Rhodium complexes of PCNHCP: Oxidative addition of dichloromethane and catalytic hydrosilylation of alkynes affording (E)-alkenylsilanes

Article abstract of DOI:10.1016/j.jorganchem.2005.07.025

New rhodium complexes of PC^NHCP have been synthesized by using the silver transfer reagent, [Ag₃(PC^NHCP) ₂Cl]Cl₂ (2). In the reaction between 2 and [Rh(COD)Cl]₂ in dichloromethane, the pres

Full text of DOI:10.1016/j.jorganchem.2005.07.025

Journal of Organometallic Chemistry 690 (2005) 5662–5671
www.elsevier.com/locate/jorganchem
Rhodium complexes of PCNHCP: Oxidative addition
of dichloromethane and catalytic hydrosilylation of alkynes
affording (E)-alkenylsilanes
*
Jing Yao Zeng, Meng-Hua Hsieh, Hon Man Lee
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
Received 30 May 2005; accepted 4 July 2005
Available online 10 August 2005
Abstract
New rhodium complexes of PCNHCP have been synthesized by using the silver transfer reagent, [Ag3(PCNHCP)2Cl]Cl2 (2). In the
reaction between 2 and [Rh(COD)Cl]2 in dichloromethane, the presumably formed nucleophilic RhI(PCNHCP)Cl intermediate (A),
undergoes a C–Cl bond activation of CH2Cl2 giving cis,mer-RhIII(PCNHCP)(CH2Cl)Cl2 (3) as the final product. Attempts to isolate
A affords the oxidative degradation product of mer-RhIII(PCNHCP)Cl3 complex (4). In contrast, the rhodium(I) center in
Rh(PCNHCP)(CO)Cl (5) is stabilized by the p-back bonding of C„O ligand; a robust complex is, therefore, obtained. The solid-state
structures of 2 and 3 were determined by X-ray diffraction. Complexes 3–5 are catalyst precursors for efficient, chemoselective
hydrosilylation of alkynes. For the reaction between phenylacetylene and dimethylphenylsilane, a rapid hydrosilylation occurs, pro-
ducing isomers of alkenylsilanes; then a slow isomerization pathway converts (Z)-alkenylsilane to its (E)-isomer. For 3, under cat-
alytic condition, a facile reductive elimination of dichloromethane giving A is anticipated. The similarity in reactivity and selectivity
between 3, 4 and 5 suggests the involvement of A as the active species in a common catalytic cycle.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Rhodium complex; N-heterocyclic carbene; Phosphine; Hybrid ligand; Hydrosilylation; X-ray structural determination
1. Introduction
is also interested in preparing and exploring the poten-
tial utilities of new hybrid ligands of NHC [10]. For
example, we have synthesized the phosphine-functional-
ized NHC ligand, PCNHCP (1) [10a]. We found that,
while palladium complexes of PCNHCP is efficient Heck
catalysts [10a], the binuclear ruthenium complex, fac-
[Ru2(l-Cl)3(PCNHCP)2]Cl, is active in catalytic transfer
hydrogenation [10f]. The reactivity study also shows
that the PCNHCP ligand is capable of accommodating
extra steric requirement by switching readily from facial
to meridional chelating mode [10f].
In this paper, continuing on our interest in
PCNHCP, we report on its new rhodium complexes.
We followed our previous established procedure of
using the trinuclear silver NHC complex, [Ag3(l-Cl)-
(PCNHCP)2]Cl2 (2), as the PCNHCP transfer reagent
[10f]. We found that a reaction between 2 and
N-heterocyclic carbenes (NHCs) have attracted much
attention because their transition metal complexes dis-
play rich coordination chemistry and have wide applica-
bility in catalysis [1]. While monodentate NHC ligands
have been shown to be highly active in various catalytic
reactions [1c,1d,1e,1f], research efforts have also been
devoted to the synthesis of hybrid ligands containing
NHC and classical donors, such as nitrogen and phos-
phorus. Some of these show interesting coordination
chemistry [2–5], efficient catalytic applications [6–8],
and biological activities [9]. Currently, our laboratory
*
Corresponding author. Tel.: +886 4 7232105x3523; fax: +886 4
7211190.
E-mail address: leehm@cc.ncue.edu.tw (H.M. Lee).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.025

J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
5663
[Rh(COD)Cl]2 in dichloromethane produced cleanly
the intriguing rhodium(III) complex, cis,mer-RhIII-
(PCNHCP)(CH2Cl)Cl2 (3), formed by a dichlorometh-
ane molecule oxidatively added to the reactive
rhodium(I) intermediate, RhI(PCNHCP)Cl (A). Oxida-
tive addition of small molecules to reactive metal com-
plexes is an important step for the activation of
substrates in homogeneous catalysis. Because of the
relatively inert C–Cl bond, the activation of dichloro-
methane requires electron-rich rhodium centers. Rho-
dium complexes of phosphine [11] and nitrogen
ligands [12] and hybrid ligands containing both P-
and N-donors [13] are able to mediate the oxidative
addition of dichloromethane and in several cases the
crystal structures of RhIII(CH2Cl)Cl were obtained
[11b,12b,12c,12d,13b]. Although rhodium complexes
of NHC are numerous [1e,1g], 3 represents the first re-
ported case of an oxidative addition of dichlorometh-
ane to a rhodium(I) complex containing a NHC
moiety. We have also prepared the rhodium(III) tri-
chloride complex, mer-RhIII(PCNHCP)Cl3 (4) and the
rhodium(I) complex, mer-RhI(PCNHCP)(CO)Cl (5).
