Electrophilic activation and the formation of an unusual Tl +/Cr3+ tetranuclear ion-complex adduct

Article abstract of DOI:10.1039/c0dt00810a

Reaction of TlPF₆ with the Cr(iii) complex fac-[CrCl ₃(NPN)] (NPN = bis(2-picolyl)phenyl phosphine) did not lead to precipitation of TlCl but rather to addition of the Tl⁺ cation to the CrCl₃ moiety, which resulted in a pseudo-dimeric adduct which has an unusual tetranuclear centrosymmetric structure in the solid state.

Full text of DOI:10.1039/c0dt00810a

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www.rsc.org/dalton | Dalton Transactions
Electrophilic activation and the formation of an unusual Tl+/Cr3+
tetranuclear ion-complex adduct†
Shaofeng Liu,a,b Riccardo Peloso,†a Roberto Pattacinia and Pierre Braunstein*a
Received 8th July 2010, Accepted 16th July 2010
First published as an Advance Article on the web 3rd August 2010
DOI: 10.1039/c0dt00810a
Reaction of TlPF6 with the Cr(III) complex fac-[CrCl3(NPN)]
(NPN = bis(2-picolyl)phenyl phosphine) did not lead to
precipitation of TlCl but rather to addition of the Tl+ cation to
the CrCl3 moiety, which resulted in a pseudo-dimeric adduct
which has an unusual tetranuclear centrosymmetric structure
in the solid state.
chloride anion from 1 in the absence of a coordinating solvent
was expected to induce the formation of a cationic, Cl-bridged
dinuclear complex of the type generated in catalytic ethylene
oligomerization from similar Cr(III) complexes and AlEt2Cl.45
A
mononuclear cationic Cr(III) complex would most likely result
from reaction with TlPF6 in the presence of a donor solvent.46
We thus reacted complex 1 with one equivalent of TlPF6 in
dichloromethane and observed a colour change from blue to
purple. After filtration, a purple solid was isolated which showed
in the FTIR spectrum a very strong absorption at 880 cm-1 due to
The strong propensity of TlPF6 to react with halide-containing
coordination and organometallic complexes with the formation
of cationic complexes and poorly soluble TlX salts1 is often
exploited to enhance the electrophilic properties of the metal
centre and hence its reactivity.2–5 Moreover, compared with
analogous reagents such as AgPF6,6 TlPF6 does not readily
undergo redox reactions and is therefore particularly convenient
in inorganic and organometallic synthesis. However, coordination
of a Tl+ cation to a metal-bound halide has been reported
in some cases.7–18 We recently found that the reaction of the
-
the P–F stretching vibration of the PF6 anion.
Single crystals of this purple compound, 2, were grown by
layering Et2O on a concentrated acetone solution of the complex
and were analysed by X-ray diffraction. It is a bimetallic adduct
which exhibits the unexpected structure depicted in Fig. 1.
The crystal formula is [CrCl3(NPN)TlPF6], corresponding to a
1 : 1 stoichiometry between 1 and TlPF6. In the solid state, 2
is best described as a centrosymmetric tetranuclear compound
2
Pt(II) complex [Pt(CH2Ph)Cl(PCH2-ox)] (PCH2-ox = k -P,N-
containing two Tl+ cations, two PF6 anions, and two neutral fac-
-
(oxazolinylmethyl)diphenylphosphine) with TlPF6 selectively af-
forded an unexpected bimetallic complex containing a new benzyl-
stabilized Tl–Pt bond while retaining the Pt–Cl bond.19 In view of
the considerable interest for the study of Cr-based homogeneous
catalysts for ethylene oligomerization and polymerization,20–42 trig-
gered by the pioneering discoveries at Union Carbide20 and Phillips
Petroleum,21 we decided to prepare Cr(III) complexes with hybrid
ligands and during attempts to form a cationic complex by chloride
abstraction, we unexpectedly obtained a novel Cr(III)/Tl(I) ion-
complex adduct exhibiting unusual interactions involving the Tl+
cation of possible relevance to catalyst activation steps.
[CrCl3(NPN)] complexes (Fig. 2).
