The (PH)2nacnac ligand in organochromium chemistry

Article abstract of DOI:10.1021/om010857k

A novel class of β-diketiminato chromium complexes was prepared to serve as a homogeneous model system for the heterogeneous Phillips ethylene polymerization catalyst. (Ph)₂nacnacCrCl₂(THF)₂ (1, (Ph)₂nacnac = N,N′-diphenyl-2,4-pentanediimme anion) was found to polymerize ethylene in the presence of excess methylaluminoxane (MAO). Reaction of 1 with 1.0 equiv of an alkylzinc compound yielded dimeric, chloride-bridged monoalkyl complexes of the type [(Ph)₂nacnacCrR(μ-Cl)THF]₂ (R = Me (3a), Et (3b)). Reaction of 1 with 2.0 equiv of alkyllithium reagents resulted in a disproportionation to [(Ph)₂nacnac]₂Cr (2) and CrR₄. 2 could be oxidized by treatment with tetrachloroethane to give trigonalbipyramidal [(Ph)₂nacnac]₂CrCl (5). Attempted alkylation of 5 with aluminum alkyls yielded the ortho-metalated product (Ph)₂nacnac(η¹-C₆H₄-Ph)nacnacCr (6). However, the chromium-(III) aryl complex [(Ph)₂nacnac]₂CrPh (4) could be synthesized by reaction of CrPh₃(THF)₃ with 2 equiv of (Ph)₂nacnacH. Square-pyramidal 4 featured both a normal planar η²-nacnac and a nonplanar η³-nanac ligand. Thermolysis of 4 resulted in the loss of benzene along with the formation of 6. The crystal structures of 2, 3a, and 4-6 were determined by X-ray diffraction, Complexes 1 and 3-5 catalyzed the polymerization of ethylene when activated with MAO, but they did not catalyze the polymerization by themselves.

Full text of DOI:10.1021/om010857k

952
Organometallics 2002, 21, 952-960
Th e (P h )2n a cn a c Liga n d in Or ga n och r om iu m Ch em istr y
Leonard A. MacAdams, Woo-Kyu Kim, Louise M. Liable-Sands, Ilia A. Guzei,
Arnold L. Rheingold, and Klaus H. Theopold*
Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology,
University of Delaware, Newark, Delaware 19716
Received October 2, 2001
A novel class of â-diketiminato chromium complexes was prepared to serve as a
homogeneous model system for the heterogeneous Phillips ethylene polymerization catalyst.
(Ph)2nacnacCrCl2(THF)2 (1, (Ph)2nacnac ) N,N′-diphenyl-2,4-pentanediimine anion) was
found to polymerize ethylene in the presence of excess methylaluminoxane (MAO). Reaction
of 1 with 1.0 equiv of an alkylzinc compound yielded dimeric, chloride-bridged monoalkyl
complexes of the type [(Ph)2nacnacCrR(µ-Cl)THF]2 (R ) Me (3a ), Et (3b)). Reaction of 1
with 2.0 equiv of alkyllithium reagents resulted in a disproportionation to [(Ph)2nacnac]2Cr
(2) and CrR4. 2 could be oxidized by treatment with tetrachloroethane to give trigonal-
bipyramidal [(Ph)2nacnac]2CrCl (5). Attempted alkylation of 5 with aluminum alkyls yielded
the ortho-metalated product (Ph)2nacnac(η1-C6H4-Ph)nacnacCr (6). However, the chromium-
(III) aryl complex [(Ph)2nacnac]2CrPh (4) could be synthesized by reaction of CrPh3(THF)3
with 2 equiv of (Ph)2nacnacH. Square-pyramidal 4 featured both a normal planar η2-nacnac
and a nonplanar η3-nanac ligand. Thermolysis of 4 resulted in the loss of benzene along
with the formation of 6. The crystal structures of 2, 3a , and 4-6 were determined by X-ray
diffraction. Complexes 1 and 3-5 catalyzed the polymerization of ethylene when activated
with MAO, but they did not catalyze the polymerization by themselves.
Ch a r t 1
In tr od u ction
Beginning with work by Thomas et al.,1,2 we have
created a functional homogeneous model system of the
so-called “Union Carbide catalyst” (Cp2Cr/SiO2)3 for the
polymerization of ethylene.4,5 So faithful is this model
that it reproduces one of the apparent disadvantages
of the commercially used heterogeneous catalyst, namely
its high selectivity for ethylene over propylene or, in
other words, its sole utility for the production of high-
density polyethylene (HDPE).3 The stereoregular ho-
mopolymerization of propylene and the copolymerization
of ethylene with R-olefins produce materials (i.e. PP,
LLDPE) of considerable value, however. Such polymers
can be madesinter aliaswith the “Phillips catalyst”
(inorganic Cr/SiO2), one of the first polymerization
catalysts to be discovered.6 Despite several decades of
investigation of this deceptively simple catalyst, there
remain many open questions regarding the nature of
the chromium sites responsible for initiation and propa-
gation of polymerization.7 In an attempt to build a
homogeneous model system for the Phillips catalyst, we
have turned to the “nacnac” ligands (i.e. N,N′-disubsti-
tuted â-diketiminates), which provide for coordination
of chromium by two hard donor atoms and facilitate site
isolation by steric protection with bulky substituents.
The analogy between the presumed active site of the
heterogeneous catalyst (A) and its homogeneous model
(B) is depicted in Chart 1. The correspondence is
obviously not perfect; nevertheless, some insight into
the relevant chemistry of chromium might be expected
to result from a study of “nacnac” chromium alkyls.
Besides, any new chromium-based polymerization cata-
lysts are of interest, and the compounds to be described
fit into the larger context of our long-standing investiga-
tion of paramagnetic organometallics (i.e. “metallaradi-
cals”).8,9
Some time ago, we communicated preliminary experi-
ments confirming the catalytic activity of (Ph)2nacnac
complexes of Ti(III), V(III), and Cr(III) ((Ph)2nacnac )
N,N′-diphenyl-2,4-pentanediimine anion).10 Specifically,
(Ph)2nacnacCrCl2(THF)2/MAO catalyzed the copolym-
erization of ethylene with R-olefins. Seeking to avoid the
ambiguities of MAO chemistry, we set out to explore
the reactivity of well-defined (Ph)2nacnacCr alkyls.
Herein we report our findings with this particular
ligand.