All the new metal complexes 3–5 are efficient catalyst
precursors in hydrosilylation of alkynes affording (E)-
vinylsilanes as the major product.
(0.043 g, 0.087 mmol) was stirred at room temperature
for 1 h. A grey solid of AgCl was slowly formed, which
was filtered off through a plug of Celite. The solvent of
the filtrate was removed completely under vacuum.
Upon addition of diethyl ether, a greenish-yellow solid
was formed which was filtered on a frit and dried under
vacuum. Yield: 0.10 g (89%). Anal. Calc. for C32H32-
N2P2Cl3Rh: C, 53.69; H, 4.51; N, 3.91. Found: C,
53.60; H, 4.49; N, 3.85, m.p.: 245–249 °C. 1H NMR
(CDCl3): d 2.69–2.77 (m, 2H, PCHH), 2.96–3.04 (m,
3
2
2H, PCHH), 3.61 (td, JHP = 8.1 Hz, JRhH = 2.8 Hz,
2
2H, CH2Cl), 4.34 (pentet, JHH = 3JHH = 3JHP
=
=
11.6 Hz, 2H, NCHH), 4.78 (pentet, JHH = 3JHH
2
3JHP = 11.6 Hz, 2H, NCHH), 6.85 (s, 2H, imi-H),
7.31–7.60 (m, 14H, Ph-H and imi-H), 7.86–7.90 (m,
3H, Ph-H), 8.33–8.36 (m, 3H, Ph-H). 13C{1H} NMR
(CDCl3): d 23.9 (t, JCP = 13.5 Hz, PCH2), 46.3 (s,
2
NCH2), 121.3 (imi-C), 127.9 (dt, JRhC = 24.1,
JCP = 5.2 Hz, P–C), 130.0 (d, JCP = 15.0 Hz, Cortho),
2
132.7 (t, JCP = 4.1 Hz, Cpara), 135.7 (t, JCP = 5.4 Hz,
Cmeta), signals for the Rh–CH2Cl and Rh–carbene were
not observed. 31P{1H} NMR (CDCl3): d 6.4 (d,
1JRhP = 103.3 Hz).
2.3. Preparation of Rh(PCNHCP)Cl3 (4)
A 10 mL DMF solution of [Ag3(l-Cl)(PCNHCP)2]Cl2
(0.13 g, 0.091 mmol) and [Rh(COD)Cl]2 (0.050 g,
0.10 mmol) was heated at 60 °C overnight. A dark
brown solid of AgCl and Ag0 was slowly formed, which
was filtered off through a plug of Celite. The solvent of
the filtrate was removed completely under vacuum.
Upon addition of diethyl ether, a yellow solid was
formed which was filtered on a frit and dried under vac-
uum. Yield: 90 mg (70%). Anal. Calc for C31H30N2-
P2Cl3Rh: C, 53.05; H, 4.31; N, 3.99. Found: C, 53.15;
2. Experimental section
2.1. General procedure
All reactions were performed under a dry nitrogen
atmosphere using a Schlenk line and standard Schlenk
technique. All solvents used were purified according to
standard procedures [14]. 1H, 13C{1H}, and 31P{1H}
NMR spectra were recorded at 300.13, 75.48, and
121.49 Hz, respectively, on a Bruker AV-300 spectrom-
eter. Chemical shifts for 1H and 13C spectra were in
ppm relative to residual proton of CDCl3 (1H: d 7.24;
13C: d 77.0) or DMSO-d6 (1H: d 2.50; 13C: d 39.5). 31P
NMR chemical shifts were relative to 85% H3PO4 exter-
nal standard (31P: d 0.0). Infrared spectra were recorded
with a Digilab Scimitar FTS 2000 spectrometer. GC/MS
analyses were performed on a HP 6890 Series GC sys-
tem with a HP 5973 mass selective detector, equipped
with a HP-5MS capillary column (length: 30 mm, ID:
0.25 mm, film thickness: 0.25 lm). Elemental analyses
were performed on a Heraeus CHN-OS Rapid Elemen-
tal Analyzer at the Instrument Center, National Chung
Hsing University, Taiwan. [Rh(COD)Cl]2 and [Rh-
(CO)2Cl]2 are purchased from commercial source.
1
H, 4.21; N, 3.95, m.p.: >290 °C. H NMR (CDCl3): d
2.90 (br, 4H, PCH2), 4.58 (br q, 3JHH = 3JHP = 11.2 Hz,
4H, NCH2), 7.29–7.52 (m, 12H, Ph-H), 7.59 (s, 2H, imi-
H), 7.95–8.09 (m, 8H, Ph-H). 13C{1H} NMR (DMSO-
d6): d 24.5 (t, JCP = 13.5 Hz, PCH2), 45.2 (NCH2),
123.7 (imi-C), 127.6 (t, JCP = 4.8 Hz, Cortho), 130.1 (s,
3
Cpara), 131.8 (t, JCP = 25.1 Hz, P–C), 134.8 (t, JCP
=
4.6 Hz, Cmeta), signal for the Rh–C was not observed.
1
31P{1H} NMR (CDCl3): d 1.7 (d, JRhP = 89.1 Hz).
