Infrared Spectroscopy & Structures of Mass-Selected Rhodium Carbonyl & Rhodium Dinitrogen Cations Heather L. Abbott, 1 Antonio D. Brathwaite 2 and Michael A. Duncan 2 1 Department of Chemistry & Biochemistry, Kennesaw State University 2 Department 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 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 NO x gases to N 2 and O 2 in 3-way catalytic converter – Converts CH 3 OH to CH 3 COOH 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 d 8 metal known to form stable square planar structures (i.e., n = 4). Image credit: 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 N 2  Ar Cations are mass selected in time-of-flight mass spectrometer Photodissociation using cm -1 tunable infrared – LaserVision OPO/OPA system pumped by Spectra Physics Pro 230 Nd:YAG laser h

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(N 2 ) n +.

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.

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 N 2 frequencies in Rh(N 2 ) 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 N 2 frequencies in Rh(N 2 ) n +.

Metal-Ligand Interactions Dewar-Chatt-Duncanson model: – Donation from filled 5  orbital on ligand to empty d orbital on metal  blue-shift – Back-donation from filled d orbital of metal to empty  * 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  donation or  * 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 (5  ), but no back donation occurs. As a result, the ligand frequencies shift toward the values of their cations. CO 2143 cm -1 CO cm -1 N cm -1 N 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 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 4 th and 5 th ligands for both Rh(CO) n + and Rh(N 2 ) n +. ComplexBinding Energy (kcal/mol) 3 Rh(N 2 ) Rh(N 2 ) Rh(N 2 ) Rh(N 2 ) Rh(N 2 ) Rh(N 2 ) Rh(N 2 ) Rh(N 2 ) ComplexBinding Energy (kcal/mol) 3 Rh(CO) Rh(CO) Rh(CO) Rh(CO) Rh(CO) Rh(CO) Rh(CO) Rh(CO)

Rh(N 2 ) n + 1 st shell 2 nd shell 2.02 Å 3.23 Å 2.02 Å 3.31 Å Rh(CO) n + 1 st shell 2 nd shell 1.99 Å1.98 Å 2.43 Å 1.98 Å 2.42 Å, 4.23 Å Coordination of Rh Complexes

Concluding Remarks Rh(CO) n + and Rh(N 2 ) 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 5 th ligand is intermediate between the 1 st and 2 nd coordination shell. – Binding energy is comparable to 2 nd shell ligands (< 5 kcal/mol). – Bond length is comparable to 1 st shell ligands. Rh(N 2 ) 4 + Rh(CO) 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: cm -1 Linewidth: ~1.0 cm -1

Experiment & Calculations

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