Synthesis, structure and properties of [1,2,4]triazolo[4,3-a]pyridin-3- ylidene rhodium and palladium complexes

Article abstract of DOI:10.1039/b907043e

The reaction of [1,2,4]triazolo[4,3-a]pyridinium tetrafluoroborates with [RhCl(COD)]₂ and [PdCl(allyl)]₂ takes place under mild basic conditions (Et₃N, THF, room temperature) to afford the corresponding [RhCl(COD)(Tripy)] and [PdCl(allyl)(Tripy)] complexes, respectively (Tripy = [1,2,4]triazolo[4,3-a]pyridin-3-ylidene), and their structures were analysed by X-ray diffractometry and spectroscopic techniques. The σ-donor ability of the new ligands was estimated by comparative analysis of infrared ν_CO stretching frequencies of [RhCl(CO) ₂(Tripy)] complexes, and proved to be strongly dependent on the substitution pattern. Additionally, a first insight into the catalytic properties of the latter in the Suzuki-Miyaura cross coupling demonstrates a good catalytic activity that enables the coupling of aryl chlorides at room temperature.

Full text of DOI:10.1039/b907043e

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N-Heterocyclic Carbenes
Guest Editor Ekkehardt Hahn
Westfälische Wilhelms-Universität, Münster, Germany
Published in issue 35, 2009 of Dalton Transactions
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www.rsc.org/dalton | Dalton Transactions
Synthesis, structure and properties of [1,2,4]triazolo[4,3-a]pyridin-3-ylidene
rhodium and palladium complexes†‡
a
b
a
b
b
a
´
Javier Iglesias-Sigu¨enza, Abel Ros, Elena D´ıez, Manuel Alcarazo, Eleuterio Alvarez, Rosario Ferna´ndez*
and Jose´ M. Lassaletta*b
Received 7th April 2009, Accepted 12th June 2009
First published as an Advance Article on the web 21st July 2009
DOI: 10.1039/b907043e
The reaction of [1,2,4]triazolo[4,3-a]pyridinium tetrafluoroborates with [RhCl(COD)]2 and
[PdCl(allyl)]2 takes place under mild basic conditions (Et3N, THF, room temperature) to afford the
corresponding [RhCl(COD)(Tripy)] and [PdCl(allyl)(Tripy)] complexes, respectively (Tripy =
[1,2,4]triazolo[4,3-a]pyridin-3-ylidene), and their structures were analysed by X-ray diffractometry and
spectroscopic techniques. The s-donor ability of the new ligands was estimated by comparative analysis
of infrared nCO stretching frequencies of [RhCl(CO)2(Tripy)] complexes, and proved to be strongly
dependent on the substitution pattern. Additionally, a first insight into the catalytic properties of the
latter in the Suzuki–Miyaura cross coupling demonstrates a good catalytic activity that enables the
coupling of aryl chlorides at room temperature.
the so-called ‘abnormal’ NHCs IX,8 and the pyridine derivatives
X9 (Fig. 1).
Introduction
The design of new types of ligands has been one of the corner-
stones that enabled the formidable development of homogeneous
catalysis over the past decades. N-Heterocyclic carbenes (NHCs)
have emerged as the younger family of widely used ligands in
transition metals catalysis. The high relevance of these ligands is
founded on the number and importance of the applications, in
turn developed on the basis of their extraordinary characteristics.1
In some aspects, these compounds can be viewed as phosphane
surrogates, the s-donor ability of NHC ligands matching or
improving that of the most basic phosphines. To further exploit
their potential as C-ligands, additional tools for the fine tuning of
their electronic and steric properties are still required.
The main strategies used to tune the electronic properties
of NHCs are the modification of the heterocycle core and
their inclusion into bicyclic systems. Taking classic imidazole
and dihydroimidazole derivatives I and II as the basic systems,
structural variability according to the first group of modifications
has been achieved by: (a) substitution of one N atom by
other heteroatoms (e.g. thiazol-2-ylidenes III2), (b) inclusion of
additional heteroatoms in the heterocyclic backbone (e.g. triazole-
3-ylidenes IV),3 (c) modification of the heterocycle ring size
(e.g. four membered V),4 six-membered tetrahydropyrimidin-
2-ylidenes (VI),5 and seven-membered 1,3-diazepan-2-ylidenes
(VII),6 (d) removal of one of the stabilizing N atoms, including
not only structures such as pyrrolidin-2-ylidenes VIII,7 but also
aDepartamento de Qu´ımica Orga´nica, Univ. de Sevilla, Apdo. de Correos
1203, 41071, Seville, Spain. E-mail: ffernan@us.es; Fax: +34 954624960
Fig. 1 Structural variation of NHC architectures.
bInstituto de Investigaciones Qu´ımicas (CSIC-US), Ame´rico Vespucio 49,
41092, Seville, Spain. E-mail: jmlassa@iiq.csic.es; Fax: +34 95 4460565
† Dedicated to Professor Josep Font on the occasion of his 70th birthday.
On the other hand, the construction of bicyclic systems by
annulation is another strategy that has been often applied to
modulate the electronic and steric properties of NHC ligands.
Thus, Hahn and co-workers have shown that benzimidazole-
derived ligands XI exhibit modified properties with respect of
‡ Electronic supplementary information (ESI) available: 1H and 13C NMR
spectra for new compounds and crystallographic data for 10, 11, and 19.
CCDC reference numbers 727062–727064. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b907043e
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7113–7120 | 7113
©

analogues I.10 Selected additional examples where benzannulation
at contiguous carbon centres have an impact in the properties
of the NHC ligand include benzothiazole derivatives XII,11
perimidine tricyclic derivative XIII,12 and isoquinoline derived
monoaminocarbenes XIV.13
More recently, our group14 and Glorius et al.15 independently
developed pyrido-annulated bicyclic systems, namely imidazo[1,5-
a]pyridin-3-ylidenes XV. This new NHC architecture is char-
acterized by the inclusion of one of the N atoms as a bridge
to the additional aromatic (pyridine) ring, and thereby offers
interesting possibilities for the electronic modulation of the
carbene ligand, even by using cross-talk communication between
arenes in cyclophane-containing systems.16 A similar annulation
strategy has also been applied to the synthesis of thiazole-
annelated imidazol-2-ylidenes XVI.17
The related [1,2,4]triazolo[4,3-a]pyridin-3-ylidenes XVII were
first reported by Enders and co-workers as in situ prepared
organocatalysts in the benzoin condensation.18 Very recently You
and co-workers19 have expanded the study of this carbenes as
organocatalysts, and described a silver complex thereof. The
coordination chemistry of the new family of NHCs, however,
remains unexplored.20 We now wish to disclose our own results on
the synthesis, structure and ligand properties of Rh(I) and Pd(II)
complexes of [1,2,4]triazolo[4,3-a]pyridin-3-ylidenes XVII.
