Diaminocarbene and phosphonium ylide ligands: A systematic comparison of their donor character

Article abstract of DOI:10.1021/ja801159v

The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types have been investigated systematically through a test family of C,C-chelating ligands containing two moieties of either kind. The overall character of o-C₆H₄A_aB_b ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(η²-C₆H₄A_aB _b)Rh(CO)₂][TfO]. The test moieties A = NC ₂H₂N⁺(Me)C^- and B = Ph ₂P⁺CH₂^- were first considered. While the ligands bearing at least one diaminocarbene end (AA, a = 2 and AB, a = 1) could be generated (and trapped by complexation), the bis-ylide case BB (a = 0) proved to be awkward: treatment of the dication C₆H ₄(P⁺Ph₂Me)₂ with n-BuLi indeed lead to the Schmidbaur's carbodiphosphorane Ph₃P=C=PPh₂Me, through an unprecendented ylidopentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand type C₆H ₄B₂ could however be generated by bridging the phosphonium methyl groups by a methylene link (B₂ = (P⁺Ph ₂CH^-)₂CH₂), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(η²-C₆H₄A_aB_b)Rh(CO) ₂][TfO] were fully characterized, including by X-ray diffraction analysis and ¹⁰³Rh NMR spectroscopy. Comparison of their IR spectra indicated that the A₂ type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B₂ type bis-ylide ligand. The excellent linear variation of the ν_CO frequencies vs a (= 0, 1, 2) shows that the coordinating moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-31G**/LANL2DZ*(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.

Full text of DOI:10.1021/ja801159v

Published on Web 06/04/2008
Diaminocarbene and Phosphonium Ylide Ligands:
A Systematic Comparison of their Donor Character
Yves Canac,* Christine Lepetit, Mohammed Abdalilah, Carine Duhayon, and
Remi Chauvin*
Laboratoire de Chimie de Coordination (LCC) of CNRS, 205 route de Narbonne, 31077
Toulouse cedex 4, France
Received February 15, 2008; E-mail: chauvin@lcc-toulouse.fr; yves.canac@lcc-toulouse.fr
Abstract: The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types
have been investigated systematically through a test family of C,C-chelating ligands containing two moieties
of either kind. The overall character of o-C6H4AaBb ligands (a + b ) 2) has been analyzed from the IR CO
stretching frequencies of isostructural complexes [(η2-C6H4AaBb)Rh(CO)2][TfO]. The test moieties A )
-
NC2H2N+(Me)C- and B ) Ph2P+CH2 were first considered. While the ligands bearing at least one
diaminocarbene end (AA, a ) 2 and AB, a ) 1) could be generated (and trapped by complexation), the
bis-ylide case BB (a ) 0) proved to be awkward: treatment of the dication C6H4(P+Ph2Me)2 with n-BuLi
indeed lead to the Schmidbaur’s carbodiphosphorane Ph3PdCdPPh2Me, through an unprecendented ylido-
pentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand
type C6H4B2 could however be generated by bridging the phosphonium methyl groups by a methylene link
(B2 ) (P+Ph2CH-)2CH2), preventing the formation of the analogous highly strained carbodiphosphorane.
The three complexes [(η2-C6H4AaBb)Rh(CO)2][TfO] were fully characterized, including by X-ray diffraction
analysis and 103Rh NMR spectroscopy. Comparison of their IR spectra indicated that the A2 type bis-NHC
ligand is less donating than the hybrid AB type, which is itself less donating than the B2 type bis-ylide
ligand. The excellent linear variation of the νCO frequencies vs a () 0, 1, 2) shows that the coordinating
moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-
31G**/LANL2DZ*(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger
donor than a diaminocarbene ligand.
Introduction
ligands today compete with the numerous types of phosphorus
and nitrogen ligands.2
Beyond the efficiency of main group Lewis acids, the superior
versatility of transition metal complexes for the activation of
organic substrates, in either stoichiometric or catalytic processes,
ensues from their ability to combine dual electronic properties,
namely: electron acceptance (of substrate valence electron pairs)
and electron donation (into the antibonding orbitals of the C-X
bonds to be cleaved). For a given process, these oxymoric
properties are tuned by the design of optimal “spectator” ligands
exhibiting the right balance between their “electron deficiency”
(π/σ accepting character) and their “electron richness” (σ/π
donating character).
The carbon category has been mainly illustrated by electron-
rich unsaturated sp2-C ligands, namely cyclic diaminocarbenes
(NHCs),3 whose efficiency is generally attributed to their unique
combination of strongly σ-donating, poorly π-accepting, and
planar steric properties. The NHCs have thus been used to
promote catalytic processes such as Pd-catalyzed cross-coupling4
and Ru-catalyzed olefin metathesis,5 where the stabilization of
coordinatively unsaturated metal intermediates is crucial.
(2) Lavallo, V.; Canac, Y.; Pra¨sang, C.; Donnadieu, B.; Bertrand, G.
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J.; Kawashima, T. Angew. Chem., Int. Ed. 2008, 47, 1141.
In the past decade, major advances have focused on the
development of electron-rich ligands of late transition metal
complexes for various challenges in catalysis, in particular C-C
bond forming reactions.1 Such spectator ligands are indeed
interesting in two respects: (i) they tend to stabilize the Lewis
acidic catalytic center, and (ii) they are prone to facilitate the
slowest (limiting) steps, such as oxidative addition, transmeta-
lation, or reductive elimination. Among them, neutral carbon
(3) Marion, N.; D´ıez-Gonza´lez, S.; Nolan, S. P. Angew. Chem., Int. Ed.
2007, 46, 2988. Pugh, D.; Danopoulos, A. A. Coord. Chem. ReV. 2007,
251, 610. Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-
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Chem. ReV. 2005, 249, 829. Peris, E.; Crabtree, R. H. Coord. Chem.
ReV. 2004, 248, 2239. Crudden, C. M.; Allen, D. P. Coord. Chem.
ReV. 2004, 248, 2247. Ce´sar, V.; Bellemin-Laponnaz, S.; Gade, L. H.
Chem. Soc. ReV. 2004, 33, 619. Bourissou, D.; Guerret, O.; Gabba¨ı,
F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39.
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Angew. Chem., Int. Ed. 1995, 34, 2371. Kantchev, E. A. B.; O’Brien,
C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768.
