Formylation of Amines by CO2Hydrogenation Using Preformed Co(II)/Nickel(II) Complexes

Co(II)/Nickel(II) Complexes

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Formylation of Amines by CO Hydrogenation Using Preformed

Mohammad A. A ﬀ an, Gabriele Schatte, and Philip G. Jessop*

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sı Supporting Information

ABSTRACT: Preparation of formamides by CO hydrogenation

requires an eﬃcient catalyst and temperatures around 100 °C or

higher, but most catalysts reported so far incorporate rare and toxic

precious metals. Five cobalt(II) or nickel(II) complexes with dmpe

or PNN (dmpe = 1,2-bis(dimethylphosphino)ethane; PNN = [(2-

(

di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine)

have been evaluated as precatalysts for the hydrogenation of CO₂

to prepare formamides from the corresponding secondary amines.

The most active catalyst for these reactions was found to be

[

NiCl (dmpe)] in DMSO, producing dimethylformamide (DMF)

from CO , H , and dimethylamine in up to 6300 TON, the highest activity reported for this reaction with an abundant metal-

phosphine complex.

INTRODUCTION

amine, it may seem that catalysts for formamide production

would be the same as those for formic acid production. This is

not, however, always the case. That thermal amidation step

requires elevated temperatures of typically 100 °C, signiﬁcantly

■

Carbon dioxide is appealing as a chemical feedstock or reagent

because it is sustainable, inexpensive, and much less toxic than

other carbonyl-introducing agents such as phosgene and

higher than that typically used for CO hydrogenation to

carbon monoxide. However, a catalyst is required for most

formic acid. For that reason, catalysts for producing

formamides are often not the same as those for producing

formic acid.

reactions of CO , other than simple acid−base reactions, to

overcome the kinetic stability.

Hydrogenation of CO to formic acid or formamides has

been extensively studied: from the initial report by Farlow and

Among the literature examples of abundant metal homoge-

Adkins of the production of formamides by reaction of CO

neous catalyst precursors for CO

the (Ph P) CuCl complex, by Haynes et al., and the

Fe(BF ) ·6H O/P(CH CH PPh ) and Co(BF ) ·6H O/P-

reduction to formamides are

with H and an amine over Raney nickel catalyst giving 2

turnovers. Early examples of homogeneous catalysts for CO

4 2

2 3

4 2

15,16

hydrogenation included the work by Inoue et al. and

(CH CH PPh ) combinations, by Federsel et al.,

which

2 3

Trzebiatowska et al. in the 1970s. In general, homogeneous

were shown to convert CO and dimethylamine to N,N-

−8

catalysts have been used for many transformations of CO₂,

but almost always platinum group metals were used, which is

unfortunate due to their cost and toxicity.

dimethylformamide (DMF) in 900, 727, and 1254 TON,

respectively: still lower than the activities reported for precious

metal catalysts. In 2012, [Fe(P P)F]BF , [P P =

−11

Abundant metal catalysts, although they have been

historically under-represented in the homogeneous catalysis

literature, are certainly capable of being excellent hydro-

(

C H PPh ) P] was shown by Ziebart et al. to produce

6 4 2 3

DMF in 5100 TON. We showed that the in situ combination

of Ni(acac) and dmpe allowed morpholine to be converted

genation catalysts. Since 2003, there have been quite a few

into the corresponding formamide in 18000 TON (dmpe =

reports of abundant-metal catalysts for CO hydrogenation,

1,2-bis(dimethylphosphino)ethane).

but few of them have reported very high activities; typically the

best activity is 5000 TON or often much lower. Many years

ago, we showed using combinatorial screening that catalysts

Better progress has been made in the search for abundant

metal-catalysts for the synthesis of formic acid or formate

active for CO hydrogenation to formic acid could be formed

from many abundant metals, including Co, Cr, Fe, In, Mo, Nb,

Received: July 15, 2020

Ni, and W. For example, NiCl (dcpe) produced formic acid in

400 TON (dcpe = 1,2-bis(dicyclohexylphosphino)ethane).

Because the hydrogenation of CO to formamides proceeds

via catalytic hydrogenation to formic acid followed by a

thermal amidation of that acid with primary or secondary

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

anion, although as mentioned above, there is no guarantee that

such catalysts would necessarily be suitable for formamide

production. An in situ combination of a Fe PNP complex with

Scheme 1. Syntheses of Nickel(II) Complexes 1-3

a Lewis acid cocatalyst was found by Zhang et al. to

hydrogenate CO to formate in 59000 TON. Klankermayer’s

group reported a Ni(II) in situ complex with the tris(2-

diphenylphosphino)ethyl)amine tetradentate ligand showing

(

for the ﬁrst time that a base-metal catalyst could have suﬃcient

stability to give a TON of over 1 million. This milestone was

particularly important as stability rather than rate has been the

most signiﬁcant concern with base metal hydrogenation

catalysts.

