Molybdenum−Pterin Chemistry. 3. Use of X-ray Photoelectron Spectroscopy To Assign Oxidation States in Metal Complexes of Noninnocent Ligands

A series of molybdenum−pterin complexes produced from reactions of molybdenum and pterin reagents in various oxidation states has been investigated by X-ray photoelectron spectroscopy (XPS). Prior difficulties in making oxidation state assignments for the metal center and coordinated pterin can be resolved through comparison of Mo 3d binding energies (BE) for these new complexes with the BEs of standard molybdenum complexes. XPS analysis of molybdenum−pterin complexes produced from reactions of Mo(VI) reagents with tetrahydropterins show binding energies that are shifted by 1.5−1.8 eV to lower energies as compared to the BEs observed for the oxo-Mo(VI) reagents. The opposite shift in BE values is observed for complexes prepared from Mo(IV) chloride and fully oxidized pterins where BEs shift to higher values with respect to those for the starting Mo(IV) reagents. Remarkably, the BEs obtained for Mo−pterin complexes originating from Mo(VI)−tetrahydropterin reactions are nearly identical with those from Mo(IV)-oxidized pterin reactions. Both shifts are consistent with a Mo oxidation state of approximately +5. Both results indicate a significant delocalization of electron density over the molybdenum−pterin framework. This electronic redistribution is bidirectional since in the first system electron density flows from the reduced pterin to Mo(VI) and in the second case it flows from the Mo(IV) center to the electron-deficient oxidized pterin. Also described are syntheses of several tris(pteridine) complexes of Mo(0) that are diamagnetic molecules having intense MLCT absorptions near 500 nm. The electronic spectroscopic properties suggest that the pterin ligands in these complexes behave as strong pi-acids for Mo(0). This idea is verified by XPS analysis of Mo(piv-pterin)3, where higher BEs are observed than for standard Mo(0) or Mo(+2) compounds. X-ray photoelectron spectroscopy may be one of the optimal spectroscopic tools for studying the poorly understood electronic interactions of molybdenum and pteridine heterocycles.