Linear coinage metal complexes stabilized by a group 13 metalloid ligand

Article abstract of DOI:10.1002/ejic.201000616

The synthesis and structural characterization of the linear, cationic complex [{(DDP)Ga}₂Cu][OTf]·2C₆H₅F (2, Tf = O₂SCF₃) stabilized by the monodentate, gallium-based ligand Ga(DDP) (DDP = [HC{(CMe)N(

Full text of DOI:10.1002/ejic.201000616

SHORT COMMUNICATION
DOI: 10.1002/ejic.201000616
Linear Coinage Metal Complexes Stabilized by a Group 13 Metalloid Ligand
Ganesan Prabusankar,[a] Sandra Gonzalez-Gallardo,[a] Adinarayana Doddi,[a]
Christian Gemel,[a] Manuela Winter,[a] and Roland A. Fischer*[a]
Keywords: Gallium / Copper / Silver / Reduction / Metal–metal interactions
The synthesis and structural characterization of the linear,
cationic complex [{(DDP)Ga}2Cu][OTf]·2C6H5F (2, Tf
O2SCF3) stabilized by the monodentate, gallium-based li-
gand Ga(DDP) (DDP = [HC{(CMe)N(2,6-iPr2C6H3)}2]–), as
well as NMR spectroscopic evidence for the formation of the
corresponding silver compound [{(DDP)Ga}2Ag][Al(hfip)4] (3,
hfip = [OC(H)(CF3)2]), are reported. The remarkable steric
properties of this gallium-based ligand permit the stabiliza-
tion of 2, which exhibits an unusual linear geometry and a
coordination number of two.
=
Introduction
The chemistry of coinage metal complexes has attracted
a lot of attention in the past few years, due to their interest-
ing bonding, structural properties and diverse applica-
tions.[1] Particularly, gold complexes bearing monodentate
ancillary ligands such as N-heterocyclic carbenes or phos-
phanes have been in the focus of extensive research in recent
years, with regard to their application as homogeneous cat-
alysts for organic synthesis,[2] where the importance of cat-
ionic complexes has been highlighted.[3] Complexes of the
d10 ions of the group 11 transition metals present variable
coordination numbers. CuI and AgI complexes most com-
monly exhibit coordination numbers of three or four, while
AuI compounds are mostly linear, two-coordinate species.
It has been suggested that this difference may be ascribed
to relativistic effects, which allow the efficient hybridization
of s, p and d orbitals of gold atoms.[4] In 2004, the first
examples of complexes containing Au–Ga bonds were
reported, namely the trinuclear cluster [Au3(µ-GaI2)3-
(Cp*Ga)5][5a] and the mononuclear, linear complexes
[Ph3PAu–Ga(Cl)DDP] and [DDPGa–Au–Ga(Cl)DDP]
(DDP = [HC{(CMe)N(2,6-iPr2C6H3)}2]).[5b] Despite the ex-
tensive work that has been done on the study of the coordi-
nation chemistry of gallium-based ligands, only a handful
of complexes of the coinage metals have been isolated. Both
neutral GaCp* and Ga(DDP), and anionic [Ga{[N(Ar)-
C(H)]2}]– (Ar = 2,6-iPr2C6H3) ligands have been used to
stabilize such species.[6] All low-valent RGa compounds are
strong σ-donors, but the nature of the R groups influences
their steric and π-acceptor properties strongly, leading to
distinctly different coordination behaviours. Therefore, the
Figure 1. Coinage metal complexes stabilized by gallium-based li-
gands.
[a] Anorganische Chemie II, Organometallics and Materials
Chemistry, Ruhr-Universität Bochum
Universtitätsstraße 150, 44780, Bochum, Germany
Fax: +49-234 321 4174
nature of the RGa–M (Cu, Ag, Au) complexes charac-
terized so far is very different from one another (Fig-
ure 1).[5–7]
E-mail: roland.fischer@rub.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000616.
View this journal online at
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R. A. Fischer et al.