Scheme 1 Synthesis of triazolium salts 5–8.
Results and discussion
The most common strategy to the synthesis of NHCs and
their metal complexes require the synthesis of the corresponding
azolium salts as the starting materials. The synthesis of the
required [1,2,4]triazolo[4,3-a]pyridin-2-ium salts 5–8 was accom-
plished in a single step from substituted pyridines 1–4 according
to a modification of the original procedure by Eicher and co-
workers21 (Scheme 1).
In order to have a first insight into the coordination properties
of the [1,2,4]triazolo[4,3-a]pyridin-3-ylidenes (Tripys), we decide
to prepare the corresponding Rh(I) compounds. Compared to the
above-mentioned imidazopyridinium analogues,14 the additional
N-atom is expected to confer a relatively high acidity to the
[1,2,4]triazolo[4,3-a]pyridin-2-ium salts, a difference that is also
found between the monocyclic triazolium and imidazolium salts.
In fact, this higher acidity makes possible their reaction with
[RhCl(COD)]2 at room temperature in the presence of a weak
base as Et3N. In this way, neutral [RhCl(COD)(Tripy)] complexes
9–12 were easily obtained in high (83–93%) yields as robust, bench-
stable compounds, that resisted chromatographic purification on
silica-gel (Scheme 2). Of interest, the slightly lower s-donor
ability of these carbenes results also in a different behaviour
with respect to the imidazopyridine (Impy) analogues. Thus,
starting from azolium salts with a non-coordinating counteranion
(hexafluorophosphate), the latter afforded exclusively cationic bis-
carbene [Rh(ImPy)2(COD)]+ complexes in their reaction with
[RhCl(COD)]2,14a even though the Rh : carbene precursor ratio
was 1 : 1.
Scheme 2 Synthesis of [RhCl(COD)(Tripy)].
diffusion of hexane into a solution of the complexes in CH2Cl2
(Fig. 2 and 3). Both structures show the expected slightly distorted
square-planar geometry at the Rh centres [C(1)–Rh(1)–Cl(1) =
87.9(3) and 85.56(9)◦, respectively], with the heterocycle plane
oriented near orthogonal to the coordination plane. This geometry
enables Rh–H interactions with H(9) and H(7), respectively,
that, according to the observed Rh–H bond distances (2.761
The structures of compounds 9–12 were assigned on the basis
of their analytical and spectroscopic data. The 13C NMR spectra
showed the characteristic C(2) doublets at d 180–183 (2JRh,C
50–52 Hz). Additionally, good quality crystals of compounds 10
=
◦
˚
and 2.497 A, respectively), C–H–Rh angles (112.1 and 141.3 ,
and 11 suitable for X-ray diffractometry were obtained by slow
respectively) and 1H chemical shifts [dH 9.31 ppm for H(9) in 10;
7114 | Dalton Trans., 2009, 7113–7120
This journal is
The Royal Society of Chemistry 2009
©

Fig.
3
ORTEP-like drawing of complex 11. Thermal ellip-
soids are drawn at the 30% probability level. Selected bond
Fig.
2
ORTEP-like drawing of complex 10. Thermal ellip-
lengths [A] and bond angles [◦]: Rh(1)–C(1) 2.019(3), N(1)–C(1)
˚
soids are drawn at the 30% probability level. Selected bond
lengths [A] and bond angles [ ]: Rh(1)–C(1) 2.041(10), N(1)–C(1)
◦
1.344(4), N(3)–C(1) 1.380(4), N(1)–N(2), 1.382(4), Rh(1)–C(20)
2.188(3), Rh(1)–C(21) 2.210(3), Rh(1)–C(17) 2.125(3), Rh(1)–C(24)
2.123(3), Rh(1)–H(7) 2.497; C(1)–Rh(1)–Cl(1) 85.56(9), C(7)–H(7)–Rh(1)
141.313, N(1)–C(1)–N(3) 102.2(3), Cl(1)–Rh(1)–C(1)–N(1) 94.4(3),
Cl(1)–Rh(1)–C(1)–N(3) -73.5(3).
˚
1.349(13), N(3)–C(1) 1.377(14), N(1)–N(2), 1.370(12), Rh(1)–C(14)
2.180(11), Rh(1)–C(15) 2.186(10), Rh(1)–C(18) 2.104(11), Rh(1)–C(19)
2.119(12), Rh(1)–H(9) 2.761; C(1)–Rh(1)–Cl(1) 87.9(3), C(9)–H(9)–Rh(1)
112.05, N(1)–C(1)–N(3) 101.8(8), Cl(1)–Rh(1)–C(1)–N(1) -98.0(9),
Cl(1)–Rh(1)–C(1)–N(3) 73.2(10).
dH 8.31 for H(7) in 11] can be classified as anagostic interactions.§22
˚
The observed C(carbene)–Rh bond lengths [2.041(12) A (10) and
˚
2.019(3) A (11)] are in the same range as those of related complexes
˚
based on diaminocarbenes [C(carbene)–Rh 2.00–2.06 A]. In both
structures, the mean Rh–C(COD) bonds trans to the carbene
˚
ligand are clearly longer (2.183 and 2.199 A, respectively) than
˚
those trans to the Cl ligand (2.107 and 2.124 A, respectively),
reflecting a strong trans influence of the Tripy ligand.
Scheme 3 Synthesis of [RhCl(Tripy)(CO)2] complexes.
Table 1 nCO stretching frequencies for complexes 13–16
We decided also to evaluate the effect of the annulation in
the [1,2,4]triazolo[4,3-a]pyridin-3-ylidene ligand properties. One
of the best established methods to evaluate the relative basicity of
a given ligand is based on the analysis of the infrared nCO stretching
frequencies of [M(ligand)X(CO)n] complexes.23 The availability of
data for a number of [RhCl(NHC)(CO)2] complexes prompted
us to synthesize the [RhCl(Tripy)(CO)2] derivatives 13–16. These
compounds were readily obtained from complexes 8–12 through
a fast and quantitative COD to CO ligand exchange, smoothly
performed by bubbling CO through a solution of the latter in
CHCl3 (Scheme 3).
