Towards Langmuir-Blodgett films of magnetically interesting materials: solution equilibria in amphiphilic iron(II) complexes of a triazole-containing ligand.

As a first step towards amphiphilic spin crossover (SCO) systems where the hydrophobic part of the system is introduced by a non-coordinating anion (i.e. where no modification of the ligands to introduce hydrophobic substituents is required), [Fe(II)(OH(2))(2)(C(16)SO(3))(2)] and [Co(II)(OH(2))(2)(C(16)SO(3))(2)] have been prepared and reacted with the triazole-containing ligands adpt and pldpt (C(16)SO(3) = hexadecanesulfonate anion, adpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole, pldpt = 4-pyrrolyl-3,5-bis(2-pyridyl)-1,2,4-triazole). In the solid state, two HS complexes of the form [Fe(II)(Rdpt)(2)(C(16)SO(3))(2)] and two of the form [Co(II)(Rdpt)(2)(CH(3)OH)(2)](C(16)SO(3))(2) are obtained, even when a six-fold excess of ligand is used (Rdpt = adpt or pldpt). In solution, the cobalt complexes remain in this form as evidenced by colour, Visible/NIR and IR spectroscopy. For the iron complexes, there is an equilibrium in solution between the neutral high-spin form of the complex [Fe(II)(Rdpt)(2)(C(16)SO(3))(2)] and the dicationic low-spin tris form [Fe(II)(Rdpt)(3)](C(16)SO(3))(2). Polar solvents favour the dicationic form, while less polar solvents favour the neutral form (as evidenced by solution colour and solution IR spectroscopy). Visible/NIR spectroscopy and Evans' method NMR spectroscopy show the equilibrium can be shifted towards the [Fe(II)(Rdpt)(3)](C(16)SO(3))(2) form by adding additional ligand to the solution. The X-ray crystal structures of [Fe(II)(adpt)(2)(C(16)SO(3))(2)] and [Co(II)(adpt)(2)(CH(3)OH)(2)](C(16)SO(3))(2).1.33CH(3)OH are presented. [Fe(II)(adpt)(2)(C(16)SO(3))(2)] has a 2D bilayer structure with alternating layers of polar Fe(adpt)(2) centres, and hydrophobic alkyl chains. The complex cations in [Co(II)(adpt)(2)(CH(3)OH)(2)](C(16)SO(3))(2).1.33CH(3)OH form 1-D columns in the solid state. The capacity of the amphiphilic complexes [Fe(II)(pldpt)(2)(C(16)SO(3))(2)] and [Fe(II)(adpt)(2)(C(16)SO(3))(2)] to self-assemble has been probed at the air-water interface using Langmuir techniques. The pertinent pressure-area isotherms reveal only a low tendency of the complexes to form films.