A Neutral RuII Hydride Complex for the Regio- and Chemoselective Reduction of N-Silylpyridinium Ions

DOI: 10.1002/chem.201705899

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Chemistry - A European Journal p. 5613 - 5622 (2018)

Update date:2022-08-11

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Authors:

B?hr, Susanne

Oestreich, Martin

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Article abstract of DOI:10.1002/chem.201705899

A detailed experimental analysis of the 1,4-selective reduction of pyridine with hydrosilanes catalyzed by a coordinatively unsaturated Ru^II thiolate complex is reported. The previously suggested intermediates, N-silylpyridinium ions and a neutral Ru^II hydride, have been independently synthesized and do indeed participate in the catalytic cycle. The resting state is not the cationic Ru^II complex initially used as the catalyst but its pyridine-coordinated congener. All Ru^II complexes, including the one resulting from hydrosilane activation, are in equilibrium with each other. The N-silylated 1,4-dihydropyridine together with the cationic Ru^II complex convert back into the corresponding N-silylpyridinium ion and the neutral Ru^II hydride (retro-hydrosilylation), followed by further backward reaction into the hydrosilane and the pyridine adduct of the cationic complex. These steps prove the overall reversibility of the transformation.

Full text of DOI:10.1002/chem.201705899

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Title: A Neutral Ru(II) Hydride Complex for the Regio- and

Chemoselective Reduction of N-Silylpyridinium Ions

Authors: Susanne Bähr and Martin Oestreich

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A Neutral Ru(II) Hydride Complex for the Regio- and

Chemoselective Reduction of N-Silylpyridinium Ions

Susanne Bähr and Martin Oestreich*^[a]

Abstract: A detailed experimental analysis of the 1,4-selective

reduction of pyridine with hydrosilanes catalyzed by a coordinatively

unsaturated Ru(II) thiolate complex is reported. The previously

suggested intermediates, N-silylpyridinium ions and a neutral Ru(II)

hydride, have been independently synthesized and do indeed

participate in the catalytic cycle. The resting state is not the cationic

Ru(II) complex initially used as the catalyst but its pyridine-

coordinated congener. All Ru(II) complexes, including the one

resulting from hydrosilane activation, are in equilibrium with each

other. The N-silylated 1,4-dihydropyridine together with the cationic

Ru(II) complex converts back into the corresponding N-

silylpyridinium ion and the neutral Ru(II) hydride (retro-

hydrosilylation), followed by further backward reaction into the

hydrosilane and the pyridine adduct of the cationic complex. These

steps prove the overall reversibility of the transformation.

transfer (SET) followed by hydrogen-atom transfer (HAT). The

pathway and also the regioselectivity depends largely on both

the substrate and the transition-metal complex.^[9a,14]

Norton and co-workers tested a series of Ru(II), Ir(III), and

Rh(III) hydrides in combination with nitrogen-containing cations,

showing that not only the transition metal but also the effect of

the ligands on the transition-metal center is crucial for reactivity,

i.e., hydricity, and regioselectivity.^[17]For example, Norton and

co-workers looked into the reduction of an N-acylpyridinium ion

several

Ru(II)

hydrides

experimentally

and

computationally.^[17d]Calculations show that the spin density

resides mainly at C4 whereas the highest charge density is at

C2 of the pyridinium ion.^[18]The Norton team was further able to

identify

transition-metal radical cation^[19]by EPR

measurements when employing Hembre’s hydride (1, dppf =

diphenylphosphinoferrocene, Figure 1) in the completely C4-

selective hydride transfer onto the N-acylpyridinium ion.^[17d]

Introduction

The redox equilibrium of NADH and NAD⁺plays an essential role

in metabolic conversions and processes.^[1]The reactivity and

properties of both redox partners and related structures^[2,3]have

been investigated in great detail. Furthermore, various transition-

metal-catalyzed transfer hydrogenations have been developed

that use either NADH itself or truncated surrogates.^[4]Moreover,

the possibility of transferring a hydride from a transition-metal

complex onto pyridinium ions continues to attract considerable

attention, and methods for the catalytic regeneration of NADH or

model compounds thereof using a variety of reducing reagents

and transition-metal catalysts were developed.^[5]In seminal

contributions, Steckhan and co-workers introduced Rh(III)

complexes,^[6]followed by extensive mechanistic work published

by Fish and coworkers.^[7]Today, the transition-metal complexes

applied in combination with pyridinium ions range from Rh(III)

and Ir(III) to Ru(II) compounds,^[8–13]and more recently also Fe(II)

hydrides were successfully used.^[14]On the other hand, a variety

of models for NAD⁺with different substituents in the C3 position

were investigated, revealing that coordination of the amide

moiety to the transition-metal center is essential for reactivity

and regioselectivity.^[7a,10]Ishitani and co-workers also establi-

shed η²-coordination of the formed 1,4-dihydropyridines to Ru(II)

centers when the Lewis-basic site at C3 is missing.^[15]Also, the

mechanism of the hydride transfer onto the respective

pyridinium ion was investigated.^[9a,14,16]It could either proceed by

a concerted ionic pathway or stepwise in a single-electron

Figure 1. Neutral Ru(II) hydrides for the regioselective reduction of N-acyl- (for

1) or N-silylpyridinium ions (for 2 and 3).

