α,β-(Phosphino)(aminocarbene) and α,ω-(phosphino)(oxyaminocarbene): New bidentate ligands for transition metal complexes

Article abstract of DOI:10.1016/j.jorganchem.2004.02.002

Direct complexation of (amino)(phosphino)carbene 1a and (amino)(oxy)carbene 1b featuring a phosphino group in position-6 to the carbene with [Rh(CO)₂Cl]₂ has been studied. With the 1,2-bidentate ligand 1a, an original cationic complex 2 featuring two (amino)(phosphino)carbenes η²-bonded to the metal has been isolated in 79% yield. In the case of the 1,6-bidentate ligand 1b, a rhodium(I) complex 3 in which the carbene is in trans position relative to the CO ligand was obtained in 85% yield. Both compounds were fully characterized including X-ray diffraction studies.

Full text of DOI:10.1016/j.jorganchem.2004.02.002

Journal of Organometallic Chemistry 689 (2004) 1431–1435
www.elsevier.com/locate/jorganchem
a; b-(Phosphino)(aminocarbene) and
a; x-(phosphino)(oxyaminocarbene): new bidentate ligands for
transition metal complexes
a
a
a,
a,b,
*
Nathalie Merceron-Saffon , Heinz Gornitzka , Antoine Baceiredo *, Guy Bertrand
a
Laboratoire Hꢀetꢀerochimie Fondamentale et Appliquꢀee, UMR 5069, Universitꢀe Paul Sabatier, 118 route de Narbonne,
F-31062 Toulouse cedex 04, France
UCR-CNRS Joint Research Chemistry Laboratory, UMR 2282, Department of Chemistry, University of California, Riverside, CA 92521-0403, USA
b
Received 21 December 2003; accepted 2 February 2004
Abstract
Direct complexation of (amino)(phosphino)carbene 1a and (amino)(oxy)carbene 1b featuring a phosphino group in position-6 to
the carbene with [Rh(CO)2Cl]2 has been studied. With the 1,2-bidentate ligand 1a, an original cationic complex 2 featuring two
(amino)(phosphino)carbenes g2-bonded to the metal has been isolated in 79% yield. In the case of the 1,6-bidentate ligand 1b, a
rhodium(I) complex 3 in which the carbene is in trans position relative to the CO ligand was obtained in 85% yield. Both compounds
were fully characterized including X-ray diffraction studies.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: (Amino)(phosphino)carbene; (Amino)(oxy)carbene; Bidentate ligands; Rhodium complexes
1. Introduction
mainly reside in the supposed inherent instability of
carbenes, which is explained by their six-valence-elec-
tron shell that defies the octet rule. Our group has pre-
pared a variety of new types of stable carbene. However,
direct complexation of the first of them, namely the
(phosphino)(silyl)carbenes [1a,5], has not yet been re-
ported, and all our attempts have failed. The apparent
reluctance of these carbenes to act as ligands has re-
cently been tentatively rationalized by theoretical studies
[6]. In marked contrast, we have found that treatment of
the [2,6-bis(trifluoromethyl)phenyl](phosphino)carbene
A [7] with half an equivalent of [RhCl(nbd)]2 in toluene
at )50 °C immediately and quantitatively affords the
corresponding carbene complex B (Scheme 1) [8]. Based
on NMR spectroscopy and X-ray diffraction study, we
concluded that the carbene–metal interaction consists
almost exclusively of donation of the carbene lone pair
into an empty metal-based orbital. Back-donation from
the metal to the carbene center is negligible compared to
that from the phosphorus lone pair. Carbene A is a
strong r-donor!
Over the years the success of homogeneous catalysis
can largely be attributed to the development of a diverse
range of ligand frameworks that have been used to tune
the behaviour of the various systems. A particularly
good example is the recent spectacular achievements
that have been made using N-heterocyclic carbene
(NHC) ligands [1]. It is noteworthy that although NHC-
transition metal complexes have been known since 1968
[2] and that their organometallic chemistry was investi-
gated by Lappert in the sixties [3], the recent develop-
ments in their application as scaffolds in catalysis have
only been made possible because of the availability of
stable N-heterocyclic carbenes [4]. Although it is possi-
ble to tune the structure of carbene ligands to some
extent, the diversity possible is by far out reached by
that possible for their phosphorus-based counterparts.
