Spin crossover in mixed ligand iron(III) complexes

Mixed ligand complexes [Fe(qsal-Cl)(qsal-Br)] Y.sol (Y = NCS, sol = MeOH 1; PF6 2; BPh4, sol = 2CH(2)Cl(2) 3; OTf, sol = 0.5MeOH 4) have been synthesized. X-ray crystallographic studies of 1, 2 and 4 at 123 K reveal that the Fe(III) centres are low spin in all cases. In 2 there are two independent Fe(III) centres in the asymmetric unit cell. The structure of 3 has been determined at 100, 123 and 269 K with the Fe centre low spin, mixed spin and mostly high spin, respectively. Packing in the complexes is dominated by pi-pi interactions which form a 1D chain. C-H center dot center dot center dot pi, C-H center dot center dot center dot Cl/ Br and C-Cl/Br center dot center dot center dot pi interactions link the chains to form highly cooperative supramolecular 3D networks mirroring the homoleptic complexes. SQUID magneto-metric studies show almost complete, 50% complete and stepped spin crossover for 1, 2 and 3 respectively. [Fe(qsal-Cl)(qsal-Br)] OTf.0.5MeOH 4 is low spin up to 350 K. Compared to the homoleptic complexes spin crossover behaviour is mostly intermediate between the [Fe(qsal-Cl)(2)](+) and [Fe(qsal-Br)(2)](+) systems except for 3 where spin crossover characteristics are improved compared to the homoleptic complexes.