Remarkable catalytic activity of dinitrogen-bridged ... · Synthesis of dinitrogen-bridged...

CMo

P

P P

P

Mo

C

IR (THF): 1945 cm–1,2041 cm–1(nNN)

Mononuclear structurein solution

(equiv/cat.) toluenert, 20 h

cat.(0.010 mmol)

72 equiv/cat. 96 equiv/cat.

N2 + + 2 NH3

1 atm

66 ReductantNH

OTf

Catalytic conversion of dinitrogen to ammonia with molybdenum-PCP[2] complex

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N Dinuclear structure is maintained in solution

Long life due to the strong bondbetween molybdenum and PCP[1] ligand

Highest catalytic activity

Mo–C : 2.153(4) Å∠P–Mo–P : 163.18(8)�∠NHC–N2(terminal) : 43.74(7)�

NN

Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

NN

NN

1 Department of Systems Innovation and 4 Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan2 School of Liberal Arts and Sciences, Daido University, Takiharu-cho, Minami-ku, Nagoya, 457-8530, Japan3 Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka, 819-0395, Japan

NN

N

N

Aya Eizawa,1 Kazuya Arashiba,1 Hiromasa Tanaka,2 Shogo Kuriyama,1 Yuki Matsuo,3 Kazunari Nakajima,4Kazunari Yoshizawa3 and Yoshiaki Nishibayashi1

This Work

Previous work

Synthesis of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing PCP ligands

Molybdenum-dinitrogen complexesbearing PCP-type pincer ligands

R = H

R = OMe

23 equiv/cat.[12 equiv/Mo]52 equiv/cat.[26 equiv/Mo]

NMoNNMoN

P

PP

P

N

NN

N

N

NN

N

RR

P = PtBu2

Schrock, R. R. et al.Science 2003, 301, 76.

Nature Chem. 2011, 3, 120.Nature Commun. 2014, 5, 3737.J. Am. Chem. Soc. 2014, 136, 9719.

[ 8 equiv/Mo ]

Mo

N

NNN

iPriPr

iPr

HIPTHIPT

HIPT

NN iPr iPr

iPrcat.

PP

P

ClMo

NPh

+

[ 63 equiv/Mo ]

J. Am. Chem. Soc.2015, 137, 5666.

P = PtBu2

cat. 2 NH3N2 + 6 e− + 6 H+

1 atm Reductant Proton Source rt

More strongly-bindingligand enabled higher catalytic activity.More electron-donating ligand

enabled higher catalytic activity.

cat.

P = PtBu2

2.0�102 equiv/cat. 3.2 equiv/cat.

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

NNN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

NMoNNMoN

P

PP

P

N

NN

N

N

NN

N

cat. 2 NH3N2 + 6 e− + 6 H+

1 atm Reductant Proton Source rt NHCStrong s donorHigh binding ability

NP

NP

Previous work This work

23 equiv/cat.

P = PtBu2

37%

Dinuclear structure is maintained in solution

: 1978 cm–1 (nNN).: 1979 cm–1 (nNN).

IR IR

(KBr)(THF)

: 1936 cm–1 (nNN).: 1944 cm–1 (nNN).

IR IR

(KBr)(THF)

cf) IR (KBr): 1911 cm–1 (nNN).

NMoNNMoN

P

PP

P

N

NN

N

N

NN

N

THFrt, 18 h

N2 (1 atm)Na-Hg

NN MoP

PCl

Cl

Cl NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

53% P = PtBu2

Nature Communications 2017, 8, 14874.

NH3

(equ

iv/ca

t.)

cat.

P = PtBu2

Reductant

(equiv/cat.)

E1/2 (V vs Ag/Ag+) in MeCN −1.15

17.6 11.85.7 −1.35 −1.78

Catalytic conversion of dinitrogen to ammonia with molybdenum-PCP[1] complex

Time (h)10 200

20

40

0

cat. Cr

toluenert, 20 h

cat.(0.0010 mmol)

1440 equiv/cat. 1920 equiv/cat.

N2 + + 2 NH3

1 atm

66NH

OTf

cat. 2 NH3N2 + 6 e− + 6 H+

1 atm 216 equiv/cat. 288 equiv/cat.toluene

rt

2.0�102 equiv/cat. [1.0�102 equiv/Mo]

P = PtBu2

17 h–1

42 h–1

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

Co Cr Co

Reductant

E1/2 (V vs Ag/Ag+) in MeCN −1.15

3.2 2.91.4 −1.35 −1.78

Co Cr Co

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

cat.

P = PtBu2

46 equiv/cat.

23 equiv/cat.

toluenert, 20 h

cat.(0.010 mmol)

72 equiv/cat. 96 equiv/cat.

N2 + + 2 NH3

1 atm

66 ReductantNH

OTf

Mo

P

P

C

46 equiv/cat.

23 equiv/cat. 0 equiv/cat.

23 equiv/cat.

cat. NH3 1st

NN Mo N N Mo

N

N

P

PP

P

NN

N

NN

N

NN

H+

e–

Dinuclear structure is essentialfor the first protonation step.

No protonation to mononuclear dinitrogen complex proceeds.

Mononuclear structure in solution

No catalytic activityProtonation to the dinitrogen ligands is inhibited.

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

NNN Mo N NP

PN

N

N

NTHFrt

+ H+

No reaction

NN Mo N NP

P

NN

NN

e– (216 equiv)H+ (288 equiv)

e– (216 equiv)H+ (288 equiv)

toluenert, 20 h

toluenert, 2 h

N2

cat.+

N2+N2+

NH3 NH3

Mo–C : 2.064(2) Å∠P–Mo–P : 153.95(3)�∠NHC–N2(terminal) : 82.35(5)�

NH3 2nd

Proposed reaction mechanism

PCP[1] ligand PCP[2] ligand

NN

NNNN

P

P

MoC

NN

N

N

Molybdenum-catalyzed reduction of N2into NH3 under ambient reaction conditions

Lifetime of the catalysts

Time profiles

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

NN

N MoP

PCl

Cl

ClTHF

rt, 18 h

N2 (1 atm)Na-Hg

recryst. frombenzene-hexane

NN Mo N NP

PN

N

N

NTHF

rt

PNP complex is deactivatedwithin 20 h.

PCP[1] complex is still active after 20 h of reaction.

NH33 H+, 4 e–

NH3

N2

2 H+, 2 e–

H+N

N Mo N N MoN

N

P

PP

P

NN

N

NN

N

NN

NN Mo N N Mo

N

N

P

PP

P

OTfN

NN

N

NN Mo N N Mo

N

N

P

PP

P

OTfN

NN

N

NN Mo N N Mo

N

N

P

PP

PNH3

NN

N

NN

N

N

NHN

NMoNNMoN

P

PP

P

N

NN

N

N

NN

N

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

cat.

NMoNNMoN

P

PP

P

N

NN

N

N

NN

N

NN Mo N N Mo

N

N

P

PP

PN

N

N

N

N

NN

N

PNP ligand