Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Rhodium 217
In general terms, three main classes of rhodium complexes have attracted
the attention of photochemists: (i) amino complexes and substituted derivatives;
(ii) multiply bridged dirhodium complexes; (iii) rhodium polypyridine
and related complexes.
Rhodium(III) halo/amino complexes have been actively investigated in the
late 1970s and in the 1980s. They have low-lying metal centered (MC) excited
stats of d – d type, and can be considered paradigmatic representatives
of the ligand-field photochemistry of d 6 metal complexes. The subject has
been summarized and clearly discussed by Ford and coworkers in their 1983
review articles [7, 8]. In more recent times, however, despite a number of interesting
investigations [9–18], the activity in the field seems to have slowed
down considerably.
Multiply bridged dirhodium complexes constitute a large family of compounds,
the structure and properties of which depend strongly upon the oxidation
state of the metals. The Rh(I)-Rh(I) species of formula Rh2(bridge)4 2+ ,
where the two d 8 metal centers are bridged by four bidentate ligands (e.g.,
diisocyanoalkanes) in square planar coordination, have dσ ∗ → pσ excited
states with a greatly shortened metal–metal bond [19] that emit efficiently in
fluid solution [20]. In the Rh(II)-Rh(II) species of formula Rh2(bridge)4X2 n+ ,
the two d 7 metal centers are bridged by four bidentate ligands (e.g., diisocyanoalkanes,
acetate) and complete their pseudo-octahedral coordination
with a metal–metal bond and two-axial monodentate ligands (e.g., X
=Cl,Brn = 2). These dirhodium complexes have long-lived excited states
of dπ ∗ → dσ ∗ type, which do not emit in fluid solution but can undergo
a variety of bimolecular energy and electron transfer reactions [21].
Dirhodium tetracarboxylato units of this type have also been used as building
blocks for a variety of supramolecular systems of photophysical interest
[22–24]. Particularly interesting triply bridged dirhodium complexes of
type X2Rh(bridge)3RhX2, LRh(bridge)3RhX2, andLRh(bridge)3RhL (bridge
= bis(difluorophosphino)methylamine, X = Br, L = PPh3) have been developed
recently by Nocera [25]. These Rh(II)-Rh(II), Rh(0)-Rh(II) and
Rh(0)-Rh(0) species, all possessing excited states of dπ ∗ → dσ ∗ type, can be
interconverted photochemically by means of two-electron redox processes.
Such two-electron photoprocesses provide the basis for a recently developed
light-driven hydrogen production system [26], with a Rh(0)-Rh(II) mixed
valence species playing the role of key photocatalysts [27]. The interest in
multiply bridged dirhodium systems is now largely driven by their potential
and implications for photocatalytic purposes.
As for other transition metals, polyimine ligands (in particular, polypyridines
and their cyclometalated analogues) have played a major role in the design
of rhodium complexes of photophysical interest. This is due to an ensemble
of factors, including chemical robustness, synthetic flexibility, electronic
structure, excited-state and redox tunability. Thus, rhodium polypyridine
and related complexes have been extensively studied from a photophysical