POCN Ni(ii) pincer complexes: Synthesis, characterization and evaluation of catalytic hydrosilylation and hydroboration activities

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POCN Ni(II) pincer complexes: synthesis,

characterization and evaluation of catalytic

Cite this: DOI: 10.1039/c8dt04854a

hydrosilylation and hydroboration activities†

b

Kristina A. Gudun,^aMedet Segizbayev,^aAssyl Adamov,^aPhilipp N. Plessow,

Konstantin A. Lyssenko, ^c,dMannix P. Balanay ^aand Andrey Y. Khalimon

*

a,e

A series of iminophosphinite POCN pincer Ni(II) complexes, (POCN)NiMe and (POCN)NiL_n(BX₄) (L =

CH₃CN, n = 0, 1; X = F, Ph, C₆F₅), have been developed and subjected to catalytic hydrosilylation of

alkenes, aldehydes and ketones and hydroboration of carbonyl compounds. The stoichiometric reactivity

of (POCN)NiMe and (POCN)Ni(BF₄) with PhSiH₃and HBPin suggests that catalytic reactions proceed via

the hydride intermediate (POCN)NiH. With regard to reactions with HBPin, eﬃcient and mild hydrobora-

tion of a variety of carbonyl compounds, including highly chemoselective hydroboration of benzaldehyde

in the presence of other common potent reductive functional groups, such as alkenes, alkynes, esters,

amides, nitriles, nitro compounds and even ketones, and the ﬁrst example of base metal catalyzed hydro-

boration of amides, including mild direct hydroborative reduction of primary and secondary amides to

borylated amines were demonstrated for (POCN)NiMe.

Received 8th December 2018,

Accepted 27th December 2018

DOI: 10.1039/c8dt04854a

rsc.li/dalton

lytic systems employing Earth abundant first-row base tran-

sition metals (such as Fe, Co, Ni) present an attractive alterna-

Introduction

Reduction of unsaturated C–C and C–E (E = heteroatom) tive to conventional precious metal catalysts.^5,6However, due

bonds with hydrosilanes and hydroboranes is of vast impor- to their small covalent radii and, as a consequence, small

tance for the modern synthetic organic chemistry, relevant to energy diﬀerence between d orbitals, the first-row transition

the production of both commodity and fine chemicals.^1,2The metal complexes often exhibit low thermal stability and/or

conventional catalytic systems developed for such transform- tend to induce one-electron transformations (or form para-

ation employ heavy late transition metals, such as Ru, Pt, Rh, magnetic species),⁷which often hinder their application in

Ir, etc., which despite their high catalytic activity are limited in catalytic transformations originally developed for two-electron

supply and expensive.^1–3Therefore, the development of more processes with the second and third row transition metals.^1–8

economical surrogates to the precious metal catalysts for

Practical approaches for designing catalytic systems with

hydrosilylation and hydroboration reactions has been an inten- first-row transition metals and to induce their precious metal-

sive area of research in the last decade.^4–6In this regard, cata- like reactivity include the use of polydentate ligands that bind

tightly to the metal as well as strong field ligands, such as

hydride, alkyl, aryl, CO, etc.^5c,7Complexes with such ligands

usually possess the necessary stability due to a strong chelat-

ing eﬀect and favor the formation of low-spin diamagnetic

species, the reactivity of which resembles that of heavy late

transition metal compounds.^7,8

One of the most popular ligand scaﬀolds employed to

promote base transition metal-catalyzed reactions are pincer

type ligands EXE (E = P, N, etc.; X = C, N, etc.).⁹The rigid

nature and easily tuneable steric and electronic properties of

pincer ligands provide a unique balance of stability vs. reactiv-

ity for transition metal complexes, often enhancing the selecti-

vity of catalytic reactions.⁹Although a series of base metal

pincer catalysts have been developed for hydrosilylation and

hydroboration of alkenes, alkynes, aldehydes, and ketones,^5,6

^aDepartment of Chemistry, School of Science and Technology, Nazarbayev University,

53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan.

E-mail: andrey.khalimon@nu.edu.kz

^bInstitute of Catalysis Research and Technology (IKFT),

Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany

^cDepartment of Chemistry, M. V. Lomonosov Moscow State University,

Leninskie Gory 1-3, Moscow 119991, Russia

^dPlekhanov Russian University of Economics, Stremyanny per. 36, Moscow 117997,

Russia

^eThe Environment & Resource Eﬃciency Cluster, Nazarbayev University, Astana,

53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan

†Electronic supplementary information (ESI) available: Complete experimental,

spectroscopic, and computational details and crystallographic data for 1-Ar,

2-Ar, 3-Ar and 6-Ar. CCDC 1883579–1883582. For ESI and crystallographic data

in CIF or other electronic format see DOI: 10.1039/c8dt04854a

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the vast majority of the reported systems concern Fe and Co

complexes,^5,6whereas the analogous chemistry of Ni pincer

compounds has been developed to a lesser extent^5c,f,g,o,6kand

the examples of such Ni-based systems for regio- and chemo-

selective hydrosilylation and hydroboration reactions are

scarce.^5g,oThe applications of Ni pincer complexes in the

hydroboration of unsaturated organic molecules are especially

undeveloped and, to the best of our knowledge, with an excep-

tion of CO₂hydroboration reactions,^5c,6h,kno examples of such

systems have been reported.¹⁰

Given the fact that nickel is the fourth most abundant tran-

sition metal on Earth after Fe, Ti and Zr¹¹and driven by our

interest in developing eﬃcient base metal catalytic systems for

hydrosilylation and hydroboration of unsaturated organic

molecules, we herein report the preparation and catalytic

activity studies of a series of novel alkyl and borate nickel(^II)

complexes with iminophosphinite POCN pincer ligands

Fig. 1 Molecular structure of 1-Ar (thermal ellipsoids are drawn at the

derived from 3-hydroxybenzaldehyde (Scheme 1). Such systems 50% probability level). Hydrogen atoms are omitted for clarity. Selected

bond distances (Å): Ni1–C1, 1.857(3); Ni1–N1, 1.977(2); Ni1–P1, 2.1148(8);

Ni1–Br1, 2.3255(5); P1–O1, 1.667(2); O1–C2, 1.395(3); N1–C13, 1.296(3).

Selected bond angles (°): C1–Ni1–Br1, 173.81(8); N1–Ni1–P1, 163.57(7);

C1–Ni1–N1, 82.80(11); C1–Ni1–P1, 80.78(9); N1–Ni1–Br1, 99.70(7); P1–

serve as pre-catalysts for a variety of hydrosilane and hydrobor-

ane addition reactions, including chemoselective hydrobora-

tion of aldehydes and the first example of base metal catalyzed

hydroborative reduction of amides to amines.

Ni1–Br1, 96.69(2).

Results and discussion

Synthesis and stoichiometric reactivity of POCN Ni(II)

complexes

First, the iminophosphinite (POCN)NiBr complexes 1 were pre-

pared using the synthetic strategy developed earlier by

Zargarian et al. for analogous compounds.¹²Complexes 1 were

fully characterized by multinuclear NMR and IR spectroscopy

and X-ray diﬀraction analysis for 1-Ar. The molecular structure

of 1-Ar is shown in Fig. 1 and its geometry is consistent with

the corresponding parameters for

(

a chloride derivative

Scheme 2 Preparation of ^i-PrPOCN^Arnickel borates 2-Ar–5-Ar.

^i-PrPOCN^Ar)NiCl, previously reported by Xia et al.¹³These

air, moisture and thermally stable bromides¹⁴can serve as

the precursors for a large variety of iminophosphinite POCN

pincer derivatives, including cationic (POCN)Ni(^II) complexes

and the borohydride and alkyl substituted derivatives. Thus,

the reaction of 1-Ar with LiBH₄in toluene instantaneously

aﬀords (^i-PrPOCN^Ar)Ni(η²-BH₄) (2-Ar; Scheme 2), isolated as a

barberry red solid in 93% yield. In contrast, the reported reac-

tions of the related (PCP)Ni halides with NaBH₄lead to the for-

mation of the corresponding hydride derivatives (PCP)NiH,¹⁵

whereas the borohydride intermediates, (PCP)Ni(BH₄), could

not be isolated and were only observed by NMR spectrosco-

py.^15bTo the best of our knowledge, the only reported isolable

borohydride pincer complex, (^t-BuPCP)Ni(η²-BH₄), prepared

from (^t-BuPCP)NiBr with an excess of LiBH₄at ambient tem-

perature,¹⁶was found to be thermally unstable and decompo-

sition to (^t-BuPCP)NiH was observed in THF at 50 °C. 2-Ar was

fully characterized by multinuclear NMR and IR spectroscopy,

and X-ray diﬀraction analysis. The ³¹P{¹H}-NMR spectrum of

2-Ar shows a singlet at δ_P208.2 ppm, characteristic of metallated

iminophosphinite POCN pincers,^12,13coupled in the ¹H–³¹P

HSQC NMR to non-equivalent ¹H resonances of the methyl

substituents of the PⁱPr₂groups at δ_H1.07 (dd, J = 6.9 and 14.8 Hz)

−

and 1.24 (dd, J = 7.2 and 18 Hz). The resonances for BH₄

1

appear in the H-NMR spectrum of 2-Ar as two broad signals

at δ_H−0.72 ppm (2H) and −0.95 ppm (2H), which correlate

with the resonance at δ_B−30.8 ppm (br s) in the ¹¹B{¹H}-NMR

−

spectrum. The presence of BH₄in 2-Ar is also supported by

the IR spectroscopy, which revealed several B–H stretches

Scheme 1 Synthesis of iminophosphinite (POCN)NiBr complexes 1.

between 1902 and 2284 cm^{−1 16}

. The molecular structure of

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of 170.31(8)° is almost identical to

(170.50(6)°).¹⁶

(

^t-BuPCP)Ni(η²-BH₄)

An analogous reaction of 1-Ar with AgBF₄in toluene gives

the tetrafluoroborate complex

^i-PrPOCN^Ar)Ni(BF₄) (3-Ar,

(

Scheme 2) as a yellow powdery compound, characterized spec-

troscopically (multinuclear NMR and IR) and by X-ray analysis

(Fig. 2B). The ¹H- and ³¹P-NMR features of 3-Ar are very similar

to the corresponding spectra of 1-Ar, implying that both

structures are quite similar. On the other hand, the tetrafluoro-

borate ligand of 3-Ar gives rise to the broad ¹¹B resonance at

δ_B−1.4 ppm and an upfield, compared to HBF₄·Et₂O

(δ_F−153 ppm in C₆D₆), and ¹⁹F resonance at δ_F−173.8 ppm in

the ¹¹B{¹H}- and ¹⁹F{¹H}-NMR spectra, respectively, suggesting

the presence of Ni–F–BF₃interactions in the Ni–BF₄moiety,

stabilizing a 16-electron valence shell. Very similar coordi-

nation of the tetrafluoroborate ligand to the nickel centre was

described for the related PCP-type borates (^t-BuPC_sp2P)Ni(η¹-

BF₄) (δ_F−177.8 ppm in THF-d₈)^15band (^i-PrPC_sp3P)Ni(η¹-BF₄)

(δ_F−179 ppm in C₆D₆),¹⁷which showed significantly upfield

shifted ¹⁹F resonances in comparison with the closely related

−

[(PCP)NiL][BF₄] complexes with noncoordinating BF₄anions

(δ_F−145 to −156.8 ppm).^15b,17

The presence of Ni-(η¹-BF₄) interaction in 3-Ar was addition-

ally supported by X-ray diﬀraction analysis (Fig. 2B), which

revealed a square-planar coordination geometry around nickel

with three positions occupied by the ^i-PrPOCN^Arligand and the

fourth site is taken by the fluorine of the BF₄anion, resulting

in an η¹-monohapto structure with α(Ni1–F1–B1) = 129.57(12)°.

