Structural and magnetic tuning from a field-induced single-ion magnet to a single-chain magnet by anions

We herein report the syntheses, crystal structures, and magnetic properties of two complexes based on the anisotropic pentagonal bipyramidal Fe-II starting material [Fe(LN3O2)](2+), namely [Fe(LN3O2)(H2O)(2)]-[MQ](2)center dot H2O (1) and [Fe(LN3O2)(CN)][ABSA]center dot 3H(2)O (2) (LN3O2 = 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo [12.3.1]octadeca-1(18),2,12,14,16-pentaene; MQ(-) = methyl orange anion; ABSA(-) = 4-aminoazobenzene-4'-sulfonic anion). Compound 1 is a mononuclear material where the [Fe(LN3O2)(H2O)(2)](2+) cations form a one-dimensional (1D) chain by the hydrogen bonds between the bulky MQ(-) anions and the coordinated water molecules. With a slightly different anion, ABSA(-), a cyano-bridged Fe-II chain is formed for compound 2. This chain has a similar structure as that in our recently reported compound [Fe(L-N5)(CN)][BF4] (Chem. Commun., 2015, 51, 4360). However, compared with the reported result where the chains interact with each other through the pi center dot center dot center dot pi interactions, the chains in 2 are well isolated by the bulky ABSA(-) anions with the shortest interchain Fe center dot center dot center dot Fe distances around 12.0 angstrom. Magnetic investigation on 1 reveals the easy-axis magnetic anisotropy of the mononuclear Fe-II centre (zero-field splitting parameter D = -3.7 cm(-1)), which leads to the field-induced slow magnetic relaxation. For compound 2, the Fe2+ spins are antiferromagnetically coupled through the cyano bridges with a coupling constant of J = -4.13(2) cm(-1) with the Hamiltonian H = -J Sigma S-i center dot Si+1. AC magnetic measurements revealed the pure single-chain magnetic (SCM) behaviour of these isolated chains with an effective energy barrier of 26.1(5) K. This system represents a good example showing that the structures and magnetic properties, such as field-induced single-ion magnets, SCMs, and SCM-based magnets, can be selectively prepared by anion modification.