The platinum(II) methyl cation [(N−N)Pt(CH3)(solv)]+BF4 - (N−N = ArNC(Me)C(Me)NAr, Ar = 2,6-(CH3)2C6H3, solv = H2O (2a) or TFE = CF3CH2OH (2b)) is prepared by treatment of (N−N)Pt(CH3)2 with 1 equiv of aqueous HBF4 in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H2O solutions cleanly affords the platinum(II) phenyl cation [(N−N)Pt(C6H5)(solv)]+BF4 - (3). Investigations of the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene coordination, reversible oxidative addition of benzene C−H bonds, reversible formation of a methane C,H-σ complex, and final dissociation of methane. Under conditions where [(N−N)Pt(CH3)(H2O)]+BF4 - (2a) is the major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic rate law (inverse first order in [H2O] and first order in [C6H6] to 3.8 M) and the small kinetic deuterium isotope effect for C6H6 vs C6D6 (k H/k D = 1.06 ± 0.05 at 25 °C). When deuterated benzenes C6D6 and 1,3,5-C6H3D3 are used, almost full statistical scrambling of deuterium from one benzene into methane is achieved, indicating that the energetic barriers for dissociating benzene and methane are considerably higher than interconversions of intermediate hydrocarbon complexes and [(N−N)Pt(C6H5)(CH3)H]+. Protonation of (N−N)Pt(CH3)(C6H5) with HBF4 in TFE, which provides an independent route into the manifold of postulated intermediates, gives a mixture of 3 + CH4 (82%) and 2 + C6H6 (18%). Protonation of (N−N)Pt(CH3)(C6H5) with triflic acid in methylene chloride/diethyl ether mixtures at −69 °C allows direct low-temperature NMR observation of a fluxional π benzene complex, [(N−N)Pt(CH3)(C,C-η2-C6H6)]+.
The C-H activation of toluene and p-xylene at cationic Pt(II) diimine complexes (N-N)Pt(CH(3))(H(2)O)(+)BF(4)(-) (N-N = Ar-N=CMe-CMe=N-Ar; 1(BF(4)(-)), N(f)-N(f), Ar = 3,5-(CF(3))(2)C(6)H(3)); 2(BF(4)(-)), N'-N', Ar = 2,6-(CH(3))(2)C(6)H(3)) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe)(+) > (N-N)Pt(p-tolyl)(NCMe)(+) > (N-N)Pt(o-tolyl)(NCMe)(+) for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N'-N')Pt(benzyl)(NCMe)(+) were concurrently formed in the C-H activation of toluene at 2. The C-H activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (N-N)Pt(xylyl)(NCMe)(+) and (N-N)Pt(p-methylbenzyl)(NCMe)(+) were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (N(f)-N(f))Pt(Tol)(2) species (3a-c; a, Tol = o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF(4) was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (N(f)-N(f))Pt(m-tolyl)(NCMe)(+) (4b) and (N(f)-N(f))Pt(p-tolyl)(NCMe)(+) (4c) from all three isomeric bis(tolyl) complexes 3a-c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl)(CH(4))(+) (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition-state structures for the solvent-induced associative elimination of methane from these intermediates.
The redox chemistry of the series of Pt(II) diimine complexes L2PtMe2 (1; L2 = Ar−NCRCRN−Ar, where Ar/R = 4-MeC6H4/H (a), 4-MeOC6H4/H (b), 4-MeC6H4/Me (c), 4-MeOC6H4/Me (d)), with particular emphasis on the oxidation processes, has been studied in detail. As seen by cyclic voltammetry, 1a−d undergo two successive, reversible one-electron reductions at the diimine ligands and an irreversible, metal-centered one-electron oxidation. The oxidation of 1b has been investigated in some detail. Chemical oxidation of 1b with Cp2Fe+PF6 - in acetonitrile yields a near 1:1 ratio of the corresponding Pt(II) and Pt(IV) cations L2Pt(NCMe)Me+ (2b) and fac-L2Pt(NCMe)Me3 + (3b). Controlled-potential electrolysis of 1b yields mixtures of 2b and 3b in a 1:1 ratio, as well as the cis,cis (4b) and one cis,trans (5b) isomer of the dicationic Pt(IV) complexes L2Pt(NCMe)2Me2 2+. The percentage of the dications 4b and 5b depended on the electrode potential. A mechanism involving methyl group transfer between two transient Pt(III) intermediates L2PtMe2 •+ is proposed to account for the generation of 2b and 3b, whereas further oxidation of the Pt(III) species at the electrode eventually provides 4b and 5b. The X-ray crystal structures of 1b and 3b(OTf-) have been determined. All Pt−Me bond distances in these two species are essentially identical, averaging 2.057(1) Å.
By use of iterative design aided by predictive models for target affinity, brain permeability, and hERG activity, novel and diverse compounds based on cyclic amidine and guanidine cores were synthesized with the goal of finding BACE-1 inhibitors as a treatment for Alzheimer's disease. Since synthesis feasibility had low priority in the design of the cores, an extensive synthesis effort was needed to make the relevant compounds. Syntheses of these compounds are reported, together with physicochemical properties and structure-activity relationships based on in vitro data. Four crystal structures of diverse amidines binding in the active site are deposited and discussed. Inhibitors of BACE-1 with 3 μM to 32 nM potencies in cells are shown, together with data on in vivo brain exposure levels for four compounds. The results presented show the importance of the core structure for the profile of the final compounds.
A series of bis(terpyridine)RuII complexes have been prepared, where one of the terpyridines is functionalized in the 4'-position by a phosphonic or carboxylic acid group for attachment to TiO2. The other is functionalized, also in the 4'-position, by a potential electron donor. In complexes 1a, 3a, and 4a,b, this donor is tyrosine or hydrogen-bonded tyrosine, while in 2a it is carotenoic amide. The synthesis and photophysical properties of the complexes are discussed. On irradiation with visible light, the formation of a long-lived charge-separated state was anticipated, via primary electron ejection into the TiO2, followed by secondary electron transfer from the donor to the photogenerated RuIII. However, such a charge-separated state could be observed with certainty only with complex 2a. To explain the result, quantum chemical calculations were performed on the different types of complexes.
The preparation of donor (D)-photosensitizer (S) arrays, consisting of a manganese complex as D and a ruthenium tris(bipyridyl) complex as S has been pursued. Two new ruthenium complexes containing coordinating sites for one (2a) and two manganese ions (3a) were prepared in order to provide models for the donor side of photosystem II in green plants. The manganese coordinating site consists of bridging and terminal phenolate as well as terminal pyridyl ligands. The corresponding ruthenium-manganese complexes, a manganese monomer 2b and dimer 3b, were obtained. For the dimer 3b, our data suggest that intramolecular electron transfer from manganese to photogenerated ruthenium(III) is fast, k(ET) > 5 x 10(7) s(-)(1).
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