The Ni1–F1 distance of 1.9431(14) Å in 3-Ar is longer than the

Ni–F bond of the related structurally characterized (^t-BuPC_sp2P)

NiF (1.867(3) Å); however, is shorter than the Ni–F distance in

the

(

^t-BuPC_sp2P)Ni(η¹-BF₄) computed by DFT methods.^15b

η¹-Coordination of BF₄in 3-Ar results in a significant elonga-

tion of the B1–F1 distance (1.435(4) Å vs. ca. 1.382 Å as a mean

value for B–F(2–4)¹⁸). Similar observations have been reported

for the theoretical structural analysis of the charge-neutral

species (^t-BuPC_sp2P)Ni(η¹-BF₄).^15b

Fig. 2 Molecular structures of complexes 2-Ar (A) and 3-Ar (B; the

other positions of the disordered BF₄group are omitted for clarity).

Thermal ellipsoids are drawn at the 50% probability level. Hydrogen

atoms, except for the BH₄⁻ligand, are omitted for clarity. Selected bond

distances (Å): 2-Ar: Ni1–C1, 1.8648(14); Ni1–N1, 1.9755(13); Ni1–P1,

2.1199(4); Ni1–B1, 2.2009(18); Ni1–H1b, 1.81(2); Ni1–H2b, 1.69(2); P1–

O1, 1.6588(11); O1–C2, 1.3933(18); N1–C13, 1.2973(19); 3-Ar: Ni1–C1,

1.842(2); Ni1–N1, 1.9719(19); Ni1–P1, 2.1492(6); Ni1–F1, 1.9431(14); P1–

O1, 1.6550(18); O1–C2, 1.393(3); B1–F1, 1.435(4). Selected bond angles

(°): 2-Ar: C1–Ni1–B1, 170.31(8); C1–Ni1–N1, 82.39(6); C1–Ni1–P1,

80.10(5); N1–Ni1–P1, 162.11(4); N1–Ni1–B1, 99.38(6); P1–Ni1–B1,

98.51(5); 3-Ar: C1–Ni1–F1, 175.79(8); C1–Ni1–N1, 83.13(9); C1–Ni1–P1,

81.01(7); N1–Ni1–P1, 163.85(4); N1–Ni1–F1, 93.41(7); P1–Ni1–F1,

102.30(5); Ni1–F1–B1, 129.57 (12).

In contrast to 3-Ar, tetraphenylborate and tetrakis(penta-

fluorophenyl)borate (BAF) derivatives can be prepared only as

charge-separated adducts [(^i-PrPOCN^Ar)Ni(CH₃CN)][X] (X

=

BPh₄, BAF). Thus, treatment of 1-Ar with NaBPh₄and

CH₃CN (ca. 20 equiv.) in toluene aﬀords [(^i-PrPOCN^Ar)Ni

(CH₃CN)][BPh₄] (4-Ar; Scheme 2), which was isolated in 83%

yield as a yellow foamy material and characterized by the ³¹P

resonance at δ_P205.5 ppm, slightly downfield of the corres-

ponding signals displayed by 1-Ar (δ_P203.1 ppm) and 3-Ar

(δ_P199.5 ppm). A similar shift of the ³¹P resonances has been

2-Ar is depicted in Fig. 2A, showing the η²-coordination mode of reported for PCP pincer complexes upon going from

the borohydride group. The complex adopts a slightly distorted

(

^i-PrPC_sp3P)NiBr (δ_P67.4 ppm) to

(

^i-PrPC_sp3P)Ni(η¹-BF₄)

square-planar geometry with the C₆H₃moiety of the pincer (δ_P63.5 ppm) and [(^i-PrPC_sp3P)Ni(CH₃CN)][BF₄] (δ_P76.7 ppm).¹⁷

−

ligand being almost coplanar with the N1–Ni1–P1 plane and BPh₄of 4-Ar gives rise to a signal at δ_B−5.8 ppm in the

with the deviation of B1 by 0.36 Å from this plane (Fig. 2A). ¹¹B{¹H}-NMR spectrum and three resonances for the phenyl

Both Ni1–C1 = 1.8648(14) Å and Ni1–B1 = 2.2009(8) Å distances groups at δ_H7.08 ppm (br t, ³J_H–H= 7.2 Hz, 4H, p-H), 7.27 ppm

in 2-Ar are slightly shorter than the corresponding bonds of (t, ³J_H–H= 7.4 Hz, 8H, m-H) and 8.04 ppm (br s, 8H, o-H) in the

the related (^t-BuPCP)Ni(η²-BH₄) complex (Ni–C: 1.9410(13) Å ¹H-NMR spectrum. The presence of coordinated acetonitrile in

and Ni–B: 2.2470 (14) Å), whereas the C1–Ni1–B1 angle 4-Ar is evident from the observation of a singlet at δ_H

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−0.12 ppm and two signals at δ_C−0.6 and 123.4 ppm in the

¹H- and ¹³C{¹H}-NMR spectra, respectively.

All attempts to obtain the BAF complex [(^i-PrPOCN^Ar)Ni

(CH₃CN)][BAF] (5-Ar) directly from the bromide 1-Ar by reac-

tions with KB(C₆F₅)₄and LiB(C₆F₅)₄·Et₂O in either PhMe or

CH₂Cl₂in the presence of CH₃CN were unsuccessful. However,

5-Ar can be conveniently prepared via ion exchange reactions

from the tetrafluoroborate and/or tetraphenylborate com-

plexes, 3-Ar and 4-Ar, with LiB(C₆F₅)₄·Et₂O in CH₂Cl₂in the

presence of CH₃CN (Scheme 2). Similarly to 4-Ar, NMR analysis

of the BAF derivative 5-Ar in CD₂Cl₂revealed a downfield ³¹P

resonance at δ_P203.9 ppm as well as a new ¹H signal at δ_H

1.68 ppm and two ¹³C signals at δ_C2.18 and 126.6 ppm for the

coordinated CH₃CN. The ¹⁹F{¹H}-NMR spectrum of 5-Ar in

CD₂Cl₂shows three resonances at δ_F−133.1 ppm (br s),

−163.8 ppm (t, ³J_F–F= 20.3 Hz) and −167.6 ppm (br m), corres-

ponding to the ortho-, para- and meta-F of the BAF anion.

Substitution reaction of 1-Ar with MeLi allows for the prepa-

ration of the corresponding methyl derivative, (^i-PrPOCN^Ar)

NiMe (6-Ar) (Scheme 3), which was fully characterized by

multinuclear NMR spectroscopy and X-ray diﬀraction analysis

(Fig. 3). The ³¹P{¹H}-NMR spectrum of 6-Ar shows an upfield

shifted, compared to 1-Ar, ³¹P resonance for the POCN ligand

at δ_P199.2 ppm. The Ni-bound CH₃ligand gives rise to a

Fig. 3 The general view of one of the independent molecules of co-

crystals of 0.81 6-Ar and 0.19 (^i-PrPOCN^Ar)NiCl illustrating the superposi-

tion of Cl(1) and Me-groups. Atoms are shown by thermal ellipsoids

(50% probability level). Hydrogen atoms are omitted for clarity. Selected

bond distances (Å) and bond angles (°) in one of the independent

molecules: Ni1–C1, 1.869(5); Ni1–N1, 1.980(4); Ni1–P1, 2.0746(14); Ni1–

Cl1, 2.210(5); Ni1–C1m, 1.959(4); C1–Ni1–C1m, 179.3(3); N1–Ni1–P1,

162.82(13); C1–Ni1–N1, 81.59(19); C1–Ni1–P1, 81.39(16); N1–Ni1–Cl1,

98.7(4); P1–Ni1–Cl1, 98.5(3); N1–Ni1–C1m, 98.7(2); P1–Ni1–C1m,

98.3(2).

1

doublet at δ_H−0.81 ppm in the H-NMR spectrum, coupled to

3

the phosphorus of the POCN ligand with J_H–P= 3.6 Hz. Other

spectroscopic characteristics of 6-Ar are very similar to those of

the bromide complex 1-Ar.

In an attempt to prepare the hydride species (^i-PrPOCN^Ar)

Single crystals of 6-Ar suitable for X-ray diﬀraction analysis NiH (7-Ar), 1-Ar was treated with either LiBHEt₃in C₆D₆or

were obtained by slow vaporization of Et₂O solution into L-selectride (lithium tri-sec-butylborohydride) in toluene-d₈

hexanes at −28 °C.³³A careful analysis of the molecular struc- (Scheme 3). Within a few hours at room temperature, both

ture of one of these crystals revealed it to be a co-crystal of 6-Ar reactions lead to the colour change from orange to dark-

and the chloride complex (^i-PrPOCN^Ar)NiCl with the superposi- brown, accompanied by the formation of a mixture of un-

tion of a Cl(1) atom and the Me group with slightly diﬀerent identified decomposition products and a large amount of

occupancies 0.23, 0.77 and 0.15, 0.85 in two independent black precipitate (presumably metallic nickel). Monitoring the

molecules (Fig. 3). The Ni–Cl and Ni–C1m bond lengths reaction of 1-Ar with ^L-selectride by NMR spectroscopy revealed

should be discussed with caution due to the low occupancy of after 10 minutes at room temperature the formation of a small

chlorine and correlations between the positional parameters. amount (16% conversion of 1-Ar) of a new POCN NI(^II) species,

It is noteworthy, however, that the structure adopts square- showing a characteristic hydride signal at δ_H−9.72 ppm (d,

planar geometry and the overall structural parameters are very ²J_H–P= 54 Hz), coupled in the ¹H–³¹P HSQC NMR to the ³¹P

similar to the structure of the bromide 1-Ar (Fig. 1) and resonance at δ_P205.1 ppm (vs. 203.1 ppm for 1-Ar). Based on

the reported structure of the chloride derivative, the NMR features this species was tentatively assigned to the

(

^i-PrPOCN^Ar)NiCl (d(Ni–Cl) = 2.1865(9) Å; α(C_POCN–Ni–Cl) = hydride complex 7-Ar.¹⁹Further NMR analysis of the reaction

174.29(8)°; α(N–Ni–P) = 163.15(7)°).¹³

for 24 h at room temperature did not reveal accumulation of

7-Ar in the reaction mixture, instead, decomposition of 7-Ar

was observed, forming a large amount of black precipitate

(metallic Ni) and

a complex mixture of unidentified

compounds, along with NMR detectable H₂and the free POCN

ligand.²⁰

An analogous NMR scale reaction of 1-Ar with LiBHEt₃in

PhMe-d₈after 10 min at room temperature leads to the colour

change from orange to green and the formation of a solution

containing two POCN Ni(^II) hydride products, 7-Ar and another

undisclosed polynuclear hydride species 8-Ar with two non-

equivalent Ni-bound hydrides (app. 1 : 4 ratio by ¹H-NMR,

respectively; 74% conversion of 1-Ar by ³¹P{¹H}-NMR). In the

Scheme 3 Preparation of 6-Ar and NMR scale generation of 7-Ar.