Obstacles to extending the range of carbenes available
* Corresponding author. Tel.: +1-909-787-2719; fax: +1-909-787-
2725.
E-mail address: gbertran@mail.ucr.edu (G. Bertrand).
Here we report the direct synthesis of transition metal
complexes from the stable (amino)(phosphino)carbene
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.02.002

1432
N. Merceron-Saffon et al. / Journal of Organometallic Chemistry 689 (2004) 1431–1435
F3C
R2N
F3C
P
1/2 [Rh(nbd)Cl]2
R = i-Pr2N
C
R2N
R2N
P
C
CF3
R2N
B
Rh
Cl
CF3
A
Scheme 1.
R
R
R
R2P
R'
R2P
R
O
O
LiHMDS
-78˚C
NR'2
C
N
X
C
NR'2
R2P
H
R'
O
C
O
1a
1b
R = t-Bu,R' = i-Pr
X = CF3SO3
Fig. 1. Solid state structure of complex 2 (ellipsoids are drawn at the
50% probability level, and hydrogen atoms are omitted for clarity).
Scheme 2.
ꢁ
Selected bond lengths [A] and angles [°]: Rh(1)–C(1) 2.000(3); Rh(1)–
C(16) 2.007(3); Rh(1)–C(31) 1.891(3); Rh(1)–P(1) 2.3480(8); Rh(1)–
P(2) 2.3262(8); C(1)–N(1) 1.299(4); C(16)–N(2) 1.302(4); P(1)–C(1)
1.791(3); P(2)–C(16) 1.798(3); C(31)–O(1) 1.135(4); P(1)–Rh(1)–C(1)
47.83(9); P(2)–Rh(1)–C(16) 48.36(9); P(1)–Rh(1)–C(31) 110.04(10);
C(1)–Rh(1)–C(31) 99.57(13); P(2)–Rh(1)–C(31) 112.05(10); C(16)–
Rh(1)–C(31) 100.35(13).
1a [9] and (amino)(oxy)carbene 1b [10] featuring a
phosphino group in position-6 with respect to the car-
bene center. Carbene 1a has been prepared by depro-
tonation of the corresponding iminium salt, while
carbene 1b has been synthesized by simple addition of
the 3,5-di-tert-butyl-ortho-quinone to carbene 1a
(Scheme 2).
are in the range typical for C–Rh single bonds, and very
similar to that observed for related N-heterocyclic car-
ꢁ
bene rhodium complexes (2.00–2.04 A) [11]. The phos-
ꢁ
phorus–rhodium bond distance (2.348 A) lies at the
2. Results and discussion
upper limit of the range typical for P–Rh single bonds
[12]. The nitrogen atom adopts a trigonal planar ge-
ꢁ
ometry with a short NC bond distance of 1.30 A, which
is identical to that of free (amino)(phosphino)carbenes
The (phosphino)(amino)carbene 1a reacted at )78 °C
with 0.25 equivalent of [Rh(CO)2Cl]2. The reaction
proceeded very cleanly, and complex 2 was isolated as
red crystals in 79% yield by recrystallization from a
THF solution at )30 °C (Scheme 3). The presence of
two phosphinocarbene ligands was indicated by the
multiplicity of the 13C NMR signals for the CO (201.0,
dt, JRh–C ¼ 86, JP–C ¼ 16 Hz) and Ccarbene nuclei (227.0,
dt, JRh–C ¼ 24, JP–C ¼ 24 Hz). The high field position
()58.6 ppm) and the large phosphorus–rhodium cou-
pling constant (JP–Rh ¼ 102 Hz) observed in the 31P
NMR spectrum suggested the formation of three-
membered metallocycles. The g2-coordination mode
was unambiguously established by an X-ray diffraction
study (Fig. 1). Complex 2 adopts a distorded square-
based pyramid structure with the CO ligand in axial
ꢁ
(1.30 A). This indicates that, the carbene–metal inter-
action results from the donation of the carbene lone pair
into an empty metal-based orbital. Back-donation from
the metal to the carbene centre is almost negligible
compared to that from the nitrogen lone pair.