2.4. Preparation of Rh(PCNHCP)(CO)Cl (5)
A 10 mL DMF solution of [Ag3(l-Cl)(PCNHCP)2]-
Cl2 (0.15 g, 0.10 mmol) and [Rh(CO)2Cl]2 (0.040 g,
0.10 mmol) was stirred at room temperature for 2 h. A
grey solid of AgCl was formed slowly, which was filtered
off through a plug of Celite. The solvent of the filtrate
was removed under vacuum. Upon addition of diethyl
ether, a dark yellow solid was formed which was filtered
on a frit and dried under vacuum. Yield: 0.11 g (77%).
2.2. Preparation of cis-Rh(PCNHCP)(CH2Cl)Cl2 (3)
A 10 mL dichloromethane solution of [Ag3(l-Cl)-
(PCNHCP)2]Cl2 (0.11 g, 0.078 mmol) and [Rh(COD)Cl]2

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J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
Table 1
Crystallographic data of 3–4
Anal. Calc for C32H30N2P2ClORh: C, 58.33; H, 4.59; N,
4.25. Found: C, 58.11; H, 4.54; N, 4.21, m.p.: 208 °C. 1H
NMR (CDCl3): d 2.69–2.72 (br, 4H, PCH2), 4.42–4.53
(br m, 4H, NCH2), 7.33 (s, 2H, imi-H), 7.40–7.50 (m,
12H, Ph-H), 7.67–7.72 (m, 8H, Ph-H). 13C{1H} NMR
(DMSO-d6): d 25.8 (t, JCP = 15.2 Hz, PCH2), 47.3
(NCH2), 123.5 (imi-C), 129.6 (t, JCP = 5.1 Hz, Cortho),
131.5 (s, Cpara), 133.3 (t, JCP = 6.7 Hz, Cmeta), 134.1 (t,
1JCP = 23.4 Hz, P–C), signals for the Rh–C and C„O
were not observed. 31P{1H} NMR (CDCl3): d 21.7 (d,
Complex 3
Complex 4
Empirical formula
C32H32Cl3N2P2Rh Æ C31H30Cl3N2P2Rh
2CH2Cl2
Formula weight
Crystal system
Space group
885.65
Monoclinic
P21/c
22.379(4)
24.663(4)
14.263(3)
90
701.77
Orthorhombic
P212121
11.9793(6)
15.6200(8)
15.6674(8)
90
˚
a (A)
˚
b (A)
˚
c (A)
1JRhP = 126.2 Hz). IR (KBr): m(C„O) 1933 cmꢀ1
.
a (°)
b (°)
c (°)
102.899(8)
90
90
90
3
˚
2.5. General procedure for the hydrosilylation of alkynes
V (A )
7674(2)
298(2)
8
2931.6(3)
150(2)
4
7772
352
0.0583
0.1378
ꢀ0.01(4)
T (K)
Z
In a typical run, a mixture of alkyne (0.77 mmol), an
appropriate amount of hydrosilane (1.1–3 equiv.), and
0.1 mol% of catalyst in 5 mL of chloroform was stirred
at 60 °C for an appropriate duration under nitrogen.
The solution was allowed to cool and filtered through
a pad of Celite. The volatiles were removed under re-
duced pressure. The residue was then re-dissolved in
No. of unique data
15066
No. of parameters refined 857
a
R1 [I > 2rI]
0.0721
0.2301
–
b
wR2 (all data)
Refined flack parameter
P
P
a
R1 = (iFoj ꢀ jFci)/ jFoj.
P
P
2
2
2
1=2
b
wR2 ¼ ½ ðj F oj ꢀ j F cj Þ = ðF 2oÞꢁ
.
1
CDCl3 for H NMR or GC/MS analyses. The assign-
ments of (Z)-, (E)-, and a-alkenylsilanes were based
3. Results and discussion
upon spectroscopic comparison with literature [15].
3.1. Preparation of cis,mer-RhIII(PCNHCP)(CH2Cl)Cl2
(3)
2.6. X-ray data collection
Crystals of 3 and 4 were obtained by vapor diffusion
of diethyl ether into their corresponding dichlorometh-
ane solution. Typically, the crystals were removed from
the vial with a small amount of mother liquor and
immediately coated with silicon grease on a weighting
paper. A suitable crystal of 4 was mounted on a glass fi-
ber with silicone grease and placed in the cold stream of
a Bruker APEX II with graphite monochromated Mo
The trinuclear silver complex, [Ag3(PCNHCP)2Cl]Cl2
(2), has been utilized successfully as the PCNHCP trans-
fer reagent in the preparation of palladium [10a] and
ruthenium complexes of PCNHCP [10f]. Here, we con-
tinue to employ 2 as a general PCNHCP transfer reagent
for the preparation of its new rhodium complexes
(Scheme 1). Initially, we stirred a mixture of 2 and
[Rh(COD)Cl]2 in dichloromethane with an anticipated
formation of a 16-electron RhI(PCNHCP)Cl complex
(A) and AgCl as by-product. In fact, grey solid of AgCl
was slowly formed and after a simple workup proce-
dure, a greenish-yellow solid was obtained. Markedly,
the subsequent NMR and structural studies reveal that
the compound produced is cis,mer-RhIII(PCNHCP)-
(CH2Cl)Cl2 (3), formed by the oxidative addition of a
dichloromethane solvent molecule to A. Complex 3 is
air-stable with a good solubility in halogenated solvent.
