A crystalline σ complex of copper

Article abstract of DOI:10.1021/ja200789z

Over the last decades, our understanding of σ-bond activation at transition metals has progressed considerably from both fundamental and synthetic points of view thanks to the preparation and characterization of a variety of σ complexes. Here we report the synthesis and structural analysis of the first σ complex involving a coinage metal. The copper(I) complex 2 derived from the diphosphine-disilane [Ph₂P(C ₆H₄)Me₂Si-SiMe₂(C₆H ₄)PPh₂] (1) has been isolated and crystallographically characterized. The coordination of the Si-Si σ bond to copper was thoroughly analyzed by quantum-chemical methods.

Full text of DOI:10.1021/ja200789z

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A Crystalline σ Complex of Copper
Pauline Gualco,† Abderrahmane Amgoune,† Karinne Miqueu,‡ Sonia Ladeira,§ and Didier Bourissou*,†
†Universitꢀe de Toulouse, UPS, LHFA, 118 Route de Narbonne, F-31062 Toulouse, France; CNRS, LHFA UMR 5069,
F-31062 Toulouse, France
‡Institut Pluridisciplinaire de Recherche sur l’Environnement et les Matꢀeriaux (UMR 5254), Equipe Chimie Physique,
Universitꢀe de Pau et des Pays de l’Adour, Hꢀelioparc, 2 Avenue du Prꢀesident Angot, 64053 Pau cedex 09, France
§Universitꢀe de Toulouse, UPS, Institut de Chimie de Toulouse, FR2599, 118 Route de Narbonne, F-31062 Toulouse, France
S Supporting Information
b
Scheme 1. Coordination of Diphosphine-Disilane 1 to
ABSTRACT: Over the last decades, our understanding of
σ-bond activation at transition metals has progressed con-
siderably from both fundamental and synthetic points of
view thanks to the preparation and characterization of a
variety of σ complexes. Here we report the synthesis and
structural analysis of the first σ complex involving a coinage
metal. The copper(I) complex 2 derived from the dipho-
sphine-disilane [Ph2P(C6H4)Me2Si-SiMe2(C6H4)PPh2]
(1) has been isolated and crystallographically characterized.
The coordination of the Si-Si σ bond to copper was
thoroughly analyzed by quantum-chemical methods.
Copper
obtained by reacting 1 with CuCl and GaCl3 in dichloromethane
at low temperature (Scheme 1).11 Decomposition occurred
upon warming to room temperature, but complex 2 could be
isolated in pure form (68% yield) by workup at -20 °C.
Complex 2 exhibits a single 31P NMR signal at 5.2 ppm,
indicating the symmetric coordination of the two phosphorus
atoms. The 29Si NMR resonance signal for complex 2 at -23.5
ppm is shifted slightly upfield relative to that of free ligand 1 at -
21.2 ppm, indicating the retention of the disilane moiety and
possibly some interaction between the Si-Si σ bond and the
metal center. The coordination of the Si-Si σ bond to the
cationic Cu center was unambiguously evidenced structurally
and theoretically.
Colorless crystals of 2 (melting point 110.9 °C) suitable for an
X-ray diffraction study were obtained from a dichloromethane/
pentane solution at -60 °C. The analysis showed that complex 2
adopts a discrete ion pair structure in the solid state (Figure 1).
The two Si atoms lie close to the metal center, with the
Cu-Si distances [2.7196(14) and 2.7212(14) Å] exceeding
the sum of the covalent radii (2.43 Å)12 by only 12%. Notably,
two methyl substituents of the SiMe2 moieties also lie close to the
copper center, with short Cu-H contacts of 2.2262(6) and
2.3615(6) Å.11 The coordination sphere is completed by the
two phosphorus atoms [P-Cu distances = 2.2079(13) and
2.2168(13) Å], whose arrangement significantly deviates from
linearity [P-Cu-P angle = 153.72(5)°]. The copper points
toward the Si-Si σ bond, and overall, its geometry is halfway
between trigonal-planar and T-shaped. Another salient feature is
the significant lengthening of the Si-Si σ bond upon coordina-
tion [from 2.3581(11) Å in free ligand 1 to 2.4505(19) Å in 2].
