The dynamic properties of molecules are a topic of current interest. High coordination-number polyhydride complexes of the transition metals are particularly dynamic. The hydride ligands of such complexes are known to exchange between inner coordination sphere sites as well as, in some complexes, participate in intermolecular exchange of protons with protons from the solvent system. Additionally, these high coordination-number complexes presumably exhibit the same possibilities for pseudorotational rearrangements that other high coordination complexes exhibit.<br/><br/>The focus of our recent research has been the dynamic properties of eight-coordinate, dodecahedral, rhenium(V) polyhydride complexes supported by either four or five hydride ligands. Our previous results with these complexes include: 1) the characterization of isomerizations that occur for certain complexes supported by unsymmetrically-substituted aromatic amines, 2) the identification of a specific hydride ligand that participates in intermolecular proton exchanges for some rhenium pentahydride complexes, 3) the identification of a pseudorotational ligand rearrangement for certain complexes that exchanges the hydride ligands within A sites and the phosphorus atoms within B sites, and 4) the discovery that certain chelating ligands can prevent the pseudorotational exchange of ligands in such complexes.<br/><br/>Our investigations to date have been hindered by the equivalence of a pair of dodecahedral B site phosphorus atoms for certain complexes. Complexes with only two chemically equivalent dodecahedral B site phosphorus atoms afford no direct information on the behavior of the set of B site ligands with respect to changes in the temperature of the NMR samples. The incorporation of a chiral center into a rhenium(V) polyhydride complex affords a simple way to gain insight into the behavior of B site ligands. Several such chiral complexes were prepared. Certain rhenium(V) tetrahydride complexes supported by a ligand with a chiral center indicate that all B site ligands exchange through a pseudorotational mechanism. The presence of inequivalent phosphorus atoms, at room temperature, on other rhenium(V) tetrahydride complexes supported by a chelating ligand with a chiral center indicate that the pseudorotational exchange is indeed stopped by the presence of the chelating ligand.

Variable temperature NMR study of fluxional processes in eight-coordinate rhenium(V) polyhydride complexes supported by a chiral ligand

ACS Spring Meeting 2021

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