Phthalazin-2-ylidenes as cyclic amino aryl carbene ligands in rhodium(I) and iridium(I) complexes

Article abstract of DOI:10.1021/om100723b

The direct metalation of phthalazinium halides with [Rh( ^tBuO)(COD)]₂ or [Ir(^tBuO)(COD)]₂ led to the isolation of stable [MCl(phthalazin-2-ylidene)X(COD)] (M = Rh, Ir) as a new family of cyclic amino aryl carben

Full text of DOI:10.1021/om100723b

Organometallics 2010, 29, 5941–5945 5941
DOI: 10.1021/om100723b
Phthalazin-2-ylidenes As Cyclic Amino Aryl Carbene Ligands in
Rhodium(I) and Iridium(I) Complexes
†
§
†
,§
ꢀ
Antonio Magriz, Silvia G oꢀ mez-Bujedo, Eleuterio Alvarez, Rosario Fern ꢀa ndez,* and
,†
Jos ꢀe M. Lassaletta*
†
Instituto de Investigaciones Quı´micas (CSIC-USe), C/Americo Vespucio 49, 41092 Seville,
§
Spain, and Departamento de Quı´mica Org aꢀ nica, Facultad de Quı´mica, Universidad de Sevilla,
Apartado de Correos 1203, 41071 Seville, Spain
Received July 24, 2010
t
The direct metalation of phthalazinium halides with [Rh( BuO)(COD)] or [Ir( BuO)(COD)] led
t
2
2
to the isolation of stable [MCl(phthalazin-2-ylidene)X(COD)] (M = Rh, Ir) as a new family of cyclic
amino aryl carbene transition metal complexes. The X-ray diffraction analysis of several representa-
˚
tives systematically revealed a relatively short C(aryl)-C(carbene) bond distance of 1.44-1.46 A,
2
2
shorter than the typical C(sp )-C(sp ) single bond, indicating some electron density delocalization
from the π system of the aryl ring over the vacant carbene orbital.
4
5
B and D (Figure 1). Despite efforts directed to their
Introduction
synthesis, free cyclic amino aryl carbenes C remain as an
unknown family of compounds.
N-Heterocyclic carbenes (NHCs) have emerged during the
past decade as one of the most useful families of ancillary
Concerning the formation of transition metal complexes
and their application in catalysis, CAAC (aliphatic) deriva-
tives are emerging as an interesting alternative for specific
1
ligands for transition-metal-catalyzed reactions. In addition
to the classic NHCs stabilized by two adjacent heteroatoms,
a growing number of related carbenes stabilized by a single
nitrogen atom are emerging as interesting alternatives that
exhibit even higher σ-donor abilities. Imidazolium-derived
ligands with the carbenic carbon at C4 or C5 as well as the
pyridinylidene family of NHCs constitute representative
6
applications. On the other hand, the aromatic analogues
(cyclic amino aryl carbenes, CAArCs) C are particularly
attractive for the opportunities that substitution at the
aromatic ring offers for a fine-tuning of the steric and
electronic properties. Though the isolation of free carbenes
C remains, as mentioned above, elusive, indirect routes to a
few transition metal complexes have been reported (C1-C4,
2
examples. In a fundamental contribution to this field,
Bertrand and co-workers first demonstrated that a single
nitrogen atom effects the required stabilization that makes
possible the isolation of stable singlet carbenes, as high-
7
Chart 1). In this context, we recently demonstrated that
isoquinoline-derived Rh(I) amino aryl carbenes C5 can be
readily synthesized from the corresponding isoquinolinium
salts by treatment with strong bases and subsequent reaction
3
lighted with the isolation of acyclic amino aryl carbenes A
and both acyclic and cyclic amino alkyl carbenes (CAACs)
with [RhCl(COD)] , in a reaction that proceeds via isoqui-
2
8
nolinium-base adducts that exhibited carbene reactivity.
We speculated that the presence of an additional nitrogen
*
To whom correspondence should be addressed. E-mail: ffernan@
us.es; jmlassa@iiq.csic.es.
1) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
b) Nolan, S. P., Ed. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH:
New York, 2006. (c) Glorius, F., Ed. N-Heterocyclic Carbenes in Transition
Metal Catalysis; Springer: Berlin, 2007. (d) Hahn, F. E.; Jahnke, M. C.
Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (e) Díez-Gonz ꢀa lez, S.;
Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612–3676.
(
(
(6) (a) Kato, T.; Maerten, E.; Baceiredo, A. In Transition Metal
Complexes of Neutral η -Carbon Ligands. Topics in Organometallic
1
Chemistry; Chauvin, R.; Canac, Y., Eds.; Springer-Verlag: Berlin, 2010, Vol.
30, pp 131-147. (b) Frey, G.D. D.; Lavallo, V.; Donnadieu, B.;
Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439–441. (c) Lavallo,
V.; Frey, G. D.; Kousar, S.; Donnadieu, B.; Bertrand, G. Proc. Natl. Acad.
Sci. U. S. A 2007, 104, 13569–13573. (d) Lavallo, V.; Frey, G. D.;
Donnadieu, B.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed.
2008, 47, 5224–5228. (e) Zeng, X.; Kinjo, R.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2010, 49, 942–945.
(2) (a) Review: Schuster, O.; Yang, L.; Raubenheimer, H. G.;
Albrecht, M. Chem. Rev. 2009, 109, 3445–3478. See also: (b) Aldeco-
Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.;
Bertrand, G. Science 2009, 326, 556–559. (c) Zeng, X.; Frey, G. D.; Kinjo,
R.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2009, 131, 8690–8696.
(
d) Jazzar, R.; Dewhurst, R. D.; Bourg, J. B.; Donnadieu, B.; Canac, Y.;
Bertrand, G. Angew. Chem., Int. Ed. 2007, 46, 2899–2902.
3) (a) Sol ꢀe , S.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.;
(7) (a) Direct metalation from imidazo[1,5-a]pyridinium salts:
Alcarazo, M.; Roseblade, S. J.; Cowley, A. R.; Fern ꢀa ndez, R.; Brown,
J. M.; Lassaletta, J. M. J. Am. Chem. Soc. 2005, 127, 3290–3291. (b)
Tautomerization from a 1,4-benzodiazepine complex: Gribble, M. W., Jr.;
Ellman, J. A.; Bergman, R. G. Organometallics 2008, 27, 2152–2155. (c)
Oxidative addition: Jazzar, R.; Bourg, J.-B.; Dewhurst, R. D.; Donnadieu, B.;
Bertrand, G. J. Org. Chem. 2007, 72, 3492–3499. (d) Direct metalation from
indazolium salts: Jothibasu, R.; Huynh, H. V. Chem. Commun. 2010, 46,
2986–2988.
(
Bertrand, G. Science 2001, 292, 1901–1903. (b) Catto €e n, X.; Gornitzka, H.;
Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 1342–1343.
4) Lavallo, V.; Mafhouz, J.; Canac, Y.; Donnadieu, B.; Schoeller,
W. W.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 8670–8671.
5) (a) Lavallo, V.; Canac, Y.; Pr €a sang, C.; Donnadieu, B.; Bertrand,
G. Angew. Chem., Int. Ed. 2005, 44, 5705–5709. (b) Lavallo, V.; Canac, Y.;
DeHope, A.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2005, 44,
7
(
(
ꢀ
(8) G oꢀ mez-Bujedo, S.; Alcarazo, M.; Pichon, C.; Alvarez, E.;
236–7239.
Fern ꢀa ndez, R.; Lassaletta, J. M. Chem. Commun. 2007, 1180–1182.
r 2010 American Chemical Society
Published on Web 10/04/2010
pubs.acs.org/Organometallics

5
942 Organometallics, Vol. 29, No. 22, 2010
Magriz et al.
Scheme 1. Synthesis of Phthalazinium Salts 2a-e
Figure 1. Cyclic and acyclic monoaminocarbenes.
Chart 1. Transition Metal Complexes of Cyclic Amino Aryl
Carbenes
Scheme 2. Synthesis of Rh(I) and Ir(I) Phthalazin-1-ylidene
Complexes 5 and 6
at position 3 in these structures should reduce the basicity of
the carbene ligand, therefore making it easier to deprotonate
the parent, more acidic azolium salts. In agreement with this
hypothesis, literature data corroborate that phthalazinium
9
salts are much more acidic than isoquinolinium analogues.
In spite of this apparent experimental advantage, a survey of
the literature data reveals that phthalazin-1-ylidene mono-
amino carbenes have never been described. There are only
reports on the dimerization of l-hydroxy-2-aryl-1,2-dihydroph-
0
thalazines yielding 1,1 -bis(2-aryl-l,2-dihydrophthalazinyli-
10
denes), a process supposed to take place via transient free
carbenes. This lack of precedents applies also to the monocyclic
series: though a growing number of studies on pyridin-2-ylidenes
2a
transition metal complexes have been reported in recent years,
1
0c
chloric acid for the introduction of tertiary [tert-butyl (2d)]
or aromatic [p-anisyl (2e)] groups.
Attempts to isolate or detect the free carbenes by deprotona-
the pyridazin-2-ylidene analogues remain unexplored. We now
report on the synthesis and structural characterization of phtha-
lazin-1-ylidene Rh(I) and Ir(I) complexes C6.
t
tion of the phthalazinium salts 2 by strong bases (KO Bu,
i
LiN Pr ) failed under a variety of conditions. Disappointingly,
Results and Discussion
2
the relatively high acidity of these salts did not enable direct
metalation reaction with Ag O according to the popular method
2
A series of phthalazinium salts 2, required as starting
materials, were synthesized following two different methods:
(a) alkylation of phthalazine 1 itself; this procedure was used
for the introduction of primary (methyl, 2a) and secondary
11
developed by Lin and co-workers. Nevertheless, the reaction
t
of azolium salts 2b-d with [Rh(O Bu)(COD)]
, generated
in situ from KO Bu and [RhCl(COD)] , afforded the expected
2
2
t
12
alkyl groups with moderate to high steric demand [isopropyl
2b) and benzhydryl (2c)] (Scheme 1); and (b) condensation
of phthalaldehyde with monosubstituted hydrazines and
subsequent reaction of the resulting pseudobases with per-
[RhX(CAArC)(COD)] complexes 5b-d in low to moderate
yields (Scheme 2). As exceptions, no product was isolated from
the N-Me and the N-anisyl derivatives 2a and 2e. Complexes
5b-d exhibited a considerable stability, which enabled even
(
1
3
purification by chromatography on silica gel. The C NMR
spectra showed characteristic C(carbene) resonances at 221-
(
9) pK
1.0: Bunting, J. W.; Chew, V. S.-F.; Sinhuatmadja, S. Can. J. Chem. 1981,
9, 3195–3199.
10) (a) Butler, R. N.; Gillan, A. M.; McArdle, P.; Cunningham., D.
a a
2-methylisoquinolinium = 16.3; pK 2-methyl phthalazinium =
2
28 ppm, slightly shifted upfield with respect to isoquinolin-1-
1
5
ylidene analogues, but still much downfield with respect to the
typical resonances in diamino carbene [RhCl(NHC)(COD)]
(
J. Chem. Soc., Chem. Commun. 1987, 1016–1018. (b) Butler, R. N.; Gillan,
A. M.; Lysaght, F. A.; McArdle, P.; Cunningham., D. J. Chem. Soc., Perkin
Trans. 1 1990, 555–564. (c) Butler, R. N.; Pyne, C. S. J. Chem. Soc., Perkin
Trans. 1 1992, 3387–3389. See also: (d) Hahn, F. E.; Wittenbecher, L.; Le
Van, D.; Fr €o hlich, R. Angew. Chem., Int. Ed. 2000, 39, 541–544.
(11) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.
(12) K o€ cher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532,
261–265.

Article
Organometallics, Vol. 29, No. 22, 2010 5943
Table 1. Selected Spectroscopic and Crystallographic Data for Complexes 5, 6, and 7
5c 5d 6b
5
b
6c
7
M
δ
Rh
Rh
227.2
9.85
Rh
223.7
10.51
Ir
Ir
217.0
9.52
Rh
1
3
C C(1) (ppm)
δ C(8)H (ppm)
221.8
9.62
6.90
213.3
9.46
6.57
228.0
9.83
7.16
δ CHMe
δ CHPh
2
(ppm)
(ppm
2
9.38
9.06
t
δ Bu (ppm)
2.29
˚
d C(1)-M (A)
d C(1)-Ar (A)
mean Rh-CCOD (trans)
mean Rh-CCOD (cis)
Δ mean Rh-CCOD (trans-cis)
C(Ar)H-Rh (A)
C(sp )H-Rh (A)
2.016(2)
1.447(3)
2.224
2.130
0.094
2.0130(18)
1.450(2)
2.224
2.129
0.095
2.0358(18)
1.461(3)
2.217
2.121
0.096
2.013(4)
1.455(5)
2.186
2.121
0.065
2.025(3)
1.453(4)
2.195
2.116
0.079
2.0242(16)
1.449(2)
2.221
2.132
0.089
˚
˚
2.826
2.615
2.770
2.594
2.688
2.365
2.835
2.626
2.783
2.618
2.768
2.555
3
˚
Figure 3. ORTEP drawing of complex 5c. Thermal ellipsoids
are shown at the 30% probability level. Most hydrogen atoms
˚
are omitted for clarity. Selected bond lengths [A] and bond
Figure 2. ORTEP drawing of complex 5b. Thermal ellipsoids
are shown at the 30% probability level. Most hydrogen
˚
atoms are omitted for clarity. Selected bond lengths [A] and
bond angles [deg]: Rh(1)-C(1) 2.016(2), C(1)-C(2) 1.447(3),
N(1)-C(1) 1.342(3), Rh(1)-I(1) 2.6699(2), Rh(1)-H(3)
angles [deg]: Rh(1)-C(1) 2.0130(18), C(1)-C(8) 1.450(2), N-
(1)-C(1) 1.343(2), Rh(1)-Br(1) 2.4973(2), Rh(1)-H(7) 2.770,
Rh(1)-H(9) 2.594, Rh(1)-C(22) 2.2289(18), Rh(1)-C(23)
2.2183(18), Rh(1)-C(26) 2.1316(18), Rh(1)-C(27) 2.1164(18),
C(1)-Rh(1)-Br(1) 90.26(5), C(7)-H(7)-Rh(1) 112.46, C(9)-
H(9)-Rh(1) 124.60.
2
2
.826, Rh(1)-H(9) 2.615, Rh(1)-C(12) 2.135(2), Rh(1)-C(13)
.126(3), Rh(1)-C(16) 2.223(3), Rh(1)-C(17) 2.224(2), C(1)-
Suitable crystals for X-ray diffraction studies could be ob-
tained for complexes 5b-d. The structure of complex 5b reveals
the expected square-planar coordination geometry about Rh,
Rh(1)-I(1) 87.56(6), C(3)-H(3)-Rh(1) 111.70, C(9)-H(9)-
Rh(1) 121.62.
˚
1
3
with a C(carbene)-Rh distance of 2.016(2) A, similar to those
observed in related acyclic (type A) and cyclic C1, C2, C4, and
complexes (180-210 ppm) (Table 1). The coupling constants
JC(1),Rh of 41-44 Hz confirmed the formation of the carbene
complexes 5. On the other hand, their H NMR spectra revealed
3b
7,8
1
C5 systems. Interestingly, the C(aryl)-C(carbene) bond
distance of 1.447(3) A is shorter than the typical distance of a
C(sp )-C(sp ) single bond, indicating some electron density
delocalization from the π system of the aryl ring over the vacant
carbene orbital. The much longer C(aryl)-C(carbene) bond
distance of 1.502(3) A reported for an acyclic derivative (type
B), where such a delocalization is not possible for geometrical
˚
a very strong downfield shift for the C(8)-H protons of the
phthalazin-2-ylidene ligand, tentatively attributed to CH-Rh
preagostic interactions. The spectra of complexes 5b and 5d
2
2
14
exhibited also a drastic deshielding for the CHMe (δ = 6.90
2
˚
ppm) and CHPh (δ = 9.38 ppm) protons, which could also be
2
attributed to significant preagostic CH-Rh interactions. Ad-
ditionally, the singlet assigned to the N- Bu group in 5c appears
3b
t
reasons, provides an adequate reference for comparison.
Finally, the Rh(I) (amino)(aryl)carbene complex E2 ob-
also clearly shifted downfield (from δ 1.80 ppm in the azolium
salt to δ2.29 ppm in the complex), suggesting also in this case the
7
tained by Ellmann, Bergman, et al. by tautomerization
b
t
presence of a Bu-metal interaction.
(
14) Accordingly, it has been demonstrated in the isoquinolin-1-
(
13) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385–
ylidene series (ref 8) that the substitution pattern in the aromatic ring
strongly influences the catalytic activity of the complex.
3
407.

5
944 Organometallics, Vol. 29, No. 22, 2010
Magriz et al.
Figure 4. ORTEP drawing of complex 5d. Thermal ellipsoids
are shown at the 30% probability level. Most hydrogen atoms
˚
are omitted for clarity. Selected bond lengths [A] and bond
angles [deg]: Rh(1)-C(1) 2.0358(18), C(1)-C(23) 1.461(3),
N(1)-C(1) 1.340(2), Rh(1)-Cl(1) 2.4005(5), Rh(1)-H(7)
.688, Rh(1)-H(10C) 2.365, Rh(1)-C(12) 2.111(2), Rh(1)-
C(13) 2.1310(19), Rh(1)-C(16) 2.198(2), Rh(1)-C(17) 2.236(2),
C(1)-Rh(1)-Cl(1) 87.80(5), C(7)-H(7)-Rh(1) 114.00, C(10)-
H(10C)-Rh(1) 122.93.
Figure 5. ORTEP drawing of complex 6b. Thermal ellipsoids are
shown at the 30% probability level. Hydrogen atoms not in-
volved in preagostic interactions are omitted for clarity. Selected
bond lengths [A] and bond angles [deg]: Ir(1)-C(1) 2.013(4),
C(1)-C(8) 1.455(5), N(1)-C(1) 1.355(5), Ir(1)-I(1) 2.6610(4),
Ir(1)-H(7) 2.835, Ir(1)-H(9) 2.626, Ir(1)-C(12) 2.177(5), Ir-
2
˚
(
1)-C(13) 2.194(4), Ir(1)-C(16) 2.103(5), Ir(1)-C(17) 2.117(5),
of a 1,4-benzodiazepine complex exhibits also a longer
˚
C(1)-Ir(1)-I(1) 89.02(12), C(7)-H(7)-Ir(1) 111.94, C(9)-
H(9)-Ir(1) 121.83.
C(aryl)-C(carbene) bond distance of 1.506(4) A, consistent
7
b
again with the absence of delocalization. Being a bicyclic
structure, the absence of delocalization in this case can be
explained by the lack of coplanarity imposed by the preferred
conformation of the seven-membered ring in the benzodia-
1
5
zepinylidene ligand. The structure of 5b shows C(8)H-Rh
˚
and CHMe -Rh bond distances of 2.826 and 2.615 A, with
2
C-H-Rh angles of 111.7° and 121.6°, respectively. These
structural data are characteristic for preagostic Rh-H
interactions, as was anticipated from the H NMR data.
A high σ-donor ability of the phthalazin-1-ylidene ligand
can be deduced from the molecular structure: the mean
C(COD)-Rh bond distances are 2.130 A (trans to iodine)
and 2.224 A (trans to the carbene ligand), the difference of
˚
ca. 0.1 A reflecting the much higher trans influence of the
1
6
1
˚
˚
monoamino carbene ligand. A direct comparison with the
i
8
isoquinolin-1-ylidene analogue [RhI(IQUI- Pr)(COD)] 7
IQUI = 2-isopropylisoquinolin-1-ylidene) reveals very simi-
(
lar structures for these isoelectronic species, which, interest-
ingly, differ in slightly stronger CH-Rh interactions for 7,
while the trans influence is unexpectedly similar in both cases.
The crystal structure of 5c presents almost identical features
with those discussed for 5b (Figure 4), exhibiting also similar
preagostic C(8)H-Rh and CHPh -Rh interactions (see data
2
3
in Table 1). In the case of 5d, the intramolecular C(sp )H-Rh
preagostic interaction involves one of the H atoms of the Bu
Figure 6. ORTEP drawing of complex 6c. Thermal ellipsoids are
shown at the 30% probability level. Hydrogen atoms not involved
in preagostic interactions are omitted for clarity. Selected bond
lengths [A] and bond angles [deg]: Ir(1)-C(1) 2.025(3), C(1)-C(8)
1.453(4), N(1)-C(1) 1.349(4), Ir(1)-Br(1) 2.4823(4), Ir(1)-H(7)
t
˚
(
15) The complex shows a torsion C(ortho)-C-(ipso)-C(carbene)-
Rh angle of 35.0(4)°.
