A Rh(I) complex with an annulated N-heterocyclic carbene ligand for E-selective alkyne hydrosilylation

Article abstract of DOI:10.1016/j.poly.2019.04.027

A Rh(I) complex supported by a fused π-conjugated imidazo[1,2-a][1,8]naphthyridine-based N-heterocyclic carbene ligand with a Dipp attachment on the imidazole nitrogen has been synthesized and structurally characterized. The title complex is found to be a

Full text of DOI:10.1016/j.poly.2019.04.027

Accepted Manuscript
A Rh(I) Complex with an Annulated N-Heterocyclic Carbene Ligand for E-
Selective Alkyne Hydrosilylation
Akshi Tyagi, Suman Yadav, Prosenjit Daw, Chitrakar Ravi, Jitendra K. Bera
PII:
S0277-5387(19)30274-8
https://doi.org/10.1016/j.poly.2019.04.027
POLY 13887
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
19 February 2019
19 April 2019
Please cite this article as: A. Tyagi, S. Yadav, P. Daw, C. Ravi, J.K. Bera, A Rh(I) Complex with an Annulated N-
Heterocyclic Carbene Ligand for E-Selective Alkyne Hydrosilylation, Polyhedron (2019), doi: https://doi.org/
10.1016/j.poly.2019.04.027
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A Rh(I) Complex with an Annulated N-Heterocyclic
Carbene Ligand for E-Selective Alkyne Hydrosilylation$
Akshi Tyagi, Suman Yadav, Prosenjit Daw, Chitrakar Ravi and Jitendra K Bera*
Department of Chemistry and Center for Environmental Sciences and Engineering,
Indian Institute of Technology Kanpur, Kanpur 208016 India.
Fax: +91–512–2596806; Tel: +91–512–2597336
E–mail: jbera@iitk.ac.in
* To whom correspondence should be addressed.
$ Special issue on “Bio-inspired Inorganic Chemistry” dedicated to Prof. Akhil R.
Chakravarty on the occasion of his 65th birthday

TOC

Abstract
A Rh(I) complex supported by a fused π-conjugated imidazo[1,2-a][1,8]naphthyridine-
based N-heterocyclic carbene ligand with a Dipp attachment on the imidazole nitrogen
has been synthesized and structurally characterized. The title complex is found to be an
excellent catalyst for accessing E-vinylsilanes. The scope of the chemoselective
hydrosilylation is examined for a range of terminal alkynes with silanes Et3SiH and
(EtO)3SiH.
Keywords: NHC, Rhodium, Catalysis, Hydrosilylation, Vinylsilanes

1. Introduction
N-heterocyclic carbenes (NHCs) constitute an important class of ligands because of
their easy accessibility, structural varieties, steric and electronic tunability, ability to bind
an array of metal ions, and for the remarkable applications of metal-NHC complexes for
a broad range of catalytic transformations.[1,2] Although the metal-NHC complexes are
well suited for oxidation chemistry,[3] the prospect of ligand dissociation from a putative
(NHC)metal-hydride intermediate via reductive elimination restricts the use of the NHC
ligands in direct hydrogenation reactions.[4] Introduction of a donor functionality on the
NHC scaffold favors the formation of the chelate complex and thus suppresses ligand
dissociation during the catalytic cycles. Several donor groups are attached to the
imidazole nitrogen either directly or through the linkers which offer wide structural
diversity and improve the stability of the metal-NHC complexes.[5-7] We reported
naphthyridine-functionalized NHC ligands which show multifaceted coordination to
different metal ions (Scheme 1a).[6] The rotational flexibility could be circumvented by
synthetic fusion of the carbene ring and the donor moiety. Suitable annulation has
resulted in a series of NHC ligands where extended π-delocalization affords the higher
conjugational stability and the enhanced σ-donor ability compared to their non-
annulated analogues.[8] An annulated π-conjugated C4/C5-bound mesoionic carbene
(MIC)[9] ligand forms a stable complex [Ru(MIC)(COD)Br2] which endures oxidative
conditions and selectively cleaves olefins to the corresponding aldehydes (Scheme
1b).[3d]

Scheme 1. (a) Multifaceted coordination of naphthyridine-
functionalized NHC ligand, (b) an annulated MIC complex of Ru.
Hydrosilylation of carbon-carbon multiple bonds is one of the important methods for
the construction of Si-C bond. Transition-metal catalyzed hydrosilylation of alkynes
gives the straightforward and atom-economical access to vinylsilanes,[10] which are
useful intermediates in organic synthesis,[11] coupling partners in Hiyama cross coupling
reactions[10c,12,13] and industrial reagents to synthesize cross-linked silicone polymers.[14]
Three possible stereo-isomeric products, namely, β-(Z), β-(E) and branched α-isomers
are formed in the hydrosilylation of terminal alkynes (Scheme 2). Despite the major
advances in recent years, the selectivity has remained a challenging issue. The nature
of the catalysts, the substrates and reagents (alkynes and silanes) and the reaction
conditions influence the product selectivity. While the Pt catalysts gave selectively β-(E)
vinylsilane,[15,16] the Ru catalysts afforded β-(Z) or α-isomers.[17-19] Several Rh catalysts
have been reported that allowed to make a few general observations.[5c,7,19-33] Neutral
Rh complexes tend to show selectivity for β-(Z) vinylsilanes whereas cationic complexes
provide β-(E) vinylsilanes.[19,32,33] Metal-assisted isomerization of β-(Z) to β-(E)
vinylsilane is reported with Rh complexes.[16,27,29,30,33] Selectivity also depends on the
nature of the silanes– electron-rich silanes give β-(Z) vinylsilanes whereas β-(E)
isomers are obtained from electron-poor silanes.[20] Further, oligomerisation,[23]

