Transition metal complexes with N-heterocyclic carbene ligands: From organometallic hydrogenation reactions toward water splitting

Accepted Manuscript Title: Transition Metal Complexes with N-heterocyclic Carbene Ligands: From Organometallic Hydrogenation Reactions towards Water Splitting Author: Simon Kaufhold Lydia Petermann Robert Staehle Sven Rau PII: DOI: Reference:

S0010-8545(14)00337-3 http://dx.doi.org/doi:10.1016/j.ccr.2014.12.004 CCR 111970

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

1-8-2014 4-12-2014 7-12-2014

Please cite this article as: S. Kaufhold, L. Petermann, R. Staehle, S. Rau, Transition Metal Complexes with N-heterocyclic Carbene Ligands: From Organometallic Hydrogenation Reactions towards Water Splitting, Coordination Chemistry Reviews (2014), http://dx.doi.org/10.1016/j.ccr.2014.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Edited Dec 7

1

Transition Metal Complexes with N-heterocyclic Carbene Ligands:

2

From Organometallic Hydrogenation Reactions towards Water

3

Splitting

5

#,a

#,a

#,a

,a

Simon Kaufhold , Lydia Petermann , Robert Staehle , Sven Rau*

7

#

8

* Corresponding author

9

a

These authors contributed equally to the manuscript

University of Ulm

12

Albert-Einstein-Allee 11

13

89081 Ulm; Germany

14

e-mail: [email protected]

15

PH:

16

FAX: +49 731 5023039

an

11

M

Materials and Catalysis

us

Institute of Inorganic Chemistry

10

+49 731 5023900

19

te

In Memoriam Karen J. Brewer.

d

17 18

cr

6

ip t

4

Content

21

1

Introduction............................................................................................................................................. 3

22

2

Hydrogenation and transfer hydrogenation reactions ............................................................................ 6

23

3

Thermal hydrogen formation from alcohols and amines...................................................................... 15

24

4

Catalytic water splitting......................................................................................................................... 20

Ac ce p

20

25

4.1

Intermolecular water reduction .................................................................................................... 21

26

4.2

27

4.3

28

4.4

Modification of the NHC unit........................................................................................................ 24

29

4.5

Modification of the terminal ligands ............................................................................................. 26

30

4.6

Water Oxidation ........................................................................................................................... 27

PMD for photocatalytic hydrogen evolution ................................................................................. 21 Modifications of the system ......................................................................................................... 24

31

5

Conclusion and Outlook ....................................................................................................................... 29

32

Acknowledgments ....................................................................................................................................... 30

33

References .................................................................................................................................................. 30

34 35 1 Page 1 of 36

Edited Dec 7 35 36

Abstract

37 N-Heterocyclic carbenes (NHCs) as a new class of ligands for the stabilization of catalytic metal

39

complexes for catalytic water splitting are discussed. An overview over current applications of NHC

40

stabilized metal catalysts in organometallic catalysis involving hydrogen in transfer hydrogenation,

41

hydrogenation and acceptorless dehydrogenation is presented. A focus is placed on the role of the NHC

42

ligand structure and utilized metal centers. The current status of NHC stabilized catalytic centers within the

43

inter- and intramolecular photocatalytic hydrogen formation and water oxidation is reviewed. The very

44

interesting photochemical properties of a new class of ruthenium complexes with NHC-carbene containing

45

potential bridging ligands are discussed. These complexes can bind catalytic metal centers at the NHC

46

sphere. The so formed photochemical devices are active photocatalysts for hydrogen evolution.

M

an

us

cr

ip t

38

47

Abbreviations

d

48 49

A = acceptor

51

AD = acceptorless dehydrogenation

52

aNHC = abnormal N-heterocyclic carbene

53

AP = artificial photosynthesis

54

bip = 1-benzyl-1H-imidazo[4,5-f][1,10]phenanthroline

55

bbip = 1,3-dibenzyl-1H-imidazo[4,5-f][1,10]phenanthrolinium

56

CAN = Cerium ammonium nitrate

57

cod = 1,5-cyclooctadiene

58

Cox = oxidation center

59

Cred = reduction center

60

D = donor

61

DABCO = 1,4-Diazabicyclo[2.2.2]octan

62

Et2BzIm = 1,3-diethyl-1H-benzo[d]imidazol-3-ium

63

erNHC = expanded ring NHC

64

FTO = Fluorine-doped tin oxide

65

ip = 1H-imidazo[4,5-f][1,10]phenanthroline

66

M = metal

67

MA = maleic anhydride

Ac ce p

te

50

2 Page 2 of 36

Edited Dec 7 mmip = 1,3-dimethyl-1H-imidazo[4,5-f][1,10]phenanthrolinium

69

N,N′ = 2,2’-bipyridine or 1,10-phenanthroline

70

NADPH = reduced nicotinamide adenine dinucleotide phosphate

71

NHC = N-heterocyclic carbene

72

P = photocenter

73

PMD = photochemical molecular device

74

tbbpy = 4,4’-di-tert-butyl-2,2’-bipyridine

75

TH = transfer hydrogentation

76

TON = turnover number

77

X = coordinated solvent

cr

ip t

68

us

78 79

Keywords: photocatalysis, N-heterocyclic carbenes, ruthenium, organometallic catalysis, water splitting

an

80 81

M

82

1 Introduction

85

It is widely agreed that climate change and shortage of fossil fuels are some of the major challenges for

86

present and future generations alike. The carbon dioxide (CO2) concentration in the atmosphere has been

87

rising steadily since the beginning of the industrial revolution [1]. As a result the current CO2 level is the

88

highest in at least the past 800 000 years and a concomitant rise in average temperature and sea level

89

has been witnessed [2–4]. Although resources of fossil fuels are expected to last something between

90

decades and centuries the availability of cheap fossil energy will undeniably come to an end and thus

91

alternative energy sources are necessary [5–7]. Of the alternative energy sources like geothermal, wind

92

and hydro power, solar energy is the most outstanding due to the tremendous amount of energy the solar

93

irradiation provides on the earth’s surface [5]. However the great challenge of efficiently capturing and

94

storing solar energy remains unsolved. One of the most promising approaches to tackle future energy

95

supply problems as well as climate issues is capturing and converting solar energy into chemical energy

96

by mimicking natural photosynthesis. In artificial photosynthesis (AP) water would be split into hydrogen

97

(H2) as the product of reduction – similar to NADPH in natural photosynthesis – and oxygen (O2) as the

98

product of oxidation. Among the different methods of AP the use of molecular assemblies in a

99

homogenous system is a very promising approach. The principle setup of a homogeneous AP system is

Ac ce p

te

d

83 84

3 Page 3 of 36

Edited Dec 7 shown in Figure 1. A photosensitizer P is brought into an electronically excited state by absorption of light.

101

By charge separation an electron can be transported over an electron relay to the reduction center Cred

102

where generation of H2 from protons can take place. The oxidized photosensitizer is reduced by electrons

103

shuttled from the oxidation center Cox where water is split into O2 and protons. The advantage of this

104

system consisting of different building blocks is that each assembly can be modified and investigated

105

separately and thus enables systematic optimization. This concept has been very successful in improving

106

a wide range of organometallic catalytic reactions [8].

