Photoelectron spectra of potassium salts of hydrotris(pyrazol-1-yl)borates: Electronic structure of the electron withdrawing scorpionates Tp ^(CF3)2, Tp*^Cl and comparison to Tp* and Tp

The electronic structures of the potassium salts of the homoscorpionates hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*, 1), hydrotris(4-chloro-3, 5-dimethylpyrazol-1-yl)borate (Tp*^Cl, 2) and hydrotris(3,5-bis(trifluoromethyl)pyrazol-1-yl)borate (Tp^(CF3)2, 3) are compared using gas-phase photoelectron spectroscopy and density functional theory (DFT). DFT calculations also are reported for the generic scorpionate potassium (hydrotris(pyrazol-1-yl)borate) (KTp). This is the first such experimental probe of the electronic structure of halogen containing scorpionate ligands and subtle differences in the ionizations from the frontier orbitals in the photoelectron spectra of 1 and 3 are observed that give insight into the influence of substituents upon metal-scorpionate bonding. Distinct assignments of the ionizations from the nitrogen σ-donor orbitals (σ_N) and σ_BH molecular orbitals are possible experimentally by the use of variable (He I and He II) excitation energies. The experimentally observed first ionization energy of 3 is stabilized by ∼2.0 eV relative to 1 due to the strong electron withdrawing effect of the trifluoromethyl groups. The photoelectron spectroscopic studies of NaTp ^(CF3)2 further confirm the assignments of ionizations from σ_N orbitals for 3 associated with the a and e sets in C ₃ symmetry. The X-ray crystal structure of 2 as the (μ-aqua) ₃(potassium hydrotris(4-chloro-3,5-dimethylpyrazol-1-yl)borate) ₂ dimer is also reported.