Chromium(II) and Chromium(III) Complexes Supported by Tris(2-pyridylmethyl)amine: Synthesis, Structures, and Reactivity

Article abstract of DOI:10.1021/ic034530i

This report describes the synthesis, structural characterization, and polymerization behavior of a series of chromium(II) and chromium(III) complexes ligated by tris(2-pyridylmethyl)amine (TPA), including chromium(III) organometallic derivatives. For instance, the combination of TPA with CrCl ₂ yields monomeric (TPA)CrCl₂ (1). A similar reaction of CrCl₂ with TPA, followed by chloride abstraction with NaBPh ₄ or NaBAr^F₄ (Ar^F = 3,5-(CF ₃)₂C₆H₃), provides the weakly associated cationic dimers [(TPA)CrCl]₂[BPh₄]₂ (2A) and [(TPA)CrCl]₂[BAr^F₄]₂ (2B), respectively. X-ray crystallographic analysis reveals that each chromium(II) center in 1, 2A, and 2B is a tetragonally elongated octahedron; such Jahn-Teller distortions are consistent with the observed high spin (S = 2) electronic configurations for these chromium(II) complexes. Likewise, reaction of CrCl₃(THF)₃ with TPA, followed by anion metathesis with NaBPh₄ or NaBAr^F₄, yields the monomeric, cationic chromium(III) complexes [(TPA)CrCl₂][BPh₄] (4A) and [(TPA)CrCl₂][BAr^F₄] (4B), respectively. Treatment of 4A with methyl and phenyl Grignard reagents produces the cationic chromium(III) organometallic derivatives [(TPA)Cr(CH₃) ₂][BPh₄] (5) and [(TPA)CrPh₂][BPh₄] (6), respectively. Similar reactions of 4A with organolithium reagents leads to intractable solids, presumably due to overreduction of the chromium(III) center. X-ray crystallographic analysis of 4A, 5, and 6 confirms that each possesses a largely undistorted octahedral chromium center, consistent with the observed S = 3/2 electronic ground states. Compounds 1, 2A, 2B, 4A, 4B, 5, and 6 are all active polymerization catalysts in the presence of methylalumoxane, producing low to moderate molecular weight high-density polyethylene.

Full text of DOI:10.1021/ic034530i

Inorg. Chem. 2003, 42, 6876−6885
Chromium(II) and Chromium(III) Complexes Supported by
Tris(2-pyridylmethyl)amine: Synthesis, Structures, and Reactivity
Nicholas J. Robertson, Michael J. Carney,* and Jason A. Halfen
Department of Chemistry, UniVersity of WisconsinsEau Claire, Eau Claire, Wisconsin 54702
Received May 16, 2003
This report describes the synthesis, structural characterization, and polymerization behavior of a series of chromium-
(II) and chromium(III) complexes ligated by tris(2-pyridylmethyl)amine (TPA), including chromium(III) organometallic
derivatives. For instance, the combination of TPA with CrCl2 yields monomeric (TPA)CrCl2 (1). A similar reaction
of CrCl2 with TPA, followed by chloride abstraction with NaBPh4 or NaBArF4 (ArF ) 3,5-(CF ) C H ), provides the
3 2
6 3
weakly associated cationic dimers [(TPA)CrCl]2[BPh4]2 (2A) and [(TPA)CrCl]2[BArF4]2 (2B), respectively. X-ray
crystallographic analysis reveals that each chromium(II) center in 1, 2A, and 2B is a tetragonally elongated octahedron;
such Jahn−Teller distortions are consistent with the observed high spin (S ) 2) electronic configurations for these
chromium(II) complexes. Likewise, reaction of CrCl3(THF)3 with TPA, followed by anion metathesis with NaBPh4 or
NaBArF4, yields the monomeric, cationic chromium(III) complexes [(TPA)CrCl2][BPh4] (4A) and [(TPA)CrCl2][BArF4]
(4B), respectively. Treatment of 4A with methyl and phenyl Grignard reagents produces the cationic chromium(III)
organometallic derivatives [(TPA)Cr(CH ) ][BPh4] (5) and [(TPA)CrPh2][BPh4] (6), respectively. Similar reactions of
3 2
4A with organolithium reagents leads to intractable solids, presumably due to overreduction of the chromium(III)
center. X-ray crystallographic analysis of 4A, 5, and 6 confirms that each possesses a largely undistorted octahedral
3
chromium center, consistent with the observed S ) /2 electronic ground states. Compounds 1, 2A, 2B, 4A, 4B,
5, and 6 are all active polymerization catalysts in the presence of methylalumoxane, producing low to moderate
molecular weight high-density polyethylene.
Introduction
catalyst presumably comprises a chromium atom covalently
bonded to the silica surface (via a Si-O-Cr linkage) and
to a single Cp ligand,4 although the metal’s oxidation state
and coordination geometry are not well defined.
These ambiguities have spurred considerable effort di-
rected toward the synthesis of discrete chromium compounds
to serve as models of the commercial systems and as potential
single-site catalysts. For example, Theopold’s group has
prepared a large family of mono-Cp chromium complexes,5
of which the coordinately unsaturated Cp* chromium(III)
alkyl derivatives (Cp* ) C5Me5) are particularly active
homogeneous catalysts.6-8 In addition, deposition of these
Chromium-based polymerization catalysts play a central
role in commercial polyethylene production. For example,
silica-supported chromium oxide catalysts1 (Phillips catalysts)
are responsible for the production of billions of pounds of
high-density polyethylene (HDPE) per year. The active site
is thought to be a low-valent chromium species bound to
silica, but the coordination sphere and mechanism of
activation are not well understood, even after years of study.2
A class of Unipol catalyst, formed by adsorption of chro-
mocene (Cp2Cr, Cp ) cyclopentadienyl) onto dehydroxylated
silica,3 is also used to prepare commercially significant
quantities of HDPE. The active site of the chromocene
(4) Karol, F. J.; Wu, C.; Reichle, W. T.; Maraschin, N. J. J. Catal. 1979,
60, 68-76.
(5) For general overviews of Cp-chromium chemistry see: (a) Theopold,
K. H. Acc. Chem. Res. 1990, 23, 263-270. (b) Theopold, K. H. Eur.
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27 (10), 26-32. (d) Pariya, C.; Theopold, K. H. Curr. Sci. 2000, 78,
1345-1351.
* To whom correspondence should be addressed. E-mail: carneymj@
uwec.edu. Phone: 715-836-3500. Fax: 715-836-4979.
(1) Hogan, J. P. J. Polym. Sci., Polym. Chem. Ed. 1970, 8, 2637.
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N.; Carrick, W. L. J. Polym. Sci., Polym. Chem. Ed. 1972, 10, 2621-
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6876 Inorganic Chemistry, Vol. 42, No. 21, 2003
10.1021/ic034530i CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/17/2003

Cr(II/III) Complexes of Tris(2-pyridylmethyl)amine
Cp* chromium alkyl complexes onto dehydroxylated silica,
alumina, and aluminum phosphate supports yields highly
active heterogeneous polymerization catalysts.9 More re-
cently, Jolly,10-13 Bazan,14,15 and others16,17 have prepared
mono-Cp chromium(III) derivatives containing an additional
neutral donor (i.e., amine or phosphine) that binds to the
chromium center. In the presence of methylalumoxane
(MAO) or borane activators, highly active catalysts are
formed, resulting in the production of polyethylene or
R-olefin products whose properties depend on the nature of
the neutral donor and the activator employed.
cane,33 bis(imidazole),34 amine bis(thioether),35 and certain
bridged-diphosphine36,37 ligands are known. When combined
with the appropriate activator, these complexes polymerize
ethylene to form polymeric or oligomeric products with
properties dependent on the identity of the chromium
compound.
