Abstract
Previous kinetic studies of photoinitiated transition metal–dinitrogen bond forming reactions are reviewed, with an emphasis on room temperature reactivity, and in particular, the techniques of time-resolved infrared (TRIR) spectroscopy and UV–vis flash photolysis. Our recent results on the reactivity of the formally 16-electron, but agostically stabilized, complex,
mer,
trans-W(CO)
3(PCy
3)
2 (
W) (Cy
=
cyclohexyl) toward N
2 in toluene and
n-hexane solution are then discussed. Laser flash photolysis of a toluene solution of
W-N
2 in the presence of excess N
2 resulted in the photoejection of N
2. The back reaction of
W with N
2 was followed by monitoring the decay of the transient absorption of
W at 600
nm. The second-order rate constant for the reaction of N
2 with
W in toluene to generate
W–N
2 was found to be (3.0
±
0.2)
×
10
5
M
−1
s
−1. The rate of the reverse reaction was found to be 100
±
10
s
−1, allowing an estimation of the equilibrium constant,
K
N
2
=
(
3.0
±
0.5
)
×
10
3
M
−
1
. Time-resolved step-scan FTIR (s
2-FTIR) spectroscopy was also used to spectroscopically characterize the
W intermediate and monitor its back-reaction with N
2 in
n-hexane solution. The rate of formation of
W–N
2 measured by s
2-FTIR agreed well with that measured by flash photolysis. Finally, density functional theory (DFT) calculations have been performed on the model complexes,
mer,
trans-W(CO)
3(PH
3)
2(L) (L
=
none and N
2) in order to understand the observed IR and UV–vis spectra of
W and
W–N
2 and to determine the nature of the frontier molecular orbitals of
W and
W–N
2, allowing their lowest energy excited states to be assigned.