Tuning Ligand Effects and Probing the Inner-Workings of Bond Activation Steps: Generation of Ruthenium Complexes with Tailor-Made Properties

Abstract

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru(II) complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex 1(2+), the pyridines were stepwise substituted with "click" triazoles (2(2+)-7(2+)). Whereas the thermo- and photoreactivity of 1(2+) are due to steric repulsion within the equatorial plane of the complex, 3(2+)-6(2+) are reactive because of triazoles in axial positions, and 4(2+) shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (2(2+) and 7(2+)) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in 1(2+), 3(2+), and 4(2+). We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches.

Photochemistry of [Ru(pytz)(btz)2]2+ and Characterization of a κ1-btz Ligand-Loss Intermediate

“…These energies can be determined or inferred spectroscopically, electrochemically and computationally and as [Ru(N^N) 3 ] 2+ complexes invariably have a highest occupied molecular orbital of predominantly metallic 4d character, and hence effectively the same ground state, these excited state energies can be readily compared). Work in our laboratory, [26][27][28] and those of others, [29][30][31][32] has shown that complexes containing 1,2,3-triazole rings that lack such steric promotion can also lead to photochemical reactivity, possibly through destabilisation of the 3 MLCT state with respect to the 3 MC state when compared to those states of the archetypal complex [Ru(bpy) 3 ] 2+ . We recently reported the series of complexes [Ru(bpy) 3-n (btz) n ] 2+ (btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 1 to 3) 33 in which an increasing number of btz ligands leads to destabilisation of the 1 MLCT bands in the visible absorption spectrum, significantly so for the homoleptic complex [Ru(btz) 3 ] 2+ .…”

How Inert are Osmium−Ligand Bonds? A Combined Thermal, Photochemical and Electrochemical Case Study With a Click Tripodal Ligand

“…For performing such reactions synthetic chemists usually use the thermal method. [6,8] On one-electron oxidation the originally Sbound DMSO ligand can dissociate and recoordinate, now Obound, to the metal center. While weakening of existing metal-ligand bonds is at the heart of all the three aforementioned methods, the ways and means in which such weakening is achieved are very different for all three methods.…”

“…[1] Less frequently electrochemical or photochemical bond activation strategies are used for the synthesis of novel transition metal complexes as well. [6,8] However, the oxidation waves of 1 and 2 (Figure 2) are fully reversible even at very low scan rates (I pc /I pa > 0.98 for the measurements at 10 mV s À1 (Fig-[a] J. Particularly intriguing is the comparison of a thermal bond activation to the photochemical bond activation of the same bond.…”

“…[1] Less frequently electrochemical or photochemical bond activation strategies are used for the synthesis of novel transition metal complexes as well. [4,5] In keeping with our interest in investigating metal-ligand bond activation reactions at divalent group 8 metal centers, [6] we asked ourselves the question: "How inert are Os-ligand bonds and to what extent is the labilization of such supposedly inert bonds linked to the method used for such labilization (thermal, electrochemical or photochemical)?". Particularly intriguing is the comparison of a thermal bond activation to the photochemical bond activation of the same bond.…”

“…[6,8] On one-electron oxidation the originally Sbound DMSO ligand can dissociate and recoordinate, now Obound, to the metal center. [6,8] On one-electron oxidation the originally Sbound DMSO ligand can dissociate and recoordinate, now Obound, to the metal center.…”