Carbon−Carbon vs Carbon−Hydrogen Bond Activation by Ruthenium(II) and Platinum(II) in Solution

Reaction of RuCl 2 (PPh 3 ) 3 with the bisphosphine {1,3,5-(CH 3 ) 3 -2,6-( i Pr 2 PCH 2 ) 2 C 6 H} ( 1 ) under 30 psi H 2 results in quantitative C−C activation of an Ar−CH 3 bond to afford Ru(Cl)(PPh 3 ){2,6-( i Pr 2 PCH 2 ) 2 -3,5-(CH 3 ) 2 C 6 H} ( 2 ) and CH 4, whereas reaction of RuCl 2 (PPh 3 ) 3 with 1 in the presence of NaO t Bu results in selective ArCH 2 −H bond activation to afford the benzylic complex Ru(Cl)(PPh 3 ){1-CH 2 -2,6-( i Pr 2 PCH 2 ) 2 -3,5-(CH 3 ) 2 C 6 H} ( 7 ). The identity of the 16-electron complex 2 was confirmed by reaction of the bisphosphine {2,6-( i Pr 2 PCH 2 ) 2 -3,5-(CH 3 ) 2 C 6 H 2 } ( 3 ), lacking the Ar−CH 3 group between the phosphine arms, with RuCl 2 (PPh 3 ) 3 . Metal insertion into an Ar−Et bond was observed as well. Follow-up of the reaction of RuHCl(PPh 3 ) 3 with 1 by NMR and deuterium labeling studies reveal that the kinetic products of ArCH 2 −H bond activation ( 7 and H 2 ) are irreversibly converted into the thermodynamically more stable products of Ar−C bond activation ( 2 and CH 4 ) via reversal of the C−H activation process. Reaction of (COD)PtCl 2 (COD = cycloocta-1,5-diene) with a stoichiometric amount of 1 at room temperature results in the exclusive formation of the benzylic Pt(II) complex Pt(Cl){1-CH 2 -2,6-( i Pr 2 PCH 2 ) 2 -3,5-(CH 3 ) 2 C 6 H} ( 8 ) and HCl. The iodide analogue of 8 has been characterized by X-ray analysis. Reaction of 8 with a 10-fold excess of HCl results in selective C C bond activation to afford Pt(Cl){2,6-( i Pr 2 PCH 2 ) 2 -3,5-(CH 3 ) 2 C 6 H} ( 10 ) and MeCl. The activation parameters for the overall process are Δ H ⧧ = 10.6 kcal/mol, Δ S ⧧ = − 40.1 eu, and Δ G ⧧ (298) = 23.1 kcal/mol in a benzene/dioxane solution (5.5:1 v/v) and Δ H ⧧ = 2.1 kcal/mol, Δ S ⧧ = −65.4 eu, and Δ G ⧧ (298) = 21.6 kcal/mol in dioxane.