Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution

Photodissociation of ironpentacarbonyl [1Fe(CO)5] in solution generates transient species in different electronic states, which we studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound 1Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl [Fe(CO)4] species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state [1Fe(CO)4] and weakly in the triplet state [3Fe(CO)4]. From the simulations, we have gained insights into intersystem crossing in solvated irontetracarbonyl based on the distinct structural differences induced by the change in multiplicity. Alternative forms of coordination between 1Fe(CO)4 and functional groups of the ethanol molecule are simulated, and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl gives information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for under-coordinated metal carbonyls in solution.

• indicate the formation of 1 Fe(CO) 4 − OH EQ.Following this region, the ligands exchange position and re-form the 1 Fe(CO) 4 − OH AX complex.Bottom: The measure of the pseudo-rotation (Θ ROT = max(θ 1 -θ 2 )) defined as the difference between the two largest angles which tends to zero as the 1 Fe(CO) 4 − OH EQ species is formed.

Solvent coordination to singlet and triplet irontetracarbonyl
The structure library used throughout the main text and summarized in Tab. 1 contains a partially optimized structure, [ 1 Fe(CO) 4 − HC α /HC β ] ‡ AX and one which contains a negligibly small imaginary frequency, 3 Fe(CO) 4 • • • OH.Table S1: Relative energies of minima and transition states, denoted with ‡, of the coordinations of an ethanol ligand with 1 Fe(CO) 4 based on structures optimized at the TPSSh/def2-TZVP + CPCM(ethanol) level of theory.The uncoordinated 1 Fe(CO) 4 AX, 1 Fe(CO) 4 EQ and 3 Fe(CO) 4 all include the energy based an optimized ethanol molecule.All structures are taken relative to the 1 Fe(CO) 4 EQ + ethanol energy.The BLYP and B3LYP energies calculated using CP2K used a cell size of 30 Å to prevent interactions with all other periodic images without CPCM.All values are reported in kcal/mol.The double lines distinguish the geometries considered in Sec.

Figure
Figure S4: A qualitative picture of the solvation of 3 Fe(CO) 4 indicates a small hydrogen bonding network of ethanol molecules around the weakly interacting metal complex.

Figure S5 :
Figure S5: The energetically relevant valence molecular orbitals of 1 Fe(CO) 5 plotted from the CP2K KS-DFT calculation using the BLYP functional and GTH pseudopotentials with the GTH-DZVP (Fe) and GTH-TZVP (C,O,H) basis sets.The HOMO is indicated by orbital label ϕ(33).

Figure S6 :
Figure S6: The energetically relevant valence molecular orbitals of 1 Fe(CO) 4 AX plotted from the CP2K KS-DFT calculation using the BLYP functional and GTH pseudopotentials with the GTH-DZVP (Fe) and GTH-TZVP (C,O,H) basis sets.The HOMO is indicated by orbital label ϕ(28).

Figure S7 :
Figure S7: The energetically relevant valence molecular orbitals of 1 Fe(CO) 4 EQ plotted from the CP2K KS-DFT calculation using the BLYP functional and GTH pseudopotentials with the GTH-DZVP (Fe) and GTH-TZVP (C,O,H) basis sets.The HOMO is indicated by orbital label ϕ(28).

Figure S10 :
Figure S10: The average Fe (d) PDOS taken every 100 fs from the partitioned 1 Fe(CO) 4 -OH AIMD simulation into distinct regions of 1 Fe(CO) 4 -OH EQ (green) and 1 Fe(CO) 4 -OH AX (blue).The gas-phase Fe PDOS shown in grey are overlaid to indicate shifts in the peak positions and intensities.

For [ 1 For 3
Fe(CO) 4 − HC α /HC β ] ‡ AX, the transition state was hard to obtain, therefore we constructed a 2D scan of the 1 Fe(CO) 4 − HC α (Fe − H = 1.82 Å, 4.06 Å) and 1 Fe(CO) 4 − HC β (Fe − H = 1.77Å, 3.18 Å) distances using ORCA 5.0.3 35 at the TPSSh/TZVP level of theory.An approximate transition state region of the potential energy surface was located and a minimum energy structure was obtained.Using this structure, the C α H − Fe − HC β angle was frozen to 57.1 • and the structure was re-optimized in Gaussian 28 at the TPSSh/def2-TZVP + CPCM level of theory to yield an approximate transition state structure with only one imaginary frequency with a value of -170.56 cm −1 .Fe(CO) 4 • • • OH, the structure was optimized without CPCM since the fragments were pushed further away from each other resulting in convergence problems.The structure that was obtained without CPCM contained one imaginary frequency with a value of -9.22 cm −1 .This imaginary mode corresponded to a small wagging of the two fragments, not involving the Fe-O distance.Nudging this structure away from the saddle point and optimization with CPCM resulted in a minimum 0.936 kcal mol −1 lower in energy at the TPSSh/def2-TZVP + CPCM level of theory.The structure that was obtained formed a bond with the hydroxyl proton and resulted in an Fe-O distance of 4.07 Å in contrast to the structure used in the paper which has an Fe-O distance of 3.29 Å.These structure are included in the structure library deposited Zenodo.