polymerization of arylalkynes[24,26,32] and the formation of alkynyl-silanes from the
dehydrogenative silylation products[25,30,31] are the side reactions that complicate the Rh
catalyzed hydrosilylation reactions.
Scheme 2. Hydrosilylation of terminal alkynes.
Jimenez, Oro and coworkers employed both neutral and cationic Rh complexes with
amino-alkyl functionalized NHC ligands (Scheme 3a).[26] High β-(Z)-selectivity was
reported for 1-hexyne whereas sterically hindered alkynes gave E-selective products. In
addition, Z to E isomerization was observed over a prolonged time, along with extensive
polymerization of the phenylacetylene. Similar Z to E isomerization of the resulting
vinylsilanes was also reported by Cassani and Mazzoni for a neutral Rh(I) complex
containing BOC-protected amino functionalized NHC ligand, although alkyne
polymerization was not observed.[27] Peris and coworkers introduced a pyridine
functionalized Rh-NHC complex for HSiMe2Ph addition to phenylacetylene (Scheme
3b).[7] A mixture of E/Z/α isomers was obtained at higher temperature (60 °C), but β-(Z)-
selectivity could be improved by carrying out the reaction at room temperature.
Recently, Salazar, Suarez and coworkers reported Rh(I) complexes containing sterically
controlled picolyl-NHC ligands which offered excellent β-(E)-selectivity for
hydrosilylation of a variety of alkynes with both electron-rich and electron-poor silanes
(Scheme 3c).[22] It is apparent from literature reports that the ligand flexibility and the
steric congestion at the metal center influence the product selectivity. To recognize the

effect of ligand rigidity, herein we report a Rh(I) complex containing an annulated π-
conjugated chelate ligand bearing bulky diisopropylphenyl (Dipp) substituent on the
imidazole ring for the selective addition of silanes to the terminal alkynes (Scheme 3d).
Jimenez and Oro,
2008
Peris, 2005
Salazar and Suarez,
2017
This work
Scheme 3. Rh(I) complexes containing N-donor functionalized ligand scaffolds
employed in hydrosilylation of alkynes.
2. Results and Discussion
2.1 Synthesis and characterization of [Rh(L)(COD)]PF6 (1)
The NHC ligand precursor 1,8-naphthyrido[1,2-a]-(2’,6’-diisopropylphenyl)imidazolium
hexaflurophosphate [LH]PF6 and its silver complex [AgL2]PF6 were synthesized using
literature procedures.[34] Transmetalation reaction of [AgL2]PF6 with [Rh(μ-Cl)(COD)]2
followed by the addition of AgPF6 in dichloromethane (DCM) at room temperature
resulted in the complex [Rh(COD)(L)]PF6 (1) in high yield (85%). The molecular
structure of 1 reveals the chelate binding of the ligand to the metal through the carbene
carbon (C2) and the naphthyridine nitrogen (N3) (Figure 1). The Rh1−C2 and Rh1−N3
bond distances are 2.032(4) and 2.137(3) Å respectively. The ligand bite angle (C2–
Rh1–N3 = 79.6°) is similar to the analogous bis-carbene (C–Rh–C = 80.7°)[8d] and
1,10-phenanthroline (N–Rh–N 79.8°) complexes,[35] but slightly shorter than that

observed in Suarez’s complexes (84.0°, 84.6° for R=H and 2,6-(OMe)2-C6H3,
respectively).[22] One COD molecule completes the distorted square planer geometry
around the metal. The Rh-C (COD) distances (Rh1–C26/C27 = 2.186(4)/2.218(4) Å)
trans to the carbene carbon are slightly longer than the corresponding distances (Rh1–
C23/C30 = 2.148(4)/2.131(4) Å) that are opposed to the pyridyl nitrogen, reflecting the
1
trans influence of the carbene carbon. The H NMR spectrum of 1 in chloroform-d
shows sets of multiplets for the COD protons (δ 1.96−5.24 ppm). A singlet resonance at
δ 8.06 ppm is observed for the imidazole proton. The naphthyridine protons appear as
four different signals in the range δ 7.45−8.42 ppm. Two doublets are observed at δ
1.31 ppm and 1.08 ppm for the chemically non-equivalent isopropyl methyl protons –
CHMe2. In addition, multiplets at δ 3.40–3.47ppm are assigned to two non-equivalent
isopropyl-CH protons. The carbene carbon appears at δ 162.3 ppm as doublet (JC-Rh
=
52 Hz) in the 13C NMR spectrum (Figure S1). ESI-MS shows a signal at m/z = 540.1867
(z = 1) which is assigned for [1 – PF6]+ (Figure S2).
Figure 1. Synthesis of 1 (left) and the molecular structure of the cationic unit
in 1 with selected atoms labeled (right). All hydrogen atoms are omitted for
the sake of clarity. Thermal ellipsoids are drawn at the 40% probability level.

2.2 Catalytic studies
The catalytic utility of 1 was evaluated for the hydrosilylation of terminal alkynes. Initial
reaction of phenylacetylene (1 mmol), Et3SiH (1.2 mmol) and 2 mol% catalyst 1 in DCM
at 40 °C afforded 97% conversion of the alkyne to the corresponding vinylsilane (85:15,
E:α) after 6 h (Table 1). Possible side products arising out of the dehydrogenative
silylation and the alkyne polymerization were not observed. Performing the same
reaction at room temperature gave only 70% of the desired product (entry 1, Table S2).
A range of solvents including CHCl3, THF, CH3CN, 1,4-dioxane, toluene and 1,1,2,2-
tetrachloroethane were screened and the best conversion (95%) could be achieved in
1,4-dioxane with good selectivity (80:20, E:α) at 80 °C. The optimization studies
revealed that the DCM was the best choice among the solvents studied. Increasing the
catalyst loading from 2 to 3 mol% did not affect the yield and the selectivity. Further,
reducing the catalyst loading from 2 to 1 mol% diminished the yield of the reaction.
Hydrosilylation of phenylacetylene was performed with other silanes, PhSiH3, Me2PhSiH
and (EtO)3SiH (Table 1). The primary silane PhSiH3 gave a mixture of β-(E) and α-
vinylsilane along with divinylsilane ((PhCH=CH)2SiPhH) in 54:40:6 ratio (entry 2).
Dimethylphenylsilane gave excellent yield with moderate E-selectivity (78:22, E:α) (entry
3). Addition of (EtO)3SiH to phenylacetylene afforded the corresponding
vinylalkoxysilane with higher E-selectivity (86:14, E:α) (entry 4).

