Oxidation State Ambiguity in f Element - PowerPoint Presentation

Slide1

Oxidation State Ambiguity in f Element

Organometallics -

a Spectroscopic and Quantum Chemical Journey

Nik Kaltsoyannis

Department of Chemistry, University College London

Slide2

Outline of presentation

Story 1: X-ray

absorption spectroscopy of Ce

compounds

Story 2: Gas-phase photoelectron spectroscopy of

CeCp

3

Story 3:

Multiconfigurational

quantum chemical calculations of M(COT)

2

(

COT =



8

-C

8

H

8

;

M =

Th

, Pa, U, Pu, Cm and Ce

)

Story 4:

Multiconfigurational

quantum chemical calculations of

CeCp

3 and CeCp3+ (Cp = 5-C5H5)

Ln(COT)

2 the “lanthanocenes”An(COT)2 the “actinocenes”

CeCp

3

Slide3

Story 1: X-ray

absorption spectroscopy of Ce compounds

Slide4

Qualitative molecular orbital diagram for M(COT)

2

M = f element

Slide5

The traditional view of Ce(COT)

2

and Th(COT)

2

Ground state is

1

A

1g

with an electronic configuration e

2u

(

p

2

)

4

f

0

M(IV) and 2 x COT

2-

Correct description of Th(COT)

2

BUT NOT Ce(COT)

2

M. Dolg, P. Fulde, H. Stoll, H. Preuss, A. Chang and R. M. Pitzer

J. Chem. Phys.

195

(1995) 71

Slide6

Dolg

et al.

’s view of Ce(COT)

2

Ground state is

1

A

1g

with

two

contributing electronic configurations e

2u

(

p

2

)

4

f

0

(20%) + e

2u

(

p

2

)

3

f



1

(80%)

Ce(III) and 2 x COT

1.5-

20%

80%

Slide7

Can we test this experimentally (how do we measure oxidation state)?

N. M. Edelstein, P. G. Allen, J. J. Bucher, D. K. Shuh, C. D. Sofield, A. Sella, M. Russo, N. Kaltsoyannis and G. Maunder

J. Am. Chem. Soc.

118

(1996) 13115

X

-ray

A

bsorption

N

ear

E

dge

S

pectroscopy (XANES)

Ce K edge (1s electrons)

Need a variable energy light source capable of delivering c. 40 keV photons (Stanford Synchrotron)

Representative K-edge spectra of Ce compounds

CeO

2

(Ce(IV))

Slide8

Ce K-edge XANES results

1

CeO

2

(solid)

8

Ce

2

(SO

4

)

3

(solid)

15

Ce(NO

3

)

3

(1.2 M HCl solution)

2

Ce(NH

4

)

4

(SO

4

)

4

.2H

2

O (solid)

9

CeSi

2 (solid)

3

Ce(NH

4

)

4

(SO

4

)

4

.2H

2

O (1.6 M HNO

3

soln.)

10

CeI

3

.(THF)

x

(THF soln.)

16

Ce[1,4(TMS)

2

C

8

H

6

]

2

(toluene soln.)

4

Ce(CH

3

C(O)CHC(O)CH

3

)

4

(toluene soln.)

11

Ce[(Me

3

C)

2

C

5

H

3

]

3

(toluene soln.)

17

Ce[1,3,6(TMS)

3

C

8

H

5

]

2

(toluene soln.)

5

CeCl

3

.6H

2

O (solid)

12

Ce

2

(SO

4

)

3

(1.6 M HNO

3

soln.)

18

Li{Ce[1,4(TMS)

2

C

8

H

6

]

2

}

(toluene soln.)

6

CeF

3

(solid)

13

Ce

2

(SO

4

)

3

(1.2 M HCl soln.)

19

K{Ce(C

8

H

8

)

2

}

(toluene soln.)

7

Ce

2

O

2

S (solid)

14

Ce(NO

3

)

3

(1.6 M HNO

3

soln.)

▼ Ce(IV) compounds

■

Ce(III) compounds

Substitued cerocenes

Ce(III) !!

Slide9

H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

Cs

Ba

La

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

Fr

Ra

Ac

Rf

Db

Sg

Bh

Hs

Mt

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

 

Th

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

 

Increasing tendency toward lanthanide-like chemistry (An(III) dominant)

Are the ground states of the later

actinocenes

multiconfigurational

?

Need: high-level

ab initio

calculations (see story 3….)

