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.