Table 1: Complex 1 catalyzed hydrosilylation of
phenylacetylene.a
Entry
Silane
Yield %b
Ratio E/αb
1.
2.
3.
4.
Et3SiH
97
93
98
98
85/15
54/40/6c
78/22
PhSiH3
(CH3)2PhSiH
(EtO)3SiH
86/14
aReaction conditions: alkyne:silane:1 = 1:1.2:0.02 mmol, in
b
CH2Cl2, 40 °C, 6 h. Yields and E/α ratio are determined by
GC-MS using mesitylene as an internal standard.
cdivinylsilane.
With an optimized catalytic condition in hand, substrate scope of 1 for alkyne
hydrosilylation was explored with two different silanes, Et3SiH and (EtO)3SiH (Table 2).
Electron-rich alkynes 4-methylphenylacetylene and 4-methoxyphenylacetylene on
reactions with Et3SiH afforded high yields (>95%) to the corresponding vinylsilanes with
good E-selectivity (86:14, E:α; entries 2,3). Electron-deficient alkynes (4-fluoro and 4-
bromophenylacetylene) afforded relatively lower yield (89% and 87% respectively, with
80:20, E:α selectivity; entries 4, 5). An excellent result was obtained for the bulky alkyne
(2-ethynyl-6-methoxynaphthalene) which showed 99% E-selectivity (entry 6). Aliphatic
alkynes 4-phenyl-1-butyne and 1-hexyne gave 86:14 and 83:17 ratio of E- and α-
vinylsilane with 85% and 89% conversions respectively (entries 7, 8). Isomerization of
vinyl silanes to allyl silanes are reported for the silylation of the aliphatic alkynes,[26,27]
but no such isomerized products are observed for catalyst 1. Hydrosilylation of
trimethylsilylacetylene gave excellent yield 97% (94:6, E:α) of the vinyldisilane (entry 9),
a useful substrate for chemoselective derivatization reactions in organic synthesis.[36]

Further, the scope of 1 was extended to (EtO)3SiH with a wide variety of aryl and alkyl
alkynes (entries 10 - 21). A list of vinylalkoxysilanes were obtained which are important
substrates in organic synthesis.[13] Electron-rich aromatic alkynes offered excellent
yields (99 and 96%) of the corresponding vinylsilanes (83:17, E:α) (entries 11, 12). 4-
fluorophenylacetylene gave 92% yield with 81:19 ratio of E:α isomers (entry 13). Similar
to Et3SiH, the (EtO)3SiH also resulted in high selectivity (>94%) and excellent yields
(>96%) with bulky substrates (entries 14 - 16). Aliphatic alkynes ethynylcyclopropane, 4-
phenyl-1-butyne and 1-heptyne gave more than 87% yields with good E-selectivity (82 -
87%) (entries 17 - 19). In addition, diynes 1,3-diethynylbenzene and 1,7-dioctyne were
also employed with 2 equivalents of (EtO)3SiH which showed 80% (80:20, E:α) and
78% (82:18, E:α) yields of the corresponding bis-vinylsilanes, respectively (entries 20,
21).
Table 2: Scope of complex 1 catalyzed hydrosilylation of terminal alkynes.a
Entry
1
Substrate
Silane
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Yield (%)b
E/α ratiob
85/15
97
95
96
89
86/14
2
3
4
5
H3CO
F
86/14
80/20
Br
Et3SiH
Et3SiH
Et3SiH
Et3SiH
87
97
85
89
80/20
99/1
H3CO
6
7
8
86/14
83/17
3

9
Si
Et3SiH
97
98
99
96
92
94/6
86/14
83/17
83/17
81/19
10
11
12
13
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
H3CO
F
14d
(EtO)3SiH
97
94/6
H3CO
15
16
17
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
98
96
92
97/3
95/5
87/13
18
19
20
21
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
87
89
80
78
82/18
83/17
80/20
82/18
4
aReaction conditions: alkyne:silane:1 = 1:1.2:0.02 mmol, in CH2Cl2, 40 °C, 6 h. bYields
and E/α ratio are determined by GC–MS using mesitylene as an internal standard.
cReaction time 10 h. d2 mmol silane.
To evaluate the chemoselectivity of the catalyst, a competitive experiment was
performed with equimolar mixture of phenylacetylene and p-methoxystyrene under the
optimized reaction conditions (Scheme 4). The silane addition product
styryltriethylsilane was obtained as expected, but the styrene remains unreacted. This
experiment confirms that the hydrosilylation of alkynes is chemoselectively achieved in
the presence of alkenes.

Scheme 4. Chemoselectivity experiment.
2.3 Proposed Mechanism
The progress of the reaction involving phenylacetylene and Et3SiH was monitored by
GC-MS (Figure 2). The substrate is gradually consumed with the appearance of the E-
vinylsilane. A small amount of α-vinylsilane was also observed but the Z-isomer was not
identified in the reaction. Therefore, the possibility of a Z to E isomerization could be
ruled out.
Figure 2. Time-conversion profile of hydrosilylation of
phenylacetylene.
To garner further insight into the reaction mechanism, a 4:1 mixture of
phenylacetylene and 1 in CDCl3 was heated inside a screw-cap NMR tube for 3 h. The
1H NMR signals corresponding to 1 and phenylacetylene remained unchanged,

