Intermolecular Interaction Tuning of Spin-Crossover Iron(III) Complexes with Aromatic Counteranions

Abstract

Iron(III) spin-crossover (SCO) complexes [Fe(qsal)]BS·MeOH·HO (1), [Fe(qsal)](NS)·MeOH (2), [Fe(qnal)](NS) (3), and [Fe(qnal)]PS·MeOH·CHCl (4) (Hqsal, N-(8-quinolinyl)salicylaldimine; Hqnal, N-(8-quinolinyl)-2-hydroxy-1-naphthaldimine; BS, benzenesulfonate; NS, 1-naphthalenesulfonate; PS, 1-pyrenesulfonate) have been synthesized and characterized by X-ray structure determinations and temperature-dependent magnetic susceptibility measurements. The aromatic counteranions BS, NS, and PS can be used for the tuning of intermolecular coupling through a variety of weak interactions. All of the complexes show temperature-dependent SCO behavior. but the light-induced excited spin-state trapping (LIESST) effect was observed only for 1, 3, and 4 when the samples were illuminated (λ 808 nm) for 1 h at 5 K. In particular, 59% of the LS form of 1 was converted to the metastable HS state by illumination, equal to the highest degree of conversion yet known for LIESST in [Fe(qsal)] derivatives. The lack of a LIESST effect for 2 may be due to the relatively limited degree of interaction between the cations and anions in the lattice, reflected in a much longer minimum Fe···Fe separation in this complex in comparison to the others.

Thermal and Light‐Activated Spin Crossover in Iron(III) qnal Complexes

“…As expected, the Fe III centre has an N 4 O 2 donor set with the two qnal ligands enforcing a cis ‐configuration. The Fe–O and Fe–N bond lengths at 123 K are 1.882 and 1.979 Å, respectively, in agreement with a LS electronic configuration (Table ) , , , . As the sample is heated to room temperature, these bond lengthen by 0.012 and 0.081 Å suggesting that while significant spin crossover has taken place it is incomplete at this temperature.…”

“…The structure of 2 is similar to the published structure of [Fe(qnal) 2 ]CF 3 SO 3 · sol in which the same supramolecular sheets form via connection of the π–π dimers by pairs of C‐H ··· π interactions, although the relative position of the chains is slightly different . [Fe(qnal) 2 ][naphthalenyl‐SO 3 ] also contains π–π dimers but these are connected by P4AE interactions . However, the structure of 2 differs from [Fe(qnal) 2 ][Pd(dmit) 2 ] 5 · acetone and [Fe(qnal) 2 ][pyrene‐SO 3 ] · MeOH · CH 2 Cl 2 where supramolecular chains are formed by orthogonal π–π interactions.…”

“…The higher energy band is assigned to a π→π* transition involving the qnal ligand. The band at 450 nm, in line with the related [Fe(qsal‐X) 2 ] + complexes,, , is most probably a ligand‐to‐metal charge‐transfer band (LMCT). It is notable when comparing these complexes with [Fe(qsal‐X) 2 ] + that the LMCT band is more intense probably due to the naphthalene moiety of the qnal ligand.…”

“…The weak bands around 3040–3057 cm –1 are aromatic C–H stretches. The complexes show a strong intensity C=N stretch for the imine of the qnal ligand at 1613–1614 cm –1 , . The N–O stretch from the nitrate is found around 1380 cm –1 , while the C≡N stretch from the NCS – anion is at 2065 cm –1 .…”

Iron(II) and Iron(III) Complexes of Tridentate NNO Schiff Base‐like Ligands – X‐ray Structures and Magnetic Properties

“…In the case of iron(II) the most frequently observed donor atom set is N 6 , and although there are examples with an N 4 O 2 , or N 4 S 2 donor atom set, they are comparatively rare. In the case of iron(III) complexes, spin crossover is more frequently observed for systems with an N 4 O 2 coordination sphere, which in their vast majority use Schiff base ligands , . So far, only few examples are reported where both, the iron(II) and iron(III) with the same ligand set are synthesized , …”