Chromium complexes supported by the multidentate monoanionic N2P2 ligand: Reduction chemistry and reactivity with ethylene

Article abstract of DOI:10.1021/om900695s

Chromium complexes supported by the multidentate monoanionic ligand [N ₂P₂] (H[N₂P₂] = BuN(H)SiMe ₂N(CH₂CH₂PⁱPr₂) ₂} are presented, and the activ

Full text of DOI:10.1021/om900695s

Organometallics 2009, 28, 6243–6253 6243
DOI: 10.1021/om900695s
Chromium Complexes Supported by the Multidentate Monoanionic N2P2
Ligand: Reduction Chemistry and Reactivity with Ethylene
Sergio S. Rozenel, Wayne A. Chomitz, and John Arnold*
Department of Chemistry, University of California, Berkeley, California 94720-1460
Received August 5, 2009
Chromium complexes supported by the multidentate monoanionic ligand [N2P2] {H[N2P2] =
tBuN(H)SiMe2N(CH2CH2PiPr2)2} are presented, and the activity of these complexes toward
ethylene oligomerization/polymerization is examined. The complexes [N2P2]CrCl2 (1) and
[N2P2]CrCl (2) polymerized ethylene after activation with MAO. Derivatives of 1 and 2 were
synthesized in order to gain insights about the active species in the ethylene oligomerization/
polymerization processes. The alkyl complexes [N2P2]CrMe (3), [N2P2]CrCH2SiMe3 (4), and
[N2P2]Cr(Cl)CH2SiMe3 (5), the cationic species {[N2P2]CrCl}BF4 (7), {[N2P2]CrCl}BPh4 (8), and
{[N2P2]CrCH2SiMe3}BF4 (9), and the Cr(II) complex [N2P2]CrOSO2CF3 (11) were not active
ethylene oligomerization/polymerization catalysts in the absence of an activator. Reaction of 1 with
2 equiv of MeLi led to reduction to 3; however, with 1 equiv of MeLi the stable mixed alkyl halide
derivative [N2P2]Cr(Cl)Me (6) was obtained. Reaction of 2 with Red-Al produced the bridging di-
hydride {([N2P2]Cr)(μ-H)}2 (10), which reacted with CO to produce the Cr(I) complex [N2P2]Cr-
(CO)2 (12). Reduction of 2 with KC8 in the presence of p-tolyl azide produced the dimeric cis μ-imido
{([N2P2]Cr)(μ-NC7H7)}2 (13). A similar reduction in the presence of ethylene resulted in the isolation
of the Cr(III) metallacyclopentane compound [N2P2]CrC4H8 (14).
Introduction
variety of polyethylenes and can be used without an activator.9
Although Phillips-type catalysts have been known for more than
50 years, the active species, as well as the mechanism of the
transformation, remain topics of debate.8-12
Of special interest is the selective oligomerization of
ethylene to 1-hexene and 1-octene due to the use of these
co-monomers in the production of LLDPE.10-19 Chromium
is the most commonly used metal for these oligomerization
processes due to its high activity and selectivity,20 although
Tetradentate monoanionic (TDMA) ligands are useful
frameworks for stabilizing reactive transition metal species.1
We have discovered that the ligand [N2P2] {H[N2P2]=tBuN-
(H)SiMe2N(CH2CH2PiPr2)2} is capable of supporting an
extensive range of transition metals in a variety of oxidation
states.2-7 The mixture of hard and soft donors and the
flexibility of the [N2P2] ligand allow it to alter its coordina-
tion mode to adhere to the electronic and geometric prefer-
ences of the metal center. Given the adaptability of the [N2P2]
ligand set, we viewed it as an appropriate ligand scaffold for
examining the proposed intermediates in the Cr-catalyzed
ethylene oligomerization/polymerization cycles.
Chromium catalysts are widely used industrially for the
polymerization of ethylene: Phillips-type catalysts (Cr/SiO2)
are responsible for the production of one-third of the polyethy-
lene sold worldwide.8 In comparison to Ziegler-Natta catalysts,
chromium catalysts have the advantage of producing a wide
(9) McDaniel, M. P. Adv. Catal. 1985, 33, 47–98.
(10) Bowen, L. E.; Charernsuk, M.; Wass, D. F. Chem. Commun.
2007, 2835–2837.
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Duchateau, R. Organometallics 2005, 24, 5214–5216.
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J. Organomet. Chem. 2004, 689, 3641–3668.
(14) Freeman, J. W. B., J. L.; Knudsen, R. D. (Phillips Petroleum
Company) US 5856257, 1999.
(15) Lashier, M. E. (Phillips Petroleum Company) EP 0780353A1,
1997.
(16) Manyik, R. M. W., W. E.; Wilson, T. P. (Union Carbide
*To whom correspondence should be addressed. E-mail: arnold@
berkeley.edu.
Corporation) U.S. 3300458, 1967.
(17) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess, F. M.;
Dixon, J. T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605–3610.
(18) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C. H.;
Englert, U.; Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334–335.
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Huyser, J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207–1222.
(20) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duch-
ateau, R. Organometallics 2006, 25, 715–718.
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2009 American Chemical Society
Published on Web 10/06/2009
pubs.acs.org/Organometallics

6244 Organometallics, Vol. 28, No. 21, 2009
Rozenel et al.
other metals have also been used.21-23 These processes are
thought to involve a metallacyclic intermediate, which is
formed by the oxidative coupling of two ethylene molecules
to the metal center.11,24-30
dried samples. X-ray crystal diffraction analyses were per-
formed at the University of California, Berkeley CHEXRAY
facility. Li[N2P2],6 [N2P2]CrCl2 (1),6 CrCl2(THF),34 and CrCl3-
(THF)335 were prepared according to literature procedures. The
remaining starting materials were obtained from Aldrich and
used without further purification. Melting points were deter-
mined using sealed capillaries prepared under nitrogen and are
uncorrected.
The oxidation state of the chromium species participating
in olefin oligomerization/polymerization processes remains
ambiguous.31 While the majority of reports attribute the
catalytic activity to the Cr(I)/Cr(III) couple, Gambarotta
and co-workers have proposed that Cr(III) is reduced by the
alkyl aluminum activator to Cr(II) in the early stages of the
catalytic cycle, and oxidative coupling of two ethylene
molecules produces a Cr(IV) metallacycle.12 McGuinness
and co-workers, studying Cr(II) and Cr(III) species sup-
ported by the ligands Ph2PCH2CH2NHCH2CH2PPh2 and
tBuSCH2CH2NHCH2CH2StBu, favor a mechanism consis-
tent with the cationic Cr(II)/Cr(IV) couple over the neutral
Cr(I)/Cr(III) couple, given that MAO is normally thought to
form cationic metal centers.31
In the course of this investigation, it was found that the
[N2P2] ligand supported both [N2P2]Cr(II) and [N2P2]Cr(III)
derivatives. Facile access to both oxidation states allowed for
the isolation of proposed intermediates along the ethylene
trimerization/polymerization process, the study of their
redox chemistry, and the investigation of the activity of each
toward ethylene oligomerization/polymerization in order to
gain more information about these types of transformations.
[N2P2]CrCl (2). A solution of Li[N2P2] (0.66 g, 1.5 mmol) in
10 mL of toluene was added dropwise to a suspension of
CrCl2(THF) (0.29 g, 1.5 mmol) in 20 mL of toluene at room
temperature. The solution turned from yellow to deep blue in
5 min. The solution was stirred overnight, filtered, concentrated,
and cooled to -40 °C, resulting in the formation of light blue,
needle-like crystals (0.45 g, 58%). Crystals suitable for an
X-ray diffraction study were grown from a concentrated solu-
1
tion of toluene cooled to -40 °C. H NMR (C6D6): 12.7 (br),
3.31 (br), 1.73 (br), 1.40 (br), 1.19 (br), 1.15 (br), 1.08 (br), 0.88
(br), 0.32(br). IR (cm-1): 1248 (m), 1219 (m), 1088 (s), 1021 (s),
936 (w), 854 (s), 802 (m), 731 (w), 608 (w). UV-vis (nm): 625,
815. Anal. Calcd: C, 50.70; H, 9.86; N, 5.38. Observed: C, 50.71;
H, 10.23; N, 5.74. Mp: 172-175 °C. μeff=4.7 μB.
[N2P2]CrMe (3). Method A. A 1.6 M solution of MeLi
(0.44 mL, 0.70 mmol) in diethyl ether was added to a solution
of [N2P2]CrCl (0.34 g, 0.64 mmol) in 15 mL of THF at -78 °C.
The solution immediately turned from blue to brown. The
reaction mixture was allowed to warm to room temperature
and was stirred for 1 h. The solvent was removed under vacuum,
and the product was extracted with pentane (2ꢀ10 mL). The
solution was concentrated and cooled to -40 °C to give purple,
block-like crystals (0.12 g, 38%).