The infrared spectra of 13–16 were recorded in solution and the
observed nCO stretching frequencies are collected in Table 1. These
values indicate a remarkable influence by the substitution pattern
of the heterocycles in the s-donor ability of some of the ligands.
All of them exhibit a higher basicity than monocyclic triazole-
3-ylidenes,24,25 and, within the series, a higher aromaticity of the
ring condensed with the triazole ring results in a better donor
ability by the ligand (entries 2–4), approaching typical values of
imidazol-2-ylidenes (Fig. 4).
Entry
Compound
n
CO sym/cm-1
n
CO asym/cm-1
n
CO average/cm-1
1
2
3
4
13
14
15
16
2083
2078
2088
2084
2007
2004
2005
2010
2045
2041
2046
2047
Taking into consideration the growing interest in palladium–
NHC complexes as catalysts in cross-coupling reactions, we
decided to prepare also a series of [PdCl(allyl)(Tripy)] complexes.
Exploiting again their relatively high acidity, salts 6–8 were made
to react with [PdCl(allyl)]2 in the presence of Et3N under mild
conditions (Scheme 4). The expected products 17 and 18 were
obtained in good yields (81 and 75%, respectively) but, in sharp
contrast, the analogous complex 19 was isolated in very poor 16%
yield under the same conditions.¶
¶ The steric properties of the carbene ligand in 19 are almost identical to
those of 18. On the other hand, the slight electronic difference (as measured
in the IR data of the corresponding [RhCl(Tripy)(CO)2] complexes 15 and
16) can hardly be invoked as a possible reason to explain the marked
difference in the observed behaviour of 19. However, lower stability of the
complex appears to be responsible for the much lower yield of the latter.
§ Anagostic interactions, also termed “preagostic”, “pregostic” or “pseu-
doagostic” interactions, are characterized by a M–H distance range of 2.3–
2.9 A, a M–H–C angle range of 110–170 , and chemical shifts typically
observed downfield of the uncoordinated hydrogen atoms. See ref. 22 for
pertinent discussions.
◦
˚
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7113–7120 | 7115
©

Fig. 5 ORTEP-like drawing of complex 19. Thermal ellipsoids are drawn
◦
˚
at the 30% probability level. Selected bond lengths [A] and bond angles [ ]:
Pd(1)–C(1) 2.031(6), N(1)–C(1) 1.386(7), N(2)–C(1) 1.360(7), N(2)–N(3)
1.376(6), Pd(1)–C(23) 2.115(6), Pd(1)–C(21) 2.172(7); Pd(1)–H(11) 2.601,
Pd(1)–H(20) 2.792, C(1)–Pd(1)–Cl(1) 95.17(16), N(2)–C(1)–N(1) 100.4(5),
Cl(1)–Pd(1)–C(1)–N(1) -79.5(6), Cl(1)–Pd(1)–C(1)–N(2) 102.4(4).
Fig. 4 Relative donor ability of Tripy ligands.
A preliminary evaluation of the catalytic activity of complexes
17–19 was attained from the Suzuki–Miyaura cross coupling of
a series of aryl halides with phenyl boronic acid. Similar results
were obtained with catalysts formed in situ from Pd(OAc)2 and
salts 6–8 as the ligand precursor in the presence of K3PO4 as the
base (Scheme 5). The results collected by using Pd(OAc)2/7 as a
representative example are summarized in Table 2. Excellent yields
of the coupling products were observed not only for aryl bromides
(entries 1, 2, 5, 6), but also for less reactive chlorides (entries 3, 4),
even with a low catalyst loading (0.5 mol%) and at relatively low
reaction temperatures.
Scheme 4 Synthesis of [PdCl(allyl)(Tripy)] complexes.
Scheme 5 Suzuki–Miyaura cross-coupling with Tripy ligands.
Table 2 Cross-coupling of aryl halides with phenyl boronic acid
The 1H NMR spectra of these compounds indicated the
presence of two sets of peaks which can be assigned to the mixture
of diastereomers that result from the exo and endo orientation of
the p-allyl ligand.
Suitable crystals of 19 for X-ray diffraction analysis were
obtained by slow diffusion of hexane into a concentrated solution
of the complex in CH2Cl2. The structure reveals the expected
square-planar geometry at the Pd(II) centre and, as in the Rh(I)
cases, the heterocycle plane is oriented near orthogonal to the
coordination plane (Fig. 5), a geometry that enables Pd–H
interactions with H(11) and H(20) that, can also be classified as
anagostic interactions according to the observed distances [Pd(1)–
Entry Starting material
T/◦C t/min Isolated yield (%)
1
2
3
4
5
6
4-Bromotoluene
2-Bromotoluene
4-Chlorotoluene
Methyl 4-chlorobenzoate 25
1-Bromonaphthalene
1-Bromonaphthalene
25
25
25
75
75
75
75
1800
120
96
97
97
99
58
96
25
65
˚
˚
H(11) 2.601 A; Pd(1)–H(20) 2.792 A] and angles [C(11)–H(11)–
Pd(1) 142.6◦; C(20)–H(20)–Pd(1) 104.2]. The Pd–C(allyl) distance
trans to the carbene ligand [Pd(1)–C(21) 2.172(7)] is longer than
that trans to the Cl ligand [Pd(1)–C(23) 2.115(6)], providing also
evidence in this case of a strong trans influence of the Tripy ligand.
Conclusions
In summary, the synthesis of Rh(I) and Pd(II) complexes
based on [1,2,4]triazolo[4,3-a]pyridin-3-ylidene ligands has been
7116 | Dalton Trans., 2009, 7113–7120
This journal is
The Royal Society of Chemistry 2009
©

◦
accomplished in only two steps from commercially available
pyridine derivatives. The analysis of infrared nCO stretching
frequencies in [RhCl(Tripy)(CO)2] complexes reveals that the
s-donor ability in these new bicyclic NHCs is dependent on the
substitution pattern of the heterocycle. From this study it was also
concluded that the benzannulation of the triazole-5-ylidene system
results in a higher basicity of the NHC ligand.
obtained as a light brown solid; mp 214–216 C; nmax(film)/cm-1
3118, 2925, 1739, 1629, 1442, 1226, 1056, 1033, 810 and 764;
dH[300 MHz; (CD3)2CO] 7.73–7.83 (3 H, m, Ar), 7.91–7.96 (1 H,
m, Ar), 8.00–8.09 (2 H, m, Ar), 8.24–8.30 (3 H, m, Ar), 8.44 (1
H, d, J 9.9, Ar), 8.74 (1 H, d, J 8.4, Ar) and 11.74 (1 H, s, 3-H);
dC[75 MHz; (CD3)2CO] 112.1, 117.8, 121.7, 124.3, 129.8, 130.4,
131.5, 132.0, 134.0, 135.8, 136.4 and 147.6; m/z (CI) 246.1038 (M+,
C16H12N3 requires 246.1031), 245 (49), 220 (11) and 128 (8%).
