Synthesis and molecular structure of the tetranuclear μ4-oxo-centered chromium complex [Cr4(CH2CH2CH2NMe2) 4(μ4-O)(μ-Cl)6]

Article abstract of DOI:10.1016/S0022-328X(98)01091-2

The complex [CrCl₃(THF)₃] reacts with Li(CH₂CH₂CH₂NMe₂) in THF at room temperature (r.t.) to form the tetranuclear complex [Cr₄(CH₂CH₂CH₂NMe₂) ₄(μ₄-O)(μ-Cl)₆] (1) where each chromium atom has a distorted octahedral coordination geometry with an oxygen atom shared among the four metal ions.

Full text of DOI:10.1016/S0022-328X(98)01091-2

Journal of Organometallic Chemistry 580 (1999) 56–59
Synthesis and molecular structure of the tetranuclear m -oxo-centered
4
chromium complex [Cr (CH CH CH NMe ) (m -O)(m-Cl) ]
4
2
2
2
2 4
4
6
Franco Ossola *, Franco Benetollo
Istituto di Chimica e Tecnologie Inorganiche e dei Materiali A6anzati, Area della Ricerca del CNR, Corso Stati Uniti 4, I-35127 Pado6a, Italy
Received 9 September 1998
Abstract
The complex [CrCl (THF) ] reacts with Li(CH CH CH NMe ) in THF at room temperature (r.t.) to form the tetranuclear
3
3
2
2
2
2
complex [Cr (CH CH CH NMe ) (m -O)(m-Cl) ] (1) where each chromium atom has a distorted octahedral coordination geometry
4
2
2
2
2 4
4
6
with an oxygen atom shared among the four metal ions. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Tetrachromium complexes; Synthesis; Crystal structure
1
. Introduction
m -oxo-centered complex [Cr (CH CH CH NMe ) (m -
4
4
2
2
2
2 4
4
O)(m-Cl) ] (1), whose synthesis and molecular structure
6
We are involved in the synthesis of organochromium
are reported here.
derivatives having alkyl ligands with terminal Lewis
basic group, as 3-(dimethylamino)propyl [1]. These
groups can behave as bidentate ligands and allow in-
tramolecular coordinative saturation for the metal, with
nitrogen as electron donor, increasing the stability of
the complex [2].
2. Results and discussion
The reaction between [CrCl (THF) ] and LiL in THF
3
3
at r.t. gave the compound 1. Compound 1 crystallizes
from THF/hexane solutions in the form of emerald
green prisms. Its molecular structure is depicted in Fig.
1. The molecule has 2-fold crystallographic symmetry
Recently we found that [CrCl (THF) ] reacted with
3
3
the tetranuclear Grignard reagent [LMg Cl (THF) ]
2
3
2 2
(
2) (L=CH CH CH NMe ) affording the trinuclear
2
2
2
2
mixed-metal compound [{L Cr(m-Cl) } Mg] (3) [1]. The
2
2 2
and appears as a cage built around the central m -oxy-
4
molecular structure of 3 showed an unusual Cr–Mg–
Cr core where the central Mg atom was chloride-
bridged to two L Cr(m-Cl) fragments in opposite
gen, which lies on the crystallographic C2 axis and
exhibits a distorted tetrahedral geometry being linked
to four chromium atoms (Cr(1)–O–Cr(2) 106.7(1)°,
2
2
positions. The stability of this core, and ease of forma-
tion of 3, seems to preclude the use of 2 in the synthesis
of other different 3-(dimethylamino)propyl chromium
derivatives not containing Mg [3]. Consequently we
decided to replace 2 with the corresponding [3-
(
a)
(a)
Cr(2)–O–Cr(1)
3.6(1)°; (a) at 1−x, −y, z).
The cage defined by Cr and Cl atoms is a distorted
130.8(1)° and Cr(1)–O–Cr(1)
9
quadratic antiprism formed by the two rectangular
faces Cr(m-Cl) Cr. In both rectangles each chlorine is
2
(
dimethylamino)propyl]lithium salt LiL [4]. The reac-
asymmetrically bridged to the corresponding metal cen-
ter (Cr(1)···Cl(1) and Cr(2)···Cl(3)), while the Cr–Cl
bond distances involving the chlorine bridging the two
rectangular faces of the antiprism have equal distances
tion of [CrCl (THF) ] with LiL gives the tetranuclear
3
3
*
Corresponding author. Fax: +39-49-8702911.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01091-2