Experimental Section
Method B. A 1.6 M solution of MeLi (1.0 mL, 1.6 mmol) in
diethyl ether was added to a suspension of [N2P2]CrCl2 (0.40 g,
0.72 mmol) in 25 mL of diethyl ether at -78 °C. The solution
immediately turned brown. The reaction mixture was allowed to
warm to room temperature and was stirred for 10 h. The solvent
was removed under vacuum, and the product was extracted with
pentane (2ꢀ10 mL) and crystallized from pentane at -40 °C to
give block-like purple crystals (0.17 g, 47%).
General Procedures. Unless otherwise noted, all reactions
were performed using standard Schlenk and N2-atmosphere
glovebox techniques. Solvents were dried by passing through a
column of activated alumina and degassed with nitrogen.32
C6D6 and CDCl3 were dried over Na/benzophenone and
CaH2, respectively, and vacuum transferred. All NMR spectra
were obtained in C6D6 or CDCl3 at ambient temperature using
Bruker AV-300 or AVQ-400 spectrometers. 1H NMR chemical
shifts (δ) were calibrated relative to residual solvent peak.
Magnetic susceptibility measurements were performed in
C6D6 or CDCl3 according to the Evans NMR method.33 Infra-
red (IR) spectra were recorded with a Mattson Genesis Series
FTIR spectrophotometer as Nujol mulls between KBr plates.
UV-vis spectra were determined with a Varian Cary 50 UV-vis
spectrophotometer using a 1 cm quartz cell. Elemental analyses
and mass spectral data (EI-MS) were obtained at the University
of California, Berkeley Microanalytical Facility, using vacuum-
Method C. A suspension of KC8 (0.11 g, 0.81 mmol) in 15 mL
of THF was added to a solution of [N2P2]CrCl (0.40 g,
0.77 mmol) in 20 mL of THF at -78 °C. A solution of AlMe3
in hexanes (1.2 mL, 2.0 M) was added while keeping the
temperature at -78 °C. The mixture was warmed to room
temperature and was stirred for 1 h. The solvent was removed
under vacuum, and the product was extracted with pentane (2 ꢀ
10 mL) and crystallized at -40 °C to give block-like purple
crystals (0.11 g, 29%). Crystals suitable for an X-ray diffraction
study were grown from a concentrated solution of pentane
1
cooled to -40 °C. H NMR (C6D6): 3.10 (br), 1.02 (br), 0.35
(br), 0.11 (br), -0.03 (br). IR (cm-1): 1259 (m), 1092 (m), 1016
(m), 800 (m), 720 (w). UV-vis (nm): 400, 620. MS: m/z 500
(Mþ). Anal. Calcd: C, 55.17; H, 10.87; N, 5.59. Observed: C,
54.81; H, 11.22; N, 5.77. Mp: 93-94 °C. μeff=4.5 μB.
(23) Santi, R. R., A. M.; Grande, M.; Sommazzi, A.; Masi, F.; Proto,
A. (ENICHEM S.P.A.) WO 0168572, 2001.
(24) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2006, 25, 2733–2742.
(25) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2007, 129, 14281–14295.
(26) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am.
[N2P2]CrCH2SiMe3 (4). A solution of LiCH2SiMe3 (0.13 g,
1.4 mmol) in 10 mL of toluene was added to a solution of
[N2P2]CrCl (0.52 g, 1.0 mmol) in 20 mL of toluene at room
temperature. The solution immediately turned from blue to
purple, and the reaction mixture was stirred overnight. The
solvent was removed under vacuum, and the product was
extracted with pentane (2ꢀ10 mL). The solution was concen-
trated and cooled to -40 °C to give purple, block-like crystals
(0.24 g, 53%). Crystals suitable for an X-ray diffraction study
were grown from a concentrated solution of pentane cooled
to -40 °C. 1H NMR (C6D6): 9.07 (br), 3.23 (br), 1.67 (br), 1.31
(br), 1.18 (br), 1.07 (br), 0.36 (br), 0.28 (br), 0.10 (br). IR
Chem. Soc. 2004, 126, 1304–1305.
(27) 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, 14712–14713.
(28) Emrich, R.; Heinemann, O.; Jolly, P. W.; Kruger, C.; Verhovnik,
G. P. J. Organometallics 1997, 16, 1511–1513.
(29) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm, S.; Was-
serscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 4337-þ.
(30) Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.; Gibson,
V. C. J. Am. Chem. Soc. 2005, 127, 10166–10167.
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J. T. Organometallics 2005, 24, 552–556.
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Educ. 2001, 78, 64–64.
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Article
Organometallics, Vol. 28, No. 21, 2009 6245
(cm-1): 1246 (m), 1211 (m), 1087 (m), 1048 (w), 1019 (m), 850
(s), 794 (m), 735 (w). Anal. Calcd: C, 54.51; H, 10.91; N, 4.89.
to a solution of [N2P2]CrCl (0.60 g, 1.1 mmol) in 25 mL of THF
at -78 °C. The solution turned from purple to dark brown. The
reaction mixture was warmed to room temperature and stirred
for 1 h, after which the solvent was removed under vacuum. The
crude product was extracted with pentane (2ꢀ10 mL), and the
solution was concentrated and cooled to -40 °C to give brown,
block-like crystals (0.47 g, 84%). Crystals suitable for an X-ray
diffraction study were grown from a concentrated solution of
Observed: C, 54.44; H, 11.23; N, 4.92. Mp: 65-68 °C. μeff
4.4 μB.
=
[N2P2]Cr(Cl)CH2SiMe3 (5). A solution of LiCH2SiMe3 (0.06g,
0.63 mmol) in 10 mL of toluene was added to a solution of
[N2P2]CrCl2 (0.35 g, 0.63 mmol) in 15 mL of toluene at room
temperature. The solution immediately turned from purple to
dark blue, and the reaction mixture was stirred overnight. The
solvent was removed under vacuum, and the product was
extracted with pentane (2ꢀ10 mL). The solution was concen-
trated and cooled to -40 °C to give purple, block-like crystals
(0.22 g, 57%). Crystals suitable for an X-ray diffraction study
were grown from a concentrated solution of diethyl ether cooled
to -40 °C. 1H NMR (C6D6): 12.2 (br), 3.21(br), 2.09 (br), 1.05
(br), 0.22 (br), 0.10 (br), -4.1 (br), -9.0 (br). IR (cm-1): 1259
(m), 1089 (w), 1050 (w), 1019 (m), 842 (w), 799 (m). Anal. Calcd:
C, 51.33; H, 10.27; N, 4.60. Observed: C, 51.31; H, 10.02;
N, 4.42. Mp: 96-99 °C. μeff=3.9 μB.
[N2P2]Cr(Cl)Me (6). A 1.6 M solution of MeLi in diethyl ether
(0.85 mL, 1.4 mmol) was added to a solution of [N2P2]CrCl2
(0.75 g, 1.3 mmol) in 40 mL of THF at -78 °C. The solution was
allowed to warm to room temperature, giving a deep purple
solution. After stirring the reaction overnight, the solvent was
removed under vacuum, and the product was extracted with
diethyl ether (2ꢀ10 mL). Cooling to -40 °C gave the product as
purple crystals (0.49 g, 68%). 1H NMR (C6D6): 11.2 (br), 3.21
(br), 1.65 (br), 1.08 (br), 0.27 (br), -7.18 (br). 31P NMR (C6D6):
45.65 ppm (br). IR (cm-1): 1259 (s), 1087 (s), 1061 (s), 1016 (s),
885 (w), 850 (w), 800 (s), 605 (m). Anal. Calcd: C, 51.52; H,
10.15; N, 5.22. Observed: C, 51.60; H, 10.08; N, 5.49. Mp:
128-130 °C. μeff=3.8 μB.
{[N2P2]CrCl}BF4 (7). A solution of AgBF4 (0.24 g, 1.2 mmol) in
20 mL of THF was added to a solution of [N2P2]CrCl (0.60 g,
1.2 mmol) in 25 mL of THF at room temperature. The solution
was stirred overnight, resulting in a blue to green color change and
precipitation of a gray solid. The solvent was removed under
vacuum. The solid was washed with diethyl ether, and the product
was extracted with THF (3ꢀ10 mL). The solution was concen-
trated and cooled to -40 °C to give green, block-like crystals
(0.44 g, 64%). Crystals suitable for an X-ray diffraction study were
grown from a concentrated solution of THF cooled to -40 °C. 1H
NMR (CDCl3): 22.2 (br), 16.0 (br), 9.7 (br), 3.77 (s), 1.86 (s), 1.14
(br), 1.16 (s), 0.08 (s), -11.4 (br). 19F NMR (CDCl3): -152 (s). IR
(cm-1): 1261 (m), 1191 (w), 1061 (s), 1014 (m), 852 (m), 756 (m),
722 (w). Anal. Calc: C, 43.46; H, 8.46; N, 4.61. Observed: C, 43.84;
H, 8.39; N, 4.24. Mp: 188-191 °C. μeff=3.8 μB.
