Chapter 1
An Overview of the Physical and Photophysical Properties of [Ru(bpy)3]2+

Daniela M. Arias-Rotondo and James K. McCusker

Michigan State University, Department of Chemistry, 578 S Shaw Lane, East Lansing, MI, 48824, United States

* An expanded discussion of these topics can be found in Chem. Soc. Rev. 2016, 45, 5803-5820.

1.1 Introduction

The photophysics and photochemistry of transition-metal coordination compounds have been studied for over half a century [1, 2]. In particular, metal polypyridyl complexes - especially those that possess visible charge transfer absorptions - have played a central role in efforts to understand fundamental aspects of excited-state electronic structure and dynamics, as well as efforts to develop a wide range of solar energy conversion strategies [3, 4]. Their footprint in the area of synthetic organic chemistry was largely nonexistent until 2008 [5], when MacMillan and coworkers [6] reported the first example of a transition-metal-based charge transfer compound, [Ru(bpy)3]2+ (where bpy is 2,2´-bipyridine), acting as a photocatalyst (PC) in an asymmetric alkylation of aldehydes; simultaneously, Yoon and coworkers [7] reported [2+2] enone cycloadditions photocatalyzed by [Ru(bpy)3]2+. Following those initial reports, several groups have explored the use of coordination compounds as photocatalysts for a variety of organic transformations [8]. These compounds engage in single-electron transfer (SET) processes with organic substrates, generating organic radicals, which play a major role in organic synthesis. This new kind of catalysis has opened the door to synthetically useful reactions that could not be performed otherwise.

The majority of the photocatalysts used nowadays are polypyridyl complexes of either Ru(II) or Ir(III) [8]. The large number of examples using [Ru(bpy)3]2+ might make this compound look like a "one size fits all" photocatalyst, when in reality, the best photocatalyst for a reaction is determined by the kinetics and thermodynamics of the system of interest. The purpose of this chapter is to provide the necessary tools to understand the different factors that come into play when choosing a photocatalyst. To this end, we will use [Ru(bpy)3]2+ as an example; it is important to note that the concepts we will discuss apply to most transition-metal polypyridyl compounds.

Scheme 1.1 shows two examples of catalytic cycles using Ru(II)-based photoredox catalysts: in both cases, the first step is the absorption of a photon by the photocatalyst to generate an excited state that then engages in redox reactions. The first cycle in Scheme 1.1, reported by Zheng and coworkers [9], is called reductive, because the excited photocatalyst is reduced. The second one, reported by Cano-Yelo and Deronzier [10], is an oxidative cycle; the photocatalyst is first oxidized and then reduced to reform its resting state.

Scheme 1.1 Examples of reductive catalytic cycle (left; see also [9]) and oxidative catalytic cycle (right; see also [10]) involving Ru(II)-based photoredox catalysts; bpz is 2,2'-bipyrazine.

As shown in Scheme 1.1, most steps in a catalytic cycle are bimolecular reactions. In a very general way, for any catalytic cycle involving [Ru(bpy)3]2+, we can write the series of reactions in Scheme 1.2 [11, 12]. The first step is the absorption of a visible light photon by the photocatalyst in its ground state and its consequent promotion to an electronic excited state (PC*); the backward reaction is the ground-state recovery (this process can be radiative (i.e., emission) and/or nonradiative, as will be discussed in Section 1.3). For the excited photocatalyst to react with a molecule (R), both species must diffuse toward each other, forming a "precursor complex." Then, the reaction takes place; of the many kinds of reactions that could happen, only electron and energy transfer are relevant for our discussion. After the reaction, the products must diffuse away from each other; if they cannot escape the solvent cage fast enough, a back reaction may take place.

Scheme 1.2 Simplified kinetic scheme for a general quenching process (see also [11, 12]).

