Two-Step versus One-Step Spin Transitions in Iron(II) 1D Chain Compounds

Eleven iron(II) 1D coordination polymers with the general formula [FeLeq(L-ax)]center dot solvent were synthesised and characterised, where L-eq = (E,E)-{diethyl 2,2'-[1,2-phenylenebis-(iminomethylidyne)]bis(3-oxobutanato) (2-)-N,N',O-3,O-3'} (L1) and {[3,3']-[1,2-phenylenebis(iminomethylidyne)]bis-(2,4-pentane-dionato)(2-)-N,N',O-2,O-2'} (L2); L-ax = 4,4'-bi-pyridine (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)]center dot MeOH (3 center dot MeOH) shows an abrupt one-step spin crossover with thermal hysteresis (27 K) and [FeL2(bppa)]center dot MeOH (2 center dot MeOH), [FeL2(bpea)] (4), [FeL2(bpea)]center dot 0.25MeOH (4 center dot 0.25MeOH) and [FeL1(bipy)]center dot MeOH (5 center dot MeOH) show a two-step spin transition with an IP at gamma(HS) approximate to 0.5 (IP is intermediate plateau and gamma(HS) is high-spin mol fraction) and up to 50 K wide hysteresis loops (5 center dot MeOH). [FeL1(bppa)] (1), [FeL2(bppa)]center dot EtOH (2 center dot EtOH) and [FeL1(bpea)]center dot 1.5Tol (3 center dot 1.5Tol) show an abrupt incomplete spin transition that stops at gamma(HS) approximate to 0.5; for [FeL1(bppa)]center dot 0.25MeOH (1 center dot 0.25MeOH) and [FeL1(bpea)] (3), the spin transition is gradual and incomplete. The X-ray crystal structures of six complexes were determined (1, 2 center dot MeOH, 3 center dot MeOH, 3 center dot 1.5Tol, 4 center dot 0.25MeOH and 5 center dot MeOH). In the case of 4 center dot 0.25MeOH, the crystal structures for the HS and LS states were dertermined; for compounds 1, 2 center dot MeOH and 3 center dot 1.5Tol, the crystal structure of the HS and the IP state was 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.