Di(2-pyridyl)methanesulfonate ligand allows for facile aerobic oxidation of a PtIIMe complex into PtIVMe(OH)2 species and clean C-O reductive elimination of methanol from the latter in either acidic or basic aqueous solutions.

Atom economy and the use of "green" reagents in organic oxidation, including oxidation of hydrocarbons, remain challenges for organic synthesis. Solutions to this problem would lead to a more sustainable economy because of improved access to energy resources such as natural gas. Although natural gas is still abundant, about a third of methane extracted in distant oil fields currently cannot be used as a chemical feedstock because of a dearth of economically and ecologically viable methodologies for partial methane oxidation. Two readily available "atom-economical" "green" oxidants are dioxygen and hydrogen peroxide, but few methodologies have utilized these oxidants effectively in selective organic transformations. Hydrocarbon oxidation and C-H functionalization reactions rely on Pd(II) and Pt(II) complexes. These reagents have practical advantages because they can tolerate moisture and atmospheric oxygen. But this tolerance for atmospheric oxygen also makes it challenging to develop novel organometallic palladium and platinum-catalyzed C-H oxidation reactions utilizing O(2) or H(2)O(2). This Account focuses on these challenges: the development of M-C bond (M = Pt(II), Pd(II)) functionalization and related selective hydrocarbon C-H oxidations with O(2) or H(2)O(2). Reactions discussed in this Account do not involve mediators, since the latter can impart low reaction selectivity and catalyst instability. As an efficient solution to the problem of direct M-C oxidation and functionalization with O(2) and H(2)O(2), this Account introduces the use of facially chelating semilabile ligands such as di(2-pyridyl)methanesulfonate and the hydrated form of di(2-pyridyl)ketone that enable selective and facile M(II)-C(sp(n)) bond functionalization with O(2) (M = Pt, n = 3; M = Pd, n = 3 (benzylic)) or H(2)O(2) (M = Pd, n = 2). The reactions proceed efficiently in protic solvents such as water, methanol, or acetic acid. With the exception of benzylic Pd(II) complexes, the organometallic substrates studied form isolable high-valent Pt(IV) or Pd(IV) intermediates as a result of an oxidant attack at the M(II) atom. The resulting high-valent M(IV) intermediates undergo C-O reductive elimination, leading to products in high yields. Guidelines for the synthesis of products containing other C-X bonds (X = OAc, Cl, Br) while using O(2) or H(2)O(2) as oxidants are also discussed. Although the M(II)-C bond functionalization reactions including high-valent intermediates are well understood, the mechanism for the aerobic functionalization of benzylic Pd(II) complexes will require a more detailed exploration. Importantly, further optimization of the systems suitable for stoichiometric M(II)-C bond functionalization led to the development of catalytic reactions, including selective acetoxylation of benzylic C-H bonds with O(2) as the oxidant and hydroxylation of aromatic C-H bonds with H(2)O(2) in acetic acid solutions. Both reactions proceed efficiently with substrates that contain a directing heteroatom. This Account also describes catal...

Monohydrocarbyl palladium(IV) complexes bearing OH, OH(2), Br, and Cl ligands at the metal and supported by facially chelating 1-hydroxy-1,1-bis(2-pyridyl)methoxide were readily prepared in water at 0 °C. These complexes reductively eliminate Ar-X (X = OH, Br, Cl) in water at room temperature in high yield, and the corresponding first-order rate constants k(OH), k(Cl), and k(Br) are on the same order of magnitude.

The ability of Pd(II) complexes derived from 2,6-pyridinedicarboxylic acids to catalyze homogeneous regioselective aerobic oxidation of 5- and 6-substituted 8-methylquinolines in AcOH-Ac(2)O solution to produce corresponding 8-quinolylmethyl acetates in high yield was demonstrated; corresponding 8-quinoline carboxylic acids are minor reaction products.

New hydrophilic ligands of the di(2-pyridyl)methanesulfonate family, L = dpms and Me-dpms, enable the synthesis of methyl platinum(IV) hydrides, LPtMe2H, the study of very fast CH reductive coupling, and reductive elimination of these complexes in water. In dichloromethane solutions, 13CH4 reacts with (Me-dpms)PtMe2H to produce isotopomeric complexes.

