Two dimensional infrared spectroscopy (2D IR) is used to study the fluxional rearrangement of cobalt organometallic complexes, Co(CNArMes2)4 (Mes=2,4,6-Me2C6H3), in solution phase. Co(CNArMes2)4 has been designed and synthesized as a stable isocyano analogue of Co(CO)4, a reactive intermediate in hydroformylation and carbonylation processes. Therefore, revealing the structure and dynamics of Co(CNArMes2)4, can help understand the transition state mechanism of Co(CO)4 in hydroformylation and carbonylation. However, based on traditional spectroscopy (FTIR, NMR), only convoluted structure information of Co(CNArMes2)4 can be obtained, due to resolution limits of these techniques. Even more, its conformational dynamics, such as fluxionality, is on the ultrafast time scale, and simply cannot be revealed by techniques that do not have sub-picosecond resolutions. Following on many previous ultrafast 2D IR works on metal carbonyls, we show that 2D IR can extend its advantages in spectral and time resolutions into Co-CN complexes as well. First, the broad CN absorption of Co(CNArMes2)4 in FTIR is further resolved into multiple peaks, which corresponds to various conformations. Second, we are able to monitor chemical exchange dynamics, by following the cross peak dynamics. We found that intensities of a few cross peaks increase within the time scale of 1.5ps, whereas the others remain unchanged over the same time frame. Combining with DFT electronic structure calculations and normal mode analysis, we conclude that the Co(CNArMes2)4 complex in solution phase have two major conformations, each has D2d and C3v symmetry, respectively. The increasing cross peak intensity simply reflects the dynamics of two conformations exchanging. The time scale is notably fast for these bulky ligands. However, only a few degrees of ligands rotation is necessary to change from one conformation to the other, which why the conformation exchange dynamics is on the picosecond time scale. Overall, we demonstrate that aside from simple metal-carbonyl complex, 2D IR can also be used to study both structure and dynamics of more complex and meta-stable metal complex intermediates, which can be a great tool to reveal microscopic mechanisms of organometallic catalysis.

Ultrafast conformational fluxionality of organometallic catalytic intermediate revealed by 2D IR spectroscopy

251st National Meeting (2016)

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