1
HMDSO cooled to -40 °C. H NMR (C6D6): 4.47 (br), 4.19
(br), 4.02 (br), 3.32 (br), 3.23 (br), 2.54 (br), 1.69 (br), 1.43 (s),
1.12 (s), 0.86 (s) 0.30 (s), 0.24 (s). IR (cm-1): 1347 (m), 1259 (m),
1208 (m), 1083 (s), 1033 (m), 1015 (s), 919 (w), 848 (m), 796 (m),
755 (w), 728 (m). Anal. Calcd: C, 54.29; H, 10.77; N, 5.76.
Observed: C, 54.09; H, 10.52; N, 5.53. Mp: 88-90 °C. μeff
2.2 μB.
=
[N2P2]CrOSO2CF3 (11). A solution of AgOSO2CF3 (0.19 g,
0.74 mmol) in 15 mL of toluene was added to a solution of
{([N2P2]Cr)(μ-H)}2 (0.35 g, 0.36 mmol) in 25 mL of THF at
room temperature. The solution was refluxed overnight, the
solvent was removed under vacuum, and the product was
extracted with toluene (3ꢀ10 mL). The solution was concen-
trated and cooled to -40 °C to give blue, needle-like crystals
(0.18 g, 39%). Crystals suitable for an X-ray diffraction study
were grown from a concentrated solution of toluene cooled
to -40 °C. 1H NMR (C6D6): 14.7 (br), 10.5 (br), 8.1 (br), 3.26
(br), 1.64 (br), 1.18 (br), 1.11 (br), 0.24 (br), -1.01 (br). IR
(cm-1): 1318 (s), 1235 (s), 1214 (s), 1171 (m), 1159 (w), 1098 (w),
1032 (s), 912 (w), 855 (m), 734 (w), 634 (s). Anal. Calcd: C, 43.52;
H, 8.10; N, 4.41: S, 5.05. Observed: C, 43.23; H, 7.89; N, 4.19: S,
5.23. Mp: 155-157 °C. μeff=4.6 μB.
[N2P2]Cr(CO)2 (12). Carbon monoxide was added into a
solution of {([N2P2]Cr)(μ-H)}2 (0.30 g, 0.31 mmol) in 25 mL
of pentane at -78 °C. The reaction mixture was allowed to warm
to room temperature and was stirred overnight, resulting in a
color change from brown to dark red. The solvent was removed
under vacuum, and the product was extracted with diethyl ether
(2 ꢀ 10 mL). The solution was concentrated and cooled to
-40 °C to give brown, block-like crystals (0.18 g, 53%). Crystals
suitable for an X-ray diffraction study were grown from a
concentrated solution of diethyl ether cooled to -40 °C. 1H
NMR (C6D6): 3.27 (s), 3.26 (s), 2.28 (br), 1.17 (br), 1.12 (br), 0.81
(br), 0.29 (br). IR (cm-1): 1872 (vs), 1742 (vs), 1352 (s), 1248 (s),
1200 (s), 1045 (s), 1018 (s), 890 (m), 845 (s), 746 (m), 670 (w).
Anal. Calcd: C, 53.21; H, 9.49; N, 5.17. Observed: C, 53.05; H,
9.84; N, 5.44. Mp: 123-124 °C. μeff=1.8 μB.
{([N2P2]Cr)(μ-NC7H7)}2 (13). A solution of p-tolyl azide (0.14 g,
1.1 mmol) dissolved in 10 mL of THF and a suspension of KC8
(0.15 g, 1.1 mmol) in 10 mL of THF were added to a solution of
[N2P2]CrCl (0.60 g, 1.1 mmol) in 30 mL of THF at -78 °C. The
solution turned from blue to dark brown and gas evolution was
observed. The reaction mixture was warmed to room temperature
and was stirred for 2 h. The solvent was removed under vacuum, and
the product was extracted with pentane (2ꢀ10 mL). The solution
was concentrated and cooled to -40 °C to give brown, block-like
crystals (0.30 g, 44%). Crystals suitable for an X-ray diffraction
study were grown from a concentrated solution of pentane cooled to
-40 °C. 1H NMR (C6D6): 7.10 (s) 3.24 (m), 2.89 (s), 1.65 (br), 1.55
(br), 1.40 (br), 1.18 (s), 1.07 (m), 0.94 (s), 0.29 (s), 0.24 (s), 0.17 (br).
31P NMR: 4.76 (s). IR (cm-1): 1595 (w), 1487 (s), 1266 (s), 1209 (s),
1101 (w), 1060 (m), 1047 (m), 903 (w), 847 (s), 814 (s), 777 (m), 756
(m). Anal. Calcd: C, 58.95; H, 9.89; N, 7.11. Observed: C, 58.70; H,
9.80; N, 6.95. Mp: 173-177 °C. μeff=0 μB.
[N2P2]CrC4H8 (14). A suspension of KC8 in THF was added
by cannula to a solution of [N2P2]CrCl (0.80 g, 1.5 mmol) in
25 mL of THF at -78 °C, while maintaining 1 atm ethylene
pressure. The solution turned from blue to dark brown, and a
dark gray precipitate was observed. The reaction mixture was
warmed to room temperature and stirred for 1 h. The solvent
was removed under vacuum, and the product was extracted with
pentane (2ꢀ10 mL) to give a green solution. Concentration and
{[N2P2]CrCl}BPh4 (8). The synthesis was carried out analo-
gously to that of 7 starting with AgBPh4 (0.33 g, 0.77 mmol)
and [N2P2]CrCl (0.40 g, 0.77 mmol), to yield dark green, block-
like crystals (0.19 g, 29%). Crystals suitable for an X-ray
diffraction study were grown from a concentrated solution
1
of THF cooled to -40 °C. H NMR (CDCl3): 22.4 (br), 16.5
(br), 7.44 (br), 7.08 (br), 3.76 (br), 3.18 (br), 1.88 (br), 1.18 (br),
0.10 (br), -12.9 (br). IR (cm-1): 1578 (m), 1256 (s), 1185 (m),
1091 (m), 1021 (s), 886 (m), 851 (s), 799 (m), 759 (m), 731 (s),
705 (s), 611 (m). Anal. Calcd: C, 65.74; H, 8.52; N, 3.33.
Observed: C, 65.55; H, 8.30; N, 3.65. Mp: 213-214 °C. μeff
3.9 μB.
=
{[N2P2]CrCH2SiMe3}BF4 (9). The synthesis was carried out
analogously to that of 7 starting with AgBF4 (0.15 g, 0.77 mmol)
and [N2P2]CrCH2SiMe3 (0.42 g, 0.73 mmol), to yield purple,
1
block-like crystals (0.32 g, 66%). H NMR (C6D6): 13.4 (br),
3.23 (br), 1.65 (br), 1.39 (br), 1.17 (br), 1.08 (br), 0.29 (br), 0.00
(br), -5.6 (br). IR (cm-1): 1260 (s), 1093 (s), 1053 (s), 1020 (s),
929 (w), 859 (m), 799 (m), 759 (m), 721 (m). Anal. Calcd: C,
47.34; H, 9.47; N, 4.25. Observed: C, 47.02; H, 9.43; N, 4.13. Mp:
76-80 °C. μeff=3.7 μB.
{([N2P2]Cr)(μ-H)}2 (10). A solution of NaAlH2(OCH2CH2-
OMe)2 (Red-Al, 3.5 M in toluene, 0.36 g, 1.2 mmol) was added

6246 Organometallics, Vol. 28, No. 21, 2009
Rozenel et al.
Scheme 1
cooling to -40 °C gave green, block-like crystals (0.27 g, 32%).
Crystals suitable for an X-ray diffraction study were grown from
a concentrated solution of pentane cooled to -40 °C. 1H NMR
(C6D6): 13.8 (br), 10.0 (br), 3.24 (br), 1.66 (br), 1.17 (br), 1.08
(br), 0.25 (br), -8.8 (br). IR (cm-1): 1350 (w), 1248 (m), 1203
(m), 1068 (s), 1051 (s), 1019 (s), 894 (w), 845 (m), 797 (s), 769 (w),
735 (m), 611 (w). MS: m/z 485 ([N2P2]Crþ), 56 (C4H8þ). Anal.
Calcd: C, 57.64; H, 10.98; N, 5.17. Observed: C, 57.48; H, 10.81;
N, 5.05. Mp: 113-115 °C. μeff=3.8 μB.