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10.1021/om010857k CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/06/2002

The (Ph)2nacnac Ligand in Organochromium Chemistry
Organometallics, Vol. 21, No. 5, 2002 953
Sch em e 1
At the time of our first preparation of a nacnac
chromium complex this class of bidentate monoanionic
nitrogen analogues of the “acac” ligands had long been
known but was effectively unexplored.11-13 However, in
the late 1990s the “nacnac” scent was apparently “in
the air”. Recent years have seen a flurry of activity in
the area, and there are now numerous nacnac deriva-
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As previously reported, slow addition of [(Ph)2nacnac]-
Li to a suspension of CrCl3(THF)3 in THF at room
temperature yielded the six-coordinate dichloride com-
plex (Ph)2nacnacCrCl2(THF)2 (1; Scheme 1). The solid-
state structure of 1 and its catalytic activity in the
presence of methylaluminoxane (MAO) for the polym-
erization of ethylene and copolymerization of ethylene
with R-olefins were described in an earlier communica-
tion.10
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In an attempt to characterize more fully the catalyti-
cally active species in 1/MAO mixtures, we set out to
explore the organometallic chemistry of the (Ph)2-
nacnacCr moiety. However, alkylation of 1 proved less
straightforward than the analogous vanadium chemis-
try, which easily yielded tetrahedral molecules of the
type (Ph)2nacnacVR2. Thus, treatment of 1 with 2.0
equiv of lithium alkyls (e.g. Me3SiCH2Li or MeLi) in
diethyl ether did not yield the desired dialkylchromium
complexes but, rather, afforded dark green [(Ph)2-
nacnac]2CrII (2) in modest yield (see Scheme 1). The
molecular structure of 2 has been determined by X-ray
diffraction; it is depicted in Figure 1, and selected
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954 Organometallics, Vol. 21, No. 5, 2002
MacAdams et al.
F igu r e 1. Molecular structure of [(Ph)2nacnac]2Cr (2).
Ta ble 1. Selected In ter a tom ic Dista n ces a n d
An gles for [(P h )2n a cn a c]2Cr (2)
Distances (Å)
Cr-N(1)
2.058(3)
2.055(4)
1.336(5)
1.314(5)
1.515(6)
1.406(6)
Cr-N(2)
2.049(4)
2.069(3)
1.451(5)
1.432(5)
1.401(6)
1.513(6)
Cr-N(3)
Cr-N(4)
F igu r e 2. Molecular structure of [(Ph)2nacnacCr(CH3)(µ-
Cl)THF]2 (3a ).
N(1)-C(2)
N(2)-C(4)
C(1)-C(2)
C(3)-C(4)
N(1)-C(11)
N(2)-C(17)
C(2)-C(3)
C(4)-C(5)
along with unidentified side products (decomposition
products of unstable CrMe4?67). Tetrahedral coordina-
tion is neither common nor energetically advantageous
for trivalent chromium (d3). The coordination of two
molecules of THF in 1 reflects the pronounced prefer-
ence of Cr(III) for octahedral coordination. However,
dialkyl derivatives are apparently not electrophilic
enough to retain the THF ligands. Even the addition of
stronger donors, such as pyridine, to the alkylation
reactions failed to prevent the disproportionation.
Angles (deg)
N(1)-Cr-N(2)
N(1)-Cr-N(4)
N(2)-Cr-N(4)
Cr-N(1)-C(2)
Cr-N(2)-C(40
Cr -N(3)-C(19)
Cr -N(4)-C(21)
88.47(14)
132.88(14)
100.69(14)
121.6(3)
126.2(3)
125.5(3)
N(1)-Cr-N(3)
N(2)-Cr-N(3)
N(3)-Cr-N(4)
Cr)-N(1)-C(11
Cr-N(2)-C(17)
Cr-N(3)-C(28)
Cr-N(4)-C(34)
105.49(14)
149.78(14)
89.35(14)
119.2(3)
110.9(3)
114.1(3)
123.1(3)
117.0(3)
interatomicdistances and angles are listed in Table 1.
The molecule crystallized in the triclinic space group
P1h and featured no crystallographic symmetry. The
coordination geometry of 2 deviates significantly from
the square-planar coordination preferred by four-
coordinate chromium(II).64 Due presumably to the steric
bulk of the phenyl substituents, 2 adopts the configu-
ration of a distorted tetrahedron. Thus, the two nacnac
planes (defined by N(1), N(2), C(2), C(3), C(4) and N(3),
N(4), C(19), C(20), C(21)) are close to perpendicular at
80.3°. The average Cr-N distance in 2 (2.06 Å) is only
marginally longer than that of 1 (2.033 Å), reflecting
the opposing effects of lower coordination number and
lower formal oxidation state. 2 is stable at ambient
temperature in an inert atmosphere but decomposes
quickly upon exposure to air. Its room-temperature
effective magnetic moment is 5.1(1) µB, consistent with
a chromium(II) compound containing four unpaired
electrons.
While 2 might simply be considered a product of
reduction of 1 by the electron-rich lithium alkyl followed
by a nacnac-ligand transfer, the detectionsby 1H NMR
spectroscopysof the homoleptic chromium(IV) alkyl Cr-
(CH2SiMe3)465,66 in the reaction mixture suggested that
2 was actually produced via disproportionation of the
unstable target molecule (Ph)2nacnacCr(CH2SiMe3)2.
The reaction of 1 with methyllithium also yielded 2,
In contrast, alkyls of the type [(Ph)2nacnacCr(THF)-
(R)(µ-Cl)]2 (3a , R ) Me; 3b, R ) Et) could be prepared
by reaction of 1 with 1.0 equiv of the corresponding
dialkyl zinc reagent (see Scheme 1). These molecules
are closely related to [(iPr2Ph)2nacnacCr(Me)(µ-Cl)]2,
reported by Gibson et al.19 3a is stable at room temper-
ature in solution for an extended period of time,
although it decomposes slowly upon exposure to air.
Crystals of 3a suitable for an X-ray structure determi-
nation were grown from a concentrated ether solution
at -30 °C. Its molecular structure is shown in Figure
2, and selected interatomic distances and angles are
listed in Table 2. The molecule lies on a crystallographic
inversion center and adopts the geometry of an edge-
sharing bioctahedron, emphasizing once again the pref-
erence of Cr(III) for octahedral coordination. The methyl
groups are positioned trans to each other, and the Cr-
Cl and Cr-C distances are within the expected ranges.
The Cr-Cr distance of 3.63 Å does not allow for any
direct metal-metal bonding; it is significantly longer
than the corresponding distances in [Cp*Cr(µ-Cl)CH3]2
(3.29 Å)68 and [Cp*Cr(µ-Cl)(CH2SiMe3)]2 (3.47 Å).69 The
effective magnetic moment of 3 (5.4 µB; i.e. 3.8(1) µΒ per
Cr) even shows that there is no magnetic coupling; it is
the value expected for magnetically independent chro-
mium(III) ions.
(64) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry; Wiley: New York, 1999; p 742 f.
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M.; Wilkinson, G. J . Chem. Soc., Dalton Trans. 1972, 533.