Likewise, Nikonov and co-workers detected such a radical

cation^[20a]in the 1,4-hydrosilylation of pyridines catalyzed by a

coordinatively unsaturated Ru(II) complex.^[20]Furthermore,

dimerized 1,4-dihydropyridines potentially resulting from radical

intermediates were observed. Isolation and characterization of

neutral Ru(II) hydride 2 remains elusive due to the low stability

of this intermediate. A radical mechanism is nevertheless

accepted for hydride transfer to C4 (to form 1,4-

dihydropyridines) while an ionic mechanism is likely operative for

hydride delivery to C2 (to form 1,2-dihydropyridines).

We recently reported the 1,4-selective hydrosilylation of

pyridines^[21]with the cationic Ru(II) thiolate complex 4⁺[BAr^F]^–as

catalyst (e.g., 5 → 6, Scheme 1).^[22]The reaction is usually

performed at room temperature, requiring reaction times of 7 to

14 h. Hydrosilanes 7b, 7c, and 7d afforded good yields whereas

7a and 7e did not react even though their activation by 4⁺is

known.^[23]The initially proposed catalytic cycle involves

hydrosilane adduct 8⁺generated by heterolytic splitting of the

Si–H bond in 7 by 4⁺.^[23a]

[a]

S. Bähr, Prof. Dr. M. Oestreich

Institut für Chemie, Technische Universität Berlin

Strasse des 17. Juni 115, 10623 Berlin (Germany)

E-mail: martin.oestreich@chem.tu-berlin.de

Homepage: http://www.organometallics.tu-berlin.de

Supporting information for this article is given via a link at the end of

the document.

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–

⁺[BAr^F

]

Table 1. Synthesis of N-silylpyridinium ions^[a,b]

appropriate:

HSiEtMe₂

(1.0 mol%)

H–Si

(

)

H–Si =

HSiMe Ph (

(1.0 equiv)

HSiMePh₂

(

)

neat

RT, 7–14 h

not working:

HSiEt₃

HSiPh₃

(

)

: 84–96%

)

[Ru]

SAr

Et₃P

py·Si⁺(9⁺)

substituents of parent hydrosilane H–Si

BAr^F

–

BAr^F

–

entry

Et₃

(7a)

EtMe₂

(7b)

Me₂Ph

(7c)

MePh₂

(7d)

Ph₃

(7e)

δ (²⁹Si) [ppm]

–

⁺[BAr^F

]

via:

py·SiEt₃

(9a⁺)



–

○



–

—

–

—

–

—

–

[Ru]

SAr

δ 44.6

Et₃P

9⁺

py·SiEtMe₂

(9b⁺)

8⁺

δ 44.5

Scheme 1. 1,4-Hydrosilylation of pyridine (5) with 4⁺and putative

intermediates [Ar^F= 3,5-bis(trifluoromethyl)phenyl].

py·SiMe₂Ph

(9c⁺)



○



–

δ 32.0

Adduct 8⁺then transfers the silicon cation from the sulfur donor

to the pyridine nitrogen atom in 5 to form the neutral Ru(II)

hydride 3 and N-silylpyridinium ion 9⁺. The reaction between

intermediates 3 and 9⁺closes the catalytic cycle by a hydride

py·SiMePh₂

(9d⁺)

–

δ 20.1

py·SiPh₃

(9e⁺)

transfer, leading to 1,4-dihydropyridine

6 and regenerated

–



–

catalyst 4⁺. Herein, we wish to disclose our efforts toward a

better understanding of the catalytic cycle. Both Ru(II) hydride 3

as well as N-silylpyridinium ions 9⁺were independently

synthesized and fully characterized. Control experiments

employing the substrates and products as well as the

δ 8.6

py·SiMe₃

(9f⁺)



–

δ 43.8

py·SiEt₂Me

(9g⁺)

synthesized intermediates allow for

mechanistic proposal.

a refinement of the

–

δ 45.1

[a] : Obtained from parent hydrosilane. : Obtained as

result of

substituent scrambling. ○: Possible but not detected. –: Not accessible from

ligand scrambling. [b] py = pyridine.

Results and Discussion

Synthesis of N-silylpyridinium ions

With regard to the assumed mechanism, we set out to prepare

various N-silylpyridinium ions 9⁺(Table 1).^[24]Following

established procedures for similar donor-stabilized silylium ions,

we intended to prepare 9b⁺from EtMe₂SiH (7b). Treatment of

7b with [Ph₃C]⁺[B(C₆F₅)₄]⁻followed by the addition of pyridine (5)

indeed led to the formation of the desired silicon cation but we

observed three instead of one resonance signals between δ 43

and 46 ppm in the ²⁹Si NMR spectrum. In turn, the use of

homoleptic hydrosilanes 7a and 7e resulted in the clean

formation of just one silicon cation. Hence, we attribute the

generation of several cations in case of heteroleptic 7b to ligand

scrambling. This phenomenon is well-known to occur with aryl

substituents and it has also been observed for trialkyl-

substituted hydrosilanes.^[25]However, it has not been

documented for Me₂PhSiH (7c) and MePh₂SiH (7d).^[26]