It was characterized by 1H, 31P and 13C NMR spectros-
copy and elemental analysis. The presence of a single
doublet at d 6.4 (1JRhP = 103.3 Hz) in its 31P{1H}
NMR spectrum shows that the two coordinating phos-
phorus atoms are chemically equivalent and, therefore,
the PCNHCP ligand is chelated in the meridional fashion.
˚
Ka radiation (k = 0.71073 A) at 150(2) K. Crystal of 3
was similarly mounted and collected on a Bruker
SMART 1000 CCD with graphite monochromated
˚
Mo Ka radiation (k = 0.71073 A) at 298(2) K at the
Instruments Center, National Chung Hsing University,
Taiwan. Crystallographic data of 3 and 4 are listed in
Table 1.
2.7. Solution and structure refinements
All structures were solved by direct methods using
SHELXS-97 and refined by full-matrix least squares meth-
ods against F2 with SHELXL-97 [16]. Tables of neutral
atom scattering factors, f0 and f00, and absorption coeffi-
cients are from a standard source [17]. Hydrogen atoms
were fixed at calculated positions, and their positions
were refined by a riding model. All atoms except hydro-
gen atoms were refined with anisotropic displacement
parameters. There are two orientations (0.5 occupancy
each) for each of the three disordered dichloromethane
solvent molecules in 3.
1
In its H NMR spectrum, the two multiplets in the up-
field region can be assigned to the diastereotopic
PCH2 protons, whereas the two apparent pentets at d
4.34 and 4.78 are due to the diastereotopic NCH2 pro-
tons. The presence of the CH2Cl moiety is evidenced

J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
5665
by the presence of a triplet of doublets at d 3.61 with
the Rh–CH2Cl distance; the Rh–Cl bond trans to the
2
3JHP and JRhH coupling constants of 8.1 and 2.8 Hz.
carbene (2.423(2) A) is slightly shorter than that trans
˚
˚
The CH2Cl moiety most likely occupies an axial position
such that the two chlorides are cis to other, as evidenced
by the disasterotopic nature of the CH2 protons. How-
ever, the 13C{1H} NMR signals due to the CH2Cl and
carbene moieties are not observed.
to the chloromethyl ligand (2.501(2) A). Similar to
those reported for Pd(PCNHCP) and Ru(PCNHCP) com-
plexes, the central imdazole ring in 3 is twisted aside
forming an overall chiral environment of the complex
in the solid state. The twist angle between the best
planes defined by the imidazole ring and the equatorial
coordination plane is 24.8°, which is the smallest
among those found in the ruthenium (27–28°) [10f]
and palladium (27–49°) complexes [10a].
To probe the structure of 3 and distinguish its cis/
trans configuration, we performed a single-crystal X-
ray structural determination. Suitable crystals of 3 were
obtained by vapor diffusion of diethyl ether into its
corresponding solution. Indeed, the X-ray analysis
reveals
3
to be cis,mer-RhIII(PCNHCP)(CH2Cl)Cl2
3.2. Preparation of mer-RhIII(PCNHCP)Cl3 (4)
(Fig. 1). Its selected bond distances and angles are tab-
ulated in Table 2. The structure is solved in the mono-
clinic space group P21/c. The asymmetric unit contains
two independent molecules of 3 and four dichloro-
methane solvent molecules, three of which are disor-
dered. Because of the almost identical geometrical
parameters of the two independent molecules, only
one of them will be used for structural discussion.
The Rh(1) is in an octahedral coordination environ-
ment with the PCNHCP chelated in a meridional fash-
ion and the chloromethyl ligand trans to a chloride.
Complex 3 contains the chloromethyl ligand which
was formed by the oxidative addition of a dichlorometh-
ane solvent molecule to the presumably very nucleo-
philic intermediate A. Therefore, we decided to carry
out the reaction between 2 and [Rh(COD)Cl]2 in nonha-
logenated solvent in the hope of isolating RhI(PCNHCP)-
Cl. Hence, stirring a solution of 2 and the rhodium
dimer in DMF at 60 °C slowly produced a turbid solu-
tion under an inert atmosphere. After filtration of the
solid AgCl, removal of the solvent gave an air-stable
yellow solid, which is slightly soluble in halogenated sol-
vent, but dissolves more readily in high polar solvents,
such as DMSO and DMF. The compound is character-
˚
The Rh–CH2Cl distance of 2.086(9) A is comparable
˚
to that found in [RhCl(CH2Cl)(edpp)2]Cl (2.078(7) A)
[13b]; shorter than that in [RhCl(CH2Cl)(dmpe)2]Cl
1
(2.161(2) A) [11b]; but longer than those found in
ized by H, 13C, and 31P NMR spectroscopy as well as
˚
[Rh(py)3(CH2Cl)Cl2] (2.045(10) A) [12c], [RhCl(CH2-
elemental analysis. Its 31P{1H} NMR spectrum displays
a doublet at d 1.7, which is slightly upfield to that of 3.