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Soc. D 1970, 1369.
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Chem. 1988, 353, 323.
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Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 5477.

The (Ph)2nacnac Ligand in Organochromium Chemistry
Organometallics, Vol. 21, No. 5, 2002 955
Ta ble 2. Selected In ter a tom ic Dista n ces a n d
An gles for [(P h )2n a cn a cCr Me(µ-Cl)THF ]2 (3)
Distances (Å)
Cr(1)-N(1)
Cr(1)-C(22)
Cr(1)-Cl(1)
N(1)-C(2)
N(2)-C(4)
C(1)-C(2)
C(3)-C(4)
2.043(2)
2.077(2)
2.4127(6)
1.334(3)
1.327(3)
1.523(3)
1.418(3)
Cr(1)-N(2)
Cr(1)-O(1)
Cr(1)-Cl(1A)
N(1)-C(6)
N(2)-C(12)
C(2)-C(3)
C(4)-C(5)
2.058(2)
2.269(2)
2.4217(6)
1.451(3)
1.453(3)
1.399(4)
1.527(4)
Angles (deg)
N(1)-Cr(1)-N(2)
N(2)-Cr(1)-C(22)
N(2)-Cr-(1)-O(1)
N(1)-Cr(1)-Cl(1)
N(2)-Cr(1)-Cl(1A)
C(22)-Cr(1)-Cl(1)
O(1)-Cr(1)-Cl(1)
91.07(8) N(1)-Cr(1)-C(22)
92.25(9) N(1)-Cr(1)-O(1)
88.53(7) C(22)-Cr(1)-O(1)
93.21(9)
85.59(7)
178.58(8)
93.08(6) N(1)-Cr(1)-Cl(1A) 171.57(6)
92.70(2) N(2)-Cr(1)-Cl(1) 173.50(6)
92.51(8) C(22)-Cr(1)-Cl(1A) 94.17(7)
86.80(5) O(1)-Cr(1)-Cl(1A) 86.97(5)
Cl(1)-Cr(1)-Cl(1A) 82.54(2) Cr(1)-Cl(1)-Cr(1A) 97.46(2)
Cr(1)-N(1)-C(2)
Cr(1)-N(2)-C(12) 119.06(14) Cr(1)-N(2)-C(4)
122.3(2)
Cr(1)-N(1)-C(6)
119.9(2)
123.3(2)
Sch em e 2
F igu r e 3. Molecular structure of [(Ph)2nacnac]2CrPh (4).
Ta ble 3. Selected In ter a tom ic Dista n ces a n d
An gles for [(P h )2n a cn a c]2Cr P h (4)
Distances (Å)
Cr-N(1)
2.114(3)
2.043(2)
2.086(3)
1.422(4)
1.427(4)
1.518(4)
1.511(5)
Cr-N(2)
2.060(3)
2.004(3)
1.306(4)
1.316(4)
1.433(5)
1.424(4)
Cr-N(3)
Cr-N(4)
Cr-C(35)
N(1)-C(6)
N(2)-C(12)
C(1)-C(4)
C(3)-C(5)
N(1)-C(1)
N(2)-C(3)
C(1)-C(2)
C(2)-C(3)
NMR spectroscopic observations suggested that 3a
adds THF reversibly. Thus, crystallization of the prod-
uct of the reaction of 1 with ZnMe2 from THF/Et2O
(rather than pure Et2O) yielded a material that exhib-
ited a 1H NMR spectrum (in C6D6) different from 3a
but very similar to 1. Trituration of this solid with
diethyl ether restored the spectrum of 3a . On the basis
of these observations we assign the formula (Ph)2-
nacnacCr(Me)(Cl)(THF)2 to this compound. We also note
that addition of excess pyridine to 3a in C6D6 solution
liberated THF and formed a new compound, tentatively
identified as (Ph)2nacnacCr(Me)(Cl)(py)2.
Angles (deg)
N(1)-Cr-N(2)
N(1)-Cr-N(4)
N(2)-Cr-N(3)
N(2)-Cr-C(35)
N(3)-Cr-C(35)
84.37 (10)
95.06 (10)
161.98 (11)
92.02 (11)
88.55 (11)
N(1)-Cr-N(3)
N(1)-Cr-C(35)
N(2)-Cr-N(4)
N(3)-Cr-N(4)
N(4)-Cr-C(35)
91.14(10)
167.23(12)
106.84(11)
90.91(11)
97.71(12)
center. Addition of 2 equiv of (Ph)2nacnacH to a stirred
solution of CrPh3(THF)3 in THF produced a slight color
change from red to brown-red. Subsequent workup of
the reaction mixture yielded red crystals of [(Ph)2-
nacnac]2CrPh (4) in good yield (72%) (see Scheme 2).
The room-temperature magnetic moment of 4.1(1) µB is
consistent with the molecule being a mononuclear
chromium(III) species. The crystal structure of 4 was
determined by X-ray diffraction, and the result is shown
in Figure 3. Selected interatomic distances and angles
are listed in Table 3.
As the targeted dialkyls were not readily accessible
by alkylation of the dichloride 1, we sought another
preparatory route to such compounds. While many
chromium alkyls of the type CrR3(THF)3 are thermally
labile (especially if R is small), CrPh3(THF)3 proved a
suitable precursor. Addition of 1 equiv of (Ph)2nacnacH
to a stirred solution of CrPh3(THF)3 in THF caused a
color change from red to dark green; however, standard
workup yielded the bis-nacnac compound 2 again. This
result is consistent with our earlier inference that
compounds of the type (Ph)2nacnacCrR2 decompose via
disproportionation. In this particular instance, forma-
tion of (Ph)2nacnacCr(Ph)2 is presumably followed by
rapid disproportionation to 2 and Cr(Ph)4 (not identified)
(Scheme 2).
4 belongs to the relatively small number of five-
coordinate Cr(III) complexes.69-74 The coordination
geometry of chromium is best described as a slightly
distorted square pyramid with N(4) occupying the apical
(70) Liang, Y.; Yap, G. P. A.; Rheingold, A. L.; Theopold, K. H.
Organometallics 1996, 15, 5284.
(71) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J . Organometallics
1997, 16, 5116.
(72) Greene, P. T.; Russ, B. J .; Wood, J . S. J . Chem. Soc. A 1971,
3636.
(73) Mu¨ller, E.; Krause, J .; Schmiedeknecht, K. J . Organomet. Chem.
1972, 44, 127.
In an attempt to avoid the disproportionation of the
chromium(III) alkyls/aryls, we next investigated the
possibility of stabilizing an organochromium(III) com-
pound by coordinating two nacnac ligands to the metal
(74) Cotton, F. A.; Czuchajowska, J .; Falvello, L. R.; Feng, X. Inorg.
