Syntheses, spectroscopy, and redox chemistry of encapsulated oxo-Mo(V) centers: implications for pyranopterin-containing molybdoenzymes

Coordination by at least four sulfur donors to an embedded molybdenum center has been found to be a common feature in the crystal structures of many mononuclear molybdenum enzymes. In an effort to model embedded molybdenum centers, we have synthesized dendritic thiolate ligands and their oxo-molybdenum complexes containing a [Mo(V)OS(4)](-) core. These compounds have been isolated in pure form as blue solids or gummy materials, and the molecular nature of these compounds has been confirmed by electrospray ionization mass spectrometry and infrared, electron paramagnetic resonance, and UV-vis spectroscopies. The dendritic complexes exhibit little variation in their broad S --> Mo charge transfer band (lambda(max) approximately 600 nm; epsilon approximately 6000 M(-)(1) cm(-)(1)), Mo=O vibration energy (941-943 cm(-)(1)), and EPR g-values (g( parallel ) approximately 2.02; g( perpendicular ) approximately 1.98; g(av) approximately 1.99). The spectroscopic data confirm the integrity of the square pyramidal [Mo(V)OS(4)](-) core with little geometric distortions, suggesting that the electronic structure at the metal center is not perturbed by the ligand architecture. The electronic structure of these complexes, calculated by the density functional theory, demonstrates a similar composition of the HOMO. In complexes 6 and 7a, the energy of the HOMO orbital might be modulated by the difference in the electronic structure of the ligands. The Mo(V/IV) reduction potentials vary as a function of the dielectric constant and the donor number of the solvent. The kinetics of the reduction is influenced by the reorganization of the geometry and the encapsulating effect. We suggest that protein structure imposed microenvironments may control the dielectric properties and hence the redox properties of the metal center in many metallobiomolecules.