The observation of only one phosphorus signal suggests
˚
˚
Cl)(edmp)2]Cl (2.050(7) A) [13b], [RhCl(CH2Cl)-
˚
(dmbpy)(DMSO)] (2.046(2) A) [12d], and [RhCl2-
1
(CH2Cl)(2,6-(C(H)@N–R)2C5H3N)] (R: i-Pr, 2.052(5)
the complex is symmetry-related. In the H NMR spec-
˚
˚
˚
A; Cy, 2.052(3) A and 2.059(3) A) [12b]. The Rh–car-
trum, the NCH2 and PCH2 protons are observed as
broad signals at d 2.90 and 4.58, respectively. The
˚
bene bond distance (2.008(9) A) is slightly shorter than
Cl3
C18
C3
C32
C19
N1
N2
C4
C5
C1
P5
Rh1
P4
Cl1
Cl2
Fig. 1. Thermal ellipsoid plot of 3 at the 30% probability level. Hydrogen atoms (except those on C32) are omitted for clarity.

5666
J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
broadening of these signals suggest a fast interconver-
sion between the left- and right-twist forms, similar to
that observed in related palladium complexes [10a].
However, separate signals cannot be obtained even if
the DMF-d6 solution was cooled down to ꢀ52 °C, indi-
cating the rapidity of the fluxional process.
unstable, which undergoes oxidative degradation in the
presence of Ag+ and chloride anions affording the
Rh(III) product. Arnold et al. [18] has shown that a
Ru(III) carbene complex can be obtained from a reac-
tion between the Ru(II) precursor, RuCl2(PPh3)3, and
the corresponding silver alkoxy carbene transfer re-
agent. Electrochemical study showed that the silver car-
bene complex itself is a poor oxidant [18].
A subsequent X-ray structural analysis shows that
the product is actually mer-RhIII(PCNHCP)Cl3 (4). Crys-
tals suitable for the structural analysis were grown fol-
lowing a similar procedure of that for 3. Fig. 2 shows
a perspective drawing of the structure. Its selected bond
length and bond distances are also listed in Table 2. The
structure is solved in the orthorhombic noncentrosym-
metric space group P212121. The asymmetric unit con-
tains a single molecule of RhIII(PCNHCP)Cl3. The
Rh(1) atom is in a slightly distorted octahedral coordi-
nation environment. The Rh(1)–Cl(2) distance of
3.3. mer-Rh(PCNHCP)(CO)Cl (5)
Because the electron rich intermediate A is not isola-
ble, it can be anticipated that this nucleophilic species
can be stabilized by the presence of a carbonyl ligand.
Hence, in order to obtain a stable Rh(I) complex of
PCNHCP, we carry out a reaction between 2 and the
Rh(I) precursor, [Rh(CO)2Cl]2. Following a similar pro-
cedure for 3, an air-stable dark yellow solid was ob-
tained. This compound is also characterized by 1H,
13C and 31P NMR spectroscopy and elemental analysis.
The 31P{1H} NMR signal for the two equivalent phos-
phorus atoms is observed at d 21.7 as a doublet, indicat-
˚
˚
2.4395(16) A is ꢂ0.07 A longer than the two mutually
trans Ru–Cl bond distances, clearly reflecting the bigger
trans influence of the carbene. The central imidazole ring
is also twisted (twist angle = 27.2°) resulting in a chiral
conformation with
a noncyrstallographic pseudo-
1
C2 rotation axis passing through the Cl–Ru–C axis. Un-
like related structures of Pd(PCNHCP) [10a] and
Ru(PCNHCP) complexes [10f] and 3 described above, 4
crystallizes in one of the enantiomeric forms in the crys-
tal lattice. In solution, however, a rapid interconversion
between the enantiomers occurs, as evidenced by the
broadening of the ethylene protons.
ing the meridional coordination of PCNHCP. In the H
NMR spectrum, the two broad multiplets in the ali-
phatic region can be assigned to the PCH2 and NCH2
protons, respectively. The broadening of these signals
suggests a fluxional process similar to that in 4. Even
though in its 13C{1H} NMR spectra, the signal due to
the carbonyl resonance is not observable, the IR spec-
trum clearly exhibits an intense absorption band for
the carbonyl ligands at 1933 cmꢀ1. The carbonyl
absorption of 5 is in accord with rhodium chlorocar-
bonyl complexes of tridentate ligands reported in the
literature. For example, the corresponding absorption
band for [RhCl(CO)(PNCHP-j3P,N,P)], (PNCHP =
Notably, the formation of mer-RhIII(PCNHCP)Cl3
from 2 and [Rh(COD)Cl]2 is peculiar. Conceivably, the
intermediate complex A is highly reactive, which was
demonstrated by its capability to activate inert C–Cl
bond to afford 3. Therefore, in the absence of carbon ha-
lide, presumably, the reactive RhI(PCNHCP)Cl is highly
C3
C2
Cl3
C4
N2
C5
P2
C1
C19
N1
Rh1
C18
P1
Cl2
Cl1
Fig. 2. Thermal ellipsoid plot of 4 at the 30% probability level. Hydrogen atoms are omitted for clarity.

J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
5667
2-(diphenylphosphino)-N-[2-(diphenylphosphino)benz-
the Rh(I) complex 5 with the hybrid ligand PCNHC
are also effective catalysts.
