The cobalt(II) complex of a new tridentate Schiff-base ligand as a catalyst for hydrosilylation of olefins

Article abstract of DOI:10.1016/j.catcom.2016.02.009

Condensation of 1-methyl-2-imidazolecarboxaldehyde with 2-(1-methylhydrazinyl)pyridine results in the synthesis of new, tridentate Schiff-base ligand L, which readily reacts with CoCl₂ to form a monometallic [CoLCl₂] complex that, upon reduction, functions as active hydrosilylation catalyst. The ligand and the [CoLCl₂] catalyst have been characterized spectroscopically (MS, NMR, FTIR) and by single crystal X-ray diffraction techniques. The results of preliminary catalytic experimentation show that the cobalt complex can induce hydrosilylation and dehydrogenative silylation of olefins, depending upon the hydrosilane substrate used.

Full text of DOI:10.1016/j.catcom.2016.02.009

The cobalt(II) complex of a new tridentate Schiff-base ligand as a catalyst for
hydrosilylation of olefins
Adam Gorczyn´ski, Maciej Zaranek, Samanta Witomska, Aleksandra Bo-
cian, Artur R. Stefankiewicz, Maciej Kubicki, Violetta Patroniak, Piotr Pawluc´
PII:
DOI:
Reference:
S1566-7367(16)30056-5
doi: 10.1016/j.catcom.2016.02.009
CATCOM 4590
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 September 2015
20 January 2016
8 February 2016
Please cite this article as: Adam Gorczyn´ski, Maciej Zaranek, Samanta Witom-
ska, Aleksandra Bocian, Artur R. Stefankiewicz, Maciej Kubicki, Violetta Patron-
iak, Piotr Pawlu´c, The cobalt(II) complex of a new tridentate Schiff-base ligand
as a catalyst for hydrosilylation of olefins, Catalysis Communications (2016), doi:
10.1016/j.catcom.2016.02.009
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ACCEPTED MANUSCRIPT
The cobalt(II) complex of a new tridentate Schiff-base ligand as a catalyst for
hydrosilylation of olefins
Adam Gorczyoski,a Maciej Zaranek,a Samanta Witomska,a Aleksandra Bocian,a Artur R. Stefankiewicz,a Maciej
Kubicki,a Violetta Patroniak,*a Piotr Pawlud*a
a Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61614 Poznao, Poland; E-mail: violetta.patroniak@amu.edu.pl,
piotr.pawluc@amu.edu.pl
Adam Gorczyoski and Maciej Zaranek contributed equally to this work.
Abstract
Condensation of 1-methyl-2-imidazolecarboxaldehyde with 2-(1-methylhydrazinyl)pyridine results in the synthesis of new, tridentate Schiff-base
ligand L, which readily reacts with CoCl2 to form a monometallic [CoLCl2] complex that, upon reduction, functions as active hydrosilylation
catalyst. The ligand and the [CoLCl2] catalyst have been characterized spectroscopically (MS, NMR, FTIR) and by single crystal X-ray diffraction
techniques. The results of preliminary catalytic experimentation show that the cobalt complex can induce hydrosilylation and dehydrogenative
silylation of olefins, depending upon the hydrosilane substrate used.
Keywords: Schiff-base; Complex; Cobalt; Hydrosilylation; Catalysis; Dehydrogenative silylation
Dedicated to Professor Janusz Jurczak on the occasion of 75th birthday
1. Introduction
Hydrosilylation is believed to be one of the most straightforward and efficient catalytic processes, important from both
academic and industrial points of view, that enables the synthesis of functionalized organosilicon compounds.[1]Since its first
description in 1947 by Sommer and coworkers[2] considerable attention and effort has been given towards broadening the
scope of the reaction involving the addition of Si-H unit to C-C multiple bonds, carbonyl and imine moieties, with much
improved tolerance of various functional groups. [1,3] Of particular importance as substrates are olefins and alkynes due to the
fact that the organosilicon products, functionalized silanes or siloxanes and their alkenyl-tailored counterparts, are valuable and
attractive scaffolds from the perspective of their further synthetic transformations [4-7] or have direct industrial applications
such as in silane-modified polymers.
Meridional tridentate ligands have been found to be useful scaffolds for transition metal ions and studies on catalytic systems
containing N,N,N chelating groups involving oxazoline[8-10], (poly)pyridine[11-13] or bis(imino)pyridine[13-15]moieties are
frequently performed. Even though the quest for cheaper and environmentally sustainable catalysts progresses[3], much still
needs to be done in order to outperform low oxidation state, platinum group metal complexes. These are particularly important
for industry, in processes such as the production of organofunctional silanes.[16] For economic reasons, iron is the most popular
alternative metal and, there have been numerous reports of its complexes’ activity[17],the most recent ones focusing on
asymmetric hydrosilylation of alkenes and carbonyl compounds.[9, 18] Conversely, complexes of cobalt are mostly recognized as
catalysts for hydrosilylation of alkynes[19-21]and they are likely to promote dehydrogenative silylation when used with
alkenes.[22]For this reason, cobalt has not been a focus of recent research[3] and any reports of its use in alkene hydrosilylation
were relatively scarce. The most recent ones are the works of Deng et al. utilising silyl-donor-functionalised NHC–cobalt
complexes in the hydrosilylation of 1-octene, [23] and of Holland et al. on β-diketiminate-supported cobalt catalysts. [24] An
excellent review on all recent advances in this field is given by Sun and Deng. [25]
Motivated by such reports of the applications of environmentally benign non-precious-metal-based catalysts in hydrosilylation
reactions, we have synthesized a new ligand and its Co(II) complex, so as to utilize the latter for studies of hydrosilylation of
alkenes, rarely undertaken due to the abovementioned reasons. It is noteworthy, that iron(0) bis(dinitrogen) complexes with
this type of ligand were successfully used by Chirik et al. as hydrosilylation catalysts.[26, 27]
2. Results and discussion
2.1 Synthesis and characterization of the Schiff-base ligand and its cobalt(II) complex

