Highly Selective and Efficient Ring Hydroxylation of Alkylbenzenes with Hydrogen Peroxide and an Osmium(VI) Nitrido Catalyst.

The OsVI nitrido complex, OsVI (N)(quin)2 (OTs) (1, quin=2-quinaldinate, OTs=tosylate), is a highly selective and efficient catalyst for the ring hydroxylation of alkylbenzenes with H2 O2 at room temperature. Oxidation of various alkylbenzenes occurs with ring/chain oxidation ratios ranging from 96.7/3.3 to 99.9/0.1, and total product yields from 93 % to 98 %. Moreover, turnover numbers up to 6360, 5670, and 3880 can be achieved for the oxidation of p-xylene, ethylbenzene, and mesitylene, respectively. Density functional theory calculations suggest that the active intermediate is an OsVIII nitrido oxo species.

Aerobic Oxidation of an Osmium(III) N-Hydroxyguanidine Complex To Give Nitric Oxide.

The aerobic oxidation of the N-hydroxyguanidinum moiety of N-hydroxyarginine to NO is a key step in the biosynthesis of NO by the enzyme nitric oxide synthase (NOS). So far, there is no chemical system that can efficiently carry out similar aerobic oxidation to give NO. We report here the synthesis and X-ray crystal structure of an osmium(III) N-hydroxyguanidine complex, mer-[Os(III){NH═C(NH2)(NHOH)}(L)(CN)3](-) (OsGOH, HL = 2-(2-hydroxyphenyl)benzoxazole), which to the best of our knowledge is the first example of a transition metal N-hydroxyguanidine complex. More significantly, this complex readily undergoes aerobic oxidation at ambient conditions to generate NO. The oxidation is pH-dependent; at pH 6.8, fac-[Os(NO)(L)(CN)3](-) is formed in which the NO produced is bound to the osmium center. On the other hand, at pH 12, aerobic oxidation of OsGOH results in the formation of the ureato complex [Os(III)(NHCONH2)(L)(CN)3](2-) and free NO. Mechanisms for this aerobic oxidation at different pH values are proposed.

Cerium(IV)-driven water oxidation catalyzed by a manganese(V)-nitrido complex.

The study of manganese complexes as water-oxidation catalysts (WOCs) is of great interest because they can serve as models for the oxygen-evolving complex of photosystem II. In most of the reported Mn-based WOCs, manganese exists in the oxidation states III or IV, and the catalysts generally give low turnovers, especially with one-electron oxidants such as Ce(IV) . Now, a different class of Mn-based catalysts, namely manganese(V)-nitrido complexes, were explored. The complex [Mn(V) (N)(CN)4 ](2-) turned out to be an active homogeneous WOC using (NH4 )2 [Ce(NO3 )6 ] as the terminal oxidant, with a turnover number of higher than 180 and a maximum turnover frequency of 6 min(-1) . The study suggests that active WOCs may be constructed based on the Mn(V) (N) platform.

Highly efficient alkane oxidation catalyzed by [Mn(V)(N)(CN)4](2-). Evidence for [Mn(VII)(N)(O)(CN)4](2-) as an active intermediate.

The oxidation of various alkanes catalyzed by [Mn(V)(N)(CN)4](2-) using various terminal oxidants at room temperature has been investigated. Excellent yields of alcohols and ketones (>95%) are obtained using H2O2 as oxidant and CF3CH2OH as solvent. Good yields (>80%) are also obtained using (NH4)2[Ce(NO3)6] in CF3CH2OH/H2O. Kinetic isotope effects (KIEs) are determined by using an equimolar mixture of cyclohexane (c-C6H12) and cyclohexane-d12 (c-C6D12) as substrate. The KIEs are 3.1 ± 0.3 and 3.6 ± 0.2 for oxidation by H2O2 and Ce(IV), respectively. On the other hand, the rate constants for the formation of products using c-C6H12 or c-C6D12 as single substrate are the same. These results are consistent with initial rate-limiting formation of an active intermediate between [Mn(N)(CN)4](2-) and H2O2 or Ce(IV), followed by H-atom abstraction from cyclohexane by the active intermediate. When PhCH2C(CH3)2OOH (MPPH) is used as oxidant for the oxidation of c-C6H12, the major products are c-C6H11OH, c-C6H10O, and PhCH2C(CH3)2OH (MPPOH), suggesting heterolytic cleavage of MPPH to generate a Mn═O intermediate. In the reaction of H2O2 with [Mn(N)(CN)4](2-) in CF3CH2OH, a peak at m/z 628.1 was observed in the electrospray ionization mass spectrometry, which is assigned to the solvated manganese nitrido oxo species, (PPh4)[Mn(N)(O)(CN)4](-)·CF3CH2OH. On the basis of the experimental results the proposed mechanism for catalytic alkane oxidation by [Mn(V)(N)(CN)4](2-)/ROOH involves initial rate-limiting O-atom transfer from ROOH to [Mn(N)(CN)4](2-) to generate a manganese(VII) nitrido oxo active species, [Mn(VII)(N)(O)(CN)4](2-), which then oxidizes alkanes (R'H) via a H-atom abstraction/O-rebound mechanism. The proposed mechanism is also supported by density functional theory calculations.

