Triplet Excited State Distortions in a Pyrazolate Bridged Platinum Dimer Measured by X-ray Transient Absorption Spectroscopy

The excited-state structure of a dinuclear platinum(II) complex with tert-butyl substituted pyrazolate bridging units, [Pt(ppy)(mu-'Bu(2)pz)](2) (ppy = 2-phenylpyridine; 'Bu(2)pz = 3,5-di-tert-butylpyrazolate) is studied by X-ray transient absorption (XTA) spectroscopy to reveal the transient electronic and nuclear geometry. DFT calculations predict that the lowest energy triplet excited state, assigned to a metal metal-to-ligand charge transfer (MMLCT) transition, has a contraction in the Pt Pt distance. The Pt-Pt bond length and other structural parameters extracted from fitting the experimental XTA difference spectra from full multiple scattering (FMS) and multidimensional interpolation calculations indicates a metal-metal distance decrease by approximately 0.2 angstrom in the triplet excited state. The advantages and challenges of this approach in resolving dynamic transient structures of nonbonding or weak-bonding dinuclear metal complexes in solution are discussed.