Organometallic derivatives of orotic acid. CO-labilizing ability of the amido group in chromium and tungsten carbonyl complexes
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Novel orotic acid and uracil derivatives of tungsten and chromium(0), [Et4N]2[Cr(CO)4(orotate)] (1), [Et4N]2-[W(CO)4(orotate)] (2), [Et4N]2[W(CO)4(dihydroorotate)] (3), and [Et4N][W(CO)5(uracilate)] (4) [where orotate = (C5H2O4N2)2-; dihydroorotate = (C5H4O4N2)2-; uracilate = (C4H3O2N2)-], have been synthesized via reaction of M(CO)5THF with the tetraethylammonium salt of the corresponding acid or uracil. These complexes have been characterized in solution by IR and 13C NMR spectroscopy and in the solid state by X-ray crystallography. The geometry of the metal dianions in 1 and 2 is that of a distorted octahedron consisting of four carbonyl ligands and a nearly planar five-membered orotate chelate ring, bound through the N1 and one of its carboxylate oxygen atoms. The uracil ring, including the exocyclic oxygens, itself deviates from pianarity by only 0.009 . However, the structure of complex 3, which closely resembles that of complexes 1 and 2, has a puckered uracil ring. The structure of complex 4 consists of the uracilate ligand bound through the deprotonated N1 to a tungsten pentacarbonyl fragment. Although the orotate complexes are resistant to thermal decarboxylation, they readily undergo decarbonylation reactions. In this regard, quantitative investigations of the lability of the carbonyl ligands on complexes 1-4 have been carried out. All complexes exhibited a low energy barrier for CO dissociation as demonstrated by 13CO exchange studies. For example, the first-order rate constants for intermolecular CO exchange in complexes 2 and 3 were measured to be 6.05 10-4 and 3.17 10-3 s-1 at 0 C, respectively. This facile CO dissociation is attributed to competition of the metal center with the uracil ring for the donation of electron density from the deprotonated N1 atom of the orotate ligand. As expected, this interaction is enhanced when the pseudoaromaticity of the uracil ring is disrupted in complex 3. The activation parameters for the intermolecular exchange of CO in complex 2 were determined to be H# = 63.2 3.8 kJ/mol and S# = - 82.8 13.0 J/molK, values consistent with a bond-making/bond-breaking (MCO/M-N) mechanistic pathway. The rate of intermolecular CO exchange was similarly examined in complex 4. The uracilate ligand displayed a donating capability comparable to that seen for chloride in the W(CO)5Cl - anion but much less donor character than the phenoxide ligand in W(CO)5OPh -. The activation parameters of the CO exchange process in complex 4 were found to be H = 106.9 4.3 kJ/mol and S# = 16.3 13.7 J/molK.