Synthesis, structures and ethylene polymerisation behaviour of low valent β-diketiminato chromium complexes

Article abstract of DOI:10.1002/1099-0682(200107)2001:7<1895::AID-EJIC1895>3.0.CO;2-S

The preparation, reaction chemistry and ethylene polymerisation behaviour of low valent β-diketiminato chromium complexes are described. [(DDP)CrCl(μ-Cl)]₂ (1) [DDPH =2-{(2,6-diisopropylphenyl)amino}-4-{(2,6-diisopropylphenyl)imino} -pent-2-ene] is formed by treatment of CrCl₃(THF)₃ with LiDDP. Alkylation of 1 with AlMe₃ results in the formation of the binuclear dimethyl complex [(DDP)CrMe(μ-Cl)]₂ (2). In contrast the attempted alkylation of 1 with benzylmagnesium chloride results in reduction to form the dichromium(II) complex [(DDP)Cr(μ-Cl)]₂ (3). Depending on the conditions of crystallisation, 3 can be obtained as the THF adduct [3(THF)₂·THF] or co-crystallised with a molecule of dibenzyl [3·Bz-Bz]. Cleavage of the dimeric unit in 1 can be achieved by the addition of carboxylates or β-diketonates to give [(DDP)CrCl(O₂CR)(THF)] (R = Me 4a, Ph 4b) and [(DDP)CrCl({O(R)C}₂CH)] (R = Me 5a, Ph 5b), respectively. Single crystal X-ray diffraction studies have been performed on 1, 2, 3(THF)₂·THF, 3·Bz-Bz, 4a, and 5b. Complexes 1, 2 and 3(THF)₂·THF are dimeric and have molecular C_2h symmetry. Complex 3·Bz Bz is also dimeric but has its potential C_2h symmetry removed by a significant tetrahedral distortion of the chromium coordination geometry. Compound 4a has an octahedral chromium centre coordinated to a single bidentate diketiminate ligand, a bidentate acetate, a chloride and a THF molecule. Complex 5b has a square pyramidal chromium with apical chloride and basal η² diketiminate and diketonate ligands. The complex contains strong intramolecular C-H···π stabilising interactions. All the complexes are active in ethylene polymerisation on treatment with suitable aluminium activators, affording high molecular weight polyethylene.

Full text of DOI:10.1002/1099-0682(200107)2001:7<1895::AID-EJIC1895>3.0.CO;2-S

FULL PAPER
Synthesis, Structures and Ethylene Polymerisation Behaviour of Low Valent
β-Diketiminato Chromium Complexes
Vernon C. Gibson,*[a] Claire Newton,[a] Carl Redshaw,[a] Gregory A. Solan,[a]
Andrew J. P. White,[a] and David J. Williams[a]
Keywords: Chromium / N ligands / Ethylene / Polymerisation
The preparation, reaction chemistry and ethylene polymeris-
ation behaviour of low valent β-diketiminato chromium com- on 1, 2, 3(THF)2·THF, 3·Bz-Bz, 4a, and 5b. Complexes 1, 2
plexes are described. [(DDP)CrCl(µ-Cl)]2 (1) [DDPH = 2-{(2,6- and 3(THF)2·THF are dimeric and have molecular C2h sym-
diisopropylphenyl)amino}-4-{(2,6-diisopropylphenyl)imino}- metry. Complex 3·Bz Bz is also dimeric but has its potential
Single crystal X-ray diffraction studies have been performed
pent-2-ene] is formed by treatment of CrCl3(THF)3 with
LiDDP. Alkylation of 1 with AlMe3 results in the formation of
the binuclear dimethyl complex [(DDP)CrMe(µ-Cl)]2 (2). In
C2h symmetry removed by a significant tetrahedral distortion
of the chromium coordination geometry. Compound 4a has
an octahedral chromium centre coordinated to a single bi-
contrast, the attempted alkylation of 1 with benzylmagnes- dentate diketiminate ligand, a bidentate acetate, a chloride
ium chloride results in reduction to form the dichromium(II)
complex [(DDP)Cr(µ-Cl)]2 (3). Depending on the conditions
and a THF molecule. Complex 5b has a square pyramidal
chromium with apical chloride and basal η2 diketiminate and
of crystallisation, 3 can be obtained as the THF adduct diketonate ligands. The complex contains strong intramolec-
[3(THF)2·THF] or co-crystallised with a molecule of dibenzyl ular C−H···π stabilising interactions. All the complexes are
[3·Bz-Bz]. Cleavage of the dimeric unit in 1 can be achieved active in ethylene polymerisation on treatment with suitable
by the addition of carboxylates or β-diketonates to give
[(DDP)CrCl(O2CR)(THF)] (R Me 4a, Ph 4b) and
aluminium activators, affording high molecular weight poly-
ethylene.
=
[(DDP)CrCl({O(R)C}2CH)] (R = Me 5a, Ph 5b), respectively.
Introduction
and R group variation and variability in bonding mode,[7c]
are particularly attractive features.
Supported chromium catalysts play a key role in the
worldwide production of polyethylene, and much effort has
been directed towards developing an understanding of these
systems from a surface science/heterogeneous catalysis
point of view.[1,2] A complementary approach is to develop
homogeneous model systems, not only to gain understand-
ing of the chromium-centred chemistry of the supported
systems, but also to advance chromium olefin polymeris-
ation catalysis into an era of well-defined, single-site cata-
lysts. To date, the majority of reports of homogeneous chro-
mium catalysts for olefin polymerisation have been on half-
sandwich chromium systems.[3Ϫ4] However, more recently
there has been growing interest in non-cyclopentadienyl
chromium catalysts.[5,6]
In this paper we are concerned with chemistry of low-
valent chromium compounds containing the monoanionic
N,N-chelate β-diketiminato ligand, [ArNC(Me)CHC(Me)-
NAr]Ϫ (Ar ϭ 2,6-iPr2C6H3, DDP). This ligand (DDP) and
members of the general family, [ArNC(R)CHC(R)NAr]Ϫ,
have been the subject of intense interest in recent years and
complexes incorporating early[7] and late[8] transition
metals, main group metals[9] and lanthanides have been re-
ported.[10] Their ease of preparation, facility for aryl (Ar)
In independent studies, Theopold and this group have
communicated chromium(III) (d3) complexes supported by
β-diketiminato ligands, albeit with different aryl groups, as
olefin polymerisation catalysts.[6h,6i] Herein we report, as
part of a general study of the olefin polymerisation capabil-
ity of non-cyclopentadienyl-containing chromium com-
plexes,[6bϪ6h] our findings on a range of DDP-supported
chromium(II) and -(III) systems (Scheme 1).
Results and Discussion
The protonated β-diketiminato ligand, 2-[(2,6-diisopro-
pylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-
ene (DDPH) was prepared using a method similar to that
reported for related ligands[11] and more recently by Feld-
man for DDPH itself.[8b] The lithium salt of DDPH was
prepared by addition of one equivalent of nBuLi to a THF
solution of DDPH at Ϫ78 °C and used without isolation.
Addition of one equivalent of CrCl3(THF)3 to LiDDP in
THF gave [(DDP)CrCl(µ-Cl)]2 (1) as a green solid in good
yield upon workup (Scheme 1). Crystals of 1 suitable for an
X-ray structure determination were grown from a concen-
trated pentane solution at Ϫ20 °C. The structure is dimeric
[a]
Department of Chemistry, Imperial College,
South Kensington, London SW7 2AY, U.K.
