Novel chromium(III) complexes containing imidazole-based chelate ligands with varying donor sets: Synthesis and reactivity

Article abstract of DOI:10.1021/om000846b

Reaction of (THF)₃RCrCl₂ (R = Cl, Me) with tri- and bidentate bis(N-methylimidazole-2-yl) ligands gave the new Cr(III) complexes [Cr(N∧N∧Y)Cl₃] (3a-c), [Cr(N∧N)Cl₃]₂ (4a-c), and [Cr(Me)(N∧N∧Y)Cl₂

Full text of DOI:10.1021/om000846b

Organometallics 2001, 20, 1247-1250
1247
Novel Ch r om iu m (III) Com p lexes Con ta in in g
Im id a zole-Ba sed Ch ela te Liga n d s w ith Va r yin g Don or
Sets: Syn th esis a n d Rea ctivity
Thomas Ru¨ther,*,† Nathalie Braussaud,† and Kingsley J . Cavell*,‡
School of Chemistry, University of Tasmania, GPO Box 252-75, Hobart,
Tasmania 7001, Australia, and Department of Chemistry, Cardiff University, P.O. Box 912,
Cardiff, CF1 3TB, U.K.
Received October 3, 2000
Summary: Reaction of (THF)3RCrCl2 (R ) Cl, Me) with
tri- and bidentate bis(N-methylimidazole-2-yl) ligands
gave the new Cr(III) complexes [Cr(N∧N∧Y)Cl3] (3a -
c), [Cr(N∧N)Cl3]2 (4a -c), and [Cr(Me)(N∧N∧Y)Cl2]
(5a ,b) in g90% yield. Toluene solutions of 3,4/ MMAO
convert ethylene into linear 1-alkenes with selectivities
of up to 79%.
diene-based and noncyclopentadiene Cr complexes con-
taining N∧N and N∧O ligand environments were
investigated by Theopold,4a,7a,b Gibson,4b,c and J olly et
al.7c From the desire to contribute to this relatively
unexplored field we recently became interested in the
synthesis and properties of transition metal complexes
bearing chelating ligands derived from the heteroaro-
matic imidazole unit.8 In this note we report the
synthesis of a number of novel chromium(III) complexes
in which the metal resides in chelate environments
bearing N-methylimidazole-based components linked to
varying donor sets. We demonstrate how changes within
the ligand system control the reactivity of the complexes
in the catalytic oligomerization of ethylene.
The ligands described herein and depicted in Scheme
1 all consist of a bridged bisimidazole framework, which
in the case of compounds 1a -c and 2b,c carries ad-
ditional P, N, or O donor centers. 1a 9 and 2a ,c10 were
prepared according to modified literature procedures.
Compound 2a proved to be a versatile building block
for the synthesis of the new mixed donor ligands 1b,c
and 2b. Details of the ligand syntheses will be published
elsewhere. Despite their obvious ligating properties,
chelating imidazoles have mainly been studied as
structural models to mimic binding sites in metalloen-
zymes.9,11 No Cr(III) complexes containing chelating
imidazole-based ligands have previously been reported.12
Chromium(III) complexes with chelating bisimidazole
ligands were prepared by adding the respective ligand
to a solution of (THF)3CrCl3 in CH2Cl2. The products
precipitate as air/moisture sensitive green or gray-green
solids and were isolated in g90% yield. The complexes
are moderately soluble in CH3CN and hot methanol. For
the complex 4c coordination of the keto-bridged ligand
The synthesis of transition metal complexes contain-
ing nitrogen-based chelates has received much attention
in recent years.1 Most notably with the discoveries by
Brookhart and Gibson of new Ni, Pd2, Co, and Fe3
catalyst systems incorporating bidentate R-diimine and
tridentate bisiminopyridine environments. These sys-
tems were found to be highly active catalysts for olefin
polymerization. With these discoveries it became evident
that a broad spectrum of rather simple coordination
compounds may find application in the conversion of
R-olefins into higher molecular weight products. These
studies on coordination chemistry and catalysis have
since been extended to include catalysts based on new
chromium complexes.4 Despite the industrial impor-
tance of chromium-based catalysts for the production
of ethylene oligomers5 (e.g., 1-hexene) and polymers,6
well-defined homogeneous model systems are scarce and
have been prepared only recently.4,7 Both cyclopenta-
† University of Tasmania.