Chim. Acta 1990, 172, 135.

956 Organometallics, Vol. 21, No. 5, 2002
MacAdams et al.
Sch em e 3
F igu r e 4. Molecular structure of one of the two indepen-
dent molecules of [(Ph)2nacnac]2CrCl (5).
site. This leaves an open coordination site trans to N(4).
The latter is probably responsible for the most remark-
able structural feature of 4, namely the nonplanarity
of the nacnac ligand containing N(1) and N(2). This
CrN2C3 ring adopts a boat configuration that brings C(2)
in relatively close contact to the metal (Cr-C(2) ) 2.56
Å). The relevant dihedral angles are 106.8° between the
Cr, N(1), N(2) plane and the N(1), N(2), C(1), C(3) plane
and 146.2° between the latter and the C(1), C(2), C(3)
plane. There are no short intermolecular contacts to
C(2). Following an argument presented by Piers et al.,48
based on calculations by Tolman and Solomon et al.,43
we suggest that the bonding of this nacnac ligand should
not be described as η5-coordination but, rather, results
from an interaction of chromium with the highest
occupied π-orbital of the nacnac ligand, which has
significant electron density on C(2) and none on C(1)
and C(3). In effect, the nacnac ligand becomes a triden-
tate σ-donor coordinating to the metal via N(1), N(2),
and C(2). The Cr-N(1) distance of 2.114 Å is longer than
the other Cr-N distances in the molecule, due to the
trans influence of the phenyl ligand.
Ta ble 4. Selected In ter a tom ic Dista n ces a n d
An gles for On e of th e Tw o In d ep en d en t Molecu les
of [(P h )2n a cn a c]2Cr Cl (5)
Distances (Å)
Cr(1)-N(21)
Cr(1)-N(2)
Cr(1)-Cl(1)
1.970(4)
2.032(4)
2.327(2)
Cr(1)-N(22)
Cr(1)-N(1)
2.021(4)
2.068(4)
Angles (deg)
N(21)-Cr(1)-N(22) 90.3 (2)
N(21)-Cr(1)-N(2)
90.9(2)
N(22)-Cr(1)-N(2) 92.8 (2)
N(21)-Cr(1)-N(1) 114.0(2)
N(22)-Cr(1)-N(1) 155.7 (2)
N(2)-Cr(1)-N(1)
88.1(2)
N(21)-Cr(1)-Cl(1)
N(2)-Cr(1)-Cl(1)
94.17 (13) N(22)-Cr(1)-Cl(1) 87.62(12)
174.95 (11) C(2)-N(1)-Cr(1)
122.8(3)
C(17)-N(2)-Cr(1) 122.3(3)
C(22)-N(21)-Cr(1) 126.7(3)
C(37)-N(22)-Cr(1) 114.9(3)
C(11)-N(1)-Cr(1) 122.8 (3)
C(4)-N(2)-Cr(1)
C(31)-N(21)-Cr(1) 113.5 (3)
C(24)-N(22)-Cr(1) 125.8 (3)
121.8 (3)
Strongly reducing alkylating reagents such as RLi
and RMgCl reduce the chloride complex 5 right back to
4. Therefore, less electron rich aluminum alkyls were
employed. Addition of 1 equiv of AlMe3 to a stirred
solution of 5 resulted in an immediate color change to
dark red. Workup of the reaction mixture with pentane
or diethyl ether gave a new paramagnetic compound (6),
With the intent of creating a starting material for
alkyls of the type [(Ph)2nacnac]2CrR, we set out to
explore the oxidation of 2. A more straightforward
synthesis of 2 involves reaction of CrCl2 with 2 equiv of
(Ph)2nacnacLi in diethyl ether. Solutions of 2 were easily
oxidized by stirring for 12 h with 1/2 equiv of 1,1,2,2-
tetrachloroethane, which results in a gradual color
change from dark green to brown-red. Slow evaporation
of a diethyl ether solution of the compound deposited
large crystals of [(Ph)2nacnac]2CrCl (5, Scheme 3) in
73% yield. The molecular structure of 5 was determined
by X-ray diffraction; it is shown in Figure 4, and selected
interatomic distances and angles are listed in Table 4.
Complex 5 is a mononuclear trigonal-bipyramidal com-
plex with the chloride atom occupying an axial site. Like
4, 5 is an example of a 5-coordinate chromium(III)
complex.69-74 However, unlike compound 4, the molecule
possesses two planar nacnac ligands bonded in a biden-
tate fashion. The Cr-Cl distance of 2.33 Å is within the
expected range and is comparable to those found in 1
(Cr-Cl, 2.346 Å).10 Although the solid-state structure
1
as indicated by H NMR spectroscopy. The compound
was initially assigned the formula [(Ph)2nacnac]2CrMe,
and in accord with that supposition it reacted with
hydrogen chloride to regenerate the starting material
5. However, several observations militated against this
assignment. Most importantly, reaction of 5 with AlEt3
1
produced the identical compound 6, as judged by its H
NMR spectrum. Also surprising was the fact that
compound 6 did not polymerize ethylene, even in the
presence of MAO. Finally, when the reactions of 5 with
aluminum alkyls AlR3 were monitored by 1H NMR
spectroscopy, resonances for methane (R ) Me) or
ethane (R ) Et) appeared along with those of 6.
Ultimately, the determination of the molecular struc-
ture of 6 by X-ray diffraction solved the puzzle. The
result is depicted in Figure 5, and selected interatomic
distances and angles of 6 are listed in Table 5. The
molecule is a five-coordinate chromium(III) aryl, with
the aryl group also being one of the N-substituents of a
nacnac moiety. In other words, 6 is the product of an
ortho metalation of the intermediate [(Ph)2nacnac]2CrR
to generate RH and 6 (Scheme 3). The room-tempera-
ture magnetic moment of 3.6(1) µB is consistent with
1
of 5 features two inequivalent nacnac ligands, the H
NMR spectrum of 5 is very simple. Apparently, in
solution at room temperature compound 5 is fluxional,
1
rendering the ligands equivalent on the H NMR time
scale.

The (Ph)2nacnac Ligand in Organochromium Chemistry
Organometallics, Vol. 21, No. 5, 2002 957
Ta ble 6. Resu lts of P olym er iza tion Exp er im en tsa
[Cr],
catalyst µM
yield,
g
Mw/
Mn
monomer
Mw
Mn
1, MAO 77.6 C2H4
10.5 1 157 039 15 892 72.8
1, MAO 77.6 C2H4/propylene
1, MAO 77.6 C2H4/1-hexene
3a , MAO 21.2 C2H4
0.3
8.7
209 900 2 050 53.9
9 659 1 402 6.9
7.8 1 629 000 9 500 171.5
22 816 4 316 5.3
3a , MAO 21.2 C2H4/1-hexene 13.2
4, MAO 12.0 C2H4
5, MAO 12.0 C2H4
5, MAO 12.0 C2H4/1-hexene
0.7
18.0
1.6
298 000 15 500 19.2
350 052 7 071 49.5
916 214 28 365 32.3
a
See the Experimental Section for details.