P
ylidene]benzeneamine (PNCHP)) [19a] and RhCl(CO)-
(triphos) (triphos = MeC(CH2PPh2)3) are at 1959 and
1920 cmꢀ1 [19b], respectively. Unfortunately, crystals
suitable for structural analysis were failed to obtain
so that the stereochemistry of 4 is not fully established;
The catalytic property of 3–5 in hydrosilylation of
terminal alkynes with hydrosilanes was summarized in
Tables 3–6. It has been shown that the selectivity in
most transition-metal catalyzed hydrosilylation is highly
dependent on factors such as substrates used,
temperature, and solvent [23]; therefore, we employ
the hydrosilylation of phenylacetylene (6) with dimeth-
ylphenylsilane (7) as the standard for the initial tuning
of reaction conditions. In general, the reaction catalyzed
by 3 affords the (E)-isomer as the major product (Table
3). The hydrosilylation is, in fact, very rapid as complete
consumption of phenylacetylene was achieved in just
15 min with a catalyst loading of 0.1 mol% and 1:3
molar ratio of 6/7 (entry 5). The E/Z/a distribution is
60:23:17. Intriguingly, the E/Z selectivity is improved
significantly with a prolonged reaction time (entries 5–
11). In the initial first 6 h, the E/Z/a distribution
remains approximately constant. But subsequently after
12 h, a stereoselective production of (E)-isomer was
achieved (entry 9). Eventually, an optimum 88% yield
of (E)-isomer was achieved at 24 h (entry 11). The
amount of the a-isomer remains approximately constant
with time. The catalytic data suggests the occurrence of
a rapid hydrosilylation reaction producing isomers of
alkenylsilanes. After the complete consumption of phe-
nylacetylene, a relatively slow isomerization pathway,
which converts the (Z)-alkenylsilane to its E-isomer is
but based on the meridional coordination of PCNHC
P
and the numerous five-coordinate rhodium(I) com-
plexes reported, a trigonal bipyramidal coordination
geometry of the rhodium(I) center can be safely
assumed.
3.4. Catalytic hydrosilylation of alkynes with hydrosilanes
Alkenylsilanes are important building blocks in or-
ganic synthesis. They are commonly prepared by the
transition metal-catalyzed hydrosilylation of alkynes
[20]. Late transition metal complexes of platinum, rho-
dium, and, ruthenium are most frequently employed.
However, the methods are usually encountered with re-
gio- and stereo-selectivity problem (Scheme 2). There-
fore, there is a continuous interest in searching metal
catalysts with improved selectivity. Rhodium catalysts,
in particular, are attracting attention because high regio-
and stereoselectivity for the alkenylsilanes can be
achieved [20a,21]. Recently, rhodium complexes of
NHC have also been employed as catalysts in the hydro-
silylation [22]. Along this direction, we are interested to
explore if the two new Rh(III) complexes 3 and 4 and
2+
N
N
2Cl-
PPh2
Ph2P
Ph2P
N
N
Cl
Ag
Ag
[Rh(COD)Cl]2
Ag
Rh
Cl
A
Ph2P
PPh2
PPh2
N
N
2
CH2Cl2
DMF/
N
[Rh(CO)2Cl]2
N
N
N
N
N
Cl
Cl
Rh
Cl
Rh
Ph2P
H2C
Cl
Ph2P
OC
Rh
PPh2
PPh2
Ph2P
Cl
PPh2
Cl
Cl
4
5
3
Scheme 1.
R
R
SiR'2R"
+
cat.
R
+
HSiR'2R"
R
+
SiR'2R"
E
SiR'2R"
Z
χ
Scheme 2.

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J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
Table 2
Table 4
˚
Selected bond lengths (A) and angles (°) for complex 3–4
Complex 3
Effect of the solvent on the hydrosilylation of PhC„CH with
HSiMe2Ph catalyzed by 3a
Rh(1)–C(1)
Rh(1)–P(1)
Rh(1)–P(2)
2.008(9)
2.332(3)
2.366(3)
Rh(1)–Cl(1)
Rh(1)–Cl(2)
Rh(1)–C(32)
2.423(2)
2.501(2)
2.076(9)
Entry
Solvent
Yield, %b
E
Z
a
1
2
3
4
5
a
CHCl3
THF
Acetone
DMF
Acetonitrile
100
100
100
100
100
83
78
61
79
89
0
17
11
29
21
6
11
10
0
C(1)–Rh(1)–Cl(1)
Cl(2)–Rh(1)–C(32) 174.9(3)
177.5(3)
Cl(1)–Rh(1)–P(2)
Cl(1)–Rh(1)–Cl(2)
Cl(1)–Rh(1)–C(32)
C32–Rh(1)–P(1)
C32–Rh(1)–P(2)
Cl(2)–Rh(1)–P(1)
Cl(2)–Rh(1)–P(2)
Rh(1)–C32–Cl(3)
88.82(8)
97.00(8)
87.1(3)
87.9(3)
91.0(3)
95.39(9)
86.09(9)
119.0(5)
5
P(1)–Rh(1)–P(1)
C(1)–Rh(1)–P(1)
C(1)–Rh(1)–P(2)
C(1)–Rh(1)–Cl(2)
C(1)–Rh(1)–C(32)
Cl(1)–Rh(1)–P(1)
175.00(9)
91.9(3)
93.1(3)
81.5(3)
94.5(4)
86.26(8)
0.1 mol% of 3, 60 °C, 12 h, 0.5 mL of solvent, alkyne/hydro-
silane = 1:3.
b
NMR yield.
