Synthesis of trans-mono(silyl)palladium(ii) bromide complexes

Article abstract of DOI:10.3390/molecules26092460

The stoichiometric reaction of cis-[Pd(ITMe)₂(SiR₃)₂], where (SiR₃ = SiMe₃ and SiMe₂Ph and ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) with allyl bromide affords the corresponding allylsilanes along with complexes of the type trans-[Pd(ITMe)₂(SiR₃)(Br)]. The structure of trans- [Pd(ITMe)₂(SiMe₂Ph)Br] 2b has been determined in the solid state and displays a slightly distorted square-planar geometry with the two N-heterocyclic carbene ligands in a trans-configuration.

Full text of DOI:10.3390/molecules26092460

molecules
Communication
Synthesis of trans-Mono(silyl)palladium(II)
Bromide Complexes
Melvyn B. Ansell , George E. Kostakis 1 , Oscar Navarro 1,2,* and John Spencer 1,*
1
1
Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, UK;
mbansell25@gmail.com (M.B.A.); G.Kostakis@sussex.ac.uk (G.E.K.)
AMPAC Fine Chemicals, Highway 50 and Hazel Avenue, Building 05-019,
Rancho Cordova, CA 95741-1718, USA
2
*
Correspondence: oscar.navarro@apfc.com (O.N.); j.spencer@sussex.ac.uk (J.S.); Tel.: +1-916-357-6928 (O.N.);
+44-1273-877-374 (J.S.)
Abstract: The stoichiometric reaction of cis-[Pd(ITMe) (SiR ) ], where (SiR = SiMe and SiMe Ph
2
3 2
3
3
2
and ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) with allyl bromide affords the corresponding
allylsilanes along with complexes of the type trans-[Pd(ITMe) (SiR )(Br)]. The structure of trans-
2
3
[
Pd(ITMe) (SiMe Ph)Br] 2b has been determined in the solid state and displays a slightly distorted
2 2
square-planar geometry with the two N-heterocyclic carbene ligands in a trans-configuration.
Keywords: palladium; silicon; N-heterocyclic carbene; allylsilane
1
. Introduction
Mono(silyl)palladium(II) halide species are purported intermediates in a number of
catalytic routes towards allylsilanes [ ]. Palladium pincer chemistry accounts of such
complexes are rather numerous, although examples of their isolation in this catalytic
cycle are rare [ ]. Trans-[PdCl(SiF Ph)(L) ] (L = PMe , PMe Ph or PMePh ) and allyl
bromide were shown to react to afford trans-[Pd(L) (SiF Ph)(Br)] and the corresponding
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1,2
Citation: Ansell, M.B.; Kostakis, G.E.;
Navarro, O.; Spencer, J. Synthesis of
trans-Mono(silyl)palladium(II)
3–
7
2
2
3
2
2
Bromide Complexes. Molecules 2021,
2
2
t
2
6, 2460. https://doi.org/10.3390/
allylsilane [
1], and [( BuPAr )Pd(SiMe )(I)] (Ar = 3,5-Me -4-OMe-C H ) was synthesized
2 3 2 6 2
t
molecules26092460
from stoichiometric quantities of [(cod)Pd(CH SiMe ) ] (cod = 1,5-cyclooctadiene), BuPAr
2
3 2
2
and Me SiI [
2
]. Analogues have been used in silyl-Negishi couplings [
8
]. We wish to report
3
Academic Editor: Vincent Ritleng
here our preliminary findings on the reaction of (ITMe) Pd(silyl) complexes with allyl
2
2
bromide (ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) [9–12].