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¹H-NMR spectrum, this new hydride complex exhibits two selective formation of a Ni(0) adduct (^i-PrPOC(H)N^Ar)Ni(COD)

diagnostic mutually coupled hydride resonances at δ_H(9-Ar) was observed at room temperature (eqn (1)).²⁹No further

−8.17 ppm (dt, J = 22 Hz, 101.5 Hz) and −15.0 ppm (dt, J = release of the cyclooctadiene (COD) ligand followed by the oxi-

22 Hz, 23.8 Hz), coupled in the ¹H–³¹P HSQC NMR to two dative addition of the C²–H bond of the pincer ligand to the Ni

³¹P-NMR signals at δ_P197.4 ppm and 208.2 ppm (s, not centre to give 7-Ar was observed even at elevated temperatures.

mutually coupled in the ³¹P{¹H}-NMR spectrum). Similar

Observation of 7-Ar and 8-Ar and a possibility of their

dinuclear nickel hydride complexes have been previously in situ generation had motivated us to study the catalytic

reported with bidentate PN and PP phosphine ligands, behaviour of complexes 1–6 as pre-catalysts in the reduction

[(PN^iPr3)Ni^II(μ₂-H)]₂and [(R₂PCH₂CH₂PR₂)Ni^I(μ₂-H)]₂(R = Pr, (hydrosilylation and hydroboration reactions) of unsaturated

^tBu),²²characterized by rather small J_H–Pvalues of 18–24 Hz. organic molecules.

Very similar J_H–Pvalues (see above) were observed for one of

21

i

the hydrides of 8-Ar, indicating its cis-position with respect to

two phosphine moieties of two POCN ligands.^21,23The second

ð1Þ

hydride, however, is most likely oriented trans- to one of the

phosphines, which is reflected in the large J_H–Pvalue of

101.5 Hz.²⁴Analogously to 7-Ar, complex 8-Ar was found to be

thermally unstable and all our attempts to isolate and fully

Catalytic hydrosilylation reactions

characterize 8-Ar were hampered by its decomposition and the

presence of 7-Ar and other unidentified impurities in the reac- To evaluate the catalytic activity of the prepared complexes, we

tion mixture.²⁵Similar lack of stability was reported for studied the hydrosilylation of alkenes with bromide and

[(PN^iPr3)Ni^II(μ₂-H)]₂and [(β-diketiminato)Ni^II(μ₂-H)]₂([ArNC methyl pre-catalysts. 6-Ar and the cationic borates 3-Ar–5-Ar

21

(CH₃)]₂CH–, Ar = 2,6-ⁱPr₂C₆H₃).²⁶The latter compound was iso- were also tested in the hydrosilylation of aldehydes and

lated and structurally characterized by Limberg et al.²⁶but was ketones. First, complexes 1-Ar and 1-Ph were subjected to the

found to be extremely sensitive to heat, and thermal decompo- hydrosilylation of alkenes (styrene and 1-octene) with PhSiH₃

sition to Ni(^I)(μ-PhMe) species via the release of H₂was (Table 1, entries 1–8). The pre-catalysts (5 mol%) were activated

observed in toluene. When [(β-diketiminato)Ni^II(μ₂-H)]₂was using either LiBHEt₃or MeLi (one equiv. to Ni).³⁰At room

heated in hexanes, the formation of a nickel mirror with an temperature, both systems showed negligible catalytic activity;

NMR detectable protonated ligand and unidentified paramag- however, at 60 °C, 28–40% conversion of styrene and 23–68%

netic species was observed,²⁶which is very similar to our conversion of 1-octene were observed in 20 h in C₆D₆. Notably,

observations for decomposition of the reaction mixture of 1-Ar despite low substrate conversions, hydrosilylation of styrene

and LiBHEt₃(or L-selectride) at room temperature (see above). with PhSiH₃is highly regioselective for both systems, produ-

Notably, the NMR scale reaction of 6-Ar with PhSiH₃at 65 °C cing exclusively the Markovnikov silane addition product,

in C₆D₆gives PhMeSiH₂in a mixture with the silane redistribu- PhCH(CH₃)SiH₂Ph. In contrast, the reaction with 1-octene

tion products, Ph₂SiH₂and SiH₄.²⁷No nickel hydrides or shows anti-Markovnikov regioselectivity to give (ⁿOct)SiH₂Ph in

nickel silyl species, akin to (^i-PrPOC(H)N^Ar)Ni(SiH₂Ph), were a mixture with 1-octene isomerization products (Table 1,

observed by NMR.²⁸

entries 2, 4, 6, and 8).³¹A decrease in the steric crowding

In another attempt to prepare the POCN Ni(II) hydride around nickel upon going from 1-Ar to 1-Ph results in slightly

species, ^i-PrPOC(H)N^Arwas reacted with Ni(COD)₂; however, the faster alkene hydrosilylation rates; however, substantially

Table 1 Hydrosilylation of alkenes with PhSiH₃using 1-Ar, 1-Ph, 6-Ar, and 9-Ar as pre-catalysts^a

Entry Pre-catalyst/additive Substrate

Time, h/temp., °C Conversion^b, % Products (ratio^c, %)

1

2

3

4

5

6

7

8

1-Ar/LiBHEt₃

1-Ph/LiBHEt₃

1-Ar/MeLi

PhCHvCH₂

1-Octene

PhCHvCH₂

1-Octene

PhCHvCH₂

1-Octene

PhCHvCH₂

1-Octene

PhCHvCH₂

PhCH₂CHvCH₂20 h/60 °C

1-Octene 20 h/60 °C

20 h/60 °C

33

23

40

34

28

34

33

68

30

>99

97

92

PhCH(Me)SiH₂Ph (33%),

(ⁿOct)SiH₂Ph (16%), 2-octene and 3-octene (7%)

PhCH(Me)SiH₂Ph (40%)

(ⁿOct)SiH₂Ph (17%), 2-octene and 3-octene (17%)

PhCH(Me)SiH₂Ph (28%)

(ⁿOct)SiH₂Ph (12%), 2-octene and 3-octene (22%)

PhCH(Me)SiH₂Ph (33%)

1-Ph/MeLi

(ⁿOct)SiH₂Ph (22%), 2-octene and 3-octene (45%)

PhCH(Me)SiH₂Ph (30%)

9

6-Ar/—

9-Ar/—

10^d

11

12

PhCH(Me)SiH₂Ph (>99%)

(PhCH₂CH₂CH₂)SiH₂Ph (62%), PhCHvCHCH₃(35%)

(ⁿOct)SiH₂Ph (50%), 2-octene and 3-octene (42%)

^a0.27 M, 5.0 mol% of [Ni], substrate : PhSiH₃= 1 : 1 unless stated otherwise, C₆D₆(solvent). For the catalytic activity of 1-Ar′/LiBHEt₃and 1-^tBu/

LiBHEt₃, see the ESI. ^bConversion of the organic substrate (determined by ¹H NMR). ^cDetermined by ¹H NMR. ^dControl experiment for Ni

(COD)₂-catalyzed (5.0 mol%) addition of PhSiH₃to styrene resulted in 58% conv. of styrene to PhCH(Me)SiH₂Ph (45%) and PhCH₂CH₂SiH₂Ph

(13%) in 20 h at 60 °C.

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larger amounts of the isomerization products were observed in route seems to arise from the unfavourable steric interactions

the 1-Ph-catalyzed hydrosilylation of 1-octene. Analogously to between the phenyl group of the incoming styrene molecule

i

the 1-Ar/MeLi-catalyzed reaction, 30% conversion of styrene and the Pr substituents in the P-arm of the POCN ligand.

with the regioselective Markovnikov addition of PhSiH₃was Although the formation of the secondary alkylnickel species C

observed in the 6-Ar-catalyzed hydrosilylation of PhCHvCH₂is preferred kinetically over the formation of E, its subsequent

(Table 1, entry 9). Based on the similarities in the hydrosilyl- reaction with PhSiH₃is unfavourable with a high barrier of

ation reactions catalysed by 1/LiBHEt₃, 1/MeLi and 6-Ar and 21.7 kcal mol⁻¹. In contrast, easier direct addition of PhSiH₃

the stoichiometric reactivity of 1-Ar with LiBHEt₃and 6-Ar with to the primary alkylnickel species E was found for the anti-

PhSiH₃, we suggest that the activation of both bromide and Markovnikov route (TS6 13.5 kcal mol⁻¹, Scheme 4), which we

methyl pre-catalysts leads to the formation of the nickel attribute to steric hindrance. The computational analysis of

hydride species 7-Ar.³²This is followed by a regioselectivity direct addition of PhSiH₃to C and E thus predicts the anti-

determining olefin migratory insertion into the Ni–H bond to Markovnikov silane addition pathway to be 5.1 kcal mol⁻¹

give either primary or secondary alkylnickel species, which more favourable than the Markovnikov reaction, in contradic-

upon further turnover with PhSiH₃results in anti-Markovnikov tion with the experimental observations. We therefore

and Markovnikov hydrosilylation products, respectively.²⁸additionally considered structures involving the dissociation

Compared to 1-Ar, large amounts of 1-octene isomerization of the N-arm of the POCN ligand. Dissociation of the N-arm is

products in hydrosilylation reactions with 1-Ph could be indeed feasible, when a η³-benzyl species is formed simul-

rationalized by the competing β-H elimination in the alkyl- taneously. This reduces the steric crowding at the Ni centre result-

nickel intermediate vs. silane addition due to the decreased ing in a significantly reduced overall barrier (15.4 kcal mol⁻¹) for

steric hindrance of the POCN ligand.

the silane addition in the Markovnikov pathway,³³slightly

To gain some insights into the observed Markovnikov regio- lower than the highest barrier of 16.6 kcal mol⁻¹for the

selectivity of the hydrosilylation of styrene, we carried out a formation of the primary alkylnickel species E in the anti-

density functional theory (DFT) study³³of the reaction of 7-Ar Markovnikov pathway (Scheme 4).

with PhCHvCH₂(Scheme 4). Gibbs free energies at 60 °C and

Switching from Ni(^II) pre-catalysts to the Ni(0) complex 9-Ar

a reference pressure of 1 bar were obtained within the rigid- results in significantly increased hydrosilylation rates for

rotator and harmonic oscillator approximations at the PBE0- styrene, 1-octene and allylbenzene (>92% conversions in 20 h

D3/def2-TZVPP//PBE-D3/dhf-SV(P) level of theory with at 60 °C; Table 1, entries 10–12) with the retention of the

TURBOMOLE, using the RI-approximation.^34,35First, no low Markovnikov regioselectivity for styrene and anti-Markovnikov

energy pathway was found for the insertion of styrene into the regioselectivity for aliphatic alkenes. It is not clear at this

Ni–H bond of 7-Ar, which involves the formation of an η³- point, whether an activation of the C_bridge–H bond of the

benzyl species, akin complex A (Scheme 4).^33,36The overall POCN ligand of 9-Ar to form the hydride species 7-Ar takes

barrier for the Markovnikov addition of styrene to 7-Ar to give place during the catalytic hydrosilylation reactions or the cata-

the secondary alkylnickel species C is somewhat lower than lysis is initiated by the oxidative addition of PhSiH₃to the

the barrier for the formation of an anti-Markovnikov inter- Ni(0) centre to give a silyl hydride intermediate. An attempted

mediate E (11.6 kcal mol⁻¹vs. 16.6 kcal mol⁻¹, respectively; stoichiometric reaction of styrene with PhSiD₃and 9-Ar

Scheme 4). In addition to the well-known electronic preference resulted in the incorporation of deuterium into both methine

for 2,1-migratory insertion of the vinyl arenes into the M–H and methyl groups of the Markovnikov styrene hydrosilylation

bonds,³⁷the energy barrier gain for the anti-Markovnikov product.³⁸However, in contrast to 9-Ar, hydrosilylation of

PhCHvCH₂using Ni(COD)₂gives substantial amounts of the

linear alkylsilane, PhCH₂CH₂SiH₂Ph (Table 1), albeit this can

be attributed to the diﬀerent steric environment provided by

POCN and COD ligands.

