Mechanism of O_2 Activation and Methanol Production by (Di(2- pyridyl)methanesulfonate)Pt^(II)Me(OH_n)^((2−n)−) Complex from Theory with Validation from Experiment

Abstract

The mechanism of the (dpms)Pt^(II)Me(OH_n)^((2–n)−) oxidation in water to form (dpms)Pt^(IV)Me(OH)_2 and (dpms)Pt_(IV)Me_2(OH) complexes was analyzed using DFT calculations. At pH < 10, (dpms)Pt^(II)Me(OH_n)^((2–n)–) reacts with O_2 to form a methyl Pt(IV)–OOH species with the methyl group trans to the pyridine nitrogen, which then reacts with (dpms)Pt^(II)Me(OH_n)^((2–n)–) to form 2 equiv of (dpms)Pt^(IV)Me(OH)_2, the major oxidation product. Both the O_2 activation and the O–O bond cleavage are pH dependent. At higher pH, O–O cleavage is inhibited whereas the Pt-to-Pt methyl transfer is not slowed down, so making the latter reaction predominant at pH > 12. The pH-independent Pt-to-Pt methyl transfer involves the isomeric methyl Pt(IV)–OOH species with the methyl group trans to the sulfonate. This methyl Pt(IV)–OOH complex is more stable and more reactive in the Pt-to-Pt methyl-transfer reaction as compared to its isomer with the methyl group trans to the pyridine nitrogen. A similar structure–reactivity relationship is also observed for the S_N2 functionalization to form methanol by two isomeric (dpms)Pt^(IV)Me(OH)_2 complexes, one featuring the methyl ligand trans to the sulfonate group and another with the methyl trans to the pyridine nitrogen. The barrier to functionalize the former isomer with the CH_3 group trans to the sulfonate group is 2–9 kcal/mol lower. The possibility of the involvement of Pt(III) species in the reactions studied was found to correspond to high-barrier reactions and is hence not viable. It is concluded that the dpms ligand facilitates Pt(II) oxidation both enthalpically and entropically.

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