Relating molecular geometry to
reaction pathways by solid-state structural correlation techniques
Complexes in which an increase in the coordination number
at silicon is achieved by intramolecular ring closure of a chelating group
gives an insight into the stereochemistry of nucleophilic substitution.
In these compounds the donor atom may play the role of captive nucleophile and the nature and behaviour of the intramolecularly coordinated species serve as models for the properties of the intermediates or transition states participating in the substitution process.
Crystallography can provide important information about
the dynamics of substitution. It might at first seem surprising that a
static structure may provide dynamic information, however by collecting
as many structures containing the molecule or structural environment of
interest as possible, and comparing appropriate structural parameters (usually
bond lengths and/or bond angles), considerable insight into the substitution
can be gained. The technique is known as structural
correlation.
The structure of a molecule in the crystal environment
is not necessarily identical to that of its equilibrium structure as an
isolated molecule. i.e. The forces exerted by the crystal environment can
deform its structure to a greater or lesser extent.
The technique of structural correlation is concerned more
with the deformation of the molecule than the forces exerted on it. As
many structures as possible containing the centre of interest are collected
and sequenced so that a gradual deformation is observed. Each of the ordered
structures represents a snapshot of a modelled molecular transformation
at a particular point of progress. Each structure is thus regarded as a
frozen point induced by its particular crystal environment.
Britton and Dunitz applied the technique to numerous 'reactions' including modelling SN2 substitution with inversion at SnVI. The study was based on a series of [R3SnX2] complex crystal structures.
By plotting dy (y axis) and dx (x axis), the differences
between the observed SnY and SnX bond lengths and the lengths
of a standard SnY and SnX bond respectively, a graph showing
the relationship between SnY bond shortening and SnX bond lengthening
as 'substitution' progressed was obtained.