Infrared Photodissociation Spectroscopy of TM+(N2)n (TM=V,Nb) Clusters
E. D. Pillai, T. D. Jaeger, M. A. Duncan
Department of Chemistry, University of Georgia
Athens, GA 30602-2556
www.arches.uga.edu/~maduncan/
U.S. Department of Energy
• Biological systems require N2 as components of proteins, nucleic acids, etc. But N2 is highly inert (IP = 15.08 eV, BE = 225 kcal/mol).
Nitrogenases catalyze N2 reduction and carry metal centers such as Fe, Mo, V.
• Large scale ammonia synthesis uses Fe as catalyst.
• N2 is isoelectronic to CO, C2H2 which are prevalent throughout inorganic and organometallic chemistry
• N2 activation gauged by change in N-N bond distance or N-N vibrational frequency
Why Study TM-Nitrogen?
N2 + H2Fe catalyst
350 - 1000 atm
300 - 500 oC
2NH3
Previous Work
• Electronic spectroscopy of M+(N2) (M = Mg, Ca) by Duncan and coworkers.
• CID studies by Armentrout and coworkers for Fe and Ni with N2
• FT-ICR studies by H.Schwarz and coworkers, and electronic spectroscopy by Brucat and coworkers on Co+
(N2)
• Theoretical studies on TM-N2 carried out by Bauschlicher
• ESR spectra for V(N2)6 and Nb(N2)6 done by Weltner.
• IR studies using matrix isolation on M(N2) (M = V, Cr, Mn, Nb, Ta, Re) done by Andrews and coworkers
Experimental Bond Energies*
Ni+(N2)n Bond Energy (kcal/mol)
n = 1 27 2 27 3 14 4 2
V+(CO)n Bond Energy (kcal/mol)
n = 1 27 2 22 3 17 4 21 5 22 6 24* Armentrout and coworkers
Fe+(N2)n Bond Energy (kcal/mol)
n = 1 13 2 19 3 10 4 13 5 15
Direct absorption in our experimentsis not possible due low ion densities.
Solution is photodissociation.
IR photon 2359 cm-1 ~ 7 kcal/mol
Small clusters may fragment via multiphoton process.Large clusters will be easier to fragment
Production of coldmetal ion complexeswith laser vaporization/supersonic expansion.
Mass selection of cationsby time-of-flight.
Tunable infrared laserphotodissociationspectroscopy.
LaserVision OPO/OPA2000-4500 cm-1
200 400 600
Nb+(N2)n
Nb+
2
4
5
6
n= 1
10
16
Mass
100 200 300 400 500
mass
Fragmentation ends atn = 6 suggesting that this cluster is more stable.
Fragmentation of Nb+
(N2)n
n = 6
7
8
9
5
6
6
67
2100 2200 2300 2400
cm-1
Free N2 mode 2359 cm-1
Infrared Photodissociation Spectra for Nb+(N2)n
Fragmentation is inefficient for the n = 1-3 clusters.
The n=4 cluster shows fragmentation 95 cm-1 red of the free N2 stretch
n = 2
n = 3
n = 42265
Dewar-Chatt-Duncanson Model of -bonding
Both factors weaken the N-N bonding in nitrogen.
The N-N stretching frequencies shift to the red.
N
NTM
NN
TM
-donation from occupied 1u or 3g N2 orbital into empty d-orbitals of the metal
- type back donation from filled dxy, dyz, dxz orbitals to g* orbitals of N2
N N
TM
N N
TM
2100 2200 2300 2400
n=3
n=4 2265
n=6
n=52204
n=7
2214
2212
cm-1
Spectra show a red shift of95 cm-1 for n=4 as compared to free N2 stretch
An additional red shift of 60 cm-1
is observed for n>4 cluster sizes
The spectra of n=6 has a lower S/N ratio suggesting the complexis harder to dissociate owing to unusual stability
B3LYP/ DGDZVP Nb+
6-311+G* NDe= 33.8 kcal/molFreq = 2291 cm-1
Osc. Strength = 55 km/mol
De= 18.6 kcal/molFreq = 2160 cm-1
Osc. Strength = 169 km/mol
De= 19.7 kcal/molFreq = 2262 cm-1
Osc. Strength = 354 km/mol
De= 8.3 kcal/molFreq = 2209 cm-1
Osc. Strength = 376 km/mol
1. DFT calculations favor linear over T-shaped structures ( De ~ 15 –20 kcal/mol
2. T-shaped complexes red-shift N-N stretch by 150-200 cm-1 whereas linear complexes red shift by 50-100 cm-1.
2100 2200 2300 2400
Nb+(N2)3
3B2
5A1
Nb+
Grnd state: 4d4 5D
1st state: 4d35s 5F
6.7 kcal/mol
2nd state: 4d4 3P
15.9 kcal/mol
Spectrum has two modesbecause there are only twoequivalent N2
2100 2200 2300 2400
3A1g
Nb+(N2)4
5B2g
cm-1
2265
DFT (B3LYP) calculations for the n = 4 complex for the 5D spin state show good correspondenceto the IR spectra.
Single peak spectrum points to a high symmetry structure.
2100 2200 2300 2400
5B1
3A2
What is causing the additional red shifts for the n>4 clusters ?
1. Other structures such as T-shaped or inserted complexes? DFT studies consistently predict linear structures overT-shaped structures. Energy differences ~ 15 kcal/mol and 20 kcal/mol.
In addition all spectra are single peaksignifying that no isomers are present.
2. A change in spin state? DFT (B3LYP) calculations for the n = 5 for triplet spin state shows better correspondence to IR spectrum than the quintet state.
Also triplet state is found to be lowerin energy by ~ 15 kcal/mol
Nb+(N2)5
2100 2200 2300 2400 2500
n=5
n=4
n=3
2258
n=7
n=6
2258
2271
2271
2288
2100 2200 2300 2400 2500
n=3
n=4 2265
n=6
n=52204
n=7
2214
2212
Comparison of Nb+(N2)n and V+(N2)n
Greater red-shifts for Nb+(N2)n than V+(N2)nNb+(N2)n V+(N2)n
N N
TM
N N
TM
1. N2 and CO are -accepting ligands and so dback donation is expected to dominate the bonding interaction.
2. d orbitals more diffuse for second row TM leading to better s-d hybridization.
3. Frequency shifts for V+(N2)n and Nb+(N2)n seems to justify this reasoning.
Conclusions
IR spectroscopy coupled with DFT calculations of Nb+(N2)n reveals the structures of these clusters.
The spectra show that N2 binds in an “end on” configuration to Nb+.
The results also reveal possible evidence for a change in multiplicity in the metal cation due to solvation effects.
The N-N stretch in Nb+(N2)n red shifts further than in V+(N2)n consistent with the previous conclusions based on various TM-(CO)n systems that -back donation is the more significant interaction in these TM-ligand systems.
2100 2200 2300 2400
n=7
2212
2214
n=8
n=6
n=9
Nb+(N2)n
n=10