Received: 13 April 2021
Accepted: 21 April 2021
Published: 23 April 2021
2. Results and Discussion
The bis(silyl)palladium complexes, cis-[Pd(ITMe) (SiR ) ] (1a: SiR = SiMe and 1b
:
2
3 2
3
3
SiMe Ph [13
,
14] were reacted with excess allylbromide at room temperature under an nitro-
2
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with regard to jurisdictional claims in
published maps and institutional affil-
iations.
gen atmosphere to yield trans-[Pd(ITMe) (SiMe )(Br)] 2a and trans-[Pd(ITMe) (SiMe Ph)Br]
2
3
2
2
2
b in 92 and 93% yield, respectively (Scheme 1). Reaction progress was monitored by
1
H NMR spectroscopy. Characteristic resonances corresponding to silanes 3a and 3b
were observed (in a 1:1 stoichiometry with 2a
/
2b, respectively, upon examination of the
crude mixtures).
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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N
N
Br
N
N
2-3 eq. allylbromide
or toluene, r.t., 1.5-2 h
N
N
N
SiR3
Pd SiR
3
Pd
SiR
+
6
C D
6
N
SiR
3
3
3
3
a : SiR3 = SiMe3
b : SiR3 = SiMe Ph
1
1
a; SiR3 = SiMe3
b; SiR3 = SiMe Ph
2a: SiR3 = SiMe3
2b : SiR3 = SiMe Ph
2
2
2
4
.0/).
Scheme 1. Stoichiometric synthesis of mono(silyl)palladium bromide complexes.
Molecules 2021, 26, 2460. https://doi.org/10.3390/molecules26092460
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2460
2 of 6
In order to further characterize the organometallic complexes, single crystals of 2b suit-
able for X-ray analysis were grown by slow evaporation of a saturated deuterated benzene
solution at room temperature. X-ray analysis revealed that 2b displays a marginally dis-
torted square-planar geometry with the two NHCs in a trans-configuration and orthogonal
to the Br-Pd-Si plane (Figure 1, Table 1).
Figure 1. Molecular structure of 2b. Hydrogen atoms are omitted for clarity. Selected bond lengths
o
[
Å] and angles [ ]: Pd1-Br1 2.6333(7), Pd-Si1 2.2948(18), Pd1-C1 2.028(5), Pd1-C8 2.025(5); C1-Pd1-Br1
9
4.97(16), C1-Pd1-Si1 89.15(17), C8-Pd1-Br1 87.62(16), C8-Pd1-Si1 88.60(17), C1-Pd1-C8 177.2(2).
Table 1. Crystal data and structure refinement for 2b.
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C H BrN PdSi
22 35 4
569.94
173
orthorhombic
P2 2 2
1
1 1
10.5467(4)
b/Å
14.3455(3)
c/Å
16.7301(4)
◦
α/
90
90
90
◦
β/
◦
γ/
3
Volume/Å
2531.23(13)
Z
g/cm
µ/mm
F(000)
Crystal size/mm
Radiation
4
3
ρ
1.496
2.374
calc
−
1
1160.0
0.22 × 0.2 × 0.15
3
MoKα (λ = 0.71073)
6.836 to 52.744
◦
2
θ range for data collection/
Index ranges
Reflections collected
Independent reflections
−13 ≤ h ≤ 8, −17 ≤ k ≤ 11, −14 ≤ l ≤ 20
7612
4799 [Rint = 0.0320, Rsigma = 0.0568]
4799/0/272
Data/restraints/parameters
2
Goodness-of-fit on F
1.018
Final R indexes [I >= 2σ (I)]
R = 0.0322, wR = 0.0614
1 2
Final R indexes [all data]
Largest diff. peak/hole/e Å
R = 0.0376, wR = 0.0640
1
2
−
3
0.51/−0.34
Flack parameter
CCDC deposition number
0.004(8)
2076437

