Preparation and characterization of a switchable single-component chromium trimerization catalyst

Article abstract of DOI:10.1021/om800563w

Reaction of [(Ar)NPN(t-Bu)]₂Cr (2) with Me₃Al afforded {[(Ar)NP(Me)N(t-Bu)]AlMe₂}Cr{ [(Ar)NP(Me)-(AlMe ₃)N(t-Bu)]AIMe(μ-Me)) (3), in which the ligand's P atom has been alkylated and one AlR₂ residue retained by the chelating framework. One of the two ligands also retained an additional Me₃Al unit via coordination to the alkylated P atom. Complex 3 provides the first case of a selective ethylene trimerization catalyst with high activity and excellent selectivity, producing 1-hexene upon exposure to ethylene at 80°C. Furthermore, upon treatment with MAO, complex 3 acts as a nonselective catalyst, producing a statistical mixture of oligomers with the highest ever observed activity. In addition, upon treatment with [(i-Bu)₂Al]₂O, the complex acts as a highly active polymerization catalyst.

Full text of DOI:10.1021/om800563w

5708
Organometallics 2008, 27, 5708–5711
Preparation and Characterization of a Switchable Single-Component
Chromium Trimerization Catalyst
Khalid Albahily,† Danya Al-Baldawi,† Sandro Gambarotta,*,† Robbert Duchateau,*,‡
Ece Koc¸,‡ and Tara J. Burchell†
Department of Chemistry, UniVersity of Ottawa, Ottawa, Ontario K1N 6N5, Canada, and Department of
Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, EindhoVen 56000MB, The Netherlands
ReceiVed June 17, 2008
Reaction of [(Ar)NPN(t-Bu)]2Cr (2) with Me3Al afforded {[(Ar)NP(Me)N(t-Bu)]AlMe2}Cr{[(Ar)NP(Me)-
(AlMe3)N(t-Bu)]AlMe(µ-Me)} (3), in which the ligand’s P atom has been alkylated and one AlR2 residue
retained by the chelating framework. One of the two ligands also retained an additional Me3Al unit via
coordination to the alkylated P atom. Complex 3 provides the first case of a selective ethylene trimerization
catalyst with high activity and excellent selectivity, producing 1-hexene upon exposure to ethylene at 80
°C. Furthermore, upon treatment with MAO, complex 3 acts as a nonselective catalyst, producing a
statistical mixture of oligomers with the highest ever observed activity. In addition, upon treatment with
[(i-Bu)2Al]2O, the complex acts as a highly active polymerization catalyst.
function of the alane activator is to reduce the metal toward a
lower oxidation state.7 Recent mechanistic work8 on the most
selective and active catalytic systems has revealed a rather
dynamic and somewhat contradictory behavior from the metal
oxidation state point of view, which, as a matter of fact, has
not yet been conclusively identified. The situation is further
complicated by the fact that some of the chromium catalysts
display extremely high activity but produce instead a statistical
distribution of oligomers.9 Therefore, questions arise as to which
mechanism the frequently observed formation of a Schultz-Flory
distribution of oligomers may belong to: namely, the nonredox
polymerization-type mechanism or the redox trimerization-type
Introduction
Catalytic selective ethylene trimerization and tetramerization
are fascinating processes increasingly attracting interest from
both industrial and academic standpoints.1 The mechanism
through which the selective trimerization2 is accomplished is
conceptually different from that of a regular polymerization.3
The ring-expansion mechanism, commonly accepted by workers
in the field,2g,h implies in fact a redox process, during which
oxidative addition of the metal center to the ethylene double
bond is followed by two subsequent insertions. The reductive
elimination of the seven-membered ring, generated during
the insertions, affords 1-hexene and restores the metal coordi-
native vacancy as well as the original low-valent configuration.
In this event, the availability of d electrons seems to be a
requirement for this catalytic cycle.
The most selective and active ethylene oligomerization
catalysts remain those based on chromium,4-6 where trivalent
complexes are by far the most frequently reported catalyst
precursors.1b This is despite its trivalent state being well known
as rather inert and certainly not inclined toward oxidative
addition. For this reason, it is also believed that a primary
(4) (a) Mimura, H.; Aoyama, T.; Yamamoto, T.; Oguri, M.; Koie, Y.
