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