E-mail: V.Gibson@ic.ac.uk
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/0707Ϫ1895 $ 17.50ϩ.50/0
1895

V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White, D. J. Williams
FULL PAPER
Figure 1. The molecular structure of 1
Scheme 1. Reagents and conditions: (i) nBuLi, THF, Ϫ78 °C; (ii)
CrCl3(THF)3, THF, Ϫ78 °C; (iii) AlMe3, THF, 20 °C; (iv) BzMgCl,
THF, Ϫ78 °C; (v) NaO2CR, THF, Ϫ78 °C; (vi) Li[{O(R)C}2CH],
THF, Ϫ78 °C
˚
Table 1. Selected bond lengths (A) and angles (°) for 1 (apical ϭ
Cl) and 2 (apical ϭ Me)
1
2[a]
(Figure 1) and has non-crystallographic C2h symmetry
about an axis passing through the two bridging chlorine
atoms (the crystallographic symmetry is Ci). The geometry
at chromium is distorted square pyramidal with the chro-
CrϪapical
CrϪCl(2)
CrϪCl(2Ј)
CrϪN(3)
2.233(2)
2.403(2)
2.397(2)
1.986(5)
1.990(5)
1.349(8)
1.395(9)
1.379(9)
1.349(8)
2.037(7)
2.3951(13)
2.3951(13)
2.028(4)
2.028(4)
1.328(6)
1.400(6)
1.400(6)
1.328(6)
2.042(8)
2.3925(13)
2.3925(13)
2.031(4)
2.031(4)
1.324(7)
1.397(6)
1.397(6)
1.324(7)
˚
mium atom being displaced by 0.35 A out of the basal plane
CrϪN(7)
in the direction of Cl(1). Although the bridging CrϪCl dis-
tances are identical within the errors of the structure deter-
mination, there is a marked asymmetry in the apical/basal
N(3)ϪC(4)
C(4)ϪC(5)
C(5)ϪC(6)
C(6)ϪN(7)
˚
CrϪCl distances, with the apical bond being ca. 0.17 A
shorter than its basal counterparts (Table 1). The CrϪN
distances are unexceptional. The nonbonded Cr···Cr sep-
aration (3.69 A) is long, and extends outside the range
N(3)ϪCrϪN(7)
N(3)ϪCrϪCl(1)
N(7)ϪCrϪCl(1)
N(3)ϪCrϪCl(2Ј)
N(7)ϪCrϪCl(2Ј)
Cl(1)ϪCrϪCl(2Ј)
N(3)ϪCrϪCl(2)
N(7)ϪCrϪCl(2)
Cl(1)ϪCrϪCl(2)
Cl(2)ϪCrϪCl(2Ј)
CrϪCl(2)ϪCrЈ
91.4(2)
99.6(2)
99.5(2)
91.2(2)
160.1(2)
99.49(7)
160.34(14)
91.9(2)
98.91(7)
79.34(6)
100.66(6)
91.3(2)
95.4(2)
95.4(2)
92.68(11)
165.73(12)
97.8(2)
165.73(12)
92.68(11)
97.8(2)
90.8(2)
96.8(2)
96.8(2)
92.68(12)
165.41(12)
96.8(2)
165.41(12)
92.68(12)
96.8(2)
˚
found in a number of other chloride-bridged chromium(III)
complexes.[12Ϫ15] This distance may in part be due to the
steric repulsion between the bulky aryl ligands. The six-
membered chelate ring has a boat conformation with the
˚
chromium and C(5) atoms lying 0.60 and 0.13 A, respect-
ively, out of the N(3)ϪC(4)ϪC(6)ϪN(7) plane (which is co-
80.31(7)
99.69(7)
80.62(7)
99.38(7)
˚
planar to within 0.01 A). There is a pronounced pattern of
delocalisation in the bond lengths of the N(3) to N(7) link-
ages. All four 2,6-diisopropylphenyl rings are oriented es-
sentially orthogonally (84 and 87°) to their respective che-
late CϪNϪCr planes, a geometry that is stabilised by pairs
[a]
The two sets of values refer to the two crystallographically inde-
pendent molecules.
of CϪH···N(pπ) interactions[16] between N(3) and N(7) and numbers has also recently been observed by Budzelaar and
their proximal isopropyl methine hydrogen atoms; the co-workers.[8a]
˚
H···N distances are in the range 2.42 to 2.54 A with associ-
The N,N-β-diketiminato chromium derivatives 2Ϫ5 have
˚
ated C···N distances of between 2.89 and 3.00 A. There are been prepared by in situ formation of 1 followed by treat-
no intermolecular packing interactions of note.
ment with the reagents shown in Scheme 1. The reaction
It is noteworthy that substitution of the 2,6-diisopropyl- with AlMe3 affords a dark oily residue. Extraction of this
phenyl groups in DDP for phenyl groups results in a residue into pentane and cooling to Ϫ20 °C gives crimson
mononuclear six-coordinate chromium(III) dichloride com- needles of [(DDP)CrMe(µ-Cl)]2 (2) in good yield. A crystal
plex in which the coordination sphere is completed by THF prepared in this manner was used for a single crystal X-ray
molecules.[6i] The preference of complexes coordinated by diffraction study. Complex 2 crystallises with two independ-
β-diketiminato ligands, with bulky groups on the 2,6-posi- ent molecules in the asymmetric unit. Both have crystallo-
tions of the aryl substituents, to exhibit low coordination graphic C2h symmetry and essentially identical conforma-
1896
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903

Low Valent β-Diketiminato Chromium Complexes
FULL PAPER
tions — the maximum deviation of the best fit of the two
molecules (excluding the isopropyl methyl groups) is 0.08
˚
A. Replacement of the apical chloride in 1 by a methyl
group to give 2 has virtually no effect on the geometry of
2, the central core (including the two chelate rings) fitting
˚
to within 0.1 A of that in 1, though the CrϪN distances are
˚
noticeably increased by ca. 0.04 A (Table 1). The CrϪMe
distances in the two independent molecules are 2.037(7) and
˚
2.042(8) A. The displacement of the chromium atom out of
the basal plane in the direction of the apical methyl group
˚
(ca. 0.25 A) is noticeably less than that towards the chloride
in 1. The nonbonded Cr···Cr separation is slightly reduced
˚
˚
(ca. 3.66 A). Within the chelate ring Cr and C(2) lie 0.52
and 0.14 A out of the N(3)ϪC(4)ϪC(6)ϪN(7) plane (see
˚
Figure 1) in one independent molecule and 0.55 and 0.15 A,
respectively, in the other. Each of the 2,6-diisopropylphenyl
rings are rotated by ca. 85° out of their respective chelate
CϪNϪCr planes, a conformation again stabilised by pairs
of CϪH···N(pπ) interactions; the C···N distances are in the
˚
range 2.87 to 3.00 A with associated H···N distances of be-
˚
tween 2.40 and 2.52 A. As with 1, there are no noteworthy
Figure 2. The molecular structure of the metal complex in
3(THF)2·THF
intermolecular packing interactions.