Ethylene Polymerization Experiments at 1 atm of Ethylene.
The precatalyst (0.03 mmol) was dissolved in 10 mL of toluene
and added to 30 mL of toluene with the requisite amount of solid
MAO (0.52 g, 300 equiv Al, if used). The flask was purged with
ethylene and then pressurized to 1 atm of ethylene. The reaction
was run for 30 min before being quenched with 120 mL of a
solution of 5% HClconc/ethanol. The solid product was washed
with ethanol and dried under vacuum overnight, affording a
white powder. A 10 mL aliquot of the liquid was analyzed by
GC-MS.
gas chromatography capillary column (30 m length, 25 mm
diameter) and an Agilent 5973N mass selective detector, adding
mesitylene (25 μL/mL) as internal standard. Working condi-
tions were 35 °C (10 min) with temperature increases of 10 °C/
min to 280 °C (10 min).
Crystallographic Analysis. Single crystals of 2-5, 7, 8, and
10-14 were coated in Paratone-N oil, mounted on a Kaptan loop,
transferred to a Siemens SMART diffractometer CCD area
detector36 centered in the beam, and cooled by a nitrogen flow
low-temperature apparatus that has been previously calibrated by
a thermocouple placed at the same position as the crystal. Pre-
liminary orientation matrixes and cell constants were determined
by collection of 60 30-s frames, followed by spot integration and
least-squares refinement. An arbitrary hemisphere of data was
collected and the raw data were integrated using SAINT.37 Cell
dimensions reported were calculated from all reflections with I >
10. The data were corrected for Lorentz and polarization effects,
but no correction for crystal decay was applied. Data were
analyzed for agreement and possible absorption using XPREP.38
An empirical absorption correction based on comparison of
redundant and equivalent reflections was applied using SA-
DABS.39 The structures were solved using SHELXS40 and refined
Ethylene Polymerization Experiments at 10 atm of Ethylene. A
Fisher-Porter tube was filled with 30 mL of toluene and MAO
(0.52 g, 300 equiv of Al). A solution of precatalyst (0.03 mmol)
dissolved in 10 mL of toluene was then added. The tube was
purged with ethylene, then pressurized to 10 atm. The reaction
was allowed to run for 30 min, keeping the pressure at 8-10 atm.
(36) SMART Area-Detector Software Package; Bruker Analytical
X-ray Systems, Inc.: Madison, WI, 2001-2003.
(37) SAINT SAX Area-Detector Integration Program, V6.40; Bruker
Analytical X-ray Systems Inc.: Madison, WI, 2003.
After quenching with 120 mL of a solution of 5% HClconc
/
ethanol, the resulting solid was filtered, washed with ethanol,
and dried under vacuum overnight. A 10 mL aliquot of the
filtrate was analyzed by GC-MS.
Characterization of the Products by GC-MS. A 10 mL aliquot
of the reaction filtrate was washed with 15 mL of deionized
water, dried over anhydrous magnesium sulfate, and analyzed
by GC-MS using an Agilent 6890N GC with a high-resolution
(38) PREP (v 6.12) Part of the SHELXTL Crystal Structure Deter-
mination Package; Bruker Analytical X-ray Systems, Inc.: Madison, WI,
2001.
(39) SADABS, Bruker-Nonius Area Detector Scaling and Absorption
v. 2.05; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2003.
(40) SHELXS, Program for the Refinement of X-ray Crystal Struc-
tures, Part of the SHELXTL Crystal Structure Determination Package;
Bruker Analytical X-ray Systems Inc.: Madison, WI, 1995-1999.

Article
Organometallics, Vol. 28, No. 21, 2009 6247
Table 1. Selected Bond Distances and Angles for Cr(II) Complexes 2, 3, 4, 10, and 11
2a 3b 4b 10c
11d
˚
˚
˚
2.127(3) A
˚
2.122(3) A
˚
1.84(2), 1.85(2) A
˚
˚
˚
Cr1-X
2.3459(15) A
2.029(5) A
˚
2.5530(10) A
˚
2.4614(8) A
˚
2.5810(8) A
Cr1-P1
2.5902(17) A
2.567(3) A
˚
2.5253(17) A
˚
2.5337(9) A
Cr1-P2
2.596(3) A
˚
˚
˚
˚
˚
Cr1-N1
2.191(3) A
2.259(2) A
2.224(2) A
2.2158(19) A
2.142(6) A
˚
2.044(3) A
˚
2.069(2) A
˚
2.001(2) A
˚
2.0437(19) A
˚
2.032(6) A
Cr1-N2
˚
2.6268(9) A
Cr1-Cr2
X-Cr-P1
X-Cr-P2
X-Cr-N1
X-Cr-N2
P1-Cr-P2
P1-Cr-N2
P1-Cr-N1
P2-Cr-N2
P2-Cr-N1
N1-Cr-N2
98.36(6)°
96.09(5)°
97.75(9)°
98.19(9)°
178.80(11)°
106.00(10)°
105.79(3)°
119.15(7)°
81.06(6)°
124.31(7)°
82.22(6)°
74.61(8)°
98.70(8)°
119.9(7)°
97.3(2)°
102.57(17)°
174.0(2)°
100.0(3)°
107.87(8)°
125.4(2)°
83.0(2)°
117.9(2)°
82.92(18)°
75.1(3)°
179.41(8)°
104.58(8)°
105.09(5)°
120.34(13)°
81.19(11)°
125.61(14)°
83.66(10)°
76.00(10)°
164.68(10)°
109.74(11)°
99.8(6)°
156.0(7)°
145.01(7)°
81.58(6)°
91.98(6)°
84.03(6)°
76.26(9)°
74.97(7)°
a X = Cl. b X = C. c X = H. d X = O.
Figure 1. Thermal ellipsoid (50%) plot of 2. Hydrogen atoms
and methyl groups have been omitted for clarity.
Figure 2. Thermal ellipsoid (50%) plot of 3. Hydrogen atoms
and methyl groups have been omitted for clarity.
on all data by full-matrix least-squares with SHELXL-97.41 Ther-
mal parameters for all non-hydrogen atoms were refined aniso-
tropically. ORTEP diagrams were created using ORTEP-3.42
in the related complexes {[(Ph2P)2NMe]2Cr(μ-Cl)AlMe3}þ
20
˚
(Cr-Cl 2.3682(19), 2.3199(13) A),
[(Ph2PCH2CH2)2-
and [N(SiMe2CH2-
17
˚
NH]CrCl2 (Cr-N 2.168(4) A),
Results and Discussion
17
˚
PPh2)2]CrMe (Cr-N 2.117(3) A). The Cr1-P1 distances
in 2 (2.5902(17) and 2.5253(17) A) correspond well with the
distances found in the chromium di(isopropyl)phosphine com-
˚
Synthesis of Cr Halide Complexes. In an earlier paper, we
reported the synthesis of the Cr(III) starting material
[N2P2]CrCl2 (1);6 the corresponding Cr(II) analogue was
prepared by reaction of Li[N2P2] and CrCl2(THF), from
which [N2P2]CrCl (2) was isolated as light blue needle-like
crystals in 58% yield (Scheme 1).
Recrystallization of 2 from toluene gave crystals suitable
for an X-ray diffraction study. Selected bond distances and
angles are shown in Table 1, and an ORTEP diagram is
provided in Figure 1.
43
pound (dippe)Cr(CH2CMe3)2 (2.556(2) A). The solution
˚
magnetic susceptibility (μeff) for complex 2 was found to be
4.7 μB, consistent with four unpaired electrons.
Substitution Chemistry of 1 and 2. The reaction of 2 with
MeLi in THF at -78 °C resulted in a color change of blue to
brown. Warming of the reaction mixture to room tempera-
ture, followed by evaporation of the solvent under vacuum,
extraction with pentane, and cooling at -40 °C, resulted in
the isolation of [N2P2]CrMe (3) as a purple solid in 38%
yield. Recrystallization from pentane at -40 °C afforded
crystals suitable for an X-ray diffraction study. Selected
bond distances and angles are shown in Table 1, and an
ORTEP diagram is provided in Figure 2. The solid-state
structures of 2 and 3 are similar, with the replacement of a
chloride with a methyl group. The Cr1-C1 bond distance
Complex 2 displays a distorted trigonal-bipyramidal geo-
metry in the solid state with the [N2P2] ligand bound κ4-N2P2
to the metal. Little distortion along the axial direction
(Cl1-Cr1-N2 179.41(8)°) is observed, although the equa-
torial positions are distorted from the expected value of 120°
(P1-Cr1-P2 105.09(5)°, P1-Cr1-N1 120.34(13)°, and
P2-Cr1-N1 125.61(14)°), likely due to steric constraints.