Slide10

Story 2: Gas-phase

photoelectron spectroscopy of CeCp

3

Slide11

Gaseousmolecules

The experiment

UV or X-ray light

e

-

e

-

e

-

Measure kinetic energy of electrons and determine ionization energy as the difference between the energy of the incident light photons and the electrons’ kinetic energy

Direct probe of electronic energy levels

Slide12

Compared with d-block complexes, very few lanthanide complexes have been studied in the gas phase,

because it is very hard to see f-based bands in spectra

Two main reasons

With

Ln(III) compounds ionizations from 4f orbitals come at similar ionization energies to those from ligand

orbitals

2. With photon energies given by discharge lamps 4f cross sections (ionization probabilities) are low

Slide13

Ionization cross sections (ionization probabilities)

Delayed maximum

At photon energies accessible with a discharge lamp, 4f electrons have very low ionization cross sections

Slide14

The “Elettra” synchrotron, Trieste, Italy

Slide15

Photoelectron spectrum of CeCp

3

At low incident photon energies only ionizations from the

Cp

rings are

visible

Cp

p

Cp

p

Slide16

Low ionization energy band (A) clearly visible BUT also a band just above 10 eV (D) showing f characteristics

Photoelectron spectrum of CeCp

3

(again)

Cp

p

Slide17

Resonance structure is observed for bands A and D

i.e.

ionization of the single 4f electron gives rise to

two

cation states with f character

Are there really two f bands?

If the incident photon energy is sufficient to excite a Ce 4d core electron to a 4f orbital, a resonance will occur. Ionization of a 4f electron can borrow intensity from this transition and the ionization cross section can show a dramatic increase



tune h

n

to the 4d ionization energy…..

A (f)

B

C

D (f)

Cp

p

Slide18

What the…..?

Assume neutral CeCp

3

has a ground state with the configuration

Lf

1

,

where L

represents the ligand electrons and f

1

is the single 4f electron

The matrix element governing the band intensity for f ionization is given by

where



is the free electron (g) wave

Note that

(a) L

e

represents a configuration with ligand electrons and no f electrons,

i.e.

Lf

0

and

(b) the ion states corresponding to bands A and D in the photoelectron spectrum must have

L

e

(Lf

0

)

as a contributing configuration

Slide19

Assume that ionization of the f electron leads to ligand to metal charge transfer, generating a

cation

configuration with a hole in the ligand orbitals and a single Ce 4f electron,

i.e.

L

-1

f

1

(sound familiar….?)

If

Lf

0

and

L

-1

f

1

have the same symmetry, mixing of the two configurations can generate two states of CeCp

3

+



g



c

1

Lf

0

+ c2L-1f1) band A – ground state of CeCp3+ ec

3

Lf0 – c4L-1f

1) band D – excited state of CeCp3

+

 Our suggestion was that the ground state of CeCp3

+ (formally Ce(IV)) is multiconfigurational, in a manner comparable with that of neutral Ce(COT)2

What the…..? (continued)

M.

Coreno

, M.

DeSimone

, J. C. Green, N. Kaltsoyannis, N.

Narband

and A. Sella,

Chemical Physics Letters

432

(2006) 17

Slide20

Story 3:

Ab

initio

quantum chemical calculations of M(COT)

2

(

M =

Th

, Pa, U, Pu, Cm and Ce)

Slide21

CASSCF/CASPT2 method

MOLCAS code

D

2h

point group

Basis sets: correlation consistent, all-electron, ANO (27s24p18d14f)/[10s9p7d5f] for An, (25s22p15d11f)/[9s8p5d4f] for Ce, VDZP for C and H

Scalar relativistic effects incorporated

via

2

nd

order Douglas-Kroll

Spin-orbit free and spin-orbit coupled calculations

Computational details

Slide22

Active spaces

Partial ground state geometry optimisations performed with ((12+

n

),16) active spaces (

n

= 0 (

Ce

,

Th

), 1 (Pa) and 2 (U

)….)

Ground and excited states calculated with ((8+

n

),14) active spaces (

n

= 0 (Ce,

Th

), 1 (

Pa), 2

(U

)….)

For the partial geometry optimisations of the ground state of Pa(COT)

2

(13,16), 11,451,440 configurations were included

Slide23

Results – Th(COT)

2

Ground state is the expected

1

A

g

(d

0

f

0

)

Metal-ring distance; 2.015

Å (

calc

), 2.004 Å (

expt

)

Two lowest energy singlet and triplet states of each D

2h

irrep

calculated (32

states)

Lowest energy dipole-allowed transition is to

1

B

1u

(d

σ1f0); 2.47 eV (calc), 2.76 eV (expt

– UV/Vis)Spin-orbit coupling makes essentially no difference to energy spectrum (<0.05 eV).

Slide24

First excited states (Th(Cp'')3)Ground stateTh(COT)2 energy level diagram

Slide25

Results – Pa(COT)

2

Ground state is a degenerate pair of spin-orbit free states

2

B

2u

/

2

B

3u

(d

0

f

f

1

)

Metal-ring distance; 1.969

Å (calc), 1.964 Å (“expt”, average of Th(COT)

2

and U(COT)

2

)

Two lowest energy doublet and quartet states of each D

2h

irrep calculated

Spin-orbit coupling makes a significant difference

Slide26

Pa(COT)2 energy level diagram(no spin-orbit coupling)

Slide27

The effect of spin-orbit coupling on the ground and lowest excited states of Pa(COT)

2

Slide28

A comparison of the spin-orbit coupled Pa(COT)

2

energy levels (eV) with those from previous calculations

State

Symmetry

This work

Chang

et al.

a

Li & Bursten

b

1

E

5/2u

0

0

0

2

E

1/2u

0.003

0.166

0.049

3

E

3/2u

0.459

0.477

0.369

4

E

7/2u

0.584

0.362

0.379

5

E

1/2u

0.642

0.569

0.541

6

E

1/2g

0.880

0.925

0.685

7

E

3/2u

1.467

1.222

1.122

a

SOCI calculations using the experimental uranocene geometry (

1.924

Å)

b

DFT calculation using the PW91 exchange-correlation functional, using an optimised geometry with ring-metal separation of 1.975

Å

Slide29

Results – U(COT)

2

Metal-ring distance; 1.944

Å (calc), 1.924 Å (expt)

Spin-orbit coupled ground state is E

3g

Dominant configuration of spin-orbit free state

Total spin of spin-orbit free state

This work

Chang

et al.

f

p

1

f

f

1

1

70.7

68.0

f

s

1

f

f

1

1

22.1

22.7

f

s

1

f

f

1

0

7.0

5.3

Slide30

Results – U(COT)

2

Comparison of experimental (UV/Vis) excitation energies (eV) with calculation

Expt

This work

1.880

1.934

1.65

2.018

1.79

Both calculated transitions are principally

f





d

σ

in character

Slide31

Results – Ce(COT)

2

Ground state is the expected

1

A

g

Metal-ring distance; 1.964

Å (calc), 1.969 Å (expt)

Lowest energy dipole-allowed transitions are to

1

B

1u

(d

0

f

σ

1

) and

1

B

2u

/

1

B

3u

(d

0

fp1); 2.47 eV (calc), 2.18 eV (expt – UV/Vis). Second dipole-allowed transition to 1B1u (d0fd

1); 2.93 eV (calc), 2.63 eV (expt)As with Th(COT)2, spin-orbit coupling makes essentially no difference to energy spectrum (<0.05 eV).

Slide32

Ce(COT)2 energy level diagram

Slide33

A look at the ground and first excited

1

A

g

states of Ce(COT)

2

58.1% f

0

, 23.4% f

d

1

, 8.7% f

d

2

84.6% f

d

1

, 6.2% f

d

2

Slide34

How can we square this result with previous theory and experiment for Ce(COT)

2

?

(number of states in state-average)

Change in ground state energy

Single state total energy = -257724.60 eV

Slide35

Experiment (XANES):

0.89

± 0.03

C.H. Booth, M.D. Walter, M. Daniel, W.W. Lukens and R.A. Andersen

Phys. Rev.

Lett

.

95

(2005) 267202

Ce(COT)

2

f electron occupancy

n

f

Calculation:

0.90

± 0.04

Slide36

Configurational

admixture of Ce(COT)

2

ground state as a function of

n

s

Slide37

Occupation (NOO) of the Ce(COT)

2

ground state natural orbitals as a function of

n

s

A. Kerridge, R. Coates and N. Kaltsoyannis

J. Phys. Chem. A

113

(2009)

2896

Slide38

What about the actinides?

Occupation of the ground state e

2u

“f



” natural orbitals in An(COT)

2

Ce(COT)

2

= 0.216

A.

Kerridge and

N. Kaltsoyannis

J. Phys. Chem. A

113

(2009)

8737

Slide39

Story 4:

Ab

initio

quantum chemical calculations of

CeCp

3

and CeCp

3

+

Slide40

A (f)

B

C

D (f)

Cp

p

Recall the PE spectrum of CeCp

3

…..

