Keywords: singlet oxygen, platinum, photosensitiser, alkane oxidation 2 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 C-H activation of alkanes, in particular electrophilic activation reactions with late transition metals. [1][2][3][4] Once C-H bond cleavage has occurred at a metal centre, a functionalisation 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 functionalisation of metal alkyl complexes should be carried out ideally with environmentally benign oxidants such as O 2 or H 2 O 2 . In a previous study, we reported the oxidation of dimethyl Pt II complexes with H 2 O 2 to generate cis-dihydroxo Pt IV complexes. [5] The unusual cis isomer was obtained by using amino-substituted bipyridine ligands, which stabilise this isomer through hydrogen bonding. When we extended our work to Pt II methyl complexes containing tridentate amino-substituted terpyridine ligands, we discovered a remarkable reactivity of the diamino substituted terpy Pt II methyl complex towards O 2 .The reaction of 6,6'-diaminoterpyridine (1) Figure 1 and the supporting material for further information). [6] This dimer formation is quite common for terpy Pt II complexes and arises from metal-metal and π-π interactions. [7] It is for example also observed in the solid state structure of [Pt ( Pt II complexes. [9][10][11] Excitation of the charge transfer region at 388 nm results in an emission spectrum with maxima at 580 and 654 nm (see Supplementary Information).Noteworthy, the diamino substituted terpyridine ligand itself also shows photoluminescence. [12,13] The luminescent behaviour of [Pt (1) [14] In addition, several Pt II and Pt IV complexes containing alkylperoxo ligands other than methylperoxo have been reported previously, but none of these have been prepared via an O 2 insertion reaction. [15][16][17][18][19][20] The O 2 insertion reaction into the Pt II -Me bond in the complex [(PN)Pt(Me)(OOMe)] is also affected by light and the authors have proposed a radical mechanism for their insertion reaction. [14] Similarly, the insertion of O 2 into a Pt IV -H bond has been reported to involve a radical pathway, [21] and insertions of O 2 into other transition metal methyl bonds such as Co and Fe methyl complexes proceed via methyl radicals. [22,23] In contrast, two reports on the direct insertion of O 2 into Pd II -H bonds have shown that these insertion reactions are not affected by light. [24,25] Mechanistic investigations suggest that these reactions proceed by a Pd I /HO 2 (triplet) radical pair intermediate or via a stepwise pathway initiated by reductive elimination of HX to form an O 2 -reactive Pd 0 intermediate. [26,27] Dioxygen, in its triplet ground state, is rather unreactive because reactions are spinforbidden and require a triplet-singlet surface crossing on the reaction c...
Platinum(II) and palladium(II) complexes [M(CH3)(L)]SbF6 with substituted terpyridine ligands L undergo light-driven oxygen insertion reactions into metal methyl bonds resulting in methylperoxo complexes [M(OOCH3)(L)]SbF6. The oxygen insertion reactions occur readily for complexes with methyl ligands that are activated due to steric interaction with substituents (NH2, NHMe or CH3) at the 6,6″-positions on the terpyridine ligand. All complexes exhibit attractive intermolecular π···π or M···M interactions in the solid state and in solution, which lead to excited triplet dinuclear M-M complexes upon irradiation. A mechanism is proposed whereby a dinuclear intermediate is generated upon irradiation that has a weakened M-C bond in the excited state, resulting in the observed oxygen insertion reactions.
Photolytic ligand displacement and salt metathesis routes have been exploited to give access to κ(1) σ-alane complexes featuring Al-H bonds bound to [W(CO)(5)] and [Cp'Mn(CO)(2)] fragments, together with a related κ(2) complex of [Cr(CO)(4)]. Spectroscopic, crystallographic, and quantum chemical studies are consistent with the alane ligands acting predominantly as σ-donors, with the resulting binding energies calculated to be marginally greater than those found for related dihydrogen complexes.
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