˚
(2.127(3) A in 3) is close to that found in (dippe)Cr-
˚
˚
The Cr1-Cl1 (2.3459(15) A), Cr1-N1 (2.044(3) A), and
˚
(CH2CMe3)2 (2.149(8) A) and in (dippe)Cr(CH2SiMe3)2
˚
Cr1-N2 (2.191(3) A) bond distances are close to those found
43
˚
(2.128(4) A). The solution magnetic susceptibility (μeff)
for complex 3 was found to be 4.5 μB, lower than the
(41) SHELXL, Program for the Refinement of X-ray Crystal Struc-
tures, Part of the SHELXT Crystal Structure Determination Package;
Bruker Analytical Systems Inc.: Madison, WI, 1995-1999.
(43) Hermes, A. R.; Morris, R. J.; Girolami, G. S. Organometallics
1988, 7, 2372–2379.
(42) Farrugia, L. J. Appl. Crystallogr. 1997, 30, 565.

6248 Organometallics, Vol. 28, No. 21, 2009
Rozenel et al.
Table 2. Selected Bond Distances and Angles for Cr(III) Com-
plexes 5, 9, and 10
5a
13b
14c
˚
2.3157(17) A
˚
˚
Cr1-X
1.856(3) A
2.081(4) A
˚
2.066(6)A
˚
1.870(3) A
˚
2.102(4) A
Cr1-Y
˚
2.5176(19) A
˚
2.5203(14) A
Cr1-P1
˚
˚
˚
Cr1-N1
2.202(4)A
2.174(3) A
2.245(3) A
˚
1.939(4)A
˚
1.900(3) A
˚
1.959(3) A
Cr1-N2
˚
2.6325(8) A
Cr1-Cr2
X-Cr1-P1
X-Cr1-N1
X-Cr1-N2
X-Cr1-Y
P1-Cr1-N2
Y-Cr1-P1
Y-Cr1-N1
N1-Cr1-N2
90.62(6)°
95.64(12)°
176.57(14)°
106.74(15)°
83.83(16)°
135.53(10)°
104.58(19)°
115.46(15)°
76.37(13)°
164.78(12)°
102.07(13)°
99.67(16)°
137.99(15)°
102.75(18)°
94.45(18)°
77.01(16)°
125.35(12)°
126.66(10)°
90.08(11)°
115.09(10)°
77.96(11)°
a X = Cl, Y = C. b X = N3, Y = N4. c X = C1, Y = C2.
Figure 3. Thermal ellipsoid (50%) plot of 4. Hydrogen atoms
and methyl groups have been omitted for clarity.
ORTEP diagram of 5 is shown in Figure 4, with selected
bond distances and angles provided in Table 2. Complex 5
crystallizes with one molecule of diethyl ether for every two
molecules of 5 in the unit cell. It displays a trigonal-bipyr-
amidal geometry in the solid state with the [N2P2] ligand
3
˚
bound κ -N2P to the metal center. The Cr1-C1 (2.066(6) A),
˚
˚
Cr1-Cl1 (2.3157(17) A), Cr1-N1 (2.202(4) A), Cr1-N2
˚
˚
(1.939(4) A), and Cr1-P1 (2.5176(19) A) metal-ligand bond
distances are consistent with distances found in the related
Cr(III) complexes [(Ph2PCH2SiMe2)2N]CrClCH2SiMe3
˚
˚
(Cr-C 2.110(6) A, Cr-Cl 2.315(2) A, Cr-Namide 2.022(4)
A, Cr-P 2.525(2) and 2.442(2) A); [κ3-(iPrNCH2)3]Cr-
44
˚
˚
˚
Cl(CH2SiMe3)2 (Cr-C 2.086(5) and 2.080(6) A, Cr-Cl
˚
2.316(1) A, Cr-Namine 2.292(4), 2.298(4), and 2.181(4)
45
˚
˚
A); and {(Me2PCH2CH2)2N}2CrCl (Cr-Cl 2.401(1) A,
˚
Cr-Namide 2.017(2) and 1.996(2) A, Cr-P 2.435(1),
2.444(1), and 2.429(1) A). Comparison of the solid-state
46
˚
structure of 5 with 1 showed consistent distances around the
metal center. The μeff for complex 5 was found to be 3.9 μB, in
accord with three unpaired electrons.
Figure 4. Thermal ellipsoid (50%) plot of 5. Hydrogen atoms,
methyl groups, and diethyl ether molecule have been omitted for
clarity.
In an attempt to synthesize a Cr(III) dialkyl derivative, 2
equiv of MeLi were added to a solution of 1 in diethyl ether.
Upon addition, the reaction mixture changed color from
purple to brown. After evaporation of the solvent, extraction
with pentane, concentration, and cooling at -40 °C, brown
crystals were obtained. Analysis of the crystalline material by
mass spectrometry showed signals consistent with complex 3
(mþ/z=500). The spectrum also showed peaks at 564 and 508
that could not be assigned, but no peak corresponding to
[N2P2]CrMe2 (Mþ=515) was observed. The identity of 3 was
also confirmed by X-ray crystallography. The reduction of
chromium complexes by alkylating reagents during ethylene
polymerization/oligomerization has been proposed as a key
step in the initiation of chromium catalysts.12,13
The reaction path to 3 likely involved the formation of
[N2P2]Cr(Cl)Me from the reaction of 1 and MeLi, followed
by reduction by a second equivalent to give the product. To
test whether [N2P2]Cr(Cl)Me was a viable intermediate, the
reaction of 1 and 1 equiv of MeLi was undertaken. Following
the addition of MeLi to a THF solution of 1, a deep purple
theoretical value (4.89 μB) but consistent with the presence of
four unpaired electrons as in the case of 2.
Addition of LiCH2SiMe3 to a solution of 2 in toluene at
room temperature resulted in a color change of the solution
from blue to purple. Following evaporation of the solvent
under vacuum, extraction with pentane, and cooling at -40 °C,
[N2P2]CrCH2SiMe3 (4) was isolated as a purple solid in 53%
yield. An X-ray diffraction study was undertaken (an ORTEP
diagram is provided in Figure 3), and selected bond distances
and angles are shown in Table 1. The chromium atom lies in a
highly distorted tetrahedral geometry (C1-Cr1-N1
164.68(10)°, C1-Cr1-N2 109.74(11)°, and C1-Cr1-P1
98.70(8)°) with the ligand bound κ3-N2P to the metal. The
Cr1-N1, Cr1-N2, and Cr1-P1 bond distances of 2.224(2),
˚
2.001(2), and 2.4614(8) A, respectively, show a constriction in
comparison with 3. The Cr1-C1 distances in both complexes,
however, are equivalent within error (2.127(3) A for 3 and
˚
˚
2.122(3) A for 4), suggesting the change in coordination mode is
likely due to steric rather than electronic effects. The μeff value
of complex 4 is 4.4 μB, consistent with the presence of four
unpaired electrons.
The metathesis reaction of 1 with 1 equiv of LiCH2SiMe3
in toluene at room temperature gave a dark blue solution,
which yielded [N2P2]Cr(Cl)CH2SiMe3 (5) as a purple solid in
57% yield after workup and crystallization from pentane. An
(44) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J.; Young, V. G.
J. Chem. Soc., Dalton Trans. 1999, 147-154.
(45) Kohn, R. D.; Kociokkohn, G. Angew. Chem., Int. Ed. Engl.
1994, 33, 1877–1878.
(46) Alsoudani, A. R. H.; Batsanov, A. S.; Edwards, P. G.; Howard,
J. A. K. J. Chem. Soc., Dalton Trans. 1994, 987–995.

Article
Organometallics, Vol. 28, No. 21, 2009 6249
Table 3. Selected Bond Distances and Angles for Cr(III) Com-
plexes 7 and 8
7
8
˚
˚
˚
˚
˚
Cr1-Cl1
2.2965(14) A
2.272(3) A
Cr1-P1
2.4691(12) A
2.465(4) A
˚
2.4554(13) A
Cr1-P2
2.436(4) A
˚
˚
Cr1-N1
2.150(3)A
2.139(7) A
˚
1.898(2)A
˚
1.889(7) A
Cr1-N2
Cl1-Cr-P1
Cl1-Cr-P2
Cl1-Cr-N1
Cl1-Cr-N2
P1-Cr-P2
P1-Cr-N2
P1-Cr-N1
P2-Cr-N2
P2-Cr-N1
N1-Cr-N2
95.98(3)°
93.53(9)°
176.36(6)°
104.09(7)°
104.77(5)°
123.83(7)°
83.70(5)°
123.83(7)°
83.07(6)°
78.95(8)°
94.37(12)°
96.10(12)°
177.5(2)°
103.1(3)°
107.72(11)°
127.8(2)°
83.2(2)°
Figure 5. Thermal ellipsoid (50%) plot of 7. Hydrogen atoms,
methyl groups, and THF molecule have been omitted for clarity.