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The C-O reductive elimination from LPt IV Me(OH) 2 (1, Me ) CH 3 ; 1-d 3 , Me ) CD 3 ; L ) di(2pyridyl)methanesulfonate) leading to methanol, dimethyl ether, and LPt II (OH 2 ) 2 BF 4 (3) was studied in acidic solutions (60 and 120 mM HBF 4 ) in H 2 O and H 2 18 O at 80 °C. In 18 O-labeled water a mixture of two isotopologous methanols, Me 18 OH and Me 16 OH, formed in 1:1 to 5:1 ratios. At a given acidity and a ∼10% conversion of 1-d 3 the Me 18 OH/Me 16 OH ratio was inversely proportional to the concentration of the complex 1-d 3 (29-120 mM). ESI-MS study showed that a slow 18 O/ 16 O exchange in hydroxo ligands of complexes 1 and 1-d 3 occurred that led to higher Me 18 OH/Me 16 OH ratios by the end of the reaction. Similarly, a mono-18 O-labeled complex, 1-18 O, reacted in H 2 16 O in the presence of HBF 4 to form a mixture of Me 16 OH and Me 18 OH. A number of intermediates of C-O elimination from 1 in acidic aqueous solution were identified, prepared independently, and characterized by NMR, X-ray diffraction, and elemental analysis: LPt IV Me(OH)(OMe) (4), sym-LPt IV Me(OMe) 2 (5), and isomeric dinuclear heterovalent cationic complexes [LPt IV Me(µ-OH) 2 Pt II L]BF 4 (cis-and trans-6). It was shown that an isomer of 4, methoxo platinum(IV) complex 13, hydrolyzed in acidic H 2 18 O solution to produce Me 16 OH isotopologue exclusively. Kinetic studies established that C-O elimination from 1 was first order in 1; it was catalyzed by acids and by one of the reaction products, complex 3. In the latter case reversible formation of intermediates 6 occurred. A suggested reaction mechanism for the formation of Me 16 OH from 1 in H 2 18 O solutions involves a bimolecular nucleophilic attack of a hydroxo ligand of 1 or 4 at the electrophilic carbon atom of the methyl group in the cationic species 1‚H + (complex 2) or 6, leading to non-18 O-labeled intermediates 4 or 5, respectively. Subsequent hydrolysis of these intermediates in H 2 18 O solution leads to Pt IV -16 OMe bond cleavage and formation of Me 16 OH. Similarly, a bimolecular nucleophilic attack of a methoxo ligand of 4 or 5 at the electrophilic carbon atom of complex 2 or 6 leads to intermediate dimethyl ether Pt IV complexes 7 and/or 8. These intermediates are responsible for the formation of dimethyl ether. The C-O coupling of 4 leading to dimethyl ether, methanol, and transient complex 5 was studied in neutral and acidic aqueous solutions. The Me 2 O/MeOH ratio was found to increase with decreasing [H + ] and increasing concentration of 4.

The ability of platinum compounds to activate methane catalytically in aqueous solution was first described more than 30 years ago. [1] Determining the mechanism of the Shilov reaction, which was considered to be an oxidative addition of an alkane to a platinum(ii) center, remained challenging for over 25 years. First, increasingly persistent alkylhydridoplatinum(iv) complexes have been discovered, [2] and then the ability of platinum(ii) species to add hydrocarbons to produce very stable platinum(iv) alkyl hydrides was demonstrated. [3] Recent work [4] has shown that the transient [LPtCH 3 ] þ , where L is a ligand that cannot structurally accommodate the square-planar geometry preferred by four-coordinate Pt II species and only modestly stabilizes the related octahedral alkane oxidative addition products, cleaves alkane CÀH bonds. Here L is [2.1.1]-(2,6)-pyridinophane, and the transient is available according to Equation (1) in Scheme 1. This transient cleaves one substrate C À H bond, but methane is eliminated easily from the Pt IV intermediate 1; b-H elimination from the derived Pt II alkyl compound then leads to net conversion of alkane to (coordinated) olefin, thus accomplishing a second substrate CÀH bond cleavage.To obtain alkane activation products different from the hydrido olefin complexes, we suggest here a modification of the starting dialkylhydridoplatinum(iv) complex to allow only an alkane single CÀH bond activation event. For this we needed a species with a single alkyl ligand attached to a platinum(iv) center which will leave one hydride in the transient [LPtX] þ (X ¼ H) species. As a synthetic source of this transient, we initially considered whether [LPtMe(H) 2 ] þ would selectively eliminate methane rather than H 2 . The same question has been addressed recently for the complex TpPtMeH 2 . [5] The free energies for methane and for H 2 elimination from [LPtMe(H) 2 ] þ were calculated (DFT, PBE functional, [6] SBK basis set, [7] and program package Priroda [8] ; Figure 1) and show that methane elimination is favored, both thermodynamically and kinetically, [9] by more than 13 kcal mol À1 relative to H 2 elimination. Due to the macrocycle constraints, Pt II cannot achieve a planar four-coordinate geometry in [LPtH] þ (Figure 1), and the best geometry is three-coordinate T-shaped, with only 14 valence electrons. The out-of-plane pendant N3 atom interacts negligibly with the Pt II center, and the longer distance of Pt to N2 than to N1 shows the trans influence of hydride. [10] Based on the energies in Figure 1, we expected to develop reversible alkane single C À H bond activation chemistry with a [LPtH] þ transient; this is unprecedented for Pt II .The synthesis of [LPtMe(H) 2 ] þ , [11] as its [BAr F 4 ] À salt (Ar F ¼ 3,5-(CF 3 ) 2 C 6 H 3 ), is summarized in Equation (2) (> 98 % yield based on NMR data, relative to signal for BAr F 4 ; yield of isolated product 85 %). The new dihydridoplatinum(iv) complex is air-and water-stable and can be recrystallized from a warm water±methanol mixture. No...