(1) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176. Zapf,
A.; Beller, M. Chem. Commun. 2005, 431. Muira, M. Angew. Chem.,
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8406 J. AM. CHEM. SOC. 2008, 130, 8406–8413
10.1021/ja801159v CCC: $40.75
2008 American Chemical Society

Diaminocarbene and Phosphonium Ylide Ligands
A R T I C L E S
Scheme 1. Test Family of C,C-Chelating Ligands and Rh(CO)2 Complexes thereof with the Three Possible Combinations of NHC and
Phosphonium Ylide Moieties
Scheme 2. Synthesis of the NHC-Phosphonium Ylide Dicarbonylrhodium Complex 2c
Neutral spectator carbon-centered ligands are also represented
by phosphonium ylides.6 The catalytic properties of these
saturated sp3-C ligands have been more punctually illustrated.6
Whereas the free ylide precursor contains an almost-planar
carbanion stabilized by an adjacent tetrahedral phosphonium
center, the ligand exclusively acts by η1-C coordination to metal
centers (to the best of our knowledge, no η2-CdP coordination
has ever been reported). Ylides share two noticeable common
features with the NHCs: (i) an intrinsic R-zwitterionic character
(-CdN+, -CsP+) resulting in a formal ꢀ-zwitterionic form
of the coordinated unit (-MsCdN+, -MsCsP+) and (ii) a
strong σ-donor vs π-acceptor character toward metals.
We recently reported on a novel type of strongly donating
C,C-chelating ligand occurring in a stable and catalytically active
NHC-phosphonium ylide complex.7 At the outset, the electron-
donating ability of soft, phosphine-like, carbene ligands was a
priori expected to be lower than that of harder ylide ligands.
Nevertheless, a more accurate understanding of the “electronics”
of these carbon ligands is desired.
A systematic investigation is tackled from a test family of
chelating C,C-ligands containing the same bridge (an ortho-
phenylene ring) and all combinations of two representatives of
the two types of ligating moiety: N-bonded N-methylimida-
zolylidenes (A) and P-bonded diphenylmethylphosphonium
ylides (B). These ligands o-C6H4AaBb, a + b ) 2, are denoted
as 1a (a ) 2), 1b (a ) 0), and 1c (a ) 1) (Scheme 1). In order
to estimate the overall σ-donating vs π-accepting properties of
1a-c, we rely on the values of the IR stretching frequencies of
“conjugated” carbonyl ligands through a given metal center.
We selected a Rh(I) center (variations of the CO stretching
frequencies of RhI(CO)2 complexes have been extensively
studied with other ligands)8 and the triflate counteranion: the
test family is thus constituted by the three complexes [(Rh-
(CO)2(η2-C6H4AaBb)][TfO], a + b ) 2, denoted as 2a (a ) 2),
2b (a ) 0), and 2c (a ) 1) (Scheme 1).
It must be emphasized that the hybrid ligands of the test
family correspond to minimal steric effects from both the NHC
and ylide moieties. Indeed, in the spirit of Tolman,9 influence
of steric effects from any ligand on RhsCdO moieties (the
selected observables) must basically appear sharply as the “size”
of the ligand arrives at the critical value corresponding to a van
der Waals contact between the ligand and a CO fragment. In
the devised rigid framework (Scheme 1), the possible steric
effect is brought down to a minimum: in the case of the
-CH2(P+Ph2Ar) ylide moiety (respectively, the CN(Ar)NMe
carbene moiety), the bulkiest ArPh2P+ (respectively NAr)
substituent is a priori constrained remote from the CO frag-
ments, and the CH2 unit (respectively the CNMe unit) is as small
as possible. In particular, it has been indeed claimed that
nonhindered NHC ligands do not exert a steric effect on the
metal center.10
Results and Discussion
The first target was complex 2c containing the hybrid NHC-
phosphonium ylide ligand 1c, recently described in a palladium
complex.7 Treatment of the known dication 4c (prepared by
dimethylation of 3c) with 0.5 equiv of [RhCl(cod)]2 in the
presence of triethylamine in acetonitrile readily afforded the
phosphoniocarbene complex 5c in 86% yield as a 60/40 mixture
of two stereoisomers (Scheme 2).
(6) Falvello, L. R.; Gine´s, J. C.; Carbo´, J. J.; Lledo´s, A.; Navarro, R.;
Soler, T.; Urriolabeitia, E. P. Inorg. Chem. 2006, 45, 6803. Vicente,
J.; hicote, M. T. Coord. Chem. ReV. 1999, 193-195, 1143. Kolodi-
azhnyi, O. I. Tetrahedron 1996, 52, 1855. Kaska, W. C.; Ostoja
Starzewski, K. A. In Ylides and Imines of Phosphorus; Johnson, A. W.,
Ed.; John Wiley & Sons: New York, 1993; Chapter 14. Schmidbaur,
H. Angew. Chem., Int. Ed. Engl. 1983, 22, 907. Canal, C.; Lepetit,
C.; Soleilhavoup, M.; Chauvin, R. Afinidad 2004, 61, 298. Zurawinski,
R.; Donnadieu, B.; Mikolajczyk, M.; Chauvin, R. J. Organomet. Chem.
2004, 689, 380. Zurawinski, R.; Donnadieu, B.; Mikolajczyk, M.;
Chauvin, R. Organometallics 2003, 22, 4810. Ohta, T.; Sasayama,
H.; Nakajima, O.; Kurahashi, N.; Fujii, T.; Furukawa, I. Tetrahedron:
Asymmetry 2003, 14, 537.
The occurrence of two stereoisomers of 5c likely results from
a restricted rotation about the N-aryl or/and Rh-CN2 bonds.
(8) Martin, D.; Baceiredo, A.; Gornitzka, H.; Schoeller, W. W.; Bertrand,
G. Angew. Chem., Int. Ed. 2005, 44, 1700. Mayr, M.; Wurst, K.;
Ongania, K.-H.; Buchmeiser, M. R. Chem.sEur. J. 2004, 10, 1256.
Herrmann, W. A.; Ofe¨le, K.; v.; Preysing, D.; Herdtweck, E. J.
Organomet. Chem. 2003, 684, 235. Enders, D.; Gielen, H.; Runsink,
J.; Breuer, K.; Brode, S.; Boehn, K. Eur. J. Inorg. Chem. 1998, 913.
(9) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(10) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.;
Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485. D´ıez-
Gonza´lez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874.
(7) Canac, Y.; Duhayon, C.; Chauvin, R. Angew. Chem., Int. Ed. 2007,
46, 6313.
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8407

A R T I C L E S
Canac et al.
Figure 1. ORTEP views of the X-ray crystal structures of complexes 2c (left) and 2a (middle), with thermal ellipsoids drawn at the 30% probability level,
and optimized geometry of complex 2b′ (the most stable conformer) (right) at the B3PW91/6-31G**/LANL2DZ*(Rh) level of calculation.