C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex.

We report experimental and computational studies of the facile oxidative C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex. We provide evidence that the initial step involves nucleophilic attack of aniline at the nitrido ligand of the ruthenium complex, which is followed by proton and electron transfer to afford a (salen)ruthenium(II) diazonium intermediate. This intermediate then undergoes unimolecular decomposition to generate benzene and N2.

Ligand-accelerated activation of strong C-H bonds of alkanes by a (salen)ruthenium(VI)-nitrido complex.

Kinetic and mechanistic studies on the intermolecular activation of strong C-H bonds of alkanes by a (salen)ruthenium(VI) nitride were performed. The initial, rate-limiting step, the hydrogen atom transfer (HAT) from the alkane to Ru(VI)≡N, generates Ru(V)=NH and RC·HCH(2)R. The following steps involve N-rebound and desaturation.

Reaction of a (Salen)ruthenium(VI) nitrido complex with thiols. C-H bond activation by (Salen)ruthenium(IV) sulfilamido species.

The reaction of [Ru(VI)(N)(L)(MeOH)](PF(6)) [1; L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion] with a stoichiometric amount of RSH in CH(3)CN gives the corresponding (salen)ruthenium(IV) sulfilamido species [Ru(IV){N(H)SR}(L)(NCCH(3))](PF(6)) (2a, R = (t)Bu; 2b, R = Ph). Metathesis of 2a with NaN(3) in methanol affords [Ru(IV){N(H)S(t)Bu}(L)(N(3))] (2c). 2a undergoes further reaction with 1 equiv of RSH to afford a (salen)ruthenium(III) sulfilamine species, [Ru(III){N(H)(2)S(t)Bu}(L)(NCCH(3))](PF(6)) (3). On the other hand, 2b reacts with 2 equiv of PhSH to give a (salen)ruthenium(III) ammine species [Ru(III)(NH(3))(L)(NCCH(3))](PF(6)) (4); this species can also be prepared by treatment of 1 with 3 equiv of PhSH. The X-ray structures of 2c and 4 have been determined. Kinetic studies of the reaction of 1 with excess RSH indicate the following schemes: 1 --> 2a --> 3 (R = (t)Bu), 1 --> 2b --> 4 (R = Ph). The conversion of 1 to 2 probably involves nucleophilic attack of RSH at the nitrido ligand, followed by a proton shift. The conversions of 2a to 3 and 2b to 4 are proposed to involve rate-limiting H-atom abstraction from RSH by 2a or 2b. 2a and 2b are also able to abstract H atoms from hydrocarbons with weak C-H bonds. These reactions occur with large deuterium isotope effects; the kinetic isotope effect values for the oxidation of 9,10-dihydroanthracene, 1,4-cyclohexadiene, and fluorene by 2a are 51, 56, and 11, respectively.

Reaction of a (salen)ruthenium(VI) nitrido complex with isocyanide.

The treatment of [Ru(VI)(N)(L)(MeOH)]PF(6) (1) (L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) with 2 equiv of RNC (R = (a) (t)Bu, (b) Cy) in CH(2)Cl(2) affords a mixture of blue cis-beta-[Ru(III)(NCNR)(L)(CNR)] (2) and green trans-[Ru(III)(L)(CNR)(2)]PF(6) (3) products. The reduction of 3 with a stoichiometric amount of Cp(2)Co in CH(3)CN gives red trans-[Ru(II)(L)(CNR)(2)] (4). Refluxing 4 with 3 equiv of RNC in methanol in the presence of 5 equiv of NH(4)PF(6) affords a yellow complex, mer-[Ru(II)(eta(3)-HL)(CNR)(3)](PF(6)) (5), in which the ligand is in an eta(3)-coordination mode. These complexes are characterized by IR, UV-vis, ESI-MS, CV, magnetic measurements, and CHN elemental analysis. The structures of 2a, 3a, 4a, 4b, and 5a have been determined by X-ray crystallography.

General synthesis of (salen)ruthenium(III) complexes via N...N coupling of (salen)ruthenium(VI) nitrides.