118.5(2)°
84.0(3)°
79.0(3)°
solution was formed, which gave purple crystals of
[N2P2]Cr(Cl)Me (6) in 68% yield following crystallization
from diethyl ether. The composition of 6 is supported by
mass spectrometry (Mþ=535, Mþ - Me=520) and elemental
analysis. Complex 6 is paramagnetic with three unpaired
electrons (μeff =3.8), and the 31P NMR spectrum shows one
broad resonance peak at 45.65 ppm.
Synthesis of Cationic Species. Two chromium(III) cationic
complexes were obtained by the oxidation of 2 with silver
salts. Addition of AgBF4 to 2 in THF produced a green
solution and precipitation of silver metal. Extraction of the
solution with THF followed by concentration and cooling
to -40 °C gave the cationic complex {[N2P2]CrCl}BF4 (7) as
green blocks in 64% yield. Analogous oxidation with
AgBPh4 produced {[N2P2]CrCl}BPh4 (8) in 29% yield as
dark green, block-like crystals. The μeff values for these
complexes are 3.8 and 3.9 μB, respectively, confirming an
oxidation state change of Cr(II) to Cr(III). Crystals suitable
for X-ray diffraction studies were grown from concentrated
THF solutions cooled to -40 °C.
Figure 6. Thermal ellipsoid (50%) plot of 8. Hydrogen atoms,
methyl groups, and THF molecule have been omitted for clarity.
Selected bond distances and angles are shown in Table 3,
and ORTEP diagrams are provided in Figures 5 and 6,
respectively. Complex 7 exhibits significant disorder around
the BF4 anion. The cationic moiety {[N2P2]CrCl}þ in 7
showed slightly larger distances around the metal center
AlEt2Cl, and MAO) were carried out. These were generally
unproductive, yielding starting materials or intractable mix-
tures. However, the reaction of 2 with KC8 in THF at -78 °C
in the presence of AlMe3 (3 equiv) produced a dark green
solution after warming to room temperature, from which the
alkylated Cr(II) species 3 (see above) was obtained in 29%
yield. It has been shown that alkyl-aluminum species can act
as alkylating agents.31
Synthesis of {([N2P2]Cr)(μ-H)}2 (10). Chromium hydride
derivatives have been postulated as intermediates formed
following β-hydride elimination of the hepta- or nonametal-
lacycle, prior to elimination of 1-hexene or 1-octene.
McGuinness and co-workers have suggested that Cr dihy-
dride species are formed in side-chain reactions during the
tetramerization process.47 In order to explore the stability of
these kinds of complexes, we targeted a [N2P2]Cr hydride
derivative.
˚
˚
(Cr1-Cl1 2.2965(14) A, Cr1-N1 2.150(2) A, Cr1-N2
˚
˚
1.898(2) A, Cr1-P1 2.4691(12) A, and Cr1-P2 2.4554(13)
˚
˚
˚
A) than in 8 (Cr1-Cl1 2.272(3) A, Cr1-N1 2.139(7) A,
Cr1-N2 1.889(7) A, Cr1-P1 2.465(4) A, and Cr1-P2
2.436(4) A).
˚
˚
˚
While the Cr atoms in 7 and 8 are in the same ligand
environment as in 2, a constriction in the distances around
the metal center is observed as expected due to the higher
oxidation state of the chromium atoms in 7 and 8. The
biggest difference is observed in the Cr1-N2 bond distance,
˚
which is approximately 0.15 A smaller in the Cr(III) species.
The reaction of AgBF4 with 4 in THF at room temperature
produced a purple solution and a silver mirror. Evaporation
of the solvent, followed by extraction with diethyl ether and
crystallization at -40 °C, gave {[N2P2]CrCH2SiMe3}BF4 (9)
as purple blocks in 66% yield. Single crystals suitable for
X-ray crystallography of 9 could not be obtained, but the 1H
NMR and IR spectra, elemental analysis, and the solution
magnetic susceptibility (μeff=3.7 μB, three unpaired electro-
ns) are consistent with the proposed formulation.
The reaction of 2 with 1.1 equiv of Red-Al (3.5 M in
toluene) in THF produced a brown solution. Following
extraction with pentane, concentration, and cooling at -40 °C,
the product {([N2P2]Cr)(μ-H)}2 (10) was isolated as brown
crystals in 84% yield. Crystals suitable for an X-ray diffract-
ion study were obtained by recrystallization of 10 from a
(47) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.; Haas-
broek, D.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D. H.
J. Am. Chem. Soc. 2005, 127, 10723–10730.
Reaction with Alkyl-Aluminum Reagents. Reactions of 2, 3,
4, and 5 with alkyl aluminum reagents (AlMe3, AlMe2Cl,

6250 Organometallics, Vol. 28, No. 21, 2009
Rozenel et al.
Figure 8. Thermal ellipsoid (50%) plot of 11. Hydrogen atoms
and methyl groups have been omitted for clarity.
Figure 7. Thermal ellipsoid (50%) plot of 10. Hydrogen atoms
and methyl groups have been omitted for clarity.
N]Cr)(μ-H)}2 (1.6 μB/Cr), {((MeC(N(2,6-Me2Ph)2)CH2C-
(N(2,6-Me2Ph)2)Me)Cr)(μ-H)}2 (1.2 μB/Cr), and {([μ-{(iBu)-
P[N(tBu)]2}Al(iBu)2]Cr)(μ-H)}2 (2.6142(16) A) (1.63 μB/
˚
Scheme 2. Reactivity of 10: Synthesis of [N2P2]CrOSO2CF3 (11)
and [N2P2]Cr(CO)2 (12)
Cr).51
Reactivity of 10. The reaction of the chromium hydride 10
with silver triflate (2 equiv) in refluxing THF overnight,
followed by evaporation of the solvent, extraction with
toluene, and crystallization at -40 °C, gave [N2P2]-
CrOSO2CF3 (11) as blue needles in 39% yield (Scheme 2).
Crystals suitable for an X-ray diffraction study were grown
from a concentrated toluene solution at -40 °C. The μeff
value for 11 was 4.6 μB, consistent with four unpaired
electrons.
concentrated solution of HMDSO at -40 °C. An ORTEP
diagram is provided in Figure 7, and selected bond distances
and angles are shown in Table 1. Complex 10 crystallizes as a
dimer with bridging hydrides and with one [N2P2] ligand bound
κ3-N2P to each metal center. The geometry around each metal
atom is distorted square pyramidal with the axial position
occupied by the phosphine ligand. The Cr1-N1 and Cr1-N2
The structure of 11 showed a distorted trigonal-bipyrami-
dal geometry very similar to that of 2, with the ligand fully
bound to the metal center and the axial positions being
occupied by the triflate and the amine nitrogen of the N2P2
ligand (selected bond distances and angles are shown in
Table 1). In 11, the {OSO2CF3}- anion is coordinated to
the chromium atom (an ORTEP diagram is shown in
Figure 8). The bond distances around the metal center are
˚
distances (2.2158(19) and 2.0437(19) A) are similar to those
˚
found in 2, 3, and 4, while the Cr1-P1 distance (2.5810(8) A) is
˚
slightly longer. The Cr1-Cr2 distance of 2.6268(9) A is con-
sistent with that found in other structurally characterized
˚
very similar to those found in 2 (Cr1-N1 2.142(6) A,
hydride-bridged chromium dimers: {((MeC(N(2,6-R2Ph)2)-
CH2C(N(2,6-R2Ph)2)Me)Cr)(μ-H)}2 (2.6207(7) A),48,49 {([(Ph2-
˚
˚
Cr1-N2 2.032(6) A, Cr1-P1 2.596(3) A, and Cr1-P2-
˚
˚
(2.567(3) A) but with a small contraction in the Cr1-N1
distance of almost 0.05 A to compensate for the weaker
50
˚
PCH2CH2SiMe2)2N]Cr)(μ-H)}2 (2.641(1) A), and {([μ-
˚
{( Bu)P[N( Bu)]2}Al( Bu)2]Cr)(μ-H)}2 (2.6142(16) A).51 In 10,
ι
t
i
˚
binding of triflate compared to chloride. The Cr1-O1 dis-
tance of 2.029(5) A is consistent with that found in Cp*Cr-
the hydride H atoms were located in the Fourier map and
˚
˚
refined. The Cr-H distances found (1.84(2) and 1.85(2) A)
may be compared to those reported above by Theopold
52
˚
(NO)2OSO2CF3 (2.030(2) A).
