A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [Ru^V(N₄O)(O)]²⁺: Electronic Structure, Redox Properties, and Oxidation Reactions Investigated by DFT Calculations

Abstract

The electronic structure and redox properties of the highly oxidizing, isolable Ru(V)=O complex [Ru(V)(N4O)(O)](2+), its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol(-1), respectively. The overall free energy activation barrier (ΔG(≠)=25.5 kcal mol(-1)) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru(III)(N4O)(HOCl)](2+) (ΔG=15.0 kcal mol(-1)) and the substitution of HOCl by a water molecule (ΔG(≠)=10.5 kcal mol(-1)). For water oxidation, nucleophilic attack on Ru(V)=O by water, leading to O-O bond formation, has a free energy barrier of 24.0 kcal mol(-1), the major component of which comes from the cleavage of the H-OH bond of water. Intermolecular self-coupling of two molecules of [Ru(V)(N4O)(O)](2+) leads to the [(N4O)Ru(IV)-O2-Ru(III)(N4O)](4+) complex with a calculated free energy barrier of 12.0 kcal mol(-1).

Arylruthenium(III) Porphyrin-Catalyzed C–H Oxidation and Epoxidation at Room Temperature and [RuV(Por)(O)(Ph)] Intermediate by Spectroscopic Analysis and Density Functional Theory Calculations

“…Previous DFT calculations by Groves, Shaik, and co-workers for the hypothetical species [Ru V (Por)­(O)­(SH)] (the ruthenium analogue of the high-valent metal-oxo species of cytochrome P450) also revealed a doublet ground state (d 3 , S = 1/2) with the quartet state lying considerably higher than the ground state. Similar findings have also been obtained by DFT calculations on mononuclear nonporphyrin Ru V -oxo complexes including [Ru V (bpy) 2 (O)­(OH)] 2+ (bpy = 2,2′-bipyridine), [Ru V (bpy)­(tpy)­(O)] 3+ (tpy = 2,2′;6′,2″-terpyridine), and [Ru V (N 4 O)­(O)] 2+ (N 4 OH = bis­(2-(2-pyridyl)­ethyl)­(2-hydroxy-2-(2-pyridyl)­ethyl)­amine) . The Ru V -oxo bond distance calculated for the six-coordinate 2 A (1.721 Å) is similar to that calculated for the six-coordinate species [Ru V (Por)­(O)­(SH)] (1.779 Å), [Ru V (bpy) 2 (O)­(OH)] 2+ (1.715 Å), [Ru V (bpy)­(tpy)­(O)] 3+ (1.76 Å), and [Ru V (N 4 O)­(O)] 2+ (1.720 Å); these Ru V -oxo distances are longer than, yet comparable to, that in the X-ray crystal structures of five-coordinate trigonal bipyramidal complexes (Pr 4 N)­[Ru V (O 2 CC­(O)­Et 2 ) 2 (O)] (1.687 Å; HO 2 CC­(OH)­Et 2 = 2-hydroxy-2-ethylbutanoic acid) and (Pr 4 N)­[Ru V (PHAB)­(O)] (1.702(3) Å; H 4 PHAB = 1,2-bis­(2,2-diphenyl-2-hydroxyethanamido)­benzene) .…”

