New organoplatinum (IV) complex with quaterpyridine ligand: Synthesis, structure and its catalytic activity in the hydrosilylation of styrene and terminal alkynes

Article abstract of DOI:10.1016/j.catcom.2013.08.005

The reaction of ligand L with PtCl₂ leads to a novel structural motif of octahedral ortho-metalated complex of formula [Pt(L_-H) Cl₃] 1. This result is a consequence of drastic conditions of reaction and preferred Pt(IV) octahedral coordination geometry. The new compound has been characterised on the basis of the spectroscopic data in solution, and its structure confirmed in the solid state by X-ray crystallography (1a - [Pt(L_-H)Cl₃]·MeOH, 1b - [Pt(L_-H)Cl ₃]·C₆H₅CH₃). This article reports organoplatinum (IV) complex 1 as effective and highly selective catalyst precursor in the hydrosilylation of styrene and terminal alkynes.

Full text of DOI:10.1016/j.catcom.2013.08.005

Catalysis Communications 42 (2013) 79–83
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
New organoplatinum (IV) complex with quaterpyridine ligand:
Synthesis, structure and its catalytic activity in the hydrosilylation of
styrene and terminal alkynes
b
Ariel Adamski a, Maciej Kubicki a, Piotr Pawluć a, , Tomasz Grabarkiewicz , Violetta Patroniak
⁎
a,b,⁎⁎
a
Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61614 Poznań, Poland
b
AdvaChemLab, Topazowa 8, 61680 Poznań, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2013
Received in revised form 7 August 2013
Accepted 8 August 2013
Available online 19 August 2013
The reaction of ligand L with PtCl2 leads to a novel structural motif of octahedral ortho-metalated complex of
formula [Pt(L-H)Cl3] 1. This result is a consequence of drastic conditions of reaction and preferred Pt(IV) octahe-
dral coordination geometry. The new compound has been characterised on the basis of the spectroscopic data in
solution, and its structure confirmed in the solid state by X-ray crystallography (1a—[Pt(L-H)Cl3]∙MeOH, 1b—
[Pt(L-H)Cl3]∙C6H5CH3). This article reports organoplatinum (IV) complex 1 as effective and highly selective
catalyst precursor in the hydrosilylation of styrene and terminal alkynes.
Keywords:
© 2013 Elsevier B.V. All rights reserved.
Quaterpyridine ligand
Platinum(IV) complex
Hydrosilylation
Orthometalation
1. Introduction
polyorganosiloxanes and unsaturated polymers [13]. Although a wide
range of catalysts have been tested for hydrosilylation, most research
The design and synthesis of complexes exhibiting novel structures
and properties has provided exciting new perspectives for the chemists
[1,2]. Cobalt(II) quaterpyridine complex is a visible light-driven catalyst
for both water oxidation and reduction [3]. Bis(imino)quaterpyridine-
bearing multimetallic late transition metal complexes are effective
catalysts in ethylene oligomerisation [4]. The application of helical
complexes of Cu(I) and Cu(II) with chiral quaterpyridines in asymmetric
catalysis is under active investigation [5].
The chemistry of platinum received considerable attention due to the
fact that platinum complexes are believed to be catalytically active in a
variety of organic syntheses [6,7]. These complexes have recently
attracted much attention because of their well-defined interesting
properties, such as photoluminescence [8], high cytotoxicity [9] and
catalytic activity [10] of potential interest in medicine [11].
Hydrosilylation is one of the most straightforward catalytic process
that allows for the synthesis of functionalised organosilicon compounds
[12]. The use of organosilicon compounds as reagents for the construc-
tion of natural products has become a powerful tool in synthesis. In
addition, the hydrosilylation reaction is used in the industrial manufac-
ture of silicon polymers, for example, for the curing and modification of
and industrial syntheses have been carried out in the presence of plati-
num complexes [12]. The Karstedt complex [Pt2{(CH2 = CHSiMe2)2O}3]
[14,15] and the Speier system (H2PtCl6/i-PrOH) [16] are the most com-
monly employed industrial catalysts. Unfortunately, the hydrosilylation
of alkenes and alkynes catalysed by platinum complexes is often accom-
panied by side reactions such as isomerisation, polymerisation and hy-
drogenation of unsaturated compounds as well as reactions in which
both substrates take part, e.g., dehydrogenative silylation. Moreover,
colloidal platinum species, which are responsible for some of the ob-
served side-reactions, are usually generated under these conditions [17].
In view of the successful synthesis of the complexes of Cu(I)
[18,19], Ag(I) [18], Mn(II) [20], Fe(II) [18], Co(II) [20,21], Zn(II)
[18,20], Cd(II) [20], Hg(II) [20], Pr(III) [22] and Eu(III) [21,22] with
dimethylquaterpyridine L, we attempted the preparation of complex
with Pt(II) (Fig. 1).
Herein, we report that our investigations into platinum coordination
chemistry of the ligand L have successfully demonstrated that it is able to
form novel organometallic Pt(IV) complex 1. We have described catalytic
activity of this complex in hydrosilylation of styrene and terminal
alkynes.
2. Results and discussion
⁎
Corresponding author.
⁎⁎ Correspondence to: V. Patroniak, Faculty of Chemistry, Adam Mickiewicz University,
Umultowska 89b, 61614 Poznań, Poland. Tel.:+48 618 291 483.
E-mail addresses: piotrpaw@amu.edu.pl (P. Pawluć), violapat@amu.edu.pl
(V. Patroniak).
In previous papers, we have carried out studies on self-assembling
the ligand L in the presence of appropriate metal ions and perceived
that the ligand is correctly preorganised for the creation of complexes
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.08.005

