Synthesis, Structures, and Solution Dynamics of Tetrasubstituted Nine-Atom Germanium Deltahedral Clusters

Reported are the rational synthesis,
structures, and solution dynamics
of three tetrasubstituted and neutral Ge9-based deltahedral
clusters [Ge9R3R′]0, where
R = Si­(SiMe3)3 and R′ = Et (1), SnnBu3 (2),
or Tl (3). The first step of the synthesis is a reaction
of an acetonitrile suspension of the intermetallic precursor compound
K4Ge9 with {Si­(SiMe3)3}Cl which produces the trisubstituted monoanions [Ge9{Si­(SiMe3)3}]−. A benzene suspension of
the latter is then reacted with SnnBu3Cl or TlCp to produce 2 and 3, respectively,
while the same acetonitrile solution is reacted with EtBr in order
to produce 1. All three structures can be viewed as tricapped
trigonal prisms of Ge9 with the three “hypersilyl”
substituents, Si­(SiMe3)3, exo-bonded to the capping atoms. The fourth substituent in 1, the ethyl group, is exo-bonded to one of the six
available Ge atoms with the Ge–C bond positioned radially to
the Ge9 core. In the case of 2, on the other
hand, the tin fragment is found above one of the triangular bases
of the prism interacting with one or more Ge atoms in three crystallographically
different molecules in the structure. Lastly, the Tl atom in the structure
of 3 is found capping a pseudosquare face between two
hypersilyl substituents. NMR spectroscopy indicates that all three
compounds are dynamic at room temperature. Variable-temperature studies
suggest that the process in 1 and 2 is intramolecular
while the process in 3 involves dissociation of the Tl+ ion from the molecule followed by association at the same
or another equivalent pseudosquare face of the molecule. Thus, the
latter compound may be considered to a large extent to be ionic as
it is made of a thallium cation and a trisubstituted cluster anion.