A focus of our research group is the synthesis of
monomeric chromium species bound by chelating, nitrogen-
based ligands. Of particular interest are tetradentate ligands
that yield monomeric octahedral transition-metal complexes
with reactive sites in a cis configuration. The cis orientation
is ideal for coordination/insertion polymerization; olefin
binding cis to the growing polymer chain allows for rapid
olefin insertion into the chain and high catalyst activity.
Tetradentate ligands, such as tris(2-pyridylmethyl)amine
(TPA), would yield such cis-ligated octahedral complexes.
The TPA ligand has a long and productive history in the
construction of functional models for various non-heme iron
mono-oxygenases and other oxygen-activating enzymes,38,39
but it has yet to be explored in the context of polymerization
catalysis. With respect to chromium chemistry, a few dimeric
chromium(III)-TPA complexes (with bridging oxo and
hydroxo groups) are known and have been structurally
characterized.40 In this report, we describe the synthesis,
X-ray structural characterization, and polymerization behav-
ior of a series of monomeric (or weakly associated dimeric)
chromium(II) and chromium(III) complexes supported by
TPA. Among these complexes are thermally robust, cationic
chromium(III) dimethyl and diphenyl derivatives. All com-
plexes possess labile or reactive coordination sites in a cis
In addition to Cp ligated compounds, transition-metal
complexes containing non-Cp ligands have enjoyed recent
success in polymerization catalysis. Several excellent reviews
have appeared on the use of non-Cp ligands to produce both
early and late metal catalysts.18-21 Chromium complexes with
non-Cp ligands include those coordinated by anionic sali-
cylaldiminato,22 reduced Schiff-base,23 imino-pyrrolide,24,25
â-diketiminate (“nacnac”),25-28 and bis(phosphoranimine)-
methanide29 ligands. Furthermore, chromium(III) derivatives
supported by neutral triazacyclohexane,30 pyridine bis-
(imine),31 pyridine bis(oxazolinyl),32 triphosphacyclodode-
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(40) (a) [Cr2(TPA)2(µ-O)(SCN)2](SCN)2: Gafford, B. G.; Holwerda, R.
A.; Schugar, H. J.; Potenza, J. A. Inorg. Chem. 1988, 27, 1128-1130.
(b) [Cr2(TPA)2(µ-OH)2]Br4‚8H2O: Gafford, B. G.; Holwerda, R. A.
Inorg. Chem. 1989, 28, 60-66. (c) [Cr2(TPA)2(µ-OH)2](ClO4)4‚4H2O:
Hodgson, D. J.; Zietlow, M. H.; Pedersen, E.; Toftlund, H. Inorg.
Chim. Acta 1988, 149, 411. (d) [Cr2(TPA)2(µ-O)(µ-O2CCH3)](ClO4)3‚
H2O: Gafford, G. G.; Marsh, R. E.; Schaefer, W. P.; Zhang, J. H.;
O’Connor, C. J.; Holwerda, R. A. Inorg. Chem. 1990, 29, 4652-
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X.; O’Connor, C. J.; Holwerda, R. A. Inorg. Chem. 1996, 35, 2196-
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Inorganic Chemistry, Vol. 42, No. 21, 2003 6877

Robertson et al.
1029, 841, 735, 702, 612 cm-1. UV-vis (CH3CN) [λmax, nm (ꢀ,
M-1 cm-1)]: 382 (2500). Anal. Calcd (found) for C84H76N8B2Cl2-
Cr2: C, 72.37 (72.12); H, 5.50 (5.47); N, 8.04 (8.15). µeff ) 4.6
µB.
arrangement and all are active polymerization catalysts in
the presence of MAO.
Experimental Section
[(TPA)CrCl]2[BArF ] (2B). Tris(2-pyridylmethyl)amine (0.200
4 2
Materials and Methods. Unless otherwise stated, all operations
were carried out in an argon-filled glovebox or using standard
Schlenk techniques. Tetrahydrofuran, diethyl ether, dichloromethane,
and acetonitrile were purified by standard drying procedures and
distilled under argon prior to use. Furthermore, acetonitrile was
stored over activated molecular sieves in the glovebox. Anhydrous
high-performance liquid chromatography grade benzene and toluene
were degassed prior to use and stored over molecular sieves.
Chlorobenzene was degassed and dried over molecular sieves.
Anhydrous chromium(II) chloride, chromium(III) chloride, sodium
tetraphenylborate (NaBPh4), 1,4-dioxane, and Grignard reagents
were purchased from Aldrich or Acros Organics and used as
received. Chromium(III) chloride tris(tetrahydrofuran),41 tris(2-
pyridylmethyl)amine (TPA),42 and sodium tetrakis(3,5-bis(trifluo-
g, 0.69 mmol), anhydrous CrCl2 (0.085 g, 0.69 mmol), and NaBArF
4
(0.610 g, 0.69 mmol) were stirred overnight in 15 mL of
tetrahydrofuran. The solvent was removed in vacuo, yielding a dark-
brown solid. The solid was dissolved in a minimum (ca. 5 mL) of
diethyl ether, and the solution was filtered and layered with an equal
volume of pentane. A dark-brown crystalline solid formed after 2
days. The solid was isolated and dried under vacuum (0.325 g,
38%). FTIR (KBr): 3072, 3040, 2929, 1785, 1610, 1438, 1356,
1279, 1123, 886, 837, 768, 715, 681, 670 cm-1. UV-vis (CH3-
CN) [λmax, nm (ꢀ, M-1 cm-1)]: 360 (1500). Anal. Calcd (found)
for C100H60N8B2Cl2F48Cr2: C, 48.39 (47.50); H, 2.43 (2.44); N,
4.51 (4.39). µeff ) 3.9 µB.
[(TPA)CrCl2][BPh4]‚MeCN (4A). Sodium tetraphenylborate
(0.236 g, 0.69 mmol), tris(2-pyridylmethyl)amine (0.200 g, 0.69
mmol), and CrCl3(THF)3 (0.258 g, 0.69 mmol) were combined in
15 mL of tetrahydrofuran. A green-brown solution formed im-
mediately, which gradually formed a flocculent purple solid after
overnight stirring. The solid was collected and dissolved in 100
mL of acetonitrile, and the solution was filtered to remove a small
amount of white solid (presumably NaCl). The resulting purple
solution was heated to 70 °C, concentrated in vacuo until the
saturation point, and slowly cooled to room temperature yielding
(0.179 g, 34%) of deep-purple crystals of 4A (acetonitrile mono-
solvate). Further concentration of the remaining filtrate deposited
an additional 0.149 g of product (total yield ) 0.328 g, 61%). FTIR
(KBr): 3049, 3031, 2985, 1958, 1888, 1830, 1608, 1479, 1444,
1281, 1159, 1053, 1030, 768, 738, 703, 610 cm-1. UV-vis (CH3-
CN) [λmax, nm (ꢀ, M-1 cm-1)]: 421 (170). Anal. Calcd (found) for
C44H41N5BCl2Cr: C, 68.32 (68.18); H, 5.34 (5.42); N, 9.05 (8.91).
µeff ) 3.5 µB.
romethyl)phenyl) borate (NaBArF )43 were prepared as described
4
in the literature. Galbraith Laboratories, Inc. (Knoxville, TN), and
Atlantic Microlab, Inc. (Norcross, GA), performed elemental
analyses. Magnetic susceptibilities were determined at room tem-
perature using a Johnson Matthey magnetic susceptibility balance,
and electronic absorption spectra were recorded on a Hewlett-
Packard 8453 spectrophotometer (190-1100 nm range). Samples
for IR were dispersed in KBR, and spectra were recorded on a
Nicolet 5DXC spectrometer with a diffuse reflectance (DRIFTS)
attachment. Polymer analysis, including gel permeation chroma-
tography (GPC), high temperature 13C NMR, and differential
scanning calorimetry (DSC) experiments were performed at W. R.
