Towards Photocatalytic Alkane Oxidation: The Insertion of Dioxygen into a Platinum(II)–Methyl Bond

Abstract

The selective catalytic oxidation of methane and higher alkanes remains one of the great challenges in catalysis research. Considerable progress has been made during the last three decades regarding the CÀH activation of alkanes, in particular electrophilic activation reactions with late transition metals. [1][2][3][4] Once C À H bond cleavage has occurred at a metal center, a functionalization of the metal-carbon bond is required, followed by release of the product and regeneration of the catalyst. In the case of oxidation reactions, this functionalization of metal-alkyl complexes should be carried out ideally with environmentally benign oxidants such as O 2 or H 2 O 2 . We previously reported the oxidation of dimethylplatinum(II) complexes with H 2 O 2 to generate cis-dihydroxoplatinum(IV) complexes.[5] The unusual cis isomer was obtained by using amino-substituted bipyridine ligands, which stabilize this isomer through hydrogen bonding. When we extended our work to methylplatinum(II) complexes containing tridentate amino-substituted terpyridine (terpy) ligands, we discovered a remarkable reactivity of the complexes towards O 2 .The reaction of 6,6'-diaminoterpyridine (1) (1)]. The rate of the O 2 insertion reaction is affected by light: Whereas the reaction proceeds within several minutes at room temperature in ambient light, the reaction is very slow in the absence of light and only 50 % conversion is obtained after 13 h at room temperature.

Insertion of Molecular Oxygen into the Metal–Methyl Bonds of Platinum(II) and Palladium(II) 1,3-Bis(2-pyridylimino)isoindolate Complexes

“…There have only been two previously reported Pt− OOCH 3 solid-state structures. 10,12 Compared to these structures, the Pt−O bond length of 3-t Bu is of similar length at 1.985(8) Å, as is the O−O bond length (1.419(10) Å).…”

“…However, mechanistic studies of these systems did not support a radical chain mechanism. 12,13 Although an experimental article proposed a mechanism where excited triplet-state dinuclear intermediates are generated upon exposure to light and then undergo reaction with O 2 to form the M−OOCH 3 species, 13 a recent computational paper has suggested that the reaction proceeds instead through a triplet-state excited monomer that reacts with oxygen to form superoxo and peroxo intermediates on their way to the M−OOCH 3 products. 14 With such a small number of reported examples of oxygen insertion into M−C bonds, it is difficult to generalize or make predictions concerning the potential reactivity of other late metal alkyl complexes with oxygen.…”

Reactions of Pd and Pt Complexes with Molecular Oxygen

“…Britovsek et al. reported the insertion of O 2 into the PtMe bond of the cationic, amino‐substituted terpyridine complex [(terpy)PtMe][SbF 6 ] ( 83 , terpy=6,6′‐diaminoterpyridine, Scheme ) to form [(terpy)PtOOMe][SbF 6 ] ( 84 ), which was crystallographically characterized 108. Notably the reaction did not proceed when a complex with an unsubstituted terpyridine ligand, [(terpy)PtMe][SbF 6 ], was used.…”

Electrophilic Pt(II) Complexes: Precision Instruments for the Initiation of Transformations Mediated by the Cation–Olefin Reaction

“…In addition to selective Pt–C fluorination reactions, we have also initiated efforts to couple cyclization with Pt–C oxygenation schemes to access C3-oxygenated polycycles. − Partial success in achieving this goal was realized by a strategy that intercepts the Pt-alkyl intermediate with a 1-electron oxidant to form a Pt­(III)-alkyl. The chemistry of such species has previously been examined by Kochi, Trogler, Baird, and others − and is characterized by rapid Pt/Pd–C bond homolysis to reform the divalent state (in a form suitable for initiating the COR) and a free radical.…”