Transcript RhCations_OSU_2013_HAbbott.pptx

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 metalcarbonyl 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
Monsato Process
• 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
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 20004000 cm-1 tunable infrared
– LaserVision OPO/OPA system
pumped by Spectra Physics Pro
230 Nd:YAG laser
hn
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+.
+
4
+
Rh(N2)n
6
Rh
9
14
0
100
200
300
m/z
400
500
600
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.
+
Rh(N2)n
5
6
4
4
8
5
7
4
6
4
5
50
100 150 200 250 300 350 400 450 500
m/z
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.
CO
2143 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)
Complex
Binding Energy
(kcal/mol)
3Rh(CO)+
41.18
3Rh(N )+
2
3Rh(CO) +
2
36.22
3Rh(N
2)2
+
24.80
1Rh(CO) +
3
37.21
3Rh(N
2)3
+
12.25
1Rh(CO) +
4
40.05
1Rh(N
2)4
+
29.20
1Rh(CO) +
5
4.85
1Rh(N
2)5
+
2.92
1Rh(CO) +
6
3.46
1Rh(N
2)6
+
2.71
1Rh(CO) +
7
3.41
1Rh(N
2)7
+
2.17
1Rh(CO) +
8
3.53
1Rh(N
2)8
+
1.94
23.20
Coordination of Rh Complexes
Rh(N2)n+
1st shell
2nd shell
2.02 Å
2.02 Å
3.23 Å
2.02 Å
3.31 Å
1.99 Å
1.98 Å
2.43 Å
1.98 Å
2.42 Å, 4.23 Å
Rh(CO)n+
1st shell
2nd shell
Concluding Remarks
• Rh(CO)n+ and Rh(N2)n+ complexes form
stable, 4-ligand, 16 electron, square
planar structures.
Rh(N2)4+
• Shifts in the bound ligand frequencies
indicate that Rh+ causes polarization
without back donation (i.e., it behaves
like a point-charge).
Rh(CO)4+
Rh(CO)5+
• 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.
2.42 Å
1.98 Å
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
Molecular Orbitals For Diatomics
N
N2
N
C
p*
2p
CO
O
p*
2p
5s
2p
5s
2p
p
2s
p
2s
2s
2s