Ruthenium(II) and Iridium(III) Complexes as Photosensitisers Towards Light-Driven Biocatalysis

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Abstract

Biocatalysis is becoming an increasingly attractive alternative to more traditional chemical transformations for use in pharmaceutical and industrial applications. This interest is primarily a consequence of the high regio-, stereo- and enantioselectivity that is associated with enzyme catalysed reactions. However, the proliferation of such techniques has been limited due to the dependence of enzyme activity upon the presence of redox cofactors, which are typically expensive and must be used in conjunction with efficient regeneration systems. Whilst numerous methods have been described, of particular potential are those that employ exogenous photosensitisers as a means of generating reducing equivalents to promote catalytic turnover. In this study the potential of transition metal complexes as photosensitisers towards the development of light-driven biocatalytic systems is evaluated. Use of such compounds gives great scope towards fine-tuning the spectral and redox properties of the sensitisers with the aim of optimising catalytic efficiency. Detailed herein is an extensive study towards the synthesis and characterisation of a range of ruthenium(II) and iridium(III) coordination compounds. Using NMR, UV-vis, and electrochemical techniques, all complexes were fully characterised and the origins of the photophysical properties further investigated using time-dependent density functional theory (TD-DFT) calculations. A series of ruthenium compounds were synthesised containing the bis(2,2′-bipyrazyl)ruthenium(II) moiety, investigating the impact of changes made to the functionality of the ancillary ligand upon the overall properties of the complex. New synthetic procedures have been developed towards the preparation of both 2,2′-bipyrazine and the related complexes, exhibiting significant benefits over previously established methods. Through manipulating the electron density of the ancillary ligand it is shown that the redox properties may be effectively tuned upon inducing changes in the energy of the metal-based HOMO. The utilisation of cyclometalated iridium(III) complexes in aqueous systems has been limited due to a poor water solubility that is typically associated with these compounds. Herein it is demonstrated that this issue may be effectively circumvented on inclusion of the positively charged pyridinium moiety. Upon incorporation of this functional group into either the cyclometalating or ancillary ligands, two distinct series of compounds were successfully prepared. Complexes of the substituted quaterpyridinium ligands possess complicated UV-vis spectra that exhibit low intensity absorbance up to 650 nm. A number of monoquaternised bipyridinium compounds were also utilised as pyridine derived N-heterocyclic carbene cyclometalating ligands, to afford a range of complexes with unique redox properties. The prepared photosensitisers were subsequently used to induce catalytic turnover in light-driven biocatalytic systems utilising the flavin dependent oxidoreductase enzymes, pentaerythritol tetranitrate reductase (PETNR) and the thermophilic old yellow enzyme (TOYE). Through an extensive investigation, optimal reaction conditions have been identified and a mechanism of electron transfer proposed. These systems were successfully implemented in the reduction of a broad range of substrates under both aqueous and biphasic conditions, delivering yields and enantiomeric excesses comparable to those obtained utilising an enzyme coupled regeneration system. This study clearly demonstrates that transition metal complexes are excellent candidates in developing practical light-driven biocatalytic systems. It is expected that, with further investigation, this approach can be readily expanded to incorporate a variety of applications and provide an effective alternative to the use of costly redox cofactors and a reliance upon complex regeneration techniques.

Keyword(s)

Biocatalysis; Iridium; Light-Driven Biocatalysis; Old Yellow Enzymes; Photosensitisers; Photosynthetic; Ruthenium

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