Synthesis of Ketonylplatinum(III) Dinuclear Complexes: Observation of the Competitive Radical vs Electrophilic Displacement in Pt(III)-Promoted C−H Bond Activation of Ketones

New ketonylplatinum(III) dinuclear complexes [Pt2((CH3)3CCONH)2(NH3)4(CH2COPh)](NO3)3 (4),
[Pt2((CH3)3CCONH)2(NH3)4(CH(CH3)COC2H5)](NO3)3 (5), and [Pt2((CH3)3CCONH)2(NH3)4(CH2COCH2COCH3)](NO3)3 (6) were prepared by treatment of platinum blue complex [Pt4(NH3)8((CH3)3CCONH)4](NO3)5
(2) with acetophenone, 3-pentanone, and acetylacetone, respectively, in the presence of concentrated HNO3.
The structures of complexes 4 and 6 have been confirmed by X-ray diffraction analysis, which revealed that
the C−H bonds of the methyl groups in acetophenone and acetylacetone have been cleaved and Pt(III)−C
bonds are formed. Formation of diketonylplatinum(III) complex 6 provides a novel example of the C−H bond
activation not at the central α-C−H but at the terminal methyl of acetylacetone. Reaction with butanone having
unsymmetrical α-H atoms led to two types of ketonylplatinum(III) complexes [Pt2((CH3)3CCONH)2(NH3)4(CH(CH3)COCH3)](NO3)3 (7a) and [Pt2((CH3)3CCONH)2(NH3)4(CH2COCH2CH3)](NO3)3 (7b) at a molar ratio
of 1.7 to 1 corresponding to the C−H bond activation of methylene and methyl groups, respectively. Use of
3-methyl-2-butanone instead of butanone gave complex [Pt2((CH3)3CCONH)2(NH3)4(CH2COCH(CH3)2)](NO3)3
(8) as a sole product via C−H bond activation in the α-methyl group. The reactivity of the ketonylplatinum(III) dinuclear complexes toward nucleophiles, such as H2O and HNEt2, was examined. The α-hydroxyl- and
α-amino-substituted ketones were generated in the reactions of [Pt2((CH3)3CCONH)2(NH3)4(CH2COCH3)](NO3)3 (1), 5, and a mixture of 7a and 7b with water and amine, which indicates that the carbon atom in the
ketonyl group bound to the Pt(III) atom can receive a nucleophilic attack. The high electrophilicity of the
ketonylplatinum(III) complexes can be accounted for by the high electron-withdrawing ability of the platinum(III) atom. A competition between the radical and electrophilic displacement pathways was observed directly
in the C−H bond activation reaction with butanone giving complexes 7a and 7b. Addition of a radical trapping
agent suppressed the radical pathway and gave complex 7b as the predominant product. On the contrary, 7a
was formed as the main product when the reaction solution was irradiated by mercury lamp light. These
results together with other mechanistic studies demonstrate that complex 7a was produced via a radical process,
whereas complex 7b is produced via electrophilic displacement of a proton by the Pt(III) atom. The competitive
processes were further observed in the reactions of platinum blue complex 2 with a mixture of acetone and
3-pentanone in the presence of HNO3. The relative molar ratio of acetonyl complex 1 to pentanoyl complex
5 was 3 to 1 under room light, whereas formation of complex 5 was almost suppressed when the reaction was
carried out in the dark with the addition of a radical trapping agent.