Grace & Co. GPC measurements were made with a Waters 150 C
GPC operating at 140 °C using 1,2,4-trichlorobenzene as solvent.
13C NMR experiments were conducted at 120 °C in 1,1,2,2-
tetrachloroethane using a Bruker ACE 300 spectrometer.
Synthesis of Chromium Complexes. (TPA)CrCl2 (1). A
mixture of anhydrous CrCl2 (0.085 g, 0.69 mmol) and tris(2-
pyridylmethyl)amine (0.200 g, 0.69 mmol) was stirred overnight
in 15 mL of tetrahydrofuran, resulting in a purple-gray precipitate.
The solid was collected, dissolved in a minimum of acetonitrile,
and filtered. Deep-purple crystals were obtained by ether diffusion
into the acetonitrile solution (0.186 g, 65%). Completely satisfactory
elemental analysis could not be obtained; however, an X-ray crystal
structure determination confirmed the identity of 1. Fourier
transform (FT)IR (KBr): 3055, 3020, 2944, 2915, 1602, 1474,
1439, 1287, 1048, 1024, 773 cm-1. UV-vis (CH3CN) [λmax, nm
(ꢀ, M-1 cm-1)]: 375 (2400). Anal. Calcd (found) for C18H18N4-
[(TPA)CrCl2][BArF ] (4B). Tris(2-pyridylmethyl)amine (0.200
4
g, 0.69 mmol), CrCl3(THF)3 (0.258 g, 0.69 mmol), and NaBArF
4
(0.610 g, 0.69 mmol) were stirred overnight in 15 mL of
tetrahydrofuran. The solvent was removed under vacuum yielding
a dark-purple solid. The solid was dissolved in a minimum of diethyl
ether, and the solution was filtered and layered with an equal volume
of pentane. A deep-purple crystalline solid formed after 2 days. It
was isolated and dried under vacuum (0.672 g, 76%). FTIR
(KBr): 3078, 2973, 2938, 1614, 1444, 1357, 1281, 1112, 884, 838,
768, 709, 668 cm-1. UV-vis (CH3CN) [λmax, nm (ꢀ, M-1 cm-1)]:
421 (180). Anal. Calcd (found) for C50H30N4BCl2F24Cr: C, 47.05
(46.82); H, 2.37 (2.38); N, 4.39 (4.27). µeff ) 3.9 µB.
[(TPA)CrMe2][BPh4]‚0.5CH2Cl2 (5). Tris(2-pyridylmethyl)-
amine (0.500 g, 1.73 mmol), CrCl3(THF)3 (0.645 g, 1.72 mmol),
and NaBPh4 (0.589 g, 1.73 mmol) were stirred overnight in 20 mL
of tetrahydrofuran. The resulting solid was collected, washed with
diethyl ether, and dried under vacuum, yielding 1.40 g of a
(presumably) 1:1 mixture of [(TPA)CrCl2][BPh4] and NaCl. This
precursor was used as a convenient starting material in the synthesis
of both 5 and 6. A 0.200 g portion of the precursor (0.25 mmol)
was slurried in tetrahydrofuran at room temperature and slowly
treated with CH3MgCl (0.52 mmol, 0.17 mL of a 3.03 M solution),
resulting in a red-brown solution. The solution was stirred at room
temperature for 1.5 h and treated with 0.50 mL of 1,4-dioxane
(99.8%, anhydrous). The resulting mixture was stirred an additional
30 min, filtered, and taken to dryness, yielding a light-brown solid.
The solid was dissolved in about 5 mL of dichloromethane and
filtered to remove a small amount of insoluble material. Slow
Cl2Cr: C, 52.32 (50.37); H, 4.39 (4.64); N, 13.56 (13.94). µeff
4.6 µB.
)
[(TPA)CrCl]2[BPh4]2 (2A). A mixture of tris(2-pyridylmethyl)-
amine (0.300 g, 1.03 mmol), anhydrous CrCl2 (0.127 g, 1.03 mmol),
and NaBPh4 (0.354 g, 1.03 mmol) was stirred overnight in 15 mL
of tetrahydrofuran, yielding a red-brown solid. The solid was
collected and dissolved in 100 mL of acetonitrile, and the solution
was heated to 70 °C, and filtered. The filtrate was concentrated in
vacuo until reaching the saturation point and slowly cooled to room
temperature. The dark-red-brown crystals that formed were collected
and dried under vacuum (0.350 g, 49%). FTIR (KBr): 3048, 2999,
2978, 1945, 1883, 1818, 1610, 1483, 1438, 1426, 1291, 1262, 1156,
(41) Herwig, W.; Zeiss, H. H. J. Org. Chem. 1958, 23, 1404.
(42) Canary, J. W.; Wang, Y.; Roy, R., Jr. Inorg. Synth. 1998, 32, 70-71.
(43) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922.
6878 Inorganic Chemistry, Vol. 42, No. 21, 2003

Cr(II/III) Complexes of Tris(2-pyridylmethyl)amine
Table 1. Summary of X-ray Crystallographic Data for Chromium-TPA Complexes 1, 2A, 2B, 4A, 5, and 6a
1
2A
2B
4A
5
6
empirical
formula
fw
cryst syst
space group
a (Å)
C
18H18Cl2CrN4
C
42H38BClCrN4
C
54H40BClCrF24N4O C44H41BCl2CrN5
C
44.50H45BClCrN4
C
56H51BCrN5
413.26
monoclinic
P21/c
15.195(3)
8.7230(10)
29.260(5)
90
97.31(2)
90
3846.8(11)
8
697.02
triclinic
P1h
1315.16
triclinic
P1h
773.53
monoclinic
P21/c
13.807(2)
10.145(1)
28.569(3)
90
95.95(1)
90
3980.2(8)
4
734.10
monoclinic
P21/c
9.839(1)
22.339(3)
18.064(2)
90
92.21(1)
90
3967.4(8)
4
856.83
monoclinic
P21
10.6190(4)
14.1215(6)
15.1598(6)
90
103.8810(10)
90
9.426(2)
11.868(1)
16.663(2)
71.02(1)
82.66(1)
82.61(1)
1740.7(4)
2
13.645(3)
13.759(3)
17.522(3)
92.43(2)
107.85(2)
114.36(2)
2798.9(10)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
2206.92(15)
2
1.289
d
calcd (mg m-3
)
1.427
1.330
1.561
1.291
1.229
cryst size (mm)
abs. coeff. (mm-1
2θ max (deg)
0.58 × 0.30 × 0.20 0.46 × 0.32 × 0.15 0.42 × 0.38 × 0.35
0.50 × 0.42 × 0.35 0.42 × 0.40 × 0.35 0.310 × 0.188 × 0.146
)
0.880
49.96
1.0-0.9199
7001
0.442
0.376
0.459
0.391
0.305
64.12
50.06
49.98
50.10
50.08
transmission range
no. reflns collected
no. indep reflns
no. obsd reflns
no. variables
1.0-0.9295
6534
1.0-0.9334
10264
1.0-0.9148
7327
1.0-0.9564
7436
28957
6743
6127
9816
7016
7001
14897
4026
4371
6625
4450
4126
7177
451
442
827
478
471
834
R1 (wR2)b [I > 2σ(I)] 0.0741 (0.1331)
0.0554 (0.0986)
1.024
0.0631 (0.1606)
1.032
0.0659 (0.1153)
1.030
0.0784 (0.1489)
1.022
0.0406 (0.0878)
1.039
GOF (F2)
1.033
diff. peaks (e-‚Å-3
)
1.004, -0.308
0.236, -0.238
0.505, -1.120
0.222, -0.369
0.332, -0.427
0.621, -0.336
a See Experimental Section for additional data collection, reduction, and structure solution and refinement details. b R1 ) ∑ ||Fo| - |Fc||/∑ |Fo|; wR2 )
[∑[w(Fo - Fc )2]]1/2 where w ) 1/σ2(Fo ) + (aP)2 + bP.