Attempted alkylation of 1 with benzyl Grignard reagent,
however, results in the reduction of 1 to give [(DDP)Cr(µ-
Cl)]2 (3) as a green solid in high yield. Recrystallisation of
this solid from THF gives 3(THF)2·THF while from pent-
ane 3·Bz-Bz is obtained. Single crystals of both forms of 3
were obtained by cooling the corresponding solutions to
Ϫ20 °C overnight. The X-ray analysis showed
3(THF)2·THF to be structurally related to 2, having crystal-
˚
Table 2. Selected bond lengths (A) and angles (°) for 3(THF)2·THF
CrϪClЈ
2.4182(6)
2.071(2)
2.442(3)
1.450(3)
1.400(3)
CrϪCl
CrϪNЈ
2.4182(6)
2.071(2)
1.336(3)
1.400(3)
CrϪN
CrϪO(20)
NϪAr
NϪC(1)
C(1)ϪC(2)
C(2)ϪC(1Ј)
lographic C2h symmetry (Figure 2) but with the apical NϪCrϪNЈ
91.26(10)
93.37(5)
171.58(6)
92.39(7)
94.44(5)
98.79(3)
NϪCrϪClЈ
NϪCrϪCl
ClЈϪCrϪCl
NЈϪCrϪO(20)
ClϪCrϪO(20)
171.58(6)
93.37(5)
81.21(3)
92.39(7)
94.44(5)
NЈϪCrϪClЈ
methyl ligand replaced by a weakly bound THF molecule
[CrϪO(20) ϭ 2.442(3) A, Table 2] which is disordered about
NЈϪCrϪCl
˚
NϪCrϪO(20)
ClЈϪCrϪO(20)
CrϪClϪCrЈ
the mirror plane which passes through Cr, CrЈ, O(20) and
O(20Ј). The geometry at chromium is still square pyramidal
˚
with the chromium atom lying 0.13 A out of the basal
˚
plane. The CrϪN distance [2.071(2) A] is increased relative
˚
˚
to that in 2 [av. 2.030 A], although it still lies within the in 2. The non-bonded Cr···Cr separation here is 3.60 A,
range observed for CrϪN bonds in the literature. The che- noticeably shorter than in 1, 2 and 3(THF)2·THF. The che-
late rings retain a boat conformation with Cr and C(2) lying late ring, whilst still having a boat conformation, is dis-
˚
0.43 and 0.12 A, respectively, out of the tinctly flattened in comparison to 1, 2 and 3(THF)2·THF,
˚
NϪC(1)ϪNЈϪC(1Ј) plane. The non-bonded Cr···Cr separa- with Cr and C(3) lying 0.30 and 0.09 A, respectively, out of
˚
tion is 3.67 A and lies at the top end of the range reported the N(1)ϪC(2)ϪC(4)ϪN(5) plane. The 2,6-diisopropyl-
for chloride-bridged dichromium(II) complexes.[17,18] The phenyl rings are rotated by 85 and 88° out of their respect-
2,6-diisopropylphenyl rings are rotated by ca. 85° out of ive chelate CϪNϪCr planes, with associated CϪH···N(pπ)
˚
their respective chelate CϪNϪCr planes, a conformation interactions (C···N distances in the range 2.87 to 2.92 A,
˚
again stabilised by pairs of CϪH···N(pπ) interactions; the H···N distances between 2.39 and 2.50 A). There are no
˚
C···N distances are in the range 2.91 to 2.94 A with associ- notable intermolecular packing interactions involving either
˚
ated H···N distances of between 2.41 and 2.50 A. There the complex or the co-crystallised dibenzyl molecules.
are no significant intermolecular close contacts between the
complexes or the included THF solvent molecule.
It is noteworthy that complex 3(THF)2·THF can also be
prepared more directly by treating CrCl2·THF with one
In the structure of 3·Bz-Bz, where there is an absence of equivalent of LiDDP in THF.
apical substituents on the chromium centres, the geometry
The pathway by which 3 is formed from 1 is uncertain.
at chromium is square planar (Figure 3), although with a However, it would seem possible that benzylation of the ap-
marked tetrahedral distortion, the NϪNϪCr and ical trans sites in 1 occurs initially to give the benzyl ana-
CrϪClϪCl planes being inclined by 18°. The complex has logue of 2 in a manner similar to the benzylation of [Cp*
crystallographic Ci symmetry and symmetric CrϪCl CrCl(µ-Cl)]2,[13e] followed by isomerisation to the cis isomer
bridges (Table 3). The CrϪN bond lengths are the same as and reductive coupling of benzyl groups to give 3 and di-
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903
1897

V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White, D. J. Williams
FULL PAPER
For example, treatment of
1
with NaO2CR
(R Me, Ph) at Ϫ78 °C gives the complexes
ϭ
[(DDP)CrCl(O2CR)(THF)] (R ϭ Me 4a, Ph 4b) in high
yield. Both 4a and 4b can be recrystallised from a concen-
trated pentane solution at Ϫ20 °C. A single crystal of 4a
was subjected to an X-ray diffraction study. The structure
of 4a consists of an octahedral chromium centre with the
oxygen atom of the THF ligand and one of the oxygen
atoms of the acetate ligand in the same plane as the nitro-
gen atoms of the bidentate ligand; the remaining oxygen
atom of the η2-chelating acetate group occupies a site trans
to the chloride ligand (Figure 4). The structure of 4a thus
resembles that of the less sterically demanding diphenyl β-
diketiminato complex [{PhNC(Me)CHC(Me)NPh}CrCl2-
(THF)2],[6i] with a THF and a chloride ligand effectively
replaced by an acetate group. In 4a the geometry at chro-
mium is distorted with cis angles in the range 63.7(1) to
101.9(1)°, the acute angle being associated with the bite of
the acetate ligand. The CrϪN distances (Table 4) are the
same as in 2 and 3·BzBz, and the CrϪCl distance [2.295(1)
Figure 3. The molecular structure of the chromium-containing spe-
cies in 3·Bz-Bz
˚
A] is similar to that to the terminal chloride in 1. The
˚
CrϪO(THF) distance [2.119(3) A] is consistent with the
˚
Table 3. Selected bond lengths (A) and angles (°) for 3·Bz-Bz
presence of a formal σ-bond, similar to that to the loosely
˚
associated apical ligand in 3(THF)2·THF [2.442(3) A]. The
CrϪCl
2.3877(9)
2.036(2)
1.331(4)
1.400(4)
CrϪClЈ
2.3851(9)
2.031(2)
1.393(4)
1.335(4)
CrϪN(1)
N(1)ϪC(2)
C(3)ϪC(4)
CrϪN(5)
C(2)ϪC(3)
C(4)ϪN(5)
six-membered chelate ring has a distinctive boat conforma-
˚
tion with the chromium and C(3) lying 0.60 and 0.11 A,
respectively, out of the N(1)ϪC(2)ϪC(4)ϪN(5) plane.
There are characteristic orthogonal orientations (83 and
87°) of the 2,6-diisopropylphenyl ring systems to the chelate
ring. Stabilisation of this conformation is again by weak
CϪH···N(pπ) interactions (C···N distances in the range 2.89
N(5)ϪCrϪN(1)
N(1)ϪCrϪClЈ
N(1)ϪCrϪCl
CrϪClϪCrЈ
90.93(10)
94.54(7)
165.75(8)
98.05(3)
N(5)ϪCrϪClЈ
N(5)ϪCrϪCl
ClϪCrϪClЈ
168.04(8)
95.21(7)
81.95(3)
˚
˚
to 2.98 A, H···N distances between 2.41 and 2.50 A). The
packing is normal van der Waals.
