  We describe a simple scheme for the implementation and control of effective
spin-spin interactions in self-assembled crystals of cold polar molecules. In
our scheme spin states are encoded in two long-lived rotational states of the
molecules and coupled via state dependent dipole-dipole forces to the lattice
vibrations. We show that by choosing an appropriate time dependent modulation
of the induced dipole moments the resulting phonon-mediated interactions
compete with the direct dipole-dipole coupling and lead to long-range and
tunable spin-spin interaction patterns. We illustrate how this technique can be
used for the generation of multi-particle entangled spin states and the
implementation of spin models with longe-range and frustrated interactions
which exhibit non-trivial phases of magnetic ordering.
