A bimetallic complex containing a cyclopentadienyl-annulated imidazol-2-ylidene

Article abstract of DOI:10.1021/ja055565f

A bimetallic carbene complex architecture that incorporates a cyclopentadienyl-annulated imidazol-2-ylidene moiety is characterized. The ligand architecture enables direct electronic interaction between the π- and σ-bonded metals. A preliminary example of aqueous Suzuki coupling employing a metallocene-fused imidazol-2-ylidene-derived catalyst is described. Copyright

Full text of DOI:10.1021/ja055565f

Published on Web 11/08/2005
A Bimetallic Complex Containing a Cyclopentadienyl-Annulated
Imidazol-2-ylidene
Anthony J. Arduengo, III,*,† Daniela Tapu,† and William J. Marshall‡
Department of Chemistry, The UniVersity of Alabama, Tuscaloosa, Alabama 35487, and DuPont Central Research,
Experimental Station, Wilmington, Delaware 19880
Received August 15, 2005; E-mail: ajarduengo@ua.edu
Variations of the basic imidazol-2-ylidene structure have prolifer-
ated since the discovery of the first isolable analogues, and these
carbenes are finding expanding application in modern catalysis.1
Pursuant to our ongoing interest in imidazol-2-ylidene chemistry,
we recently reported the synthesis of the first directly cyclopenta-
dienyl-fused imidazolium salts and imidazol-2-thiones.2 A new class
of metallocene-fused imidazol-2-ylidene complexes is available
from these precursors. The architecture of a Cp-fused imidazol-2-
ylidene enables the incorporation of multiple metals into a single
molecule through a combination of tightly coupled σ- and π-type
bonding schemes and provides a mechanism for varying the
imidazol-2-ylidene electronics. Deprotonation of an imidazolium
moiety followed by metalation is a standard method for synthesis
of carbene-metal complexes. Oxidative insertion reactions are less
frequently employed to achieve metalation, but this methodology
appears to be well-suited to 2-haloimidazolium-cyclopentadienide
precursors.3 Herein, using this approach, we report a new bimetallic
Figure 1. KANVAS drawing of the complex 3. Selected bond lengths
carbene complex architecture that incorporates a cyclopentadienyl-
(pm) and angles (deg): C2-N1(3) ) 132.9(2), 133.2(2); N1(3)-C6a(3a)
annulated imidazol-2-ylidene moiety. A preliminary example of
Suzuki coupling employing such a metallocene-fused imidazol-2-
ylidene-derived catalyst is described.
) 140.7(2), 141.0(2); C3a-C6a ) 140.7(2); C3a(6a)-C4(6) ) 142.9(2),
142.2(2); C4(6)-C5 ) 145.3(2), 144.7(3); av(Ru-C3a-6a) ) 219.04; C2-
Cl ) 168.48(19); av(Ru-CCp*) ) 217.12; C2-N1(3)-C6a(3a) ) 106.42(15),
106.51(15); C4-C5-C6 ) 110.93(15); N1-C2-N3 ) 113.33(16); N1(3)-
C6a(3a)-C3a(6a) ) 107.09(15), 106.60(15); C3a(6a)-C6a(3a)-C6(4) )
110.24(15), 110.07(15); C3a(6a)-C4(6)-C5 ) 104.15(15), 104.56(15).
Scheme 1. Synthesis of Complex 4a
The reaction of imidazolium salt 3 with an equimolar amount
of Pd(PPh3)4 in refluxing methylene chloride proceeded to complete
conversion to the mixed palladium-ruthenium complex 4. Complex
4 was isolated as a yellow solid, melting at 236-238 °C. NMR
spectroscopy indicated a cis geometry at the palladium center. Two
sets of 31P signals at δ 30.35 (d, 2JP-P ) 24.5 Hz) and δ 19.90 (d,
2JP-P ) 24.5 Hz) were observed, indicating the presence of two
phosphorus atoms at nondegenerate sites (i.e., cis isomer). The C2
center (former carbene) in 4 resonates as a doublet of doublets, δ
2
182.39 (2JC-P(cis) ) 7 Hz and JC-P(trans) ) 157 Hz).
X-ray quality crystals of 4 were grown by slow evaporation of
a saturated dichloromethane solution. The solid-state structure of
4 is depicted by the KANVAS drawing in Figure 2. The cis
geometry of the palladium center is confirmed. Conversion of the
cis isomer to a trans isomer does not take place. The stability of
the cis isomer is contrary to results with previously reported