H+

cr

e2 H+

C ox

Cred

P

107

4

108 109 110

Figure 1: General setup of an artificial photocatalytic system composed of oxidation catalyst Cox, photosensitizer P and reduction catalyst Cred.

111

This overall water splitting in a single system is very challenging as it is composed of an 4-electron

112

oxidation step and a 2-electron reduction step. Furthermore reactive intermediates can be formed during

113

the reaction and might deactivate or damage the system especially in the case of the oxidation reaction

114

[9–11]. Thus, it is helpful to simplify the operation by separating both half-reactions and investigate them

115

discretely in order to gain deeper insight into the underlying processes. Although heterogeneous systems

116

for catalytic overall water splitting are known, we will focus on concepts with separated half-reactions and

117

molecular catalysts in this article.

118

To be able to carry out the reduction or oxidation separately it is necessary to add a sacrificial electron

119

donor or acceptor to the system, respectively (Figure 2).

te

d

M

H2

Ac ce p

120

+ O2

an

2 H 2O

us

e-

ip t

100

4 Page 4 of 36

Edited Dec 7

eA

2 H2O C ox

P

4 H+ + O2

ip t

A-

2 H+

D D+

Cred H2

us

P

121

cr

e-

Figure 2: Half-reactions for water oxidation and reduction.

124

One of the first molecular system for water reduction was reported by Lehn and Sauvage in the late 70ies

125

using ruthenium and rhodium bipyridine complexes and colloidal platinum [12]. A few years later Meyer et

126

al. introduced the “blue dimer” a binuclear ruthenium bipyridine complex for water oxidation [13].

127

Since then and especially in the past decade homogeneous photocatalytic water splitting has prospered

128

significantly. This is due to deeper insight in underlying mechanisms as a result of more sophisticated

129

spectroscopic and computational methods and the tunability of these systems by advanced synthetic

130

approaches, as highlighted in several reviews [10,11,14–19].

131

All the early concepts feature intermolecular systems where each task (light absorption, electron

132

mediation and catalytic turnover) are carried out by individual molecules and thus relying on collision

133

processes for energy/electron transfer. Hence, efficiency is limited by diffusion and collision probability.

134

More recently the concept of supramolecular catalysts – where the different moieties are linked to each

135

other – found more and more favor [10,20–25]. An assembly like this can also be considered as a

136

photochemical molecular device (PMD) where each part performs a certain task [26]. The complete device

137

gives then insight into a structure and function correlation and underlying processes e.g. directional

138

electron transfer. For better comparison of the individual catalytic systems the amount of product

139

produced is correlated to the amount of catalyst used. This is usually reported in turnover numbers (TON

140

= number of product molecules per catalyst molecule).

Ac ce p

te

d

M

an

122 123

5 Page 5 of 36

Edited Dec 7 Apart from efficient photosensitizers and catalytic centers the bridging ligand plays an important role in the

142

catalytic process [22,27]. Firstly it needs to be able to bind to the photocenter without altering the optical

143

absorption behavior in order to sustain the function of the sensitizer. Secondly it has to be able to transfer

144

electrons e.g. by an extended π-system and thirdly it has to bind and stabilize the catalytic center in

145

different oxidation states to support the catalytic cycle [27]. The last aspect seems to be crucial as colloids

146

are formed under catalytic conditions in some cases, indicating insufficient binding of the bridging ligand to

147

the metal center in its reduced/oxidized state [22,28,29].

148

N-heterocyclic carbenes (NHCs) have become renowned ligands stabilizing high valence transition metal

149

centers in complexes as well as late transition metals in low oxidation states during catalysis due to their

150

extraordinary binding properties. Even though free NHCs are sensitive to oxidation or hydrolysis, if not

151

stabilized by bulky N-substituents, their transition metal complexes are astonishingly stable. This is due to

152

the strong σ-donor character of NHCs that is often stronger than that of the most basic phosphane ligands

153

as has been shown in experiments and calculations [30–32]. For electron deficient metal centers NHCs

154

can also donate electron density from π-orbitals what explains their ability to stabilize high oxidation states

155

[33]. Nevertheless, NHCs can also accept electron density into their low energy π*-orbitals to a significant

156

extent leading to further stabilization of the bond to the low valent metal center [30,34–38]. Seen as simple

157

phosphane mimics, NHCs have developed to a ligand family whose complexes often outperform those of

158

related phosphane ligands and also proved to be more stable. However the use of NHC ligands in water

159

splitting reactions remains scarce. In this review we aim to collect examples where water reduction or

160

oxidation, as well as related reactions such as dehydrogenation of alcohols and transfer hydrogenation,

161

have been carried out incorporating transition metal NHC complexes. The latter two reactions may be

162

seen as a source of inspiration of future research activities as essential components like metal hydride

163

bond formation are key elements here as well.

Ac ce p

te

d

M

an

us

cr

ip t

141

164 165

2 Hydrogenation and transfer hydrogenation reactions

166 167

NHC-M (M = metal) complexes catalyze reactions like hydrogenation, transfer hydrogenation (TH) and

168

transfer dehydrogenation. With respect to the relevance in water splitting reactions the applied catalytic

6 Page 6 of 36

Edited Dec 7 centers, the ligand environment as well as the occurring reaction mechanisms and their catalytic transition

170

states are of major importance and are addressed in more detail in the following section.

171

Hydrogenation describes the activation of molecular hydrogen accompanied by oxidation of the hydrogen

172

atoms and concomitant reduction of unsaturated organic molecules [39–41]. TH is an attractive alternative

173

to standard hydrogenation, where the use of potentially dangerous high pressure of hydrogen has to be

174

avoided [42–52]. In TH the hydrogen is transferred from a hydrogen donor, typically alcohols (e.g. 2-

175

propanol) or formic acid, to an acceptor molecule, mostly in the presence of a base [43,45,48,53,54]. This

176

concept, as a mild and environmentally friendly methodology, is already applied in large-scale industrial

177

use [42–52]. The general hydrogenation and transfer hydrogenation reactions are depicted in Scheme 1.

X

H

X

H H

X

H +

A

X = CH2, O, NR

+ D

X = O, NR

+ AH 2

X = O, NR

te

178

+ DH2

H

an

X

H

M

H2

+

X

d

X

us

cr

ip t

169

179

Scheme 1: General equations for hydrogenation (top), transfer hydrogenation (middle) and transfer dehydrogenation (bottom) reactions.

183

In the growing research field of NHC-M catalyzed reactions hydrogenation and TH have been featured

184

prominently in the past 15 years [30,42,55]. Detailed review articles already described the reduction of

185

carbonyl, imine, nitro and ester functionalities as well as of alkynes and alkenes by catalytic systems

186

containing ruthenium [54], rhodium [42], iridium [48,56], osmium [30], gold [30] and palladium [57] metal

187

centers [30,42,55] with high chemo- and enantioselectivity. An overview of reported reactions is

188

represented in Scheme 2.