Rhodium complex 3 was prepared by treatment of a
THF solution of the (amino)(oxy)carbene 1b with 0.5
equivalent of [Rh(CO)2Cl]2. It was isolated in 85% yield
as yellow crystals by slow recrystallisation from a pen-
tane solution at 0 °C (Scheme 4). The 31P NMR spec-
trum showed a sharp doublet at 202 ppm with a large
phosphorus–rhodium coupling constant (199 Hz) and in
ꢁ
position. The carbene–rhodium bond distances (2.00 A)
R
NR'2
R
C
R2P
O
O
X
[Rh(CO)2Cl]2
[Rh(CO)2Cl]2
Rh
CO
1a
C
NR'2
1b
R2P
R = t-Bu,R' = i-Pr
X = CF3SO3
R = t-Bu,R' = i-Pr
2
Rh
PR2
3
C
R'2N
OC
Scheme 4.
Cl
Scheme 3.

N. Merceron-Saffon et al. / Journal of Organometallic Chemistry 689 (2004) 1431–1435
1433
1
NMR, H NMR and 13C NMR spectra were recorded
on a Bruker AMX400 instrument. Chemical shifts are
reported in ppm downfield from Me4Si and were refer-
enced to solvent peaks (1H, 13C) or external 85% H3PO4
(31P). Coupling constants are given in Hz. [Rh(CO)2Cl]2
was used as supplied from Aldrich.
4.1. Synthesis of the (amino)(phosphino)carbene 1a
To a THF solution (5 ml) of C-(phosphino)iminium
salt (0.2 mmol) was added 1.5 equivalent of LiHMDS at
)60 °C. Immediately a red colour appeared, and the 31
P
NMR spectroscopy indicated the quantitative formation
of carbene 1a. This compound can be stored in solution
for a few days at 0 °C, and was used without any further
purification. 31P-NMR (THF-d8): 7.9 ppm. 1H-NMR
3
Fig. 2. Solid state structure of complex 3 (ellipsoids are drawn at the
50% probability level, and hydrogen atoms are omitted for clarity).
(THF-d8): 1.11 (d, 18H, JP–H ¼ 10:8, CH3CP), 1.30 (d,
3
12H, JH–H ¼ 6:4, CH3CN), 4.50 (m, 1H, CHN), 5.31
ꢁ
Selected bond lengths [A] and angles [°]: Rh(1)–C(1) 2.079(9); Rh(1)–
(m, 1H, CHN). 13C-NMR (THF-d8): 20.5 (m,
CH3CHN), 24.7 (m, CH3CHN), 29.4 (d,2JP–C ¼ 13:9,
CH3C), 32.6 (d,1JP–C ¼ 18:9, CH3CP), 52.9 (m, CHN),
P(1) 2.225(2); Rh(1)–C(8) 1.870(10); Rh(1)–Cl(1) 2.396(2); C(1)–N(1)
1.324(11); C(1)–O(1) 1.330(9); C(8)–O(3) 1.148(10); P(1)–Rh(1)–C(1)
90.0(2); C(1)–Rh(1)–Cl(1) 88.3(2); P(1)–Rh(1)–C(8) 97.0(3); C(8)–
Rh(1)–Cl(1) 85.8(3).
1
63.3 (m, CHN), 346.6 (d, JP–C ¼ 57:3, PCN).
the 13C NMR spectrum the Ccarbene nuclei appeared to
be also coupled to 103Rh (d ¼ 226 ppm, JRh–C ¼ 47 Hz),
which suggest that both the phosphine and carbene
fragments are coordinated to the metal. Interestingly
only one isomer is formed and the trans-orientation of
the carbene with respect to the CO ligand has been es-
tablished by a single crystal X-ray diffraction study
(Fig. 2). The rhodium center is in a nearly square planar
environment (sum of bond angles around Rh 361.1°).