A similar trend in the activity of 6-Ar and 9-Ar is observed

in the hydrosilylation of benzaldehyde with PhSiH₃(5.0 mol%

of Ni; Table 2, entries 1 and 2). Thus, the 6-Ar-catalyzed reac-

tion results in the full conversion of PhC(O)H to a mixture of

PhH₂Si(OBn) (14%) and PhHSi(OBn)₂(86%) in 24 h at 25 °C,

whereas for 9-Ar the reaction reaches completion in only 3 h at

room temperature.

Compared to 6-Ar, the borate complex 3-Ar showed similar

activity in the hydrosilylation of benzaldehyde and cyclohexa-

none (entries 3 and 9); however, the formation of small

amounts of the Tishchenko reaction product (also known as

the Claisen–Tishchenko reaction),³⁹PhC(O)OBn, was observed

in the reaction with PhC(O)H. Increasing the temperature of

the reaction to 60 °C results in the complete conversion of

Scheme 4 DFT-calculated mechanism for the 7-Ar-catalyzed hydro-

silylation of styrene with PhSiH₃at 60 °C (Free energies are in kcal mol⁻¹).

Free energies for the most favourable pathway are shown in green.

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Table 2 Evaluation of the catalytic activity of 3-Ar, 4-Ar, 6-Ar and 9-Ar

PhSiH₃and PhMeSiH₂catalyzed by 6-Ar and 3-Ar. Based on

the similar catalytic activity of 3-Ar and 6-Ar in the hydrosilyl-

ation of benzaldehyde, the stoichiometric reactivity of 6-Ar

with PhSiH₃and feasibility of the formation of the hydride

species 7-Ar, the intermediate observed in the hydrosilylation

of benzaldehyde was tentatively assigned to the benzoxy deriva-

tive (^i-PrPOCN^Ar)Ni(OCH₂Ph) (10-Ar), which presumably pre-

sents the resting state of the catalyst and is produced by the

reaction of PhC(O)H with the in situ generated hydride 7-Ar.

Therefore, under catalytic conditions both the methyl complex

6-Ar and the borate complex 3-Ar should generate the hydride

7-Ar, which gives 10-Ar after the migratory insertion of benz-

aldehyde into the Ni–H bond. Unfortunately, all attempts to

prepare and fully characterize the elusive benzoxy derivative

10-Ar were unsuccessful due to the thermal instability of the

hydride 7-Ar.

Taking the above observations into account and given the

highly electrophilic nature of [(^i-PrPOCN^Ar)Ni]⁺, we suggest that

the latter species acts as a Lewis acid to promote the observed

hydrosilylation reactions. By analogy to the activation of hydro-

silanes with B(C₆F₅)₃,⁴²we propose that the reactions start

with the abstraction of H⁻from PhRSiH₂(R = H, Me) by 3-Ar

to give the hydride 7-Ar and a silylium species (Scheme 5).^43,44

In the 6-Ar-catalyzed reaction, the hydride 7-Ar is likely gener-

ated via a σ-bond metathesis pathway, similar to the mecha-

nism for the hydrosilylation of styrene, discussed above. The

formation of 7-Ar is followed by the insertion of the aldehyde

into the Ni–H bond generating the benzoxy intermediate

in the hydrosilylation of benzaldehyde and cyclohexanone^a

Entry Pre-cat. Substrate

t, h/conv.^b, % Products (ratio, %)^b

1

2

3

6-Ar

9-Ar

3-Ar

PhC(O)H

24/>99

3/>99

PhH₂Si(OBn) (14)

PhHSi(OBn)₂(86)

PhH₂Si(OBn) (28)

PhHSi(OBn)₂(72)

PhH₂Si(OBn) (28)

PhHSi(OBn)₂(64)

PhC(O)OBn (8)

PhH₂Si(OBn) (49)

PhHSi(OBn)₂(39)

PhC(O)OBn (12)

PhH₂Si(OBn) (57)

PhHSi(OBn)₂(36)

PhH₂Si(OBn) (27)

PhHSi(OBn)₂(47)

PhC(O)OBn (26)

PhMeHSi(OBn) (24)

PhC(O)OBn (23)

Et₃Si(OBn) (14)

20/>99

4^c

3-Ar

PhC(O)H

1/>99

5^c,d

6^e

3-Ar

PhC(O)H

1.5/93

2/>99

7^f

3-Ar

PhC(O)H

24/47

14/14

8^c,e,g

9^h

3-Ar

Cyclohexanone 18/>99

PhH₂Si(OCy) (57)

PhHSi(OCy)₂(43)

PhH₂Si(OBn) (<2)

PhHSi(OBn)₂(>98)

10ⁱ

4-Ar

PhC(O)H 22/>99

^a0.27 M, 5.0 mol% of [Ni]; substrate : silane = 1 : 1; 25 °C unless speci-

fied otherwise; C₆D₆(solvent); PhSiH₃was used unless specified other-

wise. ^bDetermined by ¹H-NMR. ^c60 °C. ^d2.0 mol% loading of [Ni].

^e25 °C, CD₂Cl₂. ^fReaction with PhMeSiH₂. ^gReaction with Et₃SiH.

^h91% conv. of cyclohexanone to PhH₂Si(OCy) (55%) and PhHSi(OCy)₂

(36%) was observed in 2.5 h. ⁱ25 °C, THF.

PhC(O)H within 1–1.5 hours even at 2.0 mol% loading of 3-Ar 10-Ar, which reacts with another molecule of PhSiH₃to form

(Table 2, entries 4 and 5), whereas changing the solvent from the hydrosilylation product and regenerate the hydride 7-Ar.

C₆D₆to polar CD₂Cl₂allows the achievement of full conversion A control stoichiometric reaction between 3-Ar with PhSiH₃in

of PhC(O)H in 2 h even at room temperature (Table 2, entry 6). C₆D₆revealed that the silylium species, generated along with

On the other hand, reducing the acidity and somewhat increas- 7-Ar, decomposes to

a mixture of PhSiF_nH_3−nand BF₃

ing the steric hindrance of the silane leads to the significantly (detected by ¹¹B{¹H}- and ¹⁹F{¹H}-NMR). Given the strong

decreased hydrosilylation activity of 3-Ar, and only 47% conver- Lewis acidity of the latter, hydrosilylation reactions mediated

sion of PhC(O)H with the formation of substantial amounts of by 3-Ar could proceed via two concurrent mechanisms: 7-Ar-

PhC(O)OBn (23%) was observed after 24 h in the room temp- catalyzed and BF₃-catalyzed pathways (Scheme 5). Indeed,

erature reaction of benzaldehyde with PhMeSiH₂(Table 2, when the tetraphenylborate 4-Ar was employed instead of the

entry 7). The addition of Et₃SiH is even slower and results only tetrafluoroborate 3-Ar in the hydrosilylation of benzaldehyde

in 14% of Et₃Si(OBn) after 14 h at 60 °C (Table 2, entry 8).⁴⁰

Monitoring the 3-Ar-catalyzed hydrosilylation of benz-

aldehyde with PhSiH₃by ³¹P{¹H}-NMR spectroscopy in C₆D₆

revealed the formation of a new species characterized by an

upfield, compared to 3-Ar (δ_P199.4 ppm), singlet at δ_P

194.9 ppm. Interestingly, the same signal was observed in the

³¹P{¹H}-NMR spectra of 3-Ar-catalyzed reaction of PhC(O)H

with PhMeSiH₂, suggesting that this new species must form

upon reaction of 3-Ar with benzaldehyde but not with silane.

Moreover, the ³¹P{¹H}-NMR spectra taken directly from the

reaction mixture for the addition of PhSiH₃to benzaldehyde

catalyzed by the methyl complex 6-Ar (Table 2, entry 1) revealed

the presence of the same new product. On the other hand,

treatment of 3-Ar with PhC(O)H yields an adduct [(^i-PrPOCN^Ar)

Ni(η¹-OvCHPh)][BF₄]⁴¹featuring a broad ³¹P resonance at δ_P

200.3 ppm. This does not correspond to the compound

with PhSiH₃in C₆D₆, only 38% conversion of PhC(O)H to

Scheme 5 Suggested mechanism for the hydrosilylation of benz-

observed during the hydrosilylation of benzaldehyde with aldehyde catalysed by 3-Ar and 6-Ar.