Ac ce p

180 181 182

7 Page 7 of 36

Edited Dec 7

OH

O

O

OH

+

+

A

O O

R'

H2

+

R OH

O

+

R'

OH OH

cr

us KOH

+ H2

d

R'

M

CO2 + H 2

an

+ H2

N

te

+ HCO2H

189

B

O +

+

R

OH

ip t

R

KCOOK + H2

R

H N

C

D

E

F

R'

+ CO2

G

Scheme 2: Selected hydrogenation and TH reactions.

192

Many catalytic systems with different NHC ligands were already investigated including mono-NHC and

193

chelating bis-NHC, abnormal NHC (aNHC) as well as expanded ring NHC (erNHC) [30,42,55]. Catalysts

194

containing different substituents at the NHC-N and the NHC backbone significantly alter the catalytic

195

effectivity and selectivity [39,54,56–61]. However, coordinated co-ligands have a further substantial

196

influence on effective catalysis [39,54]. For better understanding of catalytic effectivity a detailed insight

197

into the single catalytic steps is important. Therefore different general pathways for hydrogenation and TH

198

reactions with different catalytically active metal centers were verified and were already summarized in

199

some review articles [43,45]. Usually in hydrogenation as well as in TH a metal hydride, a dihydride, or a

200

dihydrogen complex is involved as the active catalyst [54].

Ac ce p

190 191

201

8 Page 8 of 36

Edited Dec 7 Within the following section we want to show some representative examples of different catalytic NHC-

203

systems for hydrogenation and TH (see Figure 3) and illustrate some examples of occurring reaction

204

mechanisms.

Ac ce p

te

d

M

an

us

cr

ip t

202

9 Page 9 of 36

ip t

(arene) A

A M

X

B

N

N

N

N

a

us

B 1 : A=B=a , X=Cl, arene=C 5Me 5, M=Ir 2 : A=B=a , X=H, arene=C 5 Me5 , M=Ir 3: A=b, B=X=Cl, arene=benzene, M=Ru 4 : A= b, B=X=Cl, arene= C6 Me6 , M=Ru 5 : A= b, B=X=Cl, arene=p-cym, M=Ru 6: A=c, B=X=Cl, arene=p-cym, M=Ru 7 : d, X=Cl, arene=p-cym, M=Ru 8 : e, X=pyridine, arene=C 5 Me5 , M=Ru 9: A=f, B=X=Cl, arene=p-cym, M=Ru

10 : A=g, B=Br, M=Ir 11 : A=h, B=Br, M=Ir 12: i, M=Ir 13 : (R)-k , M=Ir 14 : (R)-k , M=Rh

N

M an

M

N g

O

b N

N

N

N

d

H 2N N

N N

SO3 -

t

Zn

N tBu

N

N

Ph2P N

N

N

KO3S 16 PPh3N N

H

N

CO N

N

CO PPh3

N

N 17

18

N

H N Ru

Ph i

N CO

N

k

Br 19

O O

O

N O solvent

O I

Ru

Ru

O Pd

I

N H

N

O Ir

15

O

Bu N

O

OMe

I

N N

Ir

N

N

N

f

Ac

e

N

O

SO3 K

Bu

ce pt

c

N

I

N

OH

t

N

N N

Bu

ed

N

N

OMe

KO3S

SO3 K

h

t

N

cr

Edited Dec 7

N

O Pd N

O

Pd

N O

O O

N

N Pd

O

O

N

O

N

O

205 206

20

21

22

23

24

Figure 3: Selected NHC-M complexes for hydrogenation and TH reactions.

10

Page 10 of 36

Edited Dec 7 207

The outcome of selected reactions is summarized in Table 1.

208 Table 1: Overview of selected catalytic systems for hydrogenation and TH and their activity. a a b Entry Type Catalyst Additives TON A

1%

10%

1

KOH

99

Ref. [62,6

ip t

1

3]

t

A

5x10 % 3

1.3% KO Bu

3

A

0.5%

4

13%

KO Bu

4

A

0.5%

5

13%

KO Bu

5

A

0.5%

6

13%

KO Bu

6

A

0.5%

7

13%

KO Bu

7

A

1%

7

100% KO Bu

8

B

7x10 % 8

9

A

2.5%

[54]

180

[54]

188

[54]

162

[54]

99

c

[54]

0.5% KO Bu, H2 (25 bar)

1480

[60]

0.8% NaOH, H2 (40 bar)

35

[61,6

cr

t t t

an

C

0.1%

11

12

A

0.5%

12

13

D

1%

13

900

[58]

989

[58]

198

[42]

H2 (50 bar)

35

[56]

poor

[56]

d

11

4]

0.5% KOH

te

10

M

9

0.1%

16

178

t

t

-2

C

15

[54]

t

10

14

845

us

-3

2

Ac ce p

209

0.5% KOH 2%

t

KO Bu

D

1%

14

H2 (50 bar)

E

1%

15

CO2/H2 (1:1, 60 bar)

E

0.5%

16

[61,6 5]

CO2/H2 (1:1, 60 bar)

d

190000

[61,6 5]

t

1000

[59]

KO Bu

t

96

[66]

t

100

[67]

H2 (100 bar)

10

[39]

21

500% HCO2H

n/a

[68]

1%

22

500% HCO2H

n/a

[68]

1%

23

1%

100

[57]

F

0.1%

17

1%

KO Bu, H2 (5 bar)

18

B

1%

18

1%

19

B

1%

19

8%

KO Bu

20

F

10%

20

21

G

1%

22

G

23

G

17

d

82300

HCO2H

11

Page 11 of 36

Edited Dec 7 24

G

1%

1%

24

HCO2H

[57]

210 211 212

a

213

Rhodium and iridium are the most commonly used metals for TH catalysts with NHC ligands [54].

214

Especially the hydrogenation/ TH of acetophenone (see reaction A in Scheme 2) has become a

215

standard reaction for testing new catalytic systems [68]. In general a base such as KO Bu or KOH is

216

present in TH reactions.

217

Catalysts 1-9 are very similar to each other. They all contain a tetrahedral structure, and besides an

218

NHC ligand contain one arene ligand. Especially comparing compounds 3-7 with one another, with

219

catalyst loads of 0.5 mol% (referring to the metal center), it can be seen that neither changing the

220

arene within the group of sterically demanding ligands like C6(CH3)6 and cymene nor the N-

221

substituents at the NHC ligands has a major influence on the TONs (TON = turnover number).

222

Catalyst 3 with a benzene ligand coordinated to the ruthenium shows in contrast a high catalytic

223

activity. Noteworthy is catalyst 7 which demonstrated good activity even in the presence of air and

224

residual moisture. Usually, “non-inert conditions” have a negative influence on catalysis [54].