ACCEPTED MANUSCRIPT
The new Schiff-base scaffold was synthesized via
a
condensation reaction between two building blocks: 2-(1-
methylhydrazinyl)pyridine A and 1-methyl-2-imidazolecarboxaldehyde B. (Fig. 1)
Figure 1
A has been recognized as moiety that effectively coordinates variety of transition metal ions, in particular having been applied
within helical nanomechanical molecular motors[28, 29], metal sensors[30], artificial model DNA nucleobases[31] and for
enantiodiscrimination of chiral organic products.[32] Its reaction with B enabled us to form an N3 tridentate Schiff-base ligand L,
with a meridional coordination motif, analogous to the classical ter- or bis(imino)pyridine ligands.[3, 12, 13] We also envisaged
that lack of steric hindrance in the o-positions of the pyridine/imidazole terminal coordinating rings should facilitate the overall
activity and selectivity of the system, as shown in the works of Chirik and co-workers.[13, 14] The new coordinating agent was
characterized by spectroscopic and analytical methods, as well as by single crystal X-ray analysis upon transformation to its
hydrochloride salt. (1)
The ligand cation (1) is almost flat and the dihedral angle between the mean planes of rings is 4.98(9)°. (Fig. 2)
Figure 2
The bond length and angles establish the double-bond character of the N8-C9 bond (1.289(2)Å); the protonation site (N14) is
confirmed by the localization of the hydrogen atom in the difference Fourier map and the successful refinement of this atom
and the bond angles pattern around N14. In the crystal structure – besides the obvious Coulombic interactions between charged
species – hydrogen bonds involving the hydration water molecule are also present and play an important role in the
determination of the internal crystal architecture. (Fig. S1 )
Direct reaction of CoCl2·6H2O salt with ligand L resulted in the synthesis of the monometallic [CoLCl2](2) complex characterized
by means of an X-ray structure determination. The Co(II) atoms are five-coordinated by three nitrogen atoms from L (meridional
binding mode) and two chlorides (Fig. 3). The latter is desirable from the catalytic point of view, bearing in mind the lability and
stereochemical flexibility of Co(II) including the possibility of adopting an octahedral binding mode.
Figure 3
The ligand has an almost flat cisoidal conformation, with dihedral angles between the main planes of terminal rings being only
3.8(6)°. The coordination of Co is best described as closer to square-pyramidal (three N atoms and Cl1B in one plane within
0.076(4) Å, Co out of this plane by 0.406(3) Å and Cl1C at apex, 2.698(3) Å) than to trigonal bipyramidal (Co placed almost
exactly in the plane formed by Cl1B, Cl1C and N9A atoms, and two remaining N atoms at equal distances from this plane: N1A
1.959(9) Å, N15A -1.965(8) Å). This observation is confirmed by the value of the parameter tau[33], defined as (β-α)/60) *where
beta and alpha are the largest X-Co-Y angles)]. In ideal cases it is equal 0 in regular square-base pyramid, and 1 in regular
trigonal bipyramid. In 1 the value of this parameter is 0.35. Relevant geometrical parameters are listed in Table S2. In the crystal
structure small voids are filled by disordered (over the twofold axis) methanol molecules, which take part in weak O-H···Cl
hydrogen bonds. (Fig. S2)
2.2 Catalytic hydrosilylation of alkenes
ESI-MS of a solution of the complex provides no evidence of disproportionation to give a [CoL2]2+ species which would be
expected to be catalytically inactive, thus indicating that the potentially active ‘open’ coordination mode present in the solid
state of [CoLCl2] is retained in the solution. Thus, due to its similarity to the active catalysts described by Chirik et al. [22], the
new cobalt complex was expected to act as hydrosilylation catalyst. However, the first attempts to hydrosilylate 1-octene with
dimethylphenylsilane under the conditions used by Chirik et al.[22] (neat, RT, 24h, [HSiMe2Ph]:[C8H16]: [Co]=1:10:0.02) did not
lead to any changes and neither did heating of the reaction mixture. Considering that the chloride complex might be a catalyst
precursor, its activation was performed in accordance with the protocol of Nakazawa et al.[12] using sodium
triethylborohydride. We expected that the key to activation might be reduction at the metal centre but a preliminary study
using [CoLCl2] in the presence of NaBHEt3, dimethylphenylsilane and 1-hexene resulted only in isomerisation of the alkene.
Seeing some activity, we switched to an olefin unable to isomerise – styrene. The conditions for an effective addition of