SHORT COMMUNICATION
During our studies on the coordination chemistry of range when compared to Ga(DDP)-supported metal com-
Ga(DDP), we have tried to obtain cationic transition metal plexes.[5b,6a] It is noteworthy that the triflate carbon atom
complexes only with limited success. Attempts to abstract of the counterion is not observed in the 13C{1H} NMR
the chlorine atoms of [(Ph3P)2Rh{Ga(DDP)}(µ-Cl)] and spectroscopic time scale. However, the 19F NMR spectrum
[(COE)(benzene)Rh-{(DDP)GaCl}] (COE = cyclooctene) presents a sharp single signal at δ = –80.5 ppm for the fluor-
with TlBArF (BArF = B[3,5-(CF3)2C6H3]4) to produce the ine atoms of the triflate group. Moreover, the NMR spectra
corresponding cationic species did not lead to stable prod- of 2 clearly indicate that 2 is stable in solution for several
ucts that could be isolated. However, their formation was hours. The triflate stretching absorptions [1261(s), 1230(m),
1
confirmed by means of H NMR spectroscopy. Only the 1163(m), 1015(s) cm–1] in the infrared spectrum suggest the
linear, symmetrical, cationic species [{(DDP)Ga}2Au]- presence of a non-coordinating triflate anion.[8]
[BArF] (1a) and [{(DDP)Ga·THF}2Au][BArF] (1b) have
To elucidate the nature of the metal–ligand bonding and
been fully characterized so far.[6a] Herein, we report on the the solid-state structure of 2, we carried out a single-crystal
synthesis of analogous complexes [{(DDP)Ga}2Cu][OTf]· X-ray diffraction study. Single crystals suitable for X-ray
2C6H5F (2) and [{(DDP)Ga}2Ag][Al(hfip)4] (3, hfip = structural determination were obtained from a fluoroben-
[OC(H)(CF3)2]), by treatment of the appropriate metallic zene/hexane solution at –30 °C over a period of 24 h. The
precursor directly with Ga(DDP).
X-ray structure, selected bond lengths and bond angles for
2 are shown in Figure 2. Complex 2 crystallizes in the space
group C2/m.[12–14] Molecule 2 contains a linear Ga–Cu–Ga
linkage [180.0(1)°] with a non-coordinating OTf group. The
Me2C3N2 backbone of Ga(DDP) ligands are nearly copla-
nar with the Ga–Cu–Ga chain. The 2,6-iPr2–C6H3 groups
attached to the DDP nitrogen atoms lie perpendicular to
the Me2C3N2 framework. It is noteworthy that there is a
limited number of known examples of linear coordination
geometry around copper centres.[4b] Furthermore, no linear
copper compounds stabilized by group 13 ligands have been
reported so far. Hence, compound 2 represents the first ex-
ample of a linear copper(I) compound supported by the
Ga(DDP) ligand.
Results and Discussion
The wide variety of available AgI and CuI starting mate-
rials allowed us to try the direct reaction of Ga(DDP) with
metal salts containing weakly coordinating anions, such as
–
TfO–, BArF–, SbF6 and [Al(hfip)4]–, to obtain the corre-
sponding ionic complexes. As previously reported,[6c] we
have also observed that the nature of the anion has a strong
influence on the formation of the desired species. Most of
the attempted reactions led to decomposition products, as
evidenced by the formation of metallic films on the walls of
the reaction flasks or precipitation of grey metallic solids.
However, we were able to isolate ionic complexes 2 and 3
(Scheme 1).
Figure 2. Molecular structure of 2. Hydrogen atoms attached to
carbon, the disordered anionic ligand OTf and two disordered fluo-
robenzene molecules are omitted for clarity. Selected bond lengths
[Å] and angles [°] for 2: Ga–Cu 2.3157(10), Ga–N 1.930(5), Ga–
Cu–Ga 180.0(1), N–Ga–N 94.9(3).
Scheme 1. Synthesis of 2 and 3.
