An Experimental and Theoretical Investigation of the Carbon Dioxide Insertion Process into the Tungsten−Nitrogen Bond of an Anionic W(0) Complex

The pyridine bound 2-aminopyridine (2APH) derivative of tungsten pentacarbonyl has been prepared from
photogenerated W(CO)5THF and 2APH. Deprotonation of the distal amine group by sodium hydride has provided
two complexes, [Na][W(CO)5(2AP)] and [Na]2[W(CO)4(2AP)]2. Both complexes have been characterized by X-ray
crystallography with the monomeric derivative being crystallized as its [Na2(18-crown-6)][W(CO)5(2AP)]2 salt
which exhibits strong Na+···-NH interactions. Photolysis of W(CO)6 in the presence of excess 2-aminopyridine
in THF has led to an efficient synthesis of the chelated neutral derivative, W(CO)4(2APH)·2APH, where the
extra equivalent of 2APH is hydrogen bonded to its bound counterpart. The 2-aminopyridine molecule of solvation
was almost quantitatively removed via aqueous washings. Deprotonation of W(CO)4(2APH) with NaH afforded
the amidopyridine derivative which was shown to rapidly undergo reaction with CO2 to yield the chelated carbamate
complex, W(CO)4(OC(O)2AP)-. Nevertheless, because of the presence of small quantities of free 2-aminopyridine
during the reactions with CO2, we have not been able to conclusively rule out participation by a ligand substitution
process involving NC5H4NHCOOH. Ab initio computations were found to substantiate many of these experimental
observations. That is, in the monodentate bound W(CO)5(2APH) derivative, binding through the pyridine nitrogen
atom is favored by about 29 kJ/mol over the amine nitrogen atom, whereas the opposite site for binding is preferred
for the deprotonated amido analogue, W(CO)5(2AP)-. Furthermore, both forms of W(CO)5(2AP)- were found to
be more stable than the chelated tungsten tetracarbonyl anion plus CO. On the other hand, CO2 insertion into the
W(CO)4(2AP)- anion to provide the chelated carbamate, W(CO)4(OC(O)2AP)-, was thermodynamically favored
by >110 kJ/mol. Finally, both experimental and theoretical studies were inconclusive with regard to identifying
reaction intermediates during the CO2 insertion pathway which involve prior interactions of CO2 at the amido
nitrogen center.