A stable P-heterocyclic carbene

Article abstract of DOI:10.1002/anie.200462239

(Chemical Equation Presented) A stable diphosphorus analogue of Enders' N-heterocyclic carbene has been prepared by taking advantage of the inherent π-donor capabilities of phosphorus, which are as good as, or better than, those of nitrogen. The P-heteroc

Full text of DOI:10.1002/anie.200462239

Communications
Heterocyclic Carbenes
have only been made possible because of the availability of
isolable NHCs.[10,11] The unusual stability of NHCs (for a long
time carbenes were considered as prototypical reactive
intermediates, their instability being due to their six-
valence-electron shell that defies the octet rule)[12] results
from the ability of nitrogen to act as a p-donor, which
decreases the electron deficiency at the adjacent carbene
center. This electronic stabilization produces the strong s-
donor and weak p-acceptor properties of NHCs that explain
their efficiency as ligands for transition-metal-based catalysts.
In a seminal paper, Schleyer et al.[13] concluded that the
inherent p-donor capabilities of the heavier elements (such as
phosphorus) are as large as, or larger than, those of their
second-row counterparts (such as nitrogen), and that their
apparently inferior donor ability is due to the difficulty in
achieving the optimum planar configuration. Therefore, one
of the obvious interesting candidates to compete with and/or
complement NHCs as ligands for transition-metal-based
catalysts are P-heterocyclic carbenes (PHCs),[14–18] providing
that there is a driving force for the planarization of the
phosphorus centers. Previous calculations have shown that
the nitrogen centers of the parent NHC AN (Scheme 1) are in
a perfectly planar environment,[19,20] whereas the phosphorus
centers of the corresponding PHC AP are strongly pyrami-
dalized, as expected.[21,22] Consequently, the singlet–triplet
A Stable P-Heterocyclic Carbene**
David Martin, Antoine Baceiredo, Heinz Gornitzka,
Wolfgang W. Schoeller, and Guy Bertrand*
The availability of catalysts that can perform specific trans-
formations is critical for both industry and academia. Over
the years, the success of homogeneous catalysis can be
attributed largely to the development of a diverse range of
ligand frameworks that have been used to tune the behavior
of a variety of systems. Recently, spectacular achievements
have been made using cyclic diaminocarbenes, usually called
N-heterocyclic carbenes (NHCs; Scheme 1), as ligands for
gap drops from 79 kcalmolÀ1 for AN to 21 kcalmolÀ1 for
[19]
AP,[22] although the latter is predicted to be highly unstable
with respect to dimerization.[21] However, there are several
ways to decrease the inversion barrier at phosphorus, as
discussed recently in a comprehensive review,[23] the simplest
being to use bulky substituents. Here, we report the synthesis
and ligand properties of a stable diphosphorus analogue of
Endersꢀ NHC B.[24]
Scheme 1. N- and P-Heterocyclic carbenes and their classical precur-
sors.
transition-metal-based catalysts,[1–3] and even as catalysts in
their own right.[4,5] Compared to most classical ligands, such as
phosphanes, NHCs bind more strongly to metal centers (thus
avoiding the necessity for the use of excess ligand), and the
resulting NHC-based catalysts are less sensitive to air and
moisture, and have proven remarkably resistant to oxida-
tion.[6]
It is noteworthy that although NHC–transition-metal
complexes have been known since the Sixties,[7–9] the recent
developments in their application as scaffolds in catalysis
The classical precursors for NHCs are the well-known
corresponding protonated species NHC(H+) (Scheme 1). In
contrast, the phosphorus analogues of such species, PHC(H+),
are unknown.[25–28] Moreover, the synthetic methods used in
the nitrogen series cannot be extended to compounds of its
heavier congener. We therefore designed an original synthetic
approach: a formal [3+2] cycloaddition of the transient
diphosphaallylic cation 2 with a dipolarophile (Scheme 2).