ACCEPTED MANUSCRIPT
hydrosilanes to styrene in the presence of the cobalt(II) complex [CoLCl2] were optimized via catalytic screening of the substrate
conversion, together with evaluation of selectivity by GC and GC-MS methods. In a typical procedure, styrene, hydrosilane and
the catalyst (1:1:0.03 molar ratio) were dissolved in dry toluene and heated in a Schlenk bomb flask fitted with a plug valve at
80oC for 30 min. Next, 0.12 equiv. of NaHBEt3 was added and the reaction mixture was again heated for 2-24h at 80oC (Table 1).
Table 1. Results of hydrosilylation of olefins using Co(II)/NaHBEt3 system.a
Olefin
Silane
Time
[h]
Conversion b
(isolated yield)
[%]
Entry
Silane
%H c(β/α)
%DS c
1.
PhCH=CH2
H3SiPh
2
82
98 (100/0)
96(100/0)
100 (100/0)
21(100/0)
1
2
2.
PhCH=CH2
HSiMe2Ph
H2SiPh2
24
24
24
24
24
24
24
24
24
48
24
24
24
24
24
24
24
24
24
79
4
3
PhCH=CH2
>99 (60)
0
4.
PhCH=CH2
HSiMe(OSiMe3)2
(HSiMe2)2Od
HSiEt3
34
79
99e
-
5.
PhCH=CH2
98
6.
PhCH=CH2
0
-
7
3-MeOC6H4CH=CH2
4-MeOC6H4CH=CH2
4-MeOC6H4CH=CH2
4-MeC6H4CH=CH2
4-MeC6H4CH=CH2
4-ClC6H4CH=CH2
4-ClC6H4CH=CH2
4-BrC6H4CH=CH2
H3SiPh
>99 (53)
98 (98/2)
100 (94/6)
25
2
8
H3SiPh
34
0
9
HSiMe(OSiMe3)2
H3SiPh
35
75
0
10
11
12
13
14
15
16
17
18
19
20
44
100 (94/6)
100 (63/37)
89
H3SiPh
65
0
HSiMe2Ph
H3SiPh
37
11
1
>99 (81)
>99
46
99f, g
H3SiPh
98g, h
2
2,4-Me2C6H3CH=CH2 H3SiPh
100 (91/9)
-
0
PhC(Me)=CH2
cis-PhCH=CHPh
Me3SiCH=CH2
Me3SiCH=CH2
PhMe2SiCH=CH2
HSiMe2Ph
0
-
H3SiPh
0
-
-
HSiMe2Ph
H2SiPh2
H3SiPh
41
85 (100/0)
98 (50/50)
100(90/10)
15
2
97
68
0

