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Alessandro Venturini

Institute for the Organic Synthesis and Photoreactivity, National Research Council of Italy

Via P. Gobetti 101, 40129 Bologna, Italy Fax: (+)39 0516398349

E-‐mail: [email protected]


Recent examples

�  “An ESR and DFT revisitation of phosphonitroxides prompted by an ineffective ESR approach to the hydrophosphination of alkynes “ A. Alberti*, M. Guerra and A. Venturini RSC Adv. 2013,3,17887-17897.

�  “Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study.” A. Venturini*, A. Barbieri, J. N. H. Reek and D. G. H. Hetterscheid Chemistry An European Journal, in press DOI: 10.1002/chem.201303796 EUROCORES – SOLARFUELTANDEM

�  “Engineering the thermostability of cellulases used in biomass conversion with computational methods. “ A. Venturini and O. U. Sezermann submitted for publication to Bilateral agreements of Scientific and Technological Cooperation (Short-‐term mobility) partner : Prof. Osman Ugur Sezermann, Sabanci Univesity Istanbul Turkey

�  Diffusion of Phospholipids in a Lipid Bilayer E. Bakalis, A. Venturini and F. Zerbetto in progress


Alessandro Venturini

Institute for the Organic Synthesis and Photoreactivity, National Research Council of Italy

Via P. Gobetti 101, 40129 Bologna, Italy Fax: (+)39 0516398349

E-mail: [email protected]


Dennis G. H. Hetterscheid and Joost N. H. Reek Chem. Commun. 2011, 47, 2712-‐2714

Catalyst TOF (mol mol-‐1 s-‐1)

TON (mol mol-‐1)

IrIV(OH)2+ 1.5 >> 2000

[IrCp*(NˆC)Cl2] * 0.9 1500

* R. H. Crabtree et al. J. Am. Chem. Soc. 2009, 131, 8730, J. Am. Chem. Soc. 2010, 132, 16017.

[Ir(Cp*)(Me2-‐NHC)OH2]+ Cp* = pentamethylcyclopentadienyl Me2-‐NHC = N-‐dimethylimidazolin-‐2-‐ylidene

2H2O --> O2 + 4H+ + 4e-


System characteristics: The combination of the pentamethylcyclopentadienyl and the N-heterocyclic carbene ligands afforded a very electron-rich iridium(III) metal center. The system is very flexible and consequently can stabilize the iridium centre particularly in the higher valence oxidation states, and in different coordination geometries.

Has two vacant sites where oxidation of water can occur, allowing for different oxidation pathways that are not accessible in case of iridium catalysts that do not possess this second free site.


Experimental data:

{Ir(OH)}+ {Ir=O}+ {Ir(OH)3}+

v  Predominant species observed by ESI-MS are:

v  Important kinetic data show that at high catalyst loadings the reaction rate is dependent both on the concentration of catalyst and that of CeIV, whereas at low catalyst loadings the reaction becomes pseudo-zeroth order in CeIV. (Cerium ammonium nitrate (CAN), a strong stoichiometric oxidant that is typically used to study water oxidation).

Ir = Ir(Cp*)(Me2-‐NHC)


Computational Methods • DFT calculations were carried out using the Truhlar’s hybrid functional M06, together with the 6-311+G(d,p) basis set, for C, N, O and H and the Stuttgart/Cologne small-core relativistic energy-consistent ECP with their correlation-consistent basis sets of double-z quality for the 5d Ir transition metal (ECP60MDF/VDZ basis set) within the framework of the Gaussian 09 suite of programs. The peculiarity of these pseudopotentials is that they incorporate both scalar and spin-orbit relativistic effects and are expected to be more appropriate to describe the transition metals. • All the molecular structures were fully optimized using the CPCM continuum model (water) and using the Berny analytical gradient optimization method where the stationary points have been characterized by frequency calculation • The intrinsic reaction coordinate (IRC) was used to trace the path of the chemical reactions. • Thermochemical analysis (ΔΔG free energy values) was performed at 298.15 K starting from the frequency calculations.


L. Vilella, P. Vidossich, D. Balcells and A. Lledos Dalton Trans., 2011, 40, 11241

In general the theoretical description is limited to the study of the O-O bond formation which is considered the rate determining step of the reaction.

Potential Energy Surface (PES)

PES is a combination of different types of mechanism: 1)  Proton Coupled Electron Transfer 2)  Coordination and elimination of water 3)  Formation of the O-‐O bond 4)  Dissociation of dioxygen from the complex


O-O bond formation


{IrCp*(OH)}+ à {IrCp*(=O)}+ + H+ + e-‐

Isodesmic Equation

SE = [E1(X) + E2(Y)] – [E2(X)+E1(Y)] X Y

{IrCp* (OH2)}+ à {IrCp*(=O) (OH}+ + H+ + e-‐

{IrCp*(OH)}+ + {IrCp*(=O) (OH}+ + H+ + e-‐ -‐> {IrCp*(=O)}+ + {IrCp* (OH2)}+ + H+ + e-‐

E2

E1


PCET steps SE (Kcal/mol.)

IrIII(OH)+ to IrIV(=O)+ 0.0

IrIII(OH)(OH2)+ to IrIV(OH)2+ 1.4

IrIV(OH)2+ to IrV(=O)(OH)+ -7.9

IrV(=O)(OH)+ to IrV(=O)(O•)+ 7.4

IrV(OH)3+to IrV(O•)(OH)2

+ -9.6

IrV(O•)(OH)2+ to IrV(O•)2(OH)+ -14.5

IrIII(OOH)+ to IrIV(η2-O2)+ 9.5

IrIII(OOH)(OH2)+ to IrIV(OOH)(OH)+ 4.3

IrIII(OOH)(OH2)+ to IrIV(OO•)(OH2)+ 14.1

IrIV(OOH)(OH)+ to IrV(η2-O2)(OH)+ 10.1

IrIV(OO•)(OH2)+ to IrV(OO•)(OH)+ -0.3

14.1 7.4 -‐14.5


IrV(=O)(O•)+

Mulliken spin densities are 0.33, 0.34 and 0.35 for the two oxygens and iridium atoms respectively. Bond orders are 1.62 and 1.61 .


X Y

ΔΔG298 = (ΔG298(X) + ΔG298(H2O)) – ΔG298(Y)

X Y ΔΔG298

IrIII(OH)+ IrIII(OH)(OH2)+ -10.0

IrIV(=O)+ IrIV(OH)2+ -8.6

IrV(=O)(OH)+ IrV(OH)3+ 4.0

IrV(=O)(O•)+ IrV(OH)2(O•)+ -12.9

IrIII(OOH)+ IrIII(OOH)(OH2)+ -10.1

IrIV(η2-O2)+ IrIV(OOH)(OH)+ -15.1

IrIV(η2-O2)+ IrIV(OO•)+(OH2)+ -5.5


O2 release


OBSERVED BY ESI-‐MS



-10.

0.0 1.4

-8.6

-7.9

4.0

-14.5

7.4 -9.6

-12.9


Potential Energy Surface

IrIII(OH)+ -> IrIV(=O)+ -> IrIV(OH)2+ -> IrV(=O)(OH)+ -> IrV(OH)3

+ -> IrV(O•)(OH)2+ -> IrV(O•)2(OH)+ = 12.9 Kcal/mol

12.4

Energy Barrier = 24.6kcal/mol

Oxidation of water with stoichiometric amounts of CAN is energetically favoured by 43.2 kcal mol-1 (water E0=1.23 V vs NHE; CAN E0= 1.5 ~ 1.7 vs NHE)





-10.

0.0 1.4

-8.6

-7.9

4.0

-14.5

7.4 -9.6

-12.9

M4

M3

M2

M1


This calculation can be described in a more formal manner using the equation:

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛ +ΔΔ++−×= ∑ ∑−

43.30

1039.2 298004 GSEEEFx CANWO

where x is the energy of the standard oxidation reaction of IrIII(OH)+ to IrIV(=O)+ (table 1), F the Faraday constant, E0

CAN the cell potential of the CeIV/CeIII couple, E0WO the standard

potential of water oxidation and ΣSE and ΣΔΔG298 the sum of individual PCET and water addition steps required to undergo one catalytic cycle (see tables 1 and 2). The term 2.39 × 10-4 is to convert from Joule to kcal/mol and 30.3 is the energy difference between IrV(O•)2(OH)+ and IrIII(OH)+ in kcal/mol.

This equation can be used to calculate the PES at any given potential, simply by replacing E0

CAN with the redox potential of the oxidant or an externally applied potential of choice.


Potential Energy Surface at 1.7 V vs NHE Potential Energy Surface at 1.23 V vs NHE

M1: O–O bond formation occurs via addition of water to IrV(=O)(OH)+, M2: O–O bond formation occurs via addition of water to IrV(=O)(O•)+, M3: O–O bond formation occurs via direct O–O bond formation from IrV(=O)(O•)+, M4: O–O bond formation occurs via direct O–O bond formation from IrV(OH)(O•)2

+


Conclusions

v Theoretical calculations in combination with experimental evidence supports a mechanism for catalytic water oxidation at harsh oxidative conditions mediated by [Ir(Cp*)(Me2-NHC)(OH)2]. v Water oxidation catalysis occurs via four sequential oxidation steps prior to formation of the O–O bond. v In the catalytic mechanism IrV(=O)(O•)+ is the more stable intermediate and formation of the O–O bond is rate limiting. Formation of the O–O bond occurs via intramolecular coupling of two oxo ligands. v The mechanism greatly differentiates from other iridium based water oxidation catalysts at which water oxidation occurs via nucleophilic attack of water on a iridium(V) oxo species

More details: A. Venturini*, A. Barbieri, J. N. H. Reek and D. G.H. Hetterscheid* “Catalytic Water Splitting with

an Iridium Carbene Complex: A Theoretical Study Chem. Eur. J. DOI: 10.1002/chem.201303796


Future work

X


Acknowledgments

Dr. A. Barbieri Institute for th Organic Synthesis and Photoreactivity, National Research Council of Italy Via P. Gobetti 101, 40129 Bologna, Italy

Dr. D. G. H. Hetterscheid Leiden Institute of Chemistry Leiden University Einsteinweg 55, Leiden, the Netherlands

Prof. Dr. J. N. H. Reek Van ‘t Hoff Institute for Molecular Sciences University of Amsterdam Sciencepark 904, Amsterdam, the Netherlands


The main objective of the project is the design and construction of a bio-inspired nano-structured organic-inorganic heterojunction tandem cell for solar to fuel conversion.

SolarFuelTandem is a Collaborative Research Project (CRP) of the European Science Foundation (ESF) EUROCORES programme EUROSOLARFUELS, involving 8 research institutions from 6 European countries and coordinated by Prof. Huub de Groot, University of Leiden (NL).

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