When we subjected Me₂PhSiH (7c) and MePh₂SiH (7d) to the

same procedure, we observed a similar behavior: multiple but

well-defined silicon cations were obtained. By comparison with

the cations obtained from homoleptic hydrosilanes 7a and 7e

and allowing for all possibilities resulting from ligand scrambling

of 7b, 7c, and 7d, we were able to unambiguously identify the N-

silylpyridinium ions 9a–g⁺on the basis of their ²⁹Si NMR shifts

(Table 1).^[27]In the mechanistic experiments discussed hereafter,

the above mixture composed of three different N-silylpyridinium

ions resulting from hydrosilane 7b was used. The three different

compounds are labelled 9b’⁺where ‘b’ stands for the originating

hydrosilane and ’ is an indicator of the mixture. Likewise, N-

silylated 1,4-dihydropyridines emerging from these N-

silylpyridinium ions are denoted the same way.

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We further noticed that the pyridine in these Lewis pairs acts as

a labile donor. Mixtures of N-silylpyridinium ions resulting from

7b, 7c, and 7d show only one set of resonance signals for the

pyridine in the ¹H and ¹³C NMR spectra. We explain this by a

fast exchange of the pyridine Lewis base between differently

substituted silicon cations on the NMR time scale.^[28]The

The fact that treatment of N-silylpyridinium ions 9b’⁺with excess

hydrosilane 7b and pyridine (5) did not lead to the formation of

the corresponding 1,4-dihydropyridines 6b’ (cf. Figure 2, middle)

attests that there is no catalysis by the silylium ion operative. A

scenario where hydride is directly transferred from the

hydrosilane 7 (possibly assisted by the Lewis base 5) onto 9⁺

resulting in the formation of 6 can be excluded. The Ru(II)

catalyst is therefore involved in the hydride delivery.

chemical shifts detected for the pyridine ligand in the ¹H and ¹³

NMR spectra therefore correspond to average resonance

signals with the predominant N-silylpyridinium ion contributing

most. This conclusion is further corroborated by an experiment

involving the homoleptic N-silylpyridinium ions 9a⁺and 9e⁺. The

combination of independently prepared samples of 9a⁺and 9e⁺

led to a shift of the pyridine resonance signals while the

characteristic resonance signals of the homoleptic silyl

fragments remained unchanged in the ¹H, ¹³C, and ²⁹Si NMR

spectra (see the Supporting Information for further details).

Aside from the aforementioned lability of the pyridine ligand, this

experiment also verifies that scrambling of alkyl and aryl groups

at the silicon atom occurs at the stage of the solvent-stabilized

silylium ions^[29]prior to the addition of the pyridine donor. No

scrambling was observed when mixing preformed N-

silylpyridinium ions.^[30]

Synthesis of Ru(II) hydrides

The preparation of the assumed hydride-transfer reagent 3 was

accomplished by

a one-pot, two-step procedure. Chloride

abstraction from Ru(II) chloride 10 with Na⁺[B₁₂Cl₁₂]^2–in the

presence of EtMe₂SiH (7b) directly furnished the hydrosilane

adduct 8b⁺in situ, and the desired Ru(II) hydride 3 was then

obtained from the reaction of the electrophilic silicon group in

8b⁺with KOMe as an external base (Table 2, entry 1).^[31]The

same procedure was successfully applied to Ru(II) chloride

complexes 11–13; these are precursors of cationic catalysts

similar to 4⁺frequently used by our group.^[32]Complexes 3 and

14–16 were characterized by means of NMR spectroscopy.^[33]

The ¹H NMR chemical shifts of the Ru(II) hydride depend on the

solvent but are generally in the range of δ –8.0 to –10 ppm. The

As another control experiment, we added excess of hydrosilane

7b (the purpose for using this substrate in excess is discussed

below) and pyridine (5) to N-silylpyridinium ions 9b’⁺obtained

large coupling constant J_H,Pto the phosphorus of about 50 Hz

indicates that the hydride is directly attached to the Ru(II) center.

from 7b. While the resonance signals in the H NMR spectrum

for both the pyridine donor shared by 9b’⁺and the free pyridine

disappeared, a new set of resonance signals appeared (Figure

2). These chemical shifts were between those observed for the

initially used sample of 9b’⁺and pyridine (5).

Table 2. Preparation and characterization of Ru(II) hydrides 3 and 14–16.

HSiEtMe₂

(7b)

Na₂B₁₂Cl₁₂

H⁴

9b'⁺

H³

H²

then KOMe

H³

R₃P

H⁴

C₆H₆, RT

1.5–3.5 h

H²

9b'⁺

+ 5

H²

δ (¹H) of Ru–

²J_H,P

[Hz]

entry

yield [%]

H [ppm]^[a]

H³

H⁴

+ 7b

H⁴

Et (10 → 3)

iPr (11 → 14)

Ph (12 → 15)

–9.32^[b]

–8.88

–8.03

–8.38

53.3

49.3

53.1

53.8

H³

H²

H⁴

4-FC₆H₄(13 → 16)

8.6

8.4

8.2

8.0

7.8

7.6

7.4 [ppm]

[a] Recorded in C₆D₆. [b] Recorded in CD₂Cl₂.