Molecules 2021, 26, 2460
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The carbenic carbon-Pd bond lengths in 2b [2.028(5) and 2.025(5) Å] are significantly
shorter than in cis-[Pd(ITMe) (SiMe Ph) ] [2.105(3) and 2.123(3) Å], suggesting SiMe Ph
2
2
2
2
exhibits a stronger trans-influence than ITMe [15]. The decreased length of the Pd-Si bond
in 2b [2.2948(18) Å] versus cis-[Pd(ITMe) (SiMe Ph) ] [2.3445(8) and 2.3346(8) Å] infers a
2
2
2
stronger Pd-Si bond in 2b and demonstrates the weak trans-influence of Br. Based on these
data, the intensity of the trans-influence in these two structures follows the sequence: Br <
ITMe < SiMe Ph. Thus, the preference for the trans-configuration observed in 2b may be
2
attributed to the high trans-influence of SiMe Ph and the large steric size of Br.
2
A possible mechanism for the formation of
2
includes either a
σ
-bond metathesis
0
between a Pd-Si, in cis-[Pd(ITMe) (SiR ) ], and Br-C bond, in allylbromide, or an S 2/S 2
2
3 2
N
N
by the nucleophilic Pd-Si bond at the electrophilic sites in the allyl halide, leading to a
trans complex. As we have previously suggested using computational studies on related
bis-ITMe complexes, an NHC would then dissociate from the palladium center followed
by a cis to trans isomerization of the Br and Si moieties (Scheme 2) [11]. Finally, the
dissociated NHC would re-coordinate, constrained by the bulk of the other ligands, in a
cis-configuration [16,17].
Scheme 2. Possible mechanistic routes for the formation of 2.
3. Experimental
The handling of air-sensitive compounds and their spectroscopic measurements
were undertaken using standard Schlenk line techniques using pre-dried Ar (using a
BASF R3-11(G) catalyst and 4 Å molecular sieves), or in a MBraun glovebox under N
2
◦
(
O < 10.0 ppm). All glassware was dried in a 160 C oven prior to use. Celite was predried
2
◦
in a 200 C oven and then dried with a heat gun under a dynamic vacuum prior to use.
◦
Filter cannulae equipped with microfiber filters were dried in an oven at 160 C prior to
use. Solvents employed in air-sensitive reactions were dried using vacuum distillation,
followed by distillation over potassium or stored over activated 4 Å molecular sieves under
an Ar atmosphere. NMR spectra were recorded on a Varian VNMRS 400 (Palo Alto, CA,
1
13
11
19
29
1
USA) ( H 399.5 MHz; C{1H} 100.5 MHz; B{1H} 128.2 MHz; F 375.9 MHz; Si{ H}