Addition of silver trifluoromethanesulfonate or gallium
trichloride to a dichloromethane solution of the readily
available phosphaalkene 1,[29,30] at À788C, in the presence of a
large excess of acetonitrile (30–45 equiv) afforded the desired
salts 3a and 3b, respectively. They were isolated after
recrystallization as white crystals (see Experimental Section).
Deprotonation of derivative 3a was then carried out in THF
at À788C with lithium bis(trimethylsilyl)amide. The resulting
carbene 4 was isolated after recrystallization from a concen-
trated THF/toluene solution at À308C as light-yellow crystals.
The X-ray diffraction analyses of compounds 3b and 4
(Figure 1 and 2) revealed the almost planar environment of
the phosphorus centers in both molecules (sum of the angles,
3b: 3548 and 3488; 4: 3538 and 3488).[31] However, the slight
deviation from planarity (trans arrangement of the 2,4,6-tri-
tert-butylphenyl substituents) makes 3b and 4 chiral in the
[*] D. Martin, G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMR 2282)
Department of Chemistry
University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)909-787-2725
E-mail: gbertran@mail.ucr.edu
D. Martin, A. Baceiredo, H. Gornitzka, G. Bertrand
Laboratoire Hꢀtꢀrochimie Fondamentale et Appliquꢀe (UMR 5069)
Universitꢀ Paul Sabatier
118, route de Narbonne, 31062 Toulouse Cedex 04 (France)
W. W. Schoeller
Fakultꢁt fꢂr Chemie der Universitꢁt
Postfach 10 01 31, 33615 Bielefeld (Germany)
1
solid state. In solution both H and 13C NMR spectroscopy
[**] We are grateful to the ACS/PRF (38192-AC4), RHODIA, and the
DFG for financial support of this work.
showed the equivalency of the diastereotopic tert-butyl
1700
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DOI: 10.1002/anie.200462239
Angew. Chem. Int. Ed. 2005, 44, 1700 –1703

Angewandte
Chemie
Scheme 2. Synthesis and complexation of P-heterocyclic carbene 4. Ar=2,4,6-tri-tert-butylphenyl, cod=cycloocta-1,5-diene, HMDS=hexamethyldi-
silazide, Tf=trifluoromethanesulfonyl.
and 1.710(6) ꢁ (4)), which are significantly shorter than
À
À
normal P C single bonds (> 1.80 ꢁ). Since the N1 C2 and
À
À
the P1 N1 and P2 C2 bonds are only marginally longer and
shorter than typical double and single bonds, respectively, the
À
À
À
interaction between the N1 C2 unit and the P1 C1 P2
fragment is weak for both 3b and 4. The same conclusions
have been drawn for the Enders-type carbene B.[24] Interest-
ingly, the same differences that are observed between NHCs
and their NHC(H+) precursors can be found between 3b and
À
À
4. The P1 C1 P2 bond angle for 4 (98.28) is more acute than
in its cationic precursor (106.28), and is in fact very similar to
the carbene bond angle recently reported for a four-mem-
bered NHC (96.78).[32] The 13C NMR signal for the carbene
carbon (d = 184 ppm) of 4 is strongly deshielded compared to
that of 3 (d = 119 ppm), and is observed at a slightly higher
field than those of NHCs (d = 205–244 ppm).
Figure 1. Molecular view of 3b in the solid state. Selected bond
À
À
À
lengths [ꢃ] and angles [8]: P1 C1 1.693(11), P2 C1 1.720(11), P1 N1
À
À
1.662(8), P2 C2 1.799(10), C2 N1 1.318(12); P1-C1-P2 106.2(5), C1-
À
À
P1-N1 106.2(5), C1-P1-C4 126.2(5), C4 P1 N1 121.7(4), C1-P2-C2
99.2(5), C1-P2-C22 129.6(5), C22-P2-C2 119.5(5), P1-N1-C2 112.6(7),
P2-C2-N1 114.7(8).
