Structural and Electrochemical Consequences of [Cp] Ligand Protonation

There are few examples of the isolation of analogous metal complexes bearing [η5-Cp*] and [η4-Cp*H] (Cp* = pentamethylcyclopentadienyl) complexes within the same metal/ligand framework, despite the relevance of such structures to catalytic applications. Recently, protonation of Cp*Rh­(bpy) (bpy = 2,2′-bipyridyl) has been shown to yield a complex bearing the uncommon [η4-Cp*H] ligand, rather than generating a [RhIII–H] complex. We now report the purification and isolation of this protonated species, as well as characterization of analogous complexes of 1,10-phenanthroline (phen). Specifically, reaction of Cp*Rh­(bpy) or Cp*Rh­(phen) with 1 equiv of Et3NH+Br– affords rhodium compounds bearing endo-η4-pentamethylcyclopentadiene (η4-Cp*H) as a ligand. NMR spectroscopy and single-crystal X-ray diffraction studies confirm protonation of the Cp* ligand, rather than formation of metal hydride complexes. Analysis of new structural data and electronic spectra suggests that phen is significantly reduced in Cp*Rh­(phen), similar to the case of Cp*Rh­(bpy). Backbonding interactions with olefinic motifs are activated by formation of [η4-Cp*H]; protonation of [Cp*] stabilizes the low-valent metal center and results in loss of reduced character on the diimine ligands. In accord with these changes in electronic structure, electrochemical studies reveal a distinct manifold of redox processes that are accessible in the [Cp*H] complexes in comparison with their [Cp*] analogues; these processes suggest new applications in catalysis for the complexes bearing endo-η4-Cp*H.