Scheme 3. Synthesis of the Bis-NHC Dicarbonylrhodium Complex 2a
The structure of 5c was definitively confirmed by X-ray
diffraction analysis of a single crystal.11 Deprotonation of the
methyl substituent of the phosphonium residue of 5c with
potassium tert-butoxide in THF gave the desired NHC-phos-
phonium ylide complex 6c in 92% yield, as a single stereoiso-
mer. Following a classical procedure, the ultimate complex 2c
was obtained in good yield (91%) by treating a solution of 6c
in THF under an atmospheric pressure of carbon monoxide. The
13C NMR spectrum of the air-stable complex 2c showed two
CO signals at +184.6 and +189.6 ppm with classical 103Rh-13C
coupling constants (JCRh ) 59.3 and 59.7 Hz, respectively). The
structure of 2c was unambiguously confirmed by X-ray dif-
fraction analysis of yellow crystals deposited from CH2Cl2/
Et2O.11 The geometry at the Rh(I) center is square-planar, with
a boat-shaped seven-membered metallacycle (Figure 1). Finally,
the IR stretching frequencies of the CO ligands of 2c in CHCl3
otherwise similar to that of 2c (quasi square-planar RhC4 unit
and boat-shaped seven-membered metallacycle). It must be
stressed that, in both 2a and 2c, a minimal steric interaction
between the CO fragments and the NHC/ylide ligands is
supported by the absence of a significant compression of the
van der Waals spheres of the CO and closest H atoms (CH2,
NCH3). The IR CO stretching frequencies of 2a occur at 2030
and 2084 cm-1 in CHCl3. As a stronger donor ligand is expected
to weaken the CdO bonds and thus result in a lower wave-
number of their stretching frequencies, the NHC-phosphonium
ylide ligand 1c is definitely a stronger donor than the bis-NHC
ligand 1a, at least with respect to the [Rh(CO)2][TfO] moiety.
The remaining target, the bis-phosphonium ylide complex 2b,
is a priori challenging for its zwitterionic rhodate nature.13 All
attempts to generate the desired bis-phosphonium ylide complex
2b in a one-pot three-step procedure from diphosphonium 4b14
failed. This could be attributed to the impossibility to generate
the bis-ylide 1b from 4b which inevitably lead to the formation
of the carbodiphosphorane 8b. Multinuclear NMR monitoring
experiments indeed showed that addition of 2 equiv of n-
butyllithium at -78 °C to a solution of 4b in THF-d8 resulted
in the immediate formation of a cyclic five-membered ylide 7b
as a mixture of two stereoisomers (95/5). This ylidophosphorane
7b results from the internal attack of the initially formed
monoylide of 4b to the adjacent phosphonium moiety (Scheme
4).
were measured at 2008 and 2071 cm-1
.
The symmetrical bis-NHC dicarbonylrhodium complex 2a
was then targeted. Dimethylation of 1,1′-(1,2-phenylene)bi-
s(imidazole) 3a12 with 2 equiv of methyl triflate (MeOTf) in
CH2Cl2 gave the corresponding dication 4a in 88% yield.
Treatment of 4a with 0.5 equiv of [RhCl(cod)]2 in the presence
of 2 equiv of triethylamine in acetonitrile cleanly afforded the
bis-NHC chelate complex 6a in 92% yield. Finally, bubbling
CO through a THF solution of 6a afforded the dicarbonyl-
rhodium complex 2a in 85% yield (Scheme 3).
By comparison with the dissymmetrical complex 2c, the main
feature of the 13C NMR spectrum of 2a is the presence of a
single CO signal at +187.5 ppm (JCRh ) 57.2 Hz), thus
suggesting a symmetrical structure. An X-ray diffraction study
of an air-stable single crystal of 2a confirmed the NMR
assignment (Figure 1).11 The coordination geometry of 2a is
The fascinating structure of 7b was first assigned on the basis
of its 31P NMR chemical shifts (δP ) -2.2 and -100.0 ppm;
JPP ) 60.8 Hz) which are indeed indicative of a PIVCPV
sequence. The CH ylide functionality was confirmed by 1H and
13C NMR spectroscopy (δCH ) +0.64 ppm, dd, JHP(IV) ) 22.3,
JHP(V) ) 13.4 Hz; δCH ) +15.1 ppm, dd, JCP(IV) ) 112.7, JCP(V)
) 163.6 Hz). The presence of two stereoisomers in 7b is
explained by its trigonal bipyramidal geometry at one phos-
(11) CCDC 676873 (5c), 676874 (2c), and 676875 (2a) contain the
supplementary 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.
(13) Chauvin, R. Eur. J. Inorg. Chem. 2000, 577.
(12) So, Y. H. Macromolecules 1992, 25, 516.
(14) Schmidbaur, H.; Herr, R.; Zybill, C. E. Chem. Ber. 1984, 117, 3374.
9
8408 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Diaminocarbene and Phosphonium Ylide Ligands
A R T I C L E S
Scheme 4. Synthesis of Carbodiphosphorane 8b via the Elusive Ylidophosphorane 7b
Scheme 5. Synthesis of the Bis-Ylide Dicarbonylrhodium Complex 2b′
phorus atom, the methyl substituent being located in either axial
or equatorial position. Pursuing the VTP NMR monitoring by
warming up the THF-d8 solution to -10 °C, the cyclic
ylidophosphorane 7b was shown to slowly (t1/2 ≈ 12 h) but
quantitatively rearrange into carbodiphosphorane 8b (Scheme
4).15
As observed for 1b′, the presence of a single signal in the
31P NMR spectrum (δp ) +41.3 ppm) indicates a symmetrical
structure for 2b′. The significant shift of the 31P signal (as
compared to the free diylide 1b′: δp ) +24.4 ppm) and the
observed multiplicity (d, JPRh ) 4.1 Hz) are both in agreement
with the ylide-rhodium complex structure of 2b′. The presence
of a Rh-CH unit was confirmed by multinuclear NMR
spectroscopy and, in particular, by the high-field 13C NMR
signal of the metalated ylidic carbon coupled to the 103Rh
nucleus: δCH ) -7.6 ppm (dd, JCP ) 33.9 and JCRh ) 13.8
Hz). As in the case of the bis-NHC dicarbonylrhodium complex
2a, the symmetry of 2b′ was confirmed by the presence of a
single 13CO signal at +188.5 ppm. The structure was further
confirmed by ESI mass spectrometry (m/z ) 645 [M]+), but all
attempts to grow high quality single crystals were unsuccessful.
Facing this difficulty, DFT calculations of complex 2b′ were
undertaken at the B3PW91/6-31G**/LANL2DZ*(Rh) level
(Figure 1). Taking into consideration the standard deviations
of XRD analyses, this level of calculation was first validated
by a reasonable agreement between the optimized and experi-
mental geometries of the complexes 2a and 2c (Table 1). The
calculations indeed reproduce the relative shortness of the
experimental NHCsRh bonds (ca 2.04 Å in 2a, 2.07 Å in 2c)
with respect to the ylidesRh bond (ca 2.16 Å in 2c). Moreover,
they are also in accord with the relative shortness of the
respective trans-RhsCO bonds and with the relative lengthening
of the respective trans-RhC)O bonds (with standard deviations
of ca 0.010 Å). Both these observations can be made consistent
by ascribing a stronger σ-donating vs π-accepting character to
the ylide moiety than to the NHC moiety. For the bis-ylide
complex 2b′, two minima of 6.4 kcal/mol relative energy were
obtained on the potential energy surface of Cs symmetry. Both
minima display a slighty distorted square-planar arrangement
at the Rh(I) center, but they differ by the conformation of the
seven-membered rhodacycle, which is envelope-shaped in the
In order to avoid the formation of the ylidophosphorane of
type 7b, we decided to study the deprotonation of a cyclic
diphosphonium 4b′.16 The steric demand in the analogous
tricyclic ylidophosphorane 7b′ is indeed unlikely. And indeed,
NMR monitoring of the deprotonation of 4b′ with 2 equiv of
n-BuLi in THF-d8 at low temperature, allowed for identifying
the expected cyclic bis-ylide 1b′. The 31P NMR spectrum of
the solution displayed a single singlet at +24.4 ppm, giving a
first indication about the nature and the symmetry of the
environment of the phosphorus atoms. The structure of 1b′ was
confirmed by 1H and 13C NMR. In particular, the simultaneous
presence of a triplet of triplets at δH ) +0.59 ppm (JHP ) 26.8,
JHH ) 3.3 Hz) and of a triplet at δC ) +21.9 ppm (JCP ) 4.4
Hz) was assigned to the bridging methylene group of 1b′. The
bis-ylide 1b′ was however quite unstable (t1/2 ≈ 45 min at +20
°C). Both the complexation and the cod-(CO)2 exchange
reactions were thus performed in situ in a one-pot sequence.
Addition of a stoichoimetric amount of [Rh(cod)2][TfO] to a
THF solution of the bis-ylide 1b′, followed by treatment with
an excess of CO, afforded the expected complex 2b′ in a 51%
overall yield (Scheme 5). In contrast to the previous NHC
complexes 2a and 2c, the bis-ylide complex 2b′ is unstable in
the air and slowly decomposes even at low temperature under
an inert atmosphere, giving a black precipitate attributable to
decomplexation. This sensitivity could be simply anticipated
from the two-bond formal charge separation of the zwitterionic
rhodate structure, which should cancel after hydrolysis.
(15) Note that 8b has been previously described starting form the same
precursor 4b; however the authors have proposed a different mech-
anism (see ref 14).
(16) The preparation of compound 4b′ will be published elsewhere.
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8409

A R T I C L E S
Canac et al.
Table 1. Selected Experimental and Calculated Structural Data of Dicarbonylrhodium Complexesa
Cylide-Rh
CNHC-Rh
Cylide-P
C-O
Rh-CO
X-Rh-Yd
s.p. dev.b
2c:
XRD
calcd
2.155(8)
2.156
2.069(8)
2.070
1.750(7)
1.776
1.147(12) (trans to NHC)
1.172(12) (trans to ylide)
1.146 (trans to NHC)
1.149 (trans to ylide)
1.864(10) (trans to NHC)
1.850(10) (trans to ylide)
1.905 (trans to NHC)
1.888 (trans to ylide)
93.9
92.8
3.9
0.5
2a:
1.117(4)
1.145
XRD
calcd
-
-
2.036(3)
2.066
-
1.899(3)
1.910
82.4
82.5
0.4
3.6
2b′c:
calcd
calcd
2.138
1.770
1.771
1.150
1.891
67.6
93.0
12.9
4.6
2b:
1.150 1.149
2.143 2.160
1.881 1.890
a Geometry optimizations were performed at the B3PW91/6-31G**/LANL2DZ*(Rh) level, under Cs symmetry constraint when possible. Bond lengths
are in Å, and valence angles in degrees. b Deviation (in degrees) from a planar RhC4 arrangement (angle between OC-Rh-CO and X-Rh-Y planes).
c Structural data for the most stable conformer. d X, Y ) A and/or B (see Scheme 1).
less stable isomer and boat-shaped in the most stable one (like
in 2a and 2c) (Figure 1).17
the bis-abnormal NHC complex 11, and the initially targeted
bis-ylide complex 2b (Table 2).
For the known complexes, a good agreement between
experimental and calculated values is obtained, with a systematic
gap of ∆av.(calcd-exp) ≈ 106-107 cm-1. These results confirm
the exceptionally high donor character of the phosphonium ylide
moieties. Accross the series, the bis-phosphine ligand of 9 is
the least donating, while the bis-ylide ligand of 2b′ is the most
donating. The latter is even a better donor than the bis-abnormal
NHC ligand of 11.19 Interestingly, the same difference in the
average νCO values is observed when substituting an NHC
moiety for a phosphonium ylide moiety: ∆avνCO(2a f 2c) )
For a direct comparison in the test family, the initially targeted
bis-ylide complex 2b was calculated (Table 1). Beyond the
difference in ylide-Rh-ylide angle and square-planar deviation,
the structure found for 2b is quite comparable with that of the
methylene-bridged homologue 2b′. The methylene bridge of 2b′
thus exerts a secondary perturbation, and this complex can
indeed be regarded as relevant within the test family.
The NMR chemical shifts of the two conformers of 2b′ were
also calculated at the B3PW91/6-31+G**/LANL2DZ*(Rh)
level using the GIAO formalism.17 The most stable conformer
gives the best agreement with the experimental values, especially
16.7 cm-1 and ∆avνCO(2c f 2b) ) 15.7 cm-1
.
1
for all H, 31P, and 13C nuclei remote from the rhodium atom
The complex 12 is deduced from 2c by removing the rigid
phenylene bridge, the separated methylene triphenylphospho-
rane, and the N-1,3-dimethyl-imidazolylidene ligands being
coordinated in cis-position. The averaged calculated νCO values
(2143.5 cm-1 for 12 and 2146.2 cm-1 for 2c) indicate that the
electronic communication, if any, between the phosphonium
ylide and the NHC units has no influence on the donor ability
of the units. This result validates a posteriori the choice of the
test family: the phenylene bridge just acts as an insulating rigid
link, allowing the cis-coordinating units of 2c to be pseudoin-
dependent.
(e.g., for the ylidic unit: δ31P(exp) ) 41.30 ppm, δ31P(calcd)
) 43.25 ppm; δ1H(exp) ) 1.73 ppm, δ1H(calcd) ) 2.05 ppm;
δ13C(exp) ) -7.60 ppm, δ13C(calcd) ) 7.37 ppm). This
excellent agreement gives further support to the structural
assignement both in the solid state and in solution.
The carbonyl IR stretching absorptions of 2b′ (in CHCl3
solution) occur at lower frequencies (νCO: 1984, 2051 cm-1
)
than those of complexes 2c (νCO: 2008, 2071 cm-1) and 2a
(νCO: 2030, 2084 cm-1). These successive shifts unambiguously
indicate that substitution of a NHC moiety for a phosphonium
ylide moiety systematically increases the donor character of the
C,C-chelating ligand. This variation is consistent with the
variation of the formal charge at the rhodium atom: +1 in 2a,
0 in 2c, -1 in 2b′, the formal oxidation state of the metal (Rh(I))
and the overall charge of the complexes (+1) remaining the
same (with the same counteranion, TfO-).
