Influence of Thiolate versus Alkoxide Ligands on the Stability of Crystallographically Characterized Mn(III)-Alkylperoxo Complexes
Alexandra N. Downing, Michael K. Coggins, Penny Chaau Yan Poon, Julie A. Kovacs
Abstract
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 (t1/2298 K = 6730 s, kobs298 K = 1.03 × 10–4 s–1) than its thiolate-ligated RS-Mn(III)-OOR derivative (t1/2293 K = 249 s, k1293 K = 2.78 × 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 π-backdonation from the thiolate competes with π-backdonation from the peroxo π*(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 π-backdonation from the antibonding peroxo π*(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 cysS-ligands.