Post on 17-Jan-2016
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