80
A. Adamski et al. / Catalysis Communications 42 (2013) 79–83
crystal packing; therefore, coordination environment will be described
on the basis of 1a—data for 1b will be given in parentheses.
The coordination of Pt(IV) ion is close to octahedral (Fig. 2). The c.n. of
six is completed by three Cl− anions, two nitrogen and one carbon atoms
from the ligand molecule.
The Pt-N, Pt-C and Pt-Cl distances are close to typical values and do
not differ significantly in the pair 1a–1b. The complexed quaterpyridine
ligand adopts non-typical syn–trans–trans conformation, surprisingly
preferring to bond with the Pt by one Pt-C bond and leaving therefore
a place for another Cl− anion to enter the coordination sphere in the
equatorial manner. The ligand as a whole is almost planar, and the
dihedral angles between the planes of consecutive rings are 1.6° (8°),
3.7° (8°) and 5.8° (8°) [for 1b, the appropriate dihedral angles are 6.5°
(1°), 3.1° (2°) and 2.3°].
Fig. 1. The ligand L—C22H18N4.
In both crystals—although they crystallise in different crystal systems
(1a in monoclinic P21/c; 1b in triclinic P-1)—the complex molecules leave
empty channels extending throughout the crystal (along c-direction in
1a, along x in 1b), which are filled by the diffused electron density,
interpreted in the refinement process as the disordered solvent guest
molecules (methanol in 1a, toluene in 1b) can occupy these channels.
Figs. 3 and 4 show the appropriate views without and with the solvent
molecules.
The crystal structures in both cases are probably organised mainly by
electrostatic and van der Waals interactions; some weak but directional
C-H · · · Cl hydrogen-bond type contacts as well as π · · · π stacking
interaction (in 1b) can be also found but their role is rather of secondary
importance.
of different types [18–22]. As a continuation of these works herein we
report the use of the ligand L in the complexation with platinum(II)
ion; we noticed that the dimethylquaterpyridine ligand L is acting as a
tridentate one and it generates mononuclear organometallic complex
[Pt(L-H)Cl3]. An equimolar reaction of platinum(II) chloride with ligand
L proceeded with orthometalation to provide new air- and moisture-
stable complexes 1a ([Pt(L-H)Cl3]∙2MeOH) and 1b ([Pt(L-H)Cl3]∙PhMe).
Unexpectedly, platinum(IV) ion in this structure is situated in a distorted
octahedral coordination environment comprising N,N-donor ligand sets,
three chlorides and C-Pt bond. The tendency of transition metal salts to
undergo orthometalation reaction with heteroaromatic ligands (mostly
including nitrogen donors) to give five-membered metallocycles has
been demonstrated with numerous metals, including, for instance
Re(I), Pt(II) and Pd(II). Cyclometalated platinum complexes have been
extensively investigated in the past years because of their interesting
photophysical properties [23].
2.2. Hydrosilylation of styrene with trisubstituted silanes
Complex 1 has been found to be highly active catalyst in the
hydrosilylation of styrene with trisubstituted silanes. The conditions for
an effective addition of hydrosilanes to styrene in the presence of 1
have been optimised via catalytic screenings of the substrates conversion
and the yield of products by using GC and GC-MS methods. In a typical
procedure, the styrene and silane (1:1 molar ratio) and complex 1
(0.01 mol%) were dissolved in dry toluene (0.5 M) and heated in a
Schlenk bomb flask fitted with a plug valve at 80 °C for 2–24 h. At a
relatively low 0.01 mol% catalyst loading, high catalytic activity was
observed. Under these conditions, complete conversion of substrates is
reached within 2 h, however, reaction with triethoxysilane required
longer time.
2.1. Crystal structures of complex 1a and 1b
In the Cambridge Structural Database (F. H. Allen, Acta Cryst B, 2002,
58, 380, version 5.34, last update from Nov. 2012), there are no examples
of Pt complexes with the ter- (or more) pyridine ligand in the NNC dispo-
sition and three Cl− anions. Drastic reaction conditions made transforma-
tion of Pt(II) to Pt(IV) oxidation state possible, with unexpected NNC
tridentate coordination motif of dimethylquaterpyridine ligand L, which
crystallised as 1a or 1b complex, both differing in guest molecules present
(1a—methanol, 1b—toluene). Pt(L-H)Cl3 appeared to be stable under
ambient conditions, and main differences were found to arose from the
The highest regioselectivities in terms of the formation of β-
addition product were observed for hydrosilylation of styrene with
Fig. 2. A perspective view of the complex 1a together with labeling scheme. The ellipsoids are drawn at the 50% probability level; hydrogen atoms are shown as spheres with arbitrary radii.