indicating a Chalk-Harrod mechanism that is initiated by silane reaction with the
catalyst.[28,37] Next, a CDCl3 solution of 1 and HSi(OEt)3 (1:4 molar ratio) was treated at
1
40 °C for 3 h and the reaction was followed by H NMR spectroscopy (Scheme 5). At
room temperature, a broad signal for Rh-H appeared at δ –17.71 ppm. However, a clear
doublet was observed with J = 30.9 Hz at lower temperature (–30 °C) (Figure S4). This
supports the plausible intermediacy of a mononuclear Rh-H species A in the catalytic
1
cycle (Scheme 6).[22,38] The H NMR spectrum further reveals one set of protons for
COD and NHC ligand in 1:1 ratio (Figure S4). Six signals corresponding to nine
aromatic ligand protons appear in the range of 7.25 – 8.42 ppm. Two doublets are
observed at δ 1.33 ppm (J = 6.9 Hz) and 1.10 ppm (J = 6.9 Hz) for the chemically non-
equivalent isopropyl methyl protons (–CHMe2) of the Dipp unit. In addition, multiplets in
the range δ 2.41–2.49 ppm are assigned for two non-equivalent isopropyl -CH protons.
The methylene and methyl protons of the metal-coordinated silyl group appear at δ 3.70
ppm (J = 6.9 Hz) and δ 1.22 ppm (J = 6.9 Hz) respectively. Eight COD methylene
protons appear as four multiplets in the range 1.93 – 2.37 ppm whereas vinylic protons
resonate at δ 4.27 and 5.24 ppm. ESI-MS analysis of the same mixture in methanol
showed a signal at m/z = 628.2420 (z = 1) which is assigned for
[Rh(L)H(Si(OEt)3)(CH3OH)]+ (Figure S5).
Scheme 5. Oxidative addition of HSi(OEt)3 to 1.

On close examination, the intensity of the Rh-H signal was found to be low relative to
the ligand protons in both CDCl3 and CD2Cl2. Literature reports reveal that the hydrido-
rhodium(III) complexes exhibit low stability in the chlorinated solvents and the hydride is
exchanged by the chloride.[39] We attribute the loss of Rh-H signal intensity to the
hydride-chloride exchange in chlorinated solvents. To circumvent this problem, we
performed the same reaction in non-chlorinated solvent CD3CN,[40] however, no Rh-H
signal was observed. Different silanes were used and the reaction conditions were
varied, but catalyst 1 failed to produce the Rh-H intermediate in CD3CN.
A GC-MS analysis of the reaction mixture of catalyst 1 and silane did not reveal free
1
COD or its hydrogenated product. H NMR spectrum of the silyl-rhodium(III)-hydride
intermediate indicated COD coordination to the metal. The weak coordination of COD to
the high-valent Rh(III) is evident from the ESI-MS which shows loss of COD in
methanol. It is conceivable that COD would be readily replaced by coordinating solvents
such as acetonitriles.22 In the absence of a coordinating solvent, the COD molecule
1
remains bound to the metal in A. An immediate product formation was detected by H
NMR on addition of phenylacetylene to A, suggesting the viability of the catalytic cycle
shown in Scheme 6.
The first step is the oxidative addition of the silane to the Rh center to generate the
hydrido-silyl-Rh species A. The COD is then extruded from the metal centre to pave the
way for alkyne coordination. The resulting intermediate B follows two pathways of
alkyne migratory insertion into the Rh-H bond. An 1,2-insertion followed by reductive
elimination directs E-vinylsilanes (path a), while in path b, α-isomer is formed through
reductive elimination following a 2,1-insertion of alkyne into the Rh-H bond. In the case

of bulky substrates, the formation of α-isomer is decreased due to steric hindrance
between the substituents on the alkyne and the silyl group.
Scheme 6. Proposed pathways for Chalk-Harrod mechanism
of
1-catalyzed
alkyne
hydrosilylation.
[Rh’]
=
[Rh(L)(COD)]PF6, [Rh] = [Rh(L)]PF6.
An alternate alkyne insertion into the Rh-Si bond[29,30] could give rise to β-(Z) along
with β-(E) and α-isomers (Figure S6). However, β-(Z) isomer was not observed in the
reaction. Furthermore, the bulky alkynes afforded E-isomers as the major products with
very little to no α-isomers. These are inconsistent with a pathway that involves alkyne
insertion into the Rh-Si bond and thus discarded.
3. Concluding Remarks
This work examines the effect of ligand rigidity for the Rh(I) catalyzed hydrosilylation of
terminal alkynes. A Rh(I) complex bearing an annulated π-conjugated chelate ligand
affords vinylsilane derivatives for a variety of terminal alkynes with good to excellent
yields and high β-(E) selectivity. Although the α-isomers are invariably obtained up to

20% of the overall products, polymerization and dehydrosilylation reactions were
avoided. The β-(E) selectivity is excellent for bulky alkynes. Further, chemoselective
hydrosilylation of alkyne was achieved in the presence of alkene. The ligand rigidity
appears to play a crucial role for the β-(E) selectivity.
4. Experimental
4.1 Instrumentation
All reactions with the metal complex were carried out under an atmosphere of purified
nitrogen using standard Schlenk–vessel and vacuum line techniques. Glasswares were
1
flame–dried under vacuum prior to use. H and 13C NMR spectra were obtained on
1
JEOL JNM–LA 400 MHz and JEOL JNM–LA 500 MHz spectrometers. H NMR
chemical shifts were referenced to the residual hydrogen signal of the deuterated
solvents. The chemical shift is given as dimensionless δ value and frequency
referenced relative to TMS for 1H and 13C NMR spectroscopy. Elemental analyses were
performed on a Thermoquest EA1110 CHNS/O analyzer. The crystallized compound
was powdered, washed several times with dry petroleum ether, and dried in vacuum for
at least 48 h prior to elemental analyses. ESI–MS were recorded on a Waters
Micromass Quattro Micro triple–quadrupole mass spectrometer. The GC-MS
experiments were performed by using an Agilent 7890A GC and 5975C MS system.
4.2 Materials
All solvents were purified and dried by conventional methods prior to use. RhCl3.xH2O
was purchased from Arora Matthey, India. All other chemicals were purchased from
[34]
[41]
Sigma–Aldrich. The ligand precursor [LH]PF6,[34] [AgL2]PF6
and [Rh(COD)(µ-Cl)]2
were prepared according to the literature procedures.