[{(DDP)Ga}2Cu][OTf]·2C6H5F (2)
Reduction of Cu(OTf)2 with 4 equiv. of Ga(DDP) in
The Ga–Cu bond length is 2.3157(10) Å, which is re-
fluorobenzene afforded bimetallic complex 2 as colourless markably shorter than the average Ga–Cu distance found in
crystals in 53% yield. Compound 2 is stable under an inert dimeric complexes [{(DDP)GaCu(OTf)}2] (av.: 2.4605 Å),
atmosphere and soluble in polar organic solvents such as [Cu2(GaCp*)(µ-GaCp*)3Ga(OTf)3] (av.: 2.412 Å) and
THF and fluorobenzene. It has been characterized by IR [Cu2(GaCp*)3(µ-GaCp*)2][OTf]2 (av.: 2.4189 Å),[7] while
and NMR (1H, 13C and 19F) spectroscopy, elemental analy- it is comparable with that in [LCu–GaR] {2.3066(6)
sis and single-crystal X-ray diffraction. The 1H and and 2.2807(5) Å with R = {N(C6H3-2,6-iPr2)CH}2, L =
13C{1H} NMR spectra in [D8]THF solution show signals {N(2,4,6-Me3–C6H2)CH}2C and L = {N(2,6-iPr2-C6H3)-
associated with the DDP ligand, which are in the expected CH}2C, respectively} and [Cu(GaCp*)4]+[BArF] [2.3517(5)
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Linear Coinage Metal Complexes with a Group 13 Metalloid Ligand
and 2.3496(5) Å] (BArF = B[3,5-(CF3)2C6H3]4).[6b,6c] The
coordination environment around gallium centres is satis-
fied with CuI atoms and nitrogen atoms from the DDP li-
gand. The Ga–N bond lengths and N–Ga–N angles of 2
are typical for (DDP)Ga-supported metal complexes.[5b]
The analogous silver compound 3 can be obtained by treat-
ment of Ag[Al(hfip)4] with 2 equiv. of Ga(DDP) in fluoro-
benzene. The product is very light-sensitive, and all
attempts to obtain satisfying elemental analysis data or sin-
gle crystals suitable for X-ray analysis failed. However, 3
was characterized by NMR spectroscopy in solution. Its
1H NMR spectrum exhibits signals corresponding to two
symmetrically coordinated Ga(DDP) moieties (γ-CH, δ =
5.45 ppm in [D8]THF, δ = 5.34 ppm in C6F5/C6D6) as well
as a signal for the [Al(hfip)4] anion in a 2:1 ratio (δ =
4.54 ppm in [D8]THF, δ = 5.13 ppm in C6F5/C6D6). The
observed chemical shifts are in good agreement with those
reported for 1a.[5b,6a]
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[{(DDP)Ga}2Cu][OTf]·2C6H5F (2)
Method A: To a mixture of Ga(DDP) (0.1 g, 0.206 mmol) and
Cu(OTf)2 (0.019 g, 0.052 mmol) was added fluorobenzene (3 mL)
under vigorous stirring at room temp. The clear pale yellow solu-
tion was heated at 60 °C for 1 h, filtered, layered with hexane
(2 mL) and stored at –30 °C for 24 h to afford colourless crystals
of 2. Yield: 53% [based on Cu(OTf)].
Method B: Cu(OTf)·4CH3CN (0.05 g, 0.132 mmol) was added to a
pale yellow solution of Ga(DDP) (0.129 g, 0.265 mmol) in fluoro-
benzene (3 mL). The resultant slurry was heated at 60 °C for 1 h.
The tan-coloured insoluble solid formed was filtered off, and the
colourless filtrate was layered with hexane to isolate colourless
crystals of 2 at –30 °C over a period of one week. Yield: 61% [based
on Cu(OTf)·4CH3CN].
The samples used for elemental analysis and spectroscopic charac-
terization were washed with hexane and completely dried under
vacuum. This procedure removes all solvents from the synthesis,
especially fluorobenzene. C59H82CuF3Ga2N4O3S (1187.36 without
solvent): calcd. C 59.68, H 6.96, N 4.71, S 2.70; found C 59.54, H
6.05, N 4.48, S 2.88. 1H NMR ([D8]THF, 250 MHz): δ = 7.30–7.07
Conclusions
Reduction of copper(II)triflate with an excess of
Ga(DDP) afforded the first copper(I) cationic complex 2
supported by a main group metalloid ligand. Similarly, the
reaction of Ag[Al(hfip)4] and Ga(DDP) is also likely to
yield the corresponding compound 3, which, however,
could only be characterized by NMR spectroscopy in solu-
tion. It is noteworthy that the cationic part of 2 can be
3
(m, 12 H, Ar), 5.35 (s, 2 H, γ-CH), 2.92 (sept, JHH = 7 Hz, 8 H,
3
CHCH3), 1.73, 1.70 (s, 12 H, CCH3), 1.10 (d, JHH = 6.5 Hz, 24
3
H, CHCH3), 0.89 (d, JHH = 6.5 Hz, 24 H, CHCH3) ppm. 13C
NMR ([D8]THF, 62.9 MHz): δ = 168.2 (CN), 144.4 (Ar), 142.0
(Ar), 124.8 (Ar), 116.1, 115.7 (Ar), 100.7 (γ-CH), 28.8 (CHCH3)
ppm. 19F NMR ([D8]THF, 235.3 MHz): δ = –80.5 (OTf) ppm. IR:
regarded as being isostructural to copper(I) cationic com- ν = 2961 (w), 2869 (w), 1594 (s), 1528 (m), 1495 (m), 1460 (m),
˜
plexes [NHC–Cu–NHC]+[Anion]– stabilized by N-hetero-
1438 (m), 1360 (s), 1317 (s), 1261 (s), 1230 (m), 1199 (m), 1176 (m),
1163 (m), 1101 (w), 1055 (m), 1015 (s), 935 (w), 867 (w), 798 (s),
cyclic carbenes [NHC, NHC = N,NЈ-bis(aryl)imidazol-2-
753 (m), 709 (w), 684 (w), 662 (vs), 632 (w), 572 (w), 531 (w), 516
ylidene], which are known to be potential catalysts for
(w), 498 (m), 439 (w), 403 (w) cm–1.
