Reactions of [Re(CO)(3)](+) with Histidylhistidine and Modified Histidines

Ac-His-OH and His-His-OH were treated with [Re(CO)(3)-(H2O)(3)](+) to yield two new compounds, which were structurally characterized. Each displays a peptide carboxamido-N donor. The complex formed with His-His-OH uses this group as a bridging, bidentate ligand; the complex resists challenge experiments showing that His-tags are a good way to deliver the [M(CO)(3)](+) core (M = Tc-99m, Re).