A Diversity of Chemical Strategies to Implement Homogeneous - PowerPoint Presentation

Slide1

A Diversity of Chemical Strategies to Implement Homogeneous Carbon Dioxide Hydrogenations

E2C 2013, Budapest

Y. Jiang, R.

Kunjanpillai

, S.

Chakraborty

,

F

. Zhou

O.

Blacque

, T. Fox and H.

BerkeSlide2

Strategies for Homogeneous

Carbon Dioxide Hydrogenations

Search

for

molecular middle transition element catalystsCheap catalysts and processesProcesses running at ambient or near ambient conditionsOxygenates (CnHmOo) as productsUse of molecular hydrogen from ’renewable’, ‘waste’ or CO2-free sources

‘Non-noble Metals for Noble Tasks’

As for methanol as the simplest oxygenate productCO2 + 3H2  CH3OH + H2O

Homogen.cat

MeOH available since the 1920s by heterogenous CO hydrogenationPotential energy carrierSmall scale and de-centralized production seems possibleFacile convertion to other oxygenates ( for instance DME) or hydrocarbonsToxicLow boiling point

Pros

ConsSlide3

Middle

Transition Elements

in

Homogeneous

Hydrogenation Catalysis Middle transition elements particularly rhenium

is bordering precious metals  may

have retained some of the „noble“

properties,

like high affinity to hydrogen and to unsaturated organic molecules-

Isoelectronic

replacements

in precious

metal

complexes

may

generate active catalysts; for instance the Ru-L unit (L= 2e- donor) replaced by the Re-NO unit Middle transition element seek stable 18e- configurations; disadvantage for catalysis, which needs permanently or temporarily vacant coordination sites Tuning of the ligand sphere is required to stabilize vacant sites via ligand effects, such as of ligands with variable electron counts, large-bite-angle-effects of bidentates, large cone angles of monodentates, cis labilization effect and the trans influence/effect and othersSlide4

Pd

125

200

275

350

Electrochemical

Volcano Plot for Hydrogen

1051M-H bond strength (kJ/mol)ThermodynamicsKineticsHydrogen evolution

reaction log(i0) (A/cm

-2)S. Trasatti, J. Electroanal. Chem. 39 (1977) 163S. Trasatti, Electrochim. Acta 39 (1994) 1739Which metals are preferred in hydrogen catalysis?Slide5

Stages

of

CO2

Reduction

by H2Carbon oxidation state from +IV to -II

Formic acid or formate stage(thermodynamically

unstable, but kinetically too stableHydroxy carbene or formaldehyde stage(joint catalytic processes to make ‘formoses’ (CnH2nO

n) seem possible)

Carbonic acid, CO2 or carbonate stageMethanol stage, potential to be converted into other oxygenates Formation of esters via the reaction of carbonic and formic acid and derivatives with alcohols including

the product methanol (

catalyzed by protons) may

help to overcome the reactivity deficit

of

formic

acid and derivatives. Esters are more easily reduced than acids or their anions Slide6

Hydrogenation

of CO

2

to Formic Acid and Formates

hydridic

protonicAs a polar molecule HCOOH would be prone for reactions with heterolysis of

H2!

Thermodynamics of the hydrogenation CO2 to formic acid:Additional driving force by salt formation

!Despite

their thermodynamic

instability, formic

acid

or

formate

salts are kinetically (too) stable!pKa = 3.8CO2(g) + H2(g) → HCOOH(l) ∆Go = 32.9 kJ/mol; ∆Ho = -31.2 kJ/mol; ∆So = -215 J/(mol K) CO2(g) + H2(g) + NH3 (aq) → HCOO- (aq) + NH4+(aq) ∆Go = -9.5 kJ/mol; ∆Ho = -84.3 kJ/mol; ∆So = -250 J/(mol K)CO2(aq) + H2(aq) + NH3 (aq) → H(CO)O- (aq) + NH4+(aq) ∆Go = -35.4 kJ/mol; ∆Ho = -59.8 kJ/mol; ∆So = -81 J/(mol K)Slide7

Cat/ (

mol

%)

Substrate

Co-catalyst

BCF = B(C6F5)3

Temp(°C)H2

(bar)Time(h)TOF(h

-1)

