Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

Article

Cher-Chiek Chia, Yeow-Chuan Teo, Ning Cham, Samuel Ying-Fu Ho, Zhe-Hua Ng, Hui-Min Toh,

Nicolas M é zailles,* and Cheuk-Wai So *

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sı Supporting Information

ABSTRACT: This study describes the ﬁrst use of a bis-

(

phosphoranyl)methanido aluminum hydride, [ClC-

PPh NMes) AlH ] (2, Mes = Me C H ), for the catalytic

hydroboration of CO . Complex 2 was synthesized by the reaction

of a lithium carbenoid [Li(Cl)C(PPh NMes) ] with 2 equiv of

AlH ·NEtMe in toluene at −78 °C. 2 (10 mol %) was able to

catalyze the reduction of CO with HBpin in C D at 110 °C for 2

days to aﬀord a mixture of methoxyborane [MeOBpin] (3a; yield:

−

8%, TOF: 0.16 h ) and bis(boryl)oxide [pinBOBpin] (3b). When more potent [BH ·SMe ] was used instead of HBpin, the

3 2

catalytic reaction was extremely pure, resulting in the formation of trimethyl borate [B(OMe) ] (3e) [catalytic loading: 1 mol % (10

mol %); reaction time: 60 min (5 min); yield: 97.6% (>99%); TOF: 292.8 h (356.4 h )] and B O (3f). Mechanistic studies

−

−1

show that the Al−H bond in complex 2 activated CO to form [ClC(PPh NMes) Al(H){OC(O)H}] (4), which was subsequently

reacted with BH ·SMe to form 3e and 3f, along with the regeneration of complex 2. Complex 2 also shows good catalytic activity

toward the hydroboration of carbonyl, nitrile, and alkyne derivatives.

3,14

INTRODUCTION

cooperation mechanism.

Nikonov et al. also demonstrated

■

that the β-diketiminato aluminum hydride complex catalyzed

the hydrosilylation of alkenes via the activation of silane by the

Lewis acidic aluminum center in the ﬁrst step of the catalysis.

Rueping et al. reported the ﬁrst asymmetric hydroboration of

ketones using aluminum complexes bearing chiral biphenol-

type ligands. These results serve as evidence that tailor-made

aluminum hydride complexes exhibit transition-metal-like

reactivity in mediating the catalytic reduction of unsaturated

polar bonds (C=O, C=N) and C−C multiple bonds. Despite

these pertinent recent developments, the implementation of

aluminum hydride complexes for catalytic CO transformation

into value-added chemicals has not been reported. In fact,

Aldridge et al. showed that β-diketiminato aluminum hydride

complexes are not eﬃcient catalysts for the hydroboration of

CO with boranes because of unfavorable kinetics of the σ-

bond metathesis mechanism. Inoue et al. reported that N-

heterocyclic imido aluminum hydride complexes promoted

CO₂reduction through a non-aluminum-containing inter-

mediate to form ill-deﬁned product mixtures.

Recently, a series of bis(phosphoranyl)methanediide and

methanide complexes of main-group elements have been

Main-group elements are considered as sustainable alternatives

to precious transition-metal species in chemical catalysis due to

their high abundancy and relatively low toxicity. In particular,

aluminum is the most abundant main-group element, and its

Lewis acidic compounds have been shown to catalyze a wide

range of classic organic reactions, such as the Meerwein−

Ponndorf−Verley reduction of carbonyls, Friedel−Crafts

acylation of aromatics, and the ring-opening polymerization

of lactams. However, aluminum does not possess donor and

acceptor valence orbitals with small energy gaps, which

classically prevents aluminum compounds from mimicking

transition-metal complexes in catalysis. In the past few years,

this limitation has been overcome by designing novel

aluminum hydride compounds with highly polarized Al−H

bonds. The hydridic H atoms are able to display reactivity

similar to transition-metal hydride complexes. For example,

Berben et al. showed that a bis(imino)pyridine complex of

aluminum catalyzed the dehydrogenative coupling of benzyl-

amine, dehydrogenation of formic acid, and electrocatalytic

production of hydrogen via a metal−ligand cooperative

mechanism. Roesky et al. illustrated that the β-diketiminato

−

shown, whereby the ylidic PN bonds (P = N ↔ P −N ; Figure

aluminum hydride complexes mediated the catalytic hydro-

boration of carbonyl compounds and alkynes.

,10

Thomas and

Received: November 30, 2020

Published: March 18, 2021

Cowley et al. reported that commercially available aluminum

hydrides such as DIBAL-H and LiAlH₄catalyzed the

hydroboration of alkenes, alkynes, and nitriles via σ-bond

1,12

metathesis.