Ab initio calculations[33] were performed on carbene 4 and
on its derivatives 4a and 4b, which feature hydrogen and
phenyl substituents, respectively, instead of 2,4,6-tri-tert-
butylphenyl substituents. As expected, the sum of the angles
around P increases significantly as the steric demand of the
substituent increases (4a: 3288 and 3428; 4b: 3418 and 3418; 4:
3518 and 3528). As a consequence of the enforced planariza-
tion of the phosphorus centers, the singlet–triplet energy gap
increases strongly (4a: 22.6; 4b: 26.7; 4: 41.4 kcalmolÀ1) as
does the energy of the HOMOs (Kohn–Sham (KS) orbital
energies) (4a: À5.9; 4b: À5.3; 4 À5.0 eV). Interestingly, the
HOMO of the experimentally observed carbene 4 is even
higher in energy than that calculated at the same level of
theory for Endersꢀ NHC B (À5.1 eV); this can be regarded as
a first indication of the strongly basic character of 4.
In initial attempts to evaluate the ligand properties of 4,
we prepared the [RhCl(cod)(4)] complex 5 and compared its
geometric parameters with those of corresponding NHC
complexes.[34–38] Complex 5 was isolated as highly thermally
stable single crystals (78% yield; m.p.: 187–1898C). Notably,
no significant decomposition was observed when a dichloro-
methane solution of 5 was stirred for several hours in air. As
observed for NHCs, the complexation induces a very small
Figure 2. Molecular view of 4 in the solid state. Selected bond lengths
À
À
À
[ꢃ] and angles [8]: P1 C1 1.673(6), P2 C1 1.710(6), P1 N1 1.708(5),
À
À
P2 C2 1.796(6), C2 N1 1.306(6); P1-C1-P2 98.2(3), C1-P1-N1
113.9(3), C1-P1-C4 124.9(3), C4-P1-N1 114.1(3), C1-P2-C2 105.7(3),
C1-P2-C22 128.7(3), C22-P2-C2 113.4(3), P1-N1-C2 107.3(4), P2-C2-N1
113.7(4).
groups, even at À1008C, thereby suggesting that the two
enantiomers are in rapid equilibrium, in agreement with the
expected low inversion barriers at the phosphorus centers.
The strong donation of the phosphorus lone-pairs to the
electron-deficient carbene center is clearly apparent from the
À
lengthening of the P C bonds, which, however, stay shorter
than those in the corresponding cation 3b (Figure 3).[31]
Importantly, the phosphorus centers are not pyramidalized,
the sum of the angles around phosphorus (3508 and 3518)
being essentially identical to those observed for the free PHC
À
P C1 bond lengths (1.693(11) and 1.720(11) ꢁ (3b); 1.673(6)
Angew. Chem. Int. Ed. 2005, 44, 1700 –1703
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Communications
Any catalytic application requiring electron-rich ligands
might benefit from the use of PHC ligands.
Experimental Section
All manipulations were performed under an inert atmosphere of
argon using standard Schlenk techniques. Dry, oxygen-free solvents
were employed. 1H, 13C, and 31P NMR spectra were recorded on
Bruker AC80, AC200, Avance 300, or AMX400 spectrometers. 1H
and 13C chemical shifts are reported in ppm relative to SiMe4 as
external standard. 31P NMR downfield chemical shifts are expressed
with a positive sign, in ppm, relative to external 85% H3PO4.