225

Most depicted complexes represent a precatalyst in the catalytic reaction. In general, for this type of

226

compounds the commonly accepted mechanism for NHC-M-catalyzed TH reactions is the

227

monohydride route [63]. A general inner-sphere monohydride mechanism for base-assisted TH from

228

alcohols to ketones by [LnM(NHC)(solvent)]-complexes is depicted in Scheme 3.

c

Load given in mol%. Theoretical value referring to the amount of substrate. ”Non-inert conditions”. TONs (TON = turnover number) based on the formation of formate. n/a not applicable.

ip t

d

b

73

Ac ce p

te

d

M

an

us

cr

t

12

Page 12 of 36

Edited Dec 7

[Ln (NHC)M(iPrOH)] I

product OH

base

Ph

+

beta-hydride elimination O

Ph

cr

OH

ip t

Ln(NHC)M O H II

+

an

us

Ln(NHC)M O H V

H Ln(NHC)M O

M

L = ligand

Ph

+

[Ln(NHC)M-H]+ III

O Ph substrate

IV

229 230

Scheme 3: Inner-sphere monohydride mechanism of base-assisted hydrogen transfer from alcohols to ketones mediated by [LnM(NHC)(solvent)] complexes [54,58,62].

234

In general, catalytic mechanisms for the TH via reduction of acetophenone occurs similar independent

235

of the involved metal. Beginning with I, a loss of ligand (e.g. arene, halide or weak coordinating

236

solvent) occurs and after addition of base deprotonation occurs and a 2-propoxide complex II can be

237

obtained. β-hydride elimination forms a hydride complex III and acetone is generated. Then

238

acetophenone coordinates to the hydride complex, resulting in IV, with subsequent attack of the

239

hydridic hydrogen at the α-C to give V with coordinated 1-phenyl-ethoxide. Finally, displacement of 1-

240

phenylethanol occurs and coordination of 2-propanol happens resulting in the 2-propoxide complex II.

241

The slowest steps are suggested to be the ligand loss and the hydride attack on the carbonyl carbon

242

[54,58,62].

243

As an example compound 1 and 2 can be mentioned. 1 but not the monohydride compound 2 showed

244

an induction period. This indicates that the hydride species is catalytically active itself or gives easier

245

access to the active species [62,63]. However, the experiments demonstrated that 2 can be an

246

intermediate in catalysis, but one or several of other related iridium hydrides must be the dominating

247

active species [63].

Ac ce p

te

d

231 232 233

13

Page 13 of 36

Edited Dec 7 Similar studies showed that the catalytic activity can be tuned by the choice of N-substituents of the

249

NHC and by the size and electronic properties of the backbone of the NHCs [39,54,56–61].

250

Furthermore, the choice of metal centers as well as the co-ligands coordinated to the catalytically

251

active metal center is of significant importance [39]. Their coordinating properties are decisive towards

252

displacement by the substrate [63]. Also the choice of substrates (e.g. ketones, imines, esters,

253

alkynes) is important for catalytic effectivity. Electronic parameters of the substrates, such as

254

electron-donating or -withdrawing groups, can also have major influence, but which functional group is

255

favored in catalysis always depends on the nature of the catalyst [54,56,57,59].

256

Besides the use of water-soluble NHC-M catalysts [61], it is desirable to apply earth-abundant metals

257

in catalysis. The first NHC-M with the redox-inactive Zn as metal center (complex 20) for the

258

hydrogenation of imines to amines was discovered [39,69].

259

More challenging substrates for hydrogenation reactions are alkenes or alkynes due to their lower

260

reactivity compared to carbonyl compounds [68]. Especially the selective hydrogenation of alkynes to

261

cis-alkenes and avoiding over-reduction is a very important task in synthetic organic chemistry [68,70].

262

Very common for partial hydrogenation of alkynes is the use of Pd-NHC-MA systems (MA = maleic

263

anhydride). For catalysts, such as 21, over-reduction of alkynes to alkanes is fully inhibited when

264

formic acid is used as hydrogen donor. This strongly hints different modes of operation for the formic

265

acid-mediated TH versus hydrogenation with molecular hydrogen [68].

266

Instead of the actual expected catalytic cycle for hydrogenation of alkynes by Pd(diimine) complexes

267

[68,71], which would consist of oxidative addition of the formic acid, further migratory insertion of the

268

hydride into the Pd-alkyne bond, decarboxylation of the formyl anion to CO2, and finally, reductive

269

elimination from a Pd(alkenyl)-hydride, the reaction mechanism differs. The mechanistic pathway for

270

Pd-NHC complexes is depicted in Scheme 4. Here, the hydrogen donor, and the nature and

271

concentration of the base are important. Both hydrogen atoms of formic acid are involved to

272

hydrogenate the alkyne and further studies of the catalyst and kinetics showed that maleic anhydride

273

is not hydrogenated off as expected previously, but is partly coordinated [68]. Furthermore, the

274

coordination of solvent versus the alkyne is competitive and strongly influences the chemoselectivity

275

[68].

Ac ce p

te

d

M

an

us

cr

ip t

248

276

14

Page 14 of 36

Edited Dec 7

O R

R

R

solvent

NHC Pd 0

O solvent

R

H

H

VII

O

HCO2 H

O

us

Pd

O

cr

O 0

R

ip t

VI

O

R R

NHC Pd 0 R

O

H R VIII

an

XI

O

O

R NHC Pd 0

NHC

O

solvent

HCO2

M

NEt 3

H-NEt 3

CO2

d

O

te

NHC Pd

0

R

H

Ac ce p

277 278

R

MA

O O

X

NHC Pd 0

H R

R IX

279 280 281

Scheme 4: Proposed catalytic cycle for TH of alkynes to Z-alkenes for Pd-NHC compounds such as 21 with triethylammonium formate as hydrogen donor [68].

282

It seems that catalysis is dependent on the substrate as well as on the nature of the NHC- and

283

co-ligands coordinated to the catalytically active metal. Both influences the catalytic activity and

284

selectivity [39].

285 286

3

Thermal hydrogen formation from alcohols and amines

287 288

Acceptorless dehydrogenation can be seen as an advanced transfer hydrogenation reaction where the

289

removed hydrogen is not captured by a sacrificial acceptor but released from the reaction. If the used

290

alcohols are derived from biomass e.g. by fermentation they can be seen as a renewable hydrogen 15

Page 15 of 36

Edited Dec 7 source. In the field of homogenous catalysis first reports on dehydrogenation of alcohols were

292

published in 1967 when Charman introduced rhodium chloride complexes that generated hydrogen

293

from refluxing isopropanol with concomitant precipitation of rhodium metal [72]. Ten years later

294

Robinson used well defined ruthenium and osmium phosphane complexes for hydrogen production

295

[73]. Since then several other systems have been presented with a focus on pincer ligands with N- or

296

P-donor moieties as highlighted and reviewed elsewhere [74–76]. But to the best of our knowledge no

297

catalysts containing carbene ligands have been reported for the sole purpose of generating hydrogen

298

gas from alcohols. There are, however, several protocols on the AD (AD = acceptorless

299

dehydrogenation) of alcohols and dehydrogenative coupling of alcohols and/or amines where NHC

300

metal complexes are used as catalysts, yielding hydrogen gas as a byproduct. An overview of

301

reported reactions can be seen in Scheme 5. Apart from reaction L all transformations use alcohols as

302

a substrate.

an

us

cr

ip t

291

303

OH

d

2 R

R

R'

te

R

O

M

OH

Ac ce p R

OH +

2 R

304

R

OH

+

H 2N R'

H

O R

OH 2 R

+ H2

R'

O

R

+ 2 H2

K

+ H2 O + H 2

L

O R

R O

R

N H

NH2

R

N

H2 N R'

R

N

2 H2

M

R

+ NH3 + H2

N

+

H2 O + H 2

O

R' +

R'

305 306

Scheme 5: Dehydrogenation reactions yielding H2 as byproduct.