benzyl (Scheme 2). It is noteworthy that Theopold has re-
Compound 4b is presumed to possess a similar structure
cently postulated a related trans-cis isomerisation of the re- to 4a. Unlike the reactions of 1 with carboxylates, treatment
lated complex [Cp*CrMe(µ-Cl)]2.[13a] However, we cannot of 1 with the more sterically demanding β-diketonates,
rule out Wurtz coupling occurring independently of the {CH[C(R)O]2}Ϫ [R ϭ Me (acac), Ph (dbm)], results in the
transition metal centre in a way that is often observed dur- five-coordinate complexes [(DDP)CrCl({O(R)C}2CH)]
ing the preparation of benzylic Grignard reagents.[19,20]
(R ϭ Me 5a, Ph 5b). Both 5a and 5b can be recrystallised
Whilst reaction of 1 with either AlMe3 or BzMgCl results from pentane at Ϫ20° C. Compound 5b has been the sub-
in preservation of the dimeric core, interaction with carb- ject of a single crystal X-ray diffraction study. This study
oxylates or β-diketonates results in cleavage of the dimer to shows 5b to have a square pyramidal coordination geo-
give discrete mononuclear chromium(III) complexes. metry at chromium (Figure 5), with the basal sites occupied
Scheme 2. Plausible pathway for the formation of 3 from 1
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Eur. J. Inorg. Chem. 2001, 1895Ϫ1903

Low Valent β-Diketiminato Chromium Complexes
FULL PAPER
˚
Table 5. Selected bond lengths (A) and angles (°) for 5b
CrϪCl
2.2294(14)
2.032(3)
1.961(3)
1.387(6)
1.329(5)
CrϪN(1)
CrϪO(6)
N(1)ϪC(2)
C(3)ϪC(4)
2.020(3)
1.958(3)
1.341(5)
1.391(6)
CrϪN(5)
CrϪO(10)
C(2)ϪC(3)
C(4)ϪN(5)
O(6)ϪCrϪO(10)
O(10)ϪCrϪN(1)
O(10)ϪCrϪN(5)
O(6)ϪCrϪCl
86.89(11)
87.83(12)
161.63(13)
96.18(10)
102.25(11)
O(6)ϪCrϪN(1)
O(6)ϪCrϪN(5)
N(1)ϪCrϪN(5)
O(10)ϪCrϪCl
N(5)ϪCrϪCl
161.27(13)
88.85(13)
90.54(13)
96.02(10)
102.20(10)
N(1)ϪCrϪCl
inate chelate ring has, as in all the other complexes, a boat
conformation with the chromium and C(3) lying 0.43 and
˚
0.11 A, respectively, out of the N(1)ϪC(2)ϪC(4)ϪN(5)
plane. The Cr(dbm) chelate ring is much flatter, with the
chromium and C(8) lying 0.23 and 0.02 A, respectively, out
Figure 4. The molecular structure of 4a
˚
of the O(6)ϪC(7)ϪC(9)ϪO(10) plane. The 2,6-diisopropyl-
phenyl ring systems are, as usual, oriented orthogonally (83
and 86°) to the chelate ring, with CϪH···N(pπ) stabilisation
˚
Table 4. Selected bond lengths (A) and angles (°) for 4a
˚
CrϪCl
2.2947(12) CrϪN(1)
2.029(3)
2.059(3)
2.119(3)
1.409(5)
1.323(5)
(C···N distances in the range 2.90 to 2.98 A, H···N distances
CrϪN(5)
CrϪO(34)
N(1)ϪC(2)
C(3)ϪC(4)
2.023(3)
2.043(3)
1.330(5)
1.383(5)
CrϪO(32)
CrϪO(36)
C(2)ϪC(3)
C(4)ϪN(5)
˚
between 2.38 and 2.49 A). In contrast, the planes of the two
phenyl rings of the dbm ligand lie close to that of their
chelate ring (twists of ca. 10 and 20°) and as a consequence
direct one of their ortho CϪH hydrogen atoms into the
faces of their proximal 2,6-diisopropylphenyl rings of the
diketiminate ligand; the H···π distances are short at 2.40
N(5)ϪCrϪN(1)
N(1)ϪCrϪO(34)
N(1)ϪCrϪO(32)
N(5)ϪCrϪO(36)
90.37(12)
N(5)ϪCrϪO(34)
90.70(12)
91.06(12)
101.90(12) N(5)ϪCrϪO(32)
165.52(12) O(34)ϪCrϪO(32) 63.67(12)
174.12(12) N(1)ϪCrϪO(36)
˚
and 2.53 A and have associated CϪH···π angles of 168 and
95.14(11)
160°, respectively. Accompanying this interaction is a no-
ticeable contraction of both the O(6)ϪC(7)ϪC(42) and
O(10)ϪC(9)ϪC(48) angles [115.8(3) and 116.2(3)°, respect-
O(34)ϪCrϪO(36) 86.16(12)
O(32)ϪCrϪO(36) 83.10(11)
N(5)ϪCrϪCl
O(34)ϪCrϪCl
O(36)ϪCrϪCl
93.10(9)
164.38(9)
88.65(9)
N(1)ϪCrϪCl
O(32)ϪCrϪCl
93.23(9)
101.08(10)
ively], and to
a
lesser extent also those for
C(7)ϪC(42)ϪC(37) and C(9)ϪC(48)ϪC(47) [117.3(2) and
118.0(2)°, respectively], from normal trigonal values. These
interactions are directly analogous to those present between
the ortho hydrogen atoms of a dbm ligand coordinated to
iridium and the phenyl rings of a pair of triphenylphos-
phane ligands coordinated to the same metal centre.[21]
There are no packing interactions of note.
by the two η2 ligands and the apical site by a chloride li-
˚
gand. The metal atom lies 0.32 A out of the basal plane
˚
˚
(which is planar to within 0.002 A) in the direction of the
chloride ligand [CrϪCl 2.229(1) A]. The CrϪN distances
(Table 5) are the same as in 2, 3·Bz-Bz and 4a. The diketim-
Examples of mononuclear five-coordinate CrIII com-
plexes are rare, due to the significant ligand-field stabilis-
ation of the d3 configuration in an octahedral geometry.[20]
5a is presumed to possess a similar structure to 5b.
All the complexes are paramagnetic, affording broad un-
informative 1H NMR spectra. The mononuclear complexes
4 and 5 exhibit magnetic moments between 3.8 and 4.1 BM
(Evans Balance at ambient temperature) consistent with
three unpaired electrons (d3) while the dimeric chro-
mium(III) complexes 1 and 2 display magnetic moments of
3.5 and 3.6 BM characteristic of antiferromagnetically
coupled S ϭ 3/2 chromium(III) centres.[7f,23] In addition,
the X-band EPR spectra of all the CrIII complexes in tolu-
ene at ambient temperature all support the complexes being
assigned CrIII with sharp signals corresponding to g values
of ca. 1.98 with no observable hyperfine coupling.[24]
The FAB mass spectra of the mononuclear complexes 4
and 5 give molecular ion peaks and/or peaks corresponding
Figure 5. The molecular structure of 5b, showing also the weak
intramolecular CϪH···π stabilising interactions
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903
1899

V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White, D. J. Williams
FULL PAPER
to fragmentation peaks. For the dimeric complexes 1Ϫ3 most active systems (runs 9 and 13) comprise a combina-
fragmentation of the dinuclear unit must readily occur in tion of one equivalent of chromium complex to thirty
the instrument as only peaks attributable to mononuclear equivalents of alkylaluminium chloride activator. We have
fragments are observed. This observation is consistent with found that activities start to fall with a greater or lower
the ready cleavage of the dimeric assembly in the presence concentration of activator. Notably, MAO (400 equiv.) re-
of carboxylates or β-diketonates (see complexes 4 and 5) sults in lower activities (runs 1, 3, 5, 8, 11, 14, 17). Thirdly,
leading to the prospect that 1Ϫ3 can be regarded as loosely the similar activities [cf. 75 (run 2), 72 g/mmol·h·bar (run
associated monomers.
6)] observed employing the DDP-supported CrIII- and CrII-
The results of Schlenk-scale ethylene polymerisation chloride complexes 1 and 3, and the purple colour of the
screening tests at 1 bar/25 °C using dual-component cata- active catalyst (in both cases), suggest a common active spe-
lyst systems (1Ϫ5 ϩ cocatalyst) are collected in Table 6. cies.