Figure 2. Segment of the ¹H NMR spectra (500 or 400 MHz, CD₂Cl₂) of N-

silylpyridinium ions 9b’⁺(top), their mixture with pyridine (5) and hydrosilane

7b (middle) as well as pure 5 (bottom).

Furthermore, we obtained single crystals of complex 15 suitable

for X-ray diffraction by slow evaporation from benzene/n-

pentane. The molecular structure of 15 (Figure 3) shows a

tetracoordinated Ru(II) center with a shortened Ru–P bond

compared to 12 (2.27 vs 2.34 Å).^[34]The Ru–S bond remains

almost unchanged, exhibiting a length of 2.38 Å in both 15 and

12.

The main conclusion emerging from these NMR measurements

is that N-silylpyridinium ions 9⁺rapidly exchange the pyridine

donor. As a result of this, the ¹H and ¹³C NMR spectra show

average chemical shifts of the pyridine unit with the major N-

silylpyridinium ion contributing most.

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exchange between the tethering thiolate ligands of 4⁺and 4⁺·py

(Figure 4).

Figure 3. Molecular structure of 15 with thermal ellipsoids at the 50%

probability level. H atoms except for H1 are omitted. Selected experimental

bond lengths (Å): Ru–H, 1.66(2); Ru–S, 2.3751(5); Ru–P, 2.2692(5).

Figure 4. Segment of the ¹H,¹H EXSY NMR spectrum (500/500 MHz, CD₂Cl₂,

_m= 600 ms) of 4⁺and 4⁺·py.

Mechanistic experiments

Before running experiments with the preformed intermediates 9⁺

and 3, we decided to collect data of the Ru–S-catalyzed

hydrosilylation of pyridine (5) under the catalytic setup. We

noticed that using excess hydrosilane leads to a faster reaction.

At short reaction times, the isolated yield of the 1,4-

dihydropyridine 6b is significantly higher when using 10 equiv of

7b compared to an equimolar amount (81% vs 31% for 5 → 6b

after 3 h). We further screened solvents such as CH₂Cl₂and

toluene but were not able to find any influence on regioselectivity,

even though the reaction proceeded much slower when any

solvent was used (see the Supporting Information for further

details). The reaction underwent a color change from blue green

to red when pyridine (5) was added to complex 4⁺, and this color

did not disappear throughout the reaction.^[35]We explain this

important observation by saturation of the vacant coordination

site in 4⁺by the pyridine donor (4⁺→ 4⁺·py). Lewis pair 4⁺·py

indeed forms quantitatively upon addition of 5 to 4⁺in CD₂Cl₂

(Scheme 2).^[36]

As expected, pyridine coordination to the Ru(II) center in 4⁺·py is

not detrimental to its catalytic activity, and 1,4-dihydropyridine

6b was seen in the NMR spectra when an excess of pyridine (5)

and hydrosilane 7b were added to 4⁺(Scheme 3A). We also

detected Ru(II) hydride 3 and N-silylpyridinium ion 9b⁺in the

reaction mixture, thereby supporting the assumption that both

are part of the catalytic cycle. Free catalyst 4⁺was never

observed in the NMR measurements, indicating that its

concentration is low in the presence of the pyridine substrate 5.

In another experiment (Scheme 3B), we combined Ru(II) hydride

3 with N-silylpyridinium ions 9b’⁺which, according to our working

hypothesis, was supposed to immediately and quantitatively

result in the formation of 1,4-dihydropyridines 6b’. However, we

observed the formation of 4⁺·py along with hydrosilane 7b and

silicon-containing byproducts but the expected dihydropyridine

resonance signals were not detected in the NMR spectra

(Scheme 3B). We therefore set out to screen for conditions that

would result in the quantitative conversion of 9b’⁺and 3 to 6b’. In

principle, both hydrosilane 7b and pyridine (5) are able to act as

Lewis-basic ligands to give coordinatively saturated Ru(II)

complexes 8b⁺and 4⁺·py, respectively. We anticipated that, to

avoid free 4⁺, coordination by either would allow for the

formation of 6b’. 1,4-Dihydropyridines 6b’ were indeed observed

when N-silylpyridinium ions 9b’⁺were added to a mixture of

Ru(II) hydride 3 and hydrosilane 7b (Scheme 3C). The amount

of 6b’ was, however, relatively small and increased only slightly

over time whereas 3 and 9b’⁺were coexisting in the presence of

hydrosilane 7b. Adduct 8b⁺was not detected but Lewis pair

4⁺·py did form; the reason for this is not understood. An

intriguing observation within experiment C is the dynamic

chemical exchange of the hydride ligand in Ru(II) complex 3 only

with the protons at C4 of 6b’ (see the Supporting Information for

Scheme 2. Formation of 4⁺·py[BAr^F₄]^–.

