Recent progress in the development of transition-metal based photoredox catalysts

Highlights

• Advances in ligand development provided access to new types of photoredox catalysts.

• Earth-abundant metal complexes are increasingly used for photoredox catalysis.

• Strong photoreductants were developed with various metals, including Mo, Ce, Ir, Pt.

Abstract

Photoredox catalysis is an old concept that has experienced a remarkable revival in the past decade, leading to important progress in organic synthesis. Many of the current photoredox applications are based on well-known Ru^II polypyridine and cyclometalated Ir^III complexes, but there is now growing interest in the development of new photoredox catalysts. We review recent examples of new coordination compounds, which were synthesized for combined applications in photophysics and photoredox catalysis. Emphasis is on conceptually new approaches in ligand and metal complex design to obtain favorable excited-state properties, whilst the discovery of new organic photoredox reactions is less in focus. Specific examples include our own Mo⁰ complexes with isocyanide chelate ligands, new types of Cr^III polypyridines, as well as novel Re^I, Ir^III, Pt^II, and Ce^III/IV complexes. Most of this work has been disclosed over the last 3 years, and parts of it were presented at the 23rd International Symposium on the Photophysics and Photochemistry of Coordination Compounds (ISPPCC) in Hong Kong.

Introduction

The use of visible light rather than heat to drive chemical reactions is attractive not only in the context of solar energy conversion but also for organic synthesis. A frequently cited example of the application of photoredox catalysis in organic synthesis is the photochemical version of the Pschorr reaction reported by Deronzier and coworkers in 1984, which relied on the absorption of 410 nm light by [Ru(bpy)₃]²⁺ [1]. Reactions that are far more sophisticated than this early example can nowadays be performed by photoredox catalysis, and specific applications in organic synthesis have been reviewed thoroughly [2], [3], [4], [5], [6], [7]. Strikingly, a very large number of photoredox studies performed until now rely on a rather small set of photoredox catalysts [8]. Among transition metal-based catalysts, Ru^II polypyridines and cyclometalated Ir^III complexes (Fig. 1a/b) still play the most important role. Such six-coordinate 4d⁶ and 5d⁶ metal complexes have low-spin electron configurations, often with long-lived ³MLCT excited states (>100 ns) in which the redox properties are drastically altered with respect to the electronic ground states, as shown on the exemplary case of [Ru(bpy)₃]²⁺ in Fig. 2. These properties, combined with their ability to absorb visible light, makes these substitution-inert second- and third-row transition metal complexes suitable for many photoredox applications [9]. Moreover, the redox, optical absorption, and excited-state properties of these complexes are tunable through ligand modification, particularly in the case of the cyclometalated Ir^III compounds [10], [11], which increases their applicability as catalysts for a wide range of chemical reactions.

Aside from the metal-based prototypes in Fig. 1a/b, the commercial Eosin Y and Rhodamine 6G dyes (Fig. 1d/e) were readily amenable to organic photoredox chemistry and therefore can be considered similarly privileged compounds in this context [12]. Other metal-free photoredox catalysts such as acridinium dyes [13], donor–acceptor cyanoarenes [14], or phenoxazines [15] have been developed. In parallel, coordination complexes made from Earth-abundant metals are gaining increasing attention as alternatives to precious metal-based systems [16], [17], stimulated by ongoing research on fundamentally new types of complexes with long-lived and emissive excited states [18]. Cu^I diimine complexes (Fig. 1c) have long been known to exhibit luminescent MLCT excited states [19], [20], and early examples of photoredox applications are known also in this case [21]. Over the past few years, there has been much progress in photoredox catalysis with Cu^I, and several topical reviews were published recently [22], [23], [24].

Herein we concentrate on photoactive metal complexes that were disclosed over the past 3 years, and which represent conceptually new types of coordination compounds that were employed for photoredox catalysis. This encompasses complexes of precious as well as Earth-abundant metals in new coordination environments, but incremental advances of traditional systems are not considered. Complexes that were included in three very recent reviews of photoredox catalysis based on Earth-abundant metal elements are not discussed in further detail [16], [17], [22].