Investigations into the Requirements for Homogeneous Platinum- and Iridium-Catalyzed Oxidative C-H Bond Functionalization

Abstract

Investigations have been performed to determine the requirements for homogeneous platinum- and iridium-catalyzed oxidative alkane functionalization. Previous platinum-catalyzed systems have involved initial C-H bond activation to generate a platinum(II)-alkyl, followed by two-electron oxidation of this species to activate the alkyl towards nucleophilic displacement from the metal center. The factors affecting C-H bond activation by platinum(II) complexes and oxidation of alkylplatinum(II) complexes have been probed, while the possibility of using a diphenolate imidazolyl-carbene ligand to stabilize iridium complexes in a variety of oxidation states has been explored. Relative oxidation and protonation rates for trichloro(methyl)platinum(II) dianion have been screened under a variety of conditions, using several different oxidants. Both one- and two-electron oxidants were shown to compete effectively with protonation of trichloro(methyl)platinum(II) dianion, including CuCl2, CuBr2, FeCl3, Na3[H3PMo9V3O40], Br2, Na2IrCl6, and (NH4)2Ce(NO3)6. Oxidation by copper(II) proved to be highly dependent on the counteranion. Disodium (2,2’-biindolyl)dimethylplatinum(II) has been synthesized for the purpose of probing the C-H bond activation chemistry of electron rich platinum(II) complexes. This complex decomposes rapidly in air, and deuterolysis of both platinum-methyls as well as the 2,2’-biindolyl ligand occurs when it is dissolved in methanol-d4. Methide abstraction from (2,2’-biindolyl)dimethylplatinum(II), either by protonolysis or by reaction with B(C6F5)3, generates monomethylplatinum(II) species capable of activating C-H and C-D bonds. In the search for ligands capable of stabilizing iridium complexes in a variety of oxidation states, the first iridium complexes containing a diphenolate imidazolyl-carbene ligand have been synthesized. 1,3-Di(2-hydroxy-5-tert-butylphenyl)imidazolium chloride was synthesized and then reacted with chloro-1,5-cyclooctadiene iridium(I) dimer to generate potassium (1,5-cyclooctadiene){1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl}iridium(I). Oxidation of this complex with two equivalents of ferrocenium(III) hexafluorophosphate generates (acetonitrile)(1,5-cyclooctadiene){1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl}iridium(III) hexafluorophosphate. Reaction of this complex with dihydrogen generates a species capable of catalyzing olefin hydrogenations. Heating (acetonitrile)(1,5-cyclooctadiene){1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl}iridium(III) hexafluorophosphate with greater than two equivalents of tricyclohexylphosphine in acetonitrile followed by treatment with tetramethylammonium chloride results in (chloro)bis(tricyclohexylphosphine){1,3-di(2-hydroxy-5-tert-butylphenyl)imidazolyl}iridium(III). As indicated by cyclic voltammetry and bulk electrolysis, this complex undergoes two reversible one-electron oxidations in methylene chloride at -0.22 V and at 0.58 V.