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PhH₂Si(OBn) (25%) and PhHSi(OBn)₂(13%) was observed in ation reactions mediated by cationic indenyl Ni(^II)

24 h at room temperature⁴⁵(vs. >99% conversion in the 3-Ar- complexes.²⁸

catalyzed reaction; Table 2, entry 5). Compared to 3-Ar, signifi-

cantly reduced catalytic activity in the hydrosilylation of benz-

Catalytic hydroboration reactions

aldehyde was also found for the borate 5-Ar. The suggestion of Taking into account the enhanced, compared to PhSiH₃, Lewis

a likely participation of BF₃in the 3-Ar-catalyzed hydrosilyl- acidity of HBPin, we envisaged that HBPin may exhibit even

ation of PhC(O)H is further supported by the formation of sub- superior reactivity patterns and elected to study the hydrobora-

stantial amounts of PhC(O)OBn (Table 2, entries 5–9), which tion catalyzed by 6-Ar. For alkenes and alkynes (styrene,

can be generated from benzaldehyde in the presence of strong phenylacetylene and 1-hexyne), 6-Ar showed low catalytic

Lewis acids via the Tishchenko reaction.^39,46In contrast, no activity in hydroboration reactions with HBPin at room temp-

Tishchenko product was detected in the 4-Ar and 5-Ar-cata- erature. Moreover, reactions of HBPin with phenylacetylene

lyzed reactions.

and 1-hexyne give complex mixtures of hydroboration and

To further probe the involvement of BF₃in the hydrosilyl- alkyne polymerization products. In contrast, hydroboration of

ation reactions with the pre-catalyst 3-Ar, we performed the fol- aldehydes, PhC(O)H, and ketones (acetone, cyclohexanone,

lowing control experiments. First, addition of PhSiH₃to benz- acetophenone and benzophenone) is fast and proceeds at

aldehyde was attempted in the presence of 5 mol% of BF₃·Et₂O room temperature to yield the corresponding borate esters

and it resulted in the formation of small amounts of PhC(O) (Table 3, entries 1–6). The activity of 6-Ar in the hydroboration

OBn with no hydrosilylation products being observed after of benzaldehyde remains high even upon reducing the catalyst

20 h at room temperature.⁴⁷In the second experiment, given loading from 5 to 0.5 mol% (Table 3, entry 2). Control experi-

the ability of strong Lewis acids to catalyze the hydrosilylation ments in the absence of 6-Ar were also performed and showed

of alkenes via the formation of [R₃Si]⁺[HBX₃]⁻species, followed no addition of HBPin to the carbonyl substrates at room temp-

by the addition of the silylium ion R₃Si⁺to the CvC bond to erature without the catalyst. Monitoring the 6-Ar-catalyzed

generate a carbocationic intermediate, which abstracts a hydroboration of benzaldehyde by ³¹P{¹H}-NMR revealed the

hydride from [HBX₃]⁻to release the alkylsilane product,^28,42eformation of the benzoxy derivative (^i-PrPOCN^Ar)Ni(OCH₂Ph)

we tested the addition of PhSiH₃to styrene in the presence of (10-Ar, δ_P194.9 ppm), previously observed in the catalytic

3-Ar (5 mol%). Assuming that the activation of 3-Ar with hydrosilylation of PhC(O)H with PhSiH₃mediated by 3-Ar and

PhSiH₃leads to the formation of the hydride 7-Ar and BF₃6-Ar, suggesting that hydroboration reactions proceed via the

(Scheme 5), diﬀerent regiochemistry of the hydrosilylation of hydride intermediate 7-Ar. 6-Ar-catalyzed hydroboration of

styrene (Markovnikov vs. anti-Markovnikov) would be expected ethyl acetate is slower than for aldehydes and ketones and

for the reactions catalyzed by 7-Ar and BF₃, respectively. This is results in 77% conversion of the substrate to give a mixture of

suggested based on the carbocationic mechanism proposed EtOBPin (54%) and MeC(O)H (23%), albeit full conversion of

for the reactions mediated by boron Lewis acids, which is con- HBPin was detected (Table 3, entry 7). Monitoring this reaction

sistent

with

the

formation

of

the

intermediate by NMR spectroscopy revealed the initial (during the first 20 h

PhC⁺HCH₂SiPh₃.^42eThus, 3-Ar-catalyzed hydrosilylation of of the reaction) accumulation of MeHC(OBPin)OEt. Within the

styrene resulted in the exclusive formation of the Markovnikov next 7 h at room temperature, the latter compound decom-

alkylsilane (63% in 24 h at room temperature), being consist- poses, presumably via transfer of the boryl moiety to the

ent with the mechanism involving the Ni–H intermediate (see ethoxy group, to the mixture of EtOBPin and MeC(O)H. This is

the discussion above and Table 1) and indicating that BF₃accompanied by the slow EtOBPin-catalyzed Tishchenko reac-

does not participate in the reaction.

tion of acetaldehyde resulting in the partial recovery of

Concerning the hydrosilylation of carbonyls in C₆D₆, the EtOAc.^39,46Such a reactivity pattern potentially explains the

observed low catalytic activity of the borates 4-Ar and 5-Ar vs. 3- observed full conversion of HBPin with 23% of ethyl acetate

Ar is also accounted for by the poor solubility of the former still being present in the reaction mixture. Formation of a very

charge-separated borate complexes. Thus, full conversion of similar mixture of products, EtOAc (27%), EtOBPin (60%) and

benzaldehyde to PhHSi(OBn)₂(>98%) and PhH₂Si(OBn) (<2%) MeC(O)H (13%), was observed when the catalytic reaction with

was observed after 22 h at room temperature when the 4-Ar cat- equimolar amounts of HBPin and EtOAc was repeated at 50 °C

alysed hydrosilylation of PhC(O)H was performed in THF for 24 h. Again, the initial formation of MeHC(OBPin)OEt

instead of C₆D₆(Table 2, entry 10). In contrast, in CD₂Cl₂, (61%), followed by its decomposition to the above-mentioned

1

despite the fact that both 4-Ar and 5-Ar are soluble and stable mixture was detected by H-NMR after 14 h at 50 °C (Table 3,

in dichloromethane, formation of only trace amounts of ben- entries 8 and 9). Notably, room temperature 6-Ar-catalyzed

zoxysilanes was detected. Given an ability of a number of late hydroboration of EtOAc with an excess of HBPin (ca. 2.0–2.5

transition metal hydride complexes to react with C–Cl bonds, equiv.) results in significantly slower decomposition of the

negligible activity of 4-Ar and 5-Ar in the benzaldehyde hydro- initial product, MeHC(OBPin)OEt, showing the exclusive for-

silylation in CD₂Cl₂is likely attributed to the deactivation of mation of EtOBPin with no NMR detectable amount of acet-

the in situ generated Ni–H species with CD₂Cl₂to form the cat- aldehyde. These observations suggest that at large HBPin con-

alytically inactive (^i-PrPOCN^Ar)NiCl.^13,30Similar observations centrations the intramolecular boryl transfer to the ethoxy

have been previously reported by Zargarian et al. for hydrosilyl- group in MeHC(OBPin)OEt is inhibited by the intermolecular

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Table 3 6-Ar-catalyzed hydroboration of carbonyl compounds^a

Entry

Substrate

Time

Product(s) (yield, %)^b

TON ^j

1

PhC(O)H

Me₂C(O)

Cyclohexanone

PhC(O)Me

Ph₂C(O)

5 min

10 min

27 h

BnOBPin (>99)

ⁱPrOBPin (>99)

CyOBPin (>99)

PhMeC(H)OBPin (>99)

Ph₂C(H)OBPin (>99)

EtOBPin (54)

20

200

20

15.4

2^c

3

4

5

6

7^d

MeC(O)OEt

MeC(O)H (23)

MeHC(OBPin)OEt (61)

EtOBPin (18)

EtOBPin (60)

MeC(O)H (13)

8^d,e

9^d,e

MeC(O)OEt

DMF

14 h

24 h

15.8

14.6

10^f

11^g

4 h

5 h

NMe₃(>99)

20

12^g,h

1 h

20

13^f

Acetamide

5 days

EtN(BPin)₂(57)

MeC(O)NH(BPin) (43)

EtN(BPin)₂(>99)

14ⁱ

Acetamide

N-Phenylpropan-amide

24 h

6 days

20

16.2

15^f

ⁿPrN(BPin)Ph (41)

EtC(O)N(BPin)Ph (40)

ⁿPrN(BPin)Ph (>99)

16ⁱ

N-Phenylpropan-amide

24 h

20

^a0.27 M, 5.0 mol% of 6-Ar, substrate : borane = 1 : 1 (unless specified otherwise), solvent: C₆D₆, 25 °C (unless specified otherwise). ^bDetermined

by ¹H-NMR. ^c0.5 mol% of [Ni]. ^d>99% conversion of HBPin. ^e50 °C. ^fAmide : HBPin = 1 : 2. ^gN-Methylsuccinimide : HBPin = 1 : 4.2. ^h60 °C.

ⁱAmide : HBPin = 1 : 5, 60 °C. ^jTurnover numbers were calculated based on the maximum conversion.

addition of HBPin across the C–O_Etbond of MeHC(OBPin)OEt. coupling reactions, and the room temperature 6-Ar-catalyzed

This also accounts for the observed initial accumulation of hydroboration of acetamide and N-phenylpropanamide result

MeHC(OBPin)OEt in the hydroboration of ethyl acetate using 1 in the mixtures of borylated amine and amide products

equiv. of HBpin.

The methyl complex 6-Ar also eﬀected the hydroboration of (ca.

(Table 3, entries 13 and 15).⁵⁰At 60 °C, with excess HBPin

equiv.), full conversion of both acetamide and

5

amides, representing, to the best of our knowledge, the first N-phenylpropanamide to the corresponding borylated amines,

example of a base metal system for such catalytic transform- EtN(BPin)₂and ⁿPrN(BPin)Ph, could be achieved within 24 h

ation.⁴⁸Thus, room temperature addition of HBPin to N,N-di- (Table 3, entries 14 and 16), presenting the first example of the

methylformamide (DMF) in the presence of 6-Ar (5 mol%) base metal catalyst for the reduction of primary and secondary

selectively aﬀords NMe₃. No intermediate product of HBPin amides to borylamines.^48,51,52

1

addition, such as H₂C(OBPin)NMe₂, was detected by H-NMR

Lastly, taking into account the observed low catalytic

spectroscopy. An analogous reaction with 2 equiv. of HBPin activity of 6-Ar in the hydroboration of alkenes and alkynes as

results in full conversion of DMF to NMe₃in only 4 h at room well as less eﬃcient, compared to aldehydes and ketones, 6-Ar-

temperature (Table 3, entry 10).⁴⁹In contrast, hydroboration of catalyzed hydroboration of esters and amides, we explored the

N-methylsuccinimide with excess HBPin (ca. 4.2 equiv.) does chemoselectivity of the hydroboration of benzaldehyde in the

not give the corresponding amine products (either N-methyl-2- presence of compounds having other potent reductive func-

pyrrolidone or N-methylpyrrolidine) and, instead, selective for- tional groups (alkenes, alkynes, esters, amides, amines,

mation of the product of single addition of HBPin to only one nitriles and nitro compounds). Chemoselectivity tests were

carbonyl moiety of N-methylsuccinimide was detected after 5 h performed on the NMR scale using benzaldehyde combined

at room temperature (Table 3, entry 11). At 60 °C, with 4.2 with 1 equiv. of a competing substrate and the results of these

equiv. of HBPin, complete conversion of N-methylsuccinimide trials are summarized in Table 4. Thus, in the presence of

is achieved in 1 h (Table 3, entry 12); however, no further alkenes (styrene, 1-octene, cyclohexene and trans-2-octene),

HBPin addition products were detected. Interestingly, no 1-hexyne, ethyl acetate, N,N-diisopropylbenzamide, aceto-

hydroboration of more deactivated (regarding the reactivity of nitrile, and nitrobenzene the reactions showed excellent che-

the carbonyl functionality) N-methyl-2-pyrrolidone with HBPin moselectivities in favour of the hydroboration of benzaldehyde

could be achieved even upon setting the reaction at 60 °C for with no NMR detectable amounts of the hydroboration pro-

24 h.

ducts for the competing substrates (Table 4, entries 1–4 and

For the primary and secondary amides, addition of HBPin 6–10). For a mixture of PhC(O)H and 4-octyne, addition of

to the carbonyl group competes with the dehydrogenative HBPin results in 91% of BnOBPin and 9% of PrCHvC(Pr)

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Table 4 Competitive 6-Ar-catalyzed hydroboration of benzaldehyde^a

veniently produced via the transfer of H⁻from the silane to

the electrophilic [(POCN)Ni]⁺species. The feasibility of hydro-

boration reactions with these cationic systems (i.e., abstraction

of H⁻from HBR₂) is currently under investigation in our

laboratories.