A. Adamski et al. / Catalysis Communications 42 (2013) 79–83
81
Fig. 3. A fragment of crystal packing as seen along z-direction; the channels within the structure of complex 1a (van der Waals representation, left) are filled by disordered methanol
molecules (sticks, right).
dimethylphenylsilane. The application of this catalytic system gives
exclusively β-(dimethylphenylsilyl)ethylbenzene with high selec-
tivity (β/α = 98/2) and good yield (Table 1). The structure of the re-
action product was confirmed by 1H NMR analysis of isolated
compound (isolated yield 92%). Disappointingly lower selectivities
were obtained using other alkoxy- and siloxy-substituted hydrosilanes;
however, in all cases, the formation of β-addition product is favored (β/
α N 60/40). In all the cases examined, the only products detected in the
reaction mixture were the hydrosilylation products (dehydrogenative
silylation or silane coupling product have not been observed), which
clearly evidences the high reactivity of hydrosilanes in the presence of
Pt(IV) quaterpyridine catalyst (Scheme 1).
The results of hydrosilylation of terminal alkynes by dimethyl-
phenylsilane and hydrosiloxanes are shown in Table 2. The optimum
conditions are 0.1 mol% catalyst at a reaction temperature of 80 °C.
Under these conditions, complete conversion of substrates is reached
within 2 h. Lowering the catalyst concentration to 0.01 mol% requires a
longer reaction time (24 h). The highest selectivities in terms of the for-
mation of β-addition products were observed for hydrosilylation of
silyl-substituted alkynes. The application of this catalytic system gives ex-
clusively unsymmetrical (E)-1,2-bis(silyl)ethenes. Stereodefined 1,2-
bis(silyl)ethenes represent a class of compounds that has attracted signif-
icant attention in recent years as valuable bifunctional building blocks in
organic synthesis [26,27]. The (E)-configuration of carbon–carbon double
bond in the bis(silyl)ethene products was determined on the basis of the
1H NMR spectra of isolated compound—(E)-1-(triethylsilyl)-2-(dimethyl-
phenylsilyl)ethene (88% yield). Hydrosilylation of phenylacetylene and 1-
octyne by dimethylphenylsilane in the presence of complex 1 yielded the
α- and β-(E)-alkenylsilanes, with predominance of the latter. The forma-
tion of two isomers has been confirmed by 1H NMR analysis of the crude
hydrosilylation products. Higher selectivities were obtained in the
hydrosilylation of 1-octyne and phenylacetylene with tetramethyl-
disiloxane or pentamethyldisiloxane in the presence of complex 1
(Table 2). Although the (E)-alkenylsilane is the major product (76–92%)
2.3. Hydrosilylation of terminal alkynes with dimethylphenylsilane and
hydrosiloxanes
We could show that complex 1 is also highly active catalyst for the
hydrosilylation of terminal alkynes. Hydrosilylation of terminal alkynes
in the presence of platinum complexes provides usually exclusive cis-
addition to produce the α- and β-(E)-alkenylsilanes, with predominance
of the latter [12]. The conventional platinum catalyst such as Speier's and
Karstedt's catalyst are usually quite nonselective in hydrosilylation of
alkynes by hydrosilanes [24]. In contrast, the platinum phosphine
complexes, especially those containing bulky ligands, show improved
regioselectivity [25].
under these experimental conditions, variable amounts of the
α
(8–24%) isomer was also found using GC-MS and 1H NMR analysis
(Scheme 2).
Fig. 4. A fragment of crystal packing as seen along z-direction; the channels within the structure of complex 1b (van der Waals representation, left) are filled by disordered toluene
molecules (sticks, right).