5/MAO was a very productive catalyst for the polym-
erization of ethylene. This observation raised questions
about the nature of the active species. One accepted
function of MAO (which always contains some AlMe3)
is that of an alkylating agent, which should transform
5 into [(Ph)2nacnac]2CrMe. However, we have shown
this compound to be unstable, decomposing to 6 (see
Scheme 3), which is catalytically inactive. What role
MAO plays in possibly suppressing the ortho-metalation
reaction of [(Ph)2nacnac]2CrMe and in its activation
remains unclear. While complete abstraction of the sole
alkyl group would seem to render an ethylene insertion
impossible, we note that polarization of a metal alkyl
by a Lewis acid has been proposed to enhance polym-
erization activity.75 An attempted copolymerization of
ethylene with 1-hexene using 5/MAO yielded merely
polyethylene without significant incorporation of comono-
mer.
F igu r e 5. Molecular structure of (Ph)2nacnac(η1-C6H4-Ph)-
nacnacCr (6).
Ta ble 5. Selected In ter a tom ic Dista n ces a n d
An gles for (P h )2n a cn a c(η1-C6H4-P h )n a cn a cCr (6)
Distances (Å)
Cr(1)-N(1)
Cr(1)-N(3)
Cr(1)-C(26)
N(4)-C(37)
N(3)-C(31)
C(21)-C(24)
C(23)-C(25)
1.976(5)
2.005(5)
2.062(7)
1.443(8)
1.396(8)
1.519(9)
1.511(10)
Cr(1)-N(2)
Cr(1)-N(4)
N(4)-C(23)
N(3)-C(21)
C(26)-C(31)
C(21)-C(22)
C(22)-C(23)
1.979(5)
2.099(5)
1.336(9)
1.339(8)
1.392(10)
1.401(10)
1.403(10)
Angles (deg)
Although we could not test the catalytic activity of
bis-nacnac chromium alkyls due to their instability, we
have tried the analogous chromium aryl compound 4.
While it is coordinatively unsaturated, it did not react
with ethylene by itself. Even in the presence of MAO
the catalytic activity of 4 was relatively low, yielding
only a small amount of polymer.
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(4)
N(2)-Cr(1)-N(3)
N(2)-Cr(1)-C(26)
N(3)-Cr(1)-C(26)
Cr(1)-N(3)-C(31)
Cr(1)-C(26)-C(31)
89.7(2) N(1)-Cr(1)-N(3)
126.0(2) N(1)-Cr(1)-C(26)
97.9(2) N(2)-Cr(1)-N(4)
102.0(3) N(3)-Cr(1)-N(4)
66.2(3) N(4)Cr(1)-C(26)
142.8(2)
99.6(3)
98.5(2)
87.5(2)
153.0(3)
95.2(4) N(3)-C(31)-C(26) 105.6(6)
92.9(5)
We note that the polymers produced in these experi-
ments featured very broad and multimodal molecular
weight distributions (Mw/Mn) not at all characteristic
of homogeneous single-site catalysts. We do not care to
speculate on the various active species generated in
these MAO-containing solutions; however, nacnac ligand
transfer from chromium to aluminum must be consid-
ered a possibility. Rather, we shall continue our search
for well-defined catalysts that do not require activation
by cocatalysts.
the molecule being a mononuclear chromium(III) com-
plex. The Cr-C distance of 2.06 Å lies within the range
found for organochromium(III) compounds. The struc-
ture of 6 also explains its reactivity toward HCl and
ethylene, as noted above.
Given the apparent propensity of alkyls of the type
[(Ph)nacnac]2CrR to suffer ortho metalation, we inves-
tigated the thermal stability of phenyl derivative 4.
Interestingly, compound 4 is stable for days at room
temperature in solution, with no apparent formation of
1
6 observed in the H NMR spectrum. However, heating
4 to 70 °C for several days cleanly generated 6 and
benzene. The enhanced kinetic stability of 4 relative to
the alkyl derivatives may well be a ground-state effect,
reflecting its greater chromium-carbon bond strength.
P olym er iza tion Exp er im en ts. Our previous work
in Cp*Cr chemistry has shown that coordinatively
unsaturated chromium(III) alkyls are catalysts for the
polymerization of ethylene.5 Accordingly, a solution of
3a showed no tendency to react with ethylene, since it
is coordinatively saturated. When activated with MAO,
the dinuclear alkyl 3a catalyzed the polymerization of
ethylene (as did 1). It did not react with propylene or
1-hexene, even when activated with MAO, nor did it
catalyze the copolymerization of 1-hexene or propylene
with ethylene. The results of the polymerization experi-
ments are listed in Table 6.
Con clu sion s
We have prepared and structurally characterized
several organometallic derivatives of the (Ph)2nacnacCrIII
fragment. None of these molecules catalyze the polym-
erization of ethylene by themselves; however, activation
by MAO transformed most of them into active catalysts.
The scope of the organometallic chemistry of chromium
with this particular nacnac ligand is limited by tenden-
cies to disproportionate and suffer ortho metalation.
These obstacles can be overcome by judicious substitu-
tion of the N-aryl groups. The results of this approach
will be the subject of future publications.
(75) Chen, E. Y. X.; Kruper, W. J .; Roof, G.; Wilson, D. R. J . Am.
Chem. Soc. 2001, 123, 745.

958 Organometallics, Vol. 21, No. 5, 2002
MacAdams et al.
[(Ph)2nacnac]2Cr (2; 210 mg, 35%) were isolated. 1H NMR
(C6D6; δ): 124.0 (6H, vb), 7.6 (4H), 4.4 (6H), -16.8 (1H, vb)
ppm. IR (KBr; cm-1): 3055 (w), 3029 (w), 2922 (w), 2879 (m),
1591 (w), 1540 (s), 1515 (m), 1482 (s), 1449 (m), 1382 (vs), 1276
(w), 1264 (w), 1021 (m), 866 (w), 842 (w), 752 (w), 700 (m).
Anal. Calcd for C34H34N4Cr: C, 74.16; H, 6.22; N, 10.17.