‡ Cardiff University.
(1) (a) Togni, A.; Venanzi, L. M. Angew. Chem. 1994, 106, 517;
Angew. Chem., Int. Ed. Engl. 1994, 33, 497. (b) Britovsek, G. J . P.;
Gibson, V. C.; Wass, D. F. Angew. Chem. 1999, 111, 448-468; Angew.
Chem., Int. Ed. 1999, 38, 429-447. (c) Gade, L. H. J . Chem. Soc., Chem.
Commun. 2000, 173-181.
(2) (a) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem.
Soc. 1995, 117, 6414-6415. (b) Killian, C. M.; J ohnson, L. K.;
Brookhart, M. Organometallics 1997, 16, 2005-2007.
(3) (a) Britovsek, G. J . P.; Gibson, V. C.; Kimberly, B. S.; Maddox,
P. J .; McTavish, S. J .; Solan, G. A.; White, A. J . P.; Williams, D. J .
Chem. Commun. 1998, 849-850. (b) Small, B. L.; Brookhart, M.;
Bennet, A. M. A. J . Am. Chem. Soc. 1998, 120, 4049-4050. (c) Small,
B. L.; Brookhart, M. J . Am. Chem. Soc. 1998, 120, 7143-7144
(4) (a) Kim, W.-K.; Fevola, M. J .; Liable-Sands, L. M.; Rheingold,
A. L.; Theopold, K. H. Organometallics 1998, 17, 4541-4543. (b)
Gibson, V. C.; Maddox, P. J .; Newton, C.; Redshaw, C.; Solan, G. A.;
White, A. J . P.; Williams, D. J . Chem. Commun. 1998, 1651-1652. (c)
Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J . P.;
Williams, D. J . J . Chem. Soc., Dalton Trans. 1999, 827-829. (d) Coles,
M. P.; Dalby, C. I.; Gibson, V. C.; Clegg, W.; Elsegood, M. R. J . Chem.
Commun. 1995, 1709-1711.
(7) (a) Theopold, K. H. CHEMTECH 1997, October, 26-32. (b)
Theopold, K. H. Eur. J . Inorg. Chem. 1998, 15-24. (c) Emmerich, R.;
Heinemann, O.; J olly, P. W.; Krueger, C.; Verhovnik, G. P. J . Orga-
nometallics 1997, 16, 1511-1513.
(8) Done, M. C.; Ru¨ther, T.; Cavell. K. J .; Kilner, M.; Peacock, E. J .;
Braussaud, N.; Skelton, B. W.; White, A. J . Organomet. Chem. 2000,
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(11) For recent examples and further references see: (a) Place, C.;
Zimmermann, J .-L.; Mulliez, E.; Guillot, G.; Bois, C.; Chottard, J .-C.
Inorg. Chem. 1998, 37, 4030-4039. (b) Sorrell, T. N.; Allen, W. E.;
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394. (b) Gonzales Garmendia, M. J .; Losada, J .; Moran, M. J . Inorg.
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(5) (a) Reagen, W. K.; Pettijohn, T. M., Freeman, J . W.; Benham,
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674-675. (c) Wu, F.-J . US-A 5,550,305, 1996.
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825, 721, 1958 [Chem. Abstr. 1958, 52, 8621h]. (b) Hogan, J . P. J .
Polym. Sci. 1970, 8, 2637-2652. (c) Karol, F. J .; Karapinaka, G. L.;
Wu, C.; Dow, A. W.; J ohnson, R. N.; Carrick, W. L. J . Polym. Sci.,
Polym. Chem. Ed. 1972, 10, 2621-2637.
10.1021/om000846b CCC: $20.00 © 2001 American Chemical Society
Publication on Web 02/10/2001

1248 Organometallics, Vol. 20, No. 6, 2001
Notes
Sch em e 1. Syn th esis of th e Com p lexes 3a -c, 4a -c,
composition product. Similar observations were reported
for [Cp*Cr(THF)2Me]PF6.7b,13
a n d 5a ,b
The complexes 4a -c, which contain bidentate ligands,
appear to be dimers with bridging and terminal chlo-
rides placing each chromium in an octahedral ligand
environment. Accordingly, the mass spectrum shows
dinuclear pentachloro molecular ions, albeit of low
intensity. Further evidence for a dinuclear structure
arises from their effective magnetical moments (3.0 µB
per Cr, 4a ,c). This value is significantly reduced from
that expected (3.87 µB) for a mononuclear Cr(III)
complex and from the values found for 3a ,b (see below).