Conv(%)Mo/0.051-hexene -14060

0.5

912

21

Mo/0.03

1-hexene

Et

3

SiH/BCF

140602525375W/0.0201-hexene Et3SiH/BCF140600.5820080Mo/0.081-octene Et3SiH/BCF14060<2350075W/0.0491-octene Et3SiH/BCF140600.5361588Mo/0.01Styrene Et3SiH/BCF140602207984W/0.037Styrene Et3SiH/BCF140600.5

2064

39

Mo/0.03

1,7-octadiene

Et

3

SiH/BCF

140

60

2

1056

65

Mo/0.07

Cyclohexene

Et

3

SiH/BCF

140

60

<10

200

100

Mo/0.07

Cyclooctene

Et

3

SiH/BCF

140

60

3

1355

55

Mo/0.07

α-methyl styrene

Et

3

SiH/BCF

140

60

16

205

60

Mo/0.09

1,5 cyclooctadiene

Et

3

SiH/BCF

140

60

14

200

74

c

at: M = Mo,W

Olefin Hydrogenations with Mo and W Nitrosyl Hydride Catalysts

S.

Chakraborty

, T. Fox, O.

Blacque

, 2013

submittedSlide8

Hydrogenation

of

CO

2

to Formic Acid Using a Tungsten ComplexBase = DBUSlide9

Large

cone

-angle „

on - off“

chelate

phosphine! Exploitation of Ligand

Effects to Make the Hydrogenation of CO2

with a Tungsten Hydride Catalytic

Carbynes

like nitrosyls (3e donors!) are trans influence and trans

effect

ligands

and activate

trans

positions

.

Tungsten has great affinity to H2!Slide10

Catalytic

CO

2

Hydrogenation

to FormateProducts [HCOO][DBUH] and HCOOH•DBU precipitate from

toluene solutionLow energy ‘Organo-catalytic

’ transition state

catSlide11

Rhenium

Based

Catalytic CO

2

Hydrogenation to Formate and MethanolSlide12

FLP Type

Activation

of

CO

2 with Rhenium

Complexes and a Lewis AcidSlide13

FLP Type

Activation

of

CO

2 with a Rhenium Hydride and B(C6F5)3

as Lewis Acid – Stoichiometric ReactionsFrustrated

Lewis pair (FLP) where the hydride plays the role of a Lewis base!Fully

characterized

by NMR in solutionCrystal structureJiang, Y.; Blacque, O.; Fox, T.; Berke, H., J. Am. Chem. Soc. 2013, 135, 7751-7760Slide14

A Stable

Formate

Dihydrogen

Complex and Subsequent Stoichiometric Reaction with Et3SiHIsolated and fully characterized

Jiang, Y.; Blacque, O.; Fox, T.; Berke, H., J. Am. Chem. Soc. 2013, 135, 7751-7760Slide15

Catalytic Reduction of CO

2

with Et3SiH

H

2

CH

2

OHOMeH2OHydrogenationHydrosilylationSlide16

Hydrogenation

of

CO2

with

a Rhenium Hydride/Lewis

Acid System in Presence of TMPStructure of [TMPH][HCOO]Slide17

Proposed Mechanism for the Rhenium Catalyzed

CO

2

Hydrogenation In Presence of B(C

6

F5)3 and TMPSlide18

Hydrogenations

with

Rhenium

Nitrosyl

Complexes Bearing Large Bite-Angle Xantphos Diphosphines

Trans NO

reactive siteKinetic ‘isomer’?

Cis NO

reactive siteThermodynamic isomer?Xantphos framework may ‘bite’ bi- or tridentate

Reactive

site

interconversion

Ammonia

complex

(NO/Br disorder)Slide19

Rhenium

Based Catalytic

CO

2

Hydrosilylation and Combined Hydrogenation/Hydrosilylation to Methanol and Derivatives

Hydrogenation/

hydrosilylation with TONs up to 330:

Hydrosilylation

with TONs up to 410:

1

2Slide20

Rhenium

Based

Catalytic CO2

Combined

Hydrogenation/Hydrosilylation to Formate DerivativesSlide21

Substrate

Catalyst (

mol

%)

Co-catalyst

SolventProductTONYield (%) 

 0.02 n-Bu

4NBr THF  PhCH

2OH

 500 100 Benzaldehyde0.02

-

Toluene

PhCH2

OH

4807

96

 Acetophenone 0.05-Toluene PhCH(OH)Me2000> 99  0.5 -THF CH3OH 112 56   0.5 n-Bu4NBr EtOH CH3OH 161 81 0.2-MeOHHCOONa285.6 0.2-D2O/THFHCOONa6813.6

0.5

n-Bu

4

NBr

EtOH

CH

3

OH

H(CO)ONa

20

24

10

12

Rhenium

Based

Ester, Aldehyde, Ketone

and

Bicarbonate

Hydrogenation

to

Alcohols

140 °C, 60 bar H

2Slide22

Co-catalyst 1 /Equiv.