307

The catalysts used for AD reactions are often generated in situ from Ru(II/III) precursors and azolium

308

salts by reaction with a base and in some cases additional donor ligands but well defined systems are

309

known as well. Some of the compounds investigated are so far displayed in Figure 4. The broad range

310

of NHCs used, including even aNHCs (aNHC = abnormal NHC), suggests that these kind of ligands

311

are only one of several factors that influence the activity of the catalytic system. 16

Page 16 of 36

Edited Dec 7 312 313 PCy3 L Ph PCy3 26

25

N

N

q-HCl q-HBr

r-HX

30

R

N R' N N

o-HI R = R' = nBu p-HI R = CH 2OSit BuMe2 R' = Mes

nBu N

X N nBu

s-HX

N N

X

t-HX

d

314

N

L

I

Cl N

M

N

Cl

an

X

X N

Ru

PPh 3 H Ru Ph 3P H PPh 3

Ph3 P

28 L = cod 29 L = p-cymene

n-HCl

m-HCl

N

Cl

27

Cl N

Ru

Ru Cl NHC/aNHC Cl

Cl

ip t

Ru

Cl

Cl

cr

Cl

us

PPh 3 a H Ru OC H PPh 3

Figure 4: Structure of catalysts, precursors and used NHC ligands as their azolium salts.

317

The outcome of selected reactions is summarized in Table 2. Usually only the most optimized

318

conditions for the investigated reactions are displayed. The reactions are typically carried out for 16-

319

36 h under refluxing conditions in high-boiling solvents such as toluene and/or a stream of argon to

320

remove the formed hydrogen gas.

321

Quite obvious is the correlation of catalyst load (referring to the metal center) and the TON. Full

322

conversion of the starting material corresponds to a TON of 20 for reactions H, L, N and O and a TON

323

of 40 for K and M at 5 mol% catalyst metal vs substrate(s). Consequently highest TONs were

324

achieved for low catalyst loadings (Table 2, Entry 6, 11, 16). This indicates that some of the catalyst

325

stays active for reasonable periods of time but for convenience in synthesis higher loads and shorter

326

reaction duration are preferred. Some trends in activity can be seen from the data. In most cases the

327

addition of excess of base was necessary to gain appreciable TONs even for catalysts were the base

328

is

Ac ce p

te

315 316

not

needed

to

deprotonate

the

NHC

precursor

(compare

Table

2).

17

Page 17 of 36

Edited Dec 7 329 Table 2: Overview of selected catalytic systems and their activity. a a Entry Type Catalyst and Ligands Additives

Ref.

-

3.4

[77]

2.5% K2CO3

5.6

[78]

ip t

5%

H

26

H

27

H

5%

27o

-

28

H

5%

27p

-

29

H

2.5% 27m

30

K

2.5% 27q + 4.5% PCy3

31

K

0.5% 27q + PCy3*HBF4

32

L

1.25% 27q + PCy3*HBF4

33

L

34

M

36

M

37

M

38

M

39

M

30 + q-HBr + MeCN 5%

27r

RuCl3 + 5% q-HBr + 5%

Ac ce p

1%

[81]

38.8

[82]

16.7% Mg3N2, 1.5% KO Bu

140

[83]

115% KOH

38.8

[83]

100% KOH

21.9

[84]

cr

KOH

us

2.5% 29 + 5% q-HBr + 5% pyridine 5%

39

K3PO4

t

27t

26 + q-HCl

[80]

10%

28 + 2% q-HCl + 2% PCyp3 5%

18

an

1%

[79]

M

M

1%

19

50%

d

35

25

2.5% [Ir(coe)2Cl]2 + n

b

TON

25

te

330

8%

t

KO Bu

[85,8 100 6]

KO Bu

t

40

[86]

15%

NaH

39.6

[87]

20%

NaH

36.8

[88]

KO Bu

t

38.8

[89]

40%

NaH

132

[90]

15%

15%

pyridine

40 41 42 43 a

M

5%

27o

20%

NaH

13.6

[79]

M

5%

27s

20%

NaH

38

[79]

-

9.5

[79]

4Å MolSieve

18

[91]

5%

N

O

5%

27o or 27s c

27q + 10% DABCO

b

331 332 333 334

Load given in mol%. Theoretical value referring to amount of H2, calculated from conversion determined by NMR, GC or isolated yield of the oxidation product. c DABCO = 1,4diazabicyclo[2.2.2]octan.

335

It is believed that the catalytically active species is a ruthenium dihydride complex as concluded by the

336

groups of Hong and Madsen from H/D-scrambling and 1H-NMR experiments and is also in line with

337

previous literature observations [82,83,89,92]. To obtain this dihydride complex (see Scheme 6) the

338

ruthenium precursor forms an alkoxide complex XII, if applicable supported by a base, followed by β-

339

hydride elimination. The substitution of the formed aldehyde by another alkoxide takes place forming 18

Page 18 of 36

Edited Dec 7 340

XIII, again followed by β-hydride elimination, leading to the reactive dihydride species XIV. If present,

341

attack of another substrate on the aldehyde leads to a hemiaminal/acetal XV and deprotonation of the

342

substrate leads to release of H2. Release of the substrate by substitution and β-hydride elimination

343

finally leads to XIV again [89,93,94].

OH

O [Ru] R

R - R

H

OH

R

O

R

H

us

H O [Ru] H E

O E

R'

R

R'

R'-EH H

- H2

an

R

O [Ru] R R'

E

XVI

H H XV

M

345

H

XIV

XIII

XII

E = O, NH

H O [Ru]

cr

[Ru]

R

H O [Ru]

+ base,

+ base,

ip t

344

Scheme 6: Formation of the active species and proposed general catalytic cycle [89,93,94].

348

The addition of base certainly facilitates the formation of the dihydride but is not essential as proven by

349

activity of catalysts 27o and 27p in reaction H under base-free conditions (Table 2, Entries 3, 4) and

350

further literature observations [95]. The aldehyde that is formed by generation of the dihydride species

351

stays coordinated to the metal center and can then react further if suitable substrates and reaction

352

conditions are present. If only alcohols are available the addition of base - especially KOH - leads to

353

formation of esters (reaction K) or partially hydrogenated aldol products (reaction L) for primary or

354

secondary alcohols, respectively (Table 2, Entries 6-9). In the case of catalyst 27m combined with

355

K3PO4, however, aldol products were only generated to a minor extend and reaction H was performed

356

instead (Table 2, Entry 5). Presence of alcohol, amine and base efficiently leads to formation of the

357

corresponding amide (reaction M) for various catalytic systems (Table 2, Entries 10-17). Interestingly,

358

when reacting aldehyde instead of alcohol with the amine formation of the amide was only observed in

359

the presence of a hydridic precatalyst like 30 and/or NaH as a base but not with KO Bu as a base.

