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(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 planarity by only 0.009 Å.
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 π 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
ΔH⧧ = 63.2 ± 3.8 kJ/mol and
ΔS⧧ = − 82.8 ± 13.0
J/mol·K, values consistent with a bond-making/bond-breaking
(M···CO/M

N)
mechanistic pathway. The rate
of intermolecular CO exchange was similarly examined in complex
4. The uracilate ligand displayed a π
donating
capability comparable to that seen for chloride in the
W(CO)5Cl - anion but much less π 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 ΔH‡ = 106.9 ± 4.3 kJ/mol and
ΔS⧧ = 16.3 ± 13.7
J/mol·K.