An Unprecedented Trinuclear Structure Involving Two High-Spin and One Spin-Crossover Iron(II) Centers

Anaerobic reaction of ferrous thiocyanate with the deprotonated form of the pentadentate dinucleating Schiff base
1,3-bis[(2-pyridylmethyl)imino]propan-2-ol (LH) yields the novel trinuclear [Fe3L2(NCS)4(H2O)] species 1. LH results
from the bis-condensation of 2-acetyl-pyridine with 1,3-diaminopropan-2-ol and includes an N4O donor set. The
X-ray crystal structure of 1 [C38H40N12O3S4Fe3, triclinic, space group P-1; a = 10.7730(10) Å, b = 12.2048(14) Å,
c = 19.0559(19) Å, α = 76.908(12)°, β = 89.106(12)°, γ = 79.637(12)°, V = 2399.8(4) Å3] can be described
either as a bent linear arrangement of ferrous centers pairwise bridged through the alkoxo oxygen atom of L- or
as a triangular FeII3 core with an Fe2−SCN−Fe3 bridge as the longer side of the Fe1−Fe2−Fe3 triangle. The
metric parameters characterizing the ligand environments of the three ferrous centers in 1 and its Mössbauer
spectra show that this unprecedented trinuclear structure involves two high-spin (Fe2 and Fe3) and one spin-crossover (Fe1) FeII centers. The donor set to the spin-crossover center (Fe1) is unprecedented:  two Npyridine, two
Nimine, and two Oalkoxo. Weak antiferromagnetic interactions transmitted through the end-to-end NCS bridge and/or
through the O1−Fe1−O2 bridge operate between Fe2 and Fe3.