360

Otherwise the corresponding imine formed instead, which is another indication for the importance of a

361

hydridic catalyst species [85,87,88]. The catalyst 27o with a more electron-donating ligand performed

362

worse than its imidazole analogue 27s in the case of amidation M but in imination reaction N they

363

gave equal turnovers (Table 2, Entries 16-18). These observations let suggest that the basicity of the

Ac ce p

te

d

346 347

t

19

Page 19 of 36

Edited Dec 7 carbene ligand can have an influence but is not of major importance. The imination from alcohols and

365

amines (reaction O) was catalyzed under base free conditions and removal of water during the

366

reaction by molecular sieves (Table 2, Entry 19). As general trend electron rich substrates tend to

367

react more readily than electron poor substrates and large steric demanding groups also hinder the

368

transformation [93].

369

A lot of progress has been made in recent years in AD and high turnover numbers and frequencies

370

have been achieved even with non-precious metal centers [96,97]. However, in comparison NHC

371

complexes are often not as active as complexes bearing more “classic” ligands with P- and N-donor

372

moieties [77,78]. Tuning the ligand properties of NHC ligands in the field of AD has so far been mainly

373

limited to steric and electronic features by introduction of more or less bulky N-substituents and use of

374

abnormal/mesoionic or saturated NHCs, respectively (compare ligand structures in Figure 4). For non-

375

NHC ligands aromatisation/dearomatisation and other cooperative ligand concepts facilitate hydrogen

376

transfer to/from the catalyst system [98–100]. Although chelation has become more prevalent and

377

applied also for NHC ligands in the last decade, there is only one example of chelating NHC ligands in

378

AD and no beneficial effects were found [80,101,102]. Certainly the investigations show that not only

379

the omnipresent rare group 10 metals (Pd, Pt) are possible catalytic centers for hydrogen gas

380

releasing reactions but that the scope could be broadened to metals like ruthenium and potentially

381

even to its first row analogue iron.

383 384

cr

us

an

M

d

te

Ac ce p

382

ip t

364

4 Catalytic water splitting

385

In artificial photosynthesis the stability of the catalytic system plays an important role to achieve high

386

turnover numbers - especially in intramolecular systems. Besides ligand dissociation from the

387

chromophore [103] and oxidation of the ligands [104] a major task is the stabilization of the oxidation

388

as well as reduction catalytic metal center during the catalytic cycle [28]. Up to now several examples

389

are known, which stabilize the redoxcatalyst via pyridine [13,105–116], phosphine [117–119], oxo

390

[25,120–123] and thiol [124,125] ligands.

391

As mentioned earlier NHCs were discovered to be very promising candidates as stabilizing ligands

392

due to their ability to stabilize low valent oxidations states [126]. Since NHCs are widely used in almost

393

all areas of organometallic catalysis (see above), it is almost surprising that they have hardly been

20

Page 20 of 36

Edited Dec 7 394

applied in water splitting. Up to now there are only a few examples known for the use of NHCs in

395

artificial water splitting. In the following water reduction and oxidation are presented separately.

396 397

4.1 Intermolecular water reduction

ip t

398

To the best of our knowledge up to now there are only very few examples known, in which the metal of

400

the hydrogen evolving catalyst is stabilized by NHCs (see Figure 5).

cr

399

I

401

N

Pd Cl

Cl

Cl Pd

N

Pd

N

N

N

31

32

I

us

Cl

N

an

OC S S CO N Fe Fe CO CO N N

N N

33

Figure 5: Selected examples of carbene stabilized oxidation catalysts. (31 [127]; 32 [128]; 33 [129])

404

Complex 31 was successfully applied as catalyst for the electrochemical reduction of protons from the

405

weak acid HOAc in CH3CN [127]. Furthermore, carbene complexes 32 and 33 were used as

406

precatalysts in photocatalytic assemblies. These systems consist of ruthenium polypyridine [128,129]

407

or the corresponding iridium complexes [130] as chromophore, triethylamine [128,130] or

408

triethanolamine [129] as electron donor and [Pd(Et2BzIm)Cl2]2 [128,130] 32 (Et2BzIm = 1,3-diethyl-1H-

409

benzo[d]imidazol-3-ium) or [cis-(NHC)2PdI2] [129] 33 as reduction catalyst.

410

However, the question of the long term stability of these reduction catalysts is not completely

411

answered, and it is assumed that at least a partial removal of the NHC ligand occurs [129,130].

413

d

te

Ac ce p

412

M

402 403

4.2 PMD for photocatalytic hydrogen evolution

414 415

Besides these intermolecular systems, the first examples of intramolecular systems ([(tbbpy)2Ru(µ-

416

bbip){AgCl}]2+

417

f][1,10]phenanthrolinium; 35); [(tbbpy)2Ru(µ-bbip){PdCl2X}]

418

[(tbbpy)2Ru(μ-bbip){Rh(cod)Cl}]2+ (cod = 1,5-cyclooctadiene; 37) for the photocatalytic water reduction

419

were recently developed in our group (Scheme 7) [131]. The bridging ligand bbip is capable of

(tbbpy =

4,4’-di-tert-butyl-2,2’-bipyridine; bbip 2+

=

1,3-(bisbenzyl)-1H-imidazo[4,5-

(X = coordinated solvent; 36), and

21

Page 21 of 36

Edited Dec 7 420

connecting the ruthenium polypyridyl fragment via a bidentate phenanthroline fragment on the one

421

hand and with another metal center via an NHC on the other hand.

N

i

N

N

N

N

N

N

ip

N

ii

iii

N N

bip

N

N

N

N

N 34

N iv

N

N

N

Ru II

v

N N

N

N

M

N N

d

N

N

N

M

N

M = PdCl2X 36 M = Rh(cod)Cl 37

35

3+

N RuII

Ag Cl

N

423 424

an

us

(bbip)Br

N

RuII

Br N

N

cr

H N

N

ip t

422

2+

Scheme 7: Synthesis of ([Ru(tbbpy)2(bbip)] 34; [(tbbpy)2Ru(µ-bbip){AgCl}] 35, [(tbbpy)2Ru(µ2+ 2+ bbip){PdCl2X}] 36 and [(tbbpy)2Ru(μ-bbip){Rh(cod)Cl}] 37 (ip = 1H-imidazo[4,5f][1,10]phenanthroline; bip = 1-benzyl-1H-imidazo[4,5-f][1,10]phenanthroline) [131].

429

The NHC structure of the bridging ligand has been unambiguously confirmed by the X-ray structure of

430

the heterodinuclear ([(tbbpy)2Ru(µ-bbip){AgCl}]

Ac ce p

te

425 426 427 428

2+

35 complex (Figure 6).