82
A. Adamski et al. / Catalysis Communications 42 (2013) 79–83
Table 1
Hydrosilylation of styrene in the presence of [Pt(L-H)Cl3].
SiR3
HSiR3 conversion Styrene conversion Time
Selectivitya B/A
(β/α)
(%)a
(%)a
[h]
Scheme 2. Hydrosilylation of terminal alkynes.
SiMe2Ph
Si(OEt)3
SiMe(OEt)2
Si(OSiMe3)2Me 99
100
92
98
100
95
99
2
24
2
98:2
60:40
70:30
74:26
100
2
Acknowledgments
[HSiR3]:[styrene]:[1] = 1:1:0.0001, toluene, 80 °C.aMeasured by GC.
This research was carried out in the frame of the Polish National
Science Center project (grant no. 2011/B/ST5/01036) and as a part of
Poznan Academic Business Incubator project “Internship—A Path to
Scientist's Success II” (Co-financed by European Union's Social Fund—
National Cohesion Strategy, Human Capital).
While platinum(0) complexes are thought to be the active catalysts
for hydrosilylation, platinum(II) derivatives may also be successfully
employed as precursors to such species. A catalytic cycle involving
Pt(II/IV) has been also proposed for hydrosilylation [28]. Pt(IV)
precatalysts can be reduced to Pt(II) or even Pt(0) species (including
colloidal platinum) in the presence of reactant silane. Two mechanisms
of the platinum-catalysed hydrosilylation are generally proposed. The
classical Chalk–Harrod mechanism (as well as its modified version)
involves oxidative addition—migratory insertion and reductive
elimination steps to explain the hydrosilylation [12,29]. More recently,
Lewis proposed the mechanism of hydrosilylation involving the
formation of the metal colloid-silane intermediate followed by a nucle-
ophilic attack of the olefin [30]. The latter mechanism explains more
easily the electronic effects on substituted silane and olefin molecules.
Lewis-type mechanism has been proposed for hydrosilylation of
functionalised alkenes in the presence of Speier catalyst [31]. Induction
period and the formation of characteristic yellow color in our reactions
could be directly linked to the formation of colloidal platinum, which
suggest that the mechanism reported for other platinum(IV) precursors
is probably also operative in our catalytic system.
Appendix A. Supplementary data
Experimental procedures and spectroscopic data of isolated
compounds. CCDC-926366 (1a) and CCDC-931522 (1b) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.08.005.
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R′
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Selectivityb β-(E)/α
Ph
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Si(i-Pr)3
n-C6H13
Ph
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