Research groups of Harder and Mulvey

reported that lithium aluminates catalyzed hydrophosphination

of alkynes and hydrogenation of imines through a Li−Al

2021 American Chemical Society

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) of the ligands promote strong electron transfer from the N

Article

atoms to main-group elements. In particular, we have shown

Figure 1. Ylidic structure of bis(phosphoranyl)methanediide and

methanide complexes of main-group elements (E).

Figure 2. X-ray crystal structure of 2 with thermal ellipsoids at 50%

probability. All H atoms except on the Al1 atom are omitted for

clarity. Selected bond lengths (Å) and angles (deg): Cl1−C1

1.785(3), C1−P2 1.725(4), P2−N2 1.623(3), N2−Al1 1.886(3),

Al1−N1 1.900(3), N1−P1 1.625(3), P1−C1 1.719(4), P2−C1−P1

133.3(2), N2−Al1−N1 103.6(1).

that the boron derivatives of bis(phosphoranyl)methanediide

and methanide, [H B −C(PPh S) Li(OEt )] and [ClC-

(

PPh NMes) BH ], were among the most eﬃcient catalysts

2 2 2

for CO reduction to methanol derivatives. We anticipated that

such electronic properties could also enhance the hydridic

presence of an AlH ⁺moiety (Al−N: 1.886(3), 1.900(3) Å).

character of an Al−H bond, resulting in the kinetically and

The C1 atom is planar, which agrees with the absence of a CH

bond reﬂected in the NMR data. The P−N bond lengths in 2

are elongated (1.623(3), 1.625(3) Å) in comparison with

thermodynamically favorable addition to CO . Herein, we

report the synthesis of a bis(phosphoranyl)methanido

aluminum hydride and its use in the catalytic hydroboration

those in 1 (1.596(3), 1.593(2) Å), which indicates that the

negative charge on the C1 atom in 2 is more delocalized into

the P−N σ* orbitals by negative hyperconjugation. Complex 2

is air- and moisture-sensitive as well as being soluble and stable

in nonpolar solvents like benzene and toluene up to 120 °C.

Decomposition into the protonated ligand [Cl(H)C-

of CO coupled with DFT calculations that rationalize our

experimental ﬁndings. We also report that this complex is an

eﬃcient catalyst for the hydroboration of a wide variety of

substrates, such as aldehydes, ketones, nitriles, and alkynes.

RESULTS AND DISCUSSION

Our starting point was the lithium carbenoid [Li(Cl)C-

(

PPh NMes) ] in hot toluene was not observed.

■

(

2 2

Next, the catalytic ability of the bis(phosphoranyl)-

methanido aluminum hydride complex 2 toward the hydro-

boration of CO with boranes was then examined. To begin

with, we veriﬁed that there was no reaction between CO and

boranes (HBpin, BH ·SMe , and HBcat) in C at 110 °C. In

addition, there was no reaction between complex 2 and HBpin

PPh NMes) ] (1, Mes = C H Me ) species, which we

reported in 2015. Upon treatment with 2 equiv of AlH₃·

NEtMe in toluene at −78 °C followed by warming to room

temperature and stirring for 1 h (Scheme 1), a white

6 6

at 110 °C for 48 h. In the presence of 2 (10 mol %), the

Scheme 1. Synthesis of the Bis(phosphoranyl)methanido

Aluminum Hydride 2

reduction of CO with HBpin in C D at 110 °C aﬀorded a

mixture of methoxyborane [MeOBpin] (3a, Scheme 2) (55.8%

−

in 24 h; 78% in 48 h, TOF: 0.16 h ) and bis(boryl)oxide

Scheme 2. 2-Catalyzed Hydroboration of CO₂

suspension was observed. After the white precipitate was

ﬁltered, which was identiﬁed as LiAlH , [ClC-

(

PPh NMes) AlH ] (2) was obtained as a colorless crystalline

2 2 2

solid (92% yield) in the concentrated ﬁltrate. The results show

that AlH ·NEtMe undergoes a disproportionation reaction to

form “AlH ”, stabilized by the carbenoid backbone, and an

−

AlH₄species in the presence of compound 1. Complex 2 was

analyzed and characterized by NMR spectroscopy and X-ray

crystallography. The H NMR spectrum shows a set of signals

for the ligand backbone while featuring a broad signal at 5.0

ppm with an integration of 2 H representing the AlH moiety.

The P NMR spectroscopy shows a new singlet at 39.0 ppm,

more than 10 ppm downﬁeld from 1 (27.2 ppm). The X-ray

crystal structure of 2 (Figure 2) features a six-membered

CPNAlNP ring in a half chair conformation due to the

Yields were calculated based on conversion of H.