Three types of aluminium co-catalyst have been employed
The mechanism by which these dual component systems
to activate pre-catalysts 1Ϫ5, namely methylaluminoxane operate is uncertain. The fact that the most active systems
(MAO), diethylaluminium chloride (DEAC) and dimethyl- combine an alkylaluminium chloride rather than MAO ra-
aluminium chloride (DMAC) (see Table 6; runs 1Ϫ19). The ther suggests that alkyl abstraction to form a cationic spe-
results reported in Table 6 were reproduced several times, cies is not occurring here. This may account for the gener-
though not optimised beyond the conditions reported.
ally lower activities observed by comparison with the Cp-
amine systems reported by Jolly where a cationic active
centre is believed to operate. The identity of the active ox-
idation state is uncertain but the similarity in productivities
and reactivity involving the CrII and CrIII complexes 1 and
3, coupled with the ready reduction of 1 on benzylation,
lends some support for a CrII species.
Table 6. Results of ethylene polymerisation runs using precata-
lysts 1Ϫ5[a]
Run Precatalyst Activator[b]
Yield
Activity
(mmol)
(mmol/ equiv.)
PE (g)[c] (g/mmol·h·bar)
1
2
3
4
5
6
7
8
1 (0.015)
1 (0.015)
2 (0.015)
2 (0.015)
3 (0.015)
3 (0.015)
3 (0.015)
4a (0.015)
4a (0.015)
4a (0.015)
4b (0.01)
4b (0.01)
4b (0.01)
5a (0.015)
5a (0.015)
5a (0.015)
5b (0.015)
5b (0.015)
5b (0.015)
MAO (6.0/400)
DEAC (0.45/30)
MAO (6.0/400)
DEAC (0.45/30)
MAO (6.0/400)
DEAC (0.45/30)
0.12
2.25
0.30
1.60
0.24
2.16
4[d]
75[d]
10[d]
54[d]
8[d]
72[d]
79[d]
3
Conclusions
The chemistry of the sterically demanding β-diketiminato
ligand DDP on low valent chromium has been explored.
Complexation of DDP with CrCl3(THF)3 leads to a readily
derivatisable coordinatively unsaturated dimeric species 1.
Preservation of the dimeric core occurs on alkylation or re-
duction of 1, while addition of chelating carboxylate or β-
diketonate ligands to 1 causes fragmentation to give discrete
mononuclear complexes. Notably, only in the case of 4 is
the more common octahedral geometry of CrIII achieved.
All the complexes polymerise ethylene on addition of alumi-
nium activators to give high molecular weight polyethylene.
DMAC (0.45/30) 2.37
MAO (6.0/400)
DEAC (0.45/30)
DMAC(0.45/30)
MAO (4.0/400)
DEAC (0.3/30)
DMAC (0.3/30)
MAO (6.0/400)
DEAC (0.45/30)
0.05
1.77
1.05
0.13
0.85
0.93
0.30
1.48
9
118[e]
70
10
11
12
13
14
15
16
17
18
19
13
85
93
20
49
54
10
56
DMAC (0.45/30) 1.61
MAO (6.0/400)
DEAC (0.45/30)
0.15
0.84
DMAC (0.45/30) 1.20
80
[a]
General conditions: 1 bar ethylene Schlenk test carried out in
Experimental Section
toluene (40 mL) at 25 °C, over 60 min., reaction quenched with dil
HCl and the solid washed with methanol (50 cm3) and dried in a
General: All manipulations were carried out under an atmosphere
of nitrogen using standard Schlenk and cannula techniques or in a
conventional nitrogen-filled glove-box. Solvents were refluxed over
an appropriate drying agent, and distilled and degassed prior to
use. Elemental analyses were performed by the microanalytical ser-
vices of the Department of Chemistry at Imperial College and Me-
dac Ltd. NMR spectra were recorded on a Bruker spectrometer at
250 MHz (1H) and 62.9 MHz (13C) at room temperature; chemical
shifts are referenced to the residual protonated impurity of the deu-
terated solvent. IR spectra (nujol mulls, KBr/CsI windows) were
recorded on PerkinϪElmer 577 and 457 grating spectrophoto-
meters. Mass spectra were obtained using fast atom bombardment
(FAB). Gel permeation chromatographs (GPC) were obtained us-
ing a Waters 150CV [columns are supplied by Shodex (807, 806 &
804) BP Chemicals Ltd.]. Magnetic Susceptibility studies were per-
formed using an Evans Balance at room temperature. Diamagnetic
corrections for the samples were determined from Pascal’s con-
[b]
[c]
vacuum oven at 40 °C. Ϫ
MAO ϭ methylaluminoxane. Ϫ
[d]
Solid polyethylene. Ϫ Activity reported per chromium centre. Ϫ
As a representative example, GPC analysis of the polyethylene
[e]
afforded Mw ϭ 925000, Mn ϭ 110000, Mw/Mn ϭ 8.4. Care should
be taken in the interpretation of these values, however, since, in
general, the polymers derived from these polymerizations are not
fully soluble in the 1,2,4-trichlorobenzene GPC solvent, even upon
heating at 160 °C for several hours.
On analysis of Table 6 several points emerge. Firstly, all
DDP-supported chromium complexes are active in the pres-
ence of alkylaluminum activator, converting ethylene to
high molecular weight polyethylene. In no case has poly-
ethylene been obtained using 1Ϫ5 in the absence of an alu-
minium activator, nor has there been any evidence for low
molecular weight oligomers (by GCMS). Secondly, the stants and literature values.[25]
1900
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903

Low Valent β-Diketiminato Chromium Complexes
FULL PAPER
and DDPH[8b] were prepared according to the 66.09, H 8.18, N 4.41; found C 66.12, H 8.41, N 4.29. Ϫ µeff
ϭ
[26]
CrCl3(THF)3
established procedures. The reagents AlMe3 (2.0  solution in tolu-
ene), Et2AlCl (1.8  solution in toluene), Me2AlCl (1.0  solution
3.80 BM.
4b: FAB-MS: m/z ϭ 696 [Mϩ], 590 [Mϩ Ϫ Cl Ϫ THF], 467 [Mϩ
in hexane), MAO (10% solution in toluene) and BzMgCl (2.0  Ϫ Cl Ϫ THF Ϫ O2CPh]. Ϫ C40H54ClCrN2 (650.33): calcd. C 68.82,
solution in THF) were purchased from Aldrich Chemical Co. All
other chemicals were obtained commercially and used as received
unless stated otherwise.
H 7.74, N 4.01; found C 68.91, H 7.89, N 3.98. Ϫ µeff ϭ 3.74 BM.
Preparation of [(DDP)CrCl({O(R)C}2CH)] (R ؍ Me 5a, Ph 5b):
To a THF (40 mL) solution of the lithium salt of DDPH (1.13 g,
2.67 mmol) at Ϫ78 °C was added CrCl3(THF)3 (1.00 g,
2.67 mmol). The solution was allowed to warm to room temper-
ature and stirred for 3 h before the addition of the lithium salts of
either 2,4-pentanedione (0.28 g, 2.67 mmol) or dibenzoylmethane
(0.61 g, 2.67 mmol) at Ϫ78 °C. The solutions were allowed to warm
to room temperature and stirred for a further 12 h. After removal
of the solvent under reduced pressure pentane (60 mL) was intro-
duced and the lithium salts filtered. Concentration of the filtrates
to half volume and cooling to Ϫ20 °C overnight gave 5a (0.72 g,
45%) and 5b (0.99 g, 51%) as red crystals.
Preparation of [(DDP)CrCl(µ-Cl)]2 (1): CrCl3(THF)3 (1.00 g,
2.67 mmol) was loaded into a Schlenk flask containing one equiva-
lent of the lithium salt of DDPH (1.13 g, 2.67 mol) in THF (40
mL) at Ϫ78 °C. The solution was stirred overnight at room temper-
ature giving an olive green solution. The volatiles were removed
under reduced pressure and pentane (50 mL) added. Filtration of
the lithium salts gave the crude product which was recrystallised
by standing at Ϫ20 °C overnight to give 1 (1.08 g, 75%). FAB-MS:
m/z ϭ 504 [Mϩ/2]. Ϫ C58H82Cl4Cr2N4 (1081.1): calcd. C 64.44, H
7.59, N 5.19; found C 64.65, H 7.81, N 5.41. Ϫ µeff ϭ 3.60 BM.
