Structural and Electrochemical Consequences of [Cp*] Ligand Protonation

Abstract

There are few examples of the isolation of analogous metal complexes bearing [η-Cp*] and [η-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 [η-Cp*H] ligand, rather than generating a [Rh-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 EtNHBr affords rhodium compounds bearing endo-η-pentamethylcyclopentadiene (η-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 [η-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-η-Cp*H.

Ligand Substituents Govern the Efficiency and Mechanistic Path of Hydrogen Production with [Cp*Rh] Catalysts

“…of acid results in transfer of the proton to the [Cp*] ring, resulting in a new rhodium(I) species bearing endo η 4 ‐pentamethylcyclopentadiene as a ligand. Our newest experiments have shown that this [(Cp*H)Rh] complex is isolable in pure form and stable in solution on a timescale of minutes to hours . Importantly, this compound is competent for H 2 evolution upon addition of a second equivalent of acid.…”

“…The 1 H NMR spectrum revealed a new set of bipyridine signals in the aromatic region with resonances at 9.23 ppm (d, 3 J H,H =5.5 Hz, 2 H), 8.77 ppm (s, 2 H), and 8.00 ppm (d, 3 J H,H =5.5 Hz, 2 H) as well as a doublet at 0.55 ppm (d, 4 J H,H =6.2 Hz, 3 H) (see Supporting Information). This spectrum is highly reminiscent of (Cp*H)Rh(bpy)Br, a known intermediate in H 2 evolution with 2 , and recently synthesized and isolated at preparative scale by our group …”

“…To rationalize the generation of an analogous compound, [(Cp*H)Rh(CF 3 ‐bpy)L] + , we imagined that 2 e − reduction of 3 would first yield Cp*Rh(CF 3 ‐bpy). Similar to the behavior we observed for Cp*Rh(bpy) and Cp*Rh(phen), generation of this compound in the presence of acid would result in the formation of the analogous [Cp*H] species . To test this theory and expand our knowledge of reduced [Cp*Rh] diimine complexes, we set out to prepare these previously unknown compounds.…”

“…The protonated complex bearing [Cp*H] and t Bu‐bpy was readily prepared and isolated ( 7 ), suggesting that this complex is reasonably stable over time, as we have found for the analogues bearing bpy and phen . Complex 9 , bearing CF 3 ‐bpy, however, could only be generated in situ, as attempts to isolate the compound resulted in formation of side products.…”

Unraveling the Light‐Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis

“…[5,13] Theb and at 440 nm is due to metal-to-ligand charge-transfer (MLCT) transitions from the Ru II center to both the tpphz and the tbbpy ligands ( Figure 2B). [14] Further evidence is pro- Thes ame was observed for the photochemical reduction of Ru(tpphz)RhCp* in presence of TEA ( Figure S17).…”