(Tosoh Corporation) JP 09268133, 1997. (b) Grove, J. J. C.; Mohamed,
H. A.; Griesel, L. (Sasol Technology (Pty) Ltd) WO 03/004158, 2002. (c)
Yoshida, T.; Yamamoto, T.; Okada, H.; Murakita, H. (Tosoh Corporation)
US2002/0035029, 2002. (d) Wass, D. F. (BP Chemicals Ltd) WO 02/04119,
2002. (e) Dixon, J. T.; Wasserscheid, P.; McGuinness, D. S.; Hess, F. M.;
Maumela, H.; Morgan, D. H.; Bollmann, A. (Sasol Technology (Pty) Ltd)
WO 03053890, 2001.
(5) (a) Mohamed, H.; Bollmann, A.; Dixon, J.; Gokul, V.; Griesel, L.;
Grove, C.; Hess, F.; Maumela, H.; Pepler, L. Appl. Catal., A 2003, 255,
355. (b) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.;
Wass, D. F. Chem. Commun. 2002, 858. (c) McGuinness, D. S.; Wasser-
scheid, P.; Keim, W.; Morgan, D. H.; Dixon, J. T.; Bollmann, A.; Maumela,
H.; Hess, F. M.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272. (d) Agapie,
T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Organo-
metallics 2006, 25, 2733. (e) Schofer, S. J.; Day, M. W.; Henling, L. M.;
Labinger, J. A.; Bercaw, J. E. Organometallics 2006, 25, 2743.
(6) (a) Reagan, W. K. (Phillips Petroleum Company) EP 0417477, 1991.
(b) Conroy, B. K.; Pettijohn, T. M.; Benham, E. A. (Phillips Petroleum
Co.) EP 0608447, 1994. (c) Freeman, J. W.; Buster, J. L.; Knudsen, R. D.
(Phillips Petroleum Co.) US 5,856,257, 1999.
(7) (a) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Inorg. Chem.
2008,ASAP. (b) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess,
F. M.; Dixon, J. T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605. (c)
Temple, C.; Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.;
Duchateau, R. Angew. Chem. 2006, 118, 7208. (d) Rucklidge, A, J.;
McGuinness, D. S.; Tooze, R. P.; Slawin, A. M. Z.; Pelletier, J. D. A.;
Hanton, M. J.; Webb, P. B. Organometallics 2007, 26, 2782. (e) Bowen,
L. E.; Haddow, M. F.; Orpen, A. G.; Wass, D. F. Dalton 2007, 1160.
(8) (a) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau,
R. Organometallic 2006, 25, 715. (b) Jabri, A.; Temple, C.; Crewdson, P.;
Gambarotta, S.; Korobkov, I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128,
9238.
* Corresponding authors. E-mail: sgambaro@uottawa.ca.
† University of Ottawa.
‡ Eindhoven University of Technology.
(1) (a) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.;
Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.;
Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem.
Soc. 2004, 126, 14712. (b) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan,
D. H. J. Organomet. Chem. 2004, 689, 3641. (c) Andes, C.; Harkins, S. B.;
Murtuza, S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423–7424.
(d) Santi, R.; Romano, A. M.; Grande, M.; Sommazzi, A.; Masi, F. Proto,
A. (ENICHEM S.P.A.) P.C.T patent WO01/68572/A1, 2001.
(2) (a) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem.
Soc. 1973, 95, 4451. (b) McDermott, X.; White, J. F.; Whitesides, G. M.
J. Am. Chem. Soc. 1976, 98, 6521. (c) Manyik, R. M.; Walker, W. E.;
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N.; Schaverien, C. J.; Orpen, A. G. Organometallics 1990, 9, 774. (g)
Emrich, R.; Heinemann, O.; Jolly, P. W.; Kru¨ger, Verhovnik, G. P. J.
Organometallics 1997, 16, 544. (h) Agapie, T.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 2007, 129, 14281.
(3) (a) Cossee, P. J. Catal. 1964, 3, 80. (b) Arlman, E. J.; Cossee, P. J.
Catal. 1964, 3, 99. (c) Skupinska, J. Chem. ReV. 1991, 91, 613.
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Duchateau, R. Organometallics 2005, 24, 5214.