A possible explanation for the low effective moments is
antiferromagnetic coupling. A similar observation was
made for bridged CpCr(III) complexes,14 and a structur-
ally characterized dimeric chloro bridged complex was
prepared by Gibson et al. from (THF)3CrCl3 and a
metalated â-diimine.4b The complexes 3a -c, which
contain tripodal ligand systems, appear to be monomeric
according to their magnetical moments (3a , 3.53 µB; 3b,
3.76 µB) and mass spectra. Mononuclear structures with
coordination through all three donor centers have been
established for other tripodal ligands linked to the CrCl3
unit.15 Considering 1a -c as neutral 6e donors, analo-
gous to Cp-, complexes 3a -c are unique isoelectronic
metal complexes.16 The variation of the third donor atom
in 1a -c provides a valuable opportunity for studying
the reactivity of complexes in which the metal center is
placed in slightly differing electronic and steric environ-
ments generated from one donor moiety only.
may occur in either an N-N or N-O fashion. Compari-
son of the IR spectra of the free ligand 2c (ν(CdO) )
1632 cm-1) and its complex 4c (ν(CdO) ) 1651 cm-1
)
suggests N-N coordination since a shift of the CdO
absorption to lower wavenumbers is expected for N-O
coordination. The electronic spectra of complexes 3 and
4 show two of the three expected absorptions (ν1,
14420-16650 cm-1; ν2, 21370-23200 cm-1) for chro-
mium(III) in an octahedral field. The third d-d transi-
tion is obscured by CT bands, a common feature
observed in many chromium(III) complexes. Good el-
emental analyses data were difficult to obtain for several
of these complexes due to high moisture sensitivity. It
was clear during the collection of data that variable
partial absorption of water and in some cases hydrolysis
occur. By deliberately exposing the complexes to moist
air, up to 6 equiv of water could be absorbed in a short
time.
While a number of homogeneous model systems for
chromium-based olefin polymerization and oligomer-
ization catalysts, based on CpCr complexes, were re-
ported by Theopold et al. and J olly et al.,7 examples of
chromium catalysts containing hard donor ligands are
scarce.4,17 Cr complexes with â-diiminato,4a,b pyrrole-
pyridine,4b reduced Schiff-base,4c and bis(imido)4d ligands
have been reported by Gibson and Theopold et al. Due
to the hardness of the donor atoms of the imidazole
framework, complexes 3 and 4 may serve as model
systems for hard donor environments present in the
Phillips catalysts (CrO3/SiO2;6a,b CrX3/pyrrole, X )
halogen, carboxylate5a). In this context, exposure of
toluene solutions of 3,4/MMAO to 40 bar of ethylene
resulted in the formation of linear 1-alkenes in the
range of C4-C30 (maximum at C8; GC-MS analysis)
with selectivities of up to 79% (Table 1). Some branched
1-alkenes are also formed. The occurrence of homologous
alkanes, an unexpected byproduct of this reaction, is
dependent on the MMAO concentration (entries 2/3 and
5/6) and is possibly also associated with the decomposi-
ton characteristics of chromium alkyls.
The first example of an alkyl Cr(III) complex with this
ligand type was prepared by reacting (THF)3CrCl2Me
with 1b in THF. 5b precipitated from THF solution as
a green solid and was characterized by MS and elemen-
tal analysis. When the ligand 1a was employed in the
same reaction, a green solid (5a ) precipitated and, as
observed for 5b, a monomer fragment at m/e 408 (M+
- CH3) was recorded in the MS. However, a satisfactory
elemental analysis could not be obtained for 5a , which
appears to be less stable at room temperature than 5b.