Molecules 2021, 26, 2460
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1
13
1
7
9.4 MHz), or 500 ( H 499.9 MHz; C{ H} 125.7 MHz). Chemical shifts are reported in
ppm. All other experimental details are outlined elsewhere [10].
Synthesis of trans-[Pd(ITMe) (SiMe )(Br)] (2a) and Allyltrimethylsilane (3a)
2
3
Allylbromide (0.032 g, 0.26 mmol) was added to a solution of cis-[Pd(ITMe) (SiMe ) ]
2
3 2
(
0.043 g, 0.09 mmol) in C D or toluene (3.0 mL) and the resulting reaction mixture was
6 6
stirred at room temperature for 1.5 h. At this stage, the volatiles were removed in vacuo
and the off-white powder was washed with hexane (3 × 4.0 mL).
1
2
a
, Yield: 0.040 g, 92%. H NMR (399.5 MHz, C D ):
δ
= 3.68 [s, 12H, N(1,3)-CH3],
6
6
H
1
1
.42 [s, 12H, C(4,5)-CH3], 0.12 [s, 9H, SiMe3]. 13C{ H} NMR (100.5 MHz, C D ):
δ
= 184.9
6
6
9
C
2
1
[
N
C
N], 124.0 [
C
(4,5)-CH ], 35.1 [N(1,3)-
C
H ], 8.5 [C(4,5)-
C
H ], 6.9 [SiMe3]. Si{ H} NMR
3
3
3
(
79.4 MHz, C D ):
δ = 7.68. Elem. Anal. Calcd. for C H N SiBrPd: C, 40.20%; H, 6.55%;
6
6
Si 17 33 4
1
N, 11.03%. Found: C, 40.15%; H, 6.54%; N, 10.95%. 3a (from crude reaction solution), H
NMR (399.5 MHz, C D ): δ = 5.77 [m, 1H, C
H
=], 4.92 [m, 1H, C
H
=], 4.89 [m, 1H, C
H
=],
6
6
H
1
1
.44 [m, 2H, CH2],
−
0.03 [s, 9H, SiMe3]. [Agrees with an independently taken H NMR
sample of commercially available allyltrimethylsilane].
Synthesis of trans-[Pd(ITMe)2(SiMe2Ph)(Br)] (2b)
Allybromide (6.0
µL, 0.07 mmol) and cis-[Pd(ITMe) (SiMe Ph)] (0.021 g, 0.03 mmol)
2 2
were dissolved in C D or toluene (1.0 mL). The resulting reaction mixture was stirred
6
6
at room temperature for 2 h under an N atmosphere. At this stage, all volatiles were
2
removed in vacuo and the resulting white solid was washed with hexane (3
×
2.0 mL).
1
Yield: 0.018 g, 93%. H NMR (399.5 MHz, C D ):
δ = 7.20 [m, 2H, SiMe2Ph], 7.07 [m,
H
6
6
3
H, SiMe2Ph], 3.51 [s, 12H, N(1,3)-CH3], 1.42 [s, 12H, C(4,5)-CH3], 0.31 [s, 6H, SiMe2Ph].
13
1
C{ H} NMR (100.5 MHz, C D ):
δ
= 183.4 [N
C
N], 149.6 [SiMe2i-Ph], 133.1 [SiMe2Ph],
(4,5)-CH ], 34.9 [N(1,3)- H ], 8.5 [C(4,5)- H3],
= 2.44. (It was not possible to obtain
6
6
C
127.0 [SiMe2Ph], 126.5 [SiMe2p-Ph], 124.2 [
C
C
C
3
3
.2 [SiMe2Ph]. 29Si{ H} NMR (79.4 MHz, C D ):
1
δ
4
6
6
Si
elemental analysis for 2b– every attempt resulted in numbers that were inconsistent with
calculated values. A possible reason for this is decomposition of 2b by exposure to air or
moisture on transit to data collection).
−
Crystal data for 2b: C H N SiBrPd, M
r
= 569.94 g mol 1, orthorhombic, space group
22
35
4
◦ ◦ ◦
β = 90 , γ = 90 ,
P2 = 2 2 , a = 10.5467(4) Å, b = 14.3455(3) Å, c = 16.7301(4) Å,
α
= 90 ,
1
1
3
V = 2531.23(13) Å , Z = 4, T = 173 K,
λMo(K
α
) = 0.71073, R [I > 2 (I)] = 0.0345, wR (all
σ
1
2
data) = 0.0677, GooF = 1.011.
1
Crude H NMR data are consistent with the formation of allyldimethylphenylsilane
(3b) as a product of this reaction. However, this was not isolated in this instance [18].
4
. Conclusions
Under mild conditions, non-pincer bis(NHC)(silyl)palladium halide complexes of the
type trans-[Pd(ITMe) (SiR )(Br)] (SiR = SiMe Ph (2a), and SiMe3 (2b)) were synthesized,
2
3
3
2
by the reaction of allylbromide with the corresponding complexes cis-[Pd(ITMe) (SiR ) ],
2
3 2
1
a
a
σ
or 1b, respectively. A possible mechanistic route for the formation of
2
involves either
0
-bond metathesis or an S 2/S 2 reaction between allybromide and
1. This would
N
N
necessitate a cis-trans isomerization via dissociation of an NHC ligand-[19]. The reactivity of
trans-[Pd(ITMe) (SiR )(Br)] is unexplored but will soon be carried out. The facile formation
2
3
and apparent stability of trans-2 may indeed hinder the catalytic silylation of ally halides
mediated by ITMe Pd-based complexes since the adoption of a cis-configuration is a
2
prerequisite for reductive elimination and involvement in a catalytic cycle. Solutions
to these unexplored questions are currently being sought, e.g., the potential for halide
abstraction, and will be reported in due course.
Author Contributions: Conceptualization, M.B.A., O.N., J.S. Writing—original draft preparation, all
authors; writing—review and editing, all authors. X-ray data acquisition and refinement: M.B.A. and
G.E.K. All authors have read and agreed to the published version of the manuscript.

Molecules 2021, 26, 2460
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Funding: This research was funded by an EPSRC Standard Research Studentship (DTG, Grant #
EP/L505109/1), to M.B.A.
Institutional Review Board Statement: Not applicable.
Acknowledgments: Dedicated to Michel Pfeffer for his contributions to C-H activation, palladium,
and ruthenium chemistry. A gentleman and a scholar.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: No samples of the compounds are available from the authors.
Disclosure: This work has been previously presented as part of a thesis available at: Ansell, Melvyn
B (2017) Novel (N-heterocyclic carbene)-palladium(0) complexes as catalysts in element-element bond additions
to unsaturated moieties. Doctoral thesis (PhD), University of Sussex, Brighton BN1 9QJ, UK; http:
/
/sro.sussex.ac.uk/id/eprint/68072/, accessed on 12 May 2017).
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