10.1021/om800563w CCC: $40.75
2008 American Chemical Society
Publication on Web 09/25/2008

Chromium Trimerization Catalyst
Scheme 1
Organometallics, Vol. 27, No. 21, 2008 5709
Figure 1. Thermal ellipsoid plot of 2 with ellipsoids drawn at the
50% probability level.
Table 1. Crystal Data and Structure Analysis Results
formula
Mw
C32H52CrN4P2,
606.72
C41H78Al3CrN4P2
821.95
space group
a (Å)
b (Å)
monoclinic, P2(1)/n
9.201(9)
21.91(2)
9.597(9)
113.947(11)
1768(3)
2
monoclinic, P2(1)/n
18.9199(16)
14.7620(12)
18.1352(15)
102.7670(10)
4939.9(7)
2
mechanism, where the intermediate metallacycle ring is suf-
ficiently stable to allow further expansion by a few subsequent
insertions.10 Or is it a completely separate mechanism? Ad-
dressing these issues is of course central to the rational design
of much wanted trimerization and tetramerization catalysts. One
possible strategy is to attempt to isolate intermediates of the
catalytic cycle capable of acting as single-component catalysts.
These species would in fact reveal the metal electronic config-
uration and steric environment and, thus, provide critical
information for process improvement and future catalyst design.
We have recently discovered a catalytic system based on the
(t-Bu)NPN(t-Bu) ligand system, which switches selectivity from
polymerization to a nonselective oligomerization and even
selective trimerization depending on the particular nature of the
alane activator.11 By replacing one of the ligand’s t-Bu groups
with an aryl substituent, we have now succeeded in obtaining
high activity10,12 for the nonselective ethylene oligomerization
and, most of all, isolated the first ever single-component
selective trimerization chromium catalyst. To the best of our
knowledge, these findings are precedented by only one claim
in the patent literature describing a lower-activity single-
component trimerization catalyst based on vanadium.1d
c (Å)
ꢀ (deg)
V (Å3)
Z
radiation
T (K)
0.781
203(2)
1.140
0.439
0.781
203(2)
1.105
0.379
1780
0.0648, 0.1383
1.069
Dcalcd (g cm-3
)
µcalcd (mm-1
)
F000
R, Rw
652
2a
0.0559, 0.1350
1.039
GoF
a R ) ∑|Fo| - |Fc|/∑|F|. Rw ) [∑(|Fo| - |Fc|)2/∑wFo
] .
2 1/2
unusual behavior. The structure of 2 is very similar to that of
the earlier reported divalent [(t-Bu)NPN(t-Bu)]2Cr11 and does
not show any relevant feature other than a divalent chromium
center in the much expected square-planar environment defined
by the four nitrogen atoms of the two NPN anions (Figure 1,
Tables 1, 2).
Following our interest in the behavior of chromium complexes
with aluminum alkyls,8,9 we have reacted 2 with a diverse series
of alanes using several different stoichiometric ratios. Whereas
the reaction of the [(t-Bu)NPN(t-Bu)]2Cr system with several
alanes afforded crystalline products,11 to this end, only treatment
of 2 with 4 equiv of Al(CH3)3 gave material in good yield that
could be characterized by X-ray single-crystal diffraction and
was found to be the novel {[(Ar)NP(Me)N(t-Bu)]AlMe2}-
Cr{[(Ar)NP(Me)(AlMe3)N(t-Bu)]AlMe(µ-Me)} (3) (Scheme 1).
A similar product was also obtained when [(t-Bu)NPN(t-
Bu)]2Cr was treated with Al(CH3)3.11 The primary role of the
alane activators appears to have been the alkylation of the P
atom. The resulting (Ar)NP(R)N(t-Bu) dianion binds to the
AlMe2 residue via the two N donor atoms, forming an
{MeP[N(t-Bu)][N(Ar)]AlMe2}- anion that is attached to the
chromium metal via a nitrogen and a phosphorus atom (Figure
2). In the second ligand in 3 the alkylated phosphorus atom
coordinates to an additional AlMe3, which prevents it from
binding to chromium. As a result, one of the two aluminum
methyl groups of the AlMe2 residue occupies the fourth
coordination site of the distorted square-planar chromium atom.