Co-catalyst 2 /Equiv

./acid

CO

2

/H2 (bar)TON--10/30

07n-Bu4NBr /5

-10/3028n-Bu4NBr /5

-

10/3026c--10/3013n-Bu4NI /100

-

10/30

11

NaI/100

-

10/30

05

n

-Bu4NBr /2-10/3020n-Bu4NBr /50-10/3030n-Bu4NBr /100-10/3030Et4NBr /100-10/301.6n-Bu4NBr /5-10/300.9-EtOH/25010/3006n-Bu4NBr /100EtOH/25010/3033n-Bu4NBr /5EtOH/100/p-TsOH10/3029gn-Bu4NBr /5-20/6088Rhenium Based and Halide Co-catalytic CO2 Hydrogenation to Methanol

Optimum

loading

of

co-cat.bromide

Improved

performance

by

EtOH

add

.

No

improvement

by

acid

add

.

H

2

pressure

matters

Is

formate

ester

formation

crucial

to

catalysis

?

What

is

the

influence of the produced water on ester formation?Slide23

Rhenium

Based

Hydrogenation of

Methyl

Formate to MethanolMethyl formate cycleFormaldehyde

cycleSlide24

Remaining

Questions

on

the CO2 Hydrogenation to MethanolRelevance of formic ester formation Can ester formation

be enhanced by protic additives?Role of the water product for the ester formation. Removal of

water required?Change type of the hydrogenation course and develop two-step hydrogenation processes? CO2  Formic acid

 formic acid

ester  methanol CO2  carbonic acid  carbonic acid diester  methanol catalysts optimizationsSlide25

Acknowledgements

Preparative

work

Y

. Jiang,

R. Kunjanpillai, S. Chakraborty, F. ZhouX-ray structures and DFTO. Blacque NMRT. Fox

Financial SupportFunds of the University of

ZurichSwiss National Science FoundationLanxess AG, Leverkusen, GermanySlide26

Utilization

of

Metal-Ligand

Activation

of CO2Slide27
Slide28

Stoichiometric

CO

2

Hydrogenation

to FormateSlide29

Cat

Base

Time (h)

Yield (%)

Mo

Na[N(SiMe3)2]154

WNa[N(SiMe3)2]

152

Catalytic

CO2 Hydrogenation to FormatePoor catalysis due to a too

stable

CO

2 complex!Slide30

Rhenium

Based

Catalytic CO2

Hydrogenation

/Hydrosilylation to Methanol and Silyl DerivativesSlide31

Reaction

Center

Mediated

Splitting

Pathways of DihydrogenMononuclear and dinuclear dihydrogen complexes prepare the H2 molecule for the splitting reaction by polarization Slide32

Single-step processes combined splitting and H-transfers (rare)Multi-step processes

separate splitting and H-tranfersActivation

of H2 = H2 splitting and H-transfers

Splitting

Homolytic splitting H

2  2 H Wilkinson/Osborn-type hydrogenation catalyses with oxidative addition of H2Heterolytic splitting H2  H- + H+ Ionic hydrogenations (protonic-hydridic reactivity, bifunctional activation). Usually achieved by deprotonation of H2 complexes with a baseH-TransfersSimultaneous (concerted

) polar transfers of polarized H atoms (H(+) protic and H(-) hydridic)Stepwise transfers: homopolar H atom transfer

via insertion into the M-H bond and reductive eliminationHeteropolar stepwise transfersH H

H2

Substrate XYHXYHPrinciples of H2 reactivityHomolytic, heterolytic splitting?

H-transfers: homopolar, heteropolar?

Sequence

of

Transfers?

Principles

of Hydrogen ReactionsH. Berke, ChemPhysChem, (2010), 11, 1837Slide33

Interconversion

of

‘

Isomeric

’ Xantphos Nitrosyl Rhenium Complexes via Anionic Trihalo Intermediates – Basis

for the Catalytic Halide Effect

I(trans) more reactive than I(cis)!Slide34

Cis P

iPr

3 ligands: P(1)-Re(1)-P(2), 103.78(2)

Structure

of 3a