5a: FAB-MS: m/z
{O(Me)C}2CH Ϫ Cr]. Ϫ C34H48ClCrN2O2 (604.21): calcd. C
ϭ Ϫ Ϫ Cl Ϫ
568 [Mϩ Cl], 419 [Mϩ
Preparation of [(DDP)CrMe(µ-Cl)]2 (2): A THF (40 mL) solution
of 1 was formed as above by stirring CrCl3(THF)3 (1.00 g,
2.67 mmol) and LiDDP (1.13 g, 2.67 mol) together overnight.
AlMe3 (2.5 mL, 5.0 mmol) was introduced to the green solution
with a syringe at room temperature. After stirring overnight the
solution was taken to dryness to give a dark oily residue. Extraction
with pentane (60 mL) followed by filtration gave a dark purple
solution. Concentration of the solution to half volume and
standing at Ϫ20 °C gave 2 as crimson needles (0.94 g, 68%). FAB-
MS: m/z ϭ 469 [Mϩ/2 Ϫ Me Ϫ Cl]. Ϫ C60H88Cl2Cr2N4 (1040.3):
calcd. C 69.30, H 8.47, N 5.39; found C 69.41, H 8.81, N 5.82. Ϫ
µeff ϭ 3.50 BM.
67.61, H 7.95, N 4.64; found C 67.30, H 8.01, N 4.44. Ϫ µeff
ϭ
4.06 BM.
5b: FAB-MS: m/z
ϭ Ϫ Ϫ Cl
692 [Mϩ Cl], 419 [Mϩ
Ϫ
{O(Ph)C}2CH Ϫ Cr]. Ϫ C44H52ClCrN2O2 (728.36): calcd. C 72.58,
H 7.15, N 3.85; found C 73.71, H 6.99, N 3.78. Ϫ µeff ϭ 3.80 BM.
Polymerisation Procedure: Results of Schlenk-line tests (1 bar) are
listed in Table 6; the polymerisation procedure is as follows: The
precatalyst (0.01Ϫ0.015 mmol of 1Ϫ5) was dissolved in toluene (40
mL) and the co-catalyst (MAO, DEAC or DMAC) added. The
Schlenk tube was purged with ethylene and the contents magnetic-
ally stirred and maintained under ethylene (1 bar) for the duration
of the polymerisation. After 1 h the polymerisation was terminated
by the addition of dil. HCl/MeOH. The solid polyethylene was re-
covered by filtration, washed with methanol (50 mL) and dried (va-
cuum oven at 50 °C).
Preparation of [(DDP)Cr(µ-Cl)]2 (3): A THF (40 mL) solution of 1
was formed as above by stirring CrCl3(THF)3 (1.00 g, 2.67 mmol)
and LiDDP (1.13 g, 2.67 mol) together overnight. Benzylmagnes-
ium chloride (2.67 mL, 5.34 mmol) was added to the green solution
at Ϫ78 °C with a syringe and the solution allowed to warm to
room temperature to give an apple green solution. Dioxane (2 mL)
was added to the solution after 12 h, stirred for a further 2 h and
filtered to give an apple green solution. The complex 3(THF)2·THF
(1.09 g, 71%) could be isolated as lime green crystals by cooling
the apple green THF/dioxane solution to Ϫ20 °C. Alternatively,
3·Bz-Bz (1.03 g, 65%) could be isolated as green crystals by strip-
ping the volatiles from the reaction mixture and extracting the res-
idue with pentane and cooling the solution to Ϫ20 °C.
3(THF)2·THF: C66H98Cl2Cr2N4O2 (1154.4): calcd. C 68.69, H 8.50,
N 4.86; found C 68.81, H 8.70, N 4.69. Ϫ FAB-MS: m/z ϭ 469
[Mϩ/2 Ϫ Cl Ϫ THF].
3·Bz-Bz: C72H96Cl2Cr2N4 (1192.5): calcd. C 72.54, H 8.06, N 4.70;
found C 72.71, H 8.34, N 4.69. Ϫ FAB-MS: m/z ϭ 469 [Mϩ/2 Ϫ
Cl Ϫ C6H5CH2].
X-ray Crystal Structure Determinations of 1, 2, 3(THF)2.THF, 3·Bz-
Bz, 4a, and 5b: Table 7 provides a summary of the crystal data,
data collection, and refinement parameters for compounds 1, 2,
3(THF)2·THF, 3·Bz-Bz, 4a and 5b.
The structures were solved by direct methods, and the major occu-
pancy non-hydrogen atoms were refined anisotropically using full-
matrix, least-squares based on F2. In 1, 2, 3(THF)2·THF and 4a
disorder was found in the solvent molecules; in each case two par-
tial occupancy orientations were identified with, in 1,
3(THF)2·THF and 4a, the major occupancy non-hydrogen atoms
refined anisotropically and the minor occupancy non-hydrogen
atoms refined isotropically, and in 2 all the disordered non-hydro-
gen atoms refined anisotropically. Throughout all six structures the
CϪH hydrogen atoms of methyl groups bound to sp2 centres were
Preparation of [(DDP)CrCl(O2CR)(THF)] (R ؍ Me 4a, Ph 4b): To located from ∆F maps, idealised, assigned isotropic thermal para-
a THF (40 mL) solution of the lithium salt of DDPH (1.13 g, meters [U(H) ϭ 1.5Ueq(C)] and allowed to ride on their parent
2.67 mmol) at Ϫ78 °C was added CrCl3(THF)3 (1.00 g, atoms. The remaining hydrogen atoms were placed in calculated
2.67 mmol). The solution was allowed to warm to room temper-
ature and stirred for 3 h before the addition of sodium acetate 1.2Ueq(C); U(H) ϭ 1.5Ueq(C-Me)], and allowed to ride on their
(0.22 g, 2.67 mmol) or sodium benzoate (0.38 g, 2.67 mmol) at Ϫ78 parent atoms. All computations were carried out using the
°C. The solutions were allowed to warm to room temperature and SHELXTL PC program system.[27]
positions, assigned isotropic thermal parameters [U(H) ϭ
stirred for a further 12 h. After removal of the solvent under re-
duced pressure, pentane (60 mL) was introduced and the lithium
Crystallographic data (excluding structure factors) have been de-
posited with the Cambridge Crystallographic Data Centre as sup-
salts filtered. Concentration of the filtrates to half volume and cool- plementary publication no’s: CCDC-150447Ϫ150452 for structures
ing to Ϫ20 °C overnight gave 4a (0.68 g, 45%) and 4b (0.85 g, 49%)
as green crystals.
1Ϫ5b, respectively (Table 7). Copies of the data can be obtained
free of charge on application to CCCD, 12 Union Road, Cam-
bridge CB2 1EZ, U.K. [Fax: (internat.) ϩ44-1223/336-033: E-mail:
4a: FAB-MS: m/z ϭ 563 [Mϩ Ϫ THF], 528 [Mϩ Ϫ Cl Ϫ THF], 467
[Mϩ Ϫ Cl Ϫ THF Ϫ O2CMe]. Ϫ C35H52ClN2O2 (568.26): calcd. C deposit@ccdc.cam.ac.uk].
