Photoelectron spectroscopy and electronic structure calculations of d1 vanadocene compounds with chelated dithiolate ligands: implications for pyranopterin Mo/W enzymes

Abstract

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp2V(dithiolate) [where dithiolate is 1,2-ethenedithiolate (S2C2H2) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d1 electron configuration at the metal center. Comparison with previously studied Cp2M(dithiolate) complexes, where M is Ti and Mo (respectively d0 and d2 electron configurations), allows evaluation of d0, d1, and d2 electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d1 metal center, allowing the d1 electron configuration and geometry to act as a low-energy electron pathway intermediate between the d0 and d2 electron configurations of the enzyme.

Figure 1

Representations of the valence orbitals of folded versus flat dithiolates. In the d⁰ case the metal orbital (M) is empty, and the S_π⁺ orbital can interact upon folding. In the d² case the metal orbital (M) is filled, and a planar orientation minimizes a filled-filled interaction. The S_π^- orbital does not have the correct symmetry to interact with the metal orbital.

Figure 2

ORTEP representation of Cp₂V(S₂C₂H₂) with 50% thermal ellipsoids.

Figure 3

Low energy valence region of the gas-phase photoelectron spectra of Cp₂V(S₂C₂H₂) and Cp₂V(bdt) with He I and He II excitation. Band C is due to the S_π^- orbital, B is the S_π⁺/V bonding orbital, and A is the singly-occupied S_π⁺ /V anti-bonding orbital.

Figure 4

The spectra of Cp₂V(S₂C₂H₂) and Cp₂V(bdt) have one more band (band A) than their respective titanocene analogs.

Figure 5

Spin correlation diagrams for Cp₂V(S₂C₂H₂) and Cp₂V(bdt) showing α- and β-spin eigenvalues, molecular orbital character and electron occupations based on ground state configuration.

Figure 6

Potential energy diagrams showing the calculated total energy with change in fold angle for the neutral (d¹) and cation (d⁰) of Cp₂V(bdt).

Figure 7

The gas-phase potential energy curve plots of the neutral (d⁰), cation (d¹) and dication (d²) of Cp₂Mo(bdt) are shown with relative total energies (eV, left-hand side) and relative potential energies with change in fold angle (kcal/mol, right-hand side). The oxidation along Path A (blue arrows and line) from the d² to d⁰ electron configurations involves a large energy barrier to oxidation and larger reorganization energies with change in fold angle. Paths B and C (green arrows and lines) show the two-step two-electron oxidation of the d² to d¹ to d⁰ electron configurations. Utilization of the shallow d¹ potential curve poises oxidation to occur with minimal reorganization energies with change in fold angle.