Nuclearity controlled cyanide-bridged bimetallic CrIII-MnII compounds:: Synthesis, crystal structures, magnetic properties and theoretical calculations

The preparation, X-ray crystallography and magnetic investigation of the compounds PPh4- [Cr(bipy)(CN)(4)].2CH(3)CN.H2O (1) (mononuclear), [{Cr(bipy)(CN)(4)}(2)Mn(H2O)(4)].4H(2)O (2) (trinuclear), [{Cr(bipy)(CN)(4)}(2)Mn(H2O)(2)] (3) (chain) and [{Cr(bipy)(CN)(4)}(2)Mn(H2O)].H2O.CH3CN (4) (double chain) [bipy=2,2'-bipyridine; PPh4+ = tetraphenylphosphonium] are described herein. The [Cr(bipy)(CN)(4)](-) unit act either as a monodentate (2) or bis-monodentate (3) ligand toward the manganese atom through one (2) or two (3) of its four cyanide groups. The manganese atom is six-coordinate with two (2) or four (3) cyanide nitrogens and four (2) or two (3) water molecules building a distorted octahedral environment. In 4, two chains of 3 are pillared through interchain Mn-N-C-Cr links which replace one of the two trans-coordinated water molecules at the manganese atom to afford a double chain structure where bis- and tris-monodenate coordination modes of [Cr(bipy)(CN)(4)](-) coexist. The magnetic properties of 1-4 were investigated in the temperature range 1.9-300 K. A Curie law behaviour for a magnetically isolated spin quartet is observed for 1. A significant antiferromagnetic interaction between Cr-III and Mn-II through the single cyanide bridge [J = -6.2 cm(-1), the Hamiltonian being defined as I = -J((S) over cap (Cr(1)).(S) over cap (Mn)+(S) over cap (Cr(2)).(S) over cap (Mn))] occurs in 2 leading to a low-lying spin doublet which is fully populated at T < 5 K. A metamagnetic behaviour is observed for 3 and 4 [the values of the critical field H-c being ca. 3000 (3) and 1500 Oe (4)] which is associated to the occurrence of weak interchain antiferromagnetic interactions between ferrimagnetic (Cr2MnII)-Mn-III chains. The analysis of the exchange pathways in 2-4 through DFT type calculations together with the magnetic bevaviour simulation using the quantum Monte Carlo methodology provided a good understanding of their magnetic properties.