The 1 : 1 reaction between [CrCl3(THF)3] and bis(2-
picolyl)phenylphosphine (NPN)43 afforded an air-stable, blue
complex in high yield.§ It was characterized by IR spectroscopy,
elemental analysis and X-ray diffraction and identified as the
mononuclear complex fac-[CrCl3(NPN)] (1). ¶Its crystal structure
has been determined (see the ESI†) for comparison with that of
the acetonitrile adduct [CrCl3(NPN)]·MeCN.44 Abstraction of a
Fig. 1 ORTEP of complex 2 (asymmetric unit) with ellipsoids drawn
at the 50% probability level. Selected bond distances (A) and angles
˚
(◦): Cr1–N1 2.122(5), Cr1–N2 2.124(5), Cr1–P1 2.3512(16), Cr1–Cl1
2.3628(16), Cr1–Cl2 2.3106(17), Cr1–Cl3 2.3280(16), Tl1–Cl1 3.1816(15),
Tl1–Cl2 3.1519(15), Tl1–Cl3 3.1381(15), Tl1–F4 3.258(6), Tl1–F5
3.223(5), Cr1–Cl1–Tl1 84.20(5), Cr1–Cl2–Tl1 85.72(5), Cr1–Cl3–Tl1
85.76(5).
aLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS), Universite´ de Strasbourg, 4 rue Blaise Pascal, F-67081, Strasbourg
Cedex, France. E-mail: braunstein@unistra.fr; Fax: (+33) 3-6885-1322
bKey Laboratory of Engineering Plastics and Beijing National Laboratory for
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190, China
Each Tl cation shows contacts with: a) three chlorides from
one molecule of 1 [Tl ◊ ◊ ◊ Cl distances: 3.1816(15), 3.1519(15) and
˚
3.1381(15) A]; b) two chlorides from the centrosymmetrically
† Electronic supplementary information (ESI) available: ORTEP plots and
experimental details. CCDC reference numbers 771851–771852. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00810a
‡ Present address: Instituto de Investigaciones Qu´ımicas – Departamento
de Qu´ımica Inorga´nica, Universidad de Sevilla – Consejo Superior de
Investigaciones Cient´ıficas, Avda. Ame´rico Vespucio 49, Isla de la Cartuja,
41092 Sevilla, Spain
˚
generated 1 [3.303(14) and 3.407(15) A]; and c) with two fluorines
˚
from the PF6 anion [3.258(6) and 3.223(5) A]. In addition, the
pseudo-dimer interacts with adjacent molecules of 1, through long
˚
intermolecular Tl ◊ ◊ ◊ C contacts [shortest: 3.459(7) A].
This pseudo-dimer
2 can be thus described, in the
solid state, as a tetranuclear, bimetallic Cr2Tl2 ion-complex,
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Fig. 2 View of the crystal structure of 2, showing the centrosymmetric
pseudo-dimer [1·(TlPF6)]2. Intermolecular Ph–P fragments interacting
with the pseudo-dimer are depicted in black. (Symmetry operation
generating equivalent atoms: -x, -y, -z).
Fig. 3 Comparison between the structures in the crystals of 1 (blue)
in 2 (red) illustrating the slight closing of the CrCl3 cone angle in the
fac-[CrCl3(NPN)] moiety of the latter. The P and Cr atom pairs are
coincident. For 2, the thallium cation is also shown.
containing a Cr2Tl2Cl6 core, showing four pseudo m3-Cl and two
pseudo m-Cl ligands. The Cr2Tl2Cl6 core shares similarities with
that of [(CpPhCrCl3)2Tl2] (CpPh = pentaphenyl-cyclopentadienyl),
prepared by addition of TlCl to [(CpPhCr(m-Cl)Cl]2.17 The
[(CpPhCrCl3)2Tl2] structure showed Tl ◊ ◊ ◊ Cl separations similar to
those in 2 and it was further stabilized by Tl ◊ ◊ ◊ ClCH2Cl (from
dichloromethane) and Tl ◊ ◊ ◊ C (from one of the phenyl groups)
contacts, in parallel to the Tl ◊ ◊ ◊ F2PF4 and Tl ◊ ◊ ◊ C ones observed
in 2.
Our results emphasize the care to be exerted when using
common halide abstractors to generate in situ reactive species
since they may not always fulfil their anticipated function. The
fact that the Tl+ cation forms bimetallic compounds instead
of removing a halide anion with precipitation of TlX may be
explained by the formation of multiple stabilizing interactions
between the Tl+ cation and its direct environment. This applies
to both stoichiometric and catalytic applications. Interestingly, a
Cr–Cl–Al linkage was recently evidenced in an adduct of AlMe3
to a [Cr(NNN)MeCl2] complex and was suggested to represent an
early stage of the catalyst activation.39
We thank the Centre National de la Recherche Scientifique
(CNRS), the Ministe`re de l’Enseignement Supe´rieur et de la
Recherche (Paris) and the Universite´ de Strasbourg for support,
the French Embassy in Beijing for a doctoral grant to S. L. and
Prof. Wen-Hua Sun (Institute of Chemistry, Chinese Academy
of Sciences, Beijing) for support. We are grateful to Mr Marc
Mermillon-Fournier for technical assistance.