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pinBOBpin] (3b). The catalytic activity in terms of TOF is in

Article

[

reaction mixture showed a single new signal at 40.5 ppm, while

the range of other main-group metal-catalyzed CO hydro-

the H NMR spectrum showed a new singlet at 8.5 ppm (1 H)

borated products with pinacolborane (HBpin) (TOF = 0.07−

suggesting the formation of a formate moiety coupled with a

set of signals representative of the bis(iminophosphoranyl)-

methanido ligand. The spectroscopic data indicates that CO₂

inserts into one Al−H bond in complex 2 to form

[ClC(PPh NMes) Al(H){OC(O)H}] (4). It was isolated as

a white precipitate by simply removing volatiles of the reaction

mixture in vacuo. Complex 4 is stable in the solid state but

−

4.5 h ). A similar reduction of CO with catecholborane

(

HBcat) using 2 as a catalyst proceeded with less eﬃciency (24

h, 28.8% conversion) to aﬀord a mixture of methoxyborane

catBOMe] (3c) and B (cat) (3d). In the catalysis, most of

[

HBcat was unreacted, while a mixture of B (cat) ( B NMR:

2.5 ppm) and unidentiﬁed four-coordinate borane com-

pounds ( B NMR: −14.5 ppm) was observed, in addtion to

c and 3d. This suggests that intermediates in the catalysis

could react with HBcat to form B (cat) and catalytically

slowly decomposes in C

made to crystallize this compound in CH

, toluene, and CH

. Eﬀorts were

, which was

unsuccessful due to its slow evolution in solution. However, a

single new species formed after a week in attempts to

crystallize 4, indicative by the appearance of a new signal at

inactive complexes, leading to low conversion of the catalysis.

When more potent and NaBH -free [BH ·SMe ] was used

instead of HBpin, the catalytic reaction was much more

42.5 ppm in the P NMR spectrum. The H NMR spectrum

showed a singlet at 8.24 ppm with an integration of 4

eﬃcient (1 mol % of 2), forming trimethyl borate [B(OMe) ]

−

(

3e) in only 1 h (yield: 97.6%, TOF: 292.8 h , Scheme 2)

indicating the formation of two OCH O moieties. In addition,

together with white precipitates. Hydrolysis of the latter in

D O quantitatively formed boric acid (by B NMR), which is

the H NMR spectrum showed a set of signals for the

bis(iminophosphoranyl)methanido ligand. Colorless crystals

indicative that the white precipitate formed during the catalysis

were obtained from the initial CH Cl solution of 4,

2 2

is B O . When the amount of complex 2 was increased to 10

crystallized at 0 °C for 1 week. The X-ray crystallographic

analysis proved the formation of the acetal derivative

mol %, the catalytic activity increased signiﬁcantly (5 min, 110

−

C, yield: >99%, TOF: 356.4 h ). Complex 2 is one of the

[LAl(OCH O) AlL] (4-D, L = ClC(PPh NMes) ). Thus, in

2 2 2 2

very few main-group element compounds that can catalyze the

the absence of any additional substrate, the second Al−H is

suﬃciently hydridic to reduce the formate moiety to an acetal

reduction of CO₂with BH , including the ambiphilic

phosphine−borane compound [1-B(OR) -2-PR′ -C H ]

moiety. Complex 4-D is stable in solution (toluene, CH

and in the solid state. The X-ray crystal structure of complex 4-

D comprises an Al(OCH O) Al eight-membered ring (Figure

2 2

Cl )

−

(

TOF: 43−257 h ; R′ = Ph, iPr; (OR) = (OMe) , catechol,

pinacol), bis(thiophosphinoyl)methanide-supported lithium

−

). The ring structure (Al −O: 1.731(4), 1.710(8) Å) is

borohydride [H B −C(PPh S) Li(OEt )] (TOF: 150 h ),

bis(iminophosphoranyl)methanido boron hydride [ClC-

PPh NMes) BH ] (TOF: 157 h ), the ring expansion

−

(

2 2 2

product arising from phosphine-derived carbenes with 9-

borabicyclo[3.3.1]nonane (9-BBN), and NHC−silyliumyli-

−

dene complex [(I ) SiH]I (TOF: 19.8 h , I = :C{N(Me)-

Me 2

C(Me)}₂). It is noteworthy that the catalytic activity of

complex 2 is more eﬃcient than these examples in terms of

both reaction time and TOF.

The catalytic mechanism was studied by performing

stoichiometric stepwise reactions with the aim of isolating

reactive intermediates. Complex 2 was reacted with CO in

toluene at room temperature for 15 min (Scheme 3), whereby

complex 2 was fully consumed. The P NMR spectrum of the

Scheme 3. Formation of Complex 4-D

Figure 3. X-ray crystal structure of 4-D with thermal ellipsoids at the

0% probability level. Selected hydrogen atoms are omitted for clarity.

