Synthesis, Characterization, and Electrochemistry of Heterometallic Dendrimers

A method for the synthesis of new heterometallic dendritic molecules containing organoplatinum centers is reported. Reaction of Pt2Me4(μ-SMe2)2 with bpy-Fc2 (1) (bpy-Fc2 = 4,4‘-bis(ferrocenylvinyl)-2,2‘-bipyridine) gave the platinum(II) complex PtMe2(bpy-Fc2) (2). Complex 2 undergoes oxidative addition with iodomethane to yield PtMe3I(bpy-Fc2) (3) and with bis(bromomethyl)benzenes to yield complexes of the type [PtMe2Br(bpy-Fc2)CH2−]2R (4, R = 1,2-C6H4; 5, R = 1,3-C6H4; 6, R = 1,4-C6H4). 1, 3, and 4 were characterized by X-ray structure determinations. For 1, a = 11.780(3) Å, b = 10.662(3) Å, c = 11.642(3) Å, β = 118.61(2)°, and V = 1283.7(6) Å3 while, for 2, a = 11.497(2) Å, b = 22.796(6) Å, c = 15.801(3) Å, β = 106.08(1)°, and V = 3979(2) Å3. Only poor quality crystals of 4 could be obtained resulting in a partial structure refinement with a = 14.46(1) Å, b = 15.14(1) Å, c = 21.32(2) Å, α = 69.45(4)°, β = 71.14(5)°, γ = 81.86(5)° and V = 4140(6) Å3. The preferred conformation for 1 places the the bipyridine groups anti with respect to each other. However, as a consequence of the coordination to the metal centers in 3 and 4, they are located cis to each other, and the cyclopentadienyl rings attached directly to the vinyl groups lie on the same plane. In compound 4 the planes containing the platinum bipyridine units lie above and below the plane of the benzene ring, presumably to minimize steric hindrance. 2 also reacts with 1,3,5-tris(bromomethyl)mesitylene, 1,2,4,5-tetrakis(bromomethyl)benzene, and 1,2,3,4,5,6-hexakis(bromomethyl)benzene in 3:1, 4:1, and 6:1 molar ratios to yield the dendritic molecules [PtMe2Br(bpy-Fc2)CH2−]nR [7, n = 3, R = 1,3,5-C6(CH3)3; 8, n = 4, R = 1,2,4,5-C6H2; 9, n = 6, R = 1,2,3,4,5,6-C6], respectively. The largest dendrimer 9 contains six platinum and 12 ferrocene centers. Each complex undergoes a single 2n-electron electrochemical oxidation (corresponding to the ferrocenium/ferrocene couple of the bpy-Fc2 ligands), confirming that the pendant ferrocenyl units are noninteracting. The number of ferrocene centers for each complex was determined both by exhaustive electrolysis and by a chronocoulometric/voltammetric assay.