Found: C, 72.42; H, 6.30; N, 10.01.78 MS (m/z (%)): 550.22
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations of compounds
were carried out using standard Schlenk, vacuum, and glove-
box techniques. Pentane, diethyl ether, tetrahydrofuran, and
toluene were distilled from purple Na benzophenone/ketyl
solutions. C6D6 and CD2Cl2 were predried with sodium and
stored under vacuum over 4 Å molecular sieves. CrCl3 (anhy-
drous) was purchased from Strem Chemical Co. ((Trimethyl-
silyl)methyl)lithium was purchased as a 1 M solution in
pentane, was crystallized from solution at -30 °C, and was
isolated as a white crystalline solid. Methyllithium, diethyl-
zinc, and dimethylzinc were purchased from Aldrich and were
used as received. CrCl3(THF)3 and CrPh3(THF)3 were prepared
according to literature procedures.76,77
(100) [M+], 301.05 (45.75) [M+ - C17H17N2]. UV-vis (Et2O; λmax
,
nm (ꢀ, M-1 cm-1)): 348 (sh) (10 445). µeff(294 K) ) 5.1(1) µB.
Mp: 220 °C dec.
In d ep en d en t Syn th esis of Bis(N,N′-d ip h en yl-2,4-p en -
ta n ed iim in a to)ch r om iu m (II), [(P h )2n a cn a c]2Cr (2). (Ph)2-
nacnacH (4.0 g, 16 mmol) was dissolved in 50 mL of THF and
cooled to -30 °C. MeLi (0.352 g, 16 mmol) was added to the
THF solution of (Ph)2nacnacH. The resulting THF solution of
(Ph)2nacnacLi was then added dropwise to the suspension of
CrCl2 (0.983 g, 8.0 mmol) in 150 mL of THF over 1 h from an
addition funnel. The reaction mixture was stirred overnight
at room temperature. The solvent was removed in vacuo, and
the residue was extracted with diethyl ether. The extract was
then filtered to remove LiCl and concentrated for crystalliza-
tion. Dark crystals of [(Ph)2nacnac]2Cr (2; 3.7 g, 7.09 mmol,
84% yield) were isolated from a concentrated diethyl ether
solution cooled to -30 °C.
NMR spectra were taken on a Bruker AM-250 spectrometer
and were referenced to the residual protons of the solvent
(C6D6, 7.15 ppm; CD2Cl2, 5.32 ppm). FTIR spectra were taken
on a Mattson Alpha Centauri spectrometer with a resolution
of 4 cm-1. UV/vis spectra were taken using a Bruins Instru-
ments Omega 20 spectrophotometer. Mass spectra were
obtained by the University of Delaware Mass Spectrometry
Facility. Elemental analyses were performed by Schwarzkopf
Microanalytical Laboratories.
P r ep a r a tion of Bis(N,N′-d ip h en yl-2,4-p en ta n ed iim in -
a to)d ich lor od im eth yld ich r om iu m (III) Bis(tetr a h yd r o-
fu r a n ) (3a ). To a stirred solution of (Ph)nacnacCrCl2(THF)2
(0.800 g, 1.5 mmol) in 40 mL of THF was added 0.852 mL of
dimethylzinc (2.0 M solution in toluene, 1.7 mmol). Stirring
for 10 min followed by removal of solvent gave a dark brown-
green solid. Crystallization from a concentrated diethyl ether
solution at -30 °C yielded 0.556 g (84%) of 3 as dark brown
crystals. 1H NMR (CD2Cl2; δ): 77.8 (vb, 6H), 12.5 (vb, 4H),
9.2 (b, 4H), 7.1 (b, 4H), 2.4 (b, 4H), -4.3 (b, 2H) ppm. IR (KBr;
cm-1): 3055 (w), 2972 (m), 2902 (m), 1572 (s), 1526 (m), 1485
(s), 1459 (vs), 1424 (s), 1257 (w), 1197 (m), 1171 (w), 1070 (w),
1014 (m), 927 (w), 865 (s), 845 (s), 755 (m), 706 (s) cm-1. Anal.
Room-temperature magnetic susceptibilities were deter-
mined using a J ohnson Matthey magnetic susceptibility bal-
ance.
P r ep a r a tion of (N,N′-Dip h en yl-2,4-p en ta n ed iim in a to)-
d ich lor och r om iu m (III) Bis(tetr a h yd r ofu r a n ), (P h )2n a c-
n a cCr Cl2(THF )2 (1). (Ph)2nacnacH (0.6 g, 2.40 mmols) was
dissolved in 50 mL of THF and cooled to -30 °C. Methyl-
lithium (2.40 mmols, 53 mg) was slowly added as a solid with
stirring. The THF solution of (Ph)2nacnacLi prepared in situ
was then slowly added over 3 h to a slurry of CrCl3(THF)3 (900
mg, 2.40 mmols) in 150 mL of THF. The color of the solution
changed from purple to red-brown. After it was stirred
overnight at room temperature, the reaction mixture was
concentrated to 50 mL and cooled to -30 °C for crystallization.
A brown microcrystalline powder was isolated by filtration.
After the powder was washed several times with cold THF
and dried under vacuum, 1.08 g (2.09 mmol, 87%) of 1 was
isolated. 1H NMR (THF-d8; δ): 15.5 (6H), 6.2 (4H) ppm. IR
(KBr; cm-1): 3050 (w), 3017 (m), 2966 (s), 2928 (s), 2883 (s),
1590 (w), 1555 (vs), 1530 (vs), 1484 (vs), 1448 (vs), 1387 (vs),
1263 (s), 1200 (s), 1065 (w), 1017 (s), 921 (m), 871 (s), 848 (s),
764 (m), 710 (s), 662 (w), 526 (m), 477 (w). Anal. Calcd. for
C25H33N2O2CrCl2: C, 58.14; H, 6.44; N, 5.42. Found: C, 57.95;
H, 6.81; N, 5.51.78 MS (m/z (%)): 370.76 (41.24) [M+ - 2THF],
335.81 (41.60) [M+ - Cl, 2THF], 300.86 (6.01) [M+ - 2Cl,
2THF]. UV/vis (Et2O; λmax, nm (ꢀ, M-1 cm-1)): 527 (572), 419
(8233), 400 (5712). µeff(294 K) ) 4.1(1) µB. Mp: 174-180 °C
dec.
Calcd for
C44H56N4O2Cl2Cr2: C, 62.33; H, 6.66; N, 6.61.
Found: C, 59.05; H, 6.61; N, 6.21.78 MS (m/z (%)): 350 (23)
[M+ - THF], 301 (1.44) [M+ - THF, Cl, Me]. UV/vis (Et2O;
λ
max, nm (ꢀ, M-1 cm-1)): 499 (413). µeff(294 K) ) 3.8(1) µB. Mp:
190 °C.