The 103Rh NMR spectrum of the complexes has also been
recorded. It is observed that an increase of the basicity of the
ligand (i.e., a decrease of the νCO value) results in an increase
of the δ103Rh value (2a, +542 ppm; 2c, +731 ppm; and 2b′,
+983 ppm). Although intriguing at first sight (if the influence
of the gross diatropic electronic effect was predominant, the
nucleus should be shielded by increasing the local electron
density), this trend is consistent with previously reported specific
tendencies of other square-planar Rh(I) complexes20 (by con-
trast, the intuitive trend applies to Rh(II) complexes20).
Nolan et al. recently discussed the electronic parameter of
NHC ligands on the basis of the experimental and DFT-
calculated νCO frequencies in the [(NHC)Ir(CO)2Cl] com-
plexes.18 In the same spirit, the νCO frequencies of 2a, 2c, and
2b′ were calculated at the DFT level and compared with the
νCO values calculated for related dicarbonylrhodium(I) com-
plexes containing other P,P-, P,C-, and C,C-chelated ligands
based on the same 1,2-phenylene bridge: the bis-diphenylphos-
phine complex 9, the diphenylphosphine-NHC complex 10,
Conclusion
In summary, we described the synthesis of C,C-chelated
rhodium complexes with NHC and phosphonium ylide moieties.
On the way to bis-ylide complexes, original ylidophosphorane
and bis-ylide were evidenced. Direct comparisons between sp2
and sp3-C ligands in dicarbonylrhodium complexes unambigu-
(17) The calculated structure of the less stable conformer of 2b′ and the
calculated NMR chemical shifts of both conformers of 2b′ are given
in the Supporting Information.
(18) Kelly, R. A.; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.;
Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2008, 27, 202.
(19) Arnold, P. L.; Pearson, S. Coord. Chem. ReV. 2007, 251, 596.
(20) Bonnaire, R.; Davoust, D.; Platzer, N. Org. Magn. Reson. 1984, 22,
80. Varshavsky, Y. S.; Cherkasova, T. G.; Podkorytov, I. S.; Lyssenko,
K. A.; Nikol’skii, A. B. J. Organomet. Chem. 2003, 665, 156.
9
8410 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Diaminocarbene and Phosphonium Ylide Ligands
A R T I C L E S
Table 2. Experimental and Calculated IR CO Stretching Frequencies (cm-1) for Complexes 2a, 2b, 2b′, 2c, 9, 10, and 11d
a Five-membered rhodacycle. b Six-membered rhodacycle. c Seven-membered rhodacycle. d Calculations were performed at the B3PW91/6-31G**/
LANL2DZ*(Rh) level.
(s, CH2cod), 31.6a (s, CH2cod), 32.4b (s, CH2cod), 32.7b (s, CH2cod),
ously demonstrated that a phosphonium ylide is more donating
33.6a (s, CH2cod), 38.4a (s, CH3N), 38.5b (s, CH3N), 67.2a (d, JCRh
than an NHC. Applications of these extremely electron-rich
ligands in homogeneous catalysis are now under investigation.
) 12.3 Hz, CHcod), 67.3a (d, JCRh ) 11.9 Hz, CHcod), 69.5b (d,
JCRh ) 13.9 Hz, CHcod), 70.1b (d, JCRh ) 14.3 Hz, CHcod), 98.6a
(m, CHcod), 99.1a,b (m, CHcod), 115.4a (d, JCP ) 84.5 Hz, Car), 117.9a
Experimental Section
(d, JCP ) 88.6 Hz, Car), 118.5b (d, JCP ) 88.1 Hz, Car), 118.9b (d,
General Remarks. THF and diethyl ether were dried and
distilled over sodium/benzophenone, pentane, dichloromethane, and
acetonitrile over P2O5. All other reagents were used as commercially
available. All reactions were carried out under an argon atmosphere,
JCP ) 87.1 Hz, Car), 119.9a (d, JCP ) 94.5 Hz, Car), 120.8 (q, JCF
) 321.1 Hz, CF3SO3-), 121.2b (d, JCP ) 90.6 Hz, Car), 122.0a (s,
CHar), 123.2b (s, CHar), 124.8b (s, CHar), 125.0a (s, CHar), 130.2b
(d, JCP ) 13.1 Hz, CHar), 130.5 (d, JCP ) 12.6 Hz, CHar), 130.6 (d,
JCP ) 12.0 Hz, CHar), 130.7 (d, JCP ) 13.3 Hz, CHar), 130.8
(d, JCP ) 13.0 Hz, CHar), 131.1 (d, JCP ) 13.1 Hz, CHar), 131.6 (d,
JCP ) 7.3 Hz, CHar), 132.5 (d, JCP ) 10.6 Hz, CHar), 132.7 (d, JCP
using Schlenk and vacuum line techniques. Column chromatography
was carried out on silica gel (60 Å, C.C 70-200 µm). The following
1
analytical instruments were used. H, 13C, 31P, and 103Rh NMR:
Bruker ARX 250, DPX 300, or AV 500. X-Ray diffraction: Ipds
STOE. Mass spectrometry: Quadrupolar Nermag R10-10H. El-
emental analyses: Perkin-Elmer 2400 CHN (flash combustion and
detection by catharometry). NMR chemical shifts δ are in ppm,
with positive values to high frequency relative to the tetramethyl-
silane reference for 1H and 13C and to H3PO4 for 31P. 103Rh
chemical shifts are given to a high frequency of ꢀ(103Rh) ) 3.16
MHz.
) 10.3 Hz, CHar), 132.8 (d, JCP ) 6.8 Hz, CHar), 133.0 (d, JCP
)
10.9 Hz, CHar), 133.6 (d, JCP ) 6.7 Hz, CHar), 134.9 (s, CHar),
135.5 (s, CHar), 136.0 (d, JCP ) 8.0 Hz, CHar), 136.2 (d, JCP ) 9.1
Hz, CHar), 142.8b (d, JCP ) 3.5 Hz, Car), 143.3a (s, Car), 183.7b (d,
JCRh ) 52.1 Hz, NCN), 184.8a (d, JCRh ) 51.4 Hz, NCN); MS(ES+):
m/z: 603 [M+]; HRMS (ES+) calcd for C31H34N2ClPRh 603.1203;
found, 603.1190. Anal. Calcd for C32H34N2ClPRhF3SO3: C, 51.04;
H, 4.55; N, 3.72. Found: C, 50.41; H, 4.76; N, 3.43.
Synthesis of 6c. A 1/1 mixture of t-BuOK and complex 5c (0.72
g, 1.0 mmol) was cooled to -78 °C, and THF (20 mL) was added.
