1. Dynamics of Metal Cyanides
Daniel Weidinger, Douglas J. Brown*, Cassidy Houchins and Jeffrey C. Owrutsky
Chemistry Division
Naval Research Laboratory, Washington, DC
* U. S. Naval Academy, Annapolis
OSU International Symposium on Molecular Spectroscopy
Special Session: Metal-Containing Molecules
June 21, 2010

2. Metal Cyanide Infrared Spectroscopy
Why study metal cyanides (MCNs)?
As a charged counterpart to metal carbonyls (MCOs)
To understand CN in other more complex environments:
Surface adsorbate
Ligand in biochemical systems – hemes, myoglobin
Infrared spectral properties (summarized in Nakamoto1)
CN vibrations around 2100 cm-1
Low frequency → High intensity
New dynamics (Vibrational Energy Relaxation = VER)
How do new MCN VER times compare?
Do dynamics trends match spectra?
How do extended structures (e.g. Prussian Blue) compare? N C Au
Dicyanogold = Au(CN)2- N C
FeII
Spell out MCN and MCO first time
Mono-metal M-CNs are model for more complicated system with CN bound to metals – could also mention building up to dinuclear and networked cyanides.
Could also point out that vibrational dynamics are important because energy disposal affects reactivity – ( use ONE of these examples: MCO have multiple CO dissociation in gas phase but only one CO comes off in solution because the energy is dissipated; on surfaces, VER affects whether adsorbates desorb or cool off first after visible excitation – geminate recombination depends on the rate of energy relaxation)
“New danamics”? – I’d say
Vibrational energy relaxation – expanding to more MCNs than just Fe
VER – requires time resolved measurement
Only for Fe – CN so far
Compare with MCOs and non-metal simple anions
Ferrocyanide = Fe(CN)64-
1 K. Nakamoto: IR and Raman Spectra of Inorg, & Coord. Comp

3. Metal Cyanide Background
Selected MCNs in H2O
Frequencies cluster around 2125 cm-1
Ru(CN)64- similar to ferrocyanide
lower frequency and
higher intensity than others
Previous ultrafast IR studies1-4 only for cyanoferrates:
Ferrocyanide, Fe(CN)64- :
T1 = 4 ps (in H2O)
Ferricyanide, Fe(CN)63- :
T1 = 7 ps (in H2O)
Center Freq.
(cm-1)
Integrated Band Intensity
(M-1cm-2)
Ru(CN)64-
2047
56300
Mn(CN)63-
2113
8200
Ni(CN)42-
2124
9000
Co(CN)63-
2127
18200
Pt(CN)42-
2134
13700
K. Ohta, H. Maekawa, K. Tominaga, Journal of Physical
Chemistry A 108 (2004) 1333.
G. M. Sando, Q. Zhong, J. C. Owrutsky, J. Chem. Phys.
121 (2004) 2158.
G. M. Sando, K. Dahl, J. C. Owrutsky, Journal of Physical
Chemistry B 109 (2005) 4084.
K. Ohta, H. Maekawa, K. Tominaga, Chem. Phys. Lett.
386 (2004) 32.
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4. IR Pump-Probe – Vibrational Relaxation
Ultrafast Pump-Probe IR setup:
4 μJ, ~350 fs IR pulses
Transient absorption probed
with 5 cm-1 resolution. 2
Transient Absorption 1
Bleach Recovery
ν = 0

5. Metal Cyanide Dynamics
Transient Absorption Spectra of
Ni(CN)42- in H2O
Transient Absorption Decays
of Selected MCNs
Explain IVR very succinctly
Point out where bleach and excited state decays are in transient spectra – say that these are specific to high frequency excitation and that is important for interpreting other (PB) results
Data fit to single exponential or
biexponential decays
Absorption, bleach indicative of
high-frequency mode excitation
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6. IR Pump-Probe – Vibrational Relaxation
Selected MCNs in H2O
Selected MCNs in D2O
I would cut this – you have too many slides already.
In H2O, VER times range from 4 ps to 80 ps
VER slows down in D2O – attributed to lower solvent friction
Isotope effect more pronounced in some species (e.g. Ru)
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7. Metal Cyanide – Spectral and Dynamical Properties Correlate
Fe(II) Ru Mn Au Ni Co Pt
Low freq, strong bands with fast relaxation
3D Plot of MCN Properties in H2O
VER Times for MCNs in H2O and D2O
Vibrational Relaxation Times (ps)
D/H
ratio
H2O
D2O
Au(CN)2-
67±15
170±24
Ni(CN)42-
38±6
140±23
2.5
Fe(CN)64- a
4.4±1
24±3
3.6
Fe(CN)63- a
7.0±1
8.0±1.5
5.5
Pt(CN)42-
34±4
100±30
1.1
Co(CN)63-
30±4
165±44
2.9
Mn(CN)63-
15±1
28±2
5.6
Ru(CN)64-
4.4±0.3
1.9
high freq, weak bands with slow relaxation
I animated the groupings for emphasis
Fe(III)
a. Results from:
G. M. Sando, Q. Zhong, J. C. Owrutsky, J. Chem. Phys. 121 (2004) 2158
K. Ohta, H. Maekawa, K. Tominaga, J. Phys. Chem. A 108 (2004) 1333.
K. Ohta, H. Maekawa, K. Tominaga, Chem. Phys. Lett. 386 (2004) 32.

