  Gaining control of the building blocks of magnetic materials and thereby
achieving particular characteristics will make possible the design and growth
of bespoke magnetic devices. While progress in the synthesis of molecular
materials, and especially coordination polymers, represents a significant step
towards this goal, the ability to tune the magnetic interactions within a
particular framework remains in its infancy. Here we demonstrate a chemical
method which achieves dimensionality selection via preferential inhibition of
the magnetic exchange in an $S=1/2$ antiferromagnet along one crystal
direction, switching the system from being quasi-two- to quasi-one-dimensional
while effectively maintaining the nearest-neighbour coupling strength.