ACCEPTED MANUSCRIPT
21
22
PhMe2SiCH=CH2
PhMe2SiCH=CH2
HSiMe2Ph
24
24
>99 (78)
55
99 (99/1)
54 (99/1)
1
HSiMe(OSiMe3)2
46
23
HSiMe2Ph
24
32
100i
0
24
H3SiPh
24
21
15 (100/0)
85
a 0.5M hydrosilane in toluene, 80 °C, [HSi]:[styrene]:[Co]:[NaHBEt3] = 1:2:0.03:0.12, 0.5 h of preactivation at 80 °C before addition
of the borohydride; b calculated by GC analysis using decane as internal standard; c based on GC-MS analysis; d 1:4 [HSi]:[styrene]
ratio was applied; e 42% of single and 63% of double addition products, f 7% of single and 93% of double addition products; g
single addition products were formed exclusively when equimolar amounts of styrene derivatives were used; h 2% of single and
98% of double addition products; imixture of isomers since the substrate consists of a mixture of isomers.
The results show that [CoLCl2] can be an active catalyst for hydrosilylation (H) and dehydrogenative silylation (DS) of styrene,
depending on the hydrosilane substrate. Hydrosilanes with phenyl substituents undergo nearly exclusively hydrosilylation, while
hydrosiloxanes give dehydrogenative silylation when a terminal alkene is used. Triethylsilane appears to be unreactive under the
given conditions, presumably due to the electronic properties of its alkyl substituents. The products were selectively formed in
1
an anti-Markovnikov manner. The structures of the selected reaction products were confirmed by the H NMR spectra of the
isolated compounds (see SI). Occurrence of dehydrogenative silylation was accompanied by formation of stoichiometric
amounts of ethylbenzene or its derivative corresponding to used styrene detectable by both GC and GC-MS analyses, and no
evolution of dihydrogen was observed. To our knowledge, such switching of the reaction product (H/DS), depending on the
chosen hydrosilane, has not been seen earlier for alkenes in the presence of a cobalt catalyst. Similar behaviour has been
demonstrated in the work of Li Yong et al. with respect to switching of regioselectivity but it was limited to terminal alkynes.[20]
The products of dehydrogenative silylation were formed exclusively as E isomers. In the case of 1,1,3,3-tetramethyldisiloxane
(Table 1, Entry 4), both single and double substitution products were formed (cf. Fig. S4). In general, substituted styrenes
bearing functional groups such as –Me, –OMe, –Cl, and –Br, irrespective of the substituents’ electronic character, reacted with
phenyl-substituted silanes to give the corresponding hydrosilylation products (Table 1, entry 7-15). In contrast, reaction of 4-
methoxystyrene with 1,1,1,3,5,5,5-heptamethyltrisiloxane in the presence of [CoLCl2] led to dehydrogenative silylation product
as the predominant one (Table 1, entry 9). The influence of the temperature and catalyst loading has been investigated for the
model hydrosilylation reaction involving styrene and dimethylphenylsilane. A decrease in the catalyst loading to 2 mol% lowered
the conversion of HSiMe2Ph to 54% after 24h. The reaction performed at a lower temperature (50oC) resulted in a decrease in
silane conversion to 29% after 24h. An attempt to use 1,4-dioxane and THF as solvents failed, resulting in precipitation of grey
metallic cobalt.
Other olefins exhibited a relatively low reactivity in the reaction with silanes. Vinylsilanes underwent a hydrosilylation with
phenyl-substituted silanes (Table 1, entry 17-20) with moderate to high conversion (41-99%) to yield mostly hydrosilylation
products. Reactions of simple 1-alkenes such as 1-hexene and 1-octene led only to the isomerisation of alkenes and no
conversion to the silane was observed. The hydrosilylation of cyclohexene by phenylsilane in the presence of Co/borohydride
system yielded predominantly the dehydrogenative silylation product.
To show that the catalyst was actually necessary for the reaction, we performed a blank experiment under similar reaction
conditions without cobalt complex. The reaction of styrene with silanes in the presence of NaHBEt3 ([HSi]:[styrene]:[NaHBEt3] =
1:2:0.12) confirmed that cobalt complex was essential to achieve high conversion of silane and to promote the hydrosilylation
process. No conversion of 1,1,3,3-tetramethyldisiloxane was observed after 24h but small amounts of hydrosilylation products
were detected when HSiMe2Ph (16% conversion of silane) was used as a reagent. Such observations seem to be consistent with
the results reported by Crabtree and co-workers [34] and Nikonov [35], who showed that borohydrides e.g. LiHBEt3 and NaBH4
as well as simple bases can be active catalysts in hydrosilylation of C=O and C=N bonds.