2
2
2
diffusion of diethyl ether into the dichloromethane solution resulted
in the formation of red-orange crystals and a small amount of off-
white solid. The crystals were separated from the off-white solid,
washed with diethyl ether, and dried under vacuum to give 5 as a
dichloromethane hemisolvate (0.123 g, 64%). FTIR (KBr): 3055,
2996, 2909, 2856, 2792, 1946, 1888, 1824, 1608, 1479, 1439, 1421,
value of 50-52°, while data for 6 were harvested from a total of
1800 frames collected with a scan width of 0.3° in ω to a maximum
2θ angle of 64.12°. Absorption corrections for 1, 2A, 2B, 4A, and
5 were applied based on azimuthal scans of several reflections for
each sample, while the absorption correction for 6 was performed
using an empirical method (SADABS). Space groups were deter-
mined based on systematic absences and intensity statistics.
Successful direct-methods solutions were calculated for each
compound using the SHELXTL suite of programs. Any non-
hydrogen atoms not identified from the initial E-map were located
after several cycles of structure expansion and full matrix least
squares refinement on F2. Hydrogen atoms were added geo-
metrically. All non-hydrogen atoms were refined with anisotropic
displacement parameters, while hydrogen atoms were refined using
a riding model with group isotropic displacement parameters.
Relevant crystallographic information for the compounds is sum-
marized in Table 1, and selected interatomic distances and angles
are provided in Table 2. Complete crystallographic data for each
compound are provided as Supporting Information in CIF and PDF
format.
The asymmetric unit of 1 contains two crystallographically
independent yet metrically similar neutral molecules. The unit cells
of 2B, 4A, 5, and 6 each contain uncoordinated solvent molecules
(2B, Et2O; 4A and 6, CH3CN; 5, 0.5CH2Cl2). These solvates are
fully ordered in 2B and 4A but disordered in 5 (statistically
disordered about an inversion center) and 6 (found in two
overlapping orientations sharing a common internal carbon atom).
Four of the eight -CF3 groups of the anion in 2B are rotationally
disordered. Split-atom models and geometric constraints were
applied to these disordered groups. The tetraphenylborate anion in
6 is disordered, occupying two positions separated by a distance
of ∼0.5 Å with site occupancy factors of 0.53 and 0.47. Geometric
constraints were applied to these two independent anions.
General Ethylene Polymerization Procedure. In a glovebox,
the chromium complex precatalyst (10-50 µmol) was dissolved
in 100 mL of the appropriate solvent (dichloromethane, chloroben-
zene, or toluene) and placed in a Fisher-Porter bottle adapted with
stainless steel valving and connections. The required cocatalyst was
1281, 1030, 768, 733, 703, 610, 487 cm-1. UV-vis (CH3CN) [λmax
nm (ꢀ, M-1 cm-1)]: 328 (sh, 1700). Anal. Calcd (found) for
44.5H45N4BClCr: C, 72.81 (72.18); H, 6.18 (6.20); N, 7.63 (7.72).
,
C
µeff ) 3.5 µB.
[(TPA)CrPh2][BPh4]‚CH3CN (6). A 0.400 g portion (0.50
mmol) of the [(TPA)CrCl2][BPh4]‚NaCl precursor, prepared as for
compound 5 above, was slurried in 20 mL of tetrahydrofuran and
treated slowly with C6H5MgCl (1.0 mmol, 0.50 mL of a 2.0 M
solution) at room temperature. A red-brown precipitate formed after
the mixture was stirred for 2 h at room temperature. The precipitate
was isolated by filtration, washed with diethyl ether, and dissolved
in a minimum of acetonitrile. The red-orange solution was heated
to 70 °C, filtered, and slowly cooled to room temperature, resulting
in the deposition of 0.141 g of red-orange blocks (33%). FTIR
(KBr): 3055, 2985, 2921, 2255, 1952, 1882, 1812, 1608, 1479,
1433, 1281, 1059, 756, 733, 709, 610, 464 cm-1. UV-vis (CH3-
CN) [λmax, nm (ꢀ, M-1 cm-1)]: 374 (sh, 1800). Anal. Calcd (found)
for C56H51N5BCr: C, 78.50 (78.45); H, 6.00 (5.99); N, 8.17 (8.27).
µeff ) 3.5 µB.
X-ray Crystallography. Single crystals of 1, 2A, 4A, and 5 were
mounted in thin-walled glass capillaries and transferred to an Enraf-
Nonius CAD4 X-ray diffractometer for data collections at 298(2)
K. Data for complex 2B were collected at 173(2) K using a Bruker-
Nonius MACH3/S diffractometer, and data for 6 were collected at
100(2) K using a Bruker SMART APEX CCD-based diffractometer
system. All data were collected using graphite monochromated Mo
KR radiation. Unit cell constants for 1, 2A, 2B, 4A, and 5 were
determined from a least-squares refinement of the setting angles
of 25 intense, high-angle reflections, while the unit cell constants
for 6 are based upon the refinement of the XYZ-centroids of 7177
reflections above 20 σ(I). Intensity data for 1, 2A, 2B, 4A, and 5
were collected using the ω/2θ scan technique to a maximum 2θ
Inorganic Chemistry, Vol. 42, No. 21, 2003 6879

Robertson et al.
Table 2. Selected Distances (Å) and Angles (deg) for Structurally Characterized Complexesa
Complex 1b
molecule 1
molecule 2
Cr(1)-N(1)
2.474(6)
2.123(6)
2.122(5)
81.7(2)
95.94(19)
75.98(19)
Cr(1)-N(4)
2.164(5)
2.619(2)
Cr(1a)-N(1a)
Cr(1a)-N(2a)
2.633(6)
Cr(1a)-N(4a)
Cr(1a)-Cl(1a)
Cr(1a)-Cl(2a)
N(2a)-Cr(1a)-Cl(2a)
N(3a)-Cr(1a)-N(4a)
N(3a)-Cr(1a)-Cl(1a)
2.152(5)
2.646(2)
2.375(2)
100.77(15)
79.52(19)
90.96(15)
100.85(16)
91.32(15)
166.13(15)
102.53(7)
Cr(1)-N(2)
Cr(1)-Cl(1)
2.105(5)
2.110(5)
92.99(18)
79.90(19)
74.92(19)
Cr(1)-N(3)
Cr(1)-Cl(2)
2.3783(19) Cr(1a)-N(3a)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
N(2)-Cr(1)-Cl(2)
N(3)-Cr(1)-N(4)
N(3)-Cr(1)-Cl(1)