Selected bond lengths (Å) and angles (deg): Cl1−C1 1.797(5), C1−

P1 1.747(6), C1−P2 1.704(6), P2−N2 1.632(4), P1−N1 1.624(4),

N1−Al1 1.881(5), N2−Al1 1.891(4), Al1−O2 1.731(4), Al1−O1

.710(8), O1−C32 1.392(11), O2−C32 1.476(8), N1−Al1−N2

04.64(19), O1A−Al1−O2 109.2(5), N1−Al1−O1A 119.5(4), N2−

Al1−O2 108.80(19), Al1−O2−C32 116.6(4), O2−C32−O1

111.2(8).

comparable with the one supported by β-diketiminate ligand

RAl(OCH O) AlR] (Al−O: 1.7123(11), 1.7239(11) Å; R =

[

HC{C(Me)NDipp} , Dipp = 2,6-iPr C H ), which was

isolated from the reaction of the germanium(II) formate

[RGeOC(O)H] and aluminum hydride [RAlH₂].

To prove that complex 4 is an intermediate in the catalysis,

0 mol % of 4 was used to catalyze the reaction of CO with

HBpin in C D at 110 °C for 48 h to aﬀord a mixture of

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Scheme 4. DFT Studies of Catalytic CO Hydroboration with BH ·SMe from Complexes A to H, Showing the Three C−H

Bond Formations from CO₂

ΔG in kcal/mol, calculated at 298 K.

methoxyborane 3a (yield: 78%) and bis(boryl)oxide 3b.

Similarly, 4-catalyzed hydroboration of CO with BH ·SMe

C H B N O AlP Cl: 799.2908 [(M + H)] ; found:

44 49 2 2 3 2

799.2938. L−M: HRMS (ESI): m/z calcd for

in C D at 110 °C formed [B(OMe) ] (3e, yield: 98%) in 60

C H B N O AlP Cl: 771.2908 [(M + H)] ; found:

44 49

min. The yields of the products are comparable with those in

771.2978.).

the catalytic CO hydroboration using 2 as a catalyst. In

On the basis of these experimental studies, a catalytic cycle

particular, complex 2 was regenerated after these catalytic runs.

On the other hand, 10 mol % of complex 4-D was unable to

catalyze the hydroboration of CO with HBpin or BH ·SMe in

for the hydroboration of CO with BH ·SMe is proposed

(Schemes 4 and 5 ) and studied by DFT calculations (see the

Supporting Information for details). Complex B results from

C D at 110 °C. In summary, these results show that complex

insertion of CO into an Al−H bond of complex A (model of

is an active catalyst and complex 4 is an intermediate in the

compound 2) in a strongly exergonic step (ΔG = −24.8 kcal/

mol). A transition state for this process was not found, despite

several attempts. BH ·SMe then coordinates with the C=O

catalytic reduction of CO , while 4-D is an oﬀ-cycle product.

Moreover, complex 4 was reacted with 3 equiv of BH ·SMe

in C D to form a mixture of bis(iminophosphoranyl)-

double bond through TS_B₋_C(ΔG = −1.8 kcal/mol) to form

compound C (ΔG = −20.5 kcal/mol). The transition state for

SMe substitution by the CO moiety at BH is accessible,

methanido compounds, conﬁrmed by P NMR spectroscopy

(

seven signals: 41.1−42.5 ppm). The reaction mixture was

then heated at 110 °C overnight. P NMR spectroscopy

showed that complex 2 was regenerated, along with the

formation of B(OMe) represented by B NMR spectroscopy.

requiring 23.0 kcal/mol. Subsequently, the B−H bond in

compound C adds to the >C=O bond via TS (ΔG = −3.5

C‑D

kcal/mol) to form complex D (ΔG = −27.8 kcal/mol). At this

point, it is obvious that the ﬁrst two C−H bond formations are

accessible. The acetal group in complex D then undergoes

rotation around the C−O bond to form complex E (ΔG =

−30.7 kcal/mol), concurrently allowing the borane to interact

with the aluminum hydride. A second BH ·SMe is then added

This indicates that the 2-catalyzed CO hydroboration with

BH ·SMe proceeds via several intermediates, and complex 2 is

the active catalyst in the mechanism. To further support our

claim that the reaction proceeds via several intermediates,

high-resolution mass spectrometry (HRMS) was performed,

and we were able to detect the presence of such intermediates

to allow for the third C−H bond to be formed. It coordinates

to the oxygen atom of the acetal group in complex E, to result

in complex F, which is almost isoenergetic (ΔG = −30.0 kcal/

(

Schemes 4 and 5; C−E: HRMS (ESI): m/z calcd for

C H BN O AlP Cl: 771.2788 [(M + H)] ; found: 771.2774.

