Azobenzene-based ruthenium(ii) catalysts for light-controlled hydrogen generation

Abstract

Eight new ruthenium(ii) half-sandwich complexes containing azobenzene-appended pyridine (1), bipyridine (2-5) and phosphine (6 and 7) ligands have been synthesized and fully characterized. UV-vis spectroscopic studies showed that the trans-to-cis photoisomerization was strongly inhibited upon coordination to the metal centre in azopyridine-derived ligands 1 and 2, but it remained efficient in azobenzene-appended bipyridine (3-5) and phosphine (6 and 7) ligands. The complexes were tested as precatalysts for photo-controlled hydrogen generation by hydrolytic decomposition of ammonia-borane (AB). In situ irradiation of the reaction mixtures of compounds [Ru(p-Cym)(6)Cl]Cl and [Ru(p-Cym)(7)Cl]Cl induced a clear change in the catalytic reaction rate, serving as a proof of concept for light-controlled hydrogen generation.

Photoswitchable phosphines in catalysis

“…Thus, coordination of E ‐ 6a to Pt(II), generates the cis ‐( E‐ 6a ) 2 PtCl 2 complex C6 that undergoes E→Z isomerization under irradiation at 350 nm in toluene for 1 h. The resulting mixture contains the (Z‐ 6a′ ) 2 PtCl 2 complex C6′ and a complex featuring both an E‐ and a Z‐ ligand ( C6“ ), together with traces of the starting material C6 . The mixture undergoes slow back isomerization in the dark at room temperature (Scheme 16) [18a] The same holds for the (p‐cymene)Ru( 9 )Cl 2 complex C9 displayed in Table 1: the half‐life time of the Z ‐isomer is roughly identical to that of the corresponding phosphine 9 c (115 min at 55 °C) [18b] …”

“…Thus, the ( p ‐cymene)ruthenium dichloride complexes C9 have been considered as catalysts for the controlled release of hydrogen from these adducts (Scheme 27). [18b] Complexes C9 bear the monodentate tris (azobenzene)phosphines 9 as photoswitchable ligands. The three azobenzene units of each ligand adopt mainly the most stable E ‐forms ( EEE ‐isomers), but small amounts of the isomeric EEZ ‐ligands may be present, with up to a 25 % amount for C9b .…”

Metal complexes bearing photochromic ligands: photocontrol of functions and processes

“…107 A ruthenium-catalyzed hydrolysis of ammonia-borane for photocontrolled hydrogen generation for chemical hydrogen storage materials was achieved. 108 Such an isomerization also led to a variation of the types of aggregates in the catalytic medium for palladium-catalyzed cleavage of allylundecylcarbonate in a heptane/water biphasic reaction medium. 109 Some example of phosphine modification with stilbene or stiffstilbene for Pd catalyzed reactions, such as the Z-E photoisomerization of the stilbene unit changing the dihedral angle of the chelating diphosphine or changing the helical chirality of the coordinating cis-diphosphine were also reported to photocontrol the stereochemical result which is particularly desirable for the pharmaceutical industry.…”

Photoswitchable catalysis based on the isomerisation of double bonds

“…30 In addition, the photocontrolled generation of hydrogen by hydrolytic decomposition of ammonia-borane was achieved using a ruthenium(II) half-sandwich complex bearing and azobenzenecontaining phosphine ligand. 31…”