Experimental
2-Phenyl[1,2,4]triazolo[4,3-f ]phenanthridinium tetrafluorobo-
rate (8). From phenanthridine (4, 466 mg, 2.6 mmol), 8 (431 mg,
56%) was obtained as a light brown solid; mp 256–258 ◦C;
General methods
n
max(film)/cm-1 1618, 1571, 1468, 1440, 1239, 1029, 970, 834, 757,
Solvents were purified and dried by standard procedures. Flash
chromatography was carried out on silica-gel (0.040–0.063 mm or
0.015–0.040 mm). Melting points were recorded in a metal block
and are uncorrected. 1H NMR spectra were recorded at 300 MHz
or 500 MHz; 13C NMR spectra were recorded at 75 MHz or
125 MHz, with the solvent peak used as the internal reference.
J values are given in Hz.
715 and 685; dH[300 MHz; (CD3)2SO] 7.67–7.83 (6 H, m, Ar),
7.89 (1 H, t, J 7.8, Ar), 8.05 (2 H, d, J 7.2, Ar), 8.29 (1 H, d, J 7.8,
Ar), 8.45–8.57 (3 H, m, Ar) and 11.71 (1 H, s, 3-H); dC[75 MHz;
(CD3)2SO] 116.9, 118.3, 121.6, 122.6, 124.1, 125.2, 125.3, 127.3,
129.6, 130.3, 130.5, 130.8, 131.1, 131.8, 134.1, 135.1, 135.3 and
146.9; m/z (CI) 296.1180 (M+, C20H14N3 requires 296.1187), 295
(47), 271 (14), 195 (26), 194 (22), 178 (17) and 104 (11%).
General procedure for the preparation of
2-phenyl[1,2,4]triazolo[4,3-a]pyridin-2-ium salts 5–8
General procedure for the synthesis of RhCl(COD)(2-
phenyl[1,2,4]triazolo[4,3-a]pyridin-3-ylidene) complexes 9–12
A Schlenk tube was charged with triethyloxonium tetrafluorob-
orate (431 mg, 2.2 mmol) under an argon atmosphere and a
solution of N-methyl-N-nitrosoaniline (300 mg, 2.2 mmol) in dry
dichloroethane (1.5 mL) was added. The reaction mixture was
stirred at room temperature for 1 h and then cooled to 0–5 ◦C.
A solution of the corresponding pyridine derivative (2.6 mmol) in
dry dichloroethane (1.0 mL) was added dropwise and the mixture
stirred for 2 h. The reaction mixture is then warmed up to room
temperature and stirred for 15 h. The solvent was removed in vacuo
and cold CH2Cl2 was added to the residue. The corresponding
triazolium salts were isolated by filtration. Starting materials,
yields and spectral and analytical data for compounds 5–8 are
as follows:
To a suspension of 2-phenyl[1,2,4]triazolo[4,3-a]pyridin-2-ium
salts 5–8 (0.13 mmol) and [RhCl(COD)]2 (33.2 mg, 0.065 mmol)
in dry THF (1 mL) under Ar was added Et3N (21 mL, 0.143 mmol)
and the mixture was stirred for 4 h at rt. The solvent was removed
in vacuo and the residue purified by flash chromatography (2 : 1
CH2Cl2–hexane → CH2Cl2 → 100 : 1 CH2Cl2–MeOH). Starting
materials, yields and spectral and analytical data for compounds
9–12 are as follows:
Rhodium(I) complex 9. From 5 (37 mg, 0.13 mmol), flash
chromatography afforded 9 (51 mg, 89%) as a yellow solid; mp
◦
222–224 C; nmax(film)/cm-1 2918, 2890, 2827, 1643, 1578, 1493,
1342, 1280, 1056, 798 and 730; dH(500 MHz; C6D6) 1.68–1.84 (4
H, m, 2 ¥ CH2(COD)), 2.17–2.30 (4 H, m, 2 ¥ CH2(COD)), 2.83–2.86 (1
H, m, CH(COD)), 3.02–3.06 (1 H, m, CH(COD)), 5.58–5.62 (1 H, m,
CH(COD)), 5.68–5.72 (1 H, m, CH(COD)), 6.04 (1 H, t, J 7.0, Ar), 6.37
(1 H, td, J 7.0, J 2.2, Ar), 6.79 (1 H, d, J 7.0, Ar), 7.11 (1 H, t, J
7.5, Ph), 7.23 (2 H, t, J 7.5, Ph), 8.95 (2 H, d, J 7.5, Ph) and 9.31
(1 H, d, J 7.0, Ar); dC(125 MHz; C6D6) 28.8, 29.2, 32.1, 33.2, 68.6
(d, JRh,C 13.8), 69.2 (d, JRh,C 12.5), 99.1 (d, JRh,C 6.3), 99.7 (d, JRh,C
7.5), 112.9, 114.4, 124.3, 128.5, 128.6, 130.5, 130.7, 140.5, 148.2
and 181.3 (d, JRh,C 52.0); m/z (CI) 441.0048 (M+, C20H21N3ClRh
requires 441.0036), 406 (96, M - Cl), 196 (48) and 195 (10%,
M - RhClCOD).
2-Phenyl[1,2,4]triazolo[4,3-a]pyridinium tetrafluoroborate (5).