4.2. Synthesis of the (amino)(oxo)carbene 1b
To a THF solution (5 ml) of (amino)(phosph-
ino)carbene 1a (0.2 mmol) was added 1 equivalent of
3,5-di-tert-butyl-ortho-quinone at )30 °C. Immediately
a deep green colour appeared, and the 31P NMR spec-
troscopy indicated the quantitative formation of carbene
1b. After evaporation of the solvent, carbene 1b was
obtained as a deep green oil (90% yield). 31P-NMR
(THF-d8, 253 K):152.5. 1H-NMR (THF-d8, 253 K):1.17
(d, 18H, JP–H ¼ 11:2, CH3CP), 1.26 (d, 6H, JH–H ¼ 6:6,
CH3CN), 1.33 (s, 9H, CH3C), 1.37 (s, 9H, CH3C), 1.42
(d, 6H, JH–H ¼ 6:3, CH3CN), 3.85 (sept, 1H, JH–H ¼ 6:3,
CHN), 5.21 (sept, 1H, JH–H ¼ 6:6, CHN), 6.97 (d, 1H,
JH–H ¼ 1:52, Haro), 7.44 (dd, 1H, JH–H ¼ 1:52 Hz,
JP–H ¼ 2:8, Haro); 13C-NMR (THF-d8, 253 K): 20.2 (s
broad, CH3CHN), 26.0 (s broad, CH3CHN), 27.4 (d,
JP–C ¼ 15:5, CH3CP), 30.5 (s, CH3C), 31.4 (s, CH3C),
34.9 (s, CH3C), 35.3 (s, CH3C), 35.8 (d, JP–C ¼ 25:7,
CH3CP), 43.2 (s, CHN), 46.8 (s, CHN), 113.9 (d,
JP–C ¼ 24:2 Hz, Caro), 115.4 (s, Caro), 140.8 (s, Caro),
145.8 (s, Caro), 146.5 (s, Caro), 152.2 (d, JP–C ¼ 9:1 Hz,
ꢁ
The Rh–P distance of 2.225 A is slightly shorter com-
pare with the expected value for a rhodium(I)–phos-
phine complex [12]. Interestingly, the Rh–Ccarbene bond
ꢁ
length of 2.079 A is very similar to the values observed
in rhodium complexes bearing a Cl and a diaminocar-
bene ligand cis to one another [13]. These observations
are in line with the observed frequency of the IR ab-
sorption m(CO) (1997 cmꢀ1) [14].
3. Conclusion
The (amino)(phosphino)carbene 1a and (amino)(oxo)
carbene 1b are certainly excellent candidates as ligand
for transition metal catalysts, in which a strong r-donor
ligand and a relatively labile ligand are desirable.
C
aro), 268.1 (s, O–C–N).
4.3. Synthesis of the (amino)(phosphino)carbene rho-
dium complex 2
4. Experimental
A THF solution (5 ml) of (amino)(phosphino)car-
bene 1a (0.2 mmol) was added to 0.25 equivalent of
[Rh(CO)2Cl]2 at )78 °C. After the solution mixture was
warmed to room temperature, 31P NMR spectroscopy
indicated the quantitative formation of complex 2,
All experiments were carried out under dry argon
using standard Schlenk or dry box techniques. THF was
distilled under argon from sodium benzophenone. 31P

1434
N. Merceron-Saffon et al. / Journal of Organometallic Chemistry 689 (2004) 1431–1435
which was obtained as a red powder after evaporation
the solvent. Red crystals of 2 were obtained from a THF
solution at )30 °C (79% yield). m.p. 155–156 °C. 31P-
NMR (THF-d8): )58.6 ppm (d, JRh–P ¼ 102). 1H-NMR
(THF-d8): 1.37 (m, 18H, CH3CP), 1.46 (m, 18H,
CH3CP), 1.47 (d, JH–H ¼ 6:2, 6H, CH3–CH), 1.55 (d,
JH–H ¼ 6:4, 6H, CH3–CH), 1.85 (d, JH–H ¼ 6:4, 6H,
CH3–CH), 1.97 (d, JH–H ¼ 6:2, 6H, CH3–CH), 4.22
(sept, JH–H ¼ 6:2, 2H, CH–N), 4.63 (sept, JH–H ¼ 6:4,
2H, CH–N). 13C-NMR (THF-d8): 19.2 (s, CH3–CH),
20.3 (s, CH3–CH), 21.2 (s, CH3–CH), 21.3 (s, CH3–CH),
30.5 (s, CH3–C), 31.8 (s, CH3–C), 36.9 (s, CH3CP), 37.1
(s, CH3CP), 52.6 (s, CHN), 74.9 (s, CHN), 201.0 (dt,
JRh–C ¼ 86, JP–C ¼ 16, CO), 227.0 (dt, JRh–C ¼ 24,
JP–C ¼ 24, PC).