The air/moisture sensitive alkyl complexes are partially
soluble in CH2Cl2, but decompose in methanol. Attempts
were made to generate a cationic complex from the
methyl complex 5b via halide abstraction with AgBF4
or Na[B{3,5-(CF3)2C6H3}4] in coordinating solvents such
as THF and CH3CN. Halogen abstraction occurs at -30
°C, as indicated by a color change from green to brown
with formation of a grayish solid and the dissolution of
the insoluble starting product. On warming to ambient
temperature, the slightly turbid solution becomes tur-
quoise and gas evolution is observed. After workup,
elemental analysis of the resulting turquoise solid
indicated the possible formation of an oligomeric de-
(13) Thomas, B. J .; Mitchell, J . F.; Theopold, K. H.; Leary, J . A. J .
Organomet. Chem. 1988, 348, 333.
(14) (a) Bhandari, G.; Kim, Y.; McFarland, J . M.; Rheingold, A. L.;
Theopold, K. H. Organometallics 1995, 14, 738-745. (b) Richeson, D.
S.; Mitchell, J . F.; Theopold, K. H. Organometallics 1989, 8, 2570-
2577.
(15) (a) Koehn, R. D.; Kociok-Koehn, G. Angew. Chem. 1994, 106,
1958; Angew. Chem., Int. Ed. Engl. 1994, 33, 1877-1878. (b) Arif, A.
M.; Hefner, J . G.; J ones, R. A.; Whittlesey, B. R. Inorg. Chem. 1986,
25, 1080-1084.
(16) (a) Hajela, S.; Schaefer, W. P.; Bercaw, J . E. J . Organomet.
Chem. 1997, 532, 45-53. (b) Wang, L.; Flood, T. C. J . Am. Chem. Soc.
1992, 114, 3169-3170.
(17) Feher, F. J .; Blanski, R. L. J . Chem. Soc., Chem. Commun.
1990, 2341-2345.

Notes
Organometallics, Vol. 20, No. 6, 2001 1249
Ta ble 1. Eth ylen e Oligom er iza tion in Tolu en ea
catalyst/
(mmol)
MMAO
(equiv)b
solid
(wt %)
entry
activityc
1-alkenes
isoalkenes
alkanesd
isoalkanes
4
1
2
3
4
5
6
7
8
3a /0.11
3b/0.1
3b/0.12
3c/0.1
4a /0.09
4a /0.06
4b/0.04
4c/0.06
400
400
182
400
400
170
408
414
208
108
90
120
56
55
90
60
79
65
77
65
50
70
60
57
9
10
11
13
16
15
20
17
12
21
12
20
25
15
18
24
3.4
9
3.3
15
1.7
2.1
2.6
5.5
2
9
2
2
a
b
d
50 mL; P(CH2dCH2) 40 bar, T ) 100 °C. Per [Cr]. c In g product mmol-1 catalyst h-1 per [Cr]. Determined by GC-MS.
Cl2 was added to a CH2Cl2 solution of (THF)3CrCl3. The
products started to precipitate within minutes. After stirring
for 12 h the solvent was separated and the crude product
stirred again in CH2Cl2 overnight. Removal of the CH2Cl2
followed by washing with CH2Cl2 and three portions of hexanes
and drying in vacuo gave the respective complexes as green
solids in g90% yield.
From the series of complexes investigated a trend
becomes evident that marks the strong impact of the
precatalyst structures on catalyst behavior. Catalyst
activities and 1-alkene selectivities differ significantly
across the various complex types studied. The highest
activities and 1-alkene selectivities were found for
monomeric complexes bearing a tripodal ligand. This
study is particularly pertinent, as it is only recently that
complexes with tridentate ligands have received in-
creased attention in the field of ethylene polymerization3
and systems in which the tridentate ligand binds in a
trigonal arrangement are less well known.1b,c,15,18 R-Ole-
fins rather than polyethylene are generated by catalysts
formed from 3 and 4 since the ligands employed create
a relatively unencumbered environment at the metal
center, leading to enhanced rates of chain transfer and
thus to the formation of ethylene oligomers. Interest-
ingly catalysts based on 3b and 3c produce considerable
amounts of polyethylene, indicating that this class of
complexes has the potential to be further developed into
polymerization catalysts.
Our preliminary results demonstrate that hard donor
bi- and tridentate imidazole ligand frameworks, carry-
ing additional donor groups to tune the electronic
properties of the ligands, react with appropriate chro-
mium(III) chloro and alkyl precursors, forming stable
complexes in high yields. The novel complexes described
herein contribute to the relatively unexplored field of
chromium imidazole complexes and to the limited
number of well-defined, homogeneous, non-cyclopenta-
diene chromium-based catalysts for ethylene conversion.