That coordination is reversible and free pyridine (5) is in dynamic

chemical exchange with the coordinating ligand (see the

an H,¹H EXSY spectrum). Also, a dynamic chemical exchange

Supporting Information for an H,¹H EXSY NMR spectrum). It is

of the three different aromatic protons of the pyridine ligand in

also possible to transfer the pyridine ligand between two cationic

9b’⁺with the protons in positions 2, 3, and

dihydropyridine 6b’ is detected (Figure 5).

4 of 1,4-

Ru(II) centers: When using an excess of Ru(II) complex 4⁺the

¹H,¹H EXSY NMR spectrum shows

a dynamic chemical

Apart from being direct evidence for the regioselectivity of the

hydride transfer,^[37]the EXSY spectrum additionally shows the

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reversibility of this particular step, a rare retro-hydrosilyla-

tion.^[20a,38]Full conversion of 3 and/or 9b’⁺to 6b’ is, however, not

achieved with excess hydrosilane 7b.

traces back to a slight excess of the hydride used in the reaction

(Scheme 3D).

From the above set of experiments, we conclude that

intermediates 3 and 9b’⁺participate in the catalytic cycle.

However, an equimolar combination of these reactants does not

furnish 1,4-dihydropyridine 6b’ as this would result in the

formation of coordinatively unsaturated Ru(II) complex 4⁺. We

intended to prove this by two more control experiments.

According to our findings, the formation of 6b only occurs with

excess pyridine (5) or hydrosilane 7b, and a 1:1:1 mixture of 4⁺

with those two substrates should not result in 1,4-

dihydropyridine formation. This behavior is indeed observed

(Scheme 4E). Apart from the starting materials, we were able to

identify pyridine adduct 4⁺·py and hydrosilane adduct 8b^+[23a]in

the reaction mixture next to N-silylpyridinium ion 9b⁺and Ru(II)

hydride 3. A similar result was obtained when adding 1,4-

dihydropyridine 6b to cationic Ru(II) complex 4⁺(Scheme 4F).

We performed this particular experiment to fulfill two main

purposes: (i) We intended to underline the conclusion drawn

from experiments B and E that the 1,4-dihydropyridine does not

exist next to the cationic Ru(II) catalyst. (ii) We searched for a

Ru(II) complex with the 1,4-dihydropyridine coordinated to the

Ru(II) center which could be another intermediate in the catalytic

cycle.^[15]

¹H

9b'⁺9b'⁺

6b'

9b'⁺

3 [ppm]

Figure 5. Segment of the ¹H,¹H EXSY NMR spectrum (500/500 MHz, CD₂Cl₂,

T_m= 600 ms) of mechanistic experiment C (see Scheme 3C).

Conversely, pyridine (5) turned out as the “favored” ligand for

the vacant site of cationic Ru(II) complex 4⁺. Combining Ru(II)

hydride 3 and 9b’⁺in the presence of 5 resulted in quantitative

consumption of the N-silylpyridinium ions to give 1,4-

dihydropyridine 6b’ concomitant with 4⁺·py. Remaining 3 likely

[Ru]

HSiEtMe₂

[Ru]

SAr

Et₃P

SiEtMe₂

9b⁺

SiEtMe₂

⁺(~1)

(~4)

⁺·py

(~4)

[Ru]

HSiEtMe₂

Et₃P

(~2)

⁺(~1)

⁺·py

chemical

exchange

[Ru]

HSiEtMe₂

SAr

Et₃P

⁺(~1)

(~1)

⁺·py

chemical

exchange

[Ru]

H–Si

SAr

Et₃P

(~1)

⁺(~1)

(~2.5)

⁺·py

Scheme 3. Mechanistic experiments A–D. Experiments were conducted by combining the reactants at room temperature in an NMR tube in CD₂Cl₂, directly

followed by NMR spectroscopic measurements (~10 min). Ratios are indicated in brackets. The formation of Et₃POSiEtMe₂⁺was observed in experiments A and

C.^[23a]Silicon-containing byproducts were obtained in experiment B. Compounds shown in grey were not observed.

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Scheme 4. Mechanistic experiments E and F. Experiments were conducted by combining the reactants at room temperature in an NMR tube in CD₂Cl₂, directly

+[23a]

followed by NMR spectroscopic measurements (~10 min). Ratios are indicated in brackets. The formation of Et₃POSiEtMe₂

observed^[33]in both E and F.

and decomposition of 3 was

We found the first assumption to be valid. The formation of

pyridine adduct 4⁺·py along with hydrosilane 7b was observed,

and N-silylpyridinium ion 9b⁺and Ru(II) hydride 3 were detected.

In turn, 1,4-dihydropyridine 6b coordinated to the cationic Ru(II)

center of 4⁺in η²fashion was not found. Interestingly, a chemical

exchange of the Ru(II) complexes 8b⁺and 4⁺·py was observed,

which exposes the ease of ligand substitution in these adduct

(Figure 6). The ¹H,¹H EXSY NMR spectrum also shows chemical

exchange of those adducts with Ru(II) hydride 3.

experiment shows the usual order of substrate addition (the

hydrosilane is added to the preformed pyridine adduct of the

catalyst): No formation of 1,4-dihydropyridine 6a was seen

(Scheme 5, top). Conversely, when the hydrosilane adduct

8a^+23ais chosen as the starting point, pyridine abstracts the

cationic silicon moiety (8a⁺→ 3, Scheme 5, bottom) to yield the

N-silylpyridinium ion (5 → 9a⁺). Under this setup, the 1,4-

dihydropyridine 6a is produced as verified by H and ²⁹Si NMR

spectroscopy. We assume that the same explanation is valid for

Ph₃SiH (7e).