4.3 Synthesis of 1
In a flame dried schlenk flask, [Rh(COD)(µ-Cl)]2 (0.032 g, 0.06 mmol) was added to a
DCM solution of [AgL2]PF6 (0.060 g, 0.06 mmol). An immediate color change from
yellow to red was observed. AgPF6 (0.016 g, 0.06 mmol) was added to the reaction
mixture and stirred for 2 h under the exclusion of light in N2 atmosphere. The resulting
suspension was filtered through a small pad of celite to remove the precipitated silver
halide. The filtrate was concentrated under reduced pressure followed by the addition of
15 mL of petroleum ether with stirring to induce precipitation. Repeated washing
followed by prolonged drying in vacuum provided 1 as an orange solid. Crystals suitable
for X-ray diffraction were grown by layering diethyl ether over a concentrated DCM
solution of the compound inside an 8 mm o.d. vacuum sealed glass tube. Yield: 85%;
ESI–MS, m/z=540.1867 (z = 1) for [1 - PF6]+; 1H-NMR (500 MHz, CDCl3): δ 8.42 (d, J =
7.1 Hz, 1H, NP), 8.06 (s, 1H, Im), 7.77 (dd, J = 7.9 Hz, 5.4 Hz, 1H, NP), 7.56 - 7.52 (m,
2H, NP), 7.45 (d, J = 9.6 Hz, 1H, NP), 7.32 - 7.29 (m, 3H, Ar), 5.24 (br, 2H, CH═ COD),
4.01 (br, 2H, CH═ COD), 2.48-2.40 (m, 2H, CHisopropyl), 2.37-2.30 (m, 2H, CHH COD),
2.25-2.18 (m, 2H, COD), 2.07 - 2.02 (m, 2H, COD), 1.99 - 1.93 (m, 2H, COD), 1.31 (d, J
= 6.8 Hz, 6H, Meisopropyl), 1.08 (d, J = 6.8 Hz, 6H, Meisopropyl); 13C NMR (125 MHz,
CDCl3): 162.3 (d, C-2, NHC), 147.7 (NCN, NP), 144.9 (CH, NP), 140.7 (NCAr), 133.4
(CH, NP), 131.6 (CH, NP), 128.6 (C, Im), 124.8 (CH, NP), 124.5 (CH, Ar), 124.2 (CH,
Ar), 120.6 (CH, Ar), 118.4 (CH, Im), 118.1(C,NP), 95.8 (d, CH═ COD), 77.8 (d, CH═
COD), 32.1 (CH2, COD), 28.4 (CHisopropyl), 22.9 (CH2, COD), 22.5 (Me); Anal. Calcd for
RhC30H35N3PF6: C, 66.64; H, 6.52; N, 7.77. Found: C, 66.43; H, 6.28; N, 7.45.
4.4 Procedure for the generation of metal hydride intermediate

A screw-cap NMR tube was charged with a solution of complex 1 (0.020 g, 0.03 mmol)
and (EtO)3SiH (21 µL, 0.12 mmol) in CDCl3 (0.5 mL). The reaction mixture was heated
1
to 40 °C for 3 h. A plausible metal-hydride intermediate was identified by H-NMR
spectroscopy.
4.5 General procedure for catalytic alkyne hydrosilylation
An oven-dried reaction vessel was charged with 1 (0.02 mmol), alkyne (1 mmol), silane
(1.2 mmol) and mesitylene (1 mmol) in 2 mL of DCM and heated at 40 °C for 6 h. The
reaction mixture was cooled, diluted with ethyl acetate and passed through a short
column of silica for GC–MS analysis to determine the yield and the E/α ratio. For NMR
characterization of the product, the solvent was evaporated under vacuum and the
residue was purified by column chromatography on silica gel (by petroleum ether/ethyl
acetate).
Acknowledgment
We thank the anonymous reviewers for valuable insights. Financial supports from the
Department of Atomic Energy and Indo-French Centre for the Promotion of Advanced
Research are gratefully appreciated. J.K.B. thanks the SERB, DST for the J C Bose
fellowship. A.T. and S.Y. thank the UGC India for fellowship. C.R. thanks IITK for a
post-doctoral fellowship.
References
[1] For Selected reviews on N-heterocyclic carbenes, see: (a) M. N. Hopkinson, C. Richter, M. Schedler,
F. Glorius, Nature, 510 (2014) 485-496; (b) S. P. Nolan, N-Heterocyclic Carbenes in Synthesis, Wiley-
VCH, Weimheim, Germany, 2006; (c) F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed., 47 (2008) 3122-
3172; (d) R. H. Crabtree, J. Organomet. Chem., 690 (2005) 5451-5471; (e) H. Jacobsen, A. Correa, A.
Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev., 253 (2009) 687-703; (f) D. Bourissou, O. Guerret, F.
P. Gabbai, G. Bertrand, Chem. Rev., 100 (2000) 39-92; (g) K. Y. Wan, F. Roelfes, A. J. Lough, F. E.