Complex 3 is a single-component trimerization catalyst
exclusively producing 1-hexene. Its activity is of the same order
of magnitude as that of other selective systems recently reported,
such as the Sasol’s [SNS]CrCl3 system.5c The selectivity also
depends on temperature. Namely, at 80 °C 1-hexene is no longer
Results and Discussion
The reaction of [(Ar)N(H)PN(t-Bu)]2 (1)13 with n-BuLi
followed by reaction with CrCl2(THF)2 afforded the divalent
[(Ar)NPN(t-Bu)]2Cr (2), where the ligand system 1 underwent
a dimer to monomer dissociation generating two (Ar)NPN(t-
Bu) monoanions (Scheme 1). The affinity of divalent chromium
for square-planar ligand fields as dictated by the d4 electronic
configuration is most probably the driving force behind this
(10) Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.; Gibson,
V. C. J. Am. Chem. Soc. 2005, 127, 10166.
(11) Albahily, K.; Koc¸, E.; Al-Baldawi, D.; Savard, D.; Gambarotta,
S.; Burchell, T. J.; Duchateau, R. Angew. Chem., Int. Ed. 2008, 47, 5816.
(12) For more highly active catalysts see: (a) Jones, D. J.; Gibson, V. C.;
Green, S. M.; Maddox, P. J. Chem. Commun. 2002, 1038. (b) Tomov, A. K.;
Chirinos, J. J.; Long, R. J.; Gibson, V. C.; Elsegood, M. R. J. J. Am. Chem.
Soc. 2006, 128, 7704. (c) McGuinness, D. S.; Gibson, V. C.; Wass, D. F.;
Steed, J. W. J. Am. Chem. Soc. 2003, 125, 12716.
(13) (a) Axenov, K. V.; Kotov, V. V.; Klinga, M.; Leskelae, M.; Repo,
T. Eur. J. Inorg. Chem. 2004, 695. (b) Axenov, K. V.; Klinga, M.; Leskelae,
M.; Kotov, V.; Repo, T. Eur. J. Inorg. Chem. 2004, 4702. (c) Axenov,
K. V.; Klinga, M.; Leskelae, M.; Repo, T. Organometallics 2005, 24 (6),
1336.

5710 Organometallics, Vol. 27, No. 21, 2008
Albahily et al.
Table 2. Selected Bond Distances (Å) and Angles (deg)
difference of reactivity (trimerization, oligomerization, and
polymerization) as a function of the presence/absence and nature
of the alane is to be ascribed to three different species.
In conclusion, we have reported a single-component catalyst
selectively producing 1-hexene with good activity. It is interest-
ing to observe that this species contains chromium in its divalent
state, as it is unlikely that the divalent state enables the selective
trimerization process.7c,11 Therefore, complex 3 can best be
regarded as a self-activating catalyst precursor. Further work
to elucidate the oxidation state of the true active species and
the origin of the switching of the catalytic selectivity of this
highly fascinating system is in progress.
2
3
Cr(1)-N(1) 2.114(3)
Cr(1)-N(3) 2.144(4)
Cr(1)-N(2) 2.086(3)
Cr(1)-N(4) 2.125(4)
N(1)-P(1) 1.628(3)
N(2)-P(1) 1.612(3)
N(1)-Cr(1)-N(2) 71.02(13)
N(1)-Cr(1)-N(2a) 108.98(13)
N(2)-Cr(2a)-N(1a) 179.998(1)
Cr(1)-C(38) 2.289(6)
Cr(1)-P(2) 2.4035(16)
N(4)-Al(2) 2.026(4)
N(2)-Al(2) 1.899(4)
Cr(1)-Al(4) 2.8098(18)
Al(2)-P(2) 2.627(2)
P(2)-N(2) 1.659(4)
P(2)-N(4) 1.717(4)
Al(4)-P(1) 2.634(2)
Al(4)-N(3) 1.951(4)
Al(4)-N(1) 1.845(4)
P(1)-N(3) 1.746(4)
Experimental Part
P(1)-N(1) 1.674(4)
N(4)-Cr(1)-N(3) 165.94(16)
N(4)-Cr(1)-C(38) 98.89(19)
N(3)-Cr(1)-C(38) 90.15(19)
N(4)-Cr(1)-P(2) 44.03(11)
N(3)-Cr(1)-P(2) 127.78(12)
C(38)-Cr(1)-P(2) 142.07(16)
Al(4)-C(38)-Cr(1) 79.9(2)
Cr(1) · · · Al(4) 2.8098(18)
All reactions were carried out under a dry nitrogen atmosphere.
