Correction; Go¨ttingen, Germany, 2000.
(19) SHELXTL version 5.03; Bruker Analytical X-ray Systems: Madison,
WI, 1997.
Crystal Structure Analyses. Diffraction-quality single crystals
were mounted on quartz fibers and X-ray data collected on a Bruker
AXS APEX diffractometer, equipped with a CCD detector, using
Mo KR radiation (λ 0.71073 Å). The data were corrected for
Lorentz and polarization effects with the SMART suite of pro-