F−I: HRMS (ESI): m/z calcd for C H B N O AlP Cl:

mol) via TS (ΔG = −12.1 kcal/mol). Notably, in complex

E‑F

85.3116 [(M + H)] ; found: 785.3167. J: HRMS (ESI): m/z

calcd for C H B N O AlP Cl: 799.3444 [(M + H)] ; found:

F, the oxygen atom and BH₂moieties are adequately

positioned to favor Al−O bond and BH bond formations

leading to complex G (ΔG = −34.3 kcal/mol), in a facile

9 9. 3 47 4 . K : HR M S ( E S I) : m/ z c a l c d fo r

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Scheme 5. DFT Studies of Catalytic CO Hydroboration with BH·SMe from H to the Formation of 3e and Regeneration of B

ΔG in kcal/mol, calculated at 298 K.

process (without TS). Complex G is a symmetrical complex

featuring two Al−O bonds and two O−BH₃moieties.

Subsequently, the B−H bond is added to the acetal group

thus pentacoordinated. Decoordination of the MeOBH₂

moiety is thus quite exergonic to form complex L (ΔG =

−82.5 kcal/mol). The σ-bond metathesis reaction between the

Al−O and B−H bonds occurs by cleaving the 3c−2e

interaction between borane moieties through complex M,

which results in forming O(BH ) and regenerating complex B.

via TS

(ΔG = −5.3 kcal/mol) resulting in the cleavage of

G‑H

the O−C bond and the formation of MeOBH coordinated to

the Al center in complex H (ΔG = −41.0 kcal/mol). The

formation of the third C−H bond is the rate-determining step

of the whole process. The computed barrier of 35.5 kcal/mol is

rather high but consistent with a reaction requiring 110 °C to

Finally, as is well-known in boron chemistry, three MeOBH₂

molecules undergo substituent rearrangements to form B-

(OMe) and BH , while three O(BH ) molecules lead to

2 2

occur. At this point, the “CH O” moiety has been generated

B O and BH . In summary, the highest kinetic barrier in the

2 3 3

from CO , and the second part of the process is dedicated to

proposed mechanism is 35.5 kcal/mol, related to the third CH

its elimination as “CH OBH ” from the Al center and

bond formation (complex TS ) between complexes G and

G‑H

reformation of Al−H moieties to initiate a second cycle as

depicted in Scheme 5. A facile rotation around the Al−O axes

of the MeOBH and OBH moieties leads to a very strong

H. The other CH bond formations are much more facile. The

calculation results are thus in accordance with the experimental

conditions and observations (formation of formyl complex 4

and acetal complex 4D at room temperature).

stabilization (complex I, ΔG = −70.9 kcal/mol). The

associated TS is found at ΔG = −33.5 kcal/mol, just 7.5

Based on the above calculations, the catalytic cycle for the

H−I

kcal/mol higher than complex H. In complex I, the BH

hydroboration of CO with HBpin is proposed (Scheme 6).

moiety forms an unsymmetrical 3c−2e interaction with the

BH group of MeOBH . Coordination of a third BH ·SMe

First, CO inserts into the Al−H bond of complex 2 to give

complex 4. Subsequently, HBpin adds across the C=O bond in

4 to form IntI. The Al−H bond then undergoes σ-bond

metathesis with the O−C bond to form MeOBpin

coordinating with the Al center. In addition, the resulting

Al−O bond coordinates with the second HBpin to form IntII.

molecule occurs via TS (ΔG = −41.6 kcal/mol), requiring

I‑J

∼

29 kcal/mol, due to increased steric crowding. Upon

coordination, a B−H bond interacts with the Al center in

complex J (ΔG = −61.1 kcal/mol). A pathway regenerating

complex B rather than the higher energy complex A was found

CO then displaces MeOBpin and reacts with HBpin to form a

from J involving a second CO molecule. In fact, the insertion

formate moiety bonded to the aluminum in IntIII. The third

HBpin undergoes σ-bond metathesis with the Al−O bond in

IntIII to form complex 4 and pinBOBpin.

of CO into the B−H bond in J occurs barrierless to form

complex K, which is isoelectronic (ΔG = −61.1 kcal/mol). In

this complex, a formate OC(O)H, a methoxyborane MeOBH ,

Following the hydroboration of CO , the catalytic ability of

and BH -coordinated OBH moieties are bound to Al, which is

complex 2 toward the hydroboration of carbonyl, alkyne, and

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Scheme 6. Catalytic Cycle for the Hydroboration of CO₂

with HBpin

Table 1. Catalytic Hydroboration of Aldehydes and Ketones

nitrile derivatives was further examined. First, we veriﬁed that

there were no reactions between substrates with HBpin in

C D at room temperature or at 110 °C without a catalyst. On

the other hand, catalyzed hydroboration of aromatic aldehyde

ArC(O)H (Ar = Ph 5a, Table 1) with HBpin as well as its

derivatives with electron-donating (Ar = MeC H 5b and 5c,

MeOC H 5f) and -withdrawing substituents (MeCO C H

d, F CC H 5e) was completed in 1 h using 5 mol % of 2 at

3 6 4

0 °C. The corresponding borate esters were obtained in high

yield. Second, moderate yield was achieved for the hydro-

boration of nonaromatic aldehydes 5g−5k. Third, a higher

catalytic loading and temperature were required for the

hydroboration of ketones 5m−5p when compared to

aldehydes in line with their reduced electrophilic character.

