A Low‐Spin Ruthenium(IV)–Oxo Complex: Does the Spin State Have an Impact on the Reactivity?

Abstract

High-valent metal-oxo complexes are key reactive species for oxidation and oxygenation of organic compounds in nature as well as in the laboratory. [1, 2] Although iron is the most common metal species among high-valent metal-oxo complexes, [3] there are also manganese-oxo, [4] ruthenium-oxo, [5] and other metal-oxo complexes.[6] High-valent metal-oxo species are produced by reductive activation of molecular oxygen coupled with proton transfer. [7][8][9] Peroxides such as hydrogen peroxide can provide a so-called "peroxide shunt" to produce high-valent metal-oxo species.[1-3] High-valent metal-oxo species can also be produced by proton-coupled electron transfer (PCET), in which deprotonation of a coordinated water molecule and oxidation of the metal center occur concertedly. [10][11][12][13][14] The reactivity of high-valent metal-oxo species varies depending on the type of metal, the oxidation state of the metal center, ligands, and the spin state. Theoretical studies proposed that the reactivity of high-valent metal-oxo species may be determined by two closely lying spin states, which have different activation barriers for the reactions with substrates. [15][16][17] The most straightforward way to clarify the effects of spin states on the reactivity of highvalent metal-oxo species is to examine the reactivity of an analogous series of metal-oxo complexes that have different spin states. There have been extensive studies on Ru IV

Comment on “A Low‐Spin Ruthenium(IV)–Oxo Complex: Does the Spin State Have an Impact on the Reactivity”

“…IntheirrecentCommunication, [1] Kojima et al describe C À H and CÀC bond activations of three simple test substrates by two closely related ruthenium(IV)-oxo complexes and conclude that "… the spin state of the Ru IV -oxo complexes does not affect their reactivity…". While the preparative work is of high quality, the general concept appears reminiscent of the seminal work of Detrich et al entitled "Can Spin State Change Slow Organometallic Reactions?"…”

“…Any observed difference would hence be the result of the coupled changes in ligand environment and spin state, the differentiation of which is nontrivial. * As noted by the authors, compound 4 has a singlet ground state only when two water molecules are coordinated to the oxo ligand, and even then the triplet state is calculated to be only 0.77 kcal mol À1 higher in energy (Table S1 in the Supporting Information of reference [1]). Without the water molecules, the triplet state is lower by 14.1 kcal mol À1 .…”

“…Thus, are selectivities of 100 and 88 %, respectively (Table 1 of Ref. [1]), to be considered as "identical"? If significant, is such a difference to be assigned to the difference in spin states or ligand structures?…”

“…[3] Specifically, in the Communication by Kojima et al, [1] several aspects have been considered insufficiently in addressing the role of spin states in chemical reactivity. * Owing to non-negligible spin-state coupling in transitionmetal compounds, different spin states can only be isolated in substance if the compounds themselves differ, for example, by having different ligands such as species 3 and 4 in reference [1]. Any observed difference would hence be the result of the coupled changes in ligand environment and spin state, the differentiation of which is nontrivial.…”

“…[3,5] The substrates chosen in reference [1] do not offer such competition. * As pointed out earlier, molecular symmetry plays a central role with regard to the reactivity of different spin states.…”