431 432 433 434

2+

Figure 6: X-ray crystal structure of the complex cation ([(tbbpy)2Ru(µ-bbip){AgCl}] 35 (ellipsoids at 50 % probability); counter ions, solvent molecules and hydrogen atoms were omitted for clarity [131]. 22

Page 22 of 36

Edited Dec 7

436

complexes (Figure 7; left).

437

The whole class of bbip-containing complexes exhibit very promising photophysical properties [131–

438

133], which become obvious comparing the quantum yields (see Figure 7; right).

100

[Ru(tbbpy)3]2+

40

0.10

20

0.05

400

500

600

0.00 800

700

an

300

wavelength [nm]

[Ru(tbbpy)2(bbip)]3+

0.20

d

0.15 0.10

te

quantum yield 

M

[Ru(bpy)3]2+

0.25

cr

0.15

us

60

emission [a.u.]

0.20

439

0.05

Ac ce p

0.00

440

0.25

[Ru(tbbpy)2(bbip)]3+

80

0 200

ip t

Absorption and emission spectra show properties typical for light harvesting ruthenium polypyridyl

extinction coefficient [103 M-1 cm-1]

435

aerated

inert

3+

2+

441 442 443 444

Figure 7: Absorption and emission spectra of [Ru(bbip)(tbbpy)2] 34 with the [Ru(tbbpy)] 38 in 3+ 2+ aerated acetonitrile [133] (top); Quantum yields of [Ru(tbbpy)2(bbip)] 31[131–133] and [Ru(bpy)3] [134] in aerated and deaerated acetonitrile (bottom).

445

Whereas the quantum yields of [Ru(tbbpy)2(bbip)]3+ 34 (Ф = 0.011 [132,133]) and [Ru(bpy)3]2+ (Ф =

446

0.018 [134]) are similar under aerated conditions, the quantum yield of [Ru(tbbpy)2(bbip)]

447

0.250 [131]) is more than twice as high compared to [Ru(bpy)3]

448

conditions.

449

Furthermore, fast intersystem crossing processes to the 3MLCT state, and long lifetimes for the

450

emissive and non-emissive dark

451

bbip){PdCl2X}]

3+

2+

2+

3

34 (Ф =

(Ф = 0.095 [134]) under inert

3+

MLCT-states in [Ru(tbbpy)2(bbip)]

34 and [(tbbpy)2Ru(µ-

36 were observed [131]. Resonance Raman investigations clearly show that the

23

Page 23 of 36

Edited Dec 7 3+

452

location of the first excited state of [Ru(tbbpy)2(bbip)]

453

terminal tbbpy ligands [133].

454

Initial photocatalytic investigations on hydrogen formation from water in the presence of triethylamine

455

as sacrificial electron donor with [(tbbpy)2Ru(µ-bbip){AgCl}]2+ 35 (TON = 4), [(tbbpy)2Ru(µ-

456

bbip){PdCl2X}]2+ 36 (TON = 36) and [(tbbpy)2Ru(μ-bbip){Rh(cod)Cl}]2+ 37 (TON = 16) showed several

457

features reminiscent of stable catalysts. For [(tbbpy)2Ru(µ-bbip){PdCl2X}]

458

frequency of 7 h over 5 hours, independence of activity on concentration of the catalyst and mainly

459

the absence of an induction phase was observed. These observations show that carbenes are very

460

promising ligands for the stabilization of redoxcatalysts in artificial water splitting [131,133].

us

461

4.3 Modifications of the system

an

462

36 a constant turnover

cr

-1

2+

ip t

34 lies on the bbip ligand and not on the

463

The combined effects of promising photophysics of the sensitizer unit and NHC-coordination of the

465

catalytic metal center form the foundations for further development of this system.

466

Thereby several possible modifications can be imagined, which may influence the catalytic activity of

467

the complex (see Figure 8). terminal ligands

te

phenanthroline

moieties at NHC

Ac ce p N

N

N

RuII

N

468

d

M

464

M

N

N

R1 N N R2

metal center

3+

469 470

Figure 8: Possible modifications starting from [Ru(tbbpy)2(bbip)]

471

Apart from modification of the photocenter by ligand exchange the system can be modified by

472

introduction of substituents at the phenanthroline or NHC moiety. In addition, a variety of other metal

473

fragments could be used as redoxcatalysts.

34.

474 475

4.4 Modification of the NHC unit

476 24

Page 24 of 36

Edited Dec 7 To further optimize catalytic turnover modifications of the subunits of the supramolecular assembly are

478

desirable. As already shown NHCs can act as ligands for redoxcatalysts. It is obvious that the nitrogen

479

bound moieties (see Figure 8; R1 and R2) of the NHC have a drastic influence on the stability and

480

reactivity of the catalytic system due to their spatial proximity to the redoxcatalyst.

481

In order to control the catalytic activity of the metal center selectively, it is fundamental to adjust the

482

ligand properties. The introduction of moieties (i.e. R1 and R2 in Figure 8) containing further stabilizing

483

ligands (e.g. phosphates, thiols, carboxylates and N-donor ligands) for the redox center would be

484

particularly interesting. In addition to reach the full range of variation symmetrical and also

485

asymmetrical (R1 ≠ R2; Figure 8) functionalization should be accessible. Besides several literature

486

known possibilities for symmetrical functionalization based on the ip scaffold [129,133,135–139] up to

487

now there is only one synthetic access known for unsymmetrical functionalization of ip (R1 ≠ R 2; Figure

488

8) [132]. In principle there are several methods to synthesize imidazolium salts with unsymmetrical

489

substituents. Because of the similarity to the synthesis of symmetrical ligands and the already gained

490

experience a similar reaction pathway as for the symmetrical ligands (see Scheme 7; i and ii for

491

comparison) is appreciable (Scheme 8).

N

d

N

te

H N

Ac ce p

N

N

N

M

an

us

cr

ip t

477

N

H N

N

N

N

R N

N

R N

N

N

N

N R'

N N

Ru

N

N

N

N

R N

N

N

N

N

Ru N

N

R N

N

N R'

Ru

N

N

492 493 494 495

Scheme 8: Possible reaction routes for unsymmetrical imidazolium salts.

25

Page 25 of 36

Edited Dec 7 496

Which route is favored depends on the alkylation reagent as the nitrogen at the phenanthroline

497

backbone can be alkylated as well [132]. The different substituents have a pronounced influence on

498

the NMR signals, but almost no influence on the photophysical properties of the various

499

[Ru(tbbpy)2(RR’ip)]

500

Since only minimal changes of the photophysical properties of the ruthenium polypyridyl fragment

501

occur when imidazole nitrogen substituents are altered a large scope for variation of these nitrogen

502

bound moieties is possible [129,132,133,135].

503

Hence, the introduction of different alkyl groups on the imidazolium unit is possible without affecting

504

the photophysics of the ruthenium polypyridyl unit, which in turn paves the way towards a ligand based

505

tuning of the catalytic center without concomitant altering the properties of the photo center.