Fourth, 10 mol % of 2 catalyzed the hydroboration of aromatic

nitriles ArC≡N (Table 2) in high yields. As shown in Table 2,

a large scope of electron-donating substituent (Ar = MeOC H

Reaction conditions: substrate (0.5 mmol), 1 equiv of HBpin (0.5

mmol), 5 mol % of 2 in C D (0.5 mL). Yields are determined by H

NMR spectroscopy on the basis of the integration of the consumed

carbonyl compound and R(R′)C(H)OBpin resonances. Isolated

yields are reported in parentheses. Reaction temperature: 110 °C

b), electron-withdrawing substituents (Ar = F CC H 7d,

Catalytic loading of 2: 10 mol %. All the catalytic trials were repeated

F C H 7e) and sterically hindered substituents (Ar =

in triplicate.

(

CH ) C H 7c) were hydroborated twice to form the

3 3 6 2

corresponding diboryl amine products [ArCH N(Bpin) ].

Replacing the aromatic substituents with alkyl groups (7f−

particular, complex 2 eﬃciently reduces CO with BH ·SMe

to form B(OMe) . Its activity is higher than that of known

h) resulted in both slower reaction rates and lower product

base-metal catalysts used for such a reaction. We have

observed that the additions to form the formate AlOCHO

yields. Finally, complex 2 was also able to catalyze the

hydroboration of aromatic alkynes (ArC≡CH, 9a−9d) with

electron-donating or -withdrawing substituents to form the

corresponding trans-boryl alkenes trans-[Ar(H)C=CH(Bpin)].

and the acetal AlOCH OAl derivatives are facile. DFT

calculations rationalized that further reduction to the methanol

derivative is rather demanding. In light of its catalytic

reactivity, we believe that complex 2 or its analogues will

ﬁnd uses in other catalytic applications. Such endeavors are

currently pursued in our laboratories, and results will be

reported in due course.

Based on the previous experiments to reduce CO , it is

suggested that the Al−H bond in complex 2 activates the

carbonyl, alkyne, or nitrile compounds, which are then reacted

with HBpin to form the corresponding hydroborated products,

along with the regeneration of complex 2.

EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out under an

inert atmosphere of argon gas by standard Schlenk techniques.

Compound 1 was prepared according to the literature procedure.

Toluene was dried over a Na/K alloy and distilled prior to use. C D

was dried over K metal and distilled prior to use. CDCl was dried

CONCLUSION

■

In conclusion, the bis(iminophosphoranyl)methanide alumi-

num complex 2 is a versatile catalyst for the hydroboration of

carbon dioxide, carbonyl compounds, alkyne, and nitrile

derivatives with HBpin to form methoxyborane, borate esters,

diboryl amines, and trans-boryl alkenes, respectively. In

over CaH and distilled prior to use. Chemicals were purchased from

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Ph-H), 7.10 (m, 4H, p-Ph-H), 7.02 (m, 8H, o-Ph-H), 6.57 (s, 2H, m-

Table 2. Catalytic Hydroboration of Nitriles

Mes-H), 6.50 (s, 2H, m-Mes-H), 2.26 (s, 6H, o-CH ), 2.12 (s, 6H, o-

CH ), 1.99 (s, 6H, p-CH ). C{ H} NMR (99.5 MHz, C D , ppm):

δ 161.6 (s, C(O)H), 138.1 (d, J_P₋_C= 53.2 Hz, Mes, ipso-C), 136.4 (s,

Mes o-C), 134.8 (s, Ph m-C), 134.3 (s, Ph o-C), 131.5 (s, Mes p-C),

31.3 (s, Ph m-C), 129.8 (d, J_P₋_C= 53.2 Hz, Ph ipso-C), 124.4 (s, Ph

p-C), 20.8 (s, Mes p-CH ), 20.6 (s, Mes p-CH ), not observable

(

PCP). P{ H} NMR (121.5 MHz, C D , ppm): δ 40.55 (s). HRMS

6 6

(ESI): m/z calcd for C H N O AlP Cl: 757.2460 [(M + H)] ;

44 45 2 2 2

found: 757.2467.