4⁺·py

¹H

4⁺·py

8b⁺

¹H

4⁺·py

8b⁺

6.0

5.6

5.2 [ppm]

Figure 6. Segment of the ¹H,¹H EXSY NMR spectrum (500/500 MHz, CD₂Cl₂,

_m= 600 ms) of mechanistic experiment F (see Scheme 4F).

Scheme 5. The role of pyridine and hydrosilane adducts.

The results of

E and F reveal that the Ru‒S-catalyzed

hydrosilylation of pyridine (5) is reversible^[38]with the equilibrium

Finally, we contemplated to clarify the question of a two-step

radical or one-step ionic hydride transfer mechanism for the

reaction of 3 with 9⁺. We indeed observed signals in the EPR

spectra when we measured the reaction of pyridine (5) with

hydrosilane 7b and catalytic amounts of 4⁺(not shown).

However, despite our efforts to identify the obtained species, we

were not able to assign the observed signals to the

corresponding pyridyl radical or the transition metal radical

cation emerging from Ru(II) complex 3 through electron transfer

on the side of the starting materials or the intermediates, that is

the Ru(II) hydride

and N-silylpyridinium ion 9⁺. The

experiments also provide an explanation for the inability of

hydrosilanes Et₃SiH (7a) and Ph₃SiH (7e) to form the

corresponding 1,4-dihydropyridines from 5. Apparently, these

hydrosilanes are not able to displace the pyridine ligand in

adducts 4⁺·py to form adducts of type 8⁺. To further prove this,

we performed the hydrosilylation of 5 with Et₃SiH (7a) and

substoichiometric amounts of 4⁺in two different ways. The first

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onto 9b⁺. The difficulty was the presence of a radical species

already in a sample of complex 4⁺. Even though being only a

minor impurity in what appeared in NMR spectra as clean

catalyst 4⁺, this species could have an impact on other radicals

formed during catalysis. Furthermore, quantum-chemical

calculations did not help to resolve this issue.

fully exclude temporary η²-coordination of 6 to the Ru(II)

center.^[15]We further think that the replacement of any ligand at

the Ru(II) center is dissociative through short-lived,

coordinatively unsaturated Ru(II) complex 4⁺. Besides, we

cannot clearly answer the question of a radical two-step or ionic

one-step hydride transfer. Even though in accordance with

literature precedent and supported with initial EPR

measurements, assignment of the resulting radical species

remains an open task.

Experimental Section

General Procedure for Synthesis of N-Silylpyridinium Ions 9⁺(GP 1)

step 1

[Ru]

SAr

[Ru]

SAr

In a glove box, [Ph₃C]⁺[B(C₆F₅)₄]^–(46 mg, 0.050 mmol, 1.0 equiv) is

placed in GLC vial and covered with C₆D₆(~0.1 mL). The corresponding

hydrosilane 7 (1.4 equiv) is added as a solution in C₆D₆(~0.1 mL). The

resulting mixture is stirred for 5 min at room temperature. The phases are

separated, and the lower phase is washed with C₆D₆(~0.1 mL). Pyridine

(5, 4.7 mg, 0.059 mmol, 1.2 equiv) is added as a solution in C₆D₆(~0.1

mL). The emulsion is stirred for 5 min at room temperature. The phases

are separated and the lower phase is washed with C₆D₆(2 0.1 mL).

The N-silylpyridinium ions 9⁺are obtained as colorless to yellowish

clathrates.

Et₃P

4⁺·py

Et₃P

4⁺

H–Si

step 4a

step 2

[Ru]

SAr

Et₃P

[Ru]

SAr

Et₃P

step 3

8⁺

General Procedure for the Synthesis of Dihydropyridine 6b (GP 2)

9⁺

a glove box, pyridine (5, 20 mg, 0.25 mmol, 1.0 equiv) and

ethyldimethylsilane (7b, 1.0–10 equiv) and if indicated the solvent

(0.125M) are placed in a GLC vial and the cationic Ru(II) complex

4⁺[BAr^F₄]^–(3.5 mg, 2.5 µmol, 1.0 mol%) is added. The resulting mixture is

stirred at room temperature for the indicated time. The reaction mixture is

diluted and filtered through a small pad of Celite^®layered with a small

amount of silica gel using a solution of cyclohexane, tert-butyl methyl

ether and triethylamine (80:5:1, 0.5 mL + 5 mL). Evaporation of the

solvents under reduced pressure and drying in high vacuum affords

dihydropyridine 6b as a yellowish, viscous oil.

H–Si

step 4b

Scheme 6. Refined catalytic cycle.

