We report 15 new Cu(ii) complexes with tridentate NNO β-acylenamino ligands derived from 2-picolylamine and bearing up to three alkyl, alkoxy, alkoxycarbonyl, or (pseudo)halide substituents.

Cooperativity among spin centres has long been the royal road to impose magnetic bistability in terms of thermal hysteresis. In this work we access magnetic multi-metastability of the iron(III) complex [Fe(L)2][BPh4] (1) at low temperature, in addition to thermal bistability. The packing of the low-spin and high-spin forms of crystalline 1 differs only marginally what ultimately leads to very minor thermal variation in the lattice constants. This indicates that the SCO-immanent breathing of the complex cation is almost fully compensated by the anion matrix. We believe that this structural conservatism is the origin of the unique cooling-rate dependence of the residual low-temperature magnetisation in 1. The system state of 1 can be continuously tuned between the trapped high-spin (ON) and the relaxed low-spin state (OFF), as a simple function of the cooling rate. That is, cooperative spin crossover can be the source of bistable and multi-metastable system states in the very same material.

Iron(II) complexes of meridional coordinating tridentate ligands have served as a very fruitful playground for spin crossover (SCO) research. In this work, we provide modular synthetic access to new meridional NN′O ligands derived from 8-aminoquinoline which vary the established Jäger-type planar N 2 O 2 motif. Solid-state spin crossover properties of three homoleptic iron(II) complexes 1–3 with the general formula [Fe(L x ) 2 ] (x denotes the remote substitution of the ligand: CH3/CH3 in HL 1 , CH3/CF3 in HL 2 , and CF3/CF3 in HL 3 ) were probed via variable temperature single-crystal X-ray diffraction, SQUID magnetometry, and 57Fe Mössbauer spectroscopy. Subtle tuning of their substitution pattern at the outer sphere of the chelate ring translates into qualitatively different SCO profiles. The transition from dynamic SCO for 1 (T TIESST = 97 K; T 1/2↑ = 124 K) to abrupt SCO with hysteresis for 2 (T 1/2↓ = 132 K; T 1/2↑ = 137 K) and continuous SCO for 3 (T 1/2 = 280 K) indicates a successive loss of cooperativity, going parallel with the exchange of CH3 for CF3 groups. The decrease of cooperativity in SCO can be traced to ever-increasing steric bulk in the crystal, which reflects in locally varied interaction pattern, particularly in fully fluorinated 3. X-ray analysis revealed an isostructural lattice for 1 and 2 (monoclinic) but lower symmetry for 3 (triclinic) because of peculiarities in the crystal packing.

In this study, a synthesis route of tri(quinolin-8yl)amine (L), a recent member of the tetradentate tris(2pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ 4 mode, two cisoriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations�bis(triflato), bis(acetonitrile), and mixed coligand species�could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mossbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time-and temperature-resolved UV/vis spectroscopy, Mossbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between highand low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)] 2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment�all of the {N 6 }-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

Dedicated to Professor Josef Breu on the Occasion of his 60 th BirthdayA Schiff base-like ligand H 2 L TerPh featuring a para-terphenyl backbone was synthesized and converted in two steps to an octahedral iron(II) coordination polymer with 1,2-bis-(4-pyridyl)ethylene as bridging ligand. Single crystal X-ray structure analysis was possible for the free ligand and the two octahedral iron(II) complexes. The Schiff base-like ligand features a [N 2 O 2 ] 2À coordination sphere for the metal center and the paraterphenyl backbone introduces intrinsic steric constraint. Char-acterization of the coordination polymer with an N 4 O 2 coordination sphere around the iron center via magnetic measurements (SQUID) and room temperature Mössbauer spectroscopy revealed HS character in the entire temperature range investigated. Structure modelling with DFT calculations supports the findings, but support also the possibility of spin crossover in solution.