Multiredox Heterometallic Carbosilane Dendrimers

A convergent growth approach for the synthesis of novel heterometallic dendritic molecules derived
from carbosilane frameworks and functionalized with silicon-bridged ferrocenyl and (η6-aryl)tricarbonylchromium moieties has been developed. The synthetic route involves the construction of the dendritic
fragments (CH2CH)MePhSiFc (1) (Fc = (η5-C5H4)Fe(η5-C5H5)) (CH2CHCH2)PhSi[(CH2)2MePhSiFc]2
(3), (CH2CH)MeSi[{(η6-C6H5)Cr(CO)3}Fc] (4), and (CH2CHCH2){(η6-C6H5)Cr(CO)3}MeSi[(CH2)2Me{(η6-C6H5)Cr(CO)3}SiFc]2 (5), containing a single reactive CC functionality, in addition to ferrocenyl
(1 and 3) and linked ferrocenyl and (η6-C6H5)Cr(CO)3 moieties (4 and 5) and their subsequent attachment
to the tetrafunctional core Si[(CH2)3Si(Me)2H]4 (6), via platinum-catalyzed hydrosilylation reactions. The
first- and second-generation dendrimers 8 and 9, containing four and eight ferrocenyl units on the dendritic
surface, respectively, have been prepared by this procedure. Thermal treatment of 8 and 9 with Cr(CO)6
afforded the targeted heterometallic dendrimers 7 and 10, carrying silicon-bridged (η5-C5H4)Fe(η5-C5H5)
and (η6-C6H5)Cr(CO)3 moieties appended at the dendrimer periphery. The molecular structures of the
vinyl-functionalized silanes 1 and 4 have been determined by single-crystal X-ray diffraction. The crystal
structure of 4 showed that the Cr(CO)3 group is disposed in a transoid configuration with respect to the
ferrocenyl moiety. The heterobimetallic 4 also shows an interesting self-assembly of the metals (Fe and
Cr) in the crystal. The electrochemical behavior of all new dendritic molecules has been investigated.
For the heterometallic 4, 7, and 10 the oxidation of the iron centers becomes more anodic compared to
the oxidation of the ferrocenyl units in the homometallic molecules 1, 8, and 9, due to the electron-withdrawing nature of the neighboring silicon-linked (η6-C6H5)Cr(CO)3 moieties. In addition, stable
electrode surfaces modified with dendrimers 8 and 9 have been prepared.