An eﬃcient hydroboration of carbonyl compounds with

HBPin was observed for the methyl pre-catalyst 6-Ar. Notably,

addition of HBPin to benzaldehyde can be performed chemo-

selectively in the presence of other common potent reductive

functional groups, such as alkenes, alkynes, esters, amides,

nitriles, nitro compounds and even ketones. Lastly, using 6-Ar,

for the first time we have demonstrated the base metal cata-

lyzed hydroboration of amides, including mild direct hydro-

borative reduction of primary and secondary amides to bory-

lated amines. Our current eﬀorts concentrate on expanding

the scope of the amide substrates and on the better under-

standing of the mechanistic aspects of these transformations.

Competing

Entry substrate

Conv. of PhC

(O)H^b, %

Conv. of competing

substrate^b, %

1

2

3

4

5

PhCHvCH₂

1-Octene

trans-2-Octene

Cyclohexene

4-Octyne

>99

91

—

6

7

8

9

10

11

12

1-Hexyne

MeC(O)OEt

PhC(O)NⁱPr₂

CH₃CN

>99

99

—

PhNO₂

—

PhC(O)Me

PhMeC(H)OBPin (1)

ⁱPr₂NH

79

ⁱPr₂NBPin (21)

^a0.27 M, 5.0 mol% of [Ni], PhC(O)H : competing substrate : borane =

1 : 1 : 1, solvent: C₆D₆, 25 °C. t = 10 min. ^bConversion to hydroboration

product(s), determined by ¹H-NMR.

BPin (Table 4, entry 5). In contrast, less selective reaction was

Experimental section

i

observed in the presence of Pr₂NH, which showed 21% con-

i

General experimental methods are described in the ESI.† The

preparation of iminophosphinite pincer ligands and the

corresponding POCN bromide complexes of Ni(^II), 1-^tBu, 1-Ph,

1-Ar′, and 1-Ar was performed according to the literature pro-

cedures for analogous compounds.¹²For full characterization

of POCN ligands and complexes 1-^tBu, 1-Ph, 1-Ar′, and 1-Ar,

see the ESI,† (^i-PrPOCN^Ar)NiCl was prepared and fully charac-

terized previously.¹³The NMR reaction of 1-Ar with LiBHEt₃

and the preparation of 9-Ar are reported in the ESI.†

version to Pr₂NBPin even before 6-Ar was introduced to the

i

reaction mixture. No further conversion of Pr₂NH was seen in

1

the H-NMR spectra when 6-Ar (5 mol%) was added; only the

formation of BnOBPin (79%) was detected (Table 4, entry 12).

Lastly, chemoselective hydroboration of benzaldehyde was

observed even in the presence of acetophenone (Table 4, entry

11), suggesting the general applicability of 6-Ar in the selective

reduction of aldehydes in the presence of other carbonyl

functionalities.

Preparation of (^i-PrPOCN^Ar)Ni(η²-BH₄) (2-Ar)

A solution of LiBH₄in THF (2.0 M, 96.3 μL, 0.1925 mmol) was

added via a syringe at room temperature to a solution of 1-Ar

Conclusions

We have prepared a series of novel iminophosphinite POCN (98.1 mg, 0.1833 mmol) in 20 mL of toluene. An immediate

pincer Ni(^II) complexes, namely the methyl derivative 6-Ar and colour change from orange-red to barberry red was observed,

the borates 2-Ar–5-Ar, which were subjected to the catalytic accompanied by the formation of a white precipitate. The reac-

hydrosilylation of alkenes, aldehydes and ketones and the tion mixture was left for stirring for 24 h, then all volatiles

hydroboration of carbonyl compounds with PhSiH₃and were pumped oﬀ and the product was extracted with hexanes

HBPin, respectively. Our studies have pointed to the intermedi- (3 × 20 mL) and dried under vacuum to give the barberry red

acy of the Ni–H derivative 7-Ar as the catalytically active species powder of 2-Ar, which was additionally recrystallized from con-

in these reactions. In the presence of 6-Ar (or with (POCN)NiBr centrated Et₂O solution (75 mg, 93%). Single crystals of 2-Ar

complexes 1-Ph and 1-Ar in combination with LiBHEt₃or suitable for X-ray diﬀraction analysis were obtained by crystalli-

MeLi), aliphatic alkenes, such as 1-ocyene and allylbenzene, zation from Et₂O solution at −30 °C. ¹H-NMR (500 MHz; C₆D₆;

are converted to the linear alkylsilanes, whereas hydrosilyl- δ, ppm): −0.95 (br s, 2H, BH₄); −0.72 (br s, 2H, BH₄); 1.07 (dd,

ation of styrene leads to the selective formation of the J = 6.9 and 14.8 Hz, 6H, 2 CH₃of ⁱPr₂P); 1.08 (d, J = 6.9 Hz, 6H,

i

Markovnikov regioisomer. Such regiospecific Markovnikov 2 CH₃of NAr); 1.24 (dd, J = 7.2 and 18 Hz, 6H, 2 CH₃of Pr₂P);

addition of PhSiH₃to styrene was rationalized by the preferen- 1.50 (d, J = 6.9 Hz, 6H, 2 CH₃of NAr); 2.06 (m, 2H, 2 CH of

tial (both steric and electronic preference) 2,1-migratory inser- ⁱPr₂P); 3.56 (sept, J = 6.9 Hz, 2H, 2 CH of NAr); 6.74 (m, 3H, aro-

tion of the styrene into the Ni–H bond of 7-Ar to give the sec- matic protons of C₆H₃/NAr); 7.12 (m, 2H, aromatic protons of

ondary alkylnickel species. DFT calculations for the turnover C₆H₃/NAr); 7.18 (m, 1H, aromatic proton of C₆H₃/NAr); 7.57 (d,

of the latter intermediate with PhSiH₃suggest the unusual J = 3.5 Hz, 1H, CHvNAr). ³¹P{¹H}-NMR (202.5 MHz; C₆D₆; δ,

chemolabile behaviour of the POCN ligand, allowing the ppm): 208.2 (s, OPⁱPr₂). ¹¹B{¹H}-NMR (160.5 MHz; C₆D₆; δ,

reduction of the steric crowding at the nickel centre.

For the hydrosilylation of benzaldehyde using cationic ppm): 16.6 (d, J = 2.5 Hz, 2 CH₃of Pr₂P); 16.9 (d, J = 4.2 Hz, 2

initiators, the catalytically active Ni–H intermediate 7-Ar is con- CH₃of Pr₂P); 23.1 (s, 2 CH₃of NAr); 25.4 (s, 2 CH₃of NAr);

ppm): −30.8 (br s, BH₄). ¹³C{¹H}-NMR (125.8 MHz; C₆D₆; δ,

i

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i

28.6 (d, J = 22.8 Hz, 2 CH of Pr₂P); 28.7 (s, 2 CH of NAr); 112.8 room temperature for 24 h and during this time the colour

(d, J = 13.1 Hz, CH of C₆H₃/NAr); 121.5 (s, CH of C₆H₃/NAr); changed from orange-red to yellow and the formation of a

123.4 (s, 2 CH of C₆H₃/NAr); 125.6 (s, CH of C₆H₃/NAr); 127.3 white precipitate was observed. The mixture was filtered and

(s, CH of C₆H₃/NAr); 141.5 (s, Cq of C₆H₃/NAr); 145.7 (s, Cq of the precipitate was washed with toluene until the filtrate

C₆H₃/NAr); 146.5 (s, Cq of C₆H₃/NAr); 156.7 (s, Cq of C₆H₃/NAr); became colourless. Toluene was removed under vacuum to

157.0 (s, Cq of C₆H₃/NAr); 166.0 (d, J = 10.1 Hz, Cq of C₆H₃); give a yellow oil, which was dried under high vacuum for 10 h

172.2 (d, J = 3.0 Hz, CHvNAr). C,H,N analysis (%): calcd for and triturated with hexanes and the hexanes/Et₂O (1/1)

C₂₅H₃₉BNNiOP (470.07): C 63.88, H 8.36, N 2.98; found: C mixture. All volatiles were pumped oﬀ to give a yellow foamy

64.18, H 8.59, N 3.06. IR (nujol, selected bands): 1902 cm⁻¹material which was dried under vacuum for an additional 5 h

(w, B–H), 1927 cm⁻¹(w, B–H), 1954 cm⁻¹(w, B–H), 2039 cm⁻¹(230 mg, 83%). ¹H-NMR (400 MHz; C₆D₆; δ, ppm): −0.12 (s,

(w, B–H), 2251 cm⁻¹(w, B–H), 2284 cm⁻¹(w, B–H).

3H, CH₃CN); 0.95 (dd, J = 6.9 and 15.2 Hz, 6H, 2 CH₃of ⁱPr₂P);

0.97 (d, J = 6.8 Hz, 6H, 2 CH₃of NAr); 1.03 (dd, J = 7.2 and 19.7

Preparation of (^i-PrPOCN^Ar)Ni(BF₄) (3-Ar)

i

Hz, 6H, 2 CH₃of Pr₂P); 1.13 (d, J = 6.8 Hz, 6H, 2 CH₃of NAr);

i

A solution of AgBF₄(76.6 mg, 0.3934 mmol) in 10 mL of 1.81 (m, 2H, 2 CH of Pr₂P); 2.98 (sept, J = 6.8 Hz, 2H, 2 CH of

toluene was added at room temperature to a solution of 1-Ar NAr); 6.42 (d, J = 8.1 Hz, 1H, C₆H₃/NAr); 6.47 (d, J = 7.4 Hz, 1H,

(200.5 mg, 0.3747 mmol) in 15 mL of toluene. The colour C₆H₃/NAr); 6.62 (t, J = 7.8 Hz, 1H, C₆H₃/NAr); 6.81 (m, 2H,

immediately changed from orange-red to yellow and the for- C₆H₃/NAr); 6.91 (m, 1H, C₆H₃/NAr); 7.08 (br t, J = 7.2 Hz, 4H,

mation of a white precipitate was observed. The reaction p-H of BPh₄); 7.27 (t, J = 7.4 Hz, 8H, m-H of BPh₄); 8.05 (br s,

mixture was stirred at room temperature for 1 h, filtered and 9H, CHvNAr and o-H of BPh₄). ³¹P{¹H}-NMR (202.5 MHz;

the residue was washed with a mixture of hexanes/toluene C₆D₆; δ, ppm): 205.5 (s, OPⁱPr₂). ¹¹B{¹H}-NMR (160.5 MHz;

