ORGANOMETALLIC DERIVATIVES OF OROTIC-ACID - CO-LABILIZING ABILITY OF THE AMIDO GROUP IN CHROMIUM AND TUNGSTEN CARBONYL-COMPLEXES

Novel orotic acid and uracil derivatives of tungsten and chromium(O), [Et4N](2)[Cr(CO)(4)(orotate)] (1), [Et4N](2-)[W(CO)(4)(orotate)] (2), [Et4N](2)[W(CO)(4)(dihydroorolate)] (3), and [Et4N] [W(CO)(5)(uracilat e)] (4) [where orotate (C5H2O4N2)(2-); dihydroorotate = (C5H4O4N2)(2-) ; uracilate = (C4H3O2N2)(-)], have been synthesized via reaction of M( C0)(5)THF with the tetraethylammonium salt of the corresponding acid o r uracil. These complexes have been characterized in solution by IR an d C-13 NMR spectroscopy and in the solid state by X-ray crystallograph y. The geometry of the metal dianions in 1 and 2 is that of a distorte d octahedron consisting of four carbonyl ligands and a nearly planar f ive-membered orotate chelate ring, bound through the N1 and one of its carboxylate oxygen atoms. The uracil ring, including the exocyclic ox ygens,itself deviates from planarity by only 0.009 Angstrom. 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 consis ts of the uracilate ligand bound through the deprotonated N1 to a tung sten pentacarbonyl fragment. Although the orotate complexes are resist ant to thermal decarboxylation, they readily undergo decarbonylation r eactions. In this regard, quantitative investigations of the lability of the carbonyl ligands on complexes 1-4 have been carried out. All co mplexes exhibited a low energy barrier for CO dissociation as demonstr ated by (CO)-C-13 exchange studies. For example, the first-order rate constants for intermolecular CO exchange in complexes 2 and 3 were mea sured to be 6.05 x 10(-4) and 3.17 x 10(-3) s(-1) at O degrees C, resp ectively. This facile CO dissociation is attributed to competition of the metal center with the uracil ring for the pi donation of electron density from the deprotonated N1 atom of the orotate Ligand. As expect ed, this interaction is enhanced when the pseudoaromaticity of the ura cil ring is disrupted in complex 3. The activation parameters for the intermolecular exchange of CO in complex 2 were determined to be Delta H = 63.2 +/- 3.8 kJ/mol and Delta S* = -82.8 +/- 13.0 J/mol.K, value s consistent with a bond-making/bond-breaking (M...CO/M-N) mechanistic pathway. The rate of intermolecular CO exchange was similarly examine d in complex 4. The uracilate ligand displayed a pi donating capabilit y comparable to that seen for chloride in the W(CO)(5)Cl- anion but mu ch less pi donor character than the phenoxide Ligand in W(C0)(5)OPh-. The activation parameters of the CO exchange process in complex 4 were found to be Delta H = 106.9 +/- 4.3 kJ/mol and Delta S* = 16.3 +/- 1 3.7 J/mol.K.