Accordingly, it is of considerable interest to accomplish this reac-
tion at ambient conditions. In principle, catalysis can be replaced
by a photochemical procedure. The activation energy is then sup-
plied by light. In favorable cases, the photoactivation is selective
and avoids interfering processes. Moreover, light may not only
provide the activation energy but also the energy for an endo-
thermic reaction that does not occur in catalysis at lower
temperatures.
In this context, it has been reported that (Mes) 3 Mo¼N¼N¼Mo
(Mes) 3 with Mes¼2,4,6-Me 3 C 6 H 2 is light sensitive and the pho-
toproducts imply the intermediate formation of (Mes) 3 MoN
( 55 ). This photolysis was carried out under conditions that are
usually applied for synthetic purposes. It may be thus difficult to
elucidate the photochemical mechanism. For example, the photol-
ysis mentioned above was performed for 18 h. In such cases, the
photoproducts could be the result of subsequent reactions that
may not be easily related to the primary photochemical steps.
Quite recently, a detailed study of the thermal and photochem-
ical reactivity of (m-N 2 )[Mo(N[tert-Bu]Ar) 3 ] 2 has been reported by
Cumminset al.( 56 ). It is rather interesting that the N 2 bridge is
thermally cleaved to generate the nitride complex NMo(N[tert-
Bu]Ar) 3 while the photolysis leads to the same result, but in a
second reaction path also liberates N 2 yielding Mo(N[tert-Bu]
Ar) 3. The ground state of them-N 2 complex has been suggested
to contain the MoNNMo fragment implying that dinitrogen is
present in a reduced form. The nature of the reactive excited
state is not immediately obvious and could be identified only by
density functional theory ( 56 ). In contrast, the reductive splitting
of the NN triple bond in a molecular complex is certainly much
more difficult to achieve and has previously not yet been
accomplished, neither thermally nor photochemically.
Accordingly, we decided to examine a binuclear complex with
a bridging N 2 ligand that largely preserves its integrity as din-
itrogen molecule. For this purpose, we selected the cation
[(NH 3 ) 5 OsII(m-N 2 )OsIII(NH 3 ) 5 ]^5 þ(57,58)(I) for a recent study ( 59 ).
A. PHOTOREDUCTION OFN 2 IN ABINUCLEAROSMIUMCOMPLEX
The cation [(NH 3 ) 5 OsII(m-N 2 )OsIII(NH 3 ) 5 ]^5 þ ( 59 ) (scheme 3)
offers several attractive features.
This complex offers several attractive features. It is easily
accessible and rather stable in aqueous solution in the absence
of light. Owing to the intense color ofIits disappearance can
be precisely monitored. Although it is a mixed-valence system
PHOTOCHEMICAL ACTIVATION AND SPLITTING OF H 2 O 361