Infrared Photodissociation Spectroscopy of Silicon Carbonyl

Cations

Antonio Brathwaite and Michael Duncan

Department of Chemistry, University of Georgia

Athens, GA 30602

maduncan.myweb.uga.edu/

Transition Metal-Carbonyls Stable transition metal-carbonyls have been studied for over a

century◦ Cr(CO)6

◦ Fe(CO)5

◦ Ni(CO)4

Stability determined by 18 electron rule

We studied cations for comparison ◦ Mn(CO)6

+ ◦ Co(CO)5

+ ◦ Ni(CO)4

+

Free molecular C-O vibration - 2143 cm-1

C-O stretching frequency shifts systematically depending on the metal atom, charge and electronic structure

There are limited studies on main group element-carbonyl systems

We use infrared photodissociation spectroscopy to study silicon carbonyls by probing the carbonyl stretching region

Classical Metal Carbonyl Bonding Dewar-Chatt-Duncanson complexation

model can be used to explain CO shifts

σ donation occurs along the CO axis into empty metal d orbitals Results in a blue-shifted CO frequency

Filled d orbitals back donate into the antibonding * orbital on CO This causes a red-shift in the CO stretching

frequency

Combined effect produces a red shifted CO frequency

5

COC O

2p

2s

2s

2p

Experiment

50 100 150 200 250 300 350 400 450

Inte

nsity

Mass (amu)

Laser on - Laser offphotofragments

parent ion depletion

50 100 150 200 250 300 350 400 450m/z

m/z

In

ten

sity

Laser on

Photodissociation

50 100 150 200 250 300 350 400 450

Inte

nsity

V(CO)+

9

V(CO)+

n

Laser off

6

m/z50 100 150 200 250 300 350 400 450

12

43

21

5

1098

Inte

nsity

full mass spectrum

h (tunable IR)

External COWeakly bound(CO-CO dimer, D0 ~100 cm-1)

Fragment ion afterCO elimination

Mass selected ionwith excess CO

Photodissociation by elimination of excess ligands

Rare Gas Tagging Bonds are sometimes too strong (D0 > 23 kcal/mol) to break with infrared light

(e.g., C-O stretch 2143 cm-1 = 5.7 kcal/mol)

We attach a weakly bound “tag” atom to enhance fragmentation.

They are eliminated when light is absorbed providing indirect evidence of

absorption

Computations on tagged and untagged ions are done

Mass selected ion,argon tagged

Si+(CO)2Ar

h (tunable IR)

Si+(CO)2

Fragment ion after argon elimination

Ions vs. Neutrals backbonding is the most important

interaction in neutral metal-carbonyls

Cations have less electron density to

disperse as the charge on the metal atom

will contract the valence electrons.

Cations have reduced backbonding and

less red shifted frequencies than their

isoelectronic neutrals.

How does Si(CO)2+ compare to its

isoelectronic neutral Al(CO)2.

J. Phys. Chem. A 2009, 113, 4701. J. Am. Soc. Mass Spectrum. 2010, 21, 5.

2000 2050 2100 2150 2200 2250 2300

cm-1

2122

2003 2143

2174 n = 7

Cr(CO)6

Mn(CO)+

n

Free CO

Comparison of carbonyl red-shift

Molecule IR frequency

Fe(CO)5 2013, 2034 cm-1

Co(CO)5+ 2140, 2150 cm-1

Cr(CO)6 2003 cm-1

Mn(CO)6+ 2114 cm-1

Asymmetric Carbonyl Coordination

Asymmetric ligand clustering has been

observed for Mg+ and Al+

Initial ion-ligand interactions cause polarization

of the occupied 3 s orbital.

Subsequent ligands tend to bind on the same

side as the first

Si+ has similar 3s orbital occupation with and

an additional electron in the 3p orbital

What kind of bonding is present in Si(CO)n +

complexes?

A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, "Theoretical Analysis of the Bonding between CO and Positively Charged Atoms," J. Phys. Chem. 101 (1997) 9551 A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, "Theoretical Analysis of the Bonding between CO and Positively Charged Atoms," J. Phys. Chem. 101 (1997) 9551

G. Gregoire, N. R. Brinkman, D. van Heijnsbergen, H. F. Shaefer, M. A. Duncan, J. Phys. Chem. A 2003, 107, 218Walters, R. S. Jaeger, T. D. Gregoire, N. R. Brinkman, H. F. Shaefer, M. A. Duncan, J. Phys. Chem. A 2003, 107, 7396

Mass Spectrum of Si(CO)n+

0 50 100 150 200 250 300 350 400 450 500 550 600

16

12

84

14

6

m/z

Si(CO)+

n

2

10

Si+

Infrared photofragmentation mass spectra

All complexes larger than n = 2,

fragment by sequential ligand

termination ending at n= 2

Weakly bound ligands are easily

eliminated by IR photons

These results are consistent with a

coordination of two0 50 100 150 200 250 300

2

m/z

3

23

4

Si(CO)+

n2 3

4

5

Infrared photodissociation Spectra of Si(CO)n

+ Ar clusters

2050 2100 2150 2200 2250

cm-1

Theory

Si(CO)+

2Ar

Experiment

2050 2100 2150 2200 2250

n=1

cm-1

2129

2154

Si(CO)+

nAr

n=2

2123Free CO

Infrared photodissociation Spectra of Si(CO)n+

clusters

Spectra detected by elimination of CO

The bands at 2123 cm-1 represent the

asymmetric stretch and the bands at 2152

cm-1 represent the symmetric stretch

The blue-shifted band at approximately 2174

cm-1 observed.

◦ This band is attributed to the weakly bound

“external” CO ligands

2050 2100 2150 2200 2250

2123

2122

2123

2123

n=3

Si(CO)+

n

cm-1

2123

2153

21772152

n=4

2176

2152

n=5

2153

n=6

2173

2171

2152

n=7

Free CO

Structures of neutrals and ions

Experiment and theory compared to Al(CO)n measured

by Douberly and co-workers

Though greater in magnitude, the frequencies observed

here are qualitatively consistent with transition metal-

carbonyl trends

Liang, T. Flynn, S. D. Morrison, A. M. Douberly, G. J. Phys. Chem. A 2009, 113, 4701

Molecule IR frequency

Asymmetric Symmetric

Si(CO)2+ 2123 cm-1 2154 cm-1

Al(CO)2 1920 cm-1 1960 cm-1

Conclusions Si(CO)2

+ is the fully coordinated

silicon carbonyl cation

Si(CO)2+ has a V-shaped structure

analogous to that of isoelectronic

Al(CO)2

Si(CO)n+ and Al(CO)n have the same

qualitative trend as transition metal-

carbonyl isoelectronic analogues

Future Work

Early transition metal-carbonyls

◦ Test the limits of metal carbonyl

coordination

Metal cluster-carbonyls ◦ Different binding sites have characteristic

frequencies

Metal oxide-carbonyls

◦ Pure metal vs. metal oxides