Reaction of 10 with excess CO in pentane at -78 °C,
followed by warming to room temperature, produced a dark
red solution with concomitant precipitation. Following
workup and cooling to -40 °C, [N2P2]Cr(CO)2 (12) was
isolated as brown crystals in 53% yield. An X-ray diffraction
study of 12 was undertaken, and an ORTEP diagram is
provided in Figure 9 (selected bond distances and angles are
shown in Table 4). The chromium atom sits in a distorted
trigonal-bipyramidal geometry, with the [N2P2] ligand
bound κ3-N2P to the metal. The axial positions are occupied
by the basal nitrogen and one CO ligand. The formation of
12 from 10 involves reduction from Cr(II) to Cr(I) by the
liberation of H2 (observed by 1H NMR). The solution
˚
(1.77(3) and 1.77(3) A) and Fryzuk (1.78(3) and 1.76(3) A)
˚
˚
and around 0.15 A longer than those reported by Gambarotta
51
˚
(1.69 and 1.68 A).
The solution magnetic susceptibility data indicate a μeff
value of 1.6 μB/Cr. This value is close to the single spin value
(1.73 μB) and is inconsistent with a d4 metal center, implying
an interaction between the chromium atoms in 10. Similar
results have been observed for {([(Ph2PCH2CH2SiMe2)2-
(48) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.; Golen, J. A.;
Rheingold, A. L.; Theopold, K. H. Chem. Commun. 2003, 1164–1165.
(49) Monillas, W. H.; Yap, G. P. A.; Theopold, K. H. Angew. Chem.,
Int. Ed. 2007, 46, 6692–6694.
(50) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J.; Thompson, R. C.
Inorg. Chem. 1994, 33, 5528–5534.
(51) Albahily, K.; Al-Baldawi, D.; Gambarotta, S.; Koc, E.; Duch-
ateau, R. Organometallics 2008, 27, 5943–5947.
(52) Legzdins, P.; Rettig, S. J.; Smith, K. M.; Tong, V.; Young, V. G.
J. Chem. Soc., Dalton Trans. 1997, 3269–3276.

Article
Organometallics, Vol. 28, No. 21, 2009 6251
Figure 9. Thermal ellipsoid (50%) plot of 12. Hydrogen atoms
and methyl groups have been omitted for clarity.
Table 4. Selected Bond Distances and Angles for 12
Figure 10. Thermal ellipsoid (50%) plot of 13. Hydrogen atoms
and methyl groups have been omitted for clarity.
˚
Cr1-C1
1.816(3) A
˚
1.830(3) A
Cr1-C2
˚
2.3471(9) A
1.922(2) A
2.200(2) A
1.176(4) A
Cr1-P1
˚
˚
˚
Cr1-N1
Cr1-N2
C1-O1
˚
1.175(4) A
C2-O2
C1-Cr1-C2
C1-Cr1-P1
C1-Cr1-N1
C1-Cr1-N2
P1-Cr1-N1
C2-Cr1-P1
C2-Cr1-N1
76.89(14)°
90.51(10)°
106.28(12)°
173.13(11)°
133.95(8)°
103.15(10)°
105.80(12)°
magnetic susceptibility of the molecule (1.8 μB) indicates one
unpaired electron, consistent with a low-spin conformation.
Figure 11. Thermal ellipsoid (50%) plot of 14. Hydrogen atoms
and methyl groups have been omitted for clarity.
˚
The Cr-C and C-O distances in 12 (1.816(3), 1.830(3) A
and 1.176(4), 1.175(4) A) are consistent with those found in
˚
other Cr(I)-CO complexes such as CpCr(CO)2PPh3 (Cr-C,
1.826(3) and 1.816(4) A; C-O, 1.16(12) and 1.16(12) A).53 A
considerable amount of π-back-bonding from the metal to
the CO ligands is reflected in the low values observed in the
IR spectrum for νCO (1742 and 1872 cm-1).
˚
˚
Reduction of 2. Several attempts to isolate a [N2P2]Cr(I)
derivative by reduction of 2 gave intractable mixtures. In an
attempt to trap a Cr(I) intermediate, 2 was reduced with KC8 in
the presence of p-tolyl azide in THF at -78 °C, with gas
evolution being observed immediately. Following evaporation
of the solvent under vacuum, extraction with diethyl ether, and
cooling at -40 °C, the bimetallic complex {([N2P2]Cr)
(μ-NC7H7)}2 (13) was isolated in 44% yield as brown blocks.
Recrystallization from diethyl ether at -40 °C afforded
crystals suitable for an X-ray diffraction study. Selected bond
distances and angles are shown in Table 2; an ORTEP
diagram is provided in Figure 10. Complex 13 is dimeric with
two bridging imido ligands. Remarkably, the [N2P2] ligand is
bound κ2-N2 in the solid state, providing the first example of
an [N2P2] complex without a metal-phosphine bond. The
geometry around the chromium atom is pseudotetrahedral
with N3-Cr1-N4 (90.08(11)°), N1-Cr1-N2 (77.96(11)°),
N1-Cr1-N4 (115.09(10)°), and N1-Cr1-N3 (125.35(12)°).
The Cr1-N3 and Cr1-N4 distances (1.856(2), 1.870(3),
Figure 12. CV for 10 showing Cr(II)-Cr(II)/Cr(II)-Cr(III)
and Cr(II)-Cr(III)/Cr(III)-Cr(III) oxidations carried out in
THF with [nBu4N][PF6] (0.1 M) as the supporting electrolyte.55
˚
and Cr2-N6 distances (1.900(2) and 1.904(2) A). These
values are consistent with those found in [Cp*Cr(μ-NR)]2
i
˚
(R=2,6- Pr2C6H3) (Cr1-N1, 1.886(2) A; Cr1-N2, 1.861(2)
A) and [Cp*Cr(μ-NR)]2 (R = 2,4,6-tBu3C6H2) (Cr1-N1,
˚
1.888(2) A; Cr1-N2, 1.905(2) A).54 Complex 13 is diamagnetic,
˚
˚
˚
1.853(2), and1.873(3) A) areslightlyshorter thanthe Cr1-N2
(54) Danopoulos, A. A.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse,
M. B. J. Chem. Soc., Dalton Trans. 1996, 271–281.
(55) Under these conditions, the peak to peak separation of the
(53) Cooley, N. A.; Watson, K. A.; Fortier, S.; Baird, M. C. Orga-
nometallics 1986, 5, 2563–2565.
Co(II)/Co(III) couple in cobaltocene (used as reference) was 0.16 mV.

6252 Organometallics, Vol. 28, No. 21, 2009
Table 5. Activity for Ethylene Polymerization of the Synthesized Complexes
Rozenel et al.
precatalyst
activator
ethylene pressure
temperature (°C)
polyethylene yield (g)a
productivity (g PE/mol Cr h)
3
1
2
MAO, 300 equiv
MAO, 300 equiv
MAO, 300 equiv
MAO, 300 equiv
MAO, 300 equiv
MAO, 300 equiv
none
MAO, 300 equiv
none
none
none
none
none
none
none
none
MAO, 300 equiv
1
1
25
25
25
25
80
80
25
25
25
25
25
80
25
25
25
25
25
0.064
0.044
2.26
0.47
1.12
3.18
0
0.119
0
0
0
0
0
0
0
0
0.16
4300
2900
151 000
31 000
75 000
212 000
0
7900
1a
2a
1a
2a
3a
3a
4a
5a
7b
8b
9b
10b
11b
14b
14a
10
10
10
10
10
1
10
10
10
10
10
10
10
10
1
0
0
0
0
0
0
0
0
10 700
a Conditions: 0.03 mmol of precatalyst, 40 mL of toluene as solvent, 30 min run time. b Conditions: 0.10 mmol of precatalysts, 40 mL of toluene as
solvent, 1 h run time.
with the 31P NMR showing a sharp singlet (4.76 ppm),
implying that the phosphine arms are equivalent in solution.
The synthesis of complex 13 highlights the ability of a
transient Cr(I) species to undergo a two-electron oxidation
process to produce a Cr(III) complex. With the aim of
directly probing specific intermediates in the catalytic oli-
gomerization of ethylene, we performed the reduction of 2
in the presence of ethylene. The reduction of 2 with KC8 in
THF at -78 °C under an atmosphere of ethylene resulted in
a green solution. Following the removal of the solvent
under vacuum, extraction with pentane, and cooling at
-40 °C, the metallacyclic complex [N2P2]CrC4H8 (14) was
isolated as a green solid in 32% yield. An ORTEP diagram
is provided in Figure 11, and selected bond distances and
angles are shown in Table 2. The chromium atom lies in a
distorted trigonal-bipyramidal geometry with the [N2P2]
ligand bound κ3-N2P to the metal. As in the case of 4, 5, 10,
and 12, one of the phosphine arms is unbound to the metal
center. All angles and distances are similar to those found in
complexes 1 and 5. The Cr1-C1 and Cr1-C2 distances
reversible one-electron oxidation for the Cr(II)/Cr(III) cou-
ple at 0.324 V vs Cp2Fe/Cp2Feþ. No reduction process
corresponding to the Cr(II)/Cr(I) couple was observed.