“…Similar findings have also been obtained by DFT calculations on mononuclear nonporphyrin Ru V -oxo complexes including [Ru V (bpy) 2 (O)­(OH)] 2+ (bpy = 2,2′-bipyridine), [Ru V (bpy)­(tpy)­(O)] 3+ (tpy = 2,2′;6′,2″-terpyridine), and [Ru V (N 4 O)­(O)] 2+ (N 4 OH = bis­(2-(2-pyridyl)­ethyl)­(2-hydroxy-2-(2-pyridyl)­ethyl)­amine) . The Ru V -oxo bond distance calculated for the six-coordinate 2 A (1.721 Å) is similar to that calculated for the six-coordinate species [Ru V (Por)­(O)­(SH)] (1.779 Å), [Ru V (bpy) 2 (O)­(OH)] 2+ (1.715 Å), [Ru V (bpy)­(tpy)­(O)] 3+ (1.76 Å), and [Ru V (N 4 O)­(O)] 2+ (1.720 Å); these Ru V -oxo distances are longer than, yet comparable to, that in the X-ray crystal structures of five-coordinate trigonal bipyramidal complexes (Pr 4 N)­[Ru V (O 2 CC­(O)­Et 2 ) 2 (O)] (1.687 Å; HO 2 CC­(OH)­Et 2 = 2-hydroxy-2-ethylbutanoic acid) and (Pr 4 N)­[Ru V (PHAB)­(O)] (1.702(3) Å; H 4 PHAB = 1,2-bis­(2,2-diphenyl-2-hydroxyethanamido)­benzene) . For the hydroxylation of 1-ethylbenzene by 2 A to give 1-phenylethanol via transition state TS1 (Figure , left), the free energy barrier of 16.6 kcal mol –1 obtained from the DFT calculations is close to that (17.0 kcal mol –1 ) inferred from the experimental k 2 value (1.99 M –1 s –1 , Table S4) via transition state theory.…”

“…Accordingly, the C2–H bond breaking and O–H bond forming could be facilitated by the hyperconjugation effect, rendering TS1 more stable than TS3 and thus resulting in a lower activation barrier. The ethylbenzene and methylcyclohexane components in TS1 and TS3 , respectively, are oriented to induce almost linear O–H–C2 angles (171.7° in Figure , 169.9° in Figure S16), which are similar to the virtually linear O–H–C angle in the transition state for the hydrogen atom abstraction of methane by [Ru V (Por)­(O)­(SH)] and comparable to the computed approaching angles in the C–H abstraction of cyclohexane and methane by [Ru V (N 4 O)­(O)] 2+ (O–H–C 170.2° and 177.6°, respectively) . For the hydroxylation of ethylbenzene and methylcyclohexane by 2 A , the computed free energy surfaces (Figure and Figure S16) revealed that the rebound process (following hydrogen atom abstraction) is barrierless, which is reminiscent of the barrierless rebound revealed by the computed energy profiles for the low-spin process of methane hydroxylation by [Ru V (Por)­(O)­(SH)]…”

“…Similar findings have also been obtained by DFT calculations on mononuclear nonporphyrin Ru V -oxo complexes including [Ru V (bpy) 2 (O)-(OH)] 2+ (bpy = 2,2′-bipyridine), 7h [Ru V (bpy)(tpy)(O)] 3+ (tpy = 2,2′;6′,2″-terpyridine), 32b and [Ru V (N 4 O)(O)] 2+ (N 4 OH = bis(2-(2-pyridyl)ethyl)(2-hydroxy-2-(2-pyridyl)ethyl)amine). 37 The Ru V -oxo bond distance calculated for the six-coordinate 2 A (1.721 Å) is similar to that calculated for the six-coordinate species 2+ (1.720 Å); 37 these Ru V -oxo distances are longer than, yet comparable to, that in the X-ray crystal structures of five-coordinate trigonal bipyramidal complexes 7e For the hydroxylation of 1ethylbenzene by 2 A to give 1-phenylethanol via transition state TS1 (Figure 10, left), the free energy barrier of 16.6 kcal mol −1 obtained from the DFT calculations is close to that (17.0 kcal mol −1 ) inferred from the experimental k 2 value (1.99 M −1 s −1 , Table S4) via transition state theory. The lower energy barrier (10.6 kcal mol −1 ) calculated for the oxidation of 1-phenylethanol to acetophenone (Figure 10, right) could account for the formation of acetophenone, instead of 1-phenylethanol, as the dominant product in catalytic ethylbenzene oxidation (e.g., entry 1, Table 1).…”

Water oxidation catalysed by iron complex of N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane. Spectroscopy of iron–oxo intermediates and density functional theory calculations

“…Hence, M06L was chosen for the subsequent DFT calculations in this work. In the literature, the M06L functional has been reported to show good performance in modelling water oxidation by ruthenium 21 and iron 12a , c complexes.…”