inorganic chemistry

This reaction is facilitated by the high oxidizing power of Fe(IV).
When suitable independent reactions for the reduction and oxi-
dation of water are available, it still would not be simple to com-
bine them to a cyclic process with Eq. (1) as net reaction.
Accordingly, it is desired to perform reduction and oxidation by
an interconvertible redox pair based on the same metal complex.
This background motivated our recent study with osmocene
(OsCp 2 , Cp
¼C 5 H 5
) as photoredox catalyst ( 12 ).

A. WATERSPLITTING BYLIGHT WITHOSMOCENE ASPHOTOCATALYST

Osmocene dissolves in water only in the presence of acids. The
protonation of osmocene leads to the formation of a well-known
hydride complexEq. (12) ( 13 ):

OsII + H+ Os

+

IV

H ð^12 Þ

Generally, osmocene is not light sensitive, but undergoes a pho-
tolysis in acidic (e.g., H 2 SO 4 or HBF 4 ) aqueous solutions. The
irradiation with UV–light (lirr¼254 nm) is accompanied by char-
acteristic spectral changes (Fig. 1) that unambiguously indicate
the formation of [Cp 2 OsIIIOsIIICp 2 ]^2 þ ( 14 ). Simultaneously,
hydrogen is released. Both photolysis products are formed
approximately in a 1:1 ratio. This photoreaction can thus be
described by a simple stoichiometric equation:

2 Cp 2 OsIVðÞH

hiþ

!

hn

Cp 2 OsIII
OsIIICp 2

hi 2 þ

þH 2 ð 13 Þ

Product formation could take place via the intermediate forma-
tion of a radical pair:

Cp 2 OsIVðÞH

hiþ

! Cp 2 OsIII

hiþ

þH
atom!

2 

Cp 2 OsIII
OsIIICp 2

hi 2 þ

þH 2

ð 14 Þ

However, this reaction path can be excluded since [Cp 2 OsIII]þ
radicals disproportionate immediately under these conditions
(see below)Eq. (15):

PHOTOCHEMICAL ACTIVATION AND SPLITTING OF H 2 O 349