The suspension was warmed to room temperature and stirred for
1 h. After evaporation of the solvent, extraction of the residue with
CH2Cl2 (20 mL) afforded 6c as a yellow oil (0.63 g, 92%). 31P{1H}
NMR (CD3CN, 25 °C): δ ) +30.8 (d, JPRh ) 2.1); 1H NMR
(CD3CN, 25 °C): δ ) 1.21-2.57 (m, 10H, CH2cod, CH2P), 3.40
(m, 1H, CHcod), 3.50 (m, 1H, CHcod), 3.73 (s, 3H, CH3), 4.37 (m,
1H, CHcod), 4.80 (m, 1H, CHcod), 6.55 (d, JHH ) 1.70 Hz, 1H, Har),
7.03 (d, JHH ) 1.70 Hz, 1H, Har), 7.33-7.96 (m, 14H, Har); 13C{1H}
NMR (CD3CN, 25 °C): δ ) -3.3 (dd, JCP ) 32.1 Hz, JCRh ) 25.4,
CH2P), 27.9 (s, CH2cod), 29.2 (s, CH2cod), 32.1 (s, CH2cod), 33.0 (s,
CH2cod), 37.2 (s, CH3N), 77.3 (d, JCRh ) 8.7 Hz, CHcod), 82.1 (d,
JCRh ) 9.5 Hz, CHcod), 91.7 (d, JCRh ) 6.9 Hz, CHcod), 93.0 (d,
JCRh ) 7.3 Hz, CHcod), 122.2 (d, JCP ) 85.3 Hz, Car), 121.2 (q, JCF
) 319.6 Hz, CF3SO3-), 122.8 (s, CHar), 124.2 (s, CHar), 125.8 (d,
Synthesis of 5c. A mixture of [{RhCl(cod)}2] (0.34 g, 0.7 mmol),
dication 4c (0.92 g, 1.4 mmol), and NEt3 (0.22 mL, 1.5 mmol)
was dissolved in CH3CN (20 mL) and stirred at room temperature
for 2 h. After evaporation of the solvent, the crude residue was
washed with water (20 mL). Then the organic layer was extracted
with CH2Cl2 (20 mL) and dried with anhydrous MgSO4. After
evaporation of the solvent, 5c was obtained as a yellow oil (0.91
g, 86%). Recrystallization of 5c at -20 °C from CH2Cl2/toluene
gave yellow crystals as a mixture (60/40) of two stereoisomers (mp
134-136 °C). NMR assignment: aminor isomer (40%), bmajor
isomer (60%): 31P{1H} NMR (CDCl3, 25 °C): δ ) +21.2a; +22.0b;
1H NMR (CDCl3, 25 °C): δ ) 1.40a (m, 0.6H, CH2cod), 1.53a (m,
0.6H, CH2cod), 1.65-2.40a,b (m, 11H, CH2cod), 2.01a (d, JHP ) 12.9
Hz, 1.8H, CH3P), 2.44b (d, JHP ) 14.2 Hz, 3H, CH3P), 2.55a (m,
0.6H, CH2cod), 3.25b (m, 1H, CHcod), 3.38b (m, 1H, CHcod), 3.43a
(m, 0.6H, CHcod), 4.05a (s, 1.8H, CH3), 4.09b (s, 3H, CH3), 4.48b
(m, 1H, CHcod), 4.76b (m, 1H, CHcod), 4.82a (m, 1.2H, CHcod), 6.10a
(s, 0.6H, Har), 6.30b (d, JHH ) 1.70 Hz, 1H, Har), 6.96a (s, 0.6H,
Har), 7.14b (d, JHH ) 1.70 Hz, 1H, Har), 7.20-7.25a,b (m, 2.2H,
Har), 7.33a (m, 0.6H, Har), 7.40-7.80a,b (m, 17H, Har), 7.96-8.05a,b
(m, 2.6H, Har), 8.74a (m, 0.6H, CHcod); 13C{1H} NMR (CDCl3, 25
°C): δ ) 7.7b (d, JCP ) 55.0 Hz, CH3P), 7.8a (d, JCP ) 57.5 Hz,
CH3P), 27.9a (s, CH2cod), 28.3b (s, CH2cod), 28.4b (s, CH2cod), 28.8a
JCP ) 98.6 Hz, Car), 126.8 (d, JCP ) 67.9 Hz, Car), 127.4 (d, JCP
6.5 Hz, CHar), 128.3 (d, JCP ) 12.2 Hz, CHar), 129.1 (d, JCP
11.8 Hz, CHar), 129.5 (d, JCP ) 10.8 Hz, CHar), 130.6 (d, JCP
)
)
)
9.1 Hz, CHar), 132.6 (d, JCP ) 9.0 Hz, CHar), 132.7 (s, CHar), 133.0
(s, CHar), 134.3 (d, JCP ) 9.4 Hz, CHar), 134.6 (s, CHar), 143.1 (s,
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8411

A R T I C L E S
Canac et al.
Car), 186.0 (dd, JCRh ) 52.2 Hz, JCP ) 3.9 Hz, NCN); MS(ES+):
m/z: 567 [M+].
Hz, CO); 103Rh NMR (CD3CN, 25 °C): δ ) +542; MS(ES+): m/z:
397 [M+]; HRMS (ES+) calcd for C16H14N4O2Rh, 397.0172; found,
397.0242.
