PTERIN CHEMISTRY .90. SYNTHESIS AND CRYSTAL-STRUCTURE OF THE 1ST QUINOID DIHYDROPTERINMOLYBDENUM(IV) COMPLEX

The first molybdenum(IV) complex with a quinoid dihydropterin, i.e. 2, was isolated and characterized by IR, UV and NMR data and single-crystal X-ray diffraction. The reaction of Mo(VI)O2Cl2 with the biologically important 6-beta-5,6,7,8-tetrahydro-L-biopterin2) (7) in MeOH gave almost quantitatively trichloro(1,5-quinoid-7,8-dihydro-6H-L-biopterin)oxomolybdenum(IV) (2). The complex crystallizes with one molecule of MeOH in the noncentrosymmetric orthorhombic space group P2(1)2(1)2(1) (No. 19) with unit cell dimensions a = 1009.3(3), b = 1104.7(3), and c = 1484.5(4) pm and Z = 4. The Mo-atom has a distorted octahedral geometry (Fig. 1). It is coordinated by N(5) and O(4) of the pterin ligand. The distance of the Mo-N(5) bond (202 pm) is unusually short compared to similar complexes. O(4) is located trans to a terminal oxo ligand. The octahedral coordination is completed by three Cl-atoms in a meridional arrangement. Complex 2, with the Mo-atom in the enzyme-relevant oxidation state + IV and the pterin in the intermediate quinoid dihydro form, should lead to an extension of the proposed 'common molybdenum cofactor' model.