From pyridine (1, 213 mL, 2.6 mmol), 5 (268 mg, 43%) was
◦
obtained as a light brown solid; mp 203–205 C; nmax(film)/cm-1
3449, 3005, 1713, 1635, 1497, 1363, 1225, 1187 and 1092;
dH[300 MHz; (CD3)2SO]: 6.61 (1 H, t, J 7.0, py), 6.79–6.88 (3
H, m, Ph), 7.03 (1 H, t, J 7.0, py), 7.16 (2 H, d, J 7.7, Ph), 7.29 (1
H, d, J 7.0, py), 7.94 (1 H, d, J 7.0, py) and 10.52 (1 H, s, 3-H);
dC[75 MHz; (CD3)2SO] 120.1, 127.5, 128.8, 130.4, 134.6, 143.1,
147.0, 148.6, 149.1 and 150.6; m/z (CI) 196.0874 (M+, C12H10N3
requires 196.0872), 108 (11) and 80 (100%)
5-Methyl-2-phenyl[1,2,4]triazolo[4,3-a]pyridinium tetrafluorobo-
rate (6). From 2-methylpyridine (2, 267 mL, 2.6 mmol), 6
(248 mg, 38%) was obtained as a white solid; mp 226–228 ◦C;
Rhodium(I) complex 10. From 6 (40 mg, 0.13 mmol), flash
chromatography afforded 10 (51 mg, 86%) as a yellow solid. X-Ray
quality crystals were grown by slow diffusion of hexane into a solu-
tion of 10 in CH2Cl2; mp 208 ◦C (decomp.); nmax(film)/cm-1 2919,
2897, 1652, 1543, 1496, 1413, 1335, 1091 and 764; dH(500 MHz;
C6D6) 1.54–1.70 (4 H, m, 2 ¥ CH2(COD)), 2.07–2.22 (4 H, m, 2 ¥
CH2(COD)), 2.67–2.70 (1 H, m, CH(COD)), 2.89–2.93 (1 H, m, CH(COD)),
3.82 (3 H, s, Me), 5.38–5.43 (1 H, m, CH(COD)), 5.59–5.64 (1 H, m,
CH(COD)), 5.83 (1 H, d, J 6.7, Ar), 6.37 (1 H, dd, J 9.0, 8.0, Ar), 6.83
(1 H, d, J 9.0, Ar), 7.12 (1 H, t, J 7.6, Ph), 7.24 (2 H, t, J 7.6, Ph)
and 8.91 (2 H, d, J 7.6, Ph); dC(125 MHz; C6D6) 23.9, 29.0, 29.1,
32.1, 33.2, 67.5 (d, JRh,C 13.9), 70.4 (d, JRh,C 14.3), 96.3 (d, JRh,C 7.5),
n
max(film)/cm-1 3153, 3126, 2344, 1656, 1540, 1506, 1420, 1287,
1053, 1028, 917, 783, 762 and 682; dH[300 MHz; (CD3)2CO] 2.82
(3 H, s, Me), 7.48 (1 H, d, J 6.9, py), 7.78–7.82 (3 H, m, Ph,
py), 7.97–8.10 (2 H, m, Ph), 8.23–8.26 (2 H, m, Ph) and 11.23 (1
H, s, 3-H); dC[75 MHz; (CD3)2CO] 17.2, 112.8, 117.8, 122.0, 130.4,
131.6, 132.3, 135.0, 135.9, 136.9 and 148.6; m/z (CI) 210.1036 (M+,
C13H12N3 requires 210.1031), 209 (63) and 80 (49%).
2-Phenyl[1,2,4]triazolo[4,3-a]quinolinium tetrafluoroborate (7).
From quinoline (3, 315 mL, 2.6 mmol), 7 (308 mg, 42%) was
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7113–7120 | 7117
©

98.1 (d, JRh,C 7.6), 112.8, 113.5, 126.3, 128.6, 128.9, 130.2, 140.5,
142.3, 150.1 and 180.0 (d, JRh,C 52.0); m/z (CI) 455.0662 (M+,
C21H23N3ClRh requires 455.0636), 420 (100, M - Cl), 312 (5, M -
COD), 210 (38), 209 (7, M - RhClCOD), 75 (58) and 61 (100%).
General procedure for the synthesis of dicarbonyl rhodium
complexes 13–16
CO was bubbled through a solution of 9–12 (0.05 mmol) in CDCl3
(0.5 mL) for 15 min. The solvent was then concentrated and the
residue was dried in vacuum to give the dicarbonyl complexes in
quantitative yields. Spectral and analytical data for compounds
13–16 are as follows:
Crystal structure determination of complex 10.
Crystal data. C21H23ClN3Rh, M = 455.78, monoclinic, space
group C2/c (no. 15), a = 23.5474(17), b = 16.3564(12), c =
Complex 13.
n
max(film)/cm-1 2910, 2083, 2007, 1648, 1540,
◦
3
˚
˚
9.7329(7) A, b = 91.310(3) , V = 3747.7(5) A , T = 100(2) K,
Z = 8, 39366 reflections measured, 5734 unique (Rint = 0.0487).
The final wR(F2) was 0.1781 (all data).
1500, 1280, 910 and 871; dH(400 MHz; CDCl3) 6.99 (1H, t, J
7.2, Ar), 7.49–7.67 (5H, m, Ar), 8.22 (1H, d, J 7.2, Ar), 8.72 (1H,
d, J 7.2, Ar) and 9.06 (1H, d, J 7.2, Ar); dC(100 MHz; CDCl3)
114.8, 115.0, 124.8 (2C), 125.5, 129.1 (2C), 130.0, 131.9, 139.3,
148.9 (Ar), 170.8 (d, JRh-C 43.5, C-3), 181.2 (d, JRh-C 72.8, CO) and
184.5(d, JRh-C 55.5, CO); m/z (CI) 391.6226 (M+, C14H11N3ClRhO2
requires 391.6221), 328 (100%, M+ - Cl - CO).
Rhodium(I) complex 11. From 7 (43 mg, 0.13 mmol), flash
chromatography afforded 11 (59 mg, 93%) as a yellow solid. X-
Ray quality crystals were grown by slow diffusion of hexane into
a solution of 11 in CH2Cl2; mp 232–234 ◦C; nmax(film)/cm-1 3343,
2917, 2880, 2833, 1630, 1595, 1561, 1497, 1450, 1356, 1333, 1252,
1064, 863, 804, 758, 729 and 690; dH(500 MHz; CDCl3) 1.59–1.68
(2 H, m, CH2(COD)), 1.87–1.98 (3 H, m, CH2(COD)), 2.19–2.27 (1
H, m, CH2(COD)), 2.34–2.42 (1 H, m, CH2(COD)), 2.47–2.54 (1 H, m,
CH2(COD)), 2.74–2.76 (1 H, m, CH(COD)), 3.17 (1 H, t, J 7.0, CH(COD)),
5.29–5.34 (2 H, m, CH(COD)), 7.39 (1 H, d, J 9.5, Ar), 7.57 (1 H,
t, J 7.0, Ar), 7.62–7.68 (4 H, m, Ar), 7.77 (1 H, dd, J 8.0, J 1.0,
Ar), 7.94 (1 H, td, J 8.0, J 1.0, Ar), 8.74 (2 H, d, J 8.5, Ar) and
11.67 (1 H, d, J 8.0, Ar); dC(125 MHz; CDCl3) 28.7, 29.1, 31.5,
33.2, 69.2 (d, JRh,C 13.8), 71.2 (d, JRh,C 13.8), 97.5 (d, JRh,C 7.5), 98.9
(d, JRh,C 7.5), 112.9, 121.5, 125.3, 127.1, 128.7, 128.9, 129.0, 130.1,
133.1, 124.1, 133.7, 141.2, 148.4 and 181.3 (d, JRh,C 52.0); m/z (CI)
491.0667 (M+, C24H23N3ClRh requires 491.0636), 456 (100, M -
Cl), 246 (41), 245 (38, M - RhClCOD) and 67 (63%).