ACCEPTED MANUSCRIPT
We believe that cobalt hydride complexes are probably the true catalysts but the paramagnetism of [CoLCl2] precluded NMR
experiments to establish the nature of catalytically active system as was possible with related Fe-based systems[12]. Co
nanoparticles are not formed under the reaction conditions (no dynamic light scattering was observed in the region of: 0.6 nm-6
m as established with Zetasizer analyzer), so that catalysis by decomplexated metallic cobalt species may be discounted. The
related ‘open species’ (i.e. those that are susceptible to the effect of external environment) iron(III) [FeLCl3] complex was found
to be inactive under similar reaction conditions.
Conclusions
The facile synthesis of a new Schiff-base ligand L has enabled the preparation of its Co(II) complex that shows activity as a
homogeneous catalyst. Preliminary results show that upon in situ reduction with NaBHEt3, we are able to isolate the products of
hydrosilylation and/or dehydrogenative silylation of alkenes depending upon the nature of the hydrosilane. Our further aim is to
broaden the scope of our new system, with respect to both the ligand framework (electronic and steric factors) and the use of
other non-precious-metals such as iron or nickel, so as to improve the reactivity and selectivity of investigated process.
Acknowledgements
This research was carried out as a part of a National Science Center project (Grant No. 2011/03/B/ST5/01036). A.G. declares that
he is a scholarship holder within the project “Scholarship support for Ph.D. students specializing in majors strategic for
Wielkopolska’s development”, Submeasure 8.2.2 Human Capital Operational Programme, cofinanced by the European Union
under the European Social Fund as well as a scholarship holder of The Adam Mickiewicz University Foundation in Poznao,
2014/2015 year edition
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Fig. 1. Synthetic protocol for new Schiff-base ligand L utilized in formation of cobalt hydrosilylation catalyst.
Fig. 2. Anisotropic ellipsoid representation of the ligand cation 1; ellipsoids are drawn at the 50% probability level, hydrogen
atoms are shown as spheres of arbitrary radii.
Fig. 3. Anisotropic ellipsoid representation of the [CoLCl2] complex; ellipsoids are drawn at the 50% probability level, hydrogen
atoms are shown as spheres of arbitrary radii.

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Figure 1

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Figure 2

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Figure 3

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Graphical abstract

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Highlights:
 Synthesis of new Schiff-base ligand and its cobalt(II) complex are reported
 Crystallographic structures of ligand L and [CoLCl2] complex are presented

Cobalt complex induces hydrosilylation and dehydrogenative silylation of alkenes