Hemanthi D Manamperi CHEM 7350 Class Presentation - PowerPoint Presentation

Slide1

Hemanthi D ManamperiCHEM 7350 Class Presentation11/13/2017

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[Ru(

bpy)3]

2+

: The most well studied coordination

compound

Highly desirable optical properties

Background/Motivation

http://en.wikipedia.org/wiki/Tris(bipyridine)ruthenium(II)_chloride

Ceroni

, P.; Credi, A.; Venturi, M.; Balzani, V., Photochem. Photobiol. Sci. 2010, 9, 1561-1573Scaltrito, D. V.; Thompson, D. W.; O'Callaghan, J. A.; Meyer, G. J., Coord. Chem. Rev. 2000, 208, 243-266

2

[Ru(bpy

)3]

2+

architecture can be modified to design complexes capable of undergoing light activated ligand exchange reactions.

[

Ru(

bpy)3]

2+ vs [Ru(tpy

)(NN)X]2+

3[Ru(bpy)3]2+ architecture can be modified to design complexes capable of undergoing light activated ligand exchange.

J. Am. Chem. Soc.

2012

, 134, 8324-8327

In photo dynamic therapy In photo isomerization reactions

Acc. Chem. Res.

2015

, 48, 2284-2287

J. Phys. Chem. Lett.

2010

, 1, 3371-3375

[Ru(

bpy)

3

]2+ vs [Ru(

tpy

)(NN)X]2+

4

Poor bite angle

introdudeced

by tpy ligand further lowers the enrgy of 3LF statesPhoto dissociation of ligands can be achieved by careful tuning of energy of 3LF states via modulating the steric hindrance at the Ru(II) center.

Thermal and photochemical reactivity of

of series of [Ru(tpy)(NN)(L)]2+ has been investigate.

Goal:

Thermodymnamic

and kinetic

evaluation of

Ru-S bond cleavage in water

5

Hmte

is coordinated to Ru(II) via its soft sulfur atom. Torsion angles Ru1-N4-C20-C21 and Ru1-N5-C21-C20 for bpy

derivative is much smaller than that for

dcbpy complex.Shorter Ru-S bond

didsrances

for bpy

complex. Stronger Ru-S bond in bpy

complex

X ray crystallography data for [5](PF

6)2 and [7](PF6)2athis workbChem. Eur. J. 2012, 18, 107216

Evaluation of thermodynamic parameters - K

i

Thermal coordination of

Hmte

to the aqua complexes with N-N=

biq

,

dcbpy

and dmbpyConditions: In D2O, pH~7, T= 297 K in dark, (a) [Ru]tot=5.13 mM, (b) [Ru]tot=12.7 mM, Slope = Ki

D

G

i

= -

RTlnK

i

D

Gi

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Kinetic

study to evaluate rate constants for thermal substitution

Kinetic measurements were performed in pure H

2

O using UV-vis spectroscopy with large excess of

Hmte

.

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Solution of [1]2+ with ex

Hmte is stable at RT and substitution with Hmte takes place only temperatures above 323 K.Plots of ln[RuOH2]/[Ru]Tot were linear for bpy

,

biq

and

dcbpy

complexes and were used to calculate

k’

i

(i

=1,2 and 3)

Kinetic Study to evaluate rate constants for thermal

substitution

Kinetic measurements were performed in pure H

2

O using UV-vis spectroscopy with large excess of

Hmte

.

9

Solution of [1]

2+ with ex. Hmte is stable at RT and substitution with Hmte takes place only temperatures above 323 K.Plots of ln[RuOH2]/[Ru]Tot were linear for bpy, biq and dcbpy complexes and were used to calculate k’i (i=1,2 and 3)For dmbpy complex a non linear plot was observed at 297 K due to thermal back substitution of Hmte by H2O

k

-

i

has been calculated using

k

i

and K

i

Slope =

k’

i

=

k

i

*[

Hmte

]

Kinetic Study to evaluate rate constants for thermal

substitution-biq

,

dcbpy and dmbpy

complexes

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kobs

= k’4 +k

-4 for dmbpy

comlexkobs = k’i = ki*[Hmte] for other complexes.Plots of kobs vs [Hmte] are straight lines, including for dmbpy complex showing that the coordination of Hmte to RuOH2 is first order in Hmte

.

