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 daniel.weidinger.ctr@nrl.navy.mil OSU International Symposium on Molecular Spectroscopy Special Session: Metal-Containing Molecules June 21, 2010

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. 2009.

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. 3 of 16

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

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 5 of 16

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) 6 of 16

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.

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) 12262. 3 P. J. Reid, C. Silva, P. F. Barbara, L. Karki, J. T. Hupp, JPC 99 (1995) 2609.

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-1 E0 = 1976 cm-1 1 D. C. Arnett, P. Vohringer, N. F. Scherer, JACS 117 (1995) 12262.

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

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 11 of 16

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

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 13 of 16

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

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 15 of 16

Metal Cyanide Anisotropy Decay Au 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. 16 of 16

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

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.