Synthesis of 2c. Carbon monoxide was bubbled (1 h) at room
temperature through a solution of complex 6c (0.25 g, 0.35 mmol)
in THF (40 mL). After evaporation of the solvent under vacuum,
purification by chromatography on silica gel (CH2Cl2/acetone) gave
2c as a yellow solid residue (0.21 g, 91%). Recrystallization at
-20 °C from CH2Cl2/Et2O afforded yellow crystals (mp 206-208
°C). 31P{1H} NMR (CDCl3, 25 °C): δ ) +33.2 (d, JPRh ) 3.9
Synthesis of 7b. Butyllithium (2.5 M, 0.11 mL, 0.27 mmol) was
added to a solution of diphosphonium 4b (0.10 g, 0.13 mmol) in
THF-d8 (1 mL) at -78 °C in an NMR tube. Monitoring the reaction
by low temperature NMR allowed the characterization of ylide 7b
as a mixture of stereoisomers (95/5). According to NMR spectros-
copy, at -10 °C, ylide 7b was quantitatively converted to
carbodiphosphorane 8b (t1/2 ≈ 12 h). Major isomer (95%): 31P{1H}
NMR (THF-d8, -10 °C): δ ) -2.2 (d, J ) 60.8 Hz, P+), -100.0
1
Hz); H NMR (CDCl3, 25 °C): δ ) 1.55 (ddd, JHH ) 13.9 Hz, J
) 12.0 Hz, J ) 2.1 Hz, 1H, CH2P), 1.94 (ddd, JHH ) 13.9 Hz, J
) 11.3 Hz, J ) 2.9 Hz, 1H, CH2P), 3.71 (s, 3H, CH3), 6.86 (d, JHH
) 1.93 Hz, 1H, Har), 7.24 (m, 1H, Har), 7.40-7.78 (m, 13H, Har);
7.93 (m, 1H, Har); 13C{1H} NMR (CD3CN, 25 °C): δ ) -5.3 (dd,
JCP ) 38.1 Hz, JCRh ) 20.9 Hz, CH2P), 38.1 (s, CH3N), 121.2 (q,
JCF ) 319.6, CF3SO3-), 121.6 (d, JCP ) 88.3 Hz, Car), 122.7 (d,
JCP ) 92.6 Hz, Car), 124.7 (s, CHar), 125.0 (s, CHar), 125.4 (d, JCP
1
(d, J ) 60.8 Hz, P); H NMR (THF-d8, -10 °C): δ ) 0.64 (dd,
JHP+ ) 22.3 Hz, JHP ) 13.4 Hz, 1H, PCHP), 2.37 (d, JHP ) 12.0
Hz, 3H, CH3P), 6.86-6.88 (m, 2H, Har), 6.94 (t, J ) 7.4 Hz, 2H,
Har); 7.02-7.16 (m, 5H, Har), 7.24 (m, 1H, Har), 7.32-7.76 (m,
10H, Har), 7.87 (dd, JHH ) 8.1 Hz, JHP+ ) 12.5, 4H, Har); 13C{1H}
NMR (THF-d8, -10 °C): δ ) 15.1 (dd, JCP+ ) 112.7 Hz, JCP
163.6 Hz, PCHP), 23.2 (d, JCP ) 20.7 Hz, CH3P), 123.6 (dd, JCP+
) 90.4 Hz, JCP ) 10.8 Hz, Car), 123.8 (s, CHar), 125.3 (d, JCP+
)
) 84.5 Hz, Car), 128.1 (d, JCP ) 6.4 Hz, CHar), 129.3 (d, JCP
12.6 Hz, CHar), 129.4 (d, JCP ) 15.1 Hz, CHar), 129.7 (d, JCP
)
)
)
11.5 Hz, CHar), 130.7 (d, JCP ) 9.4 Hz, CHar), 132.8 (d, JCP ) 9.3
Hz, CHar), 133.2 (s, CHar), 133.6 (s, CHar), 135.2 (d, JCP ) 9.2 Hz,
CHar), 135.6 (s, CHar), 141.9 (s, Car), 175.3 (dd, JCRh ) 44.6 Hz,
JCP ) 9.8 Hz, NCN), 184.6 (d, JCRh ) 59.7 Hz, CO), 189.6 (d,
JCRh ) 59.3 Hz, CO); 103Rh NMR (CD3CN, 25 °C): δ ) +731;
MS(ES+): m/z: 515 [M+]; HRMS (ES+) calcd for C25H21N2O2PRh,
515.0396; found, 515.0364.
Synthesis of 4a. Methyl trifluoromethanesulfonate (0.57 mL, 5.1
mmol), was added at -78 °C to a CH2Cl2 solution (20 mL) of 3a
(0.53 g, 2.5 mmol). Then the suspension was warmed to room
temperature and stirred for 12 h. After filtration, the solid residue
was washed with additional CH2Cl2 (40 mL) affording 4a (1.19 g,
88%) as a white microcrystalline solid (mp 163-165 °C). 1H NMR
(CD3CN, 25 °C): δ ) 3.95 (s, 6H, CH3), 7.46-7.55 (m, 4H, Har),
7.79-7.92 (m, 4H, Har), 8.89 (s, 2H, CH); 13C{1H} NMR (CD3CN,
25 °C): δ ) 36.5 (s, CH3), 121.0 (q, JCF ) 320.8 Hz, CF3SO3-),
123.3 (s, CHar), 124.8 (s, CHar), 128.5 (s, CHar), 129.9 (s, Car),
132.6 (s, CHar), 137.8 (s, CH); MS(FAB+): m/z: 389 [M -
CF3SO3-]+; HRMS (ES+) calcd for C15H16N4F3SO3, 389.0895;
found, 389.0907. Anal. Calcd for C16H16N4F6S2O6: C, 35.69; H,
3.00; N, 10.41. Found: C, 35.97; H, 2.66; N, 10.38.
Synthesis of 6a. A mixture of [{RhCl(cod)}2] (0.77 g, 1.6 mmol),
dication 4a (1.68 g, 3.1 mmol), and NEt3 (0.87 mL, 6.2 mmol)
was dissolved in CH3CN (40 mL) and stirred at 40 °C for 12 h.
After evaporation of the solvent, the crude residue was washed with
water (20 mL). Then the organic layer was extracted with CH2Cl2
(20 mL) and dried with anhydrous MgSO4. After washing with
Et2O (20 mL), 6a (1.72 g, 92%) was obtained as a beige powder
(mp 204-206 °C). 1H NMR (CD3CN, 25 °C): δ ) 1.44-1.59 (m,
2H, CH2cod), 1.76-1.99 (m, 2H, CH2cod), 2.11-2.26 (m, 2H,
CH2cod), 2.38-2.52 (m, 2H, CH2cod), 3.88 (s, 6H, CH3), 4.33 (m,
2H, CHcod), 4.60 (m, 2H, CHcod), 7.21 (d, JHH ) 1.9 Hz, 2H, Har),
7.37 (d, JHH ) 1.9 Hz, 2H, Har), 7.63-7.72 (m, 4H, Har); 13C{1H}
NMR (CD3CN, 25 °C): δ ) 29.8 (CH2cod), 30.1 (CH2cod), 37.2 (s,
CH3), 87.4 (d, JCRh ) 7.3 Hz, CHcod), 91.6 (d, JCRh ) 8.2 Hz, CHcod),
121.2 (q, JCF ) 320.8 Hz, CF3SO3-), 122.8 (s, CHar), 123.6 (s,
CHar), 126.6 (s, CHar), 129.5 (s, CHar), 135.1 (s, Car), 184.0 (d,
JCRh ) 52.2 Hz, NCN); MS(ES+): m/z: 449 [M+].
11.0 Hz, CHar), 126.1 (d, JCP ) 5.1 Hz, CHar), 126.5 (d, JCP ) 2.5
Hz, CHar), 127.3 (d, JCP+ ) 11.3 Hz, CHar), 127.9 (d, JCP+ ) 12.0
Hz, CHar), 128.5 (d, JCP+ ) 11.8 Hz, CHar), 128.6 (d, JCP+ ) 11.8
Hz, CHar), 129.0 (d, JCP+ ) 2.4 Hz, CHar), 129.7 (d, JCP ) 5.4 Hz,
CHar), 129.8 (s, CHar), 130.1 (d, JCP ) 9.5 Hz, Car), 131.1 (d, JCP+
) 2.8 Hz, CHar), 131.3 (d, JCP+ ) 2.7 Hz, CHar), 131.5 (dd, JCP+
) 16.3 Hz, JCP ) 4.9 Hz, CHar), 132.1 (d, JCP+ ) 10.5 Hz, CHar),
132.2 (d, JCP+ ) 10.2 Hz, CHar), 133.3 (d, JCP+ ) 83.3 Hz, Car),
142.7 (d, JCP ) 128.8 Hz, Car), 168.4 (dd, JCP+ ) 15.2 Hz, JCP
)
10.6 Hz, Car), 174.8 (dd, JCP+ ) 10.7 Hz, JCP ) 3.4 Hz, Car). Minor
isomer (5%): 31P{1H} NMR (THF-d8, -10 °C): δ ) -0.6 (d, J )
57.7 Hz, P+), -95.6 (d, J ) 57.7 Hz, P).
Synthesis of 1b′. Butyllithium (2.5 M, 0.16 mL, 0.40 mmol)
was added to a solution of diphosphonium 4b′ (0.15 g, 0.19 mmol)
in THF-d8 (2 mL) at -78 °C in an NMR tube. Monitoring the
reaction by low temperature NMR allowed the characterization of
1
bis-ylide 1b′. 31P{1H} NMR (THF-d8, -40 °C): δ ) +24.45; H
NMR (THF-d8, -40 °C): δ ) 0.59 (tt, JHP ) 26.8 Hz, JHH ) 3.3
Hz, 2H, CH2), 2.59 (broad t, JHP ) 22.7 Hz, 2H, CH), 7.09-7.14
(m, 2H, Har), 7.28-7.30 (m, 2H, Har), 7.35-7.50 (m, 20H, Har);
13C{1H} NMR (THF-d8, -40 °C): δ ) 21.9 (t, JCP ) 4.4 Hz, CH2),
24.4 (broad d, JCP ) 117.0 Hz, CH), 127.6 (m, CHar), 128.3 (m,
CHar), 130.0 (s, CHar), 131.5 (broad s, CHar), 133.5 (t, JCP ) 10.4
Hz, CHar), 140.4 (dd, JCP ) 86.7 Hz, JCP ) 12.3 Hz, Car), 141.1
(d, JCP ) 29.9 Hz, Car).
Synthesis of 2b′. To a solution of diphosphonium 4b′ (0.89 g,
1.13 mmol) in THF (40 mL) at -78 °C was added butyllithium
(2.5 M, 0.95 mL, 2.38 mmol). The suspension was warmed to -40
°C and stirred for 30 min. After addition of a solution of cationic
rhodium complex (0.58 g, 1.24 mmol) in THF (20 mL) at -40 °C,
the solution was stirred for 1 h. After warming up to room
temperature, carbon monoxide was bubbled during 30 min. After
filtration, the solvent was evaporated under vacuum. The remaining
residue was washed with pentane (40 mL) and then extracted with
CH2Cl2 (60 mL) affording 2b′ as a brown solid (0.54 g, 51%).
31P{1H} NMR (CDCl3, 25 °C): δ ) +41.3 (d, JPRh ) 4.1 Hz); 1H
NMR (CDCl3, 25 °C): δ ) 1.72-1.74 (m, 2H, CH), 3.32 (broad
td, JHP ) 24.7 Hz, JHH ) 13.4 Hz, 1H, CH2), 3.80-3.96 (m, 1H,
CH2), 7.14-7.18 (m, 4H, Har), 7.45-7.78 (m, 18H, Har); 7.97-8.00
Synthesis of 2a. Carbon monoxide was bubbled (1 h) at room
temperature through a solution of complex 6a (1.51 g, 2.5 mmol)
in THF (40 mL). After evaporation of the solvent, and washing
with Et2O (20 mL), 2a was obtained as a yellow solid (1.17 g,
85%). Recrystallization at -20 °C from CH2Cl2 gave yellow
(m, 2H, Har); 13C{1H} NMR (CDCl3, 25 °C): δ ) -7.6 (dd, JCP
)
33.9 Hz, JCRh ) 13.8 Hz, CH), 29.3 (broad s, CH2), 118.5 (d, JCP
) 57.9 Hz, Car), 120.7 (q, JCF ) 320.8 Hz, CF3SO3-), 124.0 (d,
JCP ) 75.5 Hz, Car), 129.6 (d, JCP ) 10.1 Hz, CHar), 130.2 (d, JCP
1
crystals (mp 174-176 °C). H NMR (CD3CN, 25 °C): δ ) 3.90
(s, 6H, CH3), 7.37 (d, JHH ) 1.9 Hz, 2H, Har), 7.46 (d, JHH ) 1.9
Hz, 2H, Har), 7.62-7.74 (m, 4H, Har); 13C{1H} NMR (CD3CN, 25
°C): δ ) 38.3 (s, CH3), 121.2 (q, JCF ) 320.8 Hz, CF3SO3-), 124.3
(s, CHar), 124.6 (s, CHar), 127.3 (s, CHar), 130.2 (s, CHar), 133.0
(s, Car), 174.1 (d, JCRh ) 44.0 Hz, NCN), 187.5 (d, JCRh ) 57.2
) 8.8 Hz, CHar), 133.3 (d, JCP ) 6.3 Hz, CHar), 133.4 (d, JCP
)
7.5 Hz, CHar), 133.6 (d, JCP ) 7.5 Hz, CHar), 133.8 (CHar), 134.3
(CHar), 137.9 (t, JCP ) 8.2 Hz, CHar), 188.5 (d, JCRh ) 64.2 Hz,
CO); 103Rh NMR (CD3CN, 25 °C): δ ) +983; MS(ES+): m/z:
9
8412 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Diaminocarbene and Phosphonium Ylide Ligands
A R T I C L E S
645 [M+]; HRMS (ES+) calcd for C35H28O2P2Rh, 645.0620; found,
645.0632.
Supporting Information Available: X-ray crystallographic
data for complexes 2a, 2c, and 5c (CIF) as well as computational
details (calculated structure of the less stable conformer of 2b′,
calculated NMR chemical shifts of both conformers of 2b′,
symmetry, Cartesian coordinates, number of imaginary vibra-
tional frequencies (NImag), total energies (a.u.) of rhodium
complexes 2a-c, 9-11). This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. The authors would like to thank the CNRS
and the University Paul-Sabatier for providing the expedient of this
work. They also thank CALMIP (Calcul intensif en Midi-Pyre´ne´es,
Toulouse - France), IDRIS (Institut du De´veloppement et des
Ressources en Informatique Scientifique - Orsay - France), and
CINES (Centre Informatique de l’Enseignement Supe´rieur- Mont-
pellier- France) for computing facilities.
JA801159V
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8413