Charge-transfer two-dimensional layers constructed from a 2 : 1 assembly of paddlewheel diruthenium(II,II) complexes and bis[1,2,5]thiadizolotetracyanoquinodimethane: bulk magnetic behavior as a function of inter-layer interactions

Abstract

Two new charge-transfer compounds, [{Ru₂(O₂CPh-x-F)₄}₂(BTDA-TCNQ)]·n(solv) (x-F-PhCO₂⁻ = x-fluorobenzoate; x = o-, 4; p-, 5; BTDA-TCNQ = bis[1,2,5]thiadizolotetracyanoquinodimethane), have been prepared from reactions of paddlewheel diruthenium(II, II) complexes, [Ru₂^II,II(O₂CPh-x-F)₄] (x = o-, p-) and BTDA-TCNQ. Compounds 4 and 5 crystallize as two-dimensional (2-D) layered structures composed of [Ru₂] units and BTDA-TCNQ in a 2 : 1 ratio. The isomer with the m-fluorobenzoate-bridged [Ru₂] unit (3) exhibits a three-dimensional (3-D) structure. The oxidation states of the constituent units were evaluated from their crystal structures, and assigned to be close to the charge-polarized state represented formally as [Ru₂⁵⁺]–[BTDA-TCNQ^˙−]–[Ru₂⁴⁺] including two types of [Ru₂] units that are electronically and structurally different and a one-electron-transferred BTDA-TCNQ radical anion. The data indicate that a 1-e⁻ transfer from one site of [Ru₂] units to BTDA-TCNQ has likely occurred. The units are paramagnetic with heterospin states for [Ru₂^II,II] (S = 1), [Ru₂^II,III] (S = 3/2), and BTDA-TCNQ^˙− (S = 1/2), which lead to magnetic ordering in the layer. In the case of 4, antiferromagnetic ordering occurs at T_N = 93 K due to inter-layer antiferromagnetic interactions which involve two steps, namely canting of the ordered spins at T_c1 = 87 K followed by rearrangement of the canted spin states at T_c2 = 13 K. Compound 5 exhibits 3-D ferromagnetic ordering at T_c = 83 K. Consequently, 4 and 5 behave as magnets although the origin of their spontaneous magnetization is different. The bulk magnetic properties of 4 and 5 are in contrast to what was observed in a similar layered compound [{Ru₂(O₂CCF₃)₄}₂TCNQF₄] (2; TCNQF₄ = tetrafluoro-7,7,8,8-tetracyanoquino-dimethane). This compound is an antiferromagnet (at H = 0) with T_N = 95 K with the behavior being strongly dependent on the environment between the layers: in particular the stacking motif can either be “in-registry” with an overlap between analogous groups (for 2) or “out-of-registry” which involves a misalignment of similar groups between layers (for 4 and 5).