P r ep a r a tion of Bis(N,N′-d ip h en yl-2,4-p en ta n ed iim in -
a to)d ich lor od ieth yld ich r om iu m (III) Bis(tetr a h yd r ofu -
r a n ) (3b). To a stirred solution of (Ph)nacnacCrCl2(THF)2
(0.800 g, 1.5 mmol) in 50 mL of THF was added 0.185 g of
diethylzinc (1.5 mmol). Stirring for 10 min, followed by removal
of solvent, gave a dark brown-green solid. Crystallization from
a concentrated diethyl ether solution at -30 °C yielded 0.520
g (76%) of 3b as dark brown crystals. 1H NMR (C6D6; δ): 90.5
(6H), 13.4 (4H), 9.1 (4H), 6.9 (4H), 2.7 (4H), -10.9 (3H) ppm.
IR (KBr; cm-1): 3055 (w), 3017 (w), 2958 (m), 2928 (m), 2895
(m), 2839 (m), 1593 (w), 1539 (s), 1484 (s), 1449 (s), 1383 (s),
1261 (s), 1190 (s), 1117 (w), 1069 (w), 1024 (m), 893 (w), 842
(m), 755 (m), 700 (s). Anal. Calcd for C46H60N4O2Cl2Cr2: C,
63.08; H, 6.90; N, 6.40. Found: C, 58.40; H, 6.54; N, 5.49.78
MS (m/z (%)): 250 (100) [-THF, Et and Cl]; 803 (0.16) [-THF].
UV/vis (Et2O; λmax, nm (ꢀ, M-1 cm-1)): 534 (534). µeff(294 K) )
3.7 (1) µB. Mp: 103-105 °C.
P r ep a r a tion of Bis(N,N′-d ip h en yl-2,4-p en ta n ed iim in -
a to)p h en ylch r om iu m (III) (4). CrPh3(THF)3 (0.400 g, 0.8
mmol) was dissolved in 50 mL of THF and 2.0 equiv of
(Ph)2nacnacH (0.400 g, 1.6 mmol) was added. The solution was
stirred for 1 h, during which time its color changed from light
red to brown-green. All volatiles were removed, and the
resulting solid was dissolved in pentane to give a dark red
solution. The pentane solution was concentrated and cooled
to -30 °C for crystallization. Dark red crystals of 4 were
isolated in 72% yield (0.362 g). 1H NMR (C6D6; δ): 36 (vb, 6H),
8.4 (12H), 5.0 (12H) ppm. IR (KBr; cm-1): 3052 (m), 2996 (w),
2959 (w), 2922 (w), 1593 (m), 1549 (s), 1523 (s), 1485 (s), 1452
Rea ction of (N,N′-Dip h en yl-2,4-p en ta n ed iim in a to)d i-
ch lor och r om iu m (III) Bis(tetr ah ydr ofu r an ) (1) with MeLi.
1 (0.600 g, 1.16 mmol) was dissolved in THF and cooled to
-30 °C. Two equivalents of MeLi (1.66 mL, 1.4 M solution in
diethyl ether) was added dropwise to the solution of 1. The
reaction mixture rapidly turned brown. After the mixture was
stirred at room temperature for 4 h, it was evaporated to
dryness. THF was removed by trituration with ether. The
resulting solid was extracted with ether and filtered to remove
LiCl. The black-green filtrate was then concentrated and
cooled to -30 °C for crystallization. Dark green crystals of
(76) Herwig, W.; Zeiss, H. H. J . Org. Chem. 1958, 9, 1404.
(77) Herwig, W.; Zeiss, H. H. J . Am. Chem. Soc. 1957, 79, 6561.
(78) Several of the compounds reported here have yielded low values
for carbon in their elemental analyses. This deficiency could not be
remedied by several crystallizations and repeated analyses. While we
are convinced of the purity of the samples submitted for analysis, we
have no explanation for the discrepancies.

The (Ph)2nacnac Ligand in Organochromium Chemistry
Organometallics, Vol. 21, No. 5, 2002 959
Ta ble 7. Cr ysta llogr a p h ic Da ta for 2, 3a , a n d 4-6
2
3a
4
5
6
formula
fw
space group
color
a, Å
b, Å
C
34H34Cr
C44H56Cl2Cr2N4O2
847.83
P21/n
C40H39CrN4
627.75
P21/c
dark brown
9.6543(2)
17.6528(2)
20.0308(3)
90
101.7650(9)
90
3342.01(4)
4
C34H34ClCrN4
586.10
P1h
brown
9.23610(2)
17.7197(2)
19.1786(3)
73.9512(6)
83.5156(2)
82.8597(6)
2982.86(5)
4
C34H33CrN4
549.64
P21/c
550.65
P1h
dark green
10.54650(10)
11.4442(2)
13.8021(2)
87.5203(9)
72.7442(8)
65.3308(3)
1439.44(5)
2
red
red
12.1282(3)
14.2684(3)
12.8048(3)
90
90.3352(4)
90
10.0898(3)
9.2347(3)
30.2026(5)
90
97.814(2)
90
c, Å
R, deg
â, deg
γ, deg
V, Å3
2215.83(4)
2
2788.03(13)
4
Z
D(calcd), g cm-3
µ(Mo KR), cm-1
temp, K
no. of data/params
radiation
GOF on F2
R(F), %
Rw(F2), %
1.270
4.27
218(2)
8660/352
1.271
6.50
213(2)
4805/244
1.248
3.76
173(2)
1.305
5.03
198(2)
11123/721
1.309
4.40
173(2)
4280/352
5099/407
Mo KR (λ ) 0.710 73 Å)
1.088
1.143
8.87a
1.036
4.49a
1.004
7.13a
1.770
9.55a
5.42a
21.76a
13.26a
14.40a
17.49a
25.78a
Quantity minimized: Rw(F2) ) ∑[w(Fo - Fc2)2]/∑[(wFo2)2]1/2; R ) ∑∆/∑(Fo), ∆ ) |(Fo - Fc)|.
a
2
(s), 1401 (vs), 1266 (m), 1189 (m), 1070 (w), 1025 (m), 920 (w),
collected at 173 K with a Siemens P4/CCD diffractometer with
graphite-monochromated Mo KR X-radiation (λ ) 0.710 73 Å).
All specimens of 2 and 6 showed diffuse diffraction character-
ized by broad line widths, which affected resolution and led
to fairly high R factors. No symmetry higher than triclinic was
observed in the diffraction data of 2 and 5. E statistics
suggested the centrosymmetric space group P1h, which yielded
chemically reasonable and computationally stable results of
refinement. The diffraction data of 3a , 4, and 6 were uniquely
consistent with the reported monoclinic space groups. Struc-
ture 3a resides on a crystallographic inversion center. There
are two chemically equivalent, but crystallographically inde-
pendent, molecules in the asymmetric unit of 5. Structures
3a , 4, 5, and 6 were solved by direct methods and structure 2
by way of the Patterson function. All structures were com-
pleted by subsequent difference Fourier syntheses and refined
by full-matrix least-squares procedures. All non-hydrogen
atoms were refined with anisotropic displacement coefficients,
and all hydrogens were treated as idealized contributions. All
software and sources of the scattering factors are contained
in the SHELXTL (version 5.1) program library (G. Sheldrick,
Siemens XRD, Madison, WI).
