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.