3a: A solid mixture of 3-chloro-1,3-bis(2,4,6-tri-tert-butylphenyl)-
1,3-diphosphaprop-1-ene (1; 1.06 g, 1.7 mmol) and silver triflate
(0.453 g, 1.7 mmol) was cooled to À788C. Acetonitrile (4 mL,
76 mmol) and dichloromethane (10 mL) were then added with
vigorous stirring and the reaction mixture was warmed to room
temperature. After 30 min the mixture was filtered and the solvent
removed under vacuum. The crude product was crystallized from a
CH2Cl2/Et2O mixture at À308C to give the title compound as white
needles (yield: 80%); m.p.: 157–1598C; 1H NMR (300 MHz, CDCl3,
298 K): d = 1.39 (s, 18H; CCH3), 1.45 (s, 36H; CCH3), 2.63 (dd, JP, H
Figure 3. Molecular view of 5 in the solid state. Selected bond lengths
À
À
À
[ꢃ] and angles [8]: C1 Rh 2.064(8), P1 C1 1.730(7), P2 C1 1.701(8),
À
À
À
P1 N1 1.657(6), P2 C2 1.782(8), C2 N1 1.325(10); P1-C1-P2
100.7(4), C1-P1-N1 109.2(4), C1-P1-C4 124.5(4), C4-P1-N1 116.2(3),
C1-P2-C2 104.6(4), C1-P2-C22 131.7(4), C22-P2-C2 114.4(4), P1-N1-C2
111.5(5), P2-C2-N1 112.6(6).
4. These data suggest a very weak p-donation from the metal
À
to the carbene ligand. This is confirmed by the Ccarbene Rh
bond length (2.064(8) ꢁ), which is at the upper limit of those
observed for [RhCl(cod)(NHC)] complexes (2.00–2.06 ꢁ),
and distinctly longer than that found for the analogous
complex with Endersꢀ NHC B as ligand (2.004 ꢁ).[36]
4
= 20 and 2 Hz, 3H; NCCH3), 7.73 (d, JP, H = 4 Hz, 2H; CHarom), 7.75
4
2
(d, JP, H = 3 Hz, 2H; CHarom), 8.60 ppm (dd, JP, H = 16 and 7 Hz, 1H;
PCHP); 13C{1H} NMR (75 MHz, CDCl3, 298 K): d = 22.2 (dd, JP, C = 25
and 30 Hz, NCCH3), 30.8 (s, CH3para), 30.9 (s, CH3para), 33.0 (s,
CH3ortho), 33.4 (s, CH3ortho), 38.5 (d, JP, C = 3 Hz, CqCH3), 38.6 (d, JP, C
= 8 Hz, CqCH3), 119.3 (dd, JP, C = 51 and 44 Hz, PCHP), 124.4 (d, JP, C
= 16 Hz, CaromH), 124.8 (d, JP, C = 14 Hz, CaromH), 159.8 (d, JP, C = 3 Hz,
The
carbonyl
stretching
frequencies
of
cis-
[RhCl(CO)2(L)] complexes are recognized as an excellent
measure of the s-donor and p-acceptor properties of the
carbene ligand L.[38,39] Complex 6 was easily prepared by
addition of half an equivalent of [{RhCl(CO)2}2] to the free
PHC 4. The values of the carbonyl stretching frequencies for
complex 6 are lower than those observed for the analogous
complex with Endersꢀ NHC B as ligand (Table 1). In fact, the
observed values for 6 are as low as those reported for
complexes featuring the most basic NHC and even acyclic
diaminocarbene ligands.[40,41]
PHC 4 is an analogue of one of the less-basic NHCs,
although its basicity already appears to be comparable to
those of saturated five- and six-membered NHCs. Therefore,
it is quite likely[42] that saturated PHCs will push upward the
electronic parameter scale characterizing NHC-type ligands.
Carom), 160.3 (d, JP, C = 4 Hz, Carom), 160.6 (dd, JP, C = 3 and 13 Hz, Carom),
160.6 (d, JP, C = 10 Hz, Carom), 172.2 ppm (dd, JP, C = 7 and 29 Hz,
NCCH3); 31P{1H} NMR (81 MHz, CDCl3, 298 K): d = 104 and 85 ppm
(2JP, P = 266 Hz).