Gen er a l Eth ylen e P olym er iza tion P r oced u r e. In a
glovebox, the precatalyst (0.012-0.21 mmol) was dissolved in
50 mL of CH2Cl2. To the precatalyst was added MAO (10 wt
% solution in toluene, Al/Cr ) 100); the mixture was placed
in a sealed Parr reactor, and the apparatus was removed from
the glovebox. The reactor was charged with ethylene at 300
psi and kept at a constant pressure. After 1 h the ethylene
supply was closed and the reaction vessel was vented. The
resulting polymer was washed with a methanol/7 M aqueous
HCl (1:1) mixture followed by deionized water. The polymer
was dried under vacuum at 60 °C for 18 h.
843 (w), 751 (m), 698 (s) cm-1. Anal. Calcd for C40H39N4Cr: C,
76.53; H, 6.26; N, 8.92. Found: C, 75.82; H, 6.12; N, 8.71.78
MS (m/z (%)): 627.3 (6) [M+], 550.23 (21) [M+ - C6H5]. UV/vis
(Et2O; λmax, nm (ꢀ, M-1 cm-1)): 547 (487), 660 (156). µeff (294
K): 4.1(1) µB. Mp: 145-147 °C.
Bis(N,N′-d ip h en yl-2,4-p en t a n ed iim in a t o)ch lor och r o-
m iu m (III), [(P h )2n a cn a c]2Cr Cl (5). [(Ph)2nacnac]2Cr (0.8 g,
1.45 mmol) was dissolved in 100 mL of diethyl ether. Addition
of tetrachloroethane (0.076 mL, 0.726 mmol) to the diethyl
ether solution with stirring caused no immediate color change.
After the mixture was stirred overnight, a brown-red solution
was obtained. Evaporation of solvent yielded a dark brown
solid that was extracted with diethyl ether. Brown needles
(0.620 g, 73% yield) were isolated by slow evaporation of
solvent at room temperature. 1H NMR (C6D6): 38.5 (12H, vb),
7.1 (20H, b) ppm. IR (KBr; cm-1): 3054 (s), 3017 (m), 2962
(w), 2920 (m), 1592 (m), 1528 (s), 1497 (s), 1410 (s), 1388 (s),
1254 (m), 1185 (m), 1070 (w), 1024 (m), 940 (w), 916 (w), 841
(w), 754 (m), 705 (s), 518 (w) cm-1. Anal. Calcd for C34H34N4-
ClCr: C, 69.7; H, 5.8; N, 9.56. Found: C, 69.41; H, 6.03; N,
9.67. MS (m/z (%)): 585.13 (78.3) [M+], 550.1522 (100)
[M+ - Cl]. UV/vis (diethyl ether; λmax, nm (ꢀ, M-1 cm-1)): 656.0
(316), 663.0 (304), 766 (178). µeff (294 K): 4.1(1) µB. Mp: 150-
152 °C.
Rea ction of 5 w ith Tr im eth yla lu m in u m . Isola tion of
(P h )2n a cn a c(η1-C6H4-P h )n a cn a cCr (6). 5 (300 mg, 0.53
mmol) was dissolved in 35 mL of diethyl ether. Slow addition
of 0.039 g of trimethylaluminum caused an immediate color
change from dark brown-red to dark red. After the mixture
was stirred for 10 min, the volatiles were removed. Dark red
crystals of 6 were isolated in 71% yield (200 mg) from a
1
concentrated pentane solution at -30 °C. H NMR (C6D6; δ):
91 (vb, 6H), 50.2 (3H), 44.6 (3H), 17.1 (2H), 13.9 (4H), 7.2 (2H),
5.1 (4H), -1.5 (3H), -6.5 (4H), -11.1 (2H) ppm. IR (KBr; cm-1):
3056 (w), 3010 (w), 2982 (w), 2923 (w), 1592 (m), 1551 (s),
1522 (s), 1482 (s), 1451 (s), 1391 (s), 1263 (m), 1191 (m), 1069
(w), 1025 (m), 915 (w), 832 (w), 754 (m), 701 (s) cm-1. Anal.
Calcd for C34H33N4Cr: C, 74.36; H, 6.06; N, 10.20. Found: C,
74.54; H, 6.42; N, 9.94. MS (m/z (%)): 550 (9.29) [M+], 301
Gen er a l Eth ylen e/Hexen e Cop olym er iza tion P r oce-
d u r e. Inside the glovebox the precatalyst was dissolved in
25 mL of CH2Cl2, and MAO (10 wt. % solution in toluene,
Al/Cr ) 100) was added to the solution. A 23.0 g amount of
dried 1-hexene was added to the reaction mixture, and the
contents were placed in a Parr reactor and removed from the
glovebox. Ethylene (300 psi) was charged into the reactor. After
1 h of stirring the ethylene supply was closed and the reactor
vented. The polymer was washed and dried as for the
polyethylene (see above).
(2.3) [M+ - (Ph)2nacnac]. UV/vis (Et2O):
λ
max, nm (ꢀ, M-1
cm-1)): 853 (203), 672 (112). µeff (294 K) ) 3.6(1) µB. Mp: 130
°C.
Cr ysta llogr a p h ic Str u ctu r e Deter m in a tion s. Crystal,
data collection, and refinement parameters are given in Table
7. Suitable crystals for data collection were selected and
mounted with epoxy cement on the tip of a fine glass fiber or
in nitrogen-flushed, thin-walled glass capillaries. All data were
Gen er a l Eth ylen e/P r op ylen e P olym er iza tion P r oce-
d u r e. Inside the glovebox the precatalyst was dissolved in 50
mL of CH2Cl2. To this solution was added MAO (Al/Cr ) 100).
The reaction mixture was placed in a Parr reactor and removed
from the glovebox. The reactor was first charged with propyl-

960 Organometallics, Vol. 21, No. 5, 2002
MacAdams et al.
ene (100 psi) for approximately 5 min, and then the propylene
supply was shut off. Ethylene (350 psi) was then charged into
the reactor for 1 h. The resulting polymer was handled as
above.
CHE-9876426) and Chevron Chemical Co. (now Chev-
ron Phillips Chemical Co. LP).
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
crystallographic data for 2, 3a , and 4-6. These data are
available free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. This research was supported by
grants from the National Science Foundation (Grant No.
OM010857K