palladium complexes whose cis isomers are the kinetic products
which easily isomerize to more stable trans isomers.7 This stability
of the cis isomer of 4 is likely due to steric congestion which would
arise in the trans isomer. The ring internal angle at C2 is 108.7-
(3)°. The palladium and phosphorus distance is slightly longer for
a Conditions: (a) (CH3)3O+BF4-; (b) Cp*Ru(CH3CN)3+CF3SO3-, 4 Å
molecular sieves; (c) Pd(PPh3)4.
Treatment of 12b with Meerwein’s salt afforded imidazolium
tetrafluoroborate 2 in 95% yield. Reaction of 2 with pentamethyl-
cyclopentadienylrutheniumtris(acetonitrile) triflate4 in the presence
of 4 Å molecular sieves leads to ruthenocene 3. The imidazolium
salt 3 was crystallized from THF to afford crystals for X-ray
diffraction studies. Selected bond distances and angles for 3 are
included in the Figure 1 caption. A KANVAS5 depiction of the
structure of 3 is shown in Figure 1. The crystal structure displays
a C2-Cl bond of 168.5(2) pm and a N1-C2-C3 angle of 113.3(2)°
that is larger than that in typical imidazolium ions.6
the atom opposite to the imidazol-2-ylidene moiety (r(Pd-P2)
238.2(1) pm) than for that trans to the chlorine atom (r(Pd-P1)
)
)
227.5(1) pm). These latter distances are consistent with a greater
trans-effect exerted by the carbene center, a stronger σ-donor than
the chlorine atom.
† The University of Alabama.
‡ DuPont.
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J. AM. CHEM. SOC. 2005, 127, 16400-16401
10.1021/ja055565f CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
catalytic activity of the metal bound to the carbene center and may
offer some advantages for aqueous-based systems.
In conclusion, we have synthesized and characterized the first
bimetallic ruthenium-palladium complex containing a cyclopen-
tadienyl-annulated imidazol-2-ylidene ligand and demonstrated its
capability to function as catalyst in the Suzuki reaction in aqueous
media. We are currently investigating this novel ligand class and
its application to a variety of catalytic processes.
Acknowledgment. We gratefully acknowledge support for this
dissertation research from Atotech U.S.A. (Fellowship to D.T.), E.
I. du Pont de Nemours and Co., and grants from the National
Science Foundation (CHE-0413521 and CHE-0115760). A.J.A.
gratefully acknowledges the Saxon Endowment of the University
of Alabama.
Supporting Information Available: Description of the preparation,
NMR spectra, and elemental analyses of 2, 3, and 4, cross-coupling
procedures; cyclic voltammograms and a complete description of the
X-ray crystallographic determination on 3 and 4, including tables of
fractional coordinates, isotropic and anisotropic thermal parameters,
bond distances and angles. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. KANVAS drawing of the complex 4. Selected bond lengths
(pm) and angles (deg): C2-N1(3) ) 134.3(4), 134.9(5); N1(3)-C6a(3a)
) 140.9(5), 141.1(4); Pd-P1(2) ) 227.48(10), 238.24(10); C3a-C6a )
139.4(5); C3a(6a)-C4(6) ) 142.5(5), 142.5(5); C4(6)-C5 ) 146.0(6),
144.7(6); av(Ru-C3a-6a) ) 216.5; C2-Pd ) 202.9(3); av(Ru-CCp*) )
219.34; C2-N1(3)-C6a(3a) ) 109.1(3), 109.4(3); C4-C5-C6 ) 110.7(3);
N1-C2-N3 ) 108.7(3); C2-Pd1-P2 ) 173.76(10); C2-Pd1-P1 )
88.95(10); N1(3)-C6a(3a)-C3a(6a) ) 106.8(3), 105.9(3); C3a(6a)-
C6a(3a)-C6(4) ) 110.2(3), 110.8(3); C3a(6a)-C4(6)-C5 ) 103.8(3),
104.5(3).
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Demonstration that the palladium-ruthenium complex 4 is an
active catalyst for aqueous Suzuki coupling reactions was provided
by preliminary experiments (Scheme 2). Cross-coupling of para-
Scheme 2. Suzuki Cross-Coupling Reactions Catalyzed by 4
(H2O:CH3CN ) 9:1, Na2CO3, 80 °C, 2 h)
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.
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