Synthesis of 4-D. Complex 2 (1.43 g, 2.0 mmol) was dissolved in

0 mL of toluene in a Schlenk ﬂask. The reaction mixture was freeze−

pump−thaw degassed thrice. CO gas (1 bar) was streamed into the

ﬂask using a Schlenk line along with vigorous stirring. Upon stirring

for 16 h, white precipitates were observed in the ﬂask. The reaction

mixture was dried under vacuum and extracted with a DCM (10 mL)

and hexane (1 mL) mixture. The resulting ﬁltrate was left to stand at

(

°C to yield colorless crystals of 4-D. Yield: 0.08 g (5.3%). H NMR

395.9 MHz, C D , ppm): δ 8.24 (s, 4H, OCH O), 8.02 (m, 16H, m-

Ph-H), 7.08 (m, 24H, o,p-Ph-H), 6.52 (s, 8H, m-Mes-H), 2.13 (s,

4H, o-CH ), 1.98 (s, 12H, p-CH ). C{ H} NMR (99.5 MHz,

C D , ppm): δ 161.3 (s, O−CH −O), 138.1 (s, Mes o-C), 135.5 (s,

Ph m-C), 135.1 (s, Ph o-C), 134.4 (t, Mes, ipso-C), 131.7 (s, Mes p-

C), 129.8 (s, Ph m-C), overlapped with solvent (Ph ipso-C), 124.4 (s,

Ph p-C), 20.5 (s, Mes p-CH ), 20.3 (s, Mes o-CH ), not observable

(

PCP). P{ H} NMR (121.5 MHz, C D , ppm): δ 40.55 (s). HRMS

6 6

(ESI): m/z calcd for C H N O Al P Cl : 1513.4842 [(M + H)] ;

88 89 4 4 2 4 2

Reaction conditions: nitrile compounds (0.2 mmol), 2 equiv of

found: 1513.4854.

Complex 2 Catalyzed Hydroboration of CO with HBPin. In

HBpin (0.4 mmol), C D (0.5 mL). Catalyst loading is relative to the

nitrile compounds. Yields are determined by H NMR spectroscopy

on the basis of the integration of the consumed nitrile compound and

RCH N(Bpin) resonances. Isolated yields are reported in paren-

a J. Young NMR tube, complex 2 (0.035 g, 0.05 mmol) and 1,3,5-tri-

tert-butylbenzene (0.0245 g, 0.10 mmol, internal standard) were

dissolved in toluene (0.5 mL). HBpin (0.064 g, 0.5 mmol) was then

added into the tube. The reaction mixture was freeze−pump−thaw

theses. All the catalytic trials were repeated in triplicate.

degassed thrice. CO gas (1 bar) was streamed into the tube using a

Sigma-Aldrich. They were degassed when received and then stored in

a glovebox. They were used directly without further puriﬁcation.

When chemicals were stored under air, they were dried, distilled, and

Schlenk line. The tube was subsequently heated under 110 °C for 48

h. The yield was calculated based on the total number of protons

converted into the methoxy product with reference to the integral of

internal standard. Yield = 78.4%.

Complex 2 Catalyzed Hydroboration of CO with BH ·SMe .

In a J. Young NMR tube, complex 2 (0.0035 g, 0.005 mmol) was

dissolved in toluene (0.5 mL). BH ·SMe (0.038 g, 0.5 mmol) was

degassed by standard Schlenk techniques before use. The H,

B{ H}, C{ H}, and P{ H} NMR spectra were recorded on a

JEOL ECA 400 spectrometer. The NMR spectra were recorded in

C D , and the chemical shifts are relative to SiMe for H, C, and

Si; BF ·Et O for B; and H PO for P respectively. Electrospray

then added into the tube. The reaction mixture was freeze−pump−

thaw degassed thrice. CO gas (1 bar) was streamed into the tube

ionization (ESI) mass spectra were obtained at the Mass

Spectrometry Laboratory at the Division of Chemistry and Biological

Chemistry, Nanyang Technological University. Control experiments

are written in the Supporting Information.

using a Schlenk line. The tube was subsequently heated under 110 °C

for 1 h. CH protons of SMe were used as the internal standard for

calculating yield. Yield was calculated based on the total number of

protons converted into a methoxy product. Yield = 97.6%.

Synthesis of 2. AlH ·EtNMe (2.0 mmol) was added dropwise to

a solution of 1 (1.0 mmol) in toluene at −78 °C in a Schlenk ﬂask.

The reaction mixture was raised to room temperature, and a white

suspension was observed. The mixture was stirred for 1 h and was

subsequently dried under vacuum to remove excess AlH ·EtNMe .

Complex 2 Catalyzed Hydroboration of CO with HBCat. In

a J. Young NMR tube, complex 2 (0.035 g, 0.05 mmol) and 1,3,5-tri-

tert-butylbenzene (0.0182 g, 0.074 mmol, internal standard) were

dissolved in toluene (0.5 mL). HBCat (0.064g, 0.5 mmol) was then

added into the tube. The reaction mixture was freeze−pump−thaw

The residue was extracted with 10 mL of toluene to yield a white

precipitate. Yield: 0.65 g, 0.92 mmol (92%, based on 1). Colorless

degassed thrice. CO gas (1 bar) was streamed into the tube using a

crystals were grown from concentrated toluene solution of 2. H

Schlenk line. The tube was subsequently heated under 110 °C for 24

h. Yield was calculated based on total number of protons converted

into the methoxy product with reference to the integral of internal

standard. Yield = 28.8%.