The CV of 6 shows three processes, an irreversible oxidation
at 1.12 V and two irreversible reductions at -1.37 and -1.87 V,
while the CV of 16 shows a one-electron reversible reduction
at -1.42 V.
The CVs of the cationic species 7 and 8 were very similar,
showing two processes, a reversible reduction for Cr(III)/
Cr(II) (at -1.28 and -1.25 V respectively) and an irreversible
oxidation for Cr(III)/Cr(IV) at 0.471 V for 7 and 0.514 V for 8.
The CV of 10 shows two reversible one-electron oxidations
in the same solvent window, at 0.071 and 0.780 V. The first
potential might indicate the oxidation of the Cr(II)-Cr(II)
dimer to a Cr(II)-Cr(III) species, and the second electro-
chemical event might correspond to the Cr(II)-Cr(III)/Cr-
(III)-Cr(III) couple (Figure 12).
Finally the CV of 12 shows two reversible, one-electron
oxidation processes at -0.637 and -0.025 V, followed by
further electrochemical inseparable and unidentified events.
The first oxidation was assigned to the Cr(I)/Cr(II) couple,
and the second one to the Cr(II)/Cr(III) couple. The second
oxidation potential is in agreement with the Cr(II)/Cr(III)
potential found in 2 and the Cr(II)-Cr(II)/Cr(II)-Cr(III)
potential for 10.
The CV experiments showed that under these conditions
the Cr(II)/Cr(III) redox process is straightforward in the
halide species, but that upon alkylation the same redox
process becomes irreversible. The cationic species also
showed an irreversible Cr(III)/Cr(IV) oxidation, supporting
the idea that the Cr(I)/Cr(III) couple is responsible for the
formation of the metallacyclopentane. Only in the case of 12
were we able to observe the Cr(I)/Cr(II) couple in the
window analyzed.
˚
(2.081(4) and 2.102(4) A) are consistent with that found in 5
˚
(2.067(5) A) and with those reported in the only other
structurally characterized chromacyclopentane, (Me2NC2-
28
H4C5Me4)CrC4H8 (2.103(1) and 2.080(1) A). The angles
˚
and distances around the carbon atoms in the metallacycle
are consistent with sp3 hybridization. The μeff value was
found to be 3.8 μB, in agreement with the presence of three
unpaired electrons. To the best of our knowledge, this
constitutes only the second example of a stable chromium
metallacyclopentane formed by oxidative coupling of ethyl-
ene.28
The formation of the metallacycle 14 is in agreement with
the mechanism proposed for ethylene trimerization by Ber-
caw and co-workers,25 in which initial reduction of the metal
center to form a Cr(I) species is followed by coordination of
two ethylene molecules to the metal center, leading to
oxidative coupling and generation of the Cr(III) metalla-
cycle. This reaction is thought to be a key step in the tri-,
tetra-, and oligomerization of ethylene by chromium cata-
lysts.
Activity toward Ethylene Oligomerization/Polymerization.
With the goal of understanding the differences in reactivity
arising from different metal oxidation states, the activities of
complexes 1 and 2 toward ethylene oligomerization were
probed. The activity was screened at 1 and 10 atm of ethylene
and at 25 and 80 °C with 30 μmol of precatalyst and 300 equiv
of MAO as an activator. The results are summarized in
Table 5.
Electrochemistry. Cyclic voltammetry (CV) studies of 2, 6,
7, 8, 10, and 12 were carried out in THF with [nBu4N][PF6]
(0.1 M) as the supporting electrolyte. The CV of 2 shows one
electrochemical process in the solvent window examined: a
A comparison of the activities of 1 and 2 shows that
the Cr(III) derivative 1 has a higher activity for ethylene

Article
Organometallics, Vol. 28, No. 21, 2009 6253
polymerization; compound 1 produced almost 50% more
polyethylene than 2 at 1 atm of ethylene at 25 °C and 5 times
more polyethylene at 10 atm of ethylene pressure at 25 °C.
Compound 2, however, did produce hexene and octene at
25 °C in a sufficient quantity to be detected by GC-MS. The
presence of hexene, octene, decene, and higher oligomers
implies that under the reaction conditions tested, the Cr(II)
derivative is capable of producing low-weight oligomers,
with a Schulz-Flory distribution. Under the same condi-
tions using 1, only polyethylene was observed.
We also tested complex 11 without adding MAO at 10 atm
of ethylene and 25 °C. Again, no reaction was observed, even
with the reduced coordinating ability of the triflate in
comparison to the chloride. Also, we saw no reaction be-
tween 10 or 14 and ethylene (10 atm) at 25 °C, indicating that
there is a large barrier to ethylene insertion for both com-
plexes. Finally, the reaction of complex 14 with ethylene
(1 atm, 25 °C, with MAO) yielded 0.16 g of polyethylene; as
in the case of 1 under similar conditions, no traces of lower
oligomers were detected.
Increasing the temperature to 80 °C while maintaining
10 atm of ethylene had a great influence on the amount of
polyethylene obtained as well as on the distribution of
products. At 80 °C, 1 produced 1.12 g of polyethylene, and
the GC-MS trace showed the presence of C14 and higher
oligomers in small quantities. No C4-C12 oligomers were
observed. Under the same conditions, 0.79 g of polyethylene
insoluble in toluene and 2.39 g of soluble olefins were
produced with 2, again, with a Schulz-Flory distribution.
Complex 2 gave a higher activity than complex 1 at 80 °C
likely due to decomposition of the Cr(III) derivative. This
phenomenon has been observed with other Cr catalysts.13
Compound 2 displayed the highest activity observed for
these complexes (2.1 ꢀ 105 g polyethylene mol Cr-1 h-1),
while running the reaction at 80 °C under 10 atm of ethylene
pressure. Other chromium catalysts for ethylene polymeri-
zation, like Cp*Cr(C6F5)(η3-benzyl) or {2,6-bis[(2,4,6-tri-
methylphenyl)iminobenzyl]pyridine}CrCl3, have shown
Conclusions
The [N2P2] ligand has shown the ability to stabilize
reactive chromium complexes by changing its coordina-
tion mode, allowing for the synthesis and characterization
of new Cr(II) and Cr(III) halide, alkyl, and hydride
derivatives. Reaction of the chromium complexes with
MAO gave active species for ethylene oligomerization/
polymerization; however, these species were not selective
for ethylene tri- or tetramerization. In order to gain insight
into the ethylene oligomerization process with chromium
catalysts, the alkyl species 3, 4, and 5 and the cationic
species 7, 8, and 9 were tested for ethylene polymerization
without the use of an activator; these complexes did not
catalyze the oligomerization/polymerization of ethylene,
supporting the theory that the neutral alkyl derivatives,
cationic Cr(III) derivatives, and cationic alkyl Cr(III)
derivatives are not the active species formed after reaction
with MAO.
The [N2P2] ligand also proved to be a useful supporting
framework for studying the redox chemistry of chromium
systems as a Phillips-type catalysis model. The reaction of 1
with 2 equiv of MeLi resulted in reduction from Cr(III) to
Cr(II), while the reaction of 10 with CO induced reduction of
the metal to form the Cr(I) derivative 12. Finally, we were
able to synthesize the chromium pentametallacycle 14 via
reduction of 2 under an ethylene atmosphere. Compounds
similar to 14 have been proposed as intermediates for
ethylene tri-, tetra-, and oligomerization.
activities of up to 1.56ꢀ108 g polyethylene mol Cr-1 h-1 56
.
The activities of complexes 3, 4, and 5 for ethylene
oligomerization were tested without the use of an activator
at 1 and 10 atm of ethylene at 25 °C. Under these conditions,
none of the complexes were active for ethylene oligomeriza-
tion. This observation suggests that MAO does more than
just alkylate the metal center.
The activities of the cationic species 7, 8, and 9 were also
tested at 10 atm of ethylene at 25 or 80 °C without an
activator. None of these complexes was an active catalyst
for ethylene polymerization, indicating that the active spe-
cies formed upon addition of MAO to 1-3 is not a simple
Cr(III) cationic alkyl derivative. The fact that the cationic
alkyl complex 9 did not react with ethylene implies that
MAO does not simply form a Cr(III) cationic alkyl complex
as the active species.
Acknowledgment. We are grateful to the NSF for
financial support.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
(56) Esteruelas, M. A.; Lopez, A. M.; Mendez, L.; Olivan, M.; Onate,
E. Organometallics 2003, 22, 395–406.