16) (a) Yao, W.; Eisenstein, O.; Crabtree, R. H. Inorg. Chim. Acta
997, 254, 105–111. (b) Lewis, J. C.; Wu, J.; Bergman, R. G.; Ellman, J. A.
2.783, Ir(1)-H(9) 2.618, Ir(1)-C(22) 2.110(4), Ir(1)-C(23)
2.122(4), Ir(1)-C(26) 2.191(4), Ir(1)-C(27) 2.199(3), Ir(1)-H(7)
2.783, Ir(1)-H(9) 2.618, C(1)-Ir(1)-Br(1) 90.92(9), C(7)-
(
1
Organometallics 2005, 24, 5737–5746. (c) Zhang, Y.; Lewis, J. C.; Bergman,
R. G.; Ellman, J. A.; Oldfield, E. Organometallics 2006, 25, 3515–3519.
H(7)-Ir(1) 112.44, C(9)-H(9)-Ir(1) 124.69.

Article
Organometallics, Vol. 29, No. 22, 2010 5945
group, resulting in a much shorter Rh-H bond distance of
bubbling CO through a solution of 5b in CH Cl for several
2
2
1
7
˚
2
.365 A (Figure 5), with a C-H-Rh angle of 122.9°.
hours. This unexpected behavior, also observed in acyclic
3
b
In a similar manner, reaction of phthalazinium salts 2b,c with
IrCl(COD)]2 under the same reaction conditions afforded
iridium analogues 6b and 6c, although in lower yields. The H
NMR data for these complexes showed also a strong downfield
shift for the C(8)H protons of the phthalazin-1-ylidene ligand
monoamino aryl carbenes, is in sharp contrast with that
exhibited by isoquinolin-2-ylidene analogues, which under-
go complete ligand exchange in a few minutes at room
temperature.
[
1
In summary, the synthesis of phthalazin-1-ylidene Rh(I)
and Ir(I) complexes has been accomplished and their struc-
tures have been analyzed by X-ray diffraction studies. This
new family of cyclic amino aryl carbenes exhibits structural
features that suggest an electronic aryl-carbene delocaliza-
tion that should allow the tuning of the ligand electronic
properties. The lack of reactivity of [RhCl(CAArC)(COD)]
with CO, however, reveals a different behavior compared to
isoquinoline analogues. These examples expand the struc-
tural variability available for cyclic amino aryl carbenes.
[δ 9.46 and 9.52 ppm for 6b and 6c, respectively] and for the
proton in the CHR at N(2) [δ 6.57 for 6b (R = Me); δ 9.06 for
2
6
c (R = Ph)], slightly weaker than for the Rh analogues. The
˚
C(aryl)-C(carbene) bond distances of 1.455(5) and 1.453(4) A,
respectively, indicate also in these cases a significant aryl/carbene
electron delocalization.
In addition to the collected structural data, the synthesis of
the [RhCl(CAArC)(CO) ] derivatives was considered of
2
interest in order to obtain an additional index of the σ-donor
ability of these ligands according to the “Rh scale”. How-
ever, no COD to CO ligand exchange was observed after
1
8
Acknowledgment. We thank the Spanish “Ministerio
de Ciencia e Innovaci oꢀ n” (grants CTQ2007-61915 and
CTQ2007-60244), the European FEDER funds and the
´
Junta de Andalucıa (grants 2009/FQM-4537 and 2008/
FQM-3833) for financial support.
1
17) As only one peak for the nine protons is observed in the H NMR
(
spectrum (see Supporting Information), a relatively fast H-Rh bond
exchange is assumed to occur in solution.
(
18) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313–348. (b) Chianese,
A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics
003, 22, 1663–1667. (c) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.;
2
Supporting Information Available: Crystallographic data for
compounds 5b-d, 6b, and 6c (CIF), experimental procedures,
Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461–2468. (d)
Kelly, R. A., III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.;
Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organo-
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and H and C NMR spectra for new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.