Table 5
Hydrosilylation of various alkynes and hydrosilanes catalyzed by 3a
Complex 4
Rh(1)–C(1)
Rh(1)–P(1)
Rh(1)–P(2)
Entry Alkyne
Hydrosilane
Yield, %b
E
Z
a
2.003(7)
2.3714(18) Rh(1)–Cl(2)
2.3589(19) Rh(1)–Cl(3)
Rh(1)–Cl(1)
2.3615(17)
2.4395(16)
2.3609(16)
1
2
3
4
PhC„CH
PhC„CH
PhC„CH
3,3-Dimethyl- HSiMe2Ph
1-butyne
3,3-Dimethyl- HSiEt3
1-butyne
3,3-Dimethyl- HSiMe2(t-Bu)
1-butyne
HSiMe2Ph
HSiEt3
HSiMe2(t-Bu) 100
100
100
88
71
0
3
12
26
C(1)–Rh(1)–Cl(2)
Cl(1)–Rh(1)–Cl(3) 171.41(6)
176.1(2)
Cl(2)–Rh(1)–P(2)
Cl(2)–Rh(1)–P(1)
Cl(1)–Rh(1)–Cl(2)
Cl(3)–Rh(1)–P(1)
Cl(3)–Rh(1)–P(2)
Cl(1)–Rh(1)–P(1)
Cl(1)–Rh(1)–P(2)
86.28(6)
89.59(6)
92.25(6)
86.02(6)
92.44(6)
95.14(6)
87.03(6)
54 26 20
83 17
100
95
0
0
P(1)–Rh(1)–P(2)
C(1)–Rh(1)–Cl(1)
C(1)–Rh(1)–Cl(3)
C(1)–Rh(1)–P(2)
C(1)–Rh(1)–P(1)
Cl(2)–Rh(1)–Cl(3)
175.41(7)
84.2(2)
87.2(2)
91.9(2)
92.3(2)
96.27(6)
5
6
32 13 55
0
0
0
a
0.1 mol% of 3, 60 °C, 24 h, 0.5 mL of CHCl3, alkyne/hydro-
silane = 1:3.
b
NMR yield.
Table 3
Hydrosilylation of PhC„CH (6) with HSiMe2Ph (7) catalyzed by 3a
Yield, %b
E
Z
a
Table 6
Entry Cat. loading, 6/7
Time
Hydrosilylation of PhC„CH with HSiR03 catalyzed by 3–5a
mole%
Entry
Catalyst
HSiR03
Yield, %b
E
Z
a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17d
18e
a
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.01
0.001
0.001
0.001c
0.001c
0.1
1/1.1 15 h
1/1.1 24 h
1/1.1 30 h
100
100
100
100
56 28 16
74 10 16
67 15 18
1
2
3
4
5
6
7
8
9
a
3
4
5
3
4
5
3
4
5
HSiMe2Ph
HSiMe2Ph
HSiMe2Ph
HSiEt3
HSiEt3
HSiEt3
HSiMe2(t-Bu)
HSiMe2(t-Bu)
HSiMe2(t-Bu)
100
100
100
100
100
100
100
100
100
88
80
85
71
81
82
54
55
54
0
0
0
3
2
12
20
15
26
17
18
20
22
25
1/2
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
1/3
24 h
15 min 100
30 min 100
1 h
6 h
12 h
18 h
24 h
1 hr
78
0
22
60 23 17
56 24 20
53 26 21
54 21 25
100
100
100
100
100
100
0
26
23
21
83
81
88
0
0
0
17
19
12
0.1 mol% of cat., 60 °C, 24 h, 0.5 mL of CHCl3, alkyne/hydro-
silane = 1:3.
38 30 32
42 19 39
30 min 100
b
NMR yield.
24 h
12 h
24 h
24 h
24 h
100
81
100
100
100
75
8
17
48 35 17
43 34 23
by the rhodium complexes with chelating and pincer
NHC. A comparison of entries 2, 4, and 11 indicates
the importance of excess hydrosilane in the isomeriza-
tion. The hydrosilylation can be catalyzed by trace
amount of catalyst as shown by entries 12–15. A catalyst
loading of 0.001 mol% can afford quantitative produc-
tion of alkenylsilanes in 30 min (entry 13) at 60 °C; how-
ever, when the reaction is conducted at room
temperature, a prolonged reaction time of 12–24 h is re-
quired (entries 15–16). A comparison of entries 14 and
16 clearly shows the temperature dependence of the
isomerization pathway. It has been shown that cationic
rhodium complexes exhibit a general (E)-selectivity
[21b,25], whereas neutral rhodium complex provides
93
93
4
3
3
4
0.1
60 °C, 0.5 mL of CHCl3.
NMR yield.
At R.T.
0.1 mol% of NaI added.