506

4.5 Modification of the terminal ligands

ip t

complexes was observed [132].

an

us

cr

3+

507

The possibility to tune the photophysical properties of ruthenium polypyridine complexes containing

509

RRip ligands could so far not be realized, (Figure 8). Preliminary investigation into a related system

510

[Ru(N,N′)(mmip)]

511

imidazo[4,5-f][1,10]phenanthrolinium) [129] showed only minor changes in the photophysical and

512

electrochemical properties changing from 2,2’-bipyridines to structurally similar phenanthroline ligands.

513

Additionally there are no significant influences on photocatalytic activity with both complexes.

514

The positive effects on the photophysics (i.e. long lifetimes and high emission quantum yield) of one

515

bbip ligand in [Ru(tbbpy)2(bbip)]

516

could enhance these trends [140,141]. Recently we could show, starting with [Ru(tbbpy)3] , that it is

517

possible to synthesize a series of complexes in which one terminal tbbpy ligand is successively

518

replaced by one bbip ligand (see Figure 9) [133].

te

d

(N,N′ = 2,2’-bipyridine or 1,10-phenanthroline; mmip = 1,3-dimethyl-1H-

Ac ce p

519

3+

M

508

3+

lead to the assumption that the introduction of further bbip ligands 2+

26

Page 26 of 36

Edited Dec 7

N

N

N

N

N

N

II

II

Ru

Ru

N

N 3

N

N

N

N

RuII

N

N

N

2

39

3 40

M

520

N

us

N RuII

an

N

34

cr

38

ip t

2

Figure 9: Series of complexes received by successive replacing of tbbpy with bbip ligands.

523

As a consequence the emission lifetimes in acetonitrile under aerobic conditions as well as the

524

quantum yields of these complexes increases significantly by the introduction of more and more bbip

525

ligands ([Ru(tbbpy)2(bbip)]3+ 34 (τ(ns) = 1050; Φ = 1,1 %) [Ru(tbbpy)(bbip)2]4+ 39 (τ(ns) = 1650; Φ =

526

2.8 %) and [Ru(bbip)3]

527

state by oxygen was investigated by comparing the lifetimes under anaerobic conditions with values

528

obtained under aerobic conditions. Thereby [Ru(bbip)3]

529

state quenching by oxygen. This makes this complex a promising candidate to act as a chromophore

530

for the artificial water oxidation and reduction under aerobic conditions [118,142–146].

532

te

5+

40 (τ(ns) = 1840; Φ = 6.1 %) [133]. Additionally the quenching of the excited

Ac ce p

531

d

521 522

5+

shows the highest resistance against excited

4.6 Water Oxidation

533 534

The application of NHC-M compounds in water oxidation only developed within the last 4 years. So far

535

only a number of catalytic systems, in which NHC ligands stabilize the metal center of the catalyst, are

536

known [147–153]. Some examples are depicted in Figure 10.

537

27

Page 27 of 36

Edited Dec 7

N

Ir Cl Cl

N

Ir Cl N

N

N 41

N 42

ip t

Ph

OH 2 N N

N

N

N

N

43

538

cr

Ir Cl

N

N Ru

us

N

44

Figure 10: Selected examples of carbene stabilized oxidation catalysts (41 [149]; 42 [153]; 43 [152]; 44 [147]).

542

Very high TONs up to 400 000 were achieved using the iridium carbene catalyst 41 and NaIO4 as

543

electron acceptor [149]. The catalysts 42 and 43 were already successfully applied in light driven

544

catalysis [152,153]. 42 was used with [Ru(bpy)3]2+ and Na2S2O8 achieving a TON of 6.4. 43 in a

545

photoelectrochemical setup with an hematite on a FTO electrode surface generated a photocurrent

546

which could be linked to water oxidation based on corresponding chemical water oxidation

547

experiments with CAN yielding a TON of 22800. Recently it has been shown that the introduction of a

548

NHC containing ligand, yielding complex 44, has a remarkable impact on the crucial O-O bond

549

formation step of the catalytic cycle [147]. Similar observation were made for other complexes

550

depicted in Figure 11 [154].

Ac ce p

te

d

M

an

539 540 541

551

N

O

552 553 554

N O

RuII N

45

N O

O

O

H N

O

H

O RuII

N

N O

O

N

46

Figure 11: Water oxidation catalyst with a monodentate carbene ligand. 28

Page 28 of 36

Edited Dec 7 555

The replacement of picoline ligands in the axial position in complex 45 to one NHC and one water

556

molecule yielding complex 46 changes the mechanism from bimolecular to monomolecular [154,155].

557

This in turn shows that NHCs as stabilizing ligands have a drastic influence on the reactivity of the

558

systems.

560

ip t

559

5 Conclusion and Outlook

cr

561

NHC-ligands are a very interesting class of activity determining ligands for organometallic catalysts.

563

The chemical structure and electronic properties of these relatively new ligands can be utilized for

564

optimal catalyst performance. The widespread application of these ligands in conventional

565

organometallic catalysis, including hydrogenation and thermal hydrogen formation from organic

566

substrates, is in strong contrast to their role in the area of photocatalysis. Especially photocatalytic

567

water splitting with the very high demand on chemical stability and activity on the employed catalyst

568

would benefit tremendously from widely tunable activity determining NHC ligands. Until very recently

569

synthetic limitations have prevented the wide spread application of NHC ligands in catalytically active

570

PMDs. The advantage of bridging ligands containing one ligand sphere for the stabilization of a

571

photochemically active metal center and a second NHC-sphere enables the development of new

572

catalysts for the photochemical water splitting. The first examples of photoredoxactive ruthenium

573

complexes containing this ligand show a highly relevant enhancement of important photochemical

574

properties such as quantum yield of emission, lifetime of excited state and resistance towards oxygen

575

induced quenching of the excited state. Furthermore, well-established design concepts for

576

organometallic NHC catalysts based on tailoring the NHC-ligand already exist. The possible transfer of

577

concepts from organometallic catalysis into the emerging area of NHC stabilized catalysts for water

578

splitting, which is easily possible due to the similarity in ligand properties, should enhance a fast and

579

efficient development of active catalysts. The already available ligand examples represent a very

580

useful starting point for the evaluation of structure activity correlation in inter- and intramolecular

581

photocatalytic systems. The envisaged high stability of NHC-stabilized catalytic centers should pave

582

the way of integrating these new ligand systems into photochemical molecular devices for catalytic

583

applications in photoelectrochemical cells. In addition a potential application of new intramolecular

584

photocatalysts within the area of organometallic catalysis would open up new avenues of

585

development.

Ac ce p

te

d

M

an

us

562

29

Page 29 of 36

Edited Dec 7 586 587 588 589

Acknowledgments

ip t

590

We thank the Elitenetzwerk Bayern, the GRK 1626, and the GSMS (FAU Erlangen-Nuremberg), the

592

SFB 583 as well as the COST Action CM1202 Perspect-H2O for financial support and inspiring

593

discussions.

cr

591

us

594

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