(5/1) until the filtrate became colourless. The solvent was C₆D₆; δ, ppm): −5.8 (s, BPh₄). ¹³C{¹H}-NMR (125.8 MHz; C₆D₆;

removed under vacuum to give a yellow powder of 3-Ar which δ, ppm): −0.6 (s, CH₃CN); 16.8 (br s, 2 CH₃of ⁱPr₂P); 17.5 (d, J =

was additionally dried under vacuum for 5 h and recrystallized 5.1 Hz, 2 CH₃of ⁱPr₂P); 22.8 (s, 2 CH₃of NAr); 23.7 (s, 2 CH₃of

i

from concentrated CH₂Cl solution (120 mg, 59%). Single crys- NAr); 28.9 (d, J = 23.5 Hz, 2 CH of Pr₂P); 29.1 (s, 2 CH of NAr);

tals of 3-Ar suitable for X-ray diﬀraction analysis were obtained 115.9 (d, J = 10.7 Hz, CH of C₆H₃/NAr); 122.2 (s, BPh₄); 123.2 (s,

by slow room temperature vaporization of Et₂O solution into CH of C₆H₃/NAr); 123.4 (s, CH₃CN); 123.8 (s, 2 CH of C₆H₃/

hexanes.^{33 1}H-NMR (500 MHz; C₆D₆; δ, ppm): 1.05 (dd, J = 7.1 NAr); 126.5 (s, BPh₄); 128.3 (s, CH of C₆H₃/NAr); 129.7 (s, CH of

and 14.7 Hz, 6H, 2 CH₃of ⁱPr₂P); 1.09 (d, J = 6.9 Hz, 6H, 2 CH₃C₆H₃/NAr); 137.0 (s, BPh₄); 140.1 (s, 2 Cq of C₆H₃/NAr); 141.2

i

of NAr); 1.41 (dd, J = 7.2 and 19.4 Hz, 6H, 2 CH₃of Pr₂P); 1.59 (s, Cq of C₆H₃/NAr); 142.9 (s, Cq of C₆H₃/NAr); 147.5 (s, Cq of

i

(d, J = 6.9 Hz, 6H, 2 CH₃of NAr); 2.26 (m, 2H, 2 CH of Pr₂P); C₆H₃/NAr); 166.2 (d, J = 7.9 Hz, Cq of C₆H₃/NAr); 176.0 (br s,

3.59 (sept, J = 6.9 Hz, 2H, 2 CH of NAr); 6.37 (d, J = 8.1 Hz, 1H, CHvNAr). C,H,N analysis (%): calcd for C₅₁H₅₈BN₂NiOP

C₆H₃/NAr); 6.43 (d, J = 7.4 Hz, 1H, C₆H₃/NAr); 6.61 (t, J = 7.8 (815.52): C 75.11, H 7.17, N 3.44; found: C 74.71, H 7.18, N

Hz, 1H, C₆H₃/NAr); 7.10 (m, 2H, C₆H₃/NAr); 7.19 (m, 1H, C₆H₃/ 4.15.

NAr); the resonance of CHvNAr is obscured by the residual

Preparation of [(^i-PrPOCN^Ar)Ni(CH₃CN)][BAF] (5-Ar)

resonance of C₆D₆(found by ¹H–¹³C HSQC NMR). ³¹P{¹H}-

NMR (202.5 MHz; C₆D₆; δ, ppm): 199.5 (s, OPⁱPr₂). ¹¹B{¹H}-

Method A. Generation of 6-Ar on the NMR scale. A yellow

NMR (160.5 MHz; C₆D₆; δ, ppm): −1.4 (br s, BF₄). ¹⁹F{¹H}-NMR solution of 3-Ar (11.5 mg, 0.0212 mmol) and CH₃CN (1.3 μL,

(470.6 MHz; C₆D₆; δ, ppm): −173.8 (br s, BF₄). ¹³C{¹H}-NMR 0.0255 mmol) in CH₂Cl₂(0.6 mL) was added at room tempera-

i

(125.8 MHz; C₆D₆; δ, ppm): 16.5 (d, J = 3.1 Hz, 2 CH₃of Pr₂P); ture to the solid LiB(C₆F₅)₄·Et₂O (16.1 mg, 0.0212 mmol). No

17.4 (d, J = 6 Hz, 2 CH₃of ⁱPr₂P); 23.0 (s, 2 CH₃of NAr); 24.7 (s, colour change and the immediate formation of a cloudy white

2 CH₃of NAr); 28.5 (d, J = 21.7 Hz, 2 CH of ⁱPr₂P); 29.5 (s, 2 CH precipitate were observed. The reaction mixture was transferred

of NAr); 114.6 (d, J = 11.3 Hz, CH of C₆H₃/NAr); 122.7 (s, CH of to an NMR tube, left at room temperature for 30 min and then

C₆H₃/NAr); 123.9 (s, 2 CH of C₆H₃/NAr); 128.6 (s, CH of C₆H₃/ checked with NMR spectroscopy, showing complete conversion

NAr); 129.3 (s, Cq of C₆H₃/NAr); 141.1 (s, 2 Cq of C₆H₃/NAr); of 3-Ar to 5-Ar. All volatiles were pumped oﬀ and the product

142.8 (s, Cq of C₆H₃/NAr); 147.2 (s, Cq of C₆H₃/NAr); 167.5 (d, was extracted with toluene (app. 2 mL) to give a yellow oily

J = 8.4 Hz, Cq of C₆H₃/NAr); 172.9 (d, J = 3.3 Hz, CHvNAr); material (22.4 mg, 90%), which was dried under vacuum and

other resonances for aromatic carbons are obscured by the dissolved in CD₂Cl₂for NMR analysis (5-Ar is poorly soluble in

signal of C₆D₆. C,H,N analysis (%): calcd for C₂₅H₃₅BF₄NNiOP C₆D₆).

(542.03): C 55.40, H 6.51, N 2.58; found: C 55.82, H 6.72, N

2.69.

Method B. A solution of CH₃CN (20 μL, 0.383 mmol) in

CH₂Cl₂(20 mL) was added to the mixture of solid 4-Ar

(117.5 mg, 0.1441 mmol) and LiB(C₆F₅)₄·Et₂O (109.6 mg,

0.1441 mmol). Formation of a yellow solution and a white pre-

Preparation of [(^i-PrPOCN^Ar)Ni(CH₃CN)][BPh₄] (4-Ar)

First, a solution of 1-Ar (182.7 mg, 0.3414 mmol) in 30 mL of cipitate was observed. The reaction mixture was left for stirring

toluene and then CH₃CN (0.5 mL, 9.57 mmol) were added at at room temperature overnight, then filtered and the residue

room temperature to the solid NaBPh₄(175.3 mg, was washed with CH₂Cl₂(5 mL). The resulting solution was

0.5121 mmol). The reaction mixture was allowed to stir at concentrated to app. 3 mL, layered with hexanes (app. 5 mL)

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Dalton Transactions

and left at −30 °C for 24 h to give a yellow oily precipitate. The 164.2 (br s, aromatic 2 C_q); 173.4 (br s, aromatic 2 C_q); 176.0 (s,

filtrate was decanted, and the residue was washed with CHvNAr). C,H,N analysis (%): calcd for the co-crystals of 0.81

hexanes/toluene (30 ml/5 ml) and dried under vacuum to give 6-Ar and 0.19 (^i-PrPOCN^Ar)NiCl: C 65.42, H 7.97, N 2.95; found:

a yellow foamy material of 5-Ar (148.3 mg, 88%). ¹H-NMR C 65.88, H 8.23, N 2.82.

(400 MHz; C₆D₆; δ, ppm): 1.24 (d, J = 6.8 Hz, 6H, 2 CH₃of

i

General procedure for catalytic hydrosilylation and

hydroboration reactions

NAr); 1.37 (m, 18H, 2 CH₃of NAr and 4 CH₃of Pr₂P); 1.68 (s,

3H, CH₃CN); 2.34 (m, 2H, 2 CH of ⁱPr₂P); 3.33 (sept, J = 6.8 Hz,

2H, 2 CH of NAr); 6.86 (m, 1H, C₆H₃/NAr); 7.2 (m, 2H, C₆H₃/ A substrate of interest or a mixture of substrates, as in the com-

NAr); 7.27 (m, 2H, C₆H₃/NAr); 7.33 (m, 1H, C₆H₃/NAr); 8.1 (d, petitive hydroboration of benzaldehyde (Table 4), each

J = 4.0, 1H CHvNAr). ³¹P{¹H}-NMR (202.5 MHz; C₆D₆; δ, ppm): 0.162 mmol, was mixed with an appropriate amount of either

203.9 (s, OPⁱPr₂). ¹⁹F{¹H}-NMR (376.5 MHz; C₆D₆; δ, ppm): PhSiH₃(see Tables 1 and 2) or HBPin (see Tables 3 and 4) in

3

−133.1 (br s, 8F, o-F of B(C₆F₅)₄); −163.8 (t, J_F–F= 20.3 Hz, 4F, 0.6 mL of C₆D₆(unless another solvent is specified, see Tables

p-F of B(C₆F₅)₄); −167.6 (br m, 8F, m-F of B(C₆F₅)₄). ¹¹B{¹H}- 1–4) at room temperature. The mixture was transferred to an

NMR (128.4 MHz; C₆D₆; δ, ppm): −16.7 (s, B(C₆F₅)₄). ¹³C{¹H}- NMR tube equipped with a Teflon valve, left at room tempera-

NMR (125.8 MHz; C₆D₆; δ, ppm): 2.18 (s, CH₃CN); 17.0 (br s, 2 ture for 15 min and checked with ¹H-NMR, confirming no

CH₃of ⁱPr₂P); 17.6 (d, J = 5.4 Hz, 2 CH₃of ⁱPr₂P); 23.0 (s, 2 CH₃reaction in the absence of the catalyst. This solution was then

of NAr); 24.0 (s, 2 CH₃of NAr); 29.2 (d, J = 24 Hz, 2 CH of added to the solid pre-catalyst, the mixture was transferred

ⁱPr₂P); 29.4 (s, 2 CH of NAr); 116.5 (d, J = 12.7 Hz, CH of C₆H₃/ back to the NMR tube and the reaction was monitored with

NAr); 124.1 (s, CH of C₆H₃/NAr); 124.2 (s, 2 CH of C₆H₃/NAr); ¹H- and ³¹P{¹H}-NMR either at room temperature or at elevated

126.6 (s, CH₃CN); 128.5 (s, CH of C₆H₃/NAr); 130.0 (s, CH of temperatures (see Tables 1–4). Conversion of substrates and

C₆H₃/NAr); 135.6 (br s, B(C₆F₅)₄); 137.6 (br s, B(C₆F₅)₄); 140.9 the yields of the hydrosilylation and hydroboration products

1

(s, 2 Cq of C₆H₃/NAr); 143.0 (s, Cq of C₆H₃/NAr); 146.9 (s, Cq of were determined by H-NMR spectroscopy using Si(SiMe₃)₄as

C₆H₃/NAr); 147.5 (br s, B(C₆F₅)₄); 147.8 (s, Cq of C₆H₃/NAr); an internal standard. Characteristic NMR signals for new

149.4 (br s, B(C₆F₅)₄); 166.3 (d, J = 8.2 Hz, Cq of C₆H₃/NAr); hydrosilylation and hydroboration products are given in the

176.4 (d, J = 3.4 Hz, CHvNAr). C,H,N analysis (%): calcd for ESI.†

C₅₁H₃₈BF₂₀N₂NiOP (1175.32): C 52.12, H 3.26, N 2.38; found: C

51.20, H 3.24, N 2.33.