3b: Compound 1 (0.8 g; 1.3 mmol) was dissolved in a mixture of
dichloromethane (10 mL) and acetonitrile (2 mL, 38 mmol). One
equivalent of GaCl3 (0.235 g) in 2 mL of dichloromethane was added,
with vigorous stirring, at À788C. The reaction mixture was then
warmed up to room temperature. The solvents were removed under
vacuum and the crude product was crystallized from a CH2Cl2/Et2O
mixture at À308C. White needles (yield: 67%); m.p.: 173–1758C.
4: A solution of 3a (1 g, 1.4 mmol) in THF (5 mL) was cooled to
À788C.
A solution of lithium hexamethyldisilazide (0.335 g,
1.4 mmol) in THF (3 mL) was added dropwise. After warming up
to room temperature, the reaction was complete. The solvent was
removed under vacuum, and the solid was extracted with pentane.
The resultant solution was concentrated and cooled to À308C to
afford a yellow powder, which was dried under vacuum. Single
crystals suitable for X-ray analysis were obtained from a concentrated
solution in THF/toluene cooled to À308C. Yellow crystals (yield:
72%); m.p.: 123–1278C; 1H NMR (400 MHz, [D8]THF; 263 K): d
= 1.35 (s, 18H; CCH3), 1.40 (s, 18H; CCH3), 1.41 (s, 18H; CCH3), 2.26
(dd, JP, H = 13 and 3 Hz, 3H; NCCH3), 7.61 (d, 4JP, H = 4 Hz, 2H;
CHarom), 7.63 ppm (d, 4JP, H = 4 Hz, 2H; CHarom); 13C{1H} NMR
Table 1: IR carbonyl stretching frequencies n˜ [cmÀ1] for cis-[RhCl(CO)2L]
complexes.
L
n(CO) I
n(CO) II
Ref.
this
work
2059
1985
=
(100 MHz, [D8]THF, 263 K): d = 19.8 (dd, JP, C = 30 and 34 Hz, N
2062
1976
[39]
CCH3), 30.6 (s, CH3), 32.0(s, CH3), 32.5(s, CH3), 35.3 (s, CCH3), 38.3
(s, CCH3), 121.1 (d, JP, C = 34 Hz, Carom), 121.3 (d, JP, C = 34 Hz, Carom),
122.3 (d, JP, C = 12 Hz, HCarom), 122.5 (d, JP, C = 11 Hz, HCarom), 153.8 (d,
2057
2081
2076
1984
1996
2006
[40]
[42]
[42]
J
P, C = 4 Hz, Carom), 153.9 (d, JP, C = 3 Hz, Carom), 157.8 (m, Carom), 179.1
=
(dd, JP, C = 16 and 31 Hz, C N), 184.4 ppm (pseudo t, JP, C = 147 Hz,
PCP); 31P{1H} NMR (162 MHz, [D8]THF, 263 K): d = 85 and 73 ppm
(2JP, P = 135 Hz).
5 and 6: A solution of 4 in THF was added to a solution of
0.5 equivalents of the corresponding chlororhodium(i) dimer
([{Rh(cod)Cl}2] for 5, [{Rh(CO)2Cl}2] for 6) in THF at À788C. After
the mixture had been warmed up to room temperature, the solvent
was removed under vacuum and the product extracted with pentane.
this
work
2089
2009
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2005, 44, 1700 –1703

Angewandte
Chemie
Single crystals suitable for X-ray analysis were obtained from a
pentane/toluene solution cooled to À308C.
isolated.[16] Metalladiphosphanyl carbenes[17] are known as
transient species, whereas several bis(iminophosphoranyl)car-
[18]
5: Deep red crystals (yield: 78%); m.p.: 187–1898C; 1H NMR
(300 MHz, C6D6, 298 K): d = 1.21 (s, 18H; CH3), 1.31 (s, 18H; CH3),
1.66 (s, 18H; CH3), 1.74 (s, 18H; CH3), 1.0–2.0 (m, 6H; H2Callyl), 2.31
bene [(R2P NSiMe3)2C] complexes have been isolated.