hydrosilylation, carbonylation and coupling reactions.[9]
Analogous Ga–Ag Complex (Attempted Synthesis of 3): A solution
of Ga(DDP) (0.245 g, 0.50 mmol) in toluene (5 mL) was added to
Ag[Al(hfip)4] (0.2 g, 0.25 mmol) in fluorobenzene (10 mL) at
–30 °C. The reaction mixture was warmed slowly to room tempera-
ture and stirred overnight. After filtration, all volatiles were re-
Experimental Section
General Considerations: All manipulations were carried out in an
atmosphere of purified argon by using standard Schlenk and glove
moved to leave an oily, clear residue, which was washed twice with
box techniques. The solvents were dried by using an mBraun Sol-
hexane to yield a white powder. The product was filtered and dried
vent Purification System. Ga(DDP)[10] and Ag[Al(hfip)4][11] were
in vacuo. Yield: 0.27 g {61% based on Ag[Al(hfip)4]}. 1H NMR
prepared according to previously reported methods. 2,6-Diisopro-
pylaniline (Aldrich), 2,4-pentanedione (Aldrich), gallium (Aldrich),
potassium hydride (Acros), iodine (Aldrich), copper(II) triflate
(ABCR), LiAlH4 (Aldrich), (CF3)2CHOH (ABCR) and AgF
(ABCR) were purchased from commercial sources. IR measure-
ments (neat) were carried out with a Bruker Alpha-P Fourier trans-
form spectrometer. Elemental analyses were performed by the Mi-
3
(C6H5F/C6D6, 250 MHz): δ = 5.34 (s, 2 H, γ-CH), 5.13 [sept, JHF
3
= 6.3 Hz, 4 H, (CF3)2C(H)O], 2.54 (sept, JHH = 6.7 Hz, 8 H,
3
CHCH3), 1.68 (s, 12 H, CCH3), 1.13 (d, JHH = 6.8 Hz, 24 H,
3
CHCH3), 0.85 (d, JHH = 6.8 Hz, 24 H, CHCH3) ppm. 13C NMR
([D8]THF, 62.9 MHz): δ = 169.2 (CN), 144.3 (Ar), 141.5 (Ar), 128.2
(Ar), 125.2 (Ar), 100.6 (γ-CH), 29.3 (CHCH3), 26.3 (CHCH3), 24.4
27
¯
(CCH3) ppm. Al NMR (C6H5F/C6D6, 65.2 MHz) δ = 61 (ω1/2
=
croanalytical Laboratory of the Ruhr University of Bochum. NMR
spectra were recorded with a Bruker Avance DPX-250 spectrometer
at 25 °C unless otherwise stated. Chemical shifts are given relative
to TMS and were referenced to the solvent resonances as internal
standards. The X-ray crystal structure of 2 was determined with an
Oxford Excalibur diffractometer. The structure was solved by direct
methods using SHELXS-97 and refined against F2 on all data by
full-matrix least-squares with SHELXL-97.[12] Two severely disor-
dered fluorobenzene molecules and the triflate anion (apart from
the sulfur) had to be “squeezed” out by using the program “Platon
1.13”[13] The fluorobenzene molecules and a triflate anion are in-
cluded in the empirical formula. CCDC-779125 contains the sup-
130 Hz) ppm.
Supporting Information (see footnote on the first page of this arti-
cle): IR and NMR spectra of 2 and 3.
Acknowledgments
G. P. is grateful for a stipend from the Alexander von Humboldt
Foundation. S. G.-G. gratefully acknowledges the Deutscher
Akademischer Austauschdienst (DAAD) for a postdoctoral sti-
pend.
Eur. J. Inorg. Chem. 2010, 4415–4418
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4417

R. A. Fischer et al.
SHORT COMMUNICATION
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Received: June 2, 2010
Published Online: August 18, 2010
4418
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Eur. J. Inorg. Chem. 2010, 4415–4418