Reaction of [Ru (VI)(N)(L (1))(MeOH)] (+) (L (1) = N, N'-bis(salicylidene)- o-cyclohexylenediamine dianion) with excess pyridine in CH 3CN produces [Ru (III)(L (1))(py) 2] (+) and N 2. The proposed mechanism involves initial equilibrium formation of [Ru (VI)(N)(L (1))(py)] (+), which undergoes rapid N...N coupling to produce [(py)(L (1))Ru (III) N N-Ru (III)(L (1))(py)] (2+); this is followed by pyridine substituion to give the final product. This ligand-induced N...N coupling of Ru (VI)N is utilized in the preparation of a series of new ruthenium(III) salen complexes, [Ru (III)(L)(X) 2] (+/-) (L = salen ligand; X = H 2O, 1-MeIm, py, Me 2SO, PhNH 2, ( t )BuNH 2, Cl (-) or CN (-)). The structures of [Ru (III)(L (1))(NH 2Ph) 2](PF 6) ( 6), K[Ru (III)(L (1))(CN) 2] ( 9), [Ru (III)(L (2))(NCCH 3) 2][Au (I)(CN) 2] ( 11) (L (2) = N, N'-bis(salicylidene)- o-phenylenediamine dianion) and [N ( n )Bu 4][Ru (III)(L (3))Cl 2] ( 12) (L (3) = N, N'-bis(salicylidene)ethylenediamine dianion) have been determined by X-ray crystallography.

Photocatalytic oxidation of alkenes and alcohols in water by a manganese(v) nitrido complex.

Mn(v) nitrido complex [Mn(N)(CN)4](2-) is an efficient catalyst for visible-light induced oxidation of alkenes and alcohols in water using [Ru(bpy)3](2+) as a photosensitizer and [Co(NH3)5Cl](2+) as a sacrificial oxidant. Alkenes are oxidized to epoxides and alcohols to carbonyl compounds.

Catalytic oxidation of water and alcohols by a robust iron(iii) complex bearing a cross-bridged cyclam ligand.

An iron(iii) complex bearing a cross-bridged cyclam ligand (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) is an efficient catalyst for the oxidation of both water and alcohols using sodium periodate as the oxidant. In catalytic water oxidation a maximum turnover number (TON) of 1030 is achieved, while in catalytic alcohol oxidation >95% conversions and yields can be obtained.

Catalytic oxidation of alkanes by a (salen)osmium(VI) nitrido complex using H2O2 as the terminal oxidant.

The osmium(VI) nitrido complex, [Os(VI)(N)(L)(CH3OH)](+) (1, L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes under ambient conditions using H2O2 as the oxidant. Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover numbers up to 2230. Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H2O2 to [Os(VI)(N)(L)](+) to generate an [Os(VIII)(N)(O)(L)](+) active intermediate.

Epoxidation of alkenes and oxidation of alcohols with hydrogen peroxide catalyzed by a manganese(V) nitrido complex.

The manganese(V) nitrido complex (PPh(4))(2)[Mn(N)(CN)(4)] is an active catalyst for alkene epoxidation and alcohol oxidation using H(2)O(2) as an oxidant. The catalytic oxidation is greatly enhanced by the addition of just one equivalent of acetic acid. The oxidation of ethene by this system has been studied computationally by the DFT method.

Oxygen evolution from BF3/MnO4-.

MnO(4)(-) is activated by BF(3) to undergo intramolecular coupling of two oxo ligands to generate O(2). DFT calculations suggest that there should be a spin intercrossing between the singlet and triplet potential energy surfaces on going from the active intermediate [MnO(2)(OBF(3))(2)](-) to the O···O coupling transition state.

BF3-activated oxidation of alkanes by MnO4 -.

The oxidation of alkanes and arylalkanes by KMnO(4) in CH(3)CN is greatly accelerated by the presence of just a few equivalents of BF(3), the reaction occurring readily at room temperature. Carbonyl compounds are the predominant products in the oxidation of secondary C-H bonds. Spectrophotometric and kinetics studies show that BF(3) forms an adduct with KMnO(4) in CH(3)CN, [BF(3).MnO(4)](-), which is the active species responsible for the oxidation of C-H bonds. The rate constant for the oxidation of toluene by [BF(3).MnO(4)](-) is over 7 orders of magnitude faster than by MnO(4)(-) alone. The kinetic isotope effects for the oxidation of cyclohexane, toluene, and ethylbenzene at 25.0 degrees C are as follows: k(C6H12)/k(C6D12) = 5.3 +/- 0.6, k(C7H8)/k(C7D8) = 6.8 +/- 0.5, k(C8H10)/k(C8D10) = 7.1 +/- 0.5. The rate-limiting step for all of these reactions is most likely hydrogen-atom transfer from the substrate to an oxo group of the adduct. A good linear correlation between log(rate constant) and C-H bond energies of the hydrocarbons is found. The accelerating effect of BF(3) on the oxidation of methane by MnO(4)(-) has been studied computationally by the Density Functional Theory (DFT) method. A significant decrease in the reaction barrier results from BF(3) coordination to MnO(4)(-). The BF(3) coordination increases the ability of the Mn metal center to achieve a d(1) Mn(VI) electron configuration in the transition state. Calculations also indicate that the species [2BF(3).MnO(4)](-) is more reactive than [BF(3).MnO(4)](-).