Complex 14.
n
max(film)/cm-1 2924, 2078, 2004, 1650, 1539,
1497, 1290, 1151 and 911; dH(300 MHz; CDCl3) 3.44 (3H, s, Me),
6.77 (1H, d, J 6.7, py), 7.41 (1H, dd, J 9.3, 6.7, py), 7.56–7.61
(4H, m, H-8, py, Ar) and 8.14–8.18 (2H, m, Ar); dC(75 MHz;
CDCl3) 24.7 (CH3), 113.1, 115.0, 126.1 (2C), 129.0 (2C), 130.0,
131.5, 139.6, 140.6, 150.2 (Ar), 169.6 (d, JRh-C 44.5, C-3), 181.6 (d,
JRh-C 73.9, CO) and 184.9 (d, JRh-C 56.5, CO); m/z (CI) 403.6406
(M+, C15H11N3ClRhO2 requires 403.6394), 340 (100%, M+ - Cl -
CO).
Complex 15.
n
max(film)/cm-1 3442, 2088, 2005, 1629, 1560, 815
and 763; dH(300 MHz; CDCl3) 7.51–7.68 (5H, m, Ar), 7.78–7.86
(3H, m, Ar), 8.19–8.24 (2H, m, Ar) and 10.49 (1H, d, J 8.4, Ar);
dC(75 MHz; CDCl3) 112.7, 121.0, 124.2, 125.9 (2C), 127.8, 129.1
(2C), 129.3, 130.0, 130.3, 133.1, 133.9, 140.6, 148.8 (Ar), 172.1 (d,
J
Rh-C 43.8, C-3), 181.3 (d, JRh-C 73.9, CO), 185.0 (d, JRh-C 56.2, CO);
m/z (CI) 439.6720 (M+, C18H11N3RhClO2 requires 439.6727), 376
(100%, M - Cl - CO).
Crystal structure determination of complex 11.
Crystal data. C25H25Cl3N3Rh [C24H23ClN3Rh·CH2Cl2], M =
Complex 16.
n
max(film)/cm-1 3433, 2084, 2047, 2010, 1650,
576.74, orthorhombic, space group Pbca (no. 61), a = 11.2461(5),
1628, 1558, 1529, 846 and 751; dH(400 MHz; CDCl3) 7.58–7.69
(5H, m, Ar), 7.74–7.81 (2H, m, Ar), 8.25 (2H, d, J 8.2, Ar), 8.39
(1H, d, J 8.2, Ar), 8.42 (1H, d, J 8.2, Ar), 8.51 (1H, d, J 7.2,
Ar) and 10.52 (1H, d, J 8.2, Ar); dC(100 MHz; CDCl3) 119.0,
121.7, 122.6, 123.8, 125.1, 126.0 (2C), 128.0, 129.1 (2C), 129.2,
129.3, 129.7, 129.9, 131.3, 132.2, 140.7, 148.6 (Ar), 173.2 (d, JRh-C
43.7, C-3), 181.3 (d, JRh-C 73.7, CO), 185.0 (d, JRh-C 55.8, CO);
m/z (CI) 491.7436 (M+, C22H15N3RhClO2 requires 491.7432), 428
(100%, M - Cl - CO).
3
˚
˚
b = 20.3332(9), c = 20.7475(9) A, V = 4744.3(4) A , T = 100(2)
K, Z = 8, 32732 reflections measured, 7229 unique (Rint = 0.0358).
The final wR(F2) was 0.1397 (all data).
Rhodium(I) complex 12. From 8 (50 mg, 0.13 mmol), flash
chro◦matography afforded 12 (58 mg, 83%) as a yellow solid; mp
244 C (decomp.); nmax(film)/cm-1 2935, 2879, 2831, 1619, 1598,
1565, 1496, 1462, 1444, 1377, 1306, 1250, 972, 754, 734 and 717;
dH(500 MHz; CDCl3) 1.60–1.68 (2 H, m, CH2(COD)), 1.88–1.98 (3
H, m, CH2(COD)), 2.18–2.27 (1 H, m, CH2(COD)), 2.36–2.43 (1 H,
m, CH2(COD)), 2.50–2.55 (1 H, m, CH2(COD)), 2.76–2.78 (1 H, m,
CH(COD)), 3.19 (1 H, t, J 7.0, CH(COD)), 5.30–5.33 (2 H, m, CH(COD)),
7.57–7.70 (2 H, m, Ar), 7.74–7.77 (3 H, m, Ar), 7.91 (1 H, t, J
7.8, Ar), 8.37 (1 H, d, J 8.0, Ar), 8.41 (1 H, d, J 8.0, Ar), 8.47
(1 H, d, J 7.5, Ar), 8.77 (2 H, d, J 8.0, Ar) and 11.75 (1 H, dd,
J 8.0, 1.0, Ar); dC(125 MHz; CDCl3) 28.7, 29.1, 31.5, 33.1, 69.3
(d, JRh,C 14.0), 71.2 (d, JRh,C 14.1), 97.3 (d, JRh,C 7.4), 98.8 (d, JRh,C
7.4), 119.5, 122.3, 122.7, 123.5, 124.9, 125.4, 127.4, 128.7, 128.8,
128.9, 129.2, 129.7, 131.6, 131.9, 141.3, 148.1 and 182.4; m/z (CI)
541.0818 (M+, C28H25N3ClRh requires 541.0792), 506 (94, M -
Cl), 457 (33), 296 (44), 295 (7, M - RhClCOD), 195 (46) and 67
(100%).