Conﬂicts of interest

Preparation of (^i-PrPOCN^Ar)NiMe (6-Ar)

There are no conflicts to declare.

A solution of MeLi in Et₂O (1.6 M, 0.2025 mmol) was added

via a syringe to a solution of 1-Ar (103.2 mg, 0.1929 mmol) in

30 mL of toluene at −80 °C. The mixture was allowed to warm

up to room temperature slowly and then stirred for an

Acknowledgements

additional 1.5 h (in total took app. 12 h). During this time the

This work was supported by Nazarbayev University through the

colour of the reaction mixture turned to crimson red. All vola-

NU Research University Development Program (grant to AYK

tiles were pumped oﬀ, the residue was dried under vacuum

2016023). MPB also thanks the Ministry of Education and

and the product was extracted with hexanes (3 × 20 mL) to give

Science of the Republic of Kazakhstan (grant AP05134515).

a dark-red solid of 6-Ar, which was recrystallized from concen-

PNP acknowledges support by the state of Baden-Württemberg

through bwHPC (bwunicluster and JUSTUS, RV bw17D011)

and financial support from the Helmholtz Association.

trated Et₂O solution (69 mg, 76%). Single crystals of 6-Ar suit-

able for X-ray diﬀraction analysis were obtained by slow vapori-

zation of Et₂O solution into hexanes at −28 °C.^{33 1}H-NMR

(500 MHz; C₆D₆; δ, ppm): −0.81 (d, J = 3.6 Hz, 3H, NiCH₃); 1.10

i

(d, J = 6.8 Hz, 6H, 2 CH₃of NAr); 1.15 (m, 6H, 2 CH₃of Pr₂P);

Notes and references

1.19 (dd, J = 3.2 and 7.1 Hz, 6H, 2 CH₃of ⁱPr₂P); 1.38 (d, J = 6.8

i

Hz, 6H, 2 CH₃of NAr); 2.05 (m, 2H, 2 CH of Pr₂P); 3.49 (sept,

J = 6.8 Hz, 2H, 2 CH of NAr); 6.84–6.90 (m, 3H, C₆H₃/NAr);

7.13–7.16 (m, 3H, C₆H₃/NAr overlapping with the residual

resonance of C₆D₆); 7.73 (d, J = 4.2 Hz, 1H, CHvNAr). ³¹P{¹H}-

NMR (202.5 MHz; C₆D₆; δ, ppm): 199.2 (s, OPⁱPr₂). ¹³C{¹H}-

NMR (125.8 MHz; C₆D₆; δ, ppm): −8.4 (d, J = 22.3 Hz, NiMe);

17.2 (s, 2 CH₃of ⁱPr₂P); 18.0 (d, J = 5.2 Hz, 2 CH₃of ⁱPr₂P); 23.0

(s, 2 CH₃of NAr); 24.5 (s, 2 CH₃of NAr); 28.3 (d, J = 23.6 Hz, 2

1 (a) B. Marciniec, J. Gulinski, W. Urbaniak and

Z. W. Kornetka, Comprehensive Handbook on

Hydrosilylation, ed. B. Marciniec, Pergamon Press, Oxford,

1992; (b) Reductions in Organic Synthesis Recent Advances

and Practical Applications, American Chemical Society,

Washington, 1996, vol. 641; (c) I. Fleming, A. Barbero and

D. Walter, Chem. Rev., 1997, 97, 2063–2192;

(d) L. S. Hegedus, Transition Metals in the Synthesis of

Complex Organic Molecules, University Science Books,

Sausalito, 2nd edn, 1999; (e) Modern Reduction Methods, ed.

P. G. Andersson and I. J. Munslow, Wiley-VCH, Weinheim,

2008; (f) B. Marciniec, H. Maciejewski, C. Pietraszuk and

i

CH of Pr₂P); 28.8 (s, 2 CH of NAr); 112.8 (d, J = 11.8 Hz,

aromatic CH); 120.9 (s, aromatic CH); 123.3 (s, aromatic 2 CH);

125.7 (s, aromatic CH); 126.6 (s, aromatic CH); 141.1 (s, aro-

matic 2 C_q); 146.0 (s, aromatic 2 C_q); 147.3 (s, aromatic 2 C_q);

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Paper

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Dalton Transactions

11 S. De, J. Zhang, R. Luque and N. Yan, Energy Environ. Sci., 28 For the similar reaction leading to the formation of a

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13 J. Sanford, C. Dent, J. D. Masuda and A. Xia, Polyhedron,

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16 S. Enthaler, A. Brück, A. Kammer, H. Junge, E. Irran and 31 Both systems, 1/LiBHEt₃and 1/MeLi were found to be inac-

S. Gülak, ChemCatChem, 2015, 7, 65–69. tive in the hydrosilylation of internal alkenes.

17 A. Castonguay, A. L. Beauchamp and D. Zargarian, Inorg. 32 In an alternative “silyl” mechanism, formation of (^i-PrPOC

Chem., 2009, 48, 3177–3184.

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18 F2, F3 and F4 in 3-Ar all occupy three positions. The mean

value of the B1–F distance was calculated as the average of

nine B1–F distances, three for each B1–F2, B1–F3 and B1–

F4.

(

^i-PrPOC(H)N^Ar)Ni(SiH₂Ph), was observed upon reaction of

1-Ar with LiBHEt₃in the presence of an excess of PhSiH₃.

19 For the preparation of

a similar (POCOP)NiH, see: 33 See the ESI† for details.

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36 Formation of an η³-benzylnickel species could potentially

explain the Markovnikov regiospecificity for the hydrosilyl-

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species also lead to the branched products in the related

23 R. H. Crabtree, The Organometallic Chemistry of the

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(1 : 2 by ³¹P{¹H}-NMR, respectively) in a mixture with 1-Ar 38 The reaction was performed at 60 °C for 4 h and showed

and unidentified decomposition products.

26 S. Pfirrmann, C. Limberg and B. Ziemer, Dalton Trans.,

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Dalton Trans.

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40 Only 6% of Et₃Si(OBn) was detected after 19

25 °C.

h

at

ence of Et₂O (commercially available BF₃·Et₂O as a 45%

solution in Et₂O was used).

41 Formation of a weak η¹-adduct was suggested based on the 48 Hydroborative reduction of amides to amines is limited by

NMR features of the product, which, compared to uncoor-

dinated PhC(O)H, revealed a slightly downfield shifted

broad benzaldehyde proton resonance (δ_H= 9.72 ppm vs.

δ_H= 9.64 ppm for PhC(O)H in C₆D₆) and the downfield

shifted benzaldehyde ¹³C resonance (δ_C= 194.7 ppm vs.

δ_C= 191.4 ppm for PhC(O)H in C₆D₆) in the ¹H-and ¹³C

two examples of Mg-catalyzed transformations, with no

examples of hydroboration of primary amides:

(a) N. L. Lampland, M. Hovey, D. Mukherjee and

A. D. Sadow, ACS Catal., 2015, 5, 4219–4226;

(b) D. Mukherjee, S. Shirase, T. P. Spaniol, K. Mashima and

J. Okuda, Chem. Commun., 2016, 52, 13155–13158.

{¹H}-NMR spectra, respectively. Moreover, the ¹⁹F{¹H}-NMR 49 For base-metal-catalyzed hydrosilative reduction of tertiary

spectrum of the benzaldehyde adduct shows a broad signal

at δ −164.6 ppm, which is significantly downfield com-

pared to the ¹⁹F resonance of 3-Ar (δ −173.8 ppm),

suggesting the presence of a noncoordinated BF₄anion

(see ref. 15b and 17).

amides, see: (a) C. M. Kelly, R. McDonald, O. L. Sydora,

M. Stradiotto and L. Turculet, Angew. Chem., Int. Ed., 2017,

56, 15901–15904; (b) A. Nurseiit, J. Janabel, K. A. Gudun,

A. Kassymbek, M. Segizbayev, T. M. Seilkhanov and

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cctc.201801605 and references therein; (c) C. M. Macaulay,

T. Ogawa, R. McDonald, O. L. Sydora, M. Stradiotto and

−

42 (a) J. B. Lambert and Y. Zhao, J. Am. Chem. Soc., 1996, 118,

7867; (b) J. B. Lambert, Y. Zhao and H. Wu, J. Org. Chem.,

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L. Turculet, Dalton Trans., 2019, DOI: 10.1039/c8dt04221g.

1

and W. E. Piers, J. Org. Chem., 1999, 64, 4887; 50 Formation of H₂was detected by H-NMR (δ 4.47 ppm in

(d) D. J. Parks, J. M. Blackwell and W. E. Piers,

J. Org. Chem., 2000, 65, 3090; (e) M. Rubin, T. Schweir and

V. Gevorgyan, J. Org. Chem., 2002, 67, 1936.

C₆D₆) during the 6-Ar-catalyzed hydroboration of

N-phenylpropanamide and acetamide.

51 The related hydrosilylation of primary and secondary

amides usually leads to the dehydration of the amides to

the corresponding nitriles and imines, respectively. For

example: (a) S. Hanada, Y. Motoyama and H. Nagashima,

Eur. J. Org. Chem., 2008, 4097–4100; (b) S. Zhu, K. Junge,

D. Addis, S. Das and M. Beller, Org. Lett., 2009, 11, 2461–

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43 For a similar mechanistic proposal for hydrosilylation reac-

tions catalysed by cationic indenyl Ni(^II) complexes, see

ref. 28.

44 In an alternative scenario, the presence of a large excess of

benzaldehyde could result in the BF₃abstraction from 3-Ar

leading to the formation of (^i-PrPOCN^Ar)NiF species, which

can generate the hydride 7-Ar upon reaction with the

hydrosilane.

45 70% conversion of benzaldehyde to form a mixture of

PhH₂Si(OBn) (41%) and PhHSi(OBn) (59%) was observed

after 45 h at room temperature.

46 For the Tishchenko reaction catalysed by boron containing

Lewis acids, see: S. P. Curran and S. J. Connon, 52 For the formal hydrogenation of secondary amides via the

Angew. Chem., Int. Ed., 2012, 51, 10866–10870.

47 The lack of hydrosilylation activity in this case could be

explained by the reduced Lewis acidity of BF₃in the pres-

formation of imines, see: Y.-Q. Zou, S. Chakraborty,

A. Nerush, D. Oren, Y. Diskin-Posner, Y. Ben-David and

D. Milstein, ACS Catal., 2018, 8, 8014–8019.

Dalton Trans.

Article Doi

POCN Ni(ii) pincer complexes: Synthesis, characterization and evaluation of catalytic hydrosilylation and hydroboration activities

DOI: 10.1039/c8dt04854a

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