Reversible modulation of the redox characteristics of acid-sensitive molybdenum and tungsten scorpionate complexes

Abstract

The large-scale synthesis of the scorpionate ligand Ttz (hydrotris(1,2,4-triazol-1-yl)borate) is reported as well as syntheses of Group VI complexes K[M(L)(CO)3] and M(L)(NO)(CO)2 (L = Ttz or Tp (hydrotris(pyrazol-1-yl)borate), M = Mo or W). The redox characteristics of the metal in these Ttz complexes are shown to be reversibly modulated by interactions between the exo-4-N lone pairs of the triazolyl rings and Brønsted or Lewis acids. The basicity of the scorpionate ligand in [M(Ttz)(CO)3]- is quantified (pKaH2O values range from 1.1 to 4.6) and found to be dependent on both the oxidation state and identity of the metal. In the presence of Brønsted acids, the observed redox behavior for the one-electron oxidation of the Group VI metal center is consistent with a proton-coupled electron transfer (PCET). Indeed, for both Mo and W derivatives, a one-electron oxidation decreases the pKa by ∼3.5 units.

Computational Analysis of Proton-Coupled Electron Transfer in Hydrotris(triazolyl)borate Mid–Late 3d and 4d Transition Metal Complexes

“…An isodesmic reaction uses a reference system that is similar to the system of interest and has been studied experimentally. , An additional benefit of isodesmic reactions is that they also avoid the necessity of determining quantities such as the free energies of the gas-phase electron and proton and the solvation free energy of the proton due to the cancellation of these quantities in isodesmic reactions . Reaction free energies for the protonation and oxidation of the complex [Mo 0 (Ttz)­(CO) 3 ] − ( Mo­(0) – ) has been reported by Machan and co-workers . Since the studied systems in this work are similar to the [Mo 0 (Ttz)­(CO) 3 ] − system, differing in the metal centers, the reactions Mo­(0) – → Mo­(0)­H (Δ G expt = −15.1 kcal/mol) and Mo­(0) – → Mo­(I) (Δ G expt = 5.0 kcal/mol) were selected as references to calculate proton transfer and electron transfer reactions, respectively.…”

“…13 Reaction free energies for the protonation and oxidation of the complex [Mo 0 (Ttz)(CO) 3 ] − (Mo(0) − ) has been reported by Machan and coworkers. 34 Since the studied systems in this work are similar to the [Mo 0 (Ttz)(CO) 3 ] − system, differing in the metal centers, the reactions Mo(0) − → Mo(0)H (ΔG expt = −15.1 kcal/mol) and Mo(0) − → Mo(I) (ΔG expt = 5.0 kcal/mol) 34 were selected as references to calculate proton transfer and electron transfer reactions, respectively. The equations in Chart 1 explain how ΔG calc values are obtained using isodesmic reactions.…”

“…Recently, Machan and co-workers have reported cyclic voltammetry experiments for tricarbonyl Group 6 metal centers (Mo and W) with the Ttz ligand. In the presence of Brønsted–Lowry acids, protonation occurs at the exo nitrogen of the Ttz ligand.…”

“…The outer- and inner-sphere reorganization energies (λ s and λ i , respectively) are calculated to estimate trends as a function of the metal for the free energy barriers of ET reactions. According to work by Machan and co-workers, adding the weak Brønsted–Lowry acid pyridinium triflate to a solution of [Mo­(Ttz)­(CO) 3 ] − results in the monoprotonation of the complex with the exo nitrogen being protonated. Hence, in all of the modeled systems, pyridine (pyridinium) plays the role of the Brønsted–Lowry base (acid) for (de)­protonation of the complex of interest.…”

“…Their research indicates the propensity of Ttz complexes of 3d metal ions to be of the form M 3d (Ttz) 2 , although this can be restricted by the use of blocking groups. 33 Recently, Machan and co-workers 34 have reported cyclic voltammetry experiments for tricarbonyl Group 6 metal centers (Mo and W) with the Ttz ligand. In the presence of Brønsted−Lowry acids, protonation occurs at the exo nitrogen of the Ttz ligand.…”