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.
The coordination of alloxazine and pterins to molybdenum(IV) is demonstrated in this study. The synthesis of MoOCl 3 (pteridineH), where pteridineH is the protonated form of 1,3,7,8-tetramethylalloxazine (tmaz), 2-pivaloyl-6,7-dimethylpterin (piv-dmp), and 6,7-dimethylpterin (dmp), proceeds readily starting from Mo(IV)Cl 4 (acetonitrile) 2 and the pteridine ligand in chloroform or methanol. X-ray crystal structures of MoOCl 3 (tmazH) (1) and MoOCl 3 -(piv-dmpH) (2) show that Mo chelates each pteridine at the carbonyl oxygen and pyrazine nitrogen and that the pteridine ligand is protonated at the other nitrogen in the pyrazine ring. A third X-ray structure for MoOCl 3 (H 3dmp) (4) is included in this work since its determination permits the comparison of metrical parameters for the oxidized and reduced forms of a pterin in identical molybdenum coordination environments. The major difference observed in the structures of 2 as compared to 4 is the Mo-N5 bond length which is significantly shorter in compound 4 containing the reduced form of the pterin. Pteridine protonation is facilitated by molybdenum(IV) coordination due to partial reduction of the pteridine ring through electronic delocalization from Mo to the pteridine ligand. Electronic spectroscopy monitoring the solution reactivity of 1, 2, and MoOCl 3 (dmpH) (3) provides evidence to support this idea. Solution conditions favoring deprotonation of the complexes 1-3 promote pteridine dissociation and complex decomposition.
The synthesis and structure of a new molybdenum complex coordinated by a reduced pterin is reported. [MoOCl-(detc)(H 3 dmp)]Cl (1) (where detc is diethyldithiocarbamate and H 3 dmp is 6,7-dimethyl-6,7,8-trihydropterin) is prepared from MoOCl 2 (detc) 2 and H 4 dmp (6,7-dimethyl-5,6,7,8-tetrahydropterin). The X-ray structure determination of [MoOCl(detc)(H 3 dmp)]Cl‚MeOH reveals an octahedral complex where the reduced pterin ligand coordinates through the carbonyl oxygen and pyrazine ring nitrogen atoms. An extensive hydrogen bonding network in the crystal lattice connects adjacent complexes, the chloride counterion, and the molecule of methanol. This hydrogen bonding persists in solution where it is identified by characteristic absorptions in the electronic spectrum. Dimethyl sulfoxide (DMSO) oxidizes [MoOCl(detc)(H 3 dmp)]Cl, producing 1 equiv of dimethylpterin and 1 / 2 equiv of oxidized dithiocarbamate, tetraethylthiuramdisulfide (TETDS), a reaction that constitutes a net five-electron oxidation of the molybdenum complex 1. Pterin oxidation is also observed for 1 in dimethylformamide (DMF) solution where it is believed to result from intermolecular electron transfer mediated by the hydrogen-bonding network.
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