Hahn, R. H. Morris, Organometallics, 37 (2018) 491-504; (h) M. C. Jahnke, F. E. Hahn, Coord. Chem.
Rev., 293 (2015) 95-115; (i) H. V. Huynh, Chem. Rev., 118 (2018) 9457-9492.
[2] For selected reviews on metal-NHC catalyzed organic transformations, see: (a) S. D. González, N.
Marion, S. P. Nolan, Chem. Rev., 109 (2009) 3612-3676; (b) E. Peris, Chem. Rev., 118 (2018) 9988-
10031; (c) F. Glorius, N-Heterocyclic Carbenes in Transition Metal Catalysis, in: Topics in Organometallic
Chemistry, Ed. Springer, Verlag Berlin, 2007; (d) V. Dragutan, I. Dragutan, L. Delaude, A. Demonceau,
Coord. Chem. Rev., 251 (2007) 765-794; (e) S. Gaillard, C. S. J. Cazin, S. P. Nolan, Acc. Chem. Res., 45
(2012) 778-787; (f) Á. Vivancos, C. Segarra, M. Albrecht, Chem. Rev., 118 (2018) 9493-9586; (g) L.
Mercs, M. Albrecht, Chem. Soc. Rev., 39 (2010) 1903-1912; (h) H. V. Huynh, The Organometallic
Chemistry of N‐heterocyclic Carbenes. John Willey & Sons Ltd., The Atrium, Southern Gate, Chichester,
West Sussex, PO19 8SQ, United Kingdom, 2017; (i) W. A. Herrmann, Angew. Chem. Int. Ed., 41 (2002)
1290-1309.
[3] For metal-NHC catalyzed oxidation reactions, see: (a) M. M. Konnick, I. A. Guzei, S. S. Stahl, J. Am.
Chem. Soc., 126 (2004) 10212-10213; (b) M. J. Schultz, S. S. Hamilton, D. R. Jensen, M. S. Sigman, J.
Org. Chem., 70, (2005) 3343-3352; (c) M. Poyatos, J. A. Mata, E. Falomir, R. H. Crabtree, E. Peris,
Organometallics, 22 (2003) 1110-1114; (d) P. Daw, R. Petakamsetty, A. Sarbajna, S. Laha, R.
Ramapanicker, J. K. Bera, J. Am. Chem. Soc., 136 (2014) 13987-13990.
[4] (a) W. S. Knowles, Angew. Chem. Int. Ed., 41 (2002) 1998-2007; (b) R. Noyori, Angew. Chem. Int.
Ed., 41 (2002) 2008-2022; (c) M. T. Powell, D. R. Hou, M. C. Perry, X. Cui, K. Burgess, J. Am. Chem.
Soc., 123 (2001) 8878-8879; (d) J. W. Sprenger, J. Wassenaaar, N. D. Clement, K. J. Cavell, C. J.
Elsevier, Angew. Chem. Int. Ed., 44 (2005) 2026-2029; (e) D. Baskakov, W. A. Herrmann, E. Herdtweck,
S. D. Hoffmann, Organometallics, 26 (2007) 626-632; (f) D. Chen, V. Banphavichit, J. Reibenspies, K.
Burgess, Organometallics, 26 (2007) 855-859; (g) K. D. Camm, N. M. Castro, Y. Liu, P. Czechura, J. L.
Snelgrove, D. E. Fogg, J. Am. Chem. Soc., 129 (2007) 4168-4169.
[5] For chelated metal-NHC complexes, see: (a) J. A. Mata, M. Poyatos, E. Peris, Coord. Chem. Rev.,
251 (2007) 841-859; (b) M. Bierenstiel, E. D. Cross, Coord. Chem. Rev., 255 (2011) 574-590; (c) A. R.
Naziruddin, C.-S. Zhuang, W.-J. Lin, W.-S. Hwang, Dalton Trans., 43 (2014) 5335-5342; (d) Z. Xi, X.
Zhang, W. Chen, S. Fu, D. Wang, Organometallics, 26 (2007) 6636-6642; (e) V. Diachenko, M. J. Page,
M. R. D. Gatus, M. Bhadbhade, B. Messerle, Organometallics, 34 (2015) 4543-4552; (f) A. A.
Danopoulos, N. Tsoureas, S. A. Macgregor, C. Smith, Organometallics 26 (2007) 253-263; (g) E. Mas-
Marzá, M. Sanaú, E. Peris, J. Organomet. Chem., 690 (2005) 5576-5580; (h) S. Winston, N. Stylianides,
A. A. D. Tulloch, J. A. Wright, A. A. Danopoulos,. Polyhedron, 23 (2004) 2813-2820; (i) M. C. Cassani, M.
A. Brucka, C. Femoni, M. Mancinelli, A. Mazzanti, R. Mazzoni, G. Solinas, New J. Chem., 38 (2014)
1768-1779; (j) G. Mancano, M. J. Page, M. Bhadbhade, B. A. Messerle, Inorg. Chem., 53 (2014) 10159-
10170.