0.2 mol% of NaI added.
b
c
d
e
in operation. The isomerization profile of 3 is consistent
with the findings of Mori et al. [24] which showed that
RhI(PPh3)3, in the presence of a hydrosilane, can cata-
lyze the isomerization of (Z)-alkenylsilanes to (E)-iso-
mers. Peris et al. [22a] reported a similar (Z)–(E)
isomerization in the hydrosilyation of alkynes catalyzed

J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
5669
(Z)-alkenylsilanes predominately [21a,26]. The addition
of NaI to the catalytic reaction should suppress the dis-
sociation of Rh–Cl bond and hence the (Z)-selectivity
should be enhanced [21d]. In contrast, entries 17 and
18 show that, in the presence of NaI, the (E)-isomer still
predominates with 93% of yield, whereas the amount of
a-isomer formed is significantly suppressed. The ob-
served (E)-selectivity can be attributed again to the pres-
ence of the isomerization pathway.
final product of cis,mer-RhIII(PCNHCP)(CH2Cl)Cl2 is
undoubtedly formed via the oxidative addition of
dichloromethane to a RhI(PCNHCP)Cl complex. Even
though attempts to isolate this intermediate A met
without success, it is conceivable from its activation
of inert C–Cl bond that the complex contains an
electron rich rhodium(I) center supported by PCNHCP.
In the absence of carbon halide, this reactive RhI-
(PCNHCP)Cl undergoes oxidative degradation affording
the mer-RhIII(PCNHCP)Cl3 complex. In contrast, the
rhodium(I) center in mer-Rh(PCNHCP)(CO)Cl is stabi-
lized by the p-back bonding of C„O ligand; conse-
quently, a stable complex can be obtained.
Complexes 3–5 are highly efficient in the hydrosilyla-
tion of alkynes with hydrosilanes producing alkenylsil-
anes in chemoselective fashion. For the reaction
between phenylacetylene and dimethylphenylsilane, a
rapid hydrosilylation occurs producing isomers of alke-
nylsilanes and then a slow isomerization pathway con-
verts (Z)-alkenylsilane to its (E)-isomer. A generally
accepted catalytic cycles for the hydrosilylation involves
a rhodium(I) catalytic species which allows the initial
oxidative addition of hydrosilane substrate [21a].
Undoubtedly, complex 3, via the reductive elimination
of dichloromethane, produces RhI(PNHCP)Cl, which is
presumably the initial active species in the catalytic
cycle. The similarity in reactivity and selectivity between
3, 4 and 5, in fact, suggests the involvement of RhI-
(PNHCP)Cl as the active species in a common catalytic
cycle. In fact, the quantitative production of alkenylsil-
anes achieved by 3 in 15 min appears to be the most effi-
cient reported in the literature (see Table 3, entry 5).
However, the overall stereoselectivity (after the isomeri-
zation step) is inferior to that catalyzed by [Rh(COD)2]
BF4/2PPh3 [21b]. Nevertheless, complex 3 represents the
most efficient catalyst among other rhodium NHC com-
plexes reported [22a,22b].
Initially, we employed chloroform as solvent. In
Table 4, we looked in different solvents and found that
both DMF and acetonitrile can replace chloroform to
achieve same level of selectivity. A partially optimized
reaction conditions: 0.1 mol% of catalyst at 60 °C with
a 1:3 molar ratio of alkyne/hydrosilane in chloroform,
is then applied for the hydrosilanes and catalyst screen-
ing (Tables 5 and 6). Table 5 shows that the selectivity
depends markedly on the hydrosilanes used. A compar-
ison of entries 1 and 3 indicates that the bulky t-Bu
group on the hydrosilane lowers the stereoselectivity.
The presence of steric bulkiness in both alkyne and
hydrosilane stops the catalytic activity (entry 6). Entries
1–3 of Table 6 clearly shows that the trivalent mer-RhIII
-
(PCNHCP)Cl3 and monovalent mer-Rh(PCNHCP)(CO)Cl
are as effective as cis,mer-RhIII(PCNHCP)(CH2Cl)Cl2 in
the stereospecific production of E-isomer. However,
consistent with the data in Table 5, use of hydrosilanes
other than HSiMe2Ph, reduces the selectivity (entries
4–9). The similarity of reactivity and selectivity between
3, 4 and 5 suggests a common active species involved.
It should be noted that there are possible competing
pathways for the consumption of alkynes; the dimeriza-
tion, cyclotrimerization, and polymerization of phenyl-
acetylene and the dehydrogenative silylation have been
observed for various catalysts [15a,15b,21c,21d] which
resulted in lower yields of the desirable alkenylsilanes.
Complexes 3–5, however, provide a high chemoselectiv-
ity for the hydrosilylation. A control experiment with a
solution of 3 and phenylacetylene without the addition
of dimethylphenylsilane shows that the alkyne is not
consumed; the GC/MS analysis of a typical catalytic
solution also confirms the absence of trimeric, dimeric
and oligomeric products of alkyne and dehydrosilyla-
tion product. In this regard, even though the catalytic
system, [Rh(COD)2]BF4/2PPh3, is superior to 3 in ste-
reo- and regio-selectivity, the hydrosilylation of phenyl-
acetylene was accompanied by polymerization [22a,21b],
whereas 3 is chemoselective.
Acknowledgement
We are grateful to the National Science Council of
Taiwan for financial support of this work.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors)
for 3 and 4 in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication numbers CCDC 270936 and 270937.
Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: +44(0)-1223-336033 or e-mail: de-
posit@ccdc.cam.ac.uk]. Supplementary data associated
4. Conclusions
We have successfully prepared new rhodium com-
plexes of PCNHCP by silver carbene transfer reaction
using [Ag3(PCNHCP)2Cl]Cl2 (2). In the reaction be-
tween 2 and [Rh(COD)Cl]2 in dichloromethane, the

5670
J.Y. Zeng et al. / Journal of Organometallic Chemistry 690 (2005) 5662–5671
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