Kinetic Study to evaluate rate constants for thermal substitution of aqua ligand with

Hmte-

bpy

complex

For

bpy

complex an indirect method was used to calculate k1, k

-1 and K1 due to its slow substitution.

For

bpy systems both k1 and k-1 were evaluated at temperatures >343 K and Eyring plot was used to calculate the corresponding data at 297 K. 11

Thermodynamic parameters:

DH#

and

DS#

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The observed rates are consistent with the expected rates based on different extents of

sterics

introduced by the substituents on

bpy

system.

Mechanism

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D

G#

1

> D

G#2

> DG

#3>

DG#4 - As expected with increasing sterics Usually the higher lability of sterically hindered complexes is explained in terms of destabilization of the ground state of the hexacoordinate complex - Rxn is always expected to follow via dissociative interchange mechanism - expected to see a significant dependence on DH# The close proximity of DH# and differences in DS

#

within the series have been observed.

For all four systems rate law is 1

st

order in

Hmte

 interchange mechanism

N-N = bpy and

biq ; D

S

#

< 0

N-N =

dcbpy

and

dmbpy

;

D

S

#

>

0

Mechanism

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N-N =

bpy and

biq

; D

S# < 0

N-N = dcbpy

and dmbpy ; D

S# > 0Interchange mechanism: H2O is still present within the coordination sphere when Ru-S bond making occurs.Ia – Ru-S bond making in synchronous with Ru-O bond breaking H bonding between Hmte and H2O can stabilize the hepta coordinate structure. More compact TS 

D

S

#

< 0

Id – Ru-S bond making occurs only when Ru-O bond is partially broken., but before H

2

O exists the second coordination sphere.

less compact TS



DS#

> 0

Photochemistry of

RuHmte to RuOH2

conversion

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Ruthenium

polypyridyl

complexes are known to undergo photosubstitution of ligands with solvent molecules.

For bpy complex ([5]

2+) full conversion to the aqua complex ([1]2+) was obtained after 30 minutes of irradiation with 452 nm light.

However, the photochemistry of the biq, dcbpy and dmbpy complexes were complicated by the thermal equilibrium between RuHmte and RuOH2 complexes. Measured in dark – account for the thermal equilibrium  k

-

i

Measured under irradiation



k

f

i

Photochemistry of

RuHmte to RuOH2

conversion

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f =

photon flux determined by

ferrioxalate

acinometry

fI = quantum yield for photosubstitutionkfi for [2]2+ and [3]2+ are order of magnitude higher than their k’i and k-I

k

f

i

for

[4]

2

+ is order of magnitude

less than their k’

4 and k-4

Increasing too much steric

hinderence

increases thermal lability

biq

and

dcbpy

represent a better compromise between thermal and photochemical lability to afford a light

sensitive

Ru-S bond in water.

Robust interconversion between

RuHmte and RuOH2

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light irradiation (520 nm) – 45 min, dark

– 90

min

The

biq

system is robust and contains only two ruthenium complexes [2]

2+ and [6]2+ that interconvert upon switching on and off of a green light.

Conclusions:

The thermodynamic, kinetic and photochemical investigation of a series of complexes of the architecture [Ru(tpy

)(N-N)(L)]

2+ has been carried out to understand the influences of steric hinderence

around Ru(II) to its reactivity.

Variable temperature kinetic data suggest that the increased reactivity is a result of entropy.

Further, the too much increase of sterics in

dmbpy complex has resulted in competing thermal equilibrium upon irradiation, making it hard to evaluate the photochemical reaction.In contrast,

biq and dcbpy

complexes provide a nice balance of steric hinderence so that, the Ru-S bond is formed spontaneously in dark and cleaved upon mild irradiation.Critiques:Could have compared ki and k-i using direct and indirect Ieyring plots) method for biq, dcbpy and dmbpy complexes to get an insight into the errors associated with the indirect method. The biq

complex may not only increase steric bulk around Ru(II), but also exert electronic effect as evidenced by its red shifted absorbance.

Conclusions and Critiques

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Thank You…!!

Questions??