Influence of Thiolate versus Alkoxide Ligands on the Stability of Crystallographically Characterized Mn(III)-Alkylperoxo Complexes

The work described herein demonstrates the exquisite control that the inner coordination sphere of metalloenzymes and transition-metal complexes can have on reactivity. We report one of few crystallographically characterized Mn-peroxo complexes and show that the tight correlations between metrical and spectroscopic parameters, established previously by our group for thiolate-ligated RS-Mn(III)-OOR complexes, can be extended to include an alkoxide-ligated RO-Mn(III)-OOR complex. We show that the alkoxide-ligated RO-Mn(III)-OOR complex is an order of magnitude more stable (t(1/2)(298 K) = 6730 s, k(obs)(298 K) = 1.03 x 10(-4) s(-1)) than its thiolate-ligated RS-Mn(III)-OOR derivative (t(1/2)(293) K = 249 s, k(1)(293) K = 2.78 x 10(-3) s(-1)). Electronic structure calculations provide insight regarding these differences in stability. The highest occupied orbital of the thiolate-ligated derivative possesses significant sulfur character and pi-backdonation from the thiolate competes with pi-backdonation from the peroxo pi*(O-O). DFT-calculated Mulliken charges show that the Mn ion Lewis acidity of alkoxide-ligated RO-Mn(III)-OOR (+0.451) is greater than that of thiolate-ligated RS-Mn(III)-OOR (+0.306), thereby facilitating pi-backdonation from the antibonding peroxo pi*(O-O) orbital and increasing its stability. This helps to explain why the photosynthetic oxygen-evolving Mn complex, which catalyzes O-O bond formation as opposed to cleavage, incorporates O- and/or N-ligands as opposed to S-cys-ligands.