[6] (a) A. Sinha, S. M. W. Rahaman, M. Sarkar, B. Saha, P. Daw, J. K. Bera, Inorg. Chem., 48 (2009)
11114-11122; (b) P. Daw, A. Sinha, S. W. M. Rahaman, S. Dinda, J. K. Bera, Organometallics, 31 (2012)
3790-3797.
[7] E. Mas-Marzá, M. Sanaú, E. Peris, Inorg. Chem., 44 (2005) 9961-9967.
[8] Selected examples: (a) D. J. Nelson, S. P. Nolan, Chem. Soc. Rev., 42 (2013) 6723-6753; (b) M.
Alcarazo, S. J. Roseblade, A. R. Cowley, R. Fernandez, J. M. Brown, J. M. Lassaletta, J. Am. Chem.
Soc., 127 (2005) 3290-3291; (c) G. Song, Y. Zhang, X. Li, Organometallics, 27 (2008) 1936-1943; (d) V.
Gierz, C. M. Mössmer, D. Künz, Organometallics, 31 (2012) 739-747; (e) A. K. Phukan, A. K. Guha, S.
Sarmah, R. D. Dewhurst, J. Org. Chem.,78 (2013) 11032-11039; (f) M. D. Sanderson, J. W. Kamplain, C.
W. Bielawski, J. Am. Chem. Soc., 128 (2006) 16514-16515; (g) C. H. Suresh, M. J. Ajitha, J. Org. Chem.,
78 (2013) 3918-3924; (h) S. R. Yetra, T. Kaicharla, S. S. Kunte, R. G. Gonnade, A. T. Biju, Org. Lett., 15
(2014) 5202-5205; (i) A. J. Arduengo, L. I. Iconaru, Dalton Trans., (2009), 6903-6914.
[9] (a) O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev., 109 (2009) 3445-3478; (b) P.
L. Arnold, S. Pearson, Coord. Chem. Rev., 251 (2007) 596–609; (c) R. H. Crabtree, Coord. Chem. Rev.,
257 (2013) 755–766.
[10] (a) K. Tamao, N. Ishida, T. Tanaka, M. Kumada, Organometallics, 2 (1983) 1694-1696; (b) I.
Fleming, R. Henning and H. Plaut, J. Chem. Soc., Chem. Commun., (1984) 29-31; (c) S. E. Denmark, C.
S. Regens, Acc. Chem. Res., 41 (2008) 1486-1499; (d) A. K. Roy, A Review of Recent Progress in
Catalyzed Homogeneous Hydrosilation (Hydrosilylation), in: Adv. Organomet. Chem.; Robert West, A. F.
H., Mark, J. F., Eds.; Academic Press: New York, 2007, 55, 1-59; (e) C. Yann, W. Christophe, B. K. Lydia,
D. J. Pierre, A. N. Francine, M. Christophe, Journal of Molecular Catalysis A: Chemical, 423, (2016) 256-
263.
[11] a) P. Somfai, B seashore-ludlow, Organosilicon reagent: Vinyl-, Alkynyl-, and arylsilanes, in:
Comprehensive Organic Synthesis Ed. Springer, Amsterdam, Netherlands, 2014; (b) B. Marciniec, M.
Majchrzak, W. Prukała, M. Kubicki, D. Chadyniak, J. org. Chem., 70, (2005) 8550-8555; (c) T.
Bunlaksananusorn, A. L. Rodriguez, P. Knochel, Chem. Commun. (2001), 745-746.
[12] For selected examples of cross coupling reactions, see: (a) T. Hiyama, in: F. Diederich, P. J. Stang
(Eds.), Metal-Catalyzed Crosscoupling Reaction, Wiley-VCH, New York, 1998, pp. 421-453; (b) C. J.
Handy, A. S. Manoso, W. T. McElroy, W. M. Seganish, P. DeShong, Tetrahedron, 61 (2005) 12201-
12225; (c) T. Hiyama, E. Shirakawa, Top. Curr. Chem., 219 (2002) 61-85; (d) T. Komiyama, Y. Minami, T.
Hiyama, ACS. Catal., 7 (2017) 631-651.
[13] (a) S. E. Denmark, M. H. Ober, Aldrichimica Acta, 36 (2003) 75-85; (b) S. E. Denmark, R. F. Sweis,
Acc. Chem. Res. 35 (2002) 835-846.
[14] E. Pouget, J. Tonnar, P. Lucas, P. L. Desmazes, F. Ganachaud, B. Boutevin, Chem. Rev., 110
(2010) 1233-1277.
[15] For selected examples, see: (a) L. N. Lewis, K. G. Sy, G. L. Bryant, P. E. Donahue, Organometallics
10 (1991) 3750-3759; (b) J. P. Murphy, J. L. Spencer, G. Procter, Tetrahedron Lett., 31 (1990) 1051-

1054; (c) T. Kyoko, Tetrahedron Lett., 34 (1993) 8263-8266; (d) S. E. Denmark, Z. Wang, Org. Lett., 3
(2001) 1073-1076; (e) K. Itami, K. Mitsudo, A. Nishino, J. Yoshida, J. Org. Chem., 67 (2002) 2645-2652;
(f) M. Blug, X. -F.Le Goff, N. Mezailles, Ṕ. Le Floch, Organometallics, 28 (2009) 2360-2362; (g) W. Wu, C.
-J. Li, Chem. Commun., (2003) 1668-1669; (h) H. Aneetha, W. Wu, J. G. Verkade, Organometallics, 24
(2005) 2590-2596; (i) G. De Bo, G. Berthon-Gelloz, B. Tinant, I. É. Markó, Organometallics, 25 (2006)
1881-1890; (j) G. Berthon-Gelloz, J. -M. Schumers, G. De Bo, I. É. Markó, J. Org. Chem., 73 (2008)
4190-4197; (j) G. F. Silbestri, J. C. Flores, E. de Jesús, Organometallics, 31 (2012) 3355-3360; (k) L.
Ortega-Moreno, R. Peloso, C. Maya, A. Suarez, É. Carmona, Chem. Commun., 51 (2015) 17008-17011;
(l) S. Dierick, E. Vercruysse, G. BerthonGelloz, I. É. Markó, Chem. Eur. J., 21 (2015) 17073-17078.
[16] M. P. Doyle, K. G. High, C. L. Nesloney, T. W. Clayton, J. Lin, Organometallics, 10 (1991)
1225−1226.
[17] For selective synthesis of β-(Z)-vinylsilane with Ru catalysts, see; (a) M. A. Esteruelas, J. Herrero, L.
A. Oro, Organometallics, 12 (1993) 2377-2379; (b) Y. Na, S. Chang, Org. Lett., 2 (2000) 1887-1889; (c)
M. Nagao, K. Asano, K. Umeda, H. Katayama, F. Ozawa, J. Org. Chem. 70 (2005) 10511-10514; (d) S.
V. Maifeld, M. N. Tran, D. Lee, Tetrahedron Lett., 46 (2005) 105-108; (e) R. Gao, D. R. Pahls, T. R.
Cundari, C. S. Yi, Organometallics, 33 (2014) 6937-6944; (f) C. Conifer, C. Gunanathan, T. Rinesch, M.
Hölscher, W. Leitner, Eur. J. Inorg. Chem., 2015, (2015) 333-339.
[18] For selective synthesis of α-vinylsilane with Ru catalysts, see: (a) B. M. Trost, Z. T. Ball, J. Am.
Chem. Soc., 123 (2001) 12726-12727; (b) Y. Kawanami, Y. Sonoda, T. Mori, K. Yamamoto, Org. Lett., 4
(2002) 2825-2827; (c) B. M. Trost, Z. T. Ball, J. Am. Chem. Soc., 127 (2005) 17644-17655.
[19] H. Katayama, K. Taniguchi, M. Kobayashi, T. Sagawa, T. Minami, F. Ozawa, J. Organomet. Chem.,
645 (2002) 192-200.
[20] I. Ojima, N. Clos, R. J. Donovan, P. Ingallina, Organometallics, 9 (1990) 3127-3133.
[21] (a) G. T. S. Andavan, E. B. Bauer, C. S. Letko, T. K. Hollis, F. S. Tham, J. Organomet. Chem., 690
(2005) 5938-5947; (b) G. Rivera, O. Elizalde, G. Roa, I. Montiel, S. Berne’s, J. Organomet. Chem., 699
(2012) 82-86; (c) A. J. Huckaba, T. K. Hollis, T. O. Howell, H. U. Valle, Y. Wu, Organometallics, 32 (2013)
63-69; (d) M. F. Lappert, R. K. Maskell, J. Organomet. Chem., 264 (1984) 217-228; (e) J. Y. Zeng, M. –H.
Hsieh, H. M. Lee, J. Organomet. Chem., 690 (2005) 5662-5671; (f) M. Poyatos, E. Mas-Marzá, J. A.
Mata, M. Sanaú, E. Peris, Eur. J. Inorg. Chem., 2003 (2003) 1215-1221; (g) J. E. Hill, T. A. Nile, J.
Organomet. Chem., 137 (1977) 293-300; (h) E. Mas-Marzá, M. Poyatos, M. Sanaú, E. Peris, Inorg.
Chem., 43 (2004) 2213-2219; (i) M. Poyatos, A. Maisse-Francois, S. B. Laponnaz, L. H. Gade,
Organometallics, 25 (2006) 2634-2641; (j) M. Iglesias, M. A. Lavrijsen, P. J. S. Miguel, F. J. Fernandez-
Álvarez, J. J. P. Torrente, L. A. Oro, Adv. Synth. Catal., 357 (2015) 350-354.
[22] J. P. Morales-Ceron, P. Lara, J. Lopez-Serrano, L. L. Santos, V. Salazaŕ, E. Álvarez, A. Suárez,
Organometallics, 36 (2017) 2460-2469.
[23] D. Field, A. J. Ward, J. Organomet. Chem., 681 (2003) 91-97.
[24] R. Takeuchi, S. Nitta, D. Watanabe, J. Org. Chem., 60 (1995) 3045-3051.

[25] (a) M. A. Esteruelas, M. Olivan, L. A. Oro, Organometallics, 15 (1996) 814-822; (b) C. Vicent, M.
Viciano, E. Mas-Marza, M. Sanau, E. Peris, Organometallics, 25 (2006) 3713-3720.
[26] M. V. Jiménez, J. J. P. Torrente, M. I. Bartolomé, V. Gierz, F. J. Lahoz, L. A. Oro, Organometallics,
27 (2008), 224-234.
[27] L. Busetto, M. C. Cassani, C. Femoni, M. Mancinelli, A. Mazzanti, R. Mazzoni, G. Solinas,
Organometallics, 30 (2011) 5258-5272.
[28] B. M. Trost, Z. T. Ball, Synthesis, 2005 (2005) 853-887.
[29] R. S. Tanke, R. H. Crabtree, J. Am. Chem. Soc., 112 (1990) 7984-7989.
[30] C. H. Jun, R. H. Crabtree, J. Organomet. Chem., 447 (1993) 177-187.
[31] M. Viciano, E. Mas-Marzá, M. Sanaú, E. Peris, Organometallics, 25 (2006) 3063−3069.
[32] (a) A. Sato, H. Kinoshita, H. Shinokubo, K. Oshima, Org. Lett., 6 (2004) 2217-2220; (b) A. Mori, E.
Takahisa, Y. Yamamura, T. Kato, A. P. Mudalige, H. Kajiro, K. Hirabayashi, Y. Nishihara, T. Hiyama,
Organometallics, 23 (2004) 1755-1765.
[33] J. W. Faller, D. G. Alliesi, Organometallics, 21 (2002) 1743-1746.
[34] M. Kriechbaum, M. List, R. J. F. Berger, M. Patzschke, U. Monkowius, Chem. Eur. J., 18 (2012)
5506-5509.
[35] C. Pettinari, F. Marchetti, A. Cingolani, G. Bianchini, A. Drozdov, V. Vertlib, S. Troyanov, J.
Organomet. Chem., 651 (2002) 5-14.
[36] S. Ding, Li-J. Song, L. Wa Chung, X. Zhang, J. Sun, Y.-Dong Wu, J. Am. Chem. Soc., 135 (2013)
13835-13842.
[37] A. J. Chalk, J. F. Harrod, J. Am. Chem. Soc., 87 (1965) 16-21.
[38] A. Bakac, L. N. Thomas, Inorg. Chem., 35 (1996) 5880-5884.
[39] (a) S. Azpeitia, M. Barquín, C. Mendicute-Fierro, M. A. Huertos, A. Rodríguez-Diéguez, J. M. Seco,
E. S. Sebastian, L. Ibarluceaa, M. A. Garralda, Dalton trans., 48 (2019) 3300-3313; (b) V. S. Nacianceno,
S. Azpeitia, L. Ibarlucea, C. Mendicute-Fierro, A. Rodríguez-Diéguez, J. M. Seco, E. S. Sebastiana, M. A.
Garralda, Dalton trans., 44 (2015) 13141-13155; (c) M. A. Garralda, R. Hernández, L. Ibarlucea, E.
Pinilla, M. R. Torres, M. Zarandona, Organometallics, 26 (2007) 1031-1038; (d) M. J. Ingleson, S. K.
Brayshaw, M. F. Mahon, G. D. Ruggiero, A. S. Weller, Inorg. Chem., 44 (2005) 3162-3171; (e) C. Tejel,
M. A. Ciriano, M. Millaruelo, J. A. López, F. J. Lahoz, L. A. Oro, Inorg. Chem., 42 (2003) 4750-4758.
[40] Suarez’s catalyst showed full-intensity signal for the corresponding Rh-H complex in CD3CN. See,
reference 22.
[41] G. Giordano, R. H. Crabtree, Inorg. Synth., 28 (1990) 90.

A Rh(I) complex supported by an annulated π-conjugated imidazo-naphthyridine based N-
heterocyclic carbene ligand featuring a Dipp attachment is an excellent catalyst for E-selective
hydrosilylation of a range of terminal alkynes.