NMR (395.9 MHz, C D , ppm): δ 8.02 (m, 8H, m-Ph-H), 7.04 (m,

2H, o,p-Ph-H), 6.55 (s, 4H, m-Mes-H), 5.00 (br, 2H, AlH ), 2.23 (s,

2H, o-CH ), 2.01 (s, 6H, p-CH ). C{ H} NMR (99.5 MHz, C D ,

ppm): δ 138.30 (t, Ph m-C), 137.81 (s, Mes m-C), 134.26 (t, Ph o-C),

33.96 (s, Mes o-C), 131.14 (s, Ph p-C), 130.75 (d, J_P₋_C= 102.95 Hz,

Ph ipso-C), 130.60 (t, P C), 129.71 (s, Mes p-C), 127.29 (t, J

General Procedures for the Complex 2 Catalyzed Hydro-

boration of Carbonyl Compounds, Nitriles, and Alkynes.

Hydroboration (Tables 1−3) were performed in a J. Young NMR

tube under Ar gas. Complex 2 was dissolved into 0.5 mL of C D .

HBPin was then added into the solution and followed by the addition

of the respective substrates. The tube was subsequently heated in an

oil bath in respective temperatures. The conversion of the substrates

were obtained based on the total number of protons converted into

their hydroborated products. The products were isolated by drying

the reaction mixture under vacuum, followed by extraction with n-

hexane. The resulting solution was dried under vacuum and

characterized by NMR spectroscopy. Isolated yields of selected

products were subsequently obtained by weight.

P−C

.72 Hz, Mes ipso-C), 20.83 (s, Mes o-CH ), 20.42 (s, Mes p-CH ).

P{ H} NMR (121.5 MHz, C D ) δ 38.98 (s). HRMS (ESI): m/z

calcd for C H N AlP Cl: 713.2562 [(M + H)] ; found: 713.2573.

Synthesis of 4. Complex 2 (1.43 g, 2.0 mmol) was dissolved in 40

mL of toluene in a Schlenk ﬂask. The reaction mixture was freeze−

pump−thaw degassed thrice. CO gas (1 bar) was streamed into the

ﬂask using a Schlenk line along with vigorous stirring. After 15 min,

the reaction mixture was dried under vacuum to yield a white

precipitate. Yield: 1.51 g (100%). H NMR (395.9 MHz, C D , ppm):

δ 8.52 (s, 1H, OC(=O)H), 8.12 (m, 4H, m-Ph-H), 7.88 (m, 4H, m-

575

Inorg. Chem. 2021, 60, 4569−4577

Inorganic Chemistry

Complete contact information is available at:

Article

Table 3. Catalytic Hydroboration of Alkynes

Nanyang Technological University, Singapore 637371;

Laboratoire Hétérochimie Fondamentale et Appliquée,

CNRS, Université Paul Sabatier, 31062 Toulouse, France

Zhe-Hua Ng − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371

Hui-Min Toh − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371

https://pubs.acs.org/10.1021/acs.inorgchem.0c03507

Author Contributions

The manuscript was contributed to equally by C.-C.C. and Y.-

C.T.

Reaction conditions: alkyne compounds (0.5 mmol), 1 equiv of

Author Contributions

All authors have given approval to the ﬁnal version of the

manuscript.

Notes

HBpin (0.5 mmol), C D (0.5 mL). Catalyst loading is relative to the

alkyne compounds. Yields are determined by H NMR spectroscopy

on the basis of the integration of consumed alkyne compound and

trans-RCH=CBpin resonances. Isolated yields are reported in

parentheses. All the catalytic trials were repeated in triplicate.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03507.

■

This work is supported by the National Research Foundation

Singapore, NRF-ANR (NRF2018-NRF-ANR026 Si-POP) and

Ministry of Education Singapore, AcRF Tier 2 (MOE2019-T2-

-129) and AcRF Tier 1 (RG 17/17) (C.-W.S.). We thank

Experimental procedures; X-ray data collection and

structural re ﬁnement; DFT calculations; and selected

spectra (PDF)

CalMip (CNRS, Toulouse, France) for access to calculation

facilities.

Theoretical studies (XYZ)

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Cheuk-Wai So − Division of Chemistry and Biological

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Nicolas Mézailles − Laboratoire Hétérochimie Fondamentale

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Cher-Chiek Chia − Division of Chemistry and Biological

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Yeow-Chuan Teo − Division of Chemistry and Biological

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Ning Cham − Division of Chemistry and Biological Chemistry,

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