Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump

Abstract

Ligand exchange of hydrated metal complexes is common in chemical and biological systems. Using the ultrafast T-jump, we examined this process, specifically the transformation of aqua cobalt (II) complexes to their fully halogenated species. The results reveal a stepwise mechanism with time scales varying from hundreds of picoseconds to nanoseconds. The dynamics are significantly faster when the structure is retained but becomes ratelimited when the octahedral-to-tetrahedral structural change bottlenecks the transformation. Evidence is presented, from bimolecular kinetics and energetics (enthalpic and entropic), for a reaction in which the ligand assists the displacement of water molecules, with the retention of the entering ligand in the activated state. The reaction time scale deviates by one to two orders of magnitude from that of ionic diffusion, suggesting the involvement of a collisional barrier between the ion and the much larger complex. C oordinated metal ions, besides being of fundamental interest in the chemical literature, are central to biological catalysis and function, as in, e.g., metabolism (1, 2). Among these, cobalt complexes are structurally prototypical systems that are biologically relevant in, e.g., vitamin B 12 function (3) and biomedical applications of cancer therapy (4, 5). Ligand substitution reactions (6-8), which involve the replacement of the ligand coordinated to the metal ion by a free one(s) from solution, are multistep processes and part of the hydration mechanism in aqueous media. The exchange, which is temperature-or ligand concentration-sensitive, in many cases is accompanied by dramatic color changes.The studies reported here focus on the aquachloro complexes of cobalt(II). Steady-state, optical, and x-ray absorption studies (9, 10) have revealed that the complex in aqueous halide solutionϪ , and [CoX 4 ] 2Ϫ ; remaining charge compensation comes from the added chloride salt, in our case LiCl. The first three species have an octahedron configuration, whereas the latter two adopt a tetrahedron configuration because of the crystal field stabilization energy (9, 11). The striking spectroscopic shift from pink to dark blue when increasing [Cl Ϫ ] or temperature is due to the shift of equilibrium from the octahedral to the tetrahedral structure, each with a characteristic absorption. Upon sudden temperature (e.g., T-jump) or concentration change, the equilibrium changes, and coordinated water molecules gradually get replaced by free chloride ions. Accompanying this change is the structural interconversion from the octahedral to the tetrahedral configuration:[1]For this complex reaction, it is important to elucidate the nature of the dynamics and the free energy surface.Here, using the ultrafast T-jump (12, 13), we systematically probed the substitution processes involved over a wide range of concentration, temperature, and time scale. By carefully selecting the reactant concentration and the initial temperature, it was possible to capture the elusive intermediates. The experiment...

Ultrafast Energy Transfer from Solvent to Solute Induced by Subpicosecond Highly Intense THz Pulses

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Photochemistry of Monochloro Complexes of Copper(II) in Methanol Probed by Ultrafast Transient Absorption Spectroscopy

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