Manganese Carbonyl Compounds Reveal Ultrafast Metal–Solvent Interactions

Herein,
we exemplify that time-resolved multiple-probe spectroscopy
(TRMPS) with infrared detection can be used to observe
and quantify the dynamic processes occurring during the solvation
of a catalytically competent manganese­(I) carbonyl compound. TRMPS has been used to demonstrate that a manganese­(I) 2-phenylpyridyl
(ppy) complex, [Mn­(ppy)­(CO)4], undergoes photochemically
induced loss of a carbonyl ligand on a sub-picosecond time scale to
give solvent (S) complexes of the type, fac-[Mn­(ppy)­(S)­(CO)3] (S = n-C7H16, CH2Cl2, NCMe, C6H5CH3, THF, 1,4-dioxane, n-Bu2O, and DMSO).
An excited state, assigned as 3[Mn­(ppy)­(CO)4], with a lifetime, τ, of ca. 5 ps is also formed as a minor
photoproduct. The vibrational modes of the carbonyl ligands in fac-[Mn­(ppy)­(S)­(CO)3] are diagnostic
of the nature of the coordinated solvent and allow for the dynamics
of solvation within the coordination of the metal to be observed.
For example, in the case of THF, initial interactions with the metal
occur through a C–H σ-interaction, assigned based on
the similarity to the bands observed for the related heptane metal
complex. Isomerization to the thermodynamically preferred O-binding
mode was observed, τ ca. 18 ps, with similar behavior evident
in 1,4-dioxane and nBu2O. The
lifetime for the isomerization shows a correlation with the number
of C–H bonds in the solvent. In 1,4-dioxane, there is no evidence
for the initial formation of an O-bound complex which indicates that
the solvent binding is not stochastic in nature and is likely determined
by the topology of the first solvation shell of the metal complex.
The insight into these ultrafast metal–solvent interactions
is enabled by the dominant photoprocess, CO dissociation, being rapid
(<1 ps). Which, taken together with prompt vibrational cooling,
enables the subtle interplay between metal and solvent during the
solvation event to be characterized and quantified.