Three of the Tl ◊ ◊ ◊ Cl distances shown in Fig. 1 compare well
with the interionic distance found in the NaCl form of TlCl
˚
(3.15 A), while the remaining two are similar to that in its CsCl
18
˚
form (3.32 A). The Tl ◊ ◊ ◊ F separations are slightly longer than
47
˚
those observed in TlPF6 [3.18(4) A]. The Tl ◊ ◊ ◊ C contacts are
slightly shorter than the sum of the van der Waals radii of Tl+ and
an aromatic ring carbon (3.70 A).15,48,49 The Tl ◊ ◊ ◊ Cr separation of
˚
˚
3.7665(8) A is too long for a direct metal–metal interaction.
Although the structure of the fac-[CrCl3(NPN)] unit in 2 is sim-
ilar to that of pure 1, interesting differences are observed (Fig. 3).
Along with slight changes in the NPN ligand arrangement, the
three chloride atoms in 2 are deviate slightly toward the Tl+ cation,
consistent with an attractive interaction (see the ESI for a detailed
comparison†).
Notes and references
§ Synthesis of fac-[CrCl3(NPN)] (1). Solid [CrCl3(THF)3] (0.438 g,
1.17 mmol) was added to a solution of NPN (0.342 g, 1.17 mmol) in
dichloromethane (20 mL) to give a blue suspension, which was stirred
overnight. The solvent was removed under vacuum affording a blue powder
which was washed with diethyl ether (2 ¥ 20 mL) and dried in vacuo
overnight (Yield: 0.508 g, 1.12 mmol, 96%). Selected IR absorptions (pure,
diamond ATR): 1606 s, 1563 w, 1485 s, 1436 s, 1380 w, 1314 w, 1264 w,
1163 w, 1110 m, 1061 w, 1020 m, 864 s, 808 w, 767 m, 743 m, 689 m cm-1.44
Synthesis of fac-[CrCl3(NPN)TlPF6] (2). Solid TlPF6 (0.116 g, 0.333 mmol)
was added to a blue suspension of fac-[CrCl3(NPN)] (0.150 g, 0.333 mmol)
in dichloromethane (10 mL). A purple suspension slowly formed, which
was stirred overnight. The volume of the solution was reduced to 5 mL
by evaporation under reduced pressure. After filtration, a purple solid was
obtained (0.260 g, 0.325 mmol, 97.6%). Selected IR absorptions (pure,
diamond ATR): 1611 m, 1570 w, 1487 m, 1438 m, 1373 w, 1317 w, 1171
w, 1111 w, 1066 w, 1026 m, 881 vs, 789 w, 752 m, 692 w cm-1. Anal. Calcd
for C18H17Cl3CrF6N2P2Tl: C, 27.02; H, 2.14; N, 3.50. Found: C, 27.22; H,
2.57; N, 3.44.
To the best of our knowledge, the bimetallic complex 2
is the first reported adduct of TlPF6 to a chromium com-
plex. It is stable in dichloromethane, but slowly converts into
unidentified products in acetone or acetonitrile with precipi-
2+
tation of TlCl. Similarly, [(P(CH2C2PPh2)3RuH(m-Cl)Tl]2 and
2+
[([9]aneS3)(PPh3)Ru(m-Cl)2Tl]2 were found to decompose in O-
coordinating solvents.8,9
After activation with methylaluminoxane (MAO), 1 was active
for ethylene oligomerization (2900 g g(Cr)-1·h-1), producing a
mixture of butenes (12%; of which 99% a-butene), hexenes (22%;
88% a-hexene), octenes (17%; 87% a-octene), decenes (9%; 72%
a-decene) and higher a-olefins. Similar activities and product
distribution were observed for the 2/MAO system, suggesting that
under these catalytic conditions, the Cr(III)/Tl(I) pseudodimer 2
may not be persistent.ꢀ
¶ Crystal data for 1: C18H17Cl3CrN2P, M = 450.66, monoclinic, a =
◦
˚
˚
˚
14.2128(5) A, b = 13.4012(5) A, c = 21.8650(7) A, b = 113.705(2) ,
7882 | Dalton Trans., 2010, 39, 7881–7883
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3
-1
˚
V = 3813.2(2) A , T = 173(2) K, space group P21/c, Z = 8, m = 1.108 mm ,
15041 reflections measured, 8300 independent reflections (Rint = 0.0608).
The final R1 values were 0.0483 (I > 2s(I)). The final wR(F2) values
were 0.1057 (I > 2s(I)). The final R1 values were 0.1178 (all data). The
final wR(F2) values were 0.1233 (all data). The goodness of fit on F2 was
0.954. Crystal data for 2: C18H17Cl3CrN2P·F6P·Tl, M = 800.00, triclinic,
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◦
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