Cooperative Iron(II) Spin Crossover Complexes with N₄O₂Coordination Sphere

Abstract

Two new spin crossover complexes [FeL(py)(2)] (1) and [FeL(DMAP)(2)] (2) with L being a tetradentate N(2)O(2)(2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O(2),O(2)'], py = pyridine and DMAP = p-dimethylaminopyridine have been investigated using temperature-dependent susceptibility and thermogravimetric and photomagnetic measurements as well as Mössbauer spectroscopy and X-ray structure analysis. Both complexes show a cooperative spin transition with an approximately 9 K wide thermal hysteresis loop in the case of 2 (T(1/2) upward arrow = 183 K and T(1/2) downward arrow = 174 K) and an approximately 2 K wide thermal hysteresis loop in the case of the pyridine diadduct 1 (T(1/2) upward arrow = 191 K and T(1/2) downward arrow = 189 K). The spin transition was additionally followed by different temperature-scanning calorimetry and Mössbauer spectroscopy for 2, and a good agreement for the transition temperatures obtained with the different methods was found. Results from X-ray structure analysis indicate that the cooperative interactions are due to elastic interactions in both compounds. They are more pronounced in the case of 2 with very short intermolecular iron-iron distances of 7.2 A and several intense C-C contacts. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bit of the equatorial ligand, from 108 degrees in the high-spin state to 90 degrees in the low-spin state. The reflectivity measurements of both compounds give at low temperature indication that at the sample surface the light-induced excited spin state trapping (LIESST) effect occurs. In bulk condition using a SQUID magnetometer the complex 2 displays some photomagnetic properties with an photoexcitation level of 60% and a T(LIESST) value of 53 K.

Complete Two‐Step Spin‐Transition in a 1D Chain Iron(II) Complex with a 110‐K Wide Intermediate Plateau

“…Upon heating, a new quadrupole split doublet appears with parameters (ΔE Q ≈ 2.0 cm -1 , δ ≈ 1.0 cm -1 ) typical for iron(II) complexes in the HS state. [20][21][22] At 280 K, the spin transition is not complete for all three compounds, in agreement with the outcomes from the susceptibility measurements. However, in the case of 1(tol), a nearly complete spin transition would be expected that is not reflected in the spectrum at 280 K. Probably, during the long measurement time (one month at 280 K), a partial loss of the included toluene occurs.…”

“…A complete list of all Tdependent Mössbauer parameters and a detailed description of the interpretation of the Mössbauer spectra is given in the Supporting Information, Tables S1-S3 and Figures S2-S4. At 80 K (1: until 150 K), the Mössbauer parameters of the three compounds are typical for octahedral iron(II) complexes of this ligand type in the LS state (ΔE Q ≈ 1.1 cm -1 , δ = 0.3-0.6 cm -1 ) [20][21][22] with no indication of a remaining HS fraction. The Mössbauer spectra of 1 and 1(temp) are very similar, both consisting of a quadrupolesplit doublet with a wide line width, which suggests two (or more) inequivalent iron centres.…”

An Iron(II) Spin‐Crossover Complex with a 70 K Wide Thermal Hysteresis Loop

“…The average bond lengths of the inner coordination sphere (2.08 (FeÀN eq ), 2.03 (FeÀO eq ), 2.22 (FeÀ N ax )) are in the region reported for similar HS iron(II) complexes-in full agreement with the results of the susceptibility measurements. [4][5][6] The observed O-Fe-O angle (1088), the so-called bite angle of the ligand, is also in the region typical for iron(II) HS complexes of this ligand type. [4][5][6] The determination of the X-ray structure of the LS state was not possible, as the crystals crumbled upon cooling.…”

“…[4] Over the last few years, we have investigated the properties of several SCO complexes with N 4 O 2 coordination spheres. [5,6] The properties of the spin transition curve correlate well with the number and intensity of the intermolecular contacts, and thermal hysteresis loops up to 18 K wide could be obtained, owing to a 3D network of short van der Waals contacts. [4] The widest hysteresis loops observed to date for a structurally characterized SCO complex are about 40 K. [7] In both examples, this hysteresis arises from p-p interactions between ligands with extended aromatic structures.…”

Ein thermisch induzierter Spinübergang an einem Eisen(II)‐Komplex mit einer 70 K breiten Hystereseschleife

“…[4] In den letzten Jahren haben wir die Eigenschaften zahlreicher SCO-Komplexe mit N 4 O 2 -Koordinationssphäre untersucht. [5,6] Dabei konnten wir feststellen, dass der Verlauf der Spinübergangskurven gut mit der Zahl und Intensität intermolekularer Kontakte korreliert. Komplexe mit einem 3D-Netzwerk aus zahlreichen kurzen Van-der-Waals-Kontakten zeigen bis zu 18 K breite Hystereseschleifen.…”