102.42(18) N(1a)-Cr(1a)-N(2a)
77.8(2)
90.97(14)
N(1a)-Cr(1a)-N(3a)
N(1a)-Cr(1a)-N(4a)
N(1)-Cr(1)-Cl(1) 165.40(14) N(3)-Cr(1)-Cl(2)
100.82(15) N(1a)-Cr(1a)-Cl(1a) 164.59(13) N(3a)-Cr(1a)-Cl(2a)
N(1)-Cr(1)-Cl(2) 89.87(14)
N(4)-Cr(1)-Cl(1)
N(4)-Cr(1)-Cl(2)
Cl(1)-Cr(1)-Cl(2)
93.02(15)
165.46(15) N(2a)-Cr(1a)-N(3a)
101.48(7) N(2a)-Cr(1a)-N(4a)
N(1a)-Cr(1a)-Cl(2a) 91.43(14)
N(4a)-Cr(1a)-Cl(1a)
N(4a)-Cr(1a)-Cl(2a)
Cl(1a)-Cr(1a)-Cl(2a)
N(2)-Cr(1)-N(3)
N(2)-Cr(1)-N(4)
156.6(2)
79.1(2)
157.4(2)
77.90(18)
N(2)-Cr(1)-Cl(1) 86.86(16)
N(2a)-Cr(1a)-Cl(1a) 90.80(14)
Complex 2Ac
Cr(1)-N(1)
2.100(3)
2.100(3)
2.370(3)
Cr(1)-N(4)
Cr(1)-Cl(1)
Cr(1)-Cl(1a)
2.131(3)
2.3528(9)
2.9388(11) N(2)-Cr(1)-Cl(1)
99.72(10)
85.66(8)
N(2)-Cr(1)-N(3)
N(2)-Cr(1)-N(4)
83.09(11)
80.18(12)
100.82(8)
86.40(8)
N(3)-Cr(1)-Cl(1)
N(3)-Cr(1)-Cl(1a)
N(4)-Cr(1)-Cl(1)
N(4)-Cr(1)-Cl(1a)
Cl(1)-Cr(1)-Cl(1a)
98.57(7)
166.83(7)
176.71(9)
92.03(8)
91.17(3)
Cr(1)-N(2)
Cr(1)-N(3)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
158.09(12) N(1)-Cr(1)-Cl(1)
101.29(11) N(1)-Cr(1)-Cl(1a)
79.73(13)
N(2)-Cr(1)-Cl(1a)
N(3)-Cr(1)-N(4)
78.40(11)
Complex 2Bc
Cr(1)-N(1)
2.129(4)
2.274(4)
2.116(4)
84.05(13)
157.65(14) N(1)-Cr(1)-Cl(1a)
79.15(14)
Cr(1)-N(4)
2.177(3)
N(2)-Cr(1)-N(3)
95.09(13)
78.32(12)
99.93(9)
167.97(9)
78.82(13)
N(3)-Cr(1)-Cl(1)
N(3)-Cr(1)-Cl(1a)
N(4)-Cr(1)-Cl(1)
N(4)-Cr(1)-Cl(1a)
Cl(1)-Cr(1)-Cl(1a)
100.45(10)
89.21(10)
178.00(10)
91.55(9)
Cr(1)-N(2)
Cr(1)-Cl(1)
2.3708(15) N(2)-Cr(1)-N(4)
2.7642(15) N(2)-Cr(1)-Cl(1)
101.69(10) N(2)-Cr(1)-Cl(1a)
Cr(1)-N(3)
Cr(1)-Cl(1a)
N(1)-Cr(1)-Cl(1)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
87.69(10)
N(3)-Cr(1)-N(4)
90.30(5)
Complex 4A
Cr(1)-N(1)
2.053(3)
2.060(3)
2.074(3)
160.35(13) N(1)-Cr(1)-Cl(1)
92.16(13)
80.12(13)
Cr(1)-N(4)
Cr(1)-Cl(1)
Cr(1)-Cl(2)
2.082(3)
N(2)-Cr(1)-N(3)
85.28(13)
80.23(13)
91.74(11)
99.50(10)
82.47(13)
N(3)-Cr(1)-Cl(1)
N(3)-Cr(1)-Cl(2)
N(4)-Cr(1)-Cl(1)
N(4)-Cr(1)-Cl(2)
Cl(1)-Cr(1)-Cl(2)
173.96(9)
92.25(9)
91.86(10)
174.71(11)
93.43(5)
Cr(1)-N(2)
2.2951(15) N(2)-Cr(1)-N(4)
Cr(1)-N(3)
2.2717(13) N(2)-Cr(1)-Cl(1)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
88.88(10)
N(2)-Cr(1)-Cl(2)
N(1)-Cr(1)-Cl(2)
100.06(10) N(3)-Cr(1)-N(4)
Complex 5
Cr(1)-N(1)
2.077(4)
2.061(4)
2.167(4)
Cr(1)-N(4)
2.157(4)
2.078(5)
2.095(4)
N(2)-Cr(1)-N(3)
N(2)-Cr(1)-N(4)
N(2)-Cr(1)-C(100)
N(2)-Cr(1)-C(101)
N(3)-Cr(1)-N(4)
83.45(16)
80.27(15)
88.55(19)
102.61(19) N(4)-Cr(1)-C(101)
79.08(14)
N(3)-Cr(1)-C(100)
N(3)-Cr(1)-C(101)
N(4)-Cr(1)-C(100)
171.04(18)
95.35(17)
95.63(18)
173.48(18)
Cr(1)-N(2)
Cr(1)-C(100)
Cr(1)-C(101)
Cr(1)-N(3)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
158.66(16) N(1)- Cr(1)-C(100) 90.55(18)
95.43(15)
78.61(15)
N(1)-Cr(1)-C(101)
98.72(19)
C(100)- Cr(1)-C(101) 90.3(2)
Complex 6
Cr(1)-N(1)
2.0797(14) Cr(1)-N(4)
2.1967(13) Cr(1)-C(19)
2.0928(13) Cr(1)-C(25)
78.66(5)
158.23(5)
80.69(5)
2.1455(14) N(2)-Cr(1)-N(3)
2.1015(16) N(2)-Cr(1)-N(4)
2.0743(14) N(2)-Cr(1)-C(19)
97.01(5)
79.23(5)
167.43(5)
95.95(5)
77.54(5)
N(3)-Cr(1)-C(19)
N(3)-Cr(1)-C(25)
N(4)-Cr(1)-C(19)
N(4)-Cr(1)-C(25)
C(19)-Cr(1)-C(25)
90.41(6)
99.80(5)
92.58(5)
174.07(5)
92.74(6)
Cr(1)-N(2)
Cr(1)-N(3)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
N(1)-Cr(1)-C(19)
N(1)-Cr(1)-C(25)
90.69(5)
N(2)-Cr(1)-C(25)
N(3)-Cr(1)-N(4)
101.86(5)
a Estimated standard deviations are indicated in parentheses. b Two crystallographically independent molecules are present in the asymmetric unit. c Atoms
labeled with an “a” are related to atoms not so labeled by a crystallographic inversion center.
Scheme 1
added to this solution, and the bottle was sealed, placed in a 40 °C
water bath, and connected to a constant 20 psig ethylene supply.
The solution was stirred with a magnetic stir bar. Polymerizations
were halted after 1 h by venting the reactor and treating the reactor
contents with 30 mL of 6 M HCl and 200 mL of methanol.
Polymeric product was collected by filtration, washed with
methanol, and dried under vacuum.
Results and Discussion
molecules in the asymmetric unit. The two molecules are
similar except for one significantly longer Cr-N bond length
in molecule 2, Cr(1a)-N(1a) ) 2.633 Å versus Cr(1)-N(1)
) 2.474 Å. At best, N(1) and N(1a) are only weakly
associated with their respective chromium centers, resulting
in a tetragonally elongated octahedral geometry around each
chromium atom with cis disposed chloride ligands. A slight
ruffling of the equatorial plane (defined by N(2), N(3), N(4),
and Cl(2)) is also observed, with the mean deviation of these
Synthesis and Structural Characterization of (TPA)-
CrCl2 (1). Treatment of anhydrous CrCl2 with 1 equiv of
tris(2-pyridylmethyl)amine (TPA) in tetrahydrofuran, fol-
lowed by recrystallization from acetonitrile, generated (TPA)-
CrCl2 (1) in good yield (Scheme 1). Figure 1 shows the
molecular structure of 1 as determined by X-ray crystal-
lography; selected bond distances and angles are summarized
in Table 2. This complex crystallizes with two independent
6880 Inorganic Chemistry, Vol. 42, No. 21, 2003

Cr(II/III) Complexes of Tris(2-pyridylmethyl)amine
Scheme 2
(alkyl or aryl) reagents at low temperature (-70 °C) resulted
in red-brown solutions that formed deep-green-blue mixtures
upon warming to room temperature. The residues, though
highly soluble in tetrahydrofuran and diethyl ether, did not
yield isolable products, perhaps hinting at the instability of
organometallic chromium(II) species containing the TPA
framework.