General procedure for the synthesis of PdCl(allyl)(2-
phenyl[1,2,4]triazolo[4,3-a]pyridin-3-ylidene) complexes 17–19
To a suspension of salts 6–8 (0.1 mmol) and [PdCl(allyl)]2 (17 mg,
0.05 mmol) in dry THF (1 mL) under Ar was slowly added Et3N
(14 mL, 0.11 mmol) and the mixture was stirred for 1 h at rt.
The solvent was removed in vacuo and the residue was purified by
flash chromatography (2 : 1 EtOAc–hexane). Starting materials,
yields and spectral and analytical data for compounds 17–19 are
as follows:
Monocarbene palladium complex 17. From
0.1 mmol), flash chromatography afforded 17 (30 mg, 81%) as
a light brown solid; dH(300 MHz; CDCl3) 1.82 (0.55 H, d, J 12.0,
6 (29 mg,
7118 | Dalton Trans., 2009, 7113–7120
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The Royal Society of Chemistry 2009
©

anti CHH, allyl), 2.32 (0.45 H, d, J 12.0, anti CHH, allyl), 3.05
(0.45, H, d, J 6.9, syn CHH, allyl), 3.22 (0.55 H, d, J 13.2, anti
CHH, allyl), 3.24 (1.65 H, s, CH3), 3.30–3.34 (1 H, m, CHH, allyl),
3.53 (1.35 H, s, CH3), 4.31 (1 H, td, J 7.2, 2.4, syn CHH, allyl),
4.91–5.04 (0.45 H, m, central CH, allyl), 5.22–5.35 (0.55 H, m,
central CH, allyl), 6.65 (0.55 H, d, J 6.6, CHArom), 6.72 (0.45 H, d,
J 6.9, CHArom), 7.35 (1 H, dd, J 15.6, 9.0, CHArom), 7.45–7.59 (4 H,
m, CHArom), 8.17 (0.9 H, d, J 8.1, CHArom) and 8.39 (1.1 H, d, J 8.1,
CHArom); dC(75 MHz; CDCl3) 22.8, 51.3, 51.8, 71.2, 113.0, 113.0,
113.6, 114.0, 114.1, 114.8, 125.1, 125.3, 128.9, 128.9, 129.2, 129.4,
131.1, 140.1, 140.8, 141.1, 150.1 and 175.6; m/z (CI) 356.0403
(M+ - Cl, C16H16N3Pd requires 356.0379) and 210 (100%).
Acknowledgements
We thank the Spanish ‘Ministerio de Ciencia y Tecnolog´ıa’
(grants CTQ2007–61915 and CTQ2007-60244) and the Junta de
Andaluc´ıa (grant 2005/FQM-658) for financial support. J. I.-S.
thanks the Ministerio de Ciencia e Innovacio´n for a predoctoral
fellowship.
Notes and references
1 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290; (b)
N-Heterocyclic Carbenes in Synthesis, ed. S. P. Nolan, Wiley-VCH,
2006; (c) N-Heterocyclic Carbenes in Transition Metal Catalysis, ed.
F. Glorius, Springer, Berlin, Heidelberg, New York, 2007; (d) E. A. B.
Kantchev, C. J. O’Brien and M. G. Organ, Angew. Chem., Int. Ed.,
2007, 46, 2768; (e) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int.
Ed., 2008, 47, 3122.
Monocarbene palladium complex 18. From
7 (54 mg,
0.1 mmol), flash chromatography afforded 18 (51 mg, 75%) as
a white solid; dH(500 MHz; CDCl3) 1.83 (0.67 H, d, J 12.0, anti
CHH, allyl), 2.41 (0.33 H, d, J 11.5, anti CHH, allyl), 3.05 (0.41 H,
d, J 6.0 Hz, syn CHH, allyl), 3.29–3.33 (1.14 H, m, CHH, allyl),
3.51 (0.45 H, d, J 13.0, anti CHH, allyl), 4.43 (1 H, d, J 7.0, syn
CHH, allyl), 5.00–5.08 (0.41 H, m, central CH, allyl), 5.35–5.47
(0.59 H, m, central CH, allyl), 7.47–7.61 (5 H, m, CHArom), 7.69–
7.79 (3 H, m, CHArom), 8.22 (0.89 H, d, J 7.5, CHArom), 8.42 (1.11 H,
d, J 7.5, CHArom), 10.00 (0.56 H, d, J 8.5, CHArom) and 10.33 (0.44
H, d, J 8.5, CHArom); dC(125 MHz; CDCl3) 51.4, 51.6, 71.8, 72.0,
77.2, 112.6, 112.7, 114.3, 115.3, 119.2, 119.4, 123.9, 124.0, 124.7,
125.0, 127.2, 127.2, 128.9, 129.0, 129.1, 129.1, 129.3, 130.4, 130.5,
133.4, 133.6, 148.6 and 177.6; m/z (FAB) 387 (14, M - allyl), 237
(100), 173 (31), 153 (31) and 131 (69%).
2 G. C. Vougioukalakis and R. H. Grubbs, J. Am. Chem. Soc., 2005, 130,
2234.
3 (a) D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J. Melder,
K. Ebel and S. Brode, Angew. Chem., Int. Ed. Engl., 1995, 34, 1021;
(b) A. Fu¨rstner, L. Ackermann, B. Gabor, R. Goddard, C. W. Lehmann,
R. Mynott, F. Stelzer and O. R. Thiel, Chem.–Eur. J., 2001, 7, 3236;
(c) Review: D. Enders and H. Gielen, J. Organomet. Chem., 2001, 617–
618, 70.
4 (a) E. Despagnet-Ayoub and R. H. Grubbs, J. Am. Chem. Soc.,
2004, 126, 10198; (b) E. Despagnet-Ayoub and R. H. Grubbs,
Organometallics, 2005, 24, 338.
5 (a) R. W. Alder, M. E. Blake, C. Bortolotti, S. Bufali, C. P. Butts,
E. Linehan, J. M. Oliva, A. G. Orpen and M. J. Quayle, Chem.
Commun., 1999, 241; (b) M. Mayr, K. Wurst, K.-H. Ongania and M. R.
Buchmeiser, Chem.–Eur. J., 2004, 10, 1256.
6 (a) R. Jazzar, H. Liang, B. Donnadieu and G. Bertrand, J. Organomet.
Chem., 2006, 691, 3201; (b) M. Iglesias, D. J. Beetstra, A. Stasch, P. N.
