Infrared Spectroscopy & Structures of Mass-Selected Rhodium Carbonyl & Rhodium Dinitrogen Cations

Heather L. Abbott,1 Antonio D. Brathwaite2 and Michael A. Duncan2

1Department of Chemistry & Biochemistry, Kennesaw State University 2Department of Chemistry, University of Georgia, Athens GA

Funding provided by:

Transition Metal Complexes

• Catalytic activity often depends upon molecular structure.

• Gas-phase model systems can improve our understanding of organometallic structure.

• The Duncan group @ UGA has investigated several metal-carbonyl complexes and found that the 18 electron rule tends to govern stability.Figures (right): Ricks, Bakker, Douberly, Duncan J. Phys. Chem. A 2009, 113, 4701.

Rhodium Complexes

• Rhodium is known to be catalytically active, albeit expensive.– Reduces NOx gases to N2 and O2 in 3-

way catalytic converter– Converts CH3OH to CH3COOH in

Monsato process– Hydrogenates alkenes as Wilkinson’s

catalyst

• Will it follow periodic trends?– According to the 18 electron rule,

Rh+ should prefer n = 5.– Rh+ is a d8 metal known to form

stable square planar structures (i.e., n = 4).

Image credit: http://en.wikipedia.org/wiki/File:Monsanto-Prozess.svg

Monsato Process

Experimental Methods

• Rh rod ablated by 355 nm laser– Spectra Physics INDI Nd:YAG

• Rh reacts w/ pulsed supersonic beam of CO or N2 Ar

• Cations are mass selected in time-of-flight mass spectrometer

• Photodissociation using 2000-4000 cm-1 tunable infrared – LaserVision OPO/OPA system

pumped by Spectra Physics Pro 230 Nd:YAG laser

h n

Time-of-Flight Spectra

• Complexes can be observed with up to 17 ligands; most of these ligands are “external”.

• Complexes with n = 4 are the most abundant for Rh(CO)n+ & Rh(N2)n

+.

0 100 200 300 400 500 600

m/z

64

9

14

Rh(N2)+n

Rh+

Photofragmentation Spectra

• Spectra are created by subtracting the “laser off” from the “laser on” TOF spectrum.

• Spectra support a coordination number of 4 for both Rh+ complexes.

50 100 150 200 250 300 350 400 450 500

5

m/z

5

64

Rh(N2)+n

8

7

5

4

64

4

Photodissociation of Large Clusters

• Only weakly bound ligands can be dissociated by infrared light (e.g., ligands in an external coordination shell).

Blue-shift is observed for the CO frequencies in Rh(CO)n

+.

Red-shift is observed for the N2 frequencies in Rh(N2)n

+.

Photodissociation of Small Clusters

• In small clusters, all the ligands are tightly bound. “Tag” atoms such as Ar are photodissociated instead.

Blue-shift is observed for the CO frequencies in Rh(CO)n

+.

Red-shift is observed for the N2 frequencies in Rh(N2)n

+.

Metal-Ligand Interactions

• Dewar-Chatt-Duncanson model:– Donation from filled 5s orbital on

ligand to empty d orbital on metal blue-shift

– Back-donation from filled d orbital of metal to empty p* orbital of ligand red-shift

– Combined effect typically results in a red-shift (i.e., lower frequency)

• Model developed by Frenking and coworkers for M+-CO– Electrostatic polarization of the ligand

evenly redistributes charge– No s donation or p* back-donation– Results in blue-shift of ligand

frequency

Lupinetti, Fau, Frenking and Strauss. J. Phys. Chem. A 1997, 101, 9551.

Metal-Ligand Interactions for Rh+

• Rh+ polarizes the ligands as it withdraws some of the electron density from the HOMO (5s), but no back donation occurs.

• As a result, the ligand frequencies shift toward the values of their cations.

CO2143 cm-1

CO+ 2184 cm-1

N2

2330 cm-1

N2+

2175 cm-1

Complimentary Calculations

• Comparison of experimental and calculated IR active vibrational modes help determine the most likely structure of the cations.

• Density functional theory: – Performed using Gaussian 03– Method: B3LYP– Basis sets:

• LANL2DZ for Rh• DZP for C, N and O• 6-311+G* for Ar

– Frequencies scaled by 0.971

Binding Energies

• Binding energies for the complexes were also calculated using DFT.

• A substantial energy difference occurs between the 4th and 5th ligands for both Rh(CO)n+ and

Rh(N2)n+.

Complex Binding Energy(kcal/mol)

3Rh(N2)+ 23.203Rh(N2)2

+ 24.803Rh(N2)3

+ 12.251Rh(N2)4

+ 29.201Rh(N2)5

+ 2.921Rh(N2)6

+ 2.711Rh(N2)7

+ 2.171Rh(N2)8

+ 1.94

Complex Binding Energy(kcal/mol)

3Rh(CO)+ 41.183Rh(CO)2

+ 36.221Rh(CO)3

+ 37.211Rh(CO)4

+ 40.051Rh(CO)5

+ 4.851Rh(CO)6

+ 3.461Rh(CO)7

+ 3.411Rh(CO)8

+ 3.53

Rh(N2)n+

1st shell2nd shell

2.02 Å 2.02 Å3.23 Å

2.02 Å3.31 Å

Rh(CO)n+

1st shell2nd shell

1.99 Å 1.98 Å2.43 Å

1.98 Å2.42 Å, 4.23 Å

Coordination of Rh Complexes

Concluding Remarks

• Rh(CO)n+ and Rh(N2)n

+ complexes form stable, 4-ligand, 16 electron, square planar structures.

• Shifts in the bound ligand frequencies indicate that Rh+ causes polarization without back donation (i.e., it behaves like a point-charge).

• For Rh(CO)n+, the 5th ligand is intermediate

between the 1st and 2nd coordination shell.– Binding energy is comparable to 2nd shell

ligands (< 5 kcal/mol).– Bond length is comparable to 1st shell ligands.

Rh(N2)4+

Rh(CO)4+

2.42 Å

1.98 Å

Rh(CO)5+

Acknowledgements

• Funding for this project was generously provided by:– Department of Energy– Air Force Office of Scientific Research

• Thanks to the members of the Duncan Group!

Department of Chemistry

Thank you for your attention.

Tunable Infrared Spectroscopy

LaserVision Tunable Infrared Laser System

designed by Dean Guyer

Pumped by a Spectra Physics Pro-230 Nd:YAG Laser

Tuning range: 600-4300 cm-1 Linewidth: ~1.0 cm-1

Experiment & Calculations

Experiment & Calculations

2s2s

2p

p

5s

p*

N NN2

2p

2s2s

2p

p

5s

p*

C OCO

2p

Molecular Orbitals For Diatomics