Title: “Exploring Vibrational Features and Coupling in Metal-Metal Multiply Bonded Systems”

Abstract: From rhodium paddlewheels to iron-nickel and iron-iron hydrogenases, metal-metal cooperativity through metal-metal bonds have generated interest in the field of catalysis. Characterization of these metal-metal bonds most often take the form of Raman spectroscopy and X-ray crystallography. While useful tools, the impact of the metal-metal bond on other groups of atoms in the structure, such as carbene in a rhodium carbene transfer catalyst, is often limited to measurements of vibrational frequency changes or bond distance changes in a crystal. These methods are a static measure of the electronic effect of the metal-metal bond. The changes measured by diffractometry or steady state vibrational spectroscopy reveal a relationship in reagent or product, but they do not impart information on whether these groups of atoms move in concert or sequentially throughout a reaction unless specific intermediates can be trapped and analyzed. To explore what motions are coupled to the metal-metal stretch, we must turn to multi-dimensional vibrational spectroscopy.

In this talk, I will discuss our work on the exploration of the multi-dimensional vibrational spectroscopy of two diruthenium azide systems as well as an extended metal atom chain with axial isothiocyanates. I will discuss Doubly Vibrationally Enhanced Four Wave Mixing Spectroscopy (DOVE-FWM) experiments as well as 2D-IR experiments performed on these complexes. For the DOVE-FWM experiments, I will show how the strong axial azide and thiocyanate modes couple into low frequency modes postulated as the metal-metal stretch and compare the features seen across the two diruthenium azides. For the 2D-IR measurements, I will discuss how polarization control studies better help us understand the relationship between multiple oscillators in a diruthenium azide system and preliminary data on the vibrational energy transfer between isothiocyanate groups. Throughout, I will highlight how multidimensional vibrational spectroscopies further our understanding of vibrational communication and structure in multiply bonded metal complexes, discuss how DOVE and 2D-IR can complement each other, and discuss steps taken to push DOVE experiments towards more non-ideal chemical systems.