8. Networked Metal Cyanides
Prussian blue (PB) well-studied1 dye
Previous ultrafast IR studies include:
Visible pump-probe of PB2
Visible pump-IR probe studies of dinuclear metal complexes3
Reverse micelles:
PB nanoparticles – remain suspended, uniform size N C
FeII
FeIII
FeII-CN-FeIII
Visible Spectra of Networked MCNs
Really think about this transition – expand to more complex systems
Keep it simple, and big picture
Easy to make with Ferrocyanide + Fe3+ salt
Mixed RMs with 2 salt solutions
For RP, use ru(II) hexacyanide instead of ferrocy
good suspension for RMs for transient optical studies
Colors look pretty similar
Ruthenium Purple (RP)
Prussian Blue (PB)
1 K. R. Dunbar, R. A. Heintz, Prog. Inorg. Chem., Vol 45, 1997, p. 283.
2 D. C. Arnett, P. Vohringer, N. F. Scherer, JACS 117 (1995)
3 P. J. Reid, C. Silva, P. F. Barbara, L. Karki, J. T. Hupp, JPC 99 (1995) 2609.

9. Prussian Blue Dynamics
800 nm pump / 800 nm probe
ground state recovery - PB in RMs
Visible pump-visible probe of PB is known1
Broad electronic MMCT band
Multiple relaxation pathways
Electronic-vibronic coupling?
FeIII-CN-FeII
FeII-CN-FeIII v1 E0 ν0
Sub-picosecond bleach
Slow (~30 ps) absorption
Absorption only on the red side of the MMCT band
v1 - ν0 = 2090 cm E0 = 1976 cm-1
1 D. C. Arnett, P. Vohringer, N. F. Scherer, JACS 117 (1995)

10. Transient IR Absorption of Networked MCNs
IR / IR (2086 cm-1) ground state
recovery - PB in RMs and film
IR / IR (2093 cm-1) ground state
recovery - RP
Not much more than “networking does not increase relaxation rate”
Td = 30 ± 2 ps
Td = 46 ± 4 ps
Vibrational lifetimes (Td) are longer than Fe precursors
PB lifetime seems independent of morphology
Two lorentzians in static spectra, possibly FeII-CN-FeIII and FeIII-CN-FeII

11. Visible pump - IR probe of Networked MCNs
800 nm pump / IR probe (2086 cm-1)
ground state recovery PB
800 nm pump / IR probe (2093 cm-1)
ground state recovery RP
Td = 29 ps
Td = 50 ps
Really emphasize that you have switched experiments – OPTICAL – PUMP!
Electronic relaxation is very fast – sub ps – all response we observed is vibrational – can we tell if it’s high or low freq modes excited
I got rid of the “T1 =“ - Optical pump does not give T1 times per se – typically relaxation from several modes and possibly different levels of excitation for each mode
Large vibrational population from visible pump
Vibrational lifetimes (Td) for 800 / IR almost the same as IR / IR
Fast back electron transfer, then vibrational relaxation
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12. IR Pump / IR Probe Transient Spectra
IR / IR Prussian Blue
IR / IR Ruthenium Purple
Time-series indicative of high-frequency mode excitation
Fits to two absorptions and two bleaches, matching static spectra

13. Visible Pump / IR Probe Transient Spectra
800 nm pump / IR Prussian Blue
800 nm pump / IR Ruthenium Purple
Again 800 / IR looks like IR / IR
High-frequency mode excitation
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14. Conclusions IR Studies of Metal Cyanides
Long decay time for MCN’s in water
VER rates strengthen, extend trends noted in static spectra: faster VER rates correlate with higher intensity, lower frequency.
Ferricyanide is anomalous, with fast VER
but low intensity and high frequency.
Visible and IR Studies of Networked Metal Cyanides
PB VER rates insensitive to morphology
Networked cyanides have long VER decays than cyanoferrates
Similar time scale VER decay
Evidence of high frequency mode excitation N C
FeII

15. Acknowledgements Scientists Funding: Office of Naval Research
Jeffrey C. Owrutsky (NRL)
Cassidy Houchins (NRL)
Douglas J. Brown (USNA)
Gerald Sando (Malvern Instruments)
Funding:
Office of Naval Research
Sponsorship:
National Research Council
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16. Metal Cyanide Anisotropy DecayAu
Anisotropy Decays
Rotation
Au(CN)2- has TR = 13 ps
Consistent with reorientation time for a linear molecule
Dipolar Exchange
Ru(CN)64- : TR = 4 ps
Pt(CN)42- : TR = 4 ps
Ni(CN)42- : TR = 3 ps
Co(CN)63- : TR = 2 ps
Due to elastic IVR among the degenerate modes.
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17. Networked MCN Dynamics
400 nm Pump / 800 nm Probe Transient Absorption Decay
PBA in w=14 AOT RMs
Each species shows three characteristic timescales:
~1 ps
~20 ps
> 100 ps

18. Integrated Band Intensity
Data for MCNs in H2O
Sodium nitroprusside (SNP)
Fe(CN)5NO
CN band shifted blue
VER rate slows to 55 ps
Center Freq.
(cm-1)
Integrated Band Intensity
(M-1cm-2)
Fe(CN)64-
2037
92000
Ru(CN)64-
2047
56300
Mn(CN)63-
2113
8200
Fe(CN)63-
2116
12000
Co(CN)63-
2127
18200
Ni(CN)42-
2124
9000
Pt(CN)42-
2134
13700
Au(CN)2-
2146
5500
Center
Frequency
(cm-1)
Integrated
Band
Intensity
(M-1cm-2)
Fe(CN)64-
2037
92000
Fe(CN)63-
2116
12000
SNP (NO)
1936
4000
SNP (CN)
2142
35000
From Jones1, integrated band intensity known to scale with d electron number.
e.g. Cr < Mn < Fe < Co
1 L. H. Jones, Inorg. Chem. Phys. 2 (1963) 777.