F. Ossola, F. Benetollo / Journal of Organometallic Chemistry 580 (1999) 56–59
57
˚
(
Cr(1)···Cl(2) 2.383(3) and Cr(2)···Cl(2) 2.385(3) A,
the unexpected m -oxygen can be only matter of
4
respectively).
speculation.
Each metal atom has a distorted octahedral coordi-
nation geometry formed by the central oxygen, three
chlorine atoms and one bidentate L ligand.
To our knowledge, only two other structurally char-
acterized examples of oxo-centered tetranuclear
chromium complexes exist: the organometallic Cr(II)
compound [Cr (C H -o-CH NMe ) (O CCH ) O] [5],
Compound 1 shows a remarkable stability. Crystals
can be treated with acidic solutions without any sign of
decomposition. In addition, thermogravimetric studies
shows 1 is stable in air up to 220°C; an exothermic
decomposition follows (225 and 375°C) giving stable
Cr O at 450°C (weight loss=61.15%; calc. for Cr O
2
3
2
3
4
6
4
2
2 4
2
3 2
6
1.08%).
Reactions of LiL with [CrCl (THF) ] and
where the oxo-ligand connects two Cr–Cr bonded frag-
ments, and the inorganic Cr(III) derivative
3
3
[
Cr(CH )Cl (THF) ], with a LiL:[Cr-reagent] ratio 2:1
3 2 3
[
Ph P] [Cr O(m -SO ) (m-Cl) Cl ]·MeCN [6]. The Cr–O
4
3
4
3
4 2
5
4
have been carried out, but elemental analysis showed
that in no case were the isolated products simple
CrL X-type complexes (X=Cl, CH ).
bonds of 1 have a mean length of 2.026(8) comparable
to the averaged value of 2.001(1) A found in [Cr (C H -
˚
4
6
4
2
3
o-CH NMe ) (O CCH ) O] for the m -O moiety and to
2
2 4
2
3 2
4
˚
the 1.987(2) and 2.004(8)
A
values found in
[
Ph P] [Cr O(m -SO ) (m-Cl) Cl ]·MeCN. The symmet-
4
3
4
3
4 2
5
4
˚
3. Experimental
ric distances Cr–Cl of 1 (averaged values 2.384(3) A)
compare to the average value of 2.388(5) A reported for
the bridging chlorine in the latter complex.
˚
3.1. General comments
At first sight, the most plausible source of the unex-
All operations were carried out in nitrogen-filled
pected m -oxygen could be inadvertent admission of
4
moisture or hydroxide ions, but we have to consider
that all the procedures have been carried out under
strictly anaerobic conditions and synthesis is reproduci-
bile, consequently it is improbable that oxygen incorpo-
ration could derive from atmospheric moisture;
glove boxes with the rigorous exclusion of oxygen and
moisture. Solvents were purified by standard methods
(THF by sodium-potassium/benzophenone procedure)
and used immediately after distillation [10]. 3-
(Dimethylamino)propyl chloride was obtained from the
commercially available HCl salt by treatment with
sodium hydroxide solution and extraction and dried
moreover, oxygen-containing impurities in [CrCl3-
(
THF) ] and LiL are very low (see Section 3). A possi-
3
ble source of oxygen could be THF, in fact, reactions
with oxygenated impurities present in the solvent, not
eliminable by standard purification procedures, and
reactions with THF leading to oxygen incorporation
are possible [7–9]. However, at this stage, the source of
over potassium carbonate. The complex [Cr(CH )-
3
Cl (THF) ] was synthesized following the procedure of
2
3
Ref. [11]. Thermogravimetric data in air were obtained
on a Netzsch STA 429 thermoanalytical equipment
3
−1
(
flux rate 200 cm min ) reference material Al O . C,
2 3
H, N and O elemental analyses were performed with a
Fisons EA 1108 (CHNS-O version) elemental analyzer.
Since commercial [CrCl (THF) ] was not pure and
3
3
the samples of [CrCl (THF) ] we obtained according to
3
3
literature procedures [12] were contaminated by a black
decomposition product generated during the high-tem-
perature extraction, we slightly modified the synthesis
reported [12]. In fact, we found that zinc-catalyzed
conversion of anhydrous chromium trichloride into its
tetrahydrofuranate also occurs at r.t. After 4 days of
stirring a dry-THF suspension of anhydrous CrCl3
(
99%) and zinc dust (ca. 50:1), the conversion to
[
CrCl (THF) ] was complete. Filtration of the solution
3
3
from zinc and evaporation of THF gave [CrCl (THF) ].
3
3
LiL was synthesized by reaction of lithium powder
with 3-(dimethylamino)propyl chloride according to
Ref. [4]. Purification by double recrystallization from
hexane-cooled solutions was performed (Anal. Calc. for
C H NLi: C, 64.52; H, 12.90; N, 15.05. Found: C,
5
12
Fig. 1. Molecular structure and atomic labelling scheme for
6
4.13; H, 12.74; N, 14.83%; oxygen determination was
[
Cr (CH CH CH NMe ) (m -O)(m-Cl) ]. The hydrogen atoms have
4
2
2
2
2 4
4
6
been omitted for clarity.
also carried out in order to exclude the presence of