=
[15] T. Cantat, N. Mezailles, N. Maigrot, L. Ricard, P. Le Floch,
Chem. Commun. 2004, 1274.
[16] E. Niecke, A. Fuchs, M. Nieger, O. Schmidt, W. W. Schoeller,
Angew. Chem. 1999, 111, 3216; Angew. Chem. Int. Ed. 1999, 38,
3031.
=
(dd, JP, H = 13 and 8 Hz, 3H; N CCH3), 3.17 (br, 2H; H2Callyl), 4.29 (br,
2H; HCvinyl), 5.14 (br, 2H; HCvinyl), 7.74 ppm (m, 4H; HCarom);
13C{1H} NMR (75 MHz, C6D6, 298 K): d = 21.4 (dd, JP, C = 25 and
=
27 Hz, N CCH3), 29.1 (s, H2Callyl), 30.7(s, H2Callyl), 30.8 (s, CH3), 31.0
[17] J. Ruiz, M. E. G. Mosquera, G. Garcꢅa, E. Patrꢆn, V. Riera, S.
Garcꢅa-Granda, F. van der Maelen, Angew. Chem. 2003, 115,
4915; Angew. Chem. Int. Ed. 2003, 42, 4767.
[18] N. D. Jones, G. Lin, R. A. Gossage, R. McDonald, R. G. Cavell,
Organometallics 2003, 22, 2832.
[19] D. A. Dixon, A. J. Arduengo III, J. Phys. Chem. 1991, 95, 4180.
[20] C. Boehme, G. Frenking, J. Am. Chem. Soc. 1996, 118, 2039.
[21] A. Fekete, L. Nyulꢇszi, J. Organomet. Chem. 2002, 643–644, 278.
[22] W. W. Schoeller, D. Eisner, Inorg. Chem. 2004, 43, 2585.
[23] L. Nyulꢇszi, Tetrahedron 2000, 56, 79.
[24] D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J.-P.
Melder, K. Ebel, S. Brode, Angew. Chem. 1995, 107, 1119;
Angew. Chem. Int. Ed. Engl. 1995, 34, 1021.
[25] Acyclic diphosphaallyl cations > P-C(R)-P < are unstable
towards rearrangement,[26] and only one C-substituted derivative
(R = SiMe3) is known with four-[27] and five-membered-ring
systems.[28]
[26] T. Kato, H. Gornitzka, A. Baceiredo, W. W. Schoeller, G.
Bertrand, J. Am. Chem. Soc. 2002, 124, 2506.
(s, CH3), 33.2 (s, H2Callyl), 33.9 (s, CH3), 34.4 (s, CH3), 35.0 (s, CCH3),
35.3 (s, CCH3), 39.3 (d, JP, C = 4 Hz, CCH3), 40.2 (d, JP, C = 2 Hz, CCH3),
69.3 (d, JC,Rh = 14 Hz, CHvinyl), 78.2 (d, JC,Rh = 14 Hz, CHvinyl), 93.8 (d,
C,Rh = 8 Hz, CHvinyl), 117.0 (dd, JP, C = 16 and 28 Hz, Carom), 119.0 (dd,
P, C = 14 and 33 Hz, Carom), 123.5 (dd, JP, C = 3 and 9 Hz, HCarom), 125.2
(dd, JP, C = 6 and 9 Hz, HCarom), 153.9 (d, JP, C = 3 Hz, Carom), 155.1 (d,
P, C = 3 Hz, Carom), 157.7 (dd, JP, C = 5 and 8 Hz, Carom), 159.2 (dd, JP, C
= 5 and 8 Hz, Carom), 170.9 (ddd, JP, C = 61 and 72 Hz, JC,Rh = 41 Hz,
CRh), 176.8 ppm (pseudo t,
(81 MHz, C6D6, 298 K): d = 89 and 88 ppm (AB system, JP, P
= 173 Hz).
