A Synthetic Oxygen Atom Transfer Photocycle from a Diruthenium Oxyanion Complex

Abstract

Three new diruthenium oxyanion complexes have been prepared, crystallographically characterized, and screened for their potential to photochemically unmask a reactive Ru–Ru=O intermediate. The most promising candidate, Ru2(chp)4ONO2 (4, chp = 6-chloro-2-hydroxypyridinate), displays a set of signals centered around m/z = 733 amu in its MALDI-TOF mass spectrum, consistent with the formation of the [Ru2(chp)4O]+ ([6]+) ion. These signals shift to 735 amu in 4*, which contains 18O-labeled nitrate. EPR spectroscopy and headspace GC-MS analysis indicate that NO2• is released upon photolysis of 4, also consistent with the formation of an Ru2(chp)4O species. Photolysis of 4 in CH2Cl2 at room temperature in the presence of excess PPh3 yields OPPh3 in 173% yield; control experiments implicate Ru2(chp)4O (6), NO2•, and free NO3− as the active oxidants. Notably, Ru2(chp)4Cl (3) is recovered after photolysis. Since 3 is the direct precursor to 4, the results described herein constitute the first example of a synthetic cycle for oxygen atom transfer that makes use of light to generate a putative metal oxo intermediate.

Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C–H Amination

“…We have carried out a series of kinetic isotope effect (KIE) experiments to interrogate the nature of the reactive nitrogen species generated during photolysis of 3 (Table 1). Photolysis of 3 in a 1:1 THF/THFd 8 mixture affords a KIE of 2.72 (8). This value is well-matched to KIEs that have been previously measured for Rh 2 -catalyzed C−H amination using iminoiodinanes as nitrene sources.…”

“…6 We have been attracted to synthetic photochemistry as a tool to access authentic reactive intermediates under conditions that allow their characterization without attenuating their reactivity. 7 In contrast to metal oxo and nitrido complexes, which can be generated by photolysis of metal-bound oxyanions 8 and azido ligands, 9 respectively, there are no readily available photoprecursors to metal nitrenoid complexes. Here, we report the development of a new photochemical synthesis of metal nitrenoids, which has enabled the first spectroscopic observation of a Rh 2 nitrenoid intermediate in C−H amination.…”

Synthesis of a terminal Ce(iv) oxo complex by photolysis of a Ce(iii) nitrate complex

“…For example, Berry and co-workers demonstrated that photolysis of Ru 2 (chp) 4 (NO 3 ) (chp = 6-chloro-2-hydroxypyridinate) generated a metal oxo with concomitant release of NO 2 . 13 Similarly, Suslick and co-workers reported that photolysis of Mn(TPP)(NO 3 ) also resulted in metal oxo generation and release of NO 2 . 15 However, it should be noted that in both of these examples the resulting metal oxo was unstable and not isolated.…”

“… 12 In our recent synthesis of [Li(12-crown-4)][(NN′ 3 )Ce(O)], the oxo ligand was formed via reduction of a nitrate ligand, in a formal 1e – O-atom transfer process. Several other examples demonstrating the use of nitrate as a 1e – O-atom source have emerged in recent years, 13 – 16 suggesting that [NO 3 ] – could be a valuable reagent for the generation of f-element oxo complexes. This mode of reactivity can also be promoted with light.…”

Synthesis, characterization and solution behavior of a systematic series of pentapyridyl-supported RuII complexes: comparison to bimetallic analogs

“…Previous research in our laboratory has focused on bimetallic complexes with metal–metal bonds that can support reactive oxo and nitrido groups capable of oxygen atom and nitrene transfer. 49–54 A topic of special interest is the role of the metal–metal bond in these complexes, which can be best elucidated by direct comparisons with monometallic analogues (Scheme 1). 53,55…”