Infrared Photodissociation Spectroscopy of TM (N2)n (TM=V,Nb) Clusters

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Infrared Photodissociation Spectroscopy of
TM(N2)n (TMV,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
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Why Study TM-Nitrogen?

• 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

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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

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Experimental Bond Energies
Direct absorption in our experiments is 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
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
Fe(N2)n Bond Energy (kcal/mol) n 1
13 2 19 3 10 4 13
5 15
Armentrout and coworkers
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Nb(N2)n
Nb
6
4
2
10
5
n 1
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Mass
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Fragmentation of Nb(N2)n
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7
9
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Fragmentation ends at n 6 suggesting that this
cluster is more stable.
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6
7
5
n 6
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Infrared Photodissociation Spectra for Nb(N2)n
Free N2 mode 2359 cm-1
2265
n 4
Fragmentation is inefficient for the n 1-3
clusters. The n4 cluster shows fragmentation
95 cm-1 red of the free N2 stretch
n 3
n 2
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Dewar-Chatt-Duncanson Model of p-bonding
s
s-donation from occupied 1pu or 3sg N2 orbital
into empty d-orbitals of the metal
p
p- type back donation from filled dxy, dyz, dxz
orbitals to pg orbitals of N2
Both factors weaken the N-N bonding in
nitrogen. The N-N stretching frequencies shift to
the red.
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Spectra show a red shift of 95 cm-1 for n4 as
compared to free N2 stretch An additional red
shift of 60 cm-1 is observed for ngt4 cluster
sizes The spectra of n6 has a lower S/N ratio
suggesting the complex is harder to dissociate
owing to unusual stability
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B3LYP/ DGDZVP? Nb 6-311G? N
De 33.8 kcal/mol Freq 2291 cm-1 Osc. Strength
55 km/mol
De 18.6 kcal/mol Freq 2160 cm-1 Osc. Strength
169 km/mol
De 8.3 kcal/mol Freq 2209 cm-1 Osc. Strength
376 km/mol
De 19.7 kcal/mol Freq 2262 cm-1 Osc. Strength
354 km/mol

• DFT calculations favor linear over T-shaped
  structures ( De 15
• 20 kcal/mol
• T-shaped complexes red-shift N-N stretch by
  150-200 cm-1 whereas
• linear complexes red shift by 50-100 cm-1.

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Nb Grnd state 4d4 5D 1st state 4d35s 5F
6.7 kcal/mol 2nd state 4d4 3P 15.9 kcal/mol
Spectrum has two modes because there are only
two equivalent N2
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2265
Single peak spectrum points to a high symmetry
structure.
DFT (B3LYP) calculations for the n 4 complex
for the 5D spin state show good
correspondence to the IR spectra.
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What is causing the additional red shifts for the
ngt4 clusters ?
Nb(N2)5

• 1. Other structures such as T-shaped or
• inserted complexes? DFT studies
• consistently predict linear structures over
• T-shaped structures. Energy differences
• 15 kcal/mol and 20 kcal/mol.
• In addition all spectra are single peak
• signifying that no isomers are present.
• 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 lower
• in energy by 15 kcal/mol

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Comparison of Nb(N2)n and V(N2)n
Greater red-shifts for Nb(N2)n than V(N2)n
V(N2)n
Nb(N2)n
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1. N2 and CO are p-accepting ligands and so
dp-back 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.
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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 p-back donation is the more
significant interaction in these TM-ligand
systems.
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