R1 ¼ 0:044 (for I > 2rðIÞ) and wR2 ¼ 0:0862 (all data).
Crystal data for 3: C30H52ClNO3PRh, M ¼ 644:06,
monoclinic, space group P21=n with a ¼ 12:438ð3Þ,
ꢁ
b ¼ 20:513ð5Þ, c ¼ 13:205ð3Þ A, b ¼ 103:907ð4Þ°, V ¼
3
ꢁ
3270:4ð12Þ A , Z ¼ 4. 12,500 reflections (3939 indepen-
dent, Rint ¼ 0:1086), largest electron density residue:
ꢁꢀ3
1.453 e A , R1 ¼ 0:0587 (for I > 2rðIÞ) and wR2 ¼
0:1468 (all data).
5. Supplementary material
CCDC-226835 [2], 226836 [3] contain the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystal-
lographic data Centre, 12 Union Road, Cambridge,
CB21EZ, UK; fax: +44-1223-336-033 or e-mail:
deposit@ccdc.cam.ac.uk).
4.4. Synthesis of (amino)(oxo)carbene rhodium complex 3
A THF solution (5 ml) of amino(oxy)carbene 1b (0.2
mmol) was added to 0.5 equivalent of [Rh(CO)2Cl]2 at
)78 °C. After the solution mixture was warmed to room
temperature, 31P NMR spectroscopy indicated the
quantitative formation of complex 3. After evaporation
the solvent, the residue was extracted with pentane (50
ml). Yellow crystals of 3 were obtained from a pentane
solution at 0 °C (85% yield). m.p. 149 °C (dec.). 31P-
NMR (CDCl3): 202 ppm (d, JRh–P ¼ 199). 1H-NMR
(CDCl3): 1.27 (s, 9H, CH3C), 1.39 (s, 9H, CH3C), 1.41
(d, JP–H ¼ 16:0, 18H, CH3CP), 1.50 (d, JH–H ¼ 6:6, 6H,
CH3–CH), 1.60 (d, JH–H ¼ 6:6, 6H, CH3–CH), 4.17
(m, 1H, CH–N), 6.50 (sept, JH–H ¼ 6:6, 1H, CH–N),
7.21 (m, 2H, Haro). 13C-NMR (CDCl3): 24.5 (s broad,
CH3–CH), 27.0 (s broad, CH3–CH), 29.7 (d, JP–C ¼ 6,
CH3–CP), 31.6 (s, CH3–C), 31.8 (s, CH3–C), 35.0 (s,
CH3C), 35.9 (s, CH3C), 39.1 (dd, JRh–C ¼ 2, JP–C ¼ 22,
CH3CP), 50.1 (s, CHN), 58.7 (s, CHN), 121.0 (s, Caro),
121.0 (d, JP–C ¼ 2, Caro), 143.0 (d, JP–C ¼ 2, Caro), 143.1
(s, Caro), 146.2 (d, JP–C ¼ 7, Caro), 147.7 (s, Caro), 193.9
(dd, JRh–C ¼ 57, JP–C ¼ 14, CO), 226.2 (d, JRh–C ¼ 47,
RhCN). IR (CH2Cl2): 1997 cmꢀ1 (CO).
Acknowledgements
We are grateful to the ACS/PRF (38192-AC4), and
RHODIA for financial support of this work.
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ꢁ
ꢁ
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¼
ꢁꢀ3
0:0487), largest electron density residue: 0.980 e A
,

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