Molybdenum-pterin chemistry. 2. Reinvestigation of molybdenum(IV) coordination by flavin gives evidence for partial pteridine reduction

The coordination of alloxazine and pterins to molybdenum(IV) is demonstrate d in this study. The synthesis of MoOCl3(pteridineH), where pteridineH is t he protonated form of 1,3,7,8-tetramethylalloxazine (tmaz), 2-pivaloyl-6,7- dimethylpterin (piv-dmp), and 6,7-dimethylpterin (dmp), proceeds readily st arting from Mo(IV)Cl-4(acetonitrile)(2) and the pteridine ligand in chlorof orm or methanol. X-ray crystal structures of MoOCl3(tmazH) (1) and MoOCl3(p iv-dmpH) (2) show that Mo chelates each pteridine at the carbonyl oxygen an d pyrazine nitrogen and that the pteridine ligand is protonated at the othe r nitrogen in the pyrazine ring. A third X-ray structure for MoOCl3(H(3)dmp ) (4) is included in this work since its determination permits the comparis on of metrical parameters for the oxidized and reduced forms of a pterin in identical molybdenum coordination environments. The major difference obser ved in the structures of 2 as compared to 4 is the Mo-NS bond length which is significantly shorter in compound 4 containing the reduced form of the p terin. Pteridine protonation is facilitated by molybdenum(IV) coordination due to partial reduction of the pteridine ring through electronic delocaliz ation from Mo to the pteridine ligand. Electronic spectroscopy monitoring t he solution reactivity of 1, 2, and MoOCl3(dmpH) (3) provides evidence to s upport this idea. Solution conditions favoring deprotonation of the complex es 1-3 promote pteridine dissociation and complex decomposition.