Synthesis and Structural Characterization of [(TPA)-
CrCl]2[BPh4]2 (2A) and [(TPA)CrCl]2[BArF ] (2B). The
4 2
long axial Cr-Cl bond in 1 suggested that this chloride might
be abstracted and replaced with a noncoordinating anion,
generating a cationic species with a vacant coordination site
cis to the remaining chloride. Halide abstraction (anion
exchange) with sodium tetraphenylborate, NaBPh4, is a
convenient way to generate coordinatively unsaturated metal
centers.19,46 Indeed, treatment of CrCl2 with 1 equiv each of
TPA and NaBPh4 (Scheme 2) in tetrahydrofuran generated
monochloro complex 2A. Substitution of the sodium salt of
Figure 1. Thermal ellipsoid representation (35% probability boundaries)
of the X-ray crystal structure of 1. Hydrogen atoms are omitted for clarity.
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBArF ) for
4
atoms from the plane being 0.14 Å. Moreover, the chromium
atom is displaced 0.14 Å out of this plane toward the axial
chloride.
NaBPh4 in Scheme 2 resulted in the formation of the
significantly more soluble BArF - analogue, 2B. For 2A, the
4
red-brown solid, isolated in nearly quantitative yield from
the reaction mixture, can be recrystallized from acetonitrile
to give analytically pure material in moderate yield (49%).
The isolated yield of 2A was reduced, in part, due to the
simultaneous formation of a cationic chromium(III) µ-oxo
dimer, {[(TPA)CrCl]2(µ-O)}[BPh4]2 (3). The dimer (whose
identity was confirmed by an X-ray crystallographic study)47
was deposited as dark-green crystals after further concentra-
tion of the filtrate remaining after crystallization of 2A.
Compound 3 presumably formed by reaction of 2A with trace
water or oxygen impurities in the solvent. Fortunately, dimer
3 is sufficiently more soluble than 2A to allow for the latter’s
isolation in analytically pure form.
The full extent of tetragonal distortion is evidenced by
the large difference in axial and equatorial Cr-N and Cr-
Cl bond lengths (Table 2). For molecule 1, the Cr-Nax bond
length is 0.34 Å longer than the average of its equatorial
counterparts, and the Cr-Clax bond is 0.24 Å longer than
Cr-Cleq. Similar statements can be made for molecule 2,
except that the difference between Cr-Nax and Cr-Neq bond
lengths is even more striking (0.51 Å difference). The
structure of 1 is consistent with the Jahn-Teller distortions44
expected for a high-spin d4 complex, and the room-
temperature magnetic moment (µeff ) 4.6 µB) of 1 agrees
with the high-spin (S ) 2) configuration. As expected, the
average Cr-Neq bond length in 1 is longer than that in the
known dimeric chromium(III)-TPA complexes,45 consistent
with the larger atomic radius of chromium(II).
The solid-state structure of 2A consists of well-separated
cations and anions. The cationic portion, shown in Figure 2,
consists of a weakly associated dimer of mononuclear
Thus far, attempts to synthesize neutral chromium(II)
organometallic derivatives, such as (TPA)CrR2 or (TPA)-
Cr(Cl)R, have led to highly soluble but intractable residues.
For example, treatment of 1 with organolithium or Grignard
(46) For a recent example, see: Aneetha, H.; Jimenez-Tenorio, M.; Puerta,
M. C.; Valerga, P.; Mereiter, K. Organometallics 2003, 22, 1779-
1782.
(47) X-ray crystallographic data for complex 3, {[(TPA)CrCl]2(µ-O)}-
[BPh4]2, C84H76B2Cl2Cr2N8O: monoclinic, P21/n, with a ) 10.137-
(2) Å, b ) 23.295(3) Å, c ) 15.472(3) Å, â ) 90.43(4)°, V )
3653.5(11) Å3, and Z ) 2 at 25 °C. Full-matrix least-squares
refinement based on F2 provided current residuals of R1 ) 0.0608
and wR2 ) 0.1097 with GOF ) 1.022 for 6441 reflections with I >
2σ(I) and 448 variables. Full details of the structure determination
will be reported elsewhere.
(44) (a) Jahn, H. A.; Teller, E. Proc. R. Soc. 1937, A161, 220. (b)
Ballhausen, C. J. Introduction to Ligand Field Theory; McGraw-Hill:
New York, 1962; p 193.
(45) For example, the average Cr-N bond length in [Cr2(TPA)2(µ-OH)2]-
Br4‚8H2O is 2.048 Å (ref 40b) compared to 2.136 Å (average of the
three Cr-Neq bonds) in 1.
Inorganic Chemistry, Vol. 42, No. 21, 2003 6881

Robertson et al.
Figure 3. Thermal ellipsoid representation (35% probability boundaries)
of the X-ray crystal structure of 4A. Hydrogen atoms, the tetraphenylborate
counterion, and the acetonitrile solvent molecule are omitted for clarity.
Figure 2. Thermal ellipsoid representation (35% probability boundaries)
of the X-ray crystal structure of 2A. Hydrogen atoms and tetraphenylborate
counterions are omitted for clarity.
solvolysis of the weakly associated dimers. Equally surpris-
ing, we observed that treatment of 2A with 1 equiv of the
strong donor ligand 4-(dimethylamino)pyridine (DMAP) left
2A unaltered. Thus, binding of neutral ligands to the cationic
chromium(II) centers appears to be weak, a feature that
forebodes relatively modest polymerization catalyst activity
(vide infra). On the basis of these observations, we believe
that crystal-packing effects are largely responsible for the
presence of bridging chloride ligands in the solid-state
structures for 2A and 2B and not any strong tendency for
chromium(II) to bind ligands at the axial site.
As found for 1, treatment of 2A with organolithium or
Grignard reagents at low temperature led to intractable, oily
residues upon warming to room temperature. Furthermore,
complexes 2A and 2B were unstable in dichloromethane,
undergoing what appeared to be oxidative halide abstraction
from the solvent to form the corresponding chromium(III)
complexes, 4A and 4B. Alternative syntheses of 4A and 4B,
along with structural characterization data, are provided in
the following section.
[(TPA)CrCl]+ fragments that are related by a crystallographi-
cally imposed inversion center. Important distances and
angles are collected in Table 2. Each monomeric chromium
center can be described as a tetragonally distorted octahedron,
with Cr(1), N(1), N(2), N(4), and Cl(1) residing in the
equatorial plane, and N(3) and the bridging Cl(1a) (from the
neighboring monomer) occupying the axial sites. Again, the
full extent of tetragonal distortion is shown by comparing
the long Cr-Clax (2.9388(11) Å) and short Cr-Cleq (2.3528-
(9) Å) bond lengths, as well as the long Cr-Nax (2.370(3)
Å) and short Cr-Neq (2.100(3), 2.100(3), and 2.131(3) Å)
distances. Alternatively, the geometry around chromium can
be described as square pyramidal, with N(1), N(2), N(4),
and Cl(1) in the basal plane and N(3) in the apical site. In
any event, the plane defined by N(1), N(2), N(4), and Cl(1)
is modestly ruffled, with the mean deviation of these atoms
from the plane being 0.10 Å. Given the asymmetry in axial
ligation, it is somewhat surprising that the chromium atom
is only raised 0.04 Å out of the plane toward N(3).