Beside abundant metal-phosphine complexes, abundant-

metal pincer complexes as precatalysts have also recently

drawn attention for the synthesis of formic acid or formate

1,22

anion.

The synthesis and characterization of a pyridine-

based iron(II)-pincer complex were reported by the Milstein

group. They utilized the isolated iron complex for the

compound 1 was characterized by ¹H, P{ H} NMR

hydrogenation of CO to formate in the presence of a base at

low-temperature with a TON of up to 788.

spectroscopy and X-ray crystallography. The H NMR

spectrum of 1 showed the characteristic resonance of the

CH group protons as a singlet at 2.14 ppm while the methyl

groups appeared at 1.69 ppm. The P{ H} NMR spectrum

showed one singlet at 43.9 ppm due to the two similar Me₂P

groups of the dmpe ligand. Crystallography revealed the

structure of 1·2H O (see Supporting Information) which was,

apart from the extra water molecule, similar to the previously

In 2016, the Bertini group reported two iron(II) pincer

complexes bearing the 2,6-diaminopyridine scaﬀold [Fe-

H i

Me i

(

PNP - Pr)(H)(CO)(Br)] and [Fe(PNP - Pr)(H)(CO)-

Br)], respectively. Both complexes showed high catalytic

(

activity as homogeneous catalysts for the hydrogenation of

CO to formate in a protic solvent at room temperature.

The cobalt(I) complex [( PrPNP)Co(CO) ] with lithium

triﬂate (LiOTf) as cocatalyst was employed for the hydro-

published structure of 1·H O.

Complex 2, cis-[NiCl (dmpe)] has also been previously

prepared by the reaction of [Ni(dmpe) Cl]Cl with NiCl ·

H O in a 2:1 ratio in ethyl alcohol under a nitrogen

atmosphere, but unfortunately, the compound was only

characterized by elemental analysis and UV−visible spectros-

copy. The P{ H} NMR spectrum had only a single peak at

0.0 ppm; this indicated that Ni(II) has a square-planar

geometry in solution. Then, we attempted to repeat the

synthesis of 2 according to Handley et al. by the treatment of

genation of CO to formate. Among the cobalt(I) catalysts,

the paired Co(I)/LiOTf system improved the catalytic

activities, obtaining a TON of up to 30000, although this

TON value is less than that of the [(iPrPNP)Fe] catalyst. Also,

solvent eﬀects were also screened under the reaction

conditions, and acetonitrile solvent aﬀorded a maximum

TON of 14,000.

Recently, Bernskoetter et al. developed a few Fe- and Co-

pincer catalysts bearing alkali metal salts for the selective CO₂

hydrogenation to formate, showing that the Lewis acid

stabilizes and increases the activity of the primary catalyst.

with NaBPh in 1:1.5 ratio in MeOH and obtained the

related nickel complex, [Ni(dmpe) Cl]BPh 3, instead.

Complex 3 was obtained in fair to good yield and was found

to be soluble in chloroform and methanol.

Structure of 2. The slow vapor diﬀusion of ether produced

crystals in a saturated solution of 2 in MeOH. The solid-state

molecular structure of [NiCl (dmpe)] 2 is shown in Figure 1

see Tables S1 −S4 for details). In the solid state, this is a cis

A low valent molybdenum-pincer complex [(κ -PN P)-

Mo(C H )(κ-O CH)] and an alkali metal salt were developed

by Zhang et al. for CO hydrogenation to formic acid,

although the TON of 35 was quite modest compared to the

results of Bernskoetter.

Thus, the catalytic activity of abundant metal catalysts has

been well demonstrated for the hydrogenation of CO to

formic acid but not nearly as well for the formylation of amines

to formamides. There is still much scope for the further

development of highly selective and cost-eﬀective catalysts for

the hydrogenation of CO to formamides. Here, we report the

synthesis and characterization of Ni(II)-dmpe and Co(II)/

Ni(II)-PNN pincer complexes and their catalytic activities

toward the hydrogenation of CO₂to formamides. The

preformed [NiCl (dmpe)] precatalyst showed the best

catalytic activity for the synthesis of DMF with the highest

TON value of 6300 among base metal catalysts.

