A. PHOTOREDUCTION OFCARBONATE IN ACOPPER(I)
PHOSPHINECOMPLEX
The photoreduction of CO 2 to CO by metal complexes has been
studied in some detail( 25 – 27 ). Frequently, complexes that contain
CO 2 as ligand have been examined. In general, such complexes are
not available by simple procedures. In this context, it is quite
surprising that the photoreactivity of carbonate complexes has
been investigated only to a rather limited extent although the pho-
toreduction of carbonate ligands has been anticipated ( 28 ).
Carbonate complexes are known since the beginning of coordi-
nation chemistry. Generally, they are rather stable and easily
accessible. The reduction of CO 32 to CO should be facilitated
in mononuclear or binuclear carbonate complexes with metal
centers that provide two excess electrons. The requested photo-
excitation corresponds to an MLCT transition that terminates
at the carbonate ligand as acceptor. However, MLCT absorptions
of this type are unknown. They are expected to occur at very
short wavelength since thep* acceptor orbital of CO 32 is cer-
tainly located at rather high energies( 29 ).
As a suitable candidate, we selected the complex (prophos)
CuI(CO 3 )CuI(prophos) (I)(Scheme 2) with prophos¼1,3-bis(diph-
enylphosphino)propane for a recent study ( 30 ).
Copper(I) is a MLCT donor and as a d^10 metal it has not avail-
able interfering LF states ( 31 ). The oxidation of both metal
centers to Cu(II) leads to the release of two electrons that are
required for the reduction of carbonate to CO. Our expectation that
complexIshould photochemically split off CO was based on various
observations. The thermal release of CO from CO 2 catalyzed by Cu
(I) at higher temperatures was already reported in 1976 (32,33).
Quite recently, a related reaction has been discovered ( 34 ). More-
over, complexIshows certain analogies to (Pf 3 ) 2 CuI(NO 3 )( 35 ). As
CT acceptor, it contains NO 3 , which is isoelectronic to CO 32 . In
this case, the irradiation leads to the population of a reactive MLCT
state that initiates the oxidation of Cu(I) and the reduction of
nitrate. The triphenylphosphine ligand does not only serve to
PPh 2PPh 2CuI(m-CO 32 - )CuIPh 2 PPh 2 P(prophos)CuI(CO 3 )CuI(prophos)SCHEME2.354 ARND VOGLER AND HORST KUNKELY