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903
1901

V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White, D. J. Williams
Table 7. Crystal data, data collection and refinement parameters for compounds 1, 2, 3(THF)2·THF, 3·Bz-Bz, 4a and 5b[a]
FULL PAPER
1
2
3(THF)2·THF
3·Bz-Bz
4a
5b
Formula
Solvent
Molecular weight
Colour, habit
C58H82N4Cl4Cr2
C5H12
1153.2
C60H88N4Cl2Cr2
0.5C5H12
1076.3
crimson prismatic
needles
C58H82N4Cl2Cr2
3C4H8O
1226.5
C58H82N4Cl2Cr2
PhCH2CH2Ph
1192.4
C35H52N2O3ClCr
Ϫ
C44H52N2O2ClCr
Ϫ
636.2
red prismatic
needles
728.3
orange/red
prisms
black blocks
green octahedra
green blocks
Crystal size/mm
Crystal system
Space group
T/K
0.23 ϫ 0.10 ϫ 0.07 0.33 ϫ 0.17 ϫ 0.13 0.73 ϫ 0.67 ϫ 0.67 0.67 ϫ 0.50 ϫ 0.33 0.90 ϫ 0.27 ϫ 0.27 0.57 ϫ 0.40 ϫ 0.23
monoclinic
P21/c (no. 14)
203
monoclinic
C2/m (no. 12)
203
tetragonal
P42/m (no 84)
203
triclinic
P1 (no. 2)
183
monoclinic
P21/c (no. 14)
203
triclinic
P1 (no. 2)
203
¯
¯
˚
a/A
13.581(3)
13.547(3)
17.553(2)
Ϫ
19.434(3)
21.764(2)
15.098(2)
Ϫ
13.036(2)
Ϫ
12.034(1)
12.564(1)
13.637(1)
97.37(1)
114.72(1)
110.28(1)
1663.9(1)
1[b]
8.935(2)
17.001(2)
23.258(2)
Ϫ
11.140(1)
12.780(2)
14.712(3)
76.52(1)
84.97(1)
80.50(1)
2006.2(5)
2
˚
b/A
˚
c/A
20.134(2)
Ϫ
α/deg
β/deg
γ/deg
104.28(2)
Ϫ
90.58(1)
Ϫ
Ϫ
97.66(1)
Ϫ
Ϫ
3
˚
V/A
3130(1)
2[b]
6386(1)
4[c]
3421.3(7)
2[d]
3501.2(8)
4
Z
Dc/g cmϪ3
F(000)
1.224
1.120
1.191
1.190
1.207
1.206
1232
Cu-Kα
4.73
2316
Cu-Kα
3.85
1320
Mo-Kα
0.44
638
Cu-Kα
3.75
1364
Mo-Kα
0.44
774
Mo-Kα
0.39
[e]
[e]
[e]
Radiation used
µ/mmϪ1
θ range/deg
No. of unique reflections
measured
3.4Ϫ58.0
2.9Ϫ60.0
1.9Ϫ30.0
6.3Ϫ63.0
1.8Ϫ25.0
1.9Ϫ25.0
4293
4866
5101
5228
6111
6954
observed, |Fo| Ͼ 4σ(|Fo|) 2823
3370
3603
4547
3963
4296
Absorption correction
Max., min. transmission 0.58, 0.41
semi-empirical
Ϫ
Ϫ
semi-empirical
0.35, 0.21
350
semi-empirical
0.90, 0.83
383
Ϫ
Ϫ
Ϫ
Ϫ
No. of variables
353
0.067, 0.153
340
0.065, 0.164
245
0.056, 0.128
427
0.058, 0.112
[f]
R1, wR2
0.056, 0.142
0.054, 0.109
[a] Details in common: graphite monochromated radiation, ω-scans, Siemens P4 diffractometer, refinement based on F2. Ϫ [b] The molecule
[c]
has crystallographic Ci symmetry. Ϫ There are two crystallographically independent C2h symmetric molecules in the asymmetric unit.
2 2
Ϫ [d] The molecule has crystallographic C2h symmetry. Ϫ [e] Rotating anode source. Ϫ [f] R1 ϭ Σ||Fo| Ϫ |Fc||/Σ|Fo|; wR2 ϭ [Σ w(Fo2 Ϫ Fc ) /
Σw(Fo2)2]1/2; wϪ1 ϭ σ2(Fo ) ϩ (aP)2 ϩ bP.
2
C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White,
D. J. Williams, J. Chem. Soc., Dalton Trans. 1999, 827Ϫ829. Ϫ
Acknowledgments
BP-Amoco Chemicals Ltd is thanked for financial support. Drs. J.
Boyle and G. Audley are thanked for NMR and GPC measure-
ments, respectively.
[6f]
V. C. Gibson, S. Mastroianni, C. Newton, C. Redshaw, G.
A. Solan, A. J. P. White, D. J. Williams, J. Chem. Soc., Dalton
Trans. 2000, 1969Ϫ1971. Ϫ [6g]V. C. Gibson, C. Newton, G. A.
Solan (BP Chemicals Ltd.), WO 99/19335, 1999 [Chem. Abstr.
1999, 130, 297099p]. Ϫ [6h]V. C. Gibson, P. J. Maddox, C. New-
[1] [1a]
F. J. Karol, G. L. Garapinka, C. Wu, A. W. Dow, R. N.
ton, C. Redshaw, G. A. Solan, A. J. P. White and D. J. Williams,
Chem. Commun. 1998, 1651Ϫ1652. Ϫ
Fevola, L. M. Liable-Sands, A. L. Rheingold, K. H. Theopold,
Johnson, W. I. Carrick, J. Polym. Sci., Part A 1972, 10,
[6i]
W.-K. Kim, M. J.
[1b]
2621Ϫ2637. Ϫ
8, 2637Ϫ2652.
J. P. Hogan, J. Polym. Sci., Part A 1972,
[6j]
Organometallics 1998, 17, 4541Ϫ4543. Ϫ
R. D. Köhn, M.
[2] [2a]
[2b]
M. P. McDaniel, Adv. Catal. 1985, 33, 47Ϫ98. Ϫ
N. Ajjou, S. L. Scott, V. Paquet, J. Am. Chem. Soc. 1998,
J. A.
Haufe, S. Mihan and D. Lilge, Chem. Commun. 2000,
1927Ϫ1928.
120, 415Ϫ416.
[7] [7a]
B. Qian, W. J. Scanlon, M. R. Smith, III, Organometallics
[3]
For recent reviews, see: K. H. Theopold, Chem. Eur. J. 1998,
3, 15Ϫ24; K. H. Theopold, CHEMTECH 1997, 27, 26Ϫ32.
[7b]
1999, 18, 1693Ϫ1698. Ϫ
P. H. M. Budzelaar, A. B. van
Oort, A. G. Orpen, Eur. J. Inorg. Chem. 1998, 1485Ϫ1494. Ϫ
[4] [4a]
R. Emrich, O. Heinemann, P. W. Jolly, C. Krüger, G. P. J.
[7c]
M. Rahim, N. J. Taylor, S. Xin, S. Collins, Organometallics
[4b]
Verhovnik, Organometallics 1997, 16, 1511Ϫ1513. Ϫ
Gohre, P. W. Jolly, B. Kryger, J. Rust, G. P. J. Verhovnik, Or-
ganometallics 2000, 19, 388Ϫ402. Ϫ
germund, P. W. Jolly, K. J. Borve, Organometallics 2000, 19,
D. A.
[7d]
1998, 17, 1315Ϫ1323. Ϫ
M. F. Lappert, D. S. Liu, J. Or-
ganomet. Chem. 1995, 500, 203Ϫ217. Ϫ [7e] P. B. Hitchcock, M.
[4c]
V. R. Jensen, K. An-
F. Lappert, S. D. Liu, J. Chem. Soc., Chem. Commun. 1994,
2637. Ϫ
[7f]
D. S. Richeson, J. F. Mitchell, K. H. Theopold,
403Ϫ410.
[7g]
Organometallics 1989, 8, 2570Ϫ2577. Ϫ
J. E. Bercaw, D. L.
[5]
G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem. Int.
Ed. 1999, 38, 428Ϫ447, and references therein.
Davies, P. T. Wolczanski, Organometallics 1986, 5, 443Ϫ450.
[8a] P. H. M. Budzelarr, R. de Gelder, A. W. Gal, Organometal-
[8]
[6] [6a]
F. J. Feher, R. L. Blanski, J. Chem. Soc., Chem. Commun.
[8b]
1990, 1614Ϫ1616. Ϫ [6b] M. P. Coles, V. C. Gibson, Polym. Bull.
1994, 33, 529Ϫ533. Ϫ
son, W. Clegg and M. R. J. Elsegood, J. Chem. Soc., Chem.
lics 1998, 17, 4121Ϫ4123. Ϫ
Parthasarathy, W. J. Marshall, J. C. Calabrese, S. D. Arthur,
J. Feldman, S. J. McLain, A.
[6c]
M. P. Coles, C. I. Dalby, V. C. Gib-
Organometallics 1997, 16, 1514Ϫ1516.
[6d]
[9] [9a]
Commun. 1995, 1709Ϫ1711. Ϫ
C. Gibson, I. R. Little, E. L. Marshall, M. H. R. da Costa, S.
Mastroianni, J. Organomet. Chem. 1999, 591, 78Ϫ87. Ϫ
1902
M. P. Coles, C. I. Dalby, V.
F. Cosledan, P. B. Hitchcock, M. F. Lappert, Chem. Com-
[9b]
mun. 1999, 705Ϫ706. Ϫ
F. Jordan, J. Am. Chem. Soc. 1998, 120, 9384Ϫ9385. Ϫ
C. E. Radzewich, M. P. Coles, R.
[6e]
[9c]
V.
B.
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903

Low Valent β-Diketiminato Chromium Complexes
FULL PAPER
M. D. Fryzuk, D. B. Leznoff, S. J.
[18c]
Qian, D. L. Ward, M. R. Smith, III, Organometallics 1998, 17,
3070Ϫ3076. Ϫ [9d] M. Cheng, E. B. Lobkovsky, G. W. Coates, J.
Am. Chem. Soc. 1998, 120, 11018Ϫ11019. Ϫ [9e] V. C. Gibson, J.
A. Segal, A. J. P. White, D. J. Williams, J. Am. Chem. Soc. 2000,
122, 7120Ϫ7121.
1990, 168, 3Ϫ4. Ϫ
Rettig, R. C. Thompson, Inorg. Chem. 1994, 33, 5528Ϫ5534.
[18d]
Ϫ
R. A. Hermtz, R. L. Ostrander, A. L. Rheingold, K.
[18e]
H. Theopold, J. Am. Chem. Soc. 1994, 116, 11387. Ϫ
B.
Bachmann, J. Heck, F. Hahn, W. Massa, J. Pebler, Z. Anorg.
Allg. Chem. 1995, 621, 2061Ϫ2064.
[10]
D. Dress, J. Magull, Z. Anorg. Allg. Chem. 1994, 620, 814Ϫ818.
[19]
[11] [11a]
H. R. Rogers, C. L. Hill, Y. Fujiwara, R. J. Rogers, H. L. Mit-
chells, G. M. Whitesides, J. Am. Chem. Soc. 1980, 102,
217Ϫ226.
S. G. McGeachin, Can. J. Chem. 1968, 46, 1903Ϫ1912. Ϫ
[11b] J. E. Parks, R. H. Holm, Inorg, Chem. 1968, 7, 1408Ϫ1416.
Range for CrIII···CrIII distances for Cr(µ-Cl)2Cr complexes:
[12]
[20] [20a]Y. Liang, G. P. A. Yap, A. L. Rheingold, K. H. Theopold,
[13Ϫ15]
˚
3.64Ϫ3.23 A. See, for example, refs.
[20b]
Organometallics 1996, 15, 5284Ϫ5286. Ϫ
P. T. Greene, B.
[13] [13a]
R. A. Heintz, S. Leelasubcharoen, L. M. Liable-Sands, A.
[20c]
J. Russ, J. S. Wood, J. Chem. Soc. A 1971, 3636Ϫ3638. Ϫ
L. Rheingold, K. H. Theopold, Organometallics 1998, 17,
M. D. Fryzuk, D. B. Leznoff, S. J. Rettig, Organometallics 1997,
[13b]
5477Ϫ5485. Ϫ
D. S. Richeson, S.-W. Hsu, N. H. Fredd,
[20d]
16, 5116Ϫ5119. Ϫ
F. A. Cotton, J. Czuchajowska, L. R.
G. Van Duyne, K. H. Theopold, J. Am. Chem. Soc. 1986, 108,
Falvello, X. Feng, Inorg. Chim. Acta 1990, 172, 135Ϫ136.
M. Cowie, M. D. Gauthier, S. J. Loeb, I. R. McKeer, Organo-
metallics 1983, 2, 1057Ϫ1059.
F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry 5th
ed.; Wiley: New York, 1988; p 689.
[13c]
8273Ϫ8274.
Ϫ
W. A. Hermann, W. R. Thiel, E.
[21]
[22]
[13d]
Herdtweck, J. Organomet. Chem. 1988, 353, 323Ϫ336. Ϫ
G. Bhandari, Y. Kim, J. M. McFarland, A. L. Rheingold, K.
H. Theopold, Organometallics 1995, 14, 738Ϫ745. Ϫ
Cotton, J. L. Eglin, R. L. Luck, K.-A. Son, Inorg. Chem. 1990,
[13e]
F. A.
[23]
[24]
C. J. Cairns, D. H. Busch, Coord. Chem. Rev. 1986, 69, 1Ϫ55.
C. Redshaw, G. Wilkinson, B. Hussain-Bates, M. B. Hurst-
house, J. Chem. Soc., Dalton Trans. 1992, 1803Ϫ1811 and
refs. therein.
29, 1802Ϫ1806.
O. Heinemann, P. W. Jolly, C. Krüger, G. P. J. Verhovnik, J.
Organomet. Chem. 1998, 553, 477Ϫ479.
F. H. Koehler, R. de Cao, K. Ackermann, J. Sedlmaier, Z. Nat-
urforsch., Teil B 1983, 38, 1406Ϫ1411.
[14]
[15]
[16]
[17]
[25] [25a]
[25b]
C. J. O’Connor, Prog. Inorg. Chem. 1982, 29, 203. Ϫ
Handbook of Chemistry and Physics (Ed.: R. C. Weast), CRC
Press, Boca Raton, FL, 70th edn., 1990, p. E134.
J. Shamir, Inorg. Chim. Acta 1989, 156, 163Ϫ164.
SHELXTL PC version 5.03, Siemens Analytical X-ray Instru-
ments, Inc., Madison, WI, 1994.
P. N. Jagg, P. F. Kelly, H. S. Rzepa, D. J. Williams, J. D. Wool-
lins, W. Wylie, J. Chem. Soc., Chem. Commun. 1991, 942Ϫ944.
Range for CrII···CrII distances for Cr(µ-Cl)2Cr complexes:
[26]
[27]
[18]
˚
3.817Ϫ2.642 A. See ref.
[18] [18a] A. R. Hermes, G. S. Girolami, Inorg. Chem. 1988, 27, 1775.
Received November 20, 2000
Ϫ
[18b] F. A. Cotton, R. L. Luck, K.-A. Son, Inorg. Chim. Acta
[I00438]
Eur. J. Inorg. Chem. 2001, 1895Ϫ1903
1903