RESULTS AND DISCUSSION

Synthesis and Characterization of Ni(II)-dmpe Com-

■

plexes. The reaction of NiCl ·6H O and a slight excess of

Me PCH CH PMe (dmpe) in ethyl alcohol at reﬂux

Figure 1. Molecular structure of [NiCl (dmpe)] (2) with the thermal

ellipsoids at 50% probability level. Hydrogen atoms were omitted for

clarity.

temperature produced a ﬁve-coordinate nickel(II) species,

NiCl(dmpe) ]Cl·2H O 1 (see Scheme 1 and Figure S1 ). The

[

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

square planar nickel(II) diphosphine complex, despite the fact

that in the literature, the same crystallographically determined

structure is described as a trans tetrahedral complex.

Complex 2 is slightly distorted from a perfect square-planar

geometry; the P(4)−Ni(1)−Cl(1) 168.588(17) and the

P(1)−Ni(1)−Cl(2) 168.013(16) angles are similar to other

nickel complexes containing dppe/dmpe in a ﬁve-membered

9,30

chelate ring.

6.614(15)° which is slightly lower than the ideal 90.00° but

similar to related complexes having a diphosphine ﬁve-

The bite angle P(4)−Ni(1)−P(1) is

membered chelate ring. The Ni−Cl bond distances

(2.2182(4) and 2.2325(4)Å)] are within the typical range

for such bonds in Ni-diphosphine complexes. The two Ni−P

bond lengths 2.1322(4)Å and 2.1394(2)Å are very close due to

the chemical equivalence of the two phosphorus atoms but

slightly shorter than that observed in nickel bidentate

[

Figure 2. Molecular structure of complex 5 with the thermal ellipsoids

at 50% probability level.

phosphine complex, [Ni(dmpe) ](PF ) ]. The solution

6 2

phase P{ H} NMR spectrum of 2 is also consistent with a

square planar geometry. If 2 had been tetrahedral in solution,

its resulting paramagnetism would have at least broadened the

NMR spectrum.

∼178.73(7)° deviate slightly from linearity, and P(14), N(1),

N(8), Ni(1) are almost coplanar, consistent with a square

planar structure of Ni(II). The two bite angles, N(1)−Ni(1)−

P(14) ∼86.48(6)° and N(1)−Ni(1)−N(8) ∼85.98(8)°, are

slightly less than 90°. The Ni−Cl distance of 2.159(7) Å is in

the range encountered for a similar square-planar nickel(II)

Synthesis and Characterization of Co(II) or Ni(II)-PNN

Complexes. Milstein’s pincer complex [Co(PNN)Cl ]·

C H O 4 and the related [Ni(PNN)Cl]Cl·H O 5 were

synthesized by the reaction of anhydrous CoCl or NiCl ,

respectively, with 1.2 equiv of the PNN ligand in THF

Scheme 2). Complex 4 was dark blue, air/moisture sensitive,

(

complex [(Me NNN )NiCl]. In complex 5, the Ni(1)−

P(14) bond length of 2.182(6) Å and the average C−C bond

lengths of the pyridine ring ∼1.374 Å are similar to those

Scheme 2. Synthesis of Complexes 4 and 5 from PNN

Ligand

previously reported for [Ni(PNP Pr)Cl)] complexes. The

Ni(1)−N(1) (pyridyl) distance of 1.876(2) Å is shorter than

the Ni(1)−N(8)(amino) bond length of 2.014 (19)Å; a

similar observation is found in complex 4.

Catalytic Activities of Ni(II) Complexes 1−5. The next

step was to test whether complexes 1−5 work as catalyst

precursors for the hydrogenation of CO to DMF in anhydrous

DMSO as a solvent. The results of these experiments are

summarized in Table 1. A blank test conﬁrmed that no activity

was observed in the absence of a Ni(II)/dmpe complex. Under

the optimized reaction conditions, Ni-complexes 1 and 2 were

catalytically active and produced the desired DMF, but

complex 3 was surprisingly inactive.

Table 1. Nickel-Catalyzed Hydrogenation of CO to DMF

and required handling under an inert atmosphere. Complex 4

displayed broad H NMR signals, which were thus not very

useful for determining the structure in solution. The solid-state

structure of complex 4 was conﬁrmed to be the same as that

reported by Milstein et al. Complex 5 was a red, air-stable

b c

_T_O_N,

precursor

Ni/Me NH

Yield

none

crystalline solid that was soluble in CHCl and DMSO but

[Ni(dmpe)

Cl]Cl 1

1:100

1:500

insoluble in common organic nonpolar solvents. The P{ H}

NMR spectrum of 5 showed a single resonance at 39.9 ppm.

