The Key Role of the Intermolecular π−π Interactions in the Presence of Spin Crossover in Neutral [Fe(abpt)₂A₂] Complexes (A = Terminal Monoanion N Ligand)

Abstract

New iron(II) complexes of formulas [Fe(abpt) 2(tcm) 2] ( 1), [Fe(abpt) 2(tcnome) 2] ( 2), and [Fe(abpt) 2(tcnoet) 2] ( 3) (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, tcm (-) = [C(CN) 3] (-) = tricyanomethanide anion; tcnome (-) = [(NC) 2CC(OCH 3)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-methoxypropenide anion; tcnoet (-) = [(NC) 2CC(OC 2H 5)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-ethoxypropenide anion) have been synthesized and characterized by infrared spectroscopy, magnetic properties and by variable-temperature single-crystal X-ray diffraction. The crystal structure determinations of 1 and 2 reveal in both cases centrosymmetric discrete iron(II) monomeric structures in which two abpt chelating ligands stand in the equatorial plane and two terminal polynitrile ligands complete the distorted octahedral environment in trans positions. For 3, the crystallographic studies revealed two polymorphs, 3- A and 3- B, exhibiting similar discrete molecular structures to those found for 1 and 2 but with different molecular arrangements. In agreement with the variable-temperature single-crystal X-ray diffraction, the magnetic susceptibility measurements, performed in the temperature range 2-400 K, showed a spin-crossover phenomenon above room temperature for complexes 1, 3- A, and 3- B with a T 1/2 of 336, 377, and 383 K, respectively, while complex 2 remains in the high-spin ground state ( S = 2) in the whole temperature range. To understand further the magnetic behaviors of 1, 3-A, and 3-B, single-crystal X-ray diffraction measurements were performed at high temperatures. The crystal structures of both polymorphs could not be obtained above 400 K because the crystals decomposed. However, single-crystal X-ray data have been collected for compound 1, which reaches the full high-spin state at lower temperatures. Its crystal structure, solved at 400 K, showed a strong modification of the iron coordination sphere (average Fe-N = 2.157(3) A vs 1.986(3) A at 293 K). In agreement with the magnetic properties. Such structural behavior is a signature of the spin-state transition from low-spin (LS) to high-spin (HS). On the basis of the intermolecular pi stacking observed for the series described in this paper and for related complexes involving similar discrete structures, we have shown that complexes displaying frontal pi stacking present spin transition such as 1, 3-A, and 3-B and those involving sideways pi stacking such as complex 2 remain in the HS state.

Investigation of the Spin Crossover Properties of Three Dinulear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type

“…A single unique helicate was present in the asymmetric unit, encompassing one high-spin and one low-spin Fe(II) centre ( Figure 5). This is confirmed by average Σ values of 90.3 and 59.4 degrees and the average Fe(II)-N distances of 2.18 and 2.00 Å for the HS and LS Fe(II) centres respectively [34,35,37], which is consistent with magnetic measurements that suggest a 53% transition to the low spin state at 100 K (χMT 7.67 cm 3 ·K·mol −1 ). The intra-helical distance in this case is 11.78 Å.…”

Structure and Electronic Configuration of an Iron(II) Complex in a LIESST State: A Pump and Probe Method

“…[13] L = 2,2'-bi-2-thiazoline (bt), X = S; [14] L = 4,6-bis(2',2''-pyridyl)pyrazine (dppt), X = S; [15] or L = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole (abpt), X = S, Se, [16] or tcnoet = 1,1,3,3-tetracyano-2-ethoxypropenide [17] ) and others such as…”

“…Both of the molecular geometries and the crystal packing at this LIESST state are very similar to those of HS-1. The p-p interactions are believed to be important for the spin-transition temperature, [12,17] as well as the LIESST phenomenon. [28] In the case of polymorph 1 A and 1 B, the p-p interactions are roughly the same except that the intralayer distances differ slightly, yet the thermal spin-transition temperature and the photorelaxation temperature TA C H T U N G T R E N N U N G (LIESST) are significantly different.…”

Photoinduced HS state in the first spin-crossover chain containing a cyanocarbanion as bridging ligand

“…In such materials, the intermolecular interactions (p-stacking, hydrogen bonding and van der Waals interactions) play a crucial role to transmit the transition. 5 Albeit, the lack of control of these non-covalent interactions hardens their organization in the crystal. To better explore the cooperative effect between the active metal sites, Kahn et al, 2 and others, 3,4 introduced an interesting and alternative approach based on the use of neutral suitable bridging ligands to covalently connect the metal centres.…”