Slide41

Active spaces for CeCp

3

and CeCp

3

+

Inclusion of all 14 MOs too costly

 (5,8) for CeCp

3

and (4,8) for CeCp

3

+

(4  a,

4 

a)

Slide42

Configurational

admixture of

CeCp

3

+

1

A



ground

state as a function of

n

s

 Use natural orbitals and their occupations

Slide43

NOOs of CeCp

3

2

A



ground state

Active space orbital

48a'

49a'

50a'

51a'

33a''

34a''

35a''

36a''

Occupation

1.967

0.001

0.027

0.005

1.966

0.029

0.005

1.000

Single

configurational

state

O

ne 4f-localised

NO

Slide44

NOOs of CeCp

3

+

1

A



ground state

Active space orbital

48a'

49a'

50a'

51a'

33a''

34a''

35a''

36a''

Occupation

1.961

0.000

0.000

0.038

1.445

0.000

0.555

0

.000

Strongly multi-

configurational

state

No 4f-localised NO (as expected following 4f ionisation)

Energy relative to CeCp

3: 7.07 eV (band A in PE spectrum 6.77 eV)

Slide45

NOOs of CeCp

3

+

fifth excited

1

A



and

1

A

 states

Active space orbital

48a'

49a'

50a'

51a'

33a''

34a''

35a''

36a''

Occupation

1

A



1.509

1.000

0.001

0.490

0.963

0.000

0.000

0.037

Occupation 1

A0.985

1.0130.000

0.0021.473

0.000

0.526

0.001

Strongly multi-

configurational

states

No 4f-localised NO (as expected following 4f ionisation)

Energy relative to CeCp

3

: 10.00 and 10.17 eV (band D in PE spectrum 9.97 eV)

R. Coates, M.

Coreno

, M.

DiSimone

, J.C. Green, N. Kaltsoyannis, A. Kerridge, N.

Narband

and A. Sella

Dalton Trans.

(

2009)

5943

Slide46

Conclusions - 1

Calculations (Dolg

et al.

) suggest that Ce(COT)

2

has a

multiconfigurational

ground state, with a dominant f

1

(Ce(III)) configuration. XANES results (us and Booth

et al.

)

appear

to support this

.

Variable

energy photoelectron spectroscopy of CeCp

3

reveals not one but two f bands during resonance; is the ground state of CeCp

3

+

multiconfigurational

?

CAS

calculations on An(COT)

2

(An =

Th

, Pa, U) yield results consistent with experiment and previous computational studies.CAS calculations on Ce(COT)2 produce excellent agreement with experiment for metal-ring separation, electronic excitation energies and f electron occupancy (nf).Total energy of Ce(COT)2 ground state, nf, the natural orbitals and their occupations are essentially invariant to the number of states included in the state-average.Description of ground state in terms of configurational admixture varies wildly as a function of state average  configurational admixture not a reliable tool to describe the electronic structure of Ce(COT)2.

Slide47

Conclusions - 2

Ce

(COT)

2

is best described as

Ce

(IV) system in which transfer of electron density from

ligand

to metal through occupation of bonding

orbitals

allows measures of the effective oxidation state to be lower than the

formal

+4 value, and indeed closer to +3 in certain cases.

Occupation of the ground state

e

2u

“f



” natural orbitals increases markedly across the actinide series, indicating that the

ground states of the later

actinocenes

are strongly

multiconfigurational

.

The ion states which give rise to bands A and D in the

photoelectron

spectrum

of

CeCp

3+ are strongly multiconfigurational, and do not possess a Ce 4f-localised natural orbital (i.e. they have the characteristics of f ionization).

Slide48

And finally……

Slide49

“The effective oxidation state of Ce in

cerocene

is intermediate between the formal Ce(IV) and Ce(III) situations. When interpreted as a Ce(IV) system the effective oxidation number is lowered toward III by strong orbital mixing, whereas when interpreted as a Ce(III) system a strong

configurational

mixing increases the effective oxidation number toward IV. The latter choice however is more compact since only two configurations…..are needed for building a sufficiently accurate zeroth-order

wavefunction

: the

cerocene

1

A

1g

ground state can be described as a…..mixture of about 70% 4f

1

p

3

and 30% 4f

0

p

4

.”

Slide50

The without whom department

National Service for Computational Chemistry Software

Berkeley

Norm Edelstein

Pat Allen

Jerry Bucher

Dave Shuh

Chad Sofield

UCL

Andy Kerridge

Rosie Coates

Andrea

Sella

Maria-Rosa Russo

Graham

Maunder

Naima Narband

Oxford

Jenny Green

Trieste (Elettra)

Monica DiSimone

Marcello Coreno

Slide51