Sulfur K-edge Spectroscopic Investigation of Second Coordination Sphere Effects in Oxomolybdenum-Thiolates: Relationship to Molybdenum−Cysteine Covalency and Electron Transfer in Sulfite Oxidase

Second-coordination sphere effects such as hydrogen bonding and steric constraints that provide for specific geometric
configurations play a critical role in tuning the electronic structure of metalloenzyme active sites and thus have a
significant effect on their catalytic efficiency. Crystallographic characterization of vertebrate and plant sulfite oxidase
(SO) suggests that an average Ooxo−Mo−SCys−C dihedral angle of ∼77° exists at the active site of these enzymes.
This angle is slightly more acute (∼72°) in the bacterial sulfite dehydrogenase (SDH) from Starkeya novella. Here
we report the synthesis, crystallographic, and electronic structural characterization of Tp*MoO(mba) (where Tp* =
(3,5-dimethyltrispyrazol-1-yl)borate; mba = 2-mercaptobenzyl alcohol), the first oxomolybdenum monothiolate to
possess an Oax−Mo−Sthiolate−C dihedral angle of ∼90°. Sulfur X-ray absorption spectroscopy clearly shows that
Oax−Mo−Sthiolate−C dihedral angles near 90° effectively eliminate covalency contributions to the Mo(xy) redox orbital
from the thiolate sulfur. Sulfur K-pre-edge X-ray absorption spectroscopy intensity ratios for the spin-allowed S(1s)
→ Sv(p) + Mo(xy) and S(1s) → Sv(p) + Mo(xz,yz) transitions have been calibrated by a direct comparison of
theory with experiment to yield thiolate Sv(p) orbital contributions,

, to the Mo(xy) redox orbital and the Mo(xz,yz)
orbital set. Furthermore, these intensity ratios are related to a second coordination sphere structural parameter, the
Ooxo−Mo−Sthiolate−C dihedral angle. The relationship between Mo−Sthiolate and Mo−Sdithiolene covalency in oxomolydenum
systems is discussed, particularly with respect to electron-transfer regeneration in SO.