Conclusions

Preparation and Analytical Data for Ru(II) Hydride 3

The mechanistic experiments described herein lead us to

propose a refined catalytic cycle (Scheme 6). Starting from 4⁺,

coordinatively saturated complex 4⁺·py is formed by reversible

coordination of pyridine (5) to the unsaturated Ru(II) center (step

1). The equilibrium is on the side of 4⁺·py, and 4⁺·py is the

resting state from which 5 eventually dissociates to release the

active catalyst 4⁺. The unsaturated complex then reversibly

activates hydrosilanes 7 to furnish adduct 8⁺(step 2).^[23a]The

ability of replacing the pyridine ligand in 4⁺·py depends on the

hydrosilane employed: For Et₃SiH (7a) and Ph₃SiH (7e), the

corresponding adducts 8⁺do not form from 4⁺·py.

In a glove box, Ru(II) chloride 10 (64 mg, 0.11 mmol, 1.0 equiv) and

Na₂B₁₂Cl₁₂(46 mg, 77 µmol, 0.75 equiv) were placed in a 5 mL glass vial

and benzene (1 mL) was added. Ethyldimethylsilane (7b, 28 mg, 0.32

mmol, 3.0 equiv) in benzene (1 mL) was added and the resulting

suspension was stirred for 30 min at room temperature, in which the

formation of a greenish solid and H₂evolution was observed.^[40]KOMe

(24 mg, 0.34 mmol, 3.1 equiv) was added in one portion, followed by

additional hydrosilane 7b (0.15 g, 1.7 mmol, 16 equiv). The suspension

was stirred at room temperature for 1 h, in which the green precipitate

disappeared. The vial was removed from the glove box and connected to

a Schlenk line under an atmosphere of nitrogen. All volatiles were

removed in high vacuum. The vial was transferred back to the glove box

and benzene (1 mL) was added. The resulting suspension was filtered

via a PTFE syringe filter into a Schlenk flask and washed with benzene

(ca. 5 mL) until the washing solution was no longer colored. The flask

was removed from the glove box and connected to a Schlenk line under

an atmosphere of nitrogen. The mixture was frozen with an ice/water

bath and the solvent was sublimed into a cooling trap. The flask was then

connected to a turbomolecular pump and silicon-containing byproducts

were removed in high vacuum (3 10^–5mbar) at 70 °C for 2 h. Ru(II)

hydride complex 3 (36 mg, 59%) was obtained as an orange powder.

For hydrosilanes 7b–7d, transfer of the silicon cation from 8⁺to

the Lewis-basic pyridine nitrogen atom (step 3) results in the

formation of N-silylpyridinium ions 9b⁺–9d⁺concomitant with

formation of Ru(II) hydride 3. These intermediates then produce

the 1,4-dihydropyridine 6 but only in the presence of donor

molecules, e.g., pyridine (5, step 4a) or less pronounced

hydrosilanes 7 (step 4b). Even this step is reversible and is as

such a rare example of a retro-hydrosilylation.^[20a,38,39]We cannot

10.1002/chem.201705899

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Hydride 3 turned out to decompose slowly in dichloromethane (see the

Supporting Information for NMR spectra). ¹H NMR (500 MHz, CD₂Cl₂): δ

= –9.32 (d, J_H,P= 53.3 Hz, 1H), 0.89–0.95 (m, 9H), 1.45–1.62 (m, 6H),

a Schlenk line under an atmosphere of nitrogen. All volatiles were

removed in high vacuum. The vial was transferred into the glove box and

benzene (1 mL) was added. The resulting suspension was filtered via a

PTFE syringe filter into a Schlenk flask and washed with benzene (ca. 5

mL) until the washing solution was no longer colored. The flask was

removed from the glove box and connected to a Schlenk line under an

atmosphere of nitrogen. The mixture was frozen with an ice/water bath

and the solvent was sublimed into a cooling trap. The flask was then

connected to a turbomolecular pump and silicon-containing byproducts

were removed in high vacuum (3 10^–5mbar) at 70 °C for 1.5 h. Ru(II)

hydride complex 15 (34 mg, 86%) was obtained as an orange solid.

Single crystals suitable for X-ray analysis were obtained by slow

evaporation from n-pentane/C₆D₆(ca. 1:1). ¹H NMR (500 MHz, C₆D₆): δ

1.88 (d, J_H,P= 2.3 Hz, 3H), 1.94 (s, 3H), 1.96 (s, 3H), 1.98 (s, 3H), 1.99 (s,

3H), 2.31 (s, 3H), 5.20 (d, J_H,P= 3.5 Hz, 1H), 5.69 (s, 1H), 6.65 (dd, ³J_H,H

= 7.3 Hz, J_H,H= 1.5 Hz, 1H), 6.88 (s, 2H), 6.89 (t, J_H,H= 7.3 Hz, 1H),

7.07 ppm (dd, ³J_H,H= 7.3 Hz, ³J_H,H= 1.5 Hz, 1H); ¹³C{¹H} NMR (126 MHz,

CD₂Cl₂): δ = 8.0 (s, 3C), 18.5 (s), 18.9 (s), 20.3 (s), 20.3 (s), 20.3 (s),

20.9 (d, J_C,P= 28.3 Hz, 3C), 21.2 (s), 85.6 (s), 87.7 (d, J_C,P= 9.0 Hz),

88.4 (d, J_C,P= 3.5 Hz), 90.5 (s), 92.6 (s), 116.9 (d, J_C,P= 3.7 Hz), 120.1

(s), 126.7 (s), 127.9 (s), 128.0 (s), 128.0 (s), 136.0 (s), 136.7 (s), 136.7

(s), 137.9 (s), 139.6 (s), 142.4 (s), 162.2 ppm (s); ³¹P{¹H} NMR (202 MHz,

CD₂Cl₂): δ = 41.7 ppm; HRMS (APCI) calculated for C₃₀H₄₁PRuS⁺[M⁺]:

566.1705; found: 566.1706.