For either description, the tetragonal distortion from purely
octahedral geometry is consistent with that expected for a
high-spin chromium(II) center. Moreover, the room-temper-
ature magnetic moment for 2A is in line with magnetically
isolated, high-spin (S ) 2) chromium centers (µeff ) 4.6 µB
per Cr). The long Cr‚‚‚Cr (3.727(1) Å) and Cr‚‚‚Cl distances
presumably preclude any significant magnetic coupling
Synthesis and Structural Characterization of [(TPA)-
CrCl2][BPh4] (4A). The relatively straightforward synthesis
of 2, via formal chloride abstraction from (TPA)CrCl2,
suggested that a similar strategy could be used in conjunction
with CrCl3 to prepare cationic chromium(III) dichloride
derivatives [(TPA)CrCl2][BAr4]. Treatment of CrCl3(THF)3
with 1 equiv each of NaBPh4 and TPA in tetrahydrofuran
produced monomeric 4A (Scheme 3), with concomitant
formation of insoluble NaCl. Recrystallization from aceto-
nitrile yielded analytically pure 4A as the acetonitrile
monosolvate. As observed in the synthesis of 2 (Scheme 2),
between the chromium centers. The structure of 2B (BArF -
4
counterion) was also determined by X-ray crystallography.
The cationic portion was found to be similar to that of 2A,
except for shorter Cr‚‚‚Cl and Cr‚‚‚Cr distances in 2B (0.17
and 0.10 Å shorter, respectively). Interestingly, the room-
temperature magnetic moment of 2B (3.9 µB per Cr) was
lower than that of 2A. Although the shorter Cr‚‚‚Cr and Cr‚
‚‚Cl distances in 2B might enhance antiferromagnetic
coupling between the chromium centers, we have not
performed a detailed magnetic susceptibility study to deter-
mine if this is indeed the case.
substitution of NaBArF for NaBPh4 in Scheme 3 afforded
4
the more soluble BArF - analogue, 4B. The structure of 4A
4
(acetonitrile solvate), as determined by X-ray crystallography,
is depicted in Figure 3; important distances and angles are
provided in Table 2. The crystal lattice consists of a roughly
octahedral cationic chromium center along with well-
-
separated BPh4 and solvent molecules. In contrast to the
For 2A and 2B, we found it somewhat surprising that
recrystallization from acetonitrile did not generate monomeric
[(TPA)CrCl(MeCN)]+ cations, products that would form by
severe tetragonal distortion exhibited by 1 and 2, complex
4A is largely octahedral with Cr-N (2.053(3)-2.082(3) Å)
and Cr-Cl (2.2717(13)-2.2951(15) Å) bond lengths varying
6882 Inorganic Chemistry, Vol. 42, No. 21, 2003

Cr(II/III) Complexes of Tris(2-pyridylmethyl)amine
Scheme 3
over a narrow range. The room-temperature magnetic mo-
3
ment (µeff ) 3.9 µB, S ) /2) and the lack of significant
tetragonal distortion are consistent with the d3 electronic
configuration for 4A. Moreover, relative to complex 1, the
equatorial plane defined by N(1), N(2), N(4), and Cl(2)
displays minimal ruffling (mean deviation from the plane is
0.04 Å) and Cr(1) is displaced only 0.06 Å from this plane
toward the axial chloride (Cl(1)). It should be noted that
compounds 1 and 4A provide examples of chromium(II) and
chromium(III) complexes with identical inner coordination
spheres, containing four nitrogens from the tetradentate TPA
framework and two chloride ligands. The striking difference
in axial Cr-N and Cr-Cl bond lengths between 1 and 4A
(see Table 2) underscores the significant structural reorgan-
ization that accompanies the chromium(III) + e- f chro-
mium(II) transition.
Synthesis and Structural Characterization of [(TPA)-
Cr(CH3)2][BPh4] (5) and [(TPA)CrPh2][BPh4] (6). At-
tempts to form cationic organometallic complexes by treating
4A with 2.0 equiv of alkyl- or aryllithium reagents led to
the formation of deep-blue-green, intractable residues, per-
haps due to reduction of the chromium(III) center by the
strongly reducing organolithium reagents. These residues
were reminiscent of those resulting from attempted alkylation
of chromium(II) derivatives 1 and 2 (vide supra). However,
use of 2.0 equiv of milder alkylating/arylating reagents, such
as MeMgCl and PhMgCl, allowed for generation and
isolation of stable, cationic organometallic derivatives [(TPA)-
Cr(CH3)2][BPh4] (5) and [(TPA)CrPh2][BPh4] (6), respec-
tively (Scheme 3). For 5, addition of 1,4-dioxane facilitated
the removal of MgCl2 from the reaction mixture. For 6, the
diphenyl complex precipitated out of tetrahydrofuran solu-
tion, allowing for its separation from the more soluble MgCl2
coproduct. In either case, the red-brown solids that were
ultimately isolated were readily recrystallized from dichlo-
romethane or acetonitrile to give analytically pure crystalline
material. The structures of dimethyl complex 5 (methylene
chloride hemisolvate) and diphenyl complex 6 (acetonitrile
solvate) were confirmed by X-ray crystallography. Selected
bond lengths and angles are given in Table 2.
Figure 4. Thermal ellipsoid representation (35% probability boundaries)
of the X-ray crystal structures of 5 (top) and 6 (bottom). Tetraphenylborate
counterions, solvent molecules, and most hydrogen atoms are omitted for
clarity.
Except for the substitution of carbon for chloride (and the
necessarily shorter Cr-C bond lengths), the basic octahedral
geometry of 5 (Figure 4) is similar to that of 4A. However,
the Cr-N bond lengths in 5 (2.061-2.167 Å) vary over a
somewhat wider range than those in 4A (2.053(3)-2.082(3)
Å) with the longest Cr-N bonds (2.157(4) and 2.167(4) Å)
Inorganic Chemistry, Vol. 42, No. 21, 2003 6883

Robertson et al.
Table 3. Results of Polymerization Experimentsa
catalyst
system
Cr,
µmol
Co-cat/Cr
mole ratio
yield,
g
Tm,
°C
run no.
activityb
Mw
Mw/Mn
1
2
3
4
5
6
7
8
9
1, MAO
50
10
10
10
10
10
50
50
10
50
50
200
200
20
200
10
1.09
0.14
trace
0.15
trace
trace
0.65
1.11
0.30
1.13
1.35
16.0
10.3
∼0
9000
11300
2.8
2.5
127.9
2A, MAOc
2A, Et2AlClc
2B, MAOc
11.0
∼0
41900
7.5
130.0
2B, TMSMLid,e
d
2B, Al(i-Bu)3
20
∼0
4A, MAO
4B, MAO
5, MAOc
5, MAO
200
200
200
200
200
9.6
3600
5800
34600
5600
2.0
2.4
7.2
2.5
3.5
123.9
126.0
131.7
16.3
22.0
16.6
19.8
10
11
6, MAO
11200
128.7
a See the Experimental Section for details. Polymerizations performed in dichloromethane unless stated otherwise. b Activity expressed in g PE produced
per (mmol Cr-atm-hr). c Polymerization performed in chlorobenzene. d Polymerization performed in toluene. e TMSMLi ) trimethylsilylmethyllithium
((CH3)3SiCH2Li).
trans to the methyl carbon atoms, C(100) and C(101). The
presence of longer Cr-N bonds opposite the methyl groups
is consistent with the methyl group’s strong σ-donor ability.
The Cr-CH3 distances in 5 (2.078(5) and 2.095(4) Å) are
fairly typical of other methylchromium(III) complexes found
in the literature,48 including those in the structurally related
chromium(III) trispyrazolylborate complex, Tp*Cr(CH3)2-
(DMAP) (Tp* ) hydrotris(3,5-dimethylpyrazolyl)borate).49
In the latter compound, the facially coordinating Tp* ligand
forces the two methyl groups into a cis arrangement with a
C-Cr-C angle of 91.2(5)°. The corresponding angle in 5
is 90.3(2)°. As observed for 5, the two longest Cr-N bonds
in Tp*Cr(CH3)2(DMAP) (2.179(9) and 2.202(9) Å) are those
trans to the Cr-methyl linkages.