Horton, M. B. Hursthouse, S. J. Coles, K. J. Cavell, A. Dervisi and I. A.
Fallis, Organometallics, 2007, 26, 4800; (c) M. Iglesias, D. J. Beetstra,
J. C. Knight, L.-L. Ooi, A. Stasch, S. Coles, L. Male, M. B. Hursthouse,
K. J. Cavell, A. Dervisi and I. A. Fallis, Organometallics, 2008, 27,
3279.
7 (a) V. Lavallo, Y. Canac, C. Pra¨sang, B. Donnadieu and G. Bertrand,
Angew. Chem., Int. Ed., 2005, 44, 5705; (b) V. Lavallo, Y. Canac, A.
DeHope, B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2005,
44, 7236.
8 (a) S. Grundemann, A. Kovacevic, M. Albrecht, J. W. Faller and
R. H. Crabtree, J. Am. Chem. Soc., 2002, 124, 10473; (b) A. R.
Chianese, A. Kovacevic, B. M. Zeglis, J. W. Faller and R. H. Crabtree,
Organometallics, 2004, 23, 2461; (c) L. N. Appelhans, D. Zuccaccia, A.
Kovacevic, A. R. Chianese, J. R. Miecznikowski, A. Macchioni, E. Clot,
O. Eisenstein and R. H. Crabtree, J. Am. Chem. Soc., 2005, 127, 16299;
(d) L. Yang, A. Kru¨ger, A. Neels and M. Albrecht, Organometallics,
2008, 27, 3161; (e) G. Song, Y. Zhang and X. Li, Organometallics, 2008,
27, 1936; (f) Review: P. Arnold and S. Pearson, Coord. Chem. Rev., 2007,
251, 596.
Monocarbene palladium complex 19. From
8 (38 mg,
0.1 mmol), flash chromatography afforded 19 (15 mg, 16%) as
a white solid; dH(500 MHz; CDCl3) 1.88 (d, J 12.5, anti CHH,
allyl), 2.47–2.27 (m, anti CHH, allyl), 3.07 (0.43H, d, J 6.7, syn
CHH, allyl), 3.33–3.29 (1.14H, m, syn CHH + anti CHH, allyl),
3.51 (0.43H, d, J 13.0, anti CHH, allyl), 4.44 (1H, m, anti CHH,
allyl), 5.10–4.95 (0.43H, m, central CH, allyl), 5.44–5.34 (0.57H,
m, central CH, allyl), 8.54–87.36 (12H, m, CHArom), 10.07 (0.57H,
dd, J 8.2, J 0.9, CHArom) and 10.41 (0.43H, d, J 8.1, CHArom);
dC(125 MHz; CDCl3) 51.5, 51.6, 71.7, 72.0, 77.4, 114.4, 115.3,
119.3, 120.0, 120.1, 123.7, 123.9, 124.9, 125.1, 127.5, 129,0, 129,3,
129,5, 129.7, 131,8, 141,0, 141.1, 148.4 and 179.0.
Crystal structure determination of complex 19.
9 (a) J. S. Owen, J. A. Labinger and J. E. Bercaw, J. Am. Chem.
Soc., 2004, 126, 8247; (b) G. Song, Y. Zhang, Y. Su, W. Deng, K.
Han and X. Li, Organometallics, 2008, 27, 6193, and references cited
therein.
Crystal data. C23H18ClN3Pd, M = 478.25, monoclinic, space
˚
group P21/c (no. 14), a = 9.5846(6), b = 30.038(2), c = 7.0204(5) A,
◦
3
˚
b = 108.420(3) , V = 1917.6(2) A , T = 100(2) K, Z = 4, 15674
reflections measured, 3931 unique (Rint = 0.0739). The final wR(F2)
10 (a) F. E. Hahn, L. Wittenbecher, R. Boese and D. Bl
a¨ser, Chem.–
Eur. J., 1999, 5, 1931; (b) F. E. Hahn, L. Wittenbecher, D. Le Van and
R. Fro¨hlich, Angew. Chem., Int. Ed., 2000, 39, 541.
11 H. V. Huynh, N. Meier, T. Pape and F. E. Hahn, Organometallics, 2006,
25, 3012.
was 0.1274 (all data).
General procedure for Suzuki–Miyaura cross-coupling
12 (a) P. Bazinet, G. P. A. Yap and D. S. Richeson, J. Am. Chem. Soc.,
2003, 125, 13314; (b) P. Bazinet, T.-G. Ong, J. S. O’Brien, N. Lavoie, E.
Bell, G. P. A. Yap, I. Korobkov and D. S. Richeson, Organometallics,
2007, 26, 2885.
A solution of 7 (0.5 mol%, 0.002 mmol) in toluene (1 mL) was
added to a mixture of the aryl halide (0.4 mmol), phenyl boronic
acid (0.6 mmol) and K3PO4 (170 mg, 0.8 mmol). A solution of
Pd(OAc)2 (0.002 mmol) in toluene (1 mL) was then added and
the mixture was stirred until consumption of the starting halide
(TLC monitoring). The resulting residue was purified by flash
chromatography using hexane as eluent.
13 S. G
o´mez-Bujedo, M. Alcarazo, C. Pichon, E. Alvarez, R. Ferna´ndez
and J. M. Lassaletta, Chem. Commun., 2007, 1180.
14 (a) M. Alcarazo, S. J. Roseblade, A. R. Cowley, R. Ferna´ndez, J. M.
Brown and J. M. Lassaletta, J. Am. Chem. Soc., 2005, 127, 3290; (b) S. J.
´
Roseblade, A. Ros, D. Monge, M. Alcarazo, E. Alvarez, J. M. Lassaletta
and R. Ferna´ndez, Organometallics, 2007, 26, 2570.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7113–7120 | 7119
©

15 C. Burstein, C. W. Lehmann and F. Glorius, Tetrahedron, 2005, 61,
6207.
22 For an overview of anagostic interactions, see: (a) M. Brookhart,
M. L. H. Green and G. Parkin, Proc. Natl. Acad. Sci. U. S. A., 2007,
104, 6908; (b) See also: M. Montag, I. Efremenko, R. Cohen, G. Leitus,
L. J. W. Shimon, Y. Diskin-Posner, Y. Ben-David, J. M. L. Martin and
D. Milstein, Chem.–Eur. J., 2008, 14, 8183.
23 (a) C. A. Tolman, Chem. Rev., 1977, 77, 313; (b) A. R. Chianese, X.
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7120 | Dalton Trans., 2009, 7113–7120
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