Photochemistry and Photophysics of Coordination Compounds

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222 M.T. Indelli et al. cases ligand-to-metal charge transfer (LMCT) photochemistry is observed. A clear example is provided by the Rh(bpy)2(ox) + complex (6) [52]. The spectrum of the colorless complex 6 is characterized by an intense band at ca. 300 nm assigned to oxalato-to-rhodium LMCT transitions. Upon UV irradiation, the following photoreaction is observed: Rh III (ox)(bpy)2 + + hν → Rh I (bpy)2 + + 2CO2 . (1) The Rh(bpy)2 + product is formed rapidly (risetime in pulsed experiments, ca. 10 ns), probably via a sequence of processes comprising photochemical intramolecular electron transfer from the oxalate ligand to Rh(III) followed by the decomposition of the oxidized ligand into CO2 and CO2 – radical (Eq. 2) and thermal reduction of the Rh(II) center to Rh(I) by the reactive CO2 – radical (Eq. 3) [52]. RhIII (ox)(bpy)2 + + hν → RhII (CO2 – )(bpy)2 + +CO2 (2) RhII (CO2 – )(bpy)2 + → RhI (bpy)2 + +CO2 . (3) The violet Rh(bpy)2 + product, with intense MLCT visible absorption, is a tetrahedrally distorted d 8 square planar complex [53]. This Rh(I) species, which can also be obtained by chemical [54], electrochemical [55], or radiation chemical [56, 57] reduction of Rh(III) complexes, is of great interest from the catalytic viewpoint. It undergoes facile oxidative addition by molecular hydrogen [58], to give the corresponding Rh(III) dihydride (Eq. 4). The reaction is fully reversible upon Rh(bpy)2 + +H2 ⇆ cis-Rh III (bpy)2(H)2 + (4) removal of molecular hydrogen from the system. Interestingly, the release of molecular hydrogen from the dihydride complex can be obtained photochemically (Eq. 5). This photoreaction provides a cis-RhIII (bpy)2(H)2 –→Rh(bpy)2 + +H2 hν (5)
Photochemistry and Photophysics of Coordination Compounds: Rhodium 223 convenient means to perturb the equilibrium of Eq. 4 and to study the kinetics of hydrogen uptake in the thermal relaxation of the perturbed system. A detailed picture of the transition state of this interesting reaction has been obtained by such type of experiments [53]. 2.2 Cyclometalated Complexes Ligands such as 2-phenylpyridine, 2-(2-thienyl)-pyridine, or benzo(h)quinoline, in their ortho-deprotonated forms (ppy, 7;thpy,8;bzq,9), can bind in a bidentate N^C fashion to a variety of transition metals, including Rh(III). These complexes are generally indicated as cyclometalated (or orthometalated) complexes. The spectroscopy and photophysics of cyclometalated complexes [59] are usually very different form those of the corresponding polypyridine complexes, the main reason being the much stronger σ donor character of C – relative to N. The consequences are (i) a high degree of covalency in the carbon–metal bond, (ii) a strongly enhanced ease of oxidation of the metal, (iii) high-energy MC excited states, (iv) relatively low-energy MLCT excited states. The photophysics of Rh(III) cyclometalated complexes, though not as developed as that of analogous Ir(III) species (see Chap. 9), has been actively investigated in the last two decades. While some tris- [60] and monocyclometalated [61] compounds have been synthesized and studied, for synthetic reasons bis-cyclometalated complexes of Rh(III) are by far more common in the literature. All the bis-cyclometalated complexes have a C,C cis geometry [62]. The Rh(ppy)2(bpy) + (10) complex can be used here to exemplify the main photophysical features of this class of compounds.
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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X Preface nation Compounds. The ven
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