Unsymmetrical A-Frame Pt2Pd Trinuclear Complexes: Site-Selective Apparent Double Insertion of Alkynes into Pd−Pt and Pd−P Bonds

Reactions of the linearly ordered Pt 2 Pd mixed-metal complex linear -[Pt 2 Pd(μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 ( 1b ) (dpmp = bis(diphenylphosphinomethyl)phenylphosphine, XylNC = 2,6-xylyl isocyanide), with electron-deficient alkynes, RC⋮CR‘, led to the formation of a series of unsymmetrical A-frame clusters formulated as [Pt 2 Pd(μ-dpmp)(μ-Ph 2 PCH 2 P(Ph)CH 2 P(Ph) 2 CRCR‘)(XylNC) 2 ](PF 6 ) 2 (R = R‘ = CO 2 Me ( 2 ); R = R‘ = CO 2 Et ( 3 ); R = H, R‘ = CO 2 Me ( 4 ); R = H, R‘ = CO 2 Et ( 5 ); R = CH 3, R‘ = CO 2 Et ( 6 )) in considerable yields, in which the alkyne molecule was site-selectively inserted into the Pt−Pd bond and was further reacted with a phosphine unit of a dpmp ligand through double bond breaking of the C⋮C unit. Complexes 2 − 6 were characterized by IR, electronic absorption, and NMR spectroscopic techniques and X-ray crystallography ( 2, 4, and 5 ) to reveal that the apparent double insertion of the acetylene unit takes place site-selectively on the Pt−Pd−P heterometallic center. In complexes 4 and 5, the P−C bond formation occurred regioselectively between the terminal alkyne carbon and phosphine unit of dpmp, resulting in a μ-η 1:η 2 -phosphorus C 2 ylide bridging structure. The reaction mechanism was discussed. In contrast, reaction of 1b with excess HBF 4 afforded [Pt 2 Pd(μ-H)(dpmp) 2 (XylNC) 2 ](BF 4 ) 3 ( 8 ), where a proton was oxidatively added to the Pt−Pt bond vs the Pt−Pd bond. From the X-ray crystal structure of 8, the structural change accompanied by the addition was revealed to be fairly small, with a slight elongation of the Pt−Pt bond from 2.690(1) Å ( 1b ) to 2.867(2) Å ( 8 ). These results demonstrated that the heterometal (Pd) was able to alter the reactivity of the linear trimetallic clusters and interestingly regulated its specificity.