Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used in this work to study the vibrational dynamics of a series of biomimetic catalysts. We set out to investigate the vibrational dynamics of catalytic compounds in systems directly relevant to molecular reactivity, specifically reactive oxidation states, catalytically relevant self-isomerizations, dendritically-induced nano-confinement, and excitonic coherence transfer. For most of the work performed for this thesis we used diiron hexacarbonyl small-molecule mimics of the [FeFe]-hydrogenase enzyme’s active site.
The vibrational dynamics of [(1,1’-bis(diphenylphosphino)ferrocene)chromium-(CO)4] (DPPFCr) in its neutral, closed-shell state were compared to the vibrational dynamics of DPPFCr as a cation radical. This comparison is possible because molecular oxidation does not significantly change the vibrational displacements of the carbonyl modes, which are studied here. Molecular oxidation induces an acceleration of the vibrational relaxation of the carbonyl modes but does not significantly affect the spectral diffusion dynamics of the carbonyl groups. We attribute this to an idiosyncrasy of the non-interacting solvent used for the experiment, CH2Cl2, which was chosen specifically for the weak nature of its solvent-solute interactions.
Unexpectedly pronounced and slow spectral diffusion in the carbonyl modes of (µ-pdt)[Fe(CO)3]2 (pdt = 1,3-propanedithiolate) was observed in alkane solvents. The contribution of solvent-solute interactions in alkane solvents to spectral diffusion is expected to be minimal, and we related the spectral diffusion to fluctuations of the carbonyl potential induced by a catalytically-relevant mode of molecular fluxionality in (µ-pdt)[Fe(CO)3]2. Comparison with a different diiron hexacarbonyl compound, (µ-edt)[Fe(CO)3]2 (edt = 1,2-ethanedithiolate), effectively ruled out isomerization of the bridging organic disulfide group, and a Boltzmann distribution of states derived from electronic structure calculations supported our hypothesis by suggesting that a significant
distribution of molecular conformations were present in at room temperature. Other fluxional organometallic complexes M3(CO)12 (M=Ru, Os) displayed similar spectral diffusion. This is the first use of spectral diffusion to study molecular conformational flexibility. We also observed an unexpected dependence of the rate of intracarbonyl IVR on the chain length of the alkane solvent.
Nano-confinement has been reported on several occasions to favorably modulate the reactivity of diiron hexacarbonyl compounds, and dendritic assemblies with diiron hexacarbonyl cores were synthesized and the vibrational dynamics of the carbonyl groups were compared to the vibrational dynamics of carbonyls on similar diiron hexacarbonyl compounds without dendritic groups. Slower IVR and an additional timescale of spectral diffusion were observed in dendritic assemblies, which are hypothesized to reflect nano-modulation of the carbonyl group’s first solvation shell by the dendritic groups.
Three diiron hexacarbonyl compounds with differing bridging disulfide groups (edt, pdt, and o-xylyldithiolate) are found to display unusual modulations of cross peak intensity which have previously been identified as spectral signatures of vibrational coherence transfer. Specific modulations of cross peak amplitude are observed in all three compounds, suggesting that certain coherence transfer events are common in diiron hexacarbonyl compounds, and an oscillatory frequency resulting from coherence transfer between bright and dark vibrational modes is identified.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145788/1/zpeckert_1.pd