Solvents were dried using an aluminum oxide solvent purification
system. Ligand 1 was prepared according to a literature procedure.12
Samples for magnetic susceptibility were preweighed inside a
drybox equipped with an analytical balance and measured on a
Johnson Matthey magnetic susceptibility balance. Data for X-ray
crystal structure determination were collected with a Bruker
diffractometer equipped with a 1K Smart CCD area detector.
CrCl2(THF)2 was prepared according to standard procedures. The
reagents DIBAC (Aldrich), TMA (Aldrich), TIBA (Aldrich),
TIBAO (Aldrich), and MAO (Chemtura) were used as received.
Preparation of [(Ar)NPN(t-Bu)]2Cr (2). A solution of cis-
{(2,6-diisopropylpheny)(H)NP[(µ-N)(t-Bu)]2PN(H)(2,6-diisopropyl-
pheny)} (1) (0.556 g, 1 mmol) in THF (20 mL) was treated with
n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes) at 0 °C. The mixture
was allowed to stir at room temperature for 20 h. The resulting
solution was added to a suspension of CrCl2 · 2THF (0.268 g, 1
mmol) in THF (10 mL). The reaction mixture was stirred at room
temperature overnight. The solvent was evaporated in Vacuo and
the residue redissolved in toluene (10 mL). The resulting suspension
was centrifuged, and the solution was stored at -30 °C. Brown
crystals separated over four days and were filtered, washed with
cold hexanes (10 mL), and dried in Vacuo to give analytically pure
2 (0.355 g, 59%). [µeff ) 4.96 µB.] Anal. Calcd (Found) for
C32H52CrN4P2: C 63.35 (63.28), H 8.64 (8.59); N 9.23 (9.19).
Preparation of {[(Ar)NP(Me)N(t-Bu)]AlMe2}Cr{[(Ar)NP(Me)-
(AlMe3)N(t-Bu)]AlMe(µ-Me)} (3). A solution of 2 (0.606 g, 1
mmol) in toluene (20 mL) was treated with trimethyl aluminum
(0.288 g, 4 mmol) at room temperature and stirred for 10 min.
Storing the resulting solution at -30 °C for 5 days afforded small
purple crystals of 3, which were filtered and washed with cold
hexanes (5 mL) and dried in Vacuo (0.455 g, 55%). [µeff ) 4.98
µB.] Anal. Calcd (Found) for C41H78Al3CrN4P2: C 59.91 (59.87);
H 9.57 (9.55), N 6.82 (6.78).
Polymerizations and Oligomerizations. Catalytic runs were
carried out in a 200 mL high-pressure Bu¨chi reactor containing a
heating/cooling jacket. A preweighed amount of catalyst was
dissolved in 10 mL of toluene under N2 and injected into the
preheated reactor already charged with cocatalyst and toluene (total
volume 100 mL). Solutions were heated using a thermostatic bath
and charged with ethylene, maintaining the pressure throughout the
run. The oligo/polymerizations were quenched by venting the
reactor (after cooling to 5 °C for oligomerizations) and addition of
EtOH and HCl. The resulting polymer was isolated by filtration,
sonicated with an acidified ethanol solution, rinsed, and thoroughly
dried prior to mass determination. Oligomers were analyzed using
GC and 1H NMR. Molecular weight and molecular weight
distributions of the resulting polymers were determined by gel
permeation chromatography on a PL-GPC210 equipped with a RI
produced and polymer is formed instead. A similar behavior
was observed by de Wet-Roos and co-workers for Sasol’s PNP-
type tetramerization catalyst system.14 At room temperature, the
catalytic activity drops and the activity is marginal. Reaction
times up to 10 h at typical reaction temperature (50 °C) did not
improve the productivity of 1-hexene, indicating rapid catalyst
deactivation (in less than 30 min). Interestingly, the analogue
of
3 with only nitrogen-t-Bu substituents, {[MeP{N(t-
Bu)}2]AlMe2}Cr{[MeP(AlMe3){N (t-Bu)}2]AlMe(µ-Me)}, af-
forded polyethylene already at 50 °C.11 This is assumed to be
due to a lower stabilizing ability of t-Bu substituents versus
aryl substituents. Upon addition of aluminum-based cocatalysts,
the selectivity of complex 3 shows a dramatic switch. In the
case of treatment with MAO, a statistical distribution of
oligomers with an enormous increment of activity was
observed.10,12 Upon activation with [(t-Bu)2Al]2O, the catalyst
selectivity switches to ethylene polymerization instead. With
Al(i-Bu)3 as cocatalyst, no oligomers and only a small amount
of polymer is formed. The switch of catalytic behavior from
selective to nonselective oligomerization is intriguing given that
the ligand and the initiating alkyl function (methyl) are the same
for the two processes in this case. The fact that at elevated
temperature 3 becomes an ethylene polymerization catalyst
indicates that it is rather facile to switch between the two
different processes.
