Understanding Spin Crossover: A Contribution

Eleven iron(II) 1D coordination polymers with the general formula [FeLeq(Lax)]· solvent were synthesised and characterised, where Leq = {diethyl (E,E)-2,2′-[1,2-phenylbis(iminomethylidyne)]bis[3-oxobutanoate] (2-)-N,N′,O,O′} (L1) and {3,3′-[1,2-phenylbis(iminomethylidyne)]bis[pentane-2,4-dione] (2-)-N,N′,O,O′} (L2); Lax = 4,4′-bipyridine (bipy), 1,2-bis(4-pyridyl)ethane (bpea) and 1,3-bis(4-pyridyl)propane (bppa) and solvent = MeOH, EtOH and toluene (tol). [FeL1(bpea)]·MeOH (3·MeOH) shows an abrupt one-step spin crossover with thermal hysteresis (27 K) and [FeL2(bppa)]·MeOH (2·MeOH), [FeL2(bpea)] (4), [FeL2(bpea)]·0.25 MeOH (4·0.25 MeOH) and [FeL1(bipy)]·MeOH (5·MeOH) show a two-step spin transition with an IP at γHS ≈ 0.5 (IP is intermediate plateau and γHS is high-spin mol fraction) and up to 50 K wide hysteresis loops (5·MeOH). 4. Two-Step versus One-Step Spin Transition in Iron(II) 1D Chain Compounds 34 [FeL1(bppa)] (1), [FeL2(bppa)]·EtOH (2·EtOH) and [FeL1(bpea)] 1.5 tol (3 1.5 tol) show an abrupt incomplete spin transition that stops at γHS ≈ 0.5; for [FeL1(bppa)]·0.25 MeOH (1·0.25 MeOH) and [FeL1(bpea)] (3) the spin transition is gradual and incomplete. The X-ray crystal structures of six complexes were determined (1, 2·MeOH, 3·MeOH, 3·1.5 tol, 4·0.25 MeOH, and 5·MeOH). In the case of 4·0.25 MeOH the crystal structures for the HS and LS states were determined; for compounds 1, 2·MeOH and 3·1.5 tol the crystal structures of the HS state and at the IP were investigated. For all complexes, the iron(II) centre is located in a distorted octahedral coordination sphere. Each axially coordinated ligand “connects” two iron(II) centres, which results in the formation of extended 1D chains with varying structures from linear (bipy) over steplike (bpea) to zigzag (bppa). Analysis of the intermolecular interactions reveals that the hysteresis width depends on both the stiffness of the axial ligand and the number of intermolecular contacts, while zigzag chains support stepwise spin transitions.