58
F. Ossola, F. Benetollo / Journal of Organometallic Chemistry 580 (1999) 56–59
Table 1
Selected
O-containing species in LiL, no oxygen was found).
The compound was found pure on NMR grade.
˚
bond
lengths
(A)
and
angles
(°)
for
[
Cr (CH CH CH NMe ) (m -O)(m-Cl) ]
4
2
2
2
2 4
4
6
3
.2. Synthesis of 1
Bond lengths
Cr(1)–Cl(1)
Cr(1)–Cl(2)
Cr(2)–Cl(3)
Cr(1)–O
2.385(3) Cr(1)–Cl(1)a
2.383(3) Cr(2)–Cl(2)
2.388(3) Cr(2)–Cl(3)
2.020(8) Cr(2)–O
2.19(1) Cr(1)–C(3)
2.20(1) Cr(2)–C(8)
2.609(3)
2.385(3)
2.613(3)
2.033(8)
2.09(1)
A solution of LiL (317 mg, 3.41 mmol) in 10 ml of
a
THF was added dropwise at r.t. to a solution of
CrCl (THF) ] (1258 mg, 3.36 mmol) in 30 ml of THF
[
3
3
Cr(1)–N(1)
Cr(2)–N(2)
while stirring. The resulting solution was stirred for 24
h. Emerald green crystals of 1 slowly grew (ca. 2 weeks)
on the walls of the vessel by repeatedly placing thin
layers of hexane over the THF solution and allowing
the layers to mix by slow diffusion. Crystals were
separated from the deep green solution and analyzed
2.08(1)
Bond angles
N(1)–Cr(1)–C(3)
O–Cr(1)–N(1)
Cl(2)–Cr(1)–N(1)
Cl(1)–Cr(1)–C(3)
Cl(1)–Cr(1)–O
N(2)–Cr(2)–C(8)
O–Cr(2)–N(2)
Cl(3)–Cr(2)–N(2)
Cl(2)–Cr(2)–C(8)
Cl(2)–Cr(2)–O
Cr(1)–Cl(2)–Cr(2)
Cr(2)–Cl(3)–Cr(2)
83.3(4)
170.9(3)
95.9(3)
94.2(3)
86.3(1)
82.3(4)
171.6(3)
94.3(3)
89.6(3)
83.5(1)
86.0(1)
72.3(1)
130.8(1)
O–Cr(1)–C(3)
Cl(2)–Cr(1)–C(3)
Cl(2)–Cr(1)–O
Cl(1)–Cr(1)–N(1)
Cl(1)–Cr(1)–Cl(2)
O–Cr(2)–C(8)
Cl(3)–Cr(2)–C(8)
Cl(3)–Cr(2)–O
Cl(2)–Cr(2)–N(2)
Cl(2)–Cr(2)–Cl(3)
Cr(1)–Cl(1)–Cr(1)
Cr(1)–O–Cr(2)
105.8(3)
90.0(3)
83.8(1)
93.7(3)
169.9(1)
106.0(4)
94.4(3)
86.4(1)
95.5(3)
169.8(1)
72.1(1)
106.7(1)
93.6(1)
(
1
156 mg, 0.20 mmol, yield 24%; significant amounts of
and increases in yield can be obtained by further
recrystallizations). Compound 1 decomposes at 220°C.
IR (KBr pellets): 1634(m), 1468(s), 1168(m), 1008(m),
64(m), 774(m), 563(m), 530(s) cm . Vis umax (THF):
51, 453 nm.
−
1
9
6
a
a
Anal. Calc. for C H N OCl Cr : C, 30.73; H, 6.15;
a
a
2
0
48
4
6
4
Cr(1)–O–Cr(2)
Cr(1)–O–Cr(1)
N, 7.17; Cl, 27.27%. Found: C, 31.54; H, 6.42; N, 7.10;
Cl, 27.35%.
a
At 1−x, −y, z.
3
.3. Crystal data and structure determination
with SHELXL-93 [16]; drawings were produced using
ORTEP II [17]. Selected bond lengths and angles are
given in Table 1.
[
Cr (CH CH CH NMe ) (m -O)(m-Cl) ], C H N -
4
2
2
2
2 4
4
6
20 48
4
OCl Cr , M =781.3, green prism (0.44×0.48×0.20
6
4
r
mm) orthorhombic, space group Aba2, a=12.949
˚
(
3), b=12.507(3), c=20.269(5) A, v(Mo–K )=18.0
Acknowledgements
a
−
1
cm , F(000)=1608, T=293 K.
A prismatic crystal was lodged in a Lindemann glass
capillary inside a drybox and centered on a four-circle
Philips PW1100 (Febo System) [13]. The orientation
matrix and cell dimensions were determined by least
squares refinement of the angular positions of 30 reflec-
tions. Three standard reflections were monitored every
We are grateful to N. Brianese for fruitful discussions
and to A. Moresco for elemental analyses.
References
2
00 reflections. There were no significant fluctuations of
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2
o
2 2
c
2
2
2
scheme
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o
2
2
analogously, the reaction of [CrCl (THF) ] with
2
3
3
o
c
(
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3
3
achieved. Furthermore, the reaction of [Cr(CH )Cl (THF) ] with
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2
3
2
([Cr(CH )Cl (THF) ]:2 ratio=1:0.5) in THF at −40°C af-
3 2 3
(
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ent carbon atom). Structure refinement was carried out
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