Polymerization Experiments. Complexes 1, 2A, 2B, 4A,
4B, 5, and 6 were evaluated as polymerization catalysts in
combination with various activators. Table 3 summarizes our
findings. It is well-known that transition-metal alkyl com-
plexes possessing vacant or accessible coordination sites cis
to the metal-carbon bond are often useful polymerization
catalysts. Olefin binding at the vacant site, followed by
insertion into the metal-carbon bond, promulgates the chain.
Thus, one might have expected that 2A and 2B, once
alkylated, would generate active catalyst sites. However,
treatment of 2A or 2B with common alkylating agents such
as trimethylsilylmethyllithium ((CH3)3SiCH2Li), diethylalu-
minum chloride, or triisobutylaluminum did not yield active
polymerization catalysts (runs 3, 5, and 6). Poor binding of
the olefin to chromium, which is consistent with the inability
of these cationic chromium(II) centers to bind even strong
donor ligands, would contribute to low catalyst activity.
Originally, we also wondered whether coordination of the
BPh4- anion to an alkylated chromium(II) site might interfere
with olefin binding and contribute to poor catalyst activity.
However, this is probably not the root cause for low activity
since complex 2B, which contains the noncoordinating
tetrakis(3,5-bis(trifluoromethyl)phenyl) borate anion, is also
inactive in the presence of alkylating cocatalysts (runs 5 and
6).
Complex 6, isolated as an acetonitrile solvate, also
possesses a slightly distorted octahedral geometry around
chromium (Figure 4). As with 5, the longest Cr-N bonds
in 6 (2.1967(13) and 2.1455(14) Å) are those trans to the
Cr-C bonds, consistent with the strong σ-donor ability of
the phenyl group. For 6, an additional slight distortion results
from the displacement of N(2) away from the C(19)-Cr(1)
axis, yielding a slightly more acute C(19)-Cr(1)-N(2) angle
(167.43(5)°) compared to the same angle in 5 (C(100)-
Cr(1)-N(3) ) 171.04(18)°). Similar to dichloro complex
4A, the equatorial planes in 5 and 6 display minimal ruffling;
the mean deviations of the three nitrogen and one carbon
atoms from their equatorial planes are 0.07 and 0.04 Å,
respectively. Furthermore, the chromium atoms in 5 and 6
are raised only slightly out of their equatorial planes (0.039
and 0.060 Å, respectively). The relatively undistorted
structures for 5 and 6, along with their room-temperature
magnetic moments (each 3.5 µB) are consistent with the d3
For complexes 1 and 4A, treatment with common alkyl-
ating agents, such as diethylaluminum chloride and triisobu-
tylaluminum, did not yield active catalysts (data not shown).
This is not surprising given that 1 and 4A are coordinatively
saturated. However, one might have predicted that a com-
petent catalyst would form by abstraction of a methyl group
from 5 with B(C6F5)3, generating a chromium(III)-methyl
complex with a vacant coordination site cis to the metal-
alkyl bond. Unfortunately, in repeated small-scale polym-
erization tests, reaction of 5 with 1 equiv of B(C6F5)3 did
not lead to catalytically active systems.
3
(S ) /2) electronic configurations expected for octahedral
chromium(III) complexes. Finally, although 5 and 6 are
formally 15-electron organometallic complexes, they are
thermally robust and are relatively air and moisture stable,
especially when compared to Cp-containing chromium(III)
compounds.6-8
However, all compounds did produce active catalysts in
the presence of MAO (runs 1, 2, 4, 7-11). In all cases,
polymerization activity (10-20 g of PE/(mmol of Cr hr atm))
was moderate according to the activity scale proposed by
Gibson.19,20 With regard to polymeric products, GPC, 13C
NMR (no branching detected), and polymer melting point
(48) For example, the Cr-Me bond length in (cyclo-C4H8NC2H4C5Me4)-
CrMe2 is 2.080(3) Å (ref 11) and that in Cp*CrMe2(pyridine) is
2.091(5) Å (ref 5d).
(49) Mashima, K.; Oshiki, T.; Tani, K.; Aoshima, T.; Urata, H. J.
Organomet. Chem. 1998, 569, 15-19.
6884 Inorganic Chemistry, Vol. 42, No. 21, 2003

Cr(II/III) Complexes of Tris(2-pyridylmethyl)amine
data were consistent with the formation of low to moderate
molecular weight high-density polyethylene. Polymers pro-
duced in dichloromethane (runs 7, 8, and 10) typically
possessed lower molecular weight than those produced in
chlorobenzene (runs 2, 4, and 9), suggesting that dichlo-
romethane acts as an effective chain termination agent.
Chromium complexes ligated by anionic and neutral non-
Cp ligands often yield low to moderate activity catalysts
(typically <100 g of PE/(mmol of Cr hr atm)) when activated
by MAO.19 However, five reported chromium(III)-ligand
combinations are exceptionally active. The five ligands, along
with the highest reported activity (in g of product/(mmol of
Cr hr atm)), are pyridine bisimine (9000),31b salicylaldiminato
with bulky ortho substituents (7000),22b aminebis(thioether)
(300),35 triazacyclohexane (700),30 and bis(diarylphosphino)-
methylamine (2600).36 Indeed, it is these exceptions that
motivates further work using various chromium-ligand
combinations. The activities of the chromium(II) and chro-
mium(III) TPA complexes reported herein are lower than
these exceptionally active catalysts but are comparable to,
or slightly higher than, MAO-activated chromium catalysts
based on â-diketiminate (30),27 imino-pyrrolide (5),24,25 bis-
(imidazole) (5),34 pyridine bis(oxazolinyl) (6),32 and reduced
Schiff base (4)23 ligands. Finally, some of the latter com-
pounds are five-coordinate and are more active with alkyl
aluminum chloride cocatalysts (e.g., Et2AlCl) than with
MAO. For example, the activity of chromium(III)-reduced
Schiff base complexes increases to 130 g/(mmol of Cr hr
atm) when activated by diethylaluminum chloride.23 In
contrast, the chromium TPA complexes, as previously stated,
require MAO to form active catalyst systems.
tetradentate TPA ligand. Both neutral (monomeric) and
cationic (weakly associated dimeric) chromium(II) complexes
were synthesized. Attempts to form stable organometallic
chromium(II) species led to intractable products. In the case
of cationic chromium(III) derivatives, dichloro as well as
dimethyl and diphenyl complexes were prepared, the latter
two demonstrating the stability of the chromium(III)-carbon
bond in the TPA ligand environment. None of the compounds
catalyzed the polymerization of ethylene in the presence of
traditional alkylating cocatalysts, but all were active catalysts
in the presence of MAO. Catalyst activity was relatively
modest, perhaps due to the inability of ethylene to bind
appreciably to the rather congested (TPA)Cr centers. Our
current research efforts are centered on the preparation of
chromium complexes supported by less sterically demanding
tridentate bis(2-pyridylmethyl)amine ligands. Results of that
work will be the focus of future reports.
Acknowledgment. We thank the donors of the Petroleum
Research Fund, administered by the American Chemical
Society (Grant 37885-GB3 to M.J.C.), Research Corporation
(Cottrell College Science Award to M.J.C.), the National
Science Foundation (CHE-0078746 to J.A.H.), and the
University of WisconsinsEau Claire for financial support
of this research. We also wish to thank Albemarle Corpora-
tion for providing MAO and tris(pentafluorophenyl)boron
and Mr. Walt Dutton, Mr. Gregory Listvoyb, and Dr. Ron
Supkowski (W. R. Grace & Co.) for polymer analyses.
Supporting Information Available: Complete X-ray crystal-
lographic information in CIF and PDF format. This material is
available free of charge on the Internet at http://pubs.acs.org.
Summary
We have prepared and structurally characterized several
chromium(II) and chromium(III) complexes containing the
IC034530I
Inorganic Chemistry, Vol. 42, No. 21, 2003 6885