Organometallic complexes of uracil and orotic acid derivatives: Coordination mode, structure, and reactivity
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Carbonyltungsten derivatives of uracilates and orotates, where the metal center is bound to a nitrogen atom of the pyrimidine ring, are described. There organometallic complexes were synthesized by the reaction of either W(CO)5THF or W(CO)5MeOH with the tetraethylammonium salt of the deprotonated monoanion of dihydrouracil (1), 5-methyluracil (2), 6-methyluracil (3), methylorotic acid (4), and N,N-dimethylorotic acid (5). The complexes were all characterized in solution by IR and 13C NMR spectroscopy, and in select instances in the solid-state by X-ray crystallography. The structures of complexes 1 and 3 consist of octahedrally coordinated anions of pentacarbonyltungsten uracilate, where the uracilate ligand is bound to the metal center by the N3 atom of the pyrimidine ring, and the tetra-ethylammonium cation for charge balance. In contrast, the anion of complex 2 reveals that the uracilate ligand is bound to the W(CO)5 fragment by the N1 atom of the pyrimidine ring. The v(CO) vibrational modes and 13C NMR chemical shifts for the carbonyl ligands of complexes 1 and 3 indicate that the metal centers are more electron-rich than those of complex 2. This conclusion is further substantiated by the rates of intermolecular CO exchange in these carbonylmetal anions with 13CO. The facile CO exchange observed in complexes 1 and 3 is faster than that seen in complex 2. The rapid CO dissociation process measured in these derivatives is attributed to the -donation of electron density from the pyrimidine ring to the tungsten center in the transition state as the M-CO bond is broken.