In addition, the metal center in these complexes resides
in a ligand environment that until now has not found
application in this type of reaction. Once activated with
MMAO, the complexes represent a new class of catalysts
for the generation of linear 1-alkenes from ethylene with
selectivities of up to approximately 80%.
Gen er a l P r oced u r e for th e Syn th esis of Ch r om iu m
Mon o Meth yl Com p lexes. One equivalent of the respective
ligand dissolved in THF was added to a THF solution of
(THF)3CrMeCl2 (5a , at -20 °C). The green products started
to precipitate within minutes. After stirring for 4 h (5a , at -15
to -10 °C) the THF phase was separated and the crude
product washed twice with THF and hexanes. Drying in vacuo
at -5 °C gave the respective complexes as green solids in g90%
yield. Despite the care in preparing and handling complex 5a ,
its reactivity toward air and moisture precluded obtaining a
satisfactory microanalysis.
[Cr {t r is(N -m e t h ylim id a zol-2-yl)m e t h oxym e t h a n e }-
Cl3] (3a ). Anal. Calcd for C14H18N6OCl3Cr: C, 37.81; H, 4.09;
N, 18.90. Found: C, 38.02; H, 5.06; N 18.26. MS (FAB): m/z
(correct isotope patterns) 408, [M - Cl]+ (80%); 373, [M - 2Cl]+
(20%); 255, [L - OCH3]+, (100%). IR (KBr): 1539, 1507, 1478
cm-1 (νimidazole ring).
[Cr {2-[2-(d ip h en ylp h osp h in o)-1-(N-m eth ylim id a zol-2-
yl)eth yl]-N-m eth ylim id a zole}Cl3] (3b). Anal. Calcd for
C
22H23N4Cl3Cr: C, 49.59; H, 4.36; N, 10.51. Found: C, 49.59;
H, 4.60; N 10.35. MS (FAB): m/z (correct isotope patterns) 496,
[M - Cl]+ (100%); 461, [M - 2Cl]+ (31%); 267, [LH
-
P-C6H5]+, (80%). IR (KBr): 1550, 1508, 1486 cm-1 (νimidazole ring);
1434 cm-1 (νP-phenyl); 739, 698 cm-1 (δarom).
[Cr {4,4-dim eth yl-1,1-bis(N-m eth ylim idazol-2-yl)pen tan -
3-on e}Cl3] (3c). Anal. Calcd for C15H22N4OCl3Cr: C, 41.62;
H, 5.13; N, 12.95. Found: C, 41.13; H, 5.41; N 12.53. MS
(FAB): m/z (correct isotope patterns) 396, [M - Cl]+ (100%);
361, [M - 2Cl]+ (38%); 275, [LH]+, (42%). IR (KBr): 1667 cm-1
(νCO); 1551, 1511, 1461 cm-1 (νimidazole ring).
[Cr {b is(N-m et h ylim id a zol-2-yl)m et h a n e}Cl3]2
(4a ).
Anal. Calcd for C18H24N8Cl6Cr2: C, 32.31; H, 3.62; N, 16.75.
Found: C, 31.86; H, 4.75; N, 14.87. MS (FAB): m/z (correct
isotope patterns) 633, [M H2 - Cl]+ (7%); 598, [M H2 - 2Cl]+
(6%); 561 [M - 3Cl]+ (4%); 298, [LCrCl2]+ (59%); 263, [LCrCl]+
(80%); 177 [LH]+, (100%). IR (KBr): 1552, 1513, 1456 cm-1
(νimidazole ring).
[Cr {2-[(d ip h en ylp h osp h in o)(N-m et h ylim id a zol-2-yl)-
m eth yl]-N-m eth ylim id a zole}Cl3]2 (4b). Anal. Calcd for
C42H42N8Cl6P2Cr2: C, 48.62; H, 4.09; N 10.79. Found: C, 47.90;
H, 4.35; N, 10.03. MS (electrospray in CH3CN): m/z (correct
isotope patterns) 1001, [MH2 - Cl]+ (98%); 523 [LCrCl2 + CH3-
CN]+ (100%). IR (KBr): 1544, 1505, 1462 cm-1 (νimidazole ring),
1435 cm-1 (νP-phenyl); 743, 696 cm-1 (δarom).
