Journal of the American Chemical Society
Communication
between 1 and [K(18c6)]N3 in thf at −80 °C for 5 h.
Crystallization from toluene at −40 °C affords crystalline
complex [K(18c6)]-2 in 72% yield. Isolated complexes
[NBu4]-2 and [K(18c6)]-2 are both stable in a thf or toluene
solution at RT for at least 1 week or at 70 °C for at least 3
days. Moreover, 1H NMR studies of [K(18c6)]-2 in thf-d8 and
toluene-d8 showed that the [K(18c6)]+ cation remains bound
to the azide in both solvents. The 2.2.2.cryptand (crypt)
analogue [K(crypt)(μ-N3)U(OSi(OtBu)3)4], [K(crypt)]-2,
could only be prepared in situ upon reaction of 1 with an
excess (5 equiv) of [K(crypt)]N3 in toluene or thf to avoid
release of the bound azide. Light blue crystals of [K(crypt)]-2
were obtained from a concentrated reaction mixture in hexane
at −40 °C.
The photolysis of thf-d8 solutions of [NBu4]-2 with a UV
lamp (253.7 nm) for 30 min yielded an orange solution whose
1H NMR spectrum showed only one major species with a
resonance at 1.44 ppm. Single crystals of the terminal nitride
complex [NBu4][U(OSi(OtBu)3)4(N)], 3 were obtained in
70% yield from a 1:1 toluene/hexane mixture at −25 °C.
Longer irradiation times (8 h) resulted in the full trans-
formation of 3 into unidentified products.
Complex 3 shows remarkable stability in thf and toluene
under ambient light for up to 3 weeks. As a comparison, 1
week was indicated as the half-life of the only other reported
terminal U(VI) nitride [U(TrenTIPS)N] in a toluene
solution.7b
However, photolysis of thf and toluene solutions of 3 with a
UV lamp (253.7 nm) resulted in the appearance of the same
unidentified decomposition products after 5 h and 30 min,
respectively, showing that the photolytic stability of 3 varies
with the solvent nature.
The molecular structures of complexes [K(crypt)]-2 (Figure
S55) and [NBu4]-2 (Figure 1) all show a similar coordination
Photolysis of toluene-d8 solutions of [NBu4]-2 with a UV
lamp (253.7 nm) for 2 to 5 h showed the simultaneous
progressive formation of 3 and other decomposition products,
also formed by photolysis of 3 in toluene, rendering impossible
the isolation of nitride 3 in these conditions. These results
show that both the photochemical stability of nitride 3 and its
rate of formation from the azide vary with the solvent,
rendering its choice crucial for the isolation of the nitride.
Notably, in thf the formation of the nitride from the azide is
much faster than its decomposition, allowing its clean
formation and isolation. We also became interested in
investigating the potential effects of cations on the photo-
chemical stability of the nitride and on the reactivity of the
terminal azide.
1H NMR studies showed that the addition of an excess (10
equiv) of [K(18c6)]I to complex 3 in toluene results in the
formation of a capped nitride, [K(18c6)]-3, which is stable in
toluene under irradiation (253.7 nm) for 5 h. These results
indicate a significantly higher photochemical stability of the
capped nitride compared to 3. In contrast, the addition of an
excess of [K(crypt)]I to 3 did not affect its decomposition rate
under irradiation, suggesting that labile binding of [K(crypt)]+
does not enhance the photochemical stability of the nitride.
1H NMR studies showed that the photolysis of toluene-d8
solutions of [K(18c6)]-2 with a UV lamp (253.7 nm) afforded
the clean formation of [K(18c6)]-3, which is complete after 5
h (Scheme 1). Thus, although the formation of the nitride
from the [K(18c6)]-capped azide is slow, as found for the
uncapped [NBu4]-2 azide in toluene, the increased stability of
the [K(18c6)]-capped nitride compared to 3 renders possible
its clean formation in toluene.
Photolysis of toluene-d8 and thf-d8 solutions of [K(crypt)]-2
led in both cases to mixtures of [K(crypt)]-2, [K(crypt)]-3,
and its decomposition products (Scheme 1). The presence of
cryptand bound to the azide results in a slower N2 elimination
compared to uncapped azide [NBu4]-2 rendering impossible
the isolation of the nitride product in these conditions.
Thus, capping alkali ions can not only increase the stability
of terminal nitride but also decrease the rate of the dinitrogen
release. Thus, the counterion choice is key in the photo-
chemical synthesis of terminal and capped nitrides.
Figure 1. Thermal ellipsoid plot of the anion [U(OSi-
t
(OtBu)3)4(N3)]− in [NBu4]-2 at 50% probability. The Bu moieties
have been omitted for clarity in all figures.
environments and only differ in the nature of the counterion.
Both [K(crypt)]+ and [K(18c6)]+ cations bind the azide ligand
in an end-to-end fashion in [K(crypt)]-2 and [K(18c6)]-2,
while [NBu4]-2 consists of a separated ion pair. The N1−K1
distance of 3.3(1) Å in [K(crypt)]-2 is much longer than the
one observed for [K(18c6)]-25h (2.562(6) Å). This difference
is in line with the cryptand being bulkier than the crown ether
and is likely to be the origin of the different stability of the
uranium-bound azides. The values of the U1−N1 in complexes
[NBu4]-2, [K(crypt)]-2, and [K(18c6)]-2, of 2.375(4),
2.379(6), and 2.351(7) Å, respectively, are on the longer
end of the range of values found in previously reported U(IV)
azide complexes (2.142(5)−2.442(6), Å).7b,11,13 The N−N
bond distances are very close to each other in all the three
complexes (1.187(5) and 1.165(6) Å for [NBu4]-2, 1.18(1)
and 1.145(6) Å for [K(crypt)]-2, and 1.180(9) and 1.226(11)
Å for [K(18c6)]-2).
These values do not necessarily suggest a low activation of
the azide moiety. Notably, a similarly long U−N distance and
similar equivalency in N−N distances were found in the
terminal U(IV) azide supported by the bulky TrenTIPS ligand,
which readily undergo photolysis to yield a cyclometalated
amide trough a nitride intermediate.7b Moreover, the higher
value found for νas(N3) of [K(18c6)]-2 (2096 cm−1)
compared to that of [NBu4]-2 (2057 cm−1) indicates a higher
degree of activation.
Compound 3 represents the first example of an isolated
terminal uranium nitride complex generated photochemically.
The molecular structure of the [U(OSi(OtBu)3)4(N)]− anion
(Figure 2), presents the uranium in a slightly distorted trigonal
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX