The well-known synthesis of the two polymorphs of the {[Fe(Htrz)2(trz)](BF4)}n spin crossover coordination polymer is explored with new template-free methods that allow a control over the local synthetic conditions. A “one-pot” synthesis approach is developed, in which the solid reactants are mixed together before the addition of the solvent, which is expected to generate instantaneous supersaturation conditions favoring the nucleation of particles over their growth. In a second method, the addition of ultrasound pulses promotes the appearance of local “hot spots” that affect the local temperature and allow exploring a different region of the concentration–temperature phase diagram, leading to an increase in the phase purity of the product. These two syntheses are compared to the classical method in which the reactants are first dissolved in separate solutions before being mixed. The use of a one-pot synthesis, with or without ultrasound pulses, induces a downsizing of the particle size by a factor of 500 on their volume. The addition of ultrasound pulses allows moving from a mixture of polymorphs I and II of this compound to the pure phase I. These approaches open the way to more studies on the control over the size or phase purity in such molecular compounds, without the use of any surfactant.