Organometallic complexes of uracil and orotic acid derivatives: Coordination mode, structure, and reactivity

Carbonyltungsten derivatives of uracilates and orotates, where the metal ce nter is bound to a nitrogen atom of the pyrimidine ring, are described. The re organometallic complexes were synthesized by the reaction of either W(CO )(5)THF or W(CO)(5)MeOH with the tetraethylammonium salt of the deprotonate d monoanion of dihydrouracil (1), 5-methyluracil (2), 6-methyluracil (3), m ethylorotic acid (4), and N,N-dimethylorotic acid (5). The complexes were a ll characterized in solution by IR and C-13 NMR spectroscopy, and in self c t instances in the solid-state by X-ray crystallography. The structures of complexes 1 and 3 consist of octahedrally coordinated anions of pentacarbon yltungsten uracilate, where the uracilate ligand is bound to the metal cent er by the N3 atom of the pyrimidine ring, and the tetraethylammonium cation for charge balance. In contrast, the anion of complex 2 reveals that the u racilate ligand is bound to the W(CO)(5) fragment by the N1 atom of the pyr imidine ring. The v(CO) vibrational modes and C-13 NMR chemical shifts for the carbonyl ligands of complexes 1 and 3 indicate that the metal centers a re more electron-rich than those of complex 2. This conclusion is further s ubstantiated by the rates of intermolecular CO exchange in these carbonylme tal anions with (CO)-C-13. The facile CO exchange observed in complexes 1 a nd 3 is faster than that seen in complex 2. The rapid CO dissociation proce ss measured in these derivatives is attributed to the re-donation of electr on density from the pyrimidine ring to the tungsten center in the transitio n state as the M-CO bond is broken.