Explaining or simply rationalizing this intriguing behavior
is not an easy task at this early stage. We speculate that the
Figure 2. Thermal ellipsoid plot of 3 with ellipsoids drawn at the
50% probability level.
(14) de Wet-Roos, D.; Du Toit, A.; Joubert, D. J. J. Polym. Sci., Polym.
Chem. 2006, 44, 6847.

Chromium Trimerization Catalyst
Organometallics, Vol. 27, No. 21, 2008 5711
Table 3. Oligomerization and Polymerization Results for Complex 3 with and without Cocatalystsa
activity
Al:Cr PE (g) oligom. (g) (g · mmol-1 · h-1) C6 ) % C8 ) % C10 ) % C12 ) % C14 ) % C16 ) % C18 ) %
cat.
(µ-mol)
cat.
cocat.
R
3
10
10
10
10
10
10
10
10
3
0.5
600
100
780
17 000
400
240
99.9
99.9
3b
3c
3
3.9
0.4
MAO
2000
85
2
39.0
9.9
27.6
15.7
8.7
4.8
2.9
1.4
0.55
3
3
[(t-Bu)2Al]2O 1000 10.5d
Al(t-Bu)3
MAO
MAO
1000
1000
300
1.2
0.2
0.4
(SNS)CrCl3
(SNS)CrCl3
7
6
1400
1200
g98
g98
2
2
a Conditions: 100 mL of toluene, 35 bar of ethylene, reaction temperature 50 °C, reaction time 30 min. b Reaction temperature 25 °C. c Reaction
temperature 80 °C. d The SEC of the polyethylene shows a bimodal distribution with an Mw for the main peak at 1.3 × 106 g · mol-1 and the Mw for the
smaller peak at 12 000 g · mol-1
.
and viscosity detector and a 3 × PLgel 10 µm Mixed-B column
set at 135 °C using 1,2,4-trichlorobenzene as solvent. The molecular
weight of the polyethylenes was referenced to polystyrene (Mw )
65500, PDI ) 1.02) standards.
with full-matrix least-squares procedures based on F2. All non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. All hydrogen atoms were treated as idealized contribu-
tions. All scattering factors and anomalous dispersion factors are
contained in the SHELXTL 6.12 program library. Relevant crystal
data and bond distances and angles are given in Tables 1 and 2
respectively.
X-ray Crystallography. Suitable crystals were selected, mounted
on a thin, glass fiber with paraffin oil, and cooled to the data
collection temperature. Data were collected on a Bruker AXS
SMART 1 k CCD diffractometer. Data collection was performed
with three batch runs at φ ) 0.00° (600 frames), at φ ) 120.00°
deg (600 frames), and at φ ) 240.00° (600 frames). Initial unit-
cell parameters were determined from 60 data frames collected at
different sections of the Ewald sphere. Semiempirical absorption
corrections based on equivalent reflections were applied. The
systematic absences and unit-cell parameters were consistent for
the reported space groups. The structures were solved by direct
methods, completed with difference Fourier syntheses, and refined
Acknowledgment. This work was supported by the
Natural Science and Engineering Council of Canada (NSERC)
and by the Dutch Polymer Institute.
Supporting Information Available: Complete crystallographic
data (as a CIF file) for complexes 2 and 3. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM800563W