Structure of 5. Single crystals suitable for the X-ray

analysis of 5 were obtained by the layering of hexane on a

″

[

″

310

2300

800

6300

1:10000

1:100

1:1000

1:20000

1:100

Ni(dmpe)Cl ] 2

saturated solution of 5 in CHCl . The solid-state structure of 5

is shown in Figure 2 (see Tables S5 and S6 for details). The

Ni(II) center adopt a slightly distorted square-planar geometry

in which the Ni(II) ion is coordinated by two N, one P, and

one Cl atoms. A second chloride is the counteranion and does

not show any close contacts with the nickel center within the

crystal packing. The N(8)−Ni(1)−P(14) bond angle

[

Ni(dmpe) Cl]BPh 3

2 4

Reaction conditions: catalyst (0.67 μmol), and dimethylamine (12.0

mmol, delivered in the form of DIMCARB) in DMSO (0.5 mL), 100

C, stirring rate of 450 min , 21 h reaction time, 60 bar CO , with

enough H added to bring the total pressure to 100 bar. Average of

the results from two trials. Determined by NMR spectroscopy.

−

∼

171.49(6)° and the N(1)−Ni(1)−Cl(1) angle

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

University, Kingston, Ontario K7L 3N6, Canada;

Article

The most active catalyst is complex 2 for the hydrogenation

Experimental methods, possible mechanisms, crystallo-

graphic data, and preliminary screening data (PDF)

of CO to DMF under our standard reaction conditions, with

the highest TON value of 6300. Because complex 2 is a 16e

Accession Codes

complex, it should be able to quickly react with H to produce

CCDC 1538716 and 1538723 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

a Ni−H intermediate. Subsequent insertion of CO into the

Ni−H bond would produce a formato compound. Because

complexes 1 and 3 are coordinatively saturated, they may be

more stable than 2, which may be why they are less

catalytically active. The TON value of 6300 is the highest

observed using a nickel(II) catalyst and comparable to the best

base metal catalysts previously reported for this reaction: the

Fe(II)/phosphine complex [TON = 5100] and Co(II)/

phosphine complex [TON = 1300] reported by Federsel et

AUTHOR INFORMATION

Corresponding Author

■

al. and Beller et al.

Philip G. Jessop − Department of Chemistry, Queen’s

orcid.org/0000-0002-5323-5095; Email: jessop@

queensu.ca

It is not clear why [Ni(dmpe) Cl]BPh 3 fails as a catalyst

for the hydrogenation of CO to formic acid or formamide

when the structurally similar complex 1 is active. In complex 1,

−

Cl is considered a coordinating counterion, but in complex 3,

−

Authors

BPh₄is considered a noncoordinating counterion. We can

Mohammad A. Aﬀan − Department of Chemistry, Queen’s

University, Kingston, Ontario K7L 3N6, Canada

Gabriele Schatte − Department of Chemistry, Queen’s

University, Kingston, Ontario K7L 3N6, Canada

speculate, therefore, that the second equivalent of chloride in

complex 1 plays an important role as a stabilizing ligand for a

catalytic intermediate or resting state, while the noncoordinat-

−

ing BPh₄anion is incapable of that role. Although the exact

reaction mechanism is unknown at this stage, based on the

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c01401

17,36

existing literature reports,

a plausible catalytic cycle for the

hydrogenation of CO using nickel(II) complex 2 is shown in

Scheme S1 . The subsequent thermal amidation step does not

require a catalyst.

Notes

The authors declare no competing ﬁnancial interest.

The isolated Co(II)/Ni(II)-PNN pincer complexes 4 and 5

were tested as precatalysts for the hydrogenation of CO to

ACKNOWLEDGMENTS

■

formamide. Unfortunately, no catalytic activity was observed

^f^o^r^t^h^e^h^y^d^r^o^g^eⁿ^a^tⁱ^oⁿ^o^f^C^O2 in the presence either of

morpholine or dimethylamine as substrates under the applied

reaction conditions (Table S7 ). Attempts to prepare hydrides

We gratefully acknowledge ﬁnancial support from the Canada

Research Chairs Program and the Natural Sciences and

Engineering Research Council, CREATE/413798-2012.

from complexes 4 and 5 by reaction with H did not give the

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