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 Pt2Pd mixed-metal complex linear-[Pt2Pd(mu -dpmp)(2)(XylNC)(2)](PFs)2 (1b) (dpmp = bis(diphenylphosphinomethyl)phenylphosphine XylNC = 2,6-xylyl isocyanide), with electron-deficient alkynes, RC drop CR', led to the formation of a series of unsymmetrical A-frame clusters formulated as [Pt2Pd(mu -dpmp)(mu -Ph2PCH2P(Ph)CH2P(Ph)(2)CRCR')(XylNC)(2)](PF6)(2) (R = R' = CO2Me (2); R = R' = CO2Et (3); R = H, R' = CO2Me (4); R = H, R' = CO2Et (5); R = CH3, R' = CO2Et (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 dropC 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 mu-eta (1):eta (2)-phosphorus C-2 ylide bridging structure. The reaction mechanism was discussed. In contrast, reaction of Ib with excess HBF4 afforded [Pt2Pd(mu -H)(dpmp)(2)(XylNC)(2)] (BF4)(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) Angstrom (1b) to 2.867(2) Angstrom (8). These results demonstrated that the heterometal (Pd) was able to alter the reactivity of the linear trimetallic clusters and interestingly regulated its specificity.