= –8.03 (d, ²J_H,P= 53.1 Hz, 1H), 1.08 (s, 3H), 1.76 (s, 3H), 2.05 (d, J_H,P

2.4 Hz, 3H), 2.17 (s, 3H), 2.25 (s, 3H), 2.36 (s, 3H), 4.40 (d, J_H,P= 3.1 Hz,

1H), 5.46 (s, 1H), 6.71 (s_br, 1H), 6.88–6.96 (m, 11H), 6.98 (t, J_H,H= 7.3

Preparation and Analytical Data for Ru(II) Hydride 14

Hz, 1H), 7.02 (dd, ³J_H,H= 7.3 Hz, ⁴J_H,H= 1.9 Hz, 1H), 7.63–7.67 ppm (m_c,

6H); ¹³C{¹H} NMR (126 MHz, C₆D₆): δ = 17.8 (s), 18.4 (s), 19.3 (s), 20.9

(s, 2C), 21.3 (s), 88.5 (s), 89.2 (d, J_C,P= 9.5 Hz), 89.6 (d, J_C,P= 3.9 Hz),

94.1 (s), 95.0 (s), 116.9 (d, J_C,P= 3.9 Hz), 120.6 (s), 126.4 (s), 127.7* (s,

In a glove box, Ru(II) chloride 11 (64 mg, 0.10 mmol, 1.0 equiv) and

Na₂B₁₂Cl₁₂(45 mg, 75 µmol, 0.75 equiv) were placed in a 5 mL glass vial

and benzene (1 mL) was added. Ethyldimethylsilane (7b, 27 mg, 0.31

mmol, 3.1 equiv) in benzene (1 mL) was added and the resulting

suspension was stirred for 30 min at room temperature, in which the

formation of a greenish solid and H₂evolution was observed.^[40]KOMe

(21 mg, 0.30 mmol, 3.0 equiv) was added in one portion, followed by

additional hydrosilane 7b (54 mg, 0.61 mmol, 6.1 equiv) in benzene (0.5

mL). The suspension was stirred at room temperature for 1.5 h, in which

the green precipitate disappeared. The vial was removed from the glove

box and connected to a Schlenk line under an atmosphere of nitrogen.

All volatiles were removed in high vacuum. The vial was transferred into

the glove box and benzene (1 mL) was added. The resulting suspension

was filtered via a PTFE syringe filter into a Schlenk flask and washed

with benzene (ca. 5 mL) until the washing solution was no longer colored.

The flask was removed from the glove box and connected to a Schlenk

line under an atmosphere of nitrogen. The mixture was frozen with an

ice/water bath and the solvent was sublimed into a cooling trap. The flask

was then connected to a turbomolecular pump and silicon-containing

byproducts were removed in high vacuum (3 10^–5mbar) at 70 °C for 2

h. Ru(II) hydride complex 14 (35 mg, 58%) was obtained as an orange

3C), 128.0* (s), 128.5* (s, 2C), 129.3 (d, J_C,P= 2.2 Hz, 6C), 134.1 (d,

²J_C,P= 11.3 Hz, 6C), 135.5 (s), 136.1 (s), 136.6 (s), 137.6 (s), 137.9 (d,

¹J_C,P= 43.9 Hz, 3C), 139.0 (s), 143.6 (s), 163.1 ppm (d, J_C,P= 3.4 Hz);

³¹P{¹H} NMR (203 MHz, C₆D₆): δ = 55.4 ppm; HRMS (APCI) calculated

for C₄₂H₄₁PRuS⁺[M⁺]: 710.1705; found: 710.1705. The crystallographic

data are available online in the CCDC database under number CCDC

1580640.

Preparation and Analytical Data for Ru(II) Hydride 16

In a glove box, Ru(II) chloride 13 (40 mg, 50 µmol, 1.0 equiv) and

Na₂B₁₂Cl₁₂(23 mg, 37 µmol, 0.74 equiv) were placed in a 5 mL glass vial

and benzene (0.5 mL) was added. Ethyldimethylsilane (7b, 13 mg, 0.15

mmol, 3.0 equiv) in benzene (0.5 mL) was added and the resulting

suspension was stirred for 30 min at room temperature, in which the

formation of a greenish solid and H₂evolution was observed.^[40]KOMe

(11 mg, 0.16 mmol, 3.1 equiv) was added in one portion, followed by

additional hydrosilane 7b (27 mg, 0.31 mmol, 6.2 equiv). The suspension

was stirred at room temperature for 3 h, in which the green precipitate