Photochemical reactions of ammineruthenium(II) complexes

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Abstract

A survey is given of the photochemical reactions displayed by ruthenium(II)–amine complexes, except for polypyridyl complexes, and including contributions from this laboratory. The latest review on the subject dates from the early 1980s. Since then, several contributions have appeared in the literature which could be brought and discussed together, giving an updated and unified view of the subject. Spectra and the nature of the excited states relevant for the discussion are described. Reactions will be discussed by type of reaction and of complexes. The reactions will mainly focus on photosubstitution, but will also include quenched and sensitized reactions, as well as eventual photooxidation reactions.

Introduction

Ruthenium chemistry experienced a great jump in development during the 1960s especially due to research performed in the laboratories of Allen and of Taube [1], [2], [3]. Improvement in the synthesis of Ru(II) and Ru(III) complexes, made them more readily available, and drew attention to their rich chemistry and spectroscopy. However, the photochemistry of ruthenium complexes was scarce, as can be seen in the 1969 monograph of Balzani and Carassiti [4], until at the very end of the 1960s, a series of fundamental papers [5] on ruthenium(II) photochemistry started to appear from the group of P.C. Ford.

Over the past 3 decades there have been a number of achievements in inorganic photochemistry both in terms of their fundamental aspects as well as application. Looking through this literature one can see an overwhelming presence of ruthenium complexes with polypyridines, the substitution inertness of which is essential for their applications. Thus, reviews on polypyridine ruthenium photochemistry and photophysics are available [6], as are reviews and a monograph on supramolecular photochemistry, which involve assemblies of basic components, many of which are ruthenium polypyridines [7].

The pentaammine and tetraamineruthenium(II) complexes, Ru(NH3)5L2+ and Ru(NH3)4LL′2+ also display a very rich photochemistry; but, in these cases, their photoreactions are dominated by substitution processes. For this reason, there is less interest in possible applications in solar energy conversion, although such components have drawn attention in terms of possible non-linear optical properties. However, the ruthenium(II) ammines have provided and continue to provide fundamental insight into the excited states reactivity of d6 metal centers. The photochemistry of such complexes was last reviewed more than 15 years ago [8], [9], [10], [11], [12], [13]. Given developments in this area since then, many in the author’s laboratory, the present CCR volume on Latin American Coordination Chemistry is a timely and appropriate opportunity to review the topic once more. This non-exhaustive review will cover mostly photosubstitution reactions of ruthenium(II) ammines and some related amines and will also include quenched and sensitized reactions, as well as eventual photooxidaton reactions.

Section snippets

Spectra and excited states of ruthenium(II) complexes

Among the excited states types, the LF, MLCT, CTTS, and IL excited states are displayed by mononuclear Ru(II) species. The former two states are of major importance to the photosubstitution reactions being reviewed. The CTTS excited state properties and systems where CTTS plays a role in photochemistry have been reviewed [11].

General aspects of the photochemical behavior of Ru(II) amine complexes

The ammine complexes of ruthenium(II) can be classified into two types. Those with saturated ligands, such as ammonia, water, and ethylenediamine, have no MLCT excited state, and the lowest energy excited state (LEES) is LF in character; at higher energies, a CTTS excited state may also be present. The other type is constituted by complexes which have at least one unsaturated ligand leading to the presence of MLCT excited states in addition to the former two; the energy of these MLCT states may

Acknowledgements

The author acknowledges the support of FAPESP, FINEP, CAPES, CNPq, and CAPES/PADCT and CNPq/PADCT for the research developed in this laboratory and research fellowship from CNPq. The author also thanks Professor Peter C. Ford for reading the manuscript.

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