Exp er im en ta l Section
Gen er a l Com m en ts. Unless otherwise stated, all manipu-
lations were carried out using standard Schlenk techniques
or in a nitrogen glovebox (Innovative Technology Inc.). All
solvents for use in an inert atmosphere were purified by
standard procedures and distilled under nitrogen immediately
prior to use. IR spectra were recorded for each complex, and
electronic spectra were recorded for complexes 3 and 4.
Elemental analysis, MS, and GC-MS were carried out by the
Central Science Laboratory (CSL), University of Tasmania.
Gen er a l P r oced u r e for Ch r om iu m Ch lor o Com p lexes.
A 1.03 equiv sample of the respective ligand dissolved in CH2-
[Cr {bis(N-m eth ylim id a zol-2-yl)k eton e}Cl3]2 (4c). Anal.
Calcd for C18H20N8O2Cl6Cr2: C, 31.01; H; 2.90; N, 16.08.
Found: C, 31.04; H 2.80; N, 16.19. MS (FAB): m/z (correct
isotope patterns): 661, [MH2 - Cl]+ (12%); 626, [MH2 - 2Cl]+
(7%); 589, [M H2 - 3Cl]+ (4%); 312, [LCrCl2]+ (95%); 277,
(18) J affart, J .; Nayral, C.; Choukroun, R.; Mathieu, R.; Etienne,
M. Eur. J . Inorg. Chem. 1998, 532, 425-428.

1250 Organometallics, Vol. 20, No. 6, 2001
Notes
[LCrCl]+ (100%); 191 [LH]+, (17%). IR (KBr): 1651 cm-1 (νCO);
1490, 1425 (νimidazole ring).
solution added via a syringe. The reactor was immersed into
a preheated oil bath and pressurized with ethylene. The
reactions were terminated by cooling the reactor in an ice salt
bath, venting of excess ethylene, and slow addition of a few
milliliters of MeOH followed by dilute HCl. The solid material
was collected on a frit, washed with MeOH, diluted HCl, and
MeOH, and dried in a high vacuum at 60 °C. The filtrate was
dried over Na2SO4 and analyzed by GC-MS.
[Cr (Me){tr is(N-m eth ylim idazol-2-yl)m eth oxym eth an e}-
Cl2]5a . MS (FAB): m/z (correct isotope patterns) 408, [M -
CH3]+ (57%); 373, [M - Cl, - CH3]+ (22%); 255, [L - OCH3],
(100%). IR (KBr): 1511, 1505, 1455 cm-1 (νimidazole ring).
[Cr (Me){2-[2-(d ip h en ylp h osp h in o)-1-(N-m eth ylim id a -
zol-2-yl)eth yl]-N-m eth ylim id a zole}Cl2] (5b). Anal. Calcd
for C23H26N4Cl2PCr: C, 53.91; H, 5.12; N, 10.94. Found: C,
53.20; H, 4.34; N, 10.22. MS (FAB): m/z (correct isotope
patterns) 512, [MH -CH3]+ (12%); 496, [M - CH3]+ (55%); 477,
[MH - Cl]+ (41%); 461, [M - CH3, - Cl]+ (55%); 442, [MH -
2Cl]+ (44%). IR (KBr): 1549, 1509, 1484 cm-1 (νimidazole ring);
1434 cm-1 (νP-phenyl); 741, 697 cm-1 (δarom).
Gen er a l P r oced u r e for th e Ca ta lysis. MMAO (1.82 M
in heptane) was added slowly with stirring to a suspension of
the selected complex in 5 mL of toluene. A 300 mL autoclave
(dried at 100 °C, cooled to RT under vacuum, and back-filled
with nitrogen) was charged with toluene and the catalyst
Ack n ow led gm en t. Australian Research Council
(SPIRT Grant) and Orica Australia are gratefully
acknowledged for financial support and for providing
postdoctoral salaries for T.R. and N.B. We also thank
staff from the Central Science Laboratory, University
of Tasmania, for their assistance with elemental analy-
ses, MS, and GC-MS.
OM000846B