The geometric features of 2 parallel those typically associated
with the coordination of σ bonds to transition metals and thus
he past decades have witnessed spectacular progress in the
T
activation of inert, apolar σ bonds (H-H as well as E-H
and E-E bonds, where E = C, Si, ...) with transition metals. The
discovery that σ bonds can coordinate to transition metals
without splitting clearly represented a major breakthrough,1-3
and the ensuing so-called σ complexes are nowadays recognized
as key intermediates. Of major interest is the characterization of
such bonding situations via spectroscopic and structural methods
in order to gain a better understanding of the factors governing
the activation of σ bonds.4 In this respect, it is striking to note that
the coordination of σ bonds has been shown structurally with
almost all transition metals except the coinage metals.
In recent years, coinage-metal complexes have emerged as
powerful catalysts for a broad range of reactions, including
σ-bond activations,5 but σ complexes species with these metals
remain very elusive. The possible existence of such a bonding
situation has rarely been supported computationally,6,7 and only
recently was experimental evidence reported. The coordination
of a C-H σ bond has been identified by pulsed electron
paramagnetic resonance spectroscopy in a copper(II) complex
of a triazamacrocylic ligand.6 Here we report experimental and
theoretical evidence for the coordination of a Si-Si σ bond to a
coinage metal. In the course of our studies on unusual metal-
ligand interactions supported by phosphine buttresses,8,9
a
crystalline σ complex of copper(I) was obtained from the diphos-
phine-disilane [Ph2P(C6H4)Me2Si-SiMe2(C6H4)PPh2] (1).
Following previously reported methodologies for the synth-
esis of diphosphine-silane ligands,10 1 was prepared by treat-
ment of 2 equiv of ortho-lithiated triphenylphosphine with 1,2-
dichlorotetramethyldisilane. The copper(I) complex 2 was
Received: January 26, 2011
Published: March 07, 2011
r
2011 American Chemical Society
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Figure 1. X-ray crystal structure of complex 2. Thermal ellipsoids have
been drawn at 50% probability, and hydrogen atoms and solvate
molecules have been omitted for clarity. Selected bond lengths (Å)
and angles (deg): P1-Cu, 2.2079 ( 13; P2-Cu, 2.2168 ( 13; Si1-Cu,
2.7212 ( 14; Si2-Cu, 2.7196 ( 14; Si1-Si2, 2.4505 ( 19; P1-Cu-
P2, 153.72 ( 5.
Figure 2. X-ray crystal structure of complex 3. Thermal ellipsoids have
been drawn at 50% probability, and hydrogen atoms and solvate
molecules have been omitted for clarity. Selected bond lengths (Å)
and angles (deg): P1-Ag, 2.4010 ( 9; P2-Ag, 2.4008 ( 9; Si1-Ag,
3.4766 ( 11; Si2-Ag, 3.3896 ( 11; Si1-Si2, 2.3633 ( 15; P1-Ag-
P2, 171.91 ( 3.
Scheme 2. Coordination of Diphosphine-Disilane 1 to
Silver
strongly argue in favor of side-on coordination of the disilane
moiety to copper.
The coordination of 1 to silver(I) was also performed via a
similar procedure to give the corresponding complex 3 in 82%
isolated yield (Scheme 2). The single-crystal X-ray diffraction
study revealed a markedly different bonding situation (Figure 2).
The metal center is surrounded by the two phosphorus atoms
[P-Ag distances = 2.4010(9) and 2.4008(9) Å]. A weak contact
with one chlorine atom of the GaCl4- counteranion [3.065(3)
Å] is present, but the geometry around Ag deviates only slightly
from linearity [P-Ag-P angle = 171.91(3)°].13 The two Si
atoms remain at much longer distances from the metal center than
in the copper complex: the Ag-Si distances in 3 [3.4766(11) and
3.3896(11) Å] exceed the sum of the covalent radii (2.56 Å)11 by
more than 32%. In addition, the Si-Si σ bond length [2.3633(15)
Å] is very similar to that in free ligand 1 and substantially shorter
than that in 2. According to these geometric data, the disilane
moiety does not coordinate to the metal center in 3.
Density functional theory calculations on the complexes 2*
and 3* (including the GaCl4- counteranion and having methyl
or phenyl substituents on the phosphorus atoms) were carried
out.11 The optimized geometries nicely reproduced those de-
termined crystallographically. In agreement with the geometric
data, natural bond orbital (NBO) analyses identified a donor-
acceptor interaction from the Si-Si σ bond to the metal center
only in the copper complex 2* (the delocalization energy found
at the second-order perturbation level was ΔENBO ∼ 14-19
kcal/mol). The coordination of the Si-Si σ bond to copper is
largely dominated by Si-Si σ to Cu donation. A donor-accep-
tor interaction associated with Cu to Si-Si σ* back-donation was
also found, but the corresponding delocalization energy was
negligible (<1 kcal/mol). Another conspicuous feature of the
Figure 3. Three-dimensional images of the NLMOs centered on the
Si-Si σ bond in complexes (a) 2a* and (b) 3a*.