This relatively simple scheme allows us to outline the main points that need to be considered when choosing a photocatalyst:

1. 1. Photocatalytic reactions make use of the enhanced reactivity of the photocatalyst in its excited state; for this reason, a photocatalyst must possess a good absorption cross section, preferably over a broad range of wavelengths that the other species in the reaction mixture do not absorb.1
2. 2. The quantum yield of formation of the reactive excited state should be as high as possible (preferably, near unity); that state must persist long enough to undergo the desired reaction with the substrate, and then cleanly regenerate in order to maintain its viability as part of a catalytic cycle. In the context of Scheme 1.2, these latter criteria mean that kd and kq must be larger than k0, so that the PC* can diffuse toward the appropriate molecule and react with it before going back to the ground state [13].
3. 3. If the catalytic cycle involves electron transfer, the excited- and ground-state redox potentials of the photocatalyst must provide for an exothermic (or at worst weakly endothermic) reaction; reversible electrochemistry is also desirable as an indicator of the stability of the photocatalyst over multiple turnovers.2
4. 4. Synthetic accessibility and, more importantly, tunability are critical in order to tailor the excited-state reactivity of the photocatalyst to the reaction of interest.

Given the various criteria just enumerated, it is no surprise that polypyridyl complexes of Ru(II) and Ir(III) have proved useful as photoredox catalystse. These compounds strongly absorb visible light, which makes it easy to selectively excite them relative to the organic substrates for typical reactions of interest. Their excited states are formed with ~100% efficiency [14] and their lifetimes range from 300 ns to 6 µs, which is long enough for them to engage in bimolecular reactions [3, 15]. As a class, these compounds are generally stable with respect to decomposition (both photochemical and thermal) and typically exhibit reversible redox behavior. They are also emissive, which facilitates mechanistic studies (as discussed in Sections 1.7 and 1.8); however, it is not a requirement. The synthesis of transition-metal polypyridyl complexes has been studied in great detail [4, 16], as well as the effect that different ligands have on the properties of the ground and excited states [17]. All these properties make these compounds the preferred choice for photocatalysts.

As mentioned above, we will discuss the properties of the ground and excited states of [Ru(bpy)3]2+, as a prototype for photoredox catalysis, describing the necessary experiments to fully understand their properties. Using this as a foundation, we will then focus on the processes that take place during a photocatalytic cycle and the experiments that allow for discriminating between various mechanistic possibilities (the main question being energy transfer versus reductive/oxidative electron transfer). In so doing, our goal is to provide a basic blueprint for how to identify, characterize, and ultimately design photocatalysts for use in a wide variety of chemical transformations.

1.2 [Ru(bpy)3]2+: Optical and Electrochemical Properties

1.2.1 Optical Properties

The electronic absorption spectrum of [Ru(bpy)3](PF6)2 in acetonitrile is shown in Figure 1.1. The intense absorption at 285 nm corresponds to a ligand-centered transition (pL pL*), which has been assigned by comparison with the absorption spectrum of the protonated ligand [18]. The band in the visible region (?max = 452 nm) corresponds to a metal-to-ligand charge transfer (MLCT) transition. As the name implies, this type of excited state can be viewed as the promotion of an electron from a metal-based orbital to a ligand-based one. Because of this spatial redistribution of electron density, this transition is responsible for the enhanced redox activity of the excited state relative to what is observed in the ground state, and makes the compound an efficient photocatalyst. Charge transfer transitions are typically very intense, with extinction coefficients in the range of 103 to 104 M-1 cm-1 [19] (in acetonitrile at room temperature, ? ~ 15 000 M-1 cm-1 for [Ru(bpy)3]2+).

Figure 1.1 Electronic absorption spectrum of [Ru(bpy)3](PF6)2 in acetonitrile at room temperature. The inset shows the metal-to-ligand charge transfer (MLCT) band.

Two additional features can be seen in the absorption spectrum of [Ru(bpy)3]2+. The origin(s) of the weaker features at 330 and 350 nm are less clear-cut and have been the subject of considerable debate over the years. They are most likely due to ligand-field (so-called "d-d") transitions within the d-orbital manifold of the metal. The inferred intensity belies this assignment to a certain extent (the symmetry-forbidden nature of d-d bands typically limits their absorptivities to the range of 10-100 M-1 cm-1) [19] but the proximity of both the ligand-centered and MLCT features influences these values in the present case....