6: Yellow crystals (yield: 69%); m.p.: 175–1788C; IR (KBr): n˜
= 1985 and 2059 cmÀ1 (CO); 1H NMR (300 MHz, C6D6, 298 K): d
= 1.23 (s, 9H; CCH3), 1.26 (s, 9H; CCH3), 1.60 (s, 18H; CCH3), 1.62
(s, 18H; CCH3), 2.33 (dd, JP, H = 17 and 4 Hz, 3H; NCCH3), 7.71 (d,
4JP, H = 5 Hz, 2H; CHarom), 7.74 ppm (d, 4JP, H = 6 Hz, 2H; CHarom);
13C{1H} NMR (75 MHz, C6D6, 298 K): d = 22.2 (dd, JP, C = 25 and
J
J
J
=
J
P, C = 14 Hz, N C) 31P{1H} NMR
2
=
330 Hz, N CCH3), 31.5 (s, CH3), 31.8 (s, CH3), 34.6(s, CH3), 34.8 (s,
CH3), 36.2 (s, CCH3), 36.0 (s, CCH3), 40.0 (d, JP, C = 4 Hz, CCH3), 40.6
(d, JP, C = 4 Hz, CCH3), 116.3 (dd, JP, C = 11 and 39 Hz, Carom), 118.6 (dd,
[27] M. Sebastian, O. Schmidt, A. Fuchs, M. Nieger, D. Szieberth, L.
Nyulaszi, E. Niecke, Phosphorus, Sulfur Silicon Relat. Elem.
2004, 179, 779.
JP, C = 4 and 58 Hz, Carom), 124.6 (d, JP, C = 14 Hz, HCarom), 125.7 (dd, JP, C
= 1 and 14 Hz, HCarom), 156.2 (d, JP, C = 4 Hz, Carom), 157.1 (d, JP, C
= 3 Hz, Carom), 159.0 (dd, JP, C = 5 and 9 Hz, Carom), 159.9 (dd, JP, C = 3
and 9 Hz, Carom), 175.9 (ddd, JP, C = 2 and 8 Hz, JC,Rh = 24 Hz, CRh),
183.9 (ddd, JC,Rh = 75 Hz, JP, C = 3 and 6 Hz, RhCO), 185.9 ppm (dt,
[28] A. Fuchs, M. Nieger, E. Niecke, L. Nyulaszi, O. Schmidt, W.
Schoeller, M. Sebastian, Phosphorus, Sulfur Silicon Relat. Elem.
2002, 177, 1605.
[29] H. H. Karsch, H.-U. Reisacher, G. Mꢈller, Angew. Chem. 1986,
98, 467; Angew. Chem. Int. Ed. Engl. 1986, 25, 454.
[30] H. H. Karsch, H. Reisacher, T. Ruprich in Synthetic Methods of
Organometallic and Inorganic Chemistry, Vol. 3 (Ed.: W. A.
Hermann), Thieme, Stuttgart, 1996, pp. 140.
J
C,Rh = 86 Hz, JP, C = 3 Hz, RhCO); 31P{1H} NMR (81 MHz, C6D6,
298 K): d = 101 and 93 ppm (2JP, P = 187 Hz).
Received: October 8, 2004
[31] CCCDC-252550 (3b), -252551 (4) and -252552 (5) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[32] E. Despagnet-Ayoub, R. H. Grubbs, J. Am. Chem. Soc. 2004,
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[33] RI-DFTwith the BP86 density functional and the SVP basis set.
Turbomole program systems. R. Ahlrichs et al.: www.turbomo-
le.com.
Published online: February 11, 2005
Keywords: carbene ligands · carbenes · heterocycles ·
.
phosphorus · rhodium
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