coordination of the disilane is the symmetry of the natural
localized molecular orbital (NLMO) centered on the Si-Si σ
bond, which is significantly distorted toward the metal in the
copper complex 2* but remains cylindrical in the silver complex
3* (Figure 3). The description of 2* as a genuine σ complex was
further supported by electron localization function (ELF)
analyses.11,14 The side-on coordination of the disilane moiety
in the copper complex 2* is associated with a trisynaptic (Cu, Si, Si)
ELF basin, whereas in the free ligand 1* and silver complex 3*, the
Si-Si σ bond is associated with a disynaptic (Si, Si) ELF basin.
On the basis of all these experimental and theoretical data, the
copper complex 2 can be unambiguously formulated as a σ
complex, and as such, it provides the first structural evidence for
such a bonding situation involving a coinage metal. The related
silver complex 3 adopts a markedly different structure without
coordination of the disilane moiety. The lower Lewis acidity of
Ag(I) and its higher propensity to accommodate linear dicoor-
dinate complexes12 most likely explain this difference. Moreover,
this indicates that diphosphine-disilane ligand 1 is relatively
flexible and that the coordination of the Si-Si σ bond in 2 is
supported but not geometrically imposed by the two phosphine
buttresses.
Notably, the formation of σ complexes is less favorable
with Si-Si bonds than with X-H bonds (X = H, C, Si, ...) on
both orbital and steric grounds.15 The directionality of the
Si-Si σ bond does not permit optimal overlap, and the
surrounding substituents induce substantial shielding. Although
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COMMUNICATION
the activation of Si-Si bonds by transition metals, which is the
key step for catalytic bis-silylation reactions, has been fairly well
documented,16 not much is known about Si-Si σ complexes.17
To date, the coordination of Si-Si σ bonds had been authenti-
cated structurally only once, in a trinuclear Pd cluster arising
from the thermal condensation of [o-C6H4(SiH2)2Pd(dmpe)]
[dmpe =1,2-bis(dimethylphosphino)ethane].18-21 The transi-
ent formation of an unsupported Si-Si σ complex was also
recently proposed to account for the fluxional behavior of the
bis-silyl Pd complex [(dcpe)Pd(SiMe2H)2] [dcpe =1,2-bis-
(dicyclohexylphosphino)ethane], as observed in solution by
NMR spectroscopy.22
In conclusion, the diphosphine-disilane copper(I) complex 2
represents the first structurally authenticated σ complex invol-
ving a coinage metal and is a very rare example of a Si-Si σ
complex. Ongoing studies are aimed at further exploration of the
coordination of σ bonds to coinage metals.
(10) Gualco, P.; Lin, T. P.; Sircoglou, M.; Mercy, M.; Ladeira, S.;
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Bourissou, D. Angew. Chem., Int. Ed. 2009, 48, 9892.
(11) See the Supporting Information for details.
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Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221. (c) Beletskaya, I.;
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’ ASSOCIATED CONTENT
(21) Because of its unique Y-shaped geometry with a relatively short
S
Supporting Information. Detailed experimental condi-
Si Si contact, the bis-silyl Pt complex [(IPr)Pt(SiMe2Ph)2] [IPr =
b
3 3 3
tions and procedures, theoretical details, and analytical data and
crystallographic data (CIF) for compounds 1, 2, and 3. This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.
bis(2,6-diisopropylphenyl)imidazol-2-ylidene] has been considered as
an intermediate in the oxidative addition/reductive elimination of Si-Si
σ bonds. See: Berthon-Gelloz, G.; de Bruin, B.; Tinant, B.; Markꢀo, I. E.
Angew. Chem., Int. Ed. 2009, 48, 3161.
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2006, 128, 9054.
’ AUTHOR INFORMATION
Corresponding Author
dbouriss@chimie.ups-tlse.fr
’ ACKNOWLEDGMENT
Financial support from the Centre National de la Recherche
Scientifique, the Universitꢀe de Toulouse, and the Agence Natio-
nale de la Recherche (ANR-10-BLAN-070901) is gratefully
acknowledged. Useful discussions with I. Fourrꢀe (UPMC)
regarding the ELF analyses are greatly appreciated. The theore-